Microstructure and strength of blended FBC-PCC fly ash geopolymer containing gypsum as an additive

This study focused on the microstructure and strength of blended fluidized bed coal combustion fly ash (FBC-FA) and pulverized coal combustion fly ash (PCC-FA) geopolymers containing gypsum as an additive. The source materials consisted of 100% FBC-FA and a blend of 75% FBC-FA and 25% PCC-FA. Gypsum was used as an additive at the dosage levels of 0, 5, and 10%wt of the source materials. NaOH and Na2SiO3 were used to activate aluminosilicate sources and temperature curing to accelerate the geopolymer reaction. The microstructures of the geopolymer pastes were examined using XRD, FTIR, MIP and SEM tests. The compressive strengths of the geopolymer mortars were also tested. Test results showed that the blending of FBC-FA and PCC-FA improved the geopolymerization and resulted in a dense matrix with reduced porosity and increased compressive strength as compared to those of the FBC-FA geopolymer. The improvement is due primarily to the high glassy phase content of PCC-FA. In addition, the use of 5% gypsum as an additive further improved the geopolymerization. The sulphate ions enhanced the leaching of alumina from the source materials forming additional aluminosilicate and increased calcium in the system which resulted in the formation of additional CSH.