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In the Chinese high-level radioactive waste geological disposal program, Gaomiaozi (GMZ) bentonite has been selected as the potential buffer/backfill material. After the closure of the repository, the Ca-OH-type alkaline solution (evolved cement water) released by cement degradation may last for more than 100,000 years. The bentonite will undergo the corrosion of evolved cement water (ECW) for a long period. This work focuses on the sorption property of GMZ bentonite altered by ECW. Firstly, the corrosion experiments on compacted GMZ specimens with the dry density of 1.70 Mg/m 3 were carried out under constant volume conditions at two temperatures. Then, the sorption of europium (Eu (III)) onto the corroded GMZ bentonite was studied by batch experiments. The results of batch sorption tests indicate that the altered GMZ bentonite keeps an effective removal property with the uptake of Eu (III) more than 99%. The effect of high-temperature conditions of the repository on the sorption property of bentonite is not significant. The results also suggest that the evolved cement water presents no detrimental effect on the long-term adsorption performance of bentonite even under higher temperature conditions.

In this study, a series of suction-control oedometer tests were conducted on unsaturated and saturated MX80 bentonite pellet/powder mixtures. The mixture samples were imposed with various suctions from 0 to 113 MPa under a low vertical stress (0.1 MPa). Then, the mixture samples were loaded in steps with controlled suction until reaching three different target stresses σv (0.1, 3.2, and 12.8 MPa) and instantaneously unloaded. The microstructure of unloaded samples was observed with mercury intrusion porosimetry (MIP) and microcomputed tomography (μCT), together with the determination of suction, water content, void ratio, and saturation degree. Results showed that the water-retention behavior of bentonite pellet/powder mixture was rather characterized by pellets instead of powder. Due to the stress release, the measured suction after unloading sme was larger than the imposed suction sim. The suction difference Δs between sme and sim depended on both suction and load. Under a given σv , the suction effect was characterized by an increase of Δs when sim decreased from 113 to 4.2 MPa, but by a noticeable decrease of Δs when sim further decreased to zero. This was found to be related to the changes of the slopes of soil-water retention curves (SWRCs): the slope was decreasing with suction increases after 4.2 MPa suction, but was increasing with suction increases before 4.2 MPa suction. As far as the stress effect was concerned, a larger Δs was identified at a higher σv for a given sim. The narrowing difference between Δs and the reduction of mean stress of pellet Δppellet with increasing load was because of the decrease of larger pores due to loading. The total mixture void ratio eTmixture was highly dependent on suction and vertical stress, whereas the total pellet void ratio eTpellet was only dependent on suction in the stress range considered. [ABSTRACT FROM AUTHOR]

A triple‐microstructure hydro‐mechanical constitutive damage model was proposed to describe the hydro‐mechanical behaviour of MX80 bentonite pellet/powder mixture, based on the results from a series of suction‐controlled oedometer tests and microstructure observations. Emphasis was put on the pellet damage behaviour. The model parameters were determined essentially based on these results. The model response was firstly verified with the obtained oedometer tests. Then, the results of Molinero‐Guerra et al. and Darde who carried out suction‐controlled oedometer tests and swelling pressure tests on similar bentonite pellet/powder mixture were employed to validate the proposed model. It appeared that the global volume behaviour and the development of swelling pressure with suction of the bentonite mixture could be well reproduced. The model allowed gaining insights into the microstructural evolutions under oedometer loading. The global compression behaviour was governed by the filling of inter‐grain pores at unsaturated state, but by the compression of grains themselves at saturated state. Conversely, when wetted under constant‐volume condition, inter‐grain pores and intra‐grain macro‐pores were closed, by the swelling of intra‐grain micro‐pores. Examination of the water‐retention property showed that water saturated the intra‐grain micro‐pores at suction 19 MPa, and then started filling intra‐grain macro‐pores. When inter‐grain pores and intra‐grain macro‐pores were all closed at suction 0.1 MPa, water finally came back to intra‐grain micro‐pores. On the whole, the hydro‐mechanical behaviour of the bentonite pellet/powder mixture can be well described by the proposed model. [ABSTRACT FROM AUTHOR]

A viscoplastic constitutive model is developed to describe the viscoplastic behavior of unsaturated soils. The proposed model accounts for the stain‐rate and suction effects on yield by adopting an unsaturated isotach concept. The nonstationary flow surface theory (NSFS) is applied for modeling the viscoplastic behavior, with the yield surface which can evolve with the viscoplastic strain, viscoplastic strain rate and suction. Meanwhile, the progressively hardening concept is adopted for reproducing the viscoplastic behavior of soil at an over‐consolidated state. For the validation, a series of loading conditions are considered based on the data from the literature. Results show that the proposed model is able to reproduce the main viscoplastic behaviors of unsaturated highly compacted Gaomiaozi (GMZ) bentonite, Glenroy silt and Qianjiangping landslide (QL) soil, including CRS compression tests, rate‐dependent triaxial shear tests, and triaxial creep tests. [ABSTRACT FROM AUTHOR]

The pore size distribution (PSD) plays an important role in the hydro-mechanical behaviour of porous materials. In this study, the PSD of MX80 bentonite pellets under various imposed suctions from 262 to 1.5 MPa was determined by both mercury intrusion porosimetry (MIP) and micro-computed tomography (μCT). The void ratios e18–344 in the overlapping pore range (18–344 μm) identified by MIP and μCT were compared to clarify the difference between the two techniques. Results showed that the MIP-detection e18–344 nearly overlapped with the μCT-detection one at suction s ≥ 57 MPa, indicating the reliability of MIP and μCT results for pellets at s ≥ 57 MPa. However, the μCT-detection e18–344 became larger than the MIP-detection one at 9 MPa ≤ s ≤ 38 MPa, while the changes of MIP-detection and μCT-detection e18–344 with suctions were opposite at s ≤ 4.2 MPa. As the difficulty of interpreting the variation in μCT attenuation coefficient at low suctions undermined the reliability of μCT thresholding, the segmentation of μCT images was suggested to be determined by making the μCT-detection e18–344 equal to the MIP-detection one. Because the reliability of MIP results at s ≤ 4.2 MPa was reduced by the difficulty of keeping the integrity of freeze-dried samples, the MIP-detection e18–344 determined in a previous study without the sample integrity problem was chosen as reference for the μCT thresholding at 1.5 MPa ≤ s ≤ 4.2 MPa. The corrected μCT-detection e18–344 calculated by the new thresholds matched well with the MIP-detection e18–344, completing the μCT-detection at suctions smaller than 57 MPa. Since the MIP-detection PSD only covers the pores smaller than 344 µm, the μCT-detection PSD with pores larger than 344 µm was then integrated into the MIP-detection PSD, enlarging the MIP-detection to millimetric pores. [ABSTRACT FROM AUTHOR]

