Concrete is a mix of cement, aggregates and water. It is the world’s most consumed material after water.
Most of concrete’s embodied carbon comes from cement, which makes up about 10% of concrete by volume, but accounts for around 75-90% of its embodied impact.
The energy-intensive process of cement manufacturing is responsible. Raw ingredients are heated to approximately 1450°C. Then, chemical reactions form the mineral phases of Portland cement clinker.
Approximately 40% of the CO2 emitted is due to fuel combustion, with the remaining 60% caused by chemical reactions.
USGS has estimated worldwide cement production at 4.1 billion metric tons in 2022. The cement industry is responsible for 5-8% of global anthropogenic CO2 emissions. Therefore, producing low carbon concrete will help to achieve worldwide carbon neutrality (net zero) by 2050.
The Carbon Leadership Forum defines embodied carbon as “the greenhouse gas emissions arising from the manufacturing, transportation, installation, maintenance, and disposal of building materials.”
Greenhouse gas emissions are converted into global warming potential (GWP), measured in kilograms of CO2 equivalent (kg CO2e), also called carbon footprint.
Approximately 860 kg CO2 is emitted directly into the atmosphere for each tonne of cement produced.
There are several methods to reduce embodied carbon that you can use, alone or combined, to produce low carbon concrete. We are going to talk about:
Reducing cement content by 50% gives a 40% reduction in its embodied carbon. Alternative cementitious materials, like ground granulated blastfurnace slag (ggbs) and fly ash, have been used for decades, representing 30-50% of the mix.
By looking at in-situ performance of your mix in real-time using concrete sensors you can measure real-world performance of your mix in your climate. Using this data you can work with your concrete supplier to design better mixes that are only as performant as you need, minimising the cement requirements. This would allow you to time the change from your summer to winter mix more optimally or even create several mixes.
Supplementary cementitious materials can replace part of the Portland cement in concrete, helping to reduce its embodied carbon.
Many SCMs are by-products of industrial processes. These include:
Other SCMs are classified as natural pozzolans, such as:
Some natural pozzolans require processing before use in concrete, which may affect the degree to which they may reduce the embodied carbon of the concrete.
Fly ash is the mineral residue from the combustion of coal, typically in the generation of electricity at a power plant.
The most common proportion of fly ash in UK cements is 25% by mass of total cementitious content, reducing embodied carbon by more than 20%.
Concrete with a large volume of fly ash can be a disadvantage if you require high early strength or pour concrete at low temperatures.
Ground granulated blastfurnace slag is a non-metallic material obtained in the production of iron.
The most common proportion of GGBS in UK-produced cements is 50% by mass of total cementitious content, reducing embodied carbon by more than 30%.
The use of GGBS can also have significant engineering benefits, such as reducing the heat of hydration. This makes it ideal for use in mass-concrete applications like concrete dams, thick raft foundations, or gravity foundations for wind turbines.
However, the use of GGBS can reduce the early strength gain of the material slightly, so it is not used in high proportions when high early strength gain is necessary.
Silica fume (also known as microsilica) is obtained from the manufacture of silicon. It is an exceptionally fine powder (as fine as smoke), used in either densified or slurry form.
The most common proportion of silica fume in UK cements is 10% by mass of total cementitious content.
Silica fume improves both strength and durability of concrete, so it is often used to obtain high strength concrete.
Limestone is a sedimentary rock composed mainly of calcium carbonate. It is abundant and serves as a highly effective SCM in powdered form.
It is limited in BS 8500 to 20% of the total cementitious content in Portland-limestone cements and 15% in limestone-containing multi-component cements.
For example, powdered limestone can be mixed with GGBS and other alternatives to produce concrete with 65% less Portland cement.
Limestone is easier to grind than clinker, requiring less energy to prepare the raw material. Cement with limestone can also increase concrete strength and reduce porosity, increasing durability. Calcination of the limestone is not necessary, thereby avoiding additional CO2 emissions.
Natural pozzolans are produced from natural mineral deposits. Some of these materials require heat treatment, known as calcining, to make them pozzolanic. Others, like volcanic ash, can be used with minimal processing (such as drying and grinding).
As with other SCMs, these pozzolans can be used in concrete to replace part of the Portland cement, enhance performance characteristics and extend service life.
A chemical admixture is a material used as an ingredient, usually in small quantities, to modify one or more of the properties of the mix in the fresh or hardened state.
Water-reducing admixtures allow you to achieve the same workability with a lower water content. Reducing the water content means that you can achieve higher strengths with the same cement content, or the same strength with a lower cement content.
