Abstract: In order to fully understand the physical and chemical changes during the hydration process of red mud-coal gangue based cementitious materials，X-ray photoelectron spectroscopy(XPS) technique was used to investigate the binding energy among Ca，Si，Al，O and Na elements of the hydration products of red mud-coal gangue based cementitious materials. The results showed that with the increasing of hydration period from 3 to 90 days，the binding energies among Ca，Si，Al，O and Na elements of the hydration products gradually increased. Especially，the increasing of Na1s binding energy represents that the red mud-coal gangue based cementitious materials can solidify Na+ during the hydration process. Si2p binding energy decreases with the increasing of Ca/Si atom ratio of C-A-S-H gel，and negative linear correlation occurs between them. As O1s binding energy of the hydration products has positive linear correlation with the relative number of bringing oxygen(RBO). Therefore，O1s binding energy can be used to evaluate the polymerization degree of ［SiO₄］ tetrahedron from hydration products.

Key words: Red mud, Coal gangue, Cementitious materials, XPS analysis, Hydration process