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When quicklime and water are combined, the product may
be dry hydrate, lime putty or milk of lime, dependent on
the increasing amount of water added.
The addition of a carefully controlled quantity of
water to quicklime produces a dry powder that is called hydrated lime, a process that is
generally undertaken in a hydrating plant.
To
produce dry powdered hydrated lime only the precise
amount of water to complete the hydration reaction is required. Addition of just
sufficient water to quicklime breaks down lumps to a fine powder. This
material has a ‘shelf life’ of
only a number of weeks, depending on storage
conditions. ‘Old’ hydrated lime would have partially carbonated and become a
less effective binder.
However, if quicklime is hydrated
with an excess of water and well agitated, it forms a milky suspension known
as milk
of lime.
Allowing the solids to settle, and drawing
off the excess water, forms a paste-like residue, termed lime putty, which is the form of lime which can be used
in building applications to best effect. This will keep almost indefinitely and, in fact, improves
with age.
In most countries, though, lime is most
widely available as a powder, due to its widespread utilization in process and
treatment industries rather than in construction. Lime putty, which needs a
stiff bag or container for transportation, is more rarely produced.
Limes with high calcium content, often called ‘fat’ or ‘white’ limes are desirable for most industries,
although the construction industry can use limes containing impurities. For
instance, limestones containing a proportion of clay are often seen as an
advantage in building as they produce hydraulic limes which will set under
water and will produce stronger mortars.
In the construction industry, lime,
in its hydrated or putty form, is mixed with aggregate and water to produce
concrete or mortar in the usual manner. Lime putties generally produce mortars
and renders of excellent quality and consistency.
Plain lime-sand mortars are quite
weak; any early adhesive strength results from drying out, and longer term
hardening occurs through the action of the air’s carbon dioxide on the lime.
Traditionally lime renders and plasters were often mixed with animal hair to improve
cohesion. Today the addition of gypsum or Portland cement and/or pozzolanas
to increase durability and give faster setting times is usual.
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