Study on degradation mechanism and life prediction of concrete mechanical properties under sulfate–carbonate composite erosion

Sulfate – carbonate composite erosion in karst groundwater threatens the long-term performance of tunnel lining concrete. This study performed 18 laboratory erosion tests under coupled ion concentration and flow velocity to quantify strength deterioration and to develop a predictive framework. A transport – reaction – damage coupled model was established to capture the staged evolution of compressive strength. In the initial stage, carbonate-related precipitation may induce local densification, while continued sulfate penetration generates expansive products that drive microcrack initiation and propagation, leading to accelerated degradation. The maximum 90-day strength reduction reached 22.5% under high concentration and high flow velocity, exceeding that under single sulfate attack. Compared with static exposure, flow increased the degradation rate by approximately 20%–30%. The model achieved high predictive accuracy across all conditions, with a mean absolute percentage error of 3.18% and ��2 > 0.90. These results support the use of the model for durability evaluation and service-life prediction of tunnel linings in composite saline groundwater.