Coal fly ash generated from thermal power plants is the largest single solid waste, and its reutilization is indispensable for the environmental protection and resource sustainability. In this work, mullite powders have been prepared from coal fly ash by the mullitization and hydrothermal processes. In the mullitization process, the milled fly ash was mixed with boehmite sol, and sintered to increase the mullite content. The mullite content firstly increased and then decreased with the increase of sintering temperature above 1200 degrees C, while it increased progressively with the increasing boehmite addition, especially above 8%. The mullite content of 63.8% was achieved with 12% boehmite at 1200 degrees C. In the subsequent hydrothermal process, the sintered powders were treated in the alkaline solution, and immersed in the acidic solution. The optimal mullite powder consisted of 96.4% mullite and 3.6% quartz. The secondary prismatic crystallites (0.3-1 mu m) coexisted with the primary spherical crystallites (similar to 0.1 mu m). The combined processing route sheds light on the potential production of mullite powders from coal fly ash. (C) 2018 Elsevier B.V. All rights reserved.