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Investigation of osmotic suction effects on the volume change behavior of clay is of great importance in evaluating the performance of engineering barrier in deep geological repository for the disposal of high-level radioactive waste. From the effective stress principle point of view, the linear relationship between the concentration of salt solution and osmotic suction described by the common van’t Hoff equation is not consistent with the experimental results, because the salt solution (or osmotic suction) effect on the swelling behavior and compressibility became insignificant when the concentration of salt solution was higher than 2.0 M. Meanwhile, the osmotic suctions described by Lang (Aust J Chem 20(9):2017–2023, 1967), Rao et al. (Geotechnique 56(10):707–713, 2006) and Witteveen et al. (Geotechnique 63(3):244–255, 2013) are also not consistent with the effective stress principle. In this study, the influence of osmotic suction on swelling behaviors was explained from the crystalline swelling and the double-layer swelling for specimens with a void ratio higher than 0.5. According to the experimental observations, a new effective stress equation was proposed with consideration of osmotic suction effects for the compacted GMZ01 bentonite specimen with a void ratio higher than 0.5. Finally, the proposed equation was verified based on the observations of the swelling and oedometer tests conducted on as-compacted GMZ01 bentonite specimens.

Compacted bentonite has been considered a suitable engineered barrier material for high-level radioactive waste (HLW) repositories for several decades. However, hyperalkaline groundwater produced by cementitious materials, combined with the heat generated by nuclear decay during the long-term storage of waste canisters, may cause the deterioration of the swelling properties of compacted bentonite. In this study, a series of swelling pressure tests and scanning electron microscopy (SEM) tests were performed on compacted Gaomiaozi (GMZ) bentonite (dry density 1.7 Mg/m3) to investigate the deterioration of the swelling pressure. Results indicated that the deterioration of the swelling pressure was facilitated by the temperature when the same concentration of NaOH solution was infiltrated, and a model of swelling pressure deterioration was developed to predict the long-term swelling pressure. Furthermore, the dissolution of montmorillonite and some silicate minerals, as well as the formation of non-expanding secondary minerals, led to transformations of the agglomeration patterns of the soil particles and structural damage to the bentonite, which controlled the long-term deterioration of the swelling pressure. Therefore, for the long-term operation of an HLW repository, the deterioration of the swelling pressure of compacted bentonite should be monitored, and safety assessments should account for the effects of heat and alkalinity.

The mineral alteration and microstructure evolution of compacted GMZ bentonite was investigated during the long-term in the Beishan area, a candidate for a high-level radioactive waste repository in China. The compacted GMZ bentonite samples at initial dry densities of 1.5 and 1.7 Mg/m3 were infiltrated by synthetic Beishan Site Water (BSW) and two synthetic cement solutions (Young Cement Water -YCW and Evolved Cement Water - ECW) under constant volume conditions for about 2 months in this study. X-Ray diffractometry (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM-EDS) tests were carried out to observe the mineralogy and microstructure of the specimens after experiencing the three types of pore solutions. There is evidence of cation exchange reactions, some zeolites and C-S-H in the GMZ bentonite after the infiltration of alkaline solutions. The hydration of alkaline solutions and mineralogy alteration result in changes to the microstructure of compacted GMZ bentonite. The generated secondary mineral on the surface of montmorillonite leads to the clogging of inter-aggregate pores. In the case of high dry density, this resulted in pore occlusion.

Gaomiaozi (GMZ) bentonite has been considered as a potential material for construction of engineering barrier in Chinese deep geological repository for disposal of high-level nuclear wastes. In the present work, thermal conductivity tests were conducted on statically compacted GMZ bentonite specimens using the needle probe method. Cubic specimens with different initial saturation degrees and dry densities were prepared and the thermal conductivity tests were conducted along the directions both parallel and perpendicular to the bedding plane of the specimen. Results show that the thermal conductivity nonlinearly increases with increasing saturation degree with a transition point that proximately corresponding to the air entry value. Meanwhile, the thermal conductivity along the parallel direction is much higher than that along the perpendicular direction. The anisotropy coefficient decreases with increasing saturation. However, the anisotropy coefficient increases with increasing dry density and the influence depends on saturation degree.

Because of its low hydraulic conductivity, high swelling capacity, and good adsorption properties, Gaomiaozi (GMZ) bentonite has been proposed as a suitable buffer/backfill material for construction of artificial barriers in deep geological repositories for high-level nuclear waste (HLW) disposal in China. By varying the concentration of NaCl solution and total suction, the swelling and shrinkage characteristics of compacted GMZ bentonite were studied under one-dimensional conditions. Results show that the swelling strain of compacted GMZ bentonite decreases as the concentration of NaCl solution increases under one-dimensional conditions. During drying with suction control by vapor phase technique, the density increases with suction. Simultaneously, the sample saturated with salt solution has a larger void ratio than that of the sample saturated with distilled water under the same suction. The void ratio and water content of the samples decrease with increasing suction. For a given suction, the sample saturated with salt solution has a higher degree of saturation than the sample saturated with distilled water. With the obtained water content (w), void ratio (e), and the degree of saturation (Sr), soil-water retention curves (SWRCs) were calculated for samples initially hydrated with different concentrations of NaCl solution. The SWRCs reveal that the sample with salt solution has higher water retention capacity than the sample with distilled water. A modified SWRC equation was proposed to account for the effect of NaCl solutions on the drying process of compacted GMZ bentonite under one-dimensional conditions. The results confirm that the modified SWRC equation can effectively describe the SWRCs of compacted GMZ bentonite considering the effects of NaCl solutions.

