Hydration of ternary Portland cement blends containing metakaolin and sodium bentonite

Although metakaolin can enhance the hydration of cement, its pozzolanic reactions occur primarily at the early age with a high heat-release rate and low degree of reaction resulting in a reduction in the composite strength especially at high substitution levels. To address this challenge, a combination of metakaolin and sodium-bentonite is adopted as a partial replacement of cement. The influence of coupled substitutions of these two clays on hydration of Portland cement is investigated by characterizing hydration kinetics and products related to microstructural and physico-chemical parameters. The results indicate that the coexistence of sodium-bentonite and metakaolin enhances the cement hydration in terms of strength development, portlandite consumption and formation of supplementary calcium silicate hydrate (C-S-H) phases with elongated molecular chain lengths. Dissolution of metakaolin is increased in the presence of bentonite. It was found that the conversion of ettringite to monosulfate, the formation of strätlingite, and the length of aluminosilicate chains increase with increasing Al/Si ratio, which is related to the amount of dissolved metakaolin. A thermodynamic simulation is also presented to support these findings.