Calcium sulfoaluminate (C4A3(S)) is the main constituent of a promising low-CO2 alternative to ordinary Portland cement.C4A3(S) hydrolyzes very quickly generating various hydration products, such as ettringite (AFt),monosulfate (AFm), amorphous aluminum hydroxide (AH3) and so on.In this paper, the phase compositions and microstructure of C4A3(S) pastes were investigated using an X-ray diffraction (XRD) instrument and a scanning electron microscope (SEM) when the water/solid ratio was 0.5 and 2.0 and the curing temperature was 20 ℃ and 60 ℃.Needle-like AFt with different sizes was easily detected only at t 20 ℃.AFm was originally flower-like and then grew into a plate-like morphology.In particular, with different water/solid ratios, compact and sparse filamentous AH3 was observed in the early stages and then the filamentous An3 gathered and grew forming smooth and villous spherical AH3.The spherical An3 usually continued to gather, forming dense or villous agglomerates in the later stages.