# Damage Characteristics of Thermally Deteriorated Carbonate Rocks: A Review
## 6 Conclusions  
This paper investigates the effect of thermal damage on the behavior of carbonate rocks in terms of their mineralogical, mechanical, dynamic, physical, and thermal properties. It covers the literature review of more than 50 years of research work to highlight the advancements in high-temperature rock mechanics to ascertain the behavior of carbonate rocks under the thermal environment. The full understanding of the engineering characteristics of rocks under varying temperature and loading conditions depends upon the theoretical framework, experimental approaches, reliable instrumentation, constitutive modeling, and numerical simulations. Temperature significantly affects the mineralogical properties of porous rocks. It alters their mineral composition, mineral strength, physical structures, textural characteristics, and bonding agents by initiating some processes, such as hydration or dehydration, redox reactions, deionization, mineral phase transformation, melting, microcracking, and deterioration of bonding agents. Unlike crystalline rocks, carbonate rocks degrade at comparatively low temperatures. At a temperature window of 400 to 500 \({}^{\circ}\)C, their primary and secondary minerals decompose, whereas clay minerals and other materials start to decay beyond the critical temperature. A high temperature (>600 \({}^{\circ}\)C) breaks the calcium carbonate, calcium bicarbonate, and magnesium carbonate; thus, emission of carbon dioxide weakens the limestone and dolostone. Under static loading conditions, the stress-strain curves of thermally deteriorated limestone describe its deformation behavior in five stages, including (1) compaction phase, (2) elastic deformation phase, (3) microcracking phase, (4) peak strength phase, and (5) post-failure phase. An increasing temperature considerably decreases the peak compressive strength and static elastic modulus. Carbonate rocks show brittle behavior above a tem  
Figure 9: Effect of temperature on thermal conductivity and thermal diffusivity of carbonate rocks tested at various temperatures. TC and TD represent the thermal conductivity and thermal diffusivity, respectively. perature of 400 \({}^{\circ}\)C. The brittle-ductile transformation phase begins at a temperature range of 400 to 500 \({}^{\circ}\)C, and, beyond this critical temperature, their ductile behavior becomes dominant because of the slippage-twining effect in the crystal lattice. High-temperature rock dynamics provides a solid base to discern the dynamic behavior of carbonate rocks subjected to varying temperatures. The variations in their dynamic properties at the macroscopic level are referred to as their fracture characteristics, including fracture initiation, fracture propagation, fracture density, fracture velocity, and energy absorption. Recent and past studies show that an increasing impact pressure and strain rate ameliorate dynamic compressive strength and elastic modulus; however, the development of thermal tension cracks at elevated temperatures appreciably reduces these dynamic properties. The variations in rocks' transport properties are mainly governed by texture (size, shape, and sorting), cementation between the particles, the orientation of particles, and natural cracks under regional or local stresses. Carbonate rocks exposed to high temperatures experience an expressive alteration in their physical response on account of the reduction in grain contact surface area, disappearance of cementation, micro-fissuring, internal defects, and intercrystalline cracking. At a temperature above 500 \({}^{\circ}\)C, the porosity and permeability of carbonate rocks improve sharply, whereas P-wave velocities decline nonlinearly. Unlike fluid transport properties for which connectivity of pores is mandatory, their thermal properties depend upon the mineral composition and grain contact surface area. Thermal transport properties (thermal conductivity and thermal diffusivity) are sensitive to temperature.