Questions about using Fly Ash in Concrete
with replies by Thomas J. Elpel
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Is fly ash toxic?
I was reading your article and noticed you made a "concrete" countertop (with sand, gravel, cement, dye, fly ash, and some cement, plus acrylic bonding agent). How did you determine that the fly ash you used was non-toxic?
Wikipedia states that: Fly ash contains trace concentrations of heavy metals and other substances that are known to be detrimental to health in sufficient quantities. Potentially toxic trace elements in coal include arsenic, beryllium, cadmium, barium, chromium, copper, lead, mercury, molybdenum, nickel, radium, selenium, thorium, uranium, vanadium, and zinc. Approximately 10 percent of the mass of coals burned in the United States consists of unburnable mineral material that becomes ash, so the concentration of most trace elements in coal ash is approximately 10 times the concentration in the original coal."
Thanks for your time.
Yes, fly ash may contain traces of heavy metals. It is difficult to know how much for sure.
Actually, I find your Wikipedia quote very encouraging for two reasons: 1) because it indicates that the quantity of heavy metals in the fly ash remains in "trace quantities," and 2) the concentration is only about ten times what it was in the original coal.
People used to burn coal in furnaces and some wood stoves, then shovel out the ashes and toss them outside someplace. That is not far off from what we used in the countertops.
My only concern in working with the fly ash is inhaling the dust from the dry powder. I wear a mask for that. Otherwise, we bind the material back into rock by making it into a countertop. We also use it when making floor tiles.
Keep in mind that just about any rock you pick up has trace quantities of heavy metals in it. Rocks are continually eroding bit by bit, releasing those toxins into the environment. It isn't a problem until the rocks are pulverised, releasing the heavy metals all at once. Locally, we have problems with acid mine drainage from pulverising rock for gold ore. The iron pyrites in the ore are normally stable, but when powdered, the sulfur in the pyrite is exposed to air and water, forming sulfuric acid, which leaches into the groundwater. As with fly ash, one of the ideal (but not always practical) ways to lock that material up is to turn it back into rock.
Thomas J. Elpel
Is fly ash radioactive?
Let me start by saying I love your site and just purchased your book which I await eagerly. I just wanted to comment on the use of fly ash in some of your things, countertops, papercrete bricks etc. It was my understanding that fly ash was highly radio active and not something that you would want to bring into an enviro home. What is your understanding of this?
Thanks for writing, and thanks for the good question! That was a new one to me.
There is a good article in Scientific American (December 13, 2007), describing the radioactivity of fly ash. According to the article,
"The waste produced by coal plants is actually more radioactive than that generated by their nuclear counterparts. In fact, the fly ash emitted by a power plant-a by-product from burning coal for electricity-carries into the surrounding environment 100 times more radiation than a nuclear power plant producing the same amount of energy."
Continuing, "At issue is coal's content of uranium and thorium, both radioactive elements. They occur in such trace amounts in natural, or "whole," coal that they aren't a problem. But when coal is burned into fly ash, uranium and thorium are concentrated at up to 10 times their
While all of that sounds pretty alarming, it is largely in the presentation of the numbers, which are then qualified later in the article, "Individuals living near coal-fired installations are exposed
to a maximum of 1.9 millirems of fly ash radiation yearly. To put these numbers in perspective, the average person encounters 360 millirems of annual "background radiation" from natural and man-made sources, including substances in Earth's crust, cosmic rays, residue from nuclear tests and smoke detectors."
Continuing, "For the average person the by-product accounts for a miniscule amount of background radiation, probably less than 0.1percent of total background radiation exposure. According to USGS
calculations, buying a house in a stack shadow-in this case within 0.6
mile [one kilometer] of a coal plant-increases the annual amount of
radiation you're exposed to by a maximum of 5 percent. But that's
still less than the radiation encountered in normal yearly exposure to
More, "In most areas, the ash contains less uranium than some common rocks."
And more, "So why does coal waste appear so radioactive? It's a matter of comparison: The chances of experiencing adverse health effects from radiation are slim for both nuclear and coal-fired power plants-they're just somewhat higher for the coal ones. "You're talking
about one chance in a billion for nuclear power plants," Christensen
says. "And it's one in 10 million to one in a hundred million for coal
So, the bottom line is that yes, fly ash is radioactive, and about 10 times more concentrated than it was in the original coal. It is a similar concern as with the heavy metal content. There are heavy metals in coal, rocks, dirt, and just about anything else. Burning the coal concentrates the heavy metals and radioactivity, making the fly ash about 10 times more dangerous than common dirt.
We do need to stop using coal, due especially to the carbon emissions. In the meantime, we have massive amounts of fly ash waste from coal, which can either blow around in the wind, be landfilled as waste, or mixed into concrete to make something useful. Substituting fly ash for
cement actually reduces the amount of coal that must be burned in the
first place. Using fly ash in concrete can also help to safely bind those heavy metals - and probably the radioactive constituents as well - into a stable form. Just as they were bound up in rock to begin with, so shall they be bound up in rock again, in the form of concrete.
Nevertheless, if you should decide to work with fly ash, I would recommend using a good quality dust mask. Fly ash is a very fine dust until it is incorporated into concrete. The greatest danger would be in inhaling heavy metals and radioactive particles with the dust. As with the radon gas we breathe in on a daily basis, the radioactive particles can damage living tissues if they decay while in your lungs or body.
I hope this helps!
Thomas J. Elpel