Compacted GMZ Bentonite Research Articles

Effects of water content on the corrosion behavior of NiCu low alloy steel embedded in compacted GMZ bentonite

Buffer material and metal disposal containers are the key engineering barriers in the geological disposal of high-level radioactive waste. The durability of disposal containers largely depends on the water content in buffer material. This work focused on investigating the corrosion evolution of NiCu low alloy steel in compacted GMZ bentonite with different water contents for 270 d by using weight loss, electrochemical measurements, and various methods for analyzing corrosion products. As the water content increased from 13% to 20%, the water in the bentonite transformed from an unsaturated to a critical saturated state, and the corrosion rate of NiCu steel clearly increased. In these two systems, the oxygen could migrate to the thin liquid film on the steel surface through the air pores in the bentonite in the gas phase and undergo cathodic reduction. Meanwhile, it oxidized the ferrous hydrolysis products into ferric corrosion products and formed a rust layer, which could block the diffusion of oxygen. At that moment, the cathodic process of NiCu steel corrosion changed to rust reduction. When the water content continually increased to 30% and 40%, the compacted bentonite was in a saturation state, and the corrosion rate of NiCu steel was significantly decreased. This was because most pores among the bentonite particles were occupied by a large amount of free water, which hindered the diffusion of oxygen and inhibited its cathodic reduction. Furthermore, it restrained the oxidation of ferrous corrosion products, which greatly weakened the cathodic depolarization of rust, leading to the cathodic process being dominated by the hydrogen evolution reaction.

Insights into anisotropic compression characteristics of unsaturated compacted GMZ bentonite

Insights into anisotropic compression characteristics of unsaturated compacted GMZ bentonite

Effects of dry density and porewater salinity on Eu(Ⅲ) diffusion in compacted GMZ bentonite: A perspective from triple pore structure

Compacted bentonite is microscopically characterized by a triple pore structure composed of inter-aggregate pores, inter-particle pores and interlayer pores. These three types of pores are available for diffusive transport of cations through different mechanisms: free water diffusion, surface diffusion, and interlayer diffusion. This study investigated the effects of dry density and porewater salinity on Eu(Ⅲ) diffusion in compacted GMZ bentonite from the perspective of triple pore structure. A series of through-diffusion (TD) experiments were conducted as a function of the density and porewater salinity. Then the triple pore structure was characterized using SEM and MIP tests. On this basis, a mathematical expression about the dependence of Eu(Ⅲ) diffusion on the triple pore structure was developed. Results indicated that increasing dry density inhibited the Eu(Ⅲ) diffusion, as evidenced by decreases in the effective diffusion coefficient De, capacity factor α, and apparent diffusion coefficient Da. This inhibition is attributed to the reduction in inter-aggregate pores, inter-particle pores, and interlayer pores with increased density, thereby suppressing free water diffusion, surface diffusion, and interlayer diffusion. Conversely, increasing porewater salinity facilitated the Eu(Ⅲ) diffusion. This is likely because the relatively high concentration of Eu(Ⅲ) in the TD tests makes free water diffusion as the dominant pathway. Although surface diffusion and interlayer diffusion were inhibited owning to the decrease of inter-particle pores and interlayer pores, their negative effects on total diffusion flux were less than the positive effects of enhanced free water diffusion resulting from the increase of inter-aggregate pores. Finally, by comparing the calculated total diffusion fluxes and experimental values, the applicability of the proposed expression for predicting Eu(Ⅲ) diffusion in compacted bentonite with varying dry densities and salinities was discussed.

Shear strength characteristics of unsaturated compacted GMZ bentonite considering anisotropy

