Transactions on Ecology and the Environment vol 18, 1998 WIT Press, ISSN

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A two-case study on the environmentallyinduced damage to materials in marine environments - Part II: Geomaterials A. Mauricio* & A.M.G. Pacheco^ *Lab. Mineralogia e Petrologia and *Dept.

1 A two-case study on the environmentallyinduced damage to materials in marine environments - Part II: Geomaterials A. Mauricio* & A.M.G. Pacheco^ *Lab. Mineralogia e Petrologia and *Dept. Engenharia Quimica, Institute Superior Tecnico (Technical University of Lisbon), Av. Rovisco Pais 7, 196 Lisboa Codex, Portugal; pcd 245@alfa. ist. utl.pt Abstract This two-part paper addresses the specific hazards that most materials are faced with in coastal areas, particularly in their atmosphere. Pretty common features like high relative humidity and airborne salts, which are inherent in such an environment, may turn into a nightmare for conservationists, architects and materials scientists, that is for everyone involved with old (historic) or new infrastructure. Two cases are presented and discussed herein. Neither of them was designed or singled out especially for the occasion: both were taken from extended programs of metal-corrosion and stone-decay monitoring in the open. The first case (Part I) deals with the implication of saline contamination for the time of wetness (TOW) of a metallic surface. The results show that standard procedures for assessing TOW from weather data can severely underestimate the duration of surface wetness and, in the final analysis, lead to some misclassification of atmospheric corrosivity. The second case (Part II) follows the evolution of salt efflorescence at an ancient building as a function of local (microclimatic) conditions, in order to get the time probability associated with deliquescence- crystallisation transitions at a given location. By doing this, it was possible to identify more-or-less risky areas in the stone monument, which could then be subjected to differential surveillance and/or care. Both studies seem pertinent to illustrating the need for establishing risk thresholds for materials selection and infrastructure maintenance that can really hold in marine environments.

88 Environmental Coastal Regions 1 S** Marij a Ta'Cwerra case study Evaporite minerals are particularly significant in the Mediterranean basin because most cultural (historic-architectonic) heritage

2 88 Environmental Coastal Regions 1 S** Marij a Ta'Cwerra case study Evaporite minerals are particularly significant in the Mediterranean basin because most cultural (historic-architectonic) heritage is concentrated in coastal areas where they can be exposed to marine spray or salt-rising damp. Their presence contributes significantly to the weathering of building stones because of their response to cycles of relative humidity. Since the critical reative humidity points of dissolution or change of state of hydration of the minerals are usually within the typical ranges of relative humidity (RH) observed in most temperate climates, they can oscillate frequently between solution and crystal phases. They can also oscillate from a crystalline phase to another [1]. The present case study is aimed at estimating when and where different pure salts may crystallise from solutions, evaluating the probability of the salt system being crystallised or deliquescent. The assessment of the salt weathering potential on the surface of the stone, by means of crystallisation/dissolution and transition probability estimations (TPE) along the year will also be considered, in a given monitored site. 1.1 Diagnosis and data collection To evaluate some effects of coastal environments on the weathering of historic buildings, an extensive study has been carried out at four pilot monuments along the east-west axis of the Mediterranean basin - Cathedrals of Cadiz (Spain) and Bari (Italy), and Church of S" Marija Ta'Cwerra (Malta). This was done with a specific interest in the action of marine salts and air pollution. The various locations of the monuments reflect dissimilar conditions of salinity, extent of marine and atmospheric pollution, and topographical aspects of the area, leading to different types and grades of weathering and decay patterns [2]. The church of S'* Marija Ta' Cwerra is located in the village Siggiwi, in the south west of the Malta island, at a distance 3 km far from the sea. It is a free standing building from the XII century, less than 1 x 1 nf plan view. The church is built entirely of Globigerina limestone. This limestone has a total porosity of 35% with mainly small pores (2-5 nm). The chemical composition of the stone is dominated by calcium carbonate (88 to 97 %). The four external walls show severe deterioration, for about twothird of their height, the lower courses are cemented. The middle courses are deteriorated in the form of alveolar weathering as well as powdering of several areas. Most of the mortar has been lost from the joints in this area. The uppermost courses are better preserved. At the inside of the building, the plaster has fallen away in several areas revealing powdering and flaking

