Impact of environmental moisture on C3A polymorphs in the absence and presence of CaSO4 . 0.5 H2O

Abstract

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, C3Ao sorbs a higher amount of water which is predominantly physically bound, whereas C 3Ac 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.