What is a pigment?

A pigment is a fine colour powder that does not dissolve in the binder. That sets it apart from a dye or stain, which does dissolve. A pigment stays in the paint, mortar or concrete as discrete particles, giving both colour and opacity. The most widely used pigment is titanium dioxide: the white, strongly opaque pigment in almost every white paint.

Types

• Inorganic pigments — metal oxides and salts: iron oxide (red, yellow, brown, black), chromium oxide (green), titanium dioxide (white), ultramarine (blue) and carbon black. They are highly light- and weather-fast and alkali resistant, which makes them suitable for cement- and lime-bound materials.
• Organic pigments — synthetic, usually brighter colours with high tinting strength, but generally less lightfast and often not alkali resistant.

For colouring cement- and lime-bound building materials, the European standard EN 12878 applies. Pigment complying with it does not impair the setting and strength of the concrete.

Applications

1. Paint, lacquer and stain — the pigment determines colour, opacity and, with white pigments, how a facade weathers.
2. Coloured concrete and mortar — through-coloured concrete for paving blocks, floors, fair-faced concrete and facade elements.
3. Pointing mortar — a grey or anthracite joint changes the look of a wall more than the brick does.
4. Plaster, render and limewash — mineral pigment is essential here because of the high pH.

Dosage in concrete and mortar

Typical dosage is between about 2 and 5 per cent of the cement weight. Beyond that the colour gains almost no intensity while the risk of strength loss rises, so more than roughly 5 per cent is rarely worthwhile. Note that the pigment colours the cement paste, not the aggregate. A light cement (white cement) gives far purer colours than standard grey Portland cement.

Common mistakes

• Varying water-cement ratio. More mixing water gives a lighter shade. Colour differences in a floor or wall come more often from differences in water content and compaction than from the pigment.
• Dosing by the bucket, by eye. Pigment must be weighed and dry-mixed long enough, otherwise you get streaks and clouding.
• Judging colour while wet. Fresh mortar and concrete are always darker; judge only after full drying, and always against a retained sample panel.
• Mixing different deliveries. Within one area, work with pigment and cement from the same batch.
• Ignoring efflorescence. Lime bloom leaves coloured concrete dull and patchy; a low water-cement ratio and proper curing largely prevent it.

Colour checks for pigmented concrete and mortar

• Weigh the pigment and dry-mix long enough: Dosing by eye gives streaks and clouding.
• Keep the water-cement ratio constant: More mixing water gives a lighter shade.
• Judge colour only after full drying: Compare against a retained sample panel; fresh mortar and concrete are always darker.
• Use one batch per area: Pigment and cement from the same batch within one area.
• Prevent efflorescence: A low water-cement ratio and proper curing largely prevent lime bloom.

Related terms

• Coloured concrete
• Admixture
• Pointing mortar
• Limewash
• Efflorescence

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.