Civil Engineering Reference
In-Depth Information
How to use lime render
and plaster on straw
be difficult to control. It must be raked and
mixed continuously and, depending on the
dampness of the sand, may not need any
extra water adding. You are aiming for a mix
that is nice and sticky; workable, but not
sloppy. It should ideally be left overnight
before using.
The internal and external faces of the straw
walls should be given a very short haircut
-trimmed down to a neat finish. All the
long, hairy, unkempt bits of straw should
be removed.
What happens to lime render
when exposed to air
The reasons for this are:
• to reduce the amount of plaster/render
required by reducing the surface area
It's essential to understand the chemical
change that starts to take place once the
render is exposed to air, in order to know
how best to care for it. Once it's on a wall,
it begins to carbonate: a chemical process
begins whereby the carbon dioxide starts to
change the calcium hydroxide back into the
original limestone (calcium carbonate).
Ca(OH)
2
+ CO
2
ᇕ CaCO
3
+ H
2
O
(calcium hydroxide + carbon dioxide ᇕ
calcium carbonate + water)
• to even out any large undulations in the
surface of the wall
•
to minimise flame spread over the
surface of the bales, in the event of a fire
before plastering.
If you are making your own plaster or render
from putty, it should be stored in a trench or
pit for a minimum of three months, covered
with sacking and straw. Alternatively, it can
be stored in airtight buckets.The lime mix
should be applied directly to the trimmed
straw.
This change happens very slowly - some
renders are hundreds of years old and even
now not all of the calcium hydroxide in
them is carbonated. Also, the change can
happen only in the presence of water. The
carbon dioxide must be in solution (dis-
solved in water), as some of it always is in
a humid atmosphere, but
too much
water
inhibits the process. It can take a pure
lime-putty/sand mix several days to harden,
which does NOT mean that all the lime is
carbonated - this process will continue for
months or years afterwards. The ideal
conditions for a lime render are high humidity
and good ventilation.
There is no need to wrap the straw in stucco
or chicken wire first, as many cement-
rendered buildings in the USA have been.
This is totally unnecessary and a waste of
time! Both lime and clay stick extremely
well to the straw, particularly if applied by
hand or sprayer.
The lime plaster or render should be beaten
and worked to a stiff consistency, so sticky
that it can be held upside-down on a trowel.
There should be no need to add water to it,
as this would increase the risk of shrinkage