Environmental Coastal Regions 89 stone underneath and even some of the carvings have almost completely disappeared [2] In the outside walls, granular disintegration and relief by rounding and

3 Environmental Coastal Regions 89 stone underneath and even some of the carvings have almost completely disappeared [2] In the outside walls, granular disintegration and relief by rounding and notching are the prevailing weathering forms. The intensity of salt weathering is basically controlled by stone characteristics, especially porous matrix properties, and degree of salt accumulation [3]. It can be seen that stone samples from the outside show clear enrichments in Na* and Cl" and a bit in SO/ On the inside, efflorescences are enriched in Na* and Cl', indicating mainly the influence of sea as a cause of chemical deterioration of the stone. Anthropogenic chemical emission impacts on the stone are of negligible importance in this church [2]. The evaporite minerals found on the monument are nitrocalcite nitromagnesite, nitratite, halite, thenardite gypsum, mirabilite and niter [4] Nitrates are almost always dissolved owing to their low deliquescence humidities when compared to the environmental relative humidity range usually found inside and outside the church. Regarding crystallisation pressures, halite is the most dangerous salt. This can be easily understood comparing crystallisation pressures (atm) of different salts, under thermodynamic conditions found in some environments: 554 for halite; 282 for gypsum; 292 for thenardite; 72 for mirabilite. Considering the molar volume (cnrmole): 22 for mirabilite, 28 for halite, 55 for gypsum, 53 for thenardite, then mirabilite can be considered the most dangerous salt. Thenardite and mirabilite are sometimes found together showing that phase transitions between anhydrous and hydrate forms easily occurs in the stone. They produce also hydration pressure in the stone porous matrix that is particularly effective because of the rapidity of the change. The transition of thenardite to mirabilite is more rapid than hydration of other salts, taking about 2 minutes at 39 C [5]. The environmental data were collected on a hourly basis at different sites (one outdoors and four indoor), from April 1994 to June 1995, by means of a monitoring station. The system comprises a network of sensors: contact thermometers attached to the surface of the stone and thermohygrometric sensors located 5cm far from the stone surface at different heights from the ground. There are four indoor contact thermometers and four indoor thermohygrometric sensors. They are positioned in Local 1, Local 2, Local 3 and Local 4 (respectively, in South wall - 3,5 m; North wall - 3,5 m; South wall -,5 m; North wall -,5 m). There is also one outdoor thermohygrometric sensor facing south/south-west, attached to the dome. In the search for interactions between atmosphere, salt-induced weathering and stone condition, accurate temperature and relative humidity measurements were carried out in the atmospheric layer close (5 cm) to the

4 9 Environmental Coastal Regions surface (Atm condition). Stone surface temperatures measurements (Surf condition) were also monitored. 1.2 Data processing In order to evaluate the potential damage on porous-stone materials resulting from pure-salt crystallisation, it is essential to become aware of their deliquescence thresholds (boundary conditions), RHeq The evaluation should be made for a given salt on the range of temperature and relative humidity existing on temperate climates. These functions are very important since they enable to establish phase diagrams for each pure salt, as well as to follow the evolution of deliquescence humidity along time as a function of ambient temperature. To deal with such an issue, a computer program was conceived, based on a few underlying hypotheses [4] in order: i) to estimate atmosphere boundary layer (thermohygrometric) conditions in equilibrium with the stone-surface; ii) to estimate the phase diagram corresponding to each pure-salt system; iii) to study the expected behaviour of each salt system upon its phase diagram, by means of scatter plots corresponding to the actual atmospheric (Atm) or to the estimated atmosphere boundary layer conditions on stone-surface (Surf); iv) to estimate the equilibrium relative humidity (RHeq) of some puresalt systems as a function of time; v) to look into the probable time-course evolution of each system, using estimates of deliquescence humidity (RHeq), monitored temperatures and relative humidities corresponding to Atm conditions, or estimated relative humidities corresponding to Surf conditions; vi) to estimate the probability of a given salt system being crystallised (f (RH < RHeq)) or deliquescent (f (RH > RHeq)), and the probability of a crystal/solution transition (TPE). This can be done for any given set of thermohygrometric data during monitoring time. The following definitions apply here: TPE - is the percentage ratio of the number of crystal/solution transitions across RHeq to the total number of data points. Both concepts refer to an interval which T and RH chronograms are available. The program can process data from different sites in a monument. However, it should be noticed that the relative humidity of the atmosphere close to a stone surface is an estimate. It is based on the

