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Calcium sulfoaluminate cements (CSA) are a potential low CO2 alternative to Ordinary Portland cement.To further reduce the CO2 emissions related to CSA, they can be blended with supplementary cementitious materials such as fly ash to reduce the amount of clinker in the cement and also to save costs.In this study, a CSA clinker mixed with anhydrite at a molar anhydrite to yeelemite ratio of 0.8 was blended with different amounts of a siliceous fly ash.The hydration kinetics of the formulations was examined by means of isothermal calorimetry, X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA) at a water/binder ratio of 0.75.Selected samples were investigated by scanning electron microscopy (SEM).The hydrate assemblage was calculated by thermodynamic modelling using the geochemical speciation code GEMS-PSI assuming a reaction degree of 20% of the amorphous part of the fly ash.Compressive strength of mortars was measured up to 28 day of hydration with water contents normalized to CSA clinker + anhydrite.