Effect of the Viscosity of Alkali Activators on the Particle Packing of Geopolymer Mortar

The packing behavior of a particle system is significantly influenced by the particle size distribution (PSD) of the mix and the viscosity of the liquid. In this study, the effect of the viscosity of the commonly used alkali activators for geopolymer binders on the packing behavior (packing factor, void ratio, optimum liquid ratio, and air ratio) of geopolymer mortar was investigated. Four different alkali activators: 10M NaOH, 14M NaOH, Na2SiO3, and a mixture of Na2SiO3 and 14M NaOH at a weight ratio of 2.5∶1.0 were employed to investigate the effect of viscosity. The Funk and Dinger distribution model was adopted to select seven different geopolymer mixes consisting of solid ingredients only. These mixes incorporated varying proportions of ground granulated blast furnace slag (GGBFS) and fly ash (FA), enabling an investigation into the influence of material type and particle shape on the particle packing. The wet packing method was employed to measure the packing behavior of the mortar mixes by mixing the activators in the solid mixes, considering various liquid ratios (ratio of the volume of all liquids to that of all solids). The packing factor of the geopolymer mortar mix increased with an increase in the liquid (alkali activator) ratio up to the optimum liquid ratio. However, further increases beyond the optimum liquid ratio resulted in a decrease in the packing factor. Below the optimum liquid ratio, the mixes remained dry and did not form a paste. The optimum liquid ratio was observed to be dependent on the viscosity of the alkali activators. The liquid ratio of 0.50 was found to be optimum for alkali activators consisting of Na2SiO3 with a high viscosity of 396.89 cP, whereas the liquid ratio of 0.40 was found to be optimum for 10M NaOH with a low viscosity of 8.79 cP. The maximum packing factor at an optimum liquid ratio reduced with an increase in the viscosity of the alkali activators, leading to an increase in the void ratio. The air ratio, on the other hand, decreased with an increase in the viscosity of alkali activators. Variations in the proportions of GGBFS and FA did not affect the optimum liquid ratio. However, for a liquid with a certain viscosity, the mixes rich in FA exhibited lower packing factors than the mixes rich in GGBFS, resulting in higher void ratios. Additionally, the air ratio increased with an increase in the FA proportion in the mixes.