Study on Mechanical and Thermal Insulation Properties of SAP Geopolymer Shotcrete in High Geo-temperature Tunnel Based on Thermal-mechanical Coupling Effect

Taking the high geo-temperature tunnels in the Gaoligong Mountains as the engineering context, laboratory simulations were conducted under high-temperature environments of 40 °C, 60 °C and 80 °C (with 20 °C serving as the control group, and relative humidity ≥95%) to investigate the mechanical and thermal properties of polymer-modified shotcrete incorporating superabsorbent polymer (SAP), as well as the mechanisms by which microstructure influences macroscopic properties. The results indicate that at 3 days of age, specimens containing 0.2 wt% SAP exhibited the highest compressive strength, whilst those containing 0.3 wt% SAP exhibited the highest splitting tensile strength. Between 7 and 28 days of curing, at 20 °C and 40 °C, both the compressive and splitting tensile strengths of the specimens were highest at 0.2 wt%, whereas at 60 °C–80 °C, the strength of the control group specimens was highest and that of the 0.3 wt% SAP group was lowest. Under constant temperature conditions, the thermal conductivity of the concrete gradually decreased as the SAP content increased. Taking all factors into account, the geopolymer concrete with a SAP content of 0.2 wt% and a curing temperature of 40 °C exhibited the optimal mechanical and thermal properties. Microstructural analysis indicated that, under standard curing conditions, SAP promotes the formation of C-A-S-H gel and enhances the degree of geopolymerisation.