LIME
AN INTRODUCTION
Lime is a remarkable and versatile material. It has a
long tradition of use in construction, agriculture,
water and waste treatment. More recently, lime has
been used in numerous manufacturing and
processing industries, most notably papermaking,
sugar processing, steel production and the
manufacture of calcium silicate bricks. This
document is an introduction to lime, how it is
produced, and what raw materials are required. It
lays particular emphasis on lime utilization in the
construction industry and the contribution it can
make towards low-cost building materials.
How lime is produced
There are two main forms of lime; quick-lime and
hydrated lime. Quicklime is produced by heating any
material containing calcium carbonate to a
temperature of around 1000°C for several hours. In
this process, known as 'calcining' or simply 'burning',
the carbon dioxide in the calcium carbonate is driven
off leaving calcium oxide plus any impurities.
Figure 1: Loading limestone for the
kiln. Chenkumbe Hills Area, Malawi.
Photo: Practical Action.
Quicklime is a chemically unstable and hazardous material and is therefore normally
hydrated, becoming not only more stable but also easier and safer to handle. Hydrated lime
is produced by adding water to quicklime in a process called 'hydration' or 'slaking', where
the calcium oxide and water combine chemically to form calcium hydroxide.
Raw materials
Limestone is the most common raw material used to produce lime, although other calcareous
materials such as marble, coral and shells are also used. With large-scale excavation of any
raw material, care must be taken to minimize environmental damage, particularly in the case
of coral and, to a lesser extent, sea shells.
Most industries specify lime that is of high purity and therefore the raw material must contain
as few impurities as possible. A calcium carbonate content of 95 per cent for the limestone
would normally be considered the minimum for production of high calcium limes. In
construction chemical purity is less important and dolomitic and hydraulic limes are also
used.
The excavation of rock for lime production is normally undertaken in a quarry
with the use of explosives or mechanical rock breaking hammers, although for very small-
scale lime production hand excavation methods may still be used. In most lime kilns, the raw
materials must be broken down to a standard size, typically between 100 and 150mm. This
is normally done by mechanical rock crushers, although again hand methods can be used for
small quantities.
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