Multiscale mineral-structural-mechanical analyses of red-bed sandstone subjected to freeze-thaw cycles

Prolonged freeze-thaw (F-T) cycling is a primary climatic driver of rock mass degradation in high-altitude cold regions. The Cenozoic red-bed sandstone of the Jungong Basin, on the northeastern Qinghai-Tibet Plateau, exhibits high susceptibility to F-T damage owing to its loose texture and weak cementation. However, the multiscale degradation mechanisms, particularly the coupling among mineralogical alteration, microstructural evolution, and mechanical weakening, remain poorly understood. This study systematically investigates these mineral-structural-mechanical linkages under progressive F-T cycling by integrating X-ray diffraction (XRD), scanning electron microscopy (SEM), computed tomography (CT), and uniaxial compression testing. XRD analysis showed continuous decreases in feldspar (from 31.6% to 23.6%) and calcite (from 12.7% to 7.2%), accompanied by a marked increase in illite (from 2.3% to 15%), indicating synergistic chemical alteration and structural loosening. SEM and CT revealed exponential increases in porosity and fracture density, characterized by rapid early-stage growth (within the first 10 cycles) followed by gradual saturation. Three-dimensional reconstructions elucidated the development of a connected pore-fracture network propagating from the periphery toward the interior. Mechanical testing indicated progressive reductions in uniaxial compressive strength and elastic modulus, while the energy dissipation ratio increased, reflecting a transition from elastic-storage dominance to dissipation dominance. Based on these findings, we propose a coupled physicochemical-mechanical framework in which mineral destabilization drives microstructural reorganization, energy redistribution, and ultimately strength attenuation. This study elucidates a multiscale mechanism for frost-induced rock damage and provides a theoretical basis for geohazard risk assessment and mitigation in high-altitude cold regions.