The effects of Na₂O content, water to binder (W/B) ratio, and the modulus (M_s) of alkaline activator solution on a fly ash-based geopolymer paste (FGP) were investigated. The microstructure of the pastes was observed and characterised by Scanning electron microscope and X-ray diffraction. The results showed that the increase of Na₂O content and M_s resulted in an increase in compressive strength: an increase in the W/B ratios led to a decreased compressive strength. It was found that the high compressive strength was achieved as M_s of the alkaline activator was 1.5, and the content of this activator was 8.0-12.0% by the proportion of Na₂O to fly ash. The compressive strength of these pastes was between 26.5-39.6 MPa at 28 days when they were cured at 85 ºC for 1 h followed by curing under standard conditions. OH^- concentration, determined by Na₂O content, M_s, and water content, seemed to be a crucial parameter influencing the compressive strength. There was an optimal OH^- concentration range, in which the appropriate strength of geopolymer paste would be achieved. An increased alkaline content promoted the reaction between the mineral phases of the fly ash with the alkaline activator and the formation of an additional sodium aluminosilicate hydrate (N-A-S-H) gel.

Keywords: Fly ash-based geopolymer pastes (FGP), Geopolymers, Alkaline activation, Compressive strength, OH^- concentration