No description available.

Yes / Increasing infrastructure growth has forced the construction industry to look for wasteful, cheap, and suitable materials for construction. An investigation into the geotechnical utilization of fly ash was carried out in the present study. Practical applications normally involve the use of large quantities of fly ash, so proper mixing of the fly ash with other materials may not be significantly achieved. Therefore, the present paper investigates the behaviour of a fly ash–bentonite layered system with different ratios. The physical properties and chemical composition of fly ash and bentonite were determined. SEM and energy dispersive X-ray experiments were also used to investigate the morphology and phase compositions of fly ash and bentonite. A series of consolidated undrained (CU) triaxial tests on fly ash–bentonite were carried out to investigate shear strength characteristics. Fly ash (F) and bentonite (B) were used in the following ratios: 1:1 (50% F:50% B), 2:1 (67% F:33% B), 3:1 (75% F:25% B), and 4:1 (80% F:20% B), with different numbers of interfaces (N), i.e., 1, 2, and 3 for each ratio. The deviator stress and cohesion value were found to increase with the number of interfaces for each ratio. The angle of shear resistance changed marginally with the increase in the fly ash–bentonite ratios and varying interfaces.

As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted; based only on the chemical, surface chemical and mineralogical characterisation of the clay. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted; based only on the chemical, surface chemical and mineralogical characterisation of the clay. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

The aim of this present research is to improve the containment properties of clayey soil by the use of bentonite and polymers as the additives. Clayey soils are generally used for the control of soil and water pollutions as they are inexpensive material. However, their applications are limited when it comes to durability of such liners in the field condition. Addition of additives to increase their efficiency by reducing the hydraulic conductivity can overcome this limitation. Generally, bentonite is used as the effective barrier additives because of their ability to render high swell and reduced hydraulic conductivity to the soil-bentonite mixtures. Due to high specific surface area and large cation exchange capacity, bentonite is always preferred for containment application. Despite their wide applicability in containment barrier, the problem with bentonite is its reduction of swell when the contaminant of interest consists of multivalent ions. Higher valency ions attach to the exchange site of bentonite and thus reduce its efficiency. To overcome these constraints the bentonite has, polymers are being used for containment applications. Normally, anionic polymers are used for such containment applications which help in increase negative charge concentration of the clayey soils. Previous studies have also proved the effectiveness of polymer in increasing the retardation factor of the soil which is due to the adsorption of contaminants to the negatively charged surface of clay. Hence attempt is also made in this thesis to study the effectiveness of super absorbent polymers when used as additive to the clayey soil. In this thesis, effectiveness of the local Bangalore soil for its use as barrier material is studied. Tests are done on the compacted sample to determine its contaminant transport properties. Because of their low hydraulic conductivities, advective flux through the barriers is negligible. In such cases, contaminant migration takes place due to the difference in concentration and this transport mechanism is known as diffusion. Laboratory test have been done to determine the diffusion coefficient of soil when the soil is saturated. For barriers and liner systems which are not fully saturated, suction adds to the hydraulic head. For unsaturated samples, although the liquid phase diffusion have been found to be nominal, head difference caused due to suction facilitates the migration of contaminants. Hence, effects of initial degree of saturation and density in contaminant transport process through soils have been examined. An attempt has been made to study unsaturated transport properties by using saturated transport parameters and unsaturated soil parameters obtained from soil-water characteristics curve. Another focus of this thesis has been towards enhancing the containment property of local soil by using additives. Effects of addition of bentonite and polymer on contaminant transport parameters of the soil have been studied. An attempt has been made to reduce the hydraulic conductivity of soil by using additives like bentonite. In addition, use of super-absorbent polymer as a novel material for barrier application has been studied to determine its suitability for local conditions. Experiments have been conducted to determine free swell, hydraulic conductivity and diffusion coefficient of