Effect of temperature on the re-structuring of the microstructure and geo-environmental behaviour of smectite

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 restructuring 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 degrees 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. (C) 2008 Elsevier B.V. All rights reserved.