In this study, time dependent ettringite formation in heat-cured mortars has been investigated. In order to clarify the effect of formation place and morphology of ettringite on expansion, secondary electron images of cracked surfaces of mortars at three ages were analysed by SEM-EDS. Also, the X-ray microtomography analysis has been performed to observe the crack formation. The expansive role of delayed formed ettringite was related with its time dependent morphology as a function of formation place. From these observations, mechanism of ettringite reformation after heat curing has been proposed. Alumina rich species were the primary sources of ettringite formation as the starting nuclei. At later ages, if S and Al sources are readily available, the mentioned alumina rich nuclei will grow up and build ball ettringite. At long term, ball type ettringites (non-expansive) converted to massive type (expansive). These conversions can only take places if the form of available space is narrow (preformed micro-cracks). Massive ettringites exert pressure in these narrow spaces and cause expansion of mortar. If the form of the available space is spherical (entrapped air voids) ball ettringites preserve their initial form and do not cause any expansion. (c) 2010 Elsevier Ltd. All rights reserved.