Civil Engineering Reference
In-Depth Information
Limestone and lime burning
However, in order to use it successfully lime
requires thought and understanding of the
processes involved in the slow carbonation
back to its original limestone. While it is
true that a carefully applied lime render
or plaster can last for hundreds of years,
there have been instances of spectacular
failure, and the reasons for these need to
be understood if we are not to repeat those
mistakes. In essence, the preparation and
practice of limework is simple, but variables
in the sand, the lime and particularly in the
weather during application and drying time
have a critical effect on the overall durability
of the material.
The raw material for all lime mortars and
renders is naturally occurring limestone,
shells or coral, which is calcium carbonate
(CaCO
3
). It is made into lime putty by a
relatively simple process. Traditionally, the
limestone is placed in a specially built kiln
(sometimes a pit or a heap) and layered
with fuel such as coal or brush and burnt for
about 12 hours. It needs to reach a tempera-
ture of 900-1200ºC; 900ºC for carbon dioxide
(CO
2
) to be driven off, and 1200ºC for the
heat to penetrate through to the centre of
the stone.
Traditionally, knowledge about lime was
passed down from one generation to the
next, and people were used to using it
continuously, and so built up a wealth of
experience based on a sound knowledge
of the material. Today, there are very few
skilled craftsmen (we haven't found any
women yet) remaining from those times,
and we are having to learn as best we can
from what we have left, and from remem-
bered histories.
As it heats up, steam is driven off first
(water, H
2
O), which is always present in
the limestone, and the following chemical
change takes place:
heat + CaCO
3
ᇕ CaO + CO
2
(heat + calcium carbonate ᇕ calcium oxide +
carbon dioxide)
At the end of the burning process, calcium
oxide in the form of whitish lumps or
powder is left with bits of burnt and unburnt
limestone. Over-burnt limestone appears
as black, glassy pieces, and these should
be removed and discarded. The chemical
reaction that takes place is usually more
complicated than this, due to other carbon-
ates and silicates being present in the
limestone, but it's important to understand
the basic changes that are taking place
at this stage. Calcium oxide, called 'lump
lime' or 'quicklime', is very reactive and
can be dangerous: it MUST be kept dry
as it reacts very quickly with water - even
the water in the air or the moisture in your
skin - to form calcium hydroxide, which is
the first step to reversing the process back
To some extent, this can result in an over-
technical approach to what was essentially a
practical and rather ad hoc building practice.
We are trying to specify exact lime/sand
mixes when most likely what happened on
site was fairly rough and ready, except for
the most prestigious jobs. And mostly, it
worked! As tens of thousands of houses in
the UK and Ireland, hundreds of years old,
can testify. So what follows is an attempt to
explain what happens in the lime burning,
slaking and mixing process, and what is
important to know, so that you can take care
of your own limework satisfactorily.