Investigation of the swelling pressure of compacted bentonite is of great importance in evaluating the performance of the engineered barrier system in deep geological repositories for disposal of high-level radioactive waste. In this study, three methods including Cui method ( Cui et al., 2013 ), swell-consolidation method and constant-volume method were employed for determining the swelling pressure of compacted GMZ01 bentonite with infiltration of different concentrations of salt solutions. The swell-consolidation method gave higher value of swelling pressure, while the other two methods produced almost the same values. A critical void ratio was determined for separating the swelling induced by combination effects of double-layer swelling and crystalline swelling from that caused by the effect of crystalline swelling. Based on this, the salt solution effect on swelling pressure can be well explained. For specimens with a void ratio higher than this critical value, swelling pressure decreases with the increase of concentration of NaCl solution, due to the inhibition of the double-layer swelling and crystalline swelling. On the contrary, in case of void ratio near this critical value, the salt solution effects become negligibly. Conclusions reached on GMZ01 bentonite in this work are useful for determination of geological engineering parameters for construction of the Chinese deep geological repository.

Different doses of carbide slag and metakaolin are used to solidify/stabilize copper-contaminated soil in accordance with its degree of contamination. This paper presents some experimental findings on the physical and chemical properties of the treated soil, including unconfined compressive strength, stress–strain state and pH, explained by series chemical and mineralogical reactions in the solidified/stabilized soil. Results showed that the unconfined compressive strength decreases and the failure strain increases with the copper ion concentration in the soil increasing. Additionally, the failure mode of the soil gradually changed from fragile failure to plastic failure. Increasing the amount of carbide slag favored improvements in unconfined compressive strength, yet decreased the failure strain. Furthermore, the pH of the soil decreased from 8.22 to 4.36 with the initial concentration of heavy metal in the contaminated soil increasing. High concentrations of Cu2+ react with OH− in pore solutions to form complexes like [Cu(OH)4]2– to decrease alkalinity in the soil. Additionally, under certain conditions, the reaction of carboxyl (–OH) and hydrated groups (–OH2) with Cu2+ release H+, which will also reduce the pH value in the soil. However, when the carbide slag content increased from 5 to 15%, the pH increased significantly, up to 10.5, demonstrating the beneficial effect of the industrial waste carbide slag in solidifying/stabilizing the copper-contaminated soil.

International audience; Knowledge of the mechanical properties of compacted bentonite is essential to assessing the performance of an engineered barrier system in a high-level radioactive waste repository. In this study, direct shear tests and uniaxial compression tests were conducted on compacted Gaomiaozi bentonite with a dry density of 1.7 g/cm3 to investigate its stiffness and strength characteristics at various initial suction levels. Specimens were found to exhibit brittle failure within an initial suction range of 11–223 MPa. The peak shear strength and shear modulus were found to increase with increasing initial suction and to begin to decrease at an initial suction of 126 MPa. The residual shear strength was found to increase with increasing initial suction within 11–223 MPa. The uniaxial compressive strength and elastic modulus were found to increase with increasing initial suction and to begin to decrease at an initial suction of 62 MPa. Models were developed to predict the stiffness and strength of the specimens as a function of the initial suction and effective stress parameter (χ). For the peak shear strength, shear modulus, uniaxial compressive strength, and elastic modulus, using the degree of saturation (Sr) and effective degree of saturation (Se) in place of χ produced the same trends which diverged from the test results at initial suction levels lower than 12 MPa and higher than 126 MPa. However, using Sr and Se in place of χ reasonably reproduced the residual shear strength within an initial suction range of 11–223 MPa. In contrast, using (Sr)α (Sr raised to an exponent α) in place of χ reproduced the test results well. For the peak shear strength, shear modulus, uniaxial compressive strength, and elastic modulus, the simulations required a value of 2.4 for the α. For the residual shear strength, the best value of α was 1.0. Based on the testing method and model approach developed in this study, the evolution of the stiffness and strength of compacted bentonite along wetting and drying paths can be further explored.

International audience; Gas breakthrough pressure is of great importance for evaluating the sealing efficiency and assessing the long-term performance of saturated buffering materials with low permeability in the deep geological disposal of high-level radioactive waste. In this study, to investigate gas breakthrough properties, step-by-step (SBS), residual capillary pressure, and mercury intrusion porosimetry tests were conducted on saturated Gaomiazoi (GMZ) bentonite specimens. Results show that, compared with the other two methods, only SBS tests can accurately measure the gas breakthrough pressure. Meanwhile, the results of gas injection tests by the SBS method indicate that gas intrusion into a specimen in the inlet accompanied by water displacement at the outlet is not an instantaneous process. Therefore, the time required for pore pressure equilibrium during each pressure step was calculated, which is in inverse proportion to the intrinsic permeability of the compacted bentonite materials. Furthermore, the gas injection tests also show that gas breakthrough could also occur at lower pressure levels, i.e., snap-off pressure, which is far below the value of the breakthrough pressure. The differences between the breakthrough pressure and the snap-off pressure could be induced by the hysteretic behavior of the bentonite material during the drainage and imbibition processes. The delayed re-imbibition of water will induce further gas migration through the interconnected pore space and a lower capillary pressure. Experimental results in this work are very useful for the engineering design and safety assessments of the repository.

Inspired by the naturally occurring anti-freeze mechanisms of biological organisms, herein we introduce an ionic compound (LiTFSI) into zwitterionic polymer networks to enhance the anti-freezing performance of gels. The ionic diffusion of the high viscosity LiTFSI aqueous solution within the aligned gel endows the reswollen polymer electrolyte with excellent freeze-tolerance and high ionic conductivity. Interestingly, ion can act as binders to promote tight adhesion between the aligned gel electrolyte and a porous electrode. The resulting well-designed supercapacitors are superior to traditional PVA gel based devices with respect to their electrochemical window (2.5 V) and energy density (52.3 W h kg−1). Furthermore, the devices are facilely assembled, possess a broad range of working temperatures (−54 to 100 °C), and exhibit remarkable flexibility under consecutive bending conditions. Ultimately, this study provides new insight and motivation for the development of robust supercapacitors suitable for complex and harsh environments.