5 Environmental Coastal Regions 91 assumption of thermal equilibrium between the stone-air boundary layer and the stone itself. An empirical approximation by Tetens to the Clausius-Clapeyron equation enables such computations to be performed [6]. The RHeq phase diagrams of each salt system were estimated through Lagrange interpolation method, from tabulated (experimental) data. Tabulated deliquescence humidities for practically all common salts that could be relevant for building materials are available from the literature. 1.3 Results Deliquescence humidity estimation As an example of an output of the computer program for a monitored site, Figures 1-a and 1-b are shown, concerning data processing for Atm monitoring conditions. Halite behaviour can be followed outside the church from April to October In Figure 1-a, the estimated phase diagram and outdoor-environment data (scatter plot) is derived. These results allow estimating deliquescence humidity along time as a function of varying local ambient temperature as it is shown in Figure 1-b. In Figures 1-b, chronograms of exterior monitored temperature and relative humidity are shown. Estimations of the equilibrium relative humidity (RHeq) of halite along monitoring time, as a function of instantaneous local temperature can be seen as well Transition Probability Estimations The classification of each monument as to its saline-risk potential can be made on the basis of TPE values. This is because such values depend simultaneously on: i) all salts present, ii) all monitoring sites and iii) all measurement conditions (Surf, Atm) [7-9]. From the set of all possible relationships that can be derived between TPE values, it is shown quantitatively the transition behaviour likely to be expected of some salt systems (ex: nitratite, halite and niter) by means of TPE values. The difference between TPE estimations for Surf and Atm conditions can also be easily evaluated (Figures 2-a, 2-b). It should be noticed that niter is expected to be always crystallised since RHeq is always above environment RH (not presented in this paper). So, there is no need to present graphically its TPE values. A summary of the computation results obtained for the three salts is presented in Table 1.

$Boundary Condition HALITE ' "'"""" '/ %'.:" lx!y%. :';'/' 7; ^:%i%%.":-y.. ' '.:J."''':;-i:"v* :.;"; :;''-\"v;'^s:-;:-c". X\. a _. f(rji < R1U,) =69.4% " "'.;.. :. ^ry/t^/y"/'' :)^-:.$

6 92 Environmental Coastal Regions Slo MARUA TA" CWERRA-MALTA - EXTERIOR OO 3OOO 35OO 4OOO K 9O ><*> ^ 7 I 6 u 5 2O 1O '% *- _ "^^"^r!::7^-vt":~ lr -^8lCJ:!y2z«-J:. Boundary Condition HALITE ' "'"""" '/ %'.:" lx!y%. :';'/' 7; ^:%i%%.":-y.. ' '.:J."''':;-i:"v* :.;"; :;''-\"v;'^s:-;:-c". X\. a _. f(rji < R1U,) =69.4% " "'.;.. :. ^ry/t^/y"/'' :)^-:.'- ' f(rii > RIU,) =3.6%....- ':' ;.:'.-':'* '.'! " "'- " -. _. -''"^^t^^r '^.i^fe^^s: %xi! ',:; 23 - oc Figure 1. Chronograms and phase diagrams corresponding to sensor location "Exterior", a: Phase diagram of Halite (calculated) and scatter plots of air temperature and relative humidity (monitored data); b: Chronograms of Halite deliquescent conditions (calculated), air temperature and relative humidity (monitored data). 1.4 Discussion of results A computer model was conceived and presented elsewere [4]. An example was given herein for some salt systems likely to be found in the Church of S* Marija Ta'Cwerra (Figures l-a,b) and (Table 1). The behaviour of pure-salts can thus be forecast on the basis of indoor and outdoor varying environment conditions if it is assumed that the kinetics of salt transitions is fast enough. However, it should be emphasised that such thermodynamically based results are merely indicative of what might happen at the stone surface of monuments under surveillance. Contamination by a single salt is very uncommon, if not at all: a mixture of salts is present in (almost) every situation, owing to air pollution and/or rising damp. Recently, Price and Brimblecombe [1], as well as Steiger [11] dealt with the thermodynamics of the much more complex case of salt mixtures, for temperature conditions of 15, 2 and 25 C They

