Thermo-mechanical responses of basalt and sandstone subjected to uniaxial compression after heating–liquid nitrogen cooling

This study investigates the strength and deformation characteristics of Rajahmundry basalt and Raniganj sandstone subjected to heating up to 800 °C, followed by air cooling (A-C) and liquid nitrogen cooling (N-C), under uniaxial compression. To validate the variations in physico-mechanical properties induced by thermal treatments, an integrated approach combining X-ray diffraction (XRD), thin-section analysis under scanning electron microscope (SEM) and petrographic microscope, and micro-computed tomography (CT) imaging was employed to characterize the microstructural changes. Results indicate that basalt undergoes a clear reduction in uniaxial compressive strength (UCS) and Young’s modulus above 500 °C, whereas sandstone undergoes a comparable decline beyond 400 °C. Stress–strain responses reveal brittle to ductile transition at these threshold temperatures, accompanied by increased peak strain, higher Poisson’s ratio, and a shift in failure mode from shear to tensile. Crack initiation and damage stresses decrease continuously up to 800 °C, indicating earlier crack onset with thermal treatment, while dilatancy appears at lower stresses, especially under LN 2 cooling. Microstructural analysis reveals that basalt loses its strength through microcrack development, whereas sandstone weakening is driven mainly by mineralogical alterations. Correlation analysis further shows that basalt strength is best predicted by ultrasonic velocities and thermal damage, while sandstone damage is more accurately reflected by temperature-driven porosity variations. LN 2 cooling resulted in more extensive structural damage than air cooling due to larger thermal gradients, leading to higher microcrack density confirmed by microstructural analyses. These findings provide an updated perspective on how engineering structures in basalt and sandstone are affected by thermal shocks in high-temperature geological environments.