Effects and mechanism of superplasticizers and precursor proportions on the fresh properties of fly ash – slag powder based geopolymers

Abstract

To promote the application of green geopolymer materials, the effects and mechanism of commercial superplasticizers on the fresh properties of one-part geopolymers prepared from fly ash (FA), slag powder (Slag) and anhydrous sodium silicate powder were studied, including flowability, rheological properties and particles packing properties. And the influence of precursor proportion (FA:Slag) i.e. the mass proportion of FA and Slag on the effectiveness of superplasticizers was also considered. The results show the fluidity of geopolymer with 2.4 % liquid naphthalene-based superplasticizer (N-L) increased by 19.20 % when FA:Slag = 85:15, and the 0.20 % one kind of solid polycarboxylate-based superplasticizer (PC-S2) increased the fluidity of geopolymer by 16.47 % when FA:Slag = 95:5. The addition of superplasticizers could improve fluidity, increase plastic viscosity, and decrease yield stress of the pastes. The particles packing properties, including excess water and water film thickness (WFT) of the geopolymer pastes with superplasticizers also increase. It is interesting to find that the effectiveness of superplasticizers varies from different types of superplasticizers. This could be understood by the mechanism of the two types of superplasticizers based on Zeta, FTIR results, and water film theory. Accordingly, naphthalene-based superplasticizers guided by the adsorption-dispersion effect, due to their stable molecular structures and short chains, could improve the workability, while the effectiveness of polycarboxylate-based superplasticizers guided by the steric hindrance effect of lateral chains may be judged by the combined effects of the interaction between particles and the effects induced by superplasticizers.