For every 20 tonnes of carbon dioxide that enter the atmosphere due to human activity, one comes from the kilns of cement factories.
It is a paradox – concrete is arguably the most sustainable building material known to man, yet the sheer quantity produced worldwide puts it among the top contributors of human CO2 emissions.
Low-carbon concretes, such as the one being developed at the Ecole Polytechnique Federale de Lausanne (EPFL) in Lausanne, Switzerland, could help to bring down these emissions.
The demand for concrete in fast-growing countries such as the UAE is already enormous, but is set to be dwarfed by that from emerging economies such as India and Brazil in the coming decades.
The sooner we can make concrete with less emissions, the better.
The basic recipe for concrete has hardly changed in millennia. Sand or gravel is mixed with a glue-like binder, composed of cement and water, to form the grey artificial material that we are now all too familiar with.
The Romans made cement by grinding volcanic ashes and lime. Today’s variant, Portland cement, is made by heating limestone in a kiln.
This step is responsible for the lion’s share of emissions – each tonne of cement produced emits roughly the same amount of CO2.
But for a lot of building purposes, nothing beats cement. So how can it be made without the environmental damage?
Over the past decade, people have got better at making concrete that contains a portion of “cement-like” materials, and thereby have shrunk concrete’s carbon footprint significantly.
But it is proving difficult to go any further using today’s most common substitutes.
Fly ash, for instance, is produced in coal-fired power plants. But would it really make sense to burn more coal to produce greener concrete?
And countries such as Switzerland and Brazil that use little or no coal to generate electricity need other options.
Other materials share similar drawbacks. They are limited geographically, or the concrete made with them simply is not as good as conventional concrete.
Seeking a viable substitute using materials that are both abundant and affordable, we have developed a formulation made by mixing heated clay and ground limestone.
It is good enough to replace just under half the cement used to make concrete without making the concrete second-rate.
We ran a full-scale test in collaboration with a factory in Cuba and a pilot test in India, deliberately trying to use low-grade raw materials in sub-optimal production conditions.
Despite that, the outcome was nothing short of a miracle – a cement good enough to be used in the same way as ordinary Portland cement, but with a 30 per cent smaller carbon footprint – largely because heating clay emits less CO2 than limestone.
And because it uses low-grade, locally available raw materials, its price stays competitive.
The biggest challenge now is winning over the building industry. It took centuries for concrete to achieve the high, reliable quality that makes it ubiquitous in our cities today.
In contrast, we do not have the luxury of centuries to fine-tune our formulation. But thankfully, technology, know-how, and a certain sense of urgency are on our side.
Karen Scrivener is a professor of construction materials at the Ecole Polytechnique Federale de Lausanne
