Additives to clay - Organic
Improving the raw material quality
Different treatments or additives, collectively known as stabilisation, can modify the
properties of soils to control their shrinkage and swelling characteristics and so improve the
binding ability of the clay in the soil. These stabilisation methods are described in this
Compaction increases the soil’s density and hence its strength and resistance to mechanical
damage. It also reduces water absorption but, with the associated reduction in porosity,
durability may be reduced.
Compaction is done in a mould or form:
statically (i.e. in a single pressing), with cylindrical rollers, wheeled rollers or presses;
dynamically (i.e. repeated), with tampers or rammers, vibrating rammers or pick
surface, with a beater – mainly for floors or roofs, although sometimes used on rammed
earth walls before they dry.
Effectiveness of compaction depends on applied pressure or energy, soil type and water
Fibres are widely used when
building with earth. Generally
fibres can be most easily mixed
in with the soil if it is in a plastic
or liquid state that is not too dry.
The fibres act to increase the
tensile strength, reduce density,
accelerate drying and reduce
cracking by dispersing stresses.
Fibres vary in shape, size,
strength, elasticity and their
bond strength with earth, so
possible improvements with
different types of fibre will vary, Figure 2: Soil-based render - stabilised with bark of Néré,
as will the amount of a particular Nigeria. Photo credit: CRATerre/EAG
fibre required. Usual proportions
range between 1 and 4% by
weight, representing in bulk a
volume which can be as high as the volume of soil.
The most common fibres used include straw, for example from wheat, rice or barley. The
chaffs or husks of these crops can also be used. Other suitable vegetable fibres include hay,
hemp, millet, sisal, filao needles, and elephant grass. Cow dung and, less frequently, horse
and camel dung have also been used as additives because they contain short fibres which
make the soil workable for plastering and rendering. Synthetic fibres such as colophane,
steel or glass wool have found very limited application. Best results are obtained with fibre
reinforcement if the wet mix is prepared several days before use.
One drawback of vegetable fibre is variable durability. Dry fibres will generally last a very long
time but when wet they are liable to rot. Also some are attacked by insects, especially
termites, but others are not and often local knowledge exists to identify the most resistant