Compacted Blocks Research Articles

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.

The role of bentonite in the repository concepts for radioactive waste in Germany: a review

Irrespective of the host rock and the repository concept, bentonite plays a central role in the geological disposal of radioactive waste by serving as the main sealing element in most repository concepts. Due to its favourable chemical, hydraulic and mechanical properties, bentonite is of paramount importance in the overall safety of a repository for radioactive waste. This work gives an overview of the use of bentonite in the German repository concepts in different host rocks. Starting with the current repository safety regulations in force in Germany, the principles for the design of the engineered barrier system are described. Among them, the principle of diverse redundancy has been established as the main design principle to fulfil the requirements of the Repository Safety Ordinance. This principle allows the arrangement of different geomaterials in the design of the geotechnical barriers where bentonite plays a key role. Bentonite is used in the German repository concepts in different host rocks as either buffer, backfill and/or sealing material. In clay and in crystalline rock, highly compacted bentonite blocks are considered for the sealing of disposal boreholes and for the construction of drift seals. Processed excavated material is mixed with bentonite to backfill the remaining voids inside the repository. For the closure of shafts, bentonite as binary mixture of briquettes and powder or as equipotential sand-bentonite-segments are considered. The present work also brings to light how bentonite is used in German repository concepts. We also discuss the differences between German and international geological disposal programmes in regards to the applications of bentonite. The innovative aspect of the German approach is demonstrated through the adoption of a dual bentonite/asphalt sealing system and the implementation of equipotential segments comprising layers of sand and bentonite. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Effect of Posidonia oceanica on the hygrothermal characterization of compacted earth blocks

A hygrothermal characterization of new construction materials, namely unstabilized and Posidonia oceanica fiber stabilized compressed earth blocks, were studied in this work. In the first part, the sorption-desorption isotherms of unstabilized blocks were compared with others stabilized with raw and treated fibers to detect the effect of adding fibers. The results revealed an improvement in hygric properties after the addition of raw fibers. Moreover, the Guggenheim-Anderson-de Boer (GAB), the Oswin, and Brunauer, Emmet, and Teller (BET) models successfully fitted experimental sorption-desorption isotherms curves. In the second step, the moisture buffer value and water vapor permeability were evaluated under dynamic conditions. According to the classification of the NORDTEST project, the innovative and new compacted earth blocks studied in this paper seem to be excellent moisture regulators because they have an MBV greater than 2 g/ (m2. %RH). In fact, BAFBB samples (block with crushed Posidonia oceanica) achieved the best results. These formulations recorded an ideal water buffer coefficient equal to 2.66 g/ (m2.%RH)

Discrete particle methods for granular bentonite material simulation

Bentonite clays are strategic materials to ensure the sealing capacity of engineered barrier systems (EBS) in deep geological repositories of high-level radioactive waste. Formerly installed in the form of compacted blocks, in recent years, there has been growing interest in EBS design using granular bentonite materials (GBM) due to improved construction processes [1]. GBM range from the powdered and granulated forms, both natural and obtained from crushed blocks, to compressed bentonite pellets, and the mixtures thereof. The effect of thermo-hydro-chemo-mechanical (THCM) conditions on the evolving gap-filling performance following installation has been investigated in full-scale experiments [2] and in laboratory-scale tests [3], with particular focus on bentonite pellet-powder mixtures [4, 5, 6].
 Numerical modelling of coupled THCM expansive soil behaviour is widespread using continuum methods, namely the finite element method [7, 8], applicable up to full-scale systems even though at the expense of resolution. At the laboratory scale, discrete particle-based methods accounting for the granular nature of bentonite materials in the early stages of hydration provide insight into pellet shape and local effects on the mechanical behaviour of dry pellet-powder systems as well as hydro-mechanical effects on the swelling capacity of GBM systems.
 The compressibility of dry pellet packings and pellet-powder mixture samples is modelled by the discrete element method (DEM). This method has been applied to study the swelling behaviour of pellets represented by spherical particles [9]. The DEM is a powerful tool for reproducing complex granular shapes by non-spherical particle surfaces [10] and also with clumps of spheres (Figure 1(a)), allowing to define mixed material and mechanical contact properties within the pellets. The deformation and fissuring of pellets under oedometer compression is tracked using a cohesive particle model [11], including damage for interparticle contacts in tension, which captures deformation associated phenomena such as surface spalling and pellet reorganisation occurring at stress levels below the diametral fracture. In addition, bentonite powder is represented by loose packings of upscaled spheres matching
 the experimental porosity of the fillings, interacting by a classical linear spring-dashpot contact model resulting in the reduced overall compressibility observed in pellet-powder mixtures [5].
 Flow through GBM systems is modelled by the new discrete particle-based XMm method [12]. In the XMm, the material is discretised into a set of bentonite units (BU) or particles acting as representative elementary volumes of a homogeneous continuum inside which the mass balance equations are solved at the megastructural (X), macrostructural (M), and microstructural (m) pore levels (Figure 1(b)). Moreover, the mechanical compatibility of the system is obtained from the coupling between strain increments and centroid displacements in each BU, where the contact stress and overlap between neighbouring particles are updated similarly to the DEM approach. The hydro-mechanical problem is posed as a set of ordinary differential equations (ODE) to be solved for each BU, and thus the efficiency of the XMm is a compromise between the typically reduced computational cost of ODE solvers and the number of BU considered. The XMm has been used to simulate the hydration of a powder sample with low heterogeneity [4], successfully capturing the evolution of the displacement field upon saturation in isochoric conditions. This method is applied to reproduce the confined hydration of a heterogeneous pellet-powder system [6]. Assuming 2D axisymmetry, the XMm allows defining different initial material properties in each BU introducing the heterogeneity of the sample. The evolving swelling pressure is reproduced as well as the hydration velocities at the different structural levels, showing the decreasing heterogeneity of the mixture.

