Deterioration and damage mechanisms of concrete under high-temperature and sulfate-rich environments

Concrete structures operating in high-temperature and sulfate-rich environments inevitably face severe durability challenges. Previous studies have mainly addressed the effects of thermal damage or sulfate corrosion on concrete separately. The mechanisms through which their coupling drives microstructural evolution and macroscopic deterioration remain underexplored. To investigate these synergistic degradation mechanisms in the newly emerging class of complex hydrothermal engineering environments with corrosive ions encountered in recent years, a series of laboratory experiments was conducted to assess mass loss, mechanical properties, microstructural evolution, and ultrasonic wave propagation in concrete under accelerated exposure conditions. The results reveal that elevated temperatures markedly accelerate corrosion onset and progression, with the 120-day mass loss rate exceeding three times that observed under standard conditions. Compressive strength and elastic modulus exhibited temperature-dependent degradation, with critical deterioration thresholds reduced by up to 47.87 % and residual service life at 80 °C falling to only 16.1 % of that at 20 °C. Microstructural analysis demonstrated that high temperatures promote ion diffusion, facilitating the formation of expansive ettringite and gypsum crystals, which generate internal stresses and initiate microcracking. These microcracks further enable ion ingress, leading to interconnected crack networks and increased porosity. The combined effects of corrosion product expansion and thermal stress cycling intensified crack propagation, ultimately compromising the load-bearing capacity of the material. Ultrasonic tests confirmed the evolution of internal defects, with wave velocity decreasing by 22.7 % and signal amplitude attenuation exceeding 80 %, alongside the disappearance of high-frequency signals. These findings clarify the damage mechanisms of concrete under coupled thermal and corrosive conditions and establish quantitative, design-oriented indicators for macroscopic performance degradation. Moreover, these outcomes in this work can also guide the optimization of construction materials for underground engineering applications exposed to hydrothermal environments containing corrosive ions.