The cement industry contributes approximately 8% of global CO₂ emissions, with each ton of clinker production for releasing nearly one ton of CO₂. Mineral admixtures such as slag, fly ash, and silica fume become essential for developing sustainable concrete, enabling a partial replacement of cement (i.e., 15%-45%, mass fraction), while enhancing durability through pozzolanic reactions. However, high-volume substitutions of exceeding 30% critically depress a pore solution alkalinity from pH value 13.3 to 12.5-12.8, destabilizing passive films on conventional carbon steel (LC) and elevating corrosion risks by 3-5 times. This alkalinity reduction has severe threats to structural integrity in marine environments and deicing salt exposure, driving global infrastructure repair costs beyond $2.5 trillion annually. A Cr10Mo1 alloy corrosion-resistant steel (LA) was developed, having a chloride threshold 10 times greater than that of LC in simulated pore solutions. However, its passivation behavior in realistic cement-mineral admixture systems with synergistic interactions of Ca²⁺, SO₄^2-, and Al³⁺ ions remains unexplored. In this study, the passivation mechanisms of the steel in complex paste extracts were investigated to provide foundational data for designing durable and sustainable infrastructure.

The passivation kinetics were analyzed by electrochemical techniques like linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis. The LA and LC electrodes (φ10 mm×10 mm) were polished to a 2000-grit finish and immersed for 10 d in extracts derived from cement pastes containing different mineral admixtures of 0%, 15%, 30%, and 45% (slag/fly ash = 1∶1), designated as 0SF, 15SF, 30SF, and 45SF. The paste extracts were prepared with P·O 42.5 cement at a water-to-binder ratio of 1∶1, cured at 25 ℃ for 14 d, filtered through 0.45 μm membranes, and stored in nitrogen to prevent carbonation. The measured pH values reduced progressively from 13.4 (0SF) to 12.5 (45SF). Electrochemical testing utilized a three-electrode cell with a platinum counter electrode and a saturated calomel reference electrode (SCE). LPR scans (±20 mV vs. open-circuit potential at 0.166 7 mV/s) and EIS measurements (10 mV amplitude, 0.01-1.00×10⁴ Hz frequency range) were conducted after passivation. The Mott-Schottky analysis was performed at 1000 Hz with potential sweeps from -1.5 V to +0.25 V vs. SCE (50 mV/s). Equivalent circuit modeling (R_s-[CPE_dl-R_ct]-[CPE_f-R_f]) was used to fit the EIS data to resolve solution resistance (R_s), passive film properties (CPE₁, R₁), and charge transfer resistance (CPE₂, R₂). X-ray photoelectron spectroscopy (XPS) was used to characterize compositional gradients by an Axis Ultra DLD spectrometer with Al K_α radiation (1486.6 eV) and Ar⁺ sputtering (0.02 nm/s). The spectra of Fe 2p₃/₂ and Cr 2p₃/₂ were deconvoluted via the Gaussian-Lorentzian peak fitting.

The steels of LA and LC both can be effectively passivated in all extracts, while demonstrating fundamentally divergent responses to increasing mineral admixture content. The LA displays significantly an enhanced passivation, evidenced by an 81% increase in polarization resistance (i.e., R_p: 348 kΩ·cm² in 0SF to 630 kΩ·cm² in 45SF) and a positive shift in corrosion potential (i.e., E_corr: -157 mV to -128 mV). Conversely, the LC suffers a degraded passivity with a 54% reduction in R_p (i.e., 348 kΩ·cm² to 160 kΩ·cm²) and a negative E_corr shift to -208 mV. The Nyquist plots corroborate these results, revealing expanded capacitive arcs for the LA and contracted arcs for the LC in low-pH value extracts. The charge transfer resistance (R_ct) for the LA increases from 5.4×10⁵ Ω·cm² (0SF) to 6.4×10⁵ Ω·cm² (45SF), which is decreased by 44% (i.e., from 5.9×10⁵ Ω·cm² in 0SF to 3.3×10⁵ Ω·cm² in 45SF) for the LC. The Mott-Schottky analysis further distinguishes their electronic properties. The LA exhibits a n-p heterojunction behavior characterized by positive slopes in the -1.00 V to +0.25 V region and negative slopes between -1.5 V and -1.0 V, with decreasing total carrier density (N_total) from 4.96×10²⁰ cm^-3 (0SF) to 1.93×10²⁰ cm^-3 (45SF). In contrast, the LC maintains a n-type semi-conductivity with increasing N_total from 4.93×10²⁰ cm^-3 to 7.09×10²⁰ cm^-3.

XPS depth profile shows critical differences in passive film architecture. The LA forms a bilayer structure comprising an outer layer (0-5 nm) of FeOOH/Fe(OH)₃ (712.2 eV) and Fe₂O₃ (710.7 eV), and an inner layer (5-15 nm) enriched with Cr₂O₃ (576.3 eV), Cr(OH)₃ (577.1 eV), and FeO (709.5 eV). Reduced alkalinity triggers an increase of 32% in Cr-oxide content (i.e., Cr_ox+hy/Cr_metal=2.8 in 45SF vs. 2.1 in 0SF), forming a dense chromium-rich barrier. The LC develops a monolayer Fe-oxide film (0-10 nm) containing Fe₂O₃, FeOOH, and FeO, with decreasing FeO content by 67% (i.e., Fe²⁺/Fe³⁺=0.18 in 45SF vs. 0.55 in 0SF). Mechanistically, a low alkalinity elevates the oxidation potential of chromium in the LA, promoting Cr³⁺ incorporation into the inner film. The resultant Cr₂O₃ (p-type semiconductor) establishes a high-energy barrier (i.e., >1.2 eV) against anion penetration, while coexisting FeO (n-type) stabilized cation transport. For the LC, FeO decomposition (Fe²⁺→Fe³⁺) at a low pH value increases oxygen vacancy density, accelerating ionic transport through the film and diminishing chloride adsorption capacity resulted from loss of [Fe²⁺-Cl^-] coordination sites.

Mineral admixtures content variation at 15%-45% could affect the passivation behavior of Cr10Mo1 alloy corrosion-resistant steel (LA) and ordinary carbon steel (LC). The two steels exhibited a good passivation in the paste extract solutions, but had different performances. As the amount of mineral admixture increased, the passivation of the LA enhanced, while the passivation effect of the LC gradually weakened. The passivation film of the LA was a bilayer structure, with the outer layer primarily composed of FeOOH/Fe(OH)₃ and Fe₂O₃, and the inner layer mainly consisting of Cr₂O₃, Cr(OH)₃, and FeO. The passivation film of LC was a monolayer structure, mainly containing Fe₂O₃, FeOOH/Fe(OH)₃, and FeO. The alkalinity of the paste extract solution decreased, and more Cr elements involved in the formation of the passivation film of the LA with the increase in mineral admixture content, enhancing its passivation. Meanwhile, for the LC, Fe oxides in the passivation film gradually decomposed, and the content of FeO that provided a decreased protection, leading to a weakened passivation. This study indicated that the introduction of mineral admixture could not hinder the passivation effect of the LA, but weaken the passivation strength of the LC.