Bentonite homogenization with technological voids upon hydration

During the construction of the nuclear waste geological repository, the inevitable creation of technological gaps will lead to the redistribution of bentonite material along the wetting path upon water intake, thus impacting its localized hydraulic and mechanical properties. It is therefore of the utmost importance to be able to understand and predict the dry density inhomogeneous distribution throughout the buffer zone. This work proposes a microscopic dynamic force balance framework, incorporating the influence of van der Waals force, diffuse double layer force, and boundary friction force, to explore the underlying mechanism of the bentonite homogenization process upon hydration. The microscopic assumption postulates that bentonite materials are an assembly of numerous parallel thin sheets with other dimensions substantially exceeding the thickness, establishing an association between the aforementioned force and the material’s volume fraction. Particular attention is directed toward exploring the mechanism of boundary friction's effects on the inhomogeneous distribution of dry density with hydration time. The application of the framework to examine the impact of engineered voids with 10% and 20% of the total volume yields a satisfactory reproduction in terms of the spatial and temporal evolution of dry density, validating the robust prediction capacity of the established model. The investigation indicates that compacted blocks in proximity to technological voids experience a rapid reduction in dry density upon hydration, leading to the homogenization process characterized by elevated values in the initial void-affected region and diminished values in the innermost area. Whereas full homogenization is yet to be achieved within the observation period of up to 2160 h, and the greater the variability of the dry density within the specimens containing larger initial voids. Further analysis reveals that as the hydration process advances, the friction force within the portion featuring the initial void exhibits a gradual increase, while emergence gradually decreases in the inner swelling zone. Notably, the high friction factor at the bentonite and steel interface allows for the maintenance of large stress gradients and the associated density gradients.

Evaluation of changes in buffer properties from an in-situ engineered barrier experiment

The In-situ Demonstration of Engineering Barrier System (In-DEBS), a field test in Korea, commenced operations in July 2016 to showcase the complex thermal-hydraulic-mechanical behavior of the engineering barrier in a high-level radioactive waste repository. To emulate the actual repository, the In-DEBS test utilized a heater and buffer material comprised of compacted bentonite blocks. From January to October 2022, the In-DEBS buffer was sequentially dismantled, and the buffer samples were collected and analyzed for physical properties such as dry density and water content based on the sample location. The findings indicated that the physical properties of the In-DEBS buffer material showed variations, which may have been caused by various factors such as technical gaps, the dismantling process, and temperature gradient caused by heaters. This study also presents several experiments conducted on the samples obtained from the In-DEBS field test and compared with laboratory experimental data of untreated samples. The thermal conductivity and the hydraulic conductivity of the field-collected samples were lower than those of the untreated samples and the swelling properties of the samples were found to be influenced by their respective locations within the buffer, Although the empirical correlation based on laboratory data on untreated samples demonstrated high accuracy, some discrepancies were observed between the properties of field samples and the predicted values, highlighting the importance of field tests.

