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Stiffening of smectite buffer clay by hydrothermal effects
Engineering Geology 116 (2010) 21–31
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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o
Stiffening of smectite buffer clay by hydrothermal effects Roland Pusch a,⁎, A.B. Drawrite a, Raymond N. Yong b, Masashi Nakano c a b c
Lund, Sweden North Saanich, Canada Masashi Nakano Tokyo University, Japan
a r t i c l e
i n f o
Article history: Received 16 February 2010 Received in revised form 30 June 2010 Accepted 6 July 2010 Available online 24 July 2010 Keywords: Buffer Cementation Clay Hydrothermal Precipitation Stiffening
a b s t r a c t Smectite clay is a major engineered “buffer” barrier in repositories for high-level radioactive waste since it provides the waste canisters with a low-permeable embedment that minimizes canister stresses caused by earth shocks and tectonics by being ductile and homogeneous. The hydrothermal conditions prevailing in deep repositories cause some loss and degradation of the buffer and stiffening by precipitation of cementing matter, like quartz, cristobalite and iron compounds emanating from the clay or from the canisters. Thus, chemical reactions leading to cementation can prevent self-sealing of fissures caused by desiccation in the early stage of maturation of the buffer and increase its stiffness so that critical stress conditions may be generated in the canisters. © 2010 Elsevier B.V. All rights reserved.
1. Scope of study The primary function of the smectite clay, i.e. the buffer surrounding heat-producing canisters with highly radioactive waste (Fig. 1) is to be less permeable than the surrounding rock and to have a capacity to adsorb possibly released radionuclides. It must also be sufficiently ductile to prevent build-up of critically high stresses in the canisters in case of seismically, tectonically or thermally induced shearing along fractures that intersect the deposition holes. The ductility is associated with a high creep potential that can cause settlement of the heavy canisters but it is also a requisite for self-sealing of voids and gaps formed at the clay/canister contacts (Fig. 2). If the gaps remain open or are filled with clay with low density they represent permeable paths that can cause fast transport of water carrying radionuclides. For closing or compressing them the swelling pressure and creep potential must be sufficient. If cementation takes place these properties can deteriorate so much that the waste-isolation capacity of the buffer is largely lost.
2. Cementation mechanisms 2.1. Basics Geology and geotechnical sciences offer numerous examples of cementation. A classical geological example is the transformation of ⁎ Corresponding author. E-mail address: [email protected] (R. Pusch). 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.07.002
clastic sediments to sedimentary rock by precipitation of elements from percolating solutions. Overconsolidation of shallow clay layers by precipitation of carbonates and iron compounds is a well-known phenomenon in Scandinavia and Canada. While many of the processes do not require elevated temperature stiffening of deep smectitic clay sediments is known to be related to heat-induced illitization (Weaver, 1979; Velde, 1991). The cementing agent is quartz, cristobalite or other silicious compounds (Savage et al., 2010). Stiffening of this type is also expected to take place in the buffer as described in the present paper, which is a synthesis of geological evidence and experience from hydrothermal experiments. 2.2. Evidence of silicious precipitation in nature We will use geological analogues and hydrothermal experiments to demonstrate that clay buffer can undergo precipitation of silicious compounds serving as cement and causing stiffening. An almost classic example of this phenomenon is the Kinnekulle case, a carefully investigated Ordovician series of sedimentary rocks in Sweden containing shales, limestones, sandstones and bentonite beds that were affected by Permian magma that penetrated some hundred meters above the clay. Detailed analysis of core samples has shown that quartz had been formed in conjunction with conversion of smectite (montmorillonite) to illite. (Thorslund, 1945, Pusch, 1983). The temperature evolution of the clay beds has been determined by analytical calculation and Conodont analysis and found to be 120– 140 °C in the first 500 years after the magma intrusion, followed by a successive drop to about 90 °C after 1000 years. The average thermal gradient was 0.02 °C/cm in the first hundreds of years (Pusch and
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An example of stronger cementation associated with conversion of smectite to illite is Silurian clay presently exposed at the southwestern coastline of the Swedish island Gotland and called “bentonite” by geologists 50 years ago. Much later the smectite-like microstructure was identified by transmission electron microscopy showing the typical wavy interwoven network of very thin lamellae (Fig. 4) (Pusch, 1968). The unique transmission electron micrographs of the clay with porewater contained in the clay were taken by use of the 1.5 MV microscope of the Laboratoire d'Optique Electronique (CNRS), Toulouse, France. The clay is presently devoid of smectite and consists of illite (40%), quartz (20%), chlorite (15%), feldspars (10%) and calcite (10%), hence suggesting that it has undergone largely isomorphous transformation although numerous well crystallized illite laths point to neoformation of this mineral. The clay has a content of minus 2 mm particles of 28%. The water content is 21%, meaning that the density at water saturation is about 2250 kg/m3, which would correspond to a swelling pressure of about 30 MPa for pure Na smectite (Pusch and Yong, 2006). This is in fact also the effective pressure exerted by the presumed overlying 2 km of Devonian sediments causing consolidation of the young Burgsvik clay but being eroded later. The temperature at this depth, 90–100 °C, prevailed for at least ten million years, starting when the compression had become significant. Examples of more recent, i.e. Tertiary, cases of silicious precipitation generated by magma intrusion in the form of diabase dikes that intersect the smectite beds, are known from Montana, Sardinia (Busachi) and Libya (Pusch and Yong, 2006). 2.3. Geochemical modelling
Fig. 1. SKB's concept for isolating highly radioactive waste, KBS-3V (Svemar, 2005).
Madsen, 1995). The present content of montmorillonite in expandable form (mixed-layer smectite/illite) in the main bentonite bed is about 25%, while illite represents about 50%. Chlorite/kaolinite, carbonates, quartz, and feldspars make up the rest. The silicification is manifested by the presence of quartz precipitates on larger particles as seen in SEM micrographs (Fig. 3) and by the dispersion caused by ultrasonic treatment (Mueller von-Moos et al., 1990).
All these cases of heat-induced cementation by precipitation of silicious matter are in agreement with observations from a number of studies of deep-sea sediments performed in conjunction with oil and gas prospection. They were performed in the last 50 years and have led to the conclusion that smectite clay, exposed to pressure and heat, undergoes conversion to illite in conjunction with uptake of potassium and release of silicons that form silica commonly assumed to be quartz (Pusch and Yong, 2006). A number of analyses of deepsea sediments and other examples of heat- and pressure-exposed smectites have yielded the following simplified reaction model (Weaver, 1979): S + ðFk + MiÞ = I + Q + Chl
ð1Þ
Fig. 2. Strain and displacement of SKB canister. Left: Schematic illustration of FEM-calculated deformation of ductile clay-embedded canister by tectonically induced instant shearing along a fracture intersecting the deposition hole. Right: Formation of heterogeneities in the clay (dark wedges) by shearing of a stiff canister. X-ray image of model test with leadshots in the clay to show the strain pattern. The bright center is the shear box arrangement (Pusch, 2008).
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where: S = Smectite, Fk = K-feldspar, Mi = K-mica, I = Illite, Q = Quartz, Chl = Chlorite. The kinetics has been investigated and the Arrhenius-type model for the rate of smectite-to-illite conversion that is most commonly referred to was early proposed (Pytte and Reynolds 1989) (Fig. 5). A more recent version of the model that is relevant to the KBS-3 concept and gives quantitative data of quartz precipitation was proposed in the early nineties by Grindrod and Takase (Grindrod and Takase, 1994). In contrast to the conventional Pytte/Reynold-type models it does not preset smectite-to-illite as the basic reaction. The model takes O10(OH)2 as a basic unit and defines the general formulae for smectite and illite as: X0:35 Mg0:33 Al1:65 Si4 O10 ðOHÞ2 and K0:5−0:75 Al2:5−2:75 Si3:25−3:5 O10 ðOHÞ2 ð2Þ
Fig. 3. Precipitated quartz in Kinnekulle bentonite (Mueller von-Moos et al., 1990).
where X is the interlamellar absorbed cation (Na for Na-montmorillonite). The reactions in the illitization process are: þ
Na0:33 Mg0:33 Al1:67 Si4 O10 ðOH Þ2 + 6H = 0:33Na + 0:33 Mg + 1:67Al
3+
2+
ð3Þ
+ 4SiO2ðaqÞ + 4H2 O
causing precipitation of illite and silicious compounds composed as: þ
K AlSi3 O10 ðOH Þ2 = K + 3Al
3+
+ 3 SiO2 ðaqÞ + 6H2
ð4Þ
(logK being a function of temperature). Grindrod and Takase used the following rate of the reaction Ea
−
R* = Ae RT K þ S2 where A = coefficient, Ea = activation energy for S/I conversion, R = universal gas constant, T = absolute temperature, K+ = potassium concentration in the porewater, and S = specific surface area for reaction. We will see that it gives results that are in agreement with observations. The problem with the Pytte/Reynold and Grindrod/Takase and similar models is that the major rate-controlling variable, the activation energy for conversion of smectite to illite, is not known with certainty. For an activation energy of around 20 kcal/mol the models would imply very significant conversion to illite in hundred years at 100 °C. However, the fact that 25% of the smectite remains in the Kinnekulle bentonite after heating to more than 100 °C for several hundred years indicates that the activation energy is higher than this
Fig. 4. Transmission electron micrographs of ultrathin sections of Burgsvik clay. Upper: Acrylate-impregnated specimen with structural features similar to those in smectiterich clay. Lower: Specimen with original water content examined in 1.5 MV electron micrograph showing a similar pattern and also the presence of small isometric bodies distributed in the matrix of thin lamellae (Pusch, 1968).
Fig. 5. Diagram showing the rate of conversion of smectite to illite for the activation energy 27 kcal/mol according to Pytte/Reynold's model.
