GMZ Bentonite Research Articles

Analyzing of the hydration crack evolution in compacted GMZ bentonite with consideration of technological void ratio

Technological voids widely exist around the bentonite blocks in the deep geological repository. After groundwater seeps into the voids, bentonite nearby expands to fill up the voids and completes the self-sealing gradually. The self-sealing ability is of highly importance, which prevents the technological voids from playing a preferential channel for groundwater migration and thus threatening the safety of the canister accommodating the high-level radioactive nuclear waste (HLW). So far, much of the research focus has been on the hydro-mechanical properties of bentonite after self-sealing, while the process itself is neglected. But actually, a cracking phenomenon will pop up in the interim of sealing process. In this paper, the dynamic evolution of self-sealing on compacted bentonite with different technological voids and sample thicknesses was observed by using a transparent observation apparatus. Digital image processing method combined with fractal theory was utilized to process, analyze and quantify the images captured. The results show that the self-sealing process contains the lifetime of the hydration crack, including its appearance, development and extinction. The technological void ratio evidently affects the time require for closing stage compared with the cracking stage. But in the range of 10–25%, there is no clear correlation between the technological void ratio and the geometry parameters or complexity of crack network. The fractal dimension introduced to quantify the complexity corresponds well with the visible image result, showing the feasibility and credibility of box-counting method. The cracking and closing mechanisms are also investigated.

Insights into the Anisotropic Swelling Pressure of Compacted ​Gmz Bentonite

Insights into the Anisotropic Swelling Pressure of Compacted ​Gmz Bentonite

Degradation of Gmz Bentonite by Diffusion of Alkaline Solutions with Various Ph

Degradation of Gmz Bentonite by Diffusion of Alkaline Solutions with Various Ph

Evolution of Hydraulic Conductivity of Unsaturated Compacted Na-Bentonite under Confined Condition-Including the Microstructure Effects.

Compacted bentonite is envisaged as engineering buffer/backfill material in geological disposal for high-level radioactive waste. In particular, Na-bentonite is characterised by lower hydraulic conductivity and higher swelling competence and cation exchange capacity, compared with other clays. A solid understanding of the hydraulic behaviour of compacted bentonite remains challenging because of the microstructure expansion of the pore system over the confined wetting path. This work proposed a novel theoretical method of pore system evolution of compacted bentonite based on its stacked microstructure, including the dynamic transfer from micro to macro porosity. Furthermore, the Kozeny–Carman equation was revised to evaluate the saturated hydraulic conductivity of compacted bentonite, taking into account microstructure effects on key hydraulic parameters such as porosity, specific surface area and tortuosity. The results show that the prediction of the revised Kozeny–Carman model falls within the acceptable range of experimental saturated hydraulic conductivity. A new constitutive relationship of relative hydraulic conductivity was also developed by considering both the pore network evolution and suction. The proposed constitutive relationship well reveals that unsaturated hydraulic conductivity undergoes a decrease controlled by microstructure evolution before an increase dominated by dropping gradient of suction during the wetting path, leading to a U-shaped relationship. The predictive outcomes of the new constitutive relationship show an excellent match with laboratory observation of unsaturated hydraulic conductivity for GMZ and MX80 bentonite over the entire wetting path, while the traditional approach overestimates the hydraulic conductivity without consideration of the microstructure effect.

Montmorillonite alteration and its influence on Sr (II) adsorption on GMZ bentonite

Montmorillonite alteration and its influence on Sr (II) adsorption on GMZ bentonite

Alkaline Degradation of GMZ Bentonite as HLW Barrier in China

Alkaline Degradation of GMZ Bentonite as HLW Barrier in China

Pore Structure Identification of GMZ Bentonite with Different Water Content Based on Digital Image

Bentonite contains many pore structures with irregular geometric shapes, with a strong adsorption to various fluids. It swells in the presence of water and could cause a significant change in its microscopic pore structures. This would lead to significant changes in its permeability, which will impact its engineering applications. This study analyses and compares the pore structure of bentonite cured under different humidity based on their microscopic images. The threshold segmentation algorithm is used to identify the pore structure in the microscopic image. Then the porosity and pore size distributions are measured, and the Hagen Poiseuille formula is used to calculate its permeability. After comparing bentonite treated with humidity of 11% and 98%, the porosity of bentonite decreased in the presence of water, while the proportion of small-sized pores increased, and the average permeability of bentonite of 98% humidity was half of that of 11%. Thus, the pore structure and permeability of bentonite changed significantly when subjected to different humidity.