soil-bentonite and soil-polymer mixes and compare them to those of local soil. To determine the effectiveness of soil-bentonite and soil-polymer mixes under the attack of concentrated solution over a long duration, chemical compatibility tests have been carried out. Micro-structural changes in the soil on addition of bentonite and polymer have been studied with the help of scanning electron microscope (SEM) images. Mineralogical changes occurring in the soil due to the additives have been studied using X-Ray Diffraction (XRD) plots. Effects of salt solution on soil-bentonite and soil-polymer mixes have also been studied by the help of SEM images and XRD plots. The final section of this thesis is on analysis of barrier performance for different conditions of densities, compaction and saturation by using numerical software CODE_BRIGHT. Transport parameters obtained from the experiments conducted in previous sections have been used as input for the analysis. A field condition of Bingipura landfill site in South Bangalore, Bangalore city has been simulated as a case study for this thesis. Time taken for contaminants to migrate from landfill to the nearby water body which is at a distance of about 122 meters is calculated for no barrier case. When barrier is provided, concentration variations of contaminant through the barrier with time are plotted and the results are compared for soil barrier, soil-bentonite barrier and soil-polymer barrier. v

Au cours de ce travail, l’objectif est d’étudier les liens entre les paramètres du procédé de séchage, la microstructure et les propriétés d’usage du matériau telles que la conductivité thermique et le module d’Young. L’étude a été effectuée sur des matériaux poreux à base d’alumine, oxyde modèle, et à base de bentonite, minéral argileux. Le procédé de séchage utilisé est le procédé de coulage-congélation, qui permet d’obtenir une microstructure orientée et de fabriquer des matériaux anisotropes. L’origine de cette anisotropie provient de la microstructure des échantillons, essentiellement la forme des pores. Les résultats obtenus ont permis de souligner l’importance des paramètres de séchage sur l’anisotropie et les propriétés d’usage du matériau considéré. Par exemple, des valeurs très faibles de la conductivité thermique (0,045 W.m-1.K-1 pour 98 % de porosité) ont été obtenues pour les matériaux à base de bentonite, tout en étant manipulables. / The aim of this thesis is to correlate the shaping process parameters, the microstructure and the material properties, like thermal conductivity and Young modulus. The materials used in this study are alumina and a bentonite, a clay material. The shaping process used is the freeze-casting, which permit to obtain a textural microstructure and anisotropic materials. The origin of this anisotropy is the samples microstructure, in particularly the pores shape. The results highlight the importance of the freeze-casting parameters on the anisotropy and on materials properties. For example, very low values for the thermal conductivity (0,045 W. m-1.K-1 for a porosity of 98 %) are obtained for bentonite material, and they are handleable.

Deep geological repository (DGR) for disposal of high-level radioactive waste (HLW) is designed to rely on successive superimposed barrier systems to isolate the waste from the biosphere. This multiple barrier system comprises the natural geological barrier provided by the repository host rock and its surrounding and an engineered barrier system (EBS). The EBS represents the synthetic, engineered materials placed within the natural barrier, comprising array of components such as waste form, waste canisters, buffer materials, backfill and seals. The buffer will enclose the waste canisters from all directions and act as a barrier between canisters and host rock of the repository. It is designed to stabilise the evolving thermo-hydro-mechanical-chemical stresses in the repository over a long period (nearly 1000 years) to retard radionuclides from reaching biosphere. Bentonite clay or bentonite-sand mix have been chosen as buffer materials in EBS design in various countries pursuing deep geological repository method. The bentonite buffer is the most important barrier among the other EBS components for a geological repository. The safety of repository depends to a large extent on proper functioning of buffer over a very long period of time during which it must remain physically, chemically and mineralogically stable. The long term stability of bentonite buffer depends on varying temperature and evolution of groundwater composition of host rocks in a complex way. The groundwater in the vicinity of deep crystalline rock is often characterized by high solute concentrations and the geotechnical engineering response of bentonite buffer could be affected by the dissolved salt concentration of the inflowing ground water. Also during the initial period, radiogenic heat produced in waste canisters would radiate into buffer and the heat generated would lead to drying and some shrinkage of bentonite buffer close