Dark brown oxide pigment is used as a dye or colourant in a variety of applications, including ceramics, paint, concrete, coatings, ink, leather, plastic, and rubber. This dry pigment powder, which is based on iron oxide, can be mixed with a binder to colour glazes and varnishes or to create paint. Iron oxide is used to coat particles in cement, mortar/and plaster. A suitable colour can be achieved using 4% by weight of oxide relative to the weight of the cement. At 8-10% saturation, no additional benefit is gained.
IRON OXIDE MIXING RATIO FOR CONCRETE/PLASTER/MORTAR
1. Determine the weight of the cement and refer to the table below to calculate the appropriate amount of oxide to use (based on 4%).
2. Mix thoroughly with aggregate/sand and half of the required water.
3. Ensure this mixture is well blended before adding the cement and the remaining water. Adequate mixing is essential to ensure the complete dispersion of colour.
4. To ensure consistent colour, all batches must be identical. Ideally, a sample should be approved before commencing the job.
| 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.
NATURAL COLOR OF THE CEMENT AND COLOR EFFECT
In colouring concrete, it is the cement paste that is pigmented rather than the aggregate. The colored cement paste coats the individual aggregate particles. Consequently, the more the colored cement paste is diluted with aggregate, the less intense the colour of the concrete becomes.
Grey cement has a muting effect on all colours, meaning it is not possible to achieve bright colours in concrete made with traditional grey cement as compared to concrete made with white cement. The enhancement of colour purity gained from using white cement, however, varies with the type of pigment added. For example, when using black pigment, there is virtually no difference between concrete made from white and grey cement. The difference is small with dark brown, red, and burnt umber, but significant with yellow, marigold, green, and blue.
PIGMENT LOADING AND COLOUR EFFECT
As pigment loading increases, colour intensity approaches a saturation point. Adding more pigment beyond this point does not significantly deepen the shade and may affect the structural strength of the concrete. The saturation point generally lies between approximately 5% and 8% based on the weight of the cement in the mix. However, a good depth of colour can be achieved with a 4% loading of our oxides.
The typical strength of ready-mix concrete is achieved using six 40 kg bags of cement per cubic meter of concrete. Therefore, 4% of this equates to approximately 9 kg of oxide. Conversely, mortar, which has a higher cement content, requires more pigment. It is important to note that pigment addition corresponds to the amount of cement in the mix, not anything else. A rough guideline is to use about 1.5 kg of oxide per 40 kg bag of cement.
BLENDING OF OXIDES FOR DIFFERENT COLOUR SHADES
Like paints, oxides can be mixed to produce a range of colours. When mixing colored concrete on-site, it is crucial to add ingredients to the mixer in the correct sequence to ensure complete pigment dispersion. For powdered colours, add the pigments along with aggregates and approximately half the required water, mixing thoroughly for a uniform blend. Then, add the cement and enough water to achieve the desired consistency.
