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Visualization of FIB-nt data acquired from a bentonite with 1.5 g/cm 3 density (Table 1). (a) Reconstruction of the volume analyzed based on BSE images. (b) 3D reconstruction of the intergranular pore space which is filled with clay gel.
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Compacted MX-80 bentonite is a potential backfill material in radioactive-waste repositories. Pore space in MX-80 has been the subject of considerable debate. 3D reconstructions of the pore space based on tomographic methods could provide new insights into the nature of the pore space
of compacted bentonites. To date, few such reconstructions have...
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During the long-term operation of a deep geological repository, infiltration of pore fluids with different chemical compositions can influence the swelling behavior of the compacted bentonite. In this paper, using a self-developed swelling deformation apparatus, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite...
Citations
... The microscopic level designates the scale of stacked flexible montmorillonite lamellae and interlayer water whose physical properties are affected by the pore walls (Marry and Turq 2003;Cheng and Hendry 2014). The mesoscopic level refers to the scale of clay gels, solid grains, and inter-aggregate pores (Keller et al. 2014;Nakashima 2003;Tomioka et al. 2010). The effective properties computed by upscaling from the microscopic to the mesoscopic scales are affected to the clay gels, while diffusion within the inter-aggregate micropores is assumed to take place without surface effects. ...
Due to their prevalence in the lithosphere and their high capability of sorbing pollutants, smectite clays play a foreground role in environmental pollution studies, waste management, and soil science. In complementarity with existing approaches at the molecular or macroscopic scales, real microstructures have been employed to investigate ionic transport by diffusion through montmorillonite and water-saturated Wyoming bentonite at intermediate scales ranging between the nanometer and the micrometer. The coupled solute transport and electrostatic phenomena investigated at the nanopore scale are upscaled using the homogenization of porous media approach. Homogenization computations rely on a hierarchical description of bentonite that acknowledges the existence of pores networks at different scales. At the scale of montmorillonite layers, digitized TEM images have been employed to simulate the diffusion of ionic solutes by considering electrostatic interactions in the vicinity of the negatively charged clay platelets’ surface. Finite element microstructures are created after extraction of the contours of the layers using dedicated image processing algorithms. Local electric potential distribution, anion exclusion, and cation inclusion are displayed by ion distribution maps. The effective diffusion tensor and the transport equation obtained through volume averaging are then used to simulate diffusion at the scale of a Wyoming bentonite sample composed of clay gels of variable density, solid grains, and micropores. Qualitative comparisons were made with existing diffusion data, and a particular attention is given to the anisotropy of the diffusion tensors at both the mesoscopic and macroscopic scales.
... The aqueous solution was developed by periodic replication of water molecules. The distances between water molecules should maintain around 3.1 Å to obtain an aqueous region of dimensions 46. which is large enough to be consistent with the real pore size distribution of a highly compacted clay with a dry density of 1.7 g/cm 3 [30]. Considering that dehydroxylation only occurred on the octahedral sheet of kaolinite during calcination, while the tetrahedral sheet remained unchanged, we consider that the improvement in absorption capacity is mostly attributable to the atomic structure change at the octahedral sheet. ...
... When char content was 10 and 15%, that is, after the incorporation of biochar, the con modulus, Mmax, increased obviously, with an increase in dry density. Gmax and M important parameters of soil stiffness [30], which reflect the deformation capac higher the compaction degree, the smaller the void ratio, the closer the contact b particles, the stronger the cementation capacity, the more stable the soil structure, stronger the resistance to deformation [31]. The Poisson's ratio, ν, was calculated from Gmax and Mmax, according to Equa Figure 7 shows the variation in Poisson's ratio of the biochar-clay mixture with d sity, under different biochar content, from which it can be seen that, when the content was 0% (pure clay), the Poisson's ratio did not change significantly with Figure 6 shows the relationship between the maximum shear modulus, G max , maximum constrained modulus, M max , and dry density ρ d of the biochar-clay mixture. ...
