The effect of sodium hydroxide molarity on setting time, workability, and compressive strength of fly ash-slag-based geopolymer mortar

This research investigates the influence of sodium hydroxide (SH) molarity on fly ash-slag-based geopolymer mortar's workability, setting time, and compressive strength. The molarity of SH varies between 1M, 2M, 4M, 6M, 8M, and 10M. The geopolymer mixture was proportioned to contain 30% slag as a substitution for fly ash, with the alkaline activator to binder (Al/Bi) ratio of 0.45 and the sodium silicate to sodium hydroxide ratio (SS/SH) of 1.5. The results showed that at a range of low molarity SH (1M, 2M, and 4M), the setting time was faster in a mixture containing higher molarities. In contrast, at a range of high molarity (6M, 8M, and 10M), the setting time is slower in a mixture with higher molarity SH. Based on the setting time, the concentration of SH that can be applied in the field is 1M, 2M, 8M, and 10M. Increasing the molarity of SH will decrease the workability but increase compressive strength. However, when SH molarity increased after optimal molarity was established, the compressive strength was reduced. The maximum compressive strength of mortar is 46.86 MPa, obtained at an SH molarity of 6M.