nuclear science and technology

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1 European Commission nuclear science and technology ECOCLAY II Effects of Cement on Clay Barrier Performance Phase II Agence nationale pour la gestion des déchets radioactifs ANDRA (FR) Bureau de recherches géologiques et minières BRGM (FR) Empresa Nacional de Residuos Radiactivos ENRESA (ES) Universidad Autónoma de Madrid UAM (ES) Consejo Superior de Investigaciones Científicas CSIC (ES) Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbh (DE) Nationale Genossenschaft für die Lagerung radioaktiver Abfälle NAGRA (CH) Paul Scherrer Institut PSI (CH) University of Bern (Uni BE) (CH) Serco Assurance (Serco) (UK) Studiecentrum voor Kernenergie Centre d'étude de l'énergie nucléaire - SCK CEN (BE) Svensk Kärnbränslehantering AB SKB (SE) POSIVA (FI) University of Helsinki HU (FI) Technical Research Centre of Finland VTT (FI) Natural Environment Research Council NERC (UK) Clay Technology CT (SE) Contract N o FIKW-CT Final report Work performed as part of the European Atomic Energy Community's R&T Specific Programme Nuclear Energy, Key Action: Nuclear Fission Safety Area: Safety of the Fuel Cycle Directorate-General for Research 2005 Euratom EUR 21921

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3 FOREWORD In 1999, as the project ECOCLAY I ended (see report EUR EN), the European Commission published its calls for proposals for the 5 th Framework Programme. ENRESA and Andra who wished to build together a second project on cement - clay interactions heard that others countries could be interested in joining a new proposal on this same subject. In a few months, the basis for the ECOCLAY II project were written and submitted to the European Commission. The ECOCLAY II project was accepted in June 2000 and officially started on October 1 st It lasted 3 years. During this time, 18 participants from 8 countries in Europe had the opportunity to work on many aspects related to cement degradation, clay alteration and performance assessment, to compare their results, to discuss them and finally to propose an assessement on the effects of an alkaline plume on clay properties. This document is the sum of their contributions and constitue the final report of the ECOCLAY II project, co-financed by the European Commission under the contract FIKW- CT It covers the period October 1 st, 2000 to September 31 th, It aims to summurize the main results of the project. As the participants wished it, the report is public. Moreover, detail articles are already published and other will follow that should complete this report, for the best. i

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5 TABLE OF CONTENTS FOREWORD...I TABLE OF CONTENTS... III EXECUTIVE SUMMARY... 1 OBJECTIVES AND STRATEGIC ASPECTS ISSUES INVOLVED AND STATE-OF-THE-ART OBJECTIVES OF THE PROJECT INNOVATION CONTRIBUTION TO PROGRAMME OBJECTIVE OBJECTIVE OF THE PROJECT - WORK PLAN SCIENTIFIC AND TECHNICAL DESCRIPTION OF THE RESULTS WORK PACKAGE 1: RETENTION Introduction Sorption Uptake of strontium, europium and curium by CSH phases in an alkaline disturbed far-field Sorption of I and Se by batch experiments on altered clay/cement systems Influence of saline groundwaters on the sorption properties of bentonite under alkaline conditions Migration Influence of alkaline conditions on diffusion of tracers in bentonite and Callovo-Oxfordian Clay Percolation and diffusion experiments on Boom Clay exposed to an alkaline perturbation Conclusions of WP1 on Retention iii

6 6.5 References of Work Package WORK PACKAGE 2: CLAY ENGINEERED BARRIER Modelling of cement / bentonite interactions Introduction Cascade experiments Swelling pressure of MX-80 in contact to high saline and alkaline solutions Conclusion Batch experiments: results on the bentonite MX Introduction Batch experiments Batch experiments : interpretation of randomly-oriented power XRD patterns Other data Conclusion Effects of an alkaline plume on the hydraulic and hydro-mechanical properties of the bentonite MX Introduction Experimental strategy The experimental apparatus and protocol Hydro-mechanical parameters Results Conclusions Geochemical reactions in FEBEX bentonite Introduction Geochemical reactions experiments Conclusions The development of the optimal concrete composition compatible with bentonite stability Objectives Work plan Dissolution precipitation phenomena of cement pastes. Acid Neutralisation Test (ANT) Permeability tests Permeability tests in the system concrete-bentonite Migration Tests General conclusions Dissolution kinetics of bentonite under alkaline conditions Introduction Materials Methods Results and discussion Conclusions iv

7 7.7 Laboratory experiments concerning compacted bentonite contacted to high ph solutions Introduction and Objectives Experimental Measurements and Analyses Results Discussion Conclusions Acknowledgments References of Work Package Appendices WORK PACKAGE 3: THE EFFECT OF ALKALINE FLUID ON CLAY HOST ROCKS Introduction The effect of high-ph cement pore waters on the mineral stability of Callovo-Oxfordian Clay Materials and Methods Results and discussion Conclusions The effect of high-ph cement pore waters on the hydrolic and hydromechanical properties of Callovo-Oxfordian Clay Objectives Hydro-Mechanical Parameters Method Results Conclusions Mass balance estimate of cement clay stone interaction with application to a HLW repository in Opalinus Clay Objectives Approach Simple model for cement degradation Simple model for the buffering capacity of Opalinus Clay Mass balance calculations Extent of rock alteration Assessment of results and conclusions Implications for performance assessment A long-term in-situ experiment for interaction of Opalinus Clay with hyperalkaline fluid at the Mont Terri URL (Switzerland) Objectives Summary of the CW experiment Overcoring of the CW experiment and sample analysis Reactive transport modelling interpretation Assessment of results and discussion Acknowledgements v

8 8.6 References of Work Package WORK PACKAGE 4: CRYSTALLINE HOST ROCK Introduction Flow-through experiments Setup and experimental Modelling simulation of the diffusion column Sampling of solids Summary of results Discussion and conclusions Mineralogical summary Cushed rock alterations MX-80 alterations References of Work Package WORK PACKAGE 5: MODELLING Modelling of Column Experiments on the InTeraction of Alkaline Fluids with Crushed Rocks Introduction Review and Qualitative Evaluation of Experimental Data Numerical Simulation of Column Experiments: Illustrative Results Uncertainties Conclusions Modelling of Experiments on the Percolation of Cement Leachates through Boom Clay Introduction Modelling Approaches PRECIP Modelling CRUNCH modelling Conclusions Geochemical processes - Modelling Experiments on the Effects of saline hyperalkaline attack on bentonite and crushed rock Introduction Predictive modelling of bentonite batches Bentonite batches - inverse mineralogical constraints Crushed rock batches - Inverse mineralogical constraints Conclusions Modelling diffusion of an alkaline plume in two types of clayey systems Introduction Construction of the model MX80: Results Callovian-Oxfordian Formation: Results Test of diffusion coefficients between 10-9 and m 2 /s vi

9 Impact of the link between number of moles of clay and number of exchange sites Conclusion Geochemical Modelling of Diffusion of an Alkaline Plume in Compacted Bentonite Introduction Experiments Evaluation of diffusivities Evaluation of swelling pressure Development of a conceptual model for alkaline attack on montmorillonite Quantification of the ph evolution in a bentonite barrier Conclusions Modelling of cement / bentonite interactions Introduction Modelling of Cement and Clay Degradation Modelling of Cement Degradation Modelling of cement / bentonite interactions Reactive transport model of cement clay stone interaction with application to a HLW repository in Opalinus Clay Summary Introduction Objectives Approach Model for cement degradation Model for Opalinus Clay Modelling of cement clay stone interaction Application to a HLW repository in Opalinus Clay Assessment of results and conclusions Implications for performance assessment Acknowledgements References of Work Package WORK PACKAGE 6: SUMMARY AND PERFORMANCE ASSESSMENT Introduction Summary of Repository concepts and alkaline plume related phenomena Repository concepts and Use of cement/concrete and clay in the repository Predicted conditions across concrete-clay/rock interfaces Possible impacts of alkaline water-clay/rock interactions Data and models Experimental data Modelling vii

10 11.4 Safety issues Introduction Possible scenarios of alkaline plume evolution Key issues for safety assessment derived from scenarios Examples of safety assessment cases for an alkaline plume spreading in fractured and diffusion dominated host rocks Recommendations and open issues Conclusions Acknowledgements References of Work Package ASSESSMENTS OF THE RESULTS AND CONCLUSIONS ACKNOWLEDGEMENTS LIST OF FIGURES LIST OF TABLES viii

11 EXECUTIVE SUMMARY

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13 Cements will be used intensively in radioactive waste repositories. During their degradation in time, in contact with geological pore water, they will release hyperalkaline fluids. This will induce transformations that will modify the properties of the geological media and the engineered barriers made of clay (EBS). In this context, waste management agencies and research organisms built a consortium around a European project in order to better understand the effects of cement on barrier performance. During the European project ECOCLAY I (4th FP), major geochemical reactions related to an interface between cement and the clay of the EBS were identified. ECOCLAY II proposed to further investigate these aspects with an aim of supplementing the undertaken studies. In this context, the main objectives of the ECOCLAY II were: The comprehension of the physicochemical behaviour of argillaceous and granitic materials altered by the effects of an hyperalkaline plume The assessment of the evolution of the confinement performances of these materials. The ECOCLAY II project consisted of: (1) a series of laboratory tests and analysis. Data were acquired on sorption and migration of radionuclides (WP1), on mineralogical transformations and modifications to properties of the bentonitic engineered barriers (WP2), of clay host rocks (WP3), and of crystalline host rocks (WP4) in contact with either alcaline fluids or different kinds of cement. (2) a hydro-geochemical modelling task (WP5). It sought definition and optimisation of a model for the hyperalkaline alteration. Simulation of the long-term behaviour of the clay and crystalline materials under the effect of an hyperalkaline plume were obtained. (3) a performance assessment work (WP6) based upon a critical review of the experimental knowledge base and the numerical results and a reflection on the relevant mechanisms to be taken into account in the performance assessment of the confinement barriers. In the Workpackage 1, the participants (CEA as ANDRA s subcontractor, PSI, GRS, SCK.CEN and the University of Helsinki) studied the sorption and transfer properties of radionuclides and alkaline ions in materials, such as bentonite (engineered barrier material; e.g. MX-80 from the Wyoming) and clay host rocks (e.g. Boom clay) under alkaline conditions. The co-precipitation of radionuclides with CSH was also investigated. The organisations contributing to Workpackage 2 (ERM, HydrASA and Eurogéomat Consulting as ANDRA s subcontractors, ENRESA, UAM, CSIC-IETcc, CSIC-EEZ, GRS, SKB) investigated the evolution of the clay engineered barrier under alkaline conditions. The work package was divided in two tasks. In the first task studies of geochemical processes in a bentonite engineered barrier under hyperalkaline conditions were performed. Therefore the quantification, time evolution and spatial progression of the mineralogical reaction including the kinetic had been investigated in batch and transport cell tests. Influences of concrete (CEM I and CAC with different additives, bentonite (FEBEX and MX-80) and solution 1

14 composition were examined. The second task dealt with the study of the consequences on bulk properties (hydro-mechanical properties) of geochemical processes. The potential host rocks of two countries were subject of investigation in Workpackage 3 (ERM, HydrASA and Eurogéomat Consulting as ANDRA s subcontractors, NAGRA and the University of Bern): the Callovo-Oxfordian argilite formation of the Bure site and the Opalinus Clay of the Mont Terri Underground Rock Laboratory. On the argilite, two types of studies were performed. Batch experiments carried out by ERM & HYDRASA covered the investigation of high-ph pore water effects on the mineral stability of the clay. Thermodynamic calculations and a kinetic evaluation supplemented these experiments. The experiments by Euro-Geomat Consulting, studied the effect of cement water on the hydraulic and hydromechanical properties of Callovo-Oxfordian argilites. Two macroscopic parameters, the permeability and the Biot coefficient, were determined in order to describe either transport processes in the host rock or the connectivity of the porous media. Two studies are reported for Opalinus Clay. A simple mass balance model for degradation of cement and the buffering capacity of clay stone has been built. The model is applied to a planned Swiss ILW repository in Opalinus Clay and predicts a negligible extent of rock alteration due to the high-ph plume emanating from cement degradation. A long-term ( ) field experiment is reported which was conducted at the Mont Terri Underground Rock Laboratory and was financed by a consortium consisting of ANDRA, NAGRA, and OBAYASHI Corp. Callovo-Oxfordian Argilite and Opalinus Clay are very similar concerning their mineral composition and hydraulic/hydro mechanical properties. In Workpackage 4, POSIVA and VTT focused their attention on the crystalline bedrock were the salinity of groundwater often increases with depth. The studied solid materials were MX-80 bentonite and site-specific crushed rock powder (Olkiluoto site, Finland). Sorption of Na and Ca on both materials was also studied in batch experiments. The objectives were to gain results on alterations of bentonite and crushed rock, to identify geochemical reactions caused by the saline alkaline water and also to evaluate the propagation of the alkaline plume by detected chemical changes. The organisations making technical contributions to Workpackage 5 on modelling were BRGM, GRS, Serco Assurance, BGS, Kemakta (subcontractor to SKB), VTT, University of Bern, and SCK.CEN. The modelling work has concerned the interaction of alkaline plumes with clays (bentonite buffer material and argillaceous host rocks) and crystalline rocks, and the degradation of cements to produce the alkaline plume. Simultaneous modelling of cement degradation and the migration of the alkaline plume into an adjacent argillaceous rock has also been carried out. The modelling has been directed both at the interpretation of experiments, most of which have been carried out as part of the project, and at the prediction of field-scale migration of an alkaline plume through bentonite buffers and clay host rocks. A variety of modelling tools was used (PHREEQC, CRUNCH, EQ3/6, PRECIP, Hydrogeochem, MINTEQ and Medusa). The variety of programs used to obtain rather similar conclusions gives confidence in the performance (verification) of these tools. The work reported to simulate the results of experiments on such systems gives confidence in current capabilities to identify the main processes contributing to the behaviour of the system and to provide an outline interpretation of that behaviour. An important conclusion, 2

15 from those studies directed at field scale modelling of the penetration of an alkaline plume into a clay buffer or host rock in a diffusion dominated system, is that the distance the plume will penetrate into the clay is very small, and not significant as far as the performance of the clay barrier is concerned. This conclusion appears to be robust to uncertainty in the modelling. The organisations contributing to Workpackage 6 on performance assessment during the course of the project were ANDRA, ENRESA, POSIVA, GRS and SERCO; although all the participants were asked to collaborate in the work. A review of all the information available on cement/clay and granite interactions and disposal designs was made. Moreover, evolution scenarios for three rocks/concepts and a list of key safety issues to be addressed in a performance assessment were built. The partners of ECOCLAY II have planned to use the results as follows: Specification for disposal design: For national waste agencies, the results will contribute to the elaboration of the design of their national repository. Site prospecting and characterisation: Countries, which have already an underground laboratory, will exploit the results to improve their knowledge. For the others, results will help to define criteria of choice of the most appropriated sites and the design of future in situ experiments. Scientific data on the hyperalkaline plume: The research organisations will exploit the results to increase their knowledge on granite or clay materials. The results will be compiled with those already available in order to develop their conceptual model. Numerical models: The model users will send to the people in charge of the development their conclusions on the calculations. Developers will have an opportunity to improve their softwares for future applications. On the international scene, the results will be exposed, by the consortium members in publications and congresses on the nuclear waste management or in Earth Sciences. By this mean, the results will be subjected to an international external review. All the participants gratefully acknowledge the support of the European Commission for this project. 3

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17 OBJECTIVES AND STRATEGIC ASPECTS 5

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19 1 ISSUES INVOLVED AND STATE-OF-THE-ART All the national concepts of radioactive waste disposal in geological formations consider the use of concrete and other cement based material, in very significant quantities, either as structural, or as confinement materials (engineered barriers, sealings, backfilling materials...). Some European concepts consider volumes of no less than m 3. Concretes have specific chemical compositions. These materials are far from equilibrium with natural media. They will be altered gradually and release in solution significant quantities of ions (mainly OH -, K +, Na + and Ca 2+ ). The resulting porewater has a high ph, ranging between 10 and The propagation of this alkaline fluid into the other repository materials, the artificial barriers (bentonite) or the geological medium (clay or granite host rocks), gives rise to a phenomenon called the hyperalkaline plume. This phenomenon, which can last a very long time (up to thousands of years), is likely to cause a range of physicochemical transformations that can modify the radionuclides confinement properties of the disposal components. The main part of the research on the hyperalkaline plume for more than ten years has been undertaken by the European countries which are concerned with the disposal of nuclear wastes, as well as by Japan. The purpose of the experiments until now were: to work out a methodology, experimental protocols etc, in order to identify the sequence of the reactive phases (dissolved or neoformed) and the mechanisms of the reactions involving these phases. It was, for example, one of the main objectives of European project ECOCLAY I, with regard to cement / bentonite interactions to possibly bring a qualitative answer to the evolution of the porosity and the permeability of the argillaceous or granitic matrix and its radionuclide sorption properties. Among the most significant processes raised include the evolution from a "young" cementitious solution (ph=13.5) to "advanced" (ph 10-11), the dissolution of bentonites and clay host rock, neo-formation of zeolites, illites, or calcium silicate hydrates (CSH), the immobilisation of the radionuclides by sorption, precipitation or co-precipitation, the migration of the alkaline front in the argillaceous matrix. Today, in order to be able to assess the evolution of the confinement performance of waste disposals due to alkaline plume, there is a need to: - characterise these reactions by specific constants (thermodynamic and kinetic) - quantify their effects on the confinement properties of the materials - seek specifications to reduce the intensity of the disturbance or to limit its effects of them 7

20 - determine the space and temporal scales of propagation of the hyperalkaline plume - make reliable the conceptual and numerical models of long-term behaviour of materials subjected to alteration ECOCLAY II will address these problems. Within the framework of the disposal design, and the safety studies, it is thus significant not only to know the mechanisms which govern these reactions, but also the effects and consequences they produce, from the scale of the materials, to that of the storage facilities, and then the geological medium. 8

21 2 OBJECTIVES OF THE PROJECT The main objectives of the ECOCLAY II project were: the comprehension of the physicochemical behaviour of argillaceous and granitic materials altered by the effects of an hyperalkaline plume the assessment of the evolution of the confinement performances of these materials. These objectives were broken up as follows: Constitution of an experimental knowledge base: This experimental knowledge base collects experimental parameters that constitute either input data set, or validation data for the models. This includes the experimental parameters related to: - the sorption, co-precipitation and migration of the radionuclides and the alkaline and alkaline-earth ions under hyperalkaline conditions - the thermodynamics and the kinetics of the reactions - the nature and the description of the reaction mechanisms (reactive and neoformed phases) - the evolution of the confinement properties of materials, in time and space, at laboratory scale and in situ - the nature of argillaceous materials (bentonite or clay host rock) The criterion for success was the effective availability of such a base that has been completed in different steps. Definition and optimisation of a model for the hyperalkaline alteration It included: - Sizing of some geochemical or hydraulic experiments based on the current knowledge. - Development of a conceptual model of hyperalkaline alteration in clay and granite host rock starting from the knowledge base brought up to date within the framework of the project. 9

22 - Numerical simulation of the alteration starting from the conceptual model and the existing software - Agreement between the conceptual models and the available numerical codes. - Choice of the best adapted model and code to the representation of the alteration The criterion for success was the agreement between numerical simulation and experimental data, and consensus of partners Numerical simulation of the long-term behaviour of the clay and crystalline materials under the effect of a hyperalkaline plume (extrapolation) was obtained by extrapolation. Performance Assessment of confinement barriers It has been based upon a critical review of the experimental knowledge base and the numerical results, and a ranking the mechanisms of alteration according to the performance evolution. At this stage, the criterion for success was the achievement of an international consensus on the choice of the most relevant mechanisms On this ground, a performance analysis of the materials of confinement barriers during deterioration, independently of the national concepts, was achieved. 10

23 3 INNOVATION The ECOCLAY II project proposed two levels of innovation: At a scientific level (experimentation and modelling): On the basis of techniques and protocols controlled by the various participants, the knowledge base on the phenomena related to the hyperalkaline plume was supplemented by means of new experiments in the field of nuclear wastes disposal. A sensitivity study was undertaken on the available data in order to evaluate the importance of uncertainties and the means of resolving it. The evaluation of existing conceptual and numerical models contributed to the study of the cement/minerals interactions, and more largely to the comprehension of the water / rocks interactions. At a methodological level: The individual members of the partnership gave access to the data of complementary experiments, and the synergy of the partnership reinforced the scientific credibility of the results which form the foundations for the performance assessment. Most of the national, European or international projects on radioactive waste management are very specific: data acquisition or modelling or performance assessment. In ECOCLAY II, the principal innovation consisted in integrating in the same project, persons responsible for these various phases and who were at the origin of the development of the international consensus around the taking into account of the hyperalkaline alteration in the models of performance. This constituted moreover, one of the principal objectives of the project. In fact, the ECOCLAY II project proposed to carry out the transition between a microscopic study of the phenomena and a macroscopic appreciation of their effects. This significant scaling was supplemented by an opening of the reactive systems, i.e. the coupling between geochemical effects and transport phenomena. This coupling was carried out at an experimental level, by the use of experimental settings where the hyperalkaline fluid circulated, and at the numerical level, by the use of coupled geochemical/transport codes. The combined resources and experience of the partners in ECOCLAY II made these goals realistic. 11

24 4 CONTRIBUTION TO PROGRAMME OBJECTIVE The European project ECOCLAY II was a contribution to the objectives of the Key action 2 Nuclear Fission Safety of the fuel cycle - of the specific programme Research and training programme (EURATOM). It gathers European organisations whose common concern is to ensure the safety and the effectiveness of the management and the final storage of the radioactive waste. Their first objective is either to ensure a mission of public interest: the management and storage of wastes, like the national radioactive waste agencies; or to study the principal scientific and technical problems related to these wastes, like the research organisations and companies. This consortium brought the expertise for the ECOCLAY II project to answer the wide aims of the research and development framework program of the European Community. To study practical solutions to the outstanding scientific and technical problems The activities envisaged within the framework of ECOCLAY II aimed at improving the scientific and technical understanding related to the hyperalkaline plume, and developing methods and techniques of properties characterisation of the geological sites, of construction of the disposal facilities (engineered barriers, sealing, backfilling...). The goal was to demonstrate that the hyperalkaline plume could be incorporated in a realistic yet conservative way in performance assessment calculations. In practice, the project allowed: - to study in laboratory and in situ (at Underground Research Laboratories) the processes modifying the behaviour of the barriers and the migration of the radionuclides released under hyperalkaline conditions (Work packages 1 to 4). - to encourage the creation and the development of a geochemical and hydraulical data base starting from the interpretation of the experimental results (Work packages 1 to 4). - to study the impact on the environment of the diffusion of the cementitious fluids (Work packages 1 to 4). - to model the geochemical and hydromechanical long-term behaviour of the engineered and natural barriers under representative conditions (Work package 5). By improving the sound basis of these particular problems, ECOCLAY II contributed to: - The reinforcement of the scientific foundations for the safety assessment of nuclear waste repository sites - The demonstration of the technical feasibility and assistance in the design of deep geological disposal. 12

25 Performance assessment of repository systems A significant part of the ECOCLAY project consisted of developing a common methodology for treating the alkaline disturbed zone in performance and safety assessment for the disposal facilities (Work Package 6). A first stage made it possible to compare the results of various studies and to examine the various levels of uncertainty. A second stage was to establish an international consensus around the major phenomena to be taken into account to represent the hyperalkaline alteration in the performance assessment models of the components of the disposal system. To develop and maintain a high level of expertise and competence on nuclear technology and safety The national agencies in charge of the management of the radioactive wastes were largely involved in ECOCLAY II. Because of their mission, they are carrying a solid experience and a high qualification level in this field. By submitting to the European scientific community their concerns, they play a role of driving force with regard to the development, the qualification, the formation and the improvement of the human resources of the research organisations and companies. The network built up around the project, ECOCLAY II was a benefit since it: - Maintained in the Community a high level of know-how and scientific and technical expertise in the nuclear field - Facilitated the exchange of information, and the analysis of the results on an international level (conferences, scientific publications). Acceptance of the public and the political leaders The ECOCLAY II project contributed to the strategy of radioactive wastes management by developing a sound basis for making it possible to evaluate the feasibility and the safety of deep geological wastes disposal. An international approach, under the control of the European Community, reinforced the credibility of the scientific results. It thus should make it possible to facilitate the policy choices on the management and the disposal of high level and long lived radioactive wastes, and to build the public confidence and trust. 13

26 5 OBJECTIVE OF THE PROJECT - WORK PLAN 5.1 The building of the framework of ECOCLAY II rested on the objectives that were defined by the partners and on the scientists, techniques and human means to reach them. The six Work Packages which were defined (except co-ordination), corresponded to the three major axes of the project (see figure 5.1.1): the missing experimental data acquisition (WP1, 2, 3 and 4), the modelling of the long-term behaviour of materials (WP5) and performance assessment of these materials (WP6). COORDINATION WP 7 RETENTION WP 1 CLAY ENGINEERED BARRIER WP 2 CLAY HOST ROCK WP 3 CRYSTALLINE HOST ROCK WP 4 Data Acquisition SUMMURY AND PERFORMANCE ASSESSMENT WP 6 MODELLING WP 5 Geochemical Processes Mechanical Processes Coupled Geochemical / Transport Processes Processes Overview Simplification Figure Work Package structure of the ECOCLAY II project The adopted sub-divisions leaded to a three-fold division: - the specification of materials being used as methodological support (bentonite of engineered barriers, clays and granite host rocks) - the type of data required (geochemical reactivity and confinement properties of materials and sorption capacity of the barriers under hyperalkaline conditions) - the degree of coupling taken into account between the phenomena (geochemical, hydraulic and coupled geochemical/transport modelling). The adopted work plan took into account the constraints related to the phasing of each objective. The experiments required for significant time because of the slow kinetics of the processes. The modelling based on the results of the experimentation was superimposed on it partly, thus showing a necessary synergy between the two. Lastly, the performance assessment arrived in the last part of the project, because it might be a synthesis of the data resulting from the experimental and numerical programs. 14

27 Lastly, the examination of the implication of each country in the various tasks shows that (table 5.1.1): - A country alone could not undertake with such a program, because of the wide scope of the total project - The weight given by the participants to each task was unequal because of difference between the national concepts (difference in the expression of the needs), and of available competence in their countries - The participants had the will to all peer review of their techniques and their results to ensure their credibility, before distributing them in the scientific community, or to their Official Authority. - All the participating countries wanted to take a part in the analysis of performance, and to profit from the review of the others. The concept of agreement was then the strong point of convergence of the ECOCLAY II project. 15

28 Table Summary of the total contribution of the Participants to each Work Packages (in person.month) WP1 WP2 WP3 WP4 WP5 WP6 TOTAL Retention Clay Engineered Barrier Clay Host Rock Crystalline Host Rock Modelling Performance Assessment ANDRA BRGM ENRESA UAM CSIC GRS NAGRA PSI Uni BE AEAT SCK.CEN SKB POSIVA HU 9 9 VTT NERC 5 5 CT TOTAL p.m in WP7 : Co-ordination 16

29 SCIENTIFIC AND TECHNICAL DESCRIPTION OF THE RESULTS 17

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31 6 WORK PACKAGE 1: RETENTION 6.1 INTRODUCTION The work package 1 which deals with retention processes has essentially two aims. The first is to study the impact of an alkaline perturbation on the retention and the transport properties of radionuclides migrating in engineered barriers and geological media affected by an alkaline plume. The second is to observe and to study the key processes (or phenomena) occurring during the reactive transport of an alkaline front in different geological media and to experimentally determine the transport parameters of the main components of the alkaline plume itself. The final goal is to better assess the thickness of the alkaline disturbed zone and to better understand the migration of radionuclides at high ph. The main objectives of WP 1 on retention are thus the following: - to gather information on the sorption of radionuclides in the disturbed near-field and farfield media (pure bentonite, mixture of bentonite with crushed granite, salts cement mixtures, and different clay host rocks); - to assess the significance of radionuclides coprecipitation with neoformed calcium silicate hydrates (CSH) phases as an extra retardation mechanism; - to assess the effect of an alkaline plume on the migration properties of the ionic species and radionuclides released from a cementitious repository into engineered barriers and/or clay formations; - to provide data to compare experimental observations with the results of chemicalcoupled transport codes used to perform the geochemical modelling of an alkaline perturbation: i.e., by measuring the chemical evolution of the fluids and by observing the mineralogical alterations of clay in different percolation experiments with various types of alkaline fluids; - to assess the extent of precipitation of calcite in clay cores containing bicarbonate-type porewater and contacted with a Ca 2+ -rich cement water. 19

32 Description of the work The retention of radionuclides in different clay and rock materials were examined by means of batch sorption tests and diffusion experiments on compacted clay affected by an alkaline plume. The significance of radionuclide co-precipitation as a retardation mechanism in a chemically disturbed far-field was assessed, compared to adsorption on the surface of solids. Sorption and co-precipitation were studied on synthetic systems. CSH phases from silica/portlandite interaction, were prepared and studied. The sorption of I and Se on altered clay/salt/cement systems were performed by means of batch experiments. The effect of increasing salinity on the extent of sorption on mixture of crushed granite and bentonite was also considered for two major cations present in the plume (Na + and Ca 2+ ). Diffusion experiments were performed on representative samples from two clay host formations: Callovo-Oxfordian Clay (COx) from the Bure site (France) and Boom Clay (BC) from the site of Mol (Belgium). For the first clay material (COx), the effects of an alkaline plume were evaluated on compacted clay cores by means of through-diffusion experiments, first with synthetic clay pore water and then with alkaline solutions. The through-diffusion experiments were conducted using radioactive and non radioactive tracers for chemical follow up. The mineralogical investigations after the experiment were also performed. To identify and to assess the phenomena and processes that influence the migration of radionuclides on the second host rock (BC), laboratory experiments (percolation tests and electromigration experiments) were also carried out to determine the apparent diffusion coefficient, the diffusion accessible porosity, and the retardation factor of Na +, Sr 2+, Ca 2+, OH. Through diffusion tests were performed to study the precipitation of calcite fronts in plastic clay which could be responsible for the hardening and the fracturing of plastic formation around the galleries. The results obtained on the effect of alkaline conditions on the retention properties of radionuclides is presented in two sections dealing respectively with sorption and migration processes: 1. sorption (or coprecipitation) processes studied by batch experiments on suspensions systems, and, 2. migration phenomena investigated by means of Through-Diffusion and percolation experiments performed onto compacted bentonite plugs and undisturbed clay cores from potential host sites. 20

33 6.2 SORPTION Uptake of strontium, europium and curium by CSH phases in an alkaline disturbed far-field Jan Tits and Erich Wieland (PSI) Introduction To date, a great number of studies have demonstrated the waste ion immobilisation potential of cement-type minerals such as calcium silicate hydrates (CSH phases), which are present in the alkaline disturbed zone around a nuclear waste repository. However, it appears that the chemical mechanisms governing immobilisation are not well documented. A mechanistic understanding of the chemical processes by which waste ions become immobilised in these types of matrices is important for long-term predictions of the performance of repositories for radioactive waste. In general, radionuclide immobilisation is viewed to occur according to an adsorption process. Adsorption describes the binding of an anion or cation at the surface of the solid material. Nevertheless, other potentially important immobilisation processes, such as coprecipitation (or incorporation) with CSH phases, may take place during secondary phase formation in the alkaline disturbed zone and, thus, exert a beneficial effect on radionuclide retardation (e.g., Cocke and Mollah, [1993] and Gougar et al., [1996]). Co-precipitation is a process by which two or more chemical components are simultaneously and permanently incorporated into a single solid phase during precipitation from solution. The compound formed may be either amorphous or crystalline. Considerable evidence has been presented that co-precipitation with amorphous and crystalline CSH phases is a possible retardation mechanism (e.g., Lieber and Gebauer, [1969], McCulloch et al., [1985], Cartledge et al., [1990], Moroni and Glasser, [1995], Johnson and Kersten, [1999]). Radionuclide immobilisation by co-precipitation with CSH phases takes place if conditions are such that the formation of secondary CSH-type minerals occurs. Such conditions may exist in the far-field due to the interaction of hyper alkaline near-field pore water with the host rock. Under these conditions, the formation of CSH phases may be driven by the dissolution of the silica-containing minerals in the presence of a Ca-containing groundwater, which allows the formation of CSH phases under weakly over-saturated conditions. The objective of the present study was to discern the interaction modes of selected radionuclides, i.e., Sr(II), Eu(III) and Cm(III), with CSH phases under conditions relevant to a repository for radioactive waste, to distinguish adsorption from co-precipitation (incorporation) processes on CSH phases, and to assess the relevance of these processes for performance assessments. Adsorption experiments were performed using batch-type sorption experiments with synthetic CSH phases. Co-precipitation experiments were performed by precipitating CSH phases in the presence of the radionuclides using both a fast and a slow precipitation procedure. 21

34 The study is divided in three parts: The first part consisted of the synthesis of CSH phases under conditions relevant for the alkaline disturbed zone around a cementitious repository and included a detailed chemical and mineralogical characterisation of the synthesised products. The second part consisted of a detailed study of the sorption of Sr(II) by CSH phases synthesised in part 1 as well as a study of the co-precipitation of Sr(II) with CSH phases. The behaviour of Sr(II) in the sorption experiments and the co-precipitation experiments was compared and an attempt was made to interpret the data in terms of a mechanistic process. In the third part, the sorption of Eu(III) and Cm(III) by CSH phases was studied as well as the co-precipitation of these trivalent cations with CSH phases using batch sorption experiments combined with Time-Resolved Laser Fluorescence Spectroscopy TRLFS) Synthesis and characterisation of CSH phases Three different series of CSH phases were synthesised: The first two series were synthesised using a fast precipitation procedure in the presence of an Artificial Cement pore Water (ACW) at ph 13.3 and in the presence of Milli-Q water. The synthesis was carried out by mixing high surface area silica fume (AEROSIL 300) with CaO in the required ratios. A third type of CSH phases was synthesised under ACW conditions using a slow precipitation procedure. In this procedure low surface area quartz was mixed with Ca(OH) 2 and the precipitation rate was controlled by the dissolution rate of the quartz particles. Five CSH phases were prepared in ACW having target Ca:Si (C:S) mol ratios of 0.75, 1.07, 1.28, 1.5 and 1.82 and four CSH phases were prepared in Milli-Q water having target C:S mol ratios of 1.07, 1.28, 1.5, and These solids were characterised by chemical analysis, scanning electron microscopy, X-ray diffraction and thermogravimetry combined with differential thermal analysis. The results show that, in ACW at ph 13.3, the synthesis reaction was complete only for Ca:Si ratios below 1.2. CSH phases with C:S ratio above 1.2 were all contaminated with unreacted Ca(OH) 2 reducing the effective C:S ratio of the synthesised CSH phases. It was observed that the CSH phases with the highest target C:S ratio (1.82) had an effective C:S ratio of 1.3. CSH phases synthesised in Milli-Q water were not contaminated with unreacted Ca(OH) 2. The effective C:S ratio of these CSH phases corresponded to their target C:S ratios. It is assumed that the lower portlandite solubility in ACW at ph 13.3 (~ M) does not allow CSH phases with C:S ratio above approximately 1.3 to be formed. When CSH phases were synthesised in Milli-Q water, the ph reaches a value of 12.5 at maximum. At this ph, the Ca(OH) 2 solubility is approximately an order of magnitude higher (~ M) than at ph 13.3, allowing for CSH phases with C:S ratios above 1.3 to be formed. The C:S ratio of the CSH phases formed in ACW using the slow precipitation procedure was determined by the aqueous Ca concentration which is controlled by the portlandite solubility (~ M) resulting in C:S ratios of approximately

35 Uptake of Sr(II) by CSH phases The uptake of Sr(II) by CSH phases was investigated by batch-type sorption and coprecipitation experiments. Sr was added to the experimental systems in the form of Sr(OH) 2. Reaction kinetics and isotherms were measured. In addition the effect of the CSH composition (C:S ratio) and the ph were investigated. Finally, the reversibility of Sr(II) sorption and co-precipitation processes was tested with the help of desorption studies. The data clearly show that the Sr(II) adsorption and co-precipitation processes under all experimental conditions give raise to similar R d values. Furthermore, it was observed that the precipitation rate had no effect on the R d values [Tits et al., 2003a]. These observations suggest that the same types of sorption sites on the CSH phases are accessible to Sr(II) in both adsorption and co-precipitation experiments. Two types of sorption sites can be identified on CSH phases: silanol groups in the interlayer space (interlayer sites) and silanol groups at the end of the silica chains (edge sites). Sorption and co-precipitation isotherms were linear over a concentration range between 10-6 M and 10-3 M suggesting that either only one type of sites is occupied by Sr(II) or that both types of Sr(II) sorption sites have similar affinities for Sr(II). Maximally 0.2 mol kg -1 Sr(II) was sorbed on the CSH phases but no clear indication for a site saturation was observed. Based on the 14 Å-tobermorite-type structure (Ca 5 [Si 6 O 16 (OH) 2 ] 8 H 2 O), 2 sorption sites (Ca atoms which can be replaced by Sr) can be assigned in the interlayer for 6 Si tetrahedra. Taking an operational molecular weight of 14Å-tobermorite of g mol -1, based on the weight after calcination (i.e. 14 Å-tobermorite = 0.83 CaO 1 SiO 2 ), the interlayer contains 3.13 mol kg -1 sites. With increasing C:S ratio, silica chains become increasingly shorter and more edge sites develop. The maximum number of edge sites is reached when the C:S ratio of the CSH phases is 2; i.e., when only silicate dimers are present. In this case, 4 edge sites are created for 6 Si tetrahedra, meaning 3.88 mol kg -1 edge sites assuming the operational molecular weight of a CSH with C:S = 2 to be 172 g mol -1, based on the weight after calcinations (i.e. 2 CaO 1 SiO 2 ). These theoretical surface site densities are a factor 15 to 20 higher than the maximum amount of Sr(II) sorbed on the CSH phases in the isotherm experiments suggesting that site saturation is no issue in these experiments. Both under ACW conditions (ph 13.3) and in the absence of Na and K (11.5<pH<12.5), Sr(II) sorption and co-precipitation depended strongly on the C:S ratio of the CSH phases with R d values decreasing with increasing C:S ratio. This decrease of the R d values suggests that the sorption of Sr(II) is controlled by a Sr(II) Ca(II) ion-exchange process as increasing C:S ratios are coupled with higher aqueous Ca concentrations [Tits et al, 2003a]. In ACW (ph 13.3), increasing amounts of Sr(II) sorbed on the CSH phases did not affect the composition of the aqueous phase (i.e. the Na, K and Ca(II) concentration in solution). An increase of the Ca(II) or Na, K concentration would be expected if ion exchange with one of these cations were the uptake-controlling mechanism. It is speculated that, under ACW conditions, the sorption sites of CSH phases with C:S ratios below 1.0 are saturated with Na and K. The sorption of Sr(II) by these CSH phases is controlled by an ion-exchange process with these alkalis. The increase of the Na and K 23

36 concentration induced by this ion-exchange process is too small to be detected against the high alkali background concentration in ACW. With increasing C:S ratios the aqueous Ca(II) concentration increases and the alkalis on the sorption sites of the CSH phases are replaced by Ca(II). Under these conditions, the Sr(II) sorption is controlled by an ion-exchange process with Ca(II) on the sorption sites. In the absence of Na and K, at ph s between 11.5 and 12.5, Sr(II) sorption is controlled by an ion-exchange process with Ca(II) as well. However, an increase of the Ca(II) concentration induced by this ion-exchange process is too small to be detected against the high Ca(II) background concentration originating from the high Ca solubility of CSH phases with high C:S ratios. The reversibility of Sr(II) sorption was tested by conducting desorption experiments. The equilibrium solution after sorption was replaced with a Sr(II) free solution followed by a measurement of the new Sr(II) equilibrium concentration in solution. The sorption process is reversible if the R d value determined based on the new Sr(II) equilibrium concentration equals the sorption R d value. The results of the desorption tests were not entirely consistent but taking into account a realistic uncertainty on the data the general trend suggests that the Sr(II) sorption process on CSH phases and the Sr(II) co-precipitation process with CSH phases are reversible. Summarising this study shows that Sr(II) uptake by CSH phases under different experimental conditions can be explained by a cation exchange process. The R d values depend on the concentrations of the cations competing for the sorption sites Uptake of Eu(III) and Cm(III) by CSH phases a) Batch sorption studies with Eu(III) The uptake of Eu(III) and Cm(III) by CSH phases was investigated in batch-type sorption and co-precipitation studies and by Time-Resolved Laser Fluorescence Spectroscopy (TRLFS). Batch sorption experiments with Eu(III) were performed to measure reaction kinetics and isotherms as well as the effect of the C:S ratio and the solution composition on the uptake processes. The measured R d 's were very high with values ranging from dm 3 kg -1 to dm 3 kg -1 [Tits et al., 2003b]. Isotherms obtained from sorption and co-precipitation studies were linear over a wide equilibrium concentration range (10-11 M and 10-7 M). Above 10-7 M the slope of the isotherms exceeded a value of 1.0, suggesting that an Eu(III) precipitate was formed. Neither the C:S ratio nor the solution composition had a significant effect on the R d values of Eu(III). Furthermore both adsorption and co-precipitation experiments gave similar R d values. Note that the extremely strong sorption of Eu(III) results in very low Eu(III) concentrations in solution (~ M). Such low concentrations are very difficult to measure and lead to very large uncertainties on the R d values thereby limiting the evaluation of the effects of parameters such as kinetics, C:S ratio and solution composition. 24

37 b) Study of the sorption and co-precipitation of Cm(III) by Time-Resolved Laser Fluorescence Spectroscopy The coordination of Cm(III) in the CSH-phases was probed using Time-Resolved Laser Fluorescence Spectroscopy (TRLFS). This work was carried out in collaboration with the Forschungszentrum Rossendorf, Germany and the Forschungszentrum Karlsruhe, Germany. This technique provides information about the number of H 2 O molecules in the first coordination sphere of Cm(III). Cm(III) and Eu(III) are analogue elements and have similar chemical properties. This actinide was chosen for this study because of its much higher fluorescence spectroscopic sensitivity which allows sorption studies at very low concentrations (10-7 M) to be conducted. The study revealed that in both the co-precipitation and sorption experiments Cm(III) becomes incorporated in the CSH structure with time [Tits et al., 2003b]. Two differently incorporated Cm(III) species could be detected: It is proposed that the first incorporated species replaces Ca(II) in the interlayer space of the CSH phases, and that the second incorporated species replaces Ca(II) in the Ca-octahedral layer of the CSH phases. This study shows that trivalent lanthanides and actinides are very strongly retained by CSH phases. Furthermore, they rapidly become incorporated in the CSH structure Conclusions Concerning application of the results from the present study to performance assessment of a cementitious nuclear waste repository the following conclusions can be drawn with respect to the alkaline earth metals and trivalent actinides and lanthanides: The formation of CSH phases in the alkaline disturbed zone will improve its retention capabilities for radionuclides of the alkaline earth series and for actinides and lanthanides compared to the undisturbed system due to the high affinity of CSH phases for these two types of radionuclides. Co-precipitation processes have no beneficial effect on the overall R d values for alkaline earth metals and actinides suggesting that all the Ca(II) sites in the CSH structure are available for ion-exchange with Sr(II). The alkaline earth metals sorb on CSH phases via an ion-exchange process. Therefore, the pore water composition (i.e. concentrations of competing cations, especially Ca(II), but also Na and K) can significantly affect the R d value for Sr(II) and other alkaline earth metal cations. The pore water composition has no effect on the sorption of actinides and lanthanides. Actinides and lanthanides are incorporated in the CSH structure thereby replacing Ca(II) both in the interlayers and in the octahedral sheets. It is however still an open question whether or not the immobilisation process is reversible. 25

