Article

Effects of hyperalkaline solutions on the swelling pressure of compacted Gaomiaozi (GMZ) bentonite from the viewpoint of Na+ cations and OH– anions

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Abstract

In China, Gaomiaozi (GMZ) bentonite has been selected as a suitable buffer material for high-level nuclear waste disposal. Groundwater in Beishan area, which has been considered as a potential site for the construction of the repository, may reach a high pH value because of its chemical background and possible concrete degradation during the operation of the repository. In order to investigate the independent effects of Na⁺ cations and OH– anions on the behavior of compacted GMZ bentonite, a series of swelling pressure tests with NaOH and NaCl solutions infiltration was conducted. The mercury intrusion porosimeter (MIP) and the X-ray diffraction (XRD) explorations were performed on the specimens before and after experiencing the swelling pressure tests in order to find out the influences of infiltration solution chemistry on the microstructure and mineralogy of compacted GMZ bentonite. The results showed that high Na⁺ cations concentration can inhibit the crystalline swelling and the double-layer swelling; high OH– anions concentration can facilitate the double-layer swelling and fabric re-arrangement; the effects of NaOH solutions with different concentrations on the swelling pressure present the interaction of Na⁺ cations and OH– anions.

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... For compacted bentonite, the swelling pressure is originated from crystalline swelling and diffuse double-layer (DDL) swelling, both of which can be weakened by the water salinity [12,27,36]. The cation ions of high concentration on the saline solutions inhibit the crystalline swelling and the double-layer swelling of compacted bentonite [20,59], and the attenuation effects were less significant for calcium ions than sodium ions [4,58]. Specifically, DDL swelling is both associated with the salt concentration in electrolyte solution and the compaction degree of specimen. ...
... The weighed mixtures were poured into a stainless steel ring and compacted with a shaft at a constant rate of 0.2 mm/min until reaching the given displacement. Once this was reached, the shaft was locked in place for more than one hour to obtain a homogeneous specimen [20,34]. The compacted mixtures were pie-shaped, with a diameter of 50 mm, height of 10 mm, and dry densities of 1.8, 1.9, and 2.0 g/cm 3 . ...
... Finally, the samples were placed in a color-changing silicone container to ensure their dryness [59]. For MIP tests, the injection pressure limit for the Micromeritics-AutoPore IV 9500 Mercury intrusion meter was 3.448 kPa and 413.79 MPa, and thus, pore sizes from 3 nm to 361,087 nm could be effectively detected [20]. ...
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Compacted Gaomiaozi bentonite–sand mixtures are regarded as attractive buffer/backfill materials for nuclear waste deep geological disposal. When the mixture blocks are emplaced, the bentonite dry density as well as the salinity of groundwater is of primary importance for their hydro-mechanical behavior. In this study, the influences of bentonite dry density and water salinity on the swelling pressure and saturated hydraulic conductivity of such mixtures are investigated using the constant volume method. Results indicated that for mixtures having a low sand content, the swelling pressure was only associated with the bentonite dry density, while the hydraulic conductivity was influenced by the presence of sand particles. Exponential relationships were noted among the final swelling pressure, the saturated hydraulic conductivity, and bentonite dry density on the case of distilled water and salt solutions infiltrating. As the salt concentration increased, the swelling pressure and hydraulic conductivity decreased and increased, respectively, and the effects became less significant as the bentonite dry density increased. The higher swelling pressure and lower hydraulic conductivity on specimens infiltrated with calcium chloride solutions were a result of cation exchange reactions and a competition between the swelling potential change caused by the calcium ions in the interlayer of smectite and electrolyte solutions. The present work can provide a reference for the design of dry density of bentonite–sand mixtures as buffer/backfill materials for deep geological disposal.
... Bentonite is generally recognized as an ideal buffer/backfill material between the canister containing HLRW and the host rock in deep geological repositories due to its low hydraulic conductivity, high swelling characteristics and good self-sealing capacity (Dixon and Gray, 1985;Komine and Ogata, 2004;Ye et al., 2010;Arellano-Cárdenas et al., 2013;Xiang et al., 2019a). The high swelling capacity of bentonite can form an enclosed environment to effectively restrict the migration of nuclides released from the repository (Ye et al., 2017;Zhu et al., 2013;Liu et al., 2018). High volumes of concrete will be used in the construction of a repository, such as in the Swedish KBS3 repository, where the amount of cement is planned as about 900,000 kg (Karnland et al., 2007;Zhao et al., 2018). ...
... High volumes of concrete will be used in the construction of a repository, such as in the Swedish KBS3 repository, where the amount of cement is planned as about 900,000 kg (Karnland et al., 2007;Zhao et al., 2018). However, concrete components will decay during the long-term operation of a repository, producing a highly alkaline solution (Nakayama et al., 2004;Fernández et al., 2010;Chen et al., 2016;Liu et al., 2018). The alkaline solution can react with bentonite near the concrete in the enclosed environment for a long time, leading to a significant local degradation in the mechanical properties of the buffer. ...
... The swelling of bentonite affected by alkaline solutions was also investigated through microstructure studies. Several studies reported the dissolution of montmorillonite and an increase in the total porosity in a synthetic alkaline groundwater environment (Karnland et al., 2007;Cuisinier et al., 2008;Chen et al., 2016;Liu et al., 2018). Karnland et al. (2007) reported that the microstructures of bentonite samples before and after contact with 1.0 mol/L NaOH solution for several months were significantly different. ...
Article
In a high-level radioactive waste repository, bentonite may react with the alkaline solution produced by cement degradation. In this study, bentonite was mixed with alkaline solution in a closed system and reacted for 3–24 months. Furthermore, swelling tests were conducted on the alkaline-dissolved bentonite immersed in distilled water. The swelling deformation decreased significantly with increases in the concentration of NaOH solution and reaction time, and this was mainly due to montmorillonite dissolution. The fractal e – p relationship ( e is the void ratio and p is the vertical pressure) with two calculation coefficients (the swelling coefficient and the fractal dimension) was employed to determine the swelling of alkaline-dissolved bentonite. The fractal dimension increased slightly with increasing reaction time or concentration of NaOH solution, as the dissolution traces caused by the alkaline solution favoured an increase in the irregularity and fractality of the bentonite surface. The swelling coefficient decreased linearly with decreasing montmorillonite content. In addition, the swelling coefficient and the fractal dimension were related exponentially to the reaction time in alkaline solution. A relationship between the swelling of alkaline-dissolved samples and the reaction time was proposed, which might be used to assess the swelling properties of bentonite barriers that would be affected by long-term dissolution of the alkaline solution in a closed repository.
... For compacted bentonite, the swelling pressure is originated from crystalline swelling and diffuse double-layer (DDL) swelling, both of which can be weakened by the water salinity [12,27,36]. The cation ions of high concentration on the saline solutions inhibit the crystalline swelling and the double-layer swelling of compacted bentonite [20,59], and the attenuation effects were less significant for calcium ions than sodium ions [4,58]. Specifically, DDL swelling is both associated with the salt concentration in electrolyte solution and the compaction degree of specimen. ...
... The weighed mixtures were poured into a stainless steel ring and compacted with a shaft at a constant rate of 0.2 mm/min until reaching the given displacement. Once this was reached, the shaft was locked in place for more than one hour to obtain a homogeneous specimen [20,34]. The compacted mixtures were pie-shaped, with a diameter of 50 mm, height of 10 mm, and dry densities of 1.8, 1.9, and 2.0 g/cm 3 . ...
... Finally, the samples were placed in a color-changing silicone container to ensure their dryness [59]. For MIP tests, the injection pressure limit for the Micromeritics-AutoPore IV 9500 Mercury intrusion meter was 3.448 kPa and 413.79 MPa, and thus, pore sizes from 3 nm to 361,087 nm could be effectively detected [20]. ...
... A swelling pressure of 3.02-5.28 MPa, along with a hydraulic conductivity of (2-20) Â 10 À14 m/s, was produced when compacted GMZ bentonite with an initial dry density of around 1.7 Mg/m 3 reacted with deionized water (Zhu et al., 2013;Ye et al., 2013;Sun et al., 2013;Zhang et al., 2014;Chen et al., 2016Chen et al., , 2017Liu et al., 2018;Xiang et al., 2019). ...
... From these observations, Bauer et al. (1998), Cama et al. (2000), Karnland et al. (2007), and Chen et al. (2016) concluded that the surface morphology of compacted bentonite is seriously deteriorated due to the dissolution of montmorillonite in the alkaline solution. Gates and Bouazza (2010), Sawaguchi et al. (2016), and Liu et al. (2018) demonstrated that the dissolution of montmorillonite and some silicate minerals, as well as the resulting mineral transformations, frequently occur in hyperalkaline solutions. ...
... However, these previous researches only focused on the effects of the short-term hydration processes on the swelling pressure of compacted GMZ bentonite while it is infiltrated by saline or alkaline solutions. Liu et al. (2018) and Sun et al. (2018) conducted preliminary experiments on the long-term swelling pressure of compacted GMZ bentonite under heat combined with chemical conditions, and the deterioration of the swelling pressure was observed. ...
Article
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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.
... Swelling pressure decreases with the increase in NaOH concentration owing to high ionic strength and strong mineral dissolution (Chen et al. 2016). The interaction between Na + cations and OH − anions leads to the continuous decrease of swelling pressure in bentonite, which indicates that the structural integrity of bentonite is deteriorated, and thus induces structural damage (Liu et al. 2018). Sun et al. (2018), examined the effects of three types of pore water (site groundwater and two cement solutions) on swelling pressure and found that the deterioration of swelling pressure is caused by montmorillonite dissolution and secondary mineral formation. ...
... The chemical components in groundwater vary along with the concrete degradation; and meanwhile, different hydration cations (Na, K and Ca, etc.) ratios and changes in the pH value will be presented in groundwater (Liu et al. 2018). The concentration of hyperalkaline solution will severely affect the swelling pressure of compacted GMZ bentonite that used as buffer material in full field-size, mainly due to its high pH of over 13 (Ramıŕez et al. 2002, Sánchez et al. 2006, Fernández et al. 2009. ...
... Sample placement and testing as well as data collection and processing were conducted sequentially. These test parameters include a Cu Kα radiation source, wavelength of 0.154 nm, diffraction angle of 4°to 8°, scan rate of 8°2 θ/min, tube voltage of 40 KV, tube current of 100 mA, and data acquisition step size of 0.02°2θ to control the XRD test (Liu et al. 2018). The JADE software was used for processing the data after data collection (Jade Software Corporation). ...
Article
Full-text available
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.
