How to make low-carbon concrete from old cement

The world gets a little greyer every year. According to a paper published in 2014 concrete—an aggregate material made by mixing cement, sand and gravel—is the second-most consumed substance in the world after water. Around three tonnes of the stuff are poured each year for every person on the planet.All that building affects not just the surface of the Earth but its atmosphere as well. The annual production of around 5bn tonnes of cement, the essential ingredient in concrete, is responsible for around 8% of the world’s man-made emissions of carbon dioxide, the main greenhouse gas. Were the cement industry a country, it would be the third-biggest carbon-dioxide polluter after China and America.