to canister. This could alter the dry density, moisture content and in turn the hydro-mechanical properties of bentonite buffer in DGR conditions. India has variety of bentonite deposits in North-Western states of Rajasthan and Gujarat. Previous studies on Indian bentonites suggest that bentonite from Barmer district of Rajasthan (termed as Barmer-1 bentonite) is suitable to serve as buffer material in DGR conditions. Nuclear power agencies of several countries have identified suitable bentonites for use as buffer in DGR through laboratory experiments and large scale underground testing facilities. Physico-chemical, mineralogical and engineering properties of Kunigel VI, Kyungju, GMZ, FoCa clay, MX-80, FEBEX and Avonseal bentonites have been extensively studied by Japan, South Korea, China, Belgium, Sweden, Spain, Canada. It is hence essential to examine the suitability of Barmer-1 bentonite as potential buffer in DGR and compare its physico-chemical and hydromechanical properties with bentonite buffers identified by other countries. The significant factors that impact the long-term stability of bentonite buffer in DGR include variations in moisture content, dry density and pore water chemistry. With a view to address these issues, the hydromechanical response of 70 % Barmer-1 bentonite + 30 % river sand mix (termed bentonite enhanced sand, BES specimens) under varying moisture content, dry density and pore water salt concentration conditions have been examined. The broad scope of the work includes: 1) Characterise the physico-chemical and hydro-mechanical properties of Barmer-1 bentonite from Rajasthan, India and compare its properties with bentonite buffers reported in literature. 2) Examine the influence of variations in dissolved salt concentration (of infiltrating solution), dry density and moisture content of compacted BES specimens on their hydro-mechanical response; the hydro-mechanical properties include, swell pressure, soil water characteristic curve (SWCC), unsaturated hydraulic conductivity, moisture diffusivity and unconfined compression strength. Organization of thesis: After the first introductory chapter, a detailed review of literature is performed to highlight the need for detailed characterisation of physico-chemical and hydromechanical properties of Barmer-1 bentonite for its possible application in DGR in the Indian context. Further, existing literature on hydro-mechanical response of bentonite buffer to changes in physical (degree of saturation/moisture content, dry density) and physico-chemical (solute concentration in pore water) is reviewed to define the scope and objectives of the present thesis in Chapter 2. Chapter 3 presents a detailed experimental programme of the study. Chapter 4 characterises Barmer-1 bentonite for physico-chemical (cation exchange capacity, pore water salinity, exchangeable sodium percentage) and hydro-mechanical properties, such as, swell pressure, saturated permeability, soil water characteristic curve (SWCC) and unconfined compression strength. The properties of Barmer-1 bentonite are compared with bentonite buffers reported in literature and generalized equations for determining swell pressure and saturated permeability coefficient of bentonite buffers are arrived at. Chapter 5 describes a method to determine solute concentrations in the inter-lamellar and free-solutions of compacted BES (bentonite enhanced sand) specimens. The solute concentrations in micro and macro pore solutions are used to examine the role of osmotic flow on swell pressures developed by compacted BES specimens (dry density 1.50-2.00 Mg/m3) inundated with distilled water and NaCl solutions (1000-5000 mg/L). The number of hydration layers developed by the compacted BES specimens on inundation with salt solutions in constant volume swell pressure tests is controlled by cation hydration/osmotic flow. The cation hydration of specimens compacted to dry density of 2.00 Mg/m3 is mainly driven by matric suction prevailing in the clay microtructure as the number of hydration layers developed at wetting equilibrium are independent of the total dissolved solids (TDS) of the wetting solution. Consequently, the swell pressures of specimens compacted to 2.00 Mg/m3 were insensitive to the salt concentration of the inundating solution. The cation hydration of specimens compacted to dry density of 1.50 Mg/m3 is driven by both matric suction (prevailing in the clay micro-structure) and osmotic flow as the number of hydration layers developed at wetting equilibrium is sensitive to the TDS of the wetting solution. Expectedly, the swell pressures of specimens compacted to 1.50 Mg/m3 responded to changes in salt concentration of the inundating solution. The 1.75 Mg/m3 specimens show behaviour that is intermediate to the 1.50 and 2.00 Mg/m3 series specimens. Chapter 6 examines the influence of initial degree of saturation on swell pressures developed by the compacted BES specimens (dry density range: 1.40- 2.00 Mg/m3) on wetting with distilled water from micro-structural considerations. The micro-structure