This paper deals with the heat release and paste microstructure during the early hydration of calcium alu- minate cement-Portland cement-calcium sulfate (anhydrite or α-hemihydrate) blends by means of calorimetry,X- ray diffraction,environmental scanning electronic microscopy and mercury intrusion porosimetry.The results show that (1) when calcium sulfate is blended with the binder,the early hydration accelerates and is dominated by the formation of ettringite as well as the corresponding transition into hydrated mono-sulphate; (2) in the first 30 mi- nutes of hydration,short and coarse ettringite in a length of about 1μm forms in the matrix,which leads to the oc- currence of the first heat evolution peak; (3) in the following 8 hours,another two obvious heat evolution peaks occur,one resulting from the dissolution of calcium aluminates and anhydrite as well as the formation of etttingite in the paste with anhydrite,the other resulting from the growth of etttingite and the formation of mono-sulphate; (4) for the paste with α-hemihydrate,both of these two peaks are attributed to the formation and growth of etttingite; and (5) thanks to its rapider dissolution than anhydrite,α-hemihydrate matches well with the dissolution of calcium aluminate,so that the paste with α-hemihydrate contains more ettringite and exhibits lower porosity.