Sustainable geopolymer concrete: Iraqi steel slag as a high-performance aggregate for improved strength and durability

The construction industry's reliance on ordinary Portland cement (OPC) contributes significantly to CO₂ emissions and resource depletion. Geopolymer concrete (GPC) offers a sustainable alternative, but natural aggregates (NAs) still harm the environment. This study investigates recycled coarse steel slag aggregate (RCSA), a metallurgical byproduct, as a partial replacement (10, 20, 30, and 40% by volume) for natural coarse aggregate (NCA) in metakaolin-based GPC. The geopolymer paste was formulated with optimal molar ratios of 3.65 for SiO₂/Al₂O₃, 2.9 for sodium hydroxide/ sodium silicate, and 0.62 for water/metakaolin. Results showed that incorporating RCSA enhances both mechanical and physical properties of GPC. Optimal performance was achieved with a 30% RCSA substitution, showing remarkable improvements over conventional GPC at 28 days of curing: a 29.56% increase in compressive strength (95.1 MPa vs. 73.4 MPa), along with 26.12 and 41.07% enhancements in splitting tensile and flexural strengths, respectively. Bulk density increased by 8.15% (2371.96 kg/m³ vs. 2193.12 kg/m³). In comparison, physical analysis revealed a 26.32% reduction in porosity and a 9.18% decrease in water absorption, indicating improved interfacial transition zone (ITZ) between the RCSA and geopolymer paste, facilitated by calcium leaching from the slag particles, which promotes densification and reduced permeability. Microstructural analyses (XRD and FTIR) confirmed successful geopolymerization and a robust amorphous geopolymer network. Findings confirm RCSA mitigates environmental impacts through waste valorization and significantly enhances GPC performance, offering a viable pathway toward more sustainable construction materials.