In the alumina extraction process of high-alumina fly ash, alkali was used to remove amorphous silica and then recovered through causticization. Focusing on the causticization process, the coupling control of CO32- conversion and CaCO3 crystallization is important to secondary-pollution reduction and high-value utilization. Influences of temperature, stirring rate, feeding time, Ca(OH)(2) concentration, and calcium/carbonate (Ca2+/CO32-) ratio on the conversion and CaCO3 morphology were investigated. Feeding time and Ca2+/CO32- ratio showed crucial effects on the conversion and crystallization of CaCO3 particles. Under optimized conditions, the conversion of CO32- can reach 95.67%, and the alkali can be well recycled with the generation of superfine calcium carbonate. Furthermore, the morphology evolution of CaCO3 was investigated in the reaction process. At the beginning of causticization, a combination of amorphous and crystalline structures can be observed. Subsequently, cubic and homogeneous morphologies were formed. At the end of the reaction, the edges of the cubic particles were dissolved, and the large particles with irregular shapes were formed by the joint of small ones in the alkaline system. (C) 2016 Elsevier B.V. All rights reserved.