Scalable carbonation of low-reactivity steel slag monoliths using sodium-based carbonates

Internal carbonation using sodium-based carbonates is proposed as a scalable strategy to activate medium-alkalinity steel slag, which contains abundant inert phases and exhibits limited hydraulic reactivity. Sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) were used as internal CO₂ carriers under ambient curing conditions. Hydration–carbonation reactions led to the formation of stable carbonate phases, including hemicarbonate, monocarbonate, and calcite; gaylussite additionally formed at 5 wt% carbonate addition at early ages. Thermodynamic modeling suggested opposite pH trends during gaylussite formation, which explained delayed early-age hydration in NaHCO₃ incorporated samples. Both carbonates significantly enhanced the hydration of belite and ferrite, increasing the 28-day compressive strength from 5 MPa to 30–35 MPa by refining the pore structure. These findings demonstrate the feasibility of valorizing steel slag and incorporating carbon capture and utilization (CCU)-derived carbonates into sustainable binder systems without relying on an external CO₂ curing system.