Evolution of physico-mechanical degradation and microscale damage in high-temperature granite subjected to water quenching

• Degradation patterns of granite under 25–1200 °C water quenching are revealed. • Critical threshold of 600 °C for thermal damage transition in granite is identified. • Molten phase transition of granite starting from 1000 °C is observed. • Microscopic damage evolution mechanisms are elucidated by multiscale methods. This study investigates the stability degradation of thermally damaged granite subjected to water quenching, a critical challenge in deep geoengineering applications. By systematically comparing the mechanical responses of granite exposed to temperatures ranging from 25 °C to 1200 °C under natural cooling and water quenching regimes, the synergistic effects of thermo-hydraulic coupling on rock degradation were examined. A suite of multiscale characterization techniques, including acoustic emission (AE) monitoring, X-ray diffraction (XRD), and Polarizing Microscope, was employed to analyze the impact of water quenching on the mechanical performance and mineralogical characteristics of granite. The major findings are as follows:(1) Water quenching significantly intensifies the deterioration of thermally damaged granite. A critical transition in degradation mechanism occurs at 600 °C, where uniaxial compressive strength (UCS) and elastic modulus of water quenching samples sharply decrease by 49.5% and 47.1%, respectively—approximately 200 °C earlier than the natural cooling group in reaching comparable levels of degradation.(2) At temperatures exceeding 1000 °C, granite enters a melting-induced phase transition. The UCS of water quenching samples stabilizes within the range of 17.08–19.51 MPa, accompanied by anomalous rebounds in both P-wave velocity and elastic modulus.(3) AE monitoring reveals that in the 400–800 °C range, the energy attenuation rate of water quenching samples accelerates by 23.55%. Additionally, a delayed peak in ringing counts indicates that water quenching significantly prolongs the plastic deformation stage of granite.(4) Microscale damage mechanisms are dominated by accelerated crack network propagation, biotite decomposition, and the induced transformation of microcline feldspar from monoclinic to triclinic structures—collectively exacerbating macroscopic mechanical degradation. Beyond 1000 °C, mineralogical reconstitution becomes the dominant factor controlling macroscopic mechanical behavior. These insights provide essential theoretical guidance for the safe design and construction of deep rock engineering projects.