... It is an essential characteristic of bentonite as it fixes the gaps between the layers of bentonite and the fissures between the host rock and bentonite layer and suppresses the microbial activity 38 . Several reports have been published regarding the bentonite swelling under the influence of strongly alkaline solutions 39,40 . He et al. 41 found that when the compacted bentonite had come in contact with highly alkaline fluids, the swell pressure was reduced. ...
... As there was less possibility of an anion entering the narrow interlamellar space of the smectite and reacting with the cation from the specific surface, the probability of complete breaking down the structure lattice was low 53 . But the cement water (CW) being an OHdominant (pH > 12), could easily perform the dissolution of montmorillonite, resulting in lower swell pressure values 40 . ...
Article
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The use of compacted bentonite around the high-level nuclear waste canister (HLW) inside the deep geological repository (DGR) ensures the prevention of entry of active radionuclides in the atmosphere due to its noteworthy large swelling ability. In the eventual repository, the waste canister has a high (100 °C–200 °C) temperature initially, and it reduces over a vast period, which induces a thermal history over the compacted bentonite layer. The cement/concrete layer is constructed as a bulkhead or in the vaults or to support the access of galleries between a buffer and the host rock, and it degrades over the period. The hyperalkaline fluid is created when it percolates through the cement/concrete layer and comes in contact with the compacted bentonite. The contact of hyperalkaline fluid to compacted bentonite induced with thermal history can hamper the swell pressure characteristic of the bentonite. Therefore to determine the combined effect of hyperalkalinity to the thermal history induced compacted bentonite, swell pressure testing has been conducted on two compacted Barmer bentonites (B1 and B2) specimens with an initial dry density of 1.5 Mg/m3, 1.75 Mg/m3, and 2.0 Mg/m3 and saturated with distilled water as well as with hyperalkaline cement water (W/C = 1 und pH = 12.5) and heated to 110 °C and 200 °C. When the specimens were saturated with hyperalkaline cement water, the swell pressure exerted by both bentonites was noticeably reduced compared to specimens saturated with distilled water. Nevertheless, the time taken to full saturation was longer than distilled water for samples saturated with hyperalkaline cement water. Also, the decrease in swell pressure was observed in the samples subjected to thermal history than samples, which were tested without inducing thermal history in both the cases of hyperalkaline cement water and distilled water. The microstructural observations through XRD, FESEM and EDX revealed the clogging of pores due to the presence of non-swelling minerals.
... It is an essential characteristic of bentonite as it fixes the gaps between the layers of bentonite and the fissures between the host rock and bentonite layer and suppresses the microbial activity 38 . Several reports have been published regarding the bentonite swelling under the influence of strongly alkaline solutions 39,40 . He et al. 41 found that when the compacted bentonite had come in contact with highly alkaline fluids, the swell pressure was reduced. ...
... As there was less possibility of an anion entering the narrow interlamellar space of the smectite and reacting with the cation from the specific surface, the probability of complete breaking down the structure lattice was low 53 . But the cement water (CW) being an OHdominant (pH > 12), could easily perform the dissolution of montmorillonite, resulting in lower swell pressure values 40 . ...
... The chemical components of the groundwater may change over time with the geological conditions or because of the degradation of concrete (Berner, 1992;Balmer et al., 2017). Many experimental studies focused on the influence of the chemical components (Herbert et al., 2008), salt solution concentration (Karnland and Birgersson, 2006;Castellanos et al., 2008;Zhu et al., 2013;Sun et al., 2015;Chen et al., 2016), alkali solution concentration (Karnland et al., 2007;Liu et al., 2018), and pH (Ye et al., 2014) on the swelling pressure. The effects of chemical cycles on the swelling pressure were further investigated (Chen et al., 2015(Chen et al., , 2017. ...
... The pore size distributions obtained from the MIP tests are shown in Fig. 4. All the pore size distribution (PSD) curves presented a bimodal shape. A pore size of 150 nm was used for dividing the inter-aggregate pores and intra-aggregate pores (Lloret et al., 2003;Romero et al., 2011;Zhang et al., 2018;Liu et al., 2018). The final intruded mercury void ratio was smaller than the total void ratio (e T, equal to the initial void ratio of the specimen) owing to the pressure limitation of the porosimeter. ...
Article
Sufficient swelling pressure of compacted bentonite upon wetting is necessary to provide long-term stability to an engineered barrier system under conditions of thermal gradient and variable water chemistry in a high-level radioactive waste repository. To investigate the combined thermal and saline effects on the swelling pressure of Gaomiaozi (GMZ) bentonite, constant-volume swelling pressure tests were performed on densely compacted specimens inundated with deionized water, NaCl, and CaCl2 solutions at 20 °C and 60 °C. The results indicate that high temperature and saline solutions decrease the swelling pressure. Crystalline swelling dominates the constant-volume swelling of densely compacted bentonite. For a given cation type, a higher temperature or higher solution concentration resulted in a lower swelling pressure. The decreasing surface potential and increasing osmotic suction with solution concentration both weaken the clay swelling. The effects of temperature on the swelling pressure are explained by the role of the lattice contraction. The degeneration of the interlamellar adsorbed water at high temperature weakens the crystalline swelling. The effects of cation types are interpreted by their difference in chemical activity. Ca²⁺ ions possess higher exchange capacity and larger hydrated radius than those of the Na⁺ ions. When densely compacted Na-bentonite is wetted with CaCl2 solution, the possible change of the interlamellar cations from Na⁺ to Ca²⁺ ions would promote the crystalline swelling. The osmotic suction imposed by CaCl2 solution may offset the role of the cation exchange to a small extent. The effect of temperature on the swelling pressure exceeds that of the cation types. For a given solution concentration, the specimen wetted with NaCl solution developed lower swelling pressure than that with CaCl2 solution at 20 °C, whereas the opposite tendency appeared at 60 °C. In addition, specimens saturated with deionized water and NaCl solution at 20 °C obtained insignificant difference in pore size distribution, whereas identifiable differences in pore size distribution occured at 60 °C. The combined thermal and saline effects on the swelling pressure may be explained by the role of the lattice contraction at varied clay structure. When CaCl2 solution is used, the quasicrystals contain more montmorillonite layers than the case of the NaCl solution. The role of the lattice contraction at high temperature is expected to decrease the swelling pressure to a greater degree in the case of the CaCl2 solution. The remarkable increase in the osmotic suction at high temperature and high solution concentration could also enlarge the decrease in the swelling pressure. These conjectures, however, still require quantitative interpretations via microscopic tests such as X-ray powder diffraction tests on bulk samples.
... Experimental SP kinetics of different expansive soils (bentonite, bentonite/sand, and bentonite/claystone mixtures) hydrated under different conditions (temperature, saline, and alkaline solutions) (Table 1 and are borrowed from the literature (Komine and Ogata 1994;Lee et al. 2012;Ye et al. 2013Ye et al. , 2014Chen et al. 2018;Liu et al. 2018;Sun et al. 2018;Lang et al. 2019;Zeng et al. 2019;Jadda and Bag 2020;Middelhoff et al. 2020) and simulated by the proposed model [Eq. (5)]. ...
... The first decrease of SP occurs immediately after the first peak and is believed to be caused by the subdivision of particles and clogging of interaggregate voids (as discussed in the section "Swelling Mechanisms of Bentonite"). The second decrease of SP occurs after the second peak and is considered as the result of montmorillonite dissolution in the high alkaline solutions (Liu et al. 2018;Sun et al. 2018). In general, the first decrease of SP would be insignificant for bentonite and bentonite-based materials with relatively high dry density (e.g., for J1, J2, K1, M1 to M5) or high initial water content (e.g., for K2). ...
... Numerous studies have shown that increases in the salinity of the groundwater can considerably decrease the expansion capacity of bentonite which may jeopardize its use as a barrier material (Suzuki et al., 2005;Villar 2006;Karnland et al., 2007;Castellanos et al., 2008;Zhang et al., 2012;Sun et al., 2015;Chen et al., 2015Chen et al., , 2017Zheng et al., 2015Zheng et al., , 2017Navarro et al., 2017;Liu et al., 2018;He et al., 2019;Akinwunmi et al., 2020;Shehata et al., 2021;Xu et al., 2021). For example, Chen et al. (2017) investigated the swelling behaviour of Gaomiaozi (GMZ) bentonite under different cycles of cation exchange and salinization and desalinization. ...
... Moreover, Navarro et al. (2017) observed that there is an increase in the swelling rate of compacted bentonite with a corresponding decrease in the swelling strain when the salinity of the solution is increased. In addition, Liu et al. (2018) investigated the effects of hyperalkaline solutions on the swelling pressure of compacted GMZ bentonite by examining the sodium ion (Na þ ) cations and hydroxide (OH À ) anions in the solution. The results indicate that high concentrations of Na þ cations can inhibit crystalline swelling and double-layer swelling, and high concentration of OH À anions makes it easier for doublelayer swelling and changes in the fabric structure or arrangement of soil. ...
Article
Compacted bentonite-sand (B/S) mixtures have been used as a barrier material in engineered barrier systems (EBSs) of deep geological repositories (DGR) to store nuclear wastes. This study investigates the individual and combined effects of different chemical compositions of deep groundwaters (chemical factor) at potential repository sites in Canada (the Trenton and Guelph regions in Ontario), heat generated in DGRs (thermal factor), dry densities and mass ratios of bentonite and sand mixtures (physical factors) on the swelling behavior and ability of bentonite-based materials. In this study, swelling tests are conducted on B/S mixtures with different B/S mix ratios (20/80 to 70/30), compacted at different dry densities (ρd = 1.6–2 g/cm³), saturated with different types of water (distilled water and simulated deep groundwater of Trenton and Guelph) and exposed to different temperatures (20 °C–80 °C). Moreover, scanning electron microscopy (SEM) analyses, mercury intrusion porosimetry (MIP) tests and X-ray diffractometry (XRD) analyses are carried out to evaluate the morphological, microstructural and mineralogical characteristics of the B/S mixtures. The test results indicate that the swelling potential of the B/S mixtures is significantly affected by these physical and chemical factors as well as the combined effects of the chemical and thermal factors. A significant decrease in the swelling capacity is observed when the B/S materials are exposed to the aforementioned groundwaters. A large decrease in the swelling capacity is observed for higher bentonite content in the mixtures. Moreover, higher temperatures intensify the chemically-induced reduction of the swelling capacity of the B/S barrier materials. This decrease in the swelling capacity is caused by the chemical and/or microstructural changes of the materials. The results from this research will help engineers to design and build EBSs for DGRs with similar groundwater and thermal conditions.
