Jan Prusinski, P.E., executive director of the Slag Cement Association, explains why "greening" occurs and what to expect:
The color you describe is caused by complex reactions between sulfide sulfur compounds in the slag cement and portland cement compounds. It usually doesn’t occur, but when it does, exposure to air and sunlight will eventually promote oxidation reactions and eliminate the greening. Greening is prolonged (but eventually disappears) if the concrete is not exposed to air/sun during curing, such as when formwork has been left on for some time.
Of course, higher-strength concrete like you are using has reduced permeability, which slows the process of going from green to white. If a new surface is continuously wet (as in a new pool) the greening may never disappear. We do not recommend slag use in pools or other applications that will not have a chance to be exposed to air/sunlight and where color is important. Also, any sealing should not be done until the greening has been eliminated.
Ultimately, the final color of the oxidized concrete will be significantly lighter than non-slag portland cement concrete because of the light color of the slag cement, especially at the 50% level. For additional information, visit www.slagcement.org.