## 4 Methods to Improve the Performance of Shotcrete under High Temperature
### Experimental Study on Performance Optimization of Shotcrete
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Refrigeration Technology** & **Advantage** & **Disadvantage** \\ \hline Air conditioning refrigeration technology & Water refrigeration technology & Using air as a refrigeration medium, the system produced no pollution in the environment, and the system structure was simple & Large refrigeration equipment, inconvenient to move and install, poor refrigeration capacity, and high refrigeration cost \\ Air conditioning refrigeration technology & Water refrigeration technology & The refrigeration range was wide, and the cooling effect was obvious & In the process of cooling, there will be a temperature jump \\ Geothermal refrigeration technology & Geothermal refrigeration energy, environmental protection and energy saving, low operating cost & The refrigeration range was limited, so it was necessary to establish multiple refrigeration systems \\ Ice refrigeration technology & Water refrigeration technology & High cold storage capacity and high heat exchange efficiency & The conveying process was easily blocked, and the melting speed was low \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline \multirow{4}{*}{Spray-cooling technology} & It had the advantages of fast cooling rate, low cost, and good dust removal effect & It was suitable for a tunneling working face with small working range, few operators, small cooling capacity, and serious heat damage \\ Non-mechanical cooling technology & Liquid CO\({}_{2}\) refrigeration technology & The conveying distance was more than 1000 m, the cooling effect was good, and the cooling system was flexible It had high dehumidification efficiency, controllable \\ Solution dehumidification and cooling & dehumidification capacity, and low operation cost. It could remove dust, bacteria, and other harmful substances in the air \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of advantages and disadvantages of refrigeration technology. [PERSON] et al. [107; 108] studied the performance changes of shotcrete mixed with hook-end steel fiber (SF-HE), wave-shaped steel fiber (SF-W), and basalt fiber (BF) at 100 \({}^{\circ}\)C, and found that the total porosity and harmful porosity of shotcrete decreased significantly after adding fiber, and the optimization effect of steel fiber on pore structure was better than that of basalt fiber. However, the porosity of shotcrete increased after the steel fiber and silica fume were mixed, which indicated that the pozzolanic activity of silica fume fails under high temperature, as shown in Figure 20. [PERSON] et al. [109] found that the toughness of foam-fiber-reinforced shotcrete decreased, and the thermal conductivity decreased. This was because foam filled in the void of shotcrete and directly cut off the heat transfer path. [PERSON] et al. [110] studied the structural performance of shotcrete and concluded that the shotcrete containing alkaline accelerator had low thermal diffusivity and better thermal insulation performance. Some scholars found that the degree of C-S-H gel polymerization and chain length increased at the cemented surface when the shotcrete was mixed with silica fume. A large number of CH crystals generated C-S-H gel, which filled in the interface pores, which enhanced the interfacial strength [111; 112]. This could be expressed as Ca(OH)\({}_{2}\)+SiO\({}_{2}\)+H\({}_{2}\)O\(\rightarrow\)C-S-H. [PERSON] et al. [113] analyzed the vulcanization properties of waste rock and concluded that it had good safety. [PERSON] et al. [114] proposed that the performance of shotcrete could be optimized by changing the density of the material to reduce the pore pressure and the gravitational potential energy of shotcrete. Figure 19: Inorganic materials for performance optimization of shotcrete. Figure 20: Porosity of different pores under different conditions [108]. BP: non-fiber material; SF-HE: 1% hook-end steel fiber; SF-W: 1% corrugated steel fiber; BF: 0.1% basalt fiber; SF-HE+Si: 1% steel fiber and 5% silica fume.