Upcycling iron-rich industrial waste into a carbon-sequestering composite binder through optimized carbonation curing for structural applications

Abstract

Background: Steel production generates large quantities of mill scale, a by-product rich in iron oxides, with global generation estimated at 13.5 million tons annually. Simultaneously, Portland cement production, essential for concrete, contributes nearly 8% of global CO<inf>2</inf> emissions, highlighting the urgent need for low-carbon alternatives. Iron carbonate (FeCO<inf>3</inf>), typically regarded as a corrosion product, offers an underexplored opportunity for deliberate synthesis in binders to achieve both structural performance and CO<inf>2</inf> sequestration. Repurposing mill scale into carbon-sink binders thus provides a dual pathway for waste valorization and climate change mitigation, while advancing circular economy and industrial symbiosis principles. Methods: A composite binder was developed using mill scale, fly ash, metakaolin, and limestone, with oxalic acid employed as a chelating agent to promote iron dissolution and carbonate formation. Specimens were subjected to carbonation curing under controlled CO<inf>2</inf> pressures (1.5–3 bar) and analyzed using XRD, TGA/DTG, FTIR, and FESEM to evaluate phase development, carbonate formation, and microstructural features. Results: An oxalic acid dosage of 4% resulted in a 133% increase in compressive strength compared to the control. Specimens cured at 3 bar CO<inf>2</inf> achieved compressive strength exceeding 65 MPa within 7 days, whereas 1.5 bar curing required 9 days. TGA confirmed CO<inf>2</inf> uptake of approximately 10–11% by binder mass, while microstructural analysis revealed the presence of stable siderite and calcite phases. Conclusions: The carbon-sink binder, composed of more than 75% industrial by-products, substantially reduces carbon emissions and energy demand compared to cement-based systems. It shows strong potential as a low-carbon alternative for precast concrete, masonry, and pavement applications. Future work should focus on long-term durability, large-scale implementation, and life cycle performance to support its adoption in sustainable construction and policy frameworks. © 2025 Informa UK Limited, trading as Taylor & Francis Group.