Rheology of fly ash-based geopolymer: Effect of NaOH concentration

•The effect of NaOH concentration on the flow and viscoelastic properties of fly ash-based geopolymers was studied.•The dissolution and geopolymerization kinetics strongly depended on the NaOH concentration.•Two domains of NaOH concentration were defined based on the rheological behavior and FTIR spectroscopy.•Increasing the temperature of fly ash-based geopolymer at fresh state enhanced its rigidification rate and storage modulus. The objective of this paper is to study the flow and viscoelastic properties of fly ash-based geopolymers activated at different concentrations of NaOH. Although the investigated geopolymers exhibited a shear thinning behavior, their rheological properties are greatly affected by the NaOH concentrations. Increasing the concentration of NaOH up to 7 mol/L resulted in an increase of the yield stress, the storage modulus, and the rigidification rate of the geopolymers. However, the use of higher concentrations resulted in lower yield stress, storage modulus, and rigidification rate. The competition between dissolution and geopolymerization processes seems to be the key parameter affecting the evolution of rheology of the investigated geopolymers. In the case of a NaOH concentration lower than 7 mol/L, the geopolymerization process is accelerated. However, when the concentration exceeds 7 mol/L, the geopolymerization is delayed despite the higher dissolution. This is probably due to the dominance of the repulsive forces between particles, thus preventing the network formation. Geopolymers activated at a NaOH concentration of 2 mol/L and a temperature around 40 °C exhibited comparable rheological and viscoelastic properties than those activated at higher concentration of 7 mol/L and a temperature of 25 °C.