The mineralogical, morphological, physical and chemical properties of seven different fly ashes from eastern, central and western lignite and bituminous coal fields in Turkey are compared in this study. The mineral matter in the fly ashes, determined by means of X-ray diffraction, is dominated mainly by anhydrite, lime, quartz and hematite + ferrite spinel. The three low-calcium ashes-Soma, Seyitomer and Catalagzi-have the typical, relatively simple, crystalline phases Q, M, H and FS. The high-calcium fly ashes-Yenikoy and Afsin-Elbistan-have the most complex assemblage of crystalline phases. The much higher calcium concentrations in these samples result in the formation of lime (CaO), melilite [(Ca,Na)(2)(Mg,Al,Fe)(Si,Al)(2)O-7] and merwinite (possible phase) [Ca3Mg(SiO4)(2)]. The presence of anhydrite in all samples indicates that the high activity of calcium not only promotes the formation of sulfates (Ah and SO) from calcite (in the presence of sulfur and oxygen), but also the dehydration of gypsum during and after combustion, which occurs at temperatures above 400-500 degrees C. It is very important to understand the interaction of high-calcium fly ashes with water occurring in Portland cement (C(3)A,C2S), Ah, which hydrates to give gypsum and lime, with the latter hydrating to give the Ca(OH)(2) solutions that promote pozzolonic reactions. It was clear from the microscopy data that some of the particles comprised predominantly irregularly formed, vesicular particles with some well-formed individual spheres in Catalagzi and Tuncbilek fly ashes. About 55-80 wt% was less than 45 mu m in size-for Yatagan, Soma, Yenikoy and Afsin-Elbistan fly ashes. Chemical analyses of the fly ashes showed that they were mainly composed of CaO, SiO2 and AL(2)O(3). These fly ashes have a potential use in wastewater treatment since they can be easily obtained in large quantities at low price or even without price. The chemical and mineralogical compositions of the high-calcium Turkish fly ashes investigated make them a good binding agent and a possible substitute for slags, pozzolana and gypsum in the amelioration of clinker. Moreover, the fly ashes could be also used as a liming material to raise the low pH of acid soils in humid regions, such as around the Black Sea coast of Turkey.