Precast concrete materials, manufacture, properties and usage - Chapter 3 ppsx

3

PIGMENTS

A pigment can be defined as ‘a fine dry powder, or aqueous suspension

or slurry of the powder, inert to the ingredients of concrete, and intended

to impart a specific colour to the product’. The words ‘intended to

impart’ are the basis of this chapter which includes discussion on what

pigments are, how they and their performance can be assessed and their

practical deployment.

Pigments are available in a variety of particle shapes and sizes but the

property they have in common is that they all have a minimum particle

size smaller than the finest cement one would use, and rely on their

smearing power over the cement and fine aggregate fractions to achieve

coloration. Obviously the colours of the cement and the fine aggregate

play important roles in determining the final colour of the concrete or

mortar and are important in the selection of concrete ingredients. Colour

is also affected by the type of formwork or mould materials, release

agent, finishing treatment (if used) and curing conditions. To obtain

uniform appearance of the product requires stringent controls of these

variables.

Other variables associated with colour are discussed in detail later but

one general aspect worth mentioning at this stage is ‘fading’ of pigments.

By the definition given above there should be no such thing as fading.

The staining power of a pigment can be masked by lime leaching over the

pigment as well as by carbonation of that lime. In the cases of open￾textured concretes and mortars, or when poor quality soft carbon blacks

are used, pigments can be washed out by the weather. There is also some

evidence that at high temperature (e.g. 40–60° C) and high humidity

carbon becomes slowly oxidised.

Copyright Applied Science Publishers Ltd 1982