In the long-term operation of a deep geological repository for disposal of high-level radioactive wastes, bentonite material will inevitably undergo cyclic salinization–desalinization processes, which exert significant effects on performance of the engineered barrier. In this study, cyclic salinization–desalinization tests were performed on compacted bentonite specimens at a dry density of 1.7 Mg/m3 under vertical stresses of 0.15, 0.20, 0.20–0.26, 0.34 and 0.34–0.47 MPa. Meanwhile, MIP tests were conducted on an as-compacted specimen and the specimens experienced hydrations with de-ionized water (DW) and 2.0 M NaCl solution, as well as cyclic salinization–desalinization (2.0 M NaCl–DW) processes, respectively. It was found that the specimen volume change is governed by both of the salinization–desalinization processes and the vertical stress. The accumulative strain measured over cyclic salinization–desalinization processes for specimens tested under vertical stresses trends to be zero, suggesting that there exists an equilibrium state at which the shrinkage strain induced by the salinization is equal to the swelling strain generated by the subsequent desalinization. Based on these, a swelling equilibrium line and a shrinkage equilibrium line were proposed. Meanwhile, a swelling zone and a shrinkage zone were divided with a critical separating vertical stress of 0.47 MPa. In the swelling zone (< 0.47 MPa), the specimen exhibited prevailing swelling behavior, while in the shrinkage zone (0.47–12.82 MPa), the specimen was prevailed by shrinkage behavior. Finally, these phenomena were interpreted from the microstructural point of view according to the MIP test results. [ABSTRACT FROM AUTHOR]

The swelling behaviour of expansive clays is strongly related to the interaction between clay particles. In this paper, a series of constant-volume swelling pressure and mercury intrusion porosimetry (MIP) tests were carried out on MX80 bentonite/Callovo-Oxfordian (COx) claystone mixtures with different bentonite fractions and dry densities. Results show that the swelling pressure increased linearly with the increase in dry density. Additionally, at a given dry density, the larger the bentonite fraction, the larger the swelling pressure. From the MIP results, the inter-particle pore volume decreased with the increases of bentonite fraction and dry density. To link the macroscopic swelling behaviour with the microstructure features, a new method was proposed, allowing the determination of the average inter-particle distance from the pore size distribution. Moreover, a linear relationship was identified between the swelling pressure and the average inter-particle distance in a semi-logarithmic plane, regardless of the bentonite fraction and montmorillonite content. [ABSTRACT FROM AUTHOR]

This paper investigates the swelling mechanism of Callovo-Oxfordian (COx) claystone grain in its mixtures with bentonite which are potential candidate for engineered barriers in the French high-level radioactive waste disposal. The swelling pressures of MX80 bentonite/COx claystone mixtures with various bentonite fractions and dry densities were first experimentally determined by carrying out infiltration tests under constant-volume conditions. According to the experimental results, the claystone void ratio after hydration in the mixtures was estimated by taking into account the bentonite/claystone grain interaction. Then, the volume change of claystone grain under different pressures applied by swollen bentonite was determined. The volumetric strain of claystone grain was found to decrease linearly with the increase in pressure. This allowed the indirect determination of the swelling pressure of claystone grain. The determined value was found to be comparable to that estimated from the relationship between the void ratio and the swelling pressure in the direction perpendicular to the stratification of intact COx claystone, confirming the identified swelling mechanism of bentonite/claystone mixtures. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a candidate sealing material in a French deep geological repository for high-level radioactive waste. An in-depth understanding of the hydraulic property of this material is essential for accessing the over-pack corrosion and nuclide migration along the gallery. In this study, water flow in the compacted bentonite/claystone mixture was investigated by carrying out infiltration and water retention tests under constant-volume conditions alongside detailed microstructure observation. The evolution of total water hydraulic conductivity with suction was experimentally determined based on the instantaneous profile method. It was observed that the measured total hydraulic conductivity increased slowly at high suctions, then quickly at low suctions and finally approached the saturated value. A new method was developed based on the evolving pore structure, accounting for both vapour and liquid-water hydraulic conductivities. The total hydraulic conductivity predicted by the proposed method agreed well with the measured value, indicating the reliability of the predictive method. The vapour hydraulic conductivity was larger than the liquid-water one at a suction larger than 15·2 MPa, while liquid-water hydraulic conductivity became dominant at a suction lower than 15·2 MPa. In addition, based on the predicted vapour and liquid-water hydraulic conductivities, a numerical model was set up to simulate the water infiltration process. Comparison between the simulation and the measurement showed close agreement, validating the two-phase water flow mechanism in the bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]

In this study, the suction induced by stress release was investigated by measuring the suction of instantaneously unloaded MX80 bentonite specimens after wetting in an oedometer either under different vertical stresses or under a constant-volume condition. Swell-consolidation tests were also performed, and the results were compared with those of specimens wetted under different vertical stresses or a constant-volume condition. The microstructure of the unloaded specimens was investigated using mercury intrusion porosimetry (MIP). The water content and void ratio as well as the degree of saturation of specimens were also determined. Results showed that in the case of limited water exchange between macropores and micropores during stress release, the change in suction was equal to that of stress (−ds/dσv=1). By contrast, with water exchange between macropores and micropores, soil swelling occurred, and the variation of suction was higher than that of stress (−ds/dσv>1). A method was proposed allowing such water exchange between macropores and micropores to be determined (−ds/dσv−modified=1 , where σv−modified is the modified stress considering water exchange), evidencing the macropore drying due to stress release. Comparison of results from MX80 bentonite and other clayey soils showed that when the nonclay (grains such as quartz, calcite, and so on) fraction (NCF) was lower than 35% by mass, NCF had an insignificant effect on −ds/dσv−modified (equal to 1). By contrast, in the case of higher NCF (>35%), −ds/dσv−modified became lower than 1, and the higher the NCF, the lower the −ds/dσv−modified value. These findings provide useful information for better understanding the mechanism involved at the microstructure level in the stress/suction coupling during stress release in clayey soils. [ABSTRACT FROM AUTHOR]

Bentonite pellet/powder mixture has been considered as a possible sealing material in deep geological disposal of radioactive waste. As the hydro-mechanical behavior of such mixture is intimately related to that of single pellet, in this study, water retention tests and compression tests were performed on single MX-80 bentonite pellet to investigate its hydro-mechanical behavior. Mercury intrusion porosimetry (MIP) tests were also conducted to investigate the microstructure change with suction. The water retention test results show that the changes in water content and void ratio are significant in low suctions (< 59 MPa) with decreasing suction, as opposed to the case in high suction range (> 59 MPa). These significant changes are accompanied by the propagation of cracks. Results from compression tests show that the contact force of pellet rapidly increases in the beginning, reaches a peak and then decreases. Meanwhile, the contact stiffness and the breaking force of pellets show different variation tendencies in different suction ranges. Results from MIP tests indicate that the volume change of pellet upon wetting is attributed to the swelling of bentonite grain, the expansion of inter-grain pores and the generation of large cracks. At the meantime, the changes of pores at micro-scale and macro-scale, especially the wetting-induced cracks, notably affect the compression properties of pellet. These findings provide useful information for better understanding the overall behavior of bentonite pellet/powder mixture. [ABSTRACT FROM AUTHOR]

