No.1 May 2002
Lime stabilisation practice
Lime stabilisation of materials is one of the oldest forms
of stabilisation and sometimes the least understood.
The stabilisation of clay subgrades using quicklime has
a long and successful history in many urban and rural
regions of Australia, and is cost effective and a
necessary requirement for Municipalities and State
Road Authorities seeking long-life roads to minimise
future maintenance costs.
The possible applications of lime stabilisation are to:
increase subgrade stiffness (see Figure 1),
reduce the PI of insitu pavement material,
enhance volumetric stability for the top layer of
q modify subbase layers to improve stiffness of the
q produce a temporary construction platform for civil
manufactured in Australia is processed through a fluid
bed, rotary or vertical shaft kilns. Problems associated
with transporting lime by sea over long distances limits
the importation of this binder.
Table 1 The properties of lime used for soil
0.45 to 0.56
0.9 to 1.3
The manufacture of quicklime involves the heating of
excavated limestone in a lime kiln to temperatures
above 900°C resulting in carbon dioxide being driven
off and calcium oxide being produced (see Figure 2).
The chemical equation is as follows:
The material type and condition of the existing
pavement material will govern the application rate and
construction practices. In order to understand the
properties of lime and its reaction with soils, this
construction tip aims to highlight:
the types of lime being used,
the manufacture of lime,
how does lime work with soil,
application rate determination,
mixing operations, and
suppliers of lime in Australia.
Types of lime used
The word “lime” is a generic term used to describe
either quicklime or hydrated lime as listed in Table 1
(but not limestone or agricultural lime). Quicklime
AustStab Technical Note No.1 Lime stabilisation practice
[Refer to last page for document use]
CaO + CO2
(Heat of dissociation ~ 760 kcal/kg of CaO)
Figure 1 Lime stabilisation of a weak subgrade for a
light trafficked street in a new subdivision.
Limestone feedstocks for calcination are not pure
calcium carbonate and the kilning processes have
inherent inefficiencies and this means that commercial
quicklime will never be 100% CaO.
Quicklime’s ability to form alkaline solutions /
suspensions in water is key to its being able to modify
certain soils in such a way that the end result is a benefit to road engineers.
At temperatures below 350°C, the calcium oxide
component of quicklime reacts with water to produce
hydrated lime (calcium hydroxide) as well as liberating
heat. The equation below shows that
(stoichiometrically) 56 unit weights of CaO (pure) will
hydrate (be “slaked”) with 18 unit weights of water.
Conversely, it would need 320 litres of water to hydrate
one tonne of CaO.
CaO + H20
Ca(OH)2 + heat
(Heat of hydration ~ 272 kcal/kg CaO)
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Figure 2 Lime manufacturing plant.
(Diagram courtesy of Pacific Lime)
In practice, more water than the stoichiometric quantity
is usually added (up to double in some instances) to
allow for that which vents to the atmosphere as steam
after it absorbs much of the heat of the hydration
temperature gauges to assess the completion of slaking
in the field.
The above process is called “hydration” and should be
strictly differentiated from the term...