QUESTION: What factors need to be considered to achieve maximised quality and consistency when using pigments in concrete?
ANSWER: Firstly, one needs to differentiate between the different types of pigments available. For many years, powder pigments were used to colour concrete products. Waterborne pigment slurries have also established themselves in the market. Free-flowing dry pigment preparations or granules, such as Lanxess’ Bayferrox® and Chrome Oxide Green pigments, have been specifically developed for use in the concrete building products industry and allow easy, accurate and dust-free weighing and conveying.
The tinting strength of a pigment is defined as its ability to impart its natural colour into the medium being coloured. A reputable pigment supplier will be able to indicate, with validated laboratory tests, what the specific tinting strength of each pigment is.
There is a debate around whether the use of grey versus white cement produces a better end colour when pigment is used. The gain in colour clarity when using white cement is more evident when green or yellow pigments are used, whereas the difference is negligible with black, dark brown and red pigments.
When producing coloured concrete, the aggregate particles are covered by the pigmented cement paste. Since it is possible that the grains of an intensively coloured aggregate do not become completely covered, the end result of the concrete could be affected by the natural colour of the aggregates. Similarly, like cement, the natural colour of the sand will also have a more marked effect on light-coloured yellow or green concrete than it would have on brown or black concrete. Furthermore, the more the coloured cement paste is diluted with aggregate in a concrete mix, the less intense the final colour of the concrete will be.
The concentration of a high-strength pigment, like Bayferrox®, will have a direct influence on the colour intensity of concrete up until approximately 5%. However, once this level is reached, the addition of further pigment will not significantly deepen the shade of the concrete – it will only make the concrete uneconomical.
Pigments are approximately 10 to 20 times finer than cement and the addition of pigment will therefore influence the water requirement of the concrete mix. Interestingly, red and black pigments have virtually no effect on the water requirement, but iron oxide yellow pigments have a needle shaped structure than can adsorb more water on the surface. This becomes noticeable at pigment concentrations of approximately 4 to 5 %, so since this value is generally not exceeded, it should not be of concern to concrete manufacturers.
The higher the water:cement ratio of concrete, the lighter the concrete will appear. To ensure trouble-free processing of the mix in a concrete manufacturing unit, the consistency should be kept within normal limits. This can be achieved by making use of a meter to measure the water content of the mixture.
In the accurate dispersion of the pigments, it is critical to ascertain exactly when the pigment should be added to the mixer. Typically, an ideal sequence involves mixing the pigment with the aggregate for approximately 15 seconds before adding the cement. Thereafter, the mixing process is identical to that of unpigmented concrete. Under no circumstances should the components be added simultaneously. With forced circulation mixers, the minimum mixing time is calculated at 90 seconds to 2 minutes to achieve a homogenous mixture.
Automation of the mixing process, using a meter, can be extremely advantageous as there is little room for error. Chryso’s colour metering system will automatically weigh just three or four base Bayferrox® pigments in over one hundred different combinations, to create various shades of one colour or a multi-coloured concrete.
Research indicates that the hardened cement matrix forms crystals of varying sizes, depending on the temperature at which the concrete is hardened. The higher the temperature, the finer the crystal needles will be and the more pronounced the light scattering ability of the crystals. This will result in a lighter-colour concrete than a mixture that hardens in lower temperatures. However, this phenomenon only becomes noticeable when one compares concrete that is steam-hardened with concrete that is allowed to harden at normal room temperature.
Cognisance needs to be given to the future development of efflorescence on concrete products. While high-quality pigments will have no effect on the development of efflorescence, it is true that the appearance of efflorescence will be more noticeable on coloured concrete than on natural grey or white concrete.
Chryso recommends that in all instances, concrete manufacturers and end users should demand that pigments comply with the EN 12 878 and ASTM C 979 standards to ensure optimum colour quality and consistency.
More information from Kirsten Kelly, Tel: +27(0)113959700/www.chryso.com