Compacted bentonite/sand mixtures are often considered as sealing/backfilling materials in deep geological disposal for radioactive waste. This study investigates the swelling pressure of compacted bentonite/sand mixtures with different bentonite fractions and dry densities upon hydration under constant-volume conditions. It was found that the swelling pressure of bentonite/sand mixtures is strongly dependent on their bentonite fraction and final montmorillonite dry density. For the samples with a bentonite fraction larger than a critical value, there is a linear relationship between the swelling pressure and final montmorillonite dry density. By contrast, for the samples with a bentonite fraction lower than the critical value, the sand particles will be skeletonized after strong compaction and lead to a heterogeneous distribution of montmorillonite, resulting in a larger swelling pressure compared to the samples with higher bentonite fractions. The maximum sand skeleton void ratio, corresponding to that at which the sand skeleton is just formed, is used to estimate the critical montmorillonite dry density and the critical bentonite fraction. Comparison between the test data and the estimated results shows good agreement, indicating the relevance of the identified swelling mechanism for compacted bentonite/sand mixtures. [ABSTRACT FROM AUTHOR]

Compacted bentonite as an artificial barrier/backfill material is susceptible to heat when combined with the hyperalkaline conditions of deep geological repositories. In order to investigate the structural damage of compacted GMZ bentonite and its effect on swelling pressure deterioration, X-ray powder diffraction (XRD) and mercury intrusion porosimetry (MIP) tests were performed on bentonite specimens after immersed in hyperalkaline solution at 20–80°C. The results suggest that the increase in the number and pore sizes of macropores, and the decrease in the crystallinity degree of montmorillonite and silicate minerals are extremely obvious, which well characterises the structural damage of compacted bentonite on a microscopic scale. The structural damage parameter is facilitated by heat combined with hyperalkaline conditions. The structural damage of compacted bentonite is more susceptible to variations in the microscopic pore structure and the dissolution of silicate minerals than that of montmorillonite; and the structural damage of compacted bentonite will cause the long-term deterioration of swelling pressure. Hence, the long-term safety of HLW repository should be evaluated taking into account of the structural damage of compacted bentonite caused by heat combined with chemical conditions. [ABSTRACT FROM AUTHOR]

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a promising sealing/backfilling material in deep geological repository for high-level radioactive waste in France. When the pre-compacted blocks are emplaced in the gallery, technological voids can remain. After the infiltration of groundwater from the host rock, they will swell freely, filling the technological voids, and then undergo further hydration under constant-volume conditions. From the perspective of the storage safety, it is essential to understand the effects of technological voids and hydration time on the hydro-mechanical behaviour of such pre-compacted blocks. In this work, a series of infiltration tests at various hydration times was carried out on compacted MX80 bentonite/COx claystone mixture with different technological voids. The evolutions of the swelling pressures in axial and radial directions as well as the hydraulic conductivity were monitored while wetting. After the predetermined hydration times, the dry density, water content and suction at different positions were determined, together with the microstructure investigation using mercury intrusion porosimetry. It was observed that the soils close to initial voids swelled and filled the initial voids upon contact with water, with significant increases in large-pore and medium-pore void ratios. From the variation of dry density profile with time, compression and swelling zones could be identified: in the compression zone, the soils corresponding to the initial soils were subjected to compression, with decrease in large-pore and medium-pore void ratios over time, whereas in the swelling zone, the soils with a higher dry density than the expected final one underwent further swelling, with large-pore void ratio increasing until saturation and then a slight decrease due to water redistribution in the soil. Owing to the soil density heterogeneity, the axial swelling pressure and hydraulic conductivity of samples with voids were slightly larger than those of samples without voids; contrarily, the radial swelling pressure was lower than that of the samples without voids. [ABSTRACT FROM AUTHOR]

Bentonite pellets are recognized as good buffer/backfill materials for sealing technological voids in high-level radioactive waste (HLW) repository. Compared to that of a traditional compacted bentonite block, one of the most important particularities of this material is the initially discrete pellets and the inevitable heterogeneous porosity formed, leading to a distinctive water retention behaviour. In this paper, water retention and mercury intrusion porosimetry (MIP) tests were conducted on pellet mixture (constant volume), single pellet (free swelling) and compacted block (constant volume) of GMZ bentonite, water retention properties and pore structure evolutions of the specimens were comparatively investigated. Results show that the water retention properties of the three specimens are almost similar to each other in the high suction range (> 10 MPa), while the water retention capacity of pellet mixture is lower than those of the compacted block and single pellet in the low suction range (< 10 MPa). Based on the capillary water retention theory (the Young–Laplace equation), a new concept 'saturated void ratio' that was positively related to water content and dependent on pore size distribution of the specimen was defined. Then, according to the product of saturated void ratio and water density in saturated void, differences of water retention properties for the three specimens at low suctions were explained. Meanwhile, MIP tests indicate that as suction decreases, the micro- and macrovoid ratios of pellet mixture and compacted block decrease as the mesovoid ratio increases, while all the void ratios of single pellets increase. This could be explained that upon wetting, water is successively adsorbed into the inter-layer, inter-particle and inter-pellet voids, leading to the subdivision of particles and swelling of aggregates and pellets. Under constant volume condition, aggregates and pellets tend to swell and fill into the inter-aggregates or inter-pellets voids. While under free swelling condition, the particles and aggregates in a single pellet tend to swell outward rather than squeezing into the inter-aggregate voids, leading to the expansion of the pores and even formation of cracks. Results including the effects of initial conditions (initial dry density and fabric) and constraint conditions (constant volume or free swelling) on the water retention capacity and pore structure evolution reached in this work are of great importance in designing of engineering barrier systems for the HLW repository. [ABSTRACT FROM AUTHOR]

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed to backfill and seal the underground galleries for radioactive waste disposal in France. While emplacing these pre-compacted blocks, technological voids are created between the blocks and the host rock and among the blocks themselves. It is expected that homogenization process will take place over time for the structure constructed with pre-compacted blocks upon hydration. This study investigated the boundary friction effect on such a process. Results showed that after the filling of technological voids, the soil far from the technological voids would swell further, while those near the voids would be compressed under the welling pressure generated by the soil behind, resulting in an increase in homogeneity in terms of dry density distribution. However, this homogenization process would stop after a certain time. Further examination showed that the homogenization process ended when the maximum boundary friction force became equal to or higher than the vector sum of swelling forces in the radial direction. Based on the force equilibrium and the mass conservation, the final dry density distribution was estimated. Comparison between the estimation and the measurement showed a good agreement, indicating the relevance of the identified mechanism related to boundary friction. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Bentonite pellet mixtures are considered as one of the candidate sealing materials for deep geological disposals of radioactive waste. One of the particularities of this material is the initial heterogeneous distribution of pellets and porosity within the mixture, leading to complex hydro-mechanical behaviour. In this paper, the hydro-mechanical properties of GMZ bentonite pellet mixtures were investigated in the laboratory by carrying out water retention tests on pellet mixtures under constant-volume condition and single pellets under free swelling condition, as well as a infiltration test on a column specimen of pellet mixture. In the infiltration test, the relative humidity and radial swelling pressure were monitored at five heights, the axial swelling pressure was also recorded. The instantaneous profile method was applied to determine the unsaturated hydraulic conductivities. Results show that, in high suction range (> 10 MPa) the water retention curve of pellet mixture under constant-volume condition was comparable to that of a single pellet under free swelling condition, while in low suction range (< 10 MPa) the latter exhibits a much higher water retention capacity. Due to clogging of large pores, the unsaturated hydraulic conductivity decreases as suction decreases to around 25 MPa. However, with further suction decrease, the hydraulic conductivity increases continuously until the value at saturated state, as in the case of most unsaturated soils. The radial swelling pressure at different heights develops with local sudden increase and decrease, which was attributed to local rearrangement of pellets upon wetting. By contrast, as the axial swelling pressure was measured on the global surface of the specimen, it develops in a more regular fashion. [ABSTRACT FROM AUTHOR]

