Fibre Reinforced Facade Encyclopedia: Finishes
Oxide pigments are finely milled metal oxides, overwhelmingly iron oxides, dispersed through a cementitious mix so the colour is part of the material rather than a coating on it. They are inert, insoluble, alkali resistant and ultraviolet stable, which is what makes them suitable for a material that spends fifty years outdoors in a highly alkaline matrix. Everything visible about a pigmented panel is decided by three things: the pigment, the cement it sits in, and the surface finish applied afterwards.
Doubling the pigment dose from 5 to 10 percent of cementitious content does not double the colour. Most iron oxides reach practical saturation between 6 and 8 percent, beyond which more pigment buys weaker paste and a higher bill, not a deeper colour.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Pigment family | Synthetic iron oxides | Red Fe2O3, yellow FeOOH, black Fe3O4, browns are blends |
| Non-iron oxides | Chromium oxide green, cobalt blue, titanium dioxide white | Higher cost; the only stable blues and greens |
| Typical dose | 2 to 6 percent by weight of cementitious content | Not by weight of total mix |
| Practical dose ceiling | 6 to 8 percent | Above this the colour gain is negligible |
| Absolute ceiling | 10 percent | Beyond this strength and shrinkage are affected |
| Particle size | 0.1 to 1 micrometre typical | Finer pigment tints harder but needs more water |
| UV stability | Excellent; inorganic and fully oxidised | Organic pigments are not used externally in cement |
| Alkali resistance | Stable at pH 13 and above | The requirement that rules out most dyes |
| Form supplied | Powder, granulated or liquid slurry | Slurry and granules disperse more reliably |
Why iron oxides and nothing else
A pigment in concrete has to survive two attacks that a pigment in paint does not: a pore solution above pH 13, and decades of sunlight with no protective film over it. Synthetic iron oxides survive both because they are already fully oxidised inorganic compounds, so there is no further reaction for the alkali to drive and no chromophore for ultraviolet to break down. Organic pigments, which give the bright clean colours available in paint, are broken down by both and are not used in external cementitious panels.
That chemistry sets the palette. Iron oxides give reds and maroons (haematite), yellows and ochres (goethite), blacks and greys (magnetite), and every brown, buff, tan and charcoal blended from them. Outside the iron family, chromium oxide gives a stable but muted green, cobalt aluminate a stable but expensive blue, and titanium dioxide white. There is no stable, affordable bright green, blue, purple or orange in this system, so a facade needing those colours is a coated facade, a fibre reinforced polymer facade with a pigmented gel coat, or a different material.
Dose, and the ceiling that matters
Pigment dose is quoted as a percentage of the weight of cementitious content, meaning cement plus any supplementary cementitious materials, not of the total mix. Using total mix weight by mistake produces a dose three to five times lower than intended and a facade that looks washed out.
The relationship between dose and colour is not linear. It rises steeply at low doses, flattens through the middle, and is essentially flat above about 6 to 8 percent for most iron oxides, because once enough pigment is present to coat the cement grains and fill the paste, more simply puts uncoloured pigment into a matrix already as dark as it can be. Most architectural work sits between 2 and 6 percent, and a specification asking for 12 percent to get a deeper colour is asking for a weaker, more shrinkage-prone panel at a higher price with no visual gain.
Why the cement matters more than the pigment
Grey Portland cement is itself a strong, variable, greenish grey pigment present at many times the dose of anything added to it, so every colour mixed into it is a colour mixed with grey. That is why pigmented grey mixes tend toward the muted and earthy, and why pale colours in grey cement are effectively impossible: a light pink in grey cement is a dirty mauve.
White Portland cement solves both problems. Made from low iron raw materials, it is far more consistent in colour than grey and provides a neutral pale base, so pale, clean and vivid colours all require it, and so does any mix where colour consistency is critical. The trade-off is a significant cost premium. The sand does the same at a smaller scale: fine silica sand is a large volume fraction of a GRC mix and its own colour shows through, so a single sand source is as much a colour control as the pigment.
How the finish changes the colour
- Off-form: the darkest and most saturated reading, the dense surface skin intact
- Acid etched: lighter and flatter, the skin removed and the rough face scattering light
- Sandblasted: lighter again, typically one to two shades paler than off-form
- Polished: darker and more saturated than off-form, the paste cut back into the body of the mix and burnished
- Exposed aggregate: the stone dominates and the pigmented matrix is only the background
A pigment dose approved against an off-form sample will therefore look wrong on an etched or blasted facade. Colour must be selected and approved on a sample carrying the specified finish, made from the production mix. Pigment suppliers’ colour cards are a starting point for conversation and nothing more.
Specifying integral colour
Nominate the pigment by supplier, product code and colour index designation rather than by colour name or paint reference. Nominate the dose as a percentage of cementitious content, and the cement type and colour, because it does more work than the pigment. Require the pigment for the whole project from a single manufacturing batch reserved at the start, batched by weight with recorded results rather than by bag or scoop.
Standards and references
- EN 12878 Pigments for the colouring of building materials based on cement or lime, requirements and test methods
- ASTM C979 Standard Specification for Pigments for Integrally Colored Concrete
- AS 3972 General purpose and blended cements; white cement is specified by colour and iron content
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (mix control and batching)
- Colour Index International generic names, for example Pigment Red 101, Pigment Yellow 42, Pigment Black 11
- PCI MNL-117 Manual for Quality Control for Plant Produced Architectural Precast Concrete
Related terms
Questions specifiers ask
How much pigment goes into a concrete or GRC mix?
Typically 2 to 6 percent by weight of cementitious content. The practical ceiling is around 6 to 8 percent, above which additional pigment produces almost no further colour, and 10 percent is an absolute limit before strength and shrinkage are affected. Dose is a percentage of cement, not of the total mix.
Why can I not get a bright blue or green?
Because only inorganic pigments survive a pH 13 matrix and decades of ultraviolet, and the stable inorganic palette is iron oxide reds, yellows, blacks and browns plus chromium oxide green and cobalt blue. Bright colours mean a coated facade or a pigmented FRP gel coat.
Do oxide pigments fade?
Synthetic iron oxides are already fully oxidised inorganic compounds with no chromophore for ultraviolet to attack, so the pigment is stable for the life of the panel. Apparent colour change comes from the cement matrix weathering, carbonation, dirt and efflorescence, not the pigment.
Why is white cement used for pale colours?
Grey cement is itself a strong greenish grey pigment present at many times the dose of anything added to it, so every colour mixed into it is muted and no pale or clean colour is achievable. White cement gives a neutral pale base and is far more consistent in its own colour.
Should I approve colour from a supplier’s colour card?
No. Approve it from a sample made with the production mix, cement and sand, carrying the specified surface finish, because every finish shifts the reading. An off-form sample looks one to two shades darker than the same mix acid etched or sandblasted.
Matching a colour across a pigmented GRC facade? We batch pigment by weight from a single reserved batch and accept production against signed control samples. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.