This paper concentrates on valorization of the fly ash originating from thermal power plant by alkaline activation. Structural and textural characterization of raw material and prepared geopolymers was carried out. The used raw material belonged to Class F according to ASTM C618-05, and it was mainly composed of quartz, mullite, hematite and calcite, that coexisted with hydroxy-sodalite in final geopolymers. The proportion of the hydroxy-sodalite phase increased with the concentration of the activating agent. During geopolymerization, structural changes appeared including the incorporation of aluminum into the silicate network leading to their crosslinking. Textural characterization showed that the material became more compact during the geopolymerization process, which in agreement with the increase of hydroxy-sodalite phase. These changes lead to an increase of mechanical strength of geopolymers.