Effect of curing temperature and SiO₂/K₂O molar ratio on the performance of metakaolin-based geopolymers

In this study, the effect of curing temperature and SiO₂/K₂O molar ratio on the mechanical properties of metakaolin-based geopolymers were investigated. Different curing temperatures (from 50° to 90 °C) were used for curing process. A series of geopolymers with different SiO₂/K₂O molar ratio (N) were prepared, with N=2.4, 2.0, 1.6 and 1.2, respectively. The phase composition, microstructure and mechanical properties of the resulting geopolymers were studied. The results indicate that curing temperature and SiO₂/K₂O molar ratio impacts final microstructures and mechanical properties of the resulting geopolymers. Microstructures become denser with increases in curing temperature, which was beneficial to the improvement of geopolymer's mechanical properties. The optimum curing temperature was ≈ 80 °C giving the best mechanical properties with the compressive strength of 106.2 MPa and elastic modulus of 8.3 GPa, respectively. With increasing SiO₂/K₂O molar ratios, the microstructure was quite porous and contained unreacted metakaolin particles, and the corresponding mechanical properties declined sharply, attributed to low degrees of geopolymerization caused by high alkali content.