Compacted Bentonite Research Articles

Induced polarization of clay-rich materials. 4. Water content and temperature effects in bentonites

Deep geological disposals (DGDs) are widely seen to be the best solution to contain high-level radioactive wastes safely. Compacted bentonite and bentonite-sand mixtures are considered the most appropriate buffers or sealing materials for access drifts, ramps, and shafts due to their favorable physicochemical and hydro-mechanical properties. Bentonite-sand mixtures are expected to swell and seal all voids when in contact with water, forming an impermeable barrier to radioactive elements. The parameters that will most affect the hydraulic performance of these seals are their water content, dry density, water salinity, and temperature. Monitoring and assessing these parameters are, therefore, crucial to confirm that the seals’ safety functions are fulfilled during the life of a DGD. Induced polarization (IP) is a nonintrusive geophysical method able to perform this task. However, the underlying physics of bentonite sand mixtures has not been checked. The complex conductivity spectra of 42 compacted bentonite-sand mixtures were measured in the frequency range of 1 Hz–45 kHz in order to develop workable relationships between in-phase and quadrature conductivities versus water content and saturation, pore water conductivity, bentonite-sand ratio (10% to 100%), temperature (10°C–60°C), and dry density (0.97 to 1.64 g cm−3). We observe that conductivity is mostly dominated by surface conductivity associated with the Stern layer (SL) coating the surface of smectite, the main component of bentonite. At a given salinity and temperature, the in-phase and quadrature conductivities obey the power law relationships with water content and saturation. The in-phase and quadrature conductivities depend on the temperature according to a classical linear relationship with the same temperature coefficient. An SL-based model is used to explain the dependence of the complex conductivity with water content, dry density, water salinity, and temperature. It could be used to interpret the IP field data to monitor the efficiency of the seal of DGD facilities.

Surface charge characteristics of Gaomiaozi bentonite in high-level nuclear waste repositories

Gaomiaozi (GMZ) bentonite is an excellent backfill material for high-level nuclear waste repositories. The bentonite minerals possess a laminated structure and carry a negative charge. These surface charge characteristics play a crucial role in the development of the electric double layer at the liquid-solid interface and the adsorption of nuclear elements during the hydration process. Following the purification of GMZ bentonite, the surface electric potential of bentonite mineral colloids in suspension and compacted bentonite blocks was measured using atomic force microscopy (AFM). Additionally, the influence of various factors on its surface charge characteristics was examined, including the types of interlayer cations, ambient temperature, number of layers, and dry density. The findings reveal that at 20 °C, the surface electric potential of Na-bentonite quasicrystals averages around −225.2mV, while that of Ca-bentonite quasicrystals averages approximately −155.3mV. Additionally, the surface electric potential increases logarithmically with the number of layers. In the compacted state, its surface potential would decrease. An increase in the dry density of compacted bentonite results in a decrease in the surface electric potential. High ambient temperatures can increase the surface electric potential. The research findings offer valuable data and a theoretical foundation for predicting colloid formation, nuclear element adsorption, and swelling pressure development in bentonites used in high-level nuclear waste repositories.

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.

Thermal impacts on the erosion of compacted bentonite under flow rate

Thermal impacts on the erosion of compacted bentonite under flow rate

Laboratory assessment of real-time water infiltration measurement in SPV200 bentonite using TDR for high-level radioactive waste disposal design

The water content movement within compacted bentonite holds significance in appraising the thermo-hydro-mechanical (THM) performance of the engineered barrier system (EBS) for the disposal of high-level radioactive waste (HLRW), but it demands considerable time to monitor water infiltration in a representative model. Therefore, employing bentonite with varying water content is favored in the laboratory. A comprehensive 3-D phase diagram was established as a benchmark for dielectric constant, temperature, and volumetric water content (VWC) using Time Domain Reflectometry (TDR). SPV200 bentonite at different densities (1400, 1500, and 1600 kg/m3) and temperatures (25, 40, and 60 °C) were prepared with TDR, achieving real-time VWC measurements within a 6 % relative error. Furthermore, numerical outcomes exhibited trends like the TDR tests. Comparation between the simulation and test data is within a relative error ±10 %, which showcased the potential for efficient and effective estimations at a reduced cost for HLRW disposal design.