... The widening of the interlayers causes the clay to swell. This phenomenon depends on the salinity and composition of the actual clay pore water as well as most adsorptionprocesses on the clay's surface(Thury, 2002;Herbert et al., 2008;Li et al., 2009; Chang et al., 2012;Massat et al., 2016;Liu et al., 2018;He et al., 2019;Soler et al., 2019). ...
... The possible reasons for a decrease in the montmorillonite mineral reflection intensity are as follows: (1) the bentonite particles flocculated. The formation of flocculated particles provides a reduction in the reflection of incident rays compared to the oriented structure (Ouhadi et al. 2010); (2) loss of layer stacking order (Liu et al. 2018); and (3) a reduction in the mass fraction of montmorillonite (Chen et al. 2013). In contrast with the peak intensity, the effect of CaCl 2 on the peak position was more remarkable. ...
Article
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The hydraulic conductivity and chemical compatibility of bentonite slurry for grouting walls were evaluated. Bentonite slurry of different concentrations and volumes was injected into fine-sand, medium-sand and coarse-sand aquifers. The results indicated that bentonite slurry was a fast and highly efficient grouting material. After injection into aquifers, a low-permeability zone could be formed immediately near the injection point. Through 24 groups of experiments, it was determined that bentonite slurry with concentrations of 6%, 7% and 10% and volumes of 0.6 PV, 0.8 PV and 0.6 PV, respectively, was required to meet the hydraulic conductivity requirement of cut-off walls for fine-sand, medium-sand and coarse-sand aquifers. In addition, the chemical compatibility of bentonite slurry was investigated to investigate its applicability as grouting wall material at waste containment facilities. Permeation with 50 mM H2SO4 solution resulted in an increase in the hydraulic conductivity by a factor of 65–73 times. The suitable pH of the bentonite slurry grouting wall ranged from 2.0 to 13.0. The hydraulic conductivity increased by a factor of approximately 4.7–54 during permeation with 10 ~ 1000 mM CaCl2. Bentonite slurry grouting walls cannot be applied in environments with salt concentrations greater than 10 mM. In contrast, permeation with 10 mM phenol solutions resulted in no appreciable change in hydraulic conductivity. Bentonite slurry grouting walls are appropriate for environments with dissolved organic pollution.
... Measurement of cation exchange capacity (CEC) implies quantity of replaceable cations exist on the surface of clay particles. Many studies also demonstrate that the pore solution and its ionic content have remarkable influence on swelling properties (Jenny and Reitemeier, 1935;Abd-Allah et al., 2009;Wang et al., 2014;Chen et al., 2017;Liu et al., 2018). Endorsing the above statements, She et al. (2019) confirmed that the differences in swelling property with respect to varying depths are attributed to the electrochemical properties only. ...
Article
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A comprehensive understanding of the chemical parameters effect can greatly aid in selecting the most compatible additive and stabilization technique to treat expansive clays. In the present study, influence of chemical parameters (total, leachable and exchangeable forms of sodium and calcium) on swelling (swelling potential, Sa, and swelling pressure, Sp) and consistency properties is quantified by relating them in the form of histograms. It is, in general, noticed that swelling property increases, reaches a peak, and thereafter, decreases with an increase in sodium (Na) and calcium (Ca) content. The value of Ca, Na, and Ca/Na ratio induced maximum swelling to soils are measured in the range of 3–7%, 0.5–0.8%, and 8–11.5 for total, 75–103 mg/kg, 108–164 mg/kg, and 0.7–1.12 for leachable, and 0.8–2.5 and 50–100 meq/100 g, 2–6 meq/100 g, and 0.2–0.5 for exchangeable forms. Three forms including, total, leachable, and exchangeable are found to have a distinct impact on swelling property. The results revealed that Ca/Na ratio, not the individual Ca or Na content, is the better parameter to estimate Sa and Sp of soils. Pertinent to exchangeable form, it is noticed that (a) the range within which Na has pronounced effect on Sa, the impact of Ca in the same range has obscured, (b) Ca/Na ratio, which varied over wider range from 2–120, could be a potential parameter for classifying expansive soils, and (c) exchangeable Ca has no bearing on Sp regardless of its intensity. From the results of the study, it is recommended for expansive clays to accord importance to chemical parameters of the above prescribed limits to design effective foundation systems, avoid swelling induced damage to superstructure, identify the most practicable additive, and select remedial technique to mitigate swelling.
... Globally, studies of the stability of bentonites, and, in particular, smectites, in alkaline solutions have been carried out for more than 10 years. In the works of a large number of authors [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] and others, one can single out the main conclusion that pH has a key effect on the stability of smectite. As a result of various laboratory experiments, it has been shown that the reactivity of smectite increases significantly with increasing pH starting at 12 [33,36,41,46]. ...
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Bentonite clays have unique properties that determine their use as the main component of engineered barrier systems (EBS) for the isolation of radioactive waste. At present, the Russian Federation is elaborating the concept of deep geological disposal of radioactive waste in crystalline rocks of the Yeniseisky site, where bentonite clays play an important role in ensuring the safety of the repository for a long period of time. This work demonstrates the first results of short-term laboratory experiments (1 and 3 months) on the thermochemical interaction of bentonite and concrete in the presence of synthetic water at an elevated temperature. These experiments will help predict the effect of EBS materials on montmorillonite. Bentonite from the 10th Khutor deposit (Russia) and Portland cement were used in the experiments. At the first stage of the experiments, solutions were obtained after leaching the concrete with a synthetic groundwater solution for 1 month at 90 °C. At the second stage, the interactions of the obtained solutions with bentonite at 90 °C were studied. As a result of the experiments, the processes of concrete leaching were revealed, which changed the composition and acidity (an increase in the pH from 6.1 to 12.1) of the synthetic water and led to an increase in the porosity of the material in contact with the solution. However, no dissolution of montmorillonite was observed, and the changes were quite small. The research results show the high stability of bentonite from the 10th Khutor deposit under model conditions, which was confirmed by modeling. Thus, we can say that at pH ≈ 12 and at elevated temperatures, montmorillonite retains a stable structure for a long time, which is important for ensuring the safety of disposal in general.
... This large surface area in combination with the small particle size leads to a comparably large solubility and dissolution of smectites can lead to the formation of other silicates. Particularly in the field of radioactive waste repository research possible smectite alteration mechanisms have to be considered (Sellin and Leupin, 2014;Kaufhold and Dohrmann, 2016;Liu et al., 2018;Kale and Ravi, 2018;Chen et al., 2018;Samper et al., 2018). In principal solid state conversions are distinguished from dissolution precipitation reactions. ...
Article
The investigation of the release of elements/structural cations from bentonites is important for high-level radioactive waste research. Smectites, the main component of bentonites, cannot be described by thermodynamics. In the present paper, therefore, element release from 38 well characterized bentonites is investigated rather than the solubility of smectites in a thermodynamic sense. The solid/liquid ratio was 0.2 mass%, temperature and time were varied. The study focused on Mg- and Si-release because Al- and Fe-concentrations measured in solution were not reproducible probably because of polymerization, recrystallization, and/or readsorption. Bentonites contain minor amounts of minerals other than smectites which could explain differences of the concentrations of released elements. Low crystalline silica phases, however, were the only admixtures significantly affecting the results. The focus, therefore, was on samples with a low amount of low crystalline silica phases. In the temperature range between 100 and 140 °C Mg and Si dissolved stoichiometrically (corresponding to the typical range of Si/Mg in dioctahedral smectites). Below 100 °C more Mg compared to Si was dissolved and the opposite was observed above 140 °C. Extrapolating these results to higher solid/liquid rations would indicate i) that the formation of Mg/Si-phases at the heater of HLRW canisters could be explained by partial smectite dissolution and ii) that different Mg/Si-phases could form depending on temperature. The element release tests used in the present study proved to be suitable to distinguish “reactive” (released more Mg and Si) from “less reactive” bentonites.
... Expansion characteristics are affected by repulsive forces between different particles, as well as osmotic forces in small pores and micro-pores resulting from variations in the concentration of saline solution (Musso et al., 2003). High concentration of positive (sodium) cations can discourage crystalline expansion (crystalline swelling) and the expansion of the diffused double layer (Liu et al., 2018). The thickness of the double layer decreases when the salinity of pore water increases, this in turn reduces swelling pressure (Castellanos et al., 2008;Siddiqua et al., 2011Siddiqua et al., , 2014. ...
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A field experiment was conducted at Al-Rashid District-Baghdad, to study the effect of alternate irrigation of fresh and ground water, as a common farming practice, on pore size distribution and parameters of fitted () function of the soil moisture characteristic curve. Fresh water (F, first) and Groundwater (G, second) were added to two (3 m by 3 m) field plots. After the cessation of infiltration cycle, which represents the first cycle, plots were covered to prevent evaporation during 55 days of drainage. At the end of the drainage cycle G was added to the first plot (FG) and F was added to the second plot (GF), which represents the second cycle of alternate use of irrigation water quality. Undisturbed and disturbed soil samples were obtained at the end of each cycle to determine the () relationships at high suction range (0 to-500 cm H 2 O), using Tempe pressure cells and low suction range (-1000 to-15000 cm H 2 O) using pressure plate apparatus respectively for 10, 30, 50, 70, 90, 110, 130 and 150 cm depths. RETC code was used for parameters estimation of the van Genuchten equation (1980). The d/d enhanced the calculation of the mean pore size between two consecutive applied suction heads as well as volume fraction (as percent) of total porosity (f) as (d/f× 100). The results showed that  measured at high suction range was higher for F compared with G and GF treatment and lower at low suction range. Measured  values for F was almost the same as measured  for FG at both high and low suction ranges. Calculated (d/f ×100) values was greater for F compared with G, FG and GF treatments suggesting more pore spaces was drained for F at the same suction range. Almost similar pore size distribution was found between FG with G and between FG with GF. The (d/f ×100) values were 70.99, 69.43, 62.57 and 62.07% for F, FG, G and GF treatments with the following descending order: F > FG > G > GF. The results also indicated that more water was retained at pore radius less than 0.198m which corresponds to-1000cm H 2 O suction head value.
... Ye et al. (2014) argued that, Mt content is directly related to CEC value of bentonites. Xiang et al. (2019b) studied that, the diffusion of alkaline solutes into bentonite-based mixtures dissolves Mt mineral which will weaken the properties of the bentonite, as pH is raised to 13-14 (Liu et al. 2018). However, according to another study by Herbert et al. (2008), after reaction with alkaline solution the Mt aggregates were broken apart between MX-80 bentonite and alkaline solution after elapsed time of 3 years. ...
