Calcium silicate hydrate (C-S-H) is a dominant hydration product of cementitious materials. Therefore, its chemical composition and physical properties affect the performance of concrete. The purpose of this study is to investigate the chemical composition (CaO/SiO2 molar ratio (Ca/Si ratio), and H2O/SiO2 molar ratio (H2O/Si ratio)) and physical properties, such as density and specific surface area, of C-S-H. These factors are measured using synthesized C-S-H samples and C-S-H generated from various cementitious materials. Experimental results show that the H2O/Si ratio of C-S-H is proportional to the Ca/Si ratio independent of the mix proportion, curing temperature and type of binder. The density and specific surface area of C-S-H are affected by its Ca/Si ratio. A linear relationship is observed between the Ca/Si ratio and density of C-S-H independent of the mix proportion, curing temperature and type of binder. An inversely proportional relationship is found between the Ca/Si ratio and specific surface area of C-S-H.