## 3 Mechanical Properties and Microstructure of Shotcrete under High Temperature
### Bond Strength between Shotcrete and Coal (Rock) Interface  
The bond between shotcrete and coal (rock) interface has been a key problem in roadway support. It was necessary to further study the mechanical properties of the interface under high temperature [73; 74]. As shown in Figure 9, [PERSON] et al. [75] studied the  
Figure 8: SEM photos of ceramsite concrete curing at different temperatures [65]. relationship between surrounding rock and shotcrete cohesion based on a high-ground-temperature simulation experiment. By setting the temperature range at 50-90 \({}^{\circ}\)C, the bond strength decreases with the increase of rock wall temperature. At the same time, when the temperature exceeds the critical value, the bond performance of shotcrete will shrink. In addition, CT scanning was performed on the surface of shotcrete at 50 \({}^{\circ}\)C and 90 \({}^{\circ}\)C. It was observed that the surface deterioration of shotcrete at 90 \({}^{\circ}\)C was more serious than that at 50 \({}^{\circ}\)C, with a large number of pores, as shown in Figure 10.  
[PERSON] et al. [16] studied the bond strength of shotcrete under 70 \({}^{\circ}\)C working conditions in a hot and humid environment, and concluded that a dry and hot environment had the greatest impact on the performance of shotcrete, and would accelerate the retrogradation of shotcrete [19; 76]. [PERSON] et al. [77; 78; 79] carried out finite element analysis on the temperature field of shotcrete rock slab, and found that temperature determined the damage degree of bond strength between shotcrete and rock slab. The heat and the moisture in shotcrete were mainly lost and evaporated through the side wall of the specimen. As shown in Figure 11, the internal pores of shotcrete under various temperature conditions was analyzed. It was determined that the pore area is proportional to the development of the temperature. For one thing, the high temperature led to a rapid evaporation of water in the shotcrete, and pores formed during the hydration process of the cement; for another, the impact force generated in the process of shotcrete spraying caused the shotcrete to be mixed with air, thus producing bubbles, resulting in an increase in the number of pores in shotcrete under high temperature. Figure 10: CT scan image of bonding surface [75]. Figure 9: Data of bond strength between surrounding rock and shotcrete under different working conditions [75]. [PERSON] et al. [80] studied the bond strength of shotcrete under high temperature by the splitting method and drawing method, as shown in Figure 12. Because the splitting failure was accompanied by shear phenomenon, the bond strength of shotcrete measured by splitting method was better than that by drawing method under high temperature. It was also found that the cracks on the cemented surface of concrete due to autogenous shrinkage at a high temperature of 60 \({}^{\circ}\)C and thermal decomposition still occurred in the transition zone between shotcrete and rock. [PERSON] et al. [81] pointed out in the paper that relevant scholars believe that 75 \({}^{\circ}\)C is the critical temperature for thermal damage of shotcrete, and beyond this temperature, shotcrete will lose its adhesion. [PERSON] et al. [82] carried out three-dimensional CT scanning on shotcrete to observe the agglomeration phenomenon of shotcrete under different temperatures. It was found that the pore and micro crack zone of shotcrete block at 90 \({}^{\circ}\)C was much higher than that at 50 \({}^{\circ}\)C. Results showed that the high temperature accelerated the hydration reaction of cement and the disorderly overlapping between molecules, which led to the intensification of autogenous shrinkage of shotcrete and the poor adhesion of hydration products generated, as shown in Figure 13. In addition, the local stress in the shotcrete would produce new cracks again, which would weaken the bond performance of the internal structure of shotcrete.