The increase in strength with curing age was low due to the low reactive of FBC fly ash. On the one hand,the increase in strength with curing age could be noticed with the blend composite mortars.The strengths at the age of 7, 28 and 90 days of 3.75SF mortar were 13.0, 15.5 and 22.0 MPa, respectively. The strength at 90 days of 3.75SF mortar was almost double that of the control. In addition to aluminosilicate compound, silica
fume in FBC geopolymer matrix reacted with calcium hydroxide and formed calcium silicate hydrate (C–S–H) as confirmed by the XRD results. For the system with substantial amount of calcium, C–S–H was formed and co-existed with the geopolymer products and the strength of composite developed with curing age [25,26].
The strength at 90 days of 1.5SF mortar was also significantly improved to 18.0 MPa.