Comprehensive analysis of inhomogeneous carbonation and phase distribution in pour-molded steel slag/cement pastes

Steel slag, a byproduct rich in calcium and magnesium, holds significant potential for CO₂ sequestration and utilization in building materials. This study involves the preparation of steel slag/cement paste using pour molding, followed by carbonation under 0.2 MPa gas pressure for CO₂ curing. It investigates the effects of varying carbonation durations on carbonation depth, CO₂ uptake, and compressive strength of the resulting steel slag/cement composites. The findings demonstrate that both CO₂ uptake and compressive strength increase with extended carbonation times, achieving a maximum CO₂ uptake of 5.1 % and a compressive strength of 63.38 MPa after 12 h of carbonation. Carbonation depth reached approximately 4 cm after 12 h for 20 × 20 × 60 mm³ specimen (unidirectional carbonation). The samples were sectioned at 1 cm intervals along the vertical axis, labeled C1–C5, and significant variations were observed in CO₂ uptake, carbonation degree, pore structure, polymerization degree, and microhardness values across different depths. Specifically, the carbonation degree ranged from 24.3 % to 7.1 % between specimens C1 and C5. Carbonation significantly reduced porosity and enhanced pore structure, with the extent of reduction inversely related to carbonation depth. Calcite was identified as the primary carbonation product, with its formation becoming denser at higher carbonation levels. Furthermore, the unreacted calcium silicate phase was encapsulated by CaCO₃, and an outer silica-rich layer formed a protective shell, inhibiting further CaCO₃ formation.