We often talk about colouring cement but, in reality, the material we are going to paint or create is almost always concrete or cement mortar.
Cement alone is at risk of excessive shrinkage and therefore cracking, which is why it is always composed with the addition of sand and often fibres to control the shrinkage of the mixture.
When does mortar need to be coloured?
There may be many cases where it may be desirable to change the final colour of the cement. Grey is indeed a bit monotonous and the colour contributes to making concrete products more pleasing to the eye.
Mostly, it is used to colour artefacts that we may have created ourselves, for decorative purposes, such as bottles, small vases, paperweights or bookends.
Another very common use of coloured cement is to make decorative blocks for paved paths in the garden.
This last use leads us to another very important consideration which we will develop later: colouring techniques can (and in many cases must) be different depending on whether we are dealing with vertical coverings or walkable objects to be laid on the ground.
In the latter case, in fact, a simple superficial colouring could prove unsuitable.
But let us proceed step by step.
Difference between painting the surface of concrete and colouring the mix
The colour that can be achieved by adding special dyes to the concrete mix is quite different from the colour that can be achieved by simply painting the outer surface of the concrete.
The latter is a surface colour that is easily attacked by the weather (if the concrete is placed outside) and needs to be protected with a transparent paint layer to limit degradation.
In the case of powders added to the mortar mix, together with cement, sand and fibres, what is obtained is a colouring that affects the whole mix and therefore not only the external part.
There's no need to say that this type of colouring is particularly suitable for items that are subject to wear, such as paving bricks or driveway slabs.
When the colour is added to the mix the end result will be a cement with an opaque colour, whereas when only the outer surface is painted the colours that can be achieved are much more varied and can even be shiny and bright.
However, if you paint the concrete with a brush or spray, the texture of the material is lost, i.e. the concrete may no longer look like concrete, especially when viewed from a distance.
This may be desirable in many cases and a little less so in others: if we want to paint a concrete floor, we could also use special resins that give the surface a smooth, shiny appearance that is particularly suitable for garage floors and that, in recent times, is also becoming popular in residential environments.
However, if you want to colour the concrete blocks of a driveway, you need a longer-lasting solution and less conspicuous colours, and this can only be achieved by mixing special pigments into the mixture.
It goes without saying that this only makes sense if we make the blocks ourselves using moulds that can be bought online or that we have made ourselves.
Colouring the cement mortar
As mentioned above, what you are going to colour is actually cement mortar, which is a mixture of cement, sand, fibres and water.
Obviously, you will need pigments for colouring, which can also be purchased online on these portals:
To understand which percentages of materials to use in order to obtain a good final result, I invite you to watch this video that explains quite well how to proceed.
To achieve the smoothest possible surface, the sand grains should be as small as possible. In the video a powder is used but it doesn't necessarily have to be this way, if we want to "print" bricks for an outdoor path, a certain final roughness may even be desirable.
In the case of a concrete kitchen worktop, on the other hand, the smoother the surface the better.
As you can see from the video, a drill whisk is required to achieve the optimum coloured mix, as mixing with a trowel alone would otherwise be difficult to achieve an even colouration.
N.B.: Since concrete pigments are expensive, it can be advantageous not to colour the whole mix but only the surface layer.
This presupposes that two mixtures are created: one coloured to obtain a layer of one or two centimetres, which will remain visible, and then a remaining layer which can be composed of non-coloured mortar. The two layers should be applied one after the other, while the mix is still wet, so that they can bond perfectly and form a single body.
Painting the concrete on the outside
Painting concrete is more difficult than painting most other surfaces: concrete is breathable, transports moisture and sucks up paint.
For all these reasons, the surface must be very carefully prepared in order to obtain a homogeneous and resistant paint finish.
The first thing to do is to wash the surface and fill in any cracks.
After sanding the concrete surface, it must then be washed again in order to remove all residual dust.
A primer is then applied, which serves to make the surface less absorbent and better anchor the subsequent coats of paint.
Cement paint (also called elastomeric paint) is a good choice for painting concrete because it contains binders that contract and expand with the concrete.
There are paints that can be applied with a trowel, brush or roller.
If it is a walk-on surface, the floor should then be protected with a protective top coat to limit scratches and abrasions that may occur through use.
If you have to paint concrete or cement blocks that have a decorative or furnishing purpose, the advice is not to cover the entire surface but rather to use stencils or create geometric shapes that allow part of the concrete surface of the object to be visible, in order, as I said above, to make it clear that these are concrete blocks and not other materials.