The Use of Environmentally Friendly Technology in the Concrete Industry
CEMENT AND CO2 EMISSIONS
Portland cement, an essential component of concrete, is the glue that binds concrete together. In recent decades however, the production of cement has been designated as a major greenhouse gas pollutant. Worldwide, the production of Portland cement alone accounts for 6-8% of human-generated carbon dioxide (CO2), the greenhouse gas most attributed as the source of global warming.
The CO2 associated with Portland cement manufacture falls into 3 categories:
- CO2 derived from de-carbonation of limestone,
- CO2 from kiln fuel combustion, and
- CO2 produced by vehicles in cement plants and distribution.
As a result of these measures, the general rule is that the creation of one ton of cement releases one ton carbon dioxide into the atmosphere.
To combat this problem, there are a number of methods and technologies now available from U.S. Concrete across the United States that provide a multitude of environmentally friendly solutions (called EF Technologies). Among these is the use of alternate or supplementary cementitious materials (SCMs) such as fly ash and slag.
Fly ash (also known as a coal combustion product or CCP) is the mineral residue resulting from the combustion of powdered coal in electric generating plants. Fly ash consists mostly of silicon dioxide, aluminum oxide and iron oxide. It is pozzolanic in nature, meaning it reacts with calcium hydroxide and alkali to form cementitious compounds.
Most power plants are required by law to reduce their fly ash emissions to less than 1 percent. The remaining 99% is collected using electrostatic precipitators or filter bags. Initially, this collected ash was disposed of in ash ponds or landfills. Once its pozzolanic properties were discovered however, it became useful as a replacement of Portland cement in concrete.
Fly ash can replace up to 50% by mass of Portland cement. Generally less expensive than Portland cement, it also presents advantages in a host of applications. Fly ash can be used to improve workability and pumpability of concrete. Due to its generally slower rate of hydration, fly ash also lowers the heat of hydration and is important in mass concrete structures, such as large foundations, bridges, and piers. High fly ash concrete shows less bleeding and shrinkage than straight cement mixes. Fly ash is also used as a component in the production of flowable fill, which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill.
More and more fly ash is being used beneficially as a recycled material, although it is still far from fully utilized as more than 65% of fly ash produced from coal power stations is disposed in landfills. This amounts to approximately 7 million tons (Mt) disposed of annually in Australia, 40 Mt in the United States and hundreds of megatons in India and China. Though it is costly to retrofit older coal-burning power plants to filter fly ash from the atmosphere, over time the economic incentive of selling the captured fly ash pays for the initial expense of installation.
Slag is the by-product of smelting ore to purify metals. In nature, the ores of metals are found in impure states, often oxidized and mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed as slag.
Through determining its many uses, it was found that ground granulated slag reacts with water to produce cementitious properties. It could therefore be used in concrete in combination with Portland cement as part of blended cement. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability properties. Since the unit volume of Portland cement will also be reduced, concrete is less vulnerable to alkali-silica and sulfate attack.
As with fly ash, processing blast furnace slag into slag cement or slag aggregate eases the burden on our environment in a number of ways. It reduces the air emissions at the blast furnace as well as the material in landfills. Most significantly, slag decreases Portland cement usage by as much as 50 percent, thereby diminishing CO2 emissions, the amount of energy required to produce concrete, and the quantity of virgin land extraction through mining raw materials for Portland cement.
Typical mix designs for structural or paving concrete normally use substitution rates between 25 and 50 percent; high-performance and mass concrete applications can use substitution rates up to 80 percent. As stated earlier, approximately one ton of CO2 is released for every ton of Portland cement produced. Figure 2 illustrates the benefits of substituting 50 percent slag cement in various concrete mixtures. Between 165 and 374 pounds of CO2 are saved per cubic yard of concrete by using a 50 percent slag cement substitution, a 42 to 46 percent reduction in greenhouse gas emissions.
Slag cement requires nearly 90 percent less energy to produce than an equivalent amount of Portland cement. Reducing the use of Portland cement in concrete by substituting a portion of it with slag reduces the embodied energy in a cubic yard of concrete by 30 to 48 percent.
Reduced Material Extraction
Raw materials for Portland cement are gathered through mining operations. A ton of Portland cement actually requires about 1.6 tons of raw materials. Substituting 50 percent slag cement can save between 281 and 640 pounds of virgin material per cubic yard of concrete.
Engineers and architects are specifying concrete mixes with fly ash, slag cement and other supplementary cementitious materials (SCMs) to not only increase the benefits for structures, but also to qualify for points toward LEED credits. Leadership in Energy and Environmental Design (LEED) is a system developed by the United States Green Building Council to rate a building's environmental performance. This system has become the principal method by which buildings can achieve green building certification.
The primary area of credit for SCMs is through Recycled Content, though use of specific proportions of SCMs can also aid in earning a credit for Innovation in Design and Sustainable Site.
High-Fly Ash Content Concrete (concrete containing 30% or higher fly ash) can qualify for the following credits:
- Materials & Resources Credit 4.1, 4.2, 5.1, and 5.2
- Sustainable Site Credit 3
- Innovation & Design Credit 1, 2
- Sustainable Site credit for reduction of heat islands.
- Materials & Resources Credit for building reuse: Slag cement makes concrete structures more durable.
- Materials & Resources Credit for recycled content.
- Materials & Resources Credit for use of local/regional materials: Slag cement can be considered a local material in many areas.
In the U.S., the use of fly ash and slag on federal-aid projects is encouraged by their classification as a "recovered" product under the federal Resource Conservation and Recovery Act, which generally mandates use of fly ash in cement or concrete in construction projects using $10,000 or more of federal funds. For more information on the EPA's Comprehensive Procurement Guidelines, a key component of the federal government's "Buy Recycled" program, visit the EPA's Comprehensive Procurement Guidelines (CPG) website.
U.S. CONCRETE'S EF TECHNOLOGY INITIATIVE
It's your project. It's our world.
U.S. Concrete recognizes the inherent value in aggressively supporting Green Building initiatives. We are working to promote buildings and projects that are environmentally responsible, cost effective and healthy places to live and work. Our concrete helps decrease greenhouse gases. With the use of alternative cementitious materials such as fly ash and slag, U.S. Concrete prevented 328,100 TONS of CO2 from polluting our atmosphere last year alone.
What can our environmentally friendly technology do for you?
For more information, please see the links below and/or contact Wally Johnson at 713-499-6229 or David Perry at 408-404-1073.
EF Technology Brochure (PDF)
EF Technology Press Release (PDF)
EF Technology Fact Sheet (PDF)
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