The effect of additional aluminium to the strength of geopolymer paste

Alternative cementitious materials have been widely used because of the environmental issue of greenhouse gasses produced by portland cement production. A possible solution is the green concrete made from fly ash based-geopolymer paste. The natural differences of Indonesian coal ash lead to various chemical contents of fly ash. The chemical composition of alkali activated paste is also varied causing differences in the strength of the paste. Silica to alumina ratio is indicated as the greatest influencing factor to geopolymer strength. The bond among silica-alumina preserves a compact microstructure resulting in the strength. This paper proposes a Si-Al ratio between 1.5 and 2.0 by adding a synthetic aluminium substance from some sources. Using a natural aluminium substance from other fly ashes which contain excessive amounts of Al₂O₃ was one of that efforts. The result indicated that the addition of a small amount of aluminium powder showed insignificant differences in geopolymer strength. The powder addition of up to 4% of fly ash decreased the strength by up to 60% of the original one. The methods of mixing are indicated as the trigger of lower strength because.