VOLUMETRIC LIGHT TRANSFER IN FUNCTIONALLY DEFINED SCENES

In computer graphics, the main goals are high realism and rendering speed. In terms of realism, physically correct rendering is important, which calculates the transmission of light from the source to the observer. A method of modeling global illumination is presented, which takes into account light coming directly from sources, as well as light redirected through other parts of the scene. This allows you to create various optical effects, such as the flow of color from one surface to another, indirect lighting and caustics. The photon display method is designed for functionally defined scenes using graphics processors, thereby achieving an interactive rendering mode. The aim of the work is to implement a method for displaying photons in interactive rendering mode using graphics processors. It was necessary to find a compromise between physical accuracy and the speed of displaying scenes. As the number of parallel computing cores increases, traditional programming models become less efficient. GPUs based on the parallel processing model are being expanded by adding improved support for general-purpose computing to their cores. CUDA is a multicore computing platform. GPU-enabled hardware provides low-cost access to many computations. The method presented in this paper extends the photon-mapping algorithm for the CUDA platform to volumetric photon mapping. The beam brightness estimate used to efficiently calculate the density is recovered with the correct physical parameters. Photons are traced and bandwidth is selected. Performance increases due to calculations on multiple cores and the use of large compacted blocks.

Carbon steel corrosion in a bentonite buffer: A comparison between in situ exposure and lab based experiments

AbstractIn this paper, carbon steel corrosion rates from experiments performed in situ (performed in an underground rock laboratory) and ex situ (performed in a conventional laboratory) test methods in anaerobic saturated bentonite are compared. The results indicate that the long‐term corrosion rate follows a power law decay curve, with a higher initial rate and greater rate of decay at higher temperature. In compacted bentonite blocks, varying the density has no significant effect on the corrosion rate measured during in situ testing, whereas granular bentonite of an equal dry density leads to a higher corrosion rate. Precorrosion of test specimens in an aerated solution at room temperature has a negligible effect on the subsequent anaerobic corrosion rate when tested ex situ for durations up to 11.8 years. The main difference between in situ and ex situ tests is the formation of silicon‐rich corrosion products in the ex situ tests but not in the in situ tests. Despite these differences, the corrosion rates in both tests exhibit the same general temporal evolution and similar magnitudes, suggesting that the main steel degradation processes are maintained in both configurations.

Modelling the density homogenisation of a block and granular bentonite buffer upon non‐isothermal saturation

AbstractThis paper presents a numerical analysis of the mechanical performance of a bentonite clay buffer for the containment of nuclear waste in the context of deep geological disposal. The design of the buffer is based on the Swiss concept where the waste canisters are emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of highly compacted bentonite. A complete analysis of the long‐term performance of the repository requires a good understanding of the mechanical evolution of the bentonite upon heating from the radioactive waste and hydration from the host rock. Despite its importance, the implications of the initial heterogeneous bentonite layout, characterised by blocks and grains, on the final dry density at the repository scale in the steady state have not been previously studied. The present study aims to shed light into these processes by means of finite element modelling using an advanced constitutive model for the bentonite behaviour that considers several thermo‐hydro‐mechanical couplings. The constitutive model is shown to be able to reproduce several laboratory tests involving saturation of block and pellets at different dry densities. The model predictions, extended up to 100,000 years, indicate that the bentonite blocks and grains tend to homogenise in terms of dry density as the buffer reaches full saturation. Due to the different swelling potential of the block pedestal and the granular backfill, the canister is subjected to movements, although these remain relatively small. The impact of initial segregation of the granular bentonite is also studied and it is seen to not to affect substantially the mechanical evolution of the buffer, although it might reduce canister displacements.

Friction influence on constant volume saturation of bentonite mixed pellet-block samples, a numerical analysis

Bentonite-based materials are employed in some nuclear waste disposal concept designs to seal underground tunnels and shafts. In some cases, these barriers consist of two parts: highly compacted blocks and granular buffer material consisting of pellets. These two components have highly distinct initial properties in terms of dry density, water content, and microstructure, but, at full water saturation, those tend to homogenise. However, the simultaneous application of pellets and blocks in the same barrier section creates challenges for understanding and modelling system performance that must be tackled. Therefore, this work merges experimental and computational approaches to better understand the hydro-mechanical processes that take place throughout the interaction of the various assemblies with one another and with the environment during hydration.Hence, the role of a variety of material configurations, considering simultaneously compacted blocks and pellets, and hydration boundary conditions is analysed giving great insight about intermediate saturation states. Discrepancies between experimental measurements and model results were explained by the effect of friction after a sensitivity analysis performed with the finite element code LAGAMINE. Results from experiments and calculations were in good agreement and offer supplementary knowledge about a relevant amount of the numerous phenomena (for instance related to dry density evolution and water distribution inside the sample) taking place during initial heterogeneous bentonite samples resaturation in isochoric conditions.

