Isn’t it nice when things just work? 

In a moment sparking thoughts of Honda’s 2003 TV ad, ‘The Cog’, researchers at RMIT have shown that steel slag which has first been used to treat sewage could make concrete stronger than using raw slag.

When used in wastewater treatment, slag absorbs contaminants like phosphate, magnesium, iron, calcium, silica and aluminium. But over time, it loses its effectiveness to filter contaminants so it eventually becomes waste. 

So, RMIT researchers thought they would see what would happen if they could recycle sewage-treatment slag a second time: in the production of concrete.

Not only did it work, but it provided an 8% increase in concrete strength compared with concrete that uses raw steel slag (and was about 17% stronger than concrete that uses conventional aggregates).

The slag is now being dubbed ‘sewage-enhanced’ slag.

7-day and 28-day strength comparisons shows higher strength in the sewage-enhanced slag concrete than conventional aggregate and raw slag concrete.

Why does ‘sewage-enhanced’ slag make concrete stronger?

Researchers found that the chemicals that slag absorbs from wastewater are beneficial to concrete.

Water engineer Dr Biplob Pramanik said “the things that we want to remove from water are actually beneficial when it comes to concrete, so it’s a perfect match.” 

The sewage-enhanced slag has more chemical products within its pore structure to react during cement hydration and, therefore, forms a more seamless bond with the cement paste, making the concrete stronger.

Hardened concrete samples at 500×, 5000×, and 20000× magnification levels showing a more seamless bond between the cement paste and the sewage-enhanced slag (M3) compared to the gravel (M1) and raw slag (M2).

What about durability implications?

Civil engineer Dr Rajeev Roychand said that whilst the initial investigation findings were positive, further studies are needed to determine the long-term mechanical and durability properties of enhanced slag concrete.

Why we should care..

“The global steel making industry produces over 130 million tons of steel slag every year,” Dr Pramanik 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.”

Dr Roychand said one of the barriers to this becoming a practical reality is that slag is not widely used in wastewater treatment - only one New Zealand plant uses slag in its treatment process.

“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.”

Read the full study here.

Two of the researchers involved in the study: water engineer Dr Biplob Pramanik & civil engineer Dr Rajeev Roychand.