... G max and M max are important parameters of soil stiffness [30], which reflect the deformation capacity. The higher the compaction degree, the smaller the void ratio, the closer the contact between particles, the stronger the cementation capacity, the more stable the soil structure, and the stronger the resistance to deformation [31]. Figure 6 shows the relationship between the maximum shear modulus, Gmax, maximum constrained modulus, Mmax, and dry density ρd of the biochar-clay mixture. ...
... Gmax and Mmax are important parameters of soil stiffness [30], which reflect the deformation capacity. The higher the compaction degree, the smaller the void ratio, the closer the contact between particles, the stronger the cementation capacity, the more stable the soil structure, and the stronger the resistance to deformation [31]. The Poisson's ratio, ν, was calculated from Gmax and Mmax, according to Equation (6). Figure 7 shows the variation in Poisson's ratio of the biochar-clay mixture with dry density, under different biochar content, from which it can be seen that, when the biochar content was 0% (pure clay), the Poisson's ratio did not change significantly with the increase in dry density. ...
During the service of a landfill, uneven soil settlement and earthquakes can cause changes to the pore structure and cracks in the covering layer. The use of a biochar–methanotroph–clay mixture as a new landfill covering layer, can improve its engineering properties. The biochar–methanotroph–clay mixture’s shear-wave velocity and compression-wave velocity were measured by the bender–extender element test, and the elastic parameters under small-strain conditions, such as maximum shear modulus, Gmax, maximum constrained modulus, Mmax, and Poisson’s ratio, ν, were obtained. The parameters showed that the elastic characteristics and lateral deformation capacity were of great significance for settlement, seismic field, and stress–strain analysis. Based on the bender–extender element test, the effects of different compaction degrees, and biochar content on Gmax, Mmax, and ν of the biochar–clay mixture, and different methane cultivation days on the biochar–methanotroph–clay mixture, were investigated. The results showed that the Gmax, Mmax, and ν of the biochar–clay mixture increased with the increase in biochar content and dry density. When the biochar content was 15%, and the dry density was 1.64 g/cm3, the ν increased significantly. The Gmax and Mmax of the biochar–methanotroph–clay mixture tended to increase with the increase in methane cultivation days, and the higher the biochar content, the more obvious the increasing trend. The ν showed a decreasing trend with the increase in methane cultivation days, and the resistance to lateral deformation was stronger.
... where E m1 , E m2 , m 1 and m 2 are the Young modulus of pellet 1 and pellet 2 and the Poisson ratio of pellet 1 and 2 in the contact. Equation (17) highlights that the contact stiffness would be significantly smaller if the Young modulus of one of the pellets in contact were smaller. ...
Bentonite pellet mixtures are candidate material for the sealing of galleries in radioactive waste disposals. The hydromechanical behaviour of assemblies of bentonite pellets is investigated upon partial hydration through (1) suction-controlled swelling pressure tests in the laboratory and (2) discrete element method simulations. The combination of these experimental and numerical approaches highlights that, before the mixture homogenization, the swelling pressure develops in two phases. The first phase is characterized by an increase in contact forces between pellets. The second phase is characterized by plasticity at contacts between pellets and is controlled by the progressive decrease in pellet strength and stiffness upon hydration. In addition, numerical results highlight that the swelling pressure measured in the laboratory can be influenced by the sample preparation, the cell size, and the diameter of the pressure sensor.
... When the clay particle is no longer stable, a sharp decrease of the number of layers per particle is observed. For MX80 bentonite, this threshold is in the range 3-7 MPa and corresponds to the development of diffuse layers, microstructural rearrangements, modification of the pore size distribution, and change in the water retention behaviour (Keller et al., 2014;Molinero-Guerra et al., 2020;Saiyouri et al., 2004;Villar, 2007). MX80 pellets and powder (crushed pellets) are thus affected by significant microstructural rearrangement in this suction range. ...
