Cement Industry Emissions
Anita Brown
Cement industry accounts for 5-7% of anthropogenic carbon dioxide emissions.[1] Cement is the main ingredient in concrete that gives the concrete its binding properties. Concrete is second to water in total volumes consumed by society.[2] The quantities of greenhouse gases emitted are a result of the large number of construction projects occurring, not necessarily the construction practices.[3] More than 5 billion tons of concrete are poured per year worldwide.[4] Concrete demand will continue to increase as countries develop and construction continues. Because concrete will continue to be the main building material used and cement will continue to be produced, the question becomes, how to effectively reduce the CO2 emissions in the cement industry. Before a proper recommendation can be made, a thorough examination needs to be made on the life cycle of concrete.
Cement Production
Cement is a powder made from calcareous deposits.[5] These raw materials provide calcium carbonate (CaCO3). The release of CO2 is a result of a process caused calcination, when the limestone is decomposed into lime. Calcination is a thermal treatment process in the absence of air or oxygen to cause thermal decomposition.[6] In this process, calcium carbonate (limestone) is decomposed into calcium oxide (CaO, lime) and carbon dioxide. The process occurs in a kiln by burning fossil fuels at temperatures of 1,450°C.[7] The decomposition of limestone emits 60-65% of the total emissions (the rest is generated by fuel combustion, which is mostly comes from heating the kiln). However, other materials, such as slag, fly ash, and limestone can be blended in to the product to decrease the amount of product that must go through the calcination process.
EPA Study
In 2009, the USEPA did a study on the potential for reducing greenhouse gas emissions in the construction sector. Most of their approaches were related to improving fuel efficiency and recycling material.[9] By recycling material approximately 1,400,000 metric tons of CO2e emissions can be avoided and the recycled material can be used as aggregates. The purpose of using aggregates in concrete is so that a smaller volume of cement would be needed in the mix. By reducing the amount of cement needed, less would be produced, and less carbon dioxide would be released as a result of cement production. However, there is a limit to the amount of aggregates that can be added before the strength of the concrete is affected.
Geopolymer Cement
Another possible solution to mitigate the carbon footprint of the cement industry is the use of geopolymer cements. Geopolymer cements are a material that combines aluminum silicate with an alkali chemical activator. Aluminum silicates can be found in industrial byproducts such as fly ash and blast furnace slag. Fly ash is a byproduct of coal combustion, and is mostly put in landfills where it is susceptible to leaching and spills. Using fly ash in cement would promote fly ash recycling and provide a way to reduce the demand for cement created through calcination. As mentioned previously, fly ash can also be used in the cement making process to decrease the amount of material that needs to go through the calcination process and produce carbon dioxide. Solely geopolymer cements have not been applied to large scale projects because of strength restrictions.
Cement Lifecycle
By studying the carbon footprint of concrete throughout its life, from the production of cement to the recycling of the material, it will be possible to see the true impact of the construction industry on the anthropogenic emissions. The impact of the lifecycle of cement is being studied by the Cement Sustainability Initiative (CSI).[13] Concrete structures can endure for centuries, with limited cost for maintenance and repair, and can be recycled into aggregates. Concrete also slowly absorbs CO2 from the air in a process called carbon sequestration or carbonation.[14] The amount of carbon dioxide sequestered in concrete is a significant percentage of the amount released during cement calcination.[15] Although cement productions is responsible for a significant percentage of carbon emissions within industries it is important to take this process into account.
Conclusion
In comparison to other building materials, the emissions from producing cement are relatively low. However, with the large volume of concrete being produced and in demand the total emissions from the cement industry are high. The best way to approach this problem would be the continued recycling of concrete and the use of substitutes within the cement, such as fly ash, that will decrease the total amount of cement that will need to be produced. Furthermore, the continued studying of the carbon footprint of concrete will allow for a better understanding of the carbonation of the concrete and how much carbon dioxide is being captured.
[1] USEPA, Potential for Reducing Greenhouse Gas Emissions in the Construction Sector. February 2009 (accessed February 2015); available from http://www.epa.gov/sectors/pdf/construction-sector-report.pdf
[2] Ibid.
[3] Ibid.
[4] http://urbaneden.uncc.edu/house/architecture/materials/geopolymer
[5] USEPA, Potential for Reducing Greenhouse Gas Emissions in the Construction Sector. February 2009 (accessed February 2015); available from http://www.epa.gov/sectors/pdf/construction-sector-report.pdf
[6] “Calcination”, Wikipedia. Last modified January 22, 2015. (accessed February 2015); available from http://en.wikipedia.org/wiki/Calcination
[7] USEPA, Potential for Reducing Greenhouse Gas Emissions in the Construction Sector. February 2009 (accessed February 2015); available from http://www.epa.gov/sectors/pdf/construction-sector-report.pdf
[8] “Calcination”, Wikipedia. Last modified January 22, 2015. (accessed February 2015); available from http://en.wikipedia.org/wiki/Calcination
[9] Geopolymer Cement Concrete. University of North Carolina Charlotte (accessed February 2015); available from http://urbaneden.uncc.edu/house/architecture/materials/geopolymer
[10] Ibid.
[11] Ibid.
[12] USEPA, Potential for Reducing Greenhouse Gas Emissions in the Construction Sector. February 2009 (accessed February 2015); available from http://www.epa.gov/sectors/pdf/construction-sector-report.pdf
[13] Ibid.
[14] Ibid.
[15] Haselbach, Liv, “Carbon Dioxide and Global Climate Change”, Portland Cement Association. (accessed February 2015); available from http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-design-production/concrete-as-a-carbon-sink
[16] Ibid.