Composition and microstructure of fly ash geopolymer containing metakaolin

Abstract

This article reports a study of composition and microstructure of fly ash geopolymer containing metakaolin which is obtained at different firing temperatures. The mixtures of fly ash and metakaolin are activated by alkali with SiO2/Al2O3 molar ratio of 3.83, Na 2O/Al2O3 of 1.26 and W/B of 0.27. The glassy components of these starting materials are chemically transformed into glassy alumino-silicate network that can create very strong solid. The highly reactive metakaolin is attained at the temperature at which the hydroxyl groups in octahedral sites of kaolinite are totally removed. XRD-pattern of metakaolin burnt at 600°C shows the presence of highly amorphous phases which are the origins of the glassy geopolymer matrix as seen by XRD and SEM while the patterns of metakaolin burnt at 400°C and 800°C contain crystalline phases identified as kaolinite and spinel respectively. Those crystalline phases exit in the matrix of glassy geopolymer cement indicating no dissolution of the crystalline phases with alkali activation. It is found that mechanical strength of fly ash geopolymer cement using metakaolin fired at 600°C is the highest. This suggests that in order to achieve the high strength geopolymer, the optimum firing temperature of kaolin that hence the highly glassy phase should be controlled. © 2009 Taylor & Francis Group, London.