The mechanism of geopolymer gel formation investigated through seeded nucleation

Seeding of hydroxide-activated geopolymer syntheses with high surface area Al2O3 nanoparticles causes significant changes both in the kinetics of the reaction and in the structures of the products formed, as shown by in situ and ex situ ATR-FTIR spectroscopy. Nucleation at the seed surfaces leads to the formation of phase-separated gels from very early in the reaction process, eliminating the induction period prior to geopolymer gel formation that is normally observed with hydroxide activation. However, other than the formation of small regions of very high-silica gel within the seeded system, the nature of the dominant geopolymer gel phase remains largely unchanged. After an extended period of curing, both seeded and unseeded geopolymers display the formation of zeolitic phases. However, while the unseeded system contains small amounts of faujasite as the favoured crystalline product, seeding leads to the formation of zeolite Na–F, a synthetic zeolite with the edingtonite structure type that has not previously been observed in geopolymers. These observations are explained mechanistically in terms of the catalytic effects of the seed particles on gel phase formation and separation.