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 CO₂ emissions (Kestner, D. (2020). “Beyond Fly Ash: How to Optimize Your Concrete Structure to Reduce Embodied Impacts.”).

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