Compacted GMZ Bentonite Research Articles

Research on water retention and microstructure characteristics of compacted GMZ bentonite under free swelling conditions

In this study, water retention tests under free swelling conditions were performed to investigate the water intake (or loss) behaviour of compacted GMZ bentonite. First, the water retention characteristics were investigated, and then the microscopic pore structure was observed by environmental scanning electron microscope (ESEM). The results indicate that GMZ bentonite has a strong swelling (or a limited shrinkage ability) due to water intake (loss). The suction behaviour of GMZ bentonite is similar to MX80 bentonite and FEBEX bentonite. We also find that the confinement conditions can affect the suction behaviour of the material, especially at high relative humidity (RH). Additionally, a mathematic model can fit the mass change data very well. Microscopic tests show that the granular sensation of GMZ bentonite is obvious for a sample at low RH. With the increase in RH, the surface of GMZ bentonite becomes more smooth. The differences in the porosities calculated by the macroscopic and microscopic tests can be attributed to image resolution. The inter-laminar pores and intra-aggregate pores cannot be observed by the ESEM method. In addition, ESEM observation can provide an intuitive basis for the further research of the seepage property of GMZ bentonite.

Membrane behavior of compacted GMZ bentonite and its granite mixture

The membrane behavior of GMZ bentonite and its granite mixture, which consists of 70% GMZ bentonite and 30% grinded granite rock in dry weight, was quantitatively evaluated in this study. Specimens with different initial dry densities were subjected to multi-stage membrane behavior tests with concentration differences of 0.01, 0.05, 0.1 and 0.5 M of NaCl or CaCl2 solutions across the specimen in each stage. Membrane behavior was evaluated by measuring the chemico-osmotic efficiency coefficient ω. Results indicate that the membrane behavior of GMZ bentonite and its granite mixture decreases with the decrease in initial dry density and bentonite content, as well as the increases in solution concentration and cation valence. These conclusions could be explained by (1) variations in pore size distribution caused by different initial dry densities and bentonite contents; (2) increase in macro-pores in quality induced by shrinkage and collapse of DDLs resulting from the interactions between clay and solutions; (3) decrease in micropores and increase in macro-pores induced by cation exchange.

Simulation of a hydromechanical mock-up test on GMZ01 bentonite with double structure approach

This paper describes the simulation of an infiltration test conducted on GMZ bentonite with Barcelona Expansive Model. The infiltration test was carried out on compacted GMZ bentonite sample, which was embedded in a Beishan granite column. Numerical simulation was performed to model the hydromechanical behaviour of the GMZ bentonite sample. Micro- and macrostructure changes during the hydration of bentonite, as well as the interactions between bentonite and granite boundary were investigated. Results show that suction calculated by the Code_Bright software agrees well with the measured ones. The calculated axial swelling stress and the radial stress also accord well with the measured ones with consideration of the influence of the interface between bentonite and granite. This conclusion provides a better understanding of the interaction between the compacted bentonite and the host granite.

Investigation on swelling-shrinkage behavior of unsaturated compacted GMZ bentonite on wetting-drying cycles

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

Influences of salt solutions and salinization-desalinization processes on the volume change of compacted GMZ01 bentonite

Investigation on influences of cyclic salinization-desalinization processes on clay is of great importance for evaluation of the behavior of engineering barrier in deep geological repository for disposal of high-level radioactive waste. In this study, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite specimens, which has an initial dry density of 1.7Mg/m3, with cyclically infiltration of NaCl, CaCl2 or KCl solution at different concentrations and de-ionized water. Results show that the volume change of the compacted clay could be influenced by concentrations and cation types of solutions, as well as number of salinization-desalinization cycles during the salinization-desalinization processes. The total swelling strain of GMZ01 bentonite specimens measured on the first salinization with salt solutions at a same concentration follows an order: NaCl>CaCl2>KCl. This observation could be explained by the influences of basal space, DDL theory and K-linkage. A cation with higher replacing power can be replaced by another cation with lower replacing capacity when it has a higher concentration in the pore solution. The concentration effect on the swelling behavior of GMZ01 bentonite was explained using the concept of osmotic suction for specimens initially infiltrated with NaCl solutions. For specimens initially infiltrated with CaCl2 solutions, in addition to the osmotic suction effect, the distance between the unit layers of montmorillonite also played a role in explaining the swelling behavior. The swelling behavior of specimens initially infiltrated with KCl solutions could be explained by the strong K-linkage.

