Effects of elevated temperature on mineralogical/microstructural changes and mechanical responses of clay soil

High temperatures can affect soil performance and alter its engineering behavior. In this study, a series of experiments were carried out on various soil samples provided under different heating conditions (i.e., heated sample and heated clay) to evaluate the various temperatures effects (elevated from 20 to 900 °C) on physical, chemical, mechanical, and mineralogical/microstructural characteristics of clayey samples. The main objective of this study is to investigate the influence of temperature and heating conditions on permeability, unconfined compressive strength, shear wave velocity, weight, color, pH, EC, XRD, and FESEM. The results indicated that dolomite can be broken down into magnesium oxide and calcite, and alpha quartz can be broken down into beta quartz. Calcite minerals decompose into calcium oxide at 900 °C, beta-quartz into alpha-cristobalite, and kaolinite into mullite and gehlenite. The decomposition of water from the hydroxyl structures of minerals in burnt soil is carbonated on the soil's surface up to a temperature of 700 °C; thus, the compressive strength of clay in the temperature range of 500–700 °C is approximately 97% lower than its maximum. At 900 °C, the formation of mullite, gehlenite, and various oxide structures, such as calcium and magnesium oxide, changes the structure of clay to a glassy and amorphous structure. This improves the strength of clay from 0.17 to 3.7 MPa; however, the permeability increases significantly due to the unstable glass structure. The shear wave velocity increased by approximately 2.74 times at 200 °C. It decreased from 300 to 700 °C, then increased to 416.7 m/s at 900 °C.