Numerical modelling of unsaturated MX-80 bentonite subjected to two different hydration paths and subsequent loading to high-pressures

MX-80 bentonite has been considered as a suitable material for the construction of engineered barriers employed in deep geological radioactive waste repositories. These barriers are generally formed of compacted unsaturated bentonite, the latter experiencing a slow saturation due to its low permeability whileinteracting with the surrounding groundwater. In order to verify the long-term safety requirements of engineered barriers, their response to hydration has to be carefully assessed. As part of the recent European project BEACON (Bentonite Mechanical Evolution), the behaviour of MX-80 bentonite subjected to different hydration paths was investigated in a number of laboratory and field experiments and numerical studies. This paper is concerned with numerical simulations of two laboratory experiments performed during the project, with the objective of examining the predictive capabilities of the proposed numerical modelling approach. The experiments were selected due to the granular state of bentonite at its placement in the testingapparatus, which differed from the large number of previous experiments conducted on specimens of compacted bentonite blocks. The paper provides a brief introduction to the adopted modelling framework, a summary of calibrated parameters for the hydro-mechanical constitutive modelling and the results of numerical simulations, concluding that a satisfactory numerical simulation of the experiments was achieved.

On the thermal hydro mechanical chemical behavior of bentonite sand mixture

The concept to high-level radioactive waste (HLW) repository generally involves anengineering-artificial barrier system, in which hardly compacted bentonite sand mixture blocks are placed carefully around the waste canister, and the canister is of stiffness in the design. Placed the bentonite sandmixture blocks, gradually receive hydration due to groundwater. Thermal-hydraulic-mechanical-chemical phenomena of compacted bentonite sand mixture after wetting, suction change and thermal effort are a critical parameter for buffered system This study proposes the influence of suction, relative humidity and thermal effort in thermal-hydraulic on deformation properties and mechanical properties for bentonite-sand mixture. A series of testing programs are prepared such as calibration test for changing of relative humiditywith temperatures, measurement of volume change involving heating and changing of suction, determination of unconfined compressive strength, for thermal performance triaxial compression tests for both unsaturated bentonite sand mixture and saturated bentonite sand mixture. The obtained results are further interesting, and summary provide as following. To suction increment incorporation with thermal heating produce induces obviously shrinkage of the bentonite sand mixture, and lead the decrement of strength. Elevated temperature confirm further reduction for maximum deviator stress using a developed triaxial compression apparatus that is regardless of unsaturated condition and saturated condition.

Evaluating Corrosion Behaviour of Copper Under Deliquescent Drying/Wetting Cycles in Humid Air Condition

