Chief Executive's Upfront
Recarbonation: Absorbing Facts About Concrete

The concept that the world’s concrete infrastructure provides the single largest human-made carbon sink has genuine scientific merit, and must be included in New Zealand’s overall carbon budget.

Rob Gaimster, Chief Executive

EMISSIONS REDUCTION PLAN
The Ministry for the Environment (MfE) recently sought feedback on Transitioning to a Low-Emissions and Climate-Resilient Future.

The consultation was essentially a collection of policy ideas that may be in the government’s final Emissions Reduction Plan, which will set the direction for New Zealand's climate action for the next 15 years.

A key element of Concrete NZ’s response was that for New Zealand to decarbonise quickly government assistance in the form of funding for the revision of construction Standards is required to facilitate the increased uptake of low carbon Supplementary Cementitious Materials (SCMs) as a replacement for cement clinker.

In other jurisdictions, such as the UK and Australia, SCMs in the form of recycled materials (e.g. fly ash or slag) or natural pozzolans (e.g. pumicite) can typically replace between 25 and 50 percent of cement in concrete.

When adopted in New Zealand, this would see a dramatic reduction in the concrete industry’s embodied carbon.

The consultation was also an opportunity to urge government to take into consideration concrete’s carbon sequestration capabilities.

To put it differently, the fact that concrete absorbs carbon dioxide must be considered in determining a building’s or piece of infrastructure’s (e.g. bridge, wind turbine or hydro-dam) carbon footprint.

WHAT IS (RE)CARBONATION?
Often overlooked in the sustainability debate, carbonation is the absorption of carbon dioxide from the atmosphere by concrete and other cementitious materials during their service life, and secondary life following recycling.

Concrete technology uses the term ‘carbonation’ for this process. However, the term ‘recarbonation’ is more accurate.

The Global Cement and Concrete Association (GCAA) describe recarbonation is a natural process, occurring when concrete reacts with carbon dioxide in the air.

Concrete can reabsorb up to 100 percent of the carbon dioxide emitted during the calcination of limestone in the cement manufacturing process. These “process” emissions account for approximately 60 percent of the embodied carbon dioxide of concrete.

The amount of carbon dioxide uptake depends on different factors, including the resistance class, exposure conditions, concrete thickness, recycling situation and secondary use.

GCCA believe that a practical estimate of the global carbon dioxide sink provided by all concrete is 25 percent of the “process” carbon dioxide emissions released during cement production.

The absorption process also happens at different speeds, occurring relatively quickly in non-reinforced products or thin applications (renders, mortars, concrete blocks and mineral foams), but more slowly in reinforced concrete elements.

GCCA also point out that non-reinforced porous applications, such as masonry, that are exposed to air, can fully recarbonate within a few years, and it is estimated that such applications account for about two-thirds of the concrete global carbon sink.

Another substantial part of concrete's carbon dioxide uptake occurs when reinforced concrete structures are demolished, as the increased surface area and exposure to air speeds up the process.

ACCOUNTING FOR CARBONATION
In the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report ⁱ, published in 2021, the United Nations acknowledged that carbonation absorbs a significant proportion of carbon dioxide emissions over the lifecycle of concrete infrastructure.

The UK’s Mineral Products Association (MPA) report that its government has recognised that this process needs to be accounted for when calculating its greenhouse gas emissions.

As such, the Department for Business, Engineering and Industrial Strategy (BEIS) has appointed the MPA to undertake research to assess the role of concrete in the absorption of carbon dioxide across the built environment.

Concrete NZ will be closely following the MPA’s progress and reporting its findings to both MfE and the Ministry of Business, Innovation and Employment’s (MBIE) Building for Climate Change team.

CARBON ZERO ROADMAP
These are exciting times for the New Zealand concrete industry as we look to build on our achievements from recent years, which saw a 15 percent reduction in carbon dioxide emissions from cement between 2005 and 2018.

Concrete NZ is currently collaborating with its members to develop a 2050 Industry Roadmap for Net Zero Carbon Concrete, which will be based around Past, Present and Future collective actions that will guide concrete’s role in a sustainable built environment.

ⁱ United Nations Intergovernmental Panel on Climate Change (IPCC) (2021). Sixth Assessment Report.
https://www.ipcc.ch/assessment-report/ar6/