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Geopolymer concrete (GPC) has emerged as a sustainable alternative to Ordinary Portland Cement (OPC) concrete, significantly
reducing CO₂ emissions and utilizing industrial by-products such as fly ash. This study investigates the durability of fly ash-based geopolymer
concrete when Fly Ash is partially replaced by Silica Fume and Metakaolin at substitution levels of 2.5%, 5%, 7.5%, and 10% by weight.
Specimens were heat-cured at 60°C for 24 hours and subsequently immersed in 5% concentrated solutions of Hydrochloric Acid (HCl) and
Magnesium Sulphate (MgSO₄) for durations of 30, 60, and 90 days. Compressive strength and percentage weight loss were measured at each
interval. Results reveal that 5% Silica Fume replacement achieves the highest compressive strength under HCl exposure (45 N/mm² at 30
days), while 7.5% Metakaolin replacement yields optimum performance under MgSO₄ exposure (46 N/mm² at 30 days). Weight loss increased
progressively with immersion duration and higher replacement levels in both acidic and sulphate environments, with acid-induced losses
consistently exceeding sulphate-induced losses.