Concrete containing a blended cement of Portland cement and ground granulated blastfurnace slag (ggbs) will sometimes exhibit a blue tinge on the surface, after the form-face has been struck, during the first few days of exposure to air.

This occurs because of the formation during hydration of iron sulphide (FeS) and manganese sulphide (MnS). Both are blue. In contact with the air, both sulphides are oxidised to iron sulphate (FeSO₄) and manganese sulphate (MnSO₄) neither of which are blue. Once the surface of concrete is in contact with air, the blue colour fades.

Where the form-face is kept in contact with the concrete for a longer time, the blue tinge will persist longer.