Anisotropic microstructure would be generated in the bentonite block during unidirectional compactions. Working as buffer materials, the compacted bentonite will inevitably experience shearing processes during the long-term operation of geological repositories. In this paper, high-pressure triaxial tests were conducted on unsaturated compacted GMZ bentonite specimens with different water contents, dry densities and confining stresses, with the compaction surface of the specimens oriented in both horizontally (H-type) and vertically (V-type) configurations. Results demonstrate that an increase in water content leads to a reduction in both peak strength and residual strength, while higher confining stresses enhance these strength parameters. Normally consolidated and lightly over-consolidated bentonite specimens display substantial shear deformation, whereas heavily over-consolidated specimens tend to experience brittle failure. Water content plays a significant role in shaping both the critical state line (CSL) and the Hvorslev surface (HS), with increasing water content resulting in decreased slope parameters for both and an increased intercept parameter for the HS. Generally, V-type specimens demonstrate a steeper CSL and an outwardly extended HS in contrast to that of H-type specimens. The critical state ratio for V-type specimens is about 10 % higher, and the friction angle is 2.8° greater, than that of the H-type ones. Moreover, this difference appears to increase with increasing water content. The difference of the HS slope parameter between the two specimens is minor, while the intercept parameter is higher for the V-type specimens.

Thermal effects on the strain rate-dependent behavior of highly compacted GMZ01 bentonite

Investigation of thermal effects on the strain rate-dependent properties of compacted bentonite is crucial for the long-term safety assessment of deep geological repository for disposal of high-level radioactive waste. In the present work, cylindrical GMZ01 bentonite specimens were compacted with suction-controlled by the vapor equilibrium technique. Then, a series of temperature- and suction-controlled stepwise constant rate strain (CRS) tests was performed and the rate-dependent compressibility behavior of the highly compacted GMZ01 bentonite was investigated. The plastic compressibility parameter λ, the elastic compressibility parameter κ, the yield stress p0, as well as the viscous parameter α were determined. Results indicate that λ, κ and α decrease and p0 increases as suction increases. Upon heating, parameters λ, α and p0 decrease. It is also found that p0 increases linearly with increasing CRS in a double-logarithm coordinate. Based on the experimental results, a viscosity parameter α(s, T) was fitted to capture the effects of suction s and temperature T on the relationship between yield stress and strain rate. Then, an elastic-thermo-viscoplastic model for unsaturated soils was developed to describe the thermal effects on the rate-dependent behavior of highly compacted GMZ01 bentonite. Validation showed that the calculated results agreed well to the measured ones.

Gas diffusion property of compacted GMZ bentonite tested under different boundary conditions considering saturation and gas pressure

Gas diffusion property of compacted GMZ bentonite tested under different boundary conditions considering saturation and gas pressure

Site water salinity effect on the hydro-mechanical behavior of compacted GMZ bentonite with technological void

Site water salinity effect on the hydro-mechanical behavior of compacted GMZ bentonite with technological void

Swelling characteristics of compacted graphene-modified GMZ bentonite infiltrated with NaCl solution

Swelling characteristics of compacted graphene-modified GMZ bentonite infiltrated with NaCl solution

Volume change characteristic of compacted bentonite during hydration upon high suction change: Experimental study and constitutive simulation

The assessment and predicting of volume change characteristic in compacted bentonite during hydration upon high suction change are crucial considerations in the design and construction of geological repositories. However, the mechanism for volume change behavior of bentonite upon suction change path is not yet well understood. Existing elastoplastic models show some limitations for describing hydration deformation of compacted bentonite upon high suction change path. In this work, suction-controlled hydration deformation tests were conducted on compacted GMZ01 bentonite under constant vertical stresses. The various forms of void ratio-suction curves observed suggested that the swelling-collapse behavior during hydration was influenced by the over-consolidation ratio (OCR). Furthermore, it was observed that each hydration deformation curve (e-lns) exhibited a saturation suction value. When the suction decreased to the saturation suction, the bentonite reached a state where further hydration was no longer possible, resulting in the absence of additional swelling or collapse deformation. The unique relationship between saturation suction and stress applied, i.e. critical saturated state (CSS) curve was proved to existed, which dividing the stress space (s-p plane) into unsaturated and saturated zones. A model framework with the concept of CSS curve was introduced and the suction-induced volume change equation was determined based on the tests results. The simulation results of hydration deformation tests demonstrated excellent agreement with the experimental data. The test results in this work provide the important mechanical parameters for the design and construction of geological repositories. Meanwhile, the proposed model could describe volume change characteristic of compacted bentonite during hydration upon high suction change, showing great significance for the safety assessment of repositories.

