Fly Ash in Concrete and Concrete Masonry Units
What can you tell me about Fly Ash and other industrial byproducts in concrete ?
I was at home depot picking up cinder blocks recently and was sort of startled by the quality control I saw. Anytime I find myself buying an unbranded product produced poorly to sell at miniscule margins, I start to worry that I might regret my purchase later. I had a vague memory that fly ash (industrial byproduct of burning coal) was used in concrete masonry units and I started googling to try to get more info. What I found:
1. Fly ash or other industrial furnace byproducts are used in cinder blocks and (much of the time) in concrete. The use of fly ash in concrete seems to have increased as environmental regulations have made it more expensive to dispose of fly ash.
2. The amount used and type seems to vary a lot depending on the end use of the concrete use but also whatever industrial byproduct happens to be cheaply available nearby.
3. Fly ash can contain a long list of toxic heavy metals. The specific content and concentrations depends on the type of coal involved and the particulars of the facility where it is burned.
4. Under certain circumstances, it is possible to toxic substances to leach out of the cured concrete.
What I was not able to find was any reliable information that would allow me to asses risk. I can’t find hard numbers on how much fly ash is in any particular concrete product, what the maximum heavy metal content is, min and max rates of leaching, case studies, etc. Given how common concrete products are, this is worrying.
Can anyone point me to reliable information ? Most of what I am seeing online is just generalizations [“been doing it for decades and haven’t been sued yet- must be OK”] with no science to back it up.
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Replies
User ...250,
I'll leave you to draw conclusions as to whether it's safe or not, but I don't think it's fair to characterize this as something that has just been left to generalizations. A quick search shows it's been widely studied:
https://iopscience.iop.org/article/10.1088/1757-899X/133/1/012036/pdf
https://www.greenbuildermedia.com/blog/the-truth-about-fly-ash
https://www.sciencedirect.com/science/article/abs/pii/S0045653519314195
https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NRMRL&dirEntryId=305270
There is also a *very* long history of using at least fly ash in various aggregates -- the practice goes back many decades. Fiber cement siding has fly ash in it too. It's doubtful there is any real health risk here, and even if something could leach out over time, where would it go that could cause you problems? These aren't materials that will go off into the air to get breathed in, and you're probably not running your drinking water through your concrete block wall, so you should be pretty safe regardless.
Bill
There are multiple grades of fly ash, class F and class C that have different properties. Their chemistries add properties to concrete that make it better - long term strength, ion penetration resistance, workability, w/cm ratio, etc. In the appropriate proportions to cement, after curing, it's all tied up in the binder that gets formed - CSH in concrete speak, or calcium silicate hydrate.
There's really no downside of adding it to concrete. Unfortunately, its' becoming more scarce and harder to obtain as the coal fired power plants shut down.