Accelerated curing of fresh concrete using CO2 is a possible approach for value-added, high-volume usage products from waste CO2 emitted from stationary sources. The extent of CO2 uptake and the spatial distribution of the CaCO3(s) precipitates formed during accelerated carbonation curing of compacted, 4-h hydrated cement mortar (fresh concrete mixture with fine aggregates) samples were investigated in this study. The maximum carbonation efficiency achieved was 20% of the theoretical uptake. Microprobe imaging was used to analyze the composition of the compacted cement mortar microstructure and showed extensive filling of pores of diameters 4 mu m and smaller, with CaCO3(s). The carbonation efficiency, however, reached 67% when an aqueous suspension of cement was carbonated in a completely mixed reactor, where interparticle pores do not exist and a higher surface area of cement particles is exposed to dissolved CO2. The theoretical efficiency was not achieved because all reactive cement surfaces were saturated with carbonation products, as indicated by equilibrium concentrations of dissolved calcium, silica, inorganic carbon, and pH. This study shows that both deposition of CaCO3(s), on reactive surfaces, and pore filling may regulate the extent of CO2 uptake during accelerated carbonation curing of concrete.