Red mud-fly ash based cementitious material mixed with different content of oil shale calcined at 700 ℃ was investigated in this paper.The effect of active Si and Al content on the solidification of Na+ during the hydration process was determined by using XRD, 27Al and 29Si MAS NMR, IR and SEM.It is shown that the content of oil shale has a remarkable effect on the solidified content of Na+.The hydration process generates a highly reactive intermediate gel phase formed by co-polymerisation of individual alumina and silicate species.This kind of gel is primarily considered as three-dimensional framework of [SiO4] and [AlO4] tetrahedron interlinked by the shared oxygen atoms randomly.The negative charges and four-coordinated Al inside the network are mainly charge-balanced by Na+.The solidifying mechanism of Na+ is greatly attributed to the forming of this kind of gel.