Iron oxide pigment is used as a dye or colourant in various applications, including ceramic, paint, concrete, coating, ink, leather, plastic and rubber. This pigment powder with an iron oxide base can be mixed with a binder to tint glaze and varnish or make paint.
Iron oxide coats the particles in cement and mortar/plaster. You can achieve a good colour using 4% by weight of oxide to the weight of cement. At 8-10%, saturation occurs, and no additional benefit is achieved.
Iron-oxide mixing ratio for concrete/plaster/mortar, follow these steps:
- To determine the weight of the cement, use the table below to calculate the amount of oxide to use (based on 4%).
- Mix thoroughly with aggregate/sand and 1/2 the water. Mix thoroughly before adding cement and the remaining water. Adequate mixing must occur to ensure the full dispersion of colour.
- Ensure all batches are identical to achieve consistent colour. Ideally, a sample should be approved before the job begins.
| MIX | CEMENT | OXIDE | |
| READY MIX | 1m3 | 250kg | 10kg |
| SITE MIX | 40kg | 7kg | 280g |
| BAG CONCRETE MIX | 25kg | 3kg | 120g |
| PATCH KIT | 1kg | 300mg | 12g |
| MORTAR | 1kg | 300mg | 12g |
The above ratio is an indicative guide. Therefore, you need to determine your own mixing ratio by doing a few tests before proceeding with your job.
Using white cement instead of grey in the mix, pastel shades can be obtained with light browns, reds, greens, and yellows.
Tinting strength of pigments
One brand of pigment is not the same as another. Two pigments identical in appearance may have a completely different yield. The decisive property in using a pigment for colouring concrete is tinting strength, which determines the depth of colour for various pigment brands. A sample mix prepared under practical conditions can provide reliable information on this.
The natural colour of the cement and its colour effect
Colouring concrete is not pigmented aggregate; the cement paste is coloured, forming a layer on the individual aggregate particles. Therefore, the more the coloured cement paste is diluted with aggregate, the less intense the colour of the concrete will be.
Grey has a subduing effect on all colours, no matter where they are. Therefore, it is impossible to colour concrete made with standard grey cement as brightly as white cement.
However, the gain in colour purity obtained using white cement depends on which pigment is added. In the case of black, there is virtually no difference between concrete made of white cement and one made of grey cement. The difference is small with dark brown, red and burnt umber, while it is considerable with yellow, marigold, green and blue.
Pigment loading and colour effect
With increasing pigment loading, the colour intensity moves towards the saturation point. Adding more pigment at this point does not significantly deepen the shade and may affect the structural strength of the concrete.
The saturation point lies between approximately 5% and 8%, calculated based on the weight of the cement in the mix. However, a good depth of colour can be achieved at a 4% loading with our Oxides.
The standard ready-mix concrete strength is 6 x 40kg bags of cement per cubic metre of concrete. 4% of this equates to approximately 9kg of oxide. Mortar, on the other hand, with its higher cement content, requires more pigment. It is important to remember that pigment addition relates to the amount of cement in the mix and nothing else. A rough guide is to use approximately 1.5kg of oxide per 40kg bag of cement.
Blending of Oxides for Different Colour Shades
Oxides, like paints, can be mixed to provide a variety of colours. When mixing coloured concrete onsite, ingredients must be placed into the mixer in sequence to ensure the pigments disperse fully. With powdered colours, put pigments into the mixer, aggregates, and approximately half the required water. Mix well to provide a uniform blend: add cement and sufficient water to obtain the desired consistency.