of the bentonite specimens are examined in the compacted and wetted states by performing X-ray diffraction measurements. The initial degree of saturation is varied by adding requisite amount of distilled water to the oven-dried BES mix and compacting the moist mixes to the desired density. The montmorillonite fraction in the BES specimens is responsible for moisture absorption during compaction and development of swell pressure in the constant volume oedometer tests. Consequently, it was considered reasonable to calculate degree of saturation based on EMDD (effective montmorillonite dry density) values and correlate the developed swell pressure values with degree of saturation of montmorillonite voids (Sr,MF). XRD measurements with compacted and wetted specimens demonstrated that if specimens of density series developed similar number of hydration layers on wetting under constant volume condition they exhibited similar swell pressures, as was the case for specimens belonging to 1.40 and 1.50 Mg/m3 series. With specimens belonging to 1.75 and 2.00 Mg/m3 series, greater number of hydration layers were developed by specimens that were less saturated initially (smaller initial Sr,MF) and consequently such specimens developed larger swell pressures. When specimens developed similar number of hydration layers in the wetted state, the compaction dry density determined the swell pressure. Chapter 7 examines the influence of salt concentration of infiltrating solution (sodium chloride concentration ranges from 1000- 5000 mg/L) on SWCC relations, unsaturated permeability and moisture diffusivity of compacted BES specimens. Analysis of the experimental and Brooks and Corey best fit plots revealed that infiltration of sodium chloride solutions had progressively lesser influence on the micro-structure and consequently on the SWCC relations with increase in dry density of the compacted specimens. The micro-structure and SWCC relations of specimens compacted to 1.50 Mg/m3 were most affected, specimens compacted to 1.75 Mg/m3 were less affected, while specimens compacted to 2.00 Mg/m3 were unaffected by infiltration of sodium chloride solutions. Variations in dry density of compacted bentonite impacts the pore space available for moisture flow, while, salinity of wetting fluid impacts the pore structure from associated physico-chemical changes in clay structure. Experimental results showed that the unsaturated permeability coefficient is insensitive to variations in dry density and solute concentration of wetting liquid, while, the effective hydraulic diffusivity is impacted by variations in these parameters. Chapter 8 summarises the major findings of the study.

Deep geological repository (DGR) for disposal of high-level radioactive waste (HLW) is designed to rely on successive superimposed barrier systems to isolate the waste from the biosphere. This multiple barrier system comprises the natural geological barrier provided by the repository host rock and its surrounding and an engineered barrier system (EBS). The EBS represents the synthetic, engineered materials placed within the natural barrier, comprising array of components such as waste form, waste canisters, buffer materials, backfill and seals. The buffer will enclose the waste canisters from all directions and act as a barrier between canisters and host rock of the repository. It is designed to stabilise the evolving thermo-hydro-mechanical-chemical stresses in the repository over a long period (nearly 1000 years) to retard radionuclides from reaching biosphere. Bentonite clay or bentonite-sand mix have been chosen as buffer materials in EBS design in various countries pursuing deep geological repository method. The bentonite buffer is the most important barrier among the other EBS components for a geological repository. The safety of repository depends to a large extent on proper functioning of buffer over a very long period of time during which it must remain physically, chemically and mineralogically stable. The long term stability of bentonite buffer depends on varying temperature and evolution of groundwater composition of host rocks in a complex way. The groundwater in the vicinity of deep crystalline rock is often characterized by high solute concentrations and the geotechnical engineering response of bentonite buffer could be affected by the dissolved salt concentration of the inflowing ground water. Also during the initial period, radiogenic heat produced in waste canisters would radiate into buffer and the heat generated would lead to drying and some shrinkage of bentonite buffer close to canister. This could alter the dry density, moisture content and in turn the hydro-mechanical properties of bentonite buffer in DGR conditions. India has variety of bentonite deposits in North-Western states of Rajasthan and Gujarat. Previous studies on Indian bentonites suggest that bentonite from Barmer district of Rajasthan (termed as Barmer-1 bentonite) is suitable to serve as buffer material in DGR conditions. Nuclear power agencies of several countries