| Summary: | SiO<sub>2</sub> aerogel is a super-insulating material that can be used for tunnel fireproofing to eliminate high-temperature spalling and extend the safe evacuation time of personnel. This study aimed to replace traditional aggregates with SiO<sub>2</sub> aerogel in mortar preparation and evaluate its mechanical properties, thermal conductivity, and durability (freeze–thaw, water, and moisture resistance). Furthermore, the high-temperature characteristics of SiO<sub>2</sub> aerogel and the damage evolution pattern of SiO<sub>2</sub> aerogel mortar were investigated with varying fire durations (0.5, 1, 1.5, 2, 2.5, and 3 h) and fire temperatures (1000, 1100, and 1200 °C) as environmental variables. The results revealed that the critical temperature and critical time of SiO<sub>2</sub> aerogel particles from amorphous to crystalline structures were about 1100 °C and 1.5 h, respectively. SiO<sub>2</sub> aerogel mortar exhibited a compressive strength of 3.5 MPa, a bond strength of 0.36 MPa, and a thermal conductivity of 0.165 W/m·K. The residual mass ratio and residual compressive strength of SiO<sub>2</sub> aerogel mortar were 81% and 1.8 MPa after 1100 °C for 2.5 h. The incorporation of SiO<sub>2</sub> aerogel significantly improved the fire resistance of the mortar. Therefore, SiO<sub>2</sub> aerogel mortar has the potential to be used as a fireproof coating and can be applied in tunnels to reduce high-temperature spalling and extend the safe evacuation time for personnel.
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