Effect of storage conditions on the shelf life of class C Fly ash for geopolymer applications under tropical conditions

The shelf life of Class C fly ash is a critical but often overlooked factor affecting its reliability as a construction material precursor, particularly for geopolymer applications in humid tropical regions. This case study investigates the physicochemical evolution of Class C fly ash stored under open and closed conditions over a six-month period and evaluates the resulting impact on geopolymer performance. Fly ash sourced directly from an operating coal-fired power plant was stored under realistic ambient laboratory conditions representative of a tropical environment. Monthly characterization using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) revealed that open storage promotes progressive hydration and carbonation, leading to particle agglomeration, calcite formation, and reduced precursor reactivity. In contrast, closed storage effectively preserved the physicochemical stability of the fly ash. These storage-induced changes were reflected in geopolymer performance. Geopolymers produced from closed-stored fly ash achieved compressive strengths of up to ~22 MPa after 28 days of curing, whereas those derived from open-stored fly ash reached only ~17 MPa after prolonged storage. Overall, uncontrolled open storage resulted in a compressive strength reduction of approximately 20–30%. The findings demonstrate that storage conditions represent a decisive but manageable factor controlling the performance of fly ash-based geopolymers. This study provides practical guidance for fly ash handling and storage in construction material supply chains, particularly in tropical regions where high humidity and delayed utilization are common in practice.