The influence of water loss during the hydration of calcium aluminates on the phase development is investigated at 60 degrees C. This is relevant for applications in which calcium aluminate cement (CAC) based formulations are exposed to quick drying during hydration. The presented results provide new insights into the well-known conversion processes occurring in CAC pastes. Using insitu XRD two different routes of the development of initially formed C(2)AH(8) are determined: (a) transformation to C(3)AH(6)+AH(3) in the presence of sufficient free water and (b) dehydration to C(2)AH(5) at a lack of free water. Moreover, the influence of precuring of the pastes at 23 degrees C before heating to 60 degrees C is investigated. The increasing loss of free water with increasing precuring time resulting from both, precipitation of hydrate phases and evaporation, causes incomplete hydration of CA or CA(2) as well as dehydration of C(2)AH(8) instead of conversion into C(3)AH(6). Comparative investigations of sealed samples always revealed complete hydration of CA and CA(2) as well as complete conversion of C(2)AH(8).