As the race to reduce global carbon emissions continues to gain focus and momentum, many industries are exploring how and what action could be taken to creep – or leap – towards the net zero-carbon finish line, through decarbonising their operations and products.

For the construction sector, often criticised for its significant contribution to carbon emissions, one area of exploration stands out above the rest: materials with high embodied carbon, particularly concrete.

Since its adoption by the ancient Greeks and Romans, concrete has been helping humankind flourish for thousands of years and has become our most popular and versatile construction material. It is the most widely used manufactured material in existence, and the second most-consumed resource on the planet behind water.

With such widespread use, concrete production is estimated to be responsible for 8 per cent of total global carbon emissions. This gives it the unenviable title of the world’s biggest industrial polluter and equates to more carbon emissions than all the cars in the world combined.

While concrete is used across many areas of our built environment – infrastructure and buildings of all shapes and sizes – emissions are of particular concern for large and complex structures, where the concrete intensity is high and its use unavoidable.

For example, it is estimated the structure of a tall tower contains embodied carbon equivalent to between 15 and 30 years of operational carbon footprint, and this figure increases with height. Similarly, on a modern underground rail system built today on the premise of a future decarbonised electricity grid, concrete is estimated to contribute around 15-20 per cent of the total lifetime carbon footprint.

As we race towards low and zero carbon targets in many areas of our lives, we can continue to build big, but we must be much smarter about the materials we use. Already, our business-as-usual practices can reduce this embodied structural footprint by 20-40 per cent through partial cement replacement and other means. But what if we targeted 100 per cent? If we did, imagine the flow-on effect across the planet.

The pathways to low – or zero – carbon concrete

Concrete remains undeniably necessary in construction. While it is here to stay, emerging developments could define a pathway for concrete to support progress towards a low or zero carbon world. Three broad pathways have been identified: pursuing incremental efficiency gains, managing the carbon produced, and removing the source of emissions.

While none of these will reach zero carbon on their own, each achieves a varying degree of improvement and when combined, they provide a robust solution that has the potential to reach ‘very low’ emissions, or even the step change to hit zero.

Concrete and carbon emissions

For context, concrete is made from many ingredients via many different recipe variations, much like a cake. Cake ingredients such as flour, butter, sugar, fruit, and eggs are analogous to the concrete ingredients of cement/binder, water, sand, aggregate and admixtures, and everyone has their own variation on the recipe, which influences characteristics like the strength, workability, durability, shrinkage, and even colour, as shown in Figure 1.

When we talk about carbon emissions from standard concrete, we mostly focus on one of these ingredients – the cement. Although cement only makes up 8-15 per cent of the weight of concrete, it contributes a staggering 85-95 per cent of the CO ₂ emissions (Kestner, D. (2020). “Beyond Fly Ash: How to Optimize Your Concrete Structure to Reduce Embodied Impacts.”).

Almost all modern cement in construction is Ordinary Portland Cement (OPC), which has been in production since the 19^th century. Created by heating limestone and clay, among many of the complex chemical reactions produced during this process, the important one to focus on is the transition from limestone into calcium oxide (Figure 2).

CO₂ is an inherent and inescapable result of creating this cement. It accounts for more than half the total emissions, followed by the fuel needed to create the heat and electricity use (Figure 3). While it is possible to eliminate the heating, energy, and logistical emissions with modern decarbonised systems, the chemical emissions are unavoidable if OPC cement is used.

Pathways to reduce or eliminate concrete carbon emissions

So what pathways are available to reduce or eliminate carbon emissions from concrete?

1. Pursuing efficiency gains through how concrete is made and used

In recent decades, concrete suppliers and users have refined, and made significant improvements in how traditional concrete is made and used.

Partial cement substitution with industrial by-products; conversion of energy supplies in the cement plant; transition from a wet to a dry process in the cement kiln; and greater adoption of research and additives to optimise the mix performance have all contributed to incremental improvements in the performance and environmental outcomes for concrete.

