Semi-dry and aqueous carbonation of steel slag: Characteristics and properties of steel slag as supplementary cementitious materials

Accelerated carbonation of steel slag presents a viable method for CO2 sequestration and the valorized utilization of steel slag. This study aimed to investigate the impact of semi-dry and aqueous carbonation, as well as the carbonation degree, on the microstructure and properties of steel slag. Additionally, it evaluated the feasibility of utilizing carbonated steel slag as supplementary cementitious materials (SCMs). Steel slag was subjected to CO2 (20% volume percentage) at 2 bar pressure for both semi-dry and aqueous carbonation process at room temperature with water/solid ratio of 0.08 and 5, respectively. The carbonation duration was varied (10 min, 30 min, 60 min, and 120 min) to adjust the degree of carbonation. Mortars and pastes with 30 wt% carbonated steel slag were formulated and their compressive strength and volume expansion values were quantified. The findings revealed that the CO2 uptake values for steel slag after 2 hours of semi-dry and aqueous carbonation were only 3.75% and 1.50%, respectively. Compounds such as Ca(OH)2, Mg(OH)2, and Ca2SiO5 demonstrated high carbonation reactivity during the semi-dry carbonation process, whereas Ca(OH)2 and Mg(OH)2 exhibited only a marginal increase in aqueous carbonated steel slag. The specific surface area of steel slag increased with an increase in the carbonation degree, attributed to the formation of silica gel or amorphous C-S-H during carbonation. However, the hydration activity of carbonated steel slag diminished with an increased degree of carbonation due to the consumption of alkaline minerals. Conversely, pozzolanic activity was enhanced with a higher degree of carbonation, owing to the formation of silica gel. The cumulative heat release of cement containing SDC30 (semi-dry carbonation for 30 min) within 24 h and 72 hours showed increases of 16.0% and 3.5%, respectively, aligning with the compressive strength results. Semi-dry carbonation treatment significantly enhanced the early-stage cementitious properties of steel slag/cement composites. Specifically, the 3-day compressive strength of mortar containing 30 wt% SDC30 increased by 22.7% relative to the reference mortar, composed of 30 wt% uncarbonated steel slag and 70 wt% Portland cement. Carbonation markedly improved the volumetric stability of steel slag with the extent of improvement was positively correlated with the carbonation degree for both semi-dry and aqueous carbonated steel slag. Aqueous carbonation was more effective in promoting the hydration process of free CaO and MgO more effectively than semi-dry carbonation.