Colour fade in pigmented concrete
Integral coloured concrete can often be disappointing as the as-struck vibrancy fades.
Concrete is rich in calcium hydroxide from the cement. This goes into solution when water or moisture leaks through concrete or is absorbed from precipitation or condensation. As the water evaporates from the surface the calcium hydroxide is precipitated and, as it is in contact with the atmosphere, reacts with carbon dioxide to form insoluble calcium carbonate.
This ‘efflorescence’ is a chalky white salt residue and is typically un-noticed on grey concrete but can be very apparent on pigmented concrete especially the darker colours turning, for instance, red to pink, brown to tan and black to an inconsistent dull grey.
In theory the deposition should cease with time as the calcium is depleted, but this can take many years. The problem can be resolved by cleaning the surface with a mild acid or proprietary efflorescence remover and then sealing to prevent moisture movement. The use of a pigmented sealer may enhance the colour.
There are many proprietary pigment systems for integral colouring of concrete. These usually include good curing practice and sealing. Manufactures should be consulted to ensure the best long term results are achieved.