have identified suitable bentonites for use as buffer in DGR through laboratory experiments and large scale underground testing facilities. Physico-chemical, mineralogical and engineering properties of Kunigel VI, Kyungju, GMZ, FoCa clay, MX-80, FEBEX and Avonseal bentonites have been extensively studied by Japan, South Korea, China, Belgium, Sweden, Spain, Canada. It is hence essential to examine the suitability of Barmer-1 bentonite as potential buffer in DGR and compare its physico-chemical and hydromechanical properties with bentonite buffers identified by other countries. The significant factors that impact the long-term stability of bentonite buffer in DGR include variations in moisture content, dry density and pore water chemistry. With a view to address these issues, the hydromechanical response of 70 % Barmer-1 bentonite + 30 % river sand mix (termed bentonite enhanced sand, BES specimens) under varying moisture content, dry density and pore water salt concentration conditions have been examined. The broad scope of the work includes: 1) Characterise the physico-chemical and hydro-mechanical properties of Barmer-1 bentonite from Rajasthan, India and compare its properties with bentonite buffers reported in literature. 2) Examine the influence of variations in dissolved salt concentration (of infiltrating solution), dry density and moisture content of compacted BES specimens on their hydro-mechanical response; the hydro-mechanical properties include, swell pressure, soil water characteristic curve (SWCC), unsaturated hydraulic conductivity, moisture diffusivity and unconfined compression strength. Organization of thesis: After the first introductory chapter, a detailed review of literature is performed to highlight the need for detailed characterisation of physico-chemical and hydromechanical properties of Barmer-1 bentonite for its possible application in DGR in the Indian context. Further, existing literature on hydro-mechanical response of bentonite buffer to changes in physical (degree of saturation/moisture content, dry density) and physico-chemical (solute concentration in pore water) is reviewed to define the scope and objectives of the present thesis in Chapter 2. Chapter 3 presents a detailed experimental programme of the study. Chapter 4 characterises Barmer-1 bentonite for physico-chemical (cation exchange capacity, pore water salinity, exchangeable sodium percentage) and hydro-mechanical properties, such as, swell pressure, saturated permeability, soil water characteristic curve (SWCC) and unconfined compression strength. The properties of Barmer-1 bentonite are compared with bentonite buffers reported in literature and generalized equations for determining swell pressure and saturated permeability coefficient of bentonite buffers are arrived at. Chapter 5 describes a method to determine solute concentrations in the inter-lamellar and free-solutions of compacted BES (bentonite enhanced sand) specimens. The solute concentrations in micro and macro pore solutions are used to examine the role of osmotic flow on swell pressures developed by compacted BES specimens (dry density 1.50-2.00 Mg/m3) inundated with distilled water and NaCl solutions (1000-5000 mg/L). The number of hydration layers developed by the compacted BES specimens on inundation with salt solutions in constant volume swell pressure tests is controlled by cation hydration/osmotic flow. The cation hydration of specimens compacted to dry density of 2.00 Mg/m3 is mainly driven by matric suction prevailing in the clay microtructure as the number of hydration layers developed at wetting equilibrium are independent of the total dissolved solids (TDS) of the wetting solution. Consequently, the swell pressures of specimens compacted to 2.00 Mg/m3 were insensitive to the salt concentration of the inundating solution. The cation hydration of specimens compacted to dry density of 1.50 Mg/m3 is driven by both matric suction (prevailing in the clay micro-structure) and osmotic flow as the number of hydration layers developed at wetting equilibrium is sensitive to the TDS of the wetting solution. Expectedly, the swell pressures of specimens compacted to 1.50 Mg/m3 responded to changes in salt concentration of the inundating solution. The 1.75 Mg/m3 specimens show behaviour that is intermediate to the 1.50 and 2.00 Mg/m3 series specimens. Chapter 6 examines the influence of initial degree of saturation on swell pressures developed by the compacted BES specimens (dry density range: 1.40- 2.00 Mg/m3) on wetting with distilled water from micro-structural considerations. The micro-structure of the bentonite specimens are examined in the compacted and wetted states by performing X-ray diffraction measurements. The initial degree of saturation is varied by adding requisite amount of distilled water to the oven-dried BES mix and compacting the moist mixes to the desired density. The montmorillonite fraction in the BES specimens is responsible for moisture absorption during compaction and development of swell pressure in the constant volume