We would like to use cinder blocks with recycled content, but have been warned that flyash rust will bleed through.

Asked by Tim
Asheville, NC

We are building a new home in Asheville, NC. The foundation will be concrete masonry unit, or CMU. One provider includes recycled content. The competitor claims that the fly ash or cinders contain iron, which will rust and bleed through the stucco finish. Is that likely to be true?


Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

June 9, 2011

Your question was of interest to me as it highlights some of the misconceptions about fly ash.

The short answer to your question is no, but the long answer brings up some points that should be of interest to all.

With the United States producing more than 136 million tons of fly ash each year, and only 43 percent of this recycled (according to Wikipedia), it is in everyone’s interest that you use the CMU with the recycled content and that you present the CMACN paper (referenced below) to the building department with your plans and see if you can grout the CMU with fly ash as well! 

Fly ash has become controversial

Fly ash is a byproduct of the combustion of coal to make electricity. Prior to environmental regulations on the exhaust from power plants, the fine fly ash would rise with the flue gases and be expelled into the atmosphere.

Fly ash is now captured by pollution equipment and is combined with the residue left at the bottom of the furnace to make up what is known as coal ash.

Coal in its natural state contains heavy metals such as mercury and lead, along with other hazardous materials that vary depending on the coal bed.

  • These elements are not transformed into energy, leaving a concentration of them in the coal ash.
  • Fly ash when alone would be considered a hazardous waste given the concentration of hazardous materials.

Currently, coal ash is either buried in landfills or dammed in surface storage ponds. The EPA has identified several cases where coal ash leaking at dump sites has contaminated nearby ground or surface water.

This has become a controversial topic since the environmental disaster of December 2008, when a coal ash storage pond at the Tennessee Valley Authority’s Kingston Fossil Plant ruptured, spilling 5.4 million cubic yards of ash in slurry form across 300 acres adjacent to Emory and Clinch Rivers.

The event has prompted the EPA to reconsider the regulations on the disposal of coal ash as a hazardous material under the Resource Conservation and Recovery Act.

Refuting the rust allegation

After researching fly ash, my first thought was that perhaps since fly ash contains mercury, lead, and other heavy metals, this may have prompted the rust rumor you mention.

However, these heavy metals are not water-soluble, so rusting would not be possible.

Another fact that further refutes the rust issue is that in manufacturing standard gray concrete, iron, in the form of calcium ferrite, is added to the Portland cement as a way of reducing the high temperatures of the furnace when drying out the concrete.

  • So there is already iron in concrete, and there is no problem of rust.
  • Also, fly ash has been used in concrete for many years with no issue of rust either.

Why companies do or don't include fly ash in CMU blocks

I spoke to a technical person from a block company who had never heard of any rust problem with the use of fly ash in block.

He elaborated on the process of making block, explaining why some companies offer fly ash in their block and others do not -- which very likely is a cost issue. When block is manufactured, the process is automated.

  • Silos hold the various components making up the block, which are delivered to the mix by conveyors.
  • The cost to add a silo to store the fly ash and the associated conveying equipment is very high, and a company can easily opt not to do it.
  • In cases where a great quantity of block is being purchased requiring fly ash, they simply hand-measure the flyash into the concrete mix, but do not offer it as a standard.

Also, as the fly ash comes from coal, it depends somewhat on how burnt the coal is as to how black the fly ash may appear. For companies that want to make a perfectly uniform colored block this might be another reason not to include the fly ash.

Expanding the use of fly ash to masonry grout

An interesting aside is that the CMACN (Concrete Masonry Association of California and Nevada) has done a study on reducing the amount of Portland cement used for masonry grout by substituting up to 80% (by weight) Class F fly ash and ground granulated blast furnace slag.  

They report that “these grouts are a viable alternative for concrete masonry construction, with significant sustainable and economic benefits.”


For more information:

Read "The town building department doesn't want us to use concrete mixed with flyash. Do you have any information on contamination in flyash?" a Q&A answered by Anthony Addesso.

Tagged In: recycled content

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