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1. Lime allows buildings to breathe
In the search for building materials sympathetic to traditional construction,
lime was found to the one of the most important. Lime binders are promoted
by the Society for the Protection of Ancient Buildings for repairs because
they allow buildings to breathe.
The importance of
breathability for a healthy indoor environment.
2. Lime provides a comfortable environment
Hygroscopic materials such as lime plasters, mortars and renders stabilise
the internal relative humidity by absorbing and releasing moisture. This
makes for a more comfortable environment and reduces surface condensation
and mould growth.
3. Free lime enables autogenous healing by precipitation
When buildings made with lime are subjected to small movements thay are
more likely to develop many fine cracks than the individual large cracks
which occur in stiffer cement-bound buildings. Water penetration can dissolve
the 'free' lime and transport it. As the water evaporates, this lime is
deposited and begins to heal the cracks. This process is called autogenous
healing.
4. Free lime encourages the growth of calcite crystals
Calcite crystals tend to be larger than those formed by more complex compounds.
The crystals form in voids in lime-rich environments. Crystal growth therefore
adds strength over time and allows more permeability than dense mixes
with little or no free lime.
5. Lime provides good adhesion
The fine particle size of lime, far smaller than cement, is linked to
the root meaning of the word 'lime', which is 'sticky material'. Due to
the fine particle size mixes penetrate minute voids in the background
more deeply than other materials. They bind gently and the stickiness
gives good adhesion to other surfaces.
6. Lime mortars can protect adjacent materials
Lime mortars with a high free lime content have the benefit of high porosity
and high permeability. These characteristics allow lime mortars to protect
other adjacent materials by handling moisture movements through the building
fabric and protecting masonry materials from harmful salts.
7. Lime renders can assist drying out by evaporation
Dense and impermeable renders can trap moisture within the building fabric.
Trapped moisture is often the catalyst for various decay mechanisms. Dense
renders used in conjunction with softer materials or on weaker backgrounds
can cause serious problems by trapping moisture and creating high local
stresses. High calcium lime renders allow evaporation and reduce the risk
of trapped moisture and decay.
8. Lime mixes have good workability
The ability of a mortar or plaster to remain smooth and mouldable, even
against the suction it may experience from porous materials, is termed
workability. Good workability greatly assists good workmanship, helping
to achieve full joints with good bonding to the other materials. This
is what makes lime-based mixes such a pleasure to use.
9. Lime binders can be durable and have stood the test of time
When used carefully, lime is exceptionally durable. Caesar's Tower at
Warwick Castle has stood the test of time for over 600 years, and many
cathedrals have stood longer. An outstanding example is the Pantheon Temple
in Rome which has a lime concrete dome spanning over 43 meters (142 feet).
This has survived for nearly 2,000 years.
10. Lime finishes are beautiful
The double refraction of calcite crystals give a unique aesthetic combining
a soft texture with a lustre that has a liveliness and delight of its
own.
11. Lime contributes to a healthy environment
Due to its alkalinity fresh lime is caustic and has been used, often in
the form of limewash, for its disinfectant qualities. Lime is also used
for water purification.
12. The use of lime has ecological benefits
Lime stone can be burnt at relatively low temperatures to achieve full
calcinations. Kilns need to reach 900 to 1,000ºC as opposed to 1,300
to 1,400 for OPC. Free lime absorbs carbon dioxide in the setting process
of carbonation. It is possible to produce lime locally on a small scale
and if this is done the consequent reduction in long distance transport
also has ecological benefits.
13. Local limes enhance regional identity and diversity
The diversity of limestone types provides variety and local distinctiveness.
Different limes will vary in colour, texture and setting properties. Local
limes have a regional identity, they give a sense of place and provide
a continuous link with the local aesthetic. Local colour is the obvious
example in respect of limewashes.
14. Disfiguring by excess cement can be avoided by the use of lime
On site the temptation to use quick and easy solutions of short-term gain
can lead to long-term problems. The attraction of using excess cement
to be 'safe' is understandable if not desirable. The fact that it is plentiful,
inexpensive and readily available adds to the problem. There is a high
probability that over-strong and dense mixes that are not fit for purpose
will be used in excess. The physical damage and unsightly aesthetic that
results from this can be avoided by the use of lime.
This is an extract from An Introduction to Building Limes by Stafford
Holmes, presented to the Foresight Lime research Conference at Manchester
University on 19 November 2002.
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