Thermal damage and mechanical properties of high temperature sandstone with cyclic heating-cooling treatment

In the coal fire control project, the multiple water injections used to extinguish coal fires and the coal fires that reignite can cause many forms of ground collapse to occur, threatening project safety. Therefore, studying the thermal damage and the mechanical strength of sandstone, a main weight bearer in coal measure strata, exposed to heating-cooling cycles, is crucial. In this study, the sandstones were first heated to 800 degrees C and cooled using water. The sandstones were heated to a setting temperature (100 degrees C, 200 degrees C, 300 degrees C, 400 degrees C, 500 degrees C, 600 degrees C, 700 degrees C, and 800 degrees C) and cooled using water again. The ultrasonic p-wave velocity, density, and water absorption of the sandstones were tested. Uniaxial compression tests and shear tests were conducted on the sandstones. The thermal damage in sandstones was analysed using rock energy theory. The results show that, compared with the mechanical strength of the sandstone at room temperature, the mechanical strength of the sandstone exposed to 800 degrees C for the first time is decreased by less than 20%. When the sandstone is exposed to high temperature again, the critical threshold temperature is 400 degrees C. When the temperature exceeds 400 degrees C, the thermal damage aggravated again and the mechanical strength rapidly degraded.