Impact of environmental moisture on C(3)A polymorphs in the absence and presence of CaSO4 center dot 0.5 H2O

The phenomenon of water vapour sorption by anhydrous C(3)A polymorphs both in the absence and in the presence of CaSO4 center dot 0.5 H2O was studied utilising dynamic and static sorption methods. It was found that orthorhombic C(3)A starts to sorb water at 55% relative humidity (RH) and cubic C(3)A at 80% RH. Also, C(3)Ao sorbs a higher amount of water which is predominantly physically bound, whereas C3Ac preferentially interacts with water by chemical reaction. In the presence of calcium sulfate hemihydrate, ettringite was observed as the predominant pre-hydration product for both C(3)A modifications: that is, ion transport had occurred between C(3)A and sulfate. Environmental scanning electron microscopic imaging revealed that in a moist atmosphere, a liquid water film condenses on the surface of the phases as a consequence of capillary condensation between the particles. C(3)A and sulfate can then dissolve and react with each other. Seemingly, pre-hydration is mainly facilitated through capillary condensation and less through surface interaction with gaseous water molecules.