Concrete is usually a good indicator of civilisation. As an inexpensive material, it has enabled cities to be built quickly and easily. Concrete, however, also has a huge carbon footprint, releasing massive amounts of carbon dioxide into the atmosphere during its production.
As we make moves to declare a "climate emergency", how will we balance our city's need for growth with its need to reduce emissions?
The origins of concrete can be traced through history from about 6000BC in the Middle East to today's tallest skyscrapers. It has become such an important part of our lives that after water, concrete is the most widely used substance on Earth.
As a material, concrete has created the foundations for modern life. Our buildings and roads all rely on concrete, and as our population grows we are moving from landscapes of blue and green to those of black and grey.
The essential ingredient in concrete is cement, a fine powder that acts as the glue that holds concrete together when mixed with water and sand.
To put our demand for cement in perspective, the total amount of plastic produced on the planet since it was invented has been just more than eight billion tonnes, the same amount of cement is produced every two years. In 2016 alone, world cement production generated about 2.2 billion tonnes of carbon dioxide, more than 90 per cent of which came from the process of making clinker — small grey balls which are ground down to form powdered cement.
To compare this to other well-known polluters, making cement is responsible for about 8 per cent of the world's carbon dioxide emissions. This is much more than common climate enemy aviation fuel, which sits at 2.5 per cent, and only just behind the global agriculture sector, which is at 12 per cent.
The process of making cement is simple. Finely ground limestone, clay and sand are superheated and mixed together in a rotating kiln at temperatures as high as 1450C. This heated mixture is then allowed to cool, which results in small grey balls of mostly calcium oxide or clinker and the release of carbon dioxide gas.
While fossil fuels burned to create the electricity and heat are responsible for about half of the CO2 emissions, the other half comes from this clinker production gas release.
With mounting pressure to meet the requirements on the Paris Agreement on climate change, annual emissions from cement will need to fall by at least 16 per cent by 2030. This means that current concrete manufacturers are under pressure and new opportunities are being created for start-ups to produce carbon-neutral or even carbon-negative sustainable materials using more eco-friendly building designs.
Current research is attacking the problem from different angles, with some attempting to create concrete without cement by using steel slag or fly ash, both of which are industrial waste products. Carbon upcycling is another strategy. It uses CO2 emissions captured from industrial activities to help concrete set much more quickly, which reduces the vast quantities of water needed while absorbing carbon dioxide at the same time. Bacteria have been brought in to grow bio-concrete blocks in sand moulds using a similar process to how coral grows in nature. Even nanoparticles made from carrot roots are being tested and shown to make some concrete mixtures twice as strong, which could help to reduce the amount needed overall.
While the perfect solution has still not been found, the building industry should probably be pushed as much as our agriculture industry has been to think up new ways to cope with our increasing need for city infrastructure while still reducing our overall global emissions.