Mechanical Responses and Multiscale Thermal-Damage Evolution of Sandstones Subjected to Elevated Temperatures: Implications for High-Temperature Rock Engineering

High-temperature-induced thermal damage in rocks critically affects the long-term stability of surrounding rock masses. Understanding the mechanical responses and damage evolution mechanisms of rocks under elevated temperatures is essential for safety assessment and risk mitigation in deep high-temperature rock engineering. In this study, Red sandstone, Yellow sandstone, and White sandstone were subjected to uniaxial compression tests after thermal treatment at different temperatures, combined with X-ray diffraction (XRD), three-dimensional computed tomography (CT), and scanning electron microscopy (SEM) analyses. The effects of temperature on mineral composition, P-wave velocity, uniaxial compressive strength, elastic modulus, failure strain, and porosity were systematically evaluated. Furthermore, the multi-scale thermal damage evolution mechanisms of sandstone were revealed. The results show that increasing temperature induces a transition of sandstone failure behavior from brittle to ductile, with compressive strength exhibiting a two-stage evolution of exponential growth followed by linear decline. Red sandstone presents a strength transition near 750 °C, with a 34.7% increase compared with room temperature, while Yellow and White sandstones reach peak strengths at approximately 500 °C, increasing by 36.1% and 56.2%, respectively. At the microscale, temperature elevation promotes the evolution of pores and cracks from small-scale and discrete features to larger-scale interconnected networks. Both porosity and fractal dimension increase continuously with temperature, and the temperature range of rapid porosity growth corresponds well with the strength transition. Microscopically, thermally induced cracks show temperature-dependent evolution: intergranular cracking dominates at low and intermediate temperatures, whereas combined intergranular and transgranular cracking develops at high temperatures, eventually forming interconnected networks. These findings provide theoretical insights and engineering references for thermal damage assessment, surrounding rock stability evaluation, and disaster early warning in deep high-temperature rock engineering.