Why is it so carbon-intensive to produce concrete?

Despite the fact that cement only makes up 10-20% of concrete by mass, it is responsible for over 90% of concrete’s embodied carbon. This is because producing cement requires heating limestone to very high temperatures in order to decompose it into cement clinker. The main emissions sources are twofold: over 50% is a direct result of the chemical reaction required to decompose the limestone, while a further 40% comes from the energy used to generate so much heat. Thermal emissions can be reduced by decarbonising the grid and using alternative fuels, but the process emissions are unavoidable. Consequently, the highest-impact way to reduce the embodied carbon of concrete is to use less clinker – typically by replacing a portion of it with another supplementary cementitious material (SCM).

Emissions sources during cement production

The current picture

Currently, the most common SCM has been ground granulated blast-furnace slag (GGBS), a co-product of steel manufacturing. Until recently, this has been seen as sustainable solution to make use of waste products from another industry, however recent studies have found that global GGBS resources are now being fully utilised. This means that while specifying a high percentage of GGBS would provide a carbon reduction for an individual project, this would mean less GGBS available for use elsewhere, and therefore would not reduce global emissions.

What are the alternatives?

Calcined Clay

Today, one of the most promising upcoming alternatives is the use of natural materials as SCMs, such as clay. Clay has to be calcined (activated by heating) to be reactive, but the temperatures required are much lower than those required to produce clinker, and there are none of the process emissions, so the overall carbon intensity is around 60% lower than Portland cement clinker.

In reality, this is not a new technology: natural materials such as volcanic ash have been used as SCMs as far back as the Roman era, including for historic landmarks such as the Pantheon in Rome. Calcined clay has also been commonly used in concrete in sub-tropical regions, such as in Brazil. The types of clay in these regions are typically more reactive than in other parts of the world, but research into the calcination potential of local clays is also ongoing.

The latest version of the British Standard which governs cement use in the UK currently allows up to 35% of cement to be replaced with calcined clay, and concrete suppliers in the UK are now starting to supply this using clay imported from Spain. Calcined clay concrete is not yet widely available for us in the UK, but is expected to start becoming available this year.

Depending on the type of clay used, concrete made with calcined clay can also be a reddish-brown colour, quite different from the typical grey of clinker and GGBS mixes.

Calcined clay concrete

Ternary Cements

It is also possible to replace a portion of clinker with raw limestone. As this has not been treated and decomposed, it has a fraction of the carbon emissions associated with clinker. Raw limestone is largely inert, but can enhance the properties of other SCMs when used in a ternary blend, allowing for higher proportions of replacement overall.

The latest codes allow for up to 50% of clinker to be replaced with a composite of two SCMs, such as calcined clay + limestone. Up to a 40% reduction in carbon emissions can be achieved using such mixes.

Components of limestone calcined clay ternary cement

While calcined clay is still just over the horizon, a combination of GGBS + limestone can be specified today. This has a similar overall carbon reduction to GGBS alone, but reduces the amount of GGBS needed, allowing the available GGBS to stretch further.

An additional advantage of using limestone fines is that as the chemical properties are less critical than for the limestone used to make clinker, lower-quality limestone from quarries that would otherwise be discarded can now be usefully utilised.

Suppliers like Holcim are currently able to supply limestone fines widely across London, at an equivalent cost to a traditional GGBS mix.

Final Notes

Early engagement between specifiers and concrete suppliers on a project is key, to determine the best mix that can be achieved based on the available resources and the location of the project. Clients and design teams pushing for use of new SCM options will also be crucial in providing the demand for these to be supplied. Note that many of the upcoming advancements are focused around London: projects in other areas, and especially rural projects, may not be able to access the same materials for some time.

It is worth noting that research into alternative cements and different approaches to decarbonising concrete is ongoing, and the industry is moving fast. Our understanding of the best options available is constantly changing: products we think are the answer today could be superseded by something else within a year.

One option which may be promising in the future is the potential for using recycled concrete as an SCM. This is still some time away from being available for project-scale use in the UK, though small proportions have started to be used in Europe, but it is an exciting possibility which we are watching closely. In the meantime, producers are ramping up recycling of excess unused concrete – particularly the aggregates, which can have carbon sequestered in them as part of the recycling process, before being put back into new concrete.