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teel slag is frequently used as a substitute
aggregate material for making concrete.
The material absorbs contaminants such
as phosphate, magnesium, iron, calcium,
silica and aluminium during the wastewater
treatment process ‘but loses its effectiveness
over time’, explains water engineer Dr Biplob
Pramanik estimates that the global steel-
making industry produces over 130 million
tonnes of steel slag every year. ‘A lot of this by-
product already goes into concrete but we’re
missing the opportunity to wring out the full
benefits of this material,’ he notes.
‘Making stronger concrete could be as sim-
ple as enhancing the steel slag by first using it
to treat our wastewater,’ the RMIT team sug-
gests. This thought inspired them to develop
a ‘zero waste’ recycling method for the steel-
making by-product. They found that concrete
made with post-treatment steel slag is around
32% stronger than raw steel slag and roughly
8% stronger than concrete made with conven-
tional coarse aggregates.
Essentially, the slag’s chemical properties
are enhanced through the wastewater treat-
ment so it performs better when used in
concrete. And owing to the slag’s pozzolanic
nature, both the raw and treated aggregates
showed a ‘seamless monolithic bond’ at the
interfacial transition zone between the slag
and the cement matrix.
‘The things we want to remove from water
are actually beneficial when it comes to con-
crete, so it’s a perfect match,’ states Pramanik.
First, a column was set up with granular
slag as a filtering medium for wastewater
obtained from a municipal treatment plant.
Parameters such as pH, phosphate, calcium,
magnesium, aluminium and silicon con-
centrations were monitored in the influent
(original wastewater) and effluent (wastewa-
ter treated with slag) samples.
In stage two, X-ray fluorescence and X-ray
diffraction analyses were conducted on the
treated and untreated powdered slag samples
to ascertain the chemical and mineralogical
changes after water treatment.
And in the third stage, concrete samples
were cast with both the treated and untreated
slag instead of conventional coarse aggregates
at a replacement level of 50% and compared
with a control mix containing 100% conven-
tional coarse aggregates.
Potential for collaboration
This is the first study into potential appli-
cations for sewage-enhanced slag in construc-
tion material, says civil engineer Dr Rajeev
Roychand. Significant research was needed
to scale up the approach, including investigat-
ing the long-term mechanical and durability
properties of enhanced slag.
‘Steel slag is currently not in widespread
use in the wastewater treatment industry,’ he
says. ‘Just one plant based in New Zealand
uses this by-product in its treatment ap-
Roychand acknowledges some techni-
cal challenges still to be overcome but sees
great potential for collaboration between the
steelmaking, wastewater treatment and con-
struction industries. ‘Together they can reap
the maximum benefits of this by-product,’ he
For more information, contact Dr Biplob Pramanik
at: [email protected]
Steel slag aggregate
RMIT University, Australia
Dr Biplob Pramanik
steel slag from wastewater
displays powerful properties
Australia’s RMIT University has found that steel slag used to treat wastewater can
be recycled into a strong aggregate material for concrete production.
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