Swelling deformation of compacted GMZ bentonite experiencing chemical cycles of sodium-calcium exchange and salinization-desalinization effect

In China, GMZ bentonite has been recognized as the first choice of buffer material while Beishan area in Gansu province has been considered as the potential disposal site for high-level radioactive waste (HLW) repository. As the groundwater at the repository field is rich in Na+ and Ca2+, the swelling properties of compacted GMZ bentonite will be affected by the chemistry of groundwater. In this work, the swelling deformation behaviors of compacted GMZ bentonite were tested at a vertical stress of 0.1MPa to assess the influence of cation exchange between Na+ and Ca2+, as well as the salinization-desalinization effect by cyclical infiltrations of distilled water and saline solutions. Results show that Na-bentonite is partly transformed into Ca-bentonite once NaCl solution is replaced with CaCl2 solution. Comparing to NaCl solution, the inhibiting effect of CaCl2 solution is weak, and thus the additional swelling is observed. Along with chemical cycles, the total strain of compacted GMZ bentonite gradually increases, while the chemical sensitivity of strain behavior reduces. This tendency is also detected for GMZ bentonite experiencing the desalinization path of distilled water prior to each infiltration of salt solution. Regarding the relative strain of GMZ bentonite, the difference in osmotic suction between distilled water and salt solution is more relevant than the sodium-calcium replacement.

Influence of pore fluid concentration on water retention properties of compacted GMZ01 bentonite

Due to its low hydraulic conductivity, high swelling capacity and good adsorption properties, the Gaomiaozi (GMZ) bentonite has been proposed as a suitable buffer/backfill material for the construction of artificial barriers in a deep geological repository for the disposal of high-level nuclear waste (HLW) in China. Compacted GMZ01 bentonite with an initial dry density of 1.70g/cm3 was hydrated with distilled water and NaCl solutions. The swelling strain was recorded. After being saturated, suction-controlled drying tests were conducted and corresponding soil water retention curves were obtained. MIP investigations were conducted on the void ratio variation of a specimen before and after experiencing wetting and drying processes. Results show that the swelling strain of compacted GMZ01 bentonite decreases as the concentration of infiltration solution increases. The shrinkage curve of saturated compacted GMZ01 bentonite specimens evolves with controlling suctions and could be divided into three stages including a normal shrinkage stage, a residual shrinkage stage and a zero shrinkage stage. For a given suction, the measured void ratio of a specimen saturated with distilled water is slightly larger than those of specimens saturated with salt solutions after the drying equilibrium is reached. For a given suction, the degree of saturation of a compacted GMZ01 bentonite specimen increases as the salt concentration increases. According to the test results, a modified SWRC equation was proposed to account for the effect of void ratio and salt solutions on drying behaviour. The verified results revealed that the proposed equation can satisfactorily describe the SWRCs of compacted GMZ01 bentonite saturated with different solutions.

Influences of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted GMZ01 bentonite

Investigation of the effects of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted bentonite is of great importance for the assessment of the behavior of engineering barrier in deep geological repository for disposal of high-level radioactive waste. In this study, oedometer tests were conducted on densely compacted GMZ01 bentonite specimens with an initial dry density of 1.70Mg/m3, which experienced swelling under different vertical stresses with infiltration of de-ionized water or NaCl solutions at different concentrations. The effects of salt solution concentration and vertical stress during saturation on the volume change of GMZ01 bentonite specimens were investigated in terms of swelling strain, elastic compressibility parameter, plastic compressibility parameter and yield stress. Results show that as the concentrations of salt solutions increase, the swelling strain, the elastic compressibility parameter and the plastic compressibility parameter decrease, while the yield stress increases. However, the vertical stress during saturation can only affect the swelling strain and the yield stress. Based on the test results, empirical equations with consideration of salt solution concentration and vertical stress during saturation effects were proposed allowing the prediction of swelling strain, plastic compressibility parameter and yield stress. The calculated results are in good agreement with the experimental ones.

Influence of salt concentration on volume shrinkage and water retention characteristics of compacted GMZ bentonite

The chemistry of pore water plays an important role on the hydraulic and mechanical properties of compacted bentonite used as buffer/backfill material in the repository of high-level radioactive waste (HLW). In this study, a series of compacted GMZ bentonite specimen with a dry density of 1.7 Mg/m3 were prepared and initially saturated with NaCl solutions of different concentrations (0, 0.5, 1.0 and 1.5 mol/L), respectively. The drying process was brought forward on the specimens through increasing their suction by using vapor phase technique. During the drying process, the cracking behavior of the specimens was observed; the volume shrinkage and the water retention characteristics (WRCs) were investigated with the consideration of the chemistry. The results obtained show that the compacted specimen initially saturated with distilled water was observed with cracking when its suction increased to 82 MPa and others were found to crack off as their suctions increased to 139 MPa, while no cracks could be observed for the specimens below the suction of 82 MPa. For the confined specimens, volume shrinkage occurred with increase of the suction, accordingly changing their porosity; the void ratio of the specimens decreased as the suction of the soil increased and increased with the chemistry of the pore water. Meanwhile, the chemistry of the pore water has more significant impact on the WRCs of the specimens at a lower suction.

Effects of solution concentration and vertical stress on the swelling behavior of compacted GMZ01 bentonite

Compacted bentonite has been considered as buffer/backfill material in radioactive waste disposal. This study deals with the effects of the concentration of infiltration saturation and vertical stress on the swelling behavior of compacted GMZ bentonite. One-dimensional swelling tests were performed on specimens at an initial dry density of 1.7Mg/m3. NaCl solutions with concentrations of 0 (de-ionized water), 0.1, 0.5 and 1.0mol/L were used to saturate the specimens while three vertical stresses of 0.06, 0.062 and 1.85MPa were applied. Results show that, for all the infiltration solutions, the swelling strain decreases with increasing vertical stress; the impact of the vertical stress is more significant for low vertical loads while it is less patent when the high salinity solution is infiltrated. For all the vertical stresses applied, the swelling strain decreases with the increase in concentration of the infiltration solution; the impact of the concentration of infiltration solution is not only related to the vertical stress, but also to the soil permeability. Furthermore, the coefficient of primary swelling decreases with the increase in concentration and vertical stress. The concentration of infiltration solution has significant restricting effect on the secondary swelling deformation in case of low vertical stress, while it has no or few effects on the secondary swelling deformation in case of higher vertical stress.

Investigation on hydraulic properties of compacted GMZ bentonite used as buffer/backfill material

During the past decades, GMZ bentonite has been widely investigated for its use as buffer/backfill materials in China. Based on a comprehensive review of the former studies, achievements on experimental and theoretic works on the hydraulic aspects of compacted GMZ bentonite with consideration of temperature effects are presented in this paper. Water retention property of compacted GMZ bentonite depends on constraint conditions. Temperature effects on water-retention depend on constraint conditions and suction. The hysteresis behaviour is not obvious. Based on the test results, a revised water retention model was developed for considering the temperature effect. The saturated hydraulic conductivity of the densely compacted GMZ bentonite increases as dry density and temperature increases. A revised model, which considers temperature influence on water viscosity and the effective flow cross-sectional area of porous channels, for prediction of saturated hydraulic conductivity have been developed and verified. The unsaturated hydraulic conductivity of confined densely compacted GMZ bentonite samples decreases first and then increases with suction decrease from an initial value of 80 MPa to zero. With consideration of temperature effects and microstructure changes, a revised model for prediction of unsaturated hydraulic conductivity of compacted GMZ01 bentonite was proposed.

Transport of 125I in compacted GMZ bentonite containing Fe-oxides, Fe-minerals or Cu2O

A systematic investigation of the transport of 125I in compacted GMZ bentonite containing Fe-Oxides, Fe-minerals or Cu2O additive was carried out by through-diffusion method. The obtained results indicate that all of the additives have positive effects on retarding the 125I diffusion in compacted GMZ bentonite. Especially for the Cu2O additive, the corresponding effective diffusion coefficient is greatly reduced. It is mainly attributed to the particular interactions between the metal ions introduced by additives and iodide ions.

Swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite under salinization–desalinization cycle conditions

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.

Influence of strain-rate on hydromechanical behavior of highly compacted GMZ01 bentonite

During the long-term operation of a deep geological repository in crystalline rock, saturation of buffer/backfill materials will last for a long period of time. Investigation on time-dependent property of buffer/backfill materials is of great importance for long-term safety assessment of the repository. In this work, GMZ01 bentonite powder was statically compacted to specimens with a dry density of 1.70g/cm3 and suction-equilibrated to their target values. Then, a series of suction-controlled constant rate of strain (CRS) tests (including single-stage and step-wise CRS tests) were conducted for the investigation of the strain rate-dependent compression behavior of highly compacted GMZ01 bentonite. Comparisons were made between the results of the single-stage and step-wise CRS tests under given suctions for the verification of the applicability of the isotache concept on the compacted GMZ01 bentonite. Based on that, the step-wise CRS compression tests were performed and results were presented in terms of vertical stress–vertical strain curves. Based on the test results, the virgin compression index λ and the yield stress p0 were determined. Analysis indicates that λ decreases, while p0 increases with increasing suction. Furthermore, p0 increases with increasing CRS, which is consistent with previous researches on saturated soils. Finally, a logarithm function proposed between α(s) and suction s is verified and several relevant parameters are fitted, which represents the coupling of suction, time and stress of GMZ01 bentonite.

Influence of salt solutions on the swelling behavior of the compacted GMZ01 bentonite

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.

Impact of cyclically infiltration of CaCl2 solution and de-ionized water on volume change behavior of compacted GMZ01 bentonite

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 1.70g/cm3, with cyclically infiltration of CaCl2 solutions at different concentrations and de-ionized water. Four concentrations (0.1, 0.5, 1.0 and 2.0M) 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.

Influence of temperature on the water retention properties of compacted GMZ01 bentonite

Investigation of the temperature effects on the water retention properties of compacted bentonite is of great importance in the context of geological disposal of high-level radioactive waste (HLW) based on the multi-barrier concept. Gaomiaozi (GMZ) bentonite, collected from the Inner Mongolia Autonomous Region of China, has been selected as a potential buffer/backfill material for the construction of Chinese HLW deep geological repository. In this study, soil water retention curves (SWRCs) of both confined and unconfined compacted GMZ01 bentonite specimens with an initial dry density of 1.70 g/cm3 were investigated at temperatures 20, 40, 60 and 80 °C. Results indicate that the water content decreases as temperature rises, even though the temperature effects on the water retention capacity of compacted GMZ01 bentonite depend on constraint-conditions. The influence of constraint conditions is more significant at relatively low suctions. Under unconfined conditions, the temperature effect is insignificant. However, under confined conditions, the influence on the water retention capacity is found to be significantly suction dependent. The hysteresis behavior of unconfined compacted bentonite becomes less significant as temperature increases. Based on the corresponding results obtained, a modified model was developed for describing the SWRCs of confined compacted GMZ01 bentonite with consideration of temperature effects. It appears that the calculated SWRCs agree well with the experimental ones, indicating that the modified model can satisfactorily describe the effect of temperature on the water retention properties of confined compacted GMZ01 bentonite.

Effects of pH and temperature on the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite

Gaomiaozi (GMZ) bentonite has been recognized as the first choice for using as buffer/backfill materials in deep geological repository for the disposal of high-level nuclear waste (HLW) in China. Groundwater in Beishan area, which has been considered as a potential site for the construction of Chinese deep geological repository, may reach a high pH value because of its chemical background and possible cement degradation during the operation of the repository. Meanwhile, temperature may increase with decay heat released from the waste in the canister. Investigation of pH value of alkaline-solutions and temperature effects on the behavior of compacted GMZ01 bentonite is of great importance for the Chinese deep geological repository program. For this purpose, a series of swelling pressure and hydraulic conductivity tests with various pHs of NaOH solutions were conducted at different temperatures. The X-ray diffraction (XRD) exploration was performed on the GMZ01 bentonite specimens before and after experiencing the swelling pressure and hydraulic conductivity tests, in order to find out the influences of temperature and pHs on the mineralogy of GMZ01 bentonite. Results show that the swelling pressure of GMZ01 bentonite decreases as the pH value of NaOH solutions increases, while the decreasing rate significantly depends on temperature. The swelling pressure evolution curve was “double-peak” structured, which faded with the increases of pH and temperature. The hydraulic conductivity of GMZ01 bentonite increases with the increase of the pH value of NaOH solutions and the rise of temperature. All these observations were consistent with the XRD test results: the dissolution of montmorillonite in GMZ01 bentonite increases with the pH increases. This process was accelerated by the temperature rise.

Through-and Out-Diffusion of Se(IV) and Re(VII) in Compacted Bentonite

Through-diffusion and out-diffusion are good way to understand anion diffusion behavior in compacted bentonite. In the present work we investigated Re (VII) and Se (IV) diffusion behavior in 2000 kg/m3 compacted GMZ bentonite. The De values of Re (VII) and Se (IV) are 0.29 ± 0.02 and 0.77 ± 0.05× 10-11 m2/s. The result obtained from out-diffusion method was not agreed with predicted data due to the heterogeneous porosity distribution in clay boundaries and species changed when the diffusion occurred in GMZ bentonite. Comparing the Da values of Se, Tc and I in GMZ and Kunigel-F bentonites, it showed that Kunigel-F bentonite hindered the mobilities of anion more efficiently than GMZ bentonite because of the higher content of montmorillonite.

An experimental investigation on the secondary compression of unsaturated GMZ01 bentonite

The secondary compression behavior of compacted GMZ01 bentonite was experimentally investigated by conducting a series of suction-controlled high-pressure oedometer tests. GMZ01 bentonite powder was first exposed to suction controlled conditions at 110MPa and then statically compacted to a dry density of 1.70g/cm3. Then, the compacted samples were hydrated to designed suctions using the vapor equilibrium technique and finally submitted to multi-staged loading oedometer tests. Corresponding results were presented in terms of stress–strain-time curves. Based on the test results, the incremental compression index Cc∗ and secondary compression index Cα were defined. Relationships between Cc∗, Cα, vertical stress and suction were analyzed. It is observed that both Cc∗ and Cα increase linearly with increasing vertical stress, which is different from the convex relationship reported by other researchers in saturated soils. Moreover, Cc∗ and Cα also increase with decreasing suction. The relationship between Cc∗ and Cα is linear in the considered stress range and the slope of this linear relationship (Cα/Cc∗) increases with decreasing suction. Finally, a logarithm function is established between Cα∗/Cc∗ and suction s, which represents the coupling of suction, creep and stress of GMZ01 bentonite.