How steel slag can strengthen concrete

Clay Technology magazine
15 May 2020

Steel slag can both clean wastewater and 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 both treat wastewater and make cement concrete stronger. 

Biplob Pramanik, Lecturer in Water Engineering at RMIT University, explains that previous studies have only focused on making either of the 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 re-applied 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.

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 aggregate denser and stronger.’

Testing reveals that concrete made with post-water-treatment slag is in fact 8% stronger than material made with raw steel slag.

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.