Hydrothermal synthesis of zeolites from class F fly ash was carried out at solid/liquid (S/L) ratio of 4 g/dm(3) to investigate the initial dissolution stage of the process. The low S/L ratio caused a significant increase in fly ash dissolution (up to 85 wt%) during first 4 h of reaction. It was shown that for S/L ratios typically used (50-150 g/dm(3)) fly ash dissolution is largely limited. In these conditions zeolite crystals are formed on the surface of partially dissolved fly ash particles. This precludes further dissolution and results in formation of a low-crystallinity zeolite-like material. At low S/L ratio, a degree of crystallization of 80 wt% was observed, and NaP1, analcime and/or hydroxy-soclalite phases were obtained depending on the applied NaOH concentration and temperature. Single phase NaP1 zeolite was obtained in 2 M NaOH solution at temperature maintained at 104 degrees C during the stage of dissolution and reduced to 80 degrees C in the stage of crystallization. Application of low S/L ratio allows a clearer description of the relationships between the synthesis conditions and the products obtained. (c) 2008 Elsevier Ltd. All rights reserved.