Bentonite pellet-powder mixtures are candidate materials for sealing the galleries in deep geological repositories for radioactive waste. In the present work, swelling pressure tests are performed on pellet-powder mixtures with different powder contents. Results highlight the influence of the initial granular structure on the mechanical behaviour of pellet-powder mixtures. In mixtures with a low density powder in the inter-pellet porosity, the macroscopic response of pellet-powder mixtures is identical to that of a pellet assembly with no powder. A model is proposed to describe the hydromechanical behaviour of pellet-powder mixtures. The formulation considers two distinct states of the material, Granular and Continuous. In the Granular state, the pellets control the mechanical behaviour of the mixture. In the Continuous state, both pellets and powder contribute to the mechanical behaviour of the mixture. In the Granular domain, the material behaviour is described by constitutive laws proposed after a numerical study using Discrete Element Method. In the Continuous domain, the material behaviour is described by a modified Barcelona Basic Model. Transition between the two domains depends on the density of the powder phase and suction. The model is implemented in a Finite Element Method code, and the swelling pressure tests performed in the laboratory are simulated with a single set of parameters. These results improve the knowledge on the behaviour of bentonite pellet-powder mixtures during hydration in repository conditions.
... In this respect, this latter has been widely addressed over the last decades (e.g. Saiyouri et al., 2000;Blatz et al., 2002;Lloret et al., 2003;Komine and Ogata, 2003;Cuisinier and Masrouri, 2005;Agus and Schanz, 2005;Delage et al., 2006;Villar 2007;Karnland et al., 2008;Wang et al., 2012;Saba et al., 2014a;Seiphoori et al., 2014;Keller et al., 2014;Massat et al., 2016;Sun et al., 2019). ...
... A micrograph obtained after wetting and drying of the sample evidences the loss of the initial aggregated structure (Figure 2-15). Using FIB-nt, Keller et al. (2014) observed the pore space of MX80 powder compacted to three dry densities: 1.23 Mg/m 3 ; 1.46 Mg/m 3 ; 1.67 Mg/m 3 . The inter-aggregate porosity appeared to be filled with clay particles exfoliated from swelling aggregates in the course of hydration. ...
... Interestingly, this structure was found to partially fill the inter-aggregate porosity at low dry density (1.23 Mg/m 3 ); entirely fill the interaggregate porosity at intermediate dry density (1.46 Mg/m 3 ) and was not observed at high dry density (1.67 Mg/m 3 ). Keller et al. (2014) suggested that the gel could not form at high density because interaggregate porosity is not developed enough for the honeycomb-structure to form, which is consistent with MIP test results obtained at different dry densities. The mechanisms affecting the material fabric has been shown by Delage et al. (2006) to be timedependent. ...
Bentonite based materials are considered as a sealing material in radioactive waste disposal concepts because of their low permeability, radionuclide retention capacity and ability to swell upon hydration, thus filling technological gaps. Within this context, bentonite pellet mixtures have been studied owing to operational convenience. Pellets are laid in the galleries in a dry state as a granular assembly. The mixture progressively becomes more homogeneous upon hydration by the pore water of the host rock. Before homogenisation, the granular structure of the material controls the macroscopic behaviour of the mixture.The present work focuses on the experimental characterisation and numerical modelling of a mixture of bentonite pellet and crushed pellet (powder), in proportion 70/30 in dry mass, a candidate sealing material in the French concept of radioactive waste disposal. The proposition, implementation and validation of a new modelling framework, considering features such as the initial granular structure of the material or local heterogeneities of densities, is the main objective of this PhD work.The influence of the initial granular structure is evidenced by performing suction-controlled swelling pressure tests in the laboratory, using samples of various powder contents. From grain-level experimental characterisation, a simple model describing the hydromechanical behaviour of a pellet is proposed and implemented in a Discrete Element Method (DEM) code. Using DEM and the proposed model, aforementioned swelling pressure tests performed on samples containing no powder are satisfactorily simulated. The same method is used to model large granular assemblies of various pellet densities upon hydromechanical loadings. Relevant parameters involved in the macroscopic behaviour of pellet mixtures in “granular” state are identified from simulation results and constitutive laws are proposed to describe the