Synergistic ultrafine steel slag and fly ash for steel passivation in sustainable ternary cements: Temperature-dependent stability and composition regulation

Abstract

The utilization of industrial by-products like ultrafine steel slag (USS) and fly ash (FA) in cementitious systems offers a pivotal strategy for decarbonizing the construction sector and valorizing solid waste. This study investigates the synergistic role of USS and FA in designing and regulating the passive film on reinforcing steel within sustainable ternary cements, with a particular focus on its temperature-dependent stability. Through an integrated approach of pore solution chemistry, multi-scale electrochemistry, and surface characterization, we deciphered the coupled mechanisms of ion transport, passive film evolution, and defect chemistry governed by the composition. The results indicate that while FA elevates K⁺ concentration, it reduces pore solution alkalinity (pH 12.56), falling below the passivation threshold. A moderate USS content (30%) optimally compensates by dissolving Ca-bearing phases, restoring the pH to levels conducive to passivation (12.95), and facilitating the formation of a superior bilayer passive film. This regulated film exhibits high impedance, low defect density, and an inner barrier layer enriched with Fe²⁺ and lattice oxygen, offering exceptional protection. Conversely, excessive USS incorporation (50%) increases surface roughness by 63%, exacerbating heterogeneity and ionic permeability, Furthermore, the stability of this optimal formulation demonstrates a critical temperature dependence. Curing at ≥40 °C accelerates film hydration and hydroxylation. It also promotes deep-layer defect proliferation, which markedly degrades the protective performance. This work unveils the fundamental mechanisms of composition-driven passivation, providing a principle for designing sustainable low-carbon cementitious systems by tailoring properties to achieve long-term material service life.