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figure. The complexity of the conversion process, exemplified by the difficulty in distinguishing between smectite and illite particles and by the impact of clay density (Gueven, 1990) and contemporary formation of other silicates (Marty et al., 2010) makes prediction of the illitization rate and hence also of the rate of the associated precipitation of silicious compounds uncertain. 2.4. Identification of cementing silicious compounds in nature Eqs. (1) and (4) indicate that silica is a reaction product of the conversion of smectite to illite and of the nature illustrated by Fig. 3. The precipitates are often very small as in the Burgsvik clay; they have the form of dense silica nodules centered in clay particle aggregates as demonstrated by the micrographs in Fig. 4. It is reasonable to assume that the physical size of the precipitations in fact ranges between a few tens of nanometers to micrometers. The following methods have been used for identifying possible cementation of smectite by comparing untreated and hydrothermally treated materials (Pusch and Yong, 2006): 1. Change in quartz and cristobalite content by XRD, 2. Examination of STEM and EDX electron micrographs for direct identification and analysis of precipitates, 3. Determination of swelling pressure, which drops by cementation, 4. Determination of shear strain and creep, which are both reduced by cementation. While XRD and electron microscopy are valuable tools for comparison of virgin and hydrothermally treated clay the swelling pressure and shear strain and creep are the most practically useful parameters in investigation of natural analogues. Examples are offered by the Busachi, Kinnekulle, and Burgsvik cases for which shear box testing have given relationships between time and strain (Pusch and Yong, 2006). The Tertiary Busachi bentonite from Sardinia, which is estimated to have been exposed to a temperature of 500 °C for 5 days, 200 °C for 2 months, and 60 °C for 1 year by impact of magma outflow (Pusch et al., 1993), gave the jerky strain pattern in Fig. 6. This behaviour was noticed for all the samples within 2 m distance from the samples from the hot contact. The Kinnekulle bentonite show the same jerky strain leading to creep failure as the Busachi clay. The well manifested silica cementation of the Kinnekulle clay is the reason for the stepped strain at the highest shear stress leading to creep failure as for the Busachi clay (Fig. 7). The Burgsvik clay with no smectite left is more strongly cemented than the other two reference clays. Thus, the creep test illustrated in Fig. 8 shows that almost no strain occurred for shear stresses up to about 80 kPa, while 95 kPa shear stress under 300 kPa normal stress initiated significant strain (Pusch, 2002).
Fig. 6. Creep behaviour of smectite-rich bentonite from Busachi. Curve for 222 kPa shear in linear time scale showing stepped strain rate behaviour related to breakage of cementation bonds (Pusch et al., 1987).
Fig. 7. Creep behaviour of Kinnekulle bentonite. The stepped strain for 95 kPa shear stress under 300 kPa normal stress is concluded to indicate breakage of larger cemented aggregates.
2.5. Precipitation of silicious compounds in hydrothermal experiments 2.5.1. Autoclave tests The indications of silicification and conversion of smectites provided by natural analogues have called for validation of the assumed hydrothermally induced process by laboratory investigation of smectite clay buffer exposed to hydrothermal conditions in repositories. Such tests have therefore been conducted in Sweden and France and they support the hypothesis of precipitation of cementing agents in the cooling phase that follows the several thousand years long hot period. A few of these tests will be referred to here for illustrating the changes in microstructural organization and mineral composition as well as their practical significance. 2.5.1.1. First series — chemically closed conditions. In the eighties a series of autoclave tests were made with purified SWY-1 smectite clay from Crook County (Newcastle Formation), Wyoming, USA, with nearly 100% montmorillonite with the following chemical composition in percent units: SiO2:69.2, Al2O3:19.6, Fe2O3:3.35, FeO:0.32, MnO:0.006, MgO:3.05, CaO:1.68, Na2O:1.53, K2O:0.53, P2O5:0.049, S:0.05, F:0.111 (Pusch, 1993). The 4.6 ml hydrothermal cells, containing nearly completely water saturated samples of clay, were goldcoated copper tubes that could resist an internal heat-generated pressure of about 100 MPa by slight yielding. Samples that had undergone heating up to 200 °C had densities of 2000 kg/m3 and 1300 kg/m3 at water saturation giving the rates of compression
Fig. 8. Creep behaviour of Silurian bentonite from Gotland. Almost no strain occurred for shear stresses up to about 80 kPa, while 95 kPa shear stress under 300 kPa normal stress gave sudden, accelerating strain leading to brittle failure.
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Fig. 9. Unconfined compression test of SWY-1 clay heated for 0.5 years. Dry density 1590 kg/m3 corresponding to 2000 kg/m3 after cooling (Pusch and Karnland, 1988).
shown in Figs. 9 and 10. The significant drop in compressibility by hydrothermal treatment is very obvious and a measure of the increased stiffness and strength. XRD analysis of the materials treated with ethylene glycol gave a stronger and sharper 2Θ = 5o reflection for 200 °C treatment than for 150 °C, and, in turn, stronger than for 20 °C specimens (Fig. 11). The increased intensity of the 001 reflections for higher temperatures indicates improved orientation of the stacks of lamellae. The same tendency has been observed in various tests of hydrothermally treated clays and indicates creation of dense stacked aggregates that can have an effect on the deformability and strength of the microstructural network. The temperature 200 °C is higher than implied by some repository concepts but has been considered by organizations like ANDRA. 2.5.1.2. Second series — chemically closed conditions. Experiments with autoclaved SWY-1 smectite clay was made for identifying processes involving Fe, which is present in octahedral positions in the purified clay and can be present in buffer clay also in the form of accessory minerals. Hydrothermal treatment in autoclaves of this very pure montmorillonite gel with a density of 1300 kg/m3 at saturation with distilled water has been conducted at the Geological Dept. of Texas Tech University at Lubbock for investigating physico/chemical effects (Gueven and Huang, 1990). The work was intended to test the
Fig. 10. Unconfined compression test of SWY-1 clay heated for 0.5 years. Dry density 480 kg/m3 corresponding to 1300 kg/m3 after water saturation (Pusch and Karnland, 1988).
Fig. 11. Rectified XRD of EG-treated SWY-1 samples representing unheated clay, and clay autoclaved at 150 and 200 °C, respectively. The samples were saturated with distilled water before being autoclaved (Pusch and Karnland, 1988).
hypothesis that heating of Na-montmorillonite to more than 100 °C causes convergence of stacks of lamellae (Gueven, 1990) and inversion of the crystal lattice from trans- to cis-coordination of Si, i.e. from the conventionally assumed crystal atom arrangement with oxygens in hexagonal patterns exposed on the basal planes of the smectite lamellae to one with equally spaced hydroxyls (Pusch and Yong, 2006). This study, which has not been fully evaluated until recently, shows that such conversion could explain the relative ease with which Si is released from the lattice tetrahedrons (cf. Lee and Kang, 2010), yielding an increase in precipitated silica on subsequent cooling. Fig. 12 shows a transmission electron micrograph (JEOL 100 STEM microscope) of untreated SWY-1 clay exhibiting a soft montmorillonite gel with typical elemental composition in all parts. Autoclave treatment of the SWY-1 clay saturated with distilled water was made for 8 h at 150 °C and 315 °C under 20 MPa water pressure. STEM analysis of the gel that had been heated to 150 °C showed the presence of larger aggregated bodies, sized up to 2 μm, as exemplified by Fig. 13. As demonstrated by the element analysis over the entire surface area of individual aggregates, they consisted of precipitated silica with some very minor amount of Fe. The Al peak represents montmorillonite lamellae underlying the aggregate. By increasing the penetration power of the electron radiation the interior of the dense aggregates could be investigated and it turned out to be very rich in iron (Fig. 14). The 150 °C treatment tended to cause some coagulation of the montmorillonite particle system leading to alignment of the stacks of lamellae that is indicated by the change in XRD peak shape as described in the preceding text. This process was even stronger in the sample autoclaved at 315 °C. These temperature ranges are not presently foreseen for buffers but may be reconsidered and the observations are therefore still of interest. It is important to realize that the autoclave experiments involved heating of fully water saturated clay under isothermal conditions, i.e. without any thermal gradient. This condition is different from that in a repository. 2.5.1.3. Third series — hydrothermal tests of MX-80 with and without gamma radiation. Cylindrical 7 cm long MX-80 samples with a dry density of 1650 kg/m3 were confined in cells with an iron plate at the heated end and a water saturated filter at the opposite one (Pusch et al., 1993). Weakly brackish water with Na as dominant cation and very little potassium (b10 ppm) was circulated through the filter that was kept at 90 °C. The iron plate was heated to 130 °C during the
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Fig. 12. Untreated very pure montmorillonite. Left: STEM picture of untreated clay. Right: Elements of the gel with Si, Al and O as major lattice constituents. Cu and Pt originate from the grids. The Mg and Fe contents were judged to be largely intracrystalline.
1 year experiments, yielding a thermal gradient of about 6 °C/cm. The solution was pressurized to 1.5 MPa. In one of the tests a gamma radiation dose of about 3E7 Gy acted on the iron plate, the adsorbed radiation dose being 3972 Gy/h at the hot plate contact, around 700 Gy/h at half length of the sample, and 456 Gy/h at the coldest end. The investigation of the samples comprised XRD analysis (Fig. 15), electron microscopy with EDX, chemical analysis, infrared spectrometry IR, and CEC determination, the main mineral data being collected in Table 1. The analyses showed that there was nearly no difference between the sample exposed to γ radiation and the one that was not irradiated except that Fe migrated from the iron plate into the clay somewhat quicker under radiation. Comparison with virgin MX-80 clay showed that hydrothermal treatment with and without radiation gave insignificant chemical changes, which was also supported by CEC data. They showed that untreated MX-80 had CEC = 99 meq/100 g while the most strongly heated and radiated clay had CEC = 93 meq/ 100 g. However, creep testing at room temperature of samples from various distances from the hottest end gave witness of significant stiffening (Fig. 16), (Pusch et al., 1993). Thus, the shear strain of samples exposed to 130 °C was about 3 times smaller than of the one heated to 90 °C.
