Over 2 billion tonnes1 of greenhouse gases are released into the atmosphere each year during the manufacture of cement, one of the basic ingredients of concrete. How can these emissions be significantly reduced?
There is no single solution, but a range of measures that can reduce greenhouse gas (GHG) emissions in the construction sector. It is essential to take action across the entire value chain. RE2020 [editor’s note: the latest energy and environmental regulations applicable to new buildings] works towards this: it is one of the first global regulations to introduce environmental performance through life-cycle analysis to quantify a building’s impact on global warming2.
What are the most important levers for decarbonisation?
The first is moderation. In practice, this involves questioning the need for new construction or prioritising renovation. Another important lever is structural design. For example, a comparison of several large stadiums3 shows that, per seat, the quantities of concrete and steel used can be reduced by a factor of 10 – thereby reducing the environmental footprint associated with materials. It is also possible to design a building with its deconstruction and the reuse of structural elements at the end of its life in mind.
Finally, adaptable buildings that evolve with changing uses help combat planned obsolescence. For non-residential buildings, the intended use often becomes obsolete before the building reaches the end of its life due to structural reasons. Since the rise of remote working, the amount of office space required has decreased: even if it is not always technically feasible, some existing office buildings could be converted into housing rather than demolished and rebuilt.
When we talk about the transition in the construction sector, we often think of new low-carbon materials and futuristic concretes. Are their promises just greenwashing?
In France, the carbon intensity of cement has fallen from 640 kg CO2e [editor’s note: a unit encompassing all greenhouse gases] per tonne of cement in 2015 to 560 kg CO2e/t cement in 2021. This reduction reflects the industry’s efforts to comply with the National Low-Carbon Strategy and progress in research and innovation. Globally, however, the carbon intensity of cement has been stagnating for several years4. The decline in France is mainly due to the use of alternative fuels in cement plants and the reduction in the average clinker content in cement (75% in 2021).
Why does reducing the clinker content in cement lower the sector’s GHG emissions?
Clinker is the main component of cement, which itself is used in concrete – alongside aggregates and water – as a binder. Cement’s carbon footprint is mainly due to clinker production: a mixture of limestone and clay heated to 1,450°C. The energy required for heating, as well as the decarbonation of the limestone during the reaction, results in significant CO₂ emissions.
As clinker production inherently emits CO2, the only solution is therefore to reduce its quantity in the cement. Part of the clinker can be replaced by mineral additives, notably co-products from other industries, such as blast furnace slag [editor’s note: a by-product of iron production]. However, the availability of these substitutes may be limited. The other challenge is to successfully develop low-clinker cements that do not compromise the mechanical properties of concrete in the short and long term. One of the fastest-growing cements at present is LC3, which contains limestone and calcined clay. It has a clinker content of just 50%, compared to around 75% in traditional cements.
Are there other alternatives for reducing concrete’s GHG emissions?
Yes, by reducing the cement content in concrete—simply by avoiding over-dosing with cement—it is possible to save 30% to 50% on cement5. For the same strength, concrete with a poorly designed mix emits four times more CO2 than optimised concrete6. Finally, replacing some of the aggregates with recycled concrete can also help limit emissions.
Life cycle assessments are essential for evaluating the environmental and climate benefits of these new materials. The environmental footprint must be calculated at the structural level, not the material level, as it is at this level that functionality is sought.
Nevertheless, despite recent efforts, the building materials industry is still responsible for around 8% of global emissions linked to energy and industrial processes in 20237. Can we simply do without concrete to accelerate decarbonisation?
Concrete is the most widely produced material in the world. Why? It offers numerous advantages: the raw materials – limestone, clay, aggregates, water – are found everywhere on the planet; it is an economical material; its manufacturing process is simple; it is easy to use and can take any shape.
It is impossible to meet people’s vital housing needs without concrete. However, it is entirely possible to increase the use of alternative building materials – wood, stone, rammed earth, etc. – which have specific and sometimes complementary properties. Structures combining materials, such as concrete and wood, can also be designed. But these hybrid constructions can present their own design and implementation complexities.
According to its roadmap8, a large part of the sector’s decarbonisation relies on CO2 capture and sequestration. Is this not yet another illusory lever?
This lever is not yet mature or operational in the short term. Today, we can capture and sequester around 50 million tonnes of CO₂ per year, whereas emissions from cement plants amount to over 2 billion tonnes of CO₂ per year. Furthermore, Ademe estimates that only 20% of French cement plants could be connected to a sequestration site. CO2 capture and sequestration should only be implemented as a last resort, once all other decarbonisation measures have been utilised.
Finally, concrete itself acts as a CO2 sink. Atmospheric CO₂ reacts chemically with the concrete and mortar in buildings and structures: this reaction creates solid calcium carbonate which accumulates within the material. However, it has recently been shown that, between 1930 and 2023, this carbonation process captured 52% of the CO₂ emissions associated with cement production over the same period9. Although this does not offset the initial emissions, it is significant. This storage of CO2 by the material itself opens up new prospects for the sector10.
