## 4 Discussions
### Effects of High Temperatures on the Mineral Composition  
In order to make the test results easily comparable with each other, the average values of \(\sigma_{\text{d}}\) and \(E_{\text{d}}\) of limestone at different temperatures and strain rates are given in Table 3. It is well known that dolomite is a complex salt that consists of MgCO\({}_{3}\), CaCO\({}_{3}\), and other impurities, and its thermal decomposition reaction can be expressed by Equations (7) and (8) [51]. Relevant studies have shown that the temperature for the thermal decomposition of MgCO\({}_{3}\) in dolomite is approximately 600 \({}^{\circ}\)C [52]. The main component of calcite is CaCO\({}_{3}\), and its decomposition temperature is about 1000 \({}^{\circ}\)C. Moreover, its thermal decomposition reaction can be represented by Equation (9). Hence, it can be inferred that calcite does not undergo decomposition in this study. \[\text{MgCO}_{3}\cdot\text{CaCO}_{3} \rightarrow\text{MgO}\cdot\text{CaO}+2\text{CO}_{2} \tag{7}\] \[\text{MgCO}_{3} \rightarrow\text{MgO}+\text{CO}_{2}\] (8) \[\text{CaCO}_{3} \rightarrow\text{CaO}+\text{CO}_{2} \tag{9}\]  
Compared with the results obtained at a real-time temperature of 25 \({}^{\circ}\)C (Figure 11a), there is a significant increase in the proportion of CaCO\({}_{3}\) in the rock specimen when the temperature reaches 800 \({}^{\circ}\)C (Figure 11b). In contrast, there is a notable decrease in the proportion of dolomite. This indicates that a portion of the dolomite undergoes decomposition, leading to alterations in its microstructure and intensifying the initial damage caused by heating [53; 54]. \begin{table}
\begin{tabular}{c c c} \hline \hline \(T\) & \(V\) = 6.0–7.0 m/s & \(V\) = 7.0–8.0 m/s & \(V\) = 8.0–9.0 –m/s & \(V\) = 9.0–10.0 m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Variation in the macro-fracture characteristics with the impact velocity \(V\) (25 \({}^{\circ}\)C and 100 \({}^{\circ}\)C). Figure 12a-d show the micro-structural characteristics of dolomite in limestone at 800 \({}^{\circ}\)C. Compared with Figure 1a, thermal decomposition occurs in dolomite after calcination at high temperatures. The thermal decomposition degree and micro-structural characteristics of dolomite vary across different regions due to the heterogeneity of the specimen and non-uniform heating. Under the action of high temperature, the dolomite undergoes passivation at certain edges and corners, as depicted in Figure 12a,b. Additionally, some crystals become granulated, resulting in the formation of pores and micro-cracks, as presented in Figure 12c,d. This phenomenon is a result of the thermal decomposition of the dolomite accompanied by CO\({}_{2}\) release, which disrupts the original structure of the dolomite and diminishes its mechanical properties. Figure 12e presents the EDS analysis results for the dolomite featured in Figure 12b. Compared with Figure 1c, it is evident that the dolomite undergoes partial thermal decomposition following the high-temperature treatment at 800 \({}^{\circ}\)C, as indicated by a decrease in the mass proportions of C and O elements and an increase in those of Ca and Mg elements. Figure 13a shows the micro-structural characteristics of calcite at 800 \({}^{\circ}\)C, and Figure 13b shows the EDS analysis results. It can be observed that the mass proportions of C, O, Ca, and Si elements are 11.0%, 49.8%, 37.2%, and 2.0%, respectively. Under high-temperature conditions, the mass proportions of the C, O, and Ca elements in calcite undergo slight changes, suggesting that there is no thermal decomposition of calcite. However, the calcite of the rock specimens calcined at 800 \({}^{\circ}\)C has many microcracks in its mesoscopic characteristics; most of them are intergranular cracks, as indicated by arrow \(a\) in Figure 13a, while some are transgranular cracks, as indicated by arrow \(b\) in Figure 13a.