Mud plasters and renders
Practical Action
shrinkage of more than a quarter of an inch (0.64cm) over the 2-foot (0.6m) length of the
box in the shrink box test indicates a soil liable to significant cracking.
• stabilising the plaster by adding cement, lime, bitumen, or some other binder in small
quantities. Possible limitations include the cost of the stabiliser and lack of skill in its
proper use.
Bitumen cutback plaster is prepared by mixing hot bitumen with kerosene in a 5:1 ratio, and
then combining one part of that mixture with 20 parts of previously fermented soil and wheat
straw. Water is added and the whole mixed together thoroughly. This type of plaster is applied
in two layers, and the second is applied only after the first has dried.
Lime-soil plaster can be made with one
part hydrated lime mixed with two parts
of clayey soil and 3 to 6 parts sand, the
optimum amount of sand depending on
the clay content of the soil. The quality
of the plaster depends a lot on the
quality of the lime available and the
type of soil.
Another proprietary plaster is ‘dagga-
cement’, a mix of two parts sand to one
part clayey soil to 0.2 parts cement by
volume. This produces a good weather-
resistant plastering mix.To plaster the
surfaces of stabilised soil blocks, and
for these surfaces only, a coating of a
stabilised mud slurry may be painted
on. This slurry is prepared by mixing
one volume of cement with two of mud.
The final mix should have the
consistency of paint, allowing it to be
applied in a very thin coat. For
decorative appearance, the colour of the
clay stands out better if white cement is
used rather than Ordinary Portland
Cement. Red cement has sometimes
also been used for this reason.
In all cases it is preferable to
experiment with different mixes to find
which one gives the best results with a
given soil, rather than accept a general
plastering recipe.
Application: Good practice
There are general rules to follow when
applying all plasters. Firstly, the wall
surface has to be prepared well. This
can be done by scrubbing off all the
surface dust and loose material with a
metallic brush. Then the wall surface
must be moistened to stop water being
drawn out of the plaster layer into the
wall. If the plaster is applied in two
coats, the first layer must be applied
with force and be no more than 20mm
thick. Before hardening of the first
layer is complete, its surface must be
4
Figure 4: The final coat of render or plaster
can be applied to produce a decorative finish.