Optimizing mechanical and microstructural properties of concrete with steel slag aggregate using response surface methodology

• SSA enhanced concrete strength by 28% (compressive) and 33% (flexural) at 90 days. • 100% SSA mix showed superior durability under aggressive conditions. • SEM/XRD revealed denser microstructure and more C–S–H formation. • Nanoindentation confirmed higher hardness and elastic modulus in ITZ. • RSM effectively optimized and predicted concrete performance. The depletion of natural river sand has intensified the need for sustainable alternatives in concrete production. This study investigates steel slag aggregate (SSA) as a fine aggregate replacement to enhance mechanical, durability, and microstructural performance . Concrete mixes containing 0%, 25%, 50%, 75%, and 100% SSA were tested at 7, 28, 56, and 90 days for compressive, split tensile, and flexural strengths. At 90 days, the 100% SSA mix achieved compressive and flexural strengths of 51.01 MPa and 5.76 MPa, representing 28% and 33% increases over the control. Durability assessments, including chloride and sulphate resistance, water absorption, and pH stability, showed minimal weight loss (0.18%) and over 92% strength retention in the 100% SSA mix under aggressive conditions. SEM revealed a denser matrix and improved interfacial transition zone, while XRD confirmed increased C–S–H formation and reduced portlandite peaks. Response surface methodology (RSM) optimization identified an optimal blend of 73.61% SSA and 14.66% natural sand, yielding a composite desirability of 1.0. These improvements are attributed to the angular morphology and latent hydraulic activity of SSA, which enhance particle packing, interlocking, and long-term hydration. The findings confirm that SSA is a technically viable and environmentally sustainable alternative to natural sand, capable of producing high-performance concrete with superior long-term strength, durability, and microstructural integrity in harsh environmental conditions.