Effect of fly ash characteristics on delayed high-strength development of geopolymers

This paper investigates the effect of fly ash characteristics on the strength development of fly ash-based geopolymer, and reports an observation of delayed high-strength development of geopolymer. Physical and chemical characteristics of fly ashes were determined by XRF, XRD and particle size analyzer. Multi-technical characterizations using MIP, FT-IR and SEM & EDS were applied to give in-depth understanding of the relationship between reaction products, microstructure and strength development according to the maturity of geopolymer. Test results provided an insight into the delayed strength development of geopolymer which mainly occurs due to the transformation of aluminosilicate gel from Al-rich gel to Si-rich gel, rather than densification of pore structures. As particle size of fly ash increased, the strength development of geopolymer was delayed. The delayed high-strength development was more evident as the vitreous phase content and SiO2/Al2O3 ratio of fly ash increased. Furthermore, the effect of crystal growth on the strength development was discussed.