The volume change behavior of compacted GMZ bentonite: combined effects of temperature and suction

The volume change behavior of compacted GMZ bentonite: combined effects of temperature and suction

Role of microstructure evolution in temperature effects on swelling behaviour of compacted bentonite at different vertical pressures

Compacted bentonite is often proposed to construct the buffer barrier in deep geological disposal of high-level radioactive waste. Temperature effects on swelling behaviour are significant for the performance assessment of the bentonite buffer. However, the contribution of microstructure evolution to temperature effects on swelling is poorly understood. This study focused on the role of microstructure change in the swelling of compacted GMZ bentonite under various temperatures and vertical pressures. Swell strain tests were conducted using a temperature-controlled oedometer, and mercury intrusion porosimetry was selected to investigate microstructure changes. Intra-aggregate and inter-aggregate pore size distributions were separated based on fitting the cumulative mercury intrusion curve using a bimodal van Genuchten-type equation. The pores larger than 0.1 μm were found to belong not only to the inter-aggregate pore family but also to the intra-aggregate pore family. The significance of the collapse of inter-aggregate porosity under high vertical pressure was highlighted, which resulted in the deceleration in swelling strain growth in the intermediate primary stage and the compression of inter-aggregate pores upon wetting. The temperature effects on the collapse are critical to the temperature dependence of swelling development under high vertical pressure. More collapse at elevated temperature gave rise to more pronounced deceleration in the intermediate stage and a larger amount of the inter-aggregate compression, which in turn led to a greater decrease in maximum swelling strain with temperature under higher vertical pressure. The intra-aggregate pore size distribution can be affected by temperature and vertical pressure, although the total void ratio of intra-aggregate pores was insensitive to temperature and vertical pressure. The volume enlargement of the intra-aggregate pores smaller than 6.5 nm was significant and decreased with increasing temperature and vertical pressure. In contrast, the volume enlargement of the intra-aggregate pores larger than 6.5 nm was relatively small due to the filling effects and exhibited increases with increasing temperature and vertical pressure.

Effect of Beishan groundwater salinity on the self-sealing performance of compacted GMZ bentonite

Effect of Beishan groundwater salinity on the self-sealing performance of compacted GMZ bentonite

Insights into anisotropic swelling pressure of compacted GMZ bentonite

Insights into anisotropic swelling pressure of compacted GMZ bentonite

A model for describing advective and diffusive gas transport through initially saturated bentonite with consideration of temperature

Understanding the gas migration mechanism in an initially saturated bentonite is of great importance for evaluation of the long-term performance of a deep geological repository for disposal of high-level radioactive wastes. Previous researches usually neglect the gas diffusion process, but adopted the advective gas flow theory (visco-capillary flow or dilatant gas flow) to describe the gas transport in the initially water-saturated bentonite clays. In this study, gas migration process was regarded as results of a combination of diffusive transport of dissolved gas through pore water and advective flow along the gas pathways formed in the bentonite matrix. Meanwhile, dilation of the gas flow pathways with variation of the external stress and gas pressure during the advective flow was also considered. Based on this concept, a new model was proposed to describe the advective and diffusive gas transports with consideration of temperature. Validation was conducted using the gas injection tests performed under different boundary conditions in literature. Comparisons show that the diffusion and advection-controlled process provided by the proposed model can effectively simulate gas transport in the compacted GMZ bentonite. For gas migration under lower injection pressures, the contributions of advective gas flow to the gas flux are comparable to that of diffusion. With the increase of the injection pressure, the advective gas fluxes would gradually dominate over the diffusive transport. Meanwhile, predicted degree of saturation of the compacted bentonite is maintained at above 99% during the whole gas migration process, indicating that advective movement of gas was highly discrete in initially saturated bentonite.

