Effects of nanosilica and used foundry sand on the mechanical and microstructural properties of concrete: Experimental and <scp>RSM</scp> ‐based analyses

Abstract This investigation addresses the synergistic effects of nanosilica (NS) and used foundry sand (UFS) on the strength and microstructural performance of concrete. In response to the environmental concerns posed by excessive extraction of natural fine aggregates and cement production, investigations have sought to create a sustainable concrete blend incorporating industrial waste and nanomaterials. UFS was used to partially replace natural sand at 10%–40%, while NS was added at 1%–5% of the binder content. Mechanical tests, including compressive, split tensile, and flexural strength evaluations, were conducted at 7 and 28 days. Response surface methodology was used for statistical modeling and optimization of mix proportions. Microstructural characterization was performed via various techniques. The results revealed that the optimized blend, comprising 3.34% NS and 25.79% UFS, exhibited substantial improvements in mechanical strength and matrix densification. Scanning electron microscopy and x‐ray diffraction analyses revealed a more compact pore structure, enhanced generation of calcium silicate hydrate, and a significant decrease in portlandite content in the optimized mixture. These findings indicate that the synergistic use of NS and treated foundry sand constitutes a viable approach for producing eco‐friendly concrete that promotes sustainable construction and circular economic efforts.