Rheology and phase formation of cement pastes incorporating CO2-activated steel slag as a supplementary cementitious material

Abstract

With the growing interest in utilizing metallurgical slag for mineral carbonation, there is an urgent need to explore the application of flue gas-carbonated slag. This study investigated the microstructure of steel slag powder (SSP) carbonated under 10 % CO₂ concentration and its impact on the hydration, rheological behavior and compressive strength of cement pastes. The primary carbonation products of SSP were nano-CaCO₃ and amorphous aluminosilicate gel, with aragonite and calcite forming as the main calcium carbonate crystals. Nano-CaCO₃ and amorphous gel actively participated in cement hydration, forming monocarboaluminate and C-S-H gel, respectively. These reactions enhanced the degree of hydration and strengthened the interface between SSP and the hydrated cement matrix. In addition, the incorporation of moderately carbonated SSP (3 h) improved flow stability and reduced thixotropy due to better particle dispersion, while extended carbonation increased viscosity and structural recovery. The compressive strength of cement pastes containing 20 wt% SSP carbonated for 3 h was 21.6 % higher than those with uncarbonated SSP, along with a 4.5 % CO₂ uptake in SSP. However, a higher degree of carbonation reduced the reactivity of SSP as a supplementary cementitious material (SCM). This study provides scientific insights and a practical method for preparing a high-quality SCM by utilizing flue gas and SSP as a cement substitute.