Clay as a binder
Practical Action
If the clay content in a soil is too high, some minerals (sand, gravels…) or fibres (straw,
hair…) can be added.
Identification of soils
Some simple field tests can be used to estimate the proportions of components in the soil to
assess its suitability and to indicate how much stabilizer to use. They are also very useful for
quality control.
Feeling the texture of the soil gives an indication of sand and gravel proportions as well as,
when wet, its cohesiveness and strength. By letting the soil settle in water in a transparent
container the sizes of the different layers indicates proportions of the components.
To estimate cohesion make a small
biscuit or briquette of soil and
observe if shrinkage is excessive
after drying. After that, estimate its
cohesiveness by the effort needed to
break it. You can also roll a cigar
(3cm in diameter) from a moistened
and well-kneaded piece of soil and
push it gently across the palm of the
hand until a piece breaks off (see
diagrams above).
If the length of this piece is less
than 5 cm, the soil contains too
much sand, between 5 and 15 cm
the soil is good, more than 15 cm
and the soil contains too much clay.
Figure 3: Church biult in pisé (rammed earth) at
Charancieu in the Dauphiné region of France.
Photo: CRATerre/EAG
Laboratory-based tests have also been
developed and these are more accurate
indicators.
Use of clays for mortars and plasters
An important application of earth
for building, whether stabilized or
not, is in mortars, plasters and
renders. An essential requirement
of these applications is that the
materials used should not be
significantly stronger than the
background materials, otherwise
the background can be damaged.
Plasters or renders are important
in protecting walls from damage
by rain, wind and abrasion, as
well as for decorative effect, and
earth-based materials are quite
often used. Mud mortars are an
ideal jointing material for use in
earthen based walls and other
structures as well as in
Figure 4: The Kasbah of the Glaoui at Ouarzazate in
Morocco. Built by the beginning of the 20th century.
Photo: CRATerre/EAG
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