Cement—or concrete, with the addition of water and gravel or sand—is the most widely used building material in the world, an often-critical component of buildings, bridges, pavements and subways. To make cement, limestone and clay are heated together at temperatures of more than 1000ºC, releasing carbon dioxide through a process known as calcination.
The authors of an international study, published in Nature Geoscience, found that 76.2 billion tonnes of cement was created globally between 1930 and 2013, a process which emitted 38.2 billion tonnes of carbon. A large part of this was burning fossil fuels to generate the necessary heat. Today, carbon dioxide emissions from cement production contribute up to five per cent of emissions’ global total.
While it’s already understood that cement and concrete reabsorb some carbon, it wasn’t known how much. After looking at the myriad ways cement and concrete are used, the study estimated 4.5 billion tonnes has been reabsorbed over the 83-year period. If the burning of fossil fuels is excluded, cement will reabsorb 43 per cent of the carbon dioxide emitted when it was formed.
The authors conclude that, while cement production is still harmful to the climate, sustainable building practice will benefit more by reducing fossil-fuel usage in the cement-making process than by reducing cement production itself.