[Objective] A systematic investigation is conducted on the detrimental effects of high geothermal temperatures (up to 60 ℃) encountered during Sichuan-Tibet Railway tunnel construction on the performance of shotcrete mortar. The primary objectives are to elucidate the impacts of elevated temperature on hydration kinetics, mechanical strength development, and microstructural evolution, with a focus on understanding the paradoxical phenomenon of early-age strength enhancement versus long-term performance degradation. The research aims to identify critical temperature thresholds and underlying mechanisms responsible for material deterioration, providing insights essential for developing durable shotcrete formulations in geothermal environments. [Methods] Cement mortar specimens that simulated shotcrete (Ordinary Portland Cement, water-cement ratio of 0.5, sand-cement ratio of 1.5) incorporating a commercial alkali-free accelerator (8% dosage) were prepared and cured under controlled humidity at 20 ℃ (as a reference), 40 ℃, and 60 ℃. A multi-technique experimental approach was employed. Isothermal calorimetry was used to track hydration heat flow. The compressive strength was measured at 1, 3, 7, and 28 days. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were employed to identify phase composition and transformation. Low-field nuclear magnetic resonance (LF-NMR) was utilized to quantify pore structure parameters. Scanning electron microscopy/backscattered electron (SEM/BSE) imaging was applied to characterize the microstructural morphology and interfacial transition zones between cement paste and aggregate. [Results] Elevated temperatures significantly accelerated early hydration. The 60 ℃ specimens achieved 1-day compressive strength that accounted for 64% of their 28-day reference strength, which was more than double the ratio observed at 20 ℃. Calorimetry revealed intensified and earlier heat release peaks at higher temperatures, indicating rapid C₃A dissolution and AFt formation. However, temperatures ≥60 ℃ triggered severe microstructural degradation: XRD/FT-IR confirmed the destabilization of ettringite (AFt) and its conversion to weaker monosulfate (AFm), with AFm content increasing by 300% at 80 ℃. LF-NMR analysis demonstrated pore structure coarsening, with harmful pores (≥50 nm) increasing by 45% at 60 ℃ due to disordered hydration product precipitation and moisture loss. SEM/BSE imaging revealed thermal stress-induced microcracks (10-50 μm wide) at paste-aggregate interfaces and excessive CH crystallization. These synergistic effects caused pronounced strength regression—specimens at 60 ℃ peaked at 1-day strength (21.8 MPa) but declined to 34.9 MPa by 28 days, retaining only 78% of the strength exhibited by 20 ℃ controls. [Conclusion] This study identifies 60 ℃ as the critical threshold for irreversible shotcrete degradation in geothermal environments. Its principal innovation lies in decoupling the dualistic temperature response: while temperatures ≤40 ℃ enhance early strength through accelerated hydration, ≥60 ℃ induces multi-scale deterioration via AFt→AFm conversion, moisture loss-induced porosity, thermal microcracking, and pore coarsening. The quantified phase transformations and pore structure evolution provide a mechanistic explanation for long-term strength regression. These findings underscore the necessity of developing thermal-stable accelerators, optimizing binders with SCMs to suppress AFm formation, and implementing active cooling strategies for tunnels exceeding 60 ℃. The study establishes a scientific basis for designing high-performance shotcrete capable of withstanding extreme geothermal conditions in major infrastructure projects like the Sichuan-Tibet Railway.