Hydration of ternary cement pastes with metakaolin and nanosilica at early ages

Supplementary cementitious materials (SCM) have been used in cementitious systems, improving performance, sustainability and cost. Among these SCM's, metakaolin (MK) and nanosilica (NS), have been topic of many studies with promising results, increasing strength and refining microstructure. However, the combined use of these materials in cement pastes, especially regarding the behavior at early ages, has been topic of few studies. Therefore, the objective of this paper was to evaluate ternary pastes with metakaolin and nanosilica at early ages (1, 3 and 7 days). Six pastes were produced, one of reference, two with cement replacement by NS and three with 15% replacement of clinker for complementary contents of NS and MK (0+15%, 1+14% and 2.5+12.5%). These contents enabled a comparison between the behavior of ternary and the respective binary pastes, evaluating the synergy between the materials. Compressive strength, thermogravimetric analysis (TGA) and semi adiabatic calorimetry, by the Langavant method, were performed. Results show the potential of combining these materials, with great improvement of the compressive strength at the early ages. Thermogravimetry and calorimetry results were used to characterize cement hydration in this type of mixture. It was marked by an acceleration and CH consumption by the pozzolanic reaction, both potentialized in the ternary mixtures. Furthermore, a synergistic effect between the materials was observed in the hydration, resulting in the highest compressive strength value at 7 days. However, differences observed in results of the ternary pastes indicate that this synergistic effect depends on the ration between metakaolin and nanosilica content in paste composition.