Influence and role of feedstock Si and Al content in Geopolymer synthesis

This study evaluates the role and relationship between SiO2 and Al2O3 contents of aluminosilicate material on the properties of Si-rich geopolymer systems using Australian fly ashes and existing published data. The selected fly ashes, with varying amounts of amorphous and total SiO2 and Al2O3 were assessed for their overall reactivity. XRD analyses yielded traces of sodalite in addition to conventional amorphous geopolymer phase for the fly ash mixtures with amorphous SiO2/Al2O3 = 3.31. The other two fly ashes with higher amorphous SiO2/Al2O3 ratio tended to produce sodium aluminosilicate zeolitic phases comprising mainly of Na-P1 zeolite and faujasite. These latter phases originate from Na2O–Al2O3–SiO2–H2O gel assemblages but the pathway appears to be governed by the available reactive SiO2 and Al2O3 in the reaction environment. However, indices based on the amorphous content or more precisely amorphous SiO2/Al2O3 ratio provided a relatively good correlation with the compressive strength of the final product.