The effect of sodium silicate and sodium hydroxide on the strength of aggregates made from coal fly ash using the geopolymerisation method

Geopolymerisation of coal fly ash to produce synthetic aggregates as a potential means of utilising coal combustion by-product has been investigated. It has been revealed that the geopolymerisation strongly depends on the physicochemical properties of fly ash, the availability of soluble silicates and aluminates, and the concentration of added sodium hydroxide. The presence of sodium hydroxide increases the amount of soluble silicates and aluminates in the mixture through fly ash solubilisation. Solubility tests on various fly ash samples have shown that solubility increases as the concentration of sodium hydroxide in the fly ash increases, which also increases the strength of the resulting geopolymer aggregates. The compression strength of the geopolymer aggregates also increases to a maximum before decreasing again as the amount of sodium silicate is increased.