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Technical Brief  > Green in Practice 102 - Concrete, Cement, and CO2
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What’s the Bottom Line?

  • Besides water, concrete is the most commonly used material on earth.
     
     
  • Cement production is responsible for about 1% of the greenhouse gases emitted in the US.
     
     
  • More than half (60%) of the CO2 emitted during cement production is due to calcination, a chemical reaction from heating limestone.
     
     
  • Concrete reabsorbs most of the CO2 emitted by calcination during the time it serves its useful life, and shortly after it is demolished and crushed.
     
     
Concrete components: cement, water, fine aggregate (sand), and coarse aggregate (gravel or crushed stone) (PCA No. 55361)
What is the Difference between
Cement and Concrete?

Although the two words “cement” and “concrete” are used interchangeably in common parlance, portland cement is actually one of the ingredients in concrete. Cement is a fine gray powder and constitutes only 7 to 15% by weight of concrete’s total mass. The water used to make concrete causes the hydration of the cement, to form a hardened mass called paste. When the paste (cement and water) is added to aggregates (sand and gravel, crushed stone, or other granular material) it acts as an adhesive and binds the aggregates together to form concrete which sets to a solid, durable mass.

The compressive strength of concrete is chiefly a function of the water-to-cement ratio of the mix. Higher compressive strengths are achieved by increasing the amount of cement and reducing the water content of the mix, while at the same time ensuring that the mix has adequate fluidity.

What is the Difference Between the
Greenhouse Effect, Global Warming and Climate Change?

Global warming is generally defined as an increase in the average temperature of the earth's atmosphere, especially a sustained increase sufficient to cause climatic change.

Climate change is generally defined as any long-term significant change in the weather patterns. Climate change can be natural or caused by changes people have made to the land or atmosphere.

“Greenhouse effect” is used to describe a scenario of how various gases cause global warming or climate change. Carbon dioxide (CO2) and other gases exist naturally in the atmosphere. These gases retain the sun’s heat and create the atmosphere that sustains life on earth. Burning fossil fuels — natural gas, gasoline, coal, and oil — adds unnatural amounts of CO2 and other gases into the air. These have the potential to trap heat, raise air temperatures, and change the balance of life on earth. These gases, in the form of pollution (emissions to air), have increased 30% in the past century.

The primary source of CO2 emissions is fossil fuel power plants, which in the US, contribute to 35% of all CO2 emissions. Cars, sport-utility vehicles and other light trucks account for another 20%. Energy efficient buildings and vehicles worldwide can have a significant affect on climate change.

Does only CO2 Affect Climate Change?
Although carbon dioxide produced by burning oil and coal is often singled out as the contributor to climate change, a number of other emissions to air (pollutants) as a result of human activities contribute to global warming. They include: methane (agriculture and burning natural gas), ground level ozone (car exhaust and power plants), water vapor (naturally occurring), nitrous oxide (fertilizer use and a pollutant) and chlorofluorocarbons (refrigerants and aerosol). Pound for pound, these other emissions to air have a much greater effect on global warming than CO2, as shown in the table.
In terms of global warming potential, one pound of methane is 21 times more potent than one pound of CO2, and one pound of N2O is 310 times more potent than one pound of CO2. Similarly, the listed refrigerants are highly potent.


Global Warming Potentials (GWP) and Atmospheric Lifetimes (Years)
Gas
Atmospheric Lifetime
GWPa
Carbon Dioxide (CO2)
50 - 200
1
Methane (CH4)†
12±3
21
Nitrous Oxide (N2O)
120
310
HFC-23
264
11,700
HFC-32
5.6
650
HFC-125 32.6
2,800
HFC-134a 14.6
1,300
HFC-143a 48.3
3,800
HFC-152a 1.5
140
HFC-227ca 36.5
2,900
HFC-236fa 209
6,300
HFC-4310mcc 17.1
1,300
CF4
50,000
6,500
C2F6
10,000
9,200
C4F10
2,600
7,000
C6F14
3,200
7,400
SF6
3,200
23,900
† The methane GWP includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The indirect effect due to the production of CO2 is not included.
Source: U.S. EPA www.epa.gov/nonco2/econ-inv/table.html (Global warming, Non-CO2 gases economic analysis and inventory, 2006).

