Red mud (RM), a typical bulk industrial solid waste from alumina production, poses risks of soil alkalization and heavy metal pollution through traditional disposal methods. Utilizing it as a cement supplementary material or partial raw material substitute can significantly reduce stockpiling needs and achieve waste valorization. Thus, the combination of calcium sulfoaluminate cement (CSA) and RM accomplishes multiple objectives: cost reduction, solid waste consumption, and environmental emission mitigation, representing a potential breakthrough for green transformation in the construction materials industry. The application scenarios for cement-based materials are diverse and complex. However, the rapid hydration and setting of CSA cement often fail to meet the varied demands of practical engineering, making the dynamic regulation of CSA concrete hydration and setting particularly important. Sodium citrate (SC), as a retarder, offers a promising solution to this industry challenge. Compared with ordinary Portland cement (OPC), CSA hydration products are primarily ettringite and aluminum gel, without significant formation of Ca(OH)2, typically resulting in a lower pore solution pH. This makes steel bars more susceptible to chloride-induced corrosion. The coupled use of RM and SC provides a potential remedy: RM contains sodium aluminosilicate minerals that slowly hydrolyze to release alkaline species, thereby increasing the pore solution pH; meanwhile, SC functions not only as a retarder but also as a corrosion inhibitor. This synergy provides a theoretical basis for enhancing the corrosion resistance of steel in CSA systems. Nevertheless, existing studies on the time-dependent corrosion performance of steel in RM-CSA (CSA cement-based material mixed with RM) systems are limited, and the influence of SC addition on corrosion behavior remains unclear.
This study comprehensively investigated the corrosion behavior of steel in CSA systems modified with SC and RM. The corrosion electrochemical response and mechanisms were studied by electrochemical analysis. Static weight loss tests, along with scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy, were employed for qualitative and quantitative analysis of corrosion rate, post-corrosion morphology, and composition, aiming to elucidate the enhancement mechanisms of SC and RM on the corrosion resistance of steel in CSA cement, and provide a theoretical basis for anti-corrosion strategies in practical engineering applications of CSA concrete.
The results showed that compared with the plain CSA solution, the concentrations of SO42-, K+, Na+, and Ca2+ increased in both the CSA-RM and CSA-RM-SC (CSA cement-based material mixed with SC and RM) solutions (especially in the CSA-RM group), while the concentrations of Si and Al decreased. The critical chloride concentration (Ccr) for steel in the CSA solution was 0.04 mmol/L, whereas the values for the CSA-RM and CSA-RM-SC groups were 0.08 mmol/L and 0.18 mmol/L, respectively. The corresponding corrosion rates (CR) were 0.492 mm·a-1, 0.363 mm·a-1, and 0.296 mm·a-1. Electrochemical studies indicated that the CSA-RM-SC group exhibited the smallest decrease in Rs, Rox, and Rct values (indicating superior resistance to chloride attack). After 7 days of corrosion, these values were 19.21 Ω·cm2, 53.14 kΩ·cm2, and 59.97 kΩ·cm2, respectively. In contrast, the plain CSA group showed the largest decrease, with final values of 10.33 Ω·cm2, 22.45 kΩ·cm2, and 35.14 kΩ·cm2, indicating the most severe steel corrosion. The corrosion products on steel in the chloride-containing CSA solution appeared as extensive, contiguous patches of dark yellow rust, with significant corrosion severity and depth (the thickness of the rust layer was slightly higher than 10 nm). In comparison, steel in the CSA-RM-SC solution showed the mildest corrosion, with only localized pitting observed on the surface (the thickness of the rust layer was about 3 nm).
The corrosion resistance of steel bars in the CSA-RM solution was improved compared with the plain CSA solution (the thickness of the rust layer was between 7-10 nm). This was attributed to the incorporation of RM, which increased the solution alkalinity and sulfate ion concentration, enhancing the competitive adsorption of OH- and SO42- ions on the steel surface and thereby weakening the depolarization effect of chloride ions. Meanwhile, citrate ions from SC formed chelates with metals such as Fe and Al (e.g., Fe-Cit and Al-Cit). These chelates adsorbed and precipitated on the steel surface during the passivation stage. Under the coordinated action of RM and SC, the chloride resistance of steel in the CSA-RM-SC solution was significantly enhanced.
Key words
red mud /
citrate /
calcium sulfoaluminate cement /
mild steel bars /
corrosion resistance
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Funding
Natural Science Foundation of China (52408258, 52271355); Henan Province Science and Technology Research Project (262102321160, 262102321150); Henan Key Laboratory of Infrastructure Corrosion and Protection (HNICP 202410); Henan Key Laboratory of Materials on Deep-Earth Engineering (Henan Polytechnic University) (MDE2025-01)
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