3. Evolution of buffers 3.1. Macrostructural processes Buffer clay is manufactured by compaction of clay granules under 100–200 MPa pressure leaving only small voids between them. The voids have the form of channels of varying aperture. At wetting the tightest parts become completely filled with capillary water and water films sorbed on the basal surfaces of the stacks while wider ones still contain air unless the piezometric pressure becomes sufficiently high to dissolve it. In the initial “isothermal” state when no water has yet been taken up by the buffer from the rock, drying of the hot part of the buffer takes place causing migration of water in liquid and vapour forms towards the rock. In the hottest part the stacks of lamellae contract, by which the individual voids and the channels formed by them become wider. Full-scale tests have demonstrated that steep, radial fractures are formed to several centimetres distance from the hot canister (Pusch et al. 1985). Once this part has ultimately become water saturated the fractures and widened channels will be closed by the swelling pressure exerted by the colder part of the buffer, and by self-sealing through redistribution of clay material if the heated clay has not lost its expandability.
Fig. 13. STEM picture of large and dense silica particle in montmorillonite sample hydrothermally treated at 150 °C.
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Fig. 14. Dense Fe nuclei in Si-rich aggregates recorded under low electron penetration power (lower micrograph) and high penetration power. Oxygen is abundant according to the element analysis, indicating that the nucleus is iron hydroxide.
3.2. Microstructural processes Smectite clay consists of stacks of 10 Å lamellae separated by up to 3 water hydrates depending on the bulk density and adsorbed cations (Pusch and Yong, 2006). The assembly of stacks is coherent for bulk densities higher than about 1050 kg/m3 if the porewater is poor in electrolytes, but must exceed about 1600 kg/m3 if the porewater is saltier than strongly brackish with calcium as dominant cation. Consolidation, implying an increase from lower to higher density,
takes place on compression while expansion takes place on unloading. Unlike non-expandable clay types compression/expansion is a nearly reversible process for smectite, except that expansion is delayed by internal friction. Voids in the buffer caused by imposed strain as in Fig. 2, or by heat-induced desiccation early after installation of the canisters, would have to be closed or sealed for acceptable performance. Closing of larger voids by expansion of the buffer is expected while sealing of small voids and fissures can take place by precipitation of silica or iron compounds. Expansion involves
Fig. 15. Schematic diffractograms of the reference MX-80 sample (20 °C) and the most heated part of the hydrothermally tested sample (130 °C). Feldspars, amphibole, some of the quartz and smectite disappeared in the hot part (Gueven and Huang, 1990).
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Table 1 Changes in one year long hydrothermal tests of MX-80. M = Montmorillonite, F = Feldspars, G = Gypsum, Q = Quartz, K = Kaolinite, Chl = Chlorite, I = Illite. +++ means strong increase, ++ significant increase, + slight increase, −−− strong loss, −− significant loss, − slight loss. 0 means no change. Treatment
125–130 °C
115–120 °C
105–110 °C
90–95 °C
Hydrothermal without radiation
M− F −−− Chl+ G ++ K ++ Q+ I+
M− F −− Chl + G +++ K+ Q+ I0
M0 F− Chl 0 G+ K0 Q0 I0
M0 F− Chl 0 G+ K− Q0 I0
exfoliation of clay particles from the dense buffer followed by reorganization of the released particles to form clay gels that become consolidated under the swelling pressure exerted by adjacent, denser clay (Pusch and Yong, 2006). Two mechanisms can cause loss of expandability: strong draught causing permanent collapse of the stacks of lamellae, and precipitation of cementing agents. Combination of the two can make the collapse permanent. It will be shown here how dissolution of smectite and accessory minerals and precipitation of cementing agents like in nature can affect the performance of buffer clay. 3.3. Cementation of the buffer 3.3.1. Processes Temporary accumulation of precipitated matter from the groundwater in the hottest part of the buffer early after installation of the buffer and canisters, i.e. formation of anhydrite and chlorides, is well known from field and mock-up tests (Pusch, 2008) but the process is expected to be reversible and of little concern in a long time perspective. In contrast, irreversible reactions caused by dissolution of clay and accessory minerals and precipitation of the solutes are processes that can cause permanent stiffening and reduced expandability. They are in focus in this paper. Application of the chemical model of Grindrod/Takase to KBS-3 repository conditions (SKB's KBS-3V concept) requires coupling with a transport model based on diffusion-dominated migration of aqueous species like silica, aluminium, sodium, magnesium, and potassium to form a set of quasi-nonlinear partial differential equations. The ones valid for aqueous species and minerals were used for calculating precipitation of silica in the direction of the thermal gradient by these investigators. It was found that almost no illite will be formed in the first 500 years despite the conservative assumption of a constant temperature of 90 °C at the buffer/canister
Fig. 16. Shear strain of specimens hydrothermally treated for 1 year without radiation. Shear box testing under 6 MPa normal effective pressure (Pusch et al., 1993).
contact, and 50 °C at the buffer/rock contact. However, for the subsequent period of time with a linear temperature drop with time to 25 °C after 10,000 years, the model predicts precipitation of quartz in the coldest part of the buffer, i.e. within about 0.1 m from the rock wall as shown in Fig. 17. The Grindrod/Takase model appears to adequately describe the chemical evolution with respect both to illitization and silicification. Quartz precipitates in the buffer and in open fractures in the walls of the deposition holes, which tends to seal them off (Pusch and Yong, 2006). 3.3.1.1. The Äspö Retrieval Test. Carefully conducted and interpreted large-scale experiments simulating the conditions in a KBS-3V repository have indicated the major chemical processes in buffer of smectite clay. Wyoming bentonite (MX-80) with 75–85% montmorillonite and Na as dominant adsorbed cation, and quartz (8–15%), feldspars (2–5%) and chlorite and micas (2–5%) as accessory minerals has been used in a number of laboratory and field experiments. Similar clays from Spain, France, Switzerland and the Czech Republic have been investigated in the same type of tests in the respective countries. A few of them have been conducted for determining physico-chemical and mineralogical changes that take place under repository-like conditions. MX-80 has been investigated in SKB's simulated KBS-3V repository at Äspö, some 300 km south of Stockholm, samples being taken from the buffer in a 5 year full-scale test called Äspö Retrieval Test (Eng 2008). The buffer was artificially wetted by being surrounded by a filter that was saturated with salt water (6000 ppm with Na/Ca ratio 1.0). The buffer in contact with the hot canister had been exposed to 78–85 °C for a couple of years and then to less than 60 °C because of malfunctioning electrical heaters. This part of the clay had a density of 1970 kg/m3 (1540 kg/m3 dry density). Geotechnical tests (Table 2) and extensive EDX and XRD analyses led to the following conclusions: • The hydrothermally treated clay had undergone substitution of Al3+ by Fe3+ in the octahedral layer causing higher lattice stresses because of the larger ion radius of Fe3+ than of Al3+ thereby reducing the resistance of the montmorillonite to dissolution, • Montmorillonite was the dominating mineral phase in both virgin and hydrothermally treated clay, the total number of samples of each type being at least 3, • The lower interlayer charge of the heated clay than of virgin MX-80 would mean that it should have a higher swelling pressure, but cementation dominated and gave the opposite effect.
Fig. 17. Evolution of quartz abundance profile (~ 10,000 y). Precipitation leading to cementation takes place in the outer, colder, 0.1 m part of the buffer (Pusch and Yong, 2006).
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was maintained at 95 °C causing a radial temperature gradient of about 2 °C/cm. As shown by Table 2 testing of samples taken at different distances from the canister showed obvious differences. One finds that the swelling pressure of the sample that had been exposed to 54–56 °C and representing the least heat-affected clay was more than twice the pressure of the more strongly heated sample. Additional samples taken in radial direction from the hot canister had physical properties intermediate to those of the most and least heated ones. The differences in mineral composition between the untreated buffer clay and the hydrothermally treated samples can be explained by 1) partial dissolution of the montmorillonite, 2) obvious formation of illite, and 3) neoformation of talc and kaolinite (cf. Fig. 19). In summary, the major mineralogical processes in the most heated smectite-rich clay mixed with graphite and quartz particles were:
Fig. 18. Main features of the Czech Mock-up test. M1, M2 etc. are samples taken in conjunction with or after the experiment.