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Knowledge of the behavior of highly compacted expansive clays, as an engineered barrier, in disposal of high-level nuclear waste (HLW) systems to prevent the pollution due to migration of radionuclide is extremely essential. The prominent properties of globally and widely used bentonites have been extensively studied during past two decades. In China, GaoMiaoZi (GMZ) bentonite is the first choice as a buffer or backfill material for deep geological repositories. This review article presents the recent progresses of knowledge on water retention properties, hydromechanical behavior, and fractal characteristics of GMZ bentonite-based materials, by reviewing 217 internationally published research articles. Firstly, the current literature regarding hydrogeochemical and mechanical characteristics of GMZ bentonite influenced by various saline solutions are critically summarized and reviewed. Then, the role of osmotic suction π alongside the application of surface fractal dimension Ds is presented from the standpoint of fractal theory. Finally, the strength characteristics of GMZ bentonites using fractal approach have been discussed. Furthermore, this study sheds light on gaps, opportunities, and further research for understanding and analyzing the long-term hydromechanical characteristics of the designed backfill material, from the standpoint of surface fractality of bentonites, and implications of sustainable buffer materials in the field of geoenvironmental engineering.
... This decreasing reaction is associated with the dissolution of montmorillonite and silicate minerals. The OHanions not only directly dissolved montmorillonite but also reacted with its hydrolyzate and even formed non-expanding secondary minerals, and then the swelling capacity decreases [29]. This observation is consistent with the conclusions reported by [14] and [30]. ...
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In China, Gaomiaozi (GMZ) bentonite is recognized as a barrier material for isolating nuclear waste. Different chemical solutions may change the hydraulic conductivity and swelling capacity of bentonite. Consequently, a series of swelling pressure and permeability experiments was carried out on bentonite-sand mixtures with various dry densities and infiltrating solutions. X-ray diffraction (XRD) and the field emission scanning electron microscope (FESEM) were carried out on the samples experiencing the tests to identify the influence of chemistry pore solutions upon the mineralogical and microstructure changes. The results show that the swelling pressure experienced rapid swelling, slow expansion, and the stable expansion stage for the specimens of infiltrating solutions except for NaOH. For the specimens infiltrated with NaOH solutions, the swelling pressure experienced rapid increases, slow decreases, and a stable development stage. With hyper-alkaline and hyper-salinity infiltration, the swelling pressure decreased, and the permeability increased. In addition, swelling pressure attained stability more quickly on contact with hyper-alkaline and hyper-salinity solutions. Comparing the test results, the results indicate that the influence of NaOH on the expansion and permeability was higher than NaCl-Na2SO4 at the same concentration.
... Still, at high dry densities, the effect of salinity may not always be negligible. For instance, Liu et al. (2018) reported swelling pressures of Na-dominated bentonite at a dry density of 1700 kg·m −3 of 5.5, 5.1, and 3.5 MPa when exposed to NaCl solutions with 0.0, 0.1 and 1.0 M, respectively. ...
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This work proposes a formulation to simulate the swelling of compacted bentonites, focusing the analysis on the modelling of the increase of macrostructural void ratio generated by the destructuration of the microstructure under low confinement conditions. First, its formulation is described, highlighting its integration into a generalised constitutive model, which is able to describe the behaviour of compacted bentonites under very different conditions of stress, salinity and water content. After, its scope is assessed, comparing the model predictions with experimental results of free swelling tests performed under different salinity conditions. The results obtained are very satisfactory, especially considering that the proposed swelling model is based on only two material parameters.
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Thermal and saline environment significantly affects the swelling behavior of montmorillonite (Mt) that is crucial for the safety of engineering barrier. However, the swelling mechanism of Mt under non-isothermal and saline environment remains poorly understood. A molecular Na-Mt model is proposed to analyze the swelling pressure change of Mt under various non-isothermal and saline conditions. The swelling mechanism of Mt at various salt solutions and temperatures is discussed based on analyses of the water concentration profile, hydration energy of Mt-salt solution system, and hydration state of interlayer cations. We find that the critical temperature that differentiates the negative and positive correlations between swelling pressure of Mt and temperature under different saline environments coincides with the hydration state saltation temperature identified from radial distribution function. Our research shows that the hydration energy of Mt and the hydration state of interlayer cations jointly control the swelling of Mt, via governing the water migration in Mt-salt solution system.
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Compacted bentonite is used as a buffer or backfill material in deep geological repositories. However, the seepage of pore water containing various salt and minerals can affect the hydro-mechanical performance of the bentonite. To investigate these effects, samples of compacted bentonite with initial dry densities of 1.50 Mg·m−3 and 1.70 Mg·m−3 were subject to saturation by distilled water, and solutions of NaCl and NaOH (0.1 M and 1.0 M), and the changes in swelling pressure and microstructure were monitored. The swelling pressure was seen to reduce with greater concentrations of the salt solution or when the initial dry density was decreased. At lower salt concentrations, the variation of the swelling pressure exhibited a double-peak, while further increases in the salt concentration caused the swelling pressure curve to exhibit only a single-peak. Compared to samples with a 1.7 Mg·m−3 dry density, the large swelling pressure decreased following the initial peak, and a smaller second peak was observed for samples with a 1.5 Mg·m−3dry density. The swelling pressure of samples saturated with NaOH solutions was notably lower, particularly after the second peak in the double-peaked curves. Applicable empirical kinetic equations were proposed to fit the relationship between the swelling pressure (p, MPa) and time (t, h) of the compacted sample saturated with DW and aqueous NaCl and NaOH. Microscopic analyses provide evidence that this reduction of swelling pressure could be attributed to the osmotic consolidation, ionic exchange, and mineral dissolution of bentonite.
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Compacted bentonite has been widely used as buffer material in the deep geological disposal of high-level radioactive waste because of its physical and chemical characteristics, such as its swelling property, which plays a crucial role in the safe operation of repositories. To enhance the properties of bentonite as buffer material, graphene has been used to modify pure bentonite. In this study, the swelling pressure of graphene-modified GMZ bentonite (GMB) was measured under different conditions to investigate the effect of the graphene content and dry density on the swelling properties of modified bentonite. The swelling pressure curve exhibits a “double-peak” pattern, which fades with the increase in the graphene content from 0% to 6% and decrease in the dry density from 1.8 to 1.6 g/cm³. The microstructure of GMB were analyzed under various conditions using mercury intrusion porosimetry and scanning electron microscopy. Results show that, in contrast to intra-aggregate pores (micropores), the distribution of inter-aggregate pores (macropores) is greatly affected by the graphene content and dry density. Based on the results, the swelling pressure of GMB under different conditions can be predicted with a logarithmic function. The present work can provide a reference for the design of GMB as buffer/backfill materials for deep geological disposal.
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The influence of alkaline aqueous solutions on the properties of bentonite was investigated to evaluate the performance of bentonitic engineered barriers when contacted with alkaline groundwater. Batch and hydraulic conductivity tests were conducted on Na-bentonite using six different alkaline aqueous solutions. For the batch tests, almost no change in the montmorillonite fraction of the bentonite was observed after reacting with alkaline solutions (pH = 8.4–13.1), regardless of the solution type. On the other hand, aluminosilicate minerals (e.g., albite) were dissolved and secondary minerals (e.g., anorthite) were formed in alkaline NaOH solutions (pH > 13). The cation (Ca or Na) concentration primarily affected the swelling properties of bentonite rather than the pH of the solution, which was comparable to the results of the hydraulic conductivity tests. For the Ca solutions, the hydraulic conductivity of the bentonite specimen to the 0.02 mol/L Ca(OH)2 solution (6.5 × 10⁻⁹ cm/s) was approximately an order of magnitude lower than that of the bentonite specimen to the 0.02 mol/L Ca(OH)2 + 1 mol/L CaCl2 solution (5.0 × 10⁻⁸ cm/s), whereas the hydraulic conductivity to the 0.02 mol/L Ca(OH)2 + 1 mol/L CaCl2 solution (pH = 11.3) (5.0 × 10⁻⁸ cm/s) was slightly higher than that to the 1 mol/L CaCl2 solution (pHi = 8.4) (4.4 × 10⁻⁸ cm/s). For the NaOH solutions with pH > 13, the hydraulic conductivity of the bentonite specimen decreased with increasing Na concentration, suggesting that the effect of Na concentration was more dominant than that of permeant pH.
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Concepts for geological disposal of high-level radioactive waste usually include bentonite buffer materials. Numerous studies have been performed with most using Wyoming bentonite. Gaomiaozi (GMZ) bentonite has been selected as a potential buffer/backfill material for the deep geological repository of high-level radioactive waste in China. In this context, the highly alkaline environment induced by cementitious materials in the repository is likely to alter montmorillonite, the main clay mineral in GMZ bentonite. This alteration may result in deterioration of the physical and/or chemical properties of the buffer material. To acquire quantitative data which would allow us to assess the dissolution of montmorillonite and changes in the diffusivity of hydroxide ions as well as their effects on the swelling pressure and permeability of the compacted GMZ bentonite, an experimental study was conducted under highly alkaline (NaOH solutions with various pH values were used), simulated groundwater conditions. The GMZ bentonite also contains cristobalite which may also have been dissolved. The microstructure of the compacted bentonite samples after the experiments was determined by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDX) was carried out to identify mineralogical changes. At pH >13, the permeability of specimens increased significantly; the swelling potential decreased with increasing pH. Furthermore, the pore volume and pore size of GMZ bentonite changed when exposed to alkaline solution, resulting in an increase in porosity and permeability. The main alteration mechanisms of compacted GMZ bentonite undergoing infiltration by highly alkaline solution are likely to be dissolution and modifications in terms of the microstructure and mineralogy.
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Gaomiaozi (GMZ) bentonite has been recognized as the first choice for using as buffer/backfill material in deep geological repository for the disposal of high-level nuclear waste in China. High salinity and high pH of the pore solutions may cause changes in the mineralogical composition, and the swelling capacity of the bentonite results in affecting the long-term performance of the engineering barrier system. In the present work, a series of hydrothermal reactivity experiments were performed using batch reactors at 20, 35 and 80 °C for evaluating the chemical and mineralogical responses of GMZ bentonites. The results show that the dissolution of smectite releases Si and Al under the conditions of 1 M KOH at 35 and 80 °C and the released Si and Al may work as a cementing agent in GMZ bentonite. XRD pattern results show that alteration of the GMZ bentonite is expected to be insignificant for reaction with NaCl, KCl and NaOH solutions under low temperatures. However, it is obviously reacted with KOH solutions especially at 80 °C, during which cementitious material was produced leading to kaolinitization. When contacted with KOH solutions, the basal spacing (d001) of the main primary diffraction peaks (001) value of GMZ bentonite increases with increasing temperature. Salinity also can influence the smectite stability and accordingly loss of swelling capacity. This conclusion could be explained by the diffusing double-layer effects induced by salt solutions and ion exchange.
