In 2019, Sydney replaced part of a busy road with concrete made from industrial waste. Five years later, the experiment had a surprising result

In 2019, the City of Sydney did something unusual on a busy road in Alexandria: it replaced part of the pavement with concrete made partly from industrial waste.

The experiment was designed to answer a simple but important question — could a lower-carbon alternative to conventional concrete survive the daily punishment of a busy road?

Five years later, the answer was encouraging.


The trial took place on Wyndham Street in Alexandria, a major route toward Sydney Airport. Instead of rebuilding the entire section with conventional concrete, the City created a 30-metre test area, splitting it into two 15-metre sections. One used traditional concrete, while the other used geopolymer concrete.

The geopolymer mix was made using fly ash from coal-fired power stations and blast-furnace slag from steel manufacturing — industrial by-products that can otherwise end up being stored or disposed of.

The idea was to replace some of the Portland cement normally used in concrete with alternative materials and, in doing so, reduce the carbon emissions associated with construction.

But the real test was the road itself.

Wyndham Street was deliberately chosen because of its traffic. Researchers from the University of New South Wales installed nine sensors beneath the pavement to monitor and compare the performance of the conventional and geopolymer concrete sections under real-world conditions.

The researchers planned to monitor the road for up to five years, looking at how the two materials responded to traffic and weather.

And the experiment lasted.

The City of Sydney's 2023/24 Green Report says the five-year trial finished in March 2024. According to the council, the results showed that geopolymer concrete could be used in real-world road conditions, and the project was later referenced in an Australian technical specification for geopolymer and alkali-activated binder concrete structures.

Earlier monitoring had already indicated no difference in wear or performance between the conventional and geopolymer sections.

That is what makes the experiment interesting.

The point wasn't simply to create a greener-looking road. Researchers needed to establish whether concrete containing industrial by-products could actually perform like the material cities already rely on for infrastructure.

The emissions potential is significant.

In the original 2019 trial, UNSW reported that the geopolymer cement generated about 300 kilograms of CO₂ per tonne, compared with approximately 900 kilograms for traditional cement production.

The City of Sydney's later reporting puts the potential reduction even higher, saying the alternative cement used in the project can generate about 180 kilograms of CO₂ per tonne, compared with 900 kilograms for traditional cement — an emissions reduction of up to 80%.

There is a reason this matters beyond one Sydney street.

Concrete is everywhere, and a large share of it ends up in roads and footpaths. The City of Sydney says around 70% of concrete produced today goes into pavements and footpaths, making lower-carbon alternatives particularly relevant to local governments that build and maintain roads.

Sydney has since continued experimenting with lower-carbon construction materials. Its current road and footpath programs include alternatives using recycled materials, recovered sands, aggregates and other substitutes for conventional cement.

So the strange-looking experiment on Wyndham Street was never really about one stretch of pavement.

It was a test of whether something normally treated as industrial waste could become part of the infrastructure cities depend on every day.

And after five years of monitoring, the experimental section had done something that mattered most to engineers: it held up.