# Damage Characteristics of Thermally Deteriorated Carbonate Rocks: A Review
## 2 Thermal Dilatancy and Alteration in Rock Fabric
### Thermal Cracking and Microstructural Variations  
Carbonate rocks exposed to a temperature window of 500 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C considerably experience an alteration in their mineral composition, mineral strength, physical structures, textural characteristics, and grain cementations [57]. Under the thermal environment, chemical processes, such as hydration or dehydration, red-ox reactions, deionization, mineral phase transformation, dissolution, and disappearance of bonding agents, alter the behavior of rocks to a great extent [58]. Rock-water interaction in geothermal systems, especially along the fault zones, recrystallizes the minerals through geochemical processes [59]. Each rock-forming mineral has a specific value of the coefficient of thermal expansion. Mineral elongation under thermal stresses increases the particle contact surface area and  
Figure 1: Percentage increase of thermal elongation in the bond length of Ca-O and Mg-O adapted from Reeder and Markgraf [52]. causes microstructural changes in the rock matrix that adversely affect the mechanical, physical, and dynamic properties of rocks [60]. The phenomenon of thermal damage to rocks can be ascertained by observing the microscopic changes in rocks, such as mineral expansion under thermal stresses, grain boundary conditions, the density of intergranular and intragranular cracks, amount of the induced thermal strain, and mineral resistance to destruction [61]. Temperature levels and time of exposure are both factors that play an important role in the thermal degradation of different rocks. For example, in the case of crystalline rocks at a low temperature, no significant changes are observed in their internal structure because of their resistant mineral composition. At a moderate level of temperature (300-500 \({}^{\circ}\)C), minerals start to expand and make closures at grain boundaries that lead to a reduction in the void spaces. At a high level of temperature (>500 \({}^{\circ}\)C), thermal stresses in the rocks exceed the threshold limit of minerals' coefficient of thermal expansion, which causes mineral damage and relaxation at their contact boundaries. Researchers observed that mineral expansion in crystalline rocks occurs at a temperature ranging from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, and, beyond this temperature, rocks start to deform plastically [62; 63]. On the other hand, in the case of carbonate rocks, at a low level of heating (<300 \({}^{\circ}\)C), the relaxation phenomenon starts at the grain contact boundaries because their minerals and bonding agents are less resistant to thermal damage. On heating limestones at temperatures > 600 \({}^{\circ}\)C, the emission of carbon dioxide with the decomposition of calcium carbonates weakens the limestone [57]. Advanced techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), computerized tomography scanning (CT), and scanning electron microscopy (SEM) have been used widely in high-temperature rock mechanics to investigate microscopic variations in rocks that give an idea to understand macroscopic changes [64; 65; 66; 67]. Furthermore, a summary of the major findings regarding the microscopic variations in thermally damaged carbonate rocks is described in Table 2.