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Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. When the compacted bentonite blocks are emplaced, technological voids related to different interfaces involving the buffer material are created, and their hydro-mechanical behaviour is of primary importance for the safety of disposal. In this study, the hydraulic resistance of the interface between compacted MX80 bentonite and Boom Clay is investigated in the laboratory using an injection cell. The results obtained show that when water is injected, the technological gap is quickly reduced due to the bentonite swelling. When water pressure reaches the hydraulic resistance of the interface, hydraulic fracturing takes place with a drastic pressure decrease. After fracturing, water injection continues and bentonite continues to swell. A higher subsequent pressure is needed to produce a new hydraulic fracturing. After a certain time, the hydraulic resistance becomes high enough so that no further fracturing occurs, suggesting that the technological gap is sealed.
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Experimental data from different testing methodologies on different compacted clayey soils, with dominant bimodal pore size distribution, are presented and analysed, to provide a comprehensive picture of the evolution of the aggregated fabric along hydraulic and mechanical paths. Fabric changes are analysed both from the porous network viewpoint, by means of careful mercury intrusion porosimetry investigation, and from the soil skeleton viewpoint, by quantifying swelling and shrinkage of the aggregates in an environmental scanning electron microscopy study. The consequences of the aggregated fabric evolution on the water retention properties of compacted soils are analysed and discussed. A new model for water retention domain is proposed, which introduces a dependence of the intra-aggregate pore volume on water content. The model succeeds in tracking correctly the evolution of the hydraulic state of the different soils investigated along generalised hydromechanical paths. The proposed approach brings to light coupling between intra-aggregate and inter-aggregate pores in the retention properties of compacted clayey soils. Dependence of the air entry and the air occlusion values on swelling and shrinking of aggregates, besides void ratio, is introduced and discussed.
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Traditional (unmodified) sodium bentonite (Na-bentonite) has been shown to exhibit semipermeable membrane behavior, which is beneficial in terms of the use of Na-bentonites as barriers or components of barriers for hydraulic containment applications. However, degradation of such membrane behavior due to the diffusion of cations into the bentonite also is known to occur, and this degradation increases with increasing salt concentration and increasing valence (charge) of the principle salt cation (e.g., Ca2+ versus K+). In this study, the membrane behavior of a polymerized bentonite referred to as a bentonite-polymer nanocomposite (BPN) was determined during multistage membrane tests using solutions of CaCl2. The BPN illustrated improved membrane behavior relative to that previously reported for both traditional and other chemically modified bentonites. However, complete degradation of the membrane behavior was ultimately observed for specimens of the BPN exposed to 10mMCaCl2 for a test conducted in a rigid-wall cell and 20mMCaCl2 for a test conducted in a flexible-wall cell. The destruction of membrane behavior for the specimen in the rigid-wallcell was attributed to short-circuiting along the side walls of the rigid-wall cell after shrinkage of the BPN, whereas destruction of the membrane behavior in the flexible-wall cell correlated with the time required to reach steadystate diffusion of calcium (Ca2+). Thus, the enhanced calcium-resistant behavior of the BPN was limited and was affected by the type of cell (rigid-wall versus flexible-wall) in which the membrane behavior was measured.
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Gaomiaozi (GMZ) Bent has been recognized as the potential buffer/backfill material for construction of Chinese deep geological repository for disposal of high-level radioactive waste (HLW). For investigation of pressure and suction effects on the thermal volumetric behavior of the highly compacted GMZ01 Bent, a new suction-temperature controlled oedometer was developed. Several suction and pressure controlled cyclic heating and cooling tests have been performed on highly compacted GMZ01 Bent. Suctions ranging from 4 to 110 MPa, temperatures from 25 to 80 degrees C and vertical loads from 0.1 to 5 MPa were applied. Results show that heating at constant suction and pressure induces either swelling or contraction. At high pressure and low suction, heating tends to induce contraction, while at low pressure and high suction, heating induces expansion. Thermal volumetric strain of GMZ01 Bent tested strongly depends on the overconsolidation ratio (OCR). Two opposite mechanisms are considered to explain the observed phenomena, that is, the volume increase induced by the thermal expansion of the mineralogical components and the adsorbed water and the double-layer thickness at the microstructure level, as well as the volume reduction caused by the macrostructural rearrangements.
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During the long-term operation of a deep geological repository, infiltration of pore fluids with different chemical compositions can influence the swelling behavior of the compacted bentonite. In this paper, using a self-developed swelling deformation apparatus, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite specimen with an initial dry density of 1.70 g/cm3, with infiltration of de-ionized water, NaCl, or CaCl2 solutions at different concentrations. Results show that the swelling strains of GMZ01 bentonite specimens infiltrated with salt solutions were significantly lower than that of those infiltrated with de-ionized water. In fact, the swelling strain decreased in a form of power function with the increasing concentration of infiltration solutions. For a given concentration, the one-dimensional swelling strain of specimens contacted with NaCl solutions was higher than that of the specimens infiltrated with CaCl2 solutions. However, the difference of the swelling strains decreased with increasing concentrations. This phenomenon could be explained by the impact of cation types on the microstructure of bentonite.
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In this study, the effects of salinity of infiltrating solutions on the swelling strain, compressibility, and hydraulic conductivity of compacted GMZ01 Bentonite were investigated. After swelling under vertical load using either distilled water or NaCl solutions with concentrations of 0.1, 0.5 M, and 1 M, laboratory oedometer tests were conducted on the compacted GMZ01 Bentonite. Based on the oedometer test results, hydraulic conductivity was determined using the Casagrande’s method. Results show that the swelling strain of highly compacted GMZ01 Bentonite decreases as the concentration of NaCl solution increases. The compression index C c* increases and then turns to decrease with an increase in the vertical stress or a decrease in the void ratio for different solutions, and the C c* decreases as the concentration of NaCl solution increases. The secondary consolidation coefficient C α increases linearly with the increase of the compression index C c*. Furthermore, a bi-linear relationship between the swelling index C s* and the secondary consolidation coefficient C α can be characterized clearly. The hydraulic conductivity increases as the concentration of NaCl solution increases, however, this increase can be prevented if a high confining stress is applied.
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Compacted expansive clays swell due to crystalline swelling and osmotic/double layer swelling mechanisms. Crystalline swelling is driven by adsorption of water molecules at clay particle surfaces that occurs at inter-layer separations of 10–22 Å. Diffuse double layer swelling occurs at inter-layer separations >22 Å. The tendency of compacted clay to develop osmotic or double layer swelling reduces with increase in solute concentration in bulk solution. This study examines the consequence of increase in solute concentration in bulk solution on the relative magnitudes of the two swelling modes. The objective is achieved by inundating compacted expansive clay specimens with distilled water and sodium chloride solutions in free-swell oedometer tests and comparing the experimental swell with predictions from Van’t Hoff equation. The results of the study indicate that swell potential of compacted expansive clay specimens wetted with relatively saline (0.4, 1 and 4 M sodium chloride) solutions are satisfied by crystalline swelling alone. Comparatively, compacted clay specimens inundated with less saline solutions (0.005–0.1 M sodium chloride) require both crystalline and osmotic swelling to satiate the swell potential.
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The swelling behaviour and hydraulic conductivity of Na-bentonite powder and bentonite-sand mixtures (10 and 20% of bentonite by dry weight) have been measured with distilled water and various salt solutions (0.01, 0.1 and 1 mol/l concentrations). It was found that in dilute solutions, the bentonite in mixtures subjected to small confining stresses swells sufficiently to separate the sand particles and reach a clay void ratio similar to that achieved by bentonite alone. At high stresses, or in strong solutions, the bentonite in a mixture has insufficient swelling capacity to force the sand particles apart and swelling is limited by the sand pore volume. The hydraulic conductivity of a mixture depends on the bentonite void ratio, and the porosity and tortuosity of the sand matrix. A design model is proposed to predict the engineering properties of a mixture over a range of confining stresses from the properties of its constituents and the permeant.
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The present study examines the influence of extraneous salt addition on pore-fluid osmotic suction of a clay soil. The dependence of swell potentials of the salt-amended clay specimens on initial pore-fluid osmotic suction is also examined. The osmotic suctions predicted by Van't Hoff's equation are in excess or smaller than the values calculated from the pore water electrical conductivity, depending on whether the Van't Hoff factor in the Van't Hoff equation is included or not. Experimental results suggest that the salt-amended specimens absorbed water and swelled in response to matric suction and chemical concentration gradients on inundation with water in oedometer cells. Salt also diffused from the soil pores of salt-amended specimens to the reservoir water in oedometer cells in response to chemical concentration gradients. Reduction in effective stress from osmotic flow into the soil specimen and increase in interparticle repulsion from reduction in pore-water salt concentration rendered the total swell potentials of salt-amended specimens independent of initial pore-fluid osmotic suction. The initial pre-fluid osmotic suction does, however, significantly affect the rate of swelling.Key words: clays, chemical properties, swell potential, suction.
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Compliance of concrete - bentonite barriers is a key issue in the performance assessment of long-term underground storage of radioactive waste. The reaction of synthetic alk. solns. (K-Na-OH and Ca-(OH)2) interacting by diffusion with a Mg-satd. compacted FEBEX-bentonite column of 2.1 cm thickness was investigated through closed-system expts. at 60 ºC over 6 and 12 mo. The first few millimeteres near the surface were grated to resolve mineralogical changes at sub-mm resoln. detd. by XRD, SEM, BET and the anal. of exchangeable cations. Alk. cations diffused beyond the mineralogical alteration zone (2-2.5 mm), and were exchanged by Mg2+ in the interlayer region of montmorillonite, but no K-silicates were formed. The presence of minor zeolites in the alteration assemblage was only detected by XRD on samples treated with ethylene-glycol. A cemented rim mixed with poorly ordered clay materials, mainly brucite, a chlorite-like phase and Mg-smectite was found at the interface. Montmorillonite was partially dissolved and a part of it remained shielded by the newly-formed cementation crusts. Cation diffusion through the interlayer of montmorillonite is inferred to be the dominant transport pathway in compacted bentonite equilibrated with external high pH solns. The high pH front diffused at a much slower rate due to buffering of mineral reactions. [on SciFinder(R)]
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Bentonite is currently designated for use as a buffer material for the repository of high-level radioactive waste because such a material requires swelling characteristics to seal the waste. A high-level radioactive waste disposal facility may be built in a coastal area of Japan because transportation of this waste by ships is feasible. Therefore, it is important to investigate the effects of seawater on a bentonite-based buffer. This study is intended to investigate the influence of seawater on the swelling pressure and swelling deformation characteristics of five common types of bentonite. The experiments described herein clarify the relations between the influence of seawater grade and compaction density, type of exchangeable cation, montmorillonite content of the bentonite, and vertical pressure condition. Based on experimental results, suitable specifications have been defined for a bentonite-based buffer that can withstand the effects of seawater.
