Ceramics-Silikáty

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 20-nm to 2- μm diameter belite particles using X-ray powder diffraction and scanning electron microscopy. Reactions were investigated at room temperature (21 °C), 80 °C and 200 °C, over periods 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 pre-dominant 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 Ca ²⁺ and OH ^- ions. When exposed to ambient air, calcium carbonate formed, in direct contrast to the hydration behaviour of Ordinary Portland cement, in which the formation of calcium car-bonate 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 additives and alloys that accelerate the hydration.