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.

Gas breakthrough in compacted Gaomiaozi bentonite under rigid boundary conditions

Predicting the gas breakthrough pressure of saturated compacted bentonite is crucial for ensuring the long-term safe operation of deep geological repositories for the disposal of high-level radioactive nuclear wastes. In this work, the swelling pressure, water injection, gas injection and mercury intrusion porosimetry (MIP) tests on saturated compacted Gaomiaozi (GMZ) bentonite specimens with a dry density of 1.3 Mg/m³, 1.4 Mg/m³, 1.5 Mg/m³, 1.6 Mg/m³ and 1.7 Mg/m³ were conducted. Subsequently, the relationships between the swelling pressure and average inter-particle distance, as well as between the gas entry pressure and the maximum effective pore size were analyzed and established. Considering that gas migration and breakthrough are all closely related to the pore structures of the tested geomaterials, a novel gas breakthrough pressure prediction model based on the pore size distribution (PSD) curve was constructed using an existing prediction model based on gas entry pressure and swelling pressure. Finally, based on the test results of the specimens 1.5 Mg/m³, 1.6 Mg/m³ and 1.7 Mg/m³, gas breakthrough pressures of the specimens with dry densities of 1.3 Mg/m³ and 1.4 Mg/m³ were predicted. The results show that the calculated gas breakthrough pressures of 0.76 MPa and 1.28 MPa are very close to the measured values of 0.80 MPa and 1.30 MPa, validating the accuracy of the proposed model.

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.

Visualization of hydraulic fracturing in compacted bentonite: The roles of dry density, water content, and pressurization rate

Deep geological repository is typically situated at depths ranging from several hundred to 1000 m below ground, making bentonite engineered barrier potentially vulnerable to high water pressure and even inducing hydraulic fracturing. This study conducted injection tests on compacted GMZ (Gaomiaozi) bentonite with a self-developed visualization set-up. The objective was to unveil the roles of dry density, water content, and pressurization rate in hydraulic fracturing from the perspective of fracturing macro-morphological dynamics and breakthrough characteristics. Moreover, the relationships between breakthrough characteristics and microstructure were examined by MIP (mercury intrusion porosimetry) analysis. Results showed that the fracturing dynamics were characterized by three stages: hydration, cracking, and fracturing stages. Compared to water content and pressurization rate, dry density exerted more pronounced effects on these stages. Increasing dry density can lead to an expansion of circular hydration zone, a more complex cracking network, and a change in fracturing patterns from long and clear to short and fuzzy. In terms of breakthrough characteristics, the breakthrough pressure was positively correlated with dry density and negatively correlated with water content. Interestingly, there is a good and unique logarithmic correlation between the breakthrough pressure and the ratio eM/em of inter-aggregate void ratio and intra-aggregate void ratio, regardless of dry density and water content. Within a certain range (i.e. 200-50 kPa/min), breakthrough pressure showed slight dependency on pressurization rate. Nevertheless, an extremely low pressurization rate of 20 kPa/min caused a transition for the specimen from quasi-brittle to plastic state owning to more water infiltration, thereby hindering fracture initiation and propagation.

Effect of Specimen Preparation on Shrinkage Behaviour of Compacted Bentonite–Sand Mixtures

Effect of Specimen Preparation on Shrinkage Behaviour of Compacted Bentonite–Sand Mixtures

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Effect of technological voids on hydro-mechanical behavior of compacted bentonite

Influence of temperature and dry density coupled effects on HTO, 36Cl, 85Sr and 133Ba diffusion through compacted bentonite

