Influence of Mineralogical and Micro-Structural Changes on the Physical and Strength Properties of Post-thermal-Treatment Clayey Rocks

This study investigates the effects of thermal treatment and exposure time on the physico-mechanical properties related to the mineralogical and structural changes of clayey rocks at temperatures of up to 1000 °C. Vitric-crystal tuffs were studied because they show different engineering behavior than most rocks at high temperatures. The samples were heated at the rate of 10 °C/min, exposed to the desired temperatures for 2 h, and cooled at room temperature. Then a series of geo-mechanical tests was performed, thin sections and SEM images were prepared, and XRD and DTA analyses were carried out. Depending on the increase in the high temperature, the strength increased by 50% in the samples exposed to 400 °C, and a higher strength value was recorded than the initial strength even at 600 °C. Thin sections and SEM images showed that while the primary void ratio decreased until 400 °C because of the melting and expansion of clay-sized particles, new micro-cracks formed at the boundaries of quartz and plagioclase. Although the secondary micro-crack ratio increased, the sintering, cementation, and fusing between clay minerals were the main reasons for the increase in strength until 600 °C. After this temperature, the strength suddenly decreased due to the transformation of minerals, especially the formation of muscovite, and the development of secondary glasses. These changes were observed in DTA peaks and XRD analysis results. The unit weight decreased until 600 °C due to dehydration of absorbed water and hydroxylation reactions of the clay minerals and then remained constant. Despite the decrease in strength, the filling of primary voids by molten clay minerals and the formation of high-density minerals like clinoptilolite and muscovite prevented decreases in unit weight after 600 °C. In addition, the investigation of the exposure time showed that 90% of the strength loss occurs within the first 20 min at the threshold temperature, after which time does not affect the change in strength.