Representative fly ash (FA) samples have been collected from the electrostatic precipitators of two different lignite-fired power plants of Greece (Megalopolis (Southern Greece) (850 MW) and Meliti (Florina, Northern Greece) (330 MW) thermal power stations), under maximum electricity load (hereinafter referred to as MG-FA and MT-FA, respectively). MG-FA and MT-FA samples underwent alkaline hydrothermal treatment at 90 °C, using 1 L NaOH 1 M per 50 g FA. The incubation period was set at 24 h and the mixing took place at 150 rounds per minute (rpm). After that period, the mixture was filtered and the solid residue was dried at 40 °C for 24 h and leached with water until no NaOH was detected.