Sustainable concrete: from gray to "green"
Despite its centuries of faithful good service, the environmental footprint of today’s concrete means that radical change is needed. Should it be systematically replaced with biobased alternatives? Or should we be putting human ingenuity to work to make concrete greener? It’s time to leave the clichés behind to understand, compare and analyze the various solutions and innovations for a more sustainable construction.
Since it was first used by the Romans more than 2,000 years ago, concrete has made itself part of modern life – employed in buildings, roads, bridges, pipelines and more... Today, it is the most widely used material in the world, after water, according to the Global Cement and Concrete Association. Multifunctional, strong, and economical, concrete has ultimately become a victim of its own success. Its visual impact has attracted criticism for many years, and now it is the turn of its environmental footprint. The latter is undeniable: concrete is responsible for nearly 8% of global CO2 emissions according to the think tank Chatham House. Although this figure falls to 2% for France and for many other countries which already have extensive infrastructure, it increases the footprint of the construction sector (responsible for 37% of the world’s CO2 emissions). Is it really reasonable, though, to hold concrete accountable for a large share of the harm being done to our planet?
It is not so much the choice of material which is causing the problem, but rather the act of (over)construction itself. In the face of a growing population and rapid urbanization, it would be difficult to build the future without concrete. Remember first of all that concrete is a structural material: there are very few other candidates that can outperform it in terms of strength, particularly when building very large structures, heavy industrial buildings and infrastructure of various types. There is no lack of tangible evidence of concrete’s extreme strength, not least over time. Many structures built from concrete have been standing for over a century, and some buildings constructed during the ancient Roman era, including the famous Pantheon, are over a thousand years old! In this respect, concrete is undeniably durable. “Concrete is also highly resistant to both fire and natural disasters,” explains Lisa Barnard, Sustainability and Customer Engagement Program Manager for CHRYSO and GCP. This is an important point at a time when these disasters are occurring more and more frequently.
A further advantage of concrete is its thermal inertia. While biobased materials heat up or cool down relatively quickly depending on the ambient temperature, concrete can withstand such fluctuations. Thanks to this thermal mass, it is a material that is capable of storing heat or coolness and releasing it gradually. In summer, concrete absorbs the cool air at night-time and releases it during the day, reducing the need for air conditioning, for example.
Construction chemicals: one solution to reduce carbon
The major criticism of concrete relates to the environmental footprint of the methods used to manufacture it. As a reminder, to make concrete, you need gravel, sand, cement and water. Since the gravel, sand and water do not undergo any processing, their carbon footprint is dependent solely on the means used to transport them. However, the cement – or more specifically one of its components, clinker – is obtained by mixing crushed limestone and clay heated to a very high temperature. It is this step which emits a lot of CO2 and is particularly energy-intensive. It is ultimately primarily responsible for concrete’s carbon footprint.
Research is therefore focused on reducing the amount of cement – or rather clinker – used in concrete. Several options for decarbonizing cement are available. The first is to try and reduce the energy consumption associated with the calcination of its raw materials, perhaps by refurbishing the industrial equipment or, more specifically, installing higher performance kilns. It is also possible to operate kilns using cleaner sources of energy, sometimes derived from biomass, as a partial replacement for fossil fuels.
CHRYSO and GCP have a few levers to minimize concrete’s CO2 footprint. First activators added to the cement that can reduce the quantity of clinker (for the same performance). A second lever is concrete admixture: “today, we can offer our clients admixtures for strong and ultra-low carbon concrete, which answer the global needs of sustainable construction” explains Frédéric Guimbal, Executive Vice President of the Construction Chemicals Business Unit at Saint-Gobain. Thanks to a dedicated admixture, CHRYSO enabled Hoffmann Green to produce a “low-carbon, 0% clinker cement” with a carbon footprint divided by five.
Why biobased materials can only supplement, not replace
Should we be looking to biobased materials to provide an alternative? Wood, along with straw, hemp and unbaked clay of course have a role to play in the mix of materials used in sustainable construction. Who could reasonably find fault with these natural, renewable and easily recyclable materials? And for some of these, at least, very little energy is required to produce them. Unbaked clay, for example, does not require any firing.
As is often the case, however, the perfect being the enemy of the good, using these options widely as a replacement for traditional materials is not desirable. In a world of rapid population growth and urbanization – don’t forget that by 2050, there will be 2 billion more people on Earth and 2.5 billion more city dwellers – it is vital to build quickly and (fairly) cheaply. Biobased materials, which are often more complex and take longer to manufacture, could risk considerably increasing construction times.
More seriously, a significant expansion of the use of wood for construction could have harmful consequences for biodiversity. A study published in the scientific journal Nature Communications in August 2022 by researchers from the Potsdam research institute showed that to cover the requirements of a transition to a city constructed primarily from wood, new planting producing a high output of wood would be needed, with an impact on land use – in terms of competition, particularly with farming – and on biodiversity, as natural ecosystems would be replaced. So these solutions, while undoubtedly useful as a supplement to concrete, cannot replace it in all instances. Finally, it is worth remembering that the carbon footprint of a material stems in part from its transportation. A biobased material from the opposite side of the world will have an enormous carbon footprint.
Being realistic, sustainable construction will be impossible without concrete, and that is good news. Two millennia of history have proven its qualities, and the progress made in engineering and construction chemicals, coupled with an awareness of the need to decarbonize the manufacture of this material, is paving the way for ever "greener" concrete.
Credits : Shutterstock/Vaidas Bucys ; Shutterstock/Peshkova