hydromechanical behaviour of these materials using a continuum mechanics approach.The transition from “granular” state to “homogenised” state is described by criteria proposed from experimental results and data available in the literature and involves suction and relative volume fractions of pellet and powder. A modified version of the Barcelona Basic Model is proposed to model the material behaviour in “homogenised” state. The proposed model is implemented in the Finite Element Method (FEM) code BIL. Using a single set of parameters, all swelling pressure tests performed in the laboratory are satisfactorily reproduced in FEM simulations along the entire hydration path.The material behaviour upon hydration in constant volume condition is finally studied at a larger scale by performing mock-up imbibition tests, using various powder contents. Cells have a square section; a glass side and a camera allow the texture to be observed during hydration. The dominance of vapour transfers in the saturation process of the material, the influence and evolution of the granular structure upon hydration, and the influence of the powder content on the macroscopic response are notably identified. Transfer laws are proposed to describe the observed material behaviour in the mock-up tests and implemented in BIL.The realisation of larger scale coupled simulations using the proposed hydromechanical model is a perspective arising from this PhD work. Predictive simulations could be performed at the structure scale, considering relevant features such as the initial granular structure and local heterogeneities of density in the sealing plugs
... Smectites are characterized by a heterogeneous arrangement of nanometerscale lamellae stacked together to form clay particles, micrometer-scale grains, water and air which saturate the various pore spaces that appear on different scales (Jozja, 2003;Holzer et al., 2010;Keller et al., 2014;Pusch and Yong, 2006;Tessier et al., 1992). As a result, clay microstructure controls most of their physical properties such as ion diffusion, swelling pressure and hydraulic conductivity. ...
... This explains the scarcity of microstructural data available to quantify the pore size distribution and pore connectivity in compacted bentonite. As pointed out in (Holzer et al., 2010;Keller et al., 2014), the proportions of free and bound porewater in compacted bentonite are the subject of ongoing discussions and investigations, and tomography techniques are still under development in order to quantify precisely the mesopore distribution in compacted swelling clays. In (Pusch and Schomburg, 1999;Pusch, 2001) Transmission Electron Microscopy (TEM) micrographs of compacted and hydrated MX-80 bentonite displayed the formation of clay gels of variable density by linking of aggregates exfoliated from the expanding clay grains. ...
... The effective conductivity was then approximated by an analytical formula combining the individual cells in parallel or in series depending on their orientation with respect to the macroscopic flow direction.Tomioka et al. (2010)used Xray microtomography for examining the morphological evolution of compacted Na-montmorillonite before and after saturation. They concluded that the outer montmorillonite sheets are likely to swell and form a gel that occupies the intergranular voids, whereas the inner sheets are not affected by the water saturation process.Holzer et al. (2010) andKeller et al. (2014)investigated the intergranular pore space (mesopores) of compacted and hydrated MX-80 bentonite using high resolution 3D imaging with Focused Ion Beam nanotomography. They observed that clay particles' hydration in non-compacted bentonite (dry density ρ dry = 0.39 g/cm 3 ) is associated with extensive exfoliation and dispersion of thin clay layers. ...
Hydraulic conductivities of compacted water-saturated bentonite were computed based on the real microstructure. The Homogenization of Periodic Media approach employed fully acknowledges the heterogeneous and multiscale microstructure of clay, as well as locally varying physical flow properties. Consequently, three levels of description were considered : the microscopic level of clay particles, the mesoscopic level of clay aggregates, mineral grains and inter-aggregate porosity, and the macroscopic level of the sample subjected to fluid pressure gradients in the laboratory. Starting from the local description of fluid flow, the expression of the effective hydraulic conductivity tensor was derived. The soft and dense gels and the open voids may form a connected flow path or remain occluded. The local problems were solved on the microstructure obtained from a digitalized micrograph by image analysis. The contribution to macroscopic flow by the soft and dense gels was investigated in various configurations, and comparisons were made with hydraulic conductivity data for MX-80 bentonite.