What is the Truth behind CO2 and the Cement Industry?
According to the World Business Council for Sustainable Development (WBCSD www.WBCSD.org), “Concrete is the most widely used material on earth apart from water, with nearly three tons used annually for each man, woman, and child.”
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 over 2500°F and CO2 is liberated from the decomposed limestone. As concrete ages, it carbonates and reabsorbs the CO2 released during calcination. Calcination is a necessary key to cement production. Therefore, the focus of reductions in CO2 emissions during cement manufacturing is on energy use.

Figure 1. Global CO2 production. (Note that in the U.S., cement accounts for 1.5 to 2%). Source: The Cement Sustainability Initiative Progress Report, June 2005. www.WBCSD.org
In the US, cement manufacturing accounts for approximately 1.5 to 2% of CO2 emissions attributable to human activities. Worldwide, cement manufacturing accounts for approximately 5% of CO2 emissions. When all greenhouse gas emissions generated by human activities are considered, the cement industry is responsible for approximately 3% of global emissions. Using the same ratio of CO2 emissions to greenhouses gases in the U.S., 1% of the greenhouse gases are attributed to cement manufacturing. In the US and elsewhere, the industry strives to further reduce that contribution.

China produces 37% of the world’s cement, followed by India with 6% and the U.S. with 5%. Most facilities in China rely on inefficient and outdated technologies; these plants contribute to 6 to 8% of the CO2 emissions in China.
The cement industry has made progress towards reducing energy associated with cement manufacturing and associated emissions. Since 1972, the cement industry has improved energy efficiencies by 33%. 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 WBCSD, over the decade of the 1990s, global cement production increased approximately 20% while unit-based cement industry CO2 emissions decreased by approximately 1.5%. Unit-based emissions vary across worldwide regions from 0.73 to 0.99 lb of CO2 per lb of cement.
 

Putting CO2 emissions into perspective

The manufacture of cement produces about 0.9 pounds 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 3900 lbs) is responsible for emitting about 400 lbs of CO2.[1] The release of 400 lbs of CO2 is about equivalent to[2]:

  • 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 9 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 U.S. 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. anthropogentic CO2 (that is, due to humans) is due to the vast quantities of concrete used in the world around us.


 


Concrete Reabsorbs CO2
During the life of a concrete structure, the concrete carbonates and absorbs the CO2 released by calcination during the cement manufacturing process. Once concrete has returned to fine particles, full carbonation occurs, and all the CO2 released by calcination is reabsorbed. A recent study indicates that in countries with the most favorable recycling practices, it is realistic to assume that approximately 86% of the concrete is carbonated after 100 years. During this time, the concrete will absorb approximately 57% of the CO2 emitted during the original calcination. About 50% of the CO2 is absorbed within a short time after concrete is crushed during recycling operations. (Nordic Innovation Centre Project 03018).

How Does Life Cycle affect Climate Change?
Concrete is a locally produced material shipped only short distances – another environmental and energy saving plus. Its primary components, sand and gravel or crushed stone, are among the most universally available materials. Accordingly, as wood and steel become scarce materials, developing nations are relying more on concrete.

Concrete can last a lifetime or longer: unlike wood, it does not rot or burn; unlike steel, it does not rust. Concrete’s low maintenance needs and long service life requires less repairs and rebuilding, and, as a result, conserves additional energy and materials.

When comparing construction alternatives, a life cycle assessment (LCA) provides a level playing field. An LCA is based on a consistent methodology applied across all products and at all stages of their production, transport, energy use, maintenance, and disposal or recycling at end of life. A number of published articles espouse the sustainability of one building product over another based on a few selected metrics instead of a full life cycle assessment (LCA). For instance, some articles representing themselves as LCA studies use only the metrics of embodied energy or embodied CO2 emissions. These comparisons are flawed because they only consider limited metrics and do not cover a full life cycle assessment of the product or building. A full LCA includes the impacts of energy use and associated emissions over the life of the product or structure, such as climate change, acidification, materials acquisition, and human health effects.

Studies show that the most significant environmental impacts are not from construction products but from the production and use of natural gas and electricity to heat, cool, and operate the buildings. For concrete houses compared to wood frame houses, the CO2 emissions from the production of the cement used in the house is more than offset by the savings in CO2 emissions from energy savings during the life of the house.

References

[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/

 
For additional resource links and reports, see the reference library links provided under "Sustainability Solutions>Stewardship"