Out of Date Flyash?

[BigH]

For portland cement, there are definite out of date provisions in most specs and in ASTM C150 under Section 11 "Rejection". For flyash used in concrete or roller compacted concrete - are there similar out of date disclaimers? I would tend to think not since flyash is a 'natural' material in a sense. I've asked this as I don't have the ASTM Flyash spec as per C150.
Thanks.

 

[SlideRuleEra]

BigH - I had no idea that fly ash for concrete could be "too old". I worked on the design and construction of several electric utility fly ash collection and recovery projects. Fly ash can have too much unburned carbon for use in concrete, refereed to as "LOI" (loss on ignition) - but that was normally the biggest problem in meeting the specification.

The age issue "might?" have something to do with either how dry it is or how compact. When fly ash is still hot, straight from an electrostatic precipitator, it has virtually no moisture content - it absorbs moisture, becoming an unworkable, "cakey" mess.

"Fresh" fly ash also has the "look" and properties of a liquid. It is normally shipped in "tanker" trucks. But, during equipment failure, I have seen the "fresh" product in normal dump truck - quite a site to see - a hot, dusty, dark, very dry solid that is literally "sloshing" around in the truck bed, just like it was water. It settles, over time, and looses the "liquid" properties. In fact, fly ash storage silos have a compressed air system (called fluidizing air) to keep the product flowable.

The fly ash used in concrete is not really a natural material. The pulverized coal (fineness of face powder) that is burned in utility boilers allows remarkably efficient combustion, creating the microscopic spheres. Then the electrostatic collection system captures the finest these. What you wind up with would not occur in nature.

Maybe you can find an accurate explanation in the scores of papers here

http://www.flyash.info/

http://www.SlideRuleEra.net

 

[BigH]

Thanks SRE - I had/have doubts too - but we are getting ready to do some very long term trial mixes on roller compacted concrete and I want to be sure - according to ASTM C618-03 under heading 'rejection' - purchaser can require flyash that has been stored more than 6 months prior to shipment to be retested and if fails in fineness requirements can reject. I think that to minimize any chances of someone coming back on us later, I will require a fresh shipment of flyash - better safe in the trials than not.