International audience; During the long term operation of a disposal repository, gas will be inevitably generated. Determination of gas permeability of compacted bentonite is of great importance for the safety assessment of the engineered barrier system. In the present work, the steady-state and residual pressure methods were employed to determine the gas permeability of GaoMiaoZi (GMZ) bentonite with consideration of variations in liquid saturation, dry density and confining pressure. Results show that, gas migration in saturated GMZ bentonite was mainly controlled by diffusion with an effective gas permeability of 1E-23 m2 - 1E-25 m2. While in unsaturated GMZ bentonite, significant gas flow rates could be observed, which increased stably with the increase of gas injection pressure. Klinkenberg effect was significant when gas flow through GMZ bentonite. The Klinkenberg corrected gas permeability decreased by 3.5–5.5 orders of magnitude as the liquid saturation increased from 10% to 70%. A decreasing magnitude of 1–2 orders in Klinkenberg corrected gas permeability was presented with the dry density increased from 1.5 Mg/m3 and 1.7 Mg/m3. The Klinkenberg corrected gas permeability decreased by 0–1 orders of magnitude as the confining pressure increased from 3 MPa to 7 MPa. By using the accessible porosity, gas measured intrinsic permeability could be determined with values ranged between 1E-15 m2 to 4E-15 m2, which was higher than the water measured one by 5 orders of magnitude. Additionally, a generalized power law was successfully adopted in this study to describe the evolution of gas relative permeability with the liquid saturation. Overall, the effective gas permeability, Klinkenberg corrected gas permeability, intrinsic and relative permeability determined in this study provided a comprehensive perspective to assess the buffering property of GMZ bentonite in multi-physical field coupling environment. The parameters obtained can be adopted in further simulation works for long-term safety analysis of the disposal repository from the viewpoint of gas migration.

Gas breakthrough is one of the key characteristics for saturated geological materials with low permeability encountered in deep geological disposal of high-level radioactive waste, shale gas extraction and CO2 capture-geological storage, etc. Although gas breakthrough phenomenon is widely investigated in previous researches, the essential mechanisms are still not clearly explained. In the present work, a new approach for determination of gas breakthrough mechanism was proposed and a series of gas injection tests were conducted for verification. According to the theoretical analysis, the capillary effect, mechanical effect, as well as the interfacial leakage can induce gas breakthrough. The capillary breakthrough indicates the displacement of liquid phase driven by the capillary effect to form continuous flow pathway across the specimen. The mechanical breakthrough indicates the mechanically induced dilatancy of flow pathways followed by a significant decrease of the capillary flow resistance in the specimen. The interfacial breakthrough indicates the failure of the sealing capacity of the lateral boundary during the gas injection test. Based on the inherent characteristics, deformation properties of the specimen, as well as the boundary conditions, relationships between the capillary/mechanical/interfacial breakthrough pressure and the confining pressure were established, which can be used to determine the triggering mechanism of the gas breakthrough. Constant-volume and constant-isotropic stress gas injection tests were subsequently conducted for verification of the proposed approach. Results showed that the mechanical breakthrough is the most likely mechanism for the low permeability specimens tested in the present work.

Bentonite, known as all-purpose soil, is a kind of clay widely used in engineering. Graphene oxide (GO) has a broad application prospect in many fields as a superior adsorbent. In this study, a new type of adsorbent, graphene oxide modified organic Gaomiaozi (GMZ) bentonite was synthesized. The adsorption behavior of Yb(III) through batch experiments on the GMZ bentonite and graphene oxide-modified organic GMZ (GO-GMZ) bentonite was studied. Before the experiments, the two adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results show that the adsorption of Yb(III) onto GMZ bentonite and GO-GMZ bentonite reaches equilibrium within 60 min. The Yb(III) adsorption process is well described by the pseudo-second-order kinetic model and the Langmuir isotherm model. The maximum Langmuir adsorption capacity of Yb (III) onto GO-GMZ bentonite is 36.90 mg/g higher than that onto GMZ bentonite of 34.48 mg/g, which demonstrates that the GO-GMZ bentonite improves the adsorption performance of GMZ bentonite. It can also be confirmed by the characterization results that the montmorillonite layers in GO-GMZ bentonite have a larger pore structure and layer spacing, which is beneficial to the intercalation of cations. Because the GO-GMZ bentonite has the main composition of bentonite, it can help to eliminate the irreversible aggregation and accumulation of graphene oxide in solutions and can improve the recyclability of bentonite. Considering the other advantages of bentonite such as low-cost, high stability, easy availability, and environmental friendliness, it is worth popularizing and applying the new GO-bentonite adsorbent.

The design of sealing/backfill elements in high level radioactive waste disposal repository often depends on the laboratory results in terms of swelling pressure and hydraulic conductivity. In this study, a series of swelling pressure test were performed on compacted specimens of MX80 bentonite and Callovo-Oxfordian (COx) claystone mixture with different initial stress state (with and without residual stress relaxation), aiming at investigating the effects of compaction induced residual lateral stress on the swelling pressure. Two methods named “Trimming” and “Transfer” were applied for the residual stress relaxation; whereas specimens were directly compacted in the hydration ring to simulate the case without residual stress relaxation. A higher swelling pressure was obtained on the specimens directly compacted in the testing ring compared to their counterparts with residual stress relaxation. Regarding to the relaxation methods, “Transfer” is more practical for specimen preparation in the laboratory, as it allows the relaxation of the stress without changes in specimens' diameter and mass.

This communication provides a facile approach to prepare a polyacrylamide polymer xerogel film which exhibits humidity-sensitive actuation and ionic conductivity.

The technological voids, which are inevitably formed during the construction of the deep geological repository for high-level radioactive nuclear waste, have negative influence on the hydro-mechanical properties of the engineered barrier. In this study, hydration tests along with image acquisition were carried out on compacted GMZ bentonite samples with simulated technological voids to investigate the self-sealing behavior and research emphasis was placed on the cracks appearing during the hydration. Results showed that with the presence of technological voids, the self-sealing process of the hydrating sample can be visually divided into cracking and closing stages, both of which were affected by the sample preparation process, the void width, and the sample height. The hydration cracks occurred at the early phase was considered as tensile cracks induced by the failure of tensile strength against the tensile stress induced by the difference of swelling strain. At the end of hydration, though the cracks were closed with the sealing of technologicalvoids, the distributions of water content and dry density along the sample diameter were found in a decreasing and an increasing trend with the increasing distance from the technological voids, respectively. By applying the image processing method, the cracking and sealing behaviors were further studied with the reciprocal verification of digital images and grey scale profiles. A good agreement between the grey scale profiles and digital images was found and the quantifiable grey values could accurately reflect the color difference on the samples which was difficult to be distinguished by the naked eye, showing the applicability and superiority of image processing technology.

