Effects of temperature on physical and mechanical properties of highly expansive mudstone

Highly expansive mudstone (HEM), which forms the roofs of several open-pit coal mines in China, is known to soften in the presence of water because of their weak lithological characteristics, resulting in overall poor stability, which can induce landslide disasters and pose a severe threat to the safe mining operations. Therefore, to study the physical and mechanical properties of calcined HEM samples, remolding samples were calcined at different temperatures, and phase composition, shrinkage ratio, acoustic velocity, uniaxial compressive strength, and micromorphology of samples were analyzed. Additionally, the uniaxial compressive failure procedures of calcined samples were tested using the deformation field inspection system. The findings reveal that kaolinite in the samples gradually transformed into metakaolinite in the temperature range of 400–600 °C, which in turn transformed into mullite at 1000 °C. The shrinkage rate increased slowly at first, with increasing temperatures of up to 800 °C, and then increased sharply at 1000 °C. The acoustic velocity and uniaxial compressive strength of samples first decreased slowly and then increased rapidly with increasing temperature. At 400 °C, both of them reached the lowest values. With increasing stress, small deformation regions of samples calcined at 1000 °C merged to form large localized deformation regions, which then evolved into strain localization bands, ending with macro-crack formation at peak stress. The crack increased rapidly when the stress exceeded the peak stress. These conclusions can provide experimental data references and valuable information for treating and evaluating open-pit mine slopes containing different calcined HEMs.