Effect of Variability of Fly Ash Obtained from the Same Source on the Characteristics of Geopolymer

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.