Ettringite is a common phase occurring during the hydration of Portland cements.It is also the main hydration product in calcium sulfoaluminate cements and calcium aluminate cement-gypsum blends.Understanding the stability of ettringite during hydration and under drying conditions is of great importance to assess the performance of systems containing large amounts of this phase.This paper presents a new study on the stability of ettringite as function of temperature and water vapor pressure.Thermodynamic properties associated to the decomposition of ettringite into metaettringite and to the reformation of metaettringite into ettringite are presented and a model to predict its stability as function of temperature proposed.A previously derived thermodynamic database (incl.different hydration states of AFm, AFt, etc.) allowed us to assess the impact of water activity on the hydrate constitution of cement paste.Based on the calculations we identified stable and metastable hydrate phase assemblages as function of water activity.The calculations suggest that at room temperature ettringite and are persistent until very low water vapor pressure.This may indicate an improved dimensional stability of cement paste containing large quantities of ettringite phases if subjected to drying.