It is well known that plants absorb carbon dioxide by photosynthesis and therefore forests act as a global sink for carbon dioxide. Far less well known is that the concrete in our built environment, in our cities and infrastructure, also absorbs carbon dioxide. Unlike forests, which release their carbon dioxide into the atmosphere when felled, concrete stores carbon dioxide permanently.
Cement, the material which binds concrete, is made by heating limestone to very high temperatures up to 1450°C. This breaks down the limestone into calcium oxide, the key ingredient of cement, and carbon dioxide. This reaction is called calcination. Concrete is the most widely used manmade material, made by mixing aggregates (crushed rocks) and sand with cement and water. After the concrete has been produced, the calcination reaction naturally reverses. The concrete starts to reabsorb carbon dioxide from the atmosphere, mineralizing the concrete and enhancing its stone-like properties. This process, called recarbonation, occurs in all concrete structures - buildings, pavements, tunnels, dams, bridges - throughout their life. Mortar, which is made by mixing sand with cement and water, also recarbonates. A recent international review showed that, as a first approximation, this is around 23% of the annual calcination emissions from cement consumed in the year.
If we want to know the true carbon footprint of the cement industry, we need to remember that cement is turned into concrete buildings and structures, cities and infrastructure, which permanently capture carbon dioxide by the natural process of recarbonation.
The science of recarbonation is well established and already included in standards for calculating the carbon footprint of concrete buildings, structure and products, although not yet in Kyoto Protocol National Greenhouse Gas Inventory Reports. For National Inventory Reporting of carbon dioxide emissions and removals to the United Nations Framework Convention on Climate Change (UNFCCC) under the Kyoto Protocol, a different, more complex, calculation is required – the amount of carbon dioxide absorbed by recarbonation for all the concrete in the reporting country in the reporting year.
Recent studies have shown that it is possible to make accurate estimations of the amount of CO2 absorbed by the building stock for a given country or region. These figures show that the production of cement has a lower CO2 footprint that commonly assumed.