Borehole seals of expandable clay with desired water content by use of dry water

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1 Borehole seals of expandable clay with desired water content by use of dry water Thomas Forsberg a, Roland Pusch b, Ting Yang c, Sven Knutsson Dept. of Civil, Environmental and Natural resources Engineering, Luleå University of Technology, Luleå, Sweden Abstract. Dense clay can be used for sealing of boreholes. Keeping the clay confined in perforated tubes it can be inserted in boreholes of nearly any length and diameter. Expansion of the clay to fill the borehole takes place by uptake of water and migration of clay through the perforation. The clay thereby exerts a swelling pressure on the confining rock causing effective sealing. For shallow boreholes to be sealed a very low initial degree of water saturation of the clay may be valuable since this makes the clay expand and seal the hole quickly, but for certain cases the clay should have a higher degree of water saturation. This can be required for moderating the rate of clay densification that may otherwise give too high wall friction for placement in very long holes. Sealing of very deep holes and holes containing highly radioactive waste makes temperature important: the heat-induced expansion of initially fully saturated clay can fracture the confining rock. The issue is therefore to prepare the clay inserts with properly selected water content. The paper describes preparation of clay seals by mixing air-dry clay powder with nanoparticles of water droplets coated with very thin shells of a hydrophobic silicious substance ( dry water ). It behaves as dry powder and is easily mixed with dry clay. On compaction to the desired density the shells break into aggregates of minute fragments while water becomes homogeneously distributed in the mass. Laboratory and benchscale testing verify that the properties of clay prepared in this way are the same as of clay saturated by sorbing water through a filter, a process that can take hundreds of years for big samples. Keywords: borehole sealing, clay blocks, degree of saturation, density, dry water, water content 1. Introduction Several concepts for sealing of boreholes and disposal of radioactive waste make use of dense smectite-rich clay, commonly called bentonite (Pusch, 1983; Yong et al, 2010). Physical characterization of such clay with respect to the rate of ion diffusion, thermal conductivity and rheological behaviour requires that the degree of water saturation is known. The state of complete water saturation is particularly important since this is usually the condition after some years or decades (Ting et al, 2015) in a longer perspective. The most common way to saturate clay samples is to confine them in cells with filters at the ends and let water be taken up by suction through the Corresponding author, Professor, Sven.Knutsson@ltu.se a Research Engineer, thomas.forsberg@ltu.se b Professor, drawrite.se@gmail.com c Ph.D. Student, ting.yang@ltu.se; yt1551@126.com

2 filters but this is a tedious diffusion-like process: saturation a 2 cm thick sample of dense smectiterich clay with uptake from two ends requires about one week, while a 4 cm sample requires 16 weeks and one with 10 cm thickness about 2 years. A 20 cm thick clay block requires about 20 years to be largely saturated. Placing the clay in powder form layer wise in cells and spraying water on the layers, followed by compaction, is possible but the time for homogenization of the clay samples will blocks will still be very long and they will not be homogeneous with respect to the distribution of water an density. We describe here a quick procedure for preparing any powdered material with homogenously distributed water content by mixing air-dry or dried clay in powder or granulate form with dry water (DW) consisting of droplets of water coated with very thin shells of a silicious substance (Forny, 2008). On compaction to the desired density the shells break into fragments that are smaller than silt grains. The released water becomes uniformly distributed in the mixture. 2. Materials 2.1 Dry water Dry water consists of solid water droplets contained in spherical, very thin shells of hydrophobic, fumed silica particles (Forny, 2009; Bomhard, 2011). The powder is dray and lyophobic, despite a water content by weight of just about 90%. It flows like flour when poured into laboratory cells or large containers for compaction to the desired dry density (ratio of mass of solid substance and total volume including voids). The angle of internal friction is reported to be at least 44 (Bomhard, 2011). The silica coating repels water and prevents the water droplets from combining at moderately high and low temperature. The material can be produced on an industrial scale by exposing volatile chlorosilanes to high temperature by flame hydrolysis and reaction with methyl chlorosilanes after cooling. Its primary use is as filling agent for plastics and as additive in food production (Lankes, 2006; Bomhard, 2011). It is available on the market, the chemical components of the material used in the described project being supplied by Wacker Chemie AG. The size distribution of the DW grains was 1-10 µm in the present study. The specific surface area is 20 to 35 % of that of smectite clay. The residual silanol content of the hydrophilic silica is 25 % and the carbon content about 2.8 % of the solid part of the DW. 2.2 Clay The tightening component of a borehole seal is dense expandable clay (smectite) that sorbs water from the confining rock or soil by its potential to bind water between the 1 nm thick Si/O and Al/Mg/OH lamellae (Pusch & Yong, 2010; Pusch, 2015). It is tightly contained in a perforated metal tube that is fitted into the hole to be sealed (Fig.1). The clay expands through the perforation and embeds the tube at a rate that depends on the density and degree of water saturation (Fig.2). The expansion, which can be up to about 3 times is caused by establishment of one, two or three water molecules thick intraparticle hydration layers. The expansion is uniquely controlled by the dry density and access to water.

