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The present study investigated the potential use and effectiveness of expansive clay stabilization using a mixture of cement and silica fume (CSF) as a possibly useful option from environmental, economic, and (or) technical perspectives. In so doing, cement and CSF blend with 10% cement replacement were separately added to a clay sample having high degree of swelling potential, and then, a set of macro and micro level tests were performed under various curing regimes to assess the responses of amendments. The results showed that at ambient condition of 20 °C, highly expansive clays need a large quantity of cement (≥ 15%) and a curing time of 28 days to get stabilized satisfactorily. In this case, although a higher temperature (40 °C) leads to a decrease in the required time of curing, there is no a remarkable reduction in the optimum content of additive. It appeared that the CSF binary system can expedite the process of stabilization and thus is more efficient in improving the clay engineering properties with a lower amount of binder (to about 33%) and shorter curing ages (by nearly 2 times) compared to the sole cement. In fact, based on the XRD and SEM analyses, the incorporation of silica fume into the cement matrix extends the formation of new cementing compounds and provides a much denser microstructure; two features which were found to be very effective in surpassing the problems associated with expansive clays. [ABSTRACT FROM AUTHOR]

Bentonite (Bent) is a favorable buffer material for waste containment barriers, although its properties may be affected by the pore fluid chemistry. Hence, this study investigates the role of different organic pollutants including methanol, acetone, acetic acid, and citric acid on the macro and microstructure responses of Na + -Bent. The chemicals at various concentrations (0 to 80% by the volume of distilled water) were separately added to the soil samples which were then subjected to a set of tests. The results indicate that an increase in the concentration of organic permeants by decreasing the dielectric constant (ε) reduces the plasticity index, compressibility and swelling potential of soil; whereas, the permeability increases. Such changes are ascribed to a tighter packing of the clay particles, as clearly confirmed by the SEM micrographs and XRD patterns. The decrease in the absolute value of soil zeta potential also supports the development of coagulated structure in the artificially contaminated samples upon reducing the ε of solution. However, the results reveal that the sensitivity of soil properties to the variations in the ε value is pronounced at the intermediate stages of concentration (i.e. the organic fluid/water ratio < 60%) when the ε is higher than 50. In addition, at relatively low concentration, the acidic permeants dramatically alter the soil fabric due to having lower pH, causing remarkable effects on the hydro-mechanical parameters as compared to the other chemicals. Overall, the organic pollutants, depending on their type and concentration, exert negative impact on the barrier performance of Na + -Bent which are not precisely comparable only with reference to the changes in the dielectric constant of pore fluid, and thereby may not be completely explained by the diffuse double layer (DDL) theory. [ABSTRACT FROM AUTHOR]

Dispersive clays may pose considerable distress if not adequately taken care of. On the other hand, the treatment of problematic soils with waste materials has been recently proved to be a useful option from economic and environmental view point. Hence, in this research, the potential use and effectiveness of dispersive soil stabilization using two types of industrial by-product, including granulated blast furnace slag (GBFS) and basic oxygen furnace slag (BOFS) were investigated. The slags were separately added (ranging from 2.5 to 30%) to a laboratory dispersed sample and a set of experiments were performed to study the physicochemical, mechanical and microstructural changes of the stabilized soil. The results indicate that the soil dispersion can be eliminated upon adding 10% BOFS. This is attributed to the exchange of interlayer sodium ions on the clay surfaces by multivalent cations from the agent. Besides, an increase in the ion concentration of soil-additive mixtures induces a more depression of the diffuse double layer that decreases the soil dispersivity potential. With increasing the curing time, an improvement in the strength of composite samples is observed. The formation of cementitious compounds due to the pozzolanic reactions is responsible for such a treatment, as confirmed by the XRD analyses and SEM micrographs. It appears that the GBFS has a lower activity as compared to BOFS, therefore causes less influence on the soil engineering parameters and a higher percentage (20–25%) of GBFS is required to govern soil dispersion. Overall, utilization of the studied slags particularly of BOFS is very effective to overcome the problems associated with dispersive soils. [ABSTRACT FROM AUTHOR]

Abstract: Dispersive soils have been responsible for failures in several geotechnical and geoenvironmental projects. Chemical treatments have been used to overcome deficiencies in the dispersive performance of these types of soils. Alum (aluminum sulfate {Al2(SO4)3·18H2O}) is commonly used for the treatment of these types of soils. The interaction of soil–additives during the treatment process is a physico-chemical phenomenon. In this paper, the fundamental aspects of interaction between dispersive soil and alum are examined and evaluated. Bentonite samples were laboratory dispersed with the use of different concentrations of sodium sulfate (Na2SO4). Alum was then used to control their dispersivity performance. A set of physico-chemical experiments including Atterberg limits, permeability, consolidation, double hydrometer, ion exchange, and pH measurement were performed to investigate the fundamental mechanism of soil–alum interaction. XRD and SEM testing were performed to support the findings. Double hydrometer testing indicates that the application of 1.5% aluminum sulfate caused a noticeable decrease in dispersivity potential. The results obtained indicate that the addition of alum causes a decrease in the pH level of dispersive bentonite. In this process, a distinguishable change in the engineering properties of soil occurred, including a reduction in liquid limit and an increase in hydraulic conductivity. In fact, the replacement of sodium ions by aluminum ions in the double layer of the clay particles resulted in a decrease in the thickness of the particles'' double layers. It is concluded that two different phenomena were responsible for overcoming soil dispersivity: ion exchange and pH effects. [Copyright &y& Elsevier]