... The studies of Churakov et al. [23,24] stressed the need for establishing clear pictures of representative porous media for swelling clay minerals, which ideally should account for the actual distributions of particle size, shape, and orientation in a system. Among the different experimental techniques that allow for such three-dimensional (3D) characterization of pure swelling clay mineral samples, x-ray tomography and/or focused-ion-beam nanotomography are the most widely considered [25][26][27][28]. These techniques, based on 3D image analysis, can in principle provide an in situ analysis of pore networks in natural or composite pure clay systems without any assumption regarding particle size and shape. ...
Swelling clay minerals play a key role in the control of water and pollutant migration in natural media such as soils. Moreover, swelling clay particles' orientational properties in porous media have significant implications for the directional dependence of fluid transfer. Herein we investigate the ability to mimic the organization of particles in natural swelling-clay porous media using a three-dimensional sequential particle deposition procedure [D. Coelho, J.-F. Thovert, and P. M. Adler, Phys. Rev. E 55, 1959 (1997)10.1103/PhysRevE.55.1959]. The algorithm considered is first used to simulate disk packings. Porosities of disk packings fall onto a single master curve when plotted against the orientational scalar order parameter value. This relation is used to validate the algorithm used in comparison with existing ones. The ellipticity degree of the particles is shown to have a negligible effect on the packing porosity for ratios ℓ_{a}/ℓ_{b} less than 1.5, whereas a significant increase in porosity is obtained for higher values. The effect of the distribution of the geometrical parameters (size, aspect ratio, and ellipticity degree) of particles on the final packing properties is also investigated. Finally, the algorithm is used to simulate particle packings for three size fractions of natural swelling-clay mineral powders. Calculated data regarding the distribution of the geometrical parameters and orientation of particles in porous media are successfully compared with experimental data obtained for the same samples. The results indicate that the obtained virtual porous media can be considered representative of natural samples and can be used to extract properties difficult to obtain experimentally, such as the anisotropic features of pore and solid phases in a system.
... At present a detailed knowledge of microstructural properties corresponding to variable bulk physical properties of MX80 bentonite is still lacking. In the context of the application as a back-fill material and the corresponding sealing behavior, microstructural investigations of the nanoscale pore space in bentonites using high-resolution imaging techniques are scarce, the work of Pusch (2001), Holzer et al. (2010) and Keller et al. (2014) being exceptions. At a larger scale of observation, other studies used X-ray computed microtomography to characterize the microstructure of bentonites ...
... In a recent study we investigated the pore structure of compacted and partially saturated MX-80 bentonite at different dry densities (Keller et al., 2014). FIB-nt was directly applied to laboratory prepared and high-pressure frozen samples in order to prevent drying artifacts (see Section 3.1.1). ...
... One potential problem with the freeze-drying step is that the samples were defrosted under high vacuum (the sample was never exposed to higher temperatures than room temperatures) which leads to a loss of the interlayer water, which might alter the microstructure. Recently, Keller et al. (2014) performed a cryo-FIB-nt study to explore the microstructure of partly saturated MX-80 bentonite. Thereby, FIB-nt was applied directly to a high-pressure frozen bentonite sample by using a cryo-stage and cryo-transfer device to transport the sample between the instruments. ...
Focused ion beam nanotomography (FIB-nt) was applied to MX80 bentonite samples from the long-term Alternative Buffer Material (ABM) experiment in order to study the evolution of the intergranular pore space under similar condition that is supposed to prevail in repositories of nuclear waste. The applied high-resolution imaging method revealed the presence of two different types of pore filler. The first type is related to corrosion of iron and is represented by newly formed heavy minerals. Extensive formation of heavy minerals occurred only near the iron parts of the experimental set up. Based on comparison with other studies, the second filler type was interpreted as clay-gel that was likely formed during water uptake and swelling. A large fraction of the initial pore space was filled with such a clay gel. By attributing filled pores to the present open porosity, the initial intergranular porosity (radii > 10 nm) of the starting material was in the range of 4.3–4.6 vol.%, which was reduced to < 1 vol.% during the experiment. A finite scaling approach was applied to the initial pore space (i.e. pores with radii > 10 nm), which yielded percolation thresholds with critical porosities ϕ in the range of 3–19 vol.%. Thus, the residual open porosity was far below the percolation threshold.
