Fly Ash vs Slag in Liquid Containing Structures

[JakeDean]

Hello,

I had a question regarding code requirements for the usage of Fly Ash (Class F) versus Ground Granulated Blast Furnace Slag (Grade 100) in liquid containing structures containing wastewater. I have reviewed ACI 350, ACI 211.1, and ACI 318 and have been unable to find any indication that would completely restrict the use of GGBF Slag in this particular application. Our client though desires only to use Class F Fly Ash due to past experience with cracks forming from concrete mixtures containing slag, so I am searching for any possible code restrictions that will support our client’s request.

At the same time, I am also looking to learn anything I can regarding the advantages and disadvantages of using Fly Ash versus GGBF Slag in the wastewater industry. Any information or assistance you could provide would be extremely beneficial. Thanks,

 

[Gumpmaster]

Well, if you haven't heard, there is somewhat of a national fly-ash shortage (which does vary greatly by region) due to natural gas prices and other variables. There is virtually zero fly ash available in the northwest right now. Your only choice is to use slag.

Check out this document from the slag cement association. Link

As far as I know, there's no large detriment to using slag over fly ash. I have a couple of examples where slag was switched out for fly ash one for one and the shrinkage was the same. The data was possibly cherry picked by the batch plant, but it is a data point nonetheless. Ensure you have a shrinkage requirement in your specification and hold the mix design to that. As long as the mix design meets your shrinkage requirement you shouldn't see any undue cracking over fly ash. There are of course many other things that affect cracking in hydraulic structures so you need to make sure those are OK as well.

 

[JedClampett]

We do predominately water and wastewater work. Our master specifications allow, but don't require fly ash. We're in the southwest, so we just don't get that many requests for slag, but if there's a shortage, maybe that's changing.
I consider fly ash in the proper proportions a miracle drug. It's cheap, it increases the chemical resistance of concrete and makes it more workable and less harsh (whatever that means). It reduces the rate of strength gain, but that's almost never an issue with us. Our concrete mixes are plenty strong.
If you're looking for a reason to refuse slag, check the heavy metal content. You can make an issue of that, but I think it's bogus, especially in wastewater.