38 6.2.2 Sorption of I and Se by batch experiments on altered clay/cement systems Thorsten Meyer (GRS) Introduction The objective of the work performed by GRS was to study the significance of Se and I sorption. The experiments are dedicated to the sorption experiments of Se and I on cement/bentonite matrix. These anionic radionuclides like 129 I (as I - ), Se (as SeO 3 or SeO 2-4 ) will be sorbed less by backfill materials like salts for the case of a solution intrusion. In addition, less precipitation of these radionuclides will be observed. Thus, these elements can be transported without any retention and give their contribution to the middle-term radiation. They are transported in long-term analysis without any retention and make up therefore a considerable contribution to the radiation exposure. The general objective of the present work is to perform a screening of potential scavengers to add to the near-field of a final disposal site to decrease the radionuclide mobility. These studies are necessary to understand the sorption properties of the mentioned ions under relevant geochemical conditions and to provide qualitative mechanisms. In the part of ECOCLAY II the sorption capacities of clay materials, MX-80 and Callovo-Oxfordian Clay, as well as salt cement mixture were investigated R d -value In many of the work concerning sorption properties of sorbent exact conditions for the experiments (solution chemistry, temperature, ph, redox conditions ) are missing. Therefore it is sometimes difficult to evaluate the given data. The distribution coefficient (R D value) frequently used in the literature is defined as follows: concentration of sorbed species[ kg / kg] R D 3 concentration in solution [ kg / m ] Eqn The distribution coefficient has theoretically a dimension of [m 3 /kg]. It is a problem that the concentration of the sorbent is indicated occasionally in concentration per unity surface and therefore the distribution coefficient has the dimension of a length. If the specific surface of a sorbent is not known, the given value cannot be converted and not used with that in [m 3 /kg]. Other authors indicate the R D value without any dimension. In such cases it has to be known if the concentrations influencing the R D -value are related to the unity volume or the unity surface, since different values come out for different dimensions. Therefore dimensionless R D values cannot be used. These difficulties were already pointed out by McKinley and Scholtis [1992]. 26

39 Experimental At the beginning the surface areas of the investigated materials were measured. The determined densities as well as the specific surface areas are given in table All examined sorbent were investigated by the N 2 -sorption method. These measurements were performed by ibmb (TU Braunschweig). The N 2 -sorption method was applied for the determination of the specific surface of porous substances by the model assumption of a correlation between the sorbed mass of gas on a surface and its surface area. A metrological basis is the determination of the adsorption isotherm which describes a relation between the amount of the adsorbed gas and the pressure of the gas in a closed vessel at constant temperature. The volume of the adsorbed gas is given by the relative gas pressure (related to the saturation pressure of the gas). The saturation pressure of the nitrogen amounts to 762 Torr (= kpa). The determination of the sorbed gas quantity in the vessel is performed by pressure measurements by applying the ideal gas law. Since the vessel surface also has an effective surface, its influence is also taken into account. This is made by regulation of the vessel constant which is determined by calibrating curves and used for the evaluation. A certain weighed mass of sorbent was filled in the vessel and evacuated with an end pressure of < 0,1 mbar. The vessel was heated with a heating rate of 1 K min -1 up to the maximum temperature of 105 C. Before the beginning of each measurement the weight of each sample was measured and the sample was cooled down. The measurements of the N 2 -sorption were performed at -196 C. Table Surface area of the investigated materials. Sample Salt Cement Callovo- Oxfordian Clay MX-80 Bentonite Fly Ash Density [g cm -3 ] Specific surface area [m 2 g -1 ] The sorption of I and Se were performed in batch tests. I-125 and Se-75 were added as tracers. The complete test took place in an incubator at 25 C and standard pressure without exclusion of oxygen. A quantity corresponding to a volume of 1 ml of each sorbent was weighed out according to its density (humid) into 30 ml centrifuge tubes (polypropylene copolymer) from NALGE whose screw tops were provided with a sealing ring. After adding of 5 ml pure balance solution the batch vessels were shaken for 7 days. In the connection of this pre-conditioning the samples were centrifuged, decanted and than weighed. After admitting the sluggish release the activation of the tests was immediately carried out with 100 μl of the tracer solution (I-125 or Se-75). The incubation period for the sorption was 28 days. The determination of the activity was performed by gamma counting with a NaI detector. For the desorption experiments unspiked equilibrium solution was added to the remaining samples. The batch vessels were shaken for 7 days. Sample preparation and measuring were the same like to the sorption experiments. 27

40 For the measurements with the bentonite MX-80 the volume-volume ratio had to be extended by 1:10 because of the strong bentonite swelling. The pre-conditioning of the salt concrete tests also had to be carried out differently than at the other variants since otherwise a considerable part (the dissolved salt of the salt concrete) of the concrete mass is lost during decantation. So only 4.5 ml of the equilibrium solution was given to the sorbent, this after the pre-conditioning not decanted and 0.5 ml tracer solution in tenfold concentration was added. Table Solutions Investigated solutions and materials. Materials IP21 solution Desionised (or bidistilled) water NaCl solution, saturated Cemented BFA Callovo-Oxfordian Clay (Bure site) Bentonite, MX Results In general the sorption of selenite on the investigated sorbents was stronger than that of iodide. Figure to figure give the sorption of I and Se in different solutions and on different investigated sorbents. In figure the sorption of I on the Callovo-Oxfordian Clay from the Bure site is depicted. It could be observed that the sorption of I on Callovo-Oxfordian Clay has a relative sorption capacity of 90 % up to an I concentration of 10-7 mol dm -3 in desionised water. At higher concentrations a loss of relative sorption capacity was observed. For concentrations of 10-6 mol dm -3 or higher the relative sorption capacity will strongly decrease. Concerning the IP21 and saturated NaCl solution there were found relative sorption capacities between 5 % and 10 %. After desorption with desionised water a relative desorption between 3 % and 10 % was found. No sorption could be observed after the desorption with NaCl and IP21 solution. In the experiments I on MX-80 (figure 6.2.2) the relative sorption in NaCl-solution was observed up to 10 % independently of I concentration in solution. No sorption could be observed for I in IP21-solution or desionised water. Therefore no sorption could be found after the desorption experiments. Figure shows the results of the sorption/desorption experiments of I on salt cement. The relative sorption of I on salt cement was observed between 4 % and 10 %. Desorption experiments was not performed. The results indicate a low sorption capacity of I on salt cement independent of the chemical solution composition. 28

41 sorption / desorption of I on Bure clay 100 relative sorption / desorption in % bidest IP-21 NaCl bidest / desorp. IP-21/ desorp. NaCl / desorp. 0 1E-12 1E-10 1E-8 1E-6 1E-4 I concentration in mol/l Figure Sorption/desorption of I in desionised (bidistilled) water, NaCl- and IP21- solution on Callovo-Oxfordian Clay from the Bure site. 50 sorption / desorption of I on MX-80 relative sorption / desorption in % bidest IP-21 NaCl bidest / desorp. IP-21 / desorp. NaCl / desorp. 0 1E-12 1E-10 1E-8 1E-6 1E-4 I concentration in mol/l Figure Sorption/desorption of I in desionised (bidistilled) water, NaCl- and IP21- solution on MX

42 sorption of I on salt cement 50 relative sorption of I in % bidest IP-21 NaCl 0 1E-12 1E-10 1E-8 1E-6 1E-4 I concentration in mol/l Figure Sorption/desorption of I in desionised (bidistilled) water, NaCl- and IP21- solution on salt cement. 100 sorption / desorption of Se on Bure clay relative sorption / desorption in % bidest IP-21 NaCl bidest / desorp. IP-21 / desorp. NaCl / desorp. 0 1E-13 1E-11 1E-9 1E-7 Se concentration in mol/l Figure Sorption/desorption of Se in desionised (bidistilled) water, NaCl- and IP21-solution on Callovo-Oxfordian Clay from the Bure site. 30

43 sorption / desorption of Se on MX relative sorption / desorption in % bidest IP-21 NaCl bidest / desorp. IP-21 / desorp. NaCl / desorp. 0 1E-13 1E-11 1E-9 1E-7 Se concentration in mol/l Figure Sorption/desorption of Se in desionised (bidistilled) water, NaCl- and IP21-solution on MX sorption of Se on salt cement relative sorption von Se in % bidest IP-21 NaCl 0 1E-13 1E-11 1E-9 1E-7 Se concentration in mol/l Figure Sorption/desorption of Se in desionised (bidistilled) water, NaCl- and IP21-solution on salt cement. 31

44 A strong sorption behaviour could be observed for the sorption of Se on Callovo-Oxfordian Clay from the Bure site (figure 6.2.4). A relative sorption between 60 and 90 % could be determined. The highest sorption potential was measured in desionised water. A level of 60 % relative sorption was measured for Se in NaCl-solution. Astonishing that in the desorption experiments the relative sorption was measured to 5 % for the Se/desionised water system in contrast to 15 % relative sorption for NaCl- and IP21-solution. The sorption of Se in NaCl- and IP21-solution on MX-80 is in the same range than on Callovo-Oxfordian Clay from the Bure site. Between 60 % and 70 % of relative sorption were measured. It is amazing that no sorption effect could be observed for the system Se/desionised water. After the desorption experiments a relative sorption of 20 % was measured for Se in NaCl- and IP21-solution, respectively. In figure the sorption experiments of Se on salt cement are depicted. In all cases more than 80 % relative sorption could be measured. Nearly all Se in desionised water is sorbed on the salt cement surface. Desorption experiments were not performed. In general the sorption of selenite of the investigated sorbents was stronger than from iodide. The expected case that the sorption of the investigated anions is strongest in desionised water in the absence of competitive ions did not occur generally. Particularly the sorption seemed to get increased for the sorption of selenite and iodide on MX-80 by saturated NaCl solution. This was also observed for iodide on salt concrete to a lower extent. Table depicts the retention of I and Se on the investigated materials: Table Retention of I and Se in desionised water, NaCl- and IP21-solution on Callovo-Oxfordian Clay (Bure site), MX-80 and salt cement. R d values (dm 3 kg -1 ) for I Concentration (mol dm -3 ) Solution used MX-80 bentonite Callovo- Oxfordian Clay Salt cement to 10-5 desionised water to 10-5 IP to 10-5 NaCl R d values (dm 3 kg -1 ) for Se Concentration (mol dm -3 ) Solution used MX-80 bentonite Callovo- Oxfordian Clay Salt cement to 10-8 desionised water to 10-8 IP to 10-8 NaCl

45 6.2.3 Influence of saline groundwaters on the sorption properties of bentonite under alkaline conditions Heini Ervanne, Martti Hakanen (VTT and UH) Experimental data a) Clay material The objective in this research was to obtain information on the possible alterations in bentonite caused by high ph of an alkaline plume and in addition to study the effect of salinity on the sorption of selected radionuclides onto bentonite before and after exposure to an alkaline perturbation under conditions relevant for saline groundwaters encountered deep in crystalline bedrock. The research work performed on the effect of alkaline conditions on the retention properties of radionuclides consists of batch experiments including sorption studies. The conditions chosen to study simulate these anticipated deep in the granitic bedrock at the final disposal site for spent nuclear fuel at Olkiluoto in Finland. In the experiments the solid material used was MX-80 bentonite. The simulated alkaline groundwater solutions used in the experiments were fresh alkaline (ph 12.5), saline alkaline (ph 12.5), and saline hyper alkaline (ph 13.5). All the experiments were conducted in anaerobic CO 2 -free atmosphere. Sorption was studied with two radionuclides, 45 Ca and 22 Na. The periods of alteration of solid material in these experiments were 1 week, 1 month, 6 month and 1.5 years. The sorption of 22 Na on bentonite was low for saline alkaline and saline hyper alkaline waters (K d ( ) 10-3 m 3 /kg). However, for the fresh water a high sorption was observed (K d ( ) 10-2 m 3 /kg). The sorption was highest for samples of shortest experimental time decreasing in samples of longer experimental time except in bentonite with fresh water. The sorption on these samples remained about the same. The sorption of 45 Ca on bentonite was very high for all water types (K d ( ) m 3 /kg). The major cations (Na, Ca, K, Fe, Mg, Al, Si) and anions (Cl, SO 4 ) were analyzed at the beginning and at the end of all alteration periods. Furthermore the evolution of the ph of the solutions contact with solid materials was followed. The ph of bentonite samples decreased considerably in case of the fresh and saline alkaline waters, but the ph for the saline hyper alkaline water was maintained. b) Granite The objective in this research is to obtain information on the possible alterations in crystalline rock caused by high ph of an alkaline plume and in addition to study the effect of salinity on the sorption of selected radionuclides onto crushed rocks and before and after exposure to an alkaline perturbation under conditions relevant for saline groundwaters 33

46 encountered in deep crystalline bedrock. The crushed rock represented the average rock composition at the planned repository Olkiluoto, Finland. The experimental set-up was same as previously described for bentonite. The sorption of 22 Na on crushed rock was low for all synthetic waters (K d ( ) 10-3 m 3 /kg). The sorption was highest for samples of shortest experimental time decreasing in samples of longer experimental time. The evolution of mass distribution ratio K d was about the same for crushed rock in all waters. For the longest experimental time the sorption was too low to be detected in crushed rock contact with fresh alkaline water. The sorption of 45 Ca on crushed rock was generally higher than for sodium (K d ( ) 10-3 m 3 /kg). The sorption increased on crushed rock with longer experimental time. The evolution of mass distribution ratio K d varied for different waters. The major cations (Na, Ca, K, Fe, Mg, Al, Si) and anions (Cl, SO 4 ) were analyzed at the beginning and at the end of all alteration periods. Furthermore the evolution of the ph of the solutions contact with solid materials was followed. The ph did not change for the crushed rock samples for fresh and saline alkaline waters. There was a slight decrease observed for the saline hyper alkaline water. 34

47 6.3 MIGRATION Influence of alkaline conditions on diffusion of tracers in bentonite and Callovo-Oxfordian Clay Melkior T., Mourzagh D., Yahiaoui S., Thoby D., Alberto J.C., and Brouard C. (CEA) Introduction The general objective of this work is to provide data to be used for the calculations of the spread of an alkaline fluid into clayey barriers and in order to evaluate its effect on radioactive waste repository. The study was applied to the case of the Bure site (Meuse/Haute-Marne, France): the argillaceous materials were the Callovo-Oxfordian Clay and compacted MX-80 bentonite samples, this clay being considered as an analogue of an engineered barrier. The clay samples were initially equilibrated with a synthetic solution representative of the site groundwater. Diffusion experiments were then performed, in the presence of cement water or not. The chemical species for which diffusion was measured were: Potassium and calcium, as these cations occur in higher concentrations in cement water compared with site water. Some chemical species chosen as reference species : HTO, Cs, Na and Cl Theory and method Diffusion of a dissolved species in a porous material is described by Fick s laws, the expression of the diffusion coefficient being modified to take into account the effect of the porous network. For 1-D transfer through an homogeneous material, the flux F j of species j crossing a surface unit of porous medium is written as (Fick s first law): F j j j C = De Eqn x Where C j j is the concentration of the diffusing species in solution, and D e is its effective diffusion coefficient. The latter is usually written as the product of a formation factor and the j diffusion coefficient of the considered species in water ( D 0 ). Traditionally, this formation factor depends on the porosity and must also account for the tortuosity of the porous medium. Various expressions of the formation factor are proposed in the literature (see for example van Brakel and Heertjes [1974]). Following the pore diffusion model, the 1-D diffusion of a dissolved species j through a porous medium coupled with sorption of this species on the solid phase is represented by the partial derivative equation: t j j j j C ( ( ) ( )) ( x, t) j C ( ) ( x, t) φ x, t C x, t + = D x, t Eqn t x e x 35

48 where C j ( x, t) is the concentration of the considered species sorbed on the solid, and Φ j is the porosity accessible to the considered species. It was assumed in this work that the transport parameters were constant throughout the clay sample. In any rigor, such an hypothesis is wrong if a reaction front extends into the material and modify the structure of the porous network as it could be the case in the presence of cement water. A macroscopic approach is adopted here and the diffusion coefficients thus deduced from experimental works in what follows must be considered as apparent values at the scale of the clay samples. The diffusion tests carried out within the framework of this study were performed according to the classical through-diffusion method. The experimental device (diffusion cell) is schematically depicted in figure The clay samples were 40 mm in diameter and 7 to 8 mm thick. They were sealed in the diffusion cell with an epoxy resin and sandwiched between two porous filter plates. Callovo-Oxfordian Clay disks were cut out with adequate dimensions using a diamond wire saw. MX-80 samples were manufactured by compacting the bentonite powder to the chosen densities. Porous filterplate Inlet reservoir Outlet reservoir Clay sample Epoxy-resin Figure Schematic representation of a diffusion cell. The diffusing species (possibly tagged by one of its radioactive isotopes) was introduced into the inlet reservoir and its concentration was maintained at a constant value during the test. The concentration in the outlet reservoir was kept at a sufficiently low value to consider it as null in comparison with the concentration in the inlet reservoir. Regular replacement of the solutions filling the reservoirs of the cell enabled the boundary conditions to stay at constant values during the experiment. In practice, when the decrease in concentration gradient reached 10 % of the initial value, the solutions were renewed. The flux passing through the clay was monitored as a function of time. Here, only effective diffusion coefficients values are discussed. In order to limit measurement errors, the presence of porous filter plates on both sides of the sample was 36

49 taken into account (see Put, [1991]). The effective diffusion coefficient D e through the clay sample was deduced from the expression: D e = D F D C F inlet L F 2 f steady state F steady state Eqn Where F steady state is the value of the flux at steady state, f is the thickness of the porous filter plates, D F is the effective diffusion coefficient in the porous filter plates, C inlet is the concentration of the diffusing species in the inlet reservoir and L is the thickness of the clay sample. All the diffusion tests were performed at 23 C. Reservoir solution sampling and renewal were carried out in a glove box swept by nitrogen, to ensure an oxygen content below 100 ppm. Dissolved oxygen concentration in the solutions introduced into the reservoirs was lower than this value. Two types of experiments are presented in what follows: the diffusion tests under site conditions correspond to experiments with site water filling both reservoirs of the cell (see figure 6.3.2a). For the diffusion tests in the presence of cement water, the inlet reservoir of the cells was filled with cement water while the outlet reservoir contained site water (see figure 6.3.2b). Again, these boundary conditions were maintained during the tests, by regular replacements of the solutions. Clay sample equilibrated with site water Clay sample equilibrated with site water Inlet reservoir : site water with considered species Outlet reservoir : site water Inlet reservoir : cement water with considered species Outlet reservoir : site water (a) Figure Schematic view of the diffusion tests in site water (a) and in the presence of cement water (b). (b) Solids and solutions a) Solids Callovo-Oxfordian Clay samples were extracted from the EST-205 borehole in the Callovo- Oxfordian Formation. Two Callovo-Oxfordian Clay samples, extracted at different depths, 37

50 were investigated. The main difference between them arises from the mineralogical composition of their clayey fraction. In the samples referred to hereafter as R 0, extracted at -488 m, smectite contents lie between 40 and 70 % and they contain no ordered illite/smectite interstratified layers (ERM, [2001]). Samples referred to as R 1 were extracted from -506 m. Their smectite contents lie between 20 and 50 % and they contain ordered illite/smectite interstratified layers. The study also involves MX-80 bentonite. Two different compaction densities were investigated here (1.7 kg dm -3 and 2.0 kg dm -3 ). b) Solutions The compositions of the solutions are given in table The site solution is supposed to represent the groundwater naturally occurring in the Callovo-Oxfordian Clay. It was degassed to drive out dissolved oxygen in order to prevent pyrite oxidation processes. To maintain a realistic bicarbonate concentration, nitrogen of atm CO 2 partial pressure was bubbled through the solution. The cement water corresponds to an evolved concrete water, having a ph controlled by portlandite solubility. This solution was also degassed to eliminate dissolved carbonates, and thus to avoid calcite precipitation. Dissolved oxygen content was measured, prior to contacting the fluids with the clay samples. It was assumed that the measurement of dissolved oxygen in the cement water gave a good idea of the state of degasification of the solution and that small amounts of O 2 implied small amounts of carbonates. Table Composition of site (Callovo-Oxfordian Clay) and cement waters. Concentration (mmol dm -3 ) Na + K + Ca 2+ Mg 2+ Cl 2 SO 4 ph Site water (Callovo-Oxfordian) Cement water Experimental Once the clay samples were set in the diffusion cells, the reservoirs were filled with site water to ensure saturation of the porous medium and equilibration of the material with this solution. Solutions contained in the reservoirs were renewed on several occasions and analyzed for major elements in order to check the equilibration kinetics. The experiment started after the equilibration step. For the diffusion tests under site conditions, the diffusing species considered were tritiated water, potassium and calcium. In the case of potassium, no radioactive tracer can be used. Diffusion was evaluated by increasing the concentration of potassium in the inlet reservoir and measuring the subsequent increase [K + ] in the outlet reservoir. The inlet concentration was thus taken equal to mol dm -3. The calcium diffusion tests were carried out by introducing 45 Ca as 45 CaCl 2 into the inlet reservoir. There was no concentration gradient in stable isotope of calcium, 45 Ca being the species that effectively diffuses. 38

51 For diffusion tests in the presence of cement water, the clay samples were at equilibrium with the site water before introducing the alkaline fluid. Two kinds of diffusion tests were undertaken in this part. Diffusion tests of some reference ionic species and a study of the evolution of HTO diffusivity. The reference ionic species considered here were Cl (as 36 Cl - ), Ca 2+ (as 45 Ca 2+ ), K +, Cs + and Na + (as 22 Na + ). For each experiment, diffusion started when cement water was introduced into the diffusion cells. These tests were designed to characterize the migration of ionic species in an alkaline plume. Some tritiated water through-diffusion experiments were performed to evaluate the potential evolution of the effective diffusivity of water with time, suspected to occur because of mineralogical changes. When the equilibration period of the clay sample with site water was completed, the alkaline solution was introduced into the inlet reservoirs of the cells, the outlet reservoir remaining filled with site water (again, these boundary conditions were maintained by regular renewals of the solutions). It is the beginning of what we call the contacting period with cement water: the values of HTO effective diffusion coefficient have been measured after contacting-periods with cement water of one, five and twelve months Results and discussion a) Preliminary equilibration For Callovo-Oxfordian Clay samples, no major changes in solution composition during this equilibration period were observed. By contrast, equilibration of bentonite samples with site water required more time and solution renewing. Each reservoir of the diffusion cells containing bentonite samples was renewed four times. The major cationic species concentrations were analyzed after each renewal for some diffusion cells. This observation was expected because site water was not designed to be equilibrated with bentonite. Sodium, the major exchangeable cation adsorbed on montmorillonite, was partly replaced by potassium, magnesium and calcium provided by the site water. As an example, the results obtained with a 1.7 kg dm -3 density sample are presented in figure

52 Relative difference with respect to site water composition Time (days) Sodium Potassium Calcium Magnesium Figure Equilibration step for a bentonite sample (density = 1.7 kg dm-3). Relative differences in major cationic species concentrations with respect to site water compositions for each renewal of the solutions in the reservoirs. The data are presented as relative concentration difference (with respect to site water) versus time. For each cationic species M n+ the relative concentration difference was calculated as: n+ n+ [ M ] solution [ M ] n+ [ M ] site _ water Eqn site _ water where [M n+ ] solution is the concentration measured in the solution that has been withdrawn and [M n+ ] site_water is the concentration in the site water. b) Diffusion tests The effective diffusivities, deduced from the diffusion experiments under site conditions and in the presence of cement water are gathered in table and table respectively. As an example, the 45 Ca diffusion through a Callovo- Oxfordian Clay sample in the presence of cement water is presented in figure

53 1.8e-3 Normalized Flux [ mol / ( mol/m 3 ) / m 2 / d ] 1.5e-3 1.2e-3 9.0e-4 6.0e-4 3.0e Time (days) Figure Calcium-45 diffusion through a Callovo-Oxfordian Clay sample (R1 level), when the inlet reservoir of the diffusion cell is filled with cement water. In the case of Ca diffusion in the presence of cement water through bentonite samples, the D e deduced from the experimental data is particularly sensitive to the value given to the diffusion coefficient in the filter plate. This latter depends on the mode of diffusion that occurs, which appears to be here complex. Therefore, the values of D e (Ca) in bentonite samples given in table should be considered with care and are associated to large uncertainties (up to an order of magnitude). Table Effective diffusion coefficients (in m2 s-1) deduced from the diffusion tests under site conditions. D e (m 2 s -1 ) Species Callovo-Oxfordian Clay MX-80 Bentonite R 0 level -488 m R 1 level -506 m Density Density 1.7 kg dm kg dm -3 HTO HTO K Ca

54 Table Effective diffusion coefficients deduced from the diffusion tests in the presence of cement water across Callovo-Oxfordian Clay and bentonite samples. D e (m 2 s -1 ) Species Callovo-Oxfordian Clay MX-80 Bentonite R 0 level -488 m R 1 level -506 m Density Density 1.7 kg dm kg dm -3 K Ca ~ ~ Cs Na Cl Whatever the considered species and the configuration of the diffusion experiment (under site conditions or in the presence of cement water), the effective diffusion coefficients obtained in the two Callovo-Oxfordian Clay samples were similar. The differences in mineralogical compositions between these two samples do not seem to affect the values of effective diffusion coefficients. This result tends to confirm the relative homogeneity of the diffusion properties in this natural medium, although such a conclusion must be supported by complementary data. The diffusion tests carried out through the bentonite samples show an effect of density on the values of effective diffusion coefficients: increasing the density overall causes a decrease in the effective diffusion coefficient. Similar trends have been observed for HTO diffusion in bentonite by Miyahara et al. [1991], Sato et al. [1992], and Oscarson [1994]. It can be noted in particular that increasing the density more specifically affects the diffusion coefficient of chloride. Clay surfaces are negatively charged because of isomorphic substitutions within the crystalline structure. Consequently anionic species in solution are repulsed from the liquid/solid interface (electrostatic processes), and do not have access to the whole porous volume in the argillaceous medium. Compaction of the bentonite causes reduction in volume and size of the pores, and has a more drastic effect on the accessibility of anions than on that of neutral or cationic species (Muurinen et al., [1989]; Oscarson et al., [1992]). The diffusion tests carried out under site conditions show that the effective diffusion coefficient of cationic species (K + and 45 Ca 2+ ) were systematically higher than those of tritiated water. High diffusivities for cationic species in clayey materials have already been 42

55 reported and discussed in the literature. Some authors assign these variations to surface diffusion processes (see for example Neretnieks, [1982]; Eriksen and Jansson, [1996]). The occurrence of this phenomenon has never been properly demonstrated and is often disputed (Oscarson, [1994]). These results could highlight the limits of the traditional model of pore diffusion to account for the migration of cations into clays. This subject is not the focus of this work, and will not be developed here. It is however essential to keep this fact in mind because of the importance of the cationic species diffusion when dealing with the spread of an alkaline fluid into a clayey material. Figure shows the HTO effective diffusion coefficient measured as a function of the contacting period of the clay sample with cement water, for Callovo-Oxfordian Clay and bentonite samples. Values at t=0 correspond to tests carried out under site conditions. Let us specify that for each type of clay sample (R 0, R 1 Callovo-Oxfordian Clay and bentonite compacted at 1.7 kg dm -3 and 2.0 kg dm -3 ), diffusion experiments were not performed through the same clay disk: different clay disks were involved to produce each point of the evolution curves. Dispersion in the results is thus possible, more particularly with Callovo- Oxfordian Clay. MX-80 bentonite samples are supposed to be more homogeneous since they were manufactured by compaction of a powder. Figure shows a slow but regular decrease of D e (HTO) in the presence of cement water. This observation could be attributed to a closure of the porous network, which still has to be confirmed by analysis of the solids. Jefferies et al. [1988] performed some diffusion experiments with a London clay and a Ca(OH) 2 solution: the diffusivity of I - dropped by one order of magnitude due to mineralogical alteration of the clay. 8e-11 7e-11 Bentonite ( d=1.7 ) Bentonite ( d=2.0 ) Bure mudrock ( -488 m ) Bure mudrock ( -506 m) 6e-11 D e ( m 2 /s ) 5e-11 4e-11 3e-11 2e-11 1e Contacting period with alkaline fluid ( months ) Figure HTO effective diffusion coefficient as a function of the contacting period with cement water. 43

56 On the other hand, effective diffusion coefficients of potassium and calcium were greater in the presence of cement water than when measured under site conditions. This observation could be explained by the differences in the diffusion modes that were considered. The potassium diffusion under site conditions corresponded to a salt diffusion while Ca diffusion under site condition was a self-diffusion. When cement water was introduced in the inlet reservoir, the diffusion system was more complex, because different concentration gradients were established in opposite directions. It is a kind of counter-diffusion system involving more than two species. The results of the tests tend to show that D 0 values of K + and Ca 2+ are greater when diffusion occurs in the presence of cement water than under site conditions. This effect then counter-balances the decrease in effective diffusion coefficient highlighted with HTO. The global assessment of these two competing effects over the duration of diffusion tests (a few months) is an increase in potassium and calcium effective diffusion coefficients compared to the measurements carried out under site conditions. For each kind of clay investigated here, effective diffusion coefficients for the various studied cations have the same order of magnitude. On the other hand, chloride exhibits an effective diffusion coefficient significantly lower than those of the cations. Some calculations about the diffusion of an alkaline plume into clayey barriers presented in the literature implement computer codes which do not make it possible to incorporate a specific diffusion coefficient for each ionic species. The results of the diffusion tests performed here show that such an assumption appears relatively realistic with regard to cation diffusion, although a parametric study will allow to validate this point definitively. It seems, on the other hand, that this assumption is very debatable concerning the anionic species (bicarbonates, sulphates...). 44

57 6.3.2 Percolation and diffusion experiments on Boom Clay exposed to an alkaline perturbation P. De Cannière, N. Maes, H. Moors, L. Wang, and D. Jacques (SCK CEN) The degradation of cementitious materials used to backfill the galleries of MLW repositories in deep geological formations will release hyperalkaline fluids rich in calcium and alkaline cations. It will modify the chemistry of the interstitial water of the host rock close to the galleries and induce mineralogical transformations that could affect the retention properties of the geological barriers. The aim of the present experimental work is to assess the effect of an alkaline plume on the chemical and mineralogical properties of the clay and on the migration of the radionuclides released from a cementitious repository into clay. To identify and to assess the phenomena and processes susceptible to influence the chemistry of the clay and the migration of radionuclides the following experiments have been performed: 1. Percolation experiments were made on 16 different clay cores with two different types of synthetic cement water: young cement water (ph ~ 13.5) and evolved cement water (ph ~ 12.5). 2. The chemical composition of the percolated water was followed as function of time. The concentration of the main components (electrical conductivity, ph, major cations, major anions, organic matter ) was measured and provided as input parameters for the work package 5 on the geochemical modelling. These experimental results were used as reference and compared with the predictions of the chemical coupled transport codes used in order to assess the various approaches followed by the different modelling teams. 3. Migration experiments have been performed with non sorbed and sorbed tracers on the clay cores exposed to an alkaline perturbation: tritiated water and anions (HTO, I, H 14 CO 3 ) and comparison with reference experiments; cations (Na +, Cs +, Ca 2+, Sr 2+ ) and comparison with available data from previous experiments; 4. Electromigration tests have been performed to determine the migration parameters of the main constituents of the plume in undisturbed clay formation: Na +, Ca Through-Diffusion experiments with evolved cement water (saturated with respect to portlandite) have been performed on the Boom Clay core to study the precipitation of calcite fronts in plastic clay formation. 6. At the end of the experiments, mineralogical analyses were done to assess the extent of mineralogical transformation of clay cores expose to young and evolved cement waters. The results were also given to WP 5 to assess the geochemical modelling results ( 10.2). 7. During the project, samples taken from an interface between a shotcrete shield and oxidized Boom Clay located in the excavation damage zone (EDZ) of the Test Drift of the HADES Underground Research laboratory in Mol have also been submitted to mineralogical analyses. 45

58 Alkaline plume through Boom Clay cores: percolation experiments a) Experimental Boom Clay cores confined between porous stainless steel filters are percolated with two types of synthetic alkaline solutions: a young cement water (mainly [Na, K]OH with a ph of 13.5), and an evolved cement water (saturated with respect to portlandite, Ca(OH) 2, with a lower ph: 12.5). The young cement water is used to simulate short-term conditions and to benefit of faster chemical kinetics, while the evolved water is more representative of the real conditions expected at long-term in the near-field of an underground repository. Figure shows the principle of the percolation experiments while table presents the chemical composition of the two types of synthetic cement waters used in the percolation experiments. ( 131 I, HTO, H 14 CO 3 ) Impulse injection Inlet (Cement Water) Inlet filter Clay core Outlet filter Outlet (Percolate) Figure Experimental set-up used to percolate synthetic cement water through Boom Clay cores and to make pulsed injections of non sorbed tracers after an alkaline perturbation. The pore volume of the clay cores is about 13.8 cm3 (with a HTO porosity of ~ 0.38). The percolation water at the outlet of the clay cores is sampled at regular interval for chemical analyses of the main dissolved species (major cations measured with ICP-AES 2 or ICP-MS 3, and major anions measured by Ion Chromatography, IC). Other parameters as ph, Electrical Conductivity (EC), UV absorbency (at 280 nm) and Dissolved Organic Carbon (DOC) were also measured. 2 ICP-AES : Inductively Coupled Plasma Atomic Emission Spectrometry. 3 ICP-MS: Inductively Coupled Plasma Mass Spectrometry. 46

59 Table Chemical composition of Boom Clay water (EGBS) and the synthetic cement water (YCW and ECW) used in the percolation experiments. EG/BS * Young Cement Water Evolved Cement Water Cations mg L -1 mmol L -1 mg L -1 mmol L -1 mg L -1 mmol L -1 K Na Ca Mg Sr Si Al <0.2 <0.007 Fe < 0.09 <0.002 Anions Cl SO < 1. < 0.01 <1.3 <0.01 Carbon (as elementary C) TIC TOC TC ph * EG/BS: Boom Clay water taken from the EGBS piezometer: water composition from Dierckx (1997). b) Effect of cement water on the chemical properties of Boom Clay The water collected at the outlet of the clay cores is analyzed at regular intervals and the main physico-chemical parameters determined. The following trends can be observed from figure and figure 6.3.9: On the duration of the experiments, the hydraulic conductivity of the clay cores percolated with young cement water increased by a twofold factor (about 90 %) while it decreased by about 20 % in the case of the evolved cement water. 47

60 Simultaneously, the electrical conductivity (EC) and ph increased for the young cement water. For the evolved cement water EC first decreased to a lower plateau before to gradually increase again after more than 15 pore volumes renewal. The ph value of the clay cores percolated with evolved cement water remained between 8.5 and 9 until 15 pore volume renewal before to increase up to after 30 pore volumes. The dissolved organic carbon (DOC) dramatically increased in the case of young cement water, indicating an important leaching of the organic matter present at the surface of clay minerals. For the clay cores percolated with evolved cement water, DOC first decreased before to be slowly released when the ph of this water begins to raise. In the case of the clay cores percolated with young cement water, some dissolved species measured in the water rapidly increased: Na +, K + evolved from the concentrations initially present in the interstitial clay water to their respective concentration in the young cement water, while the total dissolved silica and the total dissolved aluminium also increased, revealing the dissolution of clay minerals at high ph. c) Modelling of percolation experiments by SCK and Serco When comparing the model predictions of the reactive transport codes with the experimental results from the percolation experiments on Boom Clay cores, the most obvious finding is that the theoretical values predicted by the model simulations appear faster than the experimental breakthrough curves. It is especially true for the ph curves as illustrated by figure As, the ph increase predicted by the model simulations appears systematically faster than the observations in the reality, it seems that the buffering capacity of clay is systematically underestimated by the different models. The reason may be due to a process lacking in the model, or inadequately described, or to an incorrect value for some model parameters. Taking also into account phdependent cation exchange complex sites from the organic matter is not sufficient to solve this discrepancy, or may induce other discrepancies somewhere else in the model results if unrealistic values of exchange capacity are considered ph 10 8 no cation exchange single site cation exchange multisite cation exchange Pore Volume Figure Discrepancy between ph measurements and model simulations. 48

61 Young Cement Water Evolved Cement Water Hydraulic Conductivity (m/s) 1.8 E E E E-13 Young Cement Water (YCW) Hydraulic Conductivity (m/s) YCW 6.1 YCW 6.2 YCW 6.3 YCW 6.4 YCW 6.5 YCW 6.6 Hydraulic Conductivity (m/s) 2.0 E E E E E E-12 Evolved Cement Water (ECW) Hydraulic Conductivity (m/s) ECW 5.1 ECW 5.2 ECW 5.3 ECW 5.4 ECW 5.5 ECW E C E Total percolated YCW (ml) Young Cement Water (YCW) Electrical Conductivity (ms/cm) 6.0 E Total percolated ECW (ml) 3.0 Ionic Conductivity (ms/cm) YCW YCW 6.2 YCW 6.3 YCW YCW 6.5 YCW Total percolated YCW (ml) Ionic Conductivity (ms/cm) Evolved Cement Water (ECW) Electrical Conductivity (ms/cm) 17 C ECW 5.1 ECW 5.2 ECW 5.3 ECW 5.4 ECW 5.5 ECW Total percolated ECW (ml) Evolved Cement Water (ECW) ph value 12 Young Cement Water (YCW) ph value ph 11 YCW 6.1 ph 10.0 YCW YCW 6.3 YCW 6.4 YCW 6.5 YCW ECW 5.1 ECW 5.2 ECW 5.3 ECW 5.4 ECW 5.5 ECW Total percolated YCW (ml) Total percolated ECW (ml) Young Cement Water (YCW) UV Absorbency at 280 nm 10 8 Evolved Cement Water (ECW) UV Absorbency at 280 nm ECW 5.1 ECW 5.2 ECW 5.3 ECW 5.4 UV-Absorbance at 280 nm YCW 6. YCW 6.2 YCW 6.3 YCW 6.4 YCW 6.5 YCW 6.6 Absorbance at 280 nm 6 4 ECW 5.5 ECW Total percolated YCW (ml) Total percolated ECW (ml) Figure Changes of physico-chemical properties of water percolated through clay cores after injection of young and evolved synthetic cement water. 49

62 Hydraulic Conductivity (m.s-1) 2.1 E E E E E-13 Hydraulic Conductivity (m s -1 ) Young CW, Cell # 8.2 Young CW, Cell # 8.3 Evolved CW, Cell # 8.5 Conductivity [μs.cm-1] Electrical Conductivity [μs cm -1 ] Young CW, Cell # 8.2 Young CW, Cell # 8.3 Evolved CW, Cell # 8.5 Evolved CW, Cell # 8.6 Evolved CW, Cell # E Volume of percolate since switch Volume of percolate since switch Concentration [mg.l-1] Na Potassium, K Young cement water YCW, Cell # 8.2 Na YCW, Cell # 8.2 K YCW, Cell # 8.3 Na YCW, Cell # 8.3 K C0 K in YCW K ph [-] ph [-] Young CW, Cell # 8.2 Young CW, Cell # 8.3 Evolved CW, Cell # 8.5 Evolved CW, Cell # C0 Na in YCW Sodium, Na 9 ph of Boom Clay Water Volume of percolate since switch Volume of percolate since switch UV at 280 nm UV Absorbency at 280 nm (Brown Color of Dissolved Organic Matter) Young CW, Cell # 8.2 Young CW, Cell # 8.3 Evolved CW, Cell # 8.5 Evolved CW, Cell # Volume of percolate since switch DOC [mg/l] Dissolved Organic Carbon (DOC) [mg/l] Young CW, Cell # 8.2 Young CW, Cell # 8.3 Evolved CW, Cell # 8.5 Evolved CW, Cell # 8.6 DOC Volume of percolate since switch [m Concentration [mg.l-1] Silica, Si Young cement water YCW, Cell # 8.2 Si YCW, Cell # 8.3 Si C0 Si in YCW Volume of percolate since switch Si Concentration [mg.l-1] YCW, Cell # 8.2 Al YCW, Cell # 8.2 Ca YCW, Cell # 8.3 Al YCW, Cell # 8.3 Ca C0 Al in YCW C0 Ca in YCW Calcium, Ca Aluminum, Al Young cement water Volume of percolate since switch Al Ca Figure Analyses of the water percolated through clay cores after injection of young and evolved synthetic cement water. 50

63 Effect of alkaline plume on the migration behaviour of tritiated water and anions Before to start the percolation experiments with young and evolved cement waters, reference migration experiments (pulse injection) have been performed on each clay core with non-sorbed (HTO, 125 I ), or very weakly sorbed (H 14 CO 3 ), tracers to characterize the initial migration properties of the undisturbed clay. After more than two years of percolation of the clay cores with alkaline solutions, the same tracers were again injected. Their migration parameters have been determined from the breakthrough curves obtained before and after alteration and presented in figure Table and table respectively presents the values of the reference migration parameters on undisturbed Boom Clay cores and the comparison of the migration parameters for H 14 CO 3 before and after percolation with evolved cement water (ECW). On the other hand, the alkaline perturbation with young cement water (YCW) does not significantly affect the migration of HTO, 131 I and H 14 CO 3. Only the retention of carbonate ions appreciably increases in the case of evolved cement water probably by precipitation of calcium carbonate, or isotopic exchange with this latter. Table Reference migration experiments with HTO, 131I and H14CO3. Tracer V app ηr D app ηrd app ( ) (m s -1 ) ( ) (m 2 s -1 ) (m 2 s -1 ) 131 I 3.3 E E E-11 HTO 1.4 E E E-11 H 14 CO E E E-11 Table Migration experiments with H14CO3 before and after percolation with evolved cement water (ECW). Tracer V app ηr D app ηrd app ( ) (m s -1 ) ( ) (m 2 s -1 ) (m 2 s -1 ) Reference migration experiments before alkaline perturbation H 14 CO E E E-11 Migration experiments after percolation with evolved cement water H 14 CO E E E-11 Ratio of the parameter values (after/before) After/Before

64 Carbonate ηr product increases by a factor of about 6 after alteration with evolved cement water. Carbonate also appears to be 10 times more retarded than the iodide anion in undisturbed conditions. Normalized Activity [ ] Reference Migration Experiments Before Alkaline Plume 0.4 C I-131 HTO Volume percolated [ml] Normalized Activity [-] Results after Percolation of Young Cement Water (YCW) YCW Cell 2 C-14 Normalized Activity [-] Results after Percolation of Evolved Cement Water (ECW) ECW Cell 5 HTO 0.4 YCW Cell 2 HTO 0.4 ECW Cell 5 C Percolated volume [ml] Percolated volume [ml] 250 Migration of H 14 CO 3 - in evolved cement water (ECW) Migration of H 14 CO 3 - in evolved cement water (ECW) Concentration (Bq/cm 3 ) H 14 CO - 3 is moderately retarded in (ECW) η R = 1.82 if η = 0.11, then, R = 16.6 D app = m 2 s -1 D eff = m 2 s -1 Experimental data Fit result Cumulated Activity (Bq) Experimental data Fit result H 14 CO - 3 is moderately retarded in (ECW) η R = 1.82 if η = 0.11, then, R = 16.6 D app = m 2 s -1 D eff = m 2 s Time (days) Time (days) Figure Effect of alkaline conditions on the migration behaviour of tritiated water and anions. Only H 14 CO 3 appears to be retarded after percolation with Ca 2+ -rich evolved cement water (ECW). 52

65 Effect of alkaline plume on the migration behaviour of cations Spikes of strontium-85 and cesium-137 have been injected in Boom clay cores percolated during two years with young and evolved synthetic cement water. After more than one year, no activity could be detected in the water collected at the outlet of the four percolation experiments. These experiments will still be followed during a few months and then the four clay cores will be cut in thin slices to determine the activity distribution profile in the solid phase Electro-Migration with alkaline plume constituents The migration parameters of major cations as, Na +, Ca 2+ (or Sr 2+ as chemical analogue of calcium) present in the alkaline plume have been determine by means of electromigration experiments. Two clay cores sandwiching a radionuclide source are confined in the Plexiglas cell between two ceramic porous disks. Pt-electrodes are immersed into the electrolyte compartments, which are connected at each side of the cylindrical Plexiglas cell. The electrode compartments are filled with Boom Clay Water (BCW, ~ M NaHCO 3 ) (Dierckx et al., [1997]). A DC power supply connected to the electrodes provides a constant current (typically between 8-14 ma). Figure is a schematic view of the experimental set-up. During the electromigration experiment, because of water electrolysis H + and OH ions are produced respectively at the anode and at the cathode. If these ions should freely migrate into the clay cores it would induce a ph gradient inside the clay core, seriously disturbing the physicochemical conditions in the clay. To avoid this effect, the electrolyte solutions are continuously recirculated with a peristaltic pump and mixed drop by drop to preclude any electrical short circuit. After the test, the clay cores are cut in slices of ~ 1 mm thickness. The activity of each slice is measured to determine the distribution profile of the radionuclide in the clay. Cathode DC power supply - + Anode source position Porous clay core ceramic filter Cathode compartment Electromigration cell Anode compartment Peristaltic pump Acid-Base neutralization reservoir Peristaltic pump Figure Schematic representation of the electromigration set-up. 53

