Durability and mechanical behaviour of dual-sourced fly ash geopolymer concrete under aggressive environment

As the need for infrastructure development increases with a growing population, the dependence on cement consumption is rising, leading to a large amount of carbon dioxide (CO₂) emissions into the atmosphere. This study investigates the mechanical performance, durability behaviour, and environmental benefits of ambient-cured geopolymer concrete (GPC) based on fly ash for infrastructure exposed to aggressive marine environments. A dual-source fly ash binder was activated using sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solutions, and specimens were exposed to coupled chloride-sulphate environments for up to one year. A 50:50 blend of processed and unprocessed non-calcareous fly ash, activated with 10 M NaOH and Na₂SiO₃ with modulus ratio of 2 achieved 37.4 MPa compressive strength at 28 days under ambient curing. Under coupled sulphate-chloride attack, the compressive strength initially decreased due to rapid ionic ingress, and partial depolymerization of the aluminosilicate gel. A transient strength recovery at 3-6 months was attributed to continued geopolymerization and secondary gel formation that refined pore structure. Long-term degradation after 9-12 months resulted from ion exchange (Na⁺ replacement), increased pore connectivity, and microstructural relaxation. Chloride ion penetration test result (1879 C) and sorptivity analysis confirmed low ionic permeability, which was validated with mineralogical studies (SEM-EDAX and XRD analyses), confirming the formation of secondary gel formation and the absence of gypsum or ettringite phases before exposure. The developed GPC achieved 70.6% CO₂ reduction compared to OPC concrete, demonstrating durability-environment synergy for marine infrastructure applications.