While emissions are still very high, the good news is there are many opportunities to further refine and improve, particularly around:

• Optimising mix design: Research shows that through applying a specific three-stage process to optimise a mix design for its purpose, a reduction in CO₂ content is possible. By starting with high strength class cement, then maximising the structural performance through additives and mix design including limestone filler, and finally reducing the cement content back down to the target structural requirements, researchers achieved 40 MPa mixes with only 150 kg/m³ cement. They then verified through many real applications a reduction in CO₂ content of 35-60 per cent, compared with conventional concrete. The potential to find more gains through smarter mix designs is obvious.
• Improving specification: Designers often specify concrete based solely on strength using a prescriptive approach and apply that to large groupings of dissimilar structural uses. Clauses are included, which discourage environmental improvements without solid justification, and contractors then also impose extra specification constraints. While some specifications are justified, they are often applied as a blanket rule across an entire project, exacerbating issues, but also amplifying opportunities for improvement. For example, foundations do not see their full loading until long after the standard 28-day period of curing and setting; the opportunity is rarely taken to allow slower strength gain, which would enable lower cement usage. As another example, maximum cement content is rarely specified, and so in many precast and post-tensioned uses an excessively high-cement mix is used to achieve early strength. Through careful specification of performance characteristics at a granular level, appropriate to each use, and by specifying high-performance characteristics only where needed, the supplier gains the freedom and direction to optimise environmental outcomes on all other aspects of each mix design. This has the potential to enable significant emissions savings.
• Diversifying energy sources: Around one-third of cement’s emissions come from the fuel combustion for process heat. With recent technical advancements in the industrial and power generation sectors, significant reduction of these emissions appears achievable soon. Viable (researched, tested, and adopted) options include ‘green’ hydrogen, concentrated solar heating, biofuels and biomass, waste fuels, and renewable energies. These and other similar solutions show great promise to make significant cuts to concrete’s typical emissions and are being adopted across the globe – with approximately 75 per cent of US cement manufacturers already using some form of alternative fuel.

2. Managing the carbon produced through carbon capture and sequestration

Providing the same product but mitigating the net carbon entirely in the manufacturing process is the aim of emerging carbon capture and sequestration technologies. This complex process can be broken down into three key areas:

• Capture: Untreated cement plant emissions contain many gases and capturing CO₂ is no easy feat, which is why it is not yet widely practised at an industrial scale. Despite this, there are many promising trials and applications emerging across three primary areas:
  
  
  1. Pre-combustion capture from the fuel
  2. Oxyfuel combustion, to purify the emission stream
  3. Post-combustion technologies (e.g., chemical, and physical absorption, cryogenic separation, and membranes)
  Post-combustion capture trials are popular because they can be retrofitted to existing plants, and many are underway at cement plants around the globe – such as in China, India, and . Another developing area is separating and capturing the CO₂ from the chemical calcination of the limestone, the largest source of carbon emissions. Again, promising trials are underway such as the EU funded Low Emissions Intensity Lime and Cement project, which will enable Europe's cement and lime industries to reduce their emissions dramatically while retaining, or even increasing, international competitiveness Norway. Another developing area is separating and capturing the CO₂ from the chemical calcination of the limestone, the largest source of carbon emissions. Again, promising trials are underway such as the EU (European Union) funded Low Emissions Intensity Lime and Cement project, which will enable Europe's cement and lime industries to reduce their emissions dramatically while retaining, or even increasing, international competitiveness.
• Use or sequester the carbon elsewhere: Once the carbon is captured, something positive needs to be done with it – viable options include smaller scale industrial uses, to larger scale options such as injecting carbon back into depleted oil and gas reserves, as recommended by IPCC (Intergovernmental Panel on Climate Change). (Figure 4).



Figure 4: The carbon capture and storage process involves injecting CO2 into underground chambers, particularly those left behind by oil and gas extraction. © Global CCS (Carbon Capture and Storage) Institute

• Sequester carbon back into concrete: A neater and more self-contained idea is to sequester the carbon back into the concrete it produced. Curing concrete with CO₂ is still a developing area, but currently it is claimed to permanently and safely consume 240kg of CO₂ per m³ of concrete and save three trillion litres of water every year. While some limited research shows reductions in the mechanical properties of the concrete and durability of the reinforcement, the technology is still developing, and higher rates of sequestration may become possible in the future.

