Why Blue / Green colour appears on the High Slag Cement Concrete when the mould is removed?

This is the unique characteristic of High Slag Cement Concrete, and it is a normal phenomena. That means the concrete gets very good curing in its early days, and this is good for its strength development. Usually, the blue colour will automatically fade off and become normal colour after being exposed to the air for 3 – 5 days. The reason blue/green colour appears is, minimal amount of FeS and MnS are formed during the GGBS hydration process, the hydrate appears in blue colour. With the exposure to the air for a period of time, the above chemical composition will then change to FeSO4 and MnSO4, and the blue colour will then disappear.