oedometer tests. Consequently, it was considered reasonable to calculate degree of saturation based on EMDD (effective montmorillonite dry density) values and correlate the developed swell pressure values with degree of saturation of montmorillonite voids (Sr,MF). XRD measurements with compacted and wetted specimens demonstrated that if specimens of density series developed similar number of hydration layers on wetting under constant volume condition they exhibited similar swell pressures, as was the case for specimens belonging to 1.40 and 1.50 Mg/m3 series. With specimens belonging to 1.75 and 2.00 Mg/m3 series, greater number of hydration layers were developed by specimens that were less saturated initially (smaller initial Sr,MF) and consequently such specimens developed larger swell pressures. When specimens developed similar number of hydration layers in the wetted state, the compaction dry density determined the swell pressure. Chapter 7 examines the influence of salt concentration of infiltrating solution (sodium chloride concentration ranges from 1000- 5000 mg/L) on SWCC relations, unsaturated permeability and moisture diffusivity of compacted BES specimens. Analysis of the experimental and Brooks and Corey best fit plots revealed that infiltration of sodium chloride solutions had progressively lesser influence on the micro-structure and consequently on the SWCC relations with increase in dry density of the compacted specimens. The micro-structure and SWCC relations of specimens compacted to 1.50 Mg/m3 were most affected, specimens compacted to 1.75 Mg/m3 were less affected, while specimens compacted to 2.00 Mg/m3 were unaffected by infiltration of sodium chloride solutions. Variations in dry density of compacted bentonite impacts the pore space available for moisture flow, while, salinity of wetting fluid impacts the pore structure from associated physico-chemical changes in clay structure. Experimental results showed that the unsaturated permeability coefficient is insensitive to variations in dry density and solute concentration of wetting liquid, while, the effective hydraulic diffusivity is impacted by variations in these parameters. Chapter 8 summarises the major findings of the study.

It is a generally agreed internationally that high level nuclear wastes containing long-lived radioactive wastes should be disposed in deep and stable geological formations that are 500-1000 m below ground level. Deep geological disposal is based on the concept of multiple barriers to prevent deep ground-waters, present in almost all rock formations, from rapidly leaching the wastes and transporting radioactivity away from the repository. The multiple barrier system comprises of ‘engineered barriers’ that are constructed in the repository and ‘natural barriers’ in the surrounding geological environment. The engineered barrier components comprise of the vitrified solid waste, canister (to contain the vitrified waste), and a buffer or backfill material (clay or cement) that fills the annular space between the canister and the walls of the hole drilled in the floor of host-rock. The natural barrier is provided by the rocks and soils between the repository and earth’s surface. The canisters containing the hig level waste (HLW) upon placement in DGR need protection against tectonic activities and chemical attack by dissolved elements and from microbes. Densely compacted bentonite is identified suitable for this purpose owing to its large swell potential, low permeability, sufficient bearing capacity and high cation adsorption capacity. In the deep geological repository (DGR) for disposal of high level nuclear wastes, iodine-129 is one of the significant nuclides, owing to its long half-life (half life = 16 million years) and tendency to easily migrate out of the geological repository into the biosphere caused by its high solubility and poor sorption onto most geologic media. Bentonite buffer by virtue of negatively charged basal surface has negligible affinity for retention of iodide anions. Attempts have been made to improve the iodide retention capacity of bentonite by treating the clay with cationic polymers, this however occurs at the cost of reduced swelling ability of bentonite clay. The compacted bentonite employed in deep geological repositories must possess large swell potential to enable it to close fissures and cracks that form on drying of the expansive clay by the heat arising from the high level nuclear waste and thereby close pathways for migration of radionuclides (from breached canister) to the geo-environment. Therefore, it becomes important to identify an additive that enhances the iodide retention ability of the mix without significantly impairing its swelling ability. Based on the strong affinity of silver for iodide ions, the feasibility of mixing silver-kaolinite (termed AgK) clay with bentonite to improve the latter’s iodide sorption capacity and the impact of mixing AgK clay with bentonite on swelling ability of the mix forms one of the the focus of this thesis. Silver-kaolinite clay was