Rheological behavior of alkali-activated metakaolin during geopolymerization

The dynamic rheological behavior of geopolymers, inorganic materials synthesized by activation of an aluminosilicate source by an alkaline solution, is described. The pastes studied were mixtures of an activation solution (alkali + silica) and metakaolin. The influence of the activation solution (NaOH vs. KOH), the silica (Aerosil vs. Tixosil), and the temperature on the evolution of the elastic modulus (G′) and viscous modulus (G′) over time were studied in the linear viscoelastic range. The results show that the nature of the silica has little influence on the viscous and elastic moduli when the geopolymer is activated by KOH, and that the setting time is faster with sodium hydroxide and at higher temperatures regardless of the geopolymer. In addition, during geopolymerization the stepwise variation of the modulus values indicates that the formation of the 3D network occurs in several steps. Moreover, geopolymers activated by potassium hydroxide exhibit slower kinetics but the interactions between constituents are stronger, as the loss tangent (tanδ = G″/G′) is lower. Finally, the maximum loss tangent, tanδ, was also used as a criterion to determine the temperature dependence of the geopolymers synthesized. This criterion is a precursor of the transition to the glassy state. The activation energies could thus be determined for the geopolymers synthesized with potassium hydroxide or sodium hydroxide.