This article explains what low carbon concrete is, how it can be produced and what the future looks like for concrete in a decarbonised and circular construction industry. It also covers how you as a designer can influence its embodied carbon impact.

Concrete is the most consumed man-made material in the world with 14 billion m3 of concrete produced in 2020. Concrete is a critical construction material for buildings and infrastructure projects. Its popularity is due to its strength, durability, versatility and in most cases the relatively low cost.

Concrete also comes with a high embodied carbon impact, which is caused by the production of cement, one of its main constituents. This is due to cement’s energy intensive production process and carbon emissions from the chemical calcination process. Depending on the construction cycle, cement is responsible for circa 8% of the global GHG emissions. Another challenge when using concrete is reusability. Practically all concrete today is crushed into aggregates at the end of the building’s life.

Due to the very high volume used, there’s no material that can replace the use of concrete. While alternative construction materials have a valuable role in decarbonization of the built environment, their scalability is not sufficient to replace concrete. Other materials however help spur innovation in the concrete sector to improve its environmental performance.

A quick guide for design teams and contractors

A checklist for reducing embodied carbon when building with concrete

1. Minimize the concrete quantity by design. Optimize structural grids and use material efficient solutions like hollow core slabs.
2. Ensure the building adaptability is not jeopardized by the above.
3. Minimize the total clinker use in your project by using alternative binders. This also reduces cost.
4. Make the case and get buy-in from the developer / investor to implement low-carbon concrete solutions in the project. (E.g. traditional replacements for structure, new ones for paving and such)
5. Plan for construction schedule-aligned curing times to allow for longer strength evaluation time when possible.
6. Set a low carbon concrete specification as a requirement for purchasing.
7. Ask for low-carbon solutions from your suppliers.
8. Choose low carbon reinforcement bars, fibres or alternative reinforcement solutions.
9. Ask suppliers to back up their low-carbon solutions with EPDs.
10. Consider transportation related impacts when making purchase choices.

Why is concrete so carbon intensive?

Concrete is made of: cement, water, sand, aggregates and, in some cases, chemical admixtures for increased workability.

Most of concrete’s carbon emissions come from cement.

Cement in turn consists of clinker (the traditional Portland cement) and other materials.

Clinker is made by heating crushed limestone, clay and other materials in temperatures as high as 1400 degrees C.

This temperature can only be achieved with high energy content fuels, so most of its production relies on fossil fuels. In these temperatures, limestone (CaCO₃) is decomposed into lime (CaO) and CO_2.

The resulting process carbon dioxide is referred to as calcination carbon. The calcination process itself is necessary for the clinker to function as a binder, so avoiding it is not possible. Much of the time, the clinker is mixed with other materials to achieve desired properties for the cement.

Concrete is often used in monolithic structures, or cast in place, which does not allow it to be easily reused after a building’s end of life. The most circular end of life scenario is for concrete to be crushed into aggregates. Meaning that new concrete is needed for future buildings with all the additional carbon emissions that come with it.

What is low-carbon concrete?

There is no globally agreed definition for low-carbon concrete. What is commonly perceived as a low carbon concrete in the construction industry, is a concrete mix that results in lower embodied carbon compared to an average concrete mix.

However, there is no agreed benchmark in most regions and no agreed percentage reduction of embodied carbon in order for the concrete mix to be classified as low-carbon. Local attempts at doing this include the Concrete Sustainability Council (Germany), Lavkarbonbetong standard (Norway) and the UK’s Low Carbon Concrete Code.

This results in the term being used in any case where concrete has lower embodied carbon than a typical Portland cement mix, even if the reduction is minimal and at times even nominal. In most cases, the reduction of embodied carbon will be the result of cement substitution with more traditional alternative binders like Fly Ash, Ground Granulated Blast Furnace Slag (GGBS), calcined clays and in some limited cases natural pozzolans, and with innovative new solutions. This list is not an exhaustive list, many other solutions exist as well.

The problem with most of the low-carbon concrete made today is that clinker (the key ingredient of cement) is substituted with secondary materials from fossil fuel-based processes. While this is a useful transition mechanism, it cannot scale as we decarbonize power generation and other industries.

