Aqueous matrix and limiting factors in steel slag carbonation under elevated CO2 at ambient conditions

Abstract Alkaline industrial by-products have significant potential for permanent CO ₂ sequestration, but their reaction kinetics and controlling factors are often overlooked. This study examined the direct aqueous carbonation of basic oxygen furnace (BOF) slag in a slurry setup with continuous mechanical mixing and CO ₂ -enriched air bubbling in ultrapure water and natural seawater, at 25% pCO ₂ under ambient temperature and 1 atm for 24 h. CO ₂ sequestration into carbonate minerals was higher in the ultrapure water (105 ± 5 kg CO ₂ t ⁻ ¹ slag; 90% CO ₂ capture efficiency) than in seawater (78 ± 4 kg CO ₂ t ⁻ ¹ slag; 70% CO ₂ capture efficiency). Most of the rapid mineralisation potential of Ca-(hydr)oxides was realised within 4–12 h, after which the reaction rate slowed and the system approached equilibrium. The common linear extrapolation for estimating the full mineralisation potential can be misleading, as it neglects the multi-step dissolution process, starting with Ca-(hydr)oxide and followed by kinetically controlled reactions.