Investigation on compression behavior of bentonite is an important issue for designing the artificial barriers in a geological repository for nuclear waste disposal. However, significantly different from that of non-expansive clays, normal consolidation line of bentonite may be nonlinear in e-lnp plane. Obviously, the linear equation of normal consolidation line in conventional constitutive models could not describe nonlinear compression behavior of bentonite. In this work, compression tests were carried out on GMZ01 bentonite slurry and compacted specimens with suction-control (0, 1, 4.2, 9, 38 and 113 MPa). The mechanisms of nonlinear normal consolidation line for saturated and unsaturated bentonite were analyzed, respectively. According to the test results, an elastoplastic constitutive model framework with nonlinear normal consolidation line for describing compression behavior of bentonite was proposed. In which, a unified equation of normal consolidation line for saturated bentonite specimen related to water content was proposed. Meanwhile, a critical saturation curve was defined in a s-p space to describe the transformation from unsaturated to saturated states during the compression process of unsaturated bentonite specimens. Based on the critical saturation curve, a nonlinear equation of normal consolidation line for unsaturated bentonite was proposed. Finally, the compression test results for bentonite slurry and compacted specimens were simulated by the model newly proposed. Validation shows that good agreements are reached between the simulated results and experimental ones. Obviously, the nonlinear equation of normal consolidation line proposed in this paper could be used in development of constitutive models for describing nonlinear compression behavior of bentonite.

Bentonite pellet mixture is considered as a candidate buffer/backfill material for construction of artificial barrier in high-level radioactive waste (HLW) repository. Compared to that of a compacted bentonite block, one of the most obvious features of this material is its initially discrete pellets and significant structural heterogeneity, leading to a distinctive swelling behaviour. In this paper, a comparative study concerning development of swelling pressure on hydration was conducted on pellet mixture and compacted block of Gaomiaozi (GMZ) bentonite specimens with an identical dimensions (50 mm in diameter and 35 mm in height) and dry density (1.45 Mg/m3). Results show that, within the initial 72 h, the swelling pressure of pellet mixture increases at a lower rate and achieves a lower peak value than those of the compacted bentonite block. Then, the swelling pressure of the pellet mixture varies slightly while that of the compacted bentonite block decreases significantly before increasing again. Eventually, the swelling pressure for the two specimens approaches to an almost similar final stable value, 0.939 MPa and 0.947 MPa, respectively. Based on an insight into the mechanism of swelling pressure development, a new theory on competition between the accumulated and dissipated wedging pressures (AWP and DWP) within the specimen was proposed for interpreting the different developments of swelling pressure between the two specimens. As water infiltrates, the AWP continuously increases, while the DWP firstly increases and then decreases. At a given infiltration time, the AWP of pellet mixture is smaller than that of compacted bentonite block due to accommodation of the pellets swelling deformation by inter-pellet voids. While within the initial hundreds hours of infiltration, the DWP within pellet mixture could be larger than that in compacted bentonite block due to the deformation (swelling or compression), damage (cracking or collapse) and displacement (rotation or dislocation) of the pellets. For both specimens, the macro and micro void ratios decrease while the meso void ratio increases with infiltration, leading to a gradually homogenized pore structure. Taking the pore size distribution curve of the middle layer of compacted bentonite block specimen after 720 h hydration as a reference, a porosity homogenization index (PHI) was defined to reflect the degree of pore structure homogenization. Generally, the PHIs of the upper, middle and lower part of the two specimens decrease with infiltration time. At a given time, the PHIs of pellet mixture are higher than those of compacted bentonite block but the difference between them decreases, leading to corresponding decrease of the swelling pressure difference. Considering the low swelling pressure (

With the development of urbanisation, massive crushed demolished construction wastes (CDW) have been generated, leading to environmental and disposal problems. In order to recycle the waste efficiently, an engineering sustainable CDW disposal was carried in Dajiangdong area, Hangzhou China. CDW was used to stabilize the deposited loose silt for feasible constructions and liquefaction preventions. The mix comprised of silt, CDW and cement was designed to be used as subbase material with higher density and less void. The vibro-compaction was used to rearrange the grain distributions within the mixtures to improve the mechanical performance at an optimal constituent, and then the numerical simulation was performed to evaluate the internal mechanism of the admixture under compression. The result shows that the workability of CDW-silt-cement mixture has an optimum of CDW content at 50% in order to achieve the best engineering performance. Moreover, the cement, which intergrades the silt and supplies the cementation among the coarse CDW, are not only generate cohesion but also provide anti-erosion effect.

Cracking of wall surface crusts on salt-laden earthen remains at the site of Yar City, northwest China, is a serious concern. This widespread pathology is closely related to the drying shrinkage behavior of soil and the fundamental mechanisms should be investigated to better interpret the deterioration of these precious heritages. In this paper, tests were conducted on specimens that were initially slurries with NaCl contents of 0%, 2%, and 5%. The soil shrinkage characteristic curve (SSCC) was measured using the fluid displacement method during a slow air-drying process. The drying branch of the soil-water retention curve was also established by combining three common techniques (pressure plate, vapor equilibrium, and filter paper). The microstructure of specimens with varying water and NaCl contents was characterized through mercury intrusion porosimetry (MIP) testing. Results show that NaCl had negligible impact on the SSCC, which was composed of three shrinkage zones. With increasing NaCl content, matric suction was hardly influenced while the total suction increased significantly due to the pore fluid osmotic suction. The analysis indicated that drying shrinkage was actually a process of consolidation governed predominantly by matric suction. MIP test results were consistent with the macroscale observations for specimens that were initially slurries: that the total volume of pores decreased upon drying. In addition, pore-size distribution was unimodal and no significant difference was observed among specimens with different NaCl contents.

