Tartrate-induced passivation of carbon steel in belitic calcium sulfoaluminate (BCSA) cement systems

Belitic calcium sulfoaluminate (BCSA) cement is an emerging low-carbon alternative to ordinary Portland cement (OPC), but its ability to protect carbon steel remains unclear. This uncertainty is largely attributed to whether steel can be effectively passivated in the specific pore solutions of hydrated BCSA cement, which are characterized by low alkalinity and high sulfate concentration. This study investigated the passivation behavior of carbon steel in the BCSA cement system using both simulated pore solutions and reinforced mortars, as well as the effect of sodium tartrate on passivity enhancement. After being immersed in the simulated pore solution of BCSA cement for 10 days, or after being embedded in BCSA mortar for 28 days, steel fails to passivate spontaneously and falls into an active corrosion state, accompanied by the appearance of corrosion pits and corrosion products. Tartrate not only acts as a retarder for BCSA cement but also forms adsorption layers on the steel surface, thereby isolating endogenous aggressive ions that are detrimental to passivation and ultimately enhancing the steel’s ability to passivate. However, there remains a significant gap compared to the OPC counterparts, likely related to the tartrate dosage. These findings facilitate the transition of BCSA cement from a repair material to a structural one, thereby contributing to low-carbon development in construction engineering.