Influence of dry density and water salinity on the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite–sand mixtures

Influence of dry density and water salinity on the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite–sand mixtures

Experimental study on three-dimensional swelling pressure of compacted GMZ bentonite–sand mixtures

Experimental study on three-dimensional swelling pressure of compacted GMZ bentonite–sand mixtures

Thermal properties of GMZ bentonite pellet mixtures subjected to different temperatures for high-level radioactive waste repository

Thermal properties of GMZ bentonite pellet mixtures subjected to different temperatures for high-level radioactive waste repository

A nonlinear normal consolidation line for bentonite in e-logp space

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

Combined effects of temperature and salt solution on swelling of GMZ01 bentonite–sand mixtures

Combined effects of temperature and salt solution on swelling of GMZ01 bentonite–sand mixtures

Cracking and sealing behavior of the compacted bentonite upon technological voids filling

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

Insights into the water retention behaviour of GMZ bentonite pellet mixture

Insights into the water retention behaviour of GMZ bentonite pellet mixture

Pore fluid chemistry effects on the swelling behavior of compacted GMZ bentonite with an artificial annular gap

Pore fluid chemistry effects on the swelling behavior of compacted GMZ bentonite with an artificial annular gap

Anisotropic swelling behaviour of unsaturated compacted GMZ bentonite hydrated under vertical stresses

Bentonite has been considered as a potential buffer/backfill material for construction of engineering barriers in deep geological repository for disposal of high-level nuclear waste. Anisotropy will be inevitably generated during static compaction during preparation of bentonite blocks, leading to induce possible influences on buffering function formation of the engineering barrier system and even the safety of the whole repository. In the present work, swelling deformation tests along the directions both perpendicular and parallel to the compaction plane were conducted on unsaturated compacted GMZ bentonite specimens using a self-developed lateral confined swelling test setup. Influences of initial dry density and vertical pressure on the final swelling strain and its anisotropy, as well as the mechanism of evolution of anisotropy coefficient on hydration, were analysed. Results demonstrate that the swelling time, average strain rate and final swelling strain increase with increasing initial dry density and decrease with increasing vertical pressure. Meanwhile, the swelling strain along the perpendicular direction is much higher than that along the parallel direction, exhibiting an obvious anisotropic swelling characteristic. The anisotropy coefficient is relatively large at the initial swelling stage, and then gradually decreases to be stabilised. The final anisotropy coefficient increases with initial dry density and decreases with vertical pressure.

Insights into gas migration behavior in saturated GMZ bentonite under flexible constraint conditions

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

Influence of cyclic thermal processes on gas migration in saturated GMZ01 bentonite

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

Effect of granule size distribution on the compaction property of GMZ bentonite

In the current concept for disposal of high-level radioactive waste (HLW) in China, a barrier composed of blocks of highly compacted bentonite is constructed around the waste canisters. To develop this technique, uniaxial compaction of full-sized bentonite blocks has been initiated. Many factors affect the quality of bentonite blocks, including water ratio and granule size distribution of the bentonite, compaction pressure and rate, and form geometry. In the present study, compaction tests were conducted using powdered, pelleted, and mixed GMZ bentonite to evaluate the effects of granule size distribution on the integrity and uniformity of the block samples. Results show that high-quality blocks can be manufactured by adjusting the water content of the bentonite to 17%, in spite of different granule size distributions. The compaction in the mixed bentonite was better than that in the powdered and pelleted samples. However, the difference in final density was negligible when compacting at the same pressure. The block uniformity was not improved by adopting different granule size distributions when the height:diameter ratio of the bentonite samples reached 2. Blocks produced by powdered and mixed bentonite were prone to radial cracks when subjected to moisture loss; however, the cracks were healed after moisture redistribution.

Simple method for evaluating swelling of GMZ01 Na-bentonite affected by temperature at osmotic swelling

Swelling deformation tests of GaoMiaoZi (GMZ01) bentonite were performed at 25, 40 60, and 90 °C in distilled water. Experimental results suggested that the maximum swelling strain of GMZ01 bentonite increased with temperature, due to the osmotic swelling dominated the swelling. The em − σv relation for compacted bentonite at a same temperature can be expressed as em=KσvD-3 (em is montmorillonite void ratio, σv is vertical stress). The fractal dimension, D, of GMZ01 bentonite was unaffected by the temperature according to the results of N2 adsorption tests. Calculated by the diffuse double layer (DDL) model under the condition of osmotic swelling, a linearly relation was found between the montmorillonite swelling coefficient K and temperature Tc. Combing the fractal em − σv relation and linear K − Tc relation, a simple method for evaluating the swelling of GMZ01 bentonite affected by temperature was proposed.