A series of incremental loading oedometer tests was conducted on pure clays and sand–clay mixtures with various sand/clay ratios and clay mineral compositions. The void indexes Ivand IvHwere introduced to evaluate their intrinsic compressibility. Test results revealed that Ivwas more suitable for depicting the compression behavior of pure clays than IvH; whereas, for the compressibility of sand–clay mixtures, the normalized compression line by using Ivwas obviously different from that of pure clays and traditional soils due to the presence of sand particles. Therefore, a four-phase analysis framework of sand–clay mixtures was introduced to unify the intrinsic compression behavior of soils with and without sands. It was found that the updated clay void index Ivcparameter was effective to unify the soil’s compression behavior by excluding the influence of sand particles in clays before the sand skeleton formation. After the formation of the sand skeleton, the cluster particles change the stress distribution of mixtures, leading to less stress being applied on the clay matrix and thus the bifurcation of the intrinsic compression behavior.

In China, GMZ bentonite has been recognized as the first choice of buffer material while Beishan area in Gansu province has been considered as the potential disposal site for high-level radioactive waste (HLW) repository. As the groundwater at the repository field is rich in Na + and Ca 2 + , the swelling properties of compacted GMZ bentonite will be affected by the chemistry of groundwater. In this work, the swelling deformation behaviors of compacted GMZ bentonite were tested at a vertical stress of 0.1 MPa to assess the influence of cation exchange between Na + and Ca 2 + , as well as the salinization-desalinization effect by cyclical infiltrations of distilled water and saline solutions. Results show that Na-bentonite is partly transformed into Ca-bentonite once NaCl solution is replaced with CaCl 2 solution. Comparing to NaCl solution, the inhibiting effect of CaCl 2 solution is weak, and thus the additional swelling is observed. Along with chemical cycles, the total strain of compacted GMZ bentonite gradually increases, while the chemical sensitivity of strain behavior reduces. This tendency is also detected for GMZ bentonite experiencing the desalinization path of distilled water prior to each infiltration of salt solution. Regarding the relative strain of GMZ bentonite, the difference in osmotic suction between distilled water and salt solution is more relevant than the sodium-calcium replacement.

Investigation on influences of cyclic salinization-desalinization processes on clay is of great importance for evaluation of the behavior of engineering barrier in deep geological repository for disposal of high-level radioactive waste. In this study, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite specimens, which has an initial dry density of 1.7 Mg/m 3 , with cyclically infiltration of NaCl, CaCl 2 or KCl solution at different concentrations and de-ionized water. Results show that the volume change of the compacted clay could be influenced by concentrations and cation types of solutions, as well as number of salinization-desalinization cycles during the salinization-desalinization processes. The total swelling strain of GMZ01 bentonite specimens measured on the first salinization with salt solutions at a same concentration follows an order: NaCl > CaCl 2 > KCl. This observation could be explained by the influences of basal space, DDL theory and K-linkage. A cation with higher replacing power can be replaced by another cation with lower replacing capacity when it has a higher concentration in the pore solution. The concentration effect on the swelling behavior of GMZ01 bentonite was explained using the concept of osmotic suction for specimens initially infiltrated with NaCl solutions. For specimens initially infiltrated with CaCl 2 solutions, in addition to the osmotic suction effect, the distance between the unit layers of montmorillonite also played a role in explaining the swelling behavior. The swelling behavior of specimens initially infiltrated with KCl solutions could be explained by the strong K-linkage.

International audience; 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/m3 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.

International audience; This paper presents a constitutive model for simulating the swelling–shrinkage volume change of expansive soils during wetting–drying cycles. Based on the concept of the critical swelling–shrinkage state proposed by Zhao et al. [21], the elastic swelling–shrinkage deformation, plastic swelling/shrinkage deformation, and accumulative deformation during wetting–drying cycles were analyzed. Influences of the initial dry density and stress applied on the yield suction were discussed. The suction decreasing yield surface (SD) and suction increasing yield surface (SI) were defined. The variations of SD and SI yield surfaces were proposed. Calibration was performed with results of the cyclic wetting–drying tests conducted. The proposed model was validated by simulating cyclic wetting–drying tests conducted on compacted GaoMiaoZi (GMZ) bentonite. Good agreements were obtained between the simulation results and the experimental ones. Using the proposed model, influences of dry density and stress applied on the swelling–shrinkage deformation and accumulative deformation of expansive soil were well described.

International audience; In traditional soft ground improvement of cement stabilized columns, strength was paid more attention than hydraulic conductivity, but the latter can improve the settlement uniformity and long term safety. This study introduced the pore-making technology referring the foaming concrete/mortar in an attempt to improve the hydraulic conductivity of the stabilized marine clays. The flexible wall permeameter and unconfined compressive strength (UCS) tests were conducted to depict the macro behaviour, and then the mercury intrusion porosimetry (MIP) tests and scanning electron microscopy (SEM) tests were performed to clarify the micro-structures. The hydraulic conductivity of foamed cement-stabilized marine clay at a given cement ratio (20%–50%) and density (900–1400 kg/m3) was found to be approximately 100–1000 times of that of (cement stabilized) clays without foaming, whereas the strength decreased only by 2–4 times. Microstructure investigation indicated that the hydraulic conductivity of foamed cement-stabilized marine clays was mainly dominated by the macro pore volume (>1 μm). Furthermore, a greater cement ratio and more active metakaolin additives enable developing an innovative material for columns that satisfy both the strength and hydraulic conductivity requirement.

International audience; Most of the existing particle packing models were linear or only applicable for binary-size/ternary-size mixtures. In this study, based on the concept of dominant size class, a nonlinear packing model was developed allowing prediction of the void ratio of granular soils with arbitrary particle size distributions. Only two parameters (filling coefficient and embedment coefficient) were incorporated in the proposed model. Calibrations showed that both parameters were related to the particle size ratios between the dominant and non-dominant size classes. The model was verified using the experimental results on the crushed pellets of GMZ bentonite and several other granular materials from literature. Good agreement was obtained between the predictions and measurements, showing the performance of the proposed model.