Introducing water-reducing admixtures, you can meet the requirements for an exposure class at a lower cement content, reducing its embodied carbon content while enhancing long-term performance.
Aggregates are granular materials like sand, gravel and crushed stone.
Specify larger sizes to reduce cement content and obtain low carbon concrete. However, most plants and factories in the UK do not stock aggregate sizes greater than 20mm.
An RC 28/35 (BS 8500-2, Table 5) with 20 mm aggregate will reduce its carbon footprint by 34.4 kg CO2/m3 (12.5%), compared to 10 mm aggregate.
Consider also local aggregates to avoid producing additional CO2 due to transportation.
Recycled aggregates, if available locally, will further reduce the concrete's embodied carbon. If you must transport them more than 15km by road, they’ll probably give a higher carbon footprint than local primary aggregates.
Lightweight aggregates (LWAs) are construction materials that have a lower density than the most common construction aggregates.
LWAs are used to produce lightweight concrete, which BS EN 206 defines as concrete with an oven-dry density between 800-2,000 kg/m3 (Normal-weight concrete has a density between 2,000-2,600 kg/m3).
Lightweight concrete reduces the dead loads of the structure, allowing for thinner elements and foundations. This helps to save cement and other materials, reduce transportation costs and build faster, reducing the carbon footprint.
Reduce the usage of fossil fuels at the cement kiln and substitute them with alternative fuels, like waste biomass fuels, paper and wood, and tyres.
Because many alternative fuels derive from organic sources, they are effectively carbon neutral. And their use has other benefits, such as decreasing waste to landfill and reducing the amount of virgin raw materials required for cement production.
Use efficient design practices that minimise the overdesign of structural elements in terms of strength, durability, and the owner’s intended design life for the structure.
A reduction in concrete strength class will reduce cement content for the same volume unless limited by minimum cement content specifications.
For example, a reduction in strength class from C70/85 to C32/40 may reduce cement content by 150 kg/m3 of concrete, equivalent to 185 kg CO2/m3.
Reducing the cross-sectional area of reinforced concrete elements considerably reduces the volume of material used, and embodied carbon of the structure.
Choosing high strength concrete can result in even thinner concrete elements requiring less concrete volume and concreting materials. This will reduce the carbon footprint, even though the cement content can be higher.
In general, using efficient design practices allows you to do more with less.
Carbon capture, usage and storage is a process which enables carbon dioxide emissions to be captured rather than released into the atmosphere. Captured emissions are locked up in long-term storage or injected into concrete to strengthen it.
CCUS will be essential for decarbonising the construction industry and many other sectors. However, it is also a challenge: CCUS capacity is small today, and the infrastructure to transport and store CO2 is not developed.
You can specify 56-day strength, rather than the usual 28-day strength, if it’s been considered in the design. This can reduce the cement content needed by 15-20 kg/m3 or even more by using high proportions of GGBS.
A 56-day strength specification can affect the formwork striking times unless you can calculate the concrete strength. You can obtain it with:
Or you can use ConcreteDNA to see the concrete strength on your smartphone.
Speak to sales to reduce embodied carbon in concrete and deliver your project on time.
American Concrete Institute (21 December 2021). Embodied Carbon and the Concrete Industry: What you need to know [Video]. YouTube. https://www.youtube.com/watch?v=-T6cQV5wPg8&t=1s&ab_channel=AmericanConcreteInstitute
ACI Committee 130: Report on the role of materials in sustainable concrete construction ACI 130R-19. American Concrete Institute, Farmington Hills (MI), 2019.
BS EN 206:2013+A2:2021 Concrete. Specification, performance, production and conformity
BS 8500:2019 Concrete - Complementary British Standard to BS EN 206
Chen, Siwei et al.: Reducing embodied carbon in concrete materials: A state-of-the-art review. Resources, Conservation and Recycling, vol. 188, January 2023, 106653.
“Embodied Carbon 101”, Carbon Leadership Forum, https://carbonleadershipforum.org/embodied-carbon-101/ [Accessed on 27 April 2023]
National structural concrete specification for building construction (NSCS), 4th edition. The Concrete Centre, Blackwater (England), 2010.
Specifying Sustainable Concrete. MPA The Concrete Centre, London, 2020.
UK Concrete and Cement Industry - Roadmap to Beyond Net Zero. MPA The Concrete Centre, London, 2020.
U.S. Geological Survey: Mineral Commodity Summaries 2023. U.S. Geological Survey, Reston (VA), 2023.
Wilson, Michelle L.; Tennis, Paul D.: Design and Control of Concrete Mixtures, 17th Edition. Portland Cement Association, Skokie (IL), 2021.
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