Table 2 Geotechnical data of oedometer-tested samples at saturation with distilled water. The data ρsat = density at water saturation, and hydraulic conductivity = K, represent equilibrium reached after about 40 days (Pusch, 2008). Distance from heater, cm
T, °C
ρsat, kg/m3
ps, kPa
12–14 0–2
54–56 85–95
1945 1925
650 310
3.3.1.2. The Czech mock-up test. A 3 year mock-up experiment simulating the conditions in a KBS-3V hole was conducted for the Czech organization RAWRA in the years 2001 to 2004 (Pacovsky et al., 2005). The rock was simulated by a steel tube with an inner diameter of 800 mm and the canister by a heated steel cylinder with 360 mm diameter (Fig. 18). The buffer consisted of highly compacted blocks of 85% montmorillonite mixed with 10% finely ground quartz and 5% well crystallized graphite, the dry density being 1800 kg/m3. Granules of the same material were filled in the 50 mm wide space between the steel tube and the buffer, which had unlimited access to Nadominated brackish groundwater through a surrounding filter (Pusch, 2008). The temperature at mid-height of the canister surface
• Intergrowth of illite and kaolinite was obvious in the dioctahedral vermiculite in the untreated clay. In these aggregates illite dominated over kaolinite but in the heated sample most of the intergrowths of illite/kaolinite were dissolved, • Fe set free by the dissolution of the montmorillonite can have formed iron complexes resulting in cementation in the entire buffer mass, especially in the most heated part, • The quartz content, which was determined by XRD and numerous EDX analyses, increased in the direction towards the canister and is judged to be the main cementing agent, • Replacement of octahedral Al by Fe can have caused a drop in coherence of the montmorillonite crystals, thereby promoting easier dissolution, • Formation of illite in the hottest part of the buffer (sample M4) may have been triggered by uptake of K from dissolved vermiculite. Mineralogical characterization of the natural clay component was made by use of TEM with EDX and Coherent Scattering Domain (CSD) data for determining the thickness of particles or stacks of lamellae. The Koester diagrams in Fig. 20 show the charge distribution in the various layers in the natural clay component and in the most heated sample M4. The figure shows that very obvious changes in charge distribution took place in the heated sample. Significant dissolution of the Fe-rich montmorillonite and complete disappearance of the intergrowth of illite and kaolinite had taken place, suggesting migration and precipitation of released elements at different distances from the heater. The significantly lower swelling pressure of the heated sample can be explained by the formation of illite in combination with cementation by precipitated silicious compounds. The associated
Fig. 19. XRD spectra of the samples M2 to M4. Oriented mounts, air dried and ethylene glycol-treated (SEIFERT, Co Kα-radiation) with peak-area distribution by means of WinFitpeak fitting, (Pacovsky et al., 2005) (After Jörn Kasbohm, Greifswald University, Germany).
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R. Pusch et al. / Engineering Geology 116 (2010) 21–31
Fig. 21. Sediment volume of 11 g of clay in 30 ml distilled water. Left: heated clay. Right: specimen from least heated buffer (45–47 °C).
Fig. 20. Distribution of charge in tetrahedral, octahedral and interlayers in unheated clay (RMN bent) and in heated clay (M4). ML means mixed-layer minerals. The figures represent percentages (Pusch and Yong, 2006).
loss in expandability is illustrated in Fig. 21, which shows the appearance of 11 g of a specimen from the most heated part of the buffer (M4) and of a specimen from untreated buffer submerged in 30 ml distilled water. Visual examination has been made of buffer clay samples from the least heated (45–47 °C) and the most heated part after extraction from the oedometers in which they were saturated with distilled water and tested with respect to swelling pressure and hydraulic conductivity. The least heated clay was very homogeneous and ductile while the most strongly heated clay contained fissures and broke up in millimeter-sized fragments at sectioning. 4. Discussion and conclusions The experiments that included rheological measurement showed that stiffening of smectite clay takes place under isothermal conditions
as well as when thermal gradients prevail. Three of them, representing repository-like conditions with the buffer heated under chemically open conditions, involved exposure of the clay to a relatively high temperature gradient (2 °C/cm to 6 °C/cm) and a maximum temperature of 85–130 °C, while the other, having the form of autoclave testing under isothermal and chemically closed conditions exposed the clay to 90–200 °C. All the tests included chemical and mineralogical analyses and showed that silicious compounds, primarily quartz and cristobalite, had been precipitated, suggesting that they caused the stiffening and strengthening. The analysis of the autoclaved samples that had been isothermally tested at 150–315 °C tests also showed formation of quartz nodules and also indicated that they frequently had nuclei of Fe. The most comprehensive and detailed investigations of clay heated under a thermal gradient, i.e. the study of the Czech buffer clay, showed that Fe was released from the montmorillonite component and precipitated at cooling. The precipitation of quartz in the bentonite at Kinnekulle had taken place where the temperature gradient had been only 0.02 °C/cm and the temperature 140–160 °C, and in the Burgsvik clay at lower temperature but under very long time. The laboratory and mock-up tests of hydrothermally treated smectite clay as well as natural analogues represented by bentonite layers intersected or contacted by hot diabase dikes affected by hot magma show that cementation by silicious compounds has taken place. The involved processes, dissolution and precipitation of smectite and accessory minerals, occurred in a relatively short period of time while the geochemical model suggests that their comprehension is very limited in the first few tens and hundreds of years at repository temperatures below 100 °C. However, according to the Grindrod/Takase model, the conditions will be more critical after a few thousand years in the buffer of the SKB concept since the formation of cementing agents, as predicted by the model, will then become significant and affect the physical properties of the buffer to a practically important extent. The buffer clay will be stiffer and stronger as illustrated by the Kinnekulle analogue, and lose part of its self-sealing potential. After 10,000 years the cementation of the outer, colder part of the buffer is expected to make it brittle and sensitive to mechanical impact such as shearing by seismic, tectonic or thermomechanical impact. The more comprehensive cementation is in agreement with the high strength and the brittleness of the Burgsvik clay. It is important to realize that the cementation caused by precipitation of silicious and iron components in the experiments took place at the cooling that abruptly halted the heat-generated chemical processes. It remains unclear whether the stiffening was solely due precipitation of cementing agents or if microstructural changes in the form of contraction in combination with cementation caused it. References Eng A., 2008. Äspö Hard Rock Laboratory, Canister Retrieval Test, Retrieval phase. Project report IPR-08-13. SKB, Stockholm.
R. Pusch et al. / Engineering Geology 116 (2010) 21–31 Grindrod, P., Takase, H., 1994. Reactive chemical transport within engineered barriers. Proc. 4th Int. Conf. on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, Charleston, SC USA, 12–17 Dec. Oldenburg Verlag, pp. 773–779. Gueven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Engineering Geology. 28, 233–247. Gueven, N., Huang, W.-L., 1990. Effects of Mg2+ and Fe3+ substitutions on the crystallization of discrete illite and illite/smectite mixed layers. Int. rep. Dept. Geosciences Texas Tech University, Exxon Production research Co, Houston, Texas, USA. Lee, J.O., Kang, I.M., 2010. Smectite alteration and its influence on the barrier properties of smectite clay for a repository. Applied Clay Science 47 (1–2), 99–104. Marty, N.C.M., Fritz, B., Clément, A., Michau, N., 2010. Modelling the long term alteration of the engineered bentonite barrier in an underground radioactive waste repository. Applied Clay Science 47 (1–2), 82–90. Mueller von-Moos, M., Kahr, G., Bucher, F., Madsen, F.T., 1990. Investigations of the metabentonites aimed at assessing the long-term stability of bentonites under repository conditions. In: Pusch, R. (Ed.), Artificial Clay Barriers for Hih Level Radioactive Waste Repositories: Engineering Geology, Vol.28, Nos.3–4, pp. 269–280. Special Issue. Pacovsky, J., Svoboda, J., Zapletal, L., 2005. Saturation development in the bentonite barrier of the Mock-up CZ geotechnical experiment. Clay in Natural and Engineered Barriers for Radioactive Waste Confinement — Part 2: Physics and Chemistry of the Earth, 32/8-14. Elsevier Publ. Co, pp. 767–779. Pusch, R., 1968. A technique for investigation of clay microstructure. Journal de Microscopie 6, 963–986. Pusch, R., 1983. Stability of deep-sited smectite minerals in crystalline rock — chemical aspects. SKB, Stockholm. SKB Technical Report TR-83-16. Pusch, R., 1993. Evolution of models for conversion of smectite to non-expandable minerals. SKB, Stockholm. Technical Report TR-93-33. Pusch, R., 2002. The Buffer and Backfill Handbook. Part 2, Materials and Techniques. SKB, Stockholm. SKB Technical Report TR-02-12.
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Pusch, R., 2008. Geological Storage of Radioactive Waste. Springer Verlag978-3-54077332-0. Pusch, R., Karnland, O., 1988. Hydrothermal effects on montmorillonite. A preliminary study. SKB, Stockholm. SKB Technical Report TR-88-15. Pusch, R., Madsen, F., 1995. Aspects on the illitization of the Kinnekulle bentonites. Clays and Clay Minerals 43 (3), 261–270. Pusch, R., Yong, R.N., 2006. Microstructure of smectite clays and engineering performance. Taylor & Francis, London and New York. ISBN10: 0-415-36863-4. Pusch, R., Nilsson, J., Ramqvist, G., 1985. Final Report of the Buffer Mass Test — Volume I: Scope, preparative field work, and test arrangement. SKB, Stockholm. Technical Report TR-85-211. Pusch, R., Börgesson, L., Erlström, 1987. Alteration of isolating properties of dense smectite clay in repository environment as exemplified by seven pre-Quaternary clays. SKB, Stockholm. Technical Report TR-87-29. Pusch, R., Karnland, O., Lajudie, A., Decarreau, A., 1993. MX-80 Exposed to High Temperatures and Gamma Radiation. SKB, Stockholm. SKB Technical Report TR-93-03. Pytte, A.M., Reynolds, R.C., 1989. The thermal transformation of smectite to illite. In: Naeser, N.D., McCulloh, T.H. (Eds.), Thermal History of Sedimentary Basins. Springer-Verlag, New York, pp. 133–140. Savage, D., Benbow, S., Watson, C., Takase, H., Ono, K., Oda, C., Honda, A., 2010. Natural systems for the alteration of clay under alkaline conditions: an example from Searles Lake, California. Applied Clay Science 47 (1–2), 72–81. Svemar, C., 2005. Cluster Repository Project (CROP). Final Report of European Commission Contract FIR1-CT-2000–20023, Brussels, Belgium. Thorslund, P., 1945. Om bentonitlager i Sveriges kambrosilur. Geologiska Föreningens I Stockholm Förhandlingar 67, 286. Velde, B., 1991. Kinetics of I/S (illite/smectite) transformations. MRS. (Resumé NoB-II, Invited paper). Weaver, C.E., 1979. Geothermal alteration of clay minerals and shales: diagenesis. Batelle-Office on Nuclear Waste Isolation. Technical Report ONWI-21, ET-76-C061830 Contract.
