As an industrial solid waste, steel slag is produced in large quantities annually yet remains underutilized. In order to achieve resource utilization of steel slag and improve the ecological properties of artificial reef concrete,
this study investigated the effect of different combinations of steel slag and other additives plus carbonation curing on the alkalinity and strength of the concrete. Combinations include high-dosage steel slag powder used alone, and steel slag powder combined with metakaolin, silica fume, or biochar to replace 50% of cement. Techniques including X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to analyze the alkalinity reduction mechanism and the microstructural evolution. With steel slag powder replacing 50% of the cement, the pre-dried specimens were subjected to 24 hours of carbonation curing, followed by 28 days of standard curing. The results show that the pH value of the specimens reached 8.42 after curing, which complies with the alkalinity specifications for ecological reef concrete; the 28-day compressive strength was 16.80 MPa, which is adequate for engineering application demands. Alkalinity reduction was associated with the decrease of Ca(OH)2 content within the concrete. Following the replacement of 50% cement with high-dosage steel slag powder, the quantity of Ca(OH)2 generated through hydration of the composite cementitious material was substantially decreased. Furthermore, carbonation curing markedly consumed the Ca(OH)2 present in the specimens, and the pre-drying treatment prior to carbonation curing improved the efficiency of the carbonation process.