Most nuclear safety agencies around the world agree that deep geological repositories (DGRs) constructed hundreds of meters below ground in stable host rock formations are viable solutions to safely isolate used nuclear fuel. In Canada, the DGR design involves storing used nuclear fuel in sealed copper coated used fuel containers (UFC) surrounded by highly compacted bentonite blocks (HCB) ~500 m underground. Shortly after emplacement, heat generated by radioactive decay of the used fuel will warm the UFC and its surroundings, desiccating the HCB and decreasing moisture on the container surface. As water vapour is driven from the UFC surface, the presence of minor amounts of dissolved salts may cause droplet formation prior to complete evaporation, during which deposits of salts may be left behind on the UFC. Later, as conditions evolve and the relative humidity increases and the temperature decreases, condensation of liquid water on the UFC surfaces will produce droplets again, due to the deliquescence of minor salt deposits. In either the wetting or the drying scenario, secondary spreading and spatial separation of anodic and cathodic reactions and consequent non-uniform corrosion could be possible, due to the droplet geometry.To investigate this scenario, the quartz crystal microbalance (QCM) technique is being used. The QCM initially consists of a quartz plate, covered by gold electrodes on both sides where a layer of copper is deposited on the crystal by using magnetron sputtering. In this technique, the change in the resonance frequency of a vibrating quartz crystal is correlated to its mass change due to the corrosion process. To investigate the effect of salts and their deliquescence behaviours, different salts (NaCl, CaCl2, and MgCl2) and CR10-E simulated groundwater were loaded on the copper surface using an inkjet printer, and then corrosion tests were performed at different temperatures and relative humidity (RH) values. In parallel, a set of long-term vapour-phase corrosion tests was performed on wrought and cold-sprayed copper coupons at 75 °C and different RH values. As part of the post-experiment analysis, the corrosion products were analyzed using a suite of surface analysis techniques, including optical microscopy, Raman spectroscopy, and scanning electron microscopy (SEM).The results from QCM experiments at RH values below salts deliquescence point showed a low mass change that could correlate with less corrosion in comparison to the experiments at RH values above salts deliquescence point. It’s worth noting that even at RH values below the deliquescence point of the salts, a trace of corrosion was observed which could be indicative of the presence of monolayers of water on the surface. The results obtained from NaCl and CR10-E exposure conditions showed higher mass change and therefore higher corrosion in comparison to CaCl2 and MgCl2 salts. As for the long-term vapour-phase corrosion tests, Raman spectroscopy confirmed the presence of CuO and Cu2O on the surface as corrosion products. Further surface characterizations and mass loss measurements are underway to determine the extent of corrosion on the copper coupons.

Pore size distribution evolution in pellets based bentonite hydration: Comparison between experimental and numerical results

Several nuclear waste disposal concept designs take advantage of bentonite based materials to seal underground galleries and shafts. Safety assessment and long-term predictions of the material behaviour have been the main objective of a number of experimental campaigns and of constitutive models development. All these studies have underlined that the multi-porosity bentonite structure affects undeniably the strongly coupled hydro-mechanical processes taking place during water saturation. Due to this, in recent years, many classic experimental tests on unsaturated soils have been performed in conjunction with multi-scale observation techniques (for instance MIP, i.e. mercury intrusion porosimetry analises). Despite the well-known limitations of such observation methods, they provide interesting quantitative measurements in terms of pore diameters families, which differ by several orders of magnitude, and their distribution with respect to different assemblies' types (namely pellets mixtures and compacted bentonite blocks). On the other hand, very few studies have been focusing on the role of such pore size distributions with respect to the hydro-mechanical response, both from an experimental and a numerical point of view. The aim of this paper is to present the experimental campaign and the numerical modelling strategy adopted to analyse the role of different pore size distributions characterising MX-80 bentonite in different forms (i.e. 32 mm pellets mixture, 7 mm pellets mixture and compacted sample surrounded by gap) with same overall dry density during isochoric hydration tests. Taking advantage of multisensor-equipped cells and post-mortem analyses and of the finite element code LAGAMINE, the hydro-mechanical response of these bentonite assemblies is examined. Experimental and numerical outcomes result in good agreement and provide complementary information regarding the features of each assembly type.

Temperature effects on water retention behaviour and structural evolution of GMZ bentonite pellet mixtures

Mixtures of high-density, multi-sized bentonite pellets are considered as a promising buffer/backfill material for deep geological disposal of high-level radioactive waste (HLW). During the operation of a HLW repository, the pellet mixtures will be subjected to ground water infiltrated from surrounding rock and elevated temperatures induced by nuclide decay heat, leading to a complex hydro-mechanical behaviour. In this paper, water retention tests with suction controlled by vapour equilibrium and osmotic techniques, as well as structural observations by mercury intrusion porosimetry (MIP) were performed on three kinds of specimens of Gaomiaozi (GMZ) bentonite at different temperatures (20, 40, 60 and 80 °C), including constant-volume pellet mixtures (PM) and compacted blocks (CB) with an identical dry density (1.45 Mg/m3) and free-swelling single pellets with the same initial dry density (1.95 Mg/m3) as those involved in the PM specimen. For the three kinds of specimens, the water retention capacity decreased with increasing temperature. From a perspective of thermal dynamics theory, it is concluded that in high suction range (> 10 MPa), increasing temperature inhibited the water adsorption and promoted the water desorption process consequently leading to a decreasing water retention capacity. According to the capillary law (Young – Laplace equation), in the low suction range (< 10 MPa), increasing temperature reduces the capillary water content in terms of decreasing the surface tension, contact angle and density of the capillary water, as well as expanding the entrapped pore air. Meanwhile, the MIP test results illustrate that during hydration, most of the large pores (2000–360,000 nm) were gradually converted into medium pores (100–2000 nm) while the change of small pores (6.4–100 nm) and inaccessible pores (< 6.4 nm and > 360,000 nm) were limited. Compared to the effect of suction, the temperature influence on this structural evolution process was relatively slight. Therefore, the temperature-dependent water retention behaviour was mainly caused by temperature-induced changes of water-clay interactions as well as physical properties of adsorbed and capillary water during hydration, rather than the limited changes of pore structure of the specimen.