3 Fig.1 Borehole sealing by use of dense smectite clay. Left: Sealing stages (Pusch, 1983). Right: Installation of borehole seal in 500 m deep borehole. Fig.2 Appearance of 24 hour old clay plug after removing part of the clay skin formed by clay migrated from the dense clay core in the perforated tube to the narrow space between tube and rock (Pusch, 2008). 2.3 Interaction of clay and DW Fig.3 shows schematically how DW droplets are linked in the microstructure of smectite clay after mixing dry clay powder and DW. The strongly hydrophilic clay considered in this paper will suck up water given off from crushed DWs that are uniformly distributed in the DW/clay mixture. Fig.4 indicates how DW is distributed and integrated in the clay matrix.

4 Fig.3 Microstructural voids with DW droplets between 3-7 nm thick stacks of smectite lamellae in uncompacted air-dry smectite clay (Pusch, 2015). Compression of clay with DW droplets causes breakage at a pressure of kpa and subsequent, successive homogenization (cf. Fig. 4). The obtained degree of water saturation can be 100 % or lower, depending on the needs; the required amount of water is calculated and the corresponding amount of DW added by mixing. Pre-saturated blocks of dense clay for borehole sealing in a repository with canisters holding high-level radioactive waste transfer the produced heat effectively to the rock, while blocks with natural water content can give unacceptably high temperature of the canisters and clay seals. An important question dealt with in the present paper, is whether the remainders of the crushed silicious shells of the droplets can significantly affect the physical properties of the DW clay and make them deviate from those of conventionally saturated clay. Fig.4 Redistribution of DW water at compaction (left), and subsequent maturation of the clay matrix (right). 3. Experimental

5 3.1 Clay material The clay used in the study a mixed-layer clay 1 belonging to a Paleocene formation of Tertiary age (Henning and Kasbohm, 1998) with 55 % expandable minerals, mainly montmorillonite. The mineralogical and chemical compositions are given in Tables 1 and 2. Determination of the size distribution of the granules, by sieving after drying at 60 C, gave 2 mm at maximum and 40 % smaller than mm. The content of uniformly distributed residual silanol and carbon makes up less than one weight percent of the clay sample. The majority of the silicious part is believed to behave like the % amount of the silica-rich accessory minerals quartz, feldspars. Table 1 Accessory minerals in weight percentages of smectite-rich Holmehus clay identified by XRD and CEC techniques (Pusch, 2015). Muscovite Chlorite Quartz Plagioclase K-feldspar Gypsum Pyrite < Table 2 Chemical composition of Holmehus clay (weight percentages); (Kasbohm et al., 2013). Oxides SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O wt % DW material The DW material was prepared by adding 50 g pyrogenic silica powder (Whacker HD K2000) to 500 g of distilled water in a mixer and agitating it for about 2 minutes. By ru nning the mixer at 20,000 rpm for 2 minutes the DW got a suitable form it appeared as light, dry powder (Fig.5) - for being mixed with clay. Fig. 5 Manufacturing of DW by mixing 10 % (weight percentage) of distilled water with 90 % pyrogenic silica. Fig.6 illustrates the size of the droplets and the thickness of their coatings ( shells ) of 1 Illite/smectite clay provided by Dantonite A/S, Denmark