International audience; In high-level radioactive waste repositories, compacted bentonite used as buffer/backfill materials should have enough strength and stiffness to enhance the stability of the repository and to protect the waste canisters from the surrounding rock. Therefore, it is essential to understand the strength and deformation behavior of compacted bentonite. In this paper, isotropic loading tests and triaxial shear tests were carried out on unsaturated compacted GMZ bentonite. With the results of compaction and isotropic consolidation tests, the compressibility and yield behavior under isotropic condition were investigated. Based on the results of triaxial shear tests, the stress-strain relationships and dilatancy were analyzed. The overconsolidation ratios for all the specimens were determined. For normal consolidated and lightly overconsolidated soils, existence of a critical state on the compacted GMZ bentonite was confirmed. The evolution of the critical state stress ratio with change of confining pressure was analyzed. For highly overconsolidated soils, the peak strength lies above the CSL. In this regard, the Hvorslev Surface was introduced and related parameters were determined for describing the peak strength of unsaturated highly overconsolidated GMZ bentonite.

International audience; 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.

International audience; High-density bentonite pellets have been considered as an alternative sealing or buffer material for nuclear waste repository. Investigation on the packing behaviour of pellets is of great interest due to the significant impacts of global density on the hydro-mechanical behaviors of the material. In this manuscript, the packing behaviour of mono-size, binary-size, ternary-size and multi-size mixtures of crushed bentonite pellets were experimentally investigated. The effects of particle size ratio and distribution on packing dry density were analyzed. Results indicate that, for mono-size mixtures, the packing dry density can be decreased by the agglomerating effect of fine particles and wall effect of the container. For the binary-size, ternary-size and multi-size mixtures, there is an optimal particle size distribution corresponding to the peak packing dry density, which was found to be closely related to the particle size ratio of the finest size class to the coarsest one. The effects of particle size ratio and distribution can be interpreted by the ‘net positive effect’ representing the competition between the positive effects (filling and occupying effects) and the negative effects (loosening, wall, wedging and agglomeration effects) of particle interactions.

Determination of parameters for description of gas migration in saturated bentonite is of great importance for the design and construction of artificial barriers in the geological repository for the disposal of high-level radioactive nuclear waste. In this paper, temperature-controlled gas injection tests were conducted on initially water-saturated bentonite specimens using the residual capillary pressure (RCP) technique. Effective gas permeabilities in low injection pressures and the gas breakthrough pressures at temperatures 20, 40 and 60 °C were obtained. Results show that: (i) for all the temperatures tested, the intrinsic water permeabilities ( k i n ) range between 3.2 × 10−20 and 5.72 × 10−20 m2, furthermore, those values increase with rising temperature. Meanwhile, the intrinsic water permeabilities derived from the steady state tests are lower than those obtained from the non-steady state tests; (ii) for all the temperatures tested, the effective gas permeabilities ( k e f f ) corresponding to the viscous gas flow before gas breakthrough range between 4.81 × 10 - 24 and 2.74 × 10 - 22 m2, with slight fluctuations with time. The maximum effective gas permeabilities measured at the occurrence of gas breakthrough on the initially water-saturated bentonite specimens extend from 2.27 × 10 - 18 up to 3.32 × 10 - 17 m2 and increase as temperature increases, while the time required for gas breakthrough decreases as temperature increases; (iii) for all the temperatures tested, the gas breakthrough pressures measured on the initially water-saturated bentonite specimens vary from 2.74 to 4.08 MPa, while the residual capillary pressure differences, also denoted as snap-off pressures (Psnap-off), range from 0.2 to 0.38 MPa. Additionally, the gas breakthrough pressures and the residual capillary pressures decrease as temperature increases.

International audience; The swelling pressure of compacted bentonite under complex thermo-chemical conditions is a critical safety parameter of the engineering barrier system for a deep geological repository. To investigate the cyclic thermal and saline effects on the swelling pressure of densely compacted Gaomiaozi (GMZ) bentonite, a series of constant-volume swelling pressure tests were performed using deionized water and NaCl solutions. At 20 °C and 60 °C, the multi-step salinization-desalinization process generated a hysteresis of the swelling pressure, and multiple saline cycles led to its diminishing attenuation. In the framework of the double structure theory, the observed hysteresis of the swelling pressure was ascribed to the irreversible collapse of inter-aggregate pores under osmotic pressure, while the gradual attenuation was attributed to the further homogenization of the clay structure. Soaking tests were also conducted at various thermal and saline conditions to provide evidence for the cyclic saline effects. No salt precipitation was identified due to the negligible changes in the total suctions of the specimens after saline cycles conducted at 20 °C and 60 °C. The membrane effect (resistance of compacted clay to salt migration) was considered, and its degree reflected the changes in the osmotic suctions of the specimens occurred after the soaking tests. The resistance of compacted GMZ bentonite to salt migration was relatively strong during wetting with low saline solutions (0–0.5 mol/L) at 20 °C, and became weak during wetting with high saline solutions (1.0–2.0 mol/L) or at 60 °C. The variation of the swelling pressure during thermal cycling depended on the maximum temperature and concentration of the presaturation solution. By assuming the validity of the membrane effect at the maximum temperatures (valid at 40 °C and invalid at 60 °C), the inconsistent changes in the swelling pressure during thermal cycling were explained by considering the incomplete initial saturation and role of thermal osmosis. However, further verification of the proposed hypothesis by X-ray diffraction and membrane testing at elevated temperatures is required.