Discussion of parameters used to distinguish suitable from less suitable HLRW bentonites

Abstract. Bentonites will be used in the construction of some high-level radioactive waste (HLRW) repositories mostly in combination with crystalline host rocks. They will be used both as a geotechnical barrier (compacted bentonite blocks) around the canisters and for backfilling. The bentonite should be stable in contact with cement pore water, minimize metal corrosion, be stable against erosion and various salt solutions, retard radionuclides, prevent canister displacement, possess high thermal conductivity, be stable against radioactive radiation, keep its swelling capacity even when dried, and, most importantly, should have a low hydraulic conductivity. Bentonites are natural materials (clays) which are dominated by swelling clay minerals called smectites. All bentonites, therefore, possess high water uptake capacity, swelling, and cation exchange properties. Different bentonites from different deposits worldwide differ with respect to their chemical and mineralogical composition, composition and charge distribution of the smectites, particle size and morphology, microstructure (arrangement of particles relative to each other), and interlayer population. All these parameters affect the performances of bentonites in different applications. The bentonite industry, therefore, compares different bentonites based on empirical investigations to produce superior products. Specifications which could be used to select a suitable HLRW bentonite were discussed by Kaufhold and Dohrmann (2016). Additional information has been published later (Kaufhold et al., 2020a, b). First of all, some of the above listed desired bentonite properties depend more on the degree to which it is compacted compared to the natural variability. High compaction decreases the hydraulic conductivity and increases thermal conductivity. In order to prevent canister displacement only a small swelling pressure is needed which is easily achieved by compaction with all bentonites. Generally, the type of exchangeable cation is the most important parameter determining bentonite properties such as swelling and rheology. Large scale deposition tests, however, proved that the cation population will readily equilibrate with the surrounding water. The initial type of exchangeable cation is, therefore, less relevant. More important is the Fe content which negatively affects the thermal and chemical stability. Structural Fe of the smectites can be reduced or oxidized by bacteria and radiation. The Fe content of the bentonite should therefore be low. Highly charged smectites proved to be less corrosive in combination with iron canisters because they provide more reducing conditions compared to low charged ones. Bentonites containing highly charged smectites should be preferred if Fe canisters are used. In the case of Cu canisters no effect of the charge could be found. Also, soluble or at least partly soluble components such as sulphates, sulphides, carbonates, and organic matter should be absent since their possible dissolution would decrease the dry density and hence the swelling pressure. The presence of reactive silica in some bentonites proved to buffer the dissolution reactions at the cement bentonite interface and hence could have a beneficial effect.

Pellets/block bentonite barriers: Laboratory study of their evolution upon hydration

Some deep geological nuclear waste disposal concepts are considering the installation around the waste canisters of bentonite barriers made up of two components: highly compacted blocks to support the canister and a granular buffer material consisting of bentonite pellets to fill the rest of the disposal gallery. The initial characteristics of these two components in terms of dry density, water content and microstructure are very different, but some, yet scarce, experimental evidence showed that saturation with groundwater will tend to homogenise the barrier. Nevertheless, the combined use of pellets and blocks in the same section of the barrier introduces difficulties in the understanding and modelling of the system performance that have to be addressed. With the aim of obtaining qualitative and quantitative information about the hydro-mechanical evolution of initially inhomogeneous bentonite samples (pellets/blocks) upon hydration under isothermal conditions, a series of laboratory tests in decimetre-scale cells was carried out. In most tests hydration took place through the pellets part, but the effect of hydrating through the block was also checked. As well, tests were performed either under low water injection pressure or under low water inflow rate.The results highlighted the role of the large macropores on the initial stages of hydration when this took place through the pellets, and also of the influence of water availability on its redistribution and on pressure build up. Pressure developed for lower degrees of saturation and water spread more homogeneously through the two sample components if hydration was slow. After full saturation differences in dry density across the two components remained, but they were softer if saturation took place at a slow pace, either because of the low water inflow rate or because it occurred through the less permeable block. The microstructure of the bentonite in the two components was very different even after full saturation: although there was an overall trend to pore size homogenisation towards smaller sizes over time, the ratio of microstructural void ratio to macrostructural void ratio kept lower in the pellets than in the block. As well, the number of water layers in the interlayer was higher in pellets samples than in block samples.The research also highlighted the importance of the right assessment of macroporosity (particularly pores larger than 550 μm) from mercury intrusion porosimetry tests and the likely effect of local density on the pressure measured.

