Hydration kinetics of Portland cement pastes blended with C-S-H seeds and low initial reactivity supplementary cementitious materials

Abstract

Extensive research has focused on using supplementary cementitious materials (SCM) as substitutes for Portland cement to reduce the carbon footprint of composites. The combination of clinker dilution and low initial reactivity of certain SCM results in slower hydration kinetics and reduced formation of hydrated minerals during early hydration stages. However, the introduction of calcium silicate hydrate (C-S-H) seeds has been found to accelerate the precipitation of C-S-H, acting as additional nucleation sites and leading to composites with enhanced early-age strength. This study aimed to evaluate the potential of C-S-H seeds in compensating for the hydration kinetics delays caused by high levels of cement substitution with low-reactivity SCM. Three replacement levels of Portland cement (10%, 20%, and 30% by weight) by metakaolin and rice husk ash were examined, both with and without the addition of C-S-H seeds, in pastes with a water/binder ratio of 0.5. Inert #325 silica was used as a reference material. Isothermal calorimetry and ultrasonic pulse velocity tests were conducted over the initial three days of hydration to assess the effects of C-S-H seeds and SCM substitution on the hydration kinetics of the reference Portland cement paste. The results demonstrated that while SCM induced a delay in the hydration reaction, the addition of 3% C-S-H seeds effectively reduced the final time and duration of the induction period for all mixtures, irrespective of the SCM type or replacement level. Moreover, the presence of the nano-additive promoted higher heat release during the induction period and shortened the time to reach maximum heat release. Furthermore, C-S-H seeds increased the cumulative heat of the mixtures, with a more pronounced impact in the early stages of hydration. In conclusion, the utilization of C-S-H seeds shows promising potential in compensating for the issues arising from the hydration kinetics of low-reactivity SCM.