Lime: an introduction
Practical Action
One of the elements most often found in combination with calcium is magnesium and here
the raw material is known as dolomite or dolomitic limestone, depending upon the
percentage of magnesium in the rock.
When burning dolomitic limestones the temperature should be slightly lower and calcining
period longer than for high calcium limestones.
Particular attention must be given to hydration as magnesium oxide will hydrate much more
slowly than calcium oxide. If hydration is not fully complete this can lead to problems of
unsoundness (expansion) in the mortar or plaster. With dolomitic limes, pit slaking over an
extended period to form lime putty is probably the best method of hydration on a small scale.
Limestone containing clays (argillaceous limestone) will produce hydraulic limes which are
generally considered advantageous for use in the construction industry. Hydraulic limes will
gain greater strength, and at a faster rate, than high calcium limes. They will also set under
water and produce a generally more durable product.
A limestone with less than 12 per cent clay will produce a slightly hydraulic lime, while
limestone with a clay content of between 18 and 25 per cent will produce eminently
hydraulic lime (see the leaflet in this series entitled Hydraulic Lime - An Introduction).
Historical use of lime in building
History does not record where burning limestone to produce quicklime was first practised. We
do know that the Cretan civilization in the Mediterranean made use of lime as a masonry
mortar more than three thousand years ago. In China, the use of lime for mortar and plaster is
of similar antiquity. The Great Wall of China, for instance, was built with lime mortars.
That lime is an appropriate and durable binding material is well proven. The Pont du Gard at
Nimes in southern France, a Roman aqueduct built in 18AD with hydraulic lime-based
mortar, is still water- proof; the excellence of the mortar is attributed to the selection of the
materials used as well as to the time spent tamping the mix into place during construction.
Until the twentieth century, lime was the principal cementing agent used in house
construction, being widely used in concretes, mortars, plasters and renders. It was also used
extensively as a decorative finish on many buildings in the form of whitewash.
During the nineteenth century, use of Portland cement which is considerably stronger than
lime has developed. Although Portland cement is notably more complex and expensive to
produce, it was heavily promoted and by the beginning of the twentieth century it was being
extensively used. Within a few decades, Portland cement was being utilized for mortars and
render, despite its technical disadvantages in comparison to lime for these uses, and it now
dominates the cement market.
Why use and promote lime?
The use of lime as the cementing agent, particularly in mortar and plasters, has a number of
advantages over Portland cement. Mortars and plasters made with no lime and a low
percentage of Portland cement tend to have low workability (a building term meaning the
correct combination of flow, water retention and cohesiveness), are porous and lack
durability. Where the proportion of cement is increased to overcome this, other problems
such as harshness, brittleness and shrinkage tend to occur.
Lime is much better than cement in plasterwork. The setting is slow, but the result will look
better and the softer surface will be less likely to crack. Lime mortars have a high degree of
workability which is highly desirable in mortars and plasters. Lime products also set more
slowly which allows mixing in large quantities without fear of going off before use. In harsh
climatic conditions, lime mortars and plasters may not be very durable but this can easily be
overcome by the use of hydraulic limes or the addition of a small percentage of Portland
cement into the mix. Siliceous materials, known as pozzolanas (eg volcanic ash and rice husk
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