A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and Si-29 MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution. (C) 2003 Elsevier Ltd. All rights reserved.