Carbonation behavior of seawater sea-sand concrete under different recycled coarse aggregate and sea-sand replacement ratios

This study explores the carbonation behavior of seawater sea-sand recycled aggregate concrete (SSRAC), considering the effects of recycled coarse aggregate (RCA) replacement ratio, sea-sand content, and carbonation age. The influence of these parameters on carbonation depth was systematically investigated. A suite of characterization techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), and mercury intrusion porosimetry (MIP), was employed to elucidate the effects of seawater, sea sand, and RCA on hydration products, carbonation compounds, pore structure, and microstructural features of the concrete. The results revealed that increasing the RCA replacement ratio led to a decline in both compressive strength and carbonation resistance. In contrast, the incorporation of sea sand exhibited a non-monotonic effect: compressive strength initially decreased and then recovered, while carbonation resistance first improved and subsequently diminished, with optimal performance achieved at a 30 % sea-sand replacement level. Post-carbonation analysis showed a reduction in total porosity of approximately 5 %, accompanied by an increase in the most probable pore diameter by about 2 mm. A predictive model for carbonation depth in SSRAC was developed based on RCA and sea-sand replacement ratios. The model demonstrated strong predictive capability, with an average correlation coefficient of 0.92 between calculated and experimental results. Utilizing this model, the long-term carbonation depth of various concrete compositions was estimated over a 50-year service life, offering meaningful insights into the durability performance of SSRAC in practical applications.