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Colour variation in concrete is inherent to the constituent materials. Some of these materials are manufactured products (cement, admixtures, pigments) and some are natural (coarse and fine aggregate).
The colour of cement varies according to the materials from which it is manufactured. It ranges across shades of grey and differences are noted when concretes made from cements from different works are used alongside one another. The extreme is white Portland cement (PC) which is made from selected raw materials processed so as not to introduce pigmentation into the firing and grinding process. The term cement now includes other cementing materials as a combination. A combination of PC with ground granulated blastfurnace slag (ggbs) will produce a lighter colour and a combination of PC with fly ash (fa) will make the concrete darker.
Fine aggregates (sand) are either marine dredged or pit materials and they may also include crushed rock particles. They come in many different colours dependent on the local geology. Similarly coarse aggregates are either marine dredged, crushed rock or land based gravels. Particle sizes greater than 600 micron have little influence on the surface colour of concrete unless the thin outer layer of cement is removed. Finer particles (less than 150 micron) in the sand or from the crushing process, however, act as a natural pigment and influence the colour of the concrete. This effect is more noticeable when lighter coloured cements are used.
Acknowledgement: The Concrete Society