Effect of steel slag as fine aggregate on the mechanical and durability performance of concrete under acid exposure

Abstract The increasing demand for sustainable construction materials has driven research into alternative aggregates to reduce reliance on natural resources. This study comprehensively evaluates the mechanical and durability properties of concrete incorporating steel slag as a partial replacement for fine aggregates at replacement levels of 0%, 25%, 50%, 75%, and 100%. An extensive experimental program was conducted to evaluate compressive, tensile, flexural, and shear strengths, as well as acid resistance and microstructural stability using XRD. The results show that a 50% slag replacement achieved optimal mechanical performance, with the highest compressive and flexural strengths, improved tensile capacity, and superior shear resistance. Specifically, the 50% slag mix demonstrated a 7.9% increase in flexural strength (from 1090.7 psi to 1164.3 psi), maintaining over 82% of the compressive strength of the control at 28 days. The 25% and 75% mixes also demonstrated balanced strength and ductility characteristics. XRD analysis showed that slag-blended concretes retained key hydration products such as Calcium silicate hydrate (C-S-H) and Quartz after exposure to sulfuric acid, indicating improved chemical resistance. A newly proposed durability index, integrating both mass loss and strength retention, was used to quantify durability performance, with the 50% mix exhibiting a durability index DI2p of 84.3%, significantly higher than the control’s 73.8%, confirming that slag incorporation improves both mechanical and chemical resilience. These findings underscore the potential of steel slag, particularly at 25–50% replacement levels, as a sustainable and high-performance alternative to natural sand in concrete production, contributing to eco-friendly construction practices and enabling the development of more durable concrete for aggressive environments and critical infrastructure applications.