Mixtures composed of MX80 bentonite and crushed Callovo-Oxfordian (COx) claystone have been considered as one of the promising sealing/backfilling materials in French deep geological disposal for radioactive waste. One of the key issues about these mixtures is hydraulic conductivity. In this study, the swelling pressure and hydraulic conductivity of bentonite–claystone mixtures with different bentonite fractions and dry densities were determined by carrying out infiltration tests under constant volume conditions. The results indicated that as the bentonite fraction and dry density increased, the overall swelling pressure increased and the hydraulic conductivity decreased. Based on the assumption that the stress between bentonite and claystone grains was equal to the overall swelling pressure, the void ratios and volumetric fractions of bentonite and claystone in the mixture were determined. According to the relationships between hydraulic conductivity and void ratio for pure bentonite and claystone, the overall hydraulic conductivities of the bentonite–claystone mixtures were then calculated using five analytical models (parallel, series, Maxwell–Eucken 1, Maxwell–Eucken 2, and effective medium theory models) corresponding to different potential composite structures, allowing appreciation of the composite structures of bentonite–claystone mixtures with various bentonite fractions. For specimens with ≥70% bentonite, the bentonite formed a matrix with the claystone dispersed within it, for specimens with <40% bentonite, the matrix was formed by swollen claystone with the bentonite dispersed into the matrix, and for specimens with 40%–70% bentonite, the bentonite and claystone were randomly distributed, without being continuous or dispersed. Based on the defined composite structure of the bentonite–claystone mixture, a new method is proposed for estimating the overall hydraulic conductivity. A comparison between estimation and measurement showed the relevance of the proposed method. [ABSTRACT FROM AUTHOR]

Like bricks of compacted bentonite/sand mixtures, mixtures made up of pellets and powder of bentonite are considered as a possible material to make the sealing plugs used to fill up galleries and ensure long-term watertightness in deep radioactive waste disposal. Pellets/bentonite mixtures have a low permeability, high swelling capacity, good radionuclide retention capability and operational advantages in terms of placement in the galleries. Following a previous in-depth characterisation of bentonite pellets/powder mixture conducted by the same group, an investigation of the water-retention properties and microstructure changes of a bentonite pellet subjected to wetting/drying cycles under free swelling conditions was carried out by means of mercury intrusion porosimetry (MIP) and X-ray microtomography. A complete description of the changes in water content, void ratio and degree of saturation of the pellet was provided. Data showed that the free swelling of the pellet is due to the combined effect of both crack propagation at the macro scale, and the swelling of bentonite grains, governed by hydration mechanisms along the smectite faces at the nano scale. Significant development of a crack network is observed between 38 and 9 MPa. For suctions below 9 MPa, there is a significant decrease of the platelet thickness and an increase in the disorder of the platelet assembly, resulting in the average MIP entrance pore radius increasing to 0·4 μm within the expanded bentonite grains. [ABSTRACT FROM AUTHOR]

This paper investigates the swelling behaviour of compacted MX80 bentonite under hydration with the presence of initial voids that simulate the technological voids in real nuclear waste repositories. With water introduced into samples from the initial voids, the swollen bentonite first filled up the initial voids, and then swelling pressure was generated. At different hydration times, the microstructure was investigated at different heights (initial void, top, middle and bottom) using mercury intrusion porosimetry, together with the determination of dry density, water content and suction. It was observed that the microstructure was characterised by the presence of a large- and a small-pore families at bottom and middle layers, but by the creation of a new medium-pore family with dominant pore mode around 0·04–2 μm at the top and initial void layers after the void space was filled by swollen bentonite. From bottom to void, the significant increase in medium pores was accompanied by an increase in inaccessible pores and a decrease in large and small pores. Further examination showed that the sample could be divided into compression and swelling zones based on the variation of void ratio with swelling pressure: at the compression zone, the soil was compressed over time, characterised by a significant reduction of medium and large pores; whereas at the swelling zone, the soil was still undergoing swelling, represented by the increase of inaccessible pores and medium pores. In addition, the changes in microstructure along the sample were found to be well correlated with changes in dry density and water content: the water content was decreasing and the dry density was increasing from the initial void to the bottom positions; over time the dry density of the upper part was increasing due to compression, while the dry density of the lower part was decreasing due to swelling. This provided evidence for the main mechanism of compacted bentonite swelling with the presence of technological voids: the swollen bentonite fills the initial voids first and then undergoes compression by the swelling of the bentonite behind. [ABSTRACT FROM AUTHOR]

In this study, the influence of rock fracture on the swelling behavior of compacted bentonite was investigated experimentally with bentonite compacted inside a hollow cylinder of fractured Beishan granite from China. During the test, hydration was allowed by letting water flowing into the compacted bentonite from the base plate, and air being expelled from the top plate. Along the sample, swelling pressure and relative humidity were monitored at different positions. Results showed that the presence of rock fracture significantly changed the hydration process of compacted bentonite. The evolutions of swelling pressure were directly related to the relative position to the rock fracture. At the same height, for the position closer to the rock fracture, a larger decrease in local dry density occurred, resulting in a larger loss in swelling pressure. This suggests that in field conditions of radioactive waste repository involving bentonite and rock, local filling of rock fracture by bentonite may occur, and the resulted bentonite swelling pressure decrease must be accounted for in the safety evaluation of the whole storage system. • The compacted bentonite behaviour in fractured rock was investigated. • Significant rock fracture influence was identified. • Larger decrease of swelling pressure in the vicinity of fracture was observed. • The local swelling pressure decrease is correlated to the local dry density decrease. [ABSTRACT FROM AUTHOR]

