عنوان مقاله [English]
Clayey soils may be exposed to different heat gradients which cause permanent or semi-reversible variations in their engineering behaviour. The use of clayey soil in construction materials and the application of bentonite as barrier material in atomic waste disposal are among cases in which clayey soils are subjected to heat gradient from low to high ranges. The review of prior research indicates that previous studies have mainly focused on the mechanical and macro-structural behaviour of soil. In other words, there is little research emphasizing the soil microstructure in this process. This research is aimed to investigate the characteristic changes of kaolinite and bentonite due to heat treatment from micro-structural aspects. To achieve this objective, the soil samples were exposed to different heat regimes; from 25 to 1100 centigrade. After measuring the variations generated in the unconfined compression strength of the heated samples, by a series of XRD and SEM experiments, the micro-structural changes in the soil samples were examined and their impact upon the macro-structural behaviour was addressed. The results indicate that the interpretation of variations in the strength of the heated samples can be evaluated by monitoring the soil microstructure, such as de-hydroxylation, and the formation of new minerals after heat treatment. According to the achieved results, the extent of the influence of heat treatment upon soil behaviour is a function of temperature level and clay mineral type. Furthermore, by increasing the temperature up to de-hydroxylation level, an increase in soil strength is observed. This is more noticeable for bentonite, due to the presence of active clay as its main clay fraction. At the de-hydroxylation temperature, by the destruction of the clay fraction, a reduction in XRD peak intensity occurs, which is followed by a large increase in soil strength. Further increase in temperature causes a continued increase in kaolinite strength and a reduction in bentonite unconfined compression resistance. This behaviour is attributed to the type of new mineral that is formed at different levels of temperature.