Fly ash from lignite coal burning was subjected to a hydrothermal activation with NaOH and KOH (1.5 divided by 12.5 m) under different heating regimes to investigate zeolites formation. Products morphology, elemental and mineralogical composition were studied by means of SEM, EDX, and XRD. The activation with KOH (>6 m) and NaOH (>3 m) for 7 h at 373 divided by 393 K produced zeolite Linde F and hydroxy sodalite, respectively. Blend of zeolites A and P was obtained at treatment with 3.1 m NaOH at 353 K. It was found out, that in all types of zeolites Na+ had been partially replaced by Ca2+. The other impurities (e. g. Fe, Mg, and S) were detected only in hydroxy sodalite crystals and in smaller amounts. Some experiments indicated that the dominant zeolite phase in the product was determined in the first couple of hours of isothermal treatment, i.e. before the appearance of micron-sized zeolite crystals. Zeolites crystallized preferably on particles having glassy surfaces and had quite a constant composition, unrelated to that of the surface. (C) 2012 Elsevier Ltd. All rights reserved.