Contents lists available at ScienceDirect
Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o
Stiffening of smectite buffer clay by hydrothermal effects Roland Pusch a,⁎, A.B. Drawrite a, Raymond N. Yong b, Masashi Nakano c a b c
Lund, Sweden North Saanich, Canada Masashi Nakano Tokyo University, Japan
a r t i c l e
i n f o
Article history: Received 16 February 2010 Received in revised form 30 June 2010 Accepted 6 July 2010 Available online 24 July 2010 Keywords: Buffer Cementation Clay Hydrothermal Precipitation Stiffening
a b s t r a c t Smectite clay is a major engineered “buffer” barrier in repositories for high-level radioactive waste since it provides the waste canisters with a low-permeable embedment that minimizes canister stresses caused by earth shocks and tectonics by being ductile and homogeneous. The hydrothermal conditions prevailing in deep repositories cause some loss and degradation of the buffer and stiffening by precipitation of cementing matter, like quartz, cristobalite and iron compounds emanating from the clay or from the canisters. Thus, chemical reactions leading to cementation can prevent self-sealing of fissures caused by desiccation in the early stage of maturation of the buffer and increase its stiffness so that critical stress conditions may be generated in the canisters. © 2010 Elsevier B.V. All rights reserved.
1. Scope of study The primary function of the smectite clay, i.e. the buffer surrounding heat-producing canisters with highly radioactive waste (Fig. 1) is to be less permeable than the surrounding rock and to have a capacity to adsorb possibly released radionuclides. It must also be sufficiently ductile to prevent build-up of critically high stresses in the canisters in case of seismically, tectonically or thermally induced shearing along fractures that intersect the deposition holes. The ductility is associated with a high creep potential that can cause settlement of the heavy canisters but it is also a requisite for self-sealing of voids and gaps formed at the clay/canister contacts (Fig. 2). If the gaps remain open or are filled with clay with low density they represent permeable paths that can cause fast transport of water carrying radionuclides. For closing or compressing them the swelling pressure and creep potential must be sufficient. If cementation takes place these properties can deteriorate so much that the waste-isolation capacity of the buffer is largely lost.
2. Cementation mechanisms 2.1. Basics Geology and geotechnical sciences offer numerous examples of cementation. A classical geological example is the transformation of ⁎ Corresponding author. E-mail address: [email protected] (R. Pusch). 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.07.002
clastic sediments to sedimentary rock by precipitation of elements from percolating solutions. Overconsolidation of shallow clay layers by precipitation of carbonates and iron compounds is a well-known phenomenon in Scandinavia and Canada. While many of the processes do not require elevated temperature stiffening of deep smectitic clay sediments is known to be related to heat-induced illitization (Weaver, 1979; Velde, 1991). The cementing agent is quartz, cristobalite or other silicious compounds (Savage et al., 2010). Stiffening of this type is also expected to take place in the buffer as described in the present paper, which is a synthesis of geological evidence and experience from hydrothermal experiments. 2.2. Evidence of silicious precipitation in nature We will use geological analogues and hydrothermal experiments to demonstrate that clay buffer can undergo precipitation of silicious compounds serving as cement and causing stiffening. An almost classic example of this phenomenon is the Kinnekulle case, a carefully investigated Ordovician series of sedimentary rocks in Sweden containing shales, limestones, sandstones and bentonite beds that were affected by Permian magma that penetrated some hundred meters above the clay. Detailed analysis of core samples has shown that quartz had been formed in conjunction with conversion of smectite (montmorillonite) to illite. (Thorslund, 1945, Pusch, 1983). The temperature evolution of the clay beds has been determined by analytical calculation and Conodont analysis and found to be 120– 140 °C in the first 500 years after the magma intrusion, followed by a successive drop to about 90 °C after 1000 years. The average thermal gradient was 0.02 °C/cm in the first hundreds of years (Pusch and
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An example of stronger cementation associated with conversion of smectite to illite is Silurian clay presently exposed at the southwestern coastline of the Swedish island Gotland and called “bentonite” by geologists 50 years ago. Much later the smectite-like microstructure was identified by transmission electron microscopy showing the typical wavy interwoven network of very thin lamellae (Fig. 4) (Pusch, 1968). The unique transmission electron micrographs of the clay with porewater contained in the clay were taken by use of the 1.5 MV microscope of the Laboratoire d'Optique Electronique (CNRS), Toulouse, France. The clay is presently devoid of smectite and consists of illite (40%), quartz (20%), chlorite (15%), feldspars (10%) and calcite (10%), hence suggesting that it has undergone largely isomorphous transformation although numerous well crystallized illite laths point to neoformation of this mineral. The clay has a content of minus 2 mm particles of 28%. The water content is 21%, meaning that the density at water saturation is about 2250 kg/m3, which would correspond to a swelling pressure of about 30 MPa for pure Na smectite (Pusch and Yong, 2006). This is in fact also the effective pressure exerted by the presumed overlying 2 km of Devonian sediments causing consolidation of the young Burgsvik clay but being eroded later. The temperature at this depth, 90–100 °C, prevailed for at least ten million years, starting when the compression had become significant. Examples of more recent, i.e. Tertiary, cases of silicious precipitation generated by magma intrusion in the form of diabase dikes that intersect the smectite beds, are known from Montana, Sardinia (Busachi) and Libya (Pusch and Yong, 2006). 2.3. Geochemical modelling
Fig. 1. SKB's concept for isolating highly radioactive waste, KBS-3V (Svemar, 2005).
Madsen, 1995). The present content of montmorillonite in expandable form (mixed-layer smectite/illite) in the main bentonite bed is about 25%, while illite represents about 50%. Chlorite/kaolinite, carbonates, quartz, and feldspars make up the rest. The silicification is manifested by the presence of quartz precipitates on larger particles as seen in SEM micrographs (Fig. 3) and by the dispersion caused by ultrasonic treatment (Mueller von-Moos et al., 1990).
All these cases of heat-induced cementation by precipitation of silicious matter are in agreement with observations from a number of studies of deep-sea sediments performed in conjunction with oil and gas prospection. They were performed in the last 50 years and have led to the conclusion that smectite clay, exposed to pressure and heat, undergoes conversion to illite in conjunction with uptake of potassium and release of silicons that form silica commonly assumed to be quartz (Pusch and Yong, 2006). A number of analyses of deepsea sediments and other examples of heat- and pressure-exposed smectites have yielded the following simplified reaction model (Weaver, 1979): S + ðFk + MiÞ = I + Q + Chl
ð1Þ
Fig. 2. Strain and displacement of SKB canister. Left: Schematic illustration of FEM-calculated deformation of ductile clay-embedded canister by tectonically induced instant shearing along a fracture intersecting the deposition hole. Right: Formation of heterogeneities in the clay (dark wedges) by shearing of a stiff canister. X-ray image of model test with leadshots in the clay to show the strain pattern. The bright center is the shear box arrangement (Pusch, 2008).
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where: S = Smectite, Fk = K-feldspar, Mi = K-mica, I = Illite, Q = Quartz, Chl = Chlorite. The kinetics has been investigated and the Arrhenius-type model for the rate of smectite-to-illite conversion that is most commonly referred to was early proposed (Pytte and Reynolds 1989) (Fig. 5). A more recent version of the model that is relevant to the KBS-3 concept and gives quantitative data of quartz precipitation was proposed in the early nineties by Grindrod and Takase (Grindrod and Takase, 1994). In contrast to the conventional Pytte/Reynold-type models it does not preset smectite-to-illite as the basic reaction. The model takes O10(OH)2 as a basic unit and defines the general formulae for smectite and illite as: X0:35 Mg0:33 Al1:65 Si4 O10 ðOHÞ2 and K0:5−0:75 Al2:5−2:75 Si3:25−3:5 O10 ðOHÞ2 ð2Þ
Fig. 3. Precipitated quartz in Kinnekulle bentonite (Mueller von-Moos et al., 1990).
where X is the interlamellar absorbed cation (Na for Na-montmorillonite). The reactions in the illitization process are: þ
Na0:33 Mg0:33 Al1:67 Si4 O10 ðOH Þ2 + 6H = 0:33Na + 0:33 Mg + 1:67Al
3+
2+
ð3Þ
+ 4SiO2ðaqÞ + 4H2 O
causing precipitation of illite and silicious compounds composed as: þ
K AlSi3 O10 ðOH Þ2 = K + 3Al
3+
+ 3 SiO2 ðaqÞ + 6H2
ð4Þ
(logK being a function of temperature). Grindrod and Takase used the following rate of the reaction Ea
−
R* = Ae RT K þ S2 where A = coefficient, Ea = activation energy for S/I conversion, R = universal gas constant, T = absolute temperature, K+ = potassium concentration in the porewater, and S = specific surface area for reaction. We will see that it gives results that are in agreement with observations. The problem with the Pytte/Reynold and Grindrod/Takase and similar models is that the major rate-controlling variable, the activation energy for conversion of smectite to illite, is not known with certainty. For an activation energy of around 20 kcal/mol the models would imply very significant conversion to illite in hundred years at 100 °C. However, the fact that 25% of the smectite remains in the Kinnekulle bentonite after heating to more than 100 °C for several hundred years indicates that the activation energy is higher than this
Fig. 4. Transmission electron micrographs of ultrathin sections of Burgsvik clay. Upper: Acrylate-impregnated specimen with structural features similar to those in smectiterich clay. Lower: Specimen with original water content examined in 1.5 MV electron micrograph showing a similar pattern and also the presence of small isometric bodies distributed in the matrix of thin lamellae (Pusch, 1968).
Fig. 5. Diagram showing the rate of conversion of smectite to illite for the activation energy 27 kcal/mol according to Pytte/Reynold's model.
