Age of Fly Ash

[GenericName]

Hi,

I'm a final year student of civil engineering and am wanting to learn more about fly ash concretes.

I was wondering how long fly ash can be stored before the properties it provides begins to deteriorate?

I recently performed some experiments and got very poor strength results, I was wondering if this is perhaps due to the fly ash's age?

The fly ash has been stored within the lab for around 10 months or more, without being completely sealed off to air and moisture.

Cheers,
Luke

 

[Tmoose]

Have you done a chemical analysis of what you have?
Several years back I recall a conversation about what parameters were required for sale-able flyash for a coal fired power station in Wisconsin.
I'm drawing a blank as to what the chemical limit(s) and requirements were. // Might // have been carbon content ( Loss on Ignition = LOI )

Were the strength tests done not too long after the expected end of the early-stage strength stages?
"Increasing the amount of fly ash in concrete is not without shortcomings.
At high levels problems may be encountered with extended set times
and slow strength development, leading to low early-age strengths ....."

http://www.cement.org/docs/default-...48-optimizing-the-use-of-fly-ash-concrete.pdf

 

[concretemasonry]

I would have little faith in the results from fly ash stored in variable lab storage conditions for 10 months. - Was it type F or type C? - Moisture could be a major problem.

Fly ash is a waste product from burning coal and as such it is stored a minimum of time (storage facilities cost money), so it is sold for the best use and concrete is a good source of recurring costs and is somewhat predictable in term of volumes. To be sold it has to be stored in silos for conditions and loading out (truck or rail). It is not combined with another material chemically, it can deteriorate somewhat, but still usable of other purposes.

The remainder goes for uses that have different specs (size, chemistry, etc.) that can tolerate some degradation.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[GenericName]

"Have you done a chemical analysis of what you have?"

The analysis of the material was provided by the supplier/distributor -

http://www.flyashaustralia.com.au/_respub/_site/_img/content/PDS_Eraring Product Data Sheet.pdf

It is seemingly good with a LOI of 1.2%.

"Were the strength tests done not too long after the expected end of the early-stage strength stages?"

I performed tests at 1,3,7,28 and 56 days.I observed obvious errors and of course due to my naivety in a concrete lab there were a lot of them.

But I followed reviewed literature for mix designing and conformed to standards for testing and curing.
I guess I was just wondering if there was a rough "shelf life" within which fly ash should be used?

For further information the strengths were lesser for all testing days.
Also good article, as a student it is always nice to have a solid article to fall back to when wondering silly questions.

Edit: I didn't see the second reply, it is a Class F

 

[SlideRuleEra]

I've had a fair amount of first-hand experience with the "production" of fly ash at electric generating stations. The comments by Tmoose and concretemasonry are excellent.

When fly ash is collected by a coal fired plant's electrostatic precipitator it is very hot, very dry, and because it is microscopic spheres it acts exactly like a fluid. In fact, a plant's fly ash silos have an installed compressed air system (fluidizing air) to keep the fly ash "fluffed up" while it is in temporary storage. It needs to be sold and transported quickly in closed specially designed containers. Ideally, the utility will have a contract with a user to make daily pickup. Without fluidizing air, fly ash will "cake-up" and collect moisture... ruining it as a concrete additive, as you have discovered. Ten months in storage is way too long.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[GenericName]

Thanks a lot Tmoose, concretemasonry and SlideRuleEra,

Excellent to get these round about answers that help me visualize the processes of how the fly ash is produced,
always good to get a better understanding of the industry I will be entering.

 

[GenericName]

Sorry to double post,

SlideRuleEra, would you happen to have any literature that confirms what you have said?

It is always nice to have back up literature to confirm my comments and thoughts.

 

[SlideRuleEra]

GenericName - Literature? Sure. Manufacturers of ash (both coal fly ash & bottom ash) equipment in the USA include United Conveyor Corporation (UCC) and the Allen-Sherman-Hoff (ASH) division of Diamond Power. Here is a nice overview of fly ash transport and fluidizing air equipment from UCC:

http://unitedconveyor.com/uploadedFiles/Systems/M1295-115_Fly Ash Storage and Unloading Systems.pdf

Look around on the two manufacture's web pages, you may find other info of interest:

http://www.diamondpower.com/ProductsServices/AshMaterialHandling.aspx

http://unitedconveyor.com/

For general info on electrostatic precipitators, try this EPA document:

http://www.epa.state.oh.us/portals/27/engineer/eguides/electro.pdf

Much of what I told you was not learned by academic study, it's acquired by on-the-job-experience... you'll see that soon enough yourself. For example, early in my electric power career, I was responsible for initial start-up & operation of the fly ash handling system for a 500 MW coal fired generating station. We had just completed 4 years of design / construction. On start-up the concrete fly ash silo "leaked"; leaked BAD... operate the fluidizing air and ash goes EVERYWHERE. Plant start-up continues while silo concrete repairs are made. We had to use open dump trucks to temporarily haul the fly ash to a permitted onsite disposal area. Look down into a dump truck loaded with hot, dry ash, fresh from the operating precipitator, and see the fly ash "sloshing" around exactly like water. Add to that, some of this ash is becoming airborne "dust"... looks like a scene straight out of a horror movie, especially at night. That is not a sight that you forget.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net