Synthesis and characterization of carbon–silicon carbide particulate composites by powder metallurgical route

AbstractCarbon–silicon carbide composites have been prepared using powder metallurgical route. Phenolic resin, cokes, silicon powder, and silicon carbide in varying amounts were dry mixed and co‐ball milled. The co‐ball milled powders were then compacted and cured simultaneously in hot press. The compacted blocks were carbonized at 1000°C and further heat‐treated at 1400°C in an inert atmosphere. Silicon carbide was formed in carbon–ceramic composites due to the reaction of silicon with carbon matrix during heat treatment at 1400°C. The densities of the final carbon–ceramic composites obtained were in the range of 1.66–1.83 gm/cm3 with open porosities of 2–7%. Scanning electron microscopy and X‐Ray diffraction were used to characterize the microstructures and compositions, respectively, of the composites processed at 1000 and 1400°C. It is found that the sintered composites consist of β‐SiC with particle size varying from 1 to 5 μm. Mechanical strengths of the composites were found to be highly dependent on the percentage of silicon carbide formed due to the reaction of silicon with carbon matrix at 1400°C. Friction coefficient, measured by pin‐on‐disc method, was found to be higher for 1400°C heat‐treated composites as compared to 1000°C heat‐treated composites due to formation of SiC.

Homogenization of bentonite upon saturation: Density and pressure fields

Bentonite structures are a core element of engineered barriers for geological radioactive waste repositories, preventing radionuclide release into the biosphere. These are often assemblies of elements with different densities (bricks, pellets, powder), or may contain regions of different density due to the building process (technological voids, powder segregation). The performance of these barriers is largely dependent on this material reaching a final state of homogeneous hydraulic and mechanical properties upon water intake. In order to understand the coupled processes of swelling and water transport in bentonite at play at the laboratory scale, an experimental approach was designed to characterize in detail the evolution of the state of samples saturated under isochoric conditions. Firstly, three-dimensional fields of water content, solid density and displacement of a heterogeneous powder sample submitted to unidirectional hydration were measured using X-ray microtomography, as a function of time. These fields allowed to gain insight into the phenomenology of non-monotonic water and clay transport resulting from the heterogeneous and coupled evolutions of the permeability, mechanical and friction properties. Then, samples with controlled heterogeneities such as pellets, powder, technological voids or compacted blocks of different densities - but with similar average properties - were saturated in isochoric conditions with a large set of local stress measurements. The different swelling pressure average evolutions and variability, as well as post-mortem measurements provided rich data on the effect of the initial density heterogeneities on the saturation process and the final state as well as insights into the role of wall friction. In particular, hydration kinetics were shown to depend on a complex manner on the initial heterogeneities. Regions first in contact with water were shown to exhibit a non-monotonic hydromecanical response and the development of macroporosity.

Glucose amendment influences restructuring of compacted soil blocks – Results of a field experiment with varying soil-site conditions

Man-induced soil compaction is a widespread problem for soil fertility and soil usability. Negative consequences may persist for a long time. However, the burrowing activity of soil fauna (pore creation) and growth of plant roots as well as swelling and shrinking can loosen compacted soil over time. This study was focused on the question if an increased energy supply can promote the soil-loosening action of soil biota in compacted soil. We conducted an experiment where compacted soil blocks of two different substrates (different pH) with and without addition of glucose (as energy supply) were exposed to sites with contrasting potentials for restructuring (mown grassland and acidic spruce plantation). Glucose addition stimulated the burrowing activity of soil fauna in compacted soil blocks. Synergism between earthworms and plant root growth had the strongest effect on soil loosening. The site conditions (notably pore-creating biological activity) and the soil acidity of the compacted soil play a key role for the structural recovery. In soil blocks without earthworm activity glucose amendment inhibited rooting by plants, most likely because microbial metabolism led to oxygen deficit in the compacted soil block.