Importance of reactive SiO2, Al2O3 and Na2O in geopolymer formation

Geopolymer is an alternative for Portland cement based binder. It has shown better or similar engineering and durability properties compared to Portland cement based binders. The properties of geopolymer depend on the characteristics of the aluminosilicate source material used. Hence, it is important to identify the characteristics of the source material that results in a binder with good mechanical properties. This paper investigates the levels of reactive (amorphous) SiO2 and Al2O3 contents of source materials and alkali oxides content of Si-rich geopolymer binders required to achieve enhanced mechanical properties. The study also evaluates the roles of Na2O and particle size in the development of geopolymer micro-structures. We have used six different Australian Class F fly ashes as alumnosilicate source materials in geopolymer binders. These materials were characterised using XRF, XRD and particle size analyser in order to determine the bulk chemical composition, mineralogy (including content of amorphous phases) and particle size distribution. Geopolymer pastes were subsequently prepared by activating the selected fly ashes with liquid alkaline activators and the samples where cured at 60 C for 24 hrs. A constant flow was maintained for all mixtures, the contributions of Na2O, SiO2 and H2O content from the alkaline activator are different for each fly ash source. The compressive strength results showed that high strengths are given for mixtures in which the molar ratio of reactive constituents lie within the ranges SiO2/Al2O3 < 5 and Na2O/ reactive (SiO2 + Al2O3) 0.09-0.12