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The impact of alkaline solutions (pH = 13.2) on the clay mineralogy of the Callovo-Oxfordian formation hosting the French underground laboratory for nuclear waste disposal investigation (Meuse-Haute Marne site) has been studied experimentally. Initially, each of the four samples selected as representative of the mineralogical transition in this Callovo-Oxfordian formation consists of a mixture of three main clay phases: discrete illite, discrete smectite and a randomly interstratified mixed-layered mineral (MLM) containing similar to65% of non-expandable layers. Clay separates were altered in batch reactors at 60degreesC using high solution:solid ratios. The mineralogy of this clay fraction and solution chemistry were monitored as a function of reaction time. In addition, the interactions between organic matter and clay particles were investigated using scanning transmission X-ray microscopy (STXM). The clay mineralogy is little affected even though the pH is still high after 1 y reaction time. The only significant mineralogical evolution is the partial dissolution of the discrete smectite component leading to the formation of a new randomly interstratified illite-expand able MLM. Additional mineralogical transformations lead, for one sample, to the dissolution of micro-crystalline quartz and, for another sample, to the crystallization of a tobermorite-like phase, The low reactivity of clay minerals may be attributed to the presence of organic matter in the samples. In their initial state, all outer surfaces of clay particles are indeed covered with organic matter. After 1 y reaction time, STXM studies showed the basal surfaces of clay particles to be devoid of organic matter, but their edges, which are the most reactive sites, were still protected.
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In this study, the cyclic evolution of an effective stress parameter χ is determined for different swelling soils. This parameter can be modelled using the macro-, meso- and microscales in interaction, stemming from mercury intrusion porosimetry (MIP) tests. The proposed model for χ was calibrated using the experimental results from successive suction cycles for different swelling soils without a vertical stress. Finally, an effective stress model was used to describe the cyclic behaviour of both the loose and dense samples for suction cycles between 8 and 40 MPa. Comparisons between the simulations and the experimental results showed that the effective stress model could be used to accurately describe the cyclic behaviour of the swelling soils over different suction ranges.
Article
Bentonite-sand mixtures are widely accepted as the candidate sealing materials of radioactive waste repository due to their high swelling capacity and low hydraulic conductivity. These properties of the material depend largely on its microstructure, particularly, the pore space geometry. The aim of this investigation is to explore the effect of the pore fluid (NaCl and CaCl2) on the microstructure, pore geometry and hydraulic conductivity of a barrier material. Representative samples of light backfill (LBF) prepared with a 50–50 bentonite-sand mixture at a compacted dry density of 1.24 Mg/m3 were used. As a first attempt, X-ray computed tomography (X-ray CT) was used to study the LBF under the distilled water (DW) and two other pore fluid conditions. In order to acquire a good quality image with high resolution, X-ray source, detector, and a small LBF specimen (5.5 mm in diameter) were placed close together and scanned with Xradia Micro XCT- 400. The voxel of the scanned images were (1.15 × 1.15 × 1.15) μm3, indicating that the particles with a diameter greater than 2 μm could be easily observed. The porosity value estimated at the end of the consolidation test showed significantly higher values compared to the X-ray CT analysis. The interconnected pore components and absolute permeability of distilled water or salt-solution saturated LBF samples were analyzed using Avizo software. This analysis showed that the volume of interconnected pore increased due to the presence of salt solutions and resulted higher hydraulic conductivity. The salt solution increased porosity, pore size, volume of interconnected pore and hydraulic conductivity of the LBF.
Article
In order to investigate the effect of salinity on the basic swelling behavior of compacted bentonite, series of one dimensional swelling deformation test have been performed. The tests were conducted with the various initial dry densities (1.33 - 1.90 g/cm3) of compacted bentonite saturated with different concentration of NaCl solution varying from 0.5 - 4.0 mol/l. It was found that the higher initial dry density specimens show higher maximum swelling rate and the maximum swelling rate of bentonite decreased with increasing concentration of NaCl. The permeability of bentonite rapidly increases with increasing concentration of NaCl. NaCl within montmorillonite clay mineral decreases the thickness of double-layer swelling. The bentonite may change from highly swelled material to coagulate in saline water.
Article
Compacted bentonite has been used as buffer material in radioactive waste disposal. Once compacted bentonite is emplaced, the chemical composition of site water is changed due to the long-term interaction between the bentonite, surrounding rock and the concrete facility; therefore the hydraulic-mechanical behavior of compacted bentonite should be evaluated for the disposal safety. In this study, the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite were investigated under salinization-desalinization chemical cycles using a newly developed apparatus. Results show that the salinization process leads a reducing of swelling pressure and the desalination process leads to an increasing of swelling pressure; the hydraulic conductivity increases in the salinization process while decreases in the desalinization process. The variation magnitude of the swelling pressure and hydraulic conductivity is related to the solution concentration applied. Meanwhile, the initial chemical condition and chemical cycle paths have a significant effect in the swelling characteristics and hydraulic properties. Since the salinization-desalinization cycle is expected to occur over a long time during the operation of the repository, the monitoring of the buffer materials will be important for the disposal safety.
Article
Permeability and erosion experiments are carried out on Gaomiaozi (GMZ) bentonite specimens with the dry density of 1.70 g/cm3, where NaOH solution is employed to simulate hyper-alkaline pore water, which is likely be produced in highly active nuclear waste repositories. Surface properties alterations of the specimens observed with the scanning electron microscope (SEM) and the transmission electron microscope (TEM) are used to interpret the influence of hyper-alkaline solution on the swelling potential, permeability, porosity and composition of bentonite, and to assess the micro-mechanism of hyper-alkaline pore-water effect on the properties of bentonite. The results show that GMZ bentonite presents a flake layer structure, and that gels with the shape of wings are formed on the surface of montmorillonite during hydration. The dissolution of gels produced by hydration and the destruction of bentonite structure by erosion due to hyper-alkaline water are observed too. A positive relation between alterative degree and solution concentration is also observed. The surface erosion of bentonite specimens eroded by hyper-alkaline solutions is shown as the dissolution of montmorillonite, which is identical to the results obtained using X-ray diffraction (XRD) tests. Therefore, submitting the bentonite to long-term infiltration of hyper-alkaline pore-water may lead to dissolution of montmorillonite, destruction of bentonite structure, increase of porosity, decrease of swelling potential, and amplification of permeability as well, which generally weakens its sealing and buffering properties.
Article
The deep geological disposal is regarded as the most reasonable and effective way to safely dispose high-level radioactive wastes (HLW) in the world. The conceptual model of HLW geological disposal in China is based on a multi-barrier system that combines an isolating geological environment with an engineered barrier system including the vitrified HLW, canister, overpack and buffer/backfill material. The bentonite is selected as base material of the buffer/backfill material in HLW repositories, due to the very low permeability and excellent retardation of nuclides from migration, etc. GMZ deposit is selected as the candidate supplier for buffer material of HLW repositories in China. Since 2000, systematic study was conducted on GMZ-1 that is Na-bentonite produced from GMZ deposit and selected as reference material for Chinese buffer material study. The mineral composition, basic parameters of GMZ-1 bentonite and thermal conductivity, hydraulic conductivity, unconfined compression strength as function of dry density and water content are presented. The swelling stress of GMZ-1 bentonite as function of dry density is also reported. GMZ-1 bentonite is characterized by high content of montmorillonite (about 75%) and less impurities. The adequacy understanding of property and long-term behavior in deep geological condition of GMZ-1 is essential to safe dispose the high-level radioactive wastes in China.
Article
During the long-term operation of a geological repository, fluctuations of groundwater level, temperature, etc., will lead to cyclically changing of groundwater chemical components correspondingly, resulting in influencing the volume change behavior of compacted bentonite. Using an improved expansion apparatus, one-dimensional free swelling tests were conducted on as-prepared densely compacted GMZ01 bentonite specimens, which have an initial dry density of 1.70 g/cm3, with cyclically infiltration of CaCl2 solutions at different concentrations and de-ionized water. Four concentrations (0.1, 0.5, 1.0 and 2.0 M) of CaCl2 solutions and de-ionized water were employed for the infiltration tests and four salinization-desalinization cycling paths were followed. Results show that the total swelling strains of compacted GMZ01 bentonite specimens were influenced by the concentrations of infiltration CaCl2 solutions, the salinization or de-salinization processes, as well as the cycles of salinization-desalinization experienced.
Article
This work presents an insight into double-structure effects on the coupled chemo-hydro-mechanical behaviour of a compacted active clay. In the first part, selected pore size distribution curves are introduced, to highlight the influence of solute concentration on the evolution of the microstructure of compacted samples. An aggregated structure with dual-pore network is induced by compaction even at relatively high water contents. This structural arrangement is enhanced by salinisation, and has a notable influence on transient volume change behaviour - that is, the occurrence of different stages of swelling upon pore water dilution and higher volume change rates upon salinisation. A coupled chemo-hydro-mechanical model, taking into consideration double-structural features from a chemo-mechanical viewpoint, is described and then used to interpret these behavioural responses and present complementary information on local transient processes. The model is designed to identify an intra-aggregate and an inter-aggregate domain, and assigns different values of hydraulic pressure and osmotic suction to each domain. Distinct constitutive laws for both domains are formulated, and the flow of salt and water between the two domains is accounted for by a physically based mass exchange term. The model is used to simulate salt diffusion tests run in an oedometer at constant vertical stress. Parameters used in the formulation are calibrated based on separate experimental evidence, both through direct test results and through back-analyses of laboratory experiments.
Article
The producers of nuclear waste, within all countries exploring options, including Canada, have determined the long-term solution to be a deep geological repository. In the Canadian concept, within the deep geologic repository a number of clay-based barriers will separate the containers from the surrounding geosphere. Following placement the surrounding groundwater will infiltrate into the repository. In order to analyze the performance of the repository under very complex conditions, accurate material properties are required. The chemistry of the host rock is an important aspect as the behaviour of clay-based barrier materials could be affected by the saturating saline groundwater. This paper investigates the saturated mechanical behaviour of light backfill (composed of 50 % silica sand and 50 % Na-bentonite clay) and dense backfill (composed of 70 % crushed granite, 25 % glacial lake clay and 5 % Na-bentonite clay) and the quantifying the effect of pore fluid chemistry on the strength and compressibility behaviour of the materials. The results indicate that light backfill behaviour is strongly influenced by its pore fluid chemistry while dense backfill shows limited effects. The material parameters of light backfill and dense backfill are interpreted for input into numerical simulations. These results and interpretation enrich the understanding of the mechanical response of light and dense backfill, two components of the sealing system of the Canadian deep geologic repository.
