Hydraulic lime was an important cementitious material before the arrival of Portland cement
but it is little used now, except in conservation work. Production of good hydraulic lime is as
much a craft as a technology, even more than ordinary lime but, with the decline in use of
hydraulic lime, there are few skilled producers left and new producers often need to learn the
required skills from scratch.
Hydraulic lime hardens partly by reaction with water
and so differs from other types of lime which harden
by chemical reaction with carbon dioxide in the air.
Hydraulic lime has properties intermediate between
ordinary lime and Portland cement but is produced
in a similar way to ordinary lime. In addition to
containing calcium hydroxide, the chemical which
makes up ordinary lime, hydraulic lime also contains
calcium silicates similar to the main cementitious
components of Portland cement.
A cement with properties similar to hydraulic lime
can also be obtained by mixing ordinary lime with a
pozzolanic material. Hydraulic lime can be made
stronger than ordinary lime and can be used in some
applications for which ordinary lime is not suitable,
particularly where water is present.
In developing countries hydraulic lime is rarely
produced. There is probably considerable potential
for increased production in situations where
Portland cement is scarce or very expensive.
However, in general, little mapping of reserves of
raw materials or assessment of their quality has
been carried out. This information is needed before
successful production can take place.
Figure 1: Slaking. Hydrated lime is
produced by adding water to
quicklime. Chenkumbe Hills Area,
Malawi. Photo: Practical Action / David
The raw material for hydraulic lime is a limestone which contains calcium carbonate
together with a proportion of clay. Such a limestone is known as argillaceous. Most
limestones for hydraulic lime production contain between 15 and 35 per cent silica
together with alumina - two important constituents of clays.
Most argillaceous limestones are somewhat grey or blue in colour. They can also be
distinguished by having a dull surface which does not sparkle in sunlight when broken. As
with all types of limestones, argillaceous limestones will fizz when a few drops of dilute
hydrochloric or sulphuric acid are put on them. Marlstone - a soft limestone that is common
in some areas, is often a suitable raw material for producing hydraulic lime.
Most limestones used for hydraulic lime production vary in properties such as clay content
and type of clay minerals present in a single deposit. This results in the production of a lime
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