The hydrothermal alkaline activation of the oil shale fly ash was studied using SEM/EDX, XRD and Si-29 and Al-27 high-resolution MAS-NMR spectra. The silicon in the original fly ashes was completely converted into calcium-alumino-silicate hydrates, mainly into 1.1 nm tobermorite structure during 24 h treatment under hydrothermal conditions at 160 degrees C in the presence of NaOH. The local structure of synthesized tobermorite samples implies long silicate chains with small number of bridging sites. The results obtained in the study prove that the oil shale fly ash can be used for production of Al-substituted tobermorites. (c) 2006 Elsevier Ltd. All rights reserved.