Article
One of the major functions of a bentonite buffer for a HLW repository is to provide the buffer with a self-sealing capacity through its ability to swell on contact with free water. Swelling pressure measurements were carried out to investigate the swelling behavior of Ca-bentonite. Experimental results showed that the swelling pressure increased with an increase in dry density, and its dependence on dry density increased at higher dry densities beyond 1.6 Mg/m3. The effect of salinity on the swelling pressure for the Ca-bentonite was different from that for Na-bentonite. The swelling pressure of the Ca-bentonite was higher in a diluted concentration of NaCl solution (0.04 M) compared with de-mineralized water, and it then decreased with an increase in the concentration of the NaCl solution beyond 0.04 M. Such a swelling behavior of the Ca-bentonite was explained by the ion-exchange of Ca2 + for Na+ from the solution followed by hydration with water molecules and osmosis due to the differences in concentrations of the NaCl solutions.
Article
In this study, temperature controlled soil-water retention tests and unsaturated hydraulic conductivity tests for densely compacted Gaomiaozi bentonite — GMZ01 (dry density of 1.70 Mg/m3) were performed under confined conditions. Relevant soil–water retention curves (SWRCs) and unsaturated hydraulic conductivities of GMZ01 at temperatures of 40 °C and 60 °C were obtained. Based on these results as well as the previously obtained results at 20 °C, the influence of temperature on water-retention properties and unsaturated hydraulic conductivity of the densely compacted Gaomiaozi bentonite were investigated. It was observed that: (i) water retention capacity decreases as temperature increases, and the influence of temperature depends on suction; (ii) for all the temperatures tested, the unsaturated hydraulic conductivity decreases slightly in the initial stage of hydration; the value of the hydraulic conductivity becomes constant as hydration progresses and finally, the permeability increases rapidly with suction decreases as saturation is approached; (iii) under confined conditions, the hydraulic conductivity increases as temperature increases, at a decreasing rate with temperature rise. It was also observed that the influence of temperature on the hydraulic conductivity is quite suction-dependent. At high suctions (s > 60 MPa), the temperature effect is mainly due to its influence on water viscosity; by contrast, in the range of low suctions (s < 60 MPa), the temperature effect is related to both the water viscosity and the macro-pores closing phenomenon that is supposed to be temperature dependent.
Article
A mixture of bentonite and sand has been proposed for use as a buffer/backfill material in the disposal of high-level radioactive waste in many countries. In China, it is necessary to quantify the influence of the sand content ratio and the initial dry density on the swelling characteristics of GMZ bentonite–sand mixtures, and to collect test data useful in the design of buffer/backfill materials. To accomplish these goals, this study conducted laboratory tests on the swelling pressure and swelling deformation of GMZ bentonite–sand mixtures consisting of different sand contents of 0, 10, 20, 30, 40 and 50%.
Article
GMZ bentonite has been considered as a possible material for engineered barrier in the Chinese program of nuclear waste disposal at great depth. In the present work, the hydraulic conductivity of this bentonite was determined by simultaneous profile method. A specific infiltration cell equipped with five resistive relative humidity probes was designed for this purpose. The water retention properties were studied under both confined and unconfined conditions; the results show that at high suctions (> 4 MPa) the water retention capacity is independent of the confining condition, and by contrast, at low suctions (< 4 MPa) the confined condition resulted in significant low water retention. Furthermore, the microstructure was investigated at mercury intrusion porosimetry (MIP) and Environmental Scanning Electron Microscope (ESEM) in different states: on oven-dried powder, bentonite slurry, as-compacted and wetted samples. It has been observed that the soil powder is constituted of aggregates of various sizes; these aggregates are destroyed by full saturation at a water content equal to the liquid limit; compaction at the initial water content of 11–12% and a dry density of 1.7–1.75 Mg/m3 led to a microstructure characterized by a dense assembly of relatively well preserved aggregates; saturation of the compacted sample under constant volume condition defined a non-homogeneous microstructure with the presence of well preserved aggregates. This non-homogeneous microstructure would be due to the non uniform distribution of the generated swelling pressure within the soil sample upon wetting. The hydraulic conductivity determined has been found decreasing first and then increasing with suction decrease from the initial value of about 80 MPa to zero; the decrease can be attributed to the large pore clogging due to soft gel creation by exfoliation process, as observed at ESEM.
Article
The experimental and modeling studies were conducted on alteration of bentonite buffer materials. The dissolution rate of montmorillonite in, the diffusivity of hydroxide ions in and the hydraulic conductivity of compacted sand–bentonite mixtures were experimentally determined and formulated. Dissolution of montmorillonite followed the linear dependence on time under the employed experimental conditions of aOH- (activity of hydroxide ions) of 0.04–0.57moldm−3 and temperatures of 50–170°C. The slope gave the dissolution rate, RA (Mgm−3s−1). The dissolution rate was a function of pH and temperature, T (K), and expressed as RA=3.5(aOH-)1.4exp(-51000/RT), where R is the gas constant. The effective diffusivity, De (m2s−1), of hydroxide ions was found to be in the order of 10−10–10−11m2s−1 at 10–90°C and expressed as De=5.0×10−7ε2.1exp(−18600/RT) where ε is the porosity. The dependence of hydraulic conductivity, K (ms−1), of the sand–bentonite mixtures on the effective montmorillonite dry density, ρmont (Mgm−3), and on the ionic strength of the permeant, I (moldm−3), was identified as K=1.2×10−7I1.510−4.2ρmont.A PHREEQC-based, coupled mass-transport/chemical-reaction code (MC-BENT) was developed for predicting hydraulic conductivity of the bentonite buffer by using the formulae for the dissolution rate, the effective diffusivity and the hydraulic conductivity. This code was able to reproduce observed changes in concentrations of major species and montmorillonite contents in the lab-scale experiments on the bentonite alteration, which is indicative of partial verification of our calculation.
Article
GMZ bentonite has been selected as a potential material for the construction of engineered barrier in the Chinese program of geological nuclear waste disposal, for its high montmorillonite content, high cation exchange capacity (CEC) and large specific surface etc. Studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior of GMZ bentonite were performed from 1980s. Based on a review of the former studies, achievements on experimental and theoretic results obtained on compacted GMZ bentonite specimens including basic properties, thermal, hydraulic and mechanical behaviors are presented in this paper. Results show the thermal conductivity of GMZ bentonite and the bentonite-based mixtures influenced by its dry density, water content, mixture of other materials and degree of saturation etc. Water retention capacity of highly-compacted GMZ bentonite decreases as the temperature increases under confined and unconfined conditions. The hysteretic behavior in the water retention curves of the compacted GMZ bentonite is not so significant at 20 or 40°C. The unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions. This is possibly induced by the difference in the mechanism of microstructural changes during hydration under different confining conditions. The compaction curves for GMZ bentonite with different dry densities are clearly step-phased. And the optimum water content for GMZ bentonite is about 15%. An exponential relationship between swelling pressure and dry density of highly-compacted GMZ bentonite was determined for the prediction of swelling pressure. Furthermore, the void ratio after swelling for unconfined sample also can be predicted using diffuse double layer (DDL) theory.
Article
The porewater chemistry in compacted bentonite was studied in solution–bentonite interaction experiments. The parameters varied in the experiments were the bentonite density, bentonite-to-water ratio (B/W), ionic strength of the solution, and the composition of bentonite. The bentonite types used in the experiments were Volclay MX80 and artificial bentonites prepared from purified MX-80 in sodium form where CaCO3 and CaSO4 were added. At the end of the experiment, the equilibrating external solution and the porewater squeezed out of the bentonite were analyzed to give information for interpretation of the interaction. The equilibrium was modelled with the HYDRAQL code.The evolution of porewater chemistry was determined by the dissolving components initially present in the bentonite together with the ions entering with water from the surroundings. Ion-exchange processes occurred between the bentonite and the porewater. The concentrations in the external solution and porewater strongly depended on the B/W used. The concentrations in the squeezed porewaters were clearly lower than in the equilibrating waters. The modelling results reasonably fit the experimental data.
Article
Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60°C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were confined within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH∼8 were 10−14.13, 10−13.70, and 10−13.46 mol m−2 s−1, at 20, 40, and 60°C, respectively, and the activation energy for the dissolution reaction at pH ∼8 was 30.5±1.3 kJ mol−1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8–8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution:Rate=10−11.39aH+0.34pH8.5The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was directly proportional to the surface concentration of >AlOH2+ and >AlO− surface complexes, under acidic or alkaline conditions, respectively.Rate=10−8.0{>AlOH2+}pHAlO−}pH>8.5A multiple variable model is proposed to take into account simultaneously the effect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates.
Article
Compacted clay liners in landfills and brine ponds are subject to chemical concentration gradients between the soil water in the clay and the reservoir solution. Soil water is the solution residing in soil pores that is chemically composed of water $(H_2O)$ molecules and dissolved salts molecules (ISSS, 1976). These differences in salt concentration generate an osmotic suction difference: osmotic suction refers to suction arising from the presence of dissolved salts in water (Lu & Likos, 2004). This suction difference is dissipated through the diffusion of salts and flow of H2O molecules (Barbour & Fredlund, 1989). Changes in the dissolved salts concentration of soil water from salt diffusion in response to osmotic suction difference will alter the swell magnitudes of expansive clays, which are strongly influenced by salt concentration (Bolt, 1956; Mesri & Olsen, 1971; Yong & Warkentin, 1975; Mitchell, 1993). Osmotic suction differences between soil water and reservoir solution may thus alter the wetting-induced volume change behaviour of compacted clays used as liners in landfills and brine impoundments.
Article
Experiments measuring kaolinite dissolution and recrystallisation rates in KOH solutions were carried out using a batch reactor at 35° and 80°C. An untreated, sized kaolinite from St. Austell was used. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1g of kaolinite of the ≤2μm fraction and 80, 160, 240ml of 0.1–4m KOH solution. In these experiments the solution composition and mineralogy were monitored as a function of time for up to one year. A dissolution dominant stage was followed by a precipitation dominant period. The dissolution is not linear as a function of time but linear as a function of (log) time. This apparent relationship is explained by an affinity effect (approach to equilibrium).Under these high pH conditions, the sequence of reaction products observed is: illite, followed by KI-zeolite, phillipsite and finally by the stable product K-feldspar precipitation. The point in time at which crystallisation controls the concentration of Al and Si in solution and the persistence of the various metastable phases is temperature dependent.Concerning the stability of clay barriers in an alkaline solution context, low temperatures allow more material to enter solution before the crystallisation of metastable phases controls the solution composition. Also at low temperatures the dissolution phase continues for longer periods of time. Hence, low temperatures favour an actively aggressive solution (dissolving clays).