Abstract The water retention, compressibility and swelling properties of a pellet/powder bentonite mixture were investigated in the laboratory. The water retention capacity was determined under constant-volume condition using a specially designed cell allowing vapor exchange in all directions; suction controlled oedometer tests were carried out for the compressibility investigation; both constant-volume and swell-consolidation methods were applied for the swelling pressure determination. The vapor equilibrium technique was used for suction control. Comparable results were found for the water retention capacity of the pellet/powder mixture determined under constant-volume condition and one single pellet under free swelling condition for suctions higher than 4 MPa, suggesting that the suction was mainly controlled by the micro-pores inside the pellets/powder grains and the swelling of the clay particles was accommodated by the existing macro-pores. On the contrary, at lower suctions, the constant-volume conditions gave rise to a lower water retention capacity for the mixture, indicating the restricted clay particle swelling after the filling-up of the existing macro-pores. Oedometer compression curves revealed a different behavior for low and high suctions: at high suctions, the volume change behavior was governed by the pellets rearrangement combined with possible pellets crushing upon loading. By contrast, at low suctions, the mixture lost its initial granular structure during suction decrease; thus, the volume change behavior became similar to that of compacted bentonite. Compared to compacted bentonite, the pellet/powder mixture showed a lower yield stress. A lower value of swelling pressure was found with the constant-volume method, confirming the limitation of the swell-consolidation method in determining soil swelling pressure. Highlights • A new cell was designed for the determination of water rentention property. • Pellet and mixture have similar water retention properties at high suctions. • Mixture has the compressibility property of granular material at high suctions. • Irreversible swells occurs when wetting the mixture under low stress. • The swell-consolidation systematically overestimates soil swelling pressure. [ABSTRACT FROM AUTHOR]

After construction of the high-level radioactive waste repository, the bentonite undergoes corrosion of the alkaline solution produced by cement degradation. Young cement water-YCW was employed to carry out the corrosion experiments on compacted GMZ bentonite specimens. Then the removal of europium (Eu(III)) onto corroded GMZ bentonite was studied by batch experiments. The results show the YCW corroded GMZ bentonite is still an effective adsorbent. The adsorption reaction of Eu(III) on corroded GMZ bentonite is endothermic. It can be described by Langmuir isotherm model well. The bentonite corroded at higher temperature shows more stable sorption property. [ABSTRACT FROM AUTHOR]

MX80 bentonite pellet has been considered as a promising sealing/backfilling material in geological disposal facilities for high-level and intermediate-level radioactive wastes. As the sealing/backfilling material which is subjected to hydration by pore water or vapour from host rock can be damaged by wetting-induced cracking, it is essential to clarify the suction effect on the damage behaviour of the material. In this study, a combined Mercury Intrusion Porosimetry/Micro-Computed Tomography (MIP/μCT) method was applied to investigate the microstructural damage of the MX80 bentonite cylinder-shaped 32-mm pellets equilibrated under suctions s from 113 to 1.5 MPa for wetting and from 113 to 262 MPa for drying. The enlarged pore size distribution (PSD) curves by MIP/μCT revealed an insignificant drying-induced closure of cracks, as opposed to massively developed wetting-induced cracks, with the size of the largest cracks reaching several millimetres. This is confirmed by the μCT observation. Indeed, the μCT images showed a negligible effect of drying on cracking, contrary to the effect of wetting characterised by significant cracking from the marginal area to the central part of pellet, indicating significant damage of pellet by wetting. Additionally, only horizontal cracks were identified at s ≥ 82 MPa. Vertical cracks emerged and propagated only at s ≤ 57 MPa, together with further expanding of horizontal cracks, due to the preferentially horizontal orientation of clay particles resulting from the one-dimensional compaction in the pellet fabrication. A damage coefficient D was defined as the ratio of the volume of macro-pores to the total volume of pellet. D increased slightly upon drying from suction 113 to 262 MPa and significantly upon wetting from suction 113 to 3.2 MPa, but kept almost unchanged when wetting furthered from suction 3.2 to 1.5 MPa because the clogging of macro-pores by the exfoliation effect slowed down the cracking rate. • The large-range PSD curve by means of MIP/μCT was proposed; • The wetting and drying effects on the cracking evolution were revealed by MIP/μCT; • A damage coefficient was proposed to describe the cracking upon wetting/drying. [ABSTRACT FROM AUTHOR]

Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration. [ABSTRACT FROM AUTHOR]

In China, Gaomiaozi (GMZ) bentonite serves as a feasible buffer material in the high-level radioactive waste (HLW) repository, while its thermal conductivity is seen as a crucial parameter for the safety running of the HLW disposal. Due to the tremendous amount of heat released by such waste, the thermal conductivity of the buffer material is a crucial parameter for the safety running of the high-level radioactive waste disposal. For the purpose of improving its thermal conductivity, this research used the graphene oxide (GO) to modify the pure bentonite and then the nanocarbon-based bentonite (GO-GMZ) was obtained chemically. The thermal conductivity of this modified soil has been measured and investigated under various conditions in this study: the GO content, dry density, and water content. Researches confirm that the thermal conductivity of the modified bentonite is codetermined by the three conditions mentioned above, namely, the value of GO content, dry density, and water content. Besides, the study proposes an improved geometric mean model based on the special condition to predict the thermal conductivity of the compacted specimen; moreover, the calculated values are also compared with the experimental data. [ABSTRACT FROM AUTHOR]

This study investigated the adsorption of Eu(III) from simulated Beishan groundwater onto Gaomiaozi bentontie. The effects of shaking time, temperature, concentration of the adsorbent, pH value and concentration of Eu(III) were analyzed. Results showed that the adsorption could reach the equilibrium at 45 min and the maximum adsorption capacity can get to 4.52 mg/g. The optimum pH was observed above 8.5 at temperature 298 K; the adsorption capability increased with the pH value increasing from 2 to 12, which are expected to occur in the near field of Chinese repository. The adsorption process could be described with the Freundlich isotherm. [ABSTRACT FROM AUTHOR]

Compacted mixture of bentonite and sand has been proposed for using as buffer/backfill material in the disposal of high-level radioactive waste in many countries. The hydraulic behaviors and deformation properties of this material can be influenced by the chemical composition of groundwater. In the present work, the infiltration test of Pb(II) solution was conducted on compacted Gaomiaozi (GMZ) bentonite/sand mixture under unconfined conditions. The moisture migration characteristics and swelling properties were analyzed. Results show that the moisture migrates more quickly in the GMZ bentonite/sand mixture infiltrated with Pb(II) solutions than that in the pure GMZ bentonite hydrated with pure water for a same bentonite dry density. This difference could be explained by the chemical influence of Pb(II) on the thickness of diffuse double layer. For the bentonite/sand mixture with high bentonite content, its hydraulic conductivity depends on the bentonite dry density, regardless of the influence of the sand content. The hydration rate of the specimen is a function of the infiltration time and the distance from the wetting end. The largest swelling deformation was observed at the wetting end. At a given location, the vertical displacement was larger than any unilateral one of both sides. After the infiltration test, the inductively coupled plasma emission spectrometer, mercury intrusion porosimetry, and environmental scanning electron microscope tests were conducted for investigation of the Pb(II) distribution and microstructural characteristics of the tested specimen. Results show that, as the distance from the wetting end increases, the Pb(II) content nonlinearly decreases from 4.48 to 0.07 mg/g while the micro-pore increases and the macro-pore decreases. [ABSTRACT FROM AUTHOR]

