## 4 Discussions
### Effects of Temperature-Impact Loading on Microstructure  
Figure 14 shows the micro-characteristics of the limestone specimen after impact loading at different real-time temperatures and impact velocities within the range of 9.0-10.0 m/s. Based on the macro-mechanical properties, macro-micro-fracture morphology, and structural characteristics, the action mechanism of real-time temperature on the limestone can be divided into three stages. When the temperature reaches 25 \({}^{\circ}\)C, a few cracks are generated on the fracture surface, showing the brittle fracture characteristics of transgranular cracks and intergranular cracks. The presence of dolomite crystals on the fracture surface suggests that the fracture propagates along the interface of these crystals, as depicted in Figure 14a. The fracture surface becomes relatively smooth with only a few visible cracks as the real-time temperature increases to 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. The stepped brittle fractures are generated on the calcite, and the characteristic morphology of the calcite fracture is marked in the box, as illustrated in Figure 14b,c. The limestone is mainly composed of calcite, dolomite, muscovite, and clay mineral impurities. After heating, crystal expansion is caused, and primary voids are filled to a certain extent, resulting in the densification of the microstructure. Therefore, the temperature has a reinforcement effect on the mechanical properties of the specimen within the real-time temperature range of 25-200 \({}^{\circ}\)C.  
When the real-time temperature increases to 300-500 \({}^{\circ}\)C, the temperature stress inside the specimen gradually increases, and the initial damage becomes more severe. Therefore, the severity of microstructure fracture intensifies under impact loading, leading to an increase in microcracks, as depicted in Figure 14d-f. The mineral components continue to expand during this stage but are limited by the surrounding minerals, and the thermal expansion characteristics of each component are different. The occurrence of extrusion stress is prominent among the components, particularly for prismatic grains like dolomite, resulting in the generation of concentrated stress during extrusion (thermal stress). When the thermal stress reaches the fracture threshold of each component, the failure of the primary structure occurs, and new micro-cracks are generated, which gradually weakens the mechanical properties of the specimen. Consequently, elevated temperatures induce a deterioration in the mechanical characteristics, and the strength of the specimen significantly decreases. When the real-time temperature reaches 600-800 \({}^{\circ}\)C, the high stress generated by the temperature significantly impairs the microstructure, resulting in extensive formation of cracks on the fracture surface. Moreover, the quantity, width, and length of cracks escalate with the elevating temperature, as shown in Figure 14g-i. The physical properties of the mineral components in the specimens are changed due to high-temperature treatment, resulting in plastic fracture features during impact loading, as shown in the box of Figure 14h,i. It can be found that fracture features in Figure 14h,i are different from that in Figure 14b,c. In addition, dolomite also undergoes thermal decomposition and experiences a weakening of the crystal structure, causing fractures and destruction during impact loading, as shown in Figure 14i. This behavior is notably distinct from that illustrated in Figure 14a,d. The thermal expansion causes the weakening of the physical structure of the minerals. Meanwhile, dolomite minerals undergo thermal decomposition due to chemical reactions during this stage. Under the combined action of the physical and chemical effects, thermal damage in limestone rapidly increases, and the performance deterioration of the specimen increases greatly.