Exploring the Chemistry of High-Si/Al Metakaolin Geopolymers: From Synthesis to Structure

Abstract

This experimental study investigates the impact of different silica-to-alumina (Si/Al) ratios on geopolymers synthesized from metakaolin. Various ratios of Si/Al (1:1, 1.5:1, 2:1, 3:1, 4:1, and 5:1) were employed, nano-silica was the source material to alter the Si ratio. Microstructure and strength were analysed using SEM, XRD, NMR, and compressive strength testing. FTIR examined the dissolution rate of aluminium (Al) and silicon (Si) molecules during geopolymerization as well as the formation of N-A-S-H gel. The binding energy of individual silicon atoms and the overall energy produced during geopolymerization were calculated using modelling and moulding techniques. The results showed that a Si/Al ratio of 2:1 led to the highest dissolution of Si and Al, promoting the formation of Si-O-T bonds and resulting in geopolymers with superior compressive strength. The mechanical performance at different Si/Al ratios was attributed to the production of N-A-S-H gel, rather than zeolitic nuclei or silicate derivatives. These findings provide a foundation for geopolymerization of mine tailings, which often possess high Si/Al ratios.