Cement and concrete are essential to the construction of nearly all roads and buildings in the modern world. The production of these materials is responsible for substantial emissions of carbon dioxide to the atmosphere, from two sources. First, limestone (CaCO3) is roasted, using fossil fuels, to remove the CO2 from the carbonate minerals, converting them to calcium oxide (CaO). So, the carbon dioxide emissions from cement include that from the limestone and from the fossil fuel used to roast it.
Dry cement or concrete consists of calcium oxide mixed with sand (cement) or aggregate (concrete) for shipping. On site, the dry material is mixed with water and poured into forms. Of course, the shipping of sand, aggregate, and calcium oxide all entail emissions of carbon dioxide to the atmosphere. With the demand for high quality sand reaching 10 billion tons per year, some eye the glacial outwash deposits on Greenland as an attractive new source. The transport of the ingredients in cement accounts for about 10% of the movement of all materials on Earth by humans each year. See https://blogs.nicholas.duke.
Something close to 76 billion tons of cement has been produced in the past 100 years, emitting more than 38 billion tons of carbon dioxide to the atmosphere. The emission of carbon dioxide from the production of cement current accounts for about 5% of the total annual industrial emissions of CO2.Most accounts of global carbon dioxide emissions include those from the roasting of limestone and the carbon dioxide emitted from the stone itself.
What is often overlooked is the uptake or sink of carbon dioxide in cement as it sets or ages, which can take years. The reaction of calcium oxide with water produces calcium hydroxide (CaOH2), which loses water and gains carbon dioxide as cement sets—converting back to CaCO3. (The entire reaction, involving silicon is more complex, but this summarizes the basics). One recent account suggests that 43% of the carbon dioxide initially released during the roasting of limestone can be reabsorbed from the atmosphere as cement ages—what is known as carbonation. This reduces the net emissions of carbon dioxide from cement production, but not from the fossil fuels used to transport it.
Various means are proposed for reducing the carbon dioxide during cement production, including incorporating fly ash and recycled concrete materials in the supply chain. We can also avoid plastering or painting the surface of concrete which slows the carbonation process and the thus the sink for carbon dioxide.
Overall, the production and use of cement results in net emissions of carbon dioxide to the atmosphere; the use of wood as a construction material ensures a long-term net sink for carbon dioxide in the structure.
References
Haff, P.K. 2010. Hillslopes, rivers, plows, and trucks: Mass transport on Earth’s surface by natural and technological processes. Earth Surface Processes and Landforms 35: 1157-1166.
Xi, F.M., et al. 2016. Substantial global carbon uptake by cement carbonation. Nature Geoscience doi: 10.1038/ngeo2840.
