The conversion of fly ash into zeolites by incubation of the fly ash with alkaline solutions is a well known process which, however, usually results in a zeolitic product which still contains significant amounts of residual fly ash. Presented here is a method by which part of the silicon in fly ash can be used for the synthesis of a maximum of 85 g of pure zeolite per kg of fly ash prior to the residual being converted into zeolite by the traditional method. The cation exchange capacities ranged from 3.6 to 4.3 meq/g for the pure zeolites and from 2.0 to 2.5 meq/g for the zeolite containing residual fly ash. Tests showed that the pure zeolites an suitable for the removal of ammonium and heavy metal ions from waste water.