It is well known that plants absorb carbon dioxide by photosynthesis and therefore forests act as a global sink for carbon dioxide. Far less known is that the concrete in the built environment, in our cities and infrastructure, also absorbs carbon dioxide.
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 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. This process, called carbonation, 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 (re)carbonates. This phenomenon represents around 23% of the annual calcination emissions from cement consumed in the year.
The carbonation can be enhanced by injecting CO2 into recycled aggregates or during the curing process of concrete. The CO2 captured is mineralised in the aggregates or concrete in a permanent way.
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 carbonation. The absorption of CO2 turns the built environment into carbon sinks and this natural carbonation effect has been recognised in the Working Group I contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).
The science of carbonation is well established and already included in standards for calculating the carbon footprint of concrete buildings, structure and products. 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 calculation is required – the amount of carbon dioxide absorbed by carbonation for all the concrete in the reporting country in the reporting year.
CEMBUREAU believes that the enhanced carbonation should be included as a carbon removal activity within the framework of the proposed Carbon Removals Certification proposal of the European Commission.