METHODS FOR TESTING
LIME IN THE FIELD
If simple, field methods are to be used, it is easier to test the quality of ‘lime’ when it is in
the form of quicklime, i.e. calcium oxide CaO. This would be the case if the lime were
being bought direct from a kiln before the kiln operator had started to hydrate it. Usually,
though, as quicklime will deteriorate if left exposed to the air and because it can be a
hazardous material to handle, lime is more usually available and purchased as hydrated or
‘slaked’ lime, i.e. calcium hydroxide Ca(OH)2.
In either case the primary factor
that determines the quality is
the percentage of available lime
which is the actual chemical
‘lime’, either as CaO or Ca(OH)2,
though usually quoted in terms
of the equivalent CaO content.
Pure quicklime, CaO, will have
100% available CaO, whilst
pure hydrated lime, Ca(OH)2,
will contain 75.56% available
Figure 1: Loading stone/fuel through the hopper at
the chimney level, lime kiln, Chegutu, Zimbabwe
Photo: Practical Action / Kelvin Mason.
A minimum available lime
content would be specified, for
instance, by certain industrial
consumers such as sugar
refineries. A minimum available
CaO, say 60%, specified for a
hydrated lime means that it has
nearly 80% of the maximum
that would be possible. Where
lime is to be used for soil
stabilisation, such as in
roadmaking or in soil blocks, it
is still the chemical purity, as
available lime, that is important.
In this case impurities are less
critical but having less available
lime means that more ‘lime’
would have to be transported
Soils with high clay content are more in need of high available lime for their stabilisation
as the fine clay minerals react in two ways in warm climates – through cation exchange
and some pozzolanic reaction.
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