# Damage Characteristics of Thermally Deteriorated Carbonate Rocks: A Review
## 5 Thermal Deterioration of Porous Network
#### Heat Transport Properties
materials subjected to thermal stresses can be ascertained in terms of their thermal conductivity (k), thermal diffusivity (D), coefficient of thermal expansion, heat transfer capacity (C\({}_{\text{p}}\)), and thermal damage factor [120]. The variations in thermal properties of rocks at ambient temperature or beyond it have been studied by several past researchers [121; 122; 123; 124; 125; 126]. However, the imperfection of the physical contact methods often underestimated or overestimated the thermal properties of various rocks and minerals. For example, [PERSON] [127] studied the effect of temperature on the coefficient of thermal expansion of granite and limestone at a temperature range of 20 to 100 \({}^{\circ}\)C. He found that both rocks were expanded at almost the same rate. Later on, limestone's dilation was observed much lower than the granite [128]. The application of non-physical contact methods, such as laser flash analysis (LFA), in measuring the thermal transport properties of rocks has been reported by many researchers [129; 130; 131; 10]. They highlighted some problems of using the conventional approaches and found that laser flash analysis has a competitive edge over other physical contact methods. They proposed LFA as the best technique to accurately measure the thermal properties of rocks. Thermal diffusivity is the ratio of thermal conductivity to the product of density and heat capacity. This property is the function of temperature, and its value decreases as the temperature increases. Thermal diffusivity has a direct relationship with thermal conductivity and an inverse relationship with density and heat capacity. At a high temperature (>500 \({}^{\circ}\)C), due to thermal cracking and microstructural variations in rocks, thermal conductivity starts to decline, which ultimately reduces thermal diffusivity. Conversely, in the case of density and heat capacity, the increasing temperature slightly affects the density at the macroscopic level as a result of the low dilation of minerals [133]. Therefore, small variations in density do not significantly affect thermal diffusivity. Because of intercrystalline and intergranular thermal cracking, the rock begins to absorb more thermal energy, which is why heat capacity increases and thermal diffusivity depretiates with the temperature. When an intact and continuous rock specimen is heated, the initiated thermal cracking event improves its porosity and, thus, the material shows a slightly larger volume than that of an uncracked state. On reheating a thermally cracked rock specimen, expected dilation does not happen and the material becomes comparatively stiffer because of mineral expansion in the cracked spaces [124]. Unlike fluid transport properties for which connected pores are required, the thermal conductivity of limestone is mainly governed by the mineral composition, porosity, and grain contact surface area. High temperature deteriorates rock fabric, which leads to a reduction in thermal conductivity and thermal diffusivity. [PERSON] et al. [10] discussed the temperature-dependent thermal transport properties of carbonate rocks. At a temperature range of 27 to 327 \({}^{\circ}\)C, calcite-rich limestone showed lower thermal diffusivity than the thermal diffusivity of its primary mineral (calcite). Furthermore, they found that thermal conductivity dropped to 50% at 327 \({}^{\circ}\)C. [PERSON] et al. [134] studied the thermophysical characteristics of limestones and sandstones at a temperature of \(-\)30 to 1000 \({}^{\circ}\)C. In the case of limestone, they observed that, at stage 1 (\(-\)30-20 \({}^{\circ}\)C), the thermal properties were slightly improved by 8% due to freezing of pore fluids, at stage 2 (20-600 \({}^{\circ}\)C), an increasing temperature up to the 600 \({}^{\circ}\)C induced thermal tension cracks within the rock matrix that linearly reduced the thermal properties, and, at stage 3 (600-1000 \({}^{\circ}\)C), the ductile behavior of the rock sharply decreased the thermal properties.