In this study, swelling pressures of MX80 bentonite were determined by different common methods, and the microstructures were investigated by performing mercury intrusion porosimetry (MIP) tests. The obtained results and in-depth analysis allowed the microstructure-based conceptual model to be proposed for the determination of expansive soil swelling pressure. This conceptual model implied that the comparable swelling pressures determined by the swell-under-load (SUL) method, the constant-volume (CV) method, the zero-swell (ZS) method, the pre-swell (PS) method and the Cui's method were attributed to the similar macro (or micro) void ratios (e M or e m) generated during hydro-mechanical loadings. The higher swelling pressure with the swell-consolidation (SC) method corresponded to the "extra" pressure that was needed to compress the "extra" macro void ratio e M created during initial swelling under low pressure. However, the swelling pressures determined by the SC method and other methods were expected to be comparable in the case of swelling under high pressure because swelling generated similar e M (or e m) in that case. This conceptual model also provided useful information for better understanding of the swelling mechanism involved at microstructure level for expansive soils. • A microstructure-based conceptual model was proposed for the determination of soil swelling pressure. • Higher swelling pressure with SC method was due to compressing "extra" e M created during initial swelling under low pressure. • Similar swelling pressures by other methods were attributed to similar e M (or e m) generated during hydro-mechanical loadings. • Wetting generated similar e M (or e m) under very high pressure in SC and other methods, leading to similar swelling pressures. [ABSTRACT FROM AUTHOR]

• The water retention curves upon wetting/drying exhibited a significant hysteresis in high-suction range. • The bentonite pellets yielded an obvious plastic deformation after a wetting/drying cycle. • The swelling of pellets showed a significant anisotropy. • The degree of anisotropy apparently decreased during subsequent drying and rewetting processes. Bentonite pellet/powder mixtures have been proposed as the candidate sealing plugs of boreholes or galleries in deep geological disposal of high-level radioactive waste. During the long-term operation of repository, the filled bentonite pellets will be subjected to a coupled wetting/drying process, and their hydro-mechanical behaviors are accordingly expected to undergo complex changes. To clarify this point, the water retention and volume change behaviors of bentonite pellets were investigated by conducting hydration/dehydration tests under free-swell condition. Afterwards, the microstructure of bentonite pellets was studied by mercury intrusion porosimetry (MIP) tests. Results revealed that the water retention curves of pellets exhibited an obvious hysteresis in the high-suction range. The volume change of pellet was attributed to the combined actions from the expansion of micro-pores and the propagation of cracks. Meanwhile, the swelling of bentonite pellets exhibited a significant anisotropy during hydration, with a greater swelling strain in the axial direction. But this phenomenon became insignificant during the subsequent drying/rewetting processes. After a wetting/drying cycle, the bentonite pellets yielded an obvious plastic deformation, which was primarily related to the irrecoverable wetting-induced cracks. [ABSTRACT FROM AUTHOR]

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Abstract: Compacted bentonite-based materials have been proposed as possible sealing and backfill materials in geological repositories for the high-level radioactive waste disposal in several countries. During the long time lifespan of a repository, as the chemical composition of pore water can change, the swelling and sealing capacity of the material may also change. From a point of view of storage safety assessment, it is important to evaluate this possible change of swelling capacity. In this study, a mixture of bentonite and crushed Callovo-Oxfordian claystone was investigated. The long-term effect of pore water chemistry on the swelling pressure was studied at constant-volume conditions for 700days. Distilled water and synthetic water having the chemical composition similar to that of in-situ pore water were used for hydration. The results obtained in an initial period of 100h revealed no significant influence of the water composition on the swelling pressure evolution, and the maximum swelling pressures observed were close to 4.30MPa for a dry density of 1.70Mg/m3. Over a longer time period, on the contrary, the swelling pressure eventually decreased for all samples, especially for the sample saturated with synthetic water. In addition, comparison of a one-step soaking test with a multi-step soaking test showed no wetting procedure effect on the long term swelling behaviour. All the results are analysed by considering the physico-chemical interaction between the minerals of claystone, minerals of bentonite and different fluids involved. [Copyright &y& Elsevier]

Abstract: Compacted bentonite-based materials are often considered as sealing/backfill materials in deep geological repository for high level radioactive waste. A good understanding of their hydration process is essential as this process is directly related to over-pack corrosion and nuclide migration. In this study, the unsaturated hydraulic properties of MX80 bentonite/sand mixture were characterized by carrying out a series of experiments including water retention test, infiltration test as well as microstructure observation. It was found that with suction decrease under constant volume condition, the hydraulic conductivity decreased followed by an increase after a suction threshold. At suctions higher than 12.6MPa, hydration led to progressive large-pore clogging by exfoliation of clay particles. On the contrary, when saturation was approached (suction lower than 4.2MPa), the large-pore quantity increased due to the creation of two-dimensional pores. It was also observed that the soil hydraulic conductivity changed following the same tendency as the large-pore quantity during hydration. In other words, water transfer was primarily governed by the network of large-pores. [Copyright &y& Elsevier]

Abstract: Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure disposal safety. In this study, a mixture of MX80 bentonite and sand was characterised in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil are referred to as common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suction, the amount of water absorbed in the soil depends solely on suction, whereas at low suction it depends on both suction and the bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and the sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite–sand mixture yielded at a higher pre-consolidation stress. [Copyright &y& Elsevier]

A compacted mixture of Callovo-Oxfordian (COx) claystone/MX80 bentonite is a promising sealing material in the French Cigéo project for radioactive waste disposal at great depth. Over time, the sealing material will be saturated by the site water originating from the surrounding rock and the water chemistry can affect the performance of the sealing material. In this study, the effects of water chemistry and technological gaps on the hydro-mechanical property were investigated by performing hydration tests on the compacted claystone/bentonite mixture with technological gaps using deionised water and synthetic site solution for 10 and 590 days. After the tests, the dry density, water content and microstructure distributions were also determined to complement the program. Results show that the synthetic site solution did not affect the swelling pressure and hydraulic conductivity after a short term of 10 days. With the increase of hydration time, the specimens with technological gaps tended to be homogeneous, resulting in a reduction of hydraulic conductivity. Because of the cation exchanges and the decreased diffuse double layer thickness, the synthetic site solution reduced the swelling pressure and increased the hydraulic conductivity in long term, especially for the specimens with lower global dry densities. [ABSTRACT FROM AUTHOR]

Abstract: Thermal conductivity of compacted bentonite is one of the most important properties in the design of high-level radioactive waste repositories where this material is proposed for use as a buffer. In the work described here, a thermal probe based on the hot wire method was used to measure the thermal conductivity of compacted bentonite specimens. The experimental results were analyzed to observe the effects of various factors (i.e. dry density, water content, hysteresis, degree of saturation and volumetric fraction of soil constituents) on the thermal conductivity. A linear correlation was proposed to predict the thermal conductivity of compacted bentonite based on experimentally observed relationship between the volumetric fraction of air and the thermal conductivity. The relevance of this correlation was finally analyzed together with other existing methods using experimental data on several compacted bentonites. [Copyright &y& Elsevier]

