Multi-scale damage evolution of cement paste under alternating freeze-thaw and sulfate attack

In cold sulfate-rich regions, concrete structures frequently suffer from the sequential action of freeze-thaw (FT) cycles and sulfate attack, resulting in progressive degradation. To simulate this process, an alternating regime of 50 FT cycles followed by 45 days of immersion in 5% Na₂SO₄ solution was applied for four cycles, corresponding to 200 FT cycles and 180 days of sulfate exposure. The damage evolution of cement paste was investigated by macroscopic measurements including mass loss, compressive strength, open porosity, and sulfate ion concentration, and by microstructural characterizations such as SEM, XRD, MIP, and low-field NMR T₂ relaxometry. The results show that the alternating action significantly accelerates the deterioration of cement paste. The damage evolution exhibits three distinct stages, namely slow development, accelerated deterioration, and eventual near-stabilization. A key finding is that although the pore size distribution exhibits an apparent shift toward smaller pores in the later stage, as measured by MIP and LF NMR, the total porosity remains stable at 24.30–24.50% and the mechanical properties do not recover. This indicates pore structure reconfiguration rather than genuine densification, challenging the conventional assumption that porosity reduction implies improved durability. Furthermore, freeze-thaw-induced pore damage creates pathways for sulfate ingress, promoting formation and deposition of expansive products, which in turn interact with ongoing frost damage. This synergistic alternating interplay drives the continuous multi-scale deterioration of the cement paste, providing insights into non-monotonic pore structure evolution and a phenomenon referred to as apparent densification, characterized by an apparent shift toward smaller pores without corresponding recovery of mechanical properties under alternating environmental actions.