A large number of laboratory experiments about the influence of heating or heating-cooling cycles on the mechanical properties of various granites are reviewed. Both scanning electron microscopy (SEM) and particle-based discrete element modeling (DEM) are employed to quantitatively elucidate the mechanisms responsible for temperature-dependent mechanical properties of granites, from a perspective of microcracking. Both SEM observations and DEM simulations give consistent results and show that there exists a temperature threshold beyond which the thermally-induced microcracks increase drastically. Both intergranular and intragranular microcracks are observed in the granites after thermal treatment, and intergranular ones are dominant. A continuous increase in temperature can generally weaken granites, mainly by inducing significant thermal stress and generating tensile microcracks. The weakening of granites after a heating-cooling cycle is due only to the thermally induced microcracks. With increasing grain size the magnitude of Brazilian tensile strength reduction of granites due to thermal treatments becomes small, whereas with increasing heterogeneity in grain size distribution, the magnitude of Brazilian tensile strength reduction of granites due to thermal treatments becomes great. This is because the two competing mechanisms, i.e., the length and number of the thermally induced microcracks in granites.
Since the first enhanced geothermal system (EGS) was conceived at the Fenton Hill project, the United States, in the 1970s, EGS projects have been pursued around the world (McClure and Horne, 2014). EGS projects involve finding vast blocks with high temperature (> ~200 °C) and connected fracture networks. Working fluid (e.g., water or supercritical CO2) is first injected and circulated through the fracture networks in geothermal reservoirs and eventually pumped back to the surface as steam. In the world EGS projects are commonly located in granite rocks with various mineralogical properties (Zhao et al. 2018). The mechanical response of "hot granites" to cooling becomes an important question to geologists and engineers.