Effects of Anions on the Formation of Aluminosilicate Gel in Geopolymers

This work demonstrates that an increase in soluble silicate concentration over ∼200 mM in an aqueous alkaline solution (pH = ∼13.95) can significantly increase the reactivity of a Class F fly ash at room temperature. The greatly increased dissolution of the fly ash primary phase(s) and the subsequent secondary precipitation are responsible for the formation of an aluminosilicate gel similar to the major binding phase of a geopolymer. On the basis of this observation, a reaction model was developed to simulate aluminosilicate gel formation in a real geopolymeric system. In this work, potassium salts of chloride, carbonate, oxalate, and phosphate (KCl, K 2 CO 3, K 2 C 2 O 4 ·H 2 O, and K 2 HPO 4 ) were used as solution contaminants. It was found that the anions (Cl -, CO 3 2-, C 2 O 4 2-, and PO 4 3- ) significantly affected the formation kinetics as well as the nature of the aluminosilicate gel formed in the reaction model. This observation could be successfully used to mechanistically explain the retardation effects of the various salts on the gel solidification time of a geopolymer. Aluminosilicate crystallization in the chloride-affected and the oxalate-affected reaction models implies that similar processes could also occur in geopolymers. This could affect the chemical stability and durability of geopolymers and thoroughly deserves a careful future investigation.