Service life predictions of reinforced concrete structures are underpinned by the passivation film chemistry, structure, and thickness. In this work, we present how the formation of passive films at the steel–concrete interface of reinforced alkali‐ and salt‐activated slag materials can be affected by the pore solution chemistry, namely, pH, Eh, and chemical composition. Thermodynamic simulations are used to illustrate the time‐dependent changes to the pore solution chemistry, where estimated electrical conductivities of the pore solution are used as a single‐value parameter to understand the solution complexity and capacity for charge transfer. A set of passivation reactions are proposed to understand the effects of OH− and HS− (reduced sulfur species) competition on the passivation pathways. These passivation reactions become more complex considering that a reducing pore solution might not establish until 3 days into the curing process—a crucial factor explaining the significant differences in the phase composition of passive films of activated slag materials (FeOOH, FeS). This study sheds light on recent progress in understanding these initial passivation reactions, emphasizing the essential role of material design and, therefore, the pore solution chemistry of these cements, along with significant insights for vital research and development concerning the corrosion durability of activated slag materials. [ABSTRACT FROM AUTHOR]