66 The migration parameters obtained for Na +, Ca 2+, and Sr 2+ are given in table 6.3.7, table 6.3.8, and table respectively. Table Apparent molecular diffusion coefficient (D m a) obtained for 22 Na + at different electrical fields. Experiment E [V/m] V c t [10-7 m/s] D a [10-11 m²/s] D m a [10-11 m²/s] DIF-Na EM-Na EM-Na EM-Na EM-Na EM-Na4 (*) EM-Na5 (*) EM-Na6 (*) EM-Na10 (*) Linear Regression a μ c [10-9 m²/vs] Linear Regression a D m [10-11 m²/s] Average <D a m > [10-11 m²/s] All points 3.4 ± ± ± 6.4 No outlier (*) 3.8 ± ± ±

67 Table Apparent molecular diffusion coefficient (Dma) obtained for 45 Ca 2+ at different electrical fields. Experiment E [V/m] V c t [10-8 m/s] D a [10-12 m²/s] D m a [10-12 m²/s] DIF-Ca DIF-Ca EM-Ca EM-Ca EM-Ca EM-Ca EM-Ca EM-Ca Linear Regression Average a D m <D a m > [10-12 m 2 /s] [10-12 m 2 /s] 7.8 ± ± 4.7 Table Apparent molecular diffusion coefficient (D m a) obtained for 85 Sr 2+ at different electrical fields. Experiment E [V/m] V c t [10-7 m/s] D a [10-11 m²/s] DIF-Sr DIF-Sr EM-Sr EM-SR EM-SR EM-Sr EM-Sr EM-Sr Standard dev. Linear Slope α [m] = regression Intercept D m a [m²/s] =

68 Through-diffusion tests Three through-diffusion tests have been performed inside an anaerobic glove-box with a controlled argon atmosphere containing 0.4 % CO 2. The aim of these experiments was to simulate in the laboratory the precipitation of a calcite rim in the clay where a calcium-rich evolved cement water (ECW) will encounter the carbonate-bearing interstitial water of the formation. These throughdiffusion tests were started to replace an in situ piezometer initially foreseen to achieve the same objective. Unfortunately, technical difficulties in the detailed design of the piezometer and in the procedure of overcoring did not allowed to sufficiently guarantee the success of retrieval of the clay around this in situ experiment. For these reasons, the piezometer was replaced by these through diffusion tests whose principle and experimental setup are described at figure and figure respectively. Clay core Synthetic Cement Water Inlet Outlet Boom Clay Water (K, Na, Ca), OH CaCO 3 HCO 3 + CO 2 ****** Figure Schematic principle of through-diffusion test with evolved cement water (ECW). Teflon Stopper inox inox o-ring 35 ml ECW f f screw i i r l Clay Core li 35 ml t t n EG/BS Water e e g r r Figure Experimental set-up used for through-diffusion experiments. 56

69 At the end of the through diffusion tests, the clay cores are removed for mineralogical analyses to detect the precipitation of calcium carbonate Mineralogical characterization of Boom Clay exposed to an alkaline perturbation Two mineralogical characterization studies of alteration of Boom Clay by an alkaline front have been performed by Equipe Recherche et Matériaux (ERM) from Poitiers (France) in the frame of the work package 1 Retention. The first study deals with fresh Boom Clay cores from percolation experiments and through diffusion tests performed in the laboratory, while the second set of observations concerns the development of an alkaline front in oxidized Boom Clay sampled at the interface of a 10 years old shotcrete and oxidized Boom Clay from the excavation disturbed zone (EDZ). a) Fresh Boom Clay cores from percolations with YCW, ECW, and T-D test Three clay cores submitted to an alkaline perturbation during several months, or years, have been recovered at the end of the experiments. After a first cutting of each cylindrical cores into two hemicylinders parallel to the path of the alkaline fluid, one was left intact, while the second one was subdivided into five smaller pieces perpendicular to the main cylinder axis. The clay samples were dried in an anaerobic glove box with less than 1 ppm O 2 and sent to ERM for mineralogical analyses. The three clay cores were: one percolated with young cement water (YCW), one percolated with evolved cement water (ECW), and one taken from the three through-diffusion tests performed in the lab to replace an in situ piezometer aimed to detect calcium carbonate precipitation in the formation exposed to high ph fluids. The mineralogical observations made by Bouchet (2003) can be summarized as follows: 1. The only well observable alkaline perturbation is that caused by the young cement water (YCW, ph = 13.5): the dissolution of smectite was observed by X-rays diffraction (XRD) and scanning electron microscopy (SEM). This point is particularly important for the geochemical modelling work performed in the work package The sample exposed to evolved cement water (ECW, ph = 12.5) did not allow to put in evidence strong indications of alkaline alteration. 3. Finally the sample from the through-diffusion test did not reveal any trace of calcium carbonate precipitation, or any indication of alteration: it appeared as unaltered. The reason could be due to an insufficient supply of carbonate, or CO 2. The two other plugs of the through-diffusion tests were left in longer contact with the carbonate solution inside a glove box for further analyses. b) Interface Shotcrete/oxidized Boom Clay During the excavation works of the new connection gallery in the HADES underground laboratory in Mol in March 2002, the digging operations make possible an unexpected observation. The clay lying behind the thin shotcrete shield terminating the existing Test Drift was strongly oxidized on a distance of about 80 cm. Figure shows the interface between the 10 years old shotcrete shield and the oxidized Boom Clay. 57

70 Figure Interface between a shotcrete front and oxidized Boom Clay. On the photograph of figure , the excavation disturbed zone presents deep cracks filled with a yellow solid identified by Bouchet [2002] as jarosite: KFe 3 (SO 4 ) 2 (OH) 6. Jarosite is a secondary product of pyrite oxidation whose general reaction can be summarized as followed: 4 FeS O H 2 O > 4 FeO(OH) + 8 SO H Eqn Pyrite oxidation is accompanied by a strong acidification of the clay. The consequence in this case is that the alkaline perturbation did not developed in slightly alkaline intact clay (ph = 8.2), but in an acidic zone with a much lower ph strongly buffering the alkalinity of the alkaline water released by the cement of the shotcrete. A sufficiently important thickness of oxidized clay could neutralize an alkaline front, or vice et versa, the sulphuric acid released by pyrite oxidation could alter the concrete This possibility, and appropriate mass balance calculations, should also be considered for the geochemical modelling of the propagation of an alkaline perturbation in the excavation disturbed zone. The mineralogical characterization allowed to put in evidence a strong oxidation near the shotcrete, but the interface was unfortunately very fuzzy and irregular due to the typical layered structure of the shotcrete and the presence of raw sand grains in the shotcrete matrice. No clear cement/clay interface could be studied at the level of several micrometers. 58

71 6.4 CONCLUSIONS OF WP1 ON RETENTION The formation of CSH phases in the alkaline disturbed zone will improve its retention capabilities for radionuclides of the alkaline earth series and for actinides and lanthanides compared to the undisturbed system due to the high affinity of CSH phases for these two types of radionuclides. Coprecipitation processes do not increase the R d values for alkaline earth metals suggesting that they sorbe on CSH phases by ion-exchange. Therefore, the pore water composition should also significantly affect the R d value for Sr(II) and other earth alkaline metal cations. At the opposite, changes in the pore water composition have no observable effect on the sorption of actinides and lanthanides. No sorption was observed for iodide on the different sorbents studied, except a slight sorption on Callovo-Oxfordian Clay measured in desionised water. The reason should be searched for in the lab work made under air probably favouring oxidation of iodide into iodate and its subsequent sorption onto iron oxy-hydroxides from oxidized pyrite, or maybe a chemical reaction of elemental iodine with organic matter. The highest sorption of selenite was measured onto salt/cement mixture in desionised water, maybe by reaction or exchange with CSH, or with calcium carbonate. A lower sorption was observed for selenite on Callovo-Oxfordian Clay and on MX-80 bentonite in saline solutions. Batch sorption tests were performed with 22 Na and 45 Ca on MX-80 bentonite and crushed granite at high ph and high ionic strength. These conditions are representative for an alkaline plume developing in saline groundwaters present in deep crystalline bedrock. The results of these experiments can be summarized as follows. The sorption of monovalent cation as Na + was lower than that of divalent cation as Ca 2+ for all types of artificial waters and decreased in alkaline fluids due to the presence of competing cations in solution. As expected, sorption of both cations onto crushed granite was lower than on MX-80 bentonite. In general, the sorption was highest for samples of shortest experimental time and decreased with time. The sorption of 22 Na on bentonite was low for saline alkaline and saline hyper alkaline waters (K d ( ) 10-3 m 3 /kg). However, for the fresh water a higher sorption was observed (K d ( ) 10-2 m 3 /kg). The sorption of 22 Na on crushed rock was low for all synthetic waters (K d ( ) 10-3 m 3 /kg). The sorption of 45 Ca on bentonite was very high for all water types (K d ( ) m 3 /kg) while the sorption of calcium on crushed rock was generally higher than for sodium (K d ( ) 10-3 m 3 /kg). The main results of percolation and diffusion experiments made on bentonite consolidated plugs, Callovo-Oxfordian Clay, and Boom Clay cores can be summarized as follows. Important chemical changes have been observed in the alkaline fluids after percolation through the Boom Clay cores. The concentration of major dissolved species in the percolation fluids have been compared to the results of the predictions made with different models using different geochemical computer codes (see WP 5). Mineralogical analyses made on clay cores exposed to synthetic cement waters have only revealed dissolution of smectite for the Boom Clay cores exposed to fresh cement waters, in agreement with the observed increase of hydraulic conductivity. Within the time of the experiments, no mineralogical modification could be observed in the Boom Clay cores submitted to evolved cement water although the hydraulic conductivity decreased. No evidence of neo-precipitation of calcium carbonate was observed in through-diffusion experiments in which Boom Clay cores were contacted from one side with bicarbonate water and from the other side with water equilibrated with calcium hydroxide. Diffusion experiments on Boom Clay cores have also revealed a significant 59

72 retardation of H 14 CO 3 after exposure to calcium-rich evolved cement water. No change in the diffusion parameter of tritiated water have been observed on Boom Clay. The diffusion tests carried out under site conditions with Callovo-Oxfordian clay samples show that the effective diffusion coefficient of cationic species (Na +, K + and Ca 2+ ) were systematically larger than those of tritiated water suggesting an enhanced transport of cations. For the Callovo-Oxfordian Clay, the effective diffusion coefficient of Na +, K + and Ca 2+ were also greater in the presence of cement water than under the site conditions. A slow but regular decrease of effective diffusion coefficient of HTO was moreover observed in the presence of cement water. This observation could be attributed to a clogging of the porous network, but need confirmation by additional mineralogical analyses. 60

73 6.5 REFERENCES OF WORK PACKAGE 1 Atkins, M., D. Bennett,, A. Dawes, F. Glasser, A. Kindness., and D.A. Read (1991) Thermodynamic Model for Blended Cements, UK DoE Report, DoE/HMIP/RR/92/005. Bird G. W., Lopata V. J. (1980) Solution interaction of nuclear waste anions with selected geological materials. Scientific Basis for Nuclear Waste Management 2, Bouchet A. (2002) Mineralogical study of five interfaces of oxidized clay samples of Boom Clay in contact with a shotcrete front (HADES Underground Laboratory, Site of Mol Belgium). Etudes Recherches Matériaux, ERM, Poitiers. Report ERM AB 268, 89 pp. Ref. ERM: ; Affaire N SCK CEN order N 2002/ File: MOL02Had.doc. Bouchet A. (2003) Mineralogical study of three cores of Boom Clay subjected to percolation and though-diffusion experiment (Site of Mol Belgium). Etudes Recherches Matériaux, ERM, Poitiers. Report ERM AB 220, 67 pp. Ref. ERM: ; Affaire N SCK CEN order N 2003/ File: MOL03ect.doc. Cartledge, F.K., L.G. Butler, D. Chalasani, H.C. Eaton,, F.P Frey, E. Herrera, M.E. Titlebaum, and S.-L. Yang (1990) Immobilisation Mechanisms in Solidification/Stabilization of Cd and Pb Salts using Portland Cement Fixing Agents, Environ. Sci. Technol. 24, Cocke, D.L. and M.Y.A. Mollah (1993) The Chemistry and Leaching Mechanism of Hazardous Substances in Cementitious Solidification/Stabilization Systems, Lewis Publishers,. Dierckx A. (1997) Boom Clay in situ porewater chemistry. SCK CEN Report BLG-734, 3 pp. Eriksen T.E., Jansson M. (1996) Diffusion of I -, Cs + and Sr 2+ in compacted bentonite - Anion exclusion and surface diffusion, SKB Technical Report ERM, Equipe Recherche Matériaux (2001) Laboratoire souterrain de recherche Meuse/Haute- Marne, Analyses minéralogiques et géochimiques, forage EST 205, Rapport final d opération (RO), B ref. RP 0ERM /A. Poitiers. Fried S., Friedman A. M., Cohen D., Hines J. J., Strickert R. G. (1978) The migration of long-lived radioactive processing waste in selected rocks. Annual report to the office of waste handling project AN0115A FY Gougar, M.L.D., B.E. Scheetz, and D.M. Roy (1996) Ettringite and C-S-H Portland Cement Phases for Waste Ion Immobilization: A Review, Waste Management 16(4), Hingston F. J. (1981) A review of anion adsorption. From: M. A. Anderson, A. J. Rubin (eds.): Adsorption of Inorganics at Solid-Liquid Interfaces. Collingwood: Ann Arbor Science, Jefferies N.L., Tweed C.J., Wisbey S.J. (1988) The effects of changes in ph within a clay surrounding a cementitious repository, Scientific Basis for Nuclear Waste Management XI, pp

74 Johnson, C.A., and M. Kersten (1999) Solubility of Zn(II) in Association with Calcium Silicate Hydrates in Alkaline Solutions, Environ. Sci. Technol. 33, Lieber, W. and G. Gebauer (1969) Einbau von Zn in Calciumsilicathydrate, Zement-Kalk-Gips 4, Macphee, D.E., K. Luke, F.P. Glasser and E.E. Lachowski (1989) Solubility and Aging of Calcium Silicate Hydrates in Alkaline Solutions at 25 C, J. Am. Cer. Soc. 72(4), Mcculloch, C.E., M.J. Angus, R.W. Crawford, A.A. Rahman and F.P. Glasser (1985) Cements in radioactive waste disposal: some mineralogical considerations. Mineral. Magazine 49, McKinley I.G., Scholtis A. (1992) Compilation and comparison of radionuclide sorption databases used in recent performance assessments. From: Radionuclide Sorption from the Safety Evaluation Perspective, Proceedings of a NEA Workshop, Interlaken Switzerland, October 1991, Miyahara K., Ashida T., Kohara Y., Yusa Y., Sasaki N. (1991) Effect of bulk density on diffusion for cesium in compacted sodium bentonite, Radiochimica Acta 52/53, pp Moroni, L.P. and F.P. Glasser (1995) Reactions between Cement Components and U(VI) oxide. Waste Management 15, Muurinen A., Penttilä-Hiltunen P., Uusheimo K. (1989) Diffusion of chloride and uranium in compacted sodium bentonite, Materials Research Society Symposium Proceedings 127, pp Neretnieks I. (1982) Diffusivities of some dissolved constituents in compacted wet bentonite clay MX-80 and the impact on radionuclide migration in the buffer. SKB technical report Nowak E. J. (1979) Radionuclide sorption and migration studies of getters for backfill barriers. Sandia National Laboratories, Report (SAND ). Nowak E. J. (1983) Diffusion of radionuclides in brine-saturated backfill barrier materials. Mat. Res. Soc. Symp. Proc. 15, Oscarson D. W., Miller H. G., Watson R. L. (1986) The potential effectiveness of mercury minerals in decreasing the level of iodine-129 in a nuclear fuel waste disposal vault. Nuclear and Chemical Waste Management 6, Oscarson D.W. (1994) Surface diffusion: is it an important transport mechanism in compacted clays?, Clays and Clay Minerals 42(5), pp Oscarson D.W., Hume H.B., Sawatsky N.G., Cheung S.C.H. (1992) Diffusion of iodide in compacted bentonite, Soil Science Society of America Journal 56, pp Put M.J. (1991) An improved mathematical model for the interpretation of the flow-through type diffusion test with influence of filter plates., Radioactive Waste Management and the Nuclear Fuel Cycle 16(1), pp Sager M. (1993) Selenium occurrence and ecology. Stud. Environ. Sci. 55,

75 Sato H., Ashida T., Kohara Y., Yui M., Sasaki N. (1992) Effect of dry density on diffusion of some radionuclides in compacted sodium bentonite, Journal of Nuclear Science and Technology 29(9), pp Sauzéat E., Guillaume D., Villieras F., Dubessy J., François M., Pfeiffert C., Pelletier, Ruck R., Barrès O., Yvon J., Cathelineau M. (2001) Caractérisation minéralogique, cristallochimique et texturale de l argile MX-80. ANDRA Report C RP 0ENG Sazarashi M., Ikeda Y., Seki R., Yoshikawa H. (1994) Adsorption of I - ions on minerals for 129 I waste management, Journal of Nuclear Science and Technology 31, Seby F., PotinGautier M., Giffaut E., Donard O. F. X. (1998) Assessing the speciation and the biogeochemical processes affecting the mobility of selenium from a geological repository of radioactive waste to the biosphere. Analusis 26 (5), Strickert R., Friedman A. M., Fried S. (1980) The sorption of technetium and iodine radioisotopes by various minerals, Nucl. Techn. 49, Tits, J., E. Wieland, J. P. Dobler, & D. Kunz (2003a) Uptake of Strontium by calcium Silicate Hydrates under High ph Conditions: An Experimental Approach to distinguish adsorption from coprecipitation processes. Mater. Res. Soc. Symp. Proc., In press. Tits, J., T. Stumpf, T. Rabung, E. Wieland, & T. Fanghänel (2003b) Uptake of Cm(III) and Eu(III) by Calcium Silicate Hydrates: A Solution Chemistry and Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) Study. Environ. Sci. Techn., 37, van Brakel J., Heertjes P.M. (1974) Analysis of diffusion in macroporous media in terms of a porosity, a tortuosity and a constrictivity factor, Int. J. Heat Mass Transfer 17, pp

76 7 WORK PACKAGE 2: CLAY ENGINEERED BARRIER The organisations contributing to Workpackage 2 during the course of the ECOCLAY II project are ANDRA, ENRESA, UAM, IETcc, CSIC-EEZ, GRS, SKB. In this work package the evolution of the clay engineered barrier under hyperalkaline conditions had been investigated. The work package was divided in two tasks. In the first task studies of geochemical processes in a bentonite engineered barrier under hyperalkaline conditions were performed. Therefore the quantification, time evolution and spatial progression of the mineralogical reaction including the kinetic had been investigated in batch and transport cell tests. Influences of concrete, bentonite and solution composition were examined. The second task dealt with the study of the consequences on bulk properties of hydro-mechanical properties of bentonite obtained in swelling pressure measurements. Cement pore water analysis During the project the resistance of cementitious material in contact to groundwater solutions had been studied. In case of Portland cement when exposed to a granitic ground water alkali release, dissolution of portlandite as well as decalsification of the CSH-phase was observed. Dissolution of CSH-phases tends to form silicate polymers with the possibility of incorporation of aluminium in the gel to form alumino-silica gel. The investigated concretes indicate a high chemical resistivity as well as a mechanical stability against the used solutions. Salt cement in contact to high saline solutions had been investigated to determine the reaction path of these systems. The obtained reaction solutions were used for further swelling pressure experiments of MX-80 bentonite. Batch experiments Batch experiments had been performed with MX-80 and FEBEX bentonite in contact with solutions of varying composition (KOH, NaOH and CaO) within ph range ph (cement pore waters). The experiments were performed from 1 day to 18 months (temperature range T= 25 C C). The reactions were characterised by the dissolution of smectite and montmorillonite and the precipitation of neoformed phases, e.g. analcime, phillipsite, saponite, CSH, tobermorite, portlandite and gyrolite. Specific surface decrease as well as pore-size increase due to agglomerate formation could be observed. Purely siliceous minerals (quartz and cristobalite) tended to disappear in those samples that were submitted to the experiments with high ph values and high temperature values, subsequently. Transport and kinetic studies In transport cell tests the interface portland cement-bentonite was investigated. At a time scale up to 365 days (T = 25 C 200 C) NaOH solution produced only minor changes in the bentonite material. The formation of CSH-gel, analcime and magnesium clays, found in the batch experiments has been validated at the cement-bentonite interface in column experiments. The alteration of compacted bentonite was heterogeneous. Despite of the formation of a CSH-layer (tobermorite) at the interface, the precipitation of zeolithes follows 64

77 irregular patterns associated to the circulation of fluids via preferential. Diffusion of Ca 2+ from the Portland cement produced progressively Ca-exchange. Dissolution tests on smectite, volcanic glass and albite had been performed. Smectite dissolution was stoichiometric at ph 11.5 and 12.5 and quasi-sthoichiometric at ph Dissolution rates increased with temperature and ph. However, values obtained at 70 ºC were too low compared with dissolution rates at 25 and 50 ºC. Within the ph range studied, log rate was directly proportional to 0.33 ph, 0.42 ph, and 0.48 ph at 25, 50 and 70ºC, respectively. It indicated a ph dependent activation energy. The dissolution rate increased with ph and temperature. Solutions above the critical ph value of 11 may produce intensive bentonite dissolution. Volcanic glass and plagioclase dissolved one or two orders of magnitude faster than smectite. Swelling pressure measurements The swelling pressure of MX-80 in contact to 0.3 and 1.0 M NaOH solutions was strongly affected. The drop could be related to the mass loss of silica, since the swelling pressure was exponentially dependant on the clay density. In the tests, both clay and water analyses, showed a large discrepancy between the silica release and the release of other elements. The XRD analyses showed that the original cristobalite is one source for the dissolved silica. The amount of dissolved silica was somewhat larger than the possible contribution from cristobalite in the montmorillonite samples, and the large changes in CEC could not be simply be explained by the loss of cristobalite. It was therefore likely that also the montmorillonite contribute to the measured release of silica. Swelling pressure measurements were also performed using bentonite in contact with high saline solutions. In contact with cement pore water a swelling pressure of ~20 MPa could be determined. Earlier investigations could show that a less water up-take had to be observed in contact with cement solution and therefore also would have had a lower swelling pressure to be expected. The swelling pressure is largely dependant on the degree of compaction and the applied fluid pressure. The few results obtained so far can not be extrapolated to other boundary conditions. We are of the opinion, that this is true not only for our own experiments. Factors like dry density, microstructure, flooding regime, sample dimensions seem to have a much bigger influence on the swelling pressure than brine composition. 65

78 7.1 MODELLING OF CEMENT / BENTONITE INTERACTIONS Th. Meyer and H.-J. Herbert (GRS) Introduction The concepts for the final disposal of radioactive and hazardous wastes include the use of technical barriers like sealing and backfilling materials, which are based on multi-barrier concepts including clay and cemented materials. For that reason the safety assessment of the repository system implies detailed knowledge of the geochemical behaviour of cement/clay systems in contact with solutions. In the case of solution intrusion in the repository, the cemented wastes show a significant change in their structure. The changes result in dissolution and precipitation reactions, effecting a change in the solution composition and the brines ph. The resulting solution can effect changes in the clay material destructing the materials composition. The long-term behaviour of cementitious materials in brines had been investigated by means of a time accelerating leaching experiment and by the geochemical modelling of the observed reactions ( 10.6). The investigated materials had been a mixture of brown coal fly ash, cement and halite mixed with water as well as a mixture of MgO and MgCl 2 solution (oxychloride cement). The leaching experiment was developed in the GRS specifically for the boundary conditions of underground repositories e.g. in salt formations. Bentonites are considered to be favourable as sealing materials because of their swelling capacity. In salt formations however the presence of high saline brines must be considered. This fact poses certain problems concerning the use of bentonites as buffer and sealing materials. Salt solutions tend to reduce the swelling capacity and no swelling pressure will develop if the swelling capacity is reduced to an extent where the void volume is not filled by the swollen clay. For very high salt contents, the existing models are not able to predict or reproduce the experimental results. An experimental program was conducted to determine swelling pressures of compacted bentonites, under the special conditions of brine inflow into repositories situated in salt formations Cascade experiments In order to explain leaching and corrosion processes, a special laboratory scale experiment in several steps (cascades) was developed. It is a rapid experimental method for the investigation of the reaction path of the fluid-solid interactions. During the experiment that goes on for several months the temperature is kept constant and evaporation is excluded. In a cascade experiment a weighed mass of a ground down powder is reacted with a certain mass of solution (first cascade). The grinding of the cemented material leads to an increased material surface and thus a time accelerating effect can be obtained. After 2-3 days of equilibration under continuous rotation of the reaction vessel at 25 C, the solution is extruded through a pressure filter onto unreacted solid (second cascade). While these steps are repeated, the effective solid-solution ratio increases. The solid solution ratio will be kept constant for all cascades. The number of steps in the experiment ranges from 10 to 20. The number of steps 66

79 is limited by the continuously decreasing volume of leachate. The experiment is conducted in several steps towards the thermodynamical equilibrium between the leaching fluid and all involved phases. In each step the resulting chemical composition of the leaching solution is determined as well as the dissolved and precipitated phases. For each cascade the solution is analysed by ICP-MS and ICP-OES, in addition the solid material is analysed by ICP-MS, ICP-OES and XRD. Further experimental details are given in [Herbert et al. 1996]. The results of this laboratory experiment are compared with the results of the geochemical modelling ( 10.6). In figure the theoretical description of the cascade experiment is given. leachate concentration cas. 1 cascade 2 cascade 3 cascade 4 cascade 5 cascade n-1 cascade n reaction progress total leached solid reaction progress Figure Theoretical description of the cascade leaching experiment In accordance with the reaction progress the number of cascades will increase corresponding to an increase of the mass of leached material (waste). At the end of the cascade experiments only a slight change in the solutions composition is observable. The solid is in "thermodynamical equilibrium" if no further changes in the solutions composition are detectable. In most cases the limiting factor is the strong loss of solution quantity during the leaching experiment. The cascade experiment has proofed to be suitable as an experimental tool for investigation of the chemical long-term stability [Herbert et al. 1996, Herbert et al. 1998, Meyer et al. 1999a, Meyer et al. 1999b] Solid material The experiments had been carried out with two materials, a salt cement and a oxychloride cement. The materials compositions are given in table and table

80 Table Composition of salt cement in kg/m3. Component mass in kg/m 3 Type/company Salt 1380 K+S GmbH, Zielitz Cement 148 CEM III/B 32.5 NW/HS/NA Alsen Werk (Nordcement) fly ash 148 Type HKV, Safa Saarfilterasche Vertriebs GmbH & Co KG Water 267 w/z 1,8 Table Composition of oxychloride cement in kg/m3. component mass in kg/m 3 Type/company Salt 887 K+S GmbH, Zielitz dolomite 216 Rheinkalk HDW, Herzberg- Scharzfeld MgO 86,5 Type F4-200 CaSO Type A mm, Fa. Wildgrubert slate quarry material 144 Type B 0.09, Vereinigt Thüringische Schiefergruben GmbH MgCl 2 solution 468 The phase analysis of the solid material was performed using a Phillips X'Pert-MPD equipment. The X-ray pattern of the uncorroded salt cement material is given in figure The XRD pattern depicts peaks which can be attributed to the mineral friedel's salt (hydrocalumite) and halite, which is a main component of the cementitious material. The detected anhydrite derives to the salt component or eventually to the cement itself. A weak pattern is attributed to the mineral phase gypsum. Anhydrite and gypsum normally can not be detected in hardened cements, because the sulfate is bound in AFm and AFt phases. But in this case Al and Fe are bound in friedel's salt they are not available for the precipitation of e.g. ettringite or monosulfate. Likewise a small amount of quartz was detected. The minerals anhydrite and gypsum was detected in parallel, the geochemical modelling calculated the thermodynamically more stable phase, only (see 10.6). 68

81 Impulse/s CAF Hydrocalumite, syn Gypsum; Quartz Anhydrite, syn; Hydrocalumite, syn Quartz Halite, syn Gypsum Gypsum; Hydrocalumite, syn Halite, syn; Hydrocalumite, syn Gypsum; Hydrocalumite, syn Anhydrite, syn; Hydrocalumite, syn Halite, syn; Gypsum; Anhydrite, syn Anhydrite, syn Halite, syn Halite, syn Anhydrite, syn Halite, syn Halite, syn; Anhydrite, syn Halite, syn; Quartz Position [ 2Theta] Figure X-ray diffraction pattern of unreacted salt cement Solutions The experiments were carried out with three leaching fluids, NaCl solution (saturated), a half saturated NaCl solution as well as high saline salt solution in equilibrium with the salt minerals halite, carnallite, sylvite, kainite and polyhalite (IP21 solution). This solution is likely to occur in salt and potash mines which are used in Germany as repositories for radioactive and hazardous chemical wastes. The chemical compositions of these solutions are given in table Table Composition of the IP21, half and saturated NaCl solution. IP21 NaCl 50 % NaCl 100 % Element mg/kg H 2 O mg/kg H 2 O mg/kg H 2 O Na K Ca Mg SO Cl Density [g/cm 3 ] [g/cm 3 ] [g/cm 3 ] 69

82 Cascade experiment with salt cement and IP21 / NaCl solution The experiments were conducted using a solid-solution ratio determined in pre-experiments to 0.33 and kept constant during the following cascades. After 3-4 days of equilibration under continuous rotation of the reaction vessel the leachate was filtered and given on new ground down material. solid/solution-ration [kg/kg] solid/solution-ratio [kg/kg] Cl, Mg, Na [mol/kg H 2 O] Ca, K, SO 4 [mol/kg H 2 O] ,5 cascade K cascade Ca cascade SO 4 2,0 1,5 1,0 0,5 cascade Na cascade Cl cascade Mg Na EQ 3/6 Cl EQ 3/6 Mg EQ 3/ ,0 0,5 cascade Na cascade Cl EQ 3/6 Na EQ 3/6 Cl cascade K cascade Ca cascade SO 4 EQ 3/6 Ca 0, , cascade cascade Figure Reaction paths of the dissolution of salt cement in a Mg-rich high saline solution (IP21, left part) and a NaCl solution. The symbols mark the experimental data for each cascade. In the first cascade 100g of ground down material was reacted with 300g of the corroding solution. The experimental data and parameter for the reaction of salt cement and NaCl solution are listed in table

83 Table Parameter and data for the dissolution of salt cement in IP21 solution ( + calculated). Cascade Shaking time Mass of solution Mass of solid Mass of eluate Solid remaining at the bottom of the solution + Density of eluate Loss of humidity [d] [g] [g] [g] [g] [g/cm 3 ] [wt-%] 1-1 2,89 299,63 99,97 249,6 150,00 1, , ,89 300,05 100,68 249,8 150,93 1, , ,89 300,14 99,98 240,6 159,52 1, , ,52 249,6 83,2 195,9 136,90 1, , ,52 249,8 83,3 175,8 157,30 1, , ,52 240,6 80,2 191,3 129,50 1, , ,68 195,9 65,3 158,2 103,00 1, , ,68 175,8 56,6 147,5 84,90 1, , ,68 191,3 63,8 158,6 96,50 1, , ,69 158,2 52,7 128,7 82,20 1, , ,69 147,5 49,2 121,5 75,20 1, , ,69 158,6 52,9 131,6 79,90 1, , ,62 128,7 42,9 103,4 68,20 1, , ,62 121,5 40,5 97,9 64,10 1, , ,62 131, ,4 69,20 1, , ,71 103,4 34,5 84,3 53,60 1, , ,71 97,9 32,6 62,7 67,80 1, , ,71 106,4 35,50 85,6 56,30 1, , ,73 127,1 42,37 109,1 60,37 1, , ,73 105,5 35,17 77,1 63,57 1, , ,71 109,1 36,36 60,2 85,26 1, , ,71 77,1 25, ,80 1, , ,71 125,2 41,73 93,5 73,43 1, , ,71 93,5 31,17 70,6 54,07 1, , ,75 70,6 23,5 54,1 40,00 1, , ,85 54, ,3 31,80 1, , ,91 40,3 13,4 29,3 24,40 1, , ,89 29,3 9,8 17,9 21,20 1, ,908 71

84 The leachate loss varied between 14 and 45 wt-%. For each cascade the solution was analysed by ICP-MS and ICP-OES. In figure the reaction paths of the dissolution of the salt cement in Mg-rich high saline solution (IP21, left) and in saturated NaCl brine (right) are depicted. During the dissolution in Mg-rich solution a strong decrease of Mg in solution from 4.25 mol/kg H 2 O (IP21) up to 2.16 mol/kg H 2 O was observed in the reacted brine. We assume the precipitation of Mg(OH) 2, because of the absence of further Mg-phases, e.g. Mgsilicate-phases (talc, serpentine etc.). As well as Mg decreases in solution the concentration of Ca increases. In general, remaining portlandite of the cement hydration will first be dissolved in Ca-unsaturated solutions. In a second step the CSH-phases will be dissolved. Concerning the investigated material no portlandite could be detected in the hardened cement paste. From this follows the Ca increase was founded on the decrease of CSHphases. The concentration of SO 4 increased during the cascade experiment to 0.15 mol/kg H 2 O, the calculated concentrations are in accordance with the experiments. SO 4 was precipitated as gypsum (anhydrite) during the first cascade. An increase of K and Na could be observed to 0.7 mol/kg H 2 O and 0.6 mol/kg H 2 O, respectively. For the dissolution reaction of the cementitious material in contact to NaCl solution an increase of K and a slight increase of Cl were observed, whereas a slight decrease of Na could be detected. The concentration of Ca only slightly increases during the leaching experiments. Thus the CSH-phase only slightly dissolves in NaCl solution. For the dissolution reaction of the cementitious material in contact to half saturated NaCl solution a similar reaction path could be observed. During the first cascade the solution was in equilibrium with NaCl a main component in salt cement Cascade experiment with oxychloride cement and IP21 / NaCl solution For the reaction of oxychloride cement in contact with NaCl solution (saturated and half saturated) in the first cascade 100g of ground down material was reacted with 300g of the corroding solution. In case of oxychloride cement in contact with IP21 solution 30g of ground down material was reacted with 300g of the corroding solution, because of the strong loss of reacting solution. In contact with NaCl solution an increase of Mg in solution was determined. Also an increase of K, Ca, and SO 4 could be observed. Only Na was removed from the reacting solution. A dissolution of the strength building mineral phase Mg 2 Cl(OH) 3 4H 2 O could be observed. 72

85 solid-solution ratio [kg/kg] solid-solution ration [kg/kg] 0,0 0,2 0,4 0,6 0,8 1, element [mol/ H 2 O kg] Na Cl Mg Cl element [mol/ H 2 O kg] 1,4 1,2 1,0 0,8 0,6 0,4 0,2 K Ca SO 4 Mg 0,6 0,4 0,2 Ca Na SO 4 K 0, cascade 0, cascade Figure Reaction path of the dissolution of oxychloride cement in NaCl solution (saturated and half saturated (left part) and a Mg-rich IP21 solution (right part). The symbols mark the experimental data for each cascade. For the reaction of oxychloride cement in contact with IP21 only slight changes concerning the element concentrations in solution could be observed. The concentration of SO 4 decreases from 0,31 mol/kg H 2 O to 0,23 mol/kg H 2 O. A mineral phase change from Mg 3 (OH) 5 Cl 4H 2 O to Mg 2 Cl(OH) 3 4H 2 O could be observed. Results of the cascade experiments The comparison of the experimental results of the dissolution of the cementitious material with the modeled reactions ( 10.6) with a special reactant show good agreement for the ions Na+, K +, Ca 2+, Mg 2+, Cl - and SO 4 2- in solution. salt cement in contact with NaCl solution Only slight changes concerning the Na + an Cl - concentration in solution could be observed, K + decreases (dissolution from the salt) the concentration of Ca 2+ during the reaction is constant at a low concentration, no dissolution of CSH phases could be observed 73

86 salt cement BFA in contact with IP21 solution Only slight changes concerning the Na + an Cl - concentration in solution could be observed, the concentration of Mg 2+ decreases, whereas the concentration of Ca 2+ increases resulting a dissolution of C-S-H phases In the modeling Mg 2+ was calculate in Mg-Silicate phases, which could not be detected in the experiments oxychloride cement in contact with NaCl solution (reaction path s. WP 5) Mg 2+ increased in solution therefore the phase Mg 3 (OH) 5 Cl 4H 2 O was dissolved oxychloride cement in contact with IP21 solution (reaction path s. WP 5) only slight changes concerning the element concentrations in solution could be observed; slight decrease of SO 4 was determined Mg 3 (OH) 5 Cl 4H 2 O was dissolved and Mg 2 Cl(OH) 3 4H 2 O phase precipitated Swelling pressure of MX-80 in contact to high saline and alkaline solutions The swelling pressures of MX-80 in contact to solutions with high salt content and high alkalinity were measured by a method developed by GRS [Herbert & Moog 2001]. The experimental set-up for the GRS swelling pressure measurements is given in figure The GRS method uses a pressure chamber in which the volume can be kept constant during the measurement. The advantage of this method is the low cost factor and the possibility of simultaneously operating cells where many samples can be measured in a relatively short time. The bentonite is compacted within the cell with an inner diameter of 49.2 mm. The compaction is performed with a hydraulic press with a loading rate of 1 MPa/min until the proposed dry density of the sample is reached (1.6 ± 0.02 g/cm 3 ). This load is maintained for 15 minutes followed by a relaxation phase of 2 minutes to allow the compressed air to escape. A pressure gauge allows the registration of the swelling pressure and fluid pressure. The reading of this gauge before the start of the actual experiment corresponds to zero swelling pressure. The brine is pumped into the sample by a HPLC pump. The fluid pressure produced by the pump is measured within the pump and the data are transmitted to a computer. The total pressure in the compacted sample (fluid pressure + swelling pressure) is measured by the pressure gauge and the data are transmitted to a computer. A density meter for the registration of the density of the percolated brine was attached to the system. 74

87 = Sample 2= Frit 3= Piston 4= Pressure gauge 5= Support 6= Screw cap 7= Pressure tube 8= Support 9= Solution inlet 10= Solution outlet 11= Signal cable Figure Experimental set-up for the swelling pressure experiments. The investigations were performed with solutions of variable chemical composition (a fresh cement pore water solution as well as cement/solution reaction solutions; solution composition from 7 th cascade of the cement corrosion experiments). The sodium bentonite MX-80 is a highly heterogeneous material. In order to obtain reproducible results a standardized procedure for sampling was developed. The mineralogy of MX-80 had been determined by Kasbohm & Herbert [1998]. The components of MX-80 are 88% montmorillonite, 4% quartz, 2% albite, 1% calcite, 1% pyrite and less than 2% cristobalite. In figure the swelling pressure of MX-80 bentonite in contact to young cement pore solution (Na + : 0,083 mol/l; K + : 0,167 mol/l; ph: 13,2) as a function of the reduced dry density is depicted. At the beginning of the experiment the solution was pressed in the vessel and the vertical total pressure increases. When no further solution can be pressed-in the solution pressure is reduced to zero and the remaining pressure, swelling pressure can be obtained. Ten measurements for each solution were conducted. 75

88 140 total vertical pressure [bar] Eingedrückte pressed-in Lösungsmasse solution [g] [g] Figure Swelling pressure of MX-80 bentonite in contact to young pore solution as a function of the reduced dry density compared to the swelling pressure with water. The performed experiments indicate that bentonites develop swelling pressures not only in contact to water, but also in contact to high saline brines and alkaline solutions. The swelling pressures in contact to brines however are considerably lower than those with pure water. The brine composition influences the resulting swelling pressure. In earlier experiments the obtained pressures with synthetic brines containing very low concentrations of K but varying amounts of Na and Mg (solutions NaMgCl 1,6,11) increased with increasing Mg in solution. Mg with its bigger hydration sphere lead to much higher swelling pressures than Na. Mg increases and K decreases the swelling pressures, i.e. K counteracts the effect of Mg. (figure 7, IP solutions, NaMgCl-solutions, Herbert & Moog [2001]). In contact with young cement pore water a swelling pressure of ~20 MPa could be determined. For the reaction solutions in contact to salt cement swelling pressures in the order of 10 MPa were determined. The same swelling pressures were observed for the unreacted NaCl solution (saturated). Earlier investigations concerning the water up-take could show that a reduced water up-take had to be observed in contact with cement solution and therefore a lower swelling pressure would be expected. The swelling pressure is largely dependant on the degree of compaction and the applied fluid pressure. Therefore of the experiments were conducted using raw densities of about 1.6 g/cm 3. All swelling pressures 76

89 with brines or cement reaction solutions were much lower than those obtained with pure water. 40 swelling pressure [bar] Water NaCl IP-9 IP-21 IP-19 NaMgCl #1 NaMgCl #6 NaMgCl #11 new results Portland cement pore solution with MX-80 MX-80/NaCl MX-80/NaCl-cement MX-80/IP21-cement Calcigel/NaCl 0 1,30 1,35 1,40 1,45 1,50 1,55 1,60 1,65 1,70 reduced dry density [g/cm 3 ] Figure Swelling pressure of MX-80 bentonite in contact to salt solutions as a function of the reduced dry density compared to the swelling pressure with water. The few results obtained so far can not be extrapolated to other boundary conditions. We are of the opinion, that this is true not only for our own experiments. Therefore we conclude that at the time being the existing knowledge on swelling pressures in contact with brines is neither complete nor consistent enough for a successful modelling. Factors like dry density, microstructure, flooding regime, sample dimensions seem to have a much bigger influence on the swelling pressure than brine composition. The comparison of the two procedures employed for the measurement of swelling pressures reveals that not only the swelling pressure itself is important. The flooding regime of the compacted bentonites has a decisive influence on the resulting permeability and thus on the sealing capacity of the bentonites. With the same solution a very similar swelling pressure can be obtained under different brine inflow rates. Under certain conditions the build-up of a pore pressure is faster than the closure of the pores by the swelling and a relative high permeability is maintained despite a high swelling pressure. 77

90 7.1.4 Conclusion The cascade experiment is a rapid method that enables the prediction of the chemical changes in solution during the cement corrosion processes. A good agreement between the experimental data and the modelling results was obtained, but solubility data and dissolution models for CSH phases are incomplete or missing; Pitzer coefficients for Si and Al still need to be determined more accurately. A procedure for the rapid determination of swelling pressure was applied. Saturated and for geochemical scenarios relevant solutions (e.g. cement reaction solutions) give rise to significant swelling of bentonite, therefore a self sealing of MX-80 in contact with different solutions could be observed. Swelling pressure of MX-80 in contact with Portland cement pore solution is increased in comparison with NaCl and IP21 solutions (likewise their reaction solutions with cementitious material). Strong deviations of measured swelling pressures for cement pore solution ( young cement pore solution ) could be observed. 78

91 7.2 BATCH EXPERIMENTS: RESULTS ON THE BENTONITE MX-80 Alain Bouchet, Alain Cassagnabère, Jean-Claude Parneix (ERM) Introduction Hydraulic cements are likely to be used in engineered barriers or as constituent materials in wells or storage cells. The interstitial solutions from these hydraulic cements (Portland cement, FAC) are characterized by high ph values around 12.5 or even higher for certain cement. The contact of these solutions with natural materials such as bentonite or components of the site geological formations may bring about changes in the retention properties of the natural environment through a chemical imbalance. The purpose of the project was to study the interaction mechanisms between a cement water and the bentonitic clay MX80 provided by the LEM from Nancy, France. On the one hand, the transformation mechanisms of the bentonitic clay were specified and quantitatively analyzed through batch experiments. On the other hand, samples of compacted bentonite were subjected either to fluid percolation in odometric cells or to column imbibition (performed by EUROGEOMAT). Interactions with three fluids (distilled water, site water and cement water) were examined. A petrographic study of samples was scheduled in the scope of this work but results are not yet available for this final report Batch experiments Preparation of the materials The experimental work was performed on the less than 1 μm fraction of the bentonitic clay MX80 separated by centrifugation Preparation of the solutions Nine monocationic solutions (table 7.2.1) were prepared in the glove box under nitrogen from an outgassed deionized water (boiling and N 2 sparging) and the following products: 79

