Direct Dynamic Structural and Chemical Analysis of Belite Hydration Kinetics

To minimise carbon emissions from cement production, high-belite cements are being increas-ingly used to optimise the fresh and hardened cement strength. However, the hydration kinet-ics and reactions of belite remain poorly understood. A systematic, time-resolved study of belite hydration behaviour was conducted, focusing on the hydrate morphology of 20nm to 2-μm diameter belite particles using X-ray powder diffraction and scanning electron micros-copy. Reactions were investigated at room temperature (21 °C), 80 °C and 200 °C, over peri-ods from 6 hours to 42 days. The hydration rate was strongly influenced by the size and shape of the starting material, as well as the temperature. The predominant product in all the samples is a calcium silicate hydrate (C-S-H) of poor crystallinity which can be found in different morphological forms. The crystallisation of calcium hydroxide (CH) began when the solution became oversaturated with Ca2+ and OH- ions. When exposed to ambient air, calcium car-bonate formed, in direct contrast to the hydration behaviour of Ordinary Portland cement, in which the formation of calcium carbonate was insignificant. Since the hydration of pure belite proceeds slowly in water at room temperature, the development in the strength in cements with a high belite content within a reasonable time requires the incorporation of specific addi-tives and alloys that accelerate the hydration.