Additives to clay - Organic
Horse urine, added to a soil, reduces its shrinkage and makes it more resistant to erosion. It
can replace the mixing water and is sometimes mixed in with straw before adding to soil. The
strong smell disappears on drying.
Animal blood, preferably in fresh rather than powdered form, can be an effective additive
when combined with lime or polyphenols.
Proteinic casein, in the form of whey – a product formed by the souring of milk, sometimes
mixed with animal blood, can be used for stabilisation. Milk powder has also been used. One
proprietary mix is known as Poulh’s soup and is a mixture of diluted casein and brick dust
beaten to a paste.
These improve strength and water resistance. They are made by boiling the skins and bones
of animals in water.
Termite mound material can be mixed with soil for a stabilising effect. Termite mounds are
cemented with a cellulosic binder produced by the insects.
Oils and fats
Fish oil and animal fats can serve as waterproofing agents with stearates being the active
component. Proportions from 5% are used but the effect can be variable.
The need for additives
It should be noted that there is not always a need to add stabilisers. Soil properties will
dictate need and there are many examples across the world of the effective use of
unstabilised soil. Stabilisers also add significantly to cost.
If a stabiliser is deemed necessary the choice of which one to use will depend on a number of
the part of the building on which the soil is used and its exposure to the elements
the property of the soil which needs improving; e.g. dry strength, wet strength, water
erosion, abrasion resistance, surface protection, etc.
the level of improvement required
the quantity of stabiliser required
the cost and availability of the stabiliser
whether production of the stabiliser is carried out locally or whether it needs to be
The precise quantities of additives often need to be determined empirically by trial and error
for each particular situation. The results of laboratory tests often cannot be transferred
directly to field practice, although they do provide useful guidance and a starting point for
field tests. In the field, relatively simple and inexpensive tests such as the observation of the
durability of blocks on soaking in water, and the use of a simple press to assess the load a
block can carry in flexure can provide information on stabiliser requirements. As preparation
of soil mixes and their use for building is often carried out under less rigorous conditions than
for testing a judicious compensatory increase in stabiliser dosage is recommended.