Geopolymer that was made with high CaO content fly ash was found to have higher compressive strength than the low CaO fly ash, using the same mixture composition. This effect could be due to the physico-chemical properties of the fly ash, in respect to its particle size or the chemical composition. Although it was not widely published, the occurrence of flash setting of geopolymer was known to occur when using high CaO content fly ash as the precursor. Geopolymer paste may solidify within minutes after the addition of alkali activators, making it very difficult to cast in big volume. This paper investigate the effect of borax addition to the high calcium fly ash-based geopolymer mixture to reduce the occurrence of flash setting. It was found that the setting time can be extended significantly, with the addition of 5% borax, by mass, of fly ash. The addition of borax also have positive effect on increasing the compressive strength of geopolymer.

Abstract. Geopolymer properties is notably influenced by the quality of fly ash used as the precursor material. Fly ash obtained from the same source may have varying physical and chemical characteristics. The characteristics are influenced by, among other, the method and temperature of burning, the degree of grinding and the quarry sources of the coal. In this study, high calcium fly ash from a single power plant was investigated, to analyze the effect of its variability on the characteristics of geopolymer mortar. Fly ash samples were collected at ten different sampling periods. Three mixture compositions, with different sodium silicate solution to sodium hydroxide ratios, were prepared to produce geopolymer mortars. From the results, it was found that there are broad variation of fly ash chemical properties, however only small variation on the fineness and LOI was discovered. The setting time and compressive strength of geopolymer are influenced by the fly ash particle size, pH value and chemical content. Higher level of calcium oxide content increases the compressive strength, while finer particle size could have effect on the later age increase of the compressive strength. Faster setting time is also correlated with the higher pH value of the fly ash, with the tendency of flash setting occurrence for geopolymer using fly ash of high pH level.