Comprehensive study on the reactivity and mechanical properties of alkali-activated metakaolin at high H2O/Na2O ratios

This study focused on the influence of high H2O/Na2O ratios (higher than 20) on the properties of metakaolin-based geopolymers in fresh and hardened states while keeping constant Si/Al and Na/Al atomic ratios. The increase in H2O/Na2O ratio from 21 to 34 resulted in a decrease of 7-day compressive strength from 10 to 0.025 MPa. This can be attributed to the influence of water on the reactivity of the precursor, which was demonstrated by nuclear magnetic resonance (NMR) spectroscopy, and porosity evolution as the water is not chemically bound. Increasing H2O/Na2O ratio did not change the geopolymer structure of reaction products. A correlation was observed between the reaction degree deduced from NMR spectral decomposition and the cumulated released heat obtained using isothermal calorimetry. The maximum loss tangent, obtained using dynamic rheology, was linearly related to strength development.

Automated summary

This study found that increasing the H2O/Na2O ratio in metakaolin-based geopolymers resulted in decreased compressive strength. However, it did not affect the geopolymer structure but impacted the reactivity of the precursor and porosity evolution. The reaction degree, as deduced from NMR spectral analysis, correlated with the cumulated released heat determined by isothermal calorimetry. The maximum loss tangent from dynamic rheology was linearly related to strength development.