This study investigated the combined impact of temperature (from 25 to 80 °C) and dry density of bentonite (from 1.2 to 1.65 g·cm−3) on the apparent diffusion coefficient, Da, of four radionuclides (HTO, 36Cl, 85Sr and 133Ba) in a Ca-Mg bentonite. In this study, the different porewater chemistry, present under the specific experimental conditions -- a factor often overlooked in diffusion studies -- was explicitly considered. As a support to diffusion studies, batch sorption tests were carried out at different temperatures (25, 40, 60, and 80 °C) with the sorbing elements (85Sr and 133Ba).The planar source method was used for the determination of apparent diffusion coefficients, Da, which demonstrated its efficacy for both conservative (HTO and Cl) and sorbing elements. For all the investigated radionuclides, Da values decreased with increasing the clay dry density and increased with increasing temperature. Notably, the highest Da value (1·10−9 m2 s−1) was attained for HTO diffusion in the clay at 1.2 g cm−3 and 80 °C, while the lowest Da (3.6·10−12 m2 s−1) value was determined for Ba diffusion at 1.65 g cm−3 and 25 °C.The experimental Da values were analysed employing the Arrhenius law and the Stokes-Einstein equations. Results revealed that diffusion data are in a reasonable agreement with Arrhenius behaviour, but deviations from the Stokes-Einstein equation were observed for compaction densities higher than 1.4 g cm−3.

Modeling unsaturated hydraulic conductivity of compacted bentonite using a constrained CatBoost with bootstrap analysis

Accurately determining the hydraulic conductivity of unsaturated bentonite is important for modeling subsurface thermo-hydro-mechanical and chemical processes. This study introduced a new hybrid model that employs a constrained categorial boosting (CatBoost) algorithm, combined with a genetic algorithm for hyperparameter tuning, to estimate the hydraulic conductivity of unsaturated compacted bentonite The performance of the constrained CatBoost model was benchmarked against a diverse set of data-driven baseline regression models, including lasso, elastic net, polynomial, k-nearest neighbors, decision tree, bagging tree, random forest, and CatBoost. The results indicated that the constrained CatBoost model offers a superior balance between model robustness and predictive accuracy in estimating the hydraulic conductivity of compacted bentonite-based materials during the wetting phase. The model effectively captured the U-shape relationship between hydraulic conductivity and suction, a key characteristic of bentonite behavior. Additionally, bootstrapping analyses confirmed the model's reliability under data variability, further validating its applicability in environmental and geotechnical applications.

Comparative study of hydro-mechanical behaviors of compacted bentonite powder and granular bentonite

In the deep geological disposal repository of high-level radioactive waste, buffer/backfill materials typically consist of compacted bentonite block and granular bentonite. As these materials undergo a long-term hydration, it is anticipated that the two forms of bentonite materials (i.e. compacted bentonite powder (CBP) and granular bentonite (GB)) are expected to exhibit differing hydro-mechanical behaviors due to the differences in their structures. This work aims to investigate the differences in swelling pressure and compressibility through a series of swelling pressure tests, compression tests and mercury intrusion porosimetry (MIP) tests. The experimental results demonstrated that swelling pressure curves of the CBP specimens showed higher first peak values and more pronounced collapse than those of the GB specimens at a given dry density, regardless of vapor-water hydration or liquid-water hydration. The final swelling pressures of the two materials were similar at the same dry density, suggesting an independent correlation between swelling pressure and dry density. At the high suction range, the compression curves exhibited an obvious bi-linear pattern for the CBP specimens and a significant nonlinearity for the GB specimens. Meanwhile, the CBP specimens presented higher pre-consolidation pressures and larger compression indices than the GB specimens at a given suction. As suction decreased, the compression curves of the two materials gradually approached each other and their differences were reduced accordingly. After reaching saturation, a good consistency between them was observed whether for final swelling pressure or compressibility. Pore structure analysis revealed that the two materials both presented an initially double structure, and their differences were primarily manifested at the macrostructural level. Eventually, the differences in swelling pressure or compression curves of the two materials were well interpreted by combining microstructural evolutions.

Effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite for engineered barrier system

This study aims to investigate the effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite buffer material for engineered barrier system. Highly instrumented unconfined compressive strength tests were performed on cylindrical specimens prepared using cold isostatic pressing (CIP) technique at varying testing conditions: (a) CIP pressures of 29.43 MPa, 39.24 MPa, and 58.86 MPa, (b) specimen sizes of 30 × 60mm, 40 × 80mm, and 50 × 100mm, and (c) loading rates of 0.5 %/min, 1.0 %/min, and 1.5 %/min. As CIP pressure increases, the unconfined compressive strength (qu), failure strain (εf), and elastic modulus (E) of Bentonil-WRK increase similar to Gyeongju compacted bentonite (KJ-II). Increase in diameter results in a decrease in qu and εf, and increase in E. Minimal effect of loading rate was observed on qu and εf. However, increasing loading rate tends to increase E. In addition, decrease in diameter increases the elastic threshold axial strain (εe,th). Increase in diameter and loading rate slightly increases Poisson's ratio. Prediction models are provided for assessing the effects of specimen diameter on qu and εf. Furthermore, correlation models of qu to ρd and E are proposed for Bentonil-WRK bentonite buffer material.

Comparison of modeling methods for the effective diffusivities of IO3 − estimated in compacted bentonite using through-diffusion tests under aerobic conditions

Abstract Diffusion is the predominant mechanism governing the transport of 129I through geosphere. Consequently, the assessment of the experimental findings on two-compartment diffusion reservoirs of IO3 − within compacted bentonite involved the application of three distinct diffusion models: CC–CC (constant concentration), CC–VC (variable concentration), and VC–VC. To ensure the reliability of the obtained diffusion coefficients, multiple laboratory tests were performed for internal comparison. The experimental results revealed that IO3 − diffusion coefficients were ranging from 3.83 × 10−13 to 1.91 × 10−11 m2/s. These techniques using three mathematical models could be conducted to estimate the diffusion coefficients of non- or weakly-sorbing radionuclides on compacted bentonite for safety assessment of radioactive waste final disposal.

Application of machine learning in predicting the apparent diffusion coefficient of Se(IV) in compacted bentonite

Application of machine learning in predicting the apparent diffusion coefficient of Se(IV) in compacted bentonite

A CSS surface based THM bounding constitutive model for volumetric behavior of bentonite

Development of the constitutive model for bentonite under coupled thermo-hydro-mechanical (THM) condition is of great significance for the construction and safety assessment of deep geological disposal repositories for high-level radioactive waste (HLW). In this work, a new temperature-suction-mean net stress (T-s-p) space with the conception of critical saturated state (CSS) surface was defined to represent the actual stress state of bentonite under coupled THM condition. Then, based on the CSS surface, a THM constitutive model was proposed for describing the volumetric behavior of compacted bentonite. Under the THM model framework, two bounding surfaces were proposed to describe the elastoplastic volume changes induced by mechanical response of skeleton and hydration of montmorillonite, respectively. The model responses upon some typical THM paths were simulated and discussed to reveal the performance of the proposed constitutive model for bentonite. Finally, the proposed model was validated by simulating by several volume change tests carried out on different bentonites. The results confirmed that the proposed model shows more advantages in describing the THM volumetric behavior.

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.

Swelling characteristics of montmorillonite mineral particles in Gaomiaozi bentonite

Highly compacted bentonite is an ideal back-filling (buffer) material for the deep geological disposal of high-level radioactive waste. Montmorillonite is the main constituent mineral of it. The nanoscopic swelling characteristic of montmorillonite mineral particles greatly influences the macroscopic buffering performance of highly compacted bentonite blocks. Firstly, the montmorillonite suspension liquid was exfoliated by freeze-thaw cycles, the appropriate mineral particles were selected by atomic force microscope (AFM) for making colloidal probes. Then, the swelling pressure between mineral particles was measured by AFM, investigating the effects of interlayer distance, layer number, temperature, and interlayer cation type on the nanoscale swelling pressure. The experimental results showed that the swelling pressure of montmorillonite mineral particles peaks at about 0.7–0.8 nm interlayer distance. The swelling pressure of mineral particles with more layers is smaller. The higher the ambient temperature, the greater the swelling pressure. The swelling pressure of Ca-montmorillonite is significantly larger than that of Na-montmorillonite. The Laird model has a relatively good applicability with the interlayer distance of 0.6–1.2 nm; the DLVO theory has a good applicability with the interlayer distance greater than 1.5 nm. Finally, a predictive model was developed for the swelling pressure of nanoscopic mineral particles considering the effect of layer number. The research results provide data support and theoretical support for swelling pressure generation and cross-scale transmission mechanism of highly compacted bentonite hydration.