6 hydrophobic silica. Since the surface of DW particles is entirely covered by hydrophobic material the material is felt dry and performs as dry powder of sugar or flour. Fig. 6 Silicious DW droplet with coating ( shell ). The hydration energy of DW droplets is mirrored by the release and evaporation of water from them in different environments. Fig.7 shows the outcome of laboratory experiments indicating that droplets with 90 to 97 % water content lose water much slower than (free) water in a glass beaker. The loss is because the confined water diffuses slowly through the coatings of the droplets, a process that successively releases the water contained in them. In practice, only freshly prepared DW should be used for avoiding non-uniform wetting of the clay. Fig.7 Rate of loss of water from a mass of DW droplets with different water contents compared with the evaporation of ordinary water from the same volume of free water. 3.3 Mixed clay and DW DW material was prepared for reaching complete saturation of clay after compaction in oedometers. Samples with different dry densities, 1370, 1800 and 1900 kg/m 3, were made for determining the hydraulic conductivity and expandability. The conductivity was determined by applying a hydraulic gradient of 67 m/m (meter water pressure difference per meter flow length) at percolation with distilled water. The filters confining the samples at each end were connected to burettes and the pressure was adjusted to maintain constant clay volume. The oedometer cells were mounted in a compression apparatus for recording the expandability in the form of swelling pressure.

7 3.4 Results Chemical constitution Fig.8 shows the atomic composition of dried DW-clay indicating presence of iron, calcium, sulphur, silica and aluminum. The latter element represents clay particles and silica shells of DW. Pd and the strong peak to the left are signals from the detector. Fe is present both as sorbed exchangeable ions in and on the clay minerals (cf. Xiaodong, Prikryl, Pusch, 2011), in shell fragments, and in precipitated complexes in the natural clay. The atomic spectrum of the remainders of crushed droplet shows Si and Fe as important cationic components. The role of chlorine from the pyrogenic substance, if still present, is unimportant because of its coupling to the silicious component and because it makes up a very small fraction of the solid mass. Fig.8 EDX spectrum of DW-saturated clay (Warr, Greifswald University, Germany) Microstructural constitution Fig. 9 illustrates the typical microstructural appearance without visible residual shell fragments. The photo reveals the strong variation in mineral composition (quartz grains are white, feldspars brown and clay minerals greenish). The row of small black dots are organic remainders. Open voids cannot have been larger than 20 µm. The micrograph was taken of the surface of a section exposed by layerwise tape peeling. Fig. 10 shows a totally fractured but still coherent DW-grain embedded in a dense matrix of smectite particles having compressed it (cf. Fig.4, left). Such fractured and compacted shell fragments were few and remained in their original positions. The large majority of the shells were strongly fragmented and their only impact on the bulk physical properties would be to tighten very small voids and channels. Fig.9. Optical micrograph of moist DW-saturated smectite-rich Holmehus clay with a density of 1570 kg/m 3 (magnification 250x).

8 Fig. 10 SEM micrograph showing remnants of a crushed DW particle with a size of about 2 µm Hydraulic conductivity and swelling pressure The hydraulic conductivity of DW-saturated clay and clay saturated by inflow of distilled water in oedometer cells is given in Table 3. Table 3. Comparison of hydraulic conductivity (K, m/s) and swelling pressure (p s ) for tests of DW-saturated Holmehus clay and samples prepared by conventional wetting, i.e. suction of air-dry clay powder compressed and confined in oedometer cells for saturation and percolation with distilled water (RW). K, m/s p, Wetting type Dry density, Saturated density kg/m 3 kg/m 3 s MPa DW E DW E DW E RW 1) E RW 1) E RW 2) E ) Ting, 2015; 2) Equivalent clay with slightly higher montmorillonite content It is obvious that the DW-saturated clay was consistently somewhat less conductive than the conventionally wetted clay. This is believed to be caused by a more uniform distribution of water and a more homogeneous microstructural constitution of the DW-clay. The swelling pressure exhibits a similar pattern: the values are consistently somewhat higher for the DW samples than for the conventionally saturated clay. As for the conductivity this is believed to be caused by a more uniform distribution of water and a more homogeneous microstructural constitution of the DW-clay. The higher values for DW clay proved that the very fine fragments of silicious shells did not hinder the smectite stacks to hydrate and expand. They were confined in small voids in the clay.