92 Table Concentrations of the various solutions as functions of the required ph. PH NaOH solution 10-4 M 10-2 M M KOH solution 10-4 M 10-2 M M CaO solution M M 0.5M (c.f. 3.1) Note: In the case of the CaO solution, a ph value of 14 was not attainable given the solubility limit of CaO. However, the solution was prepared so as to reach the solubility limit (which corresponds approximately to a ph value of 12.6) then an amount of CaO theoretically sufficient to reach a ph value of 14 was added. This operation permits simulation of conditions close to a cement water where calcium contents in solution are low but where context may provide some calcium to the solution by dissolution of Ca(OH) 2 when the latter has been consumed Setting of the batch experiments Every operation required by the setup of the batch experiments was carried out in a glove box under nitrogen. The ratio solid mass/solution volume was set at 1/20 (in agreement with ANDRA). For the experiments performed at 60 C and 90 C, amounts of solid and solution were 2 g and 40 ml respectively. For experiments at 120 C (given the gas-tightness constraints, and consequently the sizes of available vessels allowing to meet these conditions), amounts of solid and solution were 1,5 g and 30 ml respectively. For the experiments at 60 C and 90 C, the mixture was placed in SAVILLEX-type PTFA vessels (Teflon), coated with a heatweldable plasticized aluminium film because of the gastightness constraints. For experiments at 120 C, the mixture was placed in PARR mineralization reactors (Teflon vessel coated with gas-tight stainless steel). In all cases, preparations were then placed in a thermostatically controlled oven pre-set at the various chosen temperatures and for 6 h, 24 h and 168 h Opening of the batch experiments Once out of the thermostatically controlled ovens, the preparations were immersed in water to reduce the cooling time (immersion 15 or so min). Afterwards, the preparations were opened in the glove box under nitrogen where a ph value was measured on the suspension (or gel in some cases). 80

93 a) ph measurement The following equipment was used for ph measurement: A ph-meter equipped with a temperature sensor. A magnetic stirrer for small volume (25-50 ml). A combination electrode for alkaline solution (temperature sensor included). The following reagents were used: The filling and storage solution of the electrode (KCl 3M) The buffer solution by Merck Eurolab ph= 13.0 (à 20 C) The buffer solution by Bioblock Scientific ph= 10.0 (à 25 C) Calibration and measurements were performed according to the recommendations of the BRGM (Rapport BRGM R 40918). b) Solutions The solutions were recovered (expected quantity around 10 ml) and placed in Centricon Plus-20-type centrifugation filters (10000 daltons). The ph value of the solution was measured using a combination electrode for alkaline solution (temperature sensor included). These filters were centrifuged for 15 min before they were put back in the glove box under nitrogen to be coated in a VACUMATIC-type heatweldable plasticized aluminium film. Then they were sent by mail to the BRGM for analyses. c) Solids After the recovery phase of solutions, solids were recovered by centrifugation-rinsing cycles in distilled water for NaOH experiments and in ethanol for the others Batch experiments: interpretation of randomly-oriented power XRD patterns Summary of minerals identified on the randomly-oriented powder patterns Identifications carried out have allowed the characteristic peaks of the minerals present in the initial clay MX80 to be recognized, i.e. smectite, quartz and cristobalite; these minerals are reactants. Purely siliceous minerals (quartz and cristobalite) tend to disappear in those samples that were submitted to the most aggressive experiments, i.e. with the highest ph values (and highest temperature values, subsequently). 81

94 MK646P MK644P MK648P MK946P MK944P MK948P MK1246P MK1244P MK1248P 3 9,2 15,4 21,6 27, ,2 46,4 52,6 58,8 65 Angle 2θ (radiation CuKα 1,5406) Figure Desoriented powder X-ray diffraction patterns on samples altered at ph14 with KOH solution (< 1μm fraction of MX80 bentonite). Siemens Kristalloflex Diffractomèter, radiation Cu, Kα (1,54 Å). MK646P: 60 C, 6 hours; MK644P: 60 C, 24 hours; MK648P: 60 C, 168 hours; MK946P: 90 C, 6 hours; MK944P: 90 C, 24 hours; MK948P: 90 C, 168 hours; MK1246P: 120 C, 6 hours; MK1244P: 120 C, 24 hours; MK1248P: 120 C, 24 hours. At ph 14, occurrence of minerals containing in their structure cations from the solution used for the experiment is observed: * With sodium: analcime crystallizes at ph 14, for 24 and 168 hours; * With potassium: phillipsite crystallizes at ph 14 (figure 1), for 168 hours; * With calcium: portlandite (and probably calcite, mineral whose presence needs to be confirmed) crystallizes at ph 14, for almost all temperatures and durations of experiment. In the most aggressive conditions (ph 14, 168 hours) portlandite has disappeared; on the contrary, CSH or other minerals typical of cements may be present (identification to be performed). For experiments performed at 90 and 120 C, gypsum is observed on the randomly-oriented powder patterns. For some experiments performed with calcium, amorphous or poorly-crystallized phases yield a dome on the randomly-oriented powder patterns; this is particularly well observed at ph 14 (Figure where X-ray data are given for KOH experiments). This dome is particularly clear for samples 60 C-168 hours, 90 C-24 hours and 90 C-168 hours, i.e. 82

95 moderately aggressive conditions in the frame of the time and temperature range selected for the experiments. If it is confirmed that no analytical artifact has interfered with X-ray diffraction data, it can be considered that these amorphous or poorly-crystallized phases characterize an intermediate step of the observed transformation reaction, because they do not occur as obviously in the experiments carried out at 120 C. Except for a few variations, their transient occurrence may be compared with the case of portlandite mentioned above Evolutions of reactants revealed by the randomly-oriented powder patterns Identifications of minerals performed on randomly-oriented powder patterns permit determination of the possible presence of the relicts of reactants (constituents of the initial clay MX80) and of newly-formed minerals (not present in reactants). However, these identifications do not provide any information on a possible evolution of the structure of a mineral already present among the reactants (smectite notably) whereas such evolutions are known to occur for Ecoclay-type experiments [Bauer and Velde, 1999; Rassineux et al., 2001]. In order to obtain such information about minerals present at the end of the different experiments, the intensities or positions of a few characteristic peaks have been plotted in various diagrams. This progress report presents raw data as well as the first resulting information; indeed, more detailed interpretations necessitate data from oriented-sample diffraction patterns to be taken into account (whose complete interpretation has not been performed yet). This report presents data relating to: * the (00l) peaks of swelling clay minerals, i.e. (001) peaks near Å and its harmonics near Å (002) and near 3 Å (003). * the (hkl) peaks of swelling clay minerals, i.e. the (20..,13..) peak near Å characterizing the stacking modes of layers and the (060) peak near 1.50 Å characterizing the b parameter of their mesh. * the peaks of non-clay minerals, i.e. the main peaks of quartz (near 3.34 Å) and cristobalite (near 4.1 Å) which are present among the reactants. a) Quartz and cristobalite Whatever the cation in the alkaline solution, the intensity of the main peaks of quartz (3.34 Å) and cristobalite (~4.10 Å) shows a relatively steady decrease as ph increases. The cristobalite peak is often absent from the diffraction patterns representing the samples submitted to the strongest attacks (high ph and temperature values). b) Relations between smectite (001) and (002) peaks For experiments carried out in presence of sodic and potassic solutions at ph 14, randomlyoriented powder patterns reveal a (001) peak near Å (presence of one water layer in interlayer position) and a (002) peak located near Å whereas it is located near 5.0 Å 83

96 for experiments at ph 10 and 12. At the end of the experiments carried out with calcium in solution, the distribution of (001) and (002) peak positions as a function of ph is not that contrasted, experiments at ph 14 yielding interlayer spacings contained between 14 and 16 Å (Two water layers in interfoliar position). With no humidity control during acquisition of diffraction patterns, a more thorough interpretation remains uncertain; nevertheless, note that some positions can only be explained by the presence of several hydration states among the analyzed smectite layers, whether the latter are interstratified or form clusters. For instance, for a first-order peak near 13 Å, the harmonic (if any) shall be located at 6.5 Å; this is mainly observed at ph 14 (apparent homogeneity of the size of interlayer spaces), whereas for lower ph values, the harmonic can not be found (as there is no rational series of (00l) peaks, a random interstratification 14.5/10 Å has then been analyzed). This heterogeneity may be due to the fact that the reaction observed at ph 10 and 12 is a snapshot of a reaction less advanced than at ph 14 (where it probably reaches its end). Moreover, some randomly-oriented powder patterns reveal: * two (002) peaks, one located near 6 Å and the other near 5 Å; this shows the heterogeneity (as to the number of water layers in interlayer position) of swelling clay minerals in the analyzed products. The samples implied were mainly submitted to moderately aggressive conditions (samples processed at ph10, 60 C, short durations, and samples processed at ph 14, 120 C, long durations are not involved). * a superstructure located near Å, which is characteristic of a regularly ordered interstratification of swelling layers with different thicknesses (10-12 or Å). These 3 samples were processed at 90 C with NaOH. If there is confirmation that this superstructure is significant, and that it represents a transient state of smectite (of smectites?) in course of reaction, one can imagine that the transformation model proposed by Rassineux et al. [2001] will be possibly complemented. The presence, at various experiment stages, of several swelling clay minerals characterized by interstratified layers of differing thicknesses has been confirmed by works presented by Cassagnabère & Bouchet [2000: D RP 0ERM ] from the study of oriented samples of a few samples submitted to experiments at ph14 in presence of NaOH. c) Smectite (003) peaks Whatever the cation present in the alkaline solution, the absolute intensity of this peak shows a relatively steady decrease as ph increases for experiments carried out in presence of sodium and potassium; this corresponds to a decrease in the number of elementary layers constituting the coherent domain along c*. There is no clear evolution for experiments carried out in presence of calcium, the values appearing quite stable. In the latter case, a few strong peaks were measured ; considering the probable occurrence of calcite in samples, it shall be verified that no interference occurs between the thin and a priori relatively weak peak of calcite (3.04 Å) and the broad peak of swelling clay minerals (near 3.1 Å ) that may more or less mask it. 84

97 d) Smectite (060) peaks For experiments carried out in presence of sodium and potassium, the representation of the absolute intensities of smectite (060) peaks shows a relatively steady increase as ph increases. For experiments performed in presence of calcium, this parameter slightly increases from ph 10 to ph 12 before suddenly falling at ph 14. e) Intensity ratio of smectite (001) and (060) peaks The intensity ratio of smectite (001) and (060) peaks permits a simple approach of the evolution of these minerals along the various crystallographic axes: the (001) peak is chosen to represent the stacking of elementary layers along c* while the (060) peak represents the evolution of the clay material in the ab plane. For experiments carried out in presence of sodium and potassium, the intensity ratio decreases quite steadily as ph increases; this is consistent with the decrease in the number of elementary layers constituting the coherent domain along c*. Conversely, for experiments performed in presence of calcium, this intensity ratio increases from ph 10 to ph 14. f) Intensity ratio of the peaks at 2.55 Å and 2.50 Å of the reaction products The intensity ratio of peaks located near 2.55 and 2.50 Å (the latter generally occurs as a mere shoulder on the diffraction patterns recorded at the end of experiments) shows a relatively clear evolution for experiments carried out in presence of sodium and potassium: relative stability between ph10 and ph 12, then slight increase between ph 12 and ph 14. In the latter range of ph, the evolution for experiments performed in presence of calcium is ambiguous because data split into two sets; verification is therefore necessary as a prerequisite to any interpretation. The 2.55 Å peak corresponds to 13,20 planes (combination of several hk planes: Brown, 1980). In some favorable cases, the diffraction patterns of smectites show (202) planes near Å (examples of beidellites: Brown, [1980]). It is also the case of illite/smectite mixed layers (Reynolds, [1992]). In the present case, the possible presence of a (00l) peak near 2.5 Å can not be totally excluded because of possible interstratifications (see comments in paragraph 2.2.2). For this reason, the observation of the increase of the intensity ratios of the 2.55 Å and 2.50 Å peaks from ph10 to ph14 may be unquestionable, but its interpretation as to an evolution in the ab plane or along c* can not be made with the present data only. Consequently, the presence of a relatively strong (00l) peak near 2.50Å shall be checked on the oriented samples (air-dried). Unfortunately, because of their intended use in the scope of the study, recordings of the oriented-sample diffraction patterns in our possession have been programmed up to 30 2θ only (i.e Å). This complementary recording will be performed later on a few selected samples. After this verification, the application of Reynolds s method (1992) will be possible because it corresponds to a comparison between various samples of a same series. Whatever the result of this verification, the modifications 85

98 of the diffraction pattern are an indication of the modification of the material state (even though it is not suitably interpreted to date, through lack of analytical data) that can not be ignored Other data Two main points are revealed by the cation exchange capacity (C.E.C.) measurements before and after Hofmann-Klemen tests: - for all treatments at ph 14 and all temperatures (60, 90 and 120 C) and all durations (6, 24 and 168 hours) the CEC before HK treatment tends to increase from 75 cmol kg -1 (ph 10 treatment) to about 110 cmol kg -1. ; - the CEC measured after HK treatment is linked with permanent tetrahedral charges and variable charges ; it increases of about 40 cmol kg -1 (samples altered at 60 C) and cmol kg -1 (for samples altered at 90 and 120 C). This should be due to either (1) an increase of the tetrahedral charge, or (2) an increase of variable charges, (3) or to an additionnal effect of these both phenomena. If we assume that the HK treatment does not affect the amount of variable charges, these results can be interpretated by a decrease of octahedral charge by dissolution of smectite indicated by XRD and FTIR data. But this is not exclusive of the proportional increase of tetrahedral charge by formation of beidellitic components. This seems to be corroborated by complementary results obtained with alkylammonium method giving an increase of the layer charge from 0.2 to [pers. com. A. Bauer, INE Karlsruhe] and by NMR results indicating the presence of tetrahedral Al in altered samples [pers. com. S. Ramirez in collaboration with ETI Madrid] Conclusion The objective of the results presented above is not to consider separately the particular case of each experiment, but to have a global approach of the influence of the various experimental parameters on the reaction products. This has the advantage of smoothing data and giving greater place to major trends with respect to details. Generally, sodium and potassium bring about the same trends on randomly-oriented powder patterns. The specific influence of calcium is noticeable either on absolute intensities, or on intensity ratios, or on the position of diffraction peaks. Some of these parameters are to be related to the number of water layers present in interlayer sites (1 for sodium and potassium, 2 for calcium). Other parameters characterize the transformations undergone by the smectitic material. The stacking of elementary layers along c* appears to decrease (loss of intensity of the (00l) peaks) with ph, whereas in the same time, an increase along the ab plane occurs (increase in the intensity of (hkl) peaks). So, the structure of smectite layers has been transformed. These experimental data are consistent with results published by Bauer and Velde (1999) who also observed delaminations of phyllosilicates under the influence of alkaline ph values. 86

99 Using XRD diagrams built with the Newmod program, Bauer and Velde [1999] showed that the first effect of alteration (in presence of KOH) is delamination (decrease in the number of layers in the coherent domain from to 2-4, the peak position moving from 12.5 to 13 Å). This is consistent with our observations. Then, they noted a shift of the peak from 13 Å to 11.5 Å, which they interpreted as an illitization phenomenon (I/S formation), this reaction being all the more advanced that the KOH solution is concentrated (from 0.1 to 4M). In Ecoclay II experiments presented here (in presence of KOH), the only phenomena observed are delamination and the progressive transformation of initial smectite layers into another smectite of beidellitic type. Under alkaline attack at ph 10 and 12, if we except a slight increase of CEC, residual smectitic layers keep their original properties despite the nature of the involved cation. At ph 14, whatever the nature of the cation is, starting material is drastically attacked. At 120 C and for a duration of 168 hours, the treatment is more and more aggressive in ascending order with the following cations: NaOH, KOH and Ca(OH) 2. The whole results indicate that the main cation (Na, K or Ca) in the solution directly influences the elementary reactions due to the alkaline attack. It also controls the nature of the secondary phases (zeolites) which precipitate in the system. In Ca(OH) 2 solution, initial material is intensively dissolved and CSH, calcite, gypsum precipitate. KOH and NaOH solutions the alteration is characterized by the following features: According XRD data: the Coherent Scattering Domain Size (C.S.D.S.) of smectite decreases; The amount of smectite decreases; The CEC increases after neutralization of octahedral charges indicating that tetrahedral charge remains in smectite. These results lead to the conclusion that there is a higher concentration of smectitic layers with tetrahedral charge in the remaining material. At this point the following question rises: Is there precipitation of beidellitic layers or is it a concentration of unsolved layers with tetrahedral charge already present in the starting material while montmorillonitic layers are preferentially dissolved? Several tests are on the way to answer this question: Alkylammonium tests in order to assess the variation of the layer charge (done, final interpretation on the way). Research of Al in tetrahedral site (beidellitic structure) with NMR (done, final interpretation on the way). Measurement of the variable charge (lateral charge). BET measurement of the total surface of the material. Analysis of saponite and beidellite with HRTEM. 87

100 7.3 EFFECTS OF AN ALKALINE PLUME ON THE HYDRAULIC AND HYDRO- MECHANICAL PROPERTIES OF THE BENTONITE MX-80 J.C. Robinet (Euro-Géomat Consulting) Introduction In a repository for radioactive waste, the rate of water flow into the surrounding areas has to be restricted in order to reduce potential migration of radio nuclides to the biosphere. In the case of natural geological barriers, e.g. an argilite for Bure Laboratory, groundwater flow is determined by the characteristics of the host rock. The clay barrier has the multiple purpose of providing mechanical stability for the canister, by absorbing stress and deformations, of sealing discontinuities in the host rock and principally retarding the arrival of groundwater at the canister and of retaining/retarding the migration of the radio nuclides released once damage of the canister The engineered barrier is a key element of the safety of a HLW deep geological disposal. So the phenomenological behaviour of the engineered barrier is determined to a large extent by the sealing ( low hydraulic conductivity and swelling pressure), and especially by the changes that may occur in the mechanical, hydraulic, and geo-chemical properties of the bentonite barrier due to the physico-chemical interactions with the concrete structure and host rock. The use of cementing materials is largely considered in a disposal, for example for the disposal cells of Intermediate Level Waste (French type B). These cementitious materials have a pore water chemistry that is very different (high ph from 11 to 13, high concentrations of alkalin Na +, K + ) from that of the pore water of the compacted blocks of bentonite. The behaviour of the engineered barrier under the interactions with the disposal facility, in particular that related to cementitious water in the near field of the disposal (noted alkaline disturbance or alkaline plume in following) must be evaluated. The work of Euro-Geomat Consulting (noted EGC), subcontractor to Andra, was to study the behaviour of the hydraulic and the hydro-mechanical properties of the Bentonite MX80 under an alkaline plume, in particular: To provides experimental data about the effect of an alkaline plume on the hydraulic and hydro-mechanical properties of the MX 80 at ambient temperature and at 60 c. To investigate the geo-chemical processes (ph) and their link to the hydro-mechanical processes The experimental studies undertaken by Euro-Geomat Consulting were focused on two macroscopic parameters: the permeability and the swelling pressure. On the geo-chemical level, the alkaline disturbance on clays corresponds to a whole of processes of dissolution and precipitation of solid phases. These processes involve a modification of the porosity, in particular in term of size and connectivity: the aperture and the closing of the porosity are thus conditioned respectively by the dissolution and the precipitation of solid phases. It goes 88

101 from either that this evolution of the porosity is then likely closed to a modification of the hydraulic and solute transfer properties (coefficient of permeability, coefficient of pore diffusion) and the hydro-mechanical properties (swelling pressure). These general considerations on the geo-chemical processes of an alkaline disturbance and their potential effects on the pore space resulted in directing the study of its impact on the physical properties towards two macroscopic parameters which are strongly dependent on the porosity, namely the coefficient of permeability for the hydrodynamic behaviour and the swelling pressure for the hydro-mechanical behaviour. So globally the experiments show that the no significant hydraulic (permeability) and hydromechanical (swelling pressure) effects on powder samples of MX80 at ambient temperature. Similar results are obtained at 60 C on powder samples of MX80. The variations are mainly due to the temperature. On the other hand globally the experiments show that the ph of outflow increase significantly with the ratio volume of cementitious water inflow on pore volume, although the quantity of cementitious water having percolated either remained limited (a few units of percolated pore volume). Its more significant increase of ph indicates a small interaction between the pore fluid and the solid phases Experimental strategy The experimental study of the coupling between geo-chemical processes and macroscopic physical properties requires at the base a perfect experimental control of the fluids and the geo-chemical processes, so as to measure only the effects on the required physical properties. With that, it must be considered specific experimental difficulties with hight compacted bentonite in particular the measurement of very low permeability (values lower than m/s) and the development of high swelling stress (several MPa). These considerations led: to develop an experimental strategy around experiments of percolation of the bentonite samples by a synthetic cementitious pore water (Ord. Portland Cement type CEM I) to use a specific oedometric apparatus associated to a specific apparatus of percolation. The principle of a scheme of percolation aims at obtaining the geo-chemical disturbance most homogeneous possible within the samples (dominated convection regime for solute transfer). The use of a oedometric device allows on one hand to put a device of percolation and on a other hand to determine simultaneously the required macroscopic hydraulic conductivity and the swelling pressure The experimental apparatus and protocol The measurements of the permeability and the swelling pressure require the sample to be water saturated. Moreover, fluids circulation through the samples is necessary in order to monitor the impact of geochemical interactions on the permeability and swelling pressure of the samples. For that purpose, Euro-Geomat developed an experimental device in the context of Ecoclay II on the compacted MX80 powder samples. 89

102 We will use the hydro-dynamic property highlighted in the numerous tests on the MX80 performed at E.G.C: the water content mass retention of the bentonite is the independence of the dry density and function of compaction stress. We are going to make this study in the field of unsaturated material, where the suction is higher than 10 MPa. Because of this property, the imposed relative humidity of the material can be obtained by a compaction of a stabilised powder in a controlled relative humidity atmosphere. For this first test, we used a material at a relative humidity of 66%, imposed by a nitrate sodium salt (NaNO 2 ) with a water content of 13.5%..The MX80 powder is in equilibrium in a surrounding box in which the atmosphere is controlled by saturated salts in solution in order to obtain a relative humidity of 66%. The equilibrium is reached when the weight of the powder stay constant for 1 week The apparatus The oedometer cell consists of a bronze jacket where a piston is sliding (figure 7.3.1). A radial sensor, installed in the jacket, enables to follow the evolution of the radial stress as a function of the axial one. Furthermore, a displacement measurer is placed on the piston and gives the axial strains. The compaction of the sample is provided by a piston linked to a GILSON hydraulic pump, which can reach 60 MPa. The adaptation concerns the system of injection of the fluid. Euro-Geomat has developed an injection bomb. It is a cylindrical tank in stainless steel. In the upper part of this tank, two stitching enable to apply a nitrogen pressure on the fluid inside the tank and to measure continuously this pressure. In the lower part of the injection bomb, is an exit, linked to the top of the oedometer which enables the infiltration of the fluid inside the sample. Two faucets enable to isolate the injection bomb from the percolation apparatus in order monitor the weight of the sample, which increases until the sample is saturated. Experiments are performed at constant volume. This is insured by the manual pump that provides a counter pressure on the piston, balancing the swelling pressure of the sample. By this means mechanical effects can be distinguished from the chemical ones. The measurement of the radial stress in tests of percolation under an hydraulic gradient is essential in order to maintain a total radial stress higher than fluid the injection pressure; so that the fluid percolation takes place inside the sample and not between the sample and the stainless steel jacket of the oedometer Fabric of the samples with MX80 powder at the specified dry density The powder with 13 % of water content mass is compacted in the oedometer and then it is unloaded to obtain a 20-mm high sample for two dry densities of 1.7g/cm 3 and 2g/cm 3. During the compaction stage, axial strain, radial and axial stresses are measured. These measurements enable to establish the curves in the (e-log(σ v )) plan : void ratio in function of the logarithmic vertical stress and in the (σ h -σ v ) plan : horizontal stress in function of the vertical stress 90

103 figure shows the results obtained for the dry densities 1.7 g/cm 3 and 2g/cm 3. Sensor of injection pressure Nitrogen in pressure Fluid to soak Gilson pump Radial stress measure Vertical stress Nitrogen n Manual pump Vertical strain measure Figure Schematic representation of the oedometric apparatus 1,3 2,2 1,2 1,1 2 Void ratio 1 0,9 0,8 0,7 0,6 Loading Unloading Dry density (g/cm3) 1,8 1,6 1,4 Loading Unloading 0,5 1,2 0,4 0,3 1 0, Vertical stress (MPa) 0, Vertical stress (MPa) Figure Evolutions of the void ratio and dry density with the vertical stress 91

104 Thermal loading up to 60 C Then, this powder is compacted and unloaded in order to reach a 20 mm final height of the sample. After the loading unloading path, each oedometer cell is kept at a controlled temperature. The tests have been performed at constant volume (isochore path) for decoupled the aspects mechanical hardening of thermal. The thermal evolution is an increase of 4 C a day until 60 C. This task is a continuation of some of the tests carried out at ambience temperature. Thus after equilibrium the temperature inside the cell is kept constant by automatic regulation. The evolutions of the stresses are given as a function of the temperature. Figure shows the results obtained for the dry densities 1.7 g/cm 3 and 2g/cm , Stress (MPa) 2,5 2 1,5 Total axial stress Total radial stress Stress (MPa) Total axial stress Total radial stress 1 2 0, Temperature ( C) Temperature ( C) Figure Variations of stresses during a heating for: (a) dry density 1.7g/cm 3 dry density 2g/cm 3 (b) Hydro-mechanical parameters Experiments on MX80 samples submitted to cement water percolation lasted between one and three months during which permeability swelling pressure and ph of water ar the exit of the sample are recorded. The ph of the cementitious water that is injected is checked regularly using a color-fixed indicator sticks (Baker-pHIX ph0.0-14) starting from a daily sampling. That makes it possible to draw the evolution of the ph of water at the exit of the sample, in particular to evaluate the permanent mode. 92

105 Measurement of the permeability The permeability k is calculated from the Darcy Law based on the water quantity injected during a given time interval into a saturated sample. Qw k = Ai Eqn with : Q W A i the flow of injected fluid (m 3 /s) the sectional area of the sample (m²) the applied pressure gradient. The hydraulic gradient i is given by the difference between the pressure of injection and the back-pressure, and the height of the sample H : i = (P injection P back-pressure )/H Eqn Calculation is carried out at different intervals of time which correspond to linear regime of the curve (quantity of injected water with time). The slope of the curve gives Q W in term of mass unit per time unit. Using a density of the fluid of 1 g.cm -3 both for site water and cementitious water, fluids flux is converted into m 3 /s Measurement of the swelling pressure The swelling pressure P s is the contribution of the total stresses and the pore pressure P w : P s σ = v + 2. σ 3 r P w. Eqn where : σ v σ r p w total vertical stress (MPa) total radial stress (MPa) pore pressure (MPa) 93

106 Saturation and percolation fluids used for the experiments The tests were conducted using two types of fluid. First a synthetic argilite pore water is used in order to saturate the sample before cement water percolation. The evolution of permeability is recorded from the fluid flow at a given injection pressure. The evolution of swelling pressure is recorded from the stress measurement as a function of the percolated fluid volume. The chemical compositions of the synthetic pore and cement waters used for the experiments are given in table and table respectively. Table Chemical composition of the synthetic pore water of the Callovo Oxfordian argilites. This fluid is supposed not to react at the contact of the argilite. Salt Quantity (mg/l) CaSO NaCl 333 KCl 186 CaCL MgCl NaHCO Table Chemical composition of the synthetic cementitious pore water. This composition results in a fluid of a 12,5 ph value buffered by portlandite Salt Quantity (mg/l) CaSO 4 16,3 NaCl 467,5 KCl 1640 Ca(OH)

107 7.3.5 Results References of the samples The table and table give an overview of the samples used and the experiment carried out on each of them. Table The experiments performed with MX80 at ambient temperature in 40 mm oedometers Reference test EGC Dry density (g/cm3) 1ms1co ms1co ms1co ms1co2 2 2ms1co2 2 3ms1co2 2 Type of water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water Percolation time 1: 1 month 2: 1 month 1: 1 month 2: 2 months 1: 1 month 2: 1 month 1: 1 month 2: 1.5 months 1: 1 month 2: 2 months 1: 1 month 2: 3 months Table The experiments performed with MX80 at 60 C in 40 mm oedometers Reference test EGC Dry density (g/cm3) T601ms1co T601ms2co T601ms3co T601ms1co2 2 T601ms2co2 2 T601ms3co2 2 Type of water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water 1: Site water 2: Cementitious water Percolation time 1: 1 month 2: 1 month 1: 1 month 2: 2 months 1: 1 month 2: 3 months 1: 1 month 2: 1 month 1: 1 month 2: 2 months 1: 1 month 2: 3 months 95

108 Evolution of the permeability and swelling pressure at ambient temperature and 60 C The evolutions of the quantity of fluid passing through the samples for dry density 1.7g/cm 3 and dry density 2g/cm 3 are plotted on figure and figure For all the experiments, an initial non-linear behaviour is observed, which is interpreted as the resaturation of the samples. Figure Evolution of the permeability for the samples with dry density 1.7 g/cm 3 Figure Evolution of the permeability for the samples with dry density 2 g/cm 3 Figure Evolution of the swelling pressure for the experiments with dry density 1.7g/cm 3 and figure show the evolution of the permeability with the number of renewal of the pore volume by the percolating fluid. It can be observed that the permeability remains constant with the site water and cementitious water about m/s for dry density 1.7g/cm 3 and m/s for dry density 2g/cm 3. It can be observed that the swelling pressures decreased significantly with the temperature and remaind constant with percolated with cementitious water (see figure and figure 7.3.9). That would tend to indicate that the more significant effect concerns the thermal mechanical behaviour. 96

109 All experiments undertaken dry density 1.7g/cm 3 and 2 g/cm 3 show a significant increase of the ph with the volume of percolation a cementitious pore water (see figure and figure ) Total water intake (g) Site water 1ms1co1,7 Cementitious water 1ms1co1,7 Site water 2ms1co1,7 Cementitious water 2ms1co1,7 Site water T601ms1co1,7 Cementitious water T601ms1co1,7 Site water T601ms2co1,7 Cementitious water T601ms2co1,7 Site water T601ms3co1,7 Cementitious water T601ms3co1, Time (days) Figure Evolution of the total mass of the percolated water for experiments with dry density 1.7g/cm 3 97

110 Total water intake (g) Site water 1ms1co2 Cementitious water 1ms1co2 Site water 1,5ms1co2 Cementitious water 1,5ms1co2 Site water 2ms1co2 Cementitious water 2ms1co2 Site water 3ms1co2 Cementitious water 3ms1co2 Site water T601ms1co2 Cementitious water T601ms1co2 Site water T601ms2co2 Cementitious water T601ms2co2 Site water T601ms3co2 Cementitious water T601ms3co Time (days) Figure Evolution of the total mass of the percolated water for experiments dry density 2 g/cm 3 Permeability (m/s) 1,00E-11 1,00E-12 1,00E-13 Site water 1ms1co1,7 Cementitious water 1ms1co1,7 Site water 2ms1co1,7 Cementitious water 2ms1co1,7 Site water T601ms1co1,7 Cementitious water T601ms2co1,7 Site water T601ms2co1,7 Cementitious water T601ms2co1,7 Site water T601ms3co1,7 Cementitious water T601ms3co1,7 1,00E-14 1,00E ,5 1 1,5 2 2,5 3 3,5 4 Number of percolated pore volume Figure Evolution of the permeability for the samples with dry density 1.7 g/cm 3 98

111 Permeability (m/s) 1E-11 1E-12 1E-13 Site water 1m s1co2 Cem entitious water 1m s1co2 Site water 1,5ms1co2 Cementitious water 1,5ms1co2 Site water 2m s1co2 Cem entitious water 2m s1co2 Site water 3m s1co2 Cem entitious water 3m s1co2 Site water T601ms1co2 Cementitious water T601ms1co2 Site water T601ms2co2 Cementitious water T601ms2co2 Site water T601ms3co2 Cementitious water T601ms3co2 1E-14 1E ,5 1 1,5 2 2,5 Number of percolated pore volume Figure Evolution of the permeability for the samples with dry density 2 g/cm Swelling pressure (MPa) Site water 1ms1co1,7 Cementitious w ater 1ms1co1,7 Site water 2ms1co1,7 Cementitious w ater 2ms1co1,7 Site water T601ms1co1,7 Cementitious water T601ms1co1,7 Site water T601ms2co1,7 Cementitious water T601ms2co1,7 Site water T601ms3co1,7 Cementitious water T601ms3co1,7 0 0,5 1 1,5 2 2,5 3 3,5 4 Number of percolated pore volume Figure Evolution of the swelling pressure for the experiments with dry density 1.7g/cm 3 99

112 Swelling pressure (MPa) Site water 1ms1co2 Cementitious water 1ms1co2 Site water 1,5ms1co2 Cementitious water 1,5ms1co2 Site water 2ms1co2 Cementitious water 2ms1co2 Site water 3ms1co2 Cementitious water 3ms1co2 Site water T601ms1co2 Cementitious water T601ms1co2 Site water T601ms2co2 Cementitious water T601ms2co2 Site water T601ms3co2 Cementitious water T601ms3co ,5 1 1,5 2 2,5 Number of percolated pore volume Figure Evolution of the swelling pressure for the experiments with dry density 2 g/cm ph Site water 1ms1co1,7 Cementitious water 1ms1co1,7 Site water 2ms1co1,7 Cementitious water 2ms1co1,7 Site water T601ms1co1,7 Cementitious water T601ms1co1,7 Site water T60 1ms2co1,7 Cementitious water T60 1ms2co1,7 Site water T601ms3co1,7 Cementitious water T601ms3co1,7 0 0,5 1 1,5 2 2,5 3 3,5 4 Number of percolated pore volume Figure Evolution of the ph for the experiments with dry density 1.7g/cm 3 100

113 ph Site water 1ms1co2 Cementitious water 1ms1co2 Site water 1,5ms1co2 Cementitious water 1,5ms1co2 Site water 2ms1co2 Cementitious water 2ms1co2 Site water 3ms1co2 Cementitious water 3ms1co2 Site water T 601ms1co2 Cementitious water T601ms1co2 Site water T 601ms2co2 Cementitious water T601ms2co2 Site water T 601ms3co2 Cementitious water T601ms3co2 0 0,5 1 1,5 2 2,5 Number of percolated pore volume Figure Evolutions of the ph for the experiments with dry density 2g/cm Conclusions The experimental study of the effect of an alkaline plume on the hydraulic and thermohydro-mechanical behaviours of the MX80 bentonite required the use of the particular oedometric apparatus allowing a percolation by cementitious water a ambient and at 60 C temperature. So in order to avoid the experimental artefacts, many adaptations were necessary, in particular for atmospheric insulation associated with temperature of the apparatus and the measurement of the quantities of percolated fluid. The whole apparatus made possible to measure the evolution of the permeability and swelling pressure with errors about some percents maximum, which is a very good precision for this kind of bentonite precision of measurements and in spite of the small quantities of cementitious water percolated 101

114 Permeability (m/s) 1E-11 1E-12 1E-13 Cementitious water 1ms1co1,7 Cementitious water 2ms1co1,7 Cementitious water T601ms1co1,7 Cementitious water T602ms1co1,7 Cementitious water T603ms1co1,7 Cementitious water 1ms1co2 Cementitious water 1,5ms1co2 Cementitious water 1ms2co2 Cementitious water 1ms3co2 Cementitious water T601ms1co2 Cementitious water T601ms2co2 Cementitious water T601ms3co2 1E-14 1E ,2 0,4 0,6 0,8 1 1,2 1,4 1,6 Number of percolated pore volume with cementitious water figure and figure ). So the saturated permeability k(m/s) of the MX80 depend principally of the dry density is given by the following relations: k = A exp( Bρ ). d Eqn

115 where A and B are constants the value of which are given in Table 7.3.5, ρ d represents the dry density Table Coefficients for hydraulic conductivity Temperature ( C) A B The swelling pressure depends on the dry density and the temperature due to the thermal damage and is written in the form: P swelling = C.exp(D. ρ d ) Eqn where C and D are constants the value of which are given in Table Table Coefficient for swelling pressure Temperature ( C) C D

116 Permeability (m/s) 1E-11 1E-12 1E-13 Cementitious water 1ms1co1,7 Cementitious water 2ms1co1,7 Cementitious water T601ms1co1,7 Cementitious water T602ms1co1,7 Cementitious water T603ms1co1,7 Cementitious water 1ms1co2 Cementitious water 1,5ms1co2 Cementitious water 1ms2co2 Cementitious water 1ms3co2 Cementitious water T601ms1co2 Cementitious water T601ms2co2 Cementitious water T601ms3co2 1E-14 1E ,2 0,4 0,6 0,8 1 1,2 1,4 1,6 Number of percolated pore volume with cementitious water Figure Experimental results of the evolution of the permeability with the number of percolated pore volume with cementitious water Swelling pressure (MPa) Cementitious water 1ms1co1,7 Cementitious water 2ms1co1,7 Cementitious water 1ms1co2 Cementitious water 1,5ms1co2 Cementitious water 2ms1co2 Cementitious water 3ms1co ,5 1 1,5 2 2,5 3 3,5 4 Number of percolated pore volume with cementitious water Figure Experimental results of the evolution of the swelling pressure with the number of percolated pore volume with cementitious water 104

117 7.4 GEOCHEMICAL REACTIONS IN FEBEX BENTONITE J. Cuevas (UAM) This report describes the activities carried out by the U.A.M. on the development of the laboratory tests carried out in batch reactors and transport (permeability) cells experiments using the FEBEX bentonite and Portland-type cement porewaters Introduction Two experimental designs have been implemented at UAM laboratories in order to perform Batch reactors tests and transport cell tests. Both types of tests are planned to provide basic information data about two main subjects concerning geochemical reactions: - Kinetics of the alkaline reaction of the bentonite: study of the geochemical process in the clay engineered barrier under alkaline conditions. - The reactivity at the interface Portland cement-bentonite: study of the consequences of the alkaline reaction of bentonite in the basic properties of the barrier function Geochemical reactions experiments a) Batch reactors experiments a.1 Specific objectives The batch reactor experiments were designed in order to achieve the following specific objectives: The study of nucleation and kinetics of formation of alkali zeolites in the alkaline reaction of accessory minerals and the smectite in the FEBEX bentonite The study of transformation of Al-smectite into Mg-silicates The formulation of kinetic laws for the alkaline reaction of bentonite The identification of textural aspects related to the dissolution-precipitation processes: surface properties and porosity changes a.2 Description of the experiments Experimental design A set of 48 laboratory tests conducted in sealed Teflon batch reactors have been performed. The bentonite (80g) and three alkaline solutions (0.1, 0.25 and 0.5M NaOH) (0.240 l) were mixed. The system included the presence of portlandite (6 g: about 4 x CEC of the bentonite: 100 ± 2 cmol(+)/kg). The experimental t/t grid was 1, 6, 12 and 18 months and 25, 75, 125 and 200 ºC. 105

118 The bentonite is comprised of 93 ± 3 % Ca-Mg-Na-montmorillonite, 2 ± 0.5 % quartz, 2 ± 1 % potassium feldspars, 1 ± 0.7 % plagioclase, 2 ± 0.2 % cristobalite, 1 ± 0.7 % calcite and 1.5 ± 0.1 % rhyodacitic original rock, mainly, volcanic glass [Cobeña et al., 1998; Linares et al., 1993]. The < 2 μm fraction was comprised of a mixed-layer I/S with 90 % of montmorillonite layers [Ramírez et al., 2002; Cuadros & Linares, 1995]. The average structural formulae, after Ca 2+ saturation of the clay, is (Si 7.72 Al 0.28 ) IV (Al 2.68 Fe 0.39 Mg 0.85 Ti 0.01 ) VI O 20 (OH) 4 Ca 0.62 K Analysis Both solid and aqueous phases were characterized. The analytical methods are those described in the Work-plan document delivered during the project a.3 Results a.3.1 Characterization of the aqueous phase Bentonite is able to buffer the alkaline solutions to stationary (540 days) values, between phs 12.5 and 8.5, depending on temperature and initial NaOH concentration. ph is reduced from 25 to C; at 25 ºC, at 75 ºC, at 125 ºC and at 200 ºC. This decrease of ph and alkalinity can be explained by the hydrolysis of montmorillonite in basic medium: (Si 7.72 Al 0.29 ) IV (Al 2.69 Fe Mg 0.85 ) VI O 20 (OH) 4 (Ca 0.25 Mg 0.20 Na 0.28 K 0.11 ) H 2 O (OH) Fe(OH) Al(OH) H 2 SiO Ca Mg(OH) Na K + In our case aqueous silica was demonstrated to be controlled by calcium silicate hydrate (CSH, figure 7.4.1), and alumina by the process zeolite formation ph ph initial =13.52 ph initial =13.26 ph initial = Å-toberm Ca-mont log a H 2 SiO 4 Figure Composition/activity diagram for the equilibrium solubility of Camontmorillonite (Ca-mont) and tobermorite (11Å-toberm) at 25 ºC. The following variables has been fixed according to experimental data: ph = 12.6 ( log(al(oh) 4- ): - 2.7; log(ca 2+ ): -4.4); ph = 12.2 ( log(al(oh)4-): -3.3; log(ca 2+ ): -4.15); ph = 11.9 ( log(al(oh) 4- ): -3.7; log(ca 2+ ): -4.0). 106

119 a.3.2 Characterization of the solid phase a MINERALOGY X-Ray Diffraction (XRD) data: XRD analysis of the reacted-bentonite shows the formation of analcime ((Na 2,Ca) Al 2 Si 4 O 12 2H 2 O) and 11 Å-tobermorite (Ca 5 Si 6 O 16 (OH) 2 4H 2 O)) at ºC in the whole set of tests. At lower temperatures and lower phs, amorphous calcium hydrated silicates (CSH-gel) is presumed to have formed because neither portlandite (Ca(OH) 2 ) nor nanocrystalline-csh (3.07 Å) are evidenced in XRD spectra. In addition, the smectite is significantly transformed, as can be deduced from the loss of normalized intensity in the 4.45 Å, even if no crystalline secondary phases have been formed.. Other phases identified in minor amounts were gyrolite (Ca 8 Si 12 O 30 (OH) 4 14H 2 O), at long term in the 200 ºC tests, and phillipsite (Na 2 Al 2 Si 5 O 14 5H 2 O), which has been found at 75 ºC and at ºC in the 0.1 M NaOH experiments. Temperature and ph controlled the kinetics of the analcime formation (figure 7.4.2). The reaction reach a plateau in the NaOH 0.5 M tests, and proceed at a very slow rate at lower phs. At 125 ºC, lower quantities of analcime have formed, being this phase absent at lower temperatures ºC 0.50 M 0.25 M 0.10 M 200 ºC 0.50 M 0.25 M 0.10 M % analcime t(days) Figure Analcime formation as function of time, temperature and initial NaOH concentration. Smectite is the unique clay mineral determined after the tests by means of XRD (random and oriented-ethylene-glycol solvated probes). However, there were visible changes in their structural characteristics. A Å peak evidenced the presence of a new trioctahedral phyllosilicate formed when alkalinity or temperature rises. 107

120 Scanning electron microscopy and energy dispersive X-Ray analysis (SEM-EDX) data: Time/temperature evolution of the reactions is characterized by analcime crystal growth and CSH-gel recrystallization. On the opposite, smectitic aggregates has been severely dissolved or transformed. Figure SEM aspects of new formed minerals Analcime crystals were observed first (30 days) when a > 50 μm size fraction was separated. The crystals rarely showed sizes of more than 5 μm and were located in a polycrystalline layer surrounding smectitic aggregates (figure 7.4.3). The clay surfaces acted as crystallization support. At the same time, platy tobermorite (Ca/Si: 0.8) formed isolated spherical shells. The long term experiments confirmed the formation of analcime and tobermorite. CSH-gels of fibrous or needle forms were found in the mass of clay and big polycrystals of analcime were supported in corroded plagioclase or were found isolated. It is clear that the formation of analcime need to be surface mediated by a silico-aluminate support. The zeolite formed is a sodium-type silica-rich analcime but with a non-negligible contribution of calcium and magnesium. The composition is close to the analcime described by Broxton et al. [1987] in the diagenetic alteration of volcanic tuffs in a saline alkaline environment (Na 1.02 Al 1.02 Si 2.58 O H 2 O). A typical composition of our zeolite in the same structural basis is: Na 0.74 Ca 0.07 Mg 0.10 Al 0.90 Si 2.65 O H 2 O. 108

