A sorption balance study of water vapour sorption on anhydrous cement minerals and cement constituents

The phenomenon of water vapour sorption by powdered cement constituents exposed to different relative humidities and temperatures was studied. The individual clinker phases C3S, C2S, C3A, C4AF, calcium sulfates and CaO were tested. Using a water sorption balance, the amount of chemically and physically sorbed water per unit of surface area of the powders and the relative humidity at which water sorption starts to occur on the phases were determined. Various cement clinker phases prehydrate very differently. CaO and C3A were found to be most reactive towards water vapour whereas the silicates react less. CaO starts to sorb water at very low RHs and binds it chemically. Beginning at 55% RH, orthorhombic C3A also sorbs significant amounts of water and binds it chemically and physically. Water sorption of C3S and C2S only begins at 74% RH, and the amount of water sorbed is minor. Calcium sulfates sorb water predominantly physically.