What is Lime?
Some useful definitions
Putty (non-hydraulic lime) is produced by burning relatively pure limestone
(calcium carbonate) at between 850 and 1,300 degrees C. The resulting
calcium oxide is slaked in clean water to produce lime putty (calcium
hydroxide). This form of lime cures (carbonates) by absorbing carbon dioxide,
reverting to calcium carbonate. It is usually stored under water to prevent
it curing prematurely.
Hydrate of Lime is produced from the same material and
in the same way as lime putty except that, instead of slaking under water,
the calcium oxide is hydrated with a precisely controlled amount of water
to produce a dry powder (calcium hydroxide). Unfortunately it begins to
carbonate from the moment it is produced and is stored in paper sacks.
Tests show that up to 16 per cent of the contents of an old sack of dry
hydrate may have reverted to calcium carbonate. Practitioners looking
for a pure source of calcium hydroxide tend to prefer lime putty.
Lime is also produced by a much the same method
as dry hydrate of lime but using limestone that contains a proportion
of fine clay or silica in suspension. The advantage of an hydraulic lime
is that it sets more rapidly and does not need to be in contact with the
air to set, so it can be used to fill deep voids in a wall without fear that the lime may never reach its full strength. (An hydraulic set can also be achieved by adding a fine powder
of fired clay or certain other 'pozzolanic' materials to an ordinary lime
putty.) The percentage of fine clay or silica in suspension determines
the reactivity of the material which ranges from eight per cent through
to 25 per cent and is often categorised as either feebly, moderately or
Cement is produced by burning together carefully measured
quantities of relatively pure limestone and clay which are then crushed
and fired at higher temperatures to produce the very reactive material.
A mortar made with Portland cement sets rapidly but for traditional construction
it has the disadvantage of being much harder, less flexible and less porous
than one made with lime.
This article is reproduced from The Building Conservation Directory, 1999
JONATHAN TAYLOR is the editor of The Building Conservation Directory and a co-founder of Cathedral Communications Limited. He studied architectural conservation at Heriot-Watt University, Edinburgh and has a background in architectural design, conservation and urban regeneration.
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