High stress consolidation tests (up to 30 MPa) were carried out on slurries made up of three bentonites considered as possible constituents for engineered clay barriers used for nuclear waste disposal at great depth (Na Kunigel, Na-Ca MX80 Wyoming clay, and Ca Fourges clay). Water retention curves (WRCs) were determined for a wide range of suctions (up to 305 MPa). A bilinear shape for the compression curves and the WRCs was observed in the three clays, with significant changes occurring in the 0.4–1 MPa suction–stress range. The permeability and the coefficient of consolidation were also determined during the compression tests. In the WRCs and compression curves, the two clays containing Na[sup +] exhibited a similar behaviour, which was different from that of the Ca[sup 2+] clay. The coefficient of consolidation for the three clays was decreasing in the 0.4–1 MPa stress range and increasing at higher stresses. Data were interpreted in the light of existing experimental observations on the microstructural changes of similar swelling clays carried out by various authors, which obtained compatible and coherent results. The diffuse double theory, often used in the interpretation of the volume change behaviour of swelling clays, was also considered. Under higher stresses (> 2 MPa), the compressive behaviour appeared to be linked to the expulsion water molecules that form the hydration shells around exchangeable cations inside the domains formed of staked clay plates. The permeability properties of the Ca[sup 2+] and Na[sup +] clays were linked to the microstructure features and to the WRCs, whereas the changes in the coefficients of consolidation during compression were linked to the changes in soil stiffness.Key words: consolidation, swelling clay, permeability, exchangeable cations, diffuse double layer, adsorbed water.Des essais de consolidation sous des contraintes élevées (jusqu'à 30 MPa) ont été réalisés sur des boues constituées de trois bentonites considérées comme des constituants possibles de barrières ouvragées pour l'entreposage de déchets nucléaires à grande profondeur (Na Kunigel, Na-Ca MX80 Wyoming clay et Ca Fourges clay). Les courbes de rétention d'eau ont également été déterminées sur une large plage de succion (jusqu'à 305 MPa). On a observé une forme bilinéaire des courbes de compression et de rétention d'eau pour les trois argiles, avec des changements significatifs se produisant dans la plage de 0.4–1 MPa de succion–contrainte. Les déterminations de la perméabilité et du coefficient de consolidation ont aussi été réalisées au cours des essais de compression. Dans les courbes de compression et de rétention d'eau, les deux argiles contenant du Na+ présentent un comportement similaire, différent de celui de l'argile Ca[sup 2+] . Le coefficient de consolidation des trois argiles diminue dans la plage de contraintes de 0.4–1 MPa et augmente à des contraintes plus élevées. Les données ont été interprétées à la lumière des observations expérimentales existantes sur les changements de microstructure d'argiles gonflantes similaires faites par divers auteurs qui ont obtenu des résultats compatibles et cohérents. On a aussi considéré la théorie de double couche diffuse, souvent utilisée dans l'interprétation du comportement volumique des argiles gonflantes. À des contraintes plus élevées (>2 MPa), le comportement en compression semble être relié à l'expulsion des molécules d'eau qui forment les coquilles d'hydratation autour des cations échangeables, dans les domaines formés par les feuillets argileux empilés. Les propriétés de perméabilité des argiles Ca[sup 2+] et Na+ sont reliées à des caractéristiques de microstructure et aux courbes de rétention d'eau, alors que les changements de cv durant la compression sont reliés aux changements de rigidité du sol.Mots clés : consolidation, argile gonflante, perméabilité, cations échangeables, double couche diffuse, eau adsorbée.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Compacted block composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80 bentonite has been proposed as a possible material for sealing a potential repository of radioactive waste. In this study, the compaction property of the bentonite/claystone mixtures with various bentonite fractions and water contents were first investigated to define an appropriate protocol for the fabrication of blocks. Results show that the increase of bentonite fraction decreased the overall compressibility and the achievable dry density at a given stress. As the water content increased, the compressibility at a relatively low vertical stress increased due to the increasing deformability of aggregates and the decreasing inter-aggregate friction; contrarily, at a high vertical stress, a lower compressibility was observed for the samples with a higher water content because a larger stress was required to squeeze water within aggregates. During demoulding, the required demoulding pressure increased with the increase of bentonite fraction and decreased with the increasing water content. Then, to assess the suitability of the material, the swelling pressures and hydraulic conductivities of the compacted blocks were determined. It was found that the swelling pressure increased and the hydraulic conductivity decreased with the increases of bentonite fraction and dry density. By contrast, as the water content increased, more water was intercalated in the interlayer spaces and the inter-aggregate pore volume decreased, resulting in reductions of the swelling pressure and hydraulic conductivity. • Compressibility, rebound property, demoulding pressure, swelling pressure, and hydraulic conductivity were investigated; • Effects of bentonite fraction, dry density and water content were identified; • The effect of water content on the compressibility depended on vertical stress; • The increase of water content reduced the swelling pressure and the hydraulic conductivity. [ABSTRACT FROM AUTHOR]

To ensure the performance efficiency and long-term operational safety of the engineering barrier in deep geological repositories, it is of great importance to investigate gas migration behavior in the water-saturated bentonite subjected to decay heat generated by the nuclear waste. In this work, experimental investigations were conducted to find out influences of multi-step thermal cycles on variations of the effective gas permeability, gas breakthrough pressure and microstructure of the bentonite specimens. A temperature range of 20–60 °C was selected and each temperature step was kept constant for at least 3–4 days to allow the gas flow to be equilibrated. Results show that effective gas permeability increases with heating, while decreases with cooling. The effective gas permeability decreases from 7.9 × 10−23 to 2.01 × 10−23 m2 for the specimen tested under a constant injection pressure of 1.52 MPa after experienced one to three thermal cycles. Meanwhile, the gas breakthrough pressure for GMZ bentonite after experiencing three thermal cycles was 2.43 MPa. Microstructural observations by the MIP tests and paraffin-coated tests indicate that the heating-cooling cycles induce decreases of void space, resulting from the shrinkage of the bentonite matrix. Decrease of macro-pores and a little increase of micro-pores induced by the thermal cycles applied could make it difficult for gas to break through the bentonite matrix. The interfacial effect is the dominant gas breakthrough mechanism for specimens experienced thermal cycles. • Cyclic thermal gas injection tests are conducted on initially water-saturated bentonite specimens. • Effective gas permeability increases with heating while decreases with cooling with hysteresis recorded. • Thermal cycles decrease the overall void ratio due to significant decrease of macro-pores. • Interface effect is dominant mechanism for gas breakthrough in bentonite experienced thermal cycles. [ABSTRACT FROM AUTHOR]

Pre-compacted blocks of MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture are chosen as sealing/backfilling materials in the French concept of deep geological disposal for radioactive waste. Creating technological voids is normally unavoidable during installation in the disposal galleries, in the form of axial voids between compacted blocks and radial voids between the compacted blocks and the host rock. In this work, the aeolotropic swelling behaviour of compacted mixture with axial/radial technological voids was experimentally investigated. Results showed that the aeolotropy coefficient (ratio of radial to axial pressures) of samples without voids increased from 0.46 to 0.88 with the increasing dry density from 1.6 to 2.0 Mg/m3 due to the relatively concentrated macro-pores which defined a relatively lower local dry density in the side part. For the samples with axial/radial voids, their final axial swelling pressures were in agreement with the unique correlation with final dry density, regardless of the soil density heterogeneity. Samples with axial voids had a larger aeolotropy coefficient than samples without voids, due to the reduction of the orientation of clay particles upon hydration, while samples with radial voids had a smaller aeolotropy coefficient, due to the relatively lower dry density near the cell wall. • The swelling aeolotropy of bentonite/claystone mixture were investigated; • The effects of dry density and axial/radial technological voids were identified; • Aeolotropy was due to the lower dry density in the side part and the grain orientation; • Axial technological voids decreased grain orientation and reduced the aeolotropy; • Radial technological voids increased the heterogeneity and enhanced the aeolotropy. [ABSTRACT FROM AUTHOR]

