Calcium sulfoaluminate (CSA) cements have been shown to have lower carbon footprint than that of Portland cement.In addition, the expansive nature of CSA-based cement can be utilized to enhance the resistance against shrinkage cracking by inducing compressive stress in concrete.The current paper reports the hydration and early-age volume changes of three OPC-CSA binary cement systems with varying CSA content.The CSA cement utilized in this study had a phase composition of 19% C4A3(S), 40.5% C(S), 34.8% C2S and 2.1% C4AF.Hydration and phase development was monitored through calorimetry and quantitative X-ray diffraction.Expansion characteristics of cement pastes prepared at a water-to-cement ratio of 0.44 were monitored in saturated lime water.The effect of various CSA dosages on expansion characteristics was determined.A geochemical modeling program (GEMS-PSI) was used to estimate the saturation index (SI) with respect to ettringite which had strong correlation with 1 day-expansion.The volume fraction of ettringite in a given pore volume was found to have a stronger correlation with expansion of OPC-CSA blends than the ettringite content.Furthermore, a poromechanical model was used for predicting the tensile stress in OPC-CSA blends due to crystallization.