How post-water treatment steel slag can strengthen concrete

Materials World magazine
3 Jun 2020

Steel slag used to treat wastewater can increase concrete’s strength. Idha Valeur finds out how. 

Researchers in Australia are now killing two birds with one stone using a by-product of steelmaking to treat wastewater and then re-applying it to make cement concrete stronger. 

Biplob Pramanik, Lecturer in Water Engineering at RMIT University, explains that previous studies have only focused on making either of these two happen. He notes that in water treatment slag is mainly used to remove phosphate, but it becomes less effective over time. This waste slag can then be reused as a substitute aggregate in concrete.  

He says, ‘The results of our research study show that the waste slag produced after wastewater treatment performs better than conventional aggregates in cement concrete application. Therefore, our research demonstrates that slag can have multiple applications to reap the maximum benefit out of this steelmaking industry by-product. This also supports the government’s circular economy initiative.’ 

Replacing 50% of the coarse aggregates leads to a 17% increase in strength, claim the researchers. ‘Normal aggregate has a physical bond with the cement paste, but slag from both pristine and after water treatment has a chemical bond which will provide more strength,’ says Paramanik. Testing reveals that concrete made with post-water-treatment slag is in fact 8% stronger than material created using raw steel slag. 

He adds that the aggregate’s density increases along with its change in chemical composition. ‘[It] traps some chemical component from wastewater such as calcium and silicon that makes the aggregate denser and stronger.’

Pramanik explains that to test the slag-enhanced concrete, the team used a compression test. To determine the fundamental mechanisms involved, the team used Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy and X-Ray Diffraction ‘to understand why strength is more or less, with or without using slag, before and after water treatment’. 

The team foresees the concrete being used in both structural and non-structural applications like beams, columns and pavements. Pramanik adds that there are several areas that need further investigation, such as the mechanism for removing chemical contaminants from the wastewater, additional mechanical and durability properties, complete replacement of conventional aggregates, and the economic feasibility for practical implementation.