121 a CHEMICAL COMPOSITION Chemistry of the < 0.5 μm Ca 2+ -homoionized size fraction The analysis of this size fraction shows the average composition of the smectitic crystals after the reactions. As temperature and ph rise, a significant magnesium increase can be traced according to the trioctahedral peak determined by XRD. This change is correlated linearly with the potassium and iron enrichment. These two elements indicate the presence of illitic layers within the Mg-smectite phase. Tetrahedral charge is also increased but in higher amounts respecting the potassium fixed in the structural formulae (figure 7.4.4). This means that the resulting smectite is saponitic type. The chemistry of montmorillonite to saponite reaction can be calculated because total layer charge has not changed virtually in the experiments (based on CEC and chemical analysis). (Si 7.72 Al 0.29 ) IV (Al 2.69 Fe Mg 0.85 ) VI O 20 (OH) 4 (Ca 0.25 Mg 0.20 Na 0.28 K 0.11 ) 1.29 will transform in this limit composition (full reaction): (Si 6.42 Al 1.58 ) IV (Fe Mg 3.78 ) VI O 20 (OH) 4 (Ca 0.59 K 0.40 ) Si 4+,Fe 3+,K (Fe 3+ ) VI K + (Si 4+ ) IV (Mg 2+ ) VI Figure Evolution of octahedral Mg 2+ as function of Si +4, Fe +3, and K + in the structural formulaes. The vertical line is the limit composition established for the saponitic mineral 109

122 0,0008 FEBEX 25 ºC 30 days 25 ºC 540 days ΔV/ΔΦ (cm 3 /g/a) 0,0006 0,0004 0,0002 0, pore diameter (Angstroms) Figure Pore size distribution for low temperature altered bentonites (0.5 M in NaOH) a TEXTURE (SPECIFIC SURFACE AND PORE SIZE DISTRIBUTION) (N 2 adsorption) The reduction of specific surface (60 to m 2 /g) and pore volume at low temperature and high ph is the more remarkable textural change. High temperature experiments did not showed important changes. Moreover, the dissolution of montmorillonite and the formation of analcime have destroyed a lot of smectitic surface. Then, the remaining clay should have a large specific surface in order to maintain the observed values (50-80 m 2 /g).. The pore-size distribution calculated by the BJH-method show discrete families of pores (20, 70 and 150 ) at long-term and at 25 ºC (figure 7.4.5). This new porosity is bigger in size than in the original bentonite, so that, agglomerated particles were generated a.4 Kinetics of the alkaline reaction of FEBEX bentonite a.4.1 Equilibrium status of the system In order to find a thermodynamic rationale in the formulation of a conceptual model of the hiperalkaline reaction of FEBEX bentonite, the saturation indexes for the minerals found in the experiments have been calculated. The EQ3/6 database, implemented in the PhreeqeC2 (USGS) code has been modified by including solubility constants for Na-zeolites (analcime and phillipsite (Chipera & Bish, [1997] and CSH-CSAH (HATCHES NEA13 data base, AEA Technology, [2000]). Montmorillonite is subsaturated (log (saturation index (s.i.)) < -3) and, on the opposite, saponite is supersaturated (log (s.i.) > 10) regarding the solution speciation analysis. The departure from equilibrium is more significant at low temperature and under the more alkaline conditions. The saturation index for analcime indicates equilibrium or near equilibrium conditions (-0.5 log (s.i.) 0.5) as well as CSH phases do at 25 ºC. Tobermorite is the CSH phase more supersaturated at ºC. The picture is consistent with the mineralogical assemblage produced in the experiments. At low temperature, although not observed, the same minerals can precipitate, of course at lower rates. 110

123 These data supports that montmorillonite dissolution control the overall reaction as far as CSH and zeolites will form as their components were available from the solution a.4.2 Reaction rate constraints for zeolite formation and Al-smectite to saponite reaction The kinetics of formation of analcime and saponite has been quantified as a function of [OH - ]. As far as ph is changing during the reaction, the reaction rate cannot be measured under constant OH - concentration. This fact can be overcomed by the estimation of the initial rate of the reaction at t = 0 (Chermak & Rimstidt (1990)). The reaction rate can be measured also at the quasi-stationary ph of the reactions at longer times: R(mol s -1 ) = A (m 2 ) k [OH - ] n Eqn A = reactive surface (smectite) The reaction rate for analcime has the form R(mol s -1 ) = A (m 2 ) k [OH - ] 0.3 Eqn at ºC and is very dependent on temperature. The apparent activation energy has been calculated also: ln k = (-12,23) (5176) * (1/T); Eqn E a = 43 kj/mol This data is consistent with a dissolution-precipitation process in tectosilicates [Blum and Stillings, 1995]. The reaction rate for saponite is also dependent on temperature but the order of reaction is increased as temperature drops (n = 1.5 at 75 ºC; n = 0.62 at 125 ºC; n = 0.3 at 200 ºC). This change obeys to a first rapid reaction in which the exchangeable Mg 2+ is displaced and Mgsmectite forms. When temperature rises, Al-smectite is selectively dissolved and the remaining smectite becomes of saponitic type. This second mechanism has a lower rate with the same order of reaction, at high temperature, as analcime. The same rate affecting both processes, which has been quantified by independent approaches (DRX, chemistry), confirms that montmorillonite dissolution prevails in the overall reaction rate a.4.3 Kinetic study of the alkaline alteration of FEBEX bentonite The process that allowed us to quantify the global reaction kinetics in FEBEX bentonite was the decrease of the smectite quantities as a function of time and temperature. This process has yield useful kinetic calculations in the range of 75 to 200 ºC (table 7.4.1). 111

124 Table Maxiumum values for smectite transformation under the experimental conditions (540 days) T Stationary ph % transformed smectite Initial conditions 13.5 (0.5 M NaOH) 12.9 (0.1M NaOH) 0.5 M NaOH 0.1 M NaOH 25 ºC ºC ºC ºC The global kinetics for the conversion of montmorillonite can be fitted to R(mol s -1 ) = A (m 2 ) k [OH - ] 0.5 Eqn ln k = (-20,09 ± 1.37) (2731 ± 543) * (1/T) Eqn E a = 22.7 ± 4.4 KJ/mol. (figure 7.4.6). These results are similar to previous studies for just the dissolution of montmorillonite. Our rate constant is lower, and the order of reaction is slightly higher than the data from Bauer & Berger [1998] or Huertas et al. [2001]. They find n close to 0.3. An added value of our data is that can be well extrapolated at 75 ºC to the rate of dissolution of FEBEX montmorillonite at ph = 9 obtained by Cama et al. [2000], (10-15 mol/m 2 s). Smectite transformed (mol s -1 m -2 ) 5,0x ºC 4,0x ºC 75 ºC 3,0x ,0x ,0x , [OH - ] mol/l Figure Reaction rate fits for montmorillonite transformation 112

125 b) Transport cells experiments b.1 Specific objectives Quantification of the cation exchange process and its spatial progression produced by the percolation of alkaline water through FEBEX bentonite Quantification and distribution of new-formed minerals (zeolite, Mg/Al layer silicates and calcite) and its relation to porosity evolution. Description of the time/space evolution of porosity (fissures and pore size distribution) at the cement-bentonite interface b.2 Description of the experiments Experimental design The tests consisted in the percolation of alkaline water through a compacted bentonite (1.4g/cm 3 dry density) using a cement mortar disc as the porous media for injecting the solutions. The dimension of the columns was 7 cm in diameter and 1.5/2.1 cement mortar/ bentonite thickness. The effluent composition was tested during the experiment. The tests have been performed with two solutions: 0.25M NaOH (ph 13.3) and a portlandite saturated one (0.022M Ca(OH) 2 ;ph=12.6). A CEM-I OPC type cement (high alkali) was used with the NaOH solutions and a CEM-I-SR type cement (low alkaki) was used with the Ca(OH) 2 solutions. Both cements are > 60% in C 3 S, providing a Ca(OH) 2 dominated system under hydration. The t/t conditions were 1, 6 and 12 months/ 35, 60 and 120ºC. Analysis At the end of the tests the composite cement-bentonite disc were cut in two half cilinders. One of them was reserved for a microscopic study of a longitudinal section. The other is sliced in 3 circular sections 7 mm thickness in order to perform a complete characterization of solid (subsamples S: S1, S2 and S3). These three samples are dried and grinded to prepare a characterization set equivalent to the batch reactor analysis topic b.3 Results b.3.1 Hydraulic conductivity (HC) and effluent characterization. The HC of the cement-compacted bentonite column increase as temperature rises, both with Ca(OH) 2 (saturated) or NaOH 0.25M fluids. At 120 ºC permeability oscillates between (Ca(OH) 2 ) and m/s (NaOH), being somewhat lower at 60ºC ( ) and 25 ºC ( ). Thus HC stands within the typical values of compacted FEBEX bentonite (10-13 m/s) in most of the experiments. On the other hand, the compacted bentonite was able 113

126 to buffer the alkaline plume to ph 9-8. However, at 120 ºC ((NaOH) system), ph reaches stationary values of 12.5 in parallel to the increase of HC b.3.2 Characterization of the solid phase b Mineralogy As a general rule, there were not significant changes in bentonite mineralogy at the scale of the 7 mm sections analyzed. The unique exception was found in the 0.25 M NaOH 120 ºC test at 365 days. Analcime is formed at a 17 % level in the first slice (S1), and is observed across the whole bentonite probe (10 5 %). Moreover, quartz and feldspars are partially dissolved. This is interesting as accessory mineral surfaces should be the interface for preferential transport pathways in the alteration of bentonite. Figure L section of 120 ºC NaOH permeability cells tests. CSH gel and analcime formation 114

127 SEM characterization of the cement-bentonite contact (approx 0.1 mm) has been performed ºC long-term interfaces have shown the crystallization of CSH-gel of tobermoritic compositions in both experimented systems (Ca/Si = ), mostly in the cement side. On the bentonite side, clay surfaces are composed of magnesium clays (MgO >15%) mixed with unaltered smectite. Analcime has been found only at 120 ºC in the NaOH system according to the XRD data. In the same system, but at 25ºC, brucite and analcime have been also detected. Under optical microscopy, a mm CSH cemented layer is observed. Analcime formation occurs after this layer and its aspect is heterogeneous. The crystallization is located in discrete patches (black under cross prolarized light) surrounded by oriented clays. The orientation of clays evidenced the preferential pathways in clay alteration (figure 7.4.7) b Cation exchange CEC was not changed in most experiments. However, cation exchange distribution showed a significant evolution as a function of time and the percolated fluid (figure 7.4.8). In the NaOH system sodium is retained only at long term and at 120 ºC. The effect of calcium migration from the cement interface prevailed against the NaOH fluid injection at ºC. In the Ca(OH) 2 system, sodium is progressively displaced but a very low rate. Magnesium, not shown in figure, is partially displaced when sodium or calcium enters the exchange complex. Ca, Na (cmol(+)/kg) Ca 2+ FEBEX Na + FEBEX Ca 30 days Ca 180 Ca 365 Na 30 Na 180 Ca, Na (cmol (+)/Kg) Ca 2+ FEBEX Na + FEBEX Ca 30 days Ca 180 Ca 365 Na 30s Na 180 Na(OH) 0.25 M. 120 ºC Distance from cement (mm) Na 365 Ca(OH) 2 sat. 120 ºC distance from cement (mm) Na 365 Na,Ca (cmol(+)/kg) Ca 2+ FEBEX Ca 30 days Ca 180 Ca 365 Na 30 days Na,Ca (cmol (+)/Kg) Ca 2+ FEBEX Na + FEBEX Ca 30 days Ca 180 Ca 365 Na Na + FEBEX Na Na 180 Na(OH) 0.25 M. 60 ºC Distance from cement (mm) Na 365 Ca(OH) 2 sat. 60 ºC Distance from cement (mm) Na 365 Figure Calcium and sodium in the exchange complex at 120 and 60 ºC in the transport cells tests. 115

128 b Texture and microstructure The most relevant difference between the transport cells tests and batch-tests pore-size distributions is the maintenance of the values for pore size maximums in the transport tests, despite of the important decrease of porosity in some cases. The microstructure, then, is maintained, but some populations of pores are clogged.. The impacts on porosity are very restricted to 120 ºC and S1 sections. In any case, the low-size mesopore network (claymatrix pores) seems to be partially clogged in the cement-bentonite interface. 0,0010 ΔV/ΔΦ (cm 3 /g/a) 0,0008 0,0006 0,0004 0,0002 FEBEX 59 m 2 /g 25 ºC, 540 days (batch) 35 m 2 /g 120 ºC, NaOH, S1 1.5 m 2 /g S2 4.5 m 2 /g S3 6.8 m 2 /g 120 ºC Ca(OH) 2, S1 23 m 2 /g 60 ºC, S1 42 m 2 /g 0, pore diameter (Angstroms) Figure Pore size distribution in selected transport cells tests (365 days) Conclusions Batch experiments: Kinetics of the alkaline reaction of the bentonite: study of the geochemical process in the clay engineered barrier under alkaline conditions: The hyper-alkaline reaction of bentonite acts as a OH - sink, so ph decreases as temperature rise and montmorillonite is dissolved. CSH precipitation controlled the silica activity in the order of magnitude 10-3 M. The hyperalkaline reaction of FEBEX bentonite (NaOH M) in the presence of portlandite (Ca(OH) 2 ) is characterized by the dissolution of montmorillonite and the precipitation of zeolites (analcime ( ºC) and phillipsite (75 ºC)), saponite and calcium hydrated silicates (gel-csh ( ºC), 11Å-tobermorite ( ºC) and gyrolite (200 ºC)). The reaction presents a temperature-kinetic control regulated by the dissolution of montmorillonite. 116

129 The relevant textural change in bentonite under hyperalkaline conditions takes place at low temperature (75 25 ºC). Specific surface decrease and pore-size increases due to the formation of persistent agglomerates. Transport cells tests: Reactivity at the interface Portland cement-bentonite: study of the consequences of the alkaline reaction of bentonite in the basic properties of the barrier function: Ca(OH) 2 saturated fluids has insignificant effects on mineralogy at the time scale experimented up to 365 days. NaOH fluids produced minor changes at ºC. At 120 ºC a thin tobermoritic layer of 1.5 mm precipitates in the clay aggregates surfaces at the interface. After this layer, analcime nucleates in heterogeneous patches affecting the whole compacted bentonite probe (2 cm thickness). Diffusion of Ca 2+ from the cement produces progressively Ca-exchange in the bentonite. Even when a NaOH percolating fluid was introduced in the columns. Magnesium is displaced from the exchange complex and diffuses towards the interface. Mg-Clay coatings have been detected in voids and clay surfaces. The formation of CSH-gel, analcime and magnesium clays, found in the batch experiments has been validated at the cement-bentonite interface in our column experiments. The alteration of compacted bentonite is heterogeneous. Despite of the formation of a CSH-layer (tobermorite) at the interrface, the precipitation of zeolites follows irregular patterns associated to the circulation of fluids via preferential pathways. 117

130 7.5 THE DEVELOPMENT OF THE OPTIMAL CONCRETE COMPOSITION COMPATIBLE WITH BENTONITE STABILITY A. Hidalgo, C. Andrade, C. Alonso, M. Castellote, I. Llorente (IETCC-CSIC) The study of the resistance of cementitious materials to long term water aggresion is usually based in the leaching tests of materials. This report includes results about the resistance to groundwater aggression of different cements, the evaluation of cements for use in a nuclear disposal, and the understanding of the long term effects Objectives The objectives were established to provide information about the development of the optimal concrete composition compatible with bentonite stability: To develop the optimal concrete composition compatible with bentonite stability. To study the long-term behaviour of bentonite/concrete system exposed to a granitic water attack Work plan Three leaching tests have been used in the project. Two of these methods are identified as accelerated. 1 Acid neutralisation test in powdered materials (accelerated): A classification of concretes can be made by an acid titration test, analysing different quantitative parameters or indicators of the degradation level. 2 Permeability tests: Concrete and bentonite will be placed in contact; then, a water head of 5 bars pressure will be maintained to pass water from the concrete side to the bentonite. This test reproduces a more realistic environmental scenario. 3 Migration tests (accelerated): Concretes in contact with bentonite will be tested in a migration cell. Voltages from 30 to 100V will be applied in order to accelerate the process of leaching Dissolution precipitation phenomena of cement pastes. Acid Neutralisation Test (ANT) Leaching test procedure The experimental procedure consisted of a batch test (closed system). Hydrated cement mixes are ground and sieved until a particle size less than 32 μm was obtained; then 10g of this solid (S) are mixed with deionised water (L) in a ratio S/L=1, producing an homogeneous slurry. Accelerated tests are performed in these slurries by adding an acid (HNO 3, 1N or simulant granitic water), at a rate of 0.5 ml.min -1 with an automated titrator (Net Titrino 721 from Metrohm), and stirring the sample vigorously to favour continuous mixture of leachant and sample. In the case of nitric attack, a N 2 flow was maintained during 118

131 the process to avoid carbonation. The evolution of ph was continuously recorded and liquid and solid phases were taken at different phs corresponding to progressive degradation stages. The process of hydration was interrupted in each case by freezing the sample with acetone and ethanol; then samples were vacuum filtered through a 0.45μm filter. Further analysis of the leachates and solid phases were carried out according to the requirements for each special case Materials Three Spanish cements (CEM I-SR, CEM I and CAC according to the European Standard ENV 197-1) and two types of mineral additions (fly ash and silica fume) were used for the testing. Chemical composition of mineral additions and cements used in this work is showed in table Fly ash and silica fume were added to paste mixes on a cement replacement basis. Cement pastes were fabricated with three different w/c ratios w/c=0.4, 0.35 and 0.3 and they were hydrated during 7 days in sealed conditions (98% relative humidity and 20±2 o C ). Formulation of mixes was: 1-100% CEM I-SR 2-91% CEM I-SR + 9% FA 3-77% CEM I-SR + 23% FA 4-59% CEM I-SR + 41% FA 5-82% CEM I-SR + 8% FA + 10% SF 6-73% CEM I-SR + 7% FA + 20% SF 7-73% CEM I-SR + 22% FA + 5% SF 8-100% CAC 9-100% CEM I and three w/c ratios for each formulation: 0.4, 0.35 and 0.3. Table Chemical composition of cements and mineral additions. CHEMICAL SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 Na 2 O K 2 O CaO ANALYSIS (%) (total) (free) CEM I CEM I-SR CAC Fly Ash Silica fume

132 Composition of leachant agents - HNO 3 1N. - Simulant granitic water with a chemical composition showed in table Table Chemical composition of granitic water ph [Ca 2+ ] mmol/l 7,91 ± ± 0.04 [Na + ] mmol/l 0,551 ± 0.03 [K + ] mmol/l 0,024 ± [HCO3 - ] mmol/l 2,510 ± 0.05 [Si 4+ ] mmol/l 0,351 ± [Mg 2+ ] mmol/l 0,277 ± [Cl - ] mmol/l 0,274 ± 0.01 [SO 4 2- mmol/l 0.18 ± 0.05 ] Determinations and calculations At the end of the experiment, evaluation of the samples was based on different parameters and properties: - Theoretical H 3 O + equivalents. - Na +, K +, Fe 3+, Mg 2+, Al 3+,Ca 2+, Si 4+, SO 4 2-, HCO 3 - and CO 3 = in leachates. - Saturation indexes of mineral phases with PHRQPITZ code and HATCHES database. - XRD patterns of starting and degraded materials. - Experimental H 3 O + equiv. - Chemical analysis of degraded solid materials. - Thermal analysis of starting and degraded materials Al M.A.S N.M.R in starting and degraded materials Results For Portland cement based materials, leaching of CSH gels proceeds until a constant value of Ca/Si ratio in the solid [Harris et al. 2002].In fact, for all materials, at ph=10, leachates are oversaturated in tobermorite which has, according to Taylor [1950], a Ca/Si ratio < 1.5. At ph=7, the species produced in the dissolution processes, are involved in the precipitation reactions of intermediate solid phases predicted by the calculation of saturation indexes: Chabazite: Ca Al(OH) H 4 SiO 4 CaAl 2 Si 4 O 12.6H 2 O + 6H 2 O CaP zeolite: Ca Al(OH) H 4 SiO 4 CaAl 2 Si 2.6 O H 2 O + 6H 2 O 120

133 Xu [Xu et al. 2000] suggests that geopolymers (aluminosilicates) are similar to zeolites in chemical composition, but they reveal an amorphous microstructure. They form by the copolymerisation of individual alumino and silicate species, which originate from the dissolution of silicon and aluminium containing source materials at a high ph and in the presence of soluble alkali metals. However, some authors explain the formation of CASH gels by a process of Al adsorption in the gel CSH [Hong et al. 2002]. Leaching of cement paste will produce the dissolution of Ca(OH) 2 and a drastic lowering of Ca/Si ratio in CSH until a constant value. At the same time Al, may enable low Ca/Si regions to achieve substantial Al contents. Alkali sorption is associated with these CASH clusters of low Ca/Si ratio. Differences between the reference and degraded samples are evident. XRD patterns of Portland cement based materials reveal a diffuse halo peak at 27-29º, characteristic of short order range materials as Aluminosilicates [Davidovits, 1991]. In any case, there is an important increasing on the spectra background when ph is decreasing, that can be explained by the precipitation of amorphous solid phases. Analysis of data from TGA and NMR, are also very consistent with the changes proposed. The increasing in the total loss of weigth during the leaching process, determined by TGA, could be produced by the precipitation of a gel with higher water content. Results of 27 Al MAS- NMR measurements, performed in some of the Portland cement based materials, indicate that leaching produces the displacement the 27 Al resonance to 50±20 ppm and an increasing in the ratio Altetra/Alocta. This fact is related to the substitution of Al for Silicon in tetrahedral sites typical from aluminosilicates. Faucon [Faucon et al., 1999] suggest that the silicon network of CSH with Ca/Si> 1.2 is based principally on dimers. Few of these dimers are connected by bridging silicon tetrahedra. Then, in the CSH with a Ca/Si >1.2, the substitution of Si by Al is not favoured. Increasing the silicon content of the CSH (Ca/Si<1.2) there are some changes in the silicon network. More and more dimers are connected with silicon-bridging tetrahedra; as a result, chains form, and the substitution of silicon by aluminium is favoured. The charge compensation needed after a substitution of Si 4+ by Al 3+ can be achieved by incorporating alkalis in the interlayer space. For aluminous cement pastes, when attacked by an acid, reaction of aluminates produces hydrated alumina gel. In a water rich in HCO 3 -, carbonation accelerates the leaching of calcium, due to the precipitation of calcite. However, it seems that decalcified CSH gel could also substitute Al for Si, favouring the appearing of a CASH matrix, and precipitation of chabazite which was predicted also by calculation of saturation indexes. In the case of CAC cement pastes, the carbonation of aluminium hydrates occurs also very quickly; however this fact has been considered in the literature as benefitial [García Alcocel et al., 2000], because it will help to recover the mechanical resistance of concretes that was lost during the conversion process of hydrates. 121

134 Conclusions Pastes based in Portland cement seem to form a CASH based matrix which chemical stability is well known. Then, the leaching process of Portland cement based materials, due to the decreasing of ph can be summarized as: Alkalies release. Dissolution of portlandite. Decalcification of CSH gel. Silicate polymerisation forming a gel with higher interlayer water content. Possible incorporation of tetrahedral aluminium in polymerised gel resulting in the formation of an alumino-silicate gel. In presence of HCO 3 -, precipitation of calcite is also predicted. Cements conditioned to form Alumino-silicates with a structure similar to zeolites, can act also as adsorbents for waste species.zeolitic materials are known for their abilities to adsorb toxic chemical wastes. The reaction of an acid with the CAC hydrates, produces alumina hydrate (amorphous AH 3 ), which is stable down to a ph of about 3 or 4 and can infills pores, protecting the concrete from further attack. In presence of HCO 3 -, calcite is the main precipitating solid phase. The main adavantage of using CAC cements without any mineral addition, is that the ph of CAC pore solutions seems to be more compatible with bentonite stability. However, reaction of CAC cement with silica fume, could also favour the formation of a CASH matrix, for instance, gehlenite hydrate (C 2 ASH 8 ) Permeability tests The use of accelerated bach leaching tests, in order to qualify the different concretes to be used in deep repositories, enables to acquire data about the dissolution-precipitation processes and to make comparison between different cementitious materials. However, column leach tests are considered to be more representative of field leaching conditions than the others because of the continuous flux of the leaching solution through the monolithic material. Therefore, there is an enormous concern on determining the material behaviour under the long-term action of water, in representative conditions of the real storage scenario. In practice, it is not possible to determine directly, from laboratory experiments, the degradation of the materials in real storage conditions for such a long time. Nevertheless, if the thermodynamic and kinetic processes at stake in the degradation are perfectly known, we could be able to modelling the material behaviour with the support of accelerated leaching tests. In these conditions it is possible to make very long-term predictions by means of numerical simulation. The objective in this task is to submit the selected concrete mixes to the leaching by water, representative of natural aggressive conditions. The accelerated test helps to reproduce the 122

135 sequence of degradation processes and its kinetics. Therefore, the appropriate test has to include the following characteristics, - Opened system - Control of the inflow and outflow solutions: concentrations, flow, ph. - Temperature and pressure control Leaching test procedure. Cylindrical samples are placed between two cylinders of metacrylate containing holes for water inlet and outlet. The block is sealed with an epoxy-resin in order to be sure that water pass only through the sample and measured fluxes are correct. Once the samples are placed in contact, a water head of 5 bars pressure is maintained to pass water from the concrete side to the bentonite after which, it is collected for analysis. The permeability of the samples and the pressure applied to the water regulate the water flow rate. The objective of this testing programme is to study the evolution of chemical and microstructural changes occurring when materials are subjected to a continuous flow of granitic water. The chemical composition of simulant granitic water was presented above. Characteristics of the test are: Column leaching test (open system). Unidirectional flow. Control of the inflow and outflow solutions. Material shape: monolithic (cylinders 50 mm diameter and 50 mm length). Samples will be water saturated for 24 hours, before the starting of the test. Water head of 5 bars. Length of experiments 45 days. The permeability of the samples and the pressure applied to the water regulate the water flow rate. Mix proportions for the different formulations derived from CEM I-SR and CAC cements, are shown in table Chemical composition of simulant granitic water was presented in table

136 Table Mix proportions in concrete Unit H5 H6 H7 H1 H8 H9 CEM I-SR kg/m CAC Fly Ash Water Siliceous aggregates (0-2.5) Siliceous aggregates (0-5) Siliceous aggregates (6-16) Sample characterisation a) Leachate chemical analysis Variables measured in every case were: flux, concentration of Ca, and ph of the effluent. b) Analysis of the solid The following methods were used: - X Ray diffraction. - Backscattering electron microscopy. - Mercury porosimetry. - Electrical resistivity Results Main processes that have been detected in concretes based in Portland cements, due to their interaction with groundwater, are: - Secondary hydration. - Microfisuration. - Densification of the paste due to precipitation of secondary hydrates, and secondary solid phases, mainly calcite. - Decreasing of Ca/Si ratio in CSH gels, that is, decalcification due to the leaching process. 124

137 In the case of concretes based in aluminous cements: - Sealing of cracks. - Precipitation of calcite. - Alkaline hydrolisis. - Hydrate conversion. Permeability of concrete has similar order of magnitud than permeability of compacted bentonite. Tests with concretes based in Portland cements, show an increase in hydraulic conductivity with increasing in mineral addition content, and with decreasing in cement content. Permeability of concretes based in aluminous cements, is one order of magnitude higher, and increases with the cement content. Effluent ph from concretes prepared with aluminous cements seems more compatible with bentonite stability. Self-healing of concretes based in Portland cement occur due to, secondary hydration, the progress of pozzolanic reaction and secondary minerals precipitation (i.e calcite). The presence of microcracks is due to development of microstructure of secondary solid phases. Conversion of hydrates and alkaline hydrolisis seem to occur in concretes based in aluminous cement producing an increasing in porosity. Stabilisation of cement with mineral additions seems to be necessary to avoid these problems Permeability tests in the system concrete-bentonite The evaluation of the long-term behaviour of concrete/bentonite system was initially designed to be performed with different types of concretes (see formulations in table 7.5.4) at 24 months time test and two temperatures: 20 and 60ºC. Samples of FEBEX bentonite were prepared by UAM (Universidad Autónoma de Madrid, Spain). Characteristics of the samples were the same as the employed in project ECOCLAY I (Bentonite: (Ca,Mg)-bentonite 1.2g/cm 3 dry density. Mineralogical composition of the bulk sample showed a predominance of smectite with minor quantities of quartz, potassic feldspar, plagioclase, cristobalite, opal and calcite. The clay fraction was practically pure smectite). 125

138 Sample characterisation a) Leachate chemical analysis In every leachate the following determinations have been performed: ph, Na, K, Fe, Mg, Si, S, Ca, HCO 3 - and CO 3 = and Al. b) Analysis of the solid The following methods were used: - X Ray diffraction. - Backscattering electron microscopy. - Mercury porosimetry. - Electrical resistivity. Table Formulation of concretes w/c SP nº CEMENT Dosage AGGREGATES AGGREGATES AGGREGATES T (ºC) kg/m 3 kg/m mm kg/m 3 0-5mm kg/m mm kg/m 3 H10 CAC ,60 H11 CEM I-SR (1) H12 56% CEM I-SR 35% FA 9% SF H13 83% CEM I-SR 8% FA 9% SF H14 CEM I-SR (2) H % CEM I-SR 16.65% SF H16 80% CAC 20% SF , , ,

139 Results Highest permeability corresponds to concrete based in aluminous cement, and lowest to concrete based in aluminous cement stabilised with silica fume. Concretes based in Portland cement show similar intermediate permeability values. Increasing in temperature seems to produce a decreasing in permeability values for concretes based in aluminous cement. ph of percolated waters vary between , having the lowest value waters percolated from the concrete based in aluminous cement stabilised with silica fume. Temperature increasing produces the increasing in ph values. A microstructural characterisation of concretes based in Portland cement, allowed to identify the following processes in the matrix: - Retention of Cl from water. - Decalcification of CSH gel. - Remaining anhydrous grains and fly ash particles. - Secondary hydration. - Alteration solid phases precipitation; calcite (in pores), alumino-silicates in paste and spherical particles of silicates with high Al, Fe and Mg content. - Presence of microcracks due to the development of microstructure of secondary hydrates or alteration solid phases. In concretes based in aluminous cements: - Retention of Cl from water in paste. - Increasing of porosity due to hydrate conversion. - No presence of microcraks. - Decalcification of calcium aluminate hydrates. Si and Fe are introduced in these hydrates. - Precipitation of alteration solid phases as gibbsite. Finally, concretes based in stabilised aluminous cements, show the following modifications to their microstructure: - Development of a dense paste. - Paste is composed mainly by aluminosilicates produced in the reaction of calcium aluminate hydrates with silica fume. - The final reaction product in paste is gehlenite; C 2 A 2 SH 7. - Additional precipitation of gibbsite as alteration product. 127

140 7.5.6 Migration Tests Objective The objective of the research reported here consists of the set up of an accelerated rapid methodology to evaluate the long term behaviour of bentonite in contact with cementitious materials. The methodology used is based in the application of electrical fields to force, by the establishment of an electroosmotic flux, the passage of water through the system cementitious materials/ bentonite. The establishment of the reliability of the use of accelerated tests, by application of electrical fields, to study the effect of cementitious materials on bentonite has been undertaken in three steps: Isolated bentonite, isolated cementitious material and the system bentonite/cementitious material Step 1: Isolated bentonite As a first step, isolated bentonite has been tested, with the main objective of assure that the changes in the bentonite could be representative of hydrothermal alterations and for the optimisation and control of the electroosmotic flux through the bulk of clay. In order to identify the effect of the different species in the electroosmotic flux established, a first series of tests have been carried out using different dissolutions with different ions (Na +, K + and Ca ++ ) have been forced to pass through the bentonite. The experimental details and composition of these dissolutions are given in table Table Experimental details of the first series of tests with bentonite. Experiment Voltage Electrodes Anolyte (+) Catholyte (-) nº applied (V) B-1 12 Activated Ti Distilled water Distilled water B-2 12 Activated Ti KOH 0.5 M Distilled water B-3 12 Activated Ti NaOH 0.5 M Distilled water B-4 12 Activated Ti KOH 0.5 M + CaOH sat Distilled water B-5 12 Activated Ti CaCl M Distilled water B-6 12 Activated Ti Na 2 CO M Distilled water As a second series, the tests have been carried out with synthetic specific solutions representative of the pore solution of the concretes tested in the project. In order to determine the concentration of the different species in these solutions, normalised mortar specimens were prepared and after curing them for 90 days in a humid chamber, they were squeezed at high pressure (pore-pressing technique) and the extracted liquid was analysed. The experimental details and composition of the dissolutions obtained, and used in the experiments, are summarised in table

141 Table Experimental details of the second series of tests with bentonite. Experiment nº S-1 S-2 S-3 S-4 S-5 Solution representative of OPC OPC+Fly OPC+SF OPC+Slag CAC concrete ashes ΔV (V) Na Anolyte (+) K (ppm) Ca ph Catholyte (-) Distilled water Distilled water Distilled water Distilled water Distilled water Electrodes Activated Ti Activated Ti Activated Ti Activated Ti Activated Ti Step 2: Isolated Cementitious materials As a second step, in parallel with the tests for isolated bentonite, the reliability of the electroosmotic flux through a cementitious matrix in function of the characteristics of material has been undertaken, for carbonated and non carbonated concrete, using a 1M Na 2 CO 3 dissolution in the anolyte and also with granitic water in both compartments, in order to establish the maximum rate of electroosmotic flux. Different mixes have been tested, including the concretes tested in the ECOCLAY project by different techniques. A summary of the experiments performed is given in table Table Experimental details of the tests performed with cementitious matrixes Carbonated Type ΔV Electrodes Anolyte Catholyte Yes H11 40 Activated Ti Na 2 CO 3 1 M Distilled water No H11 40 Activated Ti Na 2 CO 3 1 M Distilled water No H10 12 Activated Ti Na 2 CO 3 1 M Distilled water No H11 12 Activated Ti Na 2 CO 3 1 M Distilled water No H13 12 Activated Ti Na 2 CO 3 1 M Distilled water No H15 12 Activated Ti Na 2 CO 3 1 M Distilled water No H10 12 Activated Ti Granitic water Granitic water No H11 12 Activated Ti Granitic water Granitic water No H13 12 Activated Ti Granitic water Granitic water No H15 12 Activated Ti Granitic water Granitic water 129

142 Step 3: Concrete/bentonite system The third step in the research has consisted in the study the two components: bentonite and concrete together. The mixes of concrete studied have been: H-10, H-11, H-12, H-13 and H- 15. The solution placed in both compartments, anolite and catholite, was granitic water and the electrodes activated Titanium. A voltage drop of 30V was applied in all the cases. For making the tests, a special device was designed to be adapted to the migration cell used to test isolated concrete. It was made of a PVC disc equipped with an integrated O-Ring to allocate the slice of bentonite Results a) Isolated bentonite From this series of experiments is has been deduced that mainly K + and also Ca ++ are incorporated preferentially into the exchange complex of the smectite, which implies the change in the potential zeta of the clay and therefore an inversion of the flux. This is an important result on the point of view of the method, as the aqueous phase of the cement incorporates these ions in its composition. However, when testing with pore solutions, it has been deduced the feasibility of the establishment of the electrosmotic flux through the bentonites, during an specific period of time, with the pore solution of every concrete tested in this research. b) Isolated concrete For isolated concrete, it can be said that even with non-carbonated concrete electroosmotic flux can be established. The increasing effect of the sodium carbonate solution had been previously detected by the authors of the report. However for non-carbonated concrete, even using Na 2 CO 3 1 M, in the experiments of plain OPC concrete (without additions), no flux was noticed. It is remarkable the fact that the sign of the zeta potential of the aluminous cement (H10) is positive and therefore, the electroosmotic flux is developed through the anolyte, in the opposite direction than for the rest of matrixes. In the case of using granitic water, for every concrete tested, no electrosmotic flux was detected. c) Concrete/bentonite system When testing the system Concrete/Bentonite, the behaviour of the bentonite under the electrical field prevails over that of concrete. Therefore for every mix tested, noticeable electroosmotic flux was established, being the shape of the evolution of the zeta potential of the systems very similar to that obtained with bentonite alone, but without a reversion of the flux. The analysis of the exchange complex of bentonite, performed according to Thomas s procedure has allowed to deduced that the behaviour of bentonite is very similar for all the systems concrete/bentonite tested having deduced that the higher is the loss of Na +, the higher the uptake of Ca ++ calcium, finding an opposite tendency for potassium. No clear parallelism with magnesium have been found. Therefore, from this kind of method, a classification of concretes, with respect to their alteration on bentonite has been found. 130

143 Conclusions An accelerated methodology, based in the application of electrical fields, has been developed to force, by the establishment of an electroosmotic flux, the passage of water through the system cementitious materials/ bentonite. The preliminary results obtained exhibited very promising results, as a classification of concretes, with respect to their alteration on bentonite has been found. On this respect, it has been deduced that the higher is the loss of Na +, the higher the uptake of Ca ++ calcium, finding a not so clear opposite tendency for potassium. According to this classification, the ranking of concretes tested here is the following one: H-11>H-12>H-15>H-13>H-10, which means that: Plain OPC > 56% OPC+35% Fly ash+9% SF > 83,4% OPC +16,65% SF > 83% OPC+ 8% Fly ash + 9% SF > CAC It has to be pointed out that is it necessary to accomplish as a further step the validation and comparison of these results with those obtained by any natural method General conclusions The long term behaviour of concrete and concrete-bentonite system exposed to a granitic water attack has been studied by different leaching procedures, and for a few concrete formulations. Results from different tests are in good agreement. New concrete formulations based in stabilised aluminous cements, with low-ph, chemical stability and mechanical resistance can be proposed. 131

144 7.6 DISSOLUTION KINETICS OF BENTONITE UNDER ALKALINE CONDITIONS F. J. Huertas, M. L. Rozalen, S. Garcia-Palma, I. Iriarte and J. Linares (EEZ-CSIC) Introduction The bentonite used as an engineered barrier in the nuclear waste repositories may undergo several processes derived from its interaction with porewaters. Next to the concrete components of the barrier system, the interaction of the alkaline plume with the bentonite may induce severe chemical and mineralogical changes, which may modify the physicochemical properties of the barrier. Dissolution of the smectite and the accessory phases under barrier conditions is a process that should be included in any predictive model. Although the hydraulic properties do not allow the flux necessary to produce and maintain a massive dissolution initially, this process may start, spread and increase through fractures and discontinuities. Furthermore, this reaction can be locally accelerated in areas surrounding zones that contain some material that may produce conditions especially favourable for dissolution. The bentonite dissolution reaction releases into solution (porewater) different elements. The behaviour for major elements, as Si, Al, and Fe in solution depends on ph and temperature, being its dissolution/precipitation and transport controlled by water flow in thermal and chemical (ph) gradients. Precipitation of secondary phases of Si, Al and Fe may modify the physico-chemical and hydraulic behaviour of the barrier. These topics should be considered in detail, estimating which are the prevailing processes as a function of temperature, ph, solution composition, abundance and specific surface area of the minerals across the bentonite barrier. In conclusion, the objectives of this set of laboratory tests are the following: to determine the dissolution rates of smectite, the major component of bentonite, under alkaline conditions, as a function of ph and temperature; to estimate the dissolution rate of accessory phases in bentonite under alkaline conditions, as a function of ph and temperature; to evaluate their contribution to bentonite bulk dissolution and release of mayor elements Materials Bentonite and smectite FEBEX bentonite was extensively characterised during Febex I preliminary phase [ENRESA, 1997]. Mineralogical analysis indicates a yielding of 92 % in smectite, and the presence of volcanic glass and accessory minerals (plagioclase, K-feldspar, biotite, cristobalite, 132

145 amphiboles, pyroxenes and zeolites). The interlayer cations in the natural smectite are Na, Ca and Mg. Pure minerals were collected from bentonite suspensions in distilled water. The suspension was sieved to 37 μm and the solid fraction (> 37 μm) was stored for accessory mineral separation, whereas the residual suspension was conserved for smectite extraction sedimentation method. Smectite, μm size fraction was collected, saturated in potassium, oven dried at 60 C, and ground in agate mortar. The use of K-smectite is justified only for dissolution purpose. Osmotic swelling of Nasmectite may block the filter membranes used within the dissolution cells. On the other hand, Ca may precipitate as calcite, which requires the use of a nitrogen bench to prevent solutions from taking up from the atmosphere in alkaline conditions. Nitrogen was continuously bubbled in the reagents reservoirs, to prevent solutions from uptake of CO 2. The K-smectite was analysed for mayor elements by X-ray fluorescence (XRF), and for exchangeable cations and cation exchange capacity (CEC). The CEC yields a value of 99.8 ceq kg 1, being K + the only exchangeable cation. Mineralogical analyses were performed by X-ray diffraction (XRD), and differential thermal analysis and thermogravimetry (DTA-TG). Based on these results, the samples is composed > 99 % of a mixed layer illite/smectite containing ~15 % of illitic layers. The structural formula is the following: K 0.88 (Al 2.54 Fe Mg 1.12 ) (Si 7.91 Al 0.09 ) O 20 (OH) 4 Only 0.76 K + ions per formula unit are exchangeable, in agreement with the presence of a ~15 % of illitic layers. The specific surface area was measured by BET methods, using nitrogen adsorption isotherms. The value was of 111 m 2 s 1 (uncertainty ±10%), that corresponds to the smectite external surface. BET surface area is commonly used to normalize dissolution rates Accessory phases Accessory minerals were separate by sequential sieving of the solid fraction (> 37 μm) and density separation using sodium polytungstate (Na 6 W 12 O 39 ) [Cassagnabere, 1998]. Mineralogical composition of each fraction was analysed by XRD. Plagioclase is the most abundant accessory mineral. Taking into account its abundance and its dissolution rate in literature, it was selected for dissolution test. The structural formula of plagioclase samples, deduce from the chemical data (XRF analysis), was the following: Na K Ca Al Si O 8 The specific surface area, measured by BET method, was of 0.65 m 2 s 1. The uncertainty associated to this low vale is in the order of ±15%. 133

146 Amorphous volcanic glass (tuff) was present in the bulk bentonite. It was estimated in 1-2%. Volcanic glass is likely to dissolve faster than crystalline particles. In additions, tuffaceous materials exhibit large surface area, which contributes to an even faster specific dissolution rate. In consequence, its dissolution tests were included in the workplan. Specific samples of amorphous material (under XRD) free of minerals (especially smectite) were collected at the bentonite outcrop. The spherules of glass were crashed, ultrasonically cleaned in distilled water, ground in agate mortar and oven-dried at 60 C. It was completely amorphous by XRD and DTA-TG, with no traces of smectite. It was analysed by XRF and BET (1.51 m 2 g 1, ±15%). The following stoichiometric formula was calculated based on one Si atom: Methods Si 1 Al 0.19 Fe Ca Na K Ti (OH) O The dissolution were carried out in the flow-through reactors (figure 7.6.1), that allow us to measure the dissolution rate under fixed saturation state conditions. The solid sample is confined in the reaction chamber by membrane filters. The reactors are immersed in a water bath to control the reaction temperature. Figure Sketch of a flow-through cell for dissolution tests. Output solutions were sampled every day and analysed for ph, Si and Al. A selected group of solids were analysed for XRD, BET surface area and infrared spectroscopy. Solid analyses revelaled that dissolution was the only process and that specific surface area remained constant during the duration of the tests. Based on a simple mass balance equation, the dissolution rate (R, mol m 2 s 1 ) of a solid in a well-mixed flow-through experiment, at steady state conditions (constant value of the compositions of the output solution), is obtained from the expression [Cama et al. 2000]: 134

147 ν j R = q A (C j,out C j,inp ) Eqn where C j,inp and C j,out are the concentrations of component j in the input and the output solution, respectively (mol m 3 ), ν j is the stoichiometric coefficient of j in the dissolution reaction, t is time (s), A is the reactive surface area (m 2 ), and q is the fluid volume flux through the system (m 3 s 1 ). Note that in our formalism, the rate is defined to be negative for dissolution and positive for precipitation. For most of the experiments, the error in the calculated rate ranged between 12 and 20% and it is dominated by the uncertainty of the BET surface area measurement (±10% for smectite, and ±15% for glass and plagioclase). The temperature dependence of the dissolution rate generally follows the Arrhenius law: R = Aexp(- E a RT) Eqn where A is the pre-exponential factor, E a, is the apparent activation energy (kj mol 1 ), R is the gas constant, and T is the temperature (K). Previous studies have shown that for many minerals the dissolution rate within certain ph ranges is proportional to a fractional power of the hydrogen ion activity (proton/hydroxyl promoted dissolution rate) [Stumm & Morgan, 1981]: n R = k a H + Eqn where k is the rate coefficient and n stands for the order of reaction with respect to the activity of protons Results and discussion Some examples of the evolution of the composition of output solutions in the dissolution tests as a function of time are shown in figure Small fluctuations of the ph out with respect to ph inp due to the dissolution reaction are within the uncertainty of the ph measurement. High Al and Si concentrations are observed at the onset of most of the experiments. Afterwards, Al and Si concentrations decrease until a steady state is approached (figure 7.6.2). Dissolution rates were calculated based on Al and Si steady state concentrations using eqn , after correction of mass of solid lost during dissolution. The uncertainty associated with the measure is ±15% or 0.13 log units. Most of the rates obtained correspond to stoichiometric dissolution (R Si and R Al are equal). However, the dissolution process is so fast at 70ºC that the mass lost is too high to allow for calculation of dissolution rate from solution at ph values of Additional dissolution test were launched, but they did not reached steady-state conditions. 135

