Investigation of activation kinetics in geopolymer paste using quasielastic neutron scattering

Abstract

Quasielastic neutron scattering (QENS) has been used to investigate the binding process of water molecules in pastes of calcium geopolymer prepared with low and high calcium fly ash contents, and at two different NaOH molarities, 10 and 14M. The in situ measurements were carried at ambient and elevated curing temperatures (60°C). By carefully monitoring the time evolution of the elastic peak intensity, we infer a gelation process, followed by polymerization and a hardening in the high calcium geopolymer paste at 60°C, in agreement with previously proposed geopolymerization model. This behavior was neither observed at ambient temperature in both low and high calcium geopolymer cement paste, within the precision of the neutron instrument. Our study clearly shows that a minimal amount of heat is necessary to form gelation and polymerization during the activation process. The activation of geopolymer paste with high NaOH molarity involves more chemically bound water molecules than that at lower activator concentration. This work shows that the QENS technique can be effectively used to characterize the alkali-activation kinetics in certain geopolymer pastes, and that temperature and molarity of the activator play a vital role in controlling the gel mechanism.