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figure. The complexity of the conversion process, exemplified by the difficulty in distinguishing between smectite and illite particles and by the impact of clay density (Gueven, 1990) and contemporary formation of other silicates (Marty et al., 2010) makes prediction of the illitization rate and hence also of the rate of the associated precipitation of silicious compounds uncertain. 2.4. Identification of cementing silicious compounds in nature Eqs. (1) and (4) indicate that silica is a reaction product of the conversion of smectite to illite and of the nature illustrated by Fig. 3. The precipitates are often very small as in the Burgsvik clay; they have the form of dense silica nodules centered in clay particle aggregates as demonstrated by the micrographs in Fig. 4. It is reasonable to assume that the physical size of the precipitations in fact ranges between a few tens of nanometers to micrometers. The following methods have been used for identifying possible cementation of smectite by comparing untreated and hydrothermally treated materials (Pusch and Yong, 2006): 1. Change in quartz and cristobalite content by XRD, 2. Examination of STEM and EDX electron micrographs for direct identification and analysis of precipitates, 3. Determination of swelling pressure, which drops by cementation, 4. Determination of shear strain and creep, which are both reduced by cementation. While XRD and electron microscopy are valuable tools for comparison of virgin and hydrothermally treated clay the swelling pressure and shear strain and creep are the most practically useful parameters in investigation of natural analogues. Examples are offered by the Busachi, Kinnekulle, and Burgsvik cases for which shear box testing have given relationships between time and strain (Pusch and Yong, 2006). The Tertiary Busachi bentonite from Sardinia, which is estimated to have been exposed to a temperature of 500 °C for 5 days, 200 °C for 2 months, and 60 °C for 1 year by impact of magma outflow (Pusch et al., 1993), gave the jerky strain pattern in Fig. 6. This behaviour was noticed for all the samples within 2 m distance from the samples from the hot contact. The Kinnekulle bentonite show the same jerky strain leading to creep failure as the Busachi clay. The well manifested silica cementation of the Kinnekulle clay is the reason for the stepped strain at the highest shear stress leading to creep failure as for the Busachi clay (Fig. 7). The Burgsvik clay with no smectite left is more strongly cemented than the other two reference clays. Thus, the creep test illustrated in Fig. 8 shows that almost no strain occurred for shear stresses up to about 80 kPa, while 95 kPa shear stress under 300 kPa normal stress initiated significant strain (Pusch, 2002).
Fig. 6. Creep behaviour of smectite-rich bentonite from Busachi. Curve for 222 kPa shear in linear time scale showing stepped strain rate behaviour related to breakage of cementation bonds (Pusch et al., 1987).
Fig. 7. Creep behaviour of Kinnekulle bentonite. The stepped strain for 95 kPa shear stress under 300 kPa normal stress is concluded to indicate breakage of larger cemented aggregates.
2.5. Precipitation of silicious compounds in hydrothermal experiments 2.5.1. Autoclave tests The indications of silicification and conversion of smectites provided by natural analogues have called for validation of the assumed hydrothermally induced process by laboratory investigation of smectite clay buffer exposed to hydrothermal conditions in repositories. Such tests have therefore been conducted in Sweden and France and they support the hypothesis of precipitation of cementing agents in the cooling phase that follows the several thousand years long hot period. A few of these tests will be referred to here for illustrating the changes in microstructural organization and mineral composition as well as their practical significance. 2.5.1.1. First series — chemically closed conditions. In the eighties a series of autoclave tests were made with purified SWY-1 smectite clay from Crook County (Newcastle Formation), Wyoming, USA, with nearly 100% montmorillonite with the following chemical composition in percent units: SiO2:69.2, Al2O3:19.6, Fe2O3:3.35, FeO:0.32, MnO:0.006, MgO:3.05, CaO:1.68, Na2O:1.53, K2O:0.53, P2O5:0.049, S:0.05, F:0.111 (Pusch, 1993). The 4.6 ml hydrothermal cells, containing nearly completely water saturated samples of clay, were goldcoated copper tubes that could resist an internal heat-generated pressure of about 100 MPa by slight yielding. Samples that had undergone heating up to 200 °C had densities of 2000 kg/m3 and 1300 kg/m3 at water saturation giving the rates of compression
Fig. 8. Creep behaviour of Silurian bentonite from Gotland. Almost no strain occurred for shear stresses up to about 80 kPa, while 95 kPa shear stress under 300 kPa normal stress gave sudden, accelerating strain leading to brittle failure.
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Fig. 9. Unconfined compression test of SWY-1 clay heated for 0.5 years. Dry density 1590 kg/m3 corresponding to 2000 kg/m3 after cooling (Pusch and Karnland, 1988).
shown in Figs. 9 and 10. The significant drop in compressibility by hydrothermal treatment is very obvious and a measure of the increased stiffness and strength. XRD analysis of the materials treated with ethylene glycol gave a stronger and sharper 2Θ = 5o reflection for 200 °C treatment than for 150 °C, and, in turn, stronger than for 20 °C specimens (Fig. 11). The increased intensity of the 001 reflections for higher temperatures indicates improved orientation of the stacks of lamellae. The same tendency has been observed in various tests of hydrothermally treated clays and indicates creation of dense stacked aggregates that can have an effect on the deformability and strength of the microstructural network. The temperature 200 °C is higher than implied by some repository concepts but has been considered by organizations like ANDRA. 2.5.1.2. Second series — chemically closed conditions. Experiments with autoclaved SWY-1 smectite clay was made for identifying processes involving Fe, which is present in octahedral positions in the purified clay and can be present in buffer clay also in the form of accessory minerals. Hydrothermal treatment in autoclaves of this very pure montmorillonite gel with a density of 1300 kg/m3 at saturation with distilled water has been conducted at the Geological Dept. of Texas Tech University at Lubbock for investigating physico/chemical effects (Gueven and Huang, 1990). The work was intended to test the
Fig. 10. Unconfined compression test of SWY-1 clay heated for 0.5 years. Dry density 480 kg/m3 corresponding to 1300 kg/m3 after water saturation (Pusch and Karnland, 1988).
Fig. 11. Rectified XRD of EG-treated SWY-1 samples representing unheated clay, and clay autoclaved at 150 and 200 °C, respectively. The samples were saturated with distilled water before being autoclaved (Pusch and Karnland, 1988).
hypothesis that heating of Na-montmorillonite to more than 100 °C causes convergence of stacks of lamellae (Gueven, 1990) and inversion of the crystal lattice from trans- to cis-coordination of Si, i.e. from the conventionally assumed crystal atom arrangement with oxygens in hexagonal patterns exposed on the basal planes of the smectite lamellae to one with equally spaced hydroxyls (Pusch and Yong, 2006). This study, which has not been fully evaluated until recently, shows that such conversion could explain the relative ease with which Si is released from the lattice tetrahedrons (cf. Lee and Kang, 2010), yielding an increase in precipitated silica on subsequent cooling. Fig. 12 shows a transmission electron micrograph (JEOL 100 STEM microscope) of untreated SWY-1 clay exhibiting a soft montmorillonite gel with typical elemental composition in all parts. Autoclave treatment of the SWY-1 clay saturated with distilled water was made for 8 h at 150 °C and 315 °C under 20 MPa water pressure. STEM analysis of the gel that had been heated to 150 °C showed the presence of larger aggregated bodies, sized up to 2 μm, as exemplified by Fig. 13. As demonstrated by the element analysis over the entire surface area of individual aggregates, they consisted of precipitated silica with some very minor amount of Fe. The Al peak represents montmorillonite lamellae underlying the aggregate. By increasing the penetration power of the electron radiation the interior of the dense aggregates could be investigated and it turned out to be very rich in iron (Fig. 14). The 150 °C treatment tended to cause some coagulation of the montmorillonite particle system leading to alignment of the stacks of lamellae that is indicated by the change in XRD peak shape as described in the preceding text. This process was even stronger in the sample autoclaved at 315 °C. These temperature ranges are not presently foreseen for buffers but may be reconsidered and the observations are therefore still of interest. It is important to realize that the autoclave experiments involved heating of fully water saturated clay under isothermal conditions, i.e. without any thermal gradient. This condition is different from that in a repository. 2.5.1.3. Third series — hydrothermal tests of MX-80 with and without gamma radiation. Cylindrical 7 cm long MX-80 samples with a dry density of 1650 kg/m3 were confined in cells with an iron plate at the heated end and a water saturated filter at the opposite one (Pusch et al., 1993). Weakly brackish water with Na as dominant cation and very little potassium (b10 ppm) was circulated through the filter that was kept at 90 °C. The iron plate was heated to 130 °C during the
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Fig. 12. Untreated very pure montmorillonite. Left: STEM picture of untreated clay. Right: Elements of the gel with Si, Al and O as major lattice constituents. Cu and Pt originate from the grids. The Mg and Fe contents were judged to be largely intracrystalline.
1 year experiments, yielding a thermal gradient of about 6 °C/cm. The solution was pressurized to 1.5 MPa. In one of the tests a gamma radiation dose of about 3E7 Gy acted on the iron plate, the adsorbed radiation dose being 3972 Gy/h at the hot plate contact, around 700 Gy/h at half length of the sample, and 456 Gy/h at the coldest end. The investigation of the samples comprised XRD analysis (Fig. 15), electron microscopy with EDX, chemical analysis, infrared spectrometry IR, and CEC determination, the main mineral data being collected in Table 1. The analyses showed that there was nearly no difference between the sample exposed to γ radiation and the one that was not irradiated except that Fe migrated from the iron plate into the clay somewhat quicker under radiation. Comparison with virgin MX-80 clay showed that hydrothermal treatment with and without radiation gave insignificant chemical changes, which was also supported by CEC data. They showed that untreated MX-80 had CEC = 99 meq/100 g while the most strongly heated and radiated clay had CEC = 93 meq/ 100 g. However, creep testing at room temperature of samples from various distances from the hottest end gave witness of significant stiffening (Fig. 16), (Pusch et al., 1993). Thus, the shear strain of samples exposed to 130 °C was about 3 times smaller than of the one heated to 90 °C.
