A working procedure was developed for determining the degree of reaction of fly ash subjected to alkali activation (with 8 M NaOH) at mild temperatures. Since the reaction products dissolve in HCl, the residue left after this acid attack contains only the fraction of the original ash that failed to react with the basic solution. This residue was analysed with Rietveld XRPD quantification and NMR and the findings were compared to the results of the analyses run on the activated ash to obtain a very precise quantification of all of the (crystalline, vitreous and amorphous) phases present in the systems studied. (C) 2006 Elsevier Ltd. All rights reserved.