Effect of Activator Type and Concentration, Water-to-Solid Ratio, and Time on the Flowability of Metakaolin-Based Geopolymer Pastes

The synthesis parameters, such as alkali activator type and content and water/binder (w/b) ratio, directly affect the dissolution and geopolymerization kinetics in geopolymers, playing a major role in its flow performance over time. However, the isolated effect of each parameter on the rheology of fresh metakaolin-based geopolymer is still not completely understood. This work assessed the effect of the alkali type (Na or K), concentration (15% or 20% by weight of the activating solution), and $w/b$ ratio (from 0.70 to 0.80) on the fresh properties of metakaolin-based geopolymers. Minislump and Marsh cone tests with the use of image analysis and dynamic strain sweep rheometry were conducted at different testing times. The results showed that a high $w/b$ ratio increased the initial flowability of paste (reduced the yield stress and Marsh cone time, and increased the minislump) as expected, whereas the flowability was reduced over time. Increasing the alkali concentration also increased the flowability of pastes regardless of the alkali type used. The most important finding is the corroboration that the type of alkali (${\mathrm{Na}}^{+}$ or ${\mathrm{K}}^{+}$) was the parameter that had the greatest impact on the rheological behavior of pastes: mixes produced with potassium-based activator had higher flowability compared with those produced with sodium-based activator for the same concentration, $w/b$ ratio, and testing time. The increased yield stress reduced workability, which might negatively affect its applicability and consequently the hardened performance of the material. Therefore, if high flowability is required, the use of potassium must be considered, either through its partial or complete presence in the alkali activator.