Reducing cement clinker will reduce concrete’s embodied carbon

Concrete is used in foundations, slabs and the structural frame of most buildings across the world. With the majority of concrete’s embodied carbon coming from the production of cement the key to reducing concrete’s embodied carbon is to reduce the total amount of cement used. This must be done via:

• Avoid overdesign of structural elements. This will reduce the amount of concrete and cement being used.
• Rationalisation of live loads consideration during design. Live loads are often overestimated. Load overestimation can be avoided by detailed structural design without affecting any future adaptability of the building.
• Design for material efficiency by optimising the span of the structural grid and incorporating more material efficient concrete elements like hollow core slabs, and composite decks.
• Avoid over specifying concrete’s compressive strength. Concrete mixes can be optimized for specific parts of the building; there’s no need to use standard strength throughout.
• Avoid over specifying concrete’s compressive strength in the early days after pouring (7, 14 and 28 days). This will allow the use of concrete with alternative binders that in most cases have longer curing times.
• The use of alternative binders like Fly-Ash, GGBS, calcined clays etc.
• The use of other innovative concrete solutions, where present, that are not necessarily related to cement reduction.

A key benefit of optimizing concrete and cement clinker quantity is that it saves capital costs.

Alternative binders to cement

The most commonly used alternative binders to cement are:

• Fly Ash / Pulverized Fuel Ash
• Ground Granulated Blast Furnace Slag
• Calcined Clays

Although some are widely used by the construction industry, their availability depends on fossil fuel related processes and is expected to be limited in the future when it is hoped that fossil fuel use will decrease. As a result, the use of such binders is important today but cannot be a significant part of concrete’s zero carbon future. 

Although the binders listed here will reduce concrete’s embodied carbon, they are often viewed with scepticism by contractors since they affect the curing rate of concrete. This means that concrete mixes with such binders will need more time to reach a set level of strength compared to traditional Portland cement mixes.

A concrete mix with 50% GGBS for example will have half the strength of a Portland cement mix in the 7 days after pouring but should reach the same strength within 28 days. The slower curing time can hold construction back by a few days for each building floor added which can result in bigger delays and cost implications in taller building constructions. Contractors will have to plan for the longer curing time of low carbon concrete where possible to mitigate the delays and associated cost implications.

Other binders

Other alternative binders, albeit not used as extensively, include rice husk ash and gypsum. Rice husk ash is a by-product of rice production. When rice husks are removed from rice, they are burned. The ash resulting from this process is known as rice husk ash and has a high content of silica which allows it to produce additional cementitious compounds when reacting with carbon hydroxide which is a product of cement hydration. A key benefit of rice husk ash is that it is a renewable resource.

Other solutions

So is low carbon concrete all about alternative binders to cement? No, other low carbon concrete technologies are now emerging with some already commercially available and scaling up.

Carbon capture technologies

A significant cause of the carbon emissions of concrete is the calcinations process during the production of cement. Even if the main energy being used in the production and transportation of concrete were zero carbon, the calcination related emissions would remain the same. To address this, Heidelberg Materials is currently building the world’s first carbon capture and storage facility in a cement production plant in Brevik, Norway and planning to do the same in the Slite plant in Sweden.

Carbon Dioxide injection

Such technologies aim to use captured CO₂ by injecting it in concrete while still in liquid form. CO₂ then reacts with cement and water to produce more cementitious compounds. Such technologies already exist commercially and can be applied to existing concrete plants. CarbonCure, a Canadian company, and Carbonaide, a Finnish company, are developing technologies that can be used in existing ready mix concrete and precast concrete respectively.

CarbonBuilt has developed a similar technology for the production of ultra low carbon concrete blocks. The technology can be integrated in any existing concrete block plant and reduces the embodied carbon of the blocks by replacing the cement with another alternative material that reacts with injected CO₂ during the curing process to form CaCO₃ (limestone). The first commercial production of concrete blocks using this technology was started by Blair Block in Alabama, USA, in 2022.

Biotechnology

Research is also being undertaken on how low carbon concrete can be achieved with the use of biotechnology. Prometheus Materials in the USA has developed a concrete mix that uses algae to replace Portland cement. Their solution has been used in pilot projects and efforts are being put towards the full commercialisation of the product. Biozeroc, a startup in the UK is a biomaterials company that produces low-carbon concrete using bacteria. Their process uses bacteria to bind aggregates and sand together into a material that performs as well as conventional concrete. This BioConcrete production process emits at least 85% less carbon, and has the potential to be carbon-negative once waste materials are incorporated into the bacterial feedstocks.

Plant based solutions

Plant based solutions include materials that in many cases are a mix of lime, sand, clay and plant fibres. One such material is hempcrete which is made of lime, sand and hemp fibres. Such materials can be used for cast in place elements, shotcrete and masonry units. Although these materials are bio-based and come with ultra low embodied carbon, they cannot substitute traditional concrete other than for small buildings and non-load bearing applications.

Get early design insights with Carbon Designer 3D

How could concrete become more circular?

