Concrete 411: Replacing Portland Cement with Fly Ash

Concrete is a great and green building material with the substitution of Portland cement for fly ash.


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With applications ranging from floors, walls, and siding to kitchen countertops, shower stalls, and landscaping stones, concrete has become one of the most widely used materials in home building. Builders like it because it’s cheap and easy to work with. Homeowners are attracted to its durability (cracks and chips are easy to patch) and its good looks (it’s possible to conjure up a variety of finishes from polished to stained to acid-etched). Concrete is also one of the most inert materials available, which means no outgassing or leaching once it’s inside a house. All that said, it takes just one look into concrete’s traditional ingredients to bring its many benefits into question.

In its most basic form, hardened concrete is made up of sand, crushed stone or gravel, water, and a binding agent known as cement. During Roman times, cement was made by grinding limestone together with volcanic rock; the Pantheon in Rome was made entirely of concrete mixed with this volcanic cement. Today, by far the most commonly used type of cement is portland cement, which was patented in 1824 by English brickmason Joseph Aspdin and named for the grayish limestone found on the Isle of Portland. It’s made by heating limestone, chalk, aluminum, iron ore, and clay to 2,700 degrees Fahrenheit, a temperature at which chemical reactions occur within the mixture, fusing it into grayish-colored, marble-size pellets known as clinker. The clinker is then mixed with gypsum and ground into a superfine gray powder that’s mixed into concrete.

This sounds like a relatively harmless process until you consider that for every ton of clinker, approximately a ton of carbon dioxide (CO2)—the most damaging of the greenhouse gases—is released into the atmosphere. At last tally, the Environmental Protection Agency (EPA) estimated the cement-making industry accounts for 4 percent of all U.S. industrial CO2 emissions.

It’s no wonder, then, that green builders and green building programs have started looking into alternatives to portland cement. Blast furnace slag from steel-producing plants, crumb rubber from recycled tires, and rice hull ash from burned agricultural waste are several alternatives that are being effectively used to supplant some of the portland cement needed in concrete. The supplement that’s getting the most attention and the most use, however, is coal fly ash.

What is fly ash?

Fly ash, as it’s known in the industry, is collected from smoke stack emissions at coal-burning power plants. According to the EPA, power plants in this country generated 71.2 million tons of fly ash in 2001, 25 million tons of which was reused as a portland cement substitute. “For years, fly ash was just buried,” says Marc Richmond, a project manager at the City of Austin Green Building Program. “Now we can recycle and reuse some of it and keep it out of the landfills.”

It’s important to understand that fly ash can supplant only a percentage of the portland cement used in concrete. Most concrete mixes made with fly ash replace 20 to 25 percent of the portland cement, although some can go as high as 40 percent. That’s because properties in the ash prevent the concrete from gaining strength as quickly. “When you put fly ash into concrete, silica from the ash reacts with the calcium hydroxide in the cement to form additional cementing compounds, which slow the strength-gain process,” says Gary Shelton of San Antonio’s Boral Materials Technologies, a fly ash distributor. “If you don’t mind waiting longer, fly ash concrete becomes as strong, or stronger, than conventional concrete.”

Fly ash concrete works even better than concrete made with portland cement, says Richmond. That’s because a grain of fly ash dust is smaller and rounder than a grain of portland cement, meaning less porosity, better waterproof qualities, and stronger bonds once water and aggregate are added. Fly ash is also cheaper than portland cement—by about 60 percent, says Shelton.

Some questions about fly ash

The use of fly ash inside homes does have some architects concerned about indoor air quality. “What is the fly ash contributing in terms of chemical emissions into the space?” asks Anthony Bernheim, a U.S. Green Building Council LEED-accredited professional and principal architect at the San Francisco-based firm SMWM. “Nobody has done any testing.”

While that may be true, the EPA did issue a report in 1983 stating “it is unlikely that fly ash used in concrete would exhibit leaching characteristics.” The report, Cement and Concrete Containing Fly Ash: Guideline for Federal Procurement, points out that while fly ash does contain certain heavy metals that would be toxic in higher quantities, the actual amounts are at “trace levels.” Moreover, because fly ash used in concrete hardens into such a dense, impermeable mass, water and other “leaching mediums” can’t seep through.

Radium and its decay product, radon, are also of concern to some people. Fly ash contains miniscule amounts of these radioactive elements that occur naturally in soils and coal deposits, leaving the possibility that gamma radiation could outgas from the concrete. Others believe that radium isn’t an issue, citing another EPA report that says the amounts found in fly ash aren’t too much different from amounts found in portland cement concrete or even some soils.

Finding fly ash concrete

Unlike some of the green materials struggling to get a toehold in the building market, fly ash concrete has garnered enough demand to warrant widespread use. Home­owners can request that their concrete contractor add fly ash into their ready-mix batch for driveways, sidewalks, or concrete floors. “There’s an established network of fly ash distribution out there,” says Richmond. “It’s widely used in most parts of the country, especially in western states. You can also ask for fly ash at higher than usual levels for certain projects.”

Even some of the world’s largest portland cement manufacturers such as Lafarge Cement and Holcim have started incorporating fly ash into their products. “I think they’ve recognized that the technology is sound and that there’s a solid business there,” says Shelton. “Because we believe fly ash cement is going to be a part of concrete making in the future, we want to be the ones to supply it.”