Article
Bentonite of the Serrata de Nijar (Almeria, Spain) and concrete made with an ordinary portland cement (OPC) are candidate materials to be used as engineering barriers in the high level radioactive waste disposal in argillaceous rock. This exptl. alteration study has been conducted in order to det. the kinetics of alteration of a montmorillonitic bentonite under hyperalkaline conditions (NaOH 0.5 to 0.1 M) in the presence of portlandite. The amt. of montmorillonite destroyed and the secondary minerals formed have been measured by means of x-ray diffraction (XRD) in the solid phase after the performance of batch reactions carried out in airtight cells (3/1 liq./solid) over 30-540 days. SEM-EDX and chem. anal. of a <0.5 μm Ca-homo-ionized fraction were used to characterize the reaction byproducts, mostly zeolites (analcime and phillipsite type); calcium silicate hydrates (amorphous and 11 Å-tobermorite type) and saponite. The nature of the soln. chem. detd. in the aq. phase allowed calcn. of the chem. speciation and the satn. indexes for the obsd. minerals. The evaluation of the equil. state in the system supports the dissoln. of montmorillonite from undersatn. conditions as the driving process for the alk. reaction of bentonite. The rate of montmorillonite dissoln. has been calcd. from the mineralogical quantification of smectite in the 75-200° tests. The global kinetics for the conversion of montmorillonite can be fitted to R (mol s-1) = A(m2) k[OH-]0·5; ln k = (- 20.09 ± 1.37)-(2731 ± 543)* (1/T); Ea = 22.7 ± 4.4 kJ/mol. These data are in close agreement with the rate of dissoln. of montmorillonite obtained by methods based on soln. chem. [on SciFinder(R)]
Article
The results of an experimental program undertaken to evaluate the impact of pore fluid salinity on the hydromechanical performance of light and dense backfill materials are presented. Light and dense backfills are engineered barrier materials that are being examined in the Canadian concept for storage of spent fuel in a deep geological repository. The current research investigates the impact of pore fluid chemistry on the swelling, compressibility, stiffness, and hydraulic conductivity parameters of light and dense backfills that are required as material parameters for analysis and design. In these tests, pore fluid chemistry was selected to represent groundwater within potential host units including granite and limestone rock. Results show that the performance of light backfill is significantly affected by changes in pore fluid chemistry. The swell potential of light backfill decreases with increasing salinity of the solution. The hydraulic conductivity decreases with increasing effective montmorillonite dry density and specimens saturated with saline solution have higher hydraulic conductivity than those saturated with distilled water. Conversely, the behaviour of dense backfill is governed mainly by the crushed granite component and therefore changes to the pore fluid chemistry have relatively little effect. Results of dense backfill tests confirm the material performance as a sealing material.
Article
The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 50 and 70 °C in stirred flow-through reactors over the pH range of 1–13.5. Experiments done at very acidic and very basic pH were far from equi-librium. Near neutral pH experiments were closer to equilibrium. The Al/Si release ratio, while initially being incongruent, ultimately approached the stoichiometric value in most of the experiments. Temperature, extreme pH, and time favor congru-ency. Rates can be described by: Dissolution rates decrease with increasing pH under acid conditions, minimize at near neutral pH and increase with increasing pH under basic conditions. Apparent activation energies are pH-dependent and have a maximum of 19.3 kcal mol À1 at pH 1, a minimum of 5.3 kcal mol À1 between pH 4–7 and another maximum of 20.3 kcal mol À1 at pH 11. Protons, water, and hydroxyls promote dissolution. Their relative contribution to the overall dissolution rate depends on pH.
Article
The entry of salt into a narrow space between two charged surfaces in an ionic solution is controlled by both the electrostatic and steric effects. For a very narrow gap, the co-ion densities calculated from the Poisson-Boltzmann (PB) theory are not realistic. In situations with high surface charge, salt will not enter the electrical double layer until the separation of the surfaces is large enough to allow a formation of the third ionic layer in the mid-plane. At the time of entry, co-ion density forms a peak in the mid-plane which is significantly higher than the corresponding PB calculation and also the bulk concentration.
Article
The prerequisite for the usage of bentonites as a geotechnical barrier in HLRW repositories is the stability of the bentonite under the conditions expected. In addition to high temperature and exposure to radiation, different types of aqueous solutions could also affect the bentonite. In this respect hyperalkaline solutions from Portland cements are considered, because aluminosilicates are known to be soluble under these conditions.The aim of the present study was to compare the ‘alkaline-reactivity’ of a set of different well characterized bentonites and to identify the reasons for the differences. Theoretically, this would allow for the selection of an optimum bentonite at least with respect to the stability at the cement–bentonite interface. The ‘alkaline-reactivity’ was characterized both by measuring the dissolved structural elements and analyzing changes of the solid material (e.g. the cation exchange capacity).However, the concentration of the dissolved structural elements could not be compared systematically probably because of the unknown nature and the different amounts of precipitates. As an example, the Si concentration in the alkaline solutions strongly varied from bentonite to bentonite because it depended on the type and amount of SiO2 phases present in the different natural bentonites. Possibly, the associated SiO2 phases governed the Si/(Al + Mg + Fe) ratio in the alkaline solution. Amongst other parameters, this ratio determines which phases precipitate. This ratio obviously depends on the type and amount of SiO2 accessories.Results obtained in the present study were in accordance with published literature. More pronounced dissolution/precipitation processes than found in the present study (60–90 °C, 3–5 months, saturated solution-excess Ca(OH)2) are expected at even larger pH values, which – on the other hand – do not reflect conditions expected in a bentonite barrier in contact with cement. The most important conclusion from this study is that bentonites are surprisingly resistant against solutions at pH around 12 and up to 90 °C. Above this temperature and at larger pH values dissolution is expected to be much faster. Therefore, for a real HLRW repository it seems to be favorable to use low pH cements and keep temperature well below 100 °C.Research Highlights► Reactivity determination of different bentonites in alkaline solutions is difficult. ► Determination of solubility of bentonites is obscured by formation of precipitates. ► Formation of secondary phases depends on accessories and experimental conditions.
Article
The microstructural evolution and hydraulic conductivity of bentonite in NaCl solution is investigated by in situ laser scanning microscopy (LSM). Upon exposure to saline solution, the bentonite aggregate swells rapidly, followed by a longer period of more gradual swelling. The expansion ratio of an aggregate reached a steady state 15 min after initial exposure to the solution. The steady-state expansion ratio decreased sharply with increasing NaCl concentration up to 0.1 mol/dm3 NaCl and then decreased gradually above 0.3 mol/dm3 NaCl. The results indicate that the fraction of macropores among bentonite aggregates increases with NaCl concentration, which is likely to account for increase in hydraulic conductivity under highly saline conditions.
Article
The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7.77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. mAfter more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 × 10−53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as and respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms.
Article
The estimated quantity of cement for construction and sealing purposes is around 9E5 kg in the planned Swedish KBS3 repository for nuclear waste. The highly alkaline cement pore fluid (pH > 12) may affect other components in the repository, and especially the bentonite buffer is of concern. In this study, we simulated possible interactions between cement and bentonite by contacting highly compacted bentonite with high molar hydroxide solutions in a series of laboratory experiments. Wyoming bentonite (MX-80) and purified homo-ionic Na- and Ca-montmorillonite were used for tests with 0.1, 0.3 and 1.0 M NaOH, and saturated Ca(OH)2 solutions. Pressure cells with permeable filters were loaded with compacted discs of bentonite at the proposed buffer density (2000 kg/m³ at full water saturation). A hydroxide solution was circulated on one side of the cell and an isotonic chloride solution on the other during a minimum of 45 days. Swelling pressure and solution pH were monitored during the tests and the change in the solution composition and bentonite mineralogy were determined after completed tests.
Article
The effect of the salinity of the saturating fluid on the hydro-mechanical properties of the FEBEX compacted bentonite was investigated by means of swelling, compressibility and permeability tests in which deionised water and solutions of different concentrations and compositions were used as saturating fluids. The solutions were chosen to simulate natural and extreme conditions in a high-level radioactive waste repository excavated in crystalline or clay host rocks. The swelling capacity of the bentonite decreases with the increase in salinity of the pore water, although this difference becomes less patent for high vertical loads and high densities and when the salinity of the solution is very low. The samples saturated with solutions containing high concentration of ions are also less deformable and consolidate more rapidly than the samples saturated with low-salinity solutions. The hydraulic conductivity of the highly compacted saturated FEBEX bentonite increases when high-salinity permeants are used, especially for low densities and when the stress level is low. The influence of the composition of the solution on the hydro-mechanical properties of the bentonite was also checked.
Article
Bentonites are supposed to be suitable materials for the production of geotechnical barriers in high level radioactive waste (HLRW) repositories if the bentonites' swelling capacity is maintained at the conditions expected; e.g. in contact with different solutions which may occur during the long term storage. Among all other common cations, K+ is believed to play a special role in contact with smectites because it can be adsorbed irreversibly and may cause collapse of the interlayer space.In the present study 36 different bentonites were reacted with KCl-solutions at 60 °C with and without wetting — drying cycles and extensive drying. The study was conducted mainly to study the mechanism of irreversible K+ fixation and the resulting loss of swelling capacity (LOS).The K+ exchange for the originally present cations was to some extend buffered by carbonates (minor components of some bentonites). Such buffer reactions are supposed to be relevant for real HLRW repository situations.The expected decrease of the swelling of the bentonites depended on the adsorption energy of the probe molecules (water, EG = ethyleneglycol, Cu-triene). The results of different methods (XRDEG, water adsorption, CECCu-triene, soda-soluble silica) indicated that the smectite fraction of the products (on average) was composed of approximately 50% swelling K+ smectite, 45% non-swelling collapsed K+ smectite (50% of which with fixed K+), and 5% illite which probably formed by dissolution and precipitation. This illite was detected by the increase of soda-soluble silica. In contrast to the solvation with ethyleneglycol, which only measures the collapsed layers rather than illite particles, the determination of soluble-silica is regarded as optimum for differentiating between collapsed layer domains and real illitization.The number of collapsed layers of the K-smectites depended on the layer charge density (LCD) but not on the tetrahedral charge. A model is proposed which explains the correlation of collapsed interlayers and LCD.