Fading concrete block surfaces

Primarily metal oxide pigments are used for the colouring of concrete. They are first-order concrete additives. They are chemically inert, do not show any chemical or physical changes during the hydration of concrete; they are weather resistant as well as resistant to light, UV light or other natural influences. Users, anyhow, complain that the colour intensity of concrete block surfaces is altering in the course of time and that, in particular, dark concrete blocks are brightening. How can this phenomenon happen? This paper illustrates the changes that may occur in the pigments and on the surface of the concrete block in the course of time. Changes in the pigment or caused by the pigment only occur exceptionally, e.g. in case of the blue pigment ultramarine which may react in combi- nation with cement or in case of carbon pigment, which can escape from the hardened cement paste matrix in the course of time because of an insufficient bonding stability. The metal oxide pigments used for the colouring of concrete do not show such phenomena. A brighten- ing effect may, however, occur with such pigments too.

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