Abstract: Wetting and water uptake by partly saturated smectite clays are expected when such clays are used as buffer material surrounding embedded high level nuclear waste (HLW). Nuclear waste canisters in a repository impose elevated temperatures on the surrounding smectite buffer material for thousands of years. Generally, the behaviour of clays is markedly influenced by the type of minerals present in the clay, the nature of the exchangeable ions, and the type of clay microstructure present. Because of the different microstructures of clay soils, the corresponding macrostructure will result in different hydraulic conductivities. The published literature reveals that research has failed to pay adequate attention to the effect of temperature on the re-structuring of the microstructure of smectite, specifically after the re-hydration process. Therefore, this paper is concerned with the impact of temperature on the re-structuring of microstructure and its consequences on smectite geo-environmental performance. The effect of temperature changes on the re-structuring of interlayer-distance, micropores, macropores and the sorption forces associated with the development of the micro- and macrostructures of smectite was examined. For this purpose, the variations of position and intensity of XRD major basal spacing of smectite have been studied over a range of different temperatures and re-hydration process. In addition, a series of buffering capacity tests, and permeability tests were performed to assess the impact of heat treatment on some of the geo-environmental properties of smectite. This paper illustrates different aspects of the influence of high temperature (up to 200 °C) on the performance of smectite in respect to the temperature effects on the re-structuring of the microstructures constituting the bulk smectite clay, as evidenced by the XRD response and permeability performance. The application of XRD analyses on smectite and homoionized Ca-smectite has shown that heating the smectite sample up to 200 °C will expel the water from its basic unit layers. This causes a reduction in the basal spacing of smectite. This reduced basal spacing might be mistaken for illite''s major basal spacing. Such a decrease in the position of major basal spacing of smectite after heat treatment is attributed to the reduction in distance between its basic unit layers. It is shown that this change in the microstructure of smectite has an irreversible impact on its cation exchange capacity and permeability. However, within the temperature range and conditions studied, the geo-environmental properties of smectite remain favorable. [Copyright &y& Elsevier]

Abstract: The sorption characteristics of soils, which are fundamental to contaminant attenuation, depend on many soil-related and contaminant-related factors. Chief among the soil-related factors is soil composition. This study investigates smectite–calcite-contaminant and kaolinite–calcite-contaminant interactions for different heavy metals (HM) concentrations in order to address the impact of carbonate and heavy metals on micro-structural variations of clayey soils. To achieve the stated objectives, several batch equilibrium, consolidation and XRD (X-ray diffraction) analyses were performed. These experiments were conducted on a bentonite (smectite) sample, a kaolinite sample and several mixtures of kaolinite and carbonate. Furthermore, this study investigates the effect of HM cations in a smectite clay, with and without the benefit of sodium carbonate and sodium nitrate, with respect to the mixture''s consolidation behaviour and transmission characteristics. Characterization experiments conducted in the study included determination of changes in the buffering capacity and microstructure characteristics of the clay (smectite and kaolinite) relative to the presence of varying concentrations and types of pore fluids and physical experiments focused on the consolidation behaviour of the smectite samples. The role of buffering capacity and macrostructure and microstructure changes on the transient (evolutionary) characteristics and properties of the clays can be seen from the results obtained. Double-layer swelling forces and buffering capacity are seen to be dominant factors in the performance of the clays — apparently through their influence on formation and restructuring of microstructures in the overall clay–carbonate mixtures. Exposure of the buffered smectite to Cu2+ ions in the midst of consolidation results in the added feature of chemico-osmotic consolidation — depending on the concentration of Cu2+ ions available, thus raising the question of chemical path dependency or chemical history on consolidation performance. Furthermore, the process of interaction of the kaolinite–carbonate and HM ions was monitored through XRD analyses of the soil samples at various stages of the tests. The results reported in this paper show that the consequence of a tighter aggregation of clay particles after soil–contaminant interaction causes the development or restructuring of microstructures into tighter micro-structural units. This produces a macrostructure with more macro-pores than the original natural clay sample which causes a reduction in XRD peak intensity of laboratory contaminated soil samples. It can be seen that once the quantity of laboratory added carbonate is increased, the addition of HM to soil sample has a minimal impact on the micro-structural change of clayey soils. [Copyright &y& Elsevier]

Abstract: While the feasibility of using electrokinetics to decontaminate soils has been studied by several authors, the effects of soil composition on the efficiency of this method of decontamination has yet to be fully studied. This study focuses its attention on the effect of “calcite or carbonate” (CaCO3) on removal efficiency in electrokinetic soil remediation. Bench scale experiments were conducted on two soils: kaolinite and natural-soil of a landfill in Hamedan, Iran. Prescribed quantities of carbonates were mixed with these soils which were subsequently contaminated with zinc nitrate. After that, electrokinetic experiments were conducted to determine the efficiency of electrokinetic remediation. The results showed that an increase in the quantity of carbonate caused a noticeable increase on the contaminant retention of soil and on the resistance of soil to the contaminant removal by electrokinetic method. Because the presence of carbonates in the soil increases its buffering capacity, acidification is reduced, resulting in a decrease in the rate of heavy metal removed from the contaminant soil. This conclusion was validated by the evaluation of efficiency of electrokinetic method on a soil sample from the liner of a waste disposal site, with 28% carbonates. [Copyright &y& Elsevier]