Concrete Recycling Offers Hope for Greener Future

Each year in Australia, about 30 million cubic metres of concrete is produced for construction. A third of this, and a large part of the cost, is the gravel aggregate that is added to bulk up and strengthen the final product.  

At the same time, an estimated 5.5 million tonnes of waste concrete is produced each year as a result of demolitions. Researchers and engineers have been working for many years to re-use this concrete waste as recycled aggregate for new concrete, but the recycled product could never economically achieve the same strength or performance of so-called 'virgin' concrete. 

Distinguished Professor Vivian Tam and her team at the newly established Western Sydney University spinoff ‘CO2 Concrete’ have developed a scalable process that converts crushed recycled concrete into an aggregate that meets industrial strength standards, but at a cost that is about 10% cheaper than rock aggregate. They are the first company in Australia to demonstrate this commercially on a construction project 

"Replacing rock with normal recycled concrete aggregate reduces concrete strength by about 30%," says Tam. "Our CO2 Concrete process improves the quality of the recycled aggregate and the performance of the final concrete, and is simply added as a replacement aggregate in the normal concrete production process." 

When concrete is poured, the fluid cement undergoes a chemical reaction to form calcium silicate — a stone-like material that gives concrete its amazing strength. However, some cement fails to form silicates and instead solidifies as calcium hydroxide, which is a weak powdery material. The amount of calcium hydroxide formed depends on the curing time and other factors, but its presence is what reduces the strength of crushed concrete for reuse as aggregate. 

"In our process, we saturate the crushed recycled concrete with carbon dioxide to react with the calcium hydroxide and form calcium carbonate, which improves its strength and performance as an aggregate." 

Tam and her team have spent the past eight years doing the experimental work and required testing to verify CO2 Concrete as an industry-ready product. However, despite strong interest from industry, achieving broad uptake of the recycled aggregate is a challenge. 

In May 2022, CO2 Concrete was a winner of the Holcim Accelerator Program — an intensive 6-month programme dedicated to speeding up low-carbon innovation along the construction value chain. This gave Tam’s team the opportunity to work closely with major construction industry partners including Holcim, Mott MacDonald, AW Edwards, Sika and Bouygues Construction.  

"Through this programme, our first commercial slab was poured in October 2022 for the new Blacktown Animal Rehoming Centre in Western Sydney," says Tam. 

"CO2 Concrete was one of just nine start-ups to be selected for the programme from over 400 applications," says Evan Smith from Holcim. "It has been a pleasure working with Professor Tam over the past 12 months to bring the construction trial to a successful conclusion. The technology has the potential to be a win for reduced emissions and supporting a circular economy."