Water addition and strength of concrete.
Darker, less bright colours will be obtained with concrete of lower water/cement ratio that has been compacted by compression than concrete of higher water/cement ratio such as vibrated concrete.
The lower the water/cement ratio of a concrete, the greater its strength, providing the concrete is not very dry making it unworkable.
Also as the water/cement ratio increases, so the shade of the concrete where pigment has been added becomes lighter. It is important then to maintain a consistent water/cement ratio.
Fading of coloured concrete
Chemical reactions within concrete itself can reduce colour brightness. It is also important that the concrete is not allowed to dry too quickly. As the concrete cures, especially in hot summer weather, slow the hardening process down by keeping water on the surface. The inorganic pigments supplied by Bayer are manufactured for optimum resistance to the weathering effects of nature and will retain good colour stability during the normal life of concrete products that the pigments are used in.
This effect occurs more frequently in outdoor locations however concrete is a natural material and subject to mottling and colour variations whether pigmented or not. The causes have not been proven however it is suspected that variations in ground moisture under the concrete, partial shading of worksite and or weather variations during the pour/curing may affect the appearance.
It is essential to ensure consistency in the concrete batches, minimise added water at site, use a light powered float for consistent density and finish, keep concrete wet to cure for a week.
Efflorescence is the result of free lime formed when the cement sets and migrates to the surface of the concrete where it reacts with the carbon dioxide in the air to form insoluble calcium carbonate. This has a white powdery appearance.
Efflorescence will disappear in the course of time, normally 18 months to two years of natural weathering however this depends on the climatic conditions and on the amount of traffic the concrete has to bear.
If this natural process takes too long, the efflorescene can be treating with dilute acid in the following way:-
Soak the concrete surface thoroughly with water and then spray with a diluted acid solution (e.g. diluted Spirits of Salts - 1 part Spirits of Salts to 3 parts water). Use a common garden spray pump and wear protective clothes, acid-resistant shoes, protective gloves and safety goggles. Apply the treatment carefully to avoid a spotty result. Gently scrub with a soft broom during treatment to ensure good penetration and uniform coverage. After a short time rinse the concrete thoroughly with flowing water.
Please note that floors which are tile cut for grouting should be sealed with concrete sealer before grouting to prevent grout bleed into the coloured concrete.