148 Sm b Si (ppm) Al (ppm) ph Si, Al (ppm) Sm Si (ppm) Al (ppm) ph Si, Al (ppm) Time (h) Time (h) Ab ph Si (ppm) Si (ppm) Si, Al (ppm) Gl Si (ppm) Al (ppm) ph Si, Al (ppm) Time (h) Time (h) Figure Variation of ph, Si and Al concentration in the output solutions as a function of time. Examples of smectite (Sm), plagioclase (Ab) and volcanic glass (Gl) dissolution test Smectite Dissolution rates calculated for smectite are gathered in table Figure plots log dissolution rate vs. ph at 25, 50, and 70 C. Additional data, at ph values lower that 11 have been included for comparison. In alkaline solutions, the smectite dissolution rate increases as ph increases, showing a steeper slope for ph values higher than 11, that seems to be a critical value for the smectite dissolution and stability. The calculated reaction orders at 25 and 50 C according to Eqn are given in table The data available at 70 C do not allow for parameter calculation It is observed an increase of the reaction order, n, with temperature, as previously observed in other phyllosilicates [i.e., Carroll & Walther, 1990]. 136

149 Table Average values of ph, Si and Al concentration of the smectite output solutions in the steady-state region, and smectite dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. Series ph out Si out Al out log R Si log R Al (μmol L 1 ) (μmol L 1 ) (mol m 2 s 1 ) (mol m 2 s 1 ) Sm b Sm Sm (-13.47) Sm Sm I Sm II Sm Sm Sm Si 25¼C Si 50¼C Si 70¼C Al 25¼C Al 50¼C Al 70¼C 70¼C ¼C 25¼C ph Figure Experimental (dots) and estimated (lines) dissolution rates. Estimations were obtained using eqn and apparent activation energies (table 7.6.3). Values for ph < 11 were included for comparison. 137

150 Table Fitted parameters of eqn for alkaline solutions. Values in the ph range are included for comparison (unpublished data). ph Temp. ( C) Log k n ph Temp. ( C) Log k n > * * * Values estimated using activation energies to compute them. Apparent activation energies in the ph range 11 to 13.5 were calculated using Arrhenius equation (eqn ) (table 7.6.3). The values are typical for dissolution/precipitation reactions in silicates (~60 kj mol-1, Lasaga, [1998]). Apparent activation energy increases as ph does, as also observed for other silicates (i.e., Carroll and Walther, [1990]). They were used to estimate the dissolution rates that could not be obtained experimentally (figure and table 7.6.2). Table 7.6.3: Apparent activation energies for smectite dissolution reaction. ph Ea (kj mol 1 ) Smectite Ea (kj mol 1 ) Plagioclase The ph values of the porewaters in the barrier and in the interface with the granite host rock are in the range 7 to 8.5. Under these conditions, only few influence of ph or temperature are expected on the smectite dissolution rates, which are closed to the minimum value of R (~10 14 mol m -2 s 1 ). We may derive that these conditions provide a suitable environment to preserve the chemical stability of the bentonite. However, the alkaline plume provides geochemical conditions quite aggressive, as the ph of the solutions may reach values of Figure clearly shows that above ph 11, the smectite dissolution rate increases steeply. The effect of temperature is also important, being multiplied by the effect of the alkaline ph. 138

151 Volcanic glass No structural formula can be defined for the glass (the proposed formula is simply a stoichiometric relationship); in consequence, the dissolution rates are compiled in table were computed per mole of Si and they correspond stoichiometric dissolution processes. The fast dissolution rate found for the glass under hyperalkaline conditions made extremely difficult to get steady-state conditions, as dissolution was too massive and mass lost too high to allow for calculations in some of the series. Although additional test were launched modifying the flow rate, they did not give better results or did not approached steady-state conditions. Table Average values of ph, Si and Al concentration of the volcanic glass output solutions in the steady-state region, and volcanic glass dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. Series ph out Si out Al out log R Si log R Al (μmol L 1 ) (μmol L 1 ) (mol Si m 2 s 1 ) (mol Si m 2 s 1 ) Gl Gl (-9.72) Gl Gl Gl According to table and figure 7.6.4, the data available indicate an increase of dissolution rate of one order of magnitude from 25 to 50 C, which is twice that found in the smectite. In consequence, glass dissolution is apparently more sensitive to temperature than smectite dissolution, as revealed the value of E act at ph 12.5 (77 kj mol -1 ). The glass used for these tests consisted in small spheres (1 mm size) of compact texture, as also revealed by it low specific surface area (1.51 m 2 g 1 ). However, the glassy material in the Cortijo de Archidona outcrop is a rhyolitic tuff [de la Fuente, 2000] with composition similar to that of the spheres. It is very difficult to get an accurate value of its surface area since the vesicular texture of the tuff is modified during the dissolution process (some cavities initially inaccessible are open to solutions during reaction). If we assume for the tuff a value of surface area two orders of magnitude greater than for the compact glass 139

152 (spherules), the corresponding dissolution rates would also be two orders of magnitude greater. The tuff may dissolves to orders of magnitude faster than the smectite Plagioclase The behaviour of plagioclase during dissolution tests is similar to those observed in smectite and glass (table 7.6.5). It should be noted that dissolution reaction was frequently nonstoichiometric. This behaviour was attributed to inhomogeneity and zonation of the bulk mineral, and to the faster dissolution of the calcic plagioclase than of alkaline ones, introducing an additional scattering in the dissolution data. Dissolution tests performed at 70ºC did not provide congruent results or did not approach steady-state conditions. The plagioclase dissolution rates available exhibited an increase of dissolution rate increasing ph and temperature. The trend observed permitted to calculate reaction orders for proton (hydroxyl) promoted dissolution reaction. The n values are, respectively 0.12 and 0.19 at 25 and 50ºC (figure 7.6.5) Si 25¼C Al 25¼C Si 50¼C Al 50¼C ph Figure ph dependence of the log of the volcanic glass dissolution rate at 25 and 50 C. Rates for ph values lower than 11 were included for comparison In the range of ph and temperature studied, plagioclase dissolves approximately two orders of magnitude faster than smectite, computed on molar base, which agrees with previous studies. 140

153 Table Average values of ph, Si and Al concentration of the output solutions of plagioclase dissolution tests, in the steady-state region, and plagioclase dissolution rates derived from Si and Al concentrations. The uncertainty associated with the rate is ±15%, or 0.13 log units. Series ph out Si out Al out log R Si log R Al (μmol L 1 ) (μmol L 1 ) (mol m 2 s 1 ) (mol m 2 s 1 ) Ab I Ab II Ab Ab Ab Ab I Ab II The apparent activation energies for plagioclase (table 7.6.3) increase with ph. These values are some units higher than the corresponding values for smectite, which indicate that the temperature affect plagioclase dissolution more than it affects smectite dissolution. -9 Si 25¼C Al 25¼C -10 Si 50¼C Al 50¼C y = x y = x ph Figure ph dependence of the log of the plagioclase dissolution rate at 25 and 50. Data from solutions at ph lower than 11 are given for comparison. 141

154 7.6.5 Conclusions The experiments performed have allowed us to measure the dissolution rates of smectite, volcanic glass and plagioclase. Smectite is the main component of bentonite (92 % in FEBEX bentonite), whereas plagioclase (3 %) and volcanic glass (tuff, estimated 1-2 %) correspond to the most abundant and reactive accessory phases. The results indicate that dissolution rates are strongly affected by ph and temperature. This effect is particularly important for ph values above 11. The effect of the alkaline plume should be especially strong for young cement waters and for a repository concept in clay rocks, where the vousoirs surrounding the bentonite are closer to the canister that the concrete plug in granite concept, and therefore submitted to higher temperatures. For cool regions and slightly alkaline solutions, the bentonite components should be much more stable. The hydraulic properties in the barrier do not allow the flux necessary to produce and maintain a massive dissolution initially, but this process might start, spread and increase through fractures and discontinuities. Precipitation of secondary phases as zeolites and gels of various natures may contribute to maintain a low hydraulic conductivity in the most reactive regions, as already suggested by other partners in the project. The kinetic parameters calculated are necessary to evaluate the effective dissolution rate under barrier conditions of ph, flow, solution composition, temperature, etc. The overall dissolution rate of the bulk bentonite can be modelled as the contribution of smectite, tuff and plagioclase, because other accessory phases dissolve slowly and have small surface area (i.e., quartz) or are really trace minerals (i.e., pyroxenes, amphiboles). The contribution of minor phases to the overall dissolution of bulk bentonite is small compared with the smectite. However, they are more reactive than the smectite and their fast dissolution might be a source of silica for the precipitation of secondary phases, which contribute in turn to fill the porosity derived from dissolution reactions. The accessory minerals might act as a protective phase for the smectite and its physico-chemical properties. 142

155 7.7 LABORATORY EXPERIMENTS CONCERNING COMPACTED BENTONITE CONTACTED TO HIGH PH SOLUTIONS Ola Karnland, Clay Technology AB Introduction and Objectives Background It will be most practical to use concrete in many constructions and cement for sealing rock fractures in a repository for spent nuclear waste. In the Finnish ONKALO underground facility as an example, the estimated quantity of cement is kg [Vienno et al. ]. However, the high ph in cement pore water may affect the stability of repository components, and are thereby of concern for the performance and safety assessment analyses. In the Swedish KBS3 repository, the high ph may influence fracture minerals, the bentonite buffer, canister components and the spent fuel. The ph evolution in the bentonite is of special interest, since the bentonite buffer completely surrounds the fuel canisters and thereby may protect the canister and the fuel by a chemical buffering of the high ph. This potential ph buffering capacity of bentonite may be divided in three different groups: accessory mineral reactions montmorillonite surface reactions montmorillonite crystal lattice reactions Depending on the type of cement, type of bentonite and physico-chemical conditions, the following principle conditions may prevail in bentonite contacted to high ph solutions: Relatively slow reaction between high ph solution and all components in the bentonite compared to the transport of solutes in the pore-water. The ph propagation front will thereby be governed by diffusion only, leading to a diffuse and fast moving front. Relatively fast reaction, between high ph solution and some part of the bentonite (i.e. accessory minerals or montmorillonite surface sites), compared to the transport of solutes in the pore-water. The ph front propagation will thereby be slower compared to pure diffusion and have a steeper concentration profile. Fast reaction, between high ph solution and the major part of the bentonite (congruent dissolution of the montmorillonite lattice), compared to the transport of solutes in the pore-water. The ph front propagation will thereby be slow and have a steep concentration profile. Item one implies a well functioning buffer with respect to physical properties, but with no ph buffering capacity. The third item conditions would give a large buffering capacity but destruction of the bentonite and a reduced effective buffer thickness. Knowing the conditions make it possible to adopt the repository layout to the prevailing conditions, e.g. if the latter case prevails, then the buffer thickness can be adopted to permit a sacrificed part. 143

156 Figure shows a simplified stability diagram of montmorillonite with respect to cation/proton activity ratio and silica activity. Alkali hydroxides dissolve in fresh cement which may raise ph to values between 13 and 14, and in matured cement materials ph is governed by Portlandite (Ca(OH 2 )), which gives a ph of Compared to typical bentonite conditions this leads to an increase of the cation/proton activity ratio and into the stability field of Feldspars (figure 7.7.1, left). Further, an increase in ph will lead to a dramatic change in silica solubility. Figure 1 right shows the saturation concentrations for amorphous silica and for quartz versus ph in NaOH solutions as calculated by use of PREEQCI. The alkali hydroxides in fresh cement may consequently lead to four orders of magnitude higher silica activity compared to neutral conditions E+01 Log {Na+}/{H+} Gibbsite Kaolinite Albite Montmorillonite [SiO2], mole/litre 1.E-01 1.E-03 SiO2 (a) SiO2 (q) Log {H4SiO4} 1.E ph Figure Stability diagrams for Na-montmorillonite (left) simplified from Helgesson et al Left dotted line indicates quartz saturation and right dotted line indicates amorphous silica saturation. Solubility of silica versus ph (right). Dotted line indicates amorphous silica (a) and unbroken line indicates quartz (q) Objectives Several studies have been made concerning the interaction between bentonite and cement pore-water. Most of these have been based on batch experiments with high water solid ratio. The present study focuses on highly compacted bentonite since e.g. ion-equilibrium and transport restrictions significantly may affect possible reactions and reaction rates. The overarching objectives in this study were to gather information concerning: ph transport rate through compacted bentonite ph buffering capacity of bentonite changes in swelling pressure as a consequence of high ph effect of accessory minerals on the above items mineralogical changes in bentonite exposed to alkali and calcium hydroxide solutions dissolution rates of involved minerals. 144

157 7.7.2 Experimental Test principles The general test principle was to expose highly compacted bentonite clay to high ph solutions and to measure the response in swelling pressure, transport of the high ph plume through the bentonite, and finally to analyze the mineralogical changes in the bentonite. Both commercial bentonite and almost pure montmorillonite were used, respectively, in order to get information on the effects of the bentonite accessory minerals. After test termination, the test solutions were analyzed by use of ICP/AES and ion chromatography (IC). The clay material was analyzed by use of XRD, ICP/AES and the cation exchange capacity and the type of exchange cations were determined. In total, 24 tests were carried out. All tests were made at room temperature, and most tests were run in a closed system with limited access to air. High bentonite dissolution rate and fast transport of a high ph plume are in different ways the most unfavorable conditions from a safety assessment perspective. In order to be conservative from this respect, the concentration gradients over the test samples were kept constant, and the volume of the external solutions were large enough to keep the concentrations of dissolved species far from saturation. These conditions are assumed to minimize precipitation of dissolution products and thereby give a maximum effect on the bentonite and on the transport rates Test material Material from one batch (MX-80, ) of commercial Wyoming bentonite from American Colloid Co. was used as starting material for all tests. This bentonite grade, termed "type 6 material", is a blend of natural bentonite horizons, which is milled to millimeter sized grains and sold under the product name MX-80. In order to remove the accessory minerals and produce homo-ionic montmorillonite, ten grams of the original MX-80 material was dispersed in 1 L of de-ionized water. The fraction coarser than 2 micron was settled as calculated by Stoke s law and the supernatant was removed by decantation. The salts KCl, NaCl and CaCl 2 were added to a concentration of 1 M in the decanted dispersion, respectively. The treatment was repeated twice and the dispersion was left to settle and the supernatant was removed and new de-ionized water was added. Approximately 0.8L of the suspension was placed in dialyses membranes (Spectra/pore 3, 3500 MWCO). The membranes were placed in a Plexiglas tube with 5 L of de-ionized water, which was circulated by use of a magnetic stirrer. The electrical conductivity of the external water was measured and changed daily. After approximately one week the conductivity was below 10 μs and the clay material was removed and oven-dried at 60 C. The four test materials, MX-80 standard, MX-Na, MX-Ca and MX-K, were analyzed by use of XRD, ICP/AES, and the cation exchange capacity (CEC), and the type of exchangeable ions were determined by the technique described in

158 a) Original MX-80 material Five separate XRD scans were made of the powdered MX-80 total material and the mean intensities were used to identify and quantify the main minerals according to the technique described in section The total MX-80 material contained 83% montmorillonite, 7% albite, 5% quartz, 3% cristobalite, 1% muscovite and gypsum. In addition, grains of pyrite (FeS 2 ), calcite (CaCO 3 ), siderite (FeCO 3 ), barite (BaSO 4 ) and iron hydroxides in mean quantities less than 1% were occasionally found both by the XRD and SEM analyses. The CEC was determined to be 0.74 eq/kg for the total MX-80 material, and 0.86 eq/kg for the clay fraction. The montmorillonite fraction of the original MX-80 material, as calculated from the CEC values, is consequently 86% assuming that only montmorillonite contributes to the CEC. The mole-weight of the montmorillomite based on an O 20 (OH) 4 cell was calculated to 747 g and the layer charge The following structural formula of the montmorillonite component was calculated from the cation distribution in the original MX-80 material, and from the clay fraction analyses, by the technique described in 7.7.3: (Si 7.86 Al 0.14 ) (Al 3.11 Fe Mg 0.50 Ti 0.01 ) O 20 (OH) 4, Na 0.47 Ca 0.05 Mg 0.02 K a.1 Homo-ionic clay fractions based on MX-80 The XRD analyses of the purified MX-80 material showed minor peaks from cristobalite and quartz, which was quantified to represent less than 2% of each according to the technique described in According to the ICP/AES analyses and the calculation technique described in the SiO 2 content (cristobalite and quartz) was 3.9% in the MX-K material, 3.4% in the MX-Na material, and 2.3% in the MX-Ca material. The sodium exchanged material (MX-Na) material had a CEC of 0.85 eq/kg, and based on an O 20 (OH) 4 cell the mole-weight was calculated to be 748 g, the layer charge -0.64, and the structural formula: (Si 7.82 Al 0.18 ) (Al 3.13 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Na 0.59 Mg 0.02 Ca 0.01 The potassium exchanged material (MX-K) material had a CEC of 0.86 eq/kg, and based on an O 20 (OH) 4 cell the mole-weight was calculated to be 757 g, the layer charge -0.65, and the structural formula: (Si 7.82 Al 0.18 ) (Al 3.11 Fe Mg 0.48 Ti 0.01 ) O 20 (OH) 4 K 0.53 Na 0.02 Ca 0.02 Mg 0.01 The calcium exchanged material (MX-Ca) material had a CEC of 0.78 eq/kg, and based on an O 20 (OH) 4 cell the mole-weight was calculated to be 746 g, the layer charge -0.58, and the structural formula: (Si 7.88 Al 0.12 ) (Al 3.13 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Ca 0.28 Mg

159 b) Test solutions Alkali hydroxides dissolve in fresh cement which may raise ph to values between 13 and 14, and in matured cement materials ph is governed by Portlandite (Ca(OH 2 )). In order to cover the possible conditions the test solutions were chosen to be 0.1, 0.3 and 1.0 M NaOH, and saturated Ca(OH) 2 solution. One test was also made with 1.0 M KOH. The corresponding ph values as calculated by PHREEQC are 12.9, 13.3, 13.7 (13.8, llnl database) for the NaOH solutions, 12.4 for the Ca(OH) 2 solution, and 13.7 for the KOH solution Samples and Test Equipment A compromise between the need of reasonable fast reactions and enough material for subsequent analyses led to test samples with a diameter of 35 mm, and a height of 5 mm. The proposed KBS3 buffer density of 2000 kg/m 3 at full water saturation was aimed at in all tests. This corresponds to a dry density of 1570 kg/m 3, water ratio of 27%, a pore ratio of 0.75, and a porosity of 43%. A slightly lower density was generally measured in all samples after test termination due to the sample dimensions and yieldingness in the equipment. NaCl solution, 0.5L Force transducer Clay 5 mm NaOH solution, 0.5L ph controlled Figure Schematic drawing and photo of a sample holder. The samples were slightly over-compacted compared to the intended density and placed in the cylindrical sample holders made of titanium and PEEK ( ). The piston, load cells and upper lid were attached and fixed. The volume was initially controlled by distance pieces placed between a flange on the piston and the sample confining ring. Sensotech miniature load cells models 53 were used in order to measure the axial force. The pistons were pre-stressed to a force of around 25% of the expected final force from the sample swelling pressure. The distance pieces were removed when the measured force exceeded the applied pre-stress. 147

160 The predefined saturation solutions (0.5 L) were slowly circulated behind the two filters by means of a peristaltic pump. After reaching pressure equilibrium the test solutions were changed to a high ph solution (0.5 L) on the bottom side and a corresponding isotonic chloride solution (0.5 L) on the upper side. The test solutions were placed in polypropylene bottles and the solutions were circulated in closed loops, except for the short time required for the ph measurements. In a few tests the same solution was used on both sides and successively changed on both sides of the sample in order to ensure that the swelling pressure changes were an effect only of the solutions, respectively. At test termination the solutions were disconnected and the samples were quickly removed and split, and the samples and solutions were analysed according to the program in section Measurements and Analyses Temperature and Swelling pressure The temperature and axial force were automatically measured every half hour, and the swelling pressure (P s ) was calculated from the axial force (F) according to: F Eqn P s = A where A is the piston/bentonite contact area. The load cells were calibrated before and after each test. Temperature was measured by use of thermocouples placed outside the samples ph results A Metrohm 691 ph meter equipped with and a T-glass electrode was used for all ph measurements. Several hours long recovery time in de-ionized water followed by storage overnight in 3M KCl solution was needed after measuring in the high ph solutions. The measurements were therefore made from low to higher low ph solutions, and thorough washing of the electrode was made after each measurement in order to minimize the risk for the contamination of the solutions. The ph was measured virtually daily in the low ph solutions, and weekly in the high ph solutions. The electrode was calibrated regularly in reference solutions of ph up to Mineralogy a) General The test clay samples were split as shown in figure immediately after removal from the sample holders. The central part was used for Scanning microscopy. Part number 1 and 2 were split in the axial direction in order to give a low ph (A) and a high-ph side (B). The "1" and "2" parts were split for the subsequent analyses and in some cases used as doublets. 148

161 1 2 1A 2A Figure Principle partition of the test samples for the subsequent analyses. 1B 2B b) Cation Exchange Capacity, CEC The type of exchangeable cations was determined by exchange with ammonium ions (0,15 M NH4Cl) dissolved in 80% ethanol [Jackson, 1975]. The cation exchange capacity was determined by exchange with copper(ii) triethylenetetramine [Meier & Kahr, 1999]. The measured CEC values in the calcium saturated clay were not repeatable and generally lower than for the same clay saturated with sodium and potassium. A reference test series were therefore made in order to improvement the technique. The series included also a second montmorillonite (Milos), and Cu concentrations from 8 to 24mM. Increasing the Cu concentration to 12mM at a minimum stabilized the values for calcium saturated montmorillonite, but the analyses still gave approximately 10% lower CEC values for divalent ions (Mg and Ca). The values for the calcium saturated clay are still used since possible changes, and not the absolute values, are of main concern. However, the structural formulas for the calcium saturated clays given in are likely slightly wrong. c) X-ray diffraction analyses All scans were made with a step size of θ by use of a Seifert 3000 TT X-ray diffractometer with a variable slot and CuKα radiation produced at 50kV/30 ma. The samples were analyzed by use of X-ray diffraction of powdered samples and of oriented samples prepared according to the filter-membrane peel-off technique (Drever). The oriented mounts were scanned after pretreatment with 0.5 M MgCl 2 and after exposure to ethylene glycol (EG) vapor for 24h. Mineralogical quantification was made by use of the Siroquant Software based on the Rietveld technique [Taylor & Matulis, 1994]. The results were recalculation to fixed slot conditions in order to facilitate quantification of the clay minerals. d) Element analyses The clay samples were analysed by ACME laboratories, Vancouver, by use of ICP/AES, mainly in order to identify changes in the ratio between the main elements in the clay material, but also in order to verify ion-exchange, and to reveal possible uptake of atmospheric carbon dioxide. The tests solutions were analysed by use of ICP/AES and ion chromatography (IC) by the Ecology institution at Lund University, mainly in order to quantify dissolved elements from the clay, and secondly in order to detect possible dissolution of the equipment due to the very aggressive conditions. 149

162 d.1 Scanning Electron Microscopy (SEM) analyses SEM analyses were made by use of a Phillips 515 SEM microscope, equipped with a LINK ISIS EDX. A small piece, representing the whole section, of the central part of the samples (figure 7.7.3) was freeze dried for 24 h at -30 C and <0.1 Pa. The specimen was broken to give a fresh surface, glued on to holders and gold-sputtered to a film thickness of 10 Å in order to reduce electrical charging during the analyses. This preparation technique induces large micro-structural changes but prevail the content and distribution of dissolved species. A rather large number of positions, representing the whole section from the low ph to high ph-side were analyzed in each sample. Morphological and the chemical analyses were made in magnifications from 50 to Results General In total, 24 separate tests were run. Twelve were special tests or doublets to check repeatability, and these tests were not fully analyzed. The measurements and analyses described the previous chapter were fully carried out for the 12 separate tests shown in table and the important results are described in this chapter. Table Fully analyzed samples, solutions and calculated ph. Sample clay solution A solution B ph solution B 21 MX M NaCl 0.1M NaOH MX M NaCl 0.3M NaOH MX M NaCl 1.0M NaOH MX-Na 0.1M NaCl 0.1M NaOH MX-Na 0.3M NaCl 0.3M NaOH MX-Na 1.0M NaCl 1.0M NaOH MX M NaCl 0.1M NaCl - 32 MX M NaCl 1.0M NaCl - 33 MX M CaCl M Ca(OH) MX M CaCl M Ca(OH) MX-Ca 0.015M CaCl M Ca(OH) MX-Ca 0.015M CaCl M Ca(OH) Swelling pressure Significant reductions in swelling pressure were found for the samples contacted to high concentrations of sodium chloride and hydroxide solutions. A major difference was though that the chloride solutions gave a fast drop to a new stable pressure value, while contact to 150

163 the hydroxide solution led to a continuous lowering of the swelling pressure. Figure (left) shows the pressure response from one sample successively exposed to pure water, 1.0 M NaCl, 1.0 M NaOH, 1.0 M NaCl, and finally pure water. Figure (right) shows individual samples exposed to the same 1.0 M solutions. A significant continuous swelling pressure reduction was also observed for all samples exposed to 0.3 M NaOH solutions (figure 7.7.5). However, no swelling pressure change was noticed in the samples exposed to 0.1 M NaOH (figure 7.7.5) or to saturated Ca(OH) 2 solution (figure 7.7.6). The minor drop in the lower curve in figure 7.7.6, right, is fully explained by sensor leeway, ensured in the post-calibration, and is consequently not an effect of swelling pressure decrease. The MX-Na samples (figure and figure 7.7.5, right) generally gave higher swelling pressure compared to the MX-80 samples (figure and figure 7.7.5, left). No principle difference was noticed between the MX-80 samples and the MX-Na samples with respect to the response to NaCl or NaOH solutions, except for minor initial ion-exchange responses in the MX-80 bulk samples (figure and figure 7.7.5, left). Initial tests with 1.0M KOH and NaOH solutions gave in principle the same results and no further tests were made with KOH solutions Swelling Pressure, kpa Temperature, C Swelling Pressure, kpa H2O NaCl NaOH NaCl/NaOH Temperature, C Time, days Time, days Figure Pressure response from a MX-80 sample successively exposed to pure water, 1.0M NaCl, 1.0M NaOH, 1.0M NaCl, and finally pure water (left). Pressure response for the same exposures but in four individual MX-80 samples (right). The sample indicated NaCl/NaOH were saturated with 1.0M NaCl and exposed to 1.0M NaOH solution after 15 days. Lowest line shows temperature. 151

164 Swelling Pressure, kpa Temperature, C Swelling Pressure, kpa Temperature, C Time, days Time, days Figure Swelling pressure evolution in MX-80 bulk samples (left) and Namontmorillonite (right). All samples were initially saturated with pure water, subsequently exposed to the chloride solutions (5 days), and finally to the hydroxide solutions (60 days). Lowest line at 100 days shows results from 1.0 M solution, middle line from 0.3 M solutions, and uppermost line show 0.1 M solution Swelling Pressure, kpa Temperature, C Swelling Pressure, kpa Temperature, C Time, days Time, days Figure Swelling pressure evolution for two MX-80 bulk samples (left) and two MX-Ca samples (right). All samples were initially saturated with pure water, exposed to the 0.015M Ca(Cl) 2 after 5 days, and finally exposed to saturated Ca(OH) 2 solution in the lower filter after 60 days ph results A ph increase in the NaCl solutions was measured in the four tests exposed to 1.0M and 0.3M solutions. Logically, the increase came earlier and was more pronounced in the tests with the highest concentration (1.0M, sample 23, and 26). For both concentrations, the increase was first measured in the MX-80 samples (sample 22, 23). Figure shows the measured ph in the 1.0M NaCl solutions (left) and in the 0.3M NaCl solutions (right). No ph increase was measured after 45 days in the two tests with 0.1M solutions, nor after an exposure time of 110 days in the four tests with saturated Ca(OH) 2 solutions. 152

165 ph 9 ph Time, Days Time, Days Figure ph evolution in the NaCl solutions of samples exposed to 1.0M solutions (left), and 0.3M solution (right). Circles indicate results from MX-80 samples, triangles indicate results from MX-Na samples. Uppermost curves indicate results from the NaOH solutions, and lowest lines results from NaCl reference solutions Mineralogy a) Cation Exchange Capacity, CEC A significant increase in CEC was noticed for all samples exposed to the NaOH solutions (table 7.7.2). The increase was significant on both sides of the samples except for the NaCl side of the sample exposed to the lowest concentration (sample 24A). Table Measured ph and CEC from MX-80 and MX-Na samples exposed to 0.1, 0.3 and 1.0 M NaOH solutions on one side and corresponding NaCl solutions on the other. The CEC values show eq/kg clay. NaCl side (A) NaOH side (B) Sample ph CEC ph CEC MX-80 reference n MX-Na reference n No significant changes in CEC were measured in the samples exposed to only NaCl solutions (sample 31 and 32), or to the samples exposed to the 0.015M Ca(OH) 2 solutions (table 7.7.3). Dissolution of salt from pore-water leads to much higher content of individual ions in the test samples than in the reference samples. The results are still presented since the 153

166 distribution of ions show an almost complete ion exchange to calcium in samples 33 and 34, although the calcium concentration is as low as 0.015M in the test solutions. Table Measured CEC from MX-80 exposed to 0.1M (31AB) and 1.0M solutions (32AB), and MX-Ca samples exposed to saturated Ca(OH) 2 solutions (A side) and M CaCl 2 solutions (B side). The CEC and cation values show eq/kg clay. Sample Cu-CEC Ca K Mg Na MX-80reference AB AB A B A B MX-Ca reference A B A B a.1 X-ray diffraction analyses Significant changes were found in the samples exposed to NaOH solutions with respect to cristobalite and quartz (figure and figure 7.7.9). No cristobalite were present in the samples exposed to 1.0 M NaOH solution. Loss of cristobalite was found in the sample exposed to the 0.3M NaOH solution, and a possible decrease is indicated on the high ph side of the MX-80 sample exposed to 0.1M NaOH solution. Quartz was much less affected and the amount was almost unchanged in the MX-80 samples. A significant decrease of quartz was found in the MX-Na sample exposed to 1.0M NaOH solution. No other significant dissolution was noticed. No neoformation of minerals was found in any of the MX-Na samples. An unidentified peak pattern was found in the MX-80 samples exposed to 1.0M NaOH solution (23B) (figure 7.7.8), repeated scans did not show this pattern, indicating that the first results were accidental. A small amount of gel material was found outside the low ph side of sample 23A. This material did not a show a montmorillonite peak pattern, but a non-distinct pattern indicating an amorphous structure. The oriented and EG treated samples did not show any significant changes in swelling properties compared to the reference clay material (figure ). 154

167 23B 23A 22B 22A 21B 21A MX θ, Figure Powder XRD pattern from MX-80 samples exposed to NaOH solutions. 26B 26A 25B 25A 24B 24A MX-Na θ, Figure Powder XRD pattern from MX-Na samples exposed to NaOH solutions. 155

168 Å 21AE 21BE 22AE 22BE 23AE 23BE Å MX-80 24AE 24BE 25AE 25BE 26AE 26BE θ, 2θ, Figure XRD pattern from all samples exposed to NaOH solutions and MX-80 reference material after pre-treatment with Mg and ethylene glycol. b) Element analyses b.1 Clay samples Major results from the ICP/AES element analyses are shown in Appendix 1. The silica/aluminium ratio decreased significantly in samples exposed to the NaOH solutions, and an Si/Al gradient was found in most samples exposed to the NaOH solutions (figure ). No other elements showed a similar trend (figure , right). 3.3 MX-80 MX-Na MX-80 MX-Na MX M 0.3 M 1.0 M MX-Na 0.1 M 0.3 M 1.0 M MX M 0.3 M 1.0 M MX-Na 0.1 M 0.3 M 1.0 M Figure Si/Al ratio (left) and Fe/Al ratio (right) from ICP/AES analyses. Left bars indicate the low ph side of the samples and the right bars indicate the high ph side. The XRD analyses of the purified MX-80 material showed minor peaks from cristobalite and quartz, which was quantified to represent less than 2% of each according to the technique described in The structural formulas of the montmorillonite in the purified materials were calculated based on the ICP/AES analyses to match an O 20 (OH) 4 cell according to Newman The calculated layer charges were lower than what was calculated from the measured CEC as determined by the Cu-method. The silica content was therefore adjusted to 156

169 give the structural formula which matched the CEC values. The reduction of silica corresponded to a SiO 2 content (cristobalite and quartz) of 3.9% in the MX-K material, 3.4% in the MX-Na material, and 2.3% in the MX-Ca material. The structural formula for the samples exposed to the NaOH solutions were calculated to be: MX-Na (Si 7.82 Al 0.18 ) (Al 3.13 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Na 0.59 Mg 0.02 Ca A: (Si 7.82 Al 0.18 ) (Al 3.14 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Na B: (Si 7.83 Al 0.17 ) (Al 3.09 Fe Mg 0.51 Ti 0.01 ) O 20 (OH) 4 Na A: (Si 7.78 Al 0.22 ) (Al 3.10 Fe Mg 0.49 Ti 0.02 ) O 20 (OH) 4 Na B: (Si 7.73 Al 0.27 ) (Al 3.10 Fe Mg 0.49 Ti 0.01 ) O 20 (OH) 4 Na A: (Si 7.71 Al 0.29 ) (Al 3.09 Fe Mg 0.49 Ti 0.02 ) O 20 (OH) 4 Na B: (Si 7.65 Al 0.35 ) (Al 3.10 Fe Mg 0.49 Ti 0.02 ) O 20 (OH) 4 Na 0.80 and the structural formula for the samples exposed to the Ca(OH) 2 solutions were calculated to be: MX-Ca (Si 7.88 Al 0.12 ) (Al 3.13 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Ca 0.27 Mg /36A: (Si 7.87 Al 0.13 ) (Al 3.14 Fe Mg 0.47 Ti 0.01 ) O 20 (OH) 4 Ca 0.29 Mg /36B: (Si 7.87 Al 0.13 ) (Al 3.11 Fe Mg 0.49 Ti 0.01 ) O 20 (OH) 4 Ca 0.30 Mg b.2 Test solutions Test reference solutions and test solutions on both sides of the samples were analyses by use of ICP/AES and ion chromatography (IC) and the main elements are presented in Appendix 2. The IC analyses did not give repeatable results in the solutions with the highest ph, and the chloride concentration results should be used with care. Silica was released from all samples exposed to NaOH solutions and a clear coupling was found between silica concentration and ph also in the NaCl solutions (table 7.7.4). Small amounts of titanium were noticed in the 1.0 and 0.3M NaOH solutions, likely emanating from the sample holder filters. 157

170 Table ICP/AES analyses of solutions from tests with MX-80 exposed to NaOH solutions. Figures in mm. Sample ph Al Ca Mg K Si Ti NaCl side 21 n NaOH side Table ICP/AES analyses of solutions from tests with MX-Na exposed to NaOH solutions. Figures in mm. Sample ph Al Ca Mg K Si Ti NaCl side 24 n NaOH side c) Scanning Electron Microscopy (SEM) analyses The SEM/EDX analyses showed accessory mineral dissolution and possible minor structural changes in the exposed samples. The typical flaky montmorillonite character was, however, not change significantly in any sample (figure and figure ). Relatively large variation in silica content was found, but the natural variation is normally rather large and no clear conclusions about the silica distribution can be drawn from the present analyses. Table shows two spot analyses from gel material (figure ) found outside the filters in sample 23A (MX-80, 1.0M NaOH). 158

171 Figure Typical bentonite structure (left) in sample 23A (MX-80, NaCl side, 1.0M), and a similar structure (right) in sample 23B (NaOH side). Space bars show 50 mm (left) and 20 mm (right). Figure Pure montmorillonite (left) from sample 26B (MX-Na, NaOH side, 1.0M). Gel from the precipitation outside the filter in sample 23A (MX-80, NaCl side, 1.0M). Table SEM/EDX analyses of the gel material found outside the filters in sample 23A (MX-80, NaCl side, 1.0M) P23A1 P23A2 % % Na Mg Al Si Cl K Ca Ti Fe

172 7.7.5 Discussion The pressure response from the sample successively exposed to pure water, 1.0 M NaCl, 1.0 M NaOH, 1.0 M NaCl, and finally pure water (figure 7.7.4, left) shows several important characteristics. The fast drop to new pressure equilibrium at exposure to NaCl solutions shows the osmotic effects of the salt. The small initial response when the test solutions is changed from NaCl to NaOH, or reverse, indicate that the osmotic response is the same for the NaOH solution. The immediate stop in swelling pressure reduction, as an effect of changing the NaOH solution to NaCl, further indicates that there is a reaction between the NaOH solution and the bentonite, and not just a slow adaptation to equilibrium conditions. The final change to pure water results in an increase in swelling pressure, which is close to the initial decrease at the first contact with the NaCl solution. This strongly indicates that the swelling pressure reduction as a result of NaOH contact is permanent, and not reversible as the NaCl effect. The individual samples in figure (right) show that the final difference in pressure between pure water and NaCl solution conditions are approximately the same as the pressure difference between the NaOH solution and the NaOH/NaCl solution conditions, which shows that the pressure effect of the NaCl solution seems to be constant regardless of the impact of the NaOH solution. The pressure reduction in the two samples exposed to NaOH solution is similar, which shows that the effect of NaOH is not affected by the presence of NaCl. The pressure effects of NaOH and Ca(OH) 2 solutions are principally the same for MX-80 as for MX-Na (figure and figure 7.7.6). The ICP/AES analyses of water and clay, the CEC analyses and the XRD analyses all show that silica dissolves from the bentonite, and the resulting change in mass likely is the cause of the swelling pressure decrease. The best quantitative value is given by the ICP/AES analyses of the solutions. Sample 26, which shows the highest release, has a total loss of 200 mg Si. The total sample mass is 7.56 g of which 7.26 g is montmorillonite corresponding to 9.6 mmole montmorillonite. The structural formulas show a loss of 0.14 mole Si per mole montmorillonite in this sample, which gives 1.35 mmole Si and a corresponding mass of 38 mg. Assuming the additional loss of silica comes from SiO 2 (mainly cristobalite) gives a mass of 162*60/28 mg which gives 347 mg. The total mass loss is then = 385 mg. The final sample dry mass is then = g. The change in density of the sample was then from a dry density of 1460 to 1387 kg/m 3. The swelling pressure drop was from initial 5500 in pure water to the final 1800 kpa in the 1.0M NaOH/NaCl solution. The pressure conditions are shown in figure , which illustrates that the pressure drop may be fully explained by the mass loss and the osmotic effects of the solutions. 160

173 Swelling Pressure, kpa 1E+5 1E+4 1E+3 1E+2 1E+1 1E i 26f Swelling Pressure, kpa 1E+4 1E+3 1E i 26f Dry density, kg/m Dry density, kg/m3 Figure Measured and calculated values for swelling pressure in pure montmorillonite exposed to NaCl solutions. Initial (26i) and final (26f) pressure and density conditions in sample 26. Detail of the diagram to the right. Figures show NaCl concentrations in moles/l. It is obvious that a high ph plum may diffuse through bentonite at a sufficiently high ph gradient. The ph increase on the NaCl solution was first noticed in the MX-80 tests, which indicate that there is no major ph buffering effect from other accessory minerals than cristobalite and quartz. The diffusion can not be described by a single diffusion coefficient according to the evaluation made by Höglund in work package 5 ( 10.5). The evaluated effective diffusivity coefficients for the 1.0M solutions conditions are two orders of magnitude higher than for 0.3M solution conditions. It is obvious from the results in this study that dissolution of cristobalite takes place, which is one plausible explanation. Different effective porosity due to the test solutions is discussed by Höglund ( 10.5). The similar osmotic behavior of the hydroxide and chloride solutions in this study indicate that the ion-equilibrium conditions between bentonite and the two solutions are the similar. If this really is the case, Donnan equilibrium play an important role for the actual concentrations in the clay. The chloride concentrations in clay pore-water has been calculated to be 3 times lower than in an external solution for a 1.0M NaCl external solution, 10 times lower for a 0.3M solution and 25 times lower for a 0.1M solution (Karnland et al. 2002). The release of silica from the montmorillonite was around 40 mg from sample 26 and the exposure time was 40 days. The dissolution rate in this specific geometry is consequently 0.04/7.26/40/86400 = 1.6E-9 g*g -1 clay*s -1 in the 1.0M NaOH solution, and approximately 5e-10 g*g -1 clay*s -1 in the 0.3M solution. 161

174 7.7.6 Conclusions The swelling pressure in bentonite is strongly reduced by exposure to 0.3 and 1.0 M NaOH solutions. The decrease can to a large extent be related to dissolution of silica and the subsequent mass loss of the bentonite samples. The fast initial swelling pressure decrease is reduced after a relatively short time, and a subsequent almost constant decrease rate has been observed. The fast initial effect is likely due to the dissolution of cristobalite. A significant increase in CEC has been observed in samples exposed to the NaOH solutions. The structural formulas of the exposed montmorillonite have been calculated based on the ICP/AES and CEC data, and show a significant release of silicon from the bentonite. The release rate has been determined to be 1.6E-9 g*g -1 clay*s -1 for the 1.0M NaOH solutions and 5E-10 g*g -1 clay*s -1. Such change in the tetrahedral layers of montmorillonite may be seen as the first step towards beidellite, which can be seen as a minor problem with respect to the buffer functions. However, the properties of the clay will change dramatically if this reaction goes on until a high charges mineral is formed. No effects on swelling pressure were found in the samples exposed to 0.1M NaOH and saturated Ca(OH) 2 solutions. No mineralogical/chemical effect except ion exchange from sodium to calcium was found in the samples exposed to the Ca(OH) 2 solution. The similar osmotic behavior of bentonite exposed to NaCl and NaOH solutions indicate that Donnan equilibrium is established also for hydroxide solutions, which may help to explain the lack of chemical/mineralogical reactions in the Ca(OH) 2 solution despite the fact that silica saturation concentration is very high also at ph ACKNOWLEDGMENTS The authors wish to acknowledge that this paper is a result of work funded by the Swedish Nuclear Fuel and Waste Management Company (SKB), and the European Commission. A special thanks to Thomas McMenamin who kindly agreed to the publication of this paper. 162

175 7.9 REFERENCES OF WORK PACKAGE 2 Bauer A. & Velde B. (1999) - Smectite transformation in high molar KOH solutions. Clay Miner. 34, Bauer, A. & Berger, G. (1998). Kaolinite and smectite dissolution rate in high molar KOH solutions at 35 ºC and 80 ºC. Appl. Geochem., 33, Blum, A.E. & Stillings, L.L. (1995). Feldspar dissolution kinetics. In: White, A.F., Brantley, S.L. (Eds.), Chemical Weathering Rates of Silicate Minerals. (Rev. Miner. 31, ) Mineralogical Society of America, Washington, DC (Chapter 7). Brown G. (1980) Order-disorder in clay mineral structures. In : Crystal structures of clay minerals and their X-ray identification. Brindley G.W. & Brown G. Eds., Miner. Soc., London, Broxton, D. E., Bish, D. L. & Warren, R. G. (1987). Distribution and chemistry of diagenetic minerals at Yucca Mountain, Nye Country, Nevada. Clays and Clay Minerals 35: Cama, J., Ganor, J., Ayora, C., Lasaga, A. (2000). Smectite disssolution kinetics at 80ºC and ph 8.8. Geochimica et Cosmochimica Acta, Vol. 64, No, 15, Cama, J., J. Ganor, C. Ayora and A.C. Lasaga, Smectite dissolution kinetics at 80 C and ph 8.8,. Geochimica Cosmochimica Acta 64, , Carroll, S.A. and J.V. Walther, Kaolinite dissolution at 25, 60 and 80 C, American Journal of Science, 290, CassagnabÈre A. & Bouchet A. (2000) Programme interactions argile/ciment. MX80 ANDRA. Rapport d avancement n 2. Rapport D RP 0ERM (Réf. ERM : ERM AC 182). CassagnabÈre A. (2001) Programme interactions argile/ciment. MX80 ANDRA. Rapport d avancement Batchs n 5. Rapport D RP 0ERM (Réf. ERM : ERM AC 124). Cassagnabere, A., Caracterissation et interpretation de la transition kaolinite-dickite dans les reservoirs a hydrocarbures de Frøy et Rind (Mer du Nord, Norvege), PhD Thesis, University of Poitiers, pp 238, Chermak, J. A. & Rimstidt, J. D. (1990). The hidrotermal transformation rate of kaolinite to moscovita/illite. Geochim. Cosmo. Acta, Vol. 54, pp

