Steel slag is a kind of alkaline mixture and considered to be a potential CO2 adsorbent. In this work, the CO2-trapping characteristics of two types of steel slag, basic oxygen furnace (BOF) steel slag and electric arc furnace (EAF) steel slag, were experimentally investigated. Generally, the higher the temperature, the larger the Ca use of slag. However, the Ca use at 550 degrees C would be lower than that at 500 degrees C for a certain CO2 concentration. The CO2 concentration also has an effect on the Ca use. At higher temperatures, a larger Ca use appears at a lower CO2 concentration (<10%) or higher CO2 concentration (>75%). As the CO2 concentration decreases, the reaction rate of carbonization increases, regardless of the kind of slag used. With regard to the type of steel slag, EAF steel slag is better than BOF steel slag in reactivity and Ca use. All of the results indicate that the carbonation reaction of steel slag is controlled by not only the reaction kinetics but also the diffusion of the reactive gas CO2. Steel slag has a capacity to capture and permanently sequester CO2. However, what is more important is that it can be used in different flue gases, where the CO2 concentration is typically lower (<20%) or fairly higher (>75%). This provides the steel slag a wide application market.