Clay as a binder
Practical Action
Clay is still the most commonly used building material in developing countries. Since the
1950s, but mainly in the 1970s, many research centres developed a scientific basis for clay
technology. Many universities, training institutions and governmental or non governmental
organisations are now teaching or promoting it for almost all types of building.
Why use clay as a binder
The binding properties of clay are generally low compared with cement and, as already noted,
reversible with water. Buildings of unstabilised earth face the risk of erosion unless special
design precautions are taken to reduce exposure to rain and moisture.
On the other hand, stabilizers and other additives or physical methods such as good
compaction and grain size optimization can reduce swelling, shrinkage, and cracking, so
increasing strength and water resistance, thus allowing economy in building.
When clay is mixed with increasing quantities of water it becomes malleable, plastic or liquid,
allowing it to be shaped. When drying, clay sets and recovers its cohesive properties, and so
can bind the soil together.
Most soils consist of clay together with proportions of silt, sand and gravel. The larger
particles give structure to a soil, while the clay holds it together and to a great extent provides
the cohesion.
Earth is a ready building material and needs little further processing. Generally, a fairly wet
mix with higher proportions of clay is used in moulding and spreading applications, while a
mix with less clay is best suited to compaction in a moist or damp state.
Building design
For durability, earth should only be
used where it is not prone to water or
damp and, for maximum advantage,
appropriate designs and construction
techniques need to be selected.
Optimum designs will depend a lot
on the environment (natural
drainage, water table...), the climate
(rainfall quantity and intensity, and
strength and direction of winds
during rains...), and on the
maintenance practices of the users,
as well as on the sensitivity of the
soil to water.
Figure 2: Private house built in the town of
Amran in Yemen in 1985 Photo: CRATeere/EAG
Stabilization
To reduce or completely eliminate the reversibility of its cohesion and swelling properties on
wetting and drying, earth can be stabilized using a diverse range of physico-chemical
stabilizers.
Traditional stabilizers, which are usually obtained as or derived from naturally occurring
substances, are mainly used with traditional building techniques. They are of three different
types: glues (gum Arabic, animal glue...), oily products (sheanut butter, linseed oil...) and
tannins (horse urine, decoction of néré tree bark...). The efficiency of these products is very
variable, and often depends on local skills. Few have been scientifically studied even though
many can be very efficient.
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