## 3 Mechanical Properties and Microstructure of Shotcrete under High Temperature
### Variation Law of Compressive and Flexural Strength of Shotcrete
In addition, many scholars have explained that the increase of shotcrete compactness was due to the gradual loss of shotcrete crystal water in the gradually increasing temperature gradient, and a large amount of ettrinigite (Aft) would be transformed into monosulfide calcium sulphoaluminate (Afm) [53; 59]. Figure 6: Mechanical properties of specimens under various working conditions: **(a)** compressive strength test, **(b)** splitting tensile strength test [50]. [PERSON] et al. [60] carried out microscopic analysis of shotcrete at 90 \({}^{\circ}\)C temperature, and found that the rate of C/S in C-S-H gel increased slightly after thermal curing, while the ratio of (A + F)/C and Sulphates/C decreased, indicating that the properties of C-S-H gel changed under high temperature (mainly sulfate content), as shown in Table 1.  
A group of scholars have studied the performance of shotcrete under ultra-high temperatures (the temperatures were higher than 150 \({}^{\circ}\)C), and found that the compressive strength and flexural strength of shotcrete continue to decline with the increase of temperature, and that the damage to shotcrete's internal structure caused by ultra-high temperatures is very serious [61; 62; 63]. Under ultra-high temperatures, calcium hydroxide and wollastonite in shotcrete decompose to generate a large amount of calcium oxide, which leads to the instability of the shotcrete's internal structure. [PERSON] et al. [64] studied the influence of temperature and humidity on shotcrete, and found that low-temperature and high-humidity curing conditions were conducive to the development of shotcrete strength at later stages. The above research lacks studies on the micro void structure of shotcrete. As we all know, the change of micro void structure affects the mechanical properties of shotcrete. With the development of new detection technology, scholars use SEM, CT, and the mercury intrusion method (MIP) to measure the change of voids in shotcrete, so as to further explain the influence mechanism of high temperature on concrete deterioration. Furthermore, scholars have studied the microstructure of shotcrete under high temperature. Figure 8 show SEM images of ceramics shotcrete at different temperatures. At 20 \({}^{\circ}\)C, \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Curing**} & \multirow{2}{*}{**Concrete Type**} & \multicolumn{3}{c}{**C-S-H gels Composition**} \\ \cline{3-5}  & & **C/S** & **A + F/C** & **Sulphates/C** \\ \hline \multirow{3}{*}{Standard} & H & 1.68 \(\pm\) 0.06 & 0.145 \(\pm\) 0.033 & 0.104 \(\pm\) 0.048 \\  & F & 1.84 \(\pm\) 0.29 & 0.159 \(\pm\) 0.057 & 0.061 \(\pm\) 0.021 \\ \hline \multirow{3}{*}{Heat curing} & H & 1.84 \(\pm\) 0.12 & 0.124 \(\pm\) 0.026 & 0.074 \(\pm\) 0.011 \\  & F & 2.04 \(\pm\) 0.35 & 0.093 \(\pm\) 0.004 & 0.057 \(\pm\) 0.015 \\ \hline \end{tabular}  
* H: Cement content 100%; F: 20% limestone instead of 20% cement.  
\end{table} 
Table 1: Chemical composition of C-S-H gel formed in concrete under different conditions [60]. Figure 7: XRD spectra of different curing systems: (**a**) different curing time (**b**) Different curing temperatures [57; 58]. the shotcrete presents a particle aggregation state, and the structure is stable. At 40 \({}^{\circ}\)C, the shotcrete structure is gradually destroyed, and the particles are granular in structure. At a temperature of 60 \({}^{\circ}\)C, the concrete particles are small, there are a lot of micro pores, the structure is loose, and the degree of hydration is low, which leads to a decrease in shotcrete strength. The reason is that ceramsite is a microporous medium, and with the increase of temperature, it accelerates the water evaporation of shotcrete, so the hydration structure of the shotcrete is relatively dispersed. [PERSON] and [PERSON] et al. [66; 67] found that there was a correlation between the fractal dimension of pore volume and the strength by establishing the fractal model; that is, the more pores there are and the larger they are, the lower the strength of the shotcrete.