This study investigated the feasibility of iron-based binders under CO2 curing and the development of sustainable construction materials. Samples were prepared using varying concentrations of oxalic acid (0%, 2%, and 4%) using different mixing media (normal water and seawater), and their mechanical properties (compressive and flexural strength) and microstructural characteristics were evaluated. The results demonstrate that the best-performing oxalic acid concentrations—2% for normal water and 4% for seawater, which enhances compressive and flexural strengths by promoting carbonation reactions, leading to denser microstructures and the formation of stable carbonate phases. Excessive oxalic acid, however, hampers phase development and reduces strength due to increased acidity, porosity, and formation of weaker phases. Microstructural analysis revealed that moderate additions facilitate uniform pore distribution and controlled porosity, which accelerates carbonation and densification, while higher dosages or larger interconnected pores diminish performance. The mineral content of seawater, including Mg² ⁺, Cl ⁻, and SO₄² ⁻, synergistically interacts with oxalic acid to further enhance carbonation, resulting in superior mechanical strength and structural integrity compared to normal water. These findings emphasize the importance of optimizing additive contents and leveraging seawater chemistry to develop sustainable, high-performance iron-based binders with improved microstructural and mechanical properties.