Why add carbon dioxide to concrete?

The world is desperate for solutions to the crisis brought on by the emissions of greenhouse gases into the atmosphere.

Mainly carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrochlorofluorocarbons, and ground-level ozone (O₃), these gases trap heat, and are causing global earth and ocean temperatures to rise to unsustainable levels.

They are acknowledged by-products of mining, manufacturing, transport and agricultural industries, but these sectors cannot simply shut up shop.

So, industries seek new answers, including capturing and burying emissions, called ‘sequestration’.

One answer might be to capture CO₂ and add it to concrete, where it can be locked up forever.

Visions of bubbling, swirling cauldrons of concrete, hissing gas jets, unbreathable chaos, worthy of the witches in Shakespeare’s Macbeth. Taking it too far? Adjust the frame a little, but let’s be clear, it is a thing. Just not quite as dramatic.

Firstly, the gas is pressurised, so freezing, coming out at -80°C, pulsing mostly CO₂ snow, called dry ice, and some gas, onto that swirling grey mass. No bubbling, just a gentle process called ‘sublimating’, as our least favourite greenhouse gas bonds with the wet concrete.

Let’s step back a little. Why would you add carbon dioxide to concrete?

As we know, humans are producing too much CO₂, changing the atmosphere, including that breath you just took. And that one. 

Global atmospheric CO₂ is around 422 parts per million, a 47% increase since the beginning of the industrial age, and an 11% increase since 2000. Blanketing the earth, trapping heat and destabilising our climate – an effect multiplier. We see it all around us – breaking daily heat records, well, daily. More violent storms, flooding, bushfires, sea level rise, acidifying oceans; the list goes on.

We emit around 37 gigatonnes CO₂e (CO₂ equivalent) of greenhouse gas each year. A gigatonne is a billion metric tonnes. Convert that to dry ice and you have a 17-metre glacier over the Australian Capital Territory, an area of 2,358 square kilometres.  

The global building and construction sector accounts for about 39% of those emissions (2018 figures), with 11% of that coming from manufacturing building materials and products such as steel, cement and glass.

Concrete is a major contributor, a material we’ve been mass-producing since the Romans built the Colosseum in 70 CE. Two thousand years on, annual production is 30 billion tonnes. Concrete is the second most consumed material in the world after water.

Its most common binder — cement — contributes roughly 8% of our global carbon emissions.

Mineral carbonation might enable some of those emissions to be locked up.

Mineral carbonation is on-show any time we go for a bushwalk, or stare at a rock in the garden. It’s ‘weathering’ – formation of stable carbonates following the reaction between CO₂ and calcium (Ca) or magnesium (Mg). Carbon dioxide dissolves in rainwater, reacts with Ca- or Mg-rich silicate minerals in rock to become calcium or magnesium carbonate, and you see the result. A slow process, over geological time scales.

Mineral carbonation: two approaches to industrialising it

In 2006, Canberra entrepreneurs, Marcus Dawe and John Beever, on reading the first IPCC (Intergovernmental Panel on Climate Change) report, linking CO₂ emissions to rapid climate change, posed the question: “What can we do with CO₂ instead of burying it?”.

After much research, the pair stumbled upon mineral carbonation as a potential solution. After a further six years of university-based scientific research and industry advocacy, grants enabled a move to pilot-plant scale in 2013, and Mineral Carbonation International (MCi Carbon) was born.

MCi Carbon is now moving from pilot to demonstration plant stage. They get their CO₂ in cylinders from Orica’s Kooragang Island (NSW) manufacturing plant where it is a by-product of ammonia production. MCi mixes the CO₂ with calcium- or magnesium-rich material, such as steel slag and mine tailings, producing artificial limestone and silica products, for cement, plasterboard and other products.

The CO₂ is stored as artificial limestone to make ‘supplementary cementitious material’ (SCM). SCMs include cement fly ash, slag cement, silica fume, volcanic ash, calcined clay or shale, and diatomaceous earth. Replacing Portland cement in concrete, these materials form cements themselves when water is added, reducing material costs and energy consumption. SCMs also reduce permeability, increase strength and produce lower carbon concrete. And CO₂ emissions are lower because less cement is used.

The second approach is a partnership between Hi-Quality Concrete, ACT, and CarbonCure Technologies, Canada, and involves injecting CO₂ into wet concrete during manufacture. 

Hi-Quality Concrete’s venture was launched in the ACT in August 2022. Their carbon dioxide-storing concrete hit the market a month later, a first for Australia.

How does it work? Hi-Quality Concrete’s CO₂ is again a captured by-product of ammonia production, sourced through BOC Gases from Manildra Group’s Bomaderry fertiliser factory.

The CO₂ is injected into the wet concrete mix, reacting with the cement to produce calcium carbonate. The cement provides the calcium ions.

Read more: Negative emissions are the solution to the carbon problem

Between 85% and 94% of the CO₂ is absorbed into the wet concrete, forming artificial limestone once it cures*. The concrete’s strength is again increased, with performance maintained. Emissions are lower because about 5% less cement is used.

About 600 grams of CO₂ is injected per cubic metre of concrete.

“The technology has been received positively by everyone, with a lot of interest from architects and engineers and the ACT Government” says Mark Dawes, Director, High Quality Concrete.

Scale and challenges

Reducing emissions is a challenge. Five to 10 percent of Australia’s annual greenhouse gas (GHG) emissions are linked to the ‘embodied carbon’ produced by our $65b construction materials sector.

‘Embodied carbon’ in this context, refers to GHG emissions produced at each stage of a construction material’s lifecycle, including extracting resources, transportation to the manufacturer, manufacturing, and transportation to construction site.

Monica Richter, Project Director, Materials & Embodied Carbon Leaders’ Alliance, says challenges include the risk-averse nature of buyers, including government departments and contractors, even if the technology has been proven overseas.

“What happens if the supply chain dries up?”, she says.

There are many unanswered questions, says Professor Steve Turton, a researcher at Central Queensland University. “What are risks for incorporating carbon into concrete?

“Will the carbon remain permanently ‘locked up’ in the concrete? Is there a risk of slow leakage back into the atmosphere, hence over-estimating its efficacy for robust long-term carbon sequestration? What are the fugitive emissions associated with the process?“

*Clarification: After publication CarbonCure provided updated CO₂ absorption figures.