3. Evolution of buffers 3.1. Macrostructural processes Buffer clay is manufactured by compaction of clay granules under 100–200 MPa pressure leaving only small voids between them. The voids have the form of channels of varying aperture. At wetting the tightest parts become completely filled with capillary water and water films sorbed on the basal surfaces of the stacks while wider ones still contain air unless the piezometric pressure becomes sufficiently high to dissolve it. In the initial “isothermal” state when no water has yet been taken up by the buffer from the rock, drying of the hot part of the buffer takes place causing migration of water in liquid and vapour forms towards the rock. In the hottest part the stacks of lamellae contract, by which the individual voids and the channels formed by them become wider. Full-scale tests have demonstrated that steep, radial fractures are formed to several centimetres distance from the hot canister (Pusch et al. 1985). Once this part has ultimately become water saturated the fractures and widened channels will be closed by the swelling pressure exerted by the colder part of the buffer, and by self-sealing through redistribution of clay material if the heated clay has not lost its expandability.
Fig. 13. STEM picture of large and dense silica particle in montmorillonite sample hydrothermally treated at 150 °C.
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Fig. 14. Dense Fe nuclei in Si-rich aggregates recorded under low electron penetration power (lower micrograph) and high penetration power. Oxygen is abundant according to the element analysis, indicating that the nucleus is iron hydroxide.
3.2. Microstructural processes Smectite clay consists of stacks of 10 Å lamellae separated by up to 3 water hydrates depending on the bulk density and adsorbed cations (Pusch and Yong, 2006). The assembly of stacks is coherent for bulk densities higher than about 1050 kg/m3 if the porewater is poor in electrolytes, but must exceed about 1600 kg/m3 if the porewater is saltier than strongly brackish with calcium as dominant cation. Consolidation, implying an increase from lower to higher density,
takes place on compression while expansion takes place on unloading. Unlike non-expandable clay types compression/expansion is a nearly reversible process for smectite, except that expansion is delayed by internal friction. Voids in the buffer caused by imposed strain as in Fig. 2, or by heat-induced desiccation early after installation of the canisters, would have to be closed or sealed for acceptable performance. Closing of larger voids by expansion of the buffer is expected while sealing of small voids and fissures can take place by precipitation of silica or iron compounds. Expansion involves
Fig. 15. Schematic diffractograms of the reference MX-80 sample (20 °C) and the most heated part of the hydrothermally tested sample (130 °C). Feldspars, amphibole, some of the quartz and smectite disappeared in the hot part (Gueven and Huang, 1990).
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Table 1 Changes in one year long hydrothermal tests of MX-80. M = Montmorillonite, F = Feldspars, G = Gypsum, Q = Quartz, K = Kaolinite, Chl = Chlorite, I = Illite. +++ means strong increase, ++ significant increase, + slight increase, −−− strong loss, −− significant loss, − slight loss. 0 means no change. Treatment
125–130 °C
115–120 °C
105–110 °C
90–95 °C
Hydrothermal without radiation
M− F −−− Chl+ G ++ K ++ Q+ I+
M− F −− Chl + G +++ K+ Q+ I0
M0 F− Chl 0 G+ K0 Q0 I0
M0 F− Chl 0 G+ K− Q0 I0
exfoliation of clay particles from the dense buffer followed by reorganization of the released particles to form clay gels that become consolidated under the swelling pressure exerted by adjacent, denser clay (Pusch and Yong, 2006). Two mechanisms can cause loss of expandability: strong draught causing permanent collapse of the stacks of lamellae, and precipitation of cementing agents. Combination of the two can make the collapse permanent. It will be shown here how dissolution of smectite and accessory minerals and precipitation of cementing agents like in nature can affect the performance of buffer clay. 3.3. Cementation of the buffer 3.3.1. Processes Temporary accumulation of precipitated matter from the groundwater in the hottest part of the buffer early after installation of the buffer and canisters, i.e. formation of anhydrite and chlorides, is well known from field and mock-up tests (Pusch, 2008) but the process is expected to be reversible and of little concern in a long time perspective. In contrast, irreversible reactions caused by dissolution of clay and accessory minerals and precipitation of the solutes are processes that can cause permanent stiffening and reduced expandability. They are in focus in this paper. Application of the chemical model of Grindrod/Takase to KBS-3 repository conditions (SKB's KBS-3V concept) requires coupling with a transport model based on diffusion-dominated migration of aqueous species like silica, aluminium, sodium, magnesium, and potassium to form a set of quasi-nonlinear partial differential equations. The ones valid for aqueous species and minerals were used for calculating precipitation of silica in the direction of the thermal gradient by these investigators. It was found that almost no illite will be formed in the first 500 years despite the conservative assumption of a constant temperature of 90 °C at the buffer/canister
Fig. 16. Shear strain of specimens hydrothermally treated for 1 year without radiation. Shear box testing under 6 MPa normal effective pressure (Pusch et al., 1993).
contact, and 50 °C at the buffer/rock contact. However, for the subsequent period of time with a linear temperature drop with time to 25 °C after 10,000 years, the model predicts precipitation of quartz in the coldest part of the buffer, i.e. within about 0.1 m from the rock wall as shown in Fig. 17. The Grindrod/Takase model appears to adequately describe the chemical evolution with respect both to illitization and silicification. Quartz precipitates in the buffer and in open fractures in the walls of the deposition holes, which tends to seal them off (Pusch and Yong, 2006). 3.3.1.1. The Äspö Retrieval Test. Carefully conducted and interpreted large-scale experiments simulating the conditions in a KBS-3V repository have indicated the major chemical processes in buffer of smectite clay. Wyoming bentonite (MX-80) with 75–85% montmorillonite and Na as dominant adsorbed cation, and quartz (8–15%), feldspars (2–5%) and chlorite and micas (2–5%) as accessory minerals has been used in a number of laboratory and field experiments. Similar clays from Spain, France, Switzerland and the Czech Republic have been investigated in the same type of tests in the respective countries. A few of them have been conducted for determining physico-chemical and mineralogical changes that take place under repository-like conditions. MX-80 has been investigated in SKB's simulated KBS-3V repository at Äspö, some 300 km south of Stockholm, samples being taken from the buffer in a 5 year full-scale test called Äspö Retrieval Test (Eng 2008). The buffer was artificially wetted by being surrounded by a filter that was saturated with salt water (6000 ppm with Na/Ca ratio 1.0). The buffer in contact with the hot canister had been exposed to 78–85 °C for a couple of years and then to less than 60 °C because of malfunctioning electrical heaters. This part of the clay had a density of 1970 kg/m3 (1540 kg/m3 dry density). Geotechnical tests (Table 2) and extensive EDX and XRD analyses led to the following conclusions: • The hydrothermally treated clay had undergone substitution of Al3+ by Fe3+ in the octahedral layer causing higher lattice stresses because of the larger ion radius of Fe3+ than of Al3+ thereby reducing the resistance of the montmorillonite to dissolution, • Montmorillonite was the dominating mineral phase in both virgin and hydrothermally treated clay, the total number of samples of each type being at least 3, • The lower interlayer charge of the heated clay than of virgin MX-80 would mean that it should have a higher swelling pressure, but cementation dominated and gave the opposite effect.
Fig. 17. Evolution of quartz abundance profile (~ 10,000 y). Precipitation leading to cementation takes place in the outer, colder, 0.1 m part of the buffer (Pusch and Yong, 2006).
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was maintained at 95 °C causing a radial temperature gradient of about 2 °C/cm. As shown by Table 2 testing of samples taken at different distances from the canister showed obvious differences. One finds that the swelling pressure of the sample that had been exposed to 54–56 °C and representing the least heat-affected clay was more than twice the pressure of the more strongly heated sample. Additional samples taken in radial direction from the hot canister had physical properties intermediate to those of the most and least heated ones. The differences in mineral composition between the untreated buffer clay and the hydrothermally treated samples can be explained by 1) partial dissolution of the montmorillonite, 2) obvious formation of illite, and 3) neoformation of talc and kaolinite (cf. Fig. 19). In summary, the major mineralogical processes in the most heated smectite-rich clay mixed with graphite and quartz particles were:
Fig. 18. Main features of the Czech Mock-up test. M1, M2 etc. are samples taken in conjunction with or after the experiment.
