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Produced during the separation of molten steel from impurities, steel slag is often used as a substitute aggregate material for making concrete. It can also be used to absorb contaminants like phosphate, magnesium, iron, calcium, silica and aluminium in the wastewater treatment process, but loses its effectiveness over time.
Keen to investigate a potential contribution to the ‘circular economy’, engineering researchers at RMIT University in Melbourne, Australia, examined whether slag that had been used to treat wastewater could then be recycled as an aggregate material for concrete.
After adding post-treatment steel slag to concrete, they found it was about 17% stronger than concrete made with conventional aggregates. It was also 8% stronger than concrete made with ‘raw’ steel slag, suggesting the wastewater enhanced the slag.
The study was the first to investigate potential construction applications for sewage-enhanced slag, said water engineer Dr Biplob Pramanik.
“The global steelmaking industry produces over 130m tonnes of steel slag every year,” he said. “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. Making stronger concrete could be as simple as enhancing the steel slag by first using it to treat our wastewater.
“While there are technical challenges to overcome, we hope this research moves us one step closer to the ultimate goal of an integrated, no-waste approach to all our raw materials and by-products."
Wastewater treatment enhanced the chemical properties of the slag, the researchers found. “The things that we want to remove from water are actually beneficial when it comes to concrete, so it's a perfect match,” said Dr Pramanik.
Magnified images also show that the treated slag forms a more seamless bond with the cement paste, making the concrete stronger.
Civil engineer Dr Rajeev Roychand said the initial study was promising but further research was needed to implement the approach at a larger scale, including investigating the long-term mechanical and durability properties of enhanced slag.
“Steel slag is currently not in widespread use in the wastewater treatment industry – just one plant based in New Zealand uses this by-product in its treatment approach,” he said. “But there is great potential here for three industries to work together – steelmaking, wastewater treatment and construction – and reap the maximum benefits of this by-product.”
The research was published in Resources, Conservation and Recycling.
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