Influence of alkali molarity and recycled aggregates on sustainable geopolymer concrete

This study investigates the combined influence of activator concentration and recycled concrete aggregates (RCAs) content on the mechanical and durability performance of alkali-activated geopolymer concrete (GPC). Concrete mixes were prepared using varying fly ash (FLY) and ground granulated blast-furnace slag (GGBFS) ratios (100:0, 75:25, 50:50) and RCA replacement levels of 0%, 25% and 50%. Sodium hydroxide (NaOH) solutions with molarities of 8 M, 10 M and 12 M served as activators. Results showed that compressive and split tensile strengths were influenced by binder composition, alkali molarity and RCA content. Increasing RCA reduced strength due to a weaker interfacial transition zone, while GGBFS improved performance through calcium-induced gel formation. The optimum results occurred with 10 M sodium hydroxide and 25% RCA, achieving 62.05 MPa compressive strength at 28 days and 66 MPa at 120 days. Durability studies indicated greater deterioration in sulfuric acid (H2SO4) and magnesium sulfate (MgSO4) environments, with strength and weight losses intensifying over prolonged exposure. At 120 days, strength losses were approximately 27% in sulfuric acid, particularly in RCA-rich mixes. Mixes with higher molarity and calcium content exhibited better resistance. Microstructural analysis (scanning electron microscopy, X-ray diffraction analysis) confirmed C–A–S–H, N–A–S–H, and C–S–H gel formation, correlating with improved performance. Overall, RCA-based GPC demonstrates strong potential as a sustainable and durable construction material.