Table 2 Geotechnical data of oedometer-tested samples at saturation with distilled water. The data ρsat = density at water saturation, and hydraulic conductivity = K, represent equilibrium reached after about 40 days (Pusch, 2008). Distance from heater, cm
T, °C
ρsat, kg/m3
ps, kPa
12–14 0–2
54–56 85–95
1945 1925
650 310
3.3.1.2. The Czech mock-up test. A 3 year mock-up experiment simulating the conditions in a KBS-3V hole was conducted for the Czech organization RAWRA in the years 2001 to 2004 (Pacovsky et al., 2005). The rock was simulated by a steel tube with an inner diameter of 800 mm and the canister by a heated steel cylinder with 360 mm diameter (Fig. 18). The buffer consisted of highly compacted blocks of 85% montmorillonite mixed with 10% finely ground quartz and 5% well crystallized graphite, the dry density being 1800 kg/m3. Granules of the same material were filled in the 50 mm wide space between the steel tube and the buffer, which had unlimited access to Nadominated brackish groundwater through a surrounding filter (Pusch, 2008). The temperature at mid-height of the canister surface
• Intergrowth of illite and kaolinite was obvious in the dioctahedral vermiculite in the untreated clay. In these aggregates illite dominated over kaolinite but in the heated sample most of the intergrowths of illite/kaolinite were dissolved, • Fe set free by the dissolution of the montmorillonite can have formed iron complexes resulting in cementation in the entire buffer mass, especially in the most heated part, • The quartz content, which was determined by XRD and numerous EDX analyses, increased in the direction towards the canister and is judged to be the main cementing agent, • Replacement of octahedral Al by Fe can have caused a drop in coherence of the montmorillonite crystals, thereby promoting easier dissolution, • Formation of illite in the hottest part of the buffer (sample M4) may have been triggered by uptake of K from dissolved vermiculite. Mineralogical characterization of the natural clay component was made by use of TEM with EDX and Coherent Scattering Domain (CSD) data for determining the thickness of particles or stacks of lamellae. The Koester diagrams in Fig. 20 show the charge distribution in the various layers in the natural clay component and in the most heated sample M4. The figure shows that very obvious changes in charge distribution took place in the heated sample. Significant dissolution of the Fe-rich montmorillonite and complete disappearance of the intergrowth of illite and kaolinite had taken place, suggesting migration and precipitation of released elements at different distances from the heater. The significantly lower swelling pressure of the heated sample can be explained by the formation of illite in combination with cementation by precipitated silicious compounds. The associated
Fig. 19. XRD spectra of the samples M2 to M4. Oriented mounts, air dried and ethylene glycol-treated (SEIFERT, Co Kα-radiation) with peak-area distribution by means of WinFitpeak fitting, (Pacovsky et al., 2005) (After Jörn Kasbohm, Greifswald University, Germany).
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Fig. 21. Sediment volume of 11 g of clay in 30 ml distilled water. Left: heated clay. Right: specimen from least heated buffer (45–47 °C).
Fig. 20. Distribution of charge in tetrahedral, octahedral and interlayers in unheated clay (RMN bent) and in heated clay (M4). ML means mixed-layer minerals. The figures represent percentages (Pusch and Yong, 2006).
loss in expandability is illustrated in Fig. 21, which shows the appearance of 11 g of a specimen from the most heated part of the buffer (M4) and of a specimen from untreated buffer submerged in 30 ml distilled water. Visual examination has been made of buffer clay samples from the least heated (45–47 °C) and the most heated part after extraction from the oedometers in which they were saturated with distilled water and tested with respect to swelling pressure and hydraulic conductivity. The least heated clay was very homogeneous and ductile while the most strongly heated clay contained fissures and broke up in millimeter-sized fragments at sectioning. 4. Discussion and conclusions The experiments that included rheological measurement showed that stiffening of smectite clay takes place under isothermal conditions
as well as when thermal gradients prevail. Three of them, representing repository-like conditions with the buffer heated under chemically open conditions, involved exposure of the clay to a relatively high temperature gradient (2 °C/cm to 6 °C/cm) and a maximum temperature of 85–130 °C, while the other, having the form of autoclave testing under isothermal and chemically closed conditions exposed the clay to 90–200 °C. All the tests included chemical and mineralogical analyses and showed that silicious compounds, primarily quartz and cristobalite, had been precipitated, suggesting that they caused the stiffening and strengthening. The analysis of the autoclaved samples that had been isothermally tested at 150–315 °C tests also showed formation of quartz nodules and also indicated that they frequently had nuclei of Fe. The most comprehensive and detailed investigations of clay heated under a thermal gradient, i.e. the study of the Czech buffer clay, showed that Fe was released from the montmorillonite component and precipitated at cooling. The precipitation of quartz in the bentonite at Kinnekulle had taken place where the temperature gradient had been only 0.02 °C/cm and the temperature 140–160 °C, and in the Burgsvik clay at lower temperature but under very long time. The laboratory and mock-up tests of hydrothermally treated smectite clay as well as natural analogues represented by bentonite layers intersected or contacted by hot diabase dikes affected by hot magma show that cementation by silicious compounds has taken place. The involved processes, dissolution and precipitation of smectite and accessory minerals, occurred in a relatively short period of time while the geochemical model suggests that their comprehension is very limited in the first few tens and hundreds of years at repository temperatures below 100 °C. However, according to the Grindrod/Takase model, the conditions will be more critical after a few thousand years in the buffer of the SKB concept since the formation of cementing agents, as predicted by the model, will then become significant and affect the physical properties of the buffer to a practically important extent. The buffer clay will be stiffer and stronger as illustrated by the Kinnekulle analogue, and lose part of its self-sealing potential. After 10,000 years the cementation of the outer, colder part of the buffer is expected to make it brittle and sensitive to mechanical impact such as shearing by seismic, tectonic or thermomechanical impact. The more comprehensive cementation is in agreement with the high strength and the brittleness of the Burgsvik clay. It is important to realize that the cementation caused by precipitation of silicious and iron components in the experiments took place at the cooling that abruptly halted the heat-generated chemical processes. It remains unclear whether the stiffening was solely due precipitation of cementing agents or if microstructural changes in the form of contraction in combination with cementation caused it. References Eng A., 2008. Äspö Hard Rock Laboratory, Canister Retrieval Test, Retrieval phase. Project report IPR-08-13. SKB, Stockholm.
R. Pusch et al. / Engineering Geology 116 (2010) 21–31 Grindrod, P., Takase, H., 1994. Reactive chemical transport within engineered barriers. Proc. 4th Int. Conf. on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, Charleston, SC USA, 12–17 Dec. Oldenburg Verlag, pp. 773–779. Gueven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Engineering Geology. 28, 233–247. Gueven, N., Huang, W.-L., 1990. Effects of Mg2+ and Fe3+ substitutions on the crystallization of discrete illite and illite/smectite mixed layers. Int. rep. Dept. Geosciences Texas Tech University, Exxon Production research Co, Houston, Texas, USA. Lee, J.O., Kang, I.M., 2010. Smectite alteration and its influence on the barrier properties of smectite clay for a repository. Applied Clay Science 47 (1–2), 99–104. Marty, N.C.M., Fritz, B., Clément, A., Michau, N., 2010. Modelling the long term alteration of the engineered bentonite barrier in an underground radioactive waste repository. Applied Clay Science 47 (1–2), 82–90. Mueller von-Moos, M., Kahr, G., Bucher, F., Madsen, F.T., 1990. Investigations of the metabentonites aimed at assessing the long-term stability of bentonites under repository conditions. In: Pusch, R. (Ed.), Artificial Clay Barriers for Hih Level Radioactive Waste Repositories: Engineering Geology, Vol.28, Nos.3–4, pp. 269–280. Special Issue. Pacovsky, J., Svoboda, J., Zapletal, L., 2005. Saturation development in the bentonite barrier of the Mock-up CZ geotechnical experiment. Clay in Natural and Engineered Barriers for Radioactive Waste Confinement — Part 2: Physics and Chemistry of the Earth, 32/8-14. Elsevier Publ. Co, pp. 767–779. Pusch, R., 1968. A technique for investigation of clay microstructure. Journal de Microscopie 6, 963–986. Pusch, R., 1983. Stability of deep-sited smectite minerals in crystalline rock — chemical aspects. SKB, Stockholm. SKB Technical Report TR-83-16. Pusch, R., 1993. Evolution of models for conversion of smectite to non-expandable minerals. SKB, Stockholm. Technical Report TR-93-33. Pusch, R., 2002. The Buffer and Backfill Handbook. Part 2, Materials and Techniques. SKB, Stockholm. SKB Technical Report TR-02-12.
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Pusch, R., 2008. Geological Storage of Radioactive Waste. Springer Verlag978-3-54077332-0. Pusch, R., Karnland, O., 1988. Hydrothermal effects on montmorillonite. A preliminary study. SKB, Stockholm. SKB Technical Report TR-88-15. Pusch, R., Madsen, F., 1995. Aspects on the illitization of the Kinnekulle bentonites. Clays and Clay Minerals 43 (3), 261–270. Pusch, R., Yong, R.N., 2006. Microstructure of smectite clays and engineering performance. Taylor & Francis, London and New York. ISBN10: 0-415-36863-4. Pusch, R., Nilsson, J., Ramqvist, G., 1985. Final Report of the Buffer Mass Test — Volume I: Scope, preparative field work, and test arrangement. SKB, Stockholm. Technical Report TR-85-211. Pusch, R., Börgesson, L., Erlström, 1987. Alteration of isolating properties of dense smectite clay in repository environment as exemplified by seven pre-Quaternary clays. SKB, Stockholm. Technical Report TR-87-29. Pusch, R., Karnland, O., Lajudie, A., Decarreau, A., 1993. MX-80 Exposed to High Temperatures and Gamma Radiation. SKB, Stockholm. SKB Technical Report TR-93-03. Pytte, A.M., Reynolds, R.C., 1989. The thermal transformation of smectite to illite. In: Naeser, N.D., McCulloh, T.H. (Eds.), Thermal History of Sedimentary Basins. Springer-Verlag, New York, pp. 133–140. Savage, D., Benbow, S., Watson, C., Takase, H., Ono, K., Oda, C., Honda, A., 2010. Natural systems for the alteration of clay under alkaline conditions: an example from Searles Lake, California. Applied Clay Science 47 (1–2), 72–81. Svemar, C., 2005. Cluster Repository Project (CROP). Final Report of European Commission Contract FIR1-CT-2000–20023, Brussels, Belgium. Thorslund, P., 1945. Om bentonitlager i Sveriges kambrosilur. Geologiska Föreningens I Stockholm Förhandlingar 67, 286. Velde, B., 1991. Kinetics of I/S (illite/smectite) transformations. MRS. (Resumé NoB-II, Invited paper). Weaver, C.E., 1979. Geothermal alteration of clay minerals and shales: diagenesis. Batelle-Office on Nuclear Waste Isolation. Technical Report ONWI-21, ET-76-C061830 Contract.