Compressive strength and microstructure evolution of low calcium brown coal fly ash-based geopolymer

A comprehensive experimental study has been conducted to investigate the geopolymerisation and compressive strength development of mortar made from brown coal fly ash from two separate locations in the storage ponds of an Australian power plant. The specimens gave similar compressive strengths but had significantly different material and performance characteristics despite being from the same storage location. The Loy Yang‒A (LYA) geopolymer mortar demonstrated an approx. 30% strength increase while Loy Yang‒B (LYB) gave an approx. 18% strength drop over the period from 7 and 90 d, though both geopolymer mortars initially achieved a similar 28-d strength of approx. 23 MPa. The LYA ash had almost double the alumina content compared to LYB and a higher proportion of AlVI compared to the LYB. The lower alumina content coupled with the low quantity of AlVI in the ash and its lower conversion to AlVI during geopolymerisation is identified as the primary reason for the reduction in strength observed in the LYB geopolymer. The increase of Q4(3Al) during geopolymerisation and some conversion to Q4(4Al) coordination over time resulted in the increase in the compressive strength observed in the LYA mortar. This strength increase of LYA mortar is further correlated with an increase in Quartz phases coupled with a reduction in the Moganite phase. Formation of sodium carbonate due to atmospheric carbonation of unreacted sodium hydroxide in Loy Yang geopolymer additionally contributed to the strength development of LYA geopolymer.