The paper reports the results of slurry-phase accelerated carbonation experiments using basic oxygen surface steel slag as the source of alkalinity for CO2 sequestration. In particular, the effect of particle size distribution on the carbon sequestration performance was investigated in detail. To this aim, different size classes of the slag were separately subjected to accelerated carbonation, and the CO2 uptake, the mineralogical changes, the final particle size distribution and the chemical equilibria in the liquid phase were studied. While the total content of potentially reactive elements in the different size fractions of the slag exerted only a minor influence on the carbonation performance, particle size was found to have a dramatic role during the process, in particular during the dissolution stage. Changes by up to two orders of magnitude in CO2 sequestration (which varied in the range 4.7-465 g of CO2 sequestered per kg of slag) were observed for the different size classes investigated. Geochemical modeling of the liquid phase after the carbonation treatment showed that the system was dominated by carbonate minerals of several elements including Ca, Mn, Mg, Fe and Zn, indicating the involvement of several components in the carbonation reactions. The presence of carbonate minerals containing not only Ca, but also Mn, Mg and Fe was also detected through XRD analyses. The analysis of the pore size distribution of the slag before and after carbonation revealed the competing effects of particle erosion/dissolution and pore clogging as a result of carbonate precipitation, the relative contribution of which was found to be related to the carbonation degree attained. (C) 2016 Elsevier B.V. All rights reserved.