Influence of microsilica on the hydration of ye’elimite

One of the promising opportunities for a more environmentally friendly cement manufacture is the production of a clinker based on calcium sulfoaluminate (CSA). Although CSA clinkers may have quite variable compositions, their main component is ye’elimite. Like ordinary Portland cements, CSA cements, which are obtained by intergrinding CSA clinkers with calcium sulfate, may be blended with supplementary cementitious materials such as slag, fly ash or microsilica in order to modify their properties and to further improve their CO2 balance. In this paper the influence of microsilica on both the hydration kinetics and hydrate assemblage of synthetic ye’elimite is presented. Blends of ye’elimite and microsilica with and without addition of gypsum were examined. The hydration experiments were carried out using water or 0.1 molar KOH solution at a water/solid ratio of 2. KOH solution was chosen in order to stimulate the dissolution of microsilica. Conduction calorimetry, thermogravimetric and X-ray diffraction analyses were employed in order to follow hydration kinetics and to assess the hydration products. Thermodynamic modeling using the geochemical speciation code GEMS-PSI was carried out to calculate the stable hydrate assemblages and to compare those results with the experimental data. It was found that microsilica accelerates early hydration kinetics of ye’elimite probably due to the filler effect. This is especially the case for the systems without gypsum which exhibit much slower hydration kinetics than the systems with gypsum. Besides the effect on hydration kinetics, microsilica influences the hydration reaction of ye’elimite. Plain ye’elimite hydrates to monosulfate and aluminum hydroxide. When increasing amounts of microsilica are added, higher amounts of ettringite can additionally be observed. Besides, strätlingite occurs as further crystalline hydration product. As a result ye’elimite is able to hydrate in the presence of silica to form ettringite, strätlingite and aluminum hydroxide. This reaction path agrees well with stable phase assemblages calculated by thermodynamic modeling. KOH solution was found to accelerate early kinetics compared to the samples hydrated with water; furthermore, it promotes the dissolution of microsilica, inasmuch a higher amount of strätlingite is present after 28 days of hydration compared to reaction in pure water.