DIGEST 2006 - Sabita

A viable alternative to natural aggregates:
The use of steel slag aggregate
in asphalt mixes
Hugh Thompson
Director
WSP SA Civil and
Structural Engineers
(Pty) Ltd
Michael Bouwmeester
Associate
WSP SA Civil and Structural
Engineers (Pty) Ltd
Far from being a waste
product of the steel
production process, steel
slag is a high quality resource
for use as an aggregate in road
construction, and as such it
should be seriously considered
as a viable alternative
aggregate in hot mix asphalt.
Due to the changes in traffic
loading spectra and higher road
surface temperatures being
experienced in South Africa,
numerous investigations have
been undertaken to assess the
properties of asphalt mixes to
improve durability, and resistance
to fatigue fracture and
deformation. One such
investigation examines the use of
steel slag as an aggregate in
asphalt mixtures.
As a result of increasing focus on
the environmental requirements
for the acceptable disposal of
waste slag, as well the
considerable strain on the already
limited number of aggregate
resources, steel slag is being
increasingly considered as an
alternative to natural aggregates
in the road construction industry.
Slag production
Steel slag is produced during the
separation of molten steel from
impurities in steel-making
furnaces, and is composed of
calcium silicates together with
oxides and compounds of iron,
manganese, alumina and other
trace elements.
Free lime and magnesium oxides
that have not reacted with the
silicate structures can hydrate and
expand in humid environments,
which is precisely what happens
with steel. Consequently, slag
aggregates exhibit a tendency to
expand. Volume changes of up to
10% or more, attributable to the
hydration of calcium and
magnesium oxides, can cause
difficulties.
For this reason it is important that
further weathering of the steel
slag takes place to cause the free
calcium oxide to hydrate. If the
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