Performance optimization and evaluation of ferronickel slag concrete under carbonation using multi‐criteria decision‐making approaches

Abstract Although ferronickel slag (FNS) has been investigated as a supplementary cementitious material, its performance under accelerated carbonation, particularly under varying water to cement (w/c) ratios, remains insufficiently understood. The experimental program evaluated FNS replacement levels up to 60% at w/c ratios of 0.4, 0.5, and 0.6. Mechanical and durability properties, including compressive strength, water absorption, and rapid chloride permeability, were assessed at 28 days and following accelerated carbonation exposure. Microstructural analysis, performed using XRD, FTIR, and SEM, provided insight into phase development and morphological changes. At 28 days, results showed that both increasing w/c ratio and higher FNS replacement reduced compressive strength, primarily due to dilution and slower hydration. However, accelerated carbonation produced a 12% strength gain in mixes with 30% FNS at a w/c ratio of 0.4, exceeding the control, due to pore filling and densification by CaCO 3 and MgCO 3 . In contrast, mixes with higher w/c and FNS content exhibited strength loss up to 18%, highlighting the dominant role of pore volume. Microstructural evidence confirmed that MgO in FNS stabilized Mg‐carbonates, thereby reducing portlandite depletion and improving matrix compactness. To integrate multiple performance indicators, multi‐criteria decision‐making (MCDM) tools, such as GRA, TOPSIS, and DFA, were employed. Results showed that w/c ratio exerted the strongest influence (Δ up to 0.55), while moderate FNS replacement (30%–45%) consistently ranked higher than higher substitution levels. The combined findings demonstrate that FNSP, when used judiciously at lower w/c, can enhance durability and mechanical performance, making it suitable for structural concrete applications while contributing to sustainable cement usage.