Response of thermally treated granites are analyzed by several authors for processes like geothermal energy extraction, underground disposal nuclear waste, energy storage and structures exposed to fires. Although the conducted research helps to increase our understanding on the granites, quantification of thermal rock damage requires further studies which consequently alter the physico-mechanical response. So far, thermal damage is determined as a function of elastic modulus. However, an accurate determination of elastic modulus requires specialized laboratory facilities. When the treated granites (600°C) are allowed to cool in ambient temperature, the density decreases by 7.7%, 2.2% and 3.67% for coarse, medium and fine-grained granite, respectively. This reduction is primarily caused by the volumetric expansion of the mineral grains as seen by internal micro-cracking. In this study, the authors have quantified damage as a function of rock properties other than elastic modulus to appreciate the influence of temperature on varying grain-size.
Granite, a common rock type found in the Earth's crust, is a mixture of various rock forming minerals and can contain heat-producing radioactive isotopes (K, Th, U) that can impart temperature anomalies within the crust and elevated geothermal gradients (Shao, S.S. et al 2013). Granites are relatively stronger and possess large load bearing capacity depending on their mineralogical, morphological and weathering conditions. These attributes make granites suitable for processes such as enhanced geothermal systems (EGS), underground research laboratories (URL), nuclear waste disposal in deep geological repositories (DGR), underground energy storage and to be used as constructional materials. Since these processes generally exhibit an interaction of the strata with high temperatures, a detailed understanding of the thermal response of granite is critical in addressing the technical challenges encountered in the successful application and deployment of such processes. However, differences in mineralogical composition, grain size distribution, degree of weathering, and microstructural properties result in varied physical and mechanical response of rocks under ambient and high temperature conditions (Sirdesai et al., 2018a). Depending on the nature and conditions of thermal interaction, certain combinations of mineralogical and morphological features can result in accelerated microcracking in form of inter, intra or trans-granular microcracks, and physico-chemical degradation and deterioration of minerals (Sirdesai et al., 2018b). Since granites in nature occur in various grain sizes, it is crucial to understand the impact of size distribution and the behavior of grain boundaries on the thermal response of granites found in the aforementioned process. Consequently, this study aims to explore the influence of grain size on thermal damage in granites. Further, effect of cooling rate has been analyzed to evaluate the variation in induced thermal stresses. Thermal damage (DT) has been computed using conventional, modulus-based relation, and a comparative analysis has been performed against novel damage indices that have been developed using other physico-mechanical properties of granites.
