Experimental study on the effect of thermal treatment on the mechanical properties of clay-rich rocks (mudstone)

Temperature is one of the most important environmental factors that affect rock engineering properties. The clay-rich rocks such as mudstone are abundant in the earth's crust and have a high affinity to water. So the behavior of these rocks differs from others after thermal treatment, which might force the water out of the clay. Therefore, we investigated the effect of thermal treatment on the mechanical properties of clay-rich rock, mudstone, under three temperatures; ambient temperature, 250, and 500 degrees C. Our results showed that thermal cracks begin to appear in the mudstone at a temperature between 500 and 600 degrees C. Several mechanical properties were measured in the lab; uniaxial compressive strength (UCS), elastic modulus (E), Poisson's ratio (v), Brazilian tensile strength (BTS), fracture toughness of tensile (KIC), and shear (KIIC) cracks. Due to the drying of clay minerals through the dehydration and dihydroxylation processes, the following mechanical properties of mudstone significantly increased after treating the rock with a temperature of 500 degrees C; UCS, E, BTS, KIC, and KIIC have improved by 91%, 39%, 16%, 41%, and 94%, respectively, while v decreased by 80%. This reduction of v, due to increasing the thermal treatment temperature, caused the behavior of mudstone to transform gradually from ductile to brittle material which means that the failure of such rocks, at this temperature, might be a sudden one. This research presents new insights into the effect of temperatures below 500 degrees C on clay-rich rocks for use in different engineering fields.