Performance Evaluation of CFRP-Wrapped Concrete Made Using Magnesium Phosphate Cement (MPC) Under Sulfate Exposure

Long-term deterioration of concrete due to synergistic dry–wet cycles and sulfate attack significantly compromises the structural durability and integrity. CFRP-reinforced concrete offers a promising solution for repair and performance enhancement under such adverse conditions. In addition, binder selection is critical to the reinforcement efficacy. However, traditional epoxy (EP) binders have certain limitations in environmental adaptability and sustainability, necessitating alternative approaches. Thus, inorganic magnesium phosphate cement (MPC) binders have garnered attention due to their rapid hardening, early strength, interface compatibility, and environmental benefits. Nevertheless, the durability of MPC under combined sulfate exposure and dry–wet cycles, as well as the performance of CFRP-bonded concrete using MPC, has remained underexplored. In this study, the chemical, microstructural, pore, and mechanical properties of MPC mortar and MPC–CFRP concrete columns subjected to sulfate dry–wet cycles were systematically investigated and compared to ordinary and EP–CFRP concretes. The results indicate that MPC mortar forms struvite during sulfate dry–wet cycles, though the struvite content decreases with increasing cycles. In addition, the porosity of MPC mortar initially decreases and then increases, reaching a maximum of 1.52. The resulting compressive and flexural strengths of MPC mortar decline over cycles, reaching 28.4 MPa and 8.4 MPa, respectively, after 240 cycles. For concrete columns, sulfate dry–wet cycles result in significant sulfate crystallization on EP–CFRP surfaces, whereas MPC–CFRP columns show reduced crystallization and improved ductility. After 240-day cycles, compressive strengths of ordinary, EP–CFRP, and MPC–CFRP columns were 17.64 MPa, 53.87 MPa, and 72.41 MPa, respectively, with the strength retention rates of 56.89%, 65.57%, and 86.87%. The MPC–CFRP columns exhibited the best mechanical properties, strength retention, and ductility. Further, the degradation mechanisms of MPC and MPC/EP–CFRP concretes under sulfate dry–wet cycles were clarified, offering a reference for enhanced durability and service life of concrete structures in saline soil regions.