Concrete and CO2

By Claude Goguen, P.E., LEED AP

Carbon dioxide, also known by its chemical formula CO2, is a colorless, tasteless gas that comes from many sources – including your own lungs. You will exhale 0.0043 oz (0.12 g) in the time it takes you to get to the end of this sentence (unless you just got back from jogging, in which case it may be much more.)

This carbon dioxide from our lungs and many other natural sources exists naturally in the atmosphere. Along with other gases, CO2 helps sustain life on our planet by retaining the sun’s heat. But unnatural CO2 is also being generated by many sources, mostly from burning fossil fuels. This and other sources of CO2 are generally attributed to trapping heat and raising temperatures, thus contributing to what’s called the “greenhouse effect.” Many scientists believe this greenhouse effect causes global warming that will cause a rise in sea levels and increase the intensity of extreme weather.

Regardless of where you stand in your opinion of global warming, excess CO2 in the atmosphere is not a good thing, and it just makes sense to reduce it where we can.

The main human activity that emits unnatural CO2 is the aforementioned combustion of fossil fuels (coal, natural gas and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO2. The main sources of CO2 emissions in the United States as described by the U.S. Environmental Protection Agency (EPA) are:

1. Generation of electricity – 40% of CO2 emissions
2. Transportation – 31% of CO2 emissions
3. Industry – 14% of CO2 emissions

The manufacture of precast concrete structures falls in the Industry category. Although what you do at the plant every day, including manufacturing and shipping, contributes very little, it’s upstream from the precast plant where we see the biggest culprit: cement.

Cement facts

Carbon dioxide emissions from a cement plant are divided into two source categories: combustion and calcination. Combustion accounts for approximately 40% and calcination 60% of the total CO2 emissions from a cement manufacturing facility. The combustion-generated CO2 emissions are related to fuel use. The CO2 emissions due to calcination are formed when the raw materials (mostly limestone and clay) are heated to more than 2,500 F and CO2 is liberated from the decomposed limestone.

In the United States, cement manufacturing accounts for a little less than 1.5% of CO2 emissions attributable to human activities. Worldwide, cement manufacturing accounts for approximately 5% of CO2 emissions. In the United States and elsewhere, the industry strives to further reduce that contribution.

The cement industry has made progress toward reducing energy associated with cement manufacturing and associated emissions. Since 1972, the cement industry has improved energy efficiencies by 40%. According to the U.S. Department of Energy, U.S. cement production accounts for only 0.33% of U.S. energy consumption.

According to the World Business Council for Sustainable Development (WBCSD), throughout the 1990s, global cement production increased approximately 20% while cement industry CO2 emissions decreased by approximately 1.5%. Emissions vary across worldwide regions from 0.73 to 0.99 lb of CO2 per lb of cement.

What is the cement industry doing?

The high temperatures needed for cement manufacturing make it an energy-intensive process, as with the production of many building materials.

CO2 Output – Click to Expand

Research has led to the use of industrial byproducts in the cement manufacturing process. Pound for pound, scrap tires contain about 33% more energy than coal, and the United States generates approximately 290 millions of them each year. In 2005, about 58 million tires were consumed as fuel in cement kilns, reducing fossil fuel consumption and removing them from the waste stream.

Another recent progress involves newly introduced cement guidelines that will allow for greater use of unburned ground limestone as a component in finished cement, which will ultimately reduce calcination CO2 by more than 2.5 million tons per year.

Today, the cement industry CO2 accounts for less than 3% of U.S. industrial CO2 emissions, well below other sources such as the petroleum industry (21.8%), chemical industry (22.2%), and iron and steel mills (9%).
By 2020, the industry aims to reduce CO2 emissions by 10% from the 1990 baseline levels. To achieve this goal, the cement industry has adopted a three-part strategy:

1. Improve energy efficiency by upgrading plants with state-of-the-art equipment
2. Improve product formulation to reduce manufacturing energy consumption and minimize the use of natural resources
3. Conduct research and develop new applications for cement and concrete that improve energy efficiency and durability

Putting CO2 emissions into perspective

The manufacture of cement produces about 0.9 lbs of CO2 for every pound of cement. Since cement is only a fraction of the constituents in concrete, manufacturing a cubic yard of concrete (about 3,900 lbs) is responsible for emitting about 400 lbs of CO2. The release of 400 lbs of CO2 is about equivalent to:

• The CO2 associated with using 16 gallons of gas in a vehicle
• The CO2 associated with using a home computer for a year
• The CO2 associated with using a microwave oven in a home for a year
• The CO2 saved each year by replacing nine light bulbs in an average house with compact fluorescent light bulbs

Other sources responsible for CO2 emissions include:

• 28,400 lbs for an average U.S. house in a year
• 26,500 lbs for two family vehicles in the United States in a year
• 880,000 lbs for a 747 passenger jet traveling from New York to London

The reason concrete is responsible for 1.5 to 2% of the U.S. CO2 (due to humans) is the vast quantities of concrete used in the world around us. Concrete is the most widely used material on earth apart from water, with nearly 3 tons used annually for each man, woman and child.

Concrete reabsorbs CO2

During the life of a concrete structure, concrete can reabsorb CO2. This is done through a process known as carbonation, a chemical process where atmospheric CO2 reacts with the Calcium Oxides (CaO) in the concrete to form calcium carbonate (CaCO3). While the calcination process of cement manufacturing releases CO2, some of it is reabsorbed during the life cycle of the concrete through carbonation.

Conclusion

Implementing a sustainable manufacturing culture at your plant should include an expectation of your upstream suppliers to do the same. The cement industry as a whole is definitely working hard to reduce emissions. Ask your cement supplier what it is doing as a company. This information will hopefully provide you with the knowledge to proudly tout the inherent sustainable attributes of your product, and the continuing progress toward decreased impacts on the environment.

Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.

Resources

1. Michael A. Nisbet, Medgar L. Marceau, and Martha G. VanGeem, “Environmental Life Cycle Inventory of Portland Cement Concrete”, PCA R&D Serial No. 2137a, a report on Concrete: Sustainability and Life Cycle, PCA CD033, 2003, http://www.cement.org/ 
2. Richard Conniff, “Counting Carbons,” Discover, August 2005, http://www.discover.com/issues/aug-05/features/counting-carbons/
3. Knut O. Kjellsen, Maria Guimaraes and Åsa Nilsson, “The CO2 Balance of Concrete in a Life Cycle Perspective” 19 December 2005, ISBN: 87-7756-758-7, http://www.nordicinnovation.org/Global/_Publications/Reports/2005/CO2%20uptake%20during%20the%20concrete%20lifecycle.pdf 
4. http://www.concretethinker.com/technicalbrief/Concrete-Cement-CO2.aspx 
5. http://www.cement.org/smreport11/index.htm