The vast majority of concrete’s embodied carbon is due to the production stage (A1-A3). Even when alternative binders are used there still remains a significant A1-A3 impact. A great way to reduce these impacts in the future is to reduce the need for new concrete to be produced. As the demand for concrete is not expected to reduce in the next few decades, the only way to make this happen is to use concrete in elements that can be reused in the future.

The most likely end-of-life scenario currently for concrete is to be crushed into aggregates. However these aggregates cannot be reused indefinitely or at high rates in structural elements because of the monolithic structure of in situ concrete which makes it impossible to be disassembled and reused. Precast concrete elements when designed properly can allow for future disassembly and reuse. The key in enabling building elements like concrete columns, beams and panels to be reused is the careful design and reversible implementation of the connections. 3XN Architects are well known for their research in low carbon and circular materials. In their report Building a Circular Future, they propose reversible connections for elements made of concrete among others.

The role of the specifier in low carbon concrete

Concrete specifications are based on national standards that define the concrete grade and water to cement ratio to be used based on climate and exposure zones. Structural engineers may go a step further from national standards and specify performance-based requirements for concrete mixes.

A performance-based specification will define the durability requirements, for example resistance to carbonation, maximum acceptable shrinkage of concrete, compressive strength when concrete is fully cured and minimum early compressive strength in a set period of time as required for the continuation of construction. Early compressive strength should ideally be evaluated in longer periods than typically done with Portland cement mix to allow for the use of alternative binders.

Maximum embodied carbon may be specified for the concrete grade specified as above. The embodied carbon limits should be based on local availability and should also consider the concrete production related impacts, transportation to the site and site wastage.

The role of contractors in low carbon concrete

Contractors should aim to procure low carbon concrete as per the low carbon concrete specification. If the specification is not performance-based, they should procure concrete from manufacturers with documented low embodied carbon for the required concrete grade through the use and comparison of EPDs where available. Where EPDs might not be available, concrete mixes with maximised use of alternative binders should be preferred. Pressure should be applied to local suppliers to document their low carbon concrete mixes with EPDs.

During construction, a considerable embodied carbon reduction can be achieved by minimising the concrete wastage on site or the use of precast concrete elements where possible.

Comparing concrete mixes and how to model low carbon concrete in One Click LCA

The One Click LCA database has tens of thousands of datapoints for ready mix concrete of which approximately 6500 are generic datapoints representing typical production processes for various types of concrete. Many of these generic datapoints come from industry average EPDs like the ones produced by NRMCA in the USA and Canada. In addition, there are plenty of other generic datapoints coming from regulatory databases from European governments. Lastly, One Click LCA also provides its own generic datapoints for different types of ready-mix concrete worldwide.

The list below summarizes the ready-mix generic datapoints that are available in the One Click LCA database to help you model your concrete embodied carbon at early stages and when a concrete plant specific EPD is not available:

1. Industry average EPDs from manufacturer associations (e.g. NRMCA)
2. One Click LCA generic datapoints for Portland cement mixes and concrete grades ranging from C12/15 (1700/2200 PSI) to C60/75(8700/10900 PSI)
3. One Click LCA generic datapoints for concrete grades as above with cement substituted with PFA at a range from 10% to 50%.
4. One Click LCA generic datapoints for concrete grades as above with cement substituted with GGBS at a range from 10% to 75%.

You can also create your own concrete specification using the following datasets:

1. Portland cement
2. Ground Granulated Blast Furnace Slag
3. Pulverised Fly Ash
4. Silica Fume
5. Metakaolin
6. Standardised cement types like CEM I, CEM II, CEM III and CEM IV
7. Virgin and recycled concrete aggregates at various densities

For hempcrete-based solutions, you can use the One Click LCA generic datapoints for hempcrete, shotcrete with hempcrete and hempcrete masonry units.  

The One Click LCA database is continuously enriched with more generic datapoints to cover concrete mixes with alternative binders at different percentages. If you are considering a concrete mix with an alternative binder at a percentage at which there is no ready-to-use datapoint, you can model  your own concrete mix by using datapoints for Portland cement, other cement types, alternative binders, sand and aggregate datapoints as listed above. 

One Click LCA enables you to save these alternative concrete mixes as private constructions, store them as datasets in your library and reuse them as single datasets in your building LCAs in the future. 

Find out more about how to create private datasets and private constructions with One Click LCA via these helpdesk articles.