3. Removing the source of emissions

What if concrete could remove its largest and most difficult carbon-emitting element, the Portland cement? This is the goal of an emerging family of geopolymer concretes. While geopolymer cement is not new and has been used for centuries, modern research and development of it into a reliable and mass-producible construction material is relatively recent.

As it is research now heads towards critical mass, significant gains are likely to be made when it becomes a viable and scalable product.

Geopolymer refers to a broad range of concrete with alternative binders, typically aluminosilicate-based. These binders replace Portland cement and hence remove the chemical CO₂ emissions from limestone calcination. The two most common ingredients used for geopolymer cement today are fly ash and ground granulated blast slag, with these traditional cement supplements becoming the main ingredient themselves.

Longer term, metakaolin shows much promise in research and trials as a geopolymer cement, and as it is derived from one of the most common and widespread minerals on earth it appears to be the most likely long-term source for mass adoption, as the transition to renewables decreases the by-products from coal mining. Many other alternatives such as Bor aluminosilicates and magnesium-based cements are also in varying stages of research.

The upside to these alkali-activated concretes is strong, providing an immediate 55-75 per cent CO₂ reduction compared with ordinary concrete in most mixes. This provides the impetus for adoption. There are downsides and barriers to overcome, including the potential adverse environmental impacts of the alkali activator chemicals required, variability in the makeup of an emerging product creating market uncertainty, and lack of regulatory support, but these all appear resolvable.

While there are many encouraging developments and the outlook for massive gains is positive, there are also challenges hindering progress and slowing widespread adoption. Across all pathways to low or zero carbon concrete, supply-chain availability, commercialisation, and industry acceptance are key barriers to adoption.

For geopolymers specifically, there are also additional challenges around variability and durability, but continued research will help to understand and mitigate these issues and bring greater consistency, clarity, and confidence in the industry.

Cost and economies of scale also remain large barriers to adoption. As we have seen across many industries, emission-reducing products struggle to compete in a scale-and cost-driven market, unless there is customer demand for them. This demand is needed to drive greater industry uptake and without the customers to cover the additional costs, it can be difficult to justify the investment on economic grounds.

Concrete tomorrow: Is zero carbon concrete possible?

Despite these obstacles, the technical barriers are being broken down, and the possibilities are opening to apply these developments across the built environment. The incremental gains achieved over the past few decades look set to accelerate and lead to a reduced carbon footprint in the short-term.

In the medium-term, carbon capture and sequestration could account for a substantial proportion of the remaining emissions reduction potential on the supply side, with no onus on the end user, if economic barriers can be overcome. Investment into geopolymer concrete, while already making inroads in some corners of the construction market, appears feasible in the long-term across the entire industry.

While governments, developers, designers, suppliers, and builders involved in large infrastructure projects have much to gain from these advances in concrete, so too does the broader built environment. The key is to exploit the greatest opportunities to shape the supply chain, and to act as a beacon for change. If all parties can work together to implement low and (someday) net zero carbon concrete solutions, the gains across the built environment will be significant.

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About the author

David Law is a professional building designer and structural engineer with Aurecon. He has worked on complex structures across three continents and developed professional passions in sustainable construction, and healthcare facilities. He relishes the thrill of devising novel solutions and ideas. He has a particular interest and knowledge in driving sustainable and low-carbon concrete outcomes, through various research and industry leadership activities, including leading the Zero Carbon Concrete industry working group in Victoria, Australia.

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This article is an adapted version of David Law’s paper ‘Is a Zero Carbon concrete skyscraper possible’ published in the Council of Tall Buildings and Urban Habitat (CTBUH) Journal October 2021.

Aurecon is a proud member of the Materials Embodied Carbon Alliance (MECLA).