Hydration of calcium sulfoaluminate cement
Selected aspects relating to in-service performance of calcium sulfoaluminate cement are reported. The stability of ettringite, which is the main hydration product and major strength-giving component of calcium sulfoaluminate cement, was studied with respect to thermal stability and resistance to carbon dioxide. Using Synchrotron Radiation - Energy Dispersive Diffraction technique, the behaviour of calcium sulfoaluminate cements during the early hydration time has been investigated. Ettringite formation occurs at a very early stage of hydration, as early as about 15 minutes under the provided experimental conditions. This ettringite formation fits well with the relative heat evolution curves in terms of hydration time. Under independently controlled temperature and water vapour pressure, the formation and decomposition of synthetic ettringite are shown to be reversible but with hysteresis. The obtained data are used to predict safe conditions for the use of ettringite and ettringite-based cements in warm conditions. The partial dehydration products of ettringite typically have 10 to 13 H2O per formula weight of ettringite and still give electron diffraction patterns characteristic of ettringite although the same products are amorphous to X-ray powder diffraction. It is observed that there are important differences in carbonation mechanisms and mineralogical features between powders and pellet samples. In the interior of pellets, undergoing carbonation, escape of the excess of water is more difficult than from powders. The pore spaces tend to become flooded as a result, secondary reactions of dissolution and reprecipitation occur leading to nucleation and crystallisation of calcium monosulfoaluminate. The reactivity of two polymers, styrene-butadiene rubber (SBR) and ethylene vinyl acetate (EVA), mixed with calcium sulfoaluminate cement, Portland cement and high aluminate cement has been studied. The observation indicates there is no bulk chemical interaction between the polymers and cements. However, physical or mechanical interactions may occur between the polymers and cements.