Carbon black for cement, concrete and mortar uses.
The most commonly used pigments when colouring cement, concrete and mortar
are iron oxide black pigments. These, as well as red and yellow iron oxides, are
absolutely stable in these medium due to the great chemical resistance in alkaline
and acid medium, and UV radiation.
We can add to these characteristics of the iron oxides the size of its particles,
normally between 0.15 and 0.55 µm. Its size does not allow them to move through
the capillary lines, so they rest permanently anchored in the structure of the concrete
or mortar.
Anyway, we must take into account that, when we are talking about black iron oxide,
even if we reach the saturation dose, between 5 and 8% of the cement proportion, it
is not possible to obtain any darker colour than grey.
Iron- oxide pigmented tiles
(Chromafer 3030 at 3%
dose of the cement
proportion)
just
manufactured.
The porosity of this product
along
with the
hard
environmental conditions to
which the tiles are exposed
make carbon black pigment
unsuitable for tiles.
Once the product is dry, the
grey tone
would be
permanent.
This is why, when a “real black” colour is intended, we have to use carbon black
pigments (Pigment Black 7). These are characteristic because their composition is
mainly carbon, normally more than 90%.
Carbon black pigments used in cement have a high colorant strength compared to
iron oxides, normally 4 to 6 times higher, and it gives a completely black shade.
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Depending on the type of product, a dose of between 1,5 and 2% is enough to
create a black colour. In some cases, the dose can reach 4% or even 6%.
This is due to the small size of the particle in these pigments, which in some cases is
even 6nm.
Nevertheless, when the product is in dust format, the dessaglomeration degree
achieved in cement applications is very different from this 6nm, as it is only possible
to separate or disaggregate the fins formed by the clusters resulting from the
agglomeration of the particles.
In order to extract all the colorant strength of the carbon blacks, it is necessary to
use high-speed dispersion systems normally used to manufacture pigment paste.
The majority of carbon blacks are hydrophile. For cement uses, the biggest particle
grades are used to allow the wettability and a better dispersion in the shear stress.
The cement manufacturer must take into account the inconveniences of this product
and make the necessary tests before deciding to use this product.
WHEN USING CARBON BLACK TO COLOUR CEMENT-BASED PRODUCTS:
 Migration: the chemical stability and resistance to ultraviolet is very high,
nevertheless, the small size of its particles allows them to move to the
surface through the capillary lines. This phenomenon is called “migration of
the pigment to the surface”. It depends on the number of capillary lines, and
also on the conditions and cycles of absorption and exudation of the water,
this is, if the mortar or concrete is outside and exposed to the rain. The
consequence is that the particles arrive to the surface and they leave the
bearing, so after some time the intensity of the black diminishes, and the
bearing turns grey. The possible strategies to prevent this from happening
are:
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Hydraulic tiles pigmented with
carbon black. Exposure to the
weather has accelerated the
process of intensity loss, which is
not uniform.
This product should have been
pigmented with iron oxide.
o To make a compact manufacture, so that the number and size of the
capillary lines is reduced.
o To add some water-repellent additive will reduce the inward or outward
water flux in liquid or steam form.
o To use Super-plasticising Additives will enable to reduce the water
used in the mixing, which is anyway expelled during the setting, the
most critical moment for migration.
o A superficial sealing of the manufacture will prevent the water from
being absorbed or expelled.
o The use of mixed binders, cement and resin, reduces the number and
size of the capillary lines, improving the anchoring of the pigment.
o In certain cases, carbon black and iron oxide black can be combined.
This way, we can achieve a more intense black, with an improved
durability. The best combination is 5% iron oxide and 2,5% carbon
black.
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Upper Surface of an application of two
contiguous mixes. Left side: mortar pigmented
with Carbon Black at a 0,3% dose. Right side:
mortar pigmented with black iron oxide 3035 at
1,5%.
The same piece, observing now the surface that
has settled against the mold.
The right side, pigmented with black iron oxide,
has practically the same intensity on both sides.
Nevertheless, the left side is more intense on the
upper surface due to the migration of the carbon
black pigment to the surface.
 Dispersion. Carbon black needs a higher shear stress to obtain the optimal
dispersion, compared to iron oxides. This is why it is recommended to
enlarge the mixing time or modify the moment of addition of the components,
in order to create conditions of greater friction (for example, leaving the
thinner elements to a second mixing phase).
 Pollution: due to the small size of the particles and the lower bulk density,
carbon black is very volatile. For this reason, during the manipulation, more
particles in suspension are added to the entourage. The pigment
manufacturers try to minimize this inconvenience augmenting the humidity
content.
 Colorant strength. As we have already mentioned, the colorant strength of
this pigment is between 4 and 6 times higher than that of the iron oxide, while
the price is approximately two times the iron oxide price. This is the reason
why some manufacturers use it as a way to save money in the formula. This
could be risky: if you are trying to obtain a grey colour instead of a saturated
black, small errors in the dose can result in different intensities. This risk can
be higher if, as a consequence of a high humidity during the packaging
process, part of it is lost. For this reason, it is never a good idea to use carbon
black to obtain grey shades. It should only be used to obtain intense black
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shades.
 Mixing in humid systems. It can be produced a flotation of part of the
pigment, not being an homogenious preparation. Appart from the excessive
exudation that this will produce, it increases the risk of migration to the
surface during the setting.
 Product Waste. It must be taken into account that the products contain the
weight it had when it was packed. Subsequently, and depending on the
conditions of the storage, there is a balance between the humidity contained
and the environmental humidity. This means that it is normal that, when you
lose part of the humidity contained, a part of the weight of the packs will be
reduced. It is not a loss on the content of the product, which still has the
same pigmenting capacity. Nevertheless, it is not necessary to correct the
quantity in case it is used for saturated tones, as we explained before.
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