To investigate the occurrence of periclase in clinker aluminate and reveal the hydration mechanism, aluminates were synthesized via MgO doping at various contents. Their structure and hydrates were characterized by scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The results indicate that higher sintering temperature and MgO content improve the formation and stability of C4AF and C12F7. A phase separation occurs in molten aluminum phase. The black phase includes the main minerals like C3A and periclase, while glass and yellow phases both have a low MgO content. The hydration heat of aluminate phase in the first day decreases with the increase of MgO content, indicating that MgO doping decreases the hydration rate of aluminate phase. In addition, the layered double hydroxides with a Mg:Al ratio of 2 or 3 appear in the hydration products of aluminate incorporated with MgO.