Steam Efficiently Enhancing CO2 Direct Mineralization Steel Slag Towards Actual Production: Phase Evolution, Microstructure, and Mechanisms

About 120 million tons of steel slag are produced annually in China, making it one of the largest sources of industrial solid waste; however, its utilization rate remains only around 30%. The presence of f-CaO is the main factor in its widespread application. Currently, the carbonation of steel slag is mainly through indirect wet mineralization, which is difficult to implement on an industrial scale. Direct dry carbonation, on the other hand, consumes more energy due to its slow kinetics. In this study, steam coupled with CO2 was used to directly mineralize steel slag, a process fully compatible with existing iron and steel industry treatment processes. The required temperature can be achieved using the waste heat from hot steel slag, eliminating the need for additional heat supply. With 15% steam injection, the CaCO3 content increased to 12.02 g/100 g (52.8 kg CO2 t−1 slag utilization), representing a 16.7% improvement. After mineralization, the f-CaO decreased to 0.61%, with 91.73% of f-CaO in steel slag mineralized. The mineralization efficiency of f-CaO increased by 20.24%. This enhancement was attributed to steam entering the interior pores of steel slag, generating intermediate Ca(OH)2, causing steel slag particle breakage and fully exposing the previously enclosed f-CaO for complete carbonation. To further utilize flue gas, the effects of different CO2 concentrations on carbon fixation were investigated. At a concentration of 20% CO2, the carbon fixation reached 69.90% of that achieved at 100% CO2. This research not only addresses the stability issues of steel slag but also reduces CO2 emissions and effectively utilizes waste heat, making the process suitable for large-scale industrial application.