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179 7.10 APPENDICES Appendix 1 Figures show weight percent of total oxides. SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 LOI Tot C Tot S Summa Mx MX-Na MX-Ca MX-K MX-Mg C21A C21B C22A C22B C23A C23B C24A C24B C25A C25B C26A C26B C31A+B C32A+B C33A+34A C33B+34B C35A+36A C35B+36B

180 Appendix 2. Figures show mg/l Sample Al Ca Cu Fe K Li Mg Mo Na P S Si Sr Ti Zn Cl 1,0 M NaCl ,1 M NaCl ,0 M NaOH ,1 M NaOH CaOH S21A SAT S21B SAT S22A SAT S22B SAT S23A SAT S23B SAT S24A SAT S24B SAT S25A SAT S25B SAT S26A SAT S26B SAT A B A B A B A B

181 8 WORK PACKAGE 3: THE EFFECT OF ALKALINE FLUID ON CLAY HOST ROCKS 8.1 INTRODUCTION Cement will be used as an engineered material in the construction of deep geological repositories for radioactive waste. In such deep conditions, the contact with groundwater will cause cement degradation. According to the predicted evolution the ph of the pore water leached from cement is initially above 13 due to sodium and potassium hydroxides and will decrease to 11 during a long period of time. The effect of the high ph solutions on the mineral stability of clays and related minerals has been studied since several years in the context of the repository safety evaluation. These experimental studies made evident that quartz and clay minerals are not stable under such alkaline conditions. In this chapter investigations on the effect of cement water on argillaceous host rock are reported. The potential host rocks of two countries were subject of investigation within the ECOCLAY II project. In France the radioactive waste disposal concept foresees to dispose intermediate level waste (French type B) in a cementitious environment in a Callovo-Oxfordian Clay formation. On this argilite two types of lab studies were performed: Batch experiments carried out by ERM & HYDRASA covered the investigation of high-ph pore water effects on the mineral stability of the clay. These experiments were supplemented by thermodynamic calculations and a kinetic evaluation (section 8.2). The experiments by Euro-Geomat Consulting, reported in section 8.3, studied the effect of cement water on the hydraulic and hydromechanical properties of Callovo-Oxfordian argilites. Two macroscopic parameters, the permeability and the Biot coefficient, were determined. The permeability is of major importance for the description of transport processes in the host rock where advection is dominant. Biot coefficient is an experimental parameter that gives an account for the distribution of the fluid within the rock porosity, more precisely whether it is connected or not. The Swiss design for a potential repository for HLW and long-lived ILW in Opalinus Clay (an indurated clay stone formation of middle Jurassic age) foresees a total mass of 5000 tons of cement/concrete located in disposal tunnels apart from the disposal area for SF and HLW. A comprehensive synthesis report was filed with the Swiss government to demonstrate the feasibility of deep disposal of HLW in Opalinus Clay [Nagra 2002a, b]. A large portion of the Opalinus Clay work performed within ECOCLAY II by the University of Bern, Rock- Water Interaction, Geological Sciences did directly contribute to the description of the expected behaviour of a repository and to performance assessment. Here, two studies are reported for Opalinus Clay: Section 8.4 presents a simple mass balance model for degradation of cement and the buffering capacity of clay stone. The model is applied to a planned Swiss ILW repository in 169

182 Opalinus Clay and predicts a negligible extent of rock alteration due to the high-ph plume emanating from cement degradation. In section 8.5 a long-term ( ) field experiment is reported which was conducted at the Mont Terri Underground Rock Laboratory and was financed by a consortium consisting of ANDRA, Nagra, and OBAYASHI Corp. Callovo-Oxfordian Argilite and Opalinus Clay are very similar concerning their mineral composition and hydraulic/hydro mechanical properties. Results, described in this chapter, are therefore mutually applicable to a certain extend. 8.2 THE EFFECT OF HIGH-PH CEMENT PORE WATERS ON THE MINERAL STABILITY OF CALLOVO-OXFORDIAN CLAY Susana Ramirez and Jean-Claude Parneix (ERM) Materials and Methods Materials The Callovo-Oxfordian formation, which ranges from 420 to 550 m depth, is a complex mineral assemblage where phyllosilicates, quartz and feldspars are cemented by carbonates (calcite and dolomite). Small proportions of Fe-rich minerals (pyrite, siderite and oxide and hydroxides of iron) and organic matter are also present. The detrital minerals, little transformed by the diagenetic process, are generally in the > 2 μm fraction size whereas the diagenetic minerals, mainly phyllosilicates, are concentrated in the < 2 μm fraction size. The mineralogical composition varies as a function of burial depth [Bouchet & Rassineux, 1997; Claret, 2001]. The core sample selected for this work comes from the well EST 104 between 490,29 and 490,59 m burial depth of the foreseen French underground laboratory. The core sample was grinded to obtain a powder of 200 μm grain size. Then, the sample was decarbonated by treatment with a sodium acetate/acetic acid buffer solution (ph 5) at 70 ºC during 12 hours [Moore & Reynolds, 1989]. Finally, the < 2 μm fraction was separated by sedimentation and centrifugation procedures. Mineralogical characterization by X-ray diffraction (XRD) was performed using air dried and ethylene-glycol solvated oriented clayaggregated samples (figure 8.2.1). The sample was composed by kaolinite, chlorite, illiterich (>90% illite) interstratified mixed-layer mineral (MLM) and a randomly interstratified mixed layer mineral containing about 50 % of smectite layer, according to the Inoue et al. (1989) method. The d(001) reflection of the illite-rich MLM can hide the presence of illite s.s. and/or mica. Although phyllosilicates were predominant, quartz was also presents in small amounts (peaks at 4.26 and 3.33 Å) Batch experiments Homoionic alkaline solutions were used in the alteration tests in order to evaluate the effect of each cation in the reactivity of smectite-rich phases. The chemical compositions of initial solutions are shown in table

183 Table Chemical composition of initial solutions. solution NaOH KOH Ca(OH) 2 ph concentration cation (mol/dm 3 ) A B Figure XRD patterns of initial sample (decarbonated < 2 μm fraction of Callovo- Oxfordian clay at 490 m depth) Cu Radiation, Kα (1,54 Å). A: air dried oriented clayaggregated sample. B: ethylene-glycol solvated oriented clay-aggregated samples. 5 Å peak is attributed to ordered IS mineral; 3,57 Å peak is attributed to kaolinite. Clay samples were added to alkaline solutions, at a solid/solution ratio of 1/20. The experimental set-up was placed in a glove-box under N 2 in order to avoid solution carbonation. Batch experiments were performed in the tightly-closed PTFA reactors. The reactors were placed in a thermostated oven at 60, 90 and 120ºC for 6 to 168 hours. 171

184 After the test, reactors were cooled with water to room temperature. Solid and aqueous phases were separated by centrifugation. The aqueous phases were filtered through centrifugation filters type Centricon Plus-20 (10000 dalton) and then analysed. The solid phases were washed with ethanol and collected by centrifugation. Mineralogical characterisation of initial and altered samples was performed by X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analysis and transmission electron microscopy (TEM). The cation exchange capacity (CEC) was also measured Results and discussion Mineral transformations Non clay minerals and phyllosilicates were progressively dissolved after the alteration of the Callovo-Oxfordian clay with Ca(OH) 2 from the solid characterization after reaction by DRX, IR and thermal analysis. Dissolution was favoured by the increase of ph, time and temperature of reaction. The main reaction products formed in these experiments were the calcium silicoaluminate hydrates (CSH) phases. Experimental evidences of this neoformation were (1) the presence of the 11.7 Å reflexion in the diffractograms of randomly oriented powder preparations at ph 12.6 and 120 ºC, (2) endothermic and exothermic peaks at 690 and 890 C, respectively, which are characteristics of tobermoritelike phases and (3) the significant increase in the total CEC, from 40 meq/100 g in the initial sample to values up to 600 meq/100 g after experiments at ph The precipitation of the CSH phases could explain the low values of silica and Al concentration in solutions after experiments at ph Regarding the alteration in NaOH and KOH media, no recognizable mineralogical and structural changes were detected in samples after reaction at ph 10 and 12 with respect to the initial sample. However, dissolution-precipitation processes were evidenced combining the solution and solid data after alkaline alteration at ph 14. The product minerals were zeolites: analcime, phillipsite and chabazite. Zeolites have been previously characterized as the reactions products in the alkaline alteration of clays [Chermak, 1992, 1993; Ruiz et al., 1997; Vigil de la Villa et al., 2001; Ramírez et al., 2002a]. The starting material in the zeolites crystallisation in the Callovo-Oxfordian clay could be the poorly-crystallised phases produced in the first stage of clay alteration, since they disappeared as zeolites neoformed. This is consistent with the fact that zeolites form mainly from aluminosilicate gels or natural or synthetic glasses under neutral to alkaline conditions both in laboratory experiments and in natural systems [Barrer, 1982; Gottardi & Galli, 1985]. The reactants minerals in the alkaline reaction of the Callovo-Oxfordian clay were quartz, kaolinite and a part of smectite. Quartz and kaolinite were dissolved at 120ºC after 168 hours of reaction. Congruent dissolution of smectite layers occurred at ph 14 at 60 and 90 C from the constant Si/Al molar ratio in solution. However, comparing the Si/Al molar ratio during smectite dissolution (between 1 and 1.5) with the Si/Al molar ratio in a montmorillonite (about 2.3), one may conclude that the dissolution process is not stoichiometric, in such a way that Al-rich smectite layers were preferentially altered. The significant decrease in the Al concentration with respect to the silica one observed at high temperature at the end of

185 hours of alteration (Si/Al ratio above 3) could be due to the presence of Al complexes in solution rather than different mechanisms for dissolution at the low and high temperature. The Al 3+ released during the smectite dissolution undergoes a complexation reaction in the aqueous solutions. The dominant species at the alkaline ph measured in our experiments is - the tetrahedral Al(OH) 4 ion, which could react with several ligands, including mineral surfaces. I/S R = 0 (interlayering of high-charge and low-charge smectite) low charge i t f low charge i t f low charge high charge low charge low charge low charge high charge low charge low charge i t f low charge i t f I/S R = 1 (interlayering of I/S R = 0 and 65 % illite) amorphous phases zeolites Mg-smectite? illite illite montmorillonite montmorillonite illite illite illite preferential dissolution of montmorillonitic layers reduction of the coherent scattering domain size neoformation Figure Outline of the alkaline reaction of smectite in the Callovo-Oxfordian clay in NaO H and KOH media. The tetrahedrally-charged layers are in gray. Evidences of smectite reactivity by means of solid characterisation were observed. On the one hand, the alkaline treatment produced a reduction of the coherent scattering domain size (CSDS) of smectite in the c* direction either by a simple delamination or a dissolution process of some layers. The OH stretching region spectra obtained with infrared spectroscopy reveals a broad band centred near 3630 cm -1 corresponding to the vibration of 173

186 Al 2 OH in octahedral sheet. The absorbency decrease of this band, together with a slight broadening, indicate the partial dissolution of dioctahedral phases after alteration with NaOH and KOH and complete dissolution after the Ca(OH) 2 treatment. A decrease in the peak intensity as a function of ph, time and temperature of alteration was observed with thermal analysis confirming the dissolution process of smectite. On the other hand, preferential dissolution of octahedrally-charged smectite layers was evident from the difference in the CEC measurement before and after the Hofmann & Klemen effect. The octahedral charge from smectite layer are neutralised after the Hofmann & Klemen effect and consequently, the tetrahedrally-charge smectite layer and edge charge from phyllosilicates and neoformed phases (zeolites) contribute to the CEC. Assuming that heating at 300ºC does not modify the amount of variable charge, the difference between CEC before and after the Hofmann & Klemen test is due to octahedral charge from smectite-rich phases. Initially, CEC due to octahedrally-charged smectite layers was about 23 meq/100 g. A significant decrease was observed after alteration at ph 14 for the longest alteration time up to 6.7 and 8.2 meq/100 g at 120ºC during 168 h with NaOH and KOH, respectively. This could be attributed to the dissolution of smectite layers, specifically, layers having octahedral charge. Consequently, preferential dissolution of octahedrally-charged layers was proposed as a possible alteration process in NaOH and KOH media (figure 8.2.2). The chemical balance of such reaction should produce a release of Mg 2+, which should incorporate either in a trioctahedral smectite [Meunier et al., 1998; Beaufort et al., 2001] or in a carbonate. Actually, the formation of a trioctahedral phyllosilicate was recognizable by means of XRD and IR techniques after the reaction with NaOH 0.5 M at 200ºC of FEBEX and SAZ-1 references montmorillonites [Ramírez et al., in preparation]. However, if Mg-rich minerals were neoformed in the present alteration experiments, the amounts produced must be too low to be detected Thermodynamic modelling The thermodynamic modelling of stability relationships among minerals in the Callovo- Oxfordian clay in different chemical media was performed to validate the proposed alteration reaction from the experimental data. The degree of disequilibrium in terms of the saturation index for initial minerals dissolution after NaOH, KOH and Ca(OH) 2 alteration experiments was simulated using the solution speciation-solubility modelling code KINDIS [Madé et al., 1994] from the chemical composition of solutions. The chemical composition of smectite, interstratified illite-smectite mixed layered mineral and illite of the Callovo- Oxfordian clay are not known. In order to calculate the saturation state of the aqueous solutions with respect to these minerals, chemical compositions of equivalent minerals from similar diagenetic environment were computed. The montmorillonitic and illitic poles characterised by Meunier & Velde [1989], an interstratified illite-smectite determined by Aja [1995] and a theoretical intermediate composition of a partially tetrahedrally-charged smectite were chosen (table 8.2.2) for thermodynamic calculations. The cations saturating the smectite interlayer region were fixed as a function of the chemical composition of reacting solution. 174

187 Table Chemical composition of smectites, interstratified illite-smectite mixed layered mineral and illite used in the calculation of the saturation states. Mineral Chemical composition Reference Montmorillonite Si 4 (Al Mg ) O 10 (OH) 2 M Meunier & Velde [1989] Beidellite (Si 3.86 Al 0.14 ) (Al Mg ) Theoretical O 10 (OH) 2 M Interstratified illite/smectite (Si 3.72 Al 0.28 ) (Al 1.78 Mg 0.22 ) O 10 Aja [1995] (OH) 2 K 0.5 Illite (Si 3.25 Al 0.75 ) (Al 1.85 Mg 0.15 ) Meunier & Velde [1989] O 10 (OH) 2 K 0.9 The saturation state of the reacting solutions with respect to the considered minerals as a function of time at the highest ph is shown in figure Smectite and illite are under-saturated whatever the chemical composition of the initial solution. However, the degree of under-saturation for montmorillonite is two orders of magnitude higher than the illite one. On the other hand, both minerals and the interstratified illite/smectite phases are less stable in Ca(OH) 2 than in NaOH or KOH. This is in agreement with the experimental evidences of the smectite dissolution and conservation of illite and rich-illite interstratified after alkaline alteration at ph 14 in NaOH and KOH media and, also, with the generalised dissolution of phyllosilicates at ph 12.6 in Ca(OH) 2. The diagenetic illite is the phyllosilicate the most stable under alkaline conditions irrespective of the temperature, the reacting solution or the time (not shown). The interstratified illite/smectite is less stable than illite. This is due to differences in the tetrahedral charge and the amount of potassium, in such a way that as these parameters increase, the thermodynamical stability of a mineral increases. Finally, the most unstable phase is montmorillonite whatever the interlayer cation. The increase in the tetrahedral charge diminishes the instability in an orders of magnitude for the three reacting solutions. 175

188 2 0 KOH 1M, T=120 C K) Q/ ( g o L Calcite Quartz Phillipsite-K Chabazite- KaoliniteK Boehmit Clinoptil.-K Log t NaOH 1M, T=120 C K) Q/ ( L o g Calcite -10 Boehmit enat. Philli Heulandite N Log t K ) Q/ -6 ( g o L Calcite Kaolinite Boehmit Nat. Phillipsite Ca(OH)2 1M; T=120 C 9 8 Hydrot Gehlenite Hd Laumontit CaAlO. 19HO Log t (Y ) Figure Activity product (Q) and equilibrium constant (K) ratio of the dissolution reaction of different phyllosilicate minerals in NaOH (A), KOH (B) and Ca(OH)2 (C) as a function of time at ph 14 and 12.6, respectively, and 120 C. 1 smectite; 2 beidellite; 3 illite/smectite interstratified; 4 illite; 5 portlandite; 6 Natural phillipsite; 7 analcime; 8 CSH; 9 katoite; 10 tobermorite 11 Å

189 Smectite alteration rate Once the alteration reactions were defined and validate from the thermodynamic point of view, kinetic calculations were performed to evaluate the smectite alteration rate. This is important to estimate the variations in the confinement and retention properties of the clay, highly dependent of the mineralogical stability of smectite. Because of phyllosilicates generalised dissolution in Ca(OH) 2 media, only experiments in NaOH and KOH were considered. The extent of the preferential dissolution of smectite in NaOH at ph 14 was evaluated from the integrated intensity of the 001 reflection at 14.5 Å in the XRD patterns of randomly oriented powder preparation, which was a parameter related with the degree of progress of the reaction as a function of time (figure 8.2.4). The integrated intensity of the 001 peak varied fortuitously in samples treated with KOH solutions due to the textural effect of K-saturated clays. Consequently, kinetic calculations were not possible in this case. The rate constants for the smectite alteration in NaOH media are reported in table as a function of temperature. 4,00 d(001)/d(060) 3,00 2,00 1,00 0,00 y = -0,2041Ln(x) + 3,7393 r 2 = 0, txt Figure The d(001) and d(060) reflections integrated intensities ratio as a function of the parameter time x temperature from the XRD patterns of random powder samples after alteration with NaOH at ph 14. Table Rate constants for the preferential smectite dissolution in NaOH media at ph 14 as a function of temperature. T (ºC) rate constant (h -1 ) The apparent activation energy (Ea) of the reaction was calculated from the Arrhenius equation k = A e -Ea/RT, where A is the pre-exponential factor, R is the gas constant and T is the absolute temperature. Taking the logarithmic form of the Arrhenius equation ln k = ln A [(Ea/R) (1/T)], Ea was obtained from the slop of the straight line defined by logarithm of the rate constants against the reciprocal of the absolute temperature (figure 8.2.5). The apparent activation energy obtained for the preferential dissolution of smectite was 5 kcal mol -1. This value is similar to those determined for the transformation of low charge smectite 177

190 into high charge smectite by Howard & Roy [1985] or for the formation of the interstratified kaolinite/high charge smectite by Bouchet et al. [1992]. The reaction mechanism of the smectite alteration is controlled by diffusion since the activation energy determined is rather low (< 5 kcal/mol), according to Lasaga [1984]. But we have to take into account that only three values are available and that more data should be obtained in order to confirm such a conclusion. 0-1 ln k (k en h-1) y = -2657x + 2,7328 R 2 = 0,6786 ln(k) Linéaire (ln(k)) ,0024 0,0026 0,0028 0,003 0,0032 1/T en K-1 Figure Arrhenius plot of the rate constants for the preferential smectite dissolution in NaOH media at ph Conclusions The Callovo-Oxfordian clay is a complex assemblage with a polyphasic phyllosilicate fraction. This clay will be the host formation for the French underground research laboratory. A set of batch experiments was conducted with the Callovo-Oxfordian clay and homoionic high-ph solutions. The goal of study was determine the mineral transformations of clay minerals, in such alkaline conditions, similar to those produced by cement degradation. Two different mechanism of phyllosilicates alteration were proposed in function of the chemical composition of the reacting solution. Generalised phyllosilicates dissolution occurred in Ca(OH) 2 media whereas preferential dissolution of octahedrally-charged smectite layers was deduced from experimental evidences in NaOH and KOH media. Thermodynamic calculations corroborated that smectite-rich phases were more instable minerals under alkaline conditions than tetrahedrally Al 3+ substituted phyllosilicates such illite. Kinetic calculations were performed taking the variation of the integrated intensity of the d(001) smectite reflection in the XRD patterns of randomly oriented powder preparation as the reaction progress variable. The activation energy for the preferential dissolution of smectite after the NaOH tests at ph 14 was 5 kcal mol

191 8.3 The effect of high-ph cement pore waters on the hydrolic and hydromechanical properties of Callovo-Oxfordian Clay J.C. Robinet (Euro-Géomat Consulting) Objectives On the long term, geochemical processes resulting from the interactions between cement water and clay host rock consist in the dissolution and precipitation of solid phases. These processes lead to changes of host rock structure, in particular in terms of pore size distribution and connectivity, and are monitored by the measurement of the evolution of permeability and Biot coefficient. The objective was to determine the evolution of these parameters during the course of the interactions between the argilite and a cement alkaline fluid. For that purpose Euro-Geomat Consulting developed and used a specific oedometric apparatus with axial radial stress measurement, coupled with a device of percolation Hydro-Mechanical Parameters Experiments on argilite subjected to cement water percolation lasted between one and three months during which permeability and Biot coefficient are recorded. Permeability has been recorded both parallel and perpendicular to the natural stratification of the argilite samples. Biot coefficient is a parameter giving an account for a sample taken as a whole and the stratification is not relevant for this parameter Measurement of the permeability The permeability k is calculated from the Darcy Law based on the water quantity injected during a given time interval into a saturated sample. Q k= w. A.i Eqn with: Q W A i the flow of injected fluid (m 3 /s) the sectional area of the sample (m²) the applied pressure gradient. The hydraulic gradient i is given by the difference between the pressure of injection and the back-pressure, and the height H of the sample: i = (P injection P back-pressure )/H Eqn

192 Calculation is carried out at different intervals of time which correspond to linear regime of the curve (quantity of injected water with time). The slope of the curve gives Q W in term of mass unit per time unit. Using a density of the fluid of 1 g.cm -3 for both site water and cementitious water, fluids flux is converted into m 3 /s Measurement of the Biot coefficient The determination of the Biot coefficient is carried out at different times of the experiments and requires the fluid percolation to be stopped for a few hours (max. 12h). This measurement requires the injection of specific fluid into the sample. However, the amount of fluid injected into the sample during the determination of the Biot coefficient are very small (about 0.06 cm 3 ). Hence the general process of percolation necessary for the permeability measurement is not significantly disturbed. The Biot coefficient b is representative of the stress distribution between the solid phase and the pore fluid. It is measured by a three steps method: 1. A mechanical unloading Δσ 1 without injection of fluid. The relative deformation of the sample Δε v is then measured ; 2. A mechanical reloading until the initial state is then performed until Δε v =0 ; The axial stress is measured and maintained constant for the third step of the Biot coefficient measurement 3. Finally, the pressure of the fluid is increased until a value Δp where the deformation reaches that of the first step, the axial stress being maintained at the value measured at the second step. The Biot coefficient is deduced from the expression : k o Δσ Eqn b =. 3 ΔP where: k 0 σ 3 = σ 1 σ 1 is the axial stress measured at the fist step σ 3 is the radial stress constant along the measurement procedure Δp is the pressure variation measured at the third step The Biot coefficient ranges between 0 and 1. A value of zero indicates that the porosity of a sample is absolutely not connected whereas a value of one indicates that the porosity of a sample is fully connected Method The measurements of the permeability and the Biot coefficient require the sample to be water saturated. Moreover, fluids circulation through the samples is necessary in order to monitor the impact of geochemical interactions on the permeability and Biot coefficient of 180

193 the samples. For that purpose, Euro-Geomat developed an experimental device in the context of ECOCLAY II Apparatus The oedometer cell (Figure 8.3.1) consists of a bronze jacket where a piston is sliding. A radial sensor, installed in the jacket, enables to follow the evolution of the radial stress as a function of the axial one. Furthermore, a displacement measurer is placed on the piston and gives the axial strains. The compaction of the sample is provided by a piston linked to a GILSON hydraulic pump, which can reach 60 MPa. Sensor of injection pressure Nitrogen in pressure Fluid to soak Gilson pump Radial stress measure Vertical stress Nitrogen Manual pump Vertical strain measure Figure Schematic representation of the oedometric apparatus The adaptation concerns the system of injection of the fluid. Euro-Geomat has developed an injection bomb. It is a cylindrical tank in stainless steel. In the upper part of this tank, two stitching enable to apply a nitrogen pressure on the fluid inside the tank and to measure continuously this pressure. In the lower part of the injection bomb, is an exit, linked to the top of the oedometer which enables the infiltration of the fluid inside the sample. Two faucets enable to isolate the injection bomb from the percolation apparatus in order monitor the weight of the sample, which increases until the sample is saturated. 181

194 Experiments are performed at constant volume. This is insured by the manual pump that provides a counter pressure on the piston, balancing the swelling pressure of the sample. By this means mechanical effects can be distinguished from the chemical ones. The measurement of the radial stress in tests of percolation under an hydraulic gradient is essential in order to maintain a total radial stress higher than fluid the injection pressure; so that the fluid percolation takes place inside the sample and not between the sample and the stainless steel jacket of the oedometer Saturation and percolation fluids used for the experiments The tests were conducted using two types of fluid. First, a synthetic argilite pore water is used to saturate the sample before cement water percolation. The evolution of permeability is recorded from the fluid flow at a given injection pressure. The evolution of Biot coefficient is recorded from the stress measurement as a function of the percolated fluid volume. The chemical compositions of the synthetic pore and cement waters used for the experiments are given in table and table 8.3.2, respectively. Table Chemical composition of the synthetic pore water of the Callovo Oxfordian argilites. This fluid is supposed not to react at the contact of the argilite. Salt Quantity (mg/l) CaSO NaCl 333 KCl 186 CaCl2 755 MgCl2 619 NaHCO Table Chemical composition of the synthetic cementitious pore water. This composition results in a fluid of a 12,5 ph value buffered by portlandite Salt Quantity (mg/l) CaSO4 16,3 NaCl 467,5 KCl 1640 Ca(OH)

195 8.3.4 Results References of the samples Table gives an overview of the samples used and the experiment carried out on each of them. Table References of the samples and experiments performed Andra reference EGC reference Direction of percolation Type of water Percolation Time EST as1co Perpendicular to the stratification 1 : Site water 2 : Cement water 1: 1 month 2 : 2 months 2apps1co Parallel to the stratification 1 : Site water 2 : Cement water 1 : 1 month 2 : 2 months EST apps1co Parallel to the stratification 1 : Site water 2 : Cement water 1 : 1 month 2 : 3 months 3as1co Perpendicular to the stratification 1 : Site water 2 : Cement water 1 : 1 month 2 : 3 months EST apps1co Parallel to the stratification 1 : Site water 2 : Cement water 1 : 1 month 2 : 1 month EST as1co Perpendicular to the stratification 1 : Site water 2 : Cement water 1 : 1 month 2 : 1 month Evolution of the permeability parallel to the stratification The evolutions of the quantity of fluid passing through the samples parallel to the stratification are plotted on figure For all the experiments, an initial non-linear behaviour is observed, which is interpreted as the resaturation of the samples. Figure shows the evolution of the permeability with the number of renewal of the pore volume by the percolating fluid. It can be observed that the permeability remains quite constant with the site water but increases with the cementitious water. This increase is slight but significant with respect to the precision of the measurement. However it seems that the permeability is stabilised after ten percolated pore volumes. 183

196 70 60 Total water intake (g) Site water 1ap p s1co Cementitious water 1apps1co Site water 2ap p s1co Cementitious water 2apps1co Site water 3ap p s1co Time (days) Cementitious water 3apps1co Figure Evolutions of the total mass of the percolated water parallel to the stratification 2,5E-13 2E-13 Permeability (m/s) 1,5E-13 1E-13 5E-14 Site water 1apps1co Cementitious water 1apps1co Site water 2apps1co Cementitious water 2apps1co Site water 3apps1co Cementitious water 3apps1co Number of percolated pore volume Figure Evolutions of the permeability for the samples parallel to the stratification 184

197 Evolution of the permeability perpendicular to the stratification The evolutions of the quantity of fluid percolated for all the experiments perpendicular to the stratification are given on figure An initial non-linear behaviour is observed as previously and is also attributed to the saturation of the samples. Moreover, a significant non-linearity is observed during the transition from the site water to the cementitious water used for the percolation. This fact is interpreted as being due to the hydraulic unloading resulting from the swelling of the sample that is more pronounced when the percolation is performed perpendicular to the stratification than parallel to the stratification Total water intake (g) Time (days) Site water 1as1co Cementitious water 1as1co Site water 2as1co Cementitious water 2as1co Site water 3as1co Cementitious water 3as1co Figure Evolutions of the total mass of the intake water for the experiments perpendicular to the stratification Figure shows the evolutions of the permeability with the number of percolated pore volume. It is observed that the permeability increases slightly. The trend is consistent with the observation performed parallel to the stratification but is less pronounced. Essentially because the percolated fluid volume is quite small, the permeability perpendicular to the stratification is one order of magnitude lower than parallel to the stratification. 185

198 2,5E-13 Site water 1as1co Permeability (m/s) 2E-13 1,5E-13 1E-13 Cementitious water 1as1co Site water 2as1co Cementitious water 2as1co Site water 3as1co Cementitious water 3as1co 5E ,5 1 1,5 2 2,5 3 3,5 Number of percolated pore volume Figure Evolutions of the calculated permeability for the experiments perpendicular to the stratification Summary of the permeability evolution All the experiments undertaken parallel or perpendicular to the stratification show a significant increase of the permeability k and with the volume of percolated fluid through the samples. These increases are represented by the following relations: k = B A A + e 1+ D C V p Eqn where A, B, C and D are constants, the value of which are given in table V p represents the number of renewal of the pore volume by the percolating fluid. Table Coefficient for hydraulic conductivity Stratification A B D V P0 Parallel 1, , ,3 5 Perpendicular 3, , ,12 1,27 186

199 Evolution of the Biot coefficient The evolution of the Biot coefficient is an increase from about 0.15 up to 0.45 as a function of the injected cement water volume. The magnitude of the Biot coefficient increase is more important than the permeability increase. This indicates that alkaline perturbation has more effect on the mechanical behaviour than on the transport properties of the rock. The increase of the Biot coefficient as a function of the number of renewals of the argilite pore volume (figure 8.3.6) is described by the following law obtained from a fit on all the experimental values obtained in the present study: b = 0,39,15 + V e 1+ 0,3 0 5 p Eqn ,6 0,5 Biot's coefficient 0,4 0,3 0,2 0,1 0 Test 1as1co Test 2as1co Test 3as1co Test 1app11co Test 2apps1co Test 3apps1co Fitting Number of percolated pore volume Figure Experimental results and fitting of the evolution of the Biot s coefficient with the number of percolated pore volumes Conclusions The Biot coefficient and the permeability increases are consistent with a major process of dissolution. This is consistent with the reaction path demonstrated by some other experiments and modelling performed in the ECOCLAY II project. More precisely, the dissolution of argillaceous phases of the smectite type is the first step of the interactions between clay host rocks and cement waters. However, organic matter dissolution occurs in the clay host rock during the course of the experiments carried out by Euro-Geomat 187

200 Consulting and this may also affect the same way the permeability and to a lesser extend the Biot coefficient. Nevertheless, the percolated volumes were limited and it would be necessary to carry out experiments on longer durations in order to evaluate the evolution of the permeability and Biot coefficient at more advanced stages of the mineralogical transformations of the clay host rock. In particular the initial dissolution of the smectite is followed by a massive precipitation of CSH phases as well as calcite and a reverse trend on permeability and Biot coefficient may be observed at longer term. 8.4 MASS BALANCE ESTIMATE OF CEMENT CLAY STONE INTERACTION WITH APPLICATION TO A HLW REPOSITORY IN OPALINUS CLAY Urs Mäder and Michael Adler (University of Bern) Objectives The aim of the mass balance estimate is to put simple bounds on the expected extent of a high-ph plume emanating from cement degradation and associated rock alteration. Such an estimate can be used in combination with more sophisticated reactive transport models [Mäder & Traber, 10.7] Approach A simple approach is proposed to estimate the buffering capacity of Opalinus Clay for leachates resulting from degradation of Portland cement based solely on mass balance considerations. This will allow for an estimate of the mass of clay stone required to buffer a unit mass of concrete, and also to estimate the extent of rock alteration assuming a simple geometry and homogeneous distribution of the high-ph plume. A comparable approach had been used by Eikenberg [1990] for marl at the former Wellenberg site for LLW. The reactive component of Portland cement is represented by portlandite and Ca-Si-hydrates (CSH) in excess of Ca/Si=1. Alkali-hydroxide components are neglected, as well as CAH (aluminate component) and CFH (ferrite component). These additional reactive components could be incorporated but are subordinate in terms of mass balance. The reactive component of Opalinus Clay is restricted to kaolinite, quartz, dolomite, and chlorite. This is in agreement with experimental evidence, and additional mineral components present are subordinate in terms of their participation in degradation reactions, except for illite and illite/smectite mixed layer clays (see below). Secondary mineral precipitates forming as a result of cement clay stone interaction are calcite, brucite, laumontite, and CSH (CaH 2 SiO 4 ). These phases are models of the relevant 188

201 mineralogy and need not be the realistic phases except for their approximate chemical composition and stoichiometry. A list of minerals with relevant chemical properties is given in table 8.4.1, with data for Opalinus Clay from Nagra [2002a] and for cement from Neall [1994]. Table List of minerals and cement components Mineral Chemical formula Molecular mass Mass in OPA or available Ca(OH) 2 in hydrated cement Moles in OPA or cement per m 3 Kaolinite Al 2 Si 2 O 5 (OH) wt% 1790 Quartz SiO wt% 8100 Dolomite CaMg(CO 3 ) wt% 130 Chlorite Mg 5 Al 2 Si 3 O 10 (OH) wt% 230 Calcite CaCO wt% 3890 Brucite Mg(OH) Laumontite CaAl 2 Si 4 O 12 *4H 2 O 443 Illite (muscovite) KAl 3 Si 3 O 10 (OH) CSH CaSiO 2 (OH) 2 Portlandite Ca(OH) mol/kg 720 C 1.8 SH 1.8 Ca 1.8 SiO 2 (OH) *2.44 mol/kg 855 Note: assuming 450 kg of cement per m 3 of ILW disposal tunnel Simple model for cement degradation The following reactions are considered for cement degradation that yield the Ca(OH) 2 component to be buffered. This model is applicable to cement of SulfacemH or SulfacemN type. For chemical nomenclature of cement see e.g. Taylor [1990]. portlandite => Ca(OH) 2 C 1.8 SH 1.8 => CSH Ca(OH) 2 ( C 3 FH 6 => FH Ca(OH) 2 ) ( C 3 AH 6 => AH Ca(OH) 2 ) The latter two reactions are not considered due to the known better stability of CFH and CAH components during degradation compared to Ca-Si-hydrates. 189

202 8.4.4 Simple model for the buffering capacity of Opalinus Clay The following mass transfer buffering reactions in Opalinus Clay for the infiltrating Ca(OH) 2 component are balanced on oxide / hydroxide / carbonate components, electric charge, but not on water. Ca(OH) 2 + kaolinite + 2 quartz => laumontite Ca(OH) 2 + quartz => CSH ( Ca(OH) 2 + dolomite => 2 calcite + brucite ) ( Ca(OH) 2 + chlorite + quartz => 5 brucite + laumontite ) The latter two reactions account for only about 3% of the buffering capacity (on a molar basis) compared to kaolinite+quartz and can therefore be neglected in this estimate Mass balance calculations The model reactions for degradation and buffering can be applied to the molar mineral content of Opalinus Clay. It is possible to dissolve all of kaolinite, dolomite, quartz and chlorite to buffer Ca-hydroxide from portlandite and C 1.8 SH 1.8 dissolution. Data for cement is from Neall [1994], where for SulfacemH-type cement a portlandite content of 1.6 mol/kg (hydrated cement) is reported, and a C 1.8 SH 1.8 content of mol/kg (hydrated cement). Note that the latter quantity corresponds to 0.8*2.437=1.9 mol/kg available Ca(OH) 2 according to the degradation model above. The ILW tunnel is assumed to contain an average of 450 kg/m 3 cement paste of SulfacemH composition. A summary of the mass balance calculations is given in the table Table Mass balance calculations for Ca(OH) 2 buffering by Opalinus Clay. Mineral Molecular mass Content in OPA or cement Moles in OPA or cement Total buffering or production of Ca(OH) 2 [g/mol] [mol/m 3 ] [mol/m 3 ] OPA: kaolinite wt% 1790 OPA: quartz wt% cement: portlandite 1.6 mol/kg hc 720 cement: CSH > 1:1 1.9 mol/kg hc Note: hc = hydrated cement; ILW tunnel with 450 kg/m 3 cement paste. The buffering capacity can be obtained from the stoichiometry of the buffering reactions involving kaolinite and quartz. Note that 2*1790 moles of quartz are required to react with kaolinite and Ca(OH) 2, leaving a balance of 4520 moles of quartz to directly react with 190

203 Ca(OH) 2. The total buffering capacity for Ca(OH) 2 consists of the number of moles of kaolinite plus the remaining quartz (4520 moles), all per cubic metre of clay stone. C 1.8 SH 1.8 in cement is reduced to CSH of approximate CaSiO 2 (OH) 2 composition and therefore only 0.8 moles of Ca(OH) 2 are liberated per mole of C 1.8 SH 1.8.The total production of Ca(OH) 2 is equal to the sum of portlandite and the reactive portlandite component in CSH > 1:1, also per cubic metre of cement containing 450 kg/m 3 cement paste Extent of rock alteration Given the amounts of cement/concrete per meter of disposal tunnel an estimate of the affected volume can be made assuming an ideal radial distribution, neglecting the known anisotropy of transport properties in this approximation. It can be seen from table that 1 m 3 of Opalinus Clay is able to buffer 4 m 3 of concrete typical for an ILW disposal tunnel. It is therefore possible to easily compute the wall thickness of an hollow cylinder of altered Opalinus Clay surrounding an ILW tunnel of 8.5 m diameter (tunnel volume of 57 m 3 /m). Table also includes the thickness of the affected hostrock volume assuming incomplete progress of buffering reactions using 10% and 1% of the total capacity, respectively. Table Extent of rock alteration. Buffering capacity Ratio ILW/OPA Affected volume of OPA Thickness of hollow cylinder around ILW 100 % 4: m m 10 % 4: m m 1 % 4: m 3 17 m The actual shape of the alteration halo will be oval considering the anisotropy in transport properties parallel to bedding and perpendicular to bedding. Effective diffusion coefficients parallel to bedding are about a factor of 5 higher compared to those perpendiculars to bedding. Adopting the square root relationship for the diffusion length (x = D*t) the ratio between the large diameter of the oval to the short diameter should be approximately Assessment of results and conclusions The early alkaline phase is neglected due to its overall limited duration and smaller mass of hydroxide involved compared to the long-lasting portlandite-buffered phase. The dominant buffering reaction for the KOH-component during the early degradation phase would be the dissolution of kaolinite and the precipitation of illite (written as muscovite), according to: 1.5 kaolinite + KOH => muscovite The formation of secondary hydroxides, hydrous silicates, zeolites, and CSH represents long-lived interim storage for the hydroxide component that still is not at equilibrium with 191

204 the geosphere that lies beyond the zone affected by the developing high-ph plume. This late stage, after depletion of portlandite and C 1.8 SH 1.8 is, however, characterised by ph values < paired with small chemical gradients, and is therefore very long-lived. A major long-term buffering mechanism is provided by the diffusive influx of bicarbonate from adjacent pore-water, and the transformation of more reactive CSH and zeolites to secondary clay, silica and carbonate. The model is conservative in the sense that it neglects substantial amounts of primary illite and illite/smectite mixed-layer clay minerals (about 30%) in Opalinus Clay. These are less reactive than kaolinite but would after consumption of kaolinite also be available for buffering reactions. On the other hand, the model neglects potential Ca(OH) 2 production from degradation of CAH and CFH phases as these are assumed to be stable over a longer time period. Partial degradation might add another mol/kg hydrated cement paste, or 15-30% of the production capacity assumed in above calculations. Again, these are variations well within the order-of-magnitude estimate. The mass balance approach is a simplification of a very complex reaction history that aims solely at providing an order-of-magnitude estimate of mineral mass transfers involved. The estimate does show that the affected host rock is constrained to the immediate vicinity of the disposal tunnel Implications for performance assessment The mass balance considerations support the hypothesis that the high-ph plume does not migrate to any significant depth into the Opalinus Clay. The amount of concrete in an ILW disposal tunnel is exhausted before a high-ph plume can affect a significant volume of host rock, even if the buffering capacity is greatly reduced by conservative estimates of reaction progress. The results of this study did contribute to the discrediting of the ph plume having an adverse effect to the performance of a repository [Nagra 2002a, b]. 8.5 A LONG-TERM IN-SITU EXPERIMENT FOR INTERACTION OF OPALINUS CLAY WITH HYPERALKALINE FLUID AT THE MONT TERRI URL (SWITZERLAND) Urs Mäder and Michael Adler (University of Bern) Objectives The CW experiment aimed at recording interaction of hyperalkaline fluid with Opalinus Clay under relevant in-situ conditions over a time frame exceeding available laboratory experiments. The objectives for the ECOCLAY II contribution were to track the evidence of the interaction in the clay stone surrounding the test interval, and thus provide a data set and test case for reactive transport modelling. 192

205 8.5.2 Summary of the CW experiment Na-Ca-OH solution (ph = 13.2), heated to 30 C, and put under slight overpressure compared to the in-situ pressure of approximately 10 bars at a depth of 10 m from the underground laboratory (total overburden of 250 m). The technical installations comprised a sintered cylindrical Ti-screen to support the test interval, and an above-ground installation to maintain constant-pressure conditions and monitor mass changes of total fluid with a scale. The high-ph solution was periodically replenished, and fluid composition as well as mass transfer to the clay stone was monitored. The fluid was circulated and heated above-ground after failure of the in-situ heating device after 1.5 years. This resulted in steadier high-ph conditions in the test interval but prevented exact monitoring of mass transfer due to unknown gas volumes accumulating above or below ground. The test interval was prepared for overcoring after 2.5 years, but had to be re-connected after technical difficulties that postponed overcoring to 3 years after injection of high-ph fluid Overcoring of the CW experiment and sample analysis Overcoring of the CW experiment was performed October 1, 2001, with a diameter sufficiently large to preserve the expected alteration (few mm to cm) within clay stone surrounding the test interval. Unfortunately the effort failed after technical difficulties with the core-catching device that resulted in complete dismemberment and grinding of the clay stone within the core barrel to render subsequent analysis of solid samples virtually worthless. Visible alteration or mineralogical changes could not be detected. Any mineralogical changes were beyond the detection of X-ray diffraction of bulk samples. Variations were, however, detected within contents of leached chloride and cation exchange populations, but the data did not yield interpretable patterns due to the dismemberment and loss of sample provenance during overcoring. Also, SEM studies did not yield evidence for the formation and identity of secondary minerals, but also did not prove the absence thereof. Secondary minerals could be located within the Ti-screen used to separate the fluid volume from the clay stone, and consisted of abundant flakes of Mg-Si-hydroxide associated with Cacarbonate Reactive transport modelling interpretation A reactive transport model for Opalinus Clay high-ph fluid interaction was adapted from Adler [2001] who calibrated such a model to match observations from a 1-year laboratory experiment. This model was also used for some predictive calculations for the case of a nuclear waste repository [Adler 2001; Nagra 2002a]. Mäder & Traber ( 10.7) provide an in-depth treatment of reactive transport modelling of a high-ph plume. Known initial and boundary conditions of the CW experiment combined with independently constrained transport properties for the Mont Terri location [e.g., Pearson et al. 2003] did describe the in-diffusion of chloride into the test interval correctly (0). Assuming that diffusive transport correctly describes mass transfer of non-reactive species, the reactive 193

