Abstract: Red mud (RM), as a polluting waste residue discharged during the extraction of alumina in the aluminum industry, is expected to replace some cementitious materials and add a moderate amount of citrate retarder, which can reduce the economic cost of sulfoaluminate cement (CSA) in the field of infrastructure repair and produce favorable environmental effects. However, the effect of the addition of RM and sodium citrate (SC) retarder on the time-varying passivation performance of low carbon steel bars in CSA is still unclear. This paper systematically investigates the effects of RM and SC on the passivation behavior of steel in CSA. The time-varying behavior of passivation performance of steel was investigated through electrochemical analysis testing methods and advanced microstructure analysis testing techniques. The results indicate that CSA doped with RM participates in the formation of the passivation film in the form of reductive S²⁻, reducing the thickness of the passivation film from 5 nm to 3 nm and increasing the passivation time from 4 days to 7 days. It also weakens the integrity and corrosion resistance of the passivation film. For the addition of RM and SC to CSA, the synergistic adsorption and precipitation of RM and SC weaken the adverse effects of RM on the passivation film of steel, and enhance the passivation ability and corrosion resistance of steel in CSA (The passivation film thickness and passivation time are approximately 6 nm and 2 days, respectively).