Degradation behaviour of marine concrete with mineral admixtures under the coupled effect of multi-ionic attack and freeze-thaw deterioration

The durability of marine-oriented concrete is strongly governed by the combined effects of aggressive ions and freeze–thaw cycling in cold areas. This study investigates the degradation of concretes incorporating fly ash (FA) and silica fume (SF) under coupled multi-ionic attack (Cl⁻, SO₄²⁻, CO₃²⁻) followed by rapid freeze–thaw cycles, and develops a probabilistic service-life prediction framework. Specimens were pre-immersed in binary and ternary salt solutions and then subjected to freeze–thaw cycling. Deterioration was quantified using mass loss, relative dynamic modulus, compressive strength, and microstructural characterization (NMR and SEM). Results show a three-stage damage evolution (slow–accelerated–stable), with mass loss and dynamic modulus being sensitive indicators. Damage severity depended on ionic composition: sulfate- and carbonate-bearing solutions caused substantially greater deterioration than chloride alone, and the SO₄²⁻–CO₃²⁻–Cl⁻ ternary solution was the most aggressive. NMR/SEM analyses revealed increased meso-/macro porosity, whereas FA and SF refined the pore structure, delayed damage development, and improved freeze–thaw resistance under multi-ionic attack. Weibull modeling of degradation indices yielded service-life predictions consistent with experimental trends, supporting its applicability and highlighting the effectiveness of mineral admixtures for durable, low-carbon marine concrete. • A multi-scale approach was applied to study the degradation of marine concrete. • Coupled SO₄²⁻–CO₃²⁻–Cl⁻ ions caused the most severe freeze–thaw deterioration. • Multi-ionic attack increased harmful pores and induced honeycomb microcracking. • Fly ash and silica fume refine pores and enhance durability. • Weibull modeling provides reliable service life prediction.