Expert Advice on Green Buildings

Fly Ash: Safer than Portland Cement?

 
Question:

Kara asks: is fly ash brick a safe product? What are the benefits of using a brick product with fly ash as opposed to conventional bricks? I saw a 60 minutes episode that talked about fly ash being dangerous and toxic but then I saw your article. How can I tell if this is a problem? Thanks

Answer:

Hi Kara,

Thanks for your question.

Fly ash is a by-product of burning coal, and depending on the specific type of coal and process used for burning (i.e. power generation) can contain any number of known harmful compounds including potentially radioactive materials. The harmful effects of fly ash by itself can vary from “trace” amounts to significant quantities of known carcinogens. Fly ash and similar volcanic pozzolans have been used since Roman times in concrete with no known health concerns.

In recent years, the use of fly ash to replace Portland cement has been increasing as sustainability has grown. Using fly ash in bricks is a related step in the evolution of using replacement materials

One of the main benefits of fly ash is that it is a usable byproduct of a common process. Because coal is going to be burned for power generation for the foreseeable future, fly ash is going to be produced and needs to be disposed of or used for some other purpose.

Often fly ash is mixed into a slurry and stored in fly ash ponds where it can be exposed to weather. This manner of storage creates the potential for runoff that can potentially migrate to the environment where it can be cause for concern.

As mentioned earlier, fly ash is going to be produced as a result coal burning and needs to be disposed of or used elsewhere. A study sponsored by the National Science Foundation (NSF) stated that approximately 2/3 of fly ash generated is not used for other purposes and is stored in fly ash ponds (“Test of Mercury Vapor from Flyash Bricks” by Henry Liu, Shankha Banergi, William Burkett and Melissa Shinn, 2007). This study was conducted to determine if mercury vapor was emitted by fly ash brick, but provides some other useful information as well. The results of the test actually show that not only do fly ash bricks not emit mercury vapor, they actually absorb mercury (and quite well).

The study indicates that the EPA classifies fly ash as a non-hazardous material, even though the constituents of fly ash may be considered harmful.

I see this as no different than any other number of products used as building materials. Many materials contain compounds that when considered individually may be harmful, but when combined and manufactured into a building material are considered non-hazardous. The study further goes on to state that once the fly ash bricks are made, the fly ash is solidified and becomes inert.

Fly ash has been used for a long time as a replacement for Portland cement in concrete. The production of one ton of Portland cement produces approximately one ton of CO2. Because fly ash is already produced as result of coal burning, no additional CO2 is generated in producing the fly ash. Although it’s important to keep in mind that producing one ton of fly ash (through coal burning) produces about 20 tons of CO2. So if one can keep in mind that the fly ash is going to be produced anyways, then the additional CO2 can be disregarded. If fly ash needs to be produced specifically for use in building product, then the additional CO2 generated while producing the fly ash would also need to be considered. This would obviously make fly ash a non-sustainable product from a CO2 point of view, but unlike clay fired bricks, fly ash bricks require no additional fossil fuel burning in their production. They are a pozzolan and cementitiously bond the brick materials together. Because the process used to make fly ash bricks is different than clay fired bricks, evaluating the performance characteristics of them is important when specifying them for use on a project.

A follow-up study conducted by the same researchers as the previous study (with the addition of Jesse VonEngelenhoven, titled “Environmental Properties of Flyash Bricks”, 2009) shows that emissions of other harmful substances such as radon is less with fly ash bricks than concrete bricks.

This study also shows that fly ash bricks also store CO2 from the environment, which can be seen as an added sustainable benefit. One important experiment considered in this study, was the effect of chemical leaching from fly ash bricks as the result of rain. This test showed that there is indeed an increase in chemicals, although the levels never exceeded the EPA’s drinking water requirements. Because there is no standard test for evaluating leaching of chemicals in bricks from rain, the researchers devised their own test, by submerging the bricks in water for a period of days. As the researchers state, even though there were chemicals and metals that leached from the fly ash bricks, some of these were due to the reaction of the bricks to the acidic rain, while the test itself wasn’t a true test of bricks exposed to rain. The test submitted the bricks to constant exposure to water (by submerging) for a period of days. Typical rain would wash down the face of the brick and it would never be totally immersed in water.

A search of the internet shows various scientific studies that have been conducted on the health effects of fly ash brick. There is some discussion of the harmful compounds found in fly ash, but none of the studies that I found on fly ash bricks stated any immediate health concerns. The one problem I see with these studies is that they test whole bricks, and not bricks as they’re going to be used in a building, where they’re going to be cut, shaped and modified.

Hopefully in the next few years as the use of fly ash bricks increases, these types of studies will become available providing the A/E/C community a true evaluation of any potential health impacts arising from fly ash bricks.

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