Civil Engineering Reference
In-Depth Information
generation to the next. Opinions differ about
the quality of lime putty in the presence
of frost, although most seem to think this
can improve it. The fact that lime was kept
in pits for many years, where it must also
have been frosted, suggests that this had no
adverse effects.
sand used. Render for areas that experience
lots of wind-driven rain may contain aggregate
particles up to 10mm (
3⁄8
”) or so in size; but
inside a smoother finish is usually preferred,
using a sand of smaller grain size.
The longer a lime putty has matured, the
more solid it becomes, and the better render
it makes. It may seem hard to work at first
if it has been maturing a long time, but by
pounding and beating it with wooden mallets
or posts it soon becomes more plastic and
can be worked into the sand. This beating
part can be VERY labour intensive, but
should not be missed out. Because it's so
hard to work, it can be easier to mix the
sand with fresh lime putty, which doesn't
need beating, and then leave this mix to
mature for three months, traditionally under
a thick layer of sacking and then straw!
How to make lime render
and plaster
Lime render and plaster can be mixed
on site from lime putty and local sand,
or sometimes from quicklime and sand,
depending on local availability. Well-graded
sharp sand can be bought from a builders'
merchant, but keep it clean. If you prefer
not to make your own plaster or render, it
can be bought ready-mixed from one of a
growing number of suppliers.
Hot lime render mix
Lime putty render mix
Recipe:
2 parts quicklime powder to 9 parts
sand.
Recipe:
1 part lime putty to 3 parts sand.
This is probably the most common method
used historically for mortars. In this method,
the quicklime is added to
damp
sand in a
bath and mixed with a shovel. Very soon,
the mix starts steaming and becomes warm,
as the reactive calcium oxide hydrates with
the water in the sand. At this point, the mix
can be riddled (passed through a sieve,
usually 6mm (¼”) for renders), to get rid
of any unburnt or over-burnt limestone,
as it's easier to do this when the quicklime
has dried out the sand. This process is
dangerous because the powdered quicklime
blows in the air and can get into eyes and
lungs, reacting with the moisture there;
plus the mix gets hot very quickly and may
The sand MUST be well graded, that is,
contain particle sizes ranging from very small
(dust) to quite large (5mm or
3⁄16
”), and these
should be angular ('sharp'), not rounded.
When compressed together, the aim is to use
as much lime putty as necessary to fill the
spaces between the grains (the void spaces)
but no more.The mix is almost always 3
parts sand to 1 part lime putty (3:1), because
the void spaces take up about 33 per cent or
1⁄3
of the volume of most sands.
The only real difference between a plaster
(for inside work) and a render (for outside
work) is the fineness or coarseness of the
