Influence of aggregate and binder proportions on the microstructure and performance of fly-ash-based geopolymer concrete

A critical evaluation of the effects of mixture proportioning (binder paste content and coarse aggregate:fine aggregate ratio) on the workability, compressive strength and tensile strength of concrete made with alkali-activated fly ash (AAF) binder was undertaken. Tensile cracking in the fly-ash-based geopolymer concrete (FGC), as influenced by its fracture properties, was studied to understand the failure of the material. Microstructural analyses were conducted using scanning electron microscopy and energy-dispersive X-ray spectroscopy to ascertain product formation. The FGC mixtures demonstrated significantly higher slump and compressive strength than concrete made with Portland cement. The slump or flow of the FGC mixtures was determined by the binder paste and alkaline solution content, and the values increased with the fine aggregate content. An increasing relationship between the coarse aggregate content in the concrete mixture and its compressive strength was identified. A similar trend was observed for the splitting tensile strength, which reached 7% of the FGC compressive strength. The high strength of the FGC was attributed to the formation of sodium aluminosilicate gel and a denser interfacial transition zone (ITZ) near the aggregate interface. The dense ITZ around the aggregate contributed to an enhanced bond between the aggregate and AAF paste, resulting in aggregate fracture. The decrease in strength with binder content was attributed to the maximum paste thickness per unit volume, which contributed to a larger number of voids per unit volume of concrete.