How to compare different concrete mixes

A quick and simple way to compare different concrete mixes using One Click LCA, is to add the datapoints of interest to the “compare data” feature. This will automatically generate a graph showing the carbon emissions per life cycle module for each of the compared mixes for the required functional unit. The feature is available for both generic datapoints and manufacturer specific EPDs and can be used for any material type.

Find out more about how to use the Advanced Material Comparision feature.

Sustainable concrete reinforcement

Concrete, especially in buildings, will typically be used together with steel reinforcement. Reinforcement is necessary in order to allow concrete elements to carry tensile forces which otherwise would not be possible due to concrete’s low tensile strength. Depending on the region, the reinforcement rate and the recycled content of the steel reinforcement bars, reinforcement can be responsible for up to 50% of the reinforced concrete’s embodied carbon. (100% Portland cement mix with 60% recycled rebars and 200kg/m3 reinforcement rate).

To reduce the impact of reinforcement in concrete, there are various alternative reinforcement types that can be used. None of them can be as versatile as the traditional steel reinforcement bars, however in many cases, using one of the following alternative reinforcement types could result in significant embodied carbon savings. The alternative reinforcements can be categorised into organic fibres like hemp fibres, steel fibres which are used widely in various applications like industrial floors, and other mineral fibres or bars made for example from glass and basalt.

The following generic datapoints for alternative reinforcement can be found in the One Click LCA database in addition to the various EPDs developed by manufacturers.

1. Bi-component polyester fibre, 100% recycled content
2. Hemp fibres, straw and shives
3. Basalt fibres
4. Steel fibre for concrete reinforcement, 0% and 100% recycled content
5. Glass fibre for concrete reinforcement
6. Polypropylene fibre for concrete reinforcement, ranging 0% – 100% recycled content
7. Flax fibre
8. Jute fibre
9. Kenaf fibre
10. Basalt rebar for concrete reinforcement

How to find a lower carbon concrete mix in your region

When the time comes to specify the exact concrete manufacturer from where concrete will be procured – usually at detailed design stage or construction stage – how do you find lower impact concrete mixes and manufacturers?

One Click LCA’s green material benchmark feature enables you to identify all plant specific mixes that have lower embodied carbon than the one you have already selected. The feature is available either directly from the material query through a material’s data card or via the results page where you can ask One Click LCA to give you a list of more sustainable alternatives to your currently chosen materials.

Find out more about green material benchmarks.

Who can supply low carbon concrete solutions?

Many manufacturers are already producing low carbon ready-mix and precast concrete or cement. All manufacturers that have created EPDs for their concrete mixes or cement products can be found and compared in One Click LCA. Just some of them are listed below for reference.

What is the future of low carbon concrete?

Low or lower carbon concrete is currently made available mostly by replacing cement with alternative binders like GGBS, PFA and silica fume. With the construction industry urgently needing to decarbonise as soon as possible, the demand for such alternative binders is continuously increasing and will increase more in the future. At the same time, the supply of some binders like Fly Ash which is a by-product of coal combustion is set to decline due to the reduced demand of the primary product (e.g. coal based electricity).

Concrete must be made available at even lower embodied carbon than typically achieved now with binders like GGBS and PFA and it must be produced with new innovative manufacturing processes and binders.

In its UK concrete and cement industry roadmap to beyond net zero, the UK Concrete Centre has identified a potential to reduce the embodied carbon of concrete by 39% by 2050 due to the decarbonisation of the electricity grid and transportation, lower carbon production of cement and other binders and the switch of the main fuel used in cement production to a renewable fuel. The remaining 61% reduction required for concrete to become a zero carbon material must come via carbon capture technologies which will address mostly the calcination related emissions.

Heidelberg Materials is already building the first Carbon Capture and Storage facility in a cement production plant in Norway and is planning to do the same in another one in Sweden by 2030. Other companies like CarbonCure and Carbonaide as mentioned above reduce the embodied carbon of concrete by using CO₂ in the concrete mix itself while companies like Prometheus Materials and Biozeroc are looking into using biotechnology to eliminate the use of cement in concrete to allow reaching a zero or negative carbon building material similar to concrete.

Affordable EPDs with One Click LCA Concrete EPD Generator

One Click LCA now offers a Concrete EPD Generator specifically developed for concrete. The tool allows concrete manufacturers to create EPDs for their mixes at a fraction of the effort and cost compared to traditional EPD solutions. The tool generates EPDs that are 3rd party verified and comply with ISO 14025, EN 15804+A2 and associated standards, and it unlocks the market for more products to demonstrate their sustainability credentials.

Learn more about the One Click LCA Concrete EPD generator or watch the recording of our recent webinar on how to create fast concrete EPDs.