206 model of Adler [2001] was used to predict the to be expected alteration of the clay stone (figure 8.5.1) inclusive of calculated changes in porosity. The model predicts that diffusion of the hyperalkaline solution into the surrounding Opalinus Clay formation will cause dissolution of dolomite, kaolinite and quartz (figure 8.5.2). Calcite, hydrotalcite, illite and sepiolite are predicted to precipitate. There is no precipitation of C-S-H and zeolite phases included in the model. Magnesium in abundant secondary Mgphases is derived from dolomite dissolution and cation exchange reactions. An identical alteration mineralogy has been identified in a laboratory experiment investigating the same fluid-rock system [Adler, 2001]. The experiment of Adler [2001], Adler et al. [2001] has shown that precipitation of Mg-hydroxide phases such as hydrotalcite and sepiolite in the rock matrix effectively attenuates the ph of the infiltrating hyperalkaline solution. Predicted porosity changes during the first 3-4 years are < 1% absolute. 2.0E E E-01 Cl period 1 8.0E-02 Cl period 2 Cl period 3 4.0E-02 Cl period 4 Cl model 0.0E elapsed time (years) Figure Modelling of in-diffusion of chloride into the tests interval. Periods relate to repeat replenishment of the fluid with fresh high-ph solution. Shown is the model for the 2 nd period. 194

207 25 20 kaolinite 15 illite calcite 10 quartz dolomite 5 hydrotalcite sepiolite distance from borehole center [m] Figure Calculated mineral volume percentages around the borehole after 4 years (walls of borehole are located at and m) Assessment of results and discussion The modelling predictions cannot be assessed rigorously in the absence of direct observation of mineralogical alteration as a function of radial distance from the cement water clay stone interface. The calculations are, however, in agreement that predicted changes over 3-4 years are minor in terms of bulk rock properties, but should be detectable if suitable sample material would have been available. The absence of CSH-type phases and zeolites in the model are a simplification rooted in experimental observations [Adler et al. 1999, 2001]. More detailed reactive transport modelling ( 10.7) support the assumptions that the volumetrically dominant alteration assemblage consists of clay / calcite / Mg-Si-hydroxides. The predicted reduction in porosity very near the interface is minor but contrasts with experimental observations of Adler [2001], Adler et al. [2001] where evidence of selfsealing after little more than 1 year was observed (albeit in a more advective regime). On the other hand, the porosity permeability relationships in clay stone is as yet poorly understood and clearly poses some limits on predictive modelling Acknowledgements The authors acknowledge the support of the Mont Terri Consortium and the principle investigators Paul Bossart (Geotechnical Institute) and Bernhard Schwyn (Nagra). Thomas Fierz (SOLEXPERTS AG) contributed as engineer and Heinz Steiger (Geotechnical Institute) provided on-site support. 195

208 8.6 REFERENCES OF WORK PACKAGE 3 Adler, M. (2001): Interaction of clay stone and hyperalkaline solutions at 30 ºC: a combined experimental and modeling study. Ph D. Diss. Univ. Bern. Adler, M., Mäder, U.K. & Waber, H.N. (1999): High-pH alteration of argillaceous rocks: an experimental study. Swiss Bull. Mineral. Petrol. 79, Adler, M., Mäder, U.K. & Waber, H.N. (2001): Core infiltration experiment investigating high-ph alteration of low-permeability argillaceous rock at 30 ºC. In: R. Cidu (ed.), Proceedings WRI-10 (10th International Symp. Water Rock Interaction, Villasimius, Italy). Balkema, p Aja, S.U. (1995) Thermodynamic properties of some 2:1 layer clay minerals from solutionequilibration data. European Journal of Mineralogy, 7 (2), Barrer, R.M. (1982) Hydrothermal chemistry of zeolites. Academic Press, 360 pp. Bouchet, A. and Rassineux, F. (1997). Echantillons d argiles du forage EST 104 : Etude minéralogique approfondie. Andra, Report DR-P-0ERM A, Chatenay-Malabry, France, 107 pp. Bouchet, A., Lajudie, A., Rassineux, F., Meunier, A. and Atabek, R. (1992) Mineralogy and kinetics of a mixed-layer kaolinite/smectite in nuclear waste disposal simulation experiment (Stripa site, Sweden). Applied Clay Science, 7, Chermak, J.A. (1993) Low temperature experimental investigation of the effect of high ph KOH solutions on the Opalinus Shale, Switzerland. Clays and Clay Minerals, 41, Claret, F. (2001) Caractérisation structurale des transitions minéralogiques dans les formations argileuses : Contrôles et implications géochimiques des processus d illitisation. cas particulier d une perturbation alcaline dans le Callovo-Oxfordien, Laboratoire souterrain Meuse-Haute Marne. Thèse Université Joseph Fourier, Grenoble, 174p. Eikenberg, J. (1990): Infiltrationen von hochalkalischen Zementporenwässern in das Nahfeld eines Endlagers für radioaktive Abfälle: Geochemische Auswirkungen. Unpublished PSI Technical Note, TM , Paul Scherrer Institute, Villigen, Switzerland. Gottardi, G. and Galli, E. (1985) Natural zeolites. Springer-Verlag, 409 pp. Howard, J.J. and Roy, D.M. (1985) Development of layer charge and kinetics of experimental smectite alteration. Clays and Clay Minerals, 33, Inoue, A., Bouchet, A., Velde, B. and Meunier, A. (1989) Convenient technique for estimating smectite layer percentage in randomly interstratified illite/smectite minerals. Clays and Clay Minerals, 37, Lasaga, A.C. (1984) Chemical kinetics of water-rock interactions. Journal of Geophysical Research, 89,

209 Madé, B., Clement, A. & Fritz, B. (1994) Modélisation thermodynamique et cinétique des réactions diagénétiques dans les bassins sédimentaires. Institut Français du Pétrole, 49 (6), Meunier, A. & Velde, B. (1989) Solid solutions in I/S mixed layer minerals and illite. American Mineralogist, 74, Meunier, A., Velde, B. & Griffault, L. (1998) The reactivity of bentonites: a review. An application to clay barrier stability for nuclear waste storage. Clay Minerals, 33, Moore, D.M. and Reynolds, R.C.Jr (1989) X-ray diffraction and the identification and analysis of clay minerals. Oxford University Press, Oxford & New York, 322 pp. Nagra (2002a): Projekt Opalinuston - Synthese der geowissenschaftlichen Untersuchungsergebnisse. Entsorgungsnachweis für abgebrannte Brennelemente, verglaste hochaktive sowie langlebig-mittelaktive Abfälle. Nagra Technical Report NTB 02-03, Nagra, Wettingen, Switzerland. Nagra (2002b): Project Opalinus Clay - Safety report. Demonstration of disposal feasibility for spent fuel, vitrified high-level waste and long-lived intermediate-level waste (Entsorgungsnachweis). Nagra Technical Report NTB 02-05, Nagra, Wettingen, Switzerland. Neall, F.B. (1994): Modelling of the near-field chemistry of the SMA repository at the Wellenberg site. Nagra Technical Report NTB 94-03, Nagra, Wettingen, Switzerland. Pearson, F.J., Arcos, D., Bath, A., Boisson, J.-Y., Fernandez, A.M., Gaebler, H.E., Gaucher, E., Gautschi, A., Griffault, L., Hernan, P. & Waber, H.N. (2003): Geochemistry of water in the Opalinus Clay formation at the Mont Terri Rock Laboratory Synthesis Report. Bern, Switzerland, Berichte des Bundesamtes für Geologie und Wasser Serie Geologie, Nr. 30. Ramírez, S., Cuevas, J., Vigil, R. And Leguey, S. (2002a) Hydrothermal alteration of «La Serrata» bentonite (Almería, Spain) by alkaline solutions. Applied Clay Science, 21, Ruiz, R., Blanco, C., Pesquera, C., González, F., Benito, I. and López, J.L. (1997) Zeolitization of a bentonite and its application to the renoval of ammonium ion from waste water. Applied Clay Science, 12, Taylor, H.F.W. (1990). Cement Chemistry. - Academic Press, London. Vigil de la Villa, R., Cuevas, J., Ramírez, S. and Leguey, S. (2001) Zeolite formation during the alkaline reaction of bentonite. European Journal of Mineralogy, 13,

210 9 WORK PACKAGE 4: CRYSTALLINE HOST ROCK Ulla Vuorinen, Ari Luukkonen, Jarmo Lehikoinen, (VTT, Finland) 9.1 Introduction An alkaline attack caused by cement materials needed in the construction of a disposal vault is of concern when assessing the safety of a repository. In the crystalline bedrock the salinity of groundwater often increases with depth and therefore the knowledge of the alterations and geochemical reactions brought about by saline alkaline water in a repository as well as the propagation of such water in the bentonite buffer is of utmost importance. In this work package the studied solid materials were MX-80 bentonite and site-specific crushed rock powder (Olkiluoto site, Finland). Both these materials were also studied in the batch experiments in WP1 including measurements of sorption of Na and Ca ( 6.2.3). The mutual objectives of both studies were to gain results on alterations of bentonite and crushed rock and to identify geochemical reactions caused by the saline alkaline water and in this work package also to evaluate the propagation of the alkaline plume by detected chemical changes. The results obtained in this study are only shortly summarised here, but a more detailed report on the studies will be published in 2004 by Posiva Oy, Finland (Posiva report by Vuorinen et al.). 9.2 Flow-through experiments Setup and experimental In the flow-through system the cylinders were packed with crushed rock powder (<1.5 mm) from Olkiluoto site, Finland, in one half of the cylinder and compacted MX-80 bentonite (from Clay Technology, Sweden) in the other half. The packed cylinder system was attacked by a slow longitudinal flow of solution in the crushed rock half. The interaction within bentonite was diffusion controlled. The solution flow-rate of 2.5 ml/day, which is comparable to that in a deep repository, was controlled by a syringe pump. Figure gives a schematical representation of the flow-through test system. sinter crushed rock solution inlet solution outlet diffusion compacted bentonite sample colection Figure Flow-through system. 198

211 The 12 cm long (5 cm inner Ø) cylinder, cylinder ends and sinters (inlet and outlet) were of titanium and all the fittings and tubings were of plastic. The tests were run in an anaerobic glove box in nitrogen atmosphere free of atmospheric CO 2. The experiments were run in duplicate, three similar cylinder samples in each set. The difference between the three cylinder samples in a set was the composition of the attacking solution: fresh (ph=8.8), saline (ph=8.3) or saline alkaline (ph 12.5). More details on the experiments can be found in Vuorinen et al. [2003]. Both the solutions and solids were analysed (table 9.2.1). During the experiment the flow solutions were collected in polyethylene (Nalgene) bottles at the outlets. After completing the flow-through tests the cylinders were dismantled and the solid phases were sampled. The crushed rock powder samples and some bentonite samples were sent for mineral analysis. In addition, the bentonite samples were subjected to pore water and cationic distribution analyses. Table Plan of different analysis to be performed. 1. year 2. year cylinder 1 and 2 cylinder 3 cylinder 4 and cylinder 6 Solution type => fresh and saline saline-alkaline fresh and saline saline-alkaline 1) Flow water analysis Na, K, Ca, Mg, Al, Si, Alkalinity, Cl, SO 4, Fe, S tot - at start - between - in the end - at start - between - in the end - at start - between - in the end - at start - between - in the end ph, Eh, conductivity occasionally occasionally occasionally occasionally 2) Cationic distribution in 6-9 samples 6-9 samples 6-9 samples 6-9 samples bentonite: Na, K, Ca, Mg 3) Pore waters in bentonite Na, K, Ca, Mg, Alkalinity, Cl, SO samples 3-4 samples Modelling simulation of the diffusion column A simple modelling simulation of the flow-through experiments was performed in order to obtain a view of the propagation of the alkaline plume in the cylinders. The plume and the concomitant changes in bentonite s cation exchange chemistry were investigated by the permeation of the calcium-bearing aqueous solution through the diffusion column schematised on the left in figure

212 ~0.5 yr ~1 yr 100 % ~1.5 yr 2 yr 0 % Figure Left Cross-sectional view of the diffusion column along the symmetry plane (blue: filter plate, green: crushed rock powder, red: bentonite). Right Calculated ingress of calcium in the diffusion column. Legend: per cent of incoming calcium concentration. In the filter plates and the crushed rock powder, the transport of calcium is driven by advection and diffusion, while in compacted bentonite the fate of Ca is determined by the interplay between diffusion and adsorption (cation exchange). For the adsorption equilibrium, a non-linear Langmuir isotherm model for untreated MX-80 bentonite was applied. According to the model calculations in figure 9.2.2, the cation exchange in bentonite will not be completed in two years Sampling of solids The first set of cylinders was dismantled after about a year (360 d) of running and the second set after about 1.5 years (560 d). Swelling of bentonite occurred only in the case of fresh solution (ALL-MR) attack. In the saline alkaline case (OL-SA) some slithery bentonite was detected when pressing the solids out of the cylinders. In the cylinders which were attacked by the saline (OL-SS) solution the solids (crushed rock half and bentonite half) appeared to be separate in the sense that they could easily be parted at the interface. The solids were sectioned according to figure The crushed rock in each cylinder was sectioned in three slices (4 cm), A, B and C, whereas the bentonite half was sliced in six (PW A1, PW B1,..., PW F1), and each 2 cm-slice was further sectioned into middle upper and middle lower samples (PW A2, PW B2,..., PW F1, PW F2) as well as right side and left side upper and lower samples (K A1R, K A2R and K A1L, K A2L,...,K F1R, K F2R and K F1L, K F2L). The alphabet in bold denotes the slice in order. 200

213 4cm flow direction A B C A B C D E F K A1L K A2L PW A1 PW A2 K A1R K A2R 2 cm Figure Slicing and coding of the solids. Left slicing of crushed rock (the upper half) and bentonite (the lower half). Right further sectioning of the bentonite slices Summary of results Flow-through solutions The trends of the ph values in the out-flow solutions versus the amount of solution passed through the cylinders are shown in figure The fresh groundwater simulant (ALL-MR) showed a slightly increasing trend after the first 100 mls and thereafter the values settled at about 9.5. In the saline simulant (OL-SS) after an abrupt initial increase the ph values slightly declined and settled at about 8.0, slightly below the initial value of 8.3. The ph values in the alkaline (OL-SA) solution initially dropped about four ph units after which an increasing trend was followed, but still at the end of the experiments the ph was around 9.5, well below the initial value of The electrical conductivity in the out-flow solutions were measured as well. After an initial abrupt decrease in OL-SA and a minor initial increase in OL-SS, both saline solutions tend to approach the initial value of the saline inlet solution. In the fresh ALL-MR after an abrupt initial increase the conductivity drops quickly and continues to decrease gradually towards the value of the inlet solution. All the out-flow solution Eh values measured were around -50 mv regardless of the solution. Other analysis results of the out-flow solutions are given in table The alkalinity droped dramatically in OL-SA case whereas it increased somewhat both in OL-SS and ALL-MR cases. After an initial increase of Na + content in the case of OL-SS and ALL-MR the decreasing levels did not quite reach the level of the initial in-flow solutions, whereas in the case of OL-SA the Na + content remained at the initial level from the start of the experiment. In all three cases no marked changes in the level of Cl - was seen when compared with the initial in-flow solution levels, except for the final OL-SA sample, whose level was distinctly lower. In the OL-SA case the Ca 2+ level was well below the initial level of the in-flow solution and the trend was decreasing throughout the experiment, whereas an opposite increasing trend was seen in the case of OL-SS and ALL-MR. The initial in-flow solution Ca levels were not reached at the end of the experiment. Initially the SO 4 content 201

214 increased in all three cases and the trend throughout the experiment was decreasing. In the ALL-MR case the SO 4 2- level reached at the end the initial in-flow solution value. The levels and the trends of K +, decreasing throughout the experiments, were quite similar in both saline cases, as well as in the ALL-MR case in which the initial levels were above the inflow solution level. The highest contents of Mg 2+ were found in the OL-SS case where the level of Mg 2+ dropped only in the last sample, whereas in the OL-SS case the Mg 2+ levels decreased in the course of the experiment. In the ALL-MR case the Mg 2+ levels dropped initially from the start value and decreased thereafter. The Si(aq) trends were increasing in all three cases. The values were alike in the saline cases, but in the ALL-MR case the Si values were twice as high ALL-MR initial ALL-MR I ALL-MR II OL-SA initial OL-SA I OL-SA II OL-SS initial OL-SS I OL-SS II 10.5 ph ml Figure ph values during the experiment of the out-flow solutions versus amount of solution (ml) passed through the cylinders. 202

215 Table Analysis results (in meq/l) of the flow solutions at three points in time for both sets of cylinders (360 d and 560 d). Initial solution, (OL-SA, OL-SS, ALL-MR) values are also given. ( - denotes not analysed) Sample ph Alk Na + Ca 2+ Cl - K + Mg 2+ Si(aq) SO 4 2 OL-SA 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d d d d OL-SS d d d ALL MR 219 d d d Cation exchange In MX-80 bentonite Na-montomorillonite undergoes cation exchange; especially Na + is apt to become exchanged with Ca 2+. The major exchangeable cations (Na +, Ca 2+, K +, Mg 2+ ) of the bentonite samples were determined by extraction with NH 4 SCN in ethanol [Müller- Vonmoos & Kahr 1983]. In both the saline alkaline (OL-SA) and saline (OL-SS) cases the cation exchange capacity (CEC TOT ) value was higher when compared to the value of the initial MX-80 (table 9.2.3). In the OL-SA case a decreasing trend along the cylinder length towards the outlet end was observed, whereas in OL-SS the increase was rather even throughout the cylinder length. In the fresh case the CEC TOT values showed unsystematic variations around the initial value of MX-80. The results for the four individual elements in the saline alkaline case are shown in figure In the figure the leftmost bar (red) in the first category denotes the CEC of the initial MX-80 and in each figure the darker bar represents the duplicate cylinder results after 360 d and the lighter bar after 560 d. Each category number represents a bentonite sample along the cylinder length (see figure 9.2.4). The leftmost category represents the in-flow end and the rightmost one the out-flow end of the cylinder. For each analysed element a clear trend was seen along the cylinder length so that the more prominent effects were seen in the inflow end samples. Both the Na + and Ca 2+ contents increased when compared to the initial MX-80 values, whereas the K + and Mg 2+ contents decreased. The low Ca 2+ values in the category-3 samples belong to samples further from the interface of rock and bentonite and the values in category 1 and 2 may be reflections of some disturbances (e.g. precipitation) having occurred during the experiment. 203

216 0.8 Na 0.20 Ca [meq/g of bentonite] K 0.06 Mg [meq/g of bentonite] A1 B1 B2 C1 D1 E1 F1 F1 F Figure Measured CEC (cation exchange capacity) values (meq/g of bentonite) for the four elements in the saline alkaline case. The red bar on the left shows the initial CEC of MX-80. The text box in the figure indicates the bentonite samples (see Figure 5-4) in each category. Similar trends were observed in the saline (OL-SS) case for K + and Mg 2+, but the level of exchangeable Mg was double that seen in the OL-SA case while the K + levels matched rather well. As expected, the Na + levels dropped by about two thirds and Ca 2+ levels increased by a factor of 4 from the initial MX-80 levels and the slightly decreasing trend of Ca 2+ towards the end of the cylinder was seen for both time periods. In the fresh case (ALL- MR) no trend was seen as all the values were somewhat scattered, but altogether the Na + levels dropped and Ca 2+ levels increased from the initial MX-80 values, as expected, whereas the K + and Mg 2+ levels were scattered around the initial values of MX

217 Table Results of cation exchange determinations (meq/100g of bentonite) for both 360 d and 560 d cylinders. Na + K + Ca 2+ Mg 2+ CEC TOT Samp- meq / 100g meq / 100 g meq / 100g meq / 100g meq / 100 g le 360 d 560 d 360 d 560 d 360 d 560 d 360 d 560 d 360 d 560 d OL-SA A B B C D E F F F OL-SS B B C C D D E E F F ALL-MR A A B B C C D D E E F F MX-80 initial Bentonite pore water A detailed description of squeezing of bentonite pore water and analysing it can be found in Muurinen and Lehikoinen, [1999]. Pore waters were analysed only from the saline alkaline cylinder. The ph values in pore water increased only in the 560d samples from the interface (A1, B1, and D1) showing a decreasing trend towards the out-flow end (figure 9.2.6). The highest ph values were measured in the samples from the in-flow end but still those values (10 11) did not reach the value of the flow solution (12.5). In all the other samples (both 360 d and 560 d) the ph value remained virtually the same indicating buffering around

218 12 OL-SA ph d 560 d 0 A11 A2 2 B1 3 B2 4 D15 D2 6 F1 7 Figure Bentonite pore water ph in the saline alkaline experiment for both the 360 d and 560 d cylinder. Other analysis results on pore water samples are given in table The in-flow solution components, Na +, Ca 2+ and Cl -, all showed similar trends. At completion, after 560 days the contents of these components had decreased from those observed after 360 days, and the bentonite samples further from the interface of the solids at both time points had lower contents of the elements than the samples at the interface. Also Mg 2+ and K + showed similar behaviour; the elemental contents were higher in the interfacial samples than in the others and the contents decreased with the experimental time elapsed. However, the trends of Si(aq) and SO 4 2- contents were contrary as the contents in the interface samples were lower than in the samples further from the interface at 360 days, and for SO 4 2- also at 560 days, whereas at 560 days the Si(aq) content in the samples further from the interface had decreased below the values measured in the interface samples. Table Analysis results of the pore water in the saline alkaline (OL-SA) case. (- = below detection limit, blank = no sample). Samp- ph Alk tot Na + Ca 2+ Cl - K + Si(aq) Mg 2+ SO 4 2- le meq/l meq/l meq/l meq/l meq/l meq/l meq/l meq/l 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d 360d 560d A A B B D D F Discussion and conclusions The alkaline solution (OL-SA) affected bentonite throughout the entire column to some extent as all the determined exchangeable cation capacities either increased or decreased when compared to the initial MX-80 bentonite values. There was a clear decreasing trend in the column with more prominent changes at the in-flow end. However, the ph values in the 206

219 bentonite pore water samples supported alkaline changes only near the interface with a clear decreasing trend towards the cylinder end. The high ph value (~11) at the in-flow end decreased down to about 8, the value at which pore water was well buffered otherwise. The out-flow solution had reached a ph value around 9.5 by the end of the experiment and showed an increasing trend. If the alkaline ph was to keep its trend the estimated time needed for the out-flow solution ph value to reach a value around 11 would be about 6 years and to reach the initial value of 12.5 would take about 10 years. In the saline and fresh experiments the out-flow solution ph values levelled off at around 9.5 and about 8, respectively. The simple saline solutions, OL-SA and OL-SS, did not initially contain K +, Mg 2+, SO 4 2-, or Si(aq). The levels of K + were quite similar both in the determined exchangeable cations and in out-flow solutions indicating that interactions with bentonite were well reflected in the out-flow solution composition. This was also true for SO 4 2- and Si(aq), based on pore water and out-flow solution results. In the case of exchangeable Mg 2+ the amount left in the OL-SA bentonite samples was less than in the OL-SS samples, but this was not reflected in the Mg 2+ contents in the out-flow solutions even if pore water analysis also showed depletion of Mg 2+. This indicates that Mg 2+ remained to some extent inside the alkaline column undergoing possible reactions. The depletion of Ca 2+ in pore water and the decreasing trend in the outflow solutions of OL-SA, which was contrary to the increasing trends in the other two experiments, supports the retention of Ca 2+ in the column and possible undergoing reactions as well. There was also a distinct decrease in the Cl - content in the out-flow solution in the longer period experiment in the OL-SA case indicating Cl - undergoing reaction inside the column. An increase in the cation exchange capacity of bentonite in both saline cases was observed. The cause of the increase is not quite clear at this point, especially in the case of OL-SS, but in the case of OL-SA partial alteration of montmorillonite to beidellite can be an explanation. 9.3 Mineralogical summary Cushed rock alterations The crushed rock was prepared from Olkiluoto mica gneiss (75 wt%), granite pegmatite (20 wt%) and tonalite (5 wt%). The rock material was crushed to grain sizes <1.5 mm. All materials, including the fine fractions were mixed together. The mineralogy of the mixture is given in Table

220 Table Estimated initial mineral composition (in wt%) for crushed rock used in the experiments. Mineral wt% Mineral wt% Mineral wt% Quartz 23.2 Sillimanite 5.4 Zircon 0.2 Oligoclase 19.2 Muscovite/Sericite 2.9 Carbonate 0.1 Microcline 17.1 Hornblende 1.6 Apatite 0.1 Biotite 20.1 Chlorite 1.2 Sulphides 0.3 Cordierite 8.3 Epidote/Saussurite

221 In the batch experiments (table 9.3.2) microcline was found to react. A corresponding increase in sericite was interpreted from the hyperalkaline case. The dissolution of oligoclase was probable in all experiments. In the hyperalkaline experiment the low Ca 2+ solute concentration promoted dissolution of the anorthite part of oligoclase, and possibly minor production of saussurite. In the fresh alkaline experiment the albite part of oligoclase was believed to dissolve more readily. It seems that the high Na + solute concentrations in the saline experiments inhibited dissolution of albite. The saline experiments also indicated dissolution of hornblende and cordierite. It is suggested that a significant part of hornblende was altered to chlorite, and Mg 2+ needed for this conversion was partially produced from the decomposition of cordierite. The final water results of the batch experiments indicated a sink for Na +. In the saline experiments small precipitation of halite was a partial sink. It is believed that the newly-formed surface edge sites of the recently crushed rock were a distinct Na + -sink as well. The overall mass transfer in the crushed rock batch experiments was low. However, as a response to the small drops in ph, the batches produced minute amounts of CSH-gel. This gel gave a sink for Ca

222 In the flow-through experiments (table 9.3.2) minimal detectable changes in the crushed rock occurred. The reactivity of bentonite is much higher than that of crushed rock. In the saline alkaline flow-through experiment similar changes to those in the saline alkaline batch experiment were assumed. Interpretations were, however, subject to ambiguities because of the compositional variations in the crushed mixture. In the near-neutral-ph experiments the crushed silicates were expected to be non-reactive. Small amounts of precipitated halite were observed in the saline flow-through experiments. It was also assumed that the fresh surfaces of crushed minerals were potential for Na + adsorption. 210

223 Table Summary of the crushed rock mineralogical observations interpreted from the batch and flow-through cell experiments. + increase in final bentonite, decrease in final bentonite, () inferred but not observed. Batch Experiments Flow-through Experiments Fresh Saline Saline. Fresh Saline Saline Alkaline Alkaline Hyperalk Alkaline Hornblende ( ) Chlorite (+) (+) (+) (+) Cordierite Microcline Sericite Oligoclase Na-part (albite) Ca-part (anorthite) Saussurite (+) + (+) Halite CSH-gel (Tobermorite?) NaX MX-80 alterations In the WP1 batch experiments (table 9.3.3) the initial Na x -montmorillonite was found to alter partially to Na x -beidellite ( 6.2.3). The degree of conversion was related to the Na/Ca ratio of the initial solution and ph. The long-term (e.g. 540 days) hyperalkaline attack produced smectite that indicated good resemblance to crystallography of Na x -beidellite. The main sink for Na + was apparently the cation exchange sites of the altered smectite. However, according to Hong and Glasser [1999, 2002] also CSH gel is a Na + -sink especially in high Na + solute conditions. The CEC studies with altered solids indicated an increase in the exchange sites as a result of the alterations. The other main cations were released from bentonite. K + remained in the final solution, while Mg 2+ assumably adsorbed or precipitated into CSH gel (cf. Taubald et al. [2000]). The main sink for Ca 2+ was CSH gel and to a minor extent calcite. Especially in the hyperalkaline case quartz dissolved partially and albite almost completely. Significant amounts of halite precipitated in the saline batch experiments. In the flow-through cylinder experiments (table 9.3.3) the changes in smectite were found to be diverse. The dilute near-neutral-ph fresh water injection resulted in complexly fluctuating cation distributions in bentonite. The constant release of Na + and silica from bentonite seems to have caused interlayer deficiencies in montmorillonite. Due to the relatively low Na/Ca 211

224 ratio, the near-neutral-ph saline water caused smectite to transform towards Ca x - montmorillonite. The saline alkaline water interaction indicated comparable smectite alteration to the equivalent batch experiment. The near-neutral-ph experiments released Na + while the alkaline experiment adsorbed Na + into the cation exchange sites. In the nearneutral-ph saline experiment, the released Mg 2+ was left to final solution while in the saline alkaline experiment Mg 2+ was apparently caught by CSH gel. In all experiments, the sink for Ca 2+ was either Ca x -montmorillonite or CSH gel. Small amounts of calcite were precipitated in all cylinders. In the near-neutral-ph experiments no formation of CSH gel occurred, and apparently no significant increases in the CEC. In the saline experiments a small amount of halite precipitated in bentonite. Table Summary of the bentonite mineralogical observations interpreted from the batch and flow-through cell experiments. + increase in final bentonite, decrease in final bentonite, () inferred but not observed. Batch Experiments Flow-through Experiments Fresh Saline Saline Fresh Saline Saline Alkaline Alkaline Hyperalk Alkaline Na x -Montmorillonite Ca x -Montmorillonite + Na x -Beidellite Deficient Montmorillonite + Gypsum Albite ( ) Quartz ( ) ( ) ( ) Halite Calcite CSH-gel (Tobermorite?) NaX KX CaX 2 ++ MgX 2 CEC increase

225 9.4 REFERENCES OF WORK PACKAGE 4 Hong, S.-Y. and Glasser, F.P., Alkali binding in cement pastes. Part I. The C-S-H phase. Cement and Concrete Research 29, Hong, S.-Y. and Glasser, F.P., Alkali sorption by C-S-H and C-A-S-H gels. Part II. Role of alumina. Cement and Concrete Research 32, Muurinen, A. and Lehikoinen, J Porewater chemistry in compacted bentonite. Helsinki, Finland, Posiva Oy, Posiva 99-20, 46 p. Müller-Vonmoos, M and Kahr, G Mineralogische Untersuchungen von Wyoming Bentonit MX-80 und Montigel. Baden, Switzerland, NAGRA, Technisher Bericht 83-12, 15 p.+app. 13 p. Taubald, H., Bauer, A., Schäfer, T., Geckeis, H., Satir, M. and Kim, J.I., Experimental investigation of the effect of high-ph solutions on the Opalinus shale and the Hammerschmiede smectite. Clay Minerals 35, Vuorinen, U., Lehikoinen, J., Luukkonen, A. and Ervanne, H Effects of salinity and high ph in crushed rock and bentonite experimental work and modelling in 2001 and Olkiluoto, Finland, Posiva Oy, Working Report , 33 p. 213

226 10WORK PACKAGE 5: MODELLING The organisations making technical contributions to Workpackage 5 during the course of the ECOCLAY II project are BRGM (in association with ANDRA), GRS, Serco Assurance, BGS (Assistant Partner to Serco Assurance), Kemakta (subcontractor to SKB), VTT (assistant partner to POSIVA), University of Bern (following a change in work plan), and SCK.CEN. The modelling work has concerned the interaction of alkaline plumes with clays (bentonite buffer material and argillaceous host rocks) and crystalline rocks, and the degradation of cements to produce the alkaline plume. Simultaneous modelling of cement degradation and the migration of the alkaline plume into an adjacent argillaceous rock has also been carried out. The modelling has been directed both at the interpretation of experiments, most of which have been carried out as part of other ECOCLAY II Workpackages, and at the prediction of field-scale migration of an alkaline plume through bentonite buffers and clay host rocks. Most of the Workpackage studies have made extensive use of established geochemical modelling tools, with other approaches used where appropriate to assist in or corroborate the analysis undertaken or predictions made. The geochemical modelling program most widely used in the Workpackage is PHREEQC, although use has also been made of CRUNCH, and to a lesser extent of EQ3/6 (and related programs), PRECIP, Hydrogeochem, MINTEQ and Medusa. It is encouraging both that there is this variety of modelling tools available for carrying out the geochemical calculations needed for underground radioactive waste disposal, and that a consistency of qualitative understanding across the range of studies reported here seems to have resulted regardless of the particular simulation program used; that is, the variety of programs used to obtain rather similar conclusions gives confidence in the performance (verification) of these tools, although the facilities offered by them may suggest that one is more appropriate than other for a particular application. Both the degradation of cements, and in particular the interaction of an alkaline plume with geological materials are highly complex geochemical processes. The work reported here to simulate the results of experiments on such systems gives confidence in current capabilities to identify the main processes contributing to the behaviour of the system and to provide an outline interpretation of that behaviour. However, the result of the complexity is that it remains difficult to simulate in quantitative detail the results of experiments. These difficulties arise both from process uncertainty (for example, the relative importance of ion exchange and chemical reactions, or the contribution of organic species), and data shortages. Particularly important in this context are therefore the efforts made in some of the studies reported here to further develop the detailed understanding of the processes occurring in the experiments examined. 214

227 The geochemical calculations depend both on thermodynamic databases and in some cases on reaction rate data (including reactive surface area values). It is evident, and inevitable given the complexities of the systems being studied, that the standard geochemical thermodynamic data bases available are not complete, and in a number of the Workpackage 5 studies these data bases have been significantly augmented, for example with data for cement-derived phases. There is the perennial difficulty that thermodynamic data bases typically refer to crystalline phases, whereas in experiments secondary minerals are often precipitated as amorphous or gel phases or as solid solutions. However, it appears to be the case that it is possible to generate a thermodynamic data base for most studies that includes sufficient representative phases (and solution species) that allow the overall behaviour of the system to be reasonably represented even if it is not possible to precisely mimic the actual phases that are present. An important conclusion, from those studies directed at field scale modelling of the penetration of an alkaline plume into a clay buffer or host rock in a diffusion dominated system, is that the distance the plume will penetrate into the clay is very small, and not significant as far as the performance of the clay barrier is concerned. This conclusion appears to be robust to uncertainty in the modelling. 215

228 10.1 MODELLING OF COLUMN EXPERIMENTS ON THE INTERACTION OF ALKALINE FLUIDS WITH CRUSHED ROCKS Andrew R Hoch, Claire M Linklater 4, and William R Rodwell 5, (Serco Assurance, UK) and David J Noy 6 ( British Geological Survey, UK) Introduction Prior to the start of the ECOCLAY II, the British Geological Survey, funded by Nagra, Nirex and SKB, had carried out a series of laboratory column experiments on the interactions of alkaline cementitious fluids with repository host rock minerals [Bateman et al., 2001 (a), (b)]. The work was intended to provide data to test the capabilities of coupled geochemical models to predict the evolution of outflow fluid compositions and product solids on the flow of simulated cementitious repository porewater through host rock minerals. An overview of the experimental procedures and preliminary predictive modelling have been reported in Bateman et al. [1999]. Further work using this data has been carried out as part of ECOCLAY II with the objectives of developing a better understanding of these experiments, and to improve the correspondence between the data and the predictions of computer models. Experiments were carried out under a variety of conditions using a range of ground and sieved single minerals, synthetic rocks, and real rocks. The experiments provided quantitative results on the effluent fluid composition as a function of time. They also provided qualitative information on the mineralogical changes that had taken place (dissolution of primary minerals and precipitation of secondary minerals); this data was obtained by examination of the column contents at the end of each experiment. Here attention has been focused on the experiments with single minerals, and on simplified representations of potential repository host rocks. The single mineral columns were packed with quartz, albite, calcite, and muscovite/quartz (the quartz was included to prevent clogging). Simplified synthetic mineral assemblages were used to represent Borrowdale Volcanic Group (BVG), Äspö granite, and Wellenberg marl. Only some representative results and the overall conclusions are presented in this summary report. Full details can be found in a project technical report and journal publication [Hoch et al., 2003(a, b)]. The experiments were carried out with two alkaline fluids: a young cement leachate with a high ph and high concentrations of Na and K; and an evolved cement leachate, which was a saturated Ca(OH) 2 solution. Only the experiments with the young leachate were modelled here. 4 Now at ANSTO Environment, PMB 1 Menai, NSW 2234, Australia 5 william.rodwell@sercoassurance.com 6 djn@bgs.ac.uk 216

229 Review and Qualitative Evaluation of Experimental Data A careful review of the relevant experimental results was undertaken in order to distinguish experimental artefacts from significant trends. A representative set of effluent composition data are shown in figure (a) for a column consisting of the single mineral quartz. Examination of the effluent fluid compositions for the experiments suggests that experimental uncertainty (e.g. analytical uncertainty, fluctuations in flow rate) contributed significantly to the observed variation in effluent chemistry. After accounting for experimental uncertainty, in the cases considered here the effluent compositions can be regarded as more or less constant after a very short initial transient. (a) (b) (c) Figure Base-case results for Na-K-Ca hydroxide fluid flow through a quartz column: (a) experimental outflow fluid composition [Bateman et al., 2001 (a)]; (b)-(d) PHREEQC calculation of (a) outflow fluid chemistry, (b) primary minerals (broken lines indicate initial amounts), and (c) secondary minerals. (d) 217

230 In the case of the quartz column results illustrated in figure (a), the picture is one of steady dissolution of quartz throughout the column. Calcium in the inlet fluid reacts with the dissolved silica leading to precipitation of CSH phases. Virtually no calcium remains in the effluent fluid, which has a more or less constant Si concentration (experimental variability leads to uncertainty in the exact value of this concentration). The target flow rate in the base-case calculation corresponded to the passage through the column of 1 initial pore volume in 6.07 h. The main feature of the mineralogical analysis in many of the columns is that, while there is more change in the upstream part of the columns than further along the columns, dissolution and precipitation is spread throughout the columns with little evidence of the passage of distinct fronts along the column. This is suggestive of kinetically controlled rather than equilibrium controlled processes, so that a key parameter in the numerical simulations would be the dissolution rate of the minerals. An issue that arose during the modelling studies was related to the calcium concentration in the inlet solution. This concentration is determined by the solubility of portlandite under the experimental conditions (including the temperature of 70ºC). The experimental design did not provide for the direct measurement of this concentration, and therefore it must be calculated using thermodynamic data. The inlet Ca concentration was found to be sensitive to the choice of thermodynamic database used in its calculation Numerical Simulation of Column Experiments: Illustrative Results The numerical studies have involved a number of aspects: A brief literature review of the dissolution rates of minerals under conditions relevant to the experiments. The use of a number of different computer programs to provide a comparison between model predictions. The simulations carried out by Serco Assurance were mostly with PHREEQC (Version 2.4.2) [Parkhurst and Appelo, 1999], but HYDROGEOCHEM [Yeh et al., 1998, Yeh and Salvage, 1999] was also used. Calculations by BGS were with PRECIP [Noy, 1990, 1998]. The results shown here were obtained with PHREEQC or PRECIP. The simulation of the column experiments that had been carried out using single minerals. This work allowed the calibration of the mineral dissolution rates. (Note that uncertainty in the dissolution rates may be equivalent to uncertainty in the reactive surface areas; this needs to born in mind in connection with the comments on reaction rate adjustments discussed herein;) The simulation of experiments with synthetic representations of BVG, granite and marl. It was hoped that the dissolution rates inferred from the work on the single mineral experiments would apply also to the primary mineral constituents of the synthetic host rock lithologies. 218

231 Only some example simulations are presented in the following subsections. In selecting input parameters to best represent the experimental data, attention was focused on the effluent fluid compositions as this was the most quantitative data provided by the experiments Modelling Single Mineral Columns A good deal of attention was given to modelling a set of experiments on single mineral quartz columns under a variety of conditions (different flow rates, different mineral surface areas, different inlet fluid compositions), on the grounds that it should be possible to provide quantitatively accurate simulations of such simple systems and understand the causes for changes in results between different experiments. The calculations for the base case (see figure ) were calibrated by adjusting the quartz dissolution rate to provide a reasonable match to the experimental effluent composition. The literature (e.g. Knauss and Wollery, [1988], Lasaga [1998]) suggests that for conditions relevant to the experiments the quartz dissolution rate far from equilibrium can be written as M quartz Eqn n = k A[H ] ( 1 Ω) t where k is the rate constant, mol m -2 s -1 [Knauss and Wollery, 1988]; A is the effective surface area. The BET surface area was 110 m 2 kg -1 [Bateman et al., 2001 (a), (b)]; [H + ] is the activity of the hydrogen ion; n is the exponent of the hydrogen ion activity in solution, 0.5 [Knauss and Wollery, 1988] Ω is the saturation ratio [Lasaga, 1998]. k was subsequently reduced to mol m -2 s -1 to get a good fit to the effluent Si concentration. The CSH phases foshagite, hillebrandite and tobermorite were allowed to precipitate. Precipitation rates for these secondary minerals were assumed to be fast. Results of calculations for the base case are shown in figure Simulations of experiments undertaken under different conditions were then carried out with the same reaction rates and thermodynamic data. The results show uniform removal of quartz throughout the column and deposition of the CSH phases, foshagite and tobermorite, along the column. The effluent shows very much reduced calcium concentrations compared to the inlet fluid composition (calculated to be mol L -1 ), and the appearance of silicon. These results are consistent with the experimental results. Simulations of the experimental variations on the base case reproduce observed changes in silicon concentration in the effluent: reducing the flow rate and reducing the inlet fluid Ca content increases the silicon concentration, whereas reducing the mineral area decreases it, in all cases by amounts in reasonable agreement with the experimental results. 219

232 Modelling was also carried out on experiments using columns of albite, calcite and a muscovite/quartz mixture [Hoch et al. 2003(a, b)] Modelling Synthetic Rock Columns In this subsection, some illustrative modelling results for the column experiments using synthetic BVG and marl mineral assemblages are presented. a) Synthetic BVG Mineral Assemblage The synthetic BVG rock was made up of quartz, dolomite, K-feldspar, muscovite, calcite and haematite (the haematite was ignored in the calculations). The dissolution rates for quartz albite, calcite, and muscovite inferred from the modelling of the single mineral experiments were used in modelling the synthetic BVG column experiments, assuming that albite was representative of K-feldspar and that the dissolution rate for dolomite was the same as for calcite. The illustrative results shown for this synthetic mineral were obtained with PHREEQC. The results from the initial simulation showed that, although the predicted outflow chemistry after the first ~500 hours is similar to that observed, the mineralogical changes are inconsistent with those seen in the experiment. The calculations show that the dolomite has been completely dissolved and replaced by brucite. In contrast, the mineralogical descriptions in Bateman et al. [2001(b)] at the end of the experiment state clearly that a substantial amount of dolomite remains throughout the column, indicating that dolomite dissolution in the experiment occurs much more slowly than in the model. To try to resolve this difference, the dissolution rate constant for dolomite could be reduced arbitrarily. Better agreement with the experiments can be obtained if a reduction by a factor of 100 is applied. A similar effect may be obtained by applying a suitable inhibition to the brucite formation reaction. A possible mechanism that could cause the required dolomite dissolution rate reduction is suggested by the following observation in the mineralogical analysis in Bateman et al. [2001(b)]: The dolomite grains throughout the column are partially altered to brucite and calcite. The calcite develops as an external coating which preserves the rhombic morphology, and the brucite replaces the dolomite within. This is illustrated in figure Thus an hypothesis would be that a calcite and /or brucite layer suppresses the dissolution rate of dolomite. A simple model that suppresses the dissolution rate of dolomite using this armouring process is developed here. 220

233 Figure Backscatter electron image showing secondary calcite forming a pore lining cement around altering dolomite grains, which are being replaced by brucite and possibly MSH [Bateman et al., 2001 (b)]. For a thin layer, the surface reaction is the factor that limits the growth of the layer, and therefore the growth of the layer is linear with time. When the layer is thicker, the growth of the layer is expected to obey parabolic kinetics. Parabolic kinetics implies that the rate of growth of the layer is controlled by a gradient transport process that becomes smaller as the layer becomes thicker, e.g. M precipitate in layer 1 Eqn t α M precipitate in layer where α is a constant. Diffusion across the layer is the slowest and therefore rate-limiting step in the overall sequence of reactions. It is postulated that the thickness of the layer armouring the dolomite grains should be proportional to the amount of dissolved dolomite. A simple model for the dissolution rate of dolomite might therefore be: M t dol = k A ( 1 Ω) 1+ α 1 [ M ( t = 0) M ( t) ] dol dol Eqn where M dol is the concentration of dolomite [mol L -1 ]; k is the rate constant, mol m -2 s -1 ; A is the reactive surface area, estimated as m 2 L -1 ; is the saturation ratio; α is a constant [mol -1 L]. Provided α is large enough, the dissolution rate of dolomite can be suppressed enough so that the simulation for the synthetic BVG fault rock is consistent with the experimental data. Figure shows the results of a simulation carried out using PHREEQC assuming a value of 100 for α. It can be seen that this physically based model also gives results consistent with the experimental data. The experimental effluent chemistry data does not allow any discrimination to be made between the success of this model and the model 221