Study on microstructural evolution and brittleness parameters of thermally treated granite

Revealing the evolution of thermal damage in granite under high-temperature conditions is of great significance for accurately assessing rock stability in high-temperature underground engineering, such as geothermal development, underground coal gasification, nuclear waste disposal, and tunnel fires. In this study, Jiali granite was thermally treated at temperatures ranging from 25°C to 700°C. Variations in mineral composition, pore structure, mechanical properties, and brittleness parameters with temperature were investigated through mechanical tests and microstructural analyses. The results indicated that the thermal damage process in granite exhibited staged changes. Below 500°C, thermal treatment primarily promoted the formation of pores and intergranular cracks. The most significant transformation in pore structure occurred in the temperature range of 500-600°C, during which the NMR T 2 spectrum area increased by 74%, the proportion of macropores rose from 6% to 30%, and microscopic images indicated that the pore structure transitioned from isolated pores to a crack network. These microstructural changes led to a rapid decrease in elastic modulus and tensile strength, whereas compressive strength decreased more slowly because crack closure and frictional resistance partly maintained the load-bearing capacity under compression. The asynchronous decline in tensile and compressive strengths resulted in an anomalous increase in brittleness parameters based on strength ratios. In contrast, brittleness evaluation methods based on strain energy and stress-strain curves more accurately reflected the tendency toward brittle failure.