MX-80 Bentonite Research Articles

Erosion dynamics of compacted raw or homoionic MX80 bentonite in a low ionic strength synthetic water under quasi-stagnant flow conditions

Bentonite is planned to be used as a backfill material in deep geological repositories (DGR) notably for those to be established in crystalline rock. Potential erosion processes have to be clearly understood in order to properly assess its long-term barrier function. A likely scenario over the lifetime of a DGR in certain geographical locations is the intrusion of low mineralized glacial melt water through low transmissivity bedrock fractures that could interact with bentonite and favor release of clay colloids. The objective of this work was to determine the dynamic of erosion under a glaciation scenario for Na-exchanged, Na-Ca-exchanged, Ca-exchanged and raw bentonite. The different bentonite types cover different potential materials used for backfill and different stages of cation loading via cation exchange processes upon contact with groundwater of different composition. To this end, erosion experiments were carried out with highly compacted bentonite confined by a porous filter (20 μm) under dynamic conditions simulating the presence of a hydraulically active fracture filled with fault gauge and fracture filling minerals.The <500 μm size fractions of untreated MX80 bentonite and after Na- or Ca-homoionization, were used. Two compacted clay pellets (1.6 ± 0.1 g/cm3) of identical composition were placed in separate compartments of double-side reactors. Post-mortem visual inspection and the observed washout of water soluble MX80 components indicated full equilibration of the bentonite with simulated groundwater taking place during the experiment. The erosion was investigated by circulating a low ionic strength (1.6 10−3 M) water at pH 8.4. Bentonite swelling was confined by stainless steel filter plates.The erosion (clay colloid detachment) via a low mineralized groundwater flow was quantified by determining an average eluted mass loss rate (AMLR). AMLR values integrated over 1466 days (4 years) erosion time for the raw MX80 bentonite, the Na-MX80 and the Na-Ca-MX80 are at 0.019 ± 0.003 kg/y/m2, 0.245 ± 0.007 kg/y/m2 and 0.10 ± 0.02 kg/y/m2, respectively. Only by the sensitive ICP-MS analysis of clay-bound Th and U a tiny detachment of particles from Ca-MX80 could be identified, which shows the highest resistivity against erosion (0.004 ± 0.002 kg/y/m2). The experiments pointed to the swelling pressure as main driving force for colloid generation. A possible inhibiting effect of accessory minerals on erosion can also be deduced from this study. Assuming erosion via a groundwater flow from a single fracture intersecting a deposition hole filled with raw MX80 bentonite at an angle of 90o and with a fracture aperture of up to 200 μm, the present estimations indicated that under the given scenario the mass loss limit of 1200 kg bentonite will not be reached during the first 106 years.

Impact of initial structural heterogeneity on long-term swelling behavior of MX80 bentonite pellet/powder mixtures

To better understand results of SEALEX in situ tests carried out at Tournemire Underground Research Laboratory, the hydromechanical behavior of a pellet/powder MX80 bentonite mixtures prepared at a dry density of 1.49 Mg/m3 were investigated by means of microfocus X-ray computed tomography (μ-CT) observations and laboratory small scale infiltration tests. Radial and axial swelling pressures as well as relative humidity were monitored while wetting. Two configurations were considered: for the first, a pellet/powder mixture was prepared following a specific protocol to minimize initial structural heterogeneity; the second one was specially designed to study a strong heterogeneous mixture distribution. μ-CT observations performed on the two samples during hydration revealed an apparently homogeneous sample for the first mixture after 100 days of hydration. For the second specimen, several voids were still observed after 40 days of hydration. A comparison was made between the in situ and mock-up tests. It was observed that the evolutions of radial and axial swelling pressures depend on the initial heterogeneous distribution of the mixture. This heterogeneity is due to the different dry density values at the vicinity of the different sensors. The final values of axial swelling pressures were different for both configurations for the same global dry density.

