Valorization of copper slag as sustainable fine aggregate in geopolymer concrete: Enhancing chemical durability and long‐term performance

Abstract This study explores the use of copper slag (CS) as a sustainable alternative to natural river sand in fly ash–ground granulated blast furnace slag‐based geopolymer concrete (GPC) and mortar (GPM), with a specific focus on both mechanical and chemical durability properties. Although previous research has highlighted the mechanical performance of CS‐incorporated GPC, limited data exist on its long‐term durability, which has likely hindered its practical adoption in the construction industry. To address this gap, GPC specimens with varying CS replacement levels (0%–100%) were tested for compressive strength, split tensile strength, void content, carbonation depth, and chloride ion permeability. In parallel, GPM specimens were evaluated for compressive strength, water absorption, and alkali–silica reaction (ASR) potential. Results show that 60% replacement of natural sand with CS significantly enhances the performance of both GPC and GPM. In GPC, compressive strength increased by 25%–28%, while chloride ion permeability decreased by 54.8% and carbonation depth by 30.4%, relative to control mixes. GPM exhibited reduced water absorption (from 53 to 40 g/cm 2 ), and ASR expansion remained below the critical threshold of 0.1%. Microstructural analysis confirmed denser packing and potential pozzolanic activity of CS as contributing factors. The findings support the safe and effective use of CS as a fine aggregate replacement in GPC and GPM, offering a practical pathway toward more sustainable and durable construction materials.