Steel-making slag is a type of byproduct generated during the manufacturing of steel, which has been usually used as aggregates or supplementary cementing materials after being ground into fine powders.However the relatively low hydraulic property of the steel-making slag and high contents of free CaO and MgO included in the slags which may cause volume instability hinder the wide application of the steel-making slag in construction materials.As reported, the carbonation of slag could consume the free-CaO and MgO and meanwhile produced binding performance and thus could be used for manufacturing construction materials.However, the carbonation rate of slag under normal atmosphere condition with low CO2 concentration (e.g.0.4%) is extremely slow.In this study, paste specimens were prepared with slag alone or the slag-cement blends containing 20% Portland cement as a replacement, and then the CO2 with a pressure of 0.1 MPa was used for carbonating the specimens for various exposure times from ld to 14d.The compressive strengths,microstructures in terms of pore structure and morphologies, and carbonation degree of the slag were investigated.Results showed that after accelerated carbonation, both the specimens of slag and cement-slag blends exhibited very high compressive strength.For example, after 14d of exposure to 0.1 MPa CO2, the compressive strength of the 100% slag paste was 44.1 MPa.The longer exposure period, the greater carbonation degree of slag gained and the higher compressive strength the specimen gained.The strength gain was mainly attributed to the formation of calcite, which resulted in densification of the microstructures of paste specimens.