Steel slag as a modifier in NaOH-activated blast furnace slag: Carbonation-induced functionality change

This study investigates the role of steel slag with different carbonation degrees as a CO 2 -responsive additive in NaOH-activated blast furnace slag (AAS) systems. At a 20 % replacement level, partially carbonated slag (CS1) increased the 28-day compressive strength by 65 %, while fully carbonated slag (CS2) achieved an 83 % improvement. Calorimetry revealed that both CS1 and CS2 delayed the main hydration peak due to the buffering effect of carbonate phases, yet overall reactivity increased with carbonation through the enhanced dissolution of carbonation-induced silica gels. Mechanistically, CS1 retained residual C 2 S that hydrated gradually, resulting in moderate early strength but sustained long-term gain through the gradual formation of additional C-(A)-S-H gels. By contrast, CS2 supplied abundant reactive silica and carbonate species, which accelerated early gel polymerization, stabilized pH, and promoted matrix densification. Nanoindentation confirmed this effect, showing that the elastic modulus of C-(A)-S-H increased by over 50 %, evidencing a consistent strengthening of the gel network. These findings highlight carbonation degree as a controllable parameter to balance early and long-term performance, providing a mechanistic framework for the design of multifunctional binders with integrated CO 2 sequestration and enhanced performance potential.