Combined impact of aging and saline solution on the swelling characteristics of compacted GMZ bentonite

The swelling characteristics of compacted bentonite preferred as an ideal buffer material are of pivotal importance to the sealing functionality of the multi-barrier system in the context of nuclear waste disposal. The aging process resulted from the intermediate storage period in real case is typically ignored, while the impact of saline solution on bentonite should be generally accounted. To get a more comprehensive understanding on the swelling, this study presents an experimental investigation on the impacts of potential aging time (0–90 days) and saline solution (salinity: 0–12.3 g/L) on the swelling pressure and deformation of compacted GMZ bentonite at an initial dry density of 1.70 Mg/m3. Combined uses of Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM) were also adopted to capture the microstructure characteristics, aiming to make a reasonable interpretation from a microscopic view. Test results show that the swelling potential decreases with increasing aging time and/or solution salinity, while the aging impact appears to be ignorable over 30 days. The exfoliation of clay particles/aggregates is expected to happen before saturation due to aging, and this feature was also found to be more obvious in aged specimens, with smaller inter-aggregates and more uniform pore spaces. Clear contraction of clay aggregates is observed after immersion in saline solution, which can be attributed to the decrease in the repulsion between particles and limited inhibition of expansion in interlayer spaces, leading to a decrease in swelling potential. Specially, the aging effect is believed to be related with the phenomenon of cation demixing, during which the monovalence and bivalence cations tend to be separated from each other, resulting in a structure modification and hence a weaker swelling potential. This study highlights the importance to carefully evaluate the engineering properties of compacted bentonite considering the whole process after its manufacture.

Laboratory investigation on the membrane behaviour of highly compacted GMZ bentonite under isothermal conditions

Laboratory investigation on the membrane behaviour of highly compacted GMZ bentonite under isothermal conditions

A critical saturated state-based constitutive model for volumetric behavior of compacted bentonite

In recent years, more and more unique volumetric behaviors, including the nonlinear compression and unloading curves in the e-lnp plane, and different types of swelling and shrinkage curves obtained from suction-controlled swelling and shrinkage tests, etc., which are significantly different from those of nonexpansive clays, have been reported on compacted bentonite. More importantly, limitations were encountered when describing the unique behavior with existing constitutive models for unsaturated expansive soils. In this study, a new concept of critical saturation state (CSS) was proposed for compacted bentonite, in which, as the saturated state was reached, the matric suction still remained nonzero in the soil. Incorporated with this concept, a constitutive model was proposed. In this model, the CSS curve was defined in the suction-isotropic stress plane, delimiting the unsaturated zone. To validate the model, the volume change tests conducted on compacted GMZ bentonite under different stress paths were simulated. Good agreement confirmed that the proposed model could well describe the volume change behavior, including the swelling on hydration, shrinkage during drying, nonlinear compression, and unloading behavior.

Diffusion of Re(VII), Se(IV) and Cr(VI) in compacted GMZ bentonite

Diffusion of Re(VII), Se(IV) and Cr(VI) in compacted GMZ bentonite

Effect of aging on shear strength of compacted GMZ bentonite

The shear strength characteristics of compacted bentonite preferred as feasible buffer/backfill component is essential to the mechanical stability of the multi-barrier system in the context of nuclear waste disposal. This paper presents an investigation on the potential aging-induced changes in shear strength and microstructure of compacted GMZ bentonite at a dry density of 1.68 Mg/m3 and various water contents. For this purpose, direct shear and Mercury Intrusion Porosimetry (MIP) tests were correspondingly performed, as well as the soil-water retention tests. Statically compacted bentonite specimens were kept at constant volume and water content conditions for different periods of aging time up to 90 days, prior to experimental investigations. Results showed that the shear strength decreased with aging time, with a more noticeable trend during the first 30 days. Aging effect on shear strength was less pronounced for specimens at water content of 24%. Significant changes in microstructure after aging were observed, characterized by an increase in tiny pores (< 6 nm) and a decrease in inter-aggregate pores. The former phenomenon is related to the interlayer hydration as water redistribution is expected to occur, while the latter one is attributed to the exfoliation of clay particles that fill and reduce the inter-aggregate pores, which accounts for the dry density effect on soil-water retention curves (SWRC). Aging-induced changes in microstructure are governed by the water uptake from the non-interlayer space acting as reservoir into the interlayer pores, thereby promoting a further clay layer hydration and an arrangement of clay fabric. As such, both the effective degree of saturation (mainly considering the capillary water) and total suction decreased with aging, as well as the Bishop-type effective stress, thereby resulting in a decrease in the shear strength of compacted GMZ bentonite.

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.