Sustainability, Fly Ash Concrete

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Building Better Blocks

First things first: cement is not the same thing as concrete. Here’s the difference: cement is a substance that reacts quickly with water to form a bonding agent that holds other ingredients in place. Various forms of cement have been in use for thousands of years. The most common type of cement in use today is known as Portland cement, a mixture of limestone, clay minerals, and gypsum, heated and pulverized. Portland cement is used in grout, mortar, plaster, stucco, and concrete.

Concrete is a mixture of cement and water with fine and coarse materials, such as gravel and sand. This kind of concrete is used in exterior and interior walls, footings and floors—just about everywhere people build. In fact, more concrete is used around the world
each year than any other human-made building material. Concrete depends on natural minerals for most of its substance, plus one manufactured product—cement.

As a manufactured product, cement was first made in 1824. It is not expensive or difficult to make—but another material is proving to be still more economical. Fly ash, a by-product of combustion in a coal-burning power plant, has the same properties as cement when
used in concrete. It’s often less expensive, and its use helps keep waste products out of landfills. One other benefit is that fly ash is ready to use, which saves energy and effort compared to making cement. This reduces manufacturing waste, too.

Trash into Treasure

To help reduce airborne pollution, coal-burning power plants use equipment that removes ash from the plant’s exhaust gases. This ash was most often added to water and diverted into “ash ponds” that cost money to create and maintain. The process of creating and disposing of fly ash was only an expense for energy producers, with no apparent benefit.

Construction industry specialists looked at the properties of fly ash and found that it could be used in place of cement in many kinds of concrete. In order to fulfill the requirements of professional builders, any substitute would have to meet standards set by ASTM International, originally known as the American Society for Testing and Materials. Fly ash does.

The money earned through the sale of fly ash for use in concrete contributes to the income of a power plant that collects and sells it. What it saves in disposal costs is even greater. What had been only a waste material has become a valuable resource in two ways. Power
plants that direct fly ash toward construction use can reduce their landfill needs by 80 percent or more.

Properties of Fly Ash

Technically speaking, fly ash acts as a pozzolan—a siliceous, or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. That’s how ASTM International defines it. The word “pozzolan” comes from Pozzuoli, Italy, where volcanic ash was first used in Roman times as cement in concrete.

Most environment-friendly building materials are fairly new, and their performance over time is still unknown. If you compare fly ash to volcanic ash used in concrete, though, some examples built more than 2000 years ago are still standing: structures in Cosa, Italy, that
have withstood millennia of direct exposure to seawater; and the Pantheon in Rome, Italy, a pozzolan-and-lime concrete structure with a cast concrete dome 124 feet in diameter.

Benefits of Using Fly Ash

The noncombustible part of coal is left over in the form of ash with unique characteristics. In addition to its ability to react with water like Portland cement, fly ash particles are smaller than grains of cement, and are spherical, like microscopic ball bearings. These two physical features make fly ash better than cement in several ways:

• Fly ash has a lower unit weight than cement, so it contributes roughly 30 percent more volume of cementitious material per pound used; this helps the concrete flow better while it’s wet.
• The smaller particles of fly ash fill in gaps within the wet concrete mixture better than cement, making the final product more consistent and therefore stronger.
• This same characteristic means concrete can be made with slightly less water and sand, coating the larger aggregates more evenly and creating a more workable material.
• While concrete made with fly ash does not equal the strength of cement-based concrete for about a month, it continues to gain strength after that, becoming more durable long after cement-based concrete has reached its maximum strength.
• Concrete made with fly ash contains less lime, and converts some of the lime present into calcium silicate hydrate; this replaces one of the weakest ingredients in concrete with one of the strongest.
• Lightweight concrete made with fly ash is easier to pump, and its smooth consistency reduces unwanted air pockets; the finished surface is more even where it is used with forms.
• Fly ash makes concrete less permeable, which improves protection against corrosion; it reduces the intrusion of water, oxygen, and chemicals, protecting steel reinforcement from corrosion and expansion.
• When cement reacts with water, it generates heat very quickly,
  which helps the concrete harden and gain strength; in some situations, rapid heat gain may lead to thermal cracking and loss of strength. Fly ash produces far less heat when it reacts with water, helping reduce the damaging effects of thermal cracking.

Considering all the practical benefits of using fly ash concrete, the environmental advantages may not seem as important—but they are substantial. According to industry estimates, for each ton of fly ash used instead of cement, we save as much landfill space as an average American uses in 15 months, reduce CO2 emissions by as much as an average car produces in 2 months, and save enough electricity to power an average household for 24 days.

For the past decade in the United States, the use of fly ash in concrete has increased by more than 50 percent. At current rates, more than 12 million tons of coal fly ash are used in concrete products each year. Multiply that by the figures above, and you can see why.

One more thing: concrete itself is recyclable. Anywhere from 45 to 80 percent of crushed concrete can be used as aggregate in new construction.

Who Uses Fly Ash Concrete

Concrete made with fly ash qualifies for LEED credits from the U.S. Green Building Council. The U.S. Environmental Protection Agency requires that fly ash must be allowed on federally funded projects, and promotes the use of fly ash through its Coal Combustion Products Partnership (C2P2) program. Its use is endorsed by the U.S. Department of Energy; the U.S. Army Corps of Engineers requires fly ash concrete in most projects; and the U.S. Bureau of Reclamation has used fly ash extensively on dam projects. And all 50 states either allow or require the use of fly ash concrete in state-funded projects.