Can someone give me the downside of fly ash?

I've googled it and I think I see the benifits. Is it hard to find?
(I'm in northeast New England) Expensive? Why isn't it more
commonly used?

Thanks.

If you buy concrete regularly, you may already have been using it, as some RM suppliers keep their "formulas" to themselves. LOL
Ash is fine so long as it is kept within the guidelines. And a benefit is that it is a recycled by-product of another industry.
There have been some major lawsuits due to improper batching with fly ash, and resulting total replacements.
For commercial works there are standard testing procedures, which identify problem mixes early.
Most residential works are untested, so it is usually some time before problems become evident.
Sampling costs are cheap insurance.

Stay away from fly Ash, a pure concrete mix, with no additives, is truely the best way to go. We cannot us a release agent on our forms, so anything at all, that increases the "stickyness" of the mix, hurts the consistancy of any concrete placed. High percentage (10%+ usually depending on the type) fly ash can easlily turn your concrete into a "baby poop" constency in 15 minutes, which is not only difficult to place, however almost impossible to consolodate effectively, and sticks to everything.

Mark Ross

Last year we had a full truckload worth of concrete that would not set up by the next day. or the next day. or the next. Turns out it was batched improperly, it was all flyash and no cement at all in the mix. The 18 inch thick by 4 ft. footing (with 7,8 bar) had to be torn out and replaced.
Engineer said it would eventually set up to full strength but none of us wanted to wait that long.
Kevin

Well we all have different opinions, and as much as I respect Kevin's word, mine would differ. Kevin may have had a bad experience with fly but we love it. We have one batch plant in particular that uses it in our ICF mix and if you where to look back at allot of my post you would catch that we rave about it. This mix pours like none that we have ever poured anywhere in Michigan, and breaks out a 5200 p.s.I range. We find that it flows excellent with the proper air entrainment and takes minimal vibration techniques.

Dave

Bobke,

I would have to say I agree with some of what everyone has to say.

Portland Cement, when looked at microscopically is a flat or flake shaped particle which has excellent surface area to absorb the water you add to your concrete mix. Portland Cement also has three basic components to produce it but the formula or the exact ingredients used can vary greatly but all it has to do is meet the criteria and testing set forth by the ACI and it is considered cement. Fly Ash, which has cementitious properties, has a round or ball like shape which has poor water absorption properties. Fly Ash is a by product that is generally waste cleaned out of the coal generation plants and varies greatly in consistency and formulation and there really isn't any standard for Fly Ash at this time. Most noticeably Fly Ash can vary your air entrainment in your concrete wildly. This is due to its carbon content which varies due to one fact that the coal that is burnt in the power plants varies slightly from batch to batch as well as others.

Fly Ash can replace cement in your mixes from none to over 75% replacement. In some areas of the country, due to cement shortages and a high surplus of Fly Ash you will see more usage of Fly Ash more predominant and most people and contractors don't even know. Fly Ash will typically retard the curing of the concrete, so instead of a 28 day strength test you will have a 56 day test. Finishing of flat work produces its own subset of issues.

On the positive side, Fly Ash with its round particle works as a ball bearing in the mix and helps flow dramatically. It is used as a cement replacement, which is green.

I would have to say Mark Ross may have run into some mixes that the Fly Ash affected the air entrainment detrimentally. This can cause flow issues. Because of the inconsistency of Fly Ash I usually opt for Slag which has many of the same properties of Fly Ash but it is consistant.

ICF Contractor