Due to the large amount of CO2generated during steelmaking, to resume production as soon as possible, a fast repair material with good carbonation resistance is needed to repair the factory building. First, the performance of an ordinary Portland cement (OPC)-calcium sulfoaluminate cement (CSA) system under an accelerated carbonization environment was studied. Next, the OPC-CSA system with a CSA content of 15 wt% was selected to be modified by adding calcium hydroxide (CH). The findings showed that the addition of 15 wt% CSA to the OPC-CSA system resulted in the highest mechanical properties. Specifically, the flexural strength and compressive strength after 84 d of carbonization were 18% and 15% higher, respectively, compared to those of OPC alone. The degradation of the mechanical properties of the OPC-CSA system due to carbonation was improved by adding CH. The flexural strength (3.0 wt% CH) and the compressive strength (4.5 wt% CH) of the OPC-CSA-CH system after 84 d of carbonization were 13% and 5% higher, respectively, than those of the OPC-CSA system. The addition of CH increased the alkalinity of the OPC-CSA system and enhanced the stability of Aft, resulting in better carbonation resistance in the OPC-CSA-CH system.