Mineral composition and microstructure of fly ash-high cementitious materials were studied by such means as chemical analysis, XRD and SEM. The results show that the mineral composition is mainly 3Al2O3·2SiO2and silica-alumina gelled mineral. Fly ash_high cementitious materials are characterized by the structure of numerous big holes. The mineral composition and microstructure of these materials are quite different from those of cement concrete. The content of fly ash and the curing time affect directly the quantity of hydrated product. When the content is above 20%, the materials entirely react with the consolidating agent and form porous solid bodies.