Carbonated steel slags as supplementary cementitious materials:reaction kinetics and phase evolution

With increasing interest in utilizing metallurgical slags for mineral carbonation, there is a need to find applications for carbonated slags. The scope of this study is to explore the utilization potential of carbonated steel furnace slags (SFS) as supplementary cementitious materials (SCM) to produce low-CO₂ cement-based materials. Two different types of SFS are studied with comparable amounts of major oxides but significantly different mineral phase compositions. In the first phase of the study, several parameters that affect the CO₂ uptake during mineral carbonation are considered (temperature, CO₂ pressure, particle size, and duration) for the two slags namely, basic oxygen furnace slag (BOFS) and desulfurized slag (DeSS). Among the two slags, the DeSS exhibited significantly higher degrees of carbonation than BOFS, and the higher carbonation of DeSS is attributed to the presence of Ca as Ca(OH)₂ rather than Ca–Si in BOFS. For both the slags, increase in temperature and CO₂ pressure generally led to increase in the degree of carbonation. In the second phase of the study, 30 wt% of white Portland cement (WPC) was replaced by carbonated slags (used as SCM) in the cement mixture. The cement mixture with the BOFS carbonated at 60 °C and 1 bar CO₂ pressure, as SCM, exhibited compressive strength comparable with that of WPC. Higher degrees and rates of carbonation under different conditions appears to have reduced their reactivities as SCM.