Red mud-fly ash based cementitious material mixed with different contents of oil shale calcined at 700℃is investigated in this paper.The effect of active Si and Al content on the solidification of Na+ during the hydration process is determined by using X-ray diffraction（XRD）,27Al and 29Si magic-angle-spinning nuclear magnetic resonance（MAS-NMR）,infrared（IR）,scanning electronic microscopy（SEM） and X-ray photoelectron spectroscopy（XPS）.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 3D framework of SiO4 and AlO4 tetrahedra 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.
