The hydration and transformation of Ca-rich kerogenous oil-shale (kukersite) ashes, formed in circulating fluidized bed (CFB) combustion furnaces, were investigated during a one-year experiment. During the first stages the process was characterized by rapid hydration of free lime hydration into portlandite and dissolution of anhydrite and periclase, whereas the formation of (crystalline) ettringite was somewhat delayed. During the final process, ettringite and portlandite that are unstable under atmospheric conditions were replaced by stable calcite and Ca-sulphate. (C) 2008 Elsevier Ltd. All rights reserved.