In order to improve the cementitious activity of red mud, materials that can be easily depolymerized were added to it before co-calcination. Results from standard mechanical tests of mortars show that the cementitious activity of co-calcined red mud is significantly higher than that of the simple calcined red mud. For studying the mechanism responsible for this improved cementitious activity, analysis tools such as XRD, IR and NMR were used. XRD analysis reveals no obvious phase transitions during co-calcination. However, with IR and NMR analysis, it was found that the anti-symmetric Si-O absorption band broadened, while the NMR peaks of 29Si and 27Al not only became broader and more jagged, but also shifted in location. These results indicate a decrease in polymerization of the polyhedral Si-O in co-calcined red mud due to the process of co-calcination. Moreover, a significant increase in the dissolved quantities of SiO2 and Al2O3 in NaOH solution for co-calcined red mud is observed, which explains the increase in its cementitious activity.