prepared by heating 80% kaolinite + 20% silver nitrate mix at 400°C for 30 min, followed by washing (to remove unreacted silver nitrate) and oven-drying the resultant AgK clay. Physical mixing of AgK and bentonite was considered a viable proposition as small additions (10% to 20% on dry mass basis) besides imparting iodide sorption ability was expected to have minor influence on the swelling ability of the mix. As organo-bentonites are known to retain iodide ions, it was considered relevant to compare the iodide removal behaviour of AgK and organo¬bentonite clay. Hexadecylpyridinium-bentonite (termed as HDPy+B) is the organo¬bentonite examined in this thesis and is prepared by treating bentonite with hexadecylpyridinium chloride mono hydrate salt (C21H38ClN.H2O; molecular weight = 358.01). The hexadecylpyridinium chloride mono hydrate salt is a cationic quaternary ammonium compound and has been used by earlier researchers to prepare organo-bentonite for removal of iodide ions from aqueous solutions. The impact of mixing AgK and HDPy+B clays on the iodide retention and swelling behaviour of bentonite is also considered in the thesis. The mass-balance calculations, XRD analysis, X-ray photon emission survey spectrum and EPMA tests performed on kaolinite-silver nitrate mix/AgK/kaolinite specimen indicated that silver occurs as uniform coatings of AgO/Ag2O on kaolinite surface of the AgK specimen. The AgK clay has strong affinity for iodide ions reflected by the large distribution coefficients (Kd) values of 1367 and 293 mL/g at initial iodide concentrations of 750 mg/L and 1000 mg/L. Further, the sorption process was rapid, unaffected by the presence of co-ions, elevated temperature of sorption and was practically irreversible at range of pH conditions. The iodide retention by AgK is attributed to occurrence of hydrolysis and exchange reactions. On contacting the AgK with water, the AgO species hydrolyze to form AgOH; iodide ions are retained by replacing the hydroxyl group of AgOH leading to formation of AgI phase. The adsorption of HDPy+Cl- ions by bentonite occurs by replacement of the native exchangeable cations by HDPy+ ions and adsorption by van der Waals interactions between the organic cations and the clay surface. The adsorbed cationic polymer neutralize the negative charge of the clay surface. Zeta potential measurements of HDPy+B specimen indicated that adsorption of cationic polymer transforms the negatively charged clay particles into positively charged particles that favour anion adsorption. Sorption of iodide ions by HDPy+B specimen exhibits two distinct segments: 1) the iodide sorption increased rapidly at lower iodide concentration (91 mg/L to 475 mg/L) and are retained by Coulombic adsorption to the cationic groups contained in the loops and tails of the adsorbed polymer (primary adsorption sites) and 2) the relatively slower adsorption at higher iodide concentrations (larger than 475 mg/L) is attributed to exchange with chloride ions attached to HDPy+Cl-ion pair (secondary adsorption sites). The Kd values for iodide adsorption vary from 15 mL/g to 184 mL/g at initial iodide concentrations of 91 mg/L to 996 mg/L respectively. Comparing the iodide removal efficiencies of AgK and HDPy+B specimens revealed that the AgK clay exhibited larger iodide removal; further while the iodide removal by AgK specimen was almost instantaneous (complete in < 5 min), iodide removal by HDPy+B specimen was a slow process (18-24 h is needed to attain equilibrium). Likewise, the iodide retention capacity of the 50%B-50%HDPy+B mix (B = bentonite) is substantially smaller than of the 90%B-10%AgK and 80%B¬20%AgK mixes. Cation exchange capacity (CEC) measurements brought out that mixing AgK with bentonite besides imparting an iodide retention capacity essentially retains the large cation exchange capacity of the expansive clay. On the other hand mixing HDPy+B with bentonite imparts a smaller iodide retention capacity to the mix and leads to a notable reduction in the CEC of the expansive clay. Results of oedometer swell tests brought out that dilution of bentonite with 10% and 20% AgK specimen does not impact its swell potential and leads to some (10%) reduction in swell pressure, while dilution with 50% HDPy+B clay leads to notable (58%) reduction in swell potential and swell pressure (21%) underlining the superiority of AgK specimen as additive to bentonite in deep geological repositories. The swell pressure of the compacted 50%B-50%HDPy+B mix is 21% lower than that of the compacted bentonite specimen. Comparatively, dilution of bentonite with 10% and 20% AgK specimen induces 8-10% lower swell pressure in comparison to the undiluted counterpart. Swell pressure results of compacted 80%B-20%HDPy+B mix is not considered as this mix was unable to retain iodide ions. Superposing the field 129I concentration levels on I removal efficiency indicate that use of 90%B-10%AgK mix would suffice to provide 100% iodide removal efficiency and ensure that the swelling characteristics of bentonite is least affected by dilution.

