Deterioration behavior and mechanism of concrete under different exposure conditions of sulfate attack and carbonation

This study systematically investigated the deterioration behavior of concrete under different erosion patterns, i.e., single sulfate attack, single carbonation, and alternating cycle of sulfate attack and carbonation (alternating cycle). The effect of varying time ratios of alternating cycle, representing the proportion of time allocated to sulfate attack and carbonation within a single cycle, was analyzed for ratios of 7:3, 8:2, and 9:1. The deterioration of concrete was evaluated through changes in surface morphology, mass, compressive strength, sulfate ion distribution, and carbonation depth, and the underlying deterioration mechanisms were elucidated through microstructural analysis. Results indicate that under the single sulfate attack, the mass and compressive strength of concrete initially increased but subsequently decreased with increasing cycles, accompanied by surface cracking and sulfate ion accumulation. Under the single carbonation, the compressive strength exhibited a similar trend, while the carbonation depth progressed steadily. Under the alternating cycle patterns, a synergistic deterioration effect was observed, which became more pronounced as the carbonation duration increased. When the time ratio was 7:3, the concentration of sulfate ions in the surface rose by approximately 1.5 times, and the carbonation depth doubled compared to single exposure. Microstructural analysis revealed that the alternating cycle patterns led to extensive consumption of calcium hydroxide, reducing the pH of the system and triggering decalcification and dissolution of C-S-H gel, thereby resulting in a more porous microstructure. Concurrently, the carbonation induced the transformation of ettringite into gypsum and calcium carbonate, thereby further aggravating microstructural degradation and promoting ion ingress. This phenomenon was progressively enhanced with increasing carbonation time ratios, ultimately leading to accelerated deterioration of concrete.