Effect of thermal treatment and moisture content on the mechanical properties of soft sandstone under triaxial extension: an experimental study

The problem of deformation damage of soft rocks and the effect of temperature and moisture content on the mechanical properties of rocks has received wide attention in recent decades. To investigate the effect of thermal treatment and moisture content on the triaxial extension mechanical properties and fracture characteristics of soft rock, triaxial extension tests were conducted on sandstone specimens with good homogeneity at different heating temperatures (T = 25, 100, 200, 300, 400, 500, 800 °C) and moisture contents (Kt = 0, 20, 50, 80, 100%) with the help of MTS815 testing machine. The results showed that: For rock specimens with Kt = 0, the mass of the specimens decreased monotonically with the increase of temperature, the elastic modulus (E) showed an overall decreasing trend, and the axial ultimate strain (εamax) was Ω-like pattern. The P-wave velocity (Vp) and axial stress showed a trend of increasing first and then decreasing around T = 200 °C. All the specimens fractured instantaneously, and the SEM images showed some grain expansion before T ≤ 200 °C. At the same time, microcracks expand and become more intense after T > 200 °C. In addition, the multiple mineral compositions in the specimens showed more complex changes after different thermal treatments, which was another main reason for the deterioration of the specimens. Furthermore, the E decreases while εamax increases with the moisture content, and before T = 500 °C and Kt = 20%, the Kt significantly influences the triaxial extension strength; after that, however, the effect is minimal.