La présente étude concerne le comportement hydromécanique des matériaux compactés à base de bentonite pour le stockage des déchets radioactifs en grande profondeur. Trois matériaux candidats ont été étudiés : la bentonite (MX80) pure, le mélange bentonite/argilite broyée et le mélange bentonite/sable. Une étude expérimentale sur la pression de gonflement du mélange bentonite/argilite a été premièrement réalisée. Cette étude a mis en évidence l'effet de la salinité de l'eau, de la procédure et la durée d'hydratation, du pré-existant vide technologique et des méthodes expérimentales. Une importante relation entre la pression de gonflement et la densité sèche finale de la bentonite a été élaborée. Ensuite, des essais de rétention d'eau, des essais d'hydratation et des essais oedométriques à succion contrôlée ont été réalisés sur des échantillons à différentes porosités tout en considérant la présence du vide technologique. En introduisant les paramètres comme indice des vides de la bentonite et le ratio volume d'eau, une analyse globale des effets des vides sur la réponse hydromécanique de la barrière ouvragée a été effectuée. Pour obtenir un meilleur aperçu de l'évolution de l'étanchéité dans le cas de vide technologique, l'effet de densité sèche finale (densité après fermeture de vide technologique) et du temps d'hydratation sur la microstructure a été, de même, étudié. La perméabilité de ce matériau à l'état non saturé a été ensuite étudiée en réalisant des essais de rétention d'eau et d'infiltration ainsi que par des observations de la microstructure. Les résultats obtenus ont permis de relier la variation de la conductivité hydraulique non saturée aux changements de la microstructure. Une expérimentation en modèle réduit reproduisant à une échelle 1/10ème les essais in-situ (SEALEX) a été effectuée, et cela pour étudier la reprise des vides à long terme d'un mélange compacté bentonite/sable, tout en considérant la présence d'un vide technologique. Les résultats ont été utilisés pour interpréter les observations de l'essai in situ. A une échelle de temps réduite, cette étude fournit des informations utiles pour estimer la durée et l'efficacité de la conception en place. Finalement, les données expérimentales obtenues dans le laboratoire sur le mélange bentonite/sable ont été interprétées dans le cadre du modèle de Barcelone (BExM). Après comparaison des résultats expérimentaux avec le modèle, les performances et les limitations du modèle ont été analysées / This study deals with the hydro-mechanical behaviour of compacted bentonite-based materials used as sealing materials in high-level radioactive waste repositories. The pure MX80 bentontie, mixtures of MX80/crushed claystone and MX80/sand were used in the investigation. An experimental study on the swelling pressure of the bentonite-based materials was first performed. The results evidenced the effects of water chemistry, hydration procedure and duration, pre-existing technological void and experimental methods. Emphasis was put on the relationship between the swelling pressure and the final dry density of bentonite. Afterwards, the water retention test, hydration test and suction controlled oedometer test were conducted on samples with different voids including the technological void and the void inside the soil. By introducing the parameters as bentonite void ratio and water volume ratio, an overall analysis of the effects of voids on the hydro-mechanical response of the compacted material was performed. To get better insight into the seal evolution in case of technological void, the effects of final dry density and hydration time on the microstructure features were also characterized. Then, the hydraulic properties under unsaturated state were investigated by carrying out water retention test and infiltration test as well as the microstructure observation. The results obtained allowed relating the variation of hydraulic conductivity to the microstructure changes. A small scale (1/10) mock up test of the SEALEX in situ experiment was also performed to study the recovery capacity of bentonite-based material with consideration of a technological void. The results were used for interpreting the in-situ observations. With a reduced time scale, it provides useful information for estimating the saturation duration and sealing effectiveness of the field design. Finally, the experimental data obtained in the laboratory on bentonite/sand mixture were interpreted in the framework of the Barcelona Expansive Model (BExM). By comparing the model with the experimental results, the performance and limitation of the model were analyzed