Additives to clay - Organic
Practical Action
Vegetable Oils and Fats
The best additives of this type are those which dry, thereby harden, quickly and are insoluble
in water. Such additives include coconut, cotton and linseed oil as well as castor oil – which
is very expensive. Kapok, palmitic oil and shea butter have also been tried, but with variable
results, so local trials are recommended. Shea butter can repel termites, and an addition of
around 3% is recommended, although it can also be painted or sprayed on surfaces.
Tannins
Tannins often act to disperse clay particles so that they coat sand grains in the soil more
evenly and, also help to break up clay lumps during compaction as well as reducing
permeability of the soil and improving water resistance.
The amount of tannin required varies from a small percentage of the mixing water for the
most active types to completely replacing the mixing water in the case of decoctions –
solutions obtained by boiling the natural products. In some regions of West Africa a decoction
of the bark of the “Néré” tree (parkia biglobosa) is used for surface protection and it can also
be used to stabilise gravelly soils with good results. Other tannins are prepared from the bark
of oak, chestnut and scorpioid acacia.
Gum arabic
This is a product obtained from the acacia tree. It acts primarily as a flocculant, that is it
helps to form flocs of clay particles within the soil which help to increase dry compressive
strength and slow down water absorption, hence reducing shrinkage. However, it is soluble in
water and so offers little protection to long-term moisture exposure. It is best used inside a
building, added at 5 to 10% proportions.
Palmo copal
Copal is a resin obtained from certain tropical trees. It is usually added at 3 to 8%
concentration to sandy soils. One variety, manilla copal, has waterproofing qualities.
Sap and latexes
The latex of certain trees, such as euphorbia, hevea rubber and concentrated sisal juice,
reduces permeability slightly and improves cohesion. Proportions between 3 and 15% are
normally used and best results are achieved with neutral rather than acidic soils. The juice
squeezed from banana leaves, which is subsequently precipitated by mixing with lime to
clean it, is another material which has similar properties.
Molasses
Dehydrated sugar molasses contain aldehydes which can be converted into polymers at high
temperatures with the aid of phenolic catalysts. The resinous material obtained is similar to
asphalt and other resins in its effects. It improves the strength and reduces permeability.
Normally a proportion of about 5% is used.
Animal additives
Care must be taken with animal products. In particular it is important that the animal has not
suffered from a contagious disease for humans, such as anthrax.
Fibres
Hair and fur from animals are used with plasters and renders to reduce shrinkage and improve
adhesion and impact resistance.
Excrements
These can contain chemicals such as phosphoric acid and potassium minerals which have
beneficial effects. In addition excrements contain fibres. Additions of up to a third are
possible, or even half for a finishing mix. Cow, horse or camel dungs are normally used. Goat
dung can be used to lighten the soil. It is normal practice to leave a soil and dung mix to
ferment for several days before use.
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