# Damage Characteristics of Thermally Deteriorated Carbonate Rocks: A Review
## 2 Thermal Dilatancy and Alteration in Rock Fabric
### Thermal Decomposition of Calcite and Dolomite  
The investigation of thermal decomposition of the primary carbonate minerals is of great interest. It develops a solid background rationale to anticipate the possible reason behind microstructural variations in carbonate rocks under a thermal environment. The reaction kinetics explains the decomposition of carbonate minerals into their respective constituents at elevated temperatures. The rate of reaction is controlled by some important factors, such as heat transfer rate, mass transfer rate, or their combination [23]. It is evident from past studies that particle size, crystal structure, the order of atoms in a unit cell, possible impurities, and crystal habit substantially affect the thermal transport mechanism and mineral dilatancy [23; 24]. Considerable variations are found in the kinetic parameters of thermally decomposed carbonate minerals. For example, a great discrepancy is reported in the literature regarding the activation energy values of calcite (155-222 kJ mol\({}^{-1}\)) and dolomite (146-440 kJ mol\({}^{-1}\)) [25]. As the temperature increases, the calcite starts to deform due to thermal expansion and chemical reactions. It decomposes into calcium oxide (cubic crystal system) with the liberation of carbon dioxide at a temperature greater than 600 \({}^{\circ}\)C [26]. Chemically, it can be expressed as follows:  
\[\text{CaCO}_{3(\text{s})}\ \overset{\Delta}{\rightarrow}\ \text{CaO}_{( \text{s})}+\text{CO}_{2(\text{g})} \tag{1}\]  
The hexagonal-rhombohedral crystal system of dolomite (i.e., CaMg(CO\({}_{3}\))\({}_{2}\)) begins to deform because of the dislocation of cations and anions. Its thermal decomposition produces calcium oxide (cubic crystal system), magnesium oxide (cubic crystal system), and carbon dioxide gas [25]. Several models have been developed to understand the thermal decomposition mechanism for both natural and synthetic dolomites [27; 28; 29; 30; 31; 32]. The single-step reaction for the dolomite decomposition is described below:  
\[\text{CaMg}(\text{CO}_{3})_{2(\text{s})}\ \overset{\text{600-800}\circ\text{C}}{ \Longleftrightarrow}\ \text{CaO}_{(\text{s})}+\text{MgO}_{(\text{s})}+2\text{CO}_{2( \text{g})} \tag{2}\]  
Another proposed model suggests that the thermal decomposition of a dolomite mineral completes in more than one step [33]. In the first step, it breaks into magnesium carbonate and calcium carbonate. In the second step, unstable magnesium carbonate decomposes into magnesium oxide. Finally, calcium carbonate turns into its respective metallic oxide against increasing temperature. To study thermal damage characteristics and decomposition of carbonate minerals, several techniques have been utilized, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermo-balance, scanning electron microscopy, optical microscopy, X-ray diffraction, high-temperature X-ray diffraction, etc. [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]. A summary of major developments and employed quantitative methods regarding the decomposition of carbonate minerals is provided in Table 1.