Water absorption and compressive strength of various coal fly ash-based geopolymer pastes

This paper discusses the effects of the silicon dioxide to aluminium oxide molar ratio, sodium oxide to aluminium oxide molar ratio and concentration of sodium hydroxide on the water absorption and compressive strength of synthesised fly ash-based geopolymers. Geopolymer pastes were prepared using fly ash mixed with an alkaline activator consisting of sodium hydroxide solution and sodium silicate solution. The phase composition, microstructure and chemical bonding of the synthesised products were characterised after 7 and 28 days using X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy, respectively. The synthesised geopolymer with a silicon dioxide to aluminium oxide molar ratio of 5 and a sodium oxide to aluminium oxide molar ratio of 1.2, activated with 10 M sodium hydroxide, was found to have the highest strength and the lowest water absorption. The compressive strength was observed to increase with increasing silicon dioxide to aluminium oxide molar ratio. A high initial molar ratio of sodium oxide to aluminium oxide enhances the water absorption and decreases the strength of the geopolymer sample. The dissolution of silica and alumina from the starting materials becomes more efficient as the concentration of sodium hydroxide increases.