9 4. Discussion and conclusions The following major conclusions can be drawn from the study: At saturation by DW technique the water added to air-dry clay material by thorough mixing and subsequent compaction becomes uniformly distributed and complete homogeneity of the mixture is reached early. The technique can be used for preparing seals of boreholes and deposition holes for radioactive waste, The very thin silicious coatings of DW droplets break on compaction under less than 100 kpa pressure and create numerous fragments smaller than 1 µm, which assemble on site in small clay voids and channels in the microstructure. This does not cause any reduction of the expansion potential of the clay but has a clogging effect that explains the low bulk hydraulic conductivity compared to that of clay that is water saturated by inflow in clay confined in cells like oedometers, The physical properties of clay saturated by DW technique are slightly different from those of clays wetted by conventional one-dimensional uptake of water by suction in oedometer cells. The hydraulic conductivity is lower for DW-clay because of filling of voids and channels with shell fragments. The swelling pressure is higher since expansion and loss in density of the clay particles by expansion into microstructural voids is hindered by such fillings, an effect that contributes to the lower hydraulic conductivity of DW clay. DW wetting gives immediate saturation since it takes place in conjunction with the compaction of the clay to give blocks. The advantage of the method is that the samples instantly reach a state of uniform distribution of porewater at any desired degree of water saturation. The technique is very attractive from economical and practical points of view, especially for preparation of blocks of large dimensions for which conventional ways of saturation require many years or decades. References Bomhard, J,. (2011). Dry Water. Master of Sci. Thesis. Luleå University of Technology, Dept. Civ., Environmental and Natural resources Engineering. Forny, L., (2009). Influence of mixing characteristics for water encapsulation by self-assembling hydrophobicsilica nanoparticles. Powder Technology, Vol. 189, No2, (p.263). Henning, K. H., Kasbohm, J., (1998). Mineralbestand und Genese feinkörniger quartärer und präquartärer Sedimente in Nordostdeutschland unter besonderer Berücksichtigung des Friedländer Tones, [Berichte der Deutschen Ton- und Tonmineralgruppe e.v., Band 6], pp Kasbohm, J., Pusch, R., Nguyen, T.L., Hoang, M.T., (2013). Labscale performance of selected expandable clays under HLW repository near-field view. Environmental Earth Sciences Vol. 69, Lankes, H., (2006). Liquid absorption capacity of carriers in the food technology. Powder Technology, Vol.134, No 3, Pusch, R., (1983). Borehole sealing for underground waste storage. ASCE Proc. J. Geotechnical Engineering. Vol. 109, No.1, Pusch, R., Yong, R.N., (2010). Stiffening of smectite buffer clay by hydrothermal effects. Engineering Geology, Vol. 116, Pusch, R., (2015). Bentonite Clay. Taylor and Francis Group. ISBN-13; Pusch, R., (2008). Geological Storage of Highly Radioactive Waste, Springer Verlag. ISBN:

10 Ting Yang, (2015). Borehole sealing with expandable buffer clays in HLW disposal; Lab-scale performance. Lic. Examination thesis at the Division of Mining and Geotechnical Engineering, Luleå University of Technology, Sweden, and East China University of Technology China. Xiaodong, L., Prikryl, R., Pusch, R., (2011). THMC-testing of three expandable clays of potential use in HLW repositories. Applied Clay Science, Vol.52 (pp ). Yong, R.N., Pusch, R., Nakano, M., (2010). Containment of High-Level Radioactive and Hazardous Solid Wastes with Clay Barriers. Taylor & Francis, Spon Press, London.