## 3 Mechanical Properties and Microstructure of Shotcrete under High Temperature
### Variation Law of Compressive and Flexural Strength of Shotcrete  
With the increasing number of high-temperature construction sites, some scholars have studied the influence of formation temperature on shotcrete, firstly through the change of compressive and flexural strength. As shown in Figure 6, [PERSON] et al. [50] found that the compressive strength and splitting tensile strength of shotcrete increased with the increase of curing age through standard curing in a 40-100 \({}^{\circ}\)C dry and hot environment. Under a dry and hot environment of 60-100 \({}^{\circ}\)C, it shows an increasing trend before curing for 7 d, and has an obvious downward trend after 7 d. After curing for 28 d, the compressive strength under high temperature is lower than that under standard curing. At 7 d and 28 d, the splitting tensile strength of concrete decreases with the increase of curing temperature. Similarly, [PERSON] et al. [51] found that the initial setting time and final setting time of high-strength shotcrete shortened with the increase of curing temperature, and analyzed the compressive strength of high-strength shotcrete with the curing ages of 3 d and 28 d. The results showed that the overall compressive strength presents an increasing trend, but when the temperature is too high, the compressive strength of shotcrete has an obvious shrinkage phenomenon. Figure 5: Damage field caused by different phenomena after 360 h (heat shrinkage and self-contraction) [49]. (**a**) Creep is considered; (**b**) creep is ignored; and (**c**) only thermal shrinkage is considered. The above research only expounds the deterioration law of shotcrete as a macro phenomenon, but does not explain the causes of deterioration from the micro level. In order to deeply study the influence of high temperature on the deterioration of shotcrete, scholars analyzed the deterioration mechanism of shotcrete from the perspectives of carbonation depth and early hydration products of cement. In view of the critical size effect of concrete under high temperature, scholars have proposed using acoustic emission technology to evaluate the internal structural defects and damage of concrete [52; 53; 54]. Relevant conclusions show that there is a negative correlation between temperature and acoustic emission signal. Before the compressive strength of concrete has reached the critical point, the acoustic emission number and energy of concrete showed an increasing trend, until the concrete members were completely destroyed [55]. [PERSON] et al. [56] studied the carbonation degree of shotcrete under high temperature, and noted that the compressive strength of shotcrete decreased with the increase of temperature. Results showed that the carbonation depth of shotcrete at a high temperature of 50 \({}^{\circ}\)C was more than 4 times higher than that at a normal temperature, which easily reduced the alkalinity and damaged the sealing structure of the shotcrete. [PERSON] et al. [57] found serious cracks on the surface of 100 \({}^{\circ}\)C high-temperature shotcrete, and preliminarily explained the phenomenon of shrinkage of shotcrete compressive strength. The early hydration products formed too fast, resulting in disordered accumulation of products, which made the distribution uniformity of the shotcrete's internal structure worse. The later high temperature caused the hydration products to move strongly, and most of the hydration products deposited near the larger aggregate. Figure 7a shows the X-ray diffraction (XRD) patterns of shotcrete cured at 80 \({}^{\circ}\)C for 3 h, 4 h, 5 h, and standard curing. The AFt diffraction peak decreases and the C-S-H and C\({}_{3}\)S diffraction peaks increase with the curing time at high temperature. This showed that short-term high-temperature curing could strengthen the compactness of shotcrete. Meanwhile, [PERSON] et al. [58] also detected C-S-H phase at 0-60 \({}^{\circ}\)C, showing an increasing trend.