Grain size heterogeneity controls strengthening to weakening of granite over high-temperature treatment

Abstract Deep geological repository is usually constructed in a typical crystalline rock , granite . Therefore, comprehensive understanding on the thermo-mechanical properties of granite exposed to high temperature is crucial to repository design and risk assessment. Granite experiences a transition from strengthening to weakening as temperature increases; however, the critical temperature at which this transition occurs is still poorly known. Here we examined the physical and mechanical variations of granite specimens of three different grain grades. The specimens sourced from the same origin are heated up to 800 °C. Grain size distribution, thermal-induced cracks, and crack evolution under uniaxial compression of the heated specimens were examined by XRF (X-ray fluorescence spectrometric analysis, with a scanning capacity of 50 mm at a spatial resolution of 20 μm), SEM (Scanning electron microscope) and AE (Acoustic emission), respectively. We found that grain size heterogeneity quantified by the coefficient of variation (CV) modulates the critical temperature magnitude, and the granite specimen with a higher CV exhibits a lower critical temperature. The cracks reducing the granite strength are induced by grain mismatch due to inconsistent thermal expansion. Nonuniform distribution of grain size enhances the mismatch and broadens the favorable path for crack propagation and coalescence . Thermally-induced microcracks appear earlier in the more heterogeneous granite, which propagate and coalesce to form visible cracks earlier, leading to an earlier strength degradation. We also proposed an empirical formulation to predict the strength of granite with different grain size distribution and thermal environment . Our work is crucial for the site location and design of the deep geological repository since establishment of many construction parameters heavily depends on laboratory-determined properties of granite.