Sulfate attack mechanism and service life prediction of concrete under drying-wetting cycles synergistically regulated by hybrid fibers and CaO-MgO-based expansive agent

To address the durability degradation of concrete exposed to sulfate-rich dry-wet environments, this study investigated the synergistic effects of modified polyester fibers (PF), basalt fibers (BF), and their hybrid system (PB) combined with a calcium-magnesium composite expansive agent (CMEA). Damage evolution during exposure to a 15% Na2SO4 solution was characterized in terms of, electrical response, phase assemblage, pore structure (PS), and microstructure. Layered damage and service-life prediction models were also developed. The mechanical performance (MP) and mass changes of the specimens exhibited three distinct stages: early enhancement, intermediate deterioration, and gradual late-stage degradation. The composite containing BF and 4% CMEA (BF-4E) exhibited the best overall durability, with a compressive strength retention ratio of 54.17% after 50 cycles. Although the composite containing PB and PB-4E provided a more balanced improvement in splitting tensile resistance and resistance to surface spalling. Its splitting tensile strength (STS) retention ratio was 72.60% after 40 cycles, while its mass loss (ML) rate was only 6.21% after50 cycles. Increasing the CMEA dosage to 8% intensified late-stage crack propagation due to excessive expansion and consequently reduced durability. The R2 values of all developed models exceeded 0.96. At a critical damage depth of 30 mm, the model-predicted life of BF-4E under the adopted accelerated exposure conditions was 122 cycles, representing a 33.60% improvement over the reference mixture. These findings provide a basis for the mixture design and model-based durability assessment of fiber-CMEA reinforced concrete (FERC) exposed to sulfate environments.