Synergistic Degradation of Tunnel Lining Materials during Combined Carbonate–Sulfate Attack under Multifield Coupling
The durability of tunnel linings exposed to groundwater containing both sulfates and carbonates represents a critical challenge in karst regions. While single-ion attack has been widely studied, the coupled degradation mechanism under combined sulfate–carbonate attack remains insufficiently understood. In this work, the deterioration of concrete under such combined exposure was systematically investigated through integrated theoretical, numerical, and experimental approaches. A competitive reaction mechanism was revealed, wherein sulfate and carbonate ions jointly consume calcium ions in the pore solution. Based on this, a coupled transport model centered on calcium ion diffusion was developed and implemented via COMSOL Multiphysics, incorporating competitive ion diffusion, chemical product crystallization, and mechanical damage. Simulation results demonstrated a clear calcium ion concentration gradient consistent with Fick’s law. Strong spatial correlations were observed among the calcium ion concentration, displacement, volumetric strain, and von Mises stress fields, confirming that crystallization-induced expansion is the direct driver of deformation and damage. Microcracks were found to initiate preferentially at the interfacial transition zone and the attacked surface, illustrating the damage progression from meso- to macroscale. Experimental data from long-term exposure tests (60–300 days) showed good agreement with the simulated calcium ion distribution and compressive strength evolution, validating the model. This study provides a theoretical and numerical basis for assessing concrete durability in complex ionic environments.