Methods for testing lime in the field
Practical Action
etc.
The limestone was poorly burned so that the lime contains some of the original
limestone still as CaCO3.
The lime has been left exposed to the atmosphere so that carbon dioxide has
converted the calcium hydroxide, Ca(OH)2, back to calcium carbonate, CaCO3.
In the first two cases, the non-lime components will mostly have been removed by
screening and cycloning. Hydrated lime itself, i.e. calcium hydroxide, is very much finer
than those impurities, having a specific surface area of around 1.3 m2/g, compared with
Portland cement which is about 0.4 m2/g. So a Fineness Test can be used, passing the
lime through a fine mesh sieve, and a good quality lime hydrate should have not more
than about 1% retained on a No.85 (0.18 mm) mesh. Note also that a good hydrated
lime which does not contain significant quantities of impurities and has not carbonated
substantially would be expected to have a bulk density of around 0.5 g/ml (kg/litre), or
less, so if the lime sampled has a bulk density higher than this (unless a hydraulic lime)
then its quality would need to be questioned.
Where the lime has become carbonated,
there may be no immediately obvious
change in appearance. What tends to
happen is the lime is no longer fine, light
and soft but bags of lime become harder
and the lime lumpy. Good quality lime
should have not more than 6% calcium
carbonate. A simple test is to see if there
is a reaction with dilute hydrochloric
acid. There should be no obvious effect if
some dilute, 10%, hydrochloric acid is
dropped onto a little of the lime in a
watch glass or saucer, apart from some
dissolution of the lime. If there is ‘fizzing’
and bubbles of carbon dioxide, then it
suggests that the quality of the lime has
deteriorated due to exposure, possibly in
inadequate storage conditions over a
period of time.
Sometimes there is the risk of
unsoundness causing disruption of
plastered surfaces and possibly also
weakness in stabilised soil blocks if the
hydrated lime has in it any unhydrated
oxide, especially magnesia, MgO. This
may occur if the original raw material was Figure 2: Bagged lime in Chenkumbi, Malawi
a dolomitic limestone. A simple test for
Photo: Practical Action / Dave Mather.
soundness is the Pat Test which is one of
the two tests for soundness in the British Standard for Portland Cement BS 12, and which
can also be applied to testing the soundness of lime. In it three test pats are prepared
each in the same way. 70 g of the hydrated lime is mixed with 70 ml of water, covered
and allowed to stand for 2 hours. Then, 10 g of plaster of Paris is added and mixed with
trowels for 2 minutes and a pat with a flat surface is formed in a ring mould resting on a
flat base plate. The three pats are left for 30 minutes to set. They are then dried at 40°C
for 12 hours and placed in a steamer and subjected to saturated steam at atmospheric
pressure for 3 hours. After moving from the steamer and allowing to cool, the pats are
examined for any disintegration, popping or pitting.
The British Standard Specification for Building Limes (BS 890: 1995) does not specify
the CaO as ‘available lime’. Instead, chemical quality is based mainly on the overall CaO
+ MgO and the CO2 content. So, for instance, for Calcium Lime 90 (CL 90) equivalent to
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