Thermal damage mechanism of high geothermal tunnel lining concrete considering temperature gradient effects

High geothermal tunnels impose complex temperature and humidity gradients on concrete linings due to intense unidirectional rock heating. This study developed a novel experimental system (MUGTCS) to accurately simulate full-stage gradient curing from pouring to hardening. We comprehensively investigated the spatiotemporal thermal damage effects on concrete's hydration, microstructure, and mechanical properties under various simulated temperatures and gradients. Experimental results demonstrate that unidirectional heating creates significant non-uniform damage across the lining thickness. Optimal early strength development occurs between 70 and 80°C, a phenomenon notably enhanced by higher humidity. Mechanism analysis reveals that coupled non-uniform hydration kinetics, differential thermal expansion, and microstructural degradation (including pore structure changes and ITZ weakening) are primarily responsible for the observed reduction in mechanical properties. This research highlights the critical role of continuous gradient curing from the initial molding stage, offering crucial insights for optimizing mix design and practical curing strategies in high geothermal tunnel environments.