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 yeelimite.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 yeelimite is presented.Blends of yeelimite 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.