7 Environmental Coastal Regions 93 Table 1 - A summary of the computation results partially presented on Figures 2-a,b. \Salt Locm*---^ Level 1 Level 2 Level 3 Level 4 Atm. Surf. Surf-Atm Atm. Surf. Surf-Atm Atm. Surf. Surf-Atm Atm. Surf. Surf-Atm Nitratite M H MD+ LD+ M H D+ LD- Halite M H MD+ LD+ M H D+ HD- Niter In the Table: L low values of TPE M mean values of TPE < L < 1 %; 1< M < 2 %; H high values of TPE 2<H<4%; very high values of TPE >4%; D difference; +,-, positive, negative, or no differences between TPE; LD, MD HD, D low, mean, high, or very high differences between TPE; LD <.5 %.5 < MD < 1 % 1< HD < 1.5 %. used the approach made by Pitzer to calculate the relative humidity in equilibrium with any mixed-salt solution. Pitzer approach can in principle be extended to any situations of varying temperature. Unfortunately, the nature of the data available does not enable us to use such an approach in this paper. An index of the environmental-weathering potential should be the next step beyond, in the near future. Such an index could turn into an important tool for assessing monuments as to stone decay, provided that mineralogical, texture, porosity and interfacial (chemical and physical) characteristics could be considered and quantitatively modelled. The kinetics of deliquescence/crystallisation and crystallisation/

8 94 Environmental Coastal Regions Local 4 Malta - Nitratite Local 3 Local 2 Local 1-1,4 8,6 Malta - Halite Local 4 Local 3 Local 2 Local 1-5.O O.OO 5.O TPE % 1, Figure 2. Characteristic TPE values of two pure salt systems inside the church, a: nitratite, b: halite. hydration transitions as well as the processes of salt-solution transport for mixed-salt systems inside the porous stone should be modelled too. This allows a view to a deeper understanding of their time-course evolution and to a more accurate simulation and forecast of stone-decay patterns. Given this, an optimal choice of the sampling rate to the environment factors acting on a stone monument or any other historic-architectonic artefact, could be envisaged as well [8,9]. 2 Conclusions The results presented above show that the behaviour of the pure-salt systems conditioned to varying thermohygrometric conditions can be

9 Environmental Coastal Regions 95 significantly different when open-atmosphere or stone-surface conditions are considered. It is not enough to measure thermohygrometric variables some 5cm far from the stone surface and then extrapolates the results of RHeq estimations made thereby directly for that surface. At least, thermohygrometry of the atmosphere nearby the surface as well as surface temperature must be monitored. Considering transition-probability estimations (TPE), it is possible to ascertain which salts should be considered potentially more dangerous in a given context (monument plus environment). The projection of monitored data on the estimated phase diagram, and the monitored and estimated deliquescence humidity chronogram allows visualising immediately when and where phase transitions are likely to occur in the salt system. The state of the salt system (deliquescent or crystallised) are also easily ascertained. Some qualitative conclusions can also be made on the overall appearance of the chronograms, describing local environment behaviour along time. Further research should account for a very important aspect of saltinduced damage on historic buildings: actual salts are seldom pure. An effort must be put on adapting the presented methodology for the real situation, that is: the joint occurrence of several saline species, inside a natural, multiphase and heterogeneous porous matrix, whose behaviour is conditioned by a varying environment on every site under monitoring. To study different monuments simultaneously, it is very important that monitoring and surveillance programs are set up and carried out on a uniform (standard) basis and synchronised, for accurate data and results comparison. This should be done in what concerns either salts (origin, composition, extent) or buildings (sampling sites, material properties, etc.). Acknowledgements Research contracts PBICT/C/CTA/2127/95 and PBIC/C/QUI/2381/95 (JNICT-Portugal) assisted in meeting the production costs of the present paper (Parts I and II). References [1] Livingstone, R, Influence of evaporite minerals on gypsum crusts and alveolar weathering, Proc. of the III Int. Symp. on the Conservation of Monuments in the Mediterranean Basin, eds.. Fassina, H Ott and F. Zezza, enice, pp , 1994.

