Influence of Pore Solution Chemistry on the Evolution of Steel Passive Films in Ferrite–Aluminate Cement and Fly Ash-Blended Systems

Based on comparative experiments conducted on conventional Portland cement (OPC) systems, ferraluminate cement (FAC) systems, and FAC–fly ash (FA) composite systems, this study systematically investigates the ion evolution in pore solutions and the transformation of hydration products, as well as the compositional and microstructural evolution of the passive film formed on steel reinforcement surfaces under simulated pore solution conditions and natural passivation conditions. The results show that: ① FAC and FA-mixed systems produce different product types from OPC in the early stage (mainly AFt/AFm, C–(A)–S–H), thereby altering the pH and the evolution of the main ion concentration in the pore solution. ② An appropriate amount of FA (10%) refines the pore structure through pozzolanic reaction, enhances low-frequency electrochemical impedance, and facilitates the evolution of the steel passive film toward a more favorable composition; however, excessive incorporation (20%) induces a “dilution effect” and reduces the early-age densification rate, which is ultimately detrimental to long-term corrosion resistance. ③ The passive film exhibits a characteristic chemical gradient evolution, comprising an outer Fe3+-enriched phase and an inner Fe2+-enriched phase. Its thickness and the Fe2+/Fe3+ ratio are significantly influenced by the chemical environment of the pore solution (pH, SO42−, Al3+, Fe3+, etc.), which plays a decisive role in the protective efficiency of the steel reinforcement. These findings establish the intrinsic relationship between pore solution chemistry, hydration product evolution, and passive film development, providing new mechanistic insight into the passivation behavior of reinforcing steel in FAC–FA systems. Based on these findings, key guidelines for the proportioning and microstructural–electrochemical design of ferroaluminate cement are proposed, providing scientific support for the durability-oriented application of FAC–based materials in aggressive environments. It should be noted that the beneficial effect of incorporating 10 wt.% FA is limited to the optimization of FAC–based systems and should not be interpreted as indicating superior overall corrosion resistance compared with OPC.