Abstract: The microstructure of Friedel's (F) salt in hydrate of cement andgranulated ground blast furnace slag (GGBS) is experimentally studied. It can be seen that the maximum XRD-intensity of F salt appears at 80 nm and that an endothermic effect yields at around 360℃. The morphology of F salt is a hexagonal slice with a size of 2～3 micrometers. The influence of F salt's formation is analyzed, and the mechanism of chloride binding capability of cement with great volumes of GGBS is also explained.