Methods for testing lime in the field
Practical Action
For soils that are richer in silt and sand it may be preferable to stabilise with Portland
cement rather than lime. If lime is to be used it could with advantage be a hydraulic lime.
This is not a white lime but is usually grey and though the lime available for reaction may
not be as high as with white lime, it has an active, moderately cementitious component to
help bond the sand grains in the soil.
Other, simpler, methods for testing quicklime can give an indication of the available lime
content, but they ideally need to be done in comparison with a sample of high grade
material so as to establish a reference marker. For quicklime these include the
measurement of the temperature reached during its hydration, usually in a thermos flask.
Hydrated lime, or quicklime that has been slaked to make the dry hydrate, can be tested
by determining the strength developed by a mix of lime, pozzolana and sand but this can
take several days to complete.
Available lime
An acknowledged standard test for available lime is the ASTM Rapid Sugar Test which
uses relatively simple, portable laboratory glassware and common reagents and can be
performed wherever there is a small kitchen or table top in a quiet place. There are two
procedures depending on whether the acid reagent available is hydrochloric acid or
sulphuric acid. The methods are given in Annex 1.
Heat of hydration (for quicklime)
This simple test can also be performed ‘in the field’ with easily portable items. Though it
is primarily for comparing the reactivity of quicklimes, especially for monitoring the
burning conditions in a small lime kiln, the maximum temperature reached through the
exothermic (heat producing) reaction of quicklime with water is a good indicator of the
quality of the lime, at least in terms of the available CaO. The rate at which the
temperature rises is an indicator of how reactive it is.
Apparatus
No. 7 mesh sieve (2.83 mm)
Thermos flask
Thermometer reading to at least 100°C.
Clock or watch with seconds hand.
Scale to weigh 50 g to ± 0.5 g
A pestle and mortar or other means of crushing the quicklime to pass the No.7 mesh.
Method
Take several lumps of fresh quicklime, break them with a hammer on a clean surface,
cone and quarter to get a representative sample of small fragments. Grind 100–200 g of
this with a pestle and mortar, so that it just passes through a No.7 mesh sieve.
Into a thermos flask put 170 ml of water at the normal prevailing water temperature,
which in tropical countries may be 23°C. Carefully weigh out 50.0 g of the No.7 mesh
quicklime, put it into the thermos flask, start the stop watch and begin gently stirring the
mixture.
At one-minute intervals, record the temperature of the water and continue doing so for 24
minutes. Note the maximum temperature (and the time it was reached). By comparing
the maximum temperature, and the hydration curve of temperature against time, with
those obtained with samples of quicklime of known available lime content, the quality of
the sample can be compared and an estimate made of its available CaO content, as well
as its degree of reactivity.
Other field tests
Poor quality in hydrated lime can be due to either or all of three causes:
The original limestone had a relatively low level of calcium carbonate (CaCO3) and
there were other components containing magnesia (MgO), silica (SiO2), iron, alumina,
2