Influence of CaO/Al2O3 molar ratio of synthetic calcium aluminate hydrates on the engineering properties of metakaolin-based alkali-activated materials

This study evaluates the influence of the CaO/Al2O3 molar ratio of synthetic calcium aluminate hydrates on the properties of alkali-activated materials based on metakaolin. Calcium aluminate hydrates with CaO/Al2O3 molar ratios of 0.4, 0.6, 0.8, 1.0 and 1.2 were synthesised from bauxite and eggshells. Alkali-activated materials were prepared in which metakaolin was replaced by 0 and 10 wt.% calcium hydroxide and calcium aluminate hydrate with different CaO/Al2O3 molar ratios. Rice husk ash with a SiO2/Na2O molar ratio of 1.6 was used for the preparation of the hardener. The 28-day compressive strengths of alkali-activated materials containing 0 and 10 wt.% calcium aluminate hydrate with different CaO/Al2O3 molar ratios of 0, 0.4, 0.6, 0.8, 1.0 and 1.2 are 48.86, 63.59, 47.36, 47.89, 34.66 and 32.76 MPa, respectively. 22.08 MPa for that containing 10% by weight of calcium hydroxide. The apparent densities are 1.87, 1.79, 1.99 and 2.10 g/cm3, respectively. It has been found that the best molar ratio of CaO to Al2O3 in the structure of the 10% by weight of calcium aluminate hydrate used to replace metakaolin, which is required to produce alkali-activated materials with higher mechanical and physical properties, is about 0.4. Highlights