Impact of environmental moisture on C₃A polymorphs in the absence and presence of CaSO₄ · 0·5 H₂O

The phenomenon of water vapour sorption by anhydrous C₃A polymorphs both in the absence and in the presence of CaSO₄·0·5 H₂O was studied utilising dynamic and static sorption methods. It was found that orthorhombic C₃A starts to sorb water at 55% relative humidity (RH) and cubic C₃A at 80% RH. Also, C₃A_o sorbs a higher amount of water which is predominantly physically bound, whereas C₃A_c 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 C₃A modifications: that is, ion transport had occurred between C₃A 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. C₃A 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.