The influence of the alkaline medium on the hydrothermal activation of the oil shale fly ash with NaOH and KOH was studied using SEM/EDX, XRD, Si-29 and Al-27 high-resolution MAS-NMR spectra. In the presence of NaOH the silicon in the original fly ash was completely converted into calcium-aluminum-silicate-hydrates, mainly into 1.1 nm tobermorite structure during 24-h treatment at 160 degrees C. At similar reaction conditions, the activation with KOH resulted only to the formation of amorphous calcium-silica-hydrate gel on the surface of ash particles at temperature. The results obtained in this study indicate that the oil shale fly ash can be used for production of Al-substituted tobermorites when strongly alkaline media (NaOH) is applied. The synthesized product was used in a catalytic D-lactose isomerization reaction. (C) 2007 Elsevier Ltd. All rights reserved.