International audience; Special soft soils with notable coarse and fine fractions are not only naturally deposited in ravines or interactive marine-terrestrial sedimentation areas but also artificially formed in land reclamation projects via the hydraulic filling method and excavation cuts of different soil layers in open pit mines. The behaviour of these soils is not like that of similar normal sands or clays but rather exhibits the characteristics of transitional mixtures. A sand-clay mixture was prepared in the laboratory to simulate these special soil types, and an incremental loading oedometer test was performed. Thereafter, a secondary compression coefficient (Cα) was determined to investigate the long-term rheological properties of the mixture. The results in the Cα-log σv′, Cα-e, Cα-e/eL and Cα-Cc planes revealed that a threshold e/eL value (i.e. approximately 0.45) in the case of sands. Note that in a traditional Cα-Cc analysis system, the soils with sand exhibited the same secondary compression behaviour (SCB) as that without sand before reaching the transformation threshold, whereas after this threshold, they appeared to behave much like sandy soils. Hence, a four-phase frame of sand-clay mixture was further introduced to clarify the mechanism of the SCB of soils with and without sand. An updated Cαc/Ccc reflecting the creep mechanism of the clay phase can be effectively adopted to exclude the influence of sand particles. Because the quantitative relationship between Cαc and Ccc for soils with sand was consistent with that without sand, it can be assumed that the SCB of clay-sand mixtures was dominated by the creep of fine fractions. It is porposed that the existence pattern of the sand particles in the mixtures undergoes three stages with an increasing vertical stress, i.e. non-skeleton, partial skeleton and complete skeleton. A further discussion on the mutual relationship between phases on the behaviour of mixtures with sand revealed that isolated sand had no effect on the compressibility and SCB during the initial stage. When a partial skeleton of the sand was established in the second stage, characterized by less stress on the clay matrix, the compressibility and SCB were suppressed. When the vertical stress continuously increased or additional sand content was supplied, a complete skeleton formed. Hence, the behaviour of mixtures (regardless of compressibility and secondary compression) was completely governed by the sand fractions. Generally, the same relationship was observed between Cαc and Ccc during the first and second stages with sand, indicating that the influence of sand fractions on the compressibility and SCB can be counterweighted.

International audience; It is generally recognized that swelling/shrinkage deformation and accumulative deformation of compacted expansive soil on wetting-drying cycles are strongly influenced by the specimen’s dry density and the net stress applied. However, investigations on the coupling effects of dry density and vertical net stress on the swelling/shrinkage deformation and accumulative deformation are limited. Cyclical wetting-drying tests were conducted on unsaturated compacted GMZ bentonite specimens with three dry densities under five vertical net stresses. The influence of vertical stress on accumulative deformation was analyzed. Then, the critical vertical stress was proposed for specimens with different dry densities. The effect of dry density on accumulative deformation was studied and the critical dry density was determined for different vertical stresses. Based on these, the concept of critical swelling-shrinkage line is proposed in the e-p space. According to the relationship between the initial state of specimen and the critical swelling-shrinkage line, a state parameter is proposed for determination of the accumulative strain on wetting-drying cycles.

Gas migration behavior of bentonite-based buffer/backfill materials is one of the most important issues for safety assessment of deep geological repository for disposal of high-level radioactive wastes. In this paper, a series of gas injection tests were conducted on water-saturated bentonite specimens under stepwise increasing gas injection pressure and a flexible boundary condition. Prior to the gas injection tests, the steady state water permeability (stst kw) was determined by the water injection tests with deionized water and the stst kw values obtained were in an order of magnitude of 10-19-10-20 m2, which were slightly lower than the non-steady state ones calculated from the results obtained at the initial stage of gas injection tests. The effective gas permeability measured before and after gas breakthrough on the initially water-saturated bentonite specimens with different dry densities ranges between 5.45 × 10-25 m2 to 2.76 × 10-22 m2. The Peclet number (Pe) was adopted to determine the relative importance of advective gas transport (pressure-driven gas flow) to diffusive gas transport through saturated bentonite specimens. Calculation shows that the Pe values are significantly lower than 1, indicating the dominant role of gas diffusion across the specimens before and after gas breakthrough. As dry density of the specimen increases from 1.3 g/cm3 to 1.7 g/cm3, the gas breakthrough pressure increases from 4.92 MPa to 8.68 MPa. Meanwhile, the residual pressure differences measured after gas breakthrough ranges from 4.04 MPa to 7.88 MPa for specimens with different dry densities. A slight decrease of water content of the specimens experienced gas injection tests and a stable increase of specimen volume at the breakthrough indicate that both the displacement of water from interconnected pathways (capillary effect) and the pathway dilation (mechanical effect) play important roles in gas breakthrough for specimen tested under flexible conditions.

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.

The use of foamed concrete (FC) with cement partially replaced by fly ash (FA) for urban abandoned underground-space backfilling was examined. The influence of density, water-binder ratio, and FA content on the mechanical and chemical properties of FC and on the surrounding environment was investigated in the laboratory and in the field. FC immersed in sulfate solution exhibited increased mass and compressive strength, as well as high resistance to sulfate attack. The maximum stress generated by FC on the original structure was 75.68 kPa; settlement variation was negligible. FC is thus a promising material that minimizes environmental effects of backfilling.

Saline solution has considerable influence on the thermo-hydro-mechanical behavior of compacted bentonite. In this study, the effects of NaCl solution on the swelling deformation, oedometric compression, and thermal volume behavior of compacted Gaomiaozi bentonite were investigated. Results show that compressibility of the specimens decreases with increasing NaCl concentration of the presaturation solution. At an overconsolidation ratio OCR = 1 with a vertical pressure σ v = 0 . 1 MPa and at OCR = 3 with σ v = 0 . 1 MPa, the specimens exhibited contraction during heating below the transition temperature, followed by dilation. A larger vertical pressure led to a higher transition temperature (at which the thermal strain turns from contraction to dilation). However, the specimens exhibited only thermal dilation at OCR = 10 with σ v = 0 . 1 MPa and at OCR = 2 with σ v = 0 . 5 MPa . Additionally, for a given vertical pressure and presaturation solution, the specimens with a higher OCR exhibited a larger irreversible dilation after a thermal cycle. Moreover, the influence of the presaturation solution on the thermal volume change is non-monotonic. A dilute solution (NaCl solution of 0.1 mol/L) and high salinity solution (NaCl solution of 1.0 mol/L) markedly promote the thermal expansion. These experimental trends can be attributed to the fact that attenuation of the diffused double layer repulsion and electrostatic interaction as well as the reduction of the free water at high saline conditions harden the soil.

