Alternatives to flyash 1

[Structuralconcrete]

Recently in my area we have been having issues getting ready mix concrete. At first they said it had to do with a cement shortage more recently it has come up that the plants are experiencing issues with their flyash deliveries. Keeping in mind that flyash will not be around forever due to the decreased production of coal. Has anyone found a suitable replacement for flyash and what are the cons and pros of using said replacement compared to flash.

 

[LittleWheels]

There are lots of alternatives to fly ash, all of them idiosyncratic. Using silica fume, GGBS, limestone, pozzolanics as cement replacements depends heavily on what is locally available and has reliable quality/ effects. These cement replacements can increase or decrease strength gain rate, be susceptible to pumping or curing issues, react differently to admixtures, alter the mix's environmental resistance and so on. It is very helpful if concrete suppliers and contractors are familiar with batching and placing your intended cement replacement mix, otherwise they might be spending a lot of time learning that aspect of their trade on your coin.

 

[Structuralconcrete]

This is something I'm looking into personally. I've read a couple research reports that have been uploaded on the internet. All of them pretty insightful for the most part. One that has piqued my curiosity is CO2 injection into concrete. Have you had any experience with this? And with the use of CO2 is it better to use it in conjunction with another cementitious admixture?

 

[ACtrafficengr]

We received promotional material recently about finely ground waste glass (amorphous silica) as a pozzolan. It looks promising, but we haven't really researched in depth yet.

https://pozzotive.com/

https://www.concrete.org/publications/getarticle.aspx?m=icap&pubID=51729296

(pdf)

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[LittleWheels]

My experience doesn't include CO2 injection (looks pretty cool though).

There is potential for reactivity using some glass types, so some care is needed. It doesn't take too much contamination for problems to start, unless other aspects of the mix design compensates for it.

I am generally a fan of triple blend concrete mixes using multiple cement replacements in a single mix design. You can balance the various advantages and disadvantages to optimise the mix performance, though there are lots of ways of getting it wrong.

 

[Structuralconcrete]

I not familiar with triple blend mixes. Nor have I heard of anyone using triple blends. Can you provide an example of a triple blend and the pros and cons of using such mix design?

 

[LittleWheels]

Triple blend is a generic term for mixes using (most commonly) some combination of cement, fly ash or GGBS and silica fume (micro silica). Cement, fly ash and silica fume is probably the most common. Silica fume tends to increase high early strength, which can mitigate reduced strength gain from GGBS or fly ash. You don't need too much silica fume (often 5-10%) to get a noticeable benefit but you need an electric arc furnace making ferrosilicon alloys to get it.

Depending on the triple blend mix proportions, you can significantly alter the concrete's appearance, bleed/ finishing/ curing characteristics, pumpability, permeability, resistance to ASR, sulphate resistance and 56 day strength. Getting the right balance of concrete characteristics for a specific project (and the contractor's expertise) can be tricky and there aren't really 'standard' mix proportions.

Most of the triple blend mix designs I have been involved with over the years are high strength and use lots of superplasticiser but that isn't always the case.

https://www.devb.gov.hk/filemanager..._silica_fume_blended_with_GGBS_or_Fly_Ash.pdf

is a good basic presentation that I had nothing to do with and I can't comment with regards to the mix designs used by various projects but it might give you some ideas.

 

[BridgeSmith]

You don't need too much silica fume (often 5-10%) to get a noticeable benefit but you need an electric arc furnace making ferrosilicon alloys to get it.

Unfortunately, the silica fume also isn't as 'pure' as it used to be, and contains higher and more variable levels of unburned carbon, which apparently is still causing difficulties in getting the right amount of entrained air. We have had to look for other alternatives for our concrete bridge deck overlays, because nobody wanted to batch the stuff. It's gotten a little better in the last couple years, but it still costs nearly twice what it did 10 years ago.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

BridgeSmith,

Have you looked into Metakaolin? This admixture performs similar to Silica Fume (but typically requires double dosage) to achieve similar permeability results. The benefit is that it doesn't have the issues that silica fume has in terms of making the mix sticky and unworkable. The other issue is whether or not your local ready mix company has any experience with it.

 

[BridgeSmith]

I'll look into that, STrctPono. Thanks.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

BridgeSmith,

Have you guys developed an in-house proprietary deck overlay mix? If so, I'm curious to hear some of the admixtures? Latex? Was the silica fume added for lower permeability?

 

[BridgeSmith]

Under our current specs, the contractor can use either a silica fume modified or a latex modified mix. Our special provision is attached. I'm not sure I'd know what all of it means.

We used the silica fume for its ability to self-consolidate, bond to hardened concrete without a bonding agent, and for its high strength and low permeability. We use a minimum 1" thickness, but 1.25" is a more typical thickness.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

Thanks for the spec. It's always interesting to see how other places do certain things.

 

[JedClampett]

I'm a big fan of flyash. But saying that, the main motivation for the batch plants using it is price. As fly ash is a waste product, it is (or at least was) dirt cheap. Its enhancement properties are pretty much icing on the cake. Ground blast furnace slag is similar. So I wonder if silica fume or metakaolin will ever gain universal acceptance as they cost more than cement.
I've got serious pushback on microsilica to the point of contractors telling wild stories on how it will increase the price of the concrete XXX%. It was removed.
I'm hoping that those huge Duