The internal temperature field of shotcrete in high geothermal tunnel and its effect on microstructures and mechanical properties

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Geothermal tunnel creates one-dimensional heat conduction field which may have an uneven effect on the hydration, microstructures, and mechanical properties of shotcrete for the first lining layer. In this study, the evolution of the internal temperature gradient of shotcrete specimens subjected to the simulated geothermal environment was measured, and its effects on the heterogeneous distribution of internal microstructures, hydration products, and mechanical properties were characterized. Results showed that there was an obvious temperature gradient in the shotcrete exposed to the geothermal environment, which dramatically changes within 24 h of contact and then remains stable afterward. For the properties of shotcrete at an early age (within 24 h), the compressive and flexural strength of specimens exposed to a geothermal environment is significantly higher than that of specimens cured in standard conditions, due to the accelerating effect on hydration by the elevated temperature. The phase assemblage, hydration degree, and mechanical property in the specimens at an early age show a gradient distribution, and more hydration degree and lower porosity were formed in the internal regions with a higher temperature, resulting in a higher compressive and flexural strength.