Degradation mechanism and compressive strength prediction of polyvinyl alcohol fiber-engineered cementitious composites under coupled salt corrosion and freeze-thaw cycles

Durability of polyvinyl alcohol engineered cementitious composites (PVA-ECC) in areas with saline soils and freeze-thaw cycles is still unclear, especially the mechanism of synergistic degradation under salt corrosion and freeze-thaw action. In this study, the degradation characteristics of PVA-ECC in freshwater and composite salt solutions (Cl - SO 4 2- , CO 3 2- and other ions as main components) for 500 freeze-thaw cycles were systematically investigated. Comparative experiments evaluated apparent morphology, mass loss, relative dynamic elastic modulus, freeze-thaw damage index, nuclear magnetic resonance T 2 spectra pore evolution and scanning electron microscopy to elucidate the salt-frost interaction mechanism. The results indicate that after 500 freeze-thaw cycles, degradation of PVA-ECC in saline soil is significantly greater than in clear water. The mass loss reaches 1.21 % (≤1 % in water), the relative dynamic elastic modulus drops to 30 % (72 % in water), the damage degree increases to 0.699 (0.28 in water), and the compressive strength decreases to 50 % of its initial value (34.613 % loss in water). Under saline conditions, the pore structure deteriorates more severely: total pore area increases by 39.4 % (31.59 % in water), large pores increase by 242.79 %, multiple harmful pores rise to 36.3 % (30.0 % in water), and porosity reaches 3.258 % (2.981 % in water). Pores larger than 0.1 μm and around 0.01 μm show significant growth. Cl⁻ content increases notably, accelerating material degradation, while SO₄²⁻, CO₃²⁻, and HCO₃⁻ only slightly increase due to chemical fixation. Salt crystallization stress under combined seepage, frost heave, and chemical reactions induces microcracks, surface spalling, and fiber exposure. Compressive-strength prediction model is proposed for this environment.