Reaching net zero: toward low carbon concrete

Concrete consumption and GHG emissions 

Construction is a high carbon-emitting industry, accounting for a considerable proportion of energy use and emissions globally. Constructing assets such as buildings, roads, railways, tunnels and bridges involves emitting carbon, primarily through electricity use, fuel usage for vehicles and plant, and from the production of materials, such as steel and cement. For example, Skanska UK estimates that over 75% of its annual carbon emissions derive from the embodied carbon in materials it uses for its construction projects.

Concrete is the most used material on the planet. It is strong and durable, and the constituents are abundant almost everywhere. We rely on many forms of concrete each day, from pavements that we walk on to high-performance structural concrete used in our tall buildings and infrastructure. It is an incredible material that has supported the development of our societies and improved the quality of life for billions of people.

Concrete is made up of three main constituents: 

1. Aggregates (gravels and sands) 70%-80% 
2. Cement (the active ingredient) 10%-20% 
3. Water (which reacts with the cement) 5%-10% 

Up to 90% of the greenhouse gas (GHG) emissions associated with concrete are in the cement. Conventional Portland cement is made by heating limestone and clay and grinding them into a fine powder. The process of heating and decomposing the limestone releases about 0.86kg CO2e for every 1kg of cement produced. This is partly down to the chemical process as well as the energy involved in heating the limestone.

The challenge we face now is how to continue to use concrete when the active ingredient is such a potent source of greenhouse gas emissions. The Low Carbon Concrete Routemap, developed by the Institute of Civil Engineers, focuses on seven strands of knowledge that must be developed concurrently to reduce the embodied carbon of concrete going forward:

 

1. Defining and benchmarking carbon in concrete: A zero-carbon future for concrete can only be mapped out from an accurate starting position. The construction industry must establish appropriate boundaries to classify concrete by carbon. Further work is required to build on this data and establish a simple rating system for carbon in concrete.

2. Knowledge transfer: Knowledge transfer is crucial to addressing barriers and accelerating the use of lower-carbon concrete. There needs to be clear guidance on how to specify, design and use lower-carbon concrete within the existing standards, as well as a better understanding of performance and how and when to engage with stakeholders. Coordination between institutions and trade bodies is important to ensure guidance is effective.

3. Design and specifications: The use of concrete must be optimised within the design process regardless of its carbon intensity. Guidance that demonstrates how material savings can be made through efficient design is required. The specification of concrete and concrete products must include carbon intensity, and specifiers need to understand how they can work to reduce it while meeting other performance requirements.

4. Construction and operation: Consideration must be given to how concrete will be produced and whether in-situ or precast concrete offers greater potential carbon savings. The performance requirements, installation method and project-specific logistical constraints should all be considered during early collaboration between the concrete producer and the project team. There must also be a clear plan for verification of the material to avoid waste or an excessive testing regime.

5. Optimise existing technology: Within current standards and practice, it is possible to produce concretes that have lower embodied carbon. To achieve this, stakeholders need to work together to ensure that all options for cement types are considered. In addition, the project team must work to ensure that the cement content is optimised for a given cement type. Collectively this optimised approach will realise significant carbon savings over typical practice.

6. Adopting new technology: Concretes that use cement blends or contents outside of current standards will be part of the overall solution to reducing the carbon intensity of the industry. Some of these concretes are an extension of existing technology, while others adopt wholly different chemistry. Wherever possible and appropriate, these new technologies should be supported by the industry to allow the development of standards and an increase in commercial readiness and application.

7. Carbon sequestration: Carbon sequestration within concrete can offer some benefit in performance and the potential reduction of atmospheric CO2. Guidance on how to use novel carbon curing technology and a better understanding of how to maximise longterm carbonation is required. Carbon sequestration technology to reduce the intensity of cement production requires large-scale industry and government support and should be recognised as an end-of-pipe solution that should be considered only once other carbon-saving opportunities are maximised.