96 Environmental Coastal Regions [2] Torfs, K, an Grieken, R. & Cassar, J, Environmental effects on deterioration of monuments: case study of S" Marija Ta'Cwerra, Malta, Proc.

10 96 Environmental Coastal Regions [2] Torfs, K, an Grieken, R. & Cassar, J, Environmental effects on deterioration of monuments: case study of S" Marija Ta'Cwerra, Malta, Proc. Protection and Conservation of the European Cultural Heritage: Research Report N 4 (European Commission Research Workshop), ed. F Zezza, Bari, pp , [3] Fitzner, B, Henrichs, K. & olker, ML, Model for salt weathering at maltese globigerina limestones, Proc. Protection and Conservation of the European Cultural Heritage: Research Report N 4 (European Commission Research Workshop), ed. F. Zezza, Bari, pp , [4] Aires-Barros, L, & Mauricio, A., Chronology, probability estimations and salt efflorescence occurrence forecasts on monument building stone surfaces, Proc. 8^ Int. Cong, on Deterioration and Conservation of Stone, ed. J. Riederer, Berlin, pp , [5] Fassina,., Neoformation decay products on the monument's surface due to marine spray and polluted atmosphere in relation to indoor and outdoor climate, Proc. Protection and Conservation of the European Cultural Heritage: Research Report N 4 (European Commission Research Workshop), ed. F. Zezza, Bari, pp , [6] Monteith, J L. & Unsworth, M.H., Environmental Physics, Edward Arnold, London, pp.2-3, 199. [7] Aires-Barros, L. & Mauricio, A., Transition frequencies of evaporitic minerals on monument stone decay, Proc. of the 4^ Int. Symp. on the Conservation of Monuments in the Mediterranean Basin, eds. A. Moropoulou, F. Zezza, E Kollias and I. Papachristodoulou, Rhodes, ol. 1, pp , [8] Mauricio, A. & Aires-Barros, L., Salt systems and monument stone decay in coastal marine environment, Chemistry, Energy and the Environment, Royal Society of Chemistry, Cambridge (in the press). [9] Mauricio, A., Aires-Barros & Pacheco, A,M,G, Forecast of salt occurrences on monument stone surfaces, Chemistry, Energy and the Environment, Royal Society of Chemistry, Cambridge (in the press). [1] Price, C, & Brimblecombe, P., Preventing salt damage in porous materials, Preventing Conservation: Practice, Theory and Research, International Institute for Conservation, London, pp. 9-93, [lljsteiger, M., Crystallisation properties of mixed salt systems containing chloride and nitrate, Proceedings of the European Commission Research Workshop on the Conservation of Brick Masonry Monuments, Leuven (Belgium), pp. 1-9, 1994.

11 Section 2: Coastal Erosion

MONITORING OF ENVIRONMENTAL PARAMETERS TO EXPLAIN STONE DETERIORATION: CHURCH OF STA. MARIJA TA'CWERRA, MALTA

265 MONITORING OF ENVIRONMENTAL PARAMETERS TO EXPLAIN STONE DETERIORATION: CHURCH OF STA. MARIJA TA'CWERRA, MALTA TORFS, K. Dept. of Chemistry, University of Antwerp (U.l.A.), B - 2610 Antwerp, Belgium