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.

Rare earth elements are an important strategic resource, and it is urgent that the rare earth industry continue to explore and develop novel separation methods and technologies. Herein, we fabricated an efficient semi-IPN alginate-clay-poly(n-isopropylacrylamide) (NIPAm) hydrogel by a frozen polymerization method with the help of UV light irradiation, where alginate was employed as the main adsorption functional compound. The as-prepared hydrogel exhibits tough, sponge-like hierarchical macroporous and reversible temperature-responsive characteristics. The maximum adsorption capacity of La3+ is 182 mg/g for the hydrogel composition of 5.0% NIPAm, 4.0% clay, and 3.0% alginate. The Langmuir isotherm fits the data very well, and the adsorption follows the pseudo-second-kinetic equation. The trace of La3+ ions can be effectively separated from the coexisting metal ions. After six repeated adsorption–desorption cycles, no obvious deformation of the shape and or loss of adsorption capacity of the bulk hydrogel ...

Diffusion experiments in compacted GaoMiaoZi (GMZ) bentonite were performed with La(III) using the in-diffusion method, and the effects of dry density and pH were emphasized. The apparent diffusion coefficient (D a) was obtained at the end of the experiment. D a decreases with increasing dry density of 1.3–1.7 Mg m−3 as well as slightly decreases as pH increases from 3.6 to 8.9. The D a range is (2–16) × 10−12 m2 s−1 under the experimental conditions. The distribution coefficients (K d) for La(III) in compacted GMZ bentonite are much smaller than those on powdered bentonite obtained from batch experiments.

Metakaolin, widely applied in concretes and cement-stabilized soft clays to improve their macro performance, was evaluated for use in compacted soils in highway and backfill engineering. The compactability and strength performance of cement-modified soils with metakaolin were evaluated in the terms of their applicability and effectiveness. The cement-based modified soil with metakaolin was more insensitive to water and more convenient in the field rolling compaction. The strengths, in addition to the unconfined compressive strength and splitting tension strength, were improved significantly. Up to a threshold ratio of metakaolin to cement ranging from 1/3 to 1/2 in this case, the strengths increased gradually, while they unexpectedly decreased thereafter. This phenomenon was different from the previously reported metakaolin applications in concrete and cement-stabilized soft clays, which was probably due to the water content and the ratio of metakaolin to hydrated calcium hydroxides of the cements. Microstructure analysis by X-ray diffraction, scanning electronic microscopy, thermo-gravimetric analysis and mercury intrusion porosimetry, demonstrated that the addition of metakaolin led to a higher quantity of hydration products and a denser micro-porosity distribution.

The Site of Yar City (formerly known as the ancient city of Jiaohe) is regarded as the largest, oldest and best-preserved earthen site in the present world. However, during the long-term exposure to the environment, remaining earthen structures suffered serious deterioration caused by many influencing factors. The shallow surfaces of the earthen walls can be fully saturated by the concentrated and heavy rains to form flowing slurry, which will be turned into strongly cracked surface crusts during the following intense evaporation and peeled off by wind deflation. In the meantime, salts could be accumulated in the remains to a quite high content, which could exacerbate the deterioration conversely. In this work, desiccation tests were conducted in laboratory on the initially saturated slurry specimens with different thicknesses and NaCl contents and thicknesses. Development of crack patterns on the surface of the specimen was monitored with a digital camera and analyzed using commercialized software. Results indicated that the evaporation process of NaCl-contaminated specimens could be identified as three stages, which has the same configuration as that of salt-free specimens. Cracking behavior of soil that closely correlates to the evaporation process was also influenced. As the NaCl content increases, the CIF values, intersection number, segment number as well as the total crack length decreases. A non-orthogonal crack pattern and orthogonal crack pattern were dominant for thin and thick specimens, respectively.

International audience; Compacted bentonite has been considered as buffer/backfill material in radioactive waste disposal. This study deals with the effects of the concentration of infiltration saturation vertical stress on the swelling behavior of compacted GMZ bentonite. One-dimensional swelling tests were performed on specimens at an initial dry density of 1.7 Mg/m(3). NaCl solutions with concentrations of 0 (de-ionized water), 0.1, 0.5 1.0 mol/L were used to saturate the specimens while three vertical stresses of 0.06, 0.062 1.85 MPa were applied. Results show that, for all the infiltration solutions, the swelling strain decreases with increasing vertical stress; the impact of the vertical stress is more significant for low vertical loads while it is less patent when the high salinity solution is infiltrated. For all the vertical stresses applied, the swelling strain decreases with the increase in concentration of the infiltration solution; the impact of the concentration of infiltration solution is not only related to the vertical stress, but also to the soil permeability. Furthermore, the coefficient of primary swelling decreases with the increase in concentration vertical stress. The concentration of infiltration solution has significant restricting effect on the secondary swelling deformation in case of low vertical stress, while it has no or few effects on the secondary swelling deformation in case of higher vertical stress. (C) 2016 Elsevier B.V. All rights reserved.

The safety assessment for the long-term operation of a deep repository is an essential work for the engineering of geological disposal for high-level radioactive waste (HLW). Bentonite selected as a barrier/backfill material in repositories, can be exposed to the chemical effects of alkaline site water owing to cement degradation, which poses a great threat to the long-term safety of the repository. This work is focusing on the swelling property of bentonite with alkaline pore water in order to investigate the evolution of vertical strain of compacted Gaomiaozi bentonite under the simulated long-term environments of Chinese HLW repository. Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests were utilized to analyze the bentonite after it reacted with the pore water solutions. Results reveal that the swelling strain is significantly influenced by the composition and concentration of the solutions, and that swelling time decreases as temperature increases. In the secondary swelling stage of the bentonite hydrated by solutions containing K+, the swelling strain decays because K ions enter the montmorillonite crystal layer and generate low-swelling or non-swelling minerals. The solutions (particularly high alkaline solutions) containing K ions strongly decrease the swelling performance of bentonite, suggesting that alkaline cement water can degrade the swelling property of the buffer material in a high-level radioactive waste (HLW) disposal system.