Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation

Calcium sulfoaluminate cement (CSA) is a type of low-CO2 binder which has been widely applied in the production of concrete. To investigate the protection capability offered by CSA to keep steel from corrosion, the pore solution chemistry of CSA on steel passivation was investigated in this study. The pore solution of CSA pastes, extracted by an ex situ leaching method, was studied and compared with ordinary Portland cement (OPC). The results show that the alkalinity of the CSA pore solution is not only much lower than that of OPC, but also that a new type of ion, Al(OH)4−, and high concentration of SO42− were detected in the liquid phase of CSA. Based on pore solution chemistry analysis, a simulated pore solution (SPS) system was designed to assess the comprehensive impact of alkalinity and ion composition, featured as properties of CSA, on steel passivation. The results of the corrosion potential evolution highlight the importance of alkalinity in passivation. SO42− can cause depassivation when there is not enough hydroxyl, but Al(OH)4− is able to maintain the alkalinity of the system, enhancing the stability of the passive film.