Investigation of steel corrosion in MX80 bentonite at 120°C

AbstractCorrosion experiments were performed on two metallic substrates, a ferritic–pearlitic steel (P285NH) and a ferritic one (Armco), in silicate environment during 30 days at 120°C. Corrosion products were characterized in terms of morphology (scanning electron microscope and transmission electron microscopy), composition (energydispersive X‐ray spectroscopy) and structure (µ‐Raman, selected area electron diffraction, X‐ray absorption near edge structure). Results show a nanometric inner layer made of compact and adherent nanocrystallized magnetite with, locally a thickness up to several micrometers due to the metal microstructure. An outer layer of Fe‐rich phyllosilicate, smectite and serpentine, more porous than the inner one and poorly adherent is observed around both the samples.

Development of an upscalable HM model for representing advective gas migration through saturated bentonite

Understanding gas transport through engineered and natural clays is important for their use in buffers and seals in a range of Geological Disposal Facility (GDF) concepts. Gas migration via processes of multi-phase flow of water and bulk gas, and transport of dissolved gases, have been widely studied and are well understood for traditional porous media. However, models using only these processes are unable to represent the complex behaviour of gas flow in water-saturated clays, where gas migrates through the creation of additional flow paths (via dilatant pathways/tensile fractures). A bespoke fully-coupled hydro-mechanical model representing gas flow through dilating capillaries is presented, where gas in the model is considered to have a separate permeability to water within the saturated bentonite, dependent on the in situ stress conditions. The model permits a coarse spatial discretisation, compared to standard multiphase flow approaches, while still providing a good match to experimental datasets. This should allow for future upscaling (in space and/or time), which may be required if such modelling is necessary in a performance assessment (PA) or GDF safety case context. The model has been tested against small-scale laboratory experiments investigating gas flow through saturated MX-80 bentonite samples as part of participation in Task A of the DECOVALEX-2019 project, and has provided useful experience in the interpretation of the experimental configuration and its representation in models. Good agreement of the model results to the experimental data can be obtained after calibration of a few key parameters, which also provide a good match to an additional historical dataset. The significance of these gas flows is considered in a PA context and potential future improvements are discussed.

Experimental Investigation and Modeling of the Hydraulic Conductivity of Saturated Bentonite–Claystone Mixture

Abstract Mixtures composed of MX80 bentonite and crushed Callovo-Oxfordian (COx) claystone have been considered as one of the promising sealing/backfilling materials in French deep geological dispo...

Soil-water characteristic curves of bentonites in isochoric conditions during wetting: measurement and prediction

The determination of the soil-water characteristic curve (SWCC) of compacted bentonites in the isochoric condition is a prerequisite for unsaturated flow simulations in several geoenvironmental applications. The SWCC data are, however, not readily available for many compacted bentonites over a wide suction range due to difficulties associated with the testing. In this work, wetting SWCCs of four Indian bentonites of different plasticity were established experimentally at compaction dry densities of 1.4, 1.6, and 1.8 Mg/m3 in isochoric conditions using two independent laboratory techniques in different suction ranges. The modified Kovács (MK) model was extended to simulate the SWCC data of the Indian bentonites. Empirical relationships between MK model parameters and compaction dry density for these bentonites were established. A theoretical procedure for the estimation of SWCCs of the compacted bentonites based on the basic bentonite properties and proposed correlations was proposed and validated on the available wetting SWCC data of compacted GaoMiaoZi (GMZ), MX80, and FEBEX bentonites from the literature studies. The proposed method predicts the wetting SWCCs of compacted powder bentonites, in volume-restrained condition, from the basic properties satisfactorily. The proposed model is an approximate method for predicting the SWCCs of bentonites in the absence of measured data and in understanding the influence of dry density and plasticity.