Soil hydraulic conductivity depends not only on the state variables such as void ratio and pore size distribution, but also on its physical/geotechnical properties. In this study, Atterberg limits, hydraulic conductivity and mercury intrusion porosimetry (MIP) tests were performed on MX80 bentonite/Callovo-Oxfordian (COx) claystone mixtures. Based on the experimental results obtained in this study together with the test data compiled from literature, a general capillary model considering N p pores in series was used to evaluate the influence of soil plasticity on the relationship between hydraulic conductivity and pore size distribution. It was found that the best pore interconnection parameter N p was highly dependent on the soil property. The larger the plasticity index, the larger the best N p value and the more complex the pore interconnection. The relationship between the best N p value and plasticity index could be well described by an exponential equation. The general capillary model was then improved to estimate the hydraulic conductivity of different soils. The estimated values using the improved general capillary model were finally compared with the measured ones and the good agreement between the measurement and estimation revealed the good performance of the proposed model. • The Atterberg limits, hydraulic conductivity and pore size distribution were determined. • The effects of bentonite fraction and dry density were investigated. • The influence of soil plasticity on the hydraulic conductivity was evaluated based on a general capillary model. • An improved general capillary model was proposed to estimate the hydraulic conductivity of various soils. [ABSTRACT FROM AUTHOR]

A small-scale mock-up test was carried out on a fractured hollow granite cylinder with compacted MX80 bentonite inside, to study the interaction between engineered barrier (compacted bentonite) and natural barrier (host rock with the presence of rock fracture). The swelling pressure and relative humidity of bentonite were monitored with respect to the position of the rock fracture during 349 days of infiltration. Herein, the variation of water content, dry density, suction and microstructure along bentonite column after dismantling were reported, focusing on the changes in the vicinity of the rock fracture, to evaluate the effect of the rock fracture on the swelling behaviour of compacted bentonite. Results showed that the presence of rock fracture disturbed the water content distribution, with a lower water content at a position closer to the fracture. A significant decrease in dry density was also observed in the vicinity of the rock fracture; the closer the positions to the fracture the larger the decrease of dry density. This decrease coincided with the reduction of swelling pressure recorded by the pressure sensors, suggesting the occurrence of rock fracture filling-up by bentonite. Further examination showed that when the soil suction was higher than 9 MPa, the decrease in dry density in the near field of rock fracture was mainly attributable to the increase in large pore porosity (>2 μm). This suggests that at this suction the mechanism involving intrusion of bentonite into the rock fracture with fracture width higher than the size of bentonite gains was related to the pushing effect under the swelling of bentonite behind. By contrast, when the suction became lower than 9 MPa, the bentonite gel formed from the exfoliation of clay particles might fill up the rock fracture with smaller aperture width. • State of bentonite after 349-day of hydration inside fractured granite was reported. • Effect of rock fracture on density and microstructure of bentonite was identified. • Rock fracture cause local dry density decrease, leading to swelling pressure loss. • Bentonite filling rock fracture were discussed from a microstructure perspective. [ABSTRACT FROM AUTHOR]

MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a sealing/backfilling material in a deep geological repository of radioactive waste in France. A good understanding of the swelling behaviour of this mixture is essential when evaluating the long-term performance of the repository. In this work, the swelling pressure of MX80 bentonite/COx claystone mixture was investigated by the constant-volume method for a full range of bentonite fraction. Results show that the swelling of claystone in the mixture can be inhibited by bentonite and its contribution to the global swelling pressure depends on the bentonite fraction. For the mixture with >70% bentonite, claystone behaves as an inert material, and its contribution to the global swelling pressure can be ignored. However, for the mixture with less bentonite, the swelling of claystone will significantly contribute to the global swelling pressure. A method was proposed allowing the swelling pressure of bentonite/claystone mixture to be predicted in the full range of bentonite fraction. • Claystone's contribution to swelling pressure depends on the bentonite fraction. • The effect of bentonite on claystone's swelling can be described by an inhibition factor. • A method is proposed to predict the swelling pressure of bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]

For investigation of temperature and vertical stress influences on the swelling–shrinkage deformation of GMZ bentonite specimens, cyclic wetting–drying tests with suction ranging between 0 and 110 MPa were conducted under different constant vertical net stresses and temperatures using a suction-temperature controlled oedometer. Results show that temperature effects on the cumulative strains depend on the vertical stress. Under lower vertical stresses, rising temperature intensifies the tendency of the accumulation of expansive strains, while under higher stresses, rising temperature intensifies the tendency of accumulation of contractive ones. In addition, for a given constant temperature, the accumulative deformation recorded on the specimen tested was an expansion or a shrinkage strain depends on the vertical stress applied. Meanwhile, the corresponding vertical stress at transition was different at different temperatures. A logarithm correlation was developed between the final accumulative strain and vertical stress during wetting–drying cycles. According to the test results, the Barcelona expansive model was modified for simulation of the results of the cyclic wetting–drying tests conducted at temperatures. Simulation showed that the results calculated by the modified BExM agreed well to that measured in the present work. [ABSTRACT FROM AUTHOR]

Abstract After construction of the repository, the bentonite will undergo some chemical effects resulting from the chemistry of site water and the solution produced by cement degradation in the high-level radioactive waste repository. In this study, the swelling pressure of compacted Gaomiaozi(GMZ) bentonite was investigated with three types of pore water simulating site groundwater (synthetic Beishan Site Water - BSW) and two cement solutions (Young Cement Water – YCW and Evolved Cement Water – ECW). X-Ray diffractometry (XRD) tests were also carried out to analyse the mineralogy of the samples before and after the swelling pressure tests. Results show that the swelling pressure curve is of “double-peak” pattern, which fades with the pH of solutions increase. The swelling pressure of GMZ bentonite decreases little with BSW, significantly with YCW and slightly with ECW. At a higher dry density, the swelling pressure of compacted GMZ bentonite reaches stability later and the final swelling pressure is higher. Cation exchange takes place in the bentonite and the montmorillonite of GMZ bentonite transforms to multi-cation dominant montmorillonite with the three types of pore waters. The swelling pressure reduction affected by the cement solutions is mainly due to the montmorillonite dissolution, but the secondary minerals are in amorphous phases. The swelling property of compacted GMZ bentonite can be reduced by cement solutions in the long-term operation of repository. Highlights • The swelling pressure of bentonite decreases with cement solutions. • The swelling pressure of more compacted bentonite stabilises later. • Cation exchanges, smectite dissolution occured. • Analcime and C-S-H gels were formed after reactions. [ABSTRACT FROM AUTHOR]