Undehydrated kaolinite as materials for the preparation of geopolymer through phosphoric acid-activation

Abstract This work focuses on the geopolymerization of undehydrated kaolinite in place of the more commonly used dehydrated kaolinite via phosphoric acid-activation. The results showed that the compressive strength, microstructure, and chemical composition of acid-activated undehydrated kaolinite were greatly affected by the concentration of phosphoric acid. When kaolinite was acid-activated by 10 mol/L and 12 mol/L phosphoric acid, metavariscite and geopolymer with P–O–Al network were obtained; but the products possessed porous and inhomogeneous microstructure, which exhibited low compressive strength. When kaolinite was acid-activated by 14 mol/L phosphoric acid, geopolymer dominant with Si–O–P–O–Al networks was synthesized, which had compact and dense microstructure. The compressive strength of the geopolymer product was greatly improved. These results demonstrate the feasibility of using undehydrated kaolinite as the starting material for the preparation of phosphoric acid-based geopolymer. The findings of this study have meaning not only for extending the application field of kaolinite but for reducing the energy input in geopolymer preparation.