Modelling the long-term hydro-mechanical behaviour of a bentonite pellet/powder mixture with consideration of initial structural heterogeneities

The aim of this paper is to investigate the long-term hydro-mechanical behaviour of a highly heterogeneous MX80 bentonite pellet/powder mixture (80/20 in dry mass), which is one of the candidate sealing materials in deep geological repositories. In spite of the operational advantages related to the use of the mixture, structural heterogeneities resulting from the installation process constitute a matter of concern and require special approaches to describe adequately the material behaviour during hydration. In this study, a double structure formulation taking into account the initial structural heterogeneity of the material as well as damage to pellets upon wetting is proposed. The formulation is applied to the modelling of a 1/10 mock-up of Sealex large-scale tests. Hydraulic and mechanical parameters are determined from an extensive laboratory programme carried out on a single pellet of bentonite and on the pellet/powder mixture. To ensure an adequate analysis of the test, the initial heterogeneous structural distribution of the material is determined by image analysis of microfocus X-ray computed tomography observations. The model allows the anisotropic swelling behaviour of the mixture to be satisfactorily reproduced when accounting for the spatial variation of the material's initial porosity. In particular, the long-term (decade) hydro-mechanical behaviour of the mixture can be well described. Detailed analysis of the modelling results demonstrates the existence of dry density gradients at the long term and their influence on swelling pressure anisotropy.

Hydro-chemo-mechanical modelling of long-term evolution of bentonite swelling

The swelling properties of compacted bentonite have typically been studied by means of hydro-mechanical models. The question of how these properties are affected by changes in its chemical composition has been given much less attention. However, cation exchange reactions in montmorillonite interlayer or changes in salinity are known to affect the swelling properties of bentonite. In this paper, a hydro-chemo-mechanical (HCM) model is proposed to study the effects of reactive transport processes on the long-term evolution of swelling pressure of compacted bentonite under confined and partially saturated conditions. The HCM model is implemented in iCP, an interface between Comsol Multiphysics and PHREEQC and is calibrated using available experimental data. At the constitutive level, the hydro-mechanical Barcelona Basic model is extended to include the effects of cation exchange reactions between interlayer and bulk water, changes in salinity, and montmorillonite dissolution in highly alkaline environments. The geochemistry of bentonite is based on an advanced reactive transport model that couples solute transport by diffusion and advection with chemical reactions. An application case is presented to demonstrate the main HCM model features and its ability to predict long-term changes in bentonite swelling pressure. The performance of the sealing system for closure of the repository for radioactive waste in France is simulated under unsaturated and isothermal conditions over a period of 50,000 years. The HCM interactions of MX-80 bentonite with other near-field materials such as concrete and claystone are simulated to predict the evolution of the swelling pressure of the seal. The results show the potential of the proposed model for the performance assessment of repository components. Overall, the model predicts limited impact of geochemical alteration on the swelling pressure of the bentonite seals, which is mainly driven by cation exchange reactions between the Na-rich bentonite, the Ca-rich claystone, and the calcium and alkalis from concrete porewater.

Estimation of water retention behaviour of bentonite based on mineralogy and mercury intrusion porosimetry tests

In this paper a water retention model is developed based on soil mineralogy and microscopic features of material behaviour. The model estimates the water content at a given total suction as the sum of the bentonite minerals’ interlayer water and water in the interparticle pores. The paper proposes simple formulae to estimate the interlayer water content and interlayer void ratio based on mineralogical properties. Additionally, the model uses mercury intrusion porosimetry (MIP) measurements to approximate the water content in the larger pores. The validation of the proposed procedure relies on published data for MX-80 and GMZ bentonites and yields promising results. Even though the discussion and the validation are restricted to bentonite here, the procedure is generic in nature and can be modified easily for other geomaterials. The paper also offers insights into the links between a soil's microstructure and its macroscopic behaviour. The contribution provides a new tool for quick indirect estimation of the water retention behaviour of porous materials with known mineral composition and available microstructural data – for example, based on the MIP tests. Such a tool is of great practical interest in design – for example, nuclear waste repositories, where a large material pool may be considered initially.

Bentonite buffer blocks manufactured by isostatic compression

Isostatic compression is a technically highly specialised technology for manufacturing solid components from powder. There are only few reported experiences for isostatic compression of bentonite clay, which forms the basis of the engineered barrier system in the geological disposal concept KBS-3V, currently under investigation in Sweden and Finland. An isostatically compressed middle-scale MX-80 bentonite ring block (approximately 2/3 of full-scale diameter) was examined with regard to its dimensions, shape and surface texture. The physical and mechanical characteristics of the middle-scale ring block were analysed. The measured results were compared to previously obtained data from other isostatically compacted disc and ring blocks of similar scale, as well as to a full-scale ring block also manufactured by isostatic compression. Irrespective of the different manufacturing conditions, a consistent homogeneity among the middle-scale blocks and between the middle and full-scale blocks was determined. Results show that isostatic compression is a feasible manufacturing method that meets the high technical standards demanded in nuclear waste management.

Effects of contact time, pH, and temperature on Eu(III) sorption onto MX-80 bentonite

MX-80 bentonite is considered a popular buffer/backfill material for the construction of artificial barriers. In particular, it plays an important role in the disposal of high-level nuclear waste in deep geological repositories. In the present study, the sorption of Eu(III) on MX-80 bentonite was investigated using a batch technique; furthermore, a comprehensive analysis was performed on the influence of contact time, pH, temperature, and concentration on sorption. The results showed that the sorption regime leveled off within 30 min and the Eu(III) sorption capacity of MX-80 bentonite was approximately 46.06 mg/g under certain conditions. A second-order kinetic equation was used to fit the equilibrium sorption data. Additionally, the variation in the sorption capacity of MX-80 bentonite increased as the pH increased from 1 to 9. The optimum pH value was determined to be 7.0 at a temperature of 293.15 K. The negative value of enthalpy change (ΔH°) indicates that the temperature had a negative effect on the sorption. The sorption process was most accurately described by the Langmuir isotherm model. These results will definitely be benefit to the artificial barriers in the disposal of high-level nuclear waste.

Observed heterogeneities after hydration of MX-80 bentonite under pellet/powder form

Bentonite is considered for engineered barriers in radioactive waste geological repositories due to its low permeability, swelling pressure and radionuclide retention properties. The French concept favors the use of bentonite under granular form - pellets and crushed pellets mix - for an easier building of plugs and seals relatively to bricks. However, this introduces a significant degree of macroscopic initial heterogeneity (at several scales) that is expected to disappear via mass redistribution during resaturation in order to obtain homogeneous hydraulic and mechanical performance. This also may have practical effects on the reliability of results from tests with small volumes of bentonite compatible with the time scales manageable at the laboratory. In this study, isochoric swelling pressure tests at various densities and sample sizes were carried out and evidenced bias mainly due to the size of the cell compared to the size of the pellets, although the average density is equivalent, pointing towards incomplete homogenization. Large-scale swelling pressure tests, however, converged towards values previously measured for more finely grained materials. Local pressure measurements also evidenced some stress heterogeneities that do not seem to disappear at medium time-scales. These stress heterogeneities could be correlated to local density variations of the final state assessed using destructive measurements and X-ray computed tomography.

Correction to: Characterization of MX-80 bentonite and its sorption of radionickel in the presence of humic and fulvic acids

The original publication of the article includes mistakes on instrument types of FTIR and XRD characterization in Page 702. The corrected instrument types of FTIR and XRD characterization were shown in this correction. These corrections do not affect any of the discussion or conclusion of the paper.

Searching for bacteria in sticky situations: Methods for investigating bacterial survival at solid-air interfaces involving Wyoming MX-80 bentonite

Effective removal of prokaryotic cells from clay interfaces such as bentonite is essential for quantitative assessment of microbial communities, considering that strong bentonite clay-DNA and –RNA complexes challenge the use of molecular-based techniques. In this study, aerobic bacteria were isolated from Wyoming MX-80 bentonite and sequenced for identification (16S rRNA). A glass-bentonite substrate and sterile bentonite powder were inoculated with Arthrobacter sp. (isolated from bentonite) to test cell removal efficiency using sonication and vortexing. Manipulation of pH (pH 7 versus pH 9) did not affect cell removal efficiency, while changes in temperature within limits (15–37 °C) did affect cell removal efficiency. To evaluate microbial survival during desiccation, bacterial isolates were inoculated onto glass and bentonite-covered glass coverslip substrates, and particulate bentonite. Substrates were desiccated, and cells were removed by vortexing at different time points over 31 days. Abundance of viable cells followed a first-order rate of decrease. Vegetative desiccation-tolerant Arthrobacter sp. isolates from bentonite clay had lower loss of viable, culturable cells (0.07 d−1 to 0.89 d−1) than did a Bacillus sp. isolate (>1 d−1) or a Pseudomonas stutzeri isolate (0.79 to >1 d−1), suggesting Arthrobacter sp. may be more tolerant of these prolonged periods of desiccation on the bentonite-air interface. Tolerance to matric stress by microorganisms varies depending on the cellular adaptation of the target species, the physical and chemical properties of the given solid-air environment, as well as the employed population and community-based survival mechanisms.

Effect of technological voids on swelling behaviour of compacted bentonite–claystone mixture

The effect of technological voids on the swelling pressure of compacted MX80 bentonite–Callovo-Oxfordian (COx) claystone mixture was investigated by simulating the technological voids with a pre-defined space between the compacted disks of the mixture. Both axial and radial swelling pressures were monitored. After the tests, the microstructure of samples at different positions was investigated using mercury intrusion porosimetry (MIP), together with the determination of dry density and water content. Results showed that two main processes, filling and homogenization, occurred during soil hydration. In the filling process, the initial technological voids were gradually filled and the axial swelling pressure tended to increase. In the homogenization process, the samples had a sealing zone and a swelling zone. The sealing zone was characterized by a lower dry density than the expected final dry density while the swelling zone was characterized by a larger one. From the MIP results, the sealing zone showed larger inaccessible-pore, medium-pore, and large-pore void ratios and a lower small-pore void ratio than the swelling zone. Over time, the medium and large pores in the sealing zone were compressed, while the small pores in the swelling zone decreased. The stabilized axial swelling pressure followed a unique relationship with the expected final dry density. Moreover, the swelling pressure anisotropy was found to decrease as the technological voids increased.

State of compacted bentonite inside a fractured granite cylinder after infiltration

A small-scale mock-up test was carried out on a fractured hollow granite cylinder with compacted MX80 bentonite inside, to study the interaction between engineered barrier (compacted bentonite) and natural barrier (host rock with the presence of rock fracture). The swelling pressure and relative humidity of bentonite were monitored with respect to the position of the rock fracture during 349 days of infiltration. Herein, the variation of water content, dry density, suction and microstructure along bentonite column after dismantling were reported, focusing on the changes in the vicinity of the rock fracture, to evaluate the effect of the rock fracture on the swelling behaviour of compacted bentonite. Results showed that the presence of rock fracture disturbed the water content distribution, with a lower water content at a position closer to the fracture. A significant decrease in dry density was also observed in the vicinity of the rock fracture; the closer the positions to the fracture the larger the decrease of dry density. This decrease coincided with the reduction of swelling pressure recorded by the pressure sensors, suggesting the occurrence of rock fracture filling-up by bentonite. Further examination showed that when the soil suction was higher than 9 MPa, the decrease in dry density in the near field of rock fracture was mainly attributable to the increase in large pore porosity (>2 μm). This suggests that at this suction the mechanism involving intrusion of bentonite into the rock fracture with fracture width higher than the size of bentonite gains was related to the pushing effect under the swelling of bentonite behind. By contrast, when the suction became lower than 9 MPa, the bentonite gel formed from the exfoliation of clay particles might fill up the rock fracture with smaller aperture width.

Macroscopic effects of nano and microscopic phenomena in clayey soils and clay rocks

Based on long term investigations of the microstructure of various geomaterials as different as sensitive low plastic clays from Eastern Canada, unsaturated compacted silt from the Paris area, highly plastic compressible deep marine clay from the gulf of Guinea, compacted MX80 bentonite and the Callovo–Oxfordian claystone, considered as possible barriers in deep geological radioactive waste disposal, two fundamentals and distinct nano and micro mechanisms governing their macroscopic volume changes have been identified. In low plastic structured clays and dry compacted soils, in which an aggregate microstructure has been evidenced, the decrease in volume under mechanical compression result from the collapse of inter-aggregates pores in an ordered manner, from the larger to the smaller, with no effect on the intra-aggregate porosity. The soil skeleton can hence be modelled as an elastic fragile porous matrix, affected by the ordered collapse of its pores. Things are different in plastic soils, due to the significant reactivity of the montmorillonite minerals to changes in water content that results in significant changes of the initial porous matrix. The ordered adsorption of layers of water molecules in clay platelets with respect to the suction or stress applied, evidenced long time ago in Soil science through X-ray diffraction techniques, appear to be able to help better understanding various phenomena like the compression of plastic compressible soils and the hydration of compacted bentonites. Interestingly, it also applies to understand the effects of smectite minerals in the volume changes behaviour of the Callovo–Oxfordian claystone. These two mechanisms at nano and microscopic scales hence provide a better understanding of the macroscopic volume changes of a large variety of natural and compacted clayey soils and rocks.

Swelling behavior of unsaturated claystone/ bentonite mixtures

This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of claystone/ bentonite mixtures remained questionable.

Development of a THMC code for bentonites in COMSOL Multiphysics

The proposed use of active clays for the isolation of radioactive wastes in deep geological repositories has been followed by a deeper understanding of this type of soils. This increased knowledge has led to the need for both conceptual and numerical models capable of capturing the main trends in behaviour and the different couplings between different physical-chemical phenomena. In addition, the model must have a high degree of flexibility that enables it to accommodate future developments or new relevant phenomena. This work presents a numerical THMC code developed entirely on the COMSOL Multiphysics numerical implementation platform, which provides the required adaptability. This model includes, for the first time in this environment, a reactive transport model in unsaturated porous media for a relevant geochemical system (consistent with the MX-80 bentonite) together with a THM model based on a double porosity approach. The chemical potentials of water and solutes are used for the definition of thermodynamic equilibria between both porosity levels. Trends in the behaviour of a bentonite sample under oedometric conditions are satisfactorily simulated in response to a process of saturation and change in salinity conditions. Variations in swelling pressure, porosity distribution or dissolution/precipitation of the main accessory minerals are analysed and explained by means of the proposed conceptual model.

Microstructural and hydro-mechanical behaviour of bentonite pellets and powder mixtures

Binary mixtures of high-density MX-80 bentonite pellets (80%) and bentonite powder (20%) at hygroscopic water content have been recently considered as an alternative engineered barrier system for the long-term disposal of radioactive wastes. These mixtures display a dry density of around 1.49 Mg/m3 on pouring and present components with multi-modal pore size distributions that significantly evolve during the hydro-mechanical paths. To better understand the hydro-mechanical behaviour of this multiple porosity mixture, the contribution initially focuses on describing the initial state, as well as the main microstructural features determined by mercury intrusion porosimetry tests for the mix and each of the components (pellets and powder). Afterwards, hydro-mechanical results of both components and the mixture are presented. The hydraulic results focus on the water retention and water permeability, while the mechanical tests concentrate on the compressibility properties on loading.