Embodied carbon: what is it and how to reduce it

The building sector in Australia is responsible for one-fifth of all emissions. Concrete, steel, and aluminium contribute to nearly 70 per cent of Australia’s building’s embodied carbon – with levels currently in the millions of tonnes.

For every kilogram of concrete, steel and aluminium produced, approximately 0.2, 2.2 and 20 kg of carbon dioxide is emitted, respectively. The amount of concrete and steel used in construction dwarfs the aluminium usage.

Sustainable construction expert Dr Ali Kashani, from UNSW Civil and Environmental Engineering, says: “The construction industry is one of the biggest contributors to Australia’s total emissions but at the same time, it has great potential for decarbonisation through innovation and collaboration among different stakeholders.

“However, embodied carbon is sometimes overlooked in design and construction. We often focus on renewable energy solutions, such as solar panels, for emissions reduction [while] heating or cooling buildings.

“And that’s fantastic – but not enough.

“We should also take into account the energy spent and CO₂ emitted during manufacturing and transporting the materials, too.”

Decarbonising construction materials in Australia

Globally, concrete is the second most used material after water and makes up the bulk of Australia’s embodied carbon in construction. The sheer volume of concrete used in almost all construction projects, including buildings, roads, bridges, and tunnels, is unlikely to change anytime soon.

Portland cement, a crucial component of concrete, is produced through an energy-intensive process that emits large quantities of CO₂. Similarly, the steel industry heavily relies on coal, a major source of carbon emissions, for its production process.

Aluminium production, though not much used for construction compared to concrete and steel, is even more energy-intensive and reliant on coal-powered electricity.

The high temperatures required for these processes is one of the biggest challenges in decarbonising these industries. Despite efforts to transition to more sustainable energy sources, significantly more investment in this area is needed.

“Unfortunately, not all embodied carbon emissions can be eliminated – but thankfully, it can be reduced,” says Dr Kashani.

“Plus, we need to favour renewable energy in any stage of the production process when possible.”

Finding better alternatives

Dr Kashani encourages manufacturers to look beyond traditional methods of producing our construction materials. He says using recycled materials as alternative, low-carbon ingredients is an important step the industry can take to reduce its total embodied carbon.

“Steel that was manufactured as a building material but comes to its end-of-life can be recycled to make new steel through electric-arc furnaces which can also be powered by renewable energy,” he says.

“Additionally, transitioning from coal to using hydrogen as a renewable source could also greatly reduce carbon emissions of steel-making from iron ore – but this technology is still not well developed.

“Materials such as bamboo or engineered timber are also fast becoming popular alternatives.

“Similarly, we can partially or completely replace Portland cement with industrial by-products. For example, blast furnace slag, a by-product from steel production, and fly ash from coal power plants have been traditionally used as supplementary cementitious materials to reduce the carbon emission of concrete.

“However, as we transition to renewables, the production of these by-products will be limited in the future with more power plants expected to shut down and new methods of low-carbon steel manufacturing using green hydrogen are favoured.

“So we can’t rely on these as a long-term solution and need to explore the use of other industrial and mining by-products and wastes in concrete.”

Low-carbon concrete alternatives such as geopolymer or alkali-activated binders, which do not require Portland cement and limestone calcined clay cement, which requires lower heat for production, are also gaining global attention, he says.

“There’s a start-up company in the United States which claims to produce the first zero-emission cement through an electrolysis process using 100 per cent renewable energy.”

Carbon capturing

Dr Kashani ’s final recommendation to reduce global embodied emissions is through carbon capture and utilisation.

Carbon capturing is a three-step process that involves capturing and cleaning carbon dioxide, transporting it, and then storing it deep underground.

Some critics question whether carbon capturing ultimately reduces emissions or is a licence to ramp them up.

Dr Kashani believes we’re missing out on opportunities to utilise the captured carbon in other applications.

“We can use this process across many industries such as construction and manufacturing to lower their embodied carbon,” he says.

“For instance, captured CO₂ from steel and cement production can be used to enhance the properties of steel slag and recycled concrete aggregate which also reduces the overall emission of concrete using these recycled materials.

“I think it’s about looking at ways to reduce our overall footprint and change the way we’re thinking about carbon.

“We need to stop thinking about carbon as waste and start looking at it as a by-product that can be re-plugged back into the circular economy.”

Dr Kashani recognises that switching to renewable energy and alternative materials may not be straightforward for all industries, but says doing nothing about the carbon that is still being emitted should not be an option, either.

“I believe the solution is multi-faceted and carbon capturing and utilisation is currently essential to achieve the net-zero target by 2050; at least until a better solution comes around.”