Environment

Using steel slag to treat sewage makes it better for use in concrete

Using steel slag to treat sewa...
Microscope images showing concrete made with treated slag (center), conventional aggregates (left) and raw slag (right) – the treated slag forms a more seamless bond with the surrounding cement
Microscope images showing concrete made with treated slag (center), conventional aggregates (left) and raw slag (right) – the treated slag forms a more seamless bond with the surrounding cement
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Microscope images showing concrete made with treated slag (center), conventional aggregates (left) and raw slag (right) – the treated slag forms a more seamless bond with the surrounding cement
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Microscope images showing concrete made with treated slag (center), conventional aggregates (left) and raw slag (right) – the treated slag forms a more seamless bond with the surrounding cement

Steel slag is currently utilized both to treat wastewater, and as a concrete aggregate. New research now indicates that using it for the former makes it perform even better as the latter – so the same slag could be used twice, first for one purpose and then for the other.

Slag is a by-product of the steel-making process, produced as impurities are separated from molten steel in mill furnaces. Although much of it is still just dumped, the material is sometimes now used in place of the gravel in concrete. This not only keeps the slag out of the landfill, but also reduces the need to mine for gravel.

Additionally, slag can be utilized in wastewater treatment plants, absorbing elements such as phosphate, magnesium, iron, calcium, silica and aluminium from sewage. It does lose this ability as it becomes saturated from repeated uses, however, meaning that it eventually has to be "retired." According to scientists at Australia's RMIT University, though, the absorbed elements in that post-water-treatment slag actually add to its usefulness as concrete aggregate.

In studies performed by the researchers, concrete made with post-treatment slag was found to be 8 percent stronger than concrete containing slag that came straight from the steel mill – and it was 17 percent stronger than concrete made with traditional aggregates. This was mainly because the post-treatment slag formed a more seamless bond with the cement that acts as a binder within the concrete.

"The things that we want to remove from water are actually beneficial when it comes to concrete, so it's a perfect match," says the lead scientist, Dr. Biplob Pramanik. "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. But there is great potential here for three industries to work together – steel making, wastewater treatment and construction – and reap the maximum benefits of this by-product."

A paper on the study was recently published in the journal Resources, Conservation and Recycling.

Previous research, conducted at Philadelphia's Drexel University, has indicated that steel slag could also be used to make concrete more resistant to salt damage.

Source: RMIT University

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