Concrete’s carbon footprint still weighs heavy, so what’s the fix?

It’s difficult to imagine a world without concrete. After water, it’s the second most consumed material in the world and it’s everywhere – in our buildings, footpaths and roads. The substance is synonymous with the modern world and while it can, at times, feel grey and monotonous, it’s also incredibly flexible and can be moulded into architectural masterpieces.

But concrete has a carbon problem. Cement, concrete’s key ingredient that binds the other sand and gravel together to give it its famed strength, is the source of about 8 per cent of the world’s carbon dioxide (CO2) emissions.

Cutting the emissions in cement is hard because the emissions largely come from an unavoidable chemical reaction during its manufacture. Swapping out cement for other low carbon binding ingredients is an option but despite years of experimentation, no alternative has been found that can completely replace cement without sacrificing performance.

The material’s carbon footprint is being taken seriously. This year, the Global Cement and Concrete Association launched a roadmap to decarbonise the global industry by 2050, and closer to home, Cement, Concrete and Aggregates Australia (CCAA) and other organisations have commissioned a report on decarbonisation pathways for Australia’s concrete and cement industry.

It’s evident that all levers will need to be pulled to meet this target and even then, the industry is pinning its hopes on expensive, unproven carbon capture utilisation and storage (CCUS) technologies to squeeze emissions out of its supply chain entirely.

Materials expert Jonas Bengtsson, chief executive officer and co-founder of sustainability consultancy Edge, confirms that there’s been no real “breakthroughs” in low carbon concrete technology and that the task at hand relies on incremental change.

“We’re kind of a little bit locked in, because I don’t think we can do much without concrete. Demand is just increasing. We can’t just miniaturise the built environment,” Bengtsson says.

“So at this stage it is about looking at what are the options to minimise the impact.”

Bengtsson says there’s “a lot of healthy competition” between concrete manufacturers to lower the per cubic metre carbon intensity of the product.

But he says the concrete industry is not the only actor that can help drive down the emission profile of the sector. For a start, many emissions occur further up the supply chain in the clinker production, which may not necessarily be in the concrete manufacturer’s purview. And further down the supply chain, builders are also responsible for a lot of spillage and wastage of the grey mixture.

The design and engineering profession also have a role to play to specify less concrete in buildings and infrastructure and, better yet, developers should consider the lifespan of structures and design them in a way that they can be adapted for reuse in the future. 

By way of example, Bengtsson says car parks can be designed with higher ceilings in the expectation that in a few decades, they can be easily converted into office space or apartments rather than being demolished.

“We’re just going to be in that cycle where we need more and more concrete forever unless we build a bit smarter for adaptive reuse.”

“I don’t think, these days, we can have a conversation with a client, a developer, without carbon coming into the conversation.”

Using lower carbon materials will also play a role, as will hybrid structures where the strength and performance of materials such as concrete and steel are paired with lower carbon materials such as timber.

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George Agriogiannis, chief executive officer at Holcim Australia, which has a Science based Target and is offering a carbon neutral concrete product (achieved via offsets), says there’s a lot of demand in the market for lower carbon materials.

“I don’t think, these days, we can have a conversation with a client, a developer, without carbon coming into the conversation.”

A very stubborn source of emissions

There are two big ticket emissions-intensive processes in cement manufacturing. The first is the use of fossil fuels in heat processing, which is responsible for about 40-45 per cent of emissions, and the second is an unavoidable chemical reaction called calcination that is responsible for around 50 per cent of emissions where calcium carbonate turns into lime and carbon dioxide.

[diagram from CCAA’s report https://cement.org.au/wp-content/uploads/2021/10/Decarbonisation_Pathways_Australian_Cement_and_Concrete_Sector.pdf]

Cement Concrete & Aggregates Australia chief executive officer Ken Slattery says so far the industry has focused on reducing emissions during processing using alternative fuels, new kiln technologies that use less fuel, and improved grinding and processing activities.

He says the industry has already travelled a reasonable distance towards decarbonisation, achieving around 25 per cent reduction in emissions since 2000.

“But the opportunities for further reduction are getting harder and harder.”

With the low hanging fruit already picked, the industry is now focused on clinker emissions. Slattery says the only way to reduce clinker emissions is to avoid using as much of it and to supplement it with other materials.

The practice of substituting clinker for what are known as supplementary cementitious materials (SCMs) is now fairly common.

By adding fly ash (a bi-product from burning coal) and ground granulated blast furnace slag (a bi-product from steelmaking), concrete manufacturers have been able to lower the emissions content of their products without detracting from performance. In fact, the substitutions can sometimes improve concrete properties.

Why a low carbon cement substitute is so elusive

According to Dr Louise Keyte, Boral general manager – technology execution and co-director of the UTS Boral Centre for Sustainable Building, the primary limitation of cement replacements is around strength development.

Concrete is not a one-size-fits-all product. Different mixes have different properties. For example, for the concrete floors in a multistorey building, rapid strength development is important to keep construction tracking on schedule.

The problem is that when cement is replaced by SCMs, they react more slowly than and take longer to contribute to strength development.

Keyte explains that this delay presents challenges for tight construction timelines and budgets. Waiting an extra day for a slab to harden enough for the next phase of construction can quickly blow out costs when 20 contractors have to wait around for those extra days.

“That’s why innovation is critical, we want to be able to provide a lower carbon option without that compromise.”

The company already has a range of low carbon concrete options, each suited to different applications, which are able to achieve as high as a 50 per cent reduction in cement. Keyte says one of her research priorities is to improve on this score and whittle cement content down by 70 per cent.

Inflexible specifications and regulations stopping innovation in low carbon concrete

Slattery says there’s still more opportunities to reduce emissions through SCMs but there are some barriers.

Top of the list is overhauling Australia’s restrictive standards and specifications for concrete. Designed to ensure fit-for-purpose concrete ends up in roads, buildings and other critical infrastructure.

Holcim Australia’s Agriogiannis says that the industry tends to be very risk averse and defaults to specifications used in past projects.

To encourage low emissions innovations, the industry is calling for specifications that are performance-based.

“So there’s a big body of work that’s required to facilitate that,” Slattery says.

He says governments, state governments in particular, could unlock innovation in this area by taking on more of the risk. When around 15-20 per cent of concrete goes into major government infrastructure projects, he says governments could really help drive change in the industry through overarching procurement policies that preference low carbon materials.

“And we’ve seen that change around the world. So it’s not impossible.”

What about when fly ash and slag run out?

Another distant problem on the horizon is that common cement replacement materials are actually bi-products from other carbon intensive processes – burning coal and making steel in a blast iron furnace – that also need to be phased out as part of the decarbonisation challenge. There’s currently an abundance of fly ash around but alternatives will need to be sourced in the future.

Slattery says there are some “fairly encouraging developments” around the natural materials rather than recycled materials that are very low carbon emissions. Geopolymers are one such alternative, although Slattery says geopolymers will only work in some situations.

Competition for diminishing supplies of fly ash could also increase as Bengtsson says there are other uses for it, such as in ceramic tiles, plaster and in paints and adhesives.

Carbon capture and storage

At this point in time, carbon capture utilisation and storage technologies are set to play a pivotal role in decarbonising concrete.

Commercialising this technology has so far faced substantial barriers but unlike in the energy generation space where renewables can replace fossil fuels, Bengtsson says at this stage there are no real alternatives to CCUS to remove the stubborn emissions emitted during clinker calcination.

“CCUS is where a lot of the faith is.”

Bengtsson is apprehensive about relying so heavily on CCUS technology but recognises that there aren’t many credible alternatives to Portland cement. Even the low carbon attributes of geopolymer cement aren’t as promising as first thought and he says rigorous studies are showing the product to be only slightly better than a heavily optimised Portland cement (made with fly ash etc).

“It’s not a zero emissions alternative technology, far from it.”

This technology is still a long way from broad adoption. Global materials manufacturer HeidelbergCement is, however, upgrading its cement plant in the Swedish Islands to capture CO2 and then transport it for permanent storage site offshore several kilometres down in bedrock. The plant is expected to be the first carbon neutral cement plant, and is expected to be fully carbon neutral by 2030.

“That will really start to demonstrate what the options are for the industry.”

Agriogiannis says it’s still very early days with this technology.

“At the moment, we don’t have a lot of data to support how successful it’s going to be or how expensive it’s going to be.”

“What we do know, though, is that for Australia, about a third of carbon emitted will have to be captured.”

He says the R&D and commercialisation of this technology isn’t going to come cheap and will be an area that the industry will be seeking government support for.

Putting carbon to good use

There are also companies turning the low carbon challenge on its head.

Canadian based company CarbonCure Technologies, which is currently eyeing off the Australian market, is in the business of treating carbon as a resource rather than a nuisance waste product.

Senior director of sustainability for CarbonCure Technologies, Christie Gamble, says the company’s commercial technology, already used in North America and other parts of the globe, works by introducing carbon dioxide that’s been captured from an industrial source, typically an ethanol plant or fertiliser plant, depending on local conditions, into concrete as it’s being mixed.

When this precise dose of liquified and purified CO2 is injected into the mixing concrete, a chemical reaction occurs where the CO2 reacts with other materials in the cement to convert into a calcium carbonate mineral. Within a couple of minutes, Gamble says the CO2 becomes permanently embedded in the concrete.

The technology has the advantage of improving the concrete’s strength, which allows the use of less cement. Using less cement means lower carbon concrete, and also lower costs because cement is the most expensive ingredient in concrete.

“So it creates a financial model that works for the concrete producer to be able to decarbonise because they’re able to reduce some of their material costs.”

The technology is no silver bullet but does deliver a 5 per cent reduction in global warming potential compared to traditional carbon. Gamble also says that the technology is compatible with other decarbonising strategies, such as substituting for cement, and that the company has other technologies and innovations in R&D that will push the carbon footprint reduction even further.

The technology does have the benefit of locking away carbon that would otherwise be used in beverages or other uses that will be eventually released into the atmosphere.

“And that’s where CO2 utilisation solutions like Carbon Cure are so critical, it creates a demand for more capture of CO2.”

In terms of retrofitting the technology, it’s a fairly straightforward task of installing a tank filled with liquefied CO2. The concrete producer incurs the costs but these are offset by the material cost savings that comes from using less cement.

“In most scenarios, it’s price neutral.”

Fuels also matter

Bengtsson says that the industry has been slow to adopt lower carbon fuel sources such as waste-to-energy and biomass.

“It’s still largely based on fossil fuel based energy, which is an opportunity.”

The industry would also like to see regulatory changes to allow the use of biomass in kilns to replace fossil fuels.

Slattery says anything with calorific value is attractive, such as waste timber, sewage sludge and tyres. Take up of these waste products is “accelerating fairly quickly” but Slattery says there are some limitations to tapping into the biomass resource, including government regulations.

Closing the loop

From a circular economy perspective, one of the key benefits of concrete is that a building, designed well, will last a long time. Concrete buildings and infrastructure can also be demolished, recycled and sold back as concrete or road base.

Dwindling sand supplies have been identified as an issue for the industry, with concerns raised about ecological damage to beaches and river beds as well as links to criminal activity to mine the finite material.

There are ways to avoid using natural sand by using aggregate crushed into a substance resembling sand. Boral’s Envirocrete, for example, claims to use a “manufactured” sand made from aggregate.

There’s also a lot of innovation underway to replace raw materials with recycled alternatives.

Civil Engineering expert and Associate Professor at Macquarie University Sorn Vimonsatit and her colleagues at Macquarie University, which is hosting the SmartCrete CRC, have secured $400,000 to develop a polymer concrete using waste latex paint.

Along with reducing raw resource use in concrete manufacturing, Vimonsatit also says the circular economy creates new employment opportunities, which is a social sustainability benefit that shouldn’t be overlooked.

Trucking this heavy material around is also emission intensive

Transport is another carbon intensive component of the concrete supply chain. Slattery says it’s responsible for around 10-15 per cent of the industry’s carbon footprint and given the weight of the material, decarbonising land transport is not as simple as electrifying fleets. Slattery says this area will need “quite a lot of investment” and flags hydrogen power trucks as an option.

Carbon offtake – how significant is this really?

Holcim Australia’s head of sustainability Cyril Giraud says that concrete also offtakes carbon where, through the life of the concrete, carbon is reabsorbed into the material.

He says the industry is trying to get the embodied carbon reducing capabilities of this phenomenon recognised.

When questioned on the expected effectiveness of carbon offtakes, it could account for around 6 per cent of the total decarbonisation challenge. “So that’s definitely significant.”

From the lab to real world application

The challenged material has attracted Cooperative Research Centres (CRC) funding, which are grants for medium to long-term, industry-led research collaborations. Sustainability is part of the mandate for the SmartCrete CRC along with improving productivity in concrete construction and maintenance of concrete assets.

Vimonsatit, who is involved in SmartCrete CRC research, says that it’s important for industry, academia and government to work collaboratively to solve the decarbonisation and sustainability challenges facing concrete, and that the industry should default to sharing sustainability knowledge.

She also says that for the government to be recognised as a world leader, Australia will need a clear national roadmap to govern the industry sectors.

“And we need to see a strong commitment by the government leadership to provide the necessary resources in the form of funding.”