GMZ Bentonite Research Articles

Effects of K+ solutions on swelling behavior of compacted GMZ bentonite

During the long-term operation of a deep geological repository, the buffer/backfill properties of compacted bentonite could be affected by the chemistry of the infiltrated porewater. In this study, swelling tests and microstructural analyses were conducted on the compacted GMZ bentonite specimens with an initial dry density of 1.5 or 1.7 Mg/m3 with infiltration of distilled water, as well as, KCl and KOH solutions (0.1 and 1.0 M). Influences of K+ solutions on the swelling behavior of compacted GMZ bentonite were investigated. Results show that swelling properties of compacted GMZ bentonite specimens could be significantly influenced by concentration of K+ solutions and dry density of specimens. Swelling pressure of compacted GMZ bentonite specimens was significantly attenuated by infiltration of K+ salt/alkaline solutions. For swelling strain tests with infiltration of KCl or KOH solutions, instead of a secondary swelling stage, a volumetric collapse stage was observed. Micro-structural test results demonstrate that the attenuation of swelling behavior could be attributed to the K+ exchange or silica dissolution.

Influence of temperature on swelling deformation characteristic of compacted GMZ bentonite-sand mixtures

Laboratory swelling deformation tests were carried out on compacted GMZ bentonite and bentonite-sand mixtures with 30% and 50 % sand contents at 20, 40, 60, 80 and 90 °C with infiltration of distilled water. Influence of temperature, initial dry density, and quartz sand content on the swelling deformation characteristic of compacted bentonite specimens was analyzed. Results indicate that the swelling deformation process is accelerated, and the maximum swelling strain increases with the increase in temperature, while the maximum swelling strain tends to be stable with increasing temperature. In the meantime, the temperature effects depend on both of the sand content and the initial dry density of the specimens, the increases of the maximum swelling strain induced by increasing temperature, are enlarged by increasing sand content or initial dry density. Adding of quartz sand to bentonite not only influences the integrality of bentonite specimen, but also increase the microfissuring in area on quartz sand, which are advantageous to the heat transfer, leading to the increase of swelling deformation capacity of the specimen. The increased dry density relatively increases the bentonite content, so the swelling property is enhanced. However, no change on mineral composition of bentonite was observed when temperature was changed from 20 to 90 °C.

Removal of europium on GMZ bentonite corroded by young cement water at different temperatures

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

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.

Influences of PSD curve and vibration on the packing dry density of crushed bentonite pellet mixtures

Crushed high-density bentonite pellet mixture has been proposed for filling up gaps in the deep geological repository for disposal of high-level radioactive waste (HLW), due to its swelling capacity and easily to be handled. In this regard, for the long term safety evaluation of a deep geological repository, it is of great importance to investigate the packing behavior of the crushed bentonite pellet mixtures for achieving an adequate bulk dry density of the engineering barrier system. In the present work, crushed high-density GMZ bentonite pellets (1.94–1.95 Mg/m3) with different sizes were produced. Particle size distribution (PSD) curves of pellet mixtures were established using the distribution modulus method (DMM) and the mixing ratio method (MRM). A series of packing density tests were conducted by free fall packing with or without vibration. Results show that the PSD curves of pellet mixtures had great influences on the packing dry density, which first increased and then turned to decrease with increasing distribution modulus q (or the coarse/fine mass ratio αcf). In the meantime, vibration during packing process can significantly improve the packing dry density and this vibration effect is relative to the composition of the pellet mixture.

Stability of GMZ bentonite colloids: Aggregation kinetic and reversibility study

Abstract Colloids associating with radionuclides provide an important mechanism for radionuclide migration, the knowledge on colloidal stability is essential for understanding the migration processes. In the present work, the stability of Na-bentonite colloids from Gaomiaozi region (Na-GMZ bentonite) considering aggregation kinetic and reversibility were investigated as a function of colloids concentration, electrolyte concentration and pH by using the photon correlation spectroscopy (PCS) technique. The results showed that the stability of GMZ bentonite colloids was dependent significantly on cations concentrations and pH values, while slightly on colloidal concentrations and anions concentrations. The GMZ bentonite colloids were more stable in low cations concentrations and high pH conditions, aggregation was getting obvious and kinetically fast when increasing either salinity or acidity. The electrostatic force between colloidal particles was essential for the colloidal stability. The aggregation reversibility results revealed that the aggregation process was reversible, the aggregated colloidal bentonite clusters could return to fully disaggregated state if environmental conditions became dispersion favorable. The results in present work can provide an improved understanding on environmental behavior of bentonite colloids as well as a vital reference for safety assessment of high-level radioactive waste repository.

Modelling the hydro-mechanical behaviour of GMZ bentonite

This paper proposes a hydro-mechanical model of GMZ bentonite, based on the considerable amount of experimental research that has been developed recently. The model uses a double porosity approach based on the framework provided by the Barcelona Expansive Model to describe the deformability of the bentonite. A state surface was used to define its microstructural behaviour, based on the high-suction range data from water retention curves. The intra-aggregate (microstructural) and inter-aggregate (macrostructural) water contents were differentiated. The proposal was satisfactorily applied to simulate several constant stress swelling tests and swelling pressure tests, improving the confidence in the model. The use of this model has allowed not only reproduction of the experimental data but also improvement of our knowledge of GMZ bentonite behaviour.

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.

Effect of salt concentration on desiccation cracking behavior of GMZ bentonite

Bentonite has been proposed for use as an engineering barrier and buffer in nuclear waste repositories and has been used frequently in municipal waste landfills. The cracking behavior and deformation properties of this material can be influenced by the chemistry of pore water. In the present work, the influence of salt concentration on the desiccation cracking behavior of GMZ bentonite was investigated with laboratory experiments. Image processing techniques and SEM tests were performed on the specimens which had undergone the desiccation testing in order to analyze the cracking mechanisms. Results show that the water evaporation process can be identified by a steady rate stage, a falling rate stage and a residual stage. The water evaporation rate is strongly affected by the salt concentration of the pore water; higher salt concentrations result in lower evaporation rates; the final water content is strongly impacted by a high initial salinity; otherwise the water contents are very similar for the residual stage. During desiccating, most of cracks appeared at the steady evaporation stage. The cracking morphology and patterns were greatly affected by the salt concentration of the pore water; and larger crack lengths and lower crack densities were obtained as the initial salinity was increased.

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.

The geotechnical properties of GMZ buffer/backfill material used in high-level radioactive nuclear waste geological repository: a review

Buffer/backfill material in engineered barrier plays an important role in preventing leakage and migrating high-level radioactive nuclear waste (HLW) in geological repository. GMZ bentonite has been proposed as prior buffer/backfill material to be applied in HLW geological repository in China. It is crucial to have a better understanding of the geotechnical properties of this material for optimizing the design and ensuring the long-term stability of the repository. In this paper, the research progress on the geotechnical properties of GMZ bentonite was summarized, including hydraulic conductivity, water retention capacity, thermal physical properties, swelling properties, microstructure, compressibility, multi-fields coupling behavior and long-term characteristics. The following findings are obtained: (1) The basic geotechnical properties of GMZ bentonite are significantly related to the thermal, hydraulic and mechanical boundary conditions, which can be characterized by complex coupling of multi-phase and multi-field; (2) on the premise of meeting the basic requirements of engineered barrier, adding quartz sand and other additives to GMZ bentonite is a currently available and feasible way to improve the geotechnical properties of GMZ bentonite; (3) the observed volumetric deformation and mechanical behavior of GMZ bentonite and mixture under thermal, hydraulic, mechanical and chemical boundary conditions significantly depend on its compaction conditions, relative montmorillonite content and microstructure characteristics; (4) the current researches on GMZ bentonite are mainly limited by traditional laboratory small-scale tests under single-field and single boundary conditions in obtaining single parameter. However, there is still a gap between the simulated environmental conditions in the tests and the real environmental conditions in geological repository. Based on the above understanding and current research shortages in this field, some important research topics were proposed for future work, including multi-field/phase/component coupling characteristics and the mechanisms, long-term performance, large-scale model tests, underground in situ tests, numerical modeling and the constitutive modeling of GMZ bentonite under complex boundary conditions.

Diffusion characteristics of HTO and 99TcO4 − in compacted Gaomiaozi (GMZ) bentonite

The characteristics of diffusion are essential to the transport of radionuclides through buffer/backfill materials, such as bentonite, which are commonly found in waste repositories. This study used through-diffusion techniques to investigate the diffusion behavior of HTO and 99TcO4 − on GMZ bentonite of various densities. Diffusion rates were calculated by measuring the diffusion coefficients (D e, D a), plotting breakthrough curves and interpreting experiment data. The apparent and effective diffusion coefficients of HTO ranged from (1.68 ± 0.40) × 10−11 to (2.80 ± 0.62) × 10−11 m2/s and from (4.61 ± 1.28) × 10−12 to (16.2 ± 2.50) × 10−12 m2/s, respectively. The apparent and effective diffusion coefficients of 99TcO4 − ranged from (5.26 ± 0.16) × 10−12 to (7.78 ± 0.43) × 10−12 m2/s and from (1.49 ± 0.002) × 10−12 to (4.16 ± 0.07) × 10−12 m2/s, respectively. The distribution coefficients of HTO and 99TcO4 − ranged from (0.70 ± 0.12) × 10−2 to (1.36 ± 0.53) × 10−2 mL/g and from (1.12 ± 0.06) × 10−2 to (5.79 ± 2.22) × 10−2 mL/g, respectively. The D e and K d values were shown to decrease with an increase in the bulk dry density of compacted bentonite. Our results show that HTO and 99Tc could be considered non-sorbent radionuclides. The data obtained in this study provide a valuable reference for the safety assessment of waste repositories.

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.

Compression, swelling and rebound behavior of GMZ bentonite/additive mixture under coupled hydro-mechanical condition

The GMZ bentonite/additive mixtures have been proposed as the buffer/backfill media for the high-level radioactive waste geological disposal in China. In this study, two types of mixtures were prepared by mixing variable amounts of crushed granite powder or quartz sand (a range of 10% to 50% by dry mass of aggregate addition) with pure GMZ bentonite. To investigate the volumetric deformation characteristics of the bentonite/additive mixtures under coupled hydro-mechanical conditions, sequential oedometer tests comprising four phases were carried out, including preliminary compression, swelling under constant vertical stress, re-compression, and unloading-rebounding. Experimental results indicate that the volumetric deformation characteristics of the mixture largely depend on the initial condition, additive content and type. The evolutions of compression index and swelling index are governed by the degree of saturation and the state of soil matrix during different loading phases. During the compression phase, the soil compressibility relates to the formation of additive skeletal structure, while the rebound behavior during the unloading phase is determined by the competing mechanical and physico-chemical effects. The correlation between swelling index and bentonite dry density is not unique and dependent on the nature of the additive. The impact of additive properties on the volumetric behavior is further confirmed through tracking swelling indicators including final swelling strain and swelling pressure. This study provides an improved fundamental understanding of the mechanical behavior of GMZ bentonite/additive mixture under coupled conditions and optimizes their engineering applications for nuclear waste disposal.

Salt effects on Re(VII) and Se(IV) diffusion in bentonite

The through-diffusion method was employed to investigate the effect of the salt concentration on the diffusion behaviour of Re(VII) and Se(IV) in compacted bentonite. The accessible porosity, εacc, of Re(VII) increased from 0.25 to 0.38 with an increase of the ionic strength from 0.1 to 1.0mol/L. The dependency of the accessible porosity εacc on the salt concentration could be described approximately by a first-order decay exponential function. The interlayer porosity of GMZ bentonite was 21% of the total porosity at a bulk dry density of 1600kg/m3 at 12°C. The De values of Re(VII) and Se(IV) increased from 2.6×10−11 to 6.1×10−11m2/s and from 1.6×10−11 to 4.7×10−11m2/s in GMZ bentonite with an increase of the ionic strength from 0.1 to 1.0mol/L, respectively. In the case of Se(IV), the adsorption showed a different ionic strength dependency in GMZ bentonite and montmorillonite. The ionic strength has no significant effect on the adsorption (Kd=1.8×10−4m3/kg) on GMZ bentonite, indicating the formation of inner-sphere surface complexes. In the case of synthetic montmorillonite, the Kd values increased with increasing ionic strength. This can be explained by the fact that the electric potential of surface sites becomes less negative with increasing ionic strength resulting in an increase in the adsorption of Se(IV) at a pH above the point of zero salt effect (PZSE). The De values of both Re(VII) and Se(IV) increased with increasing salt concentration. Both anions are totally excluded from the interlayer space and partially from the diffuse double layer. Ionic strength alters the effective diffusion coefficient of anions by controlling the thickness of the diffuse double layer. Anion diffusion data from the literature (effective and apparent diffusion coefficients) were satisfactorily evaluated by Archie's relation.

On the rheological characteristics of GMZ bentonite suspension

At the interface where the buffer bentonite contacts with the surrounding rock of the HLW disposal repository, the bentonite is likely to extrude into the fractures under swelling pressure and hydrate completely, which will as a result bring about looser bentonite suspension that might face the risk of being brought away easily by ground water flow. In this paper, the rheological characteristics of GMZ bentonite suspensions under different water–solid ratios and different types of saline solutions in different concentrations were studied through a rheology experiment using a “Brookfield DV-III Ultra” rheometer. The yield stresses of these GMZ bentonite suspensions were found from the corresponding rheological curves obtained from the rheology experiment. Then, the equation estimating the critical flow velocity was derived based on the Stokes law, and the critical flow velocities corresponding to these yield stresses were calculated. The results show that the rheological curve moves lower with an increase in water–solid ratio, when the water–solid ratio is below 5.0, and meanwhile, the suspension shows a characteristic of Newtonian fluids under lower shear stresses and a characteristic of nonlinear Bingham fluids under higher shear stresses; but when water–solid ratio goes beyond 5.0, the suspension shows a characteristic of pseudo-plastic fluids; when NaCl concentration increases in bentonite suspension, bentonite suspension’s yield stress decreases firstly and then increases, while bentonite suspension’s yield stress decreases when CaCl2 concentration increases; the critical flow velocities range from 10−4 to 10−2 m/s under the conditions of this paper.

Adsorptive removal of Eu(III) from simulated groundwater by GMZ bentonite on the repository conditions

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

Surface complexation modeling of U(VI) sorption on GMZ bentonite in the presence of fulvic acid

Abstract In this work, experiments and modeling for the interactions between uranyl ion and GMZ bentonite in the presence of fulvic acid are presented. The results demonstrated that FA is strongly bound to GMZ bentonite, and these molecules have a very large effect on the U(VI) sorption. The results also demonstrated that U(VI) sorption to GMZ bentonite in the presence and absence of sorbed FA can be well predicted by combining SHM and DLM. According to the model calculations, the nature of the interactions between FA with U(VI) at GMZ bentonite surface is mainly surface complex. The first attempt to simulate clay interaction with humus by the SHM model.