Industrial mineral carbonation of alkaline wastes, an increasingly promising component of carbon capture and storage, may play an important role as a CO2 mitigation strategy in the context of climate change. Steelmaking slags are of particular interest owing to their high content of calcium. The cumulated 'effective' CO2-specific sequestration capacity (calculated on the basis of calcium and magnesium extracted to a 0.5 M HNO3 solution) of three basic oxygen and one electric arc furnace slags generated at steel mills in South Africa was 253 kt CO2 per annum, which was 25.2% lower than their cumulated 'theoretical' capacity (estimated on the basis of total calcium and magnesium content in slags). The mineralogical composition and solubility characteristics of slags conferred very distinct leaching behaviours to the slags, including differences in: (i) the amount of heat generated during their dissolution, (ii) their buffering capacity, (iii) the rate and extent of calcium and magnesium extraction from the slags, and (iv) the mineralogical composition of the non-dissolved residues. These findings suggest that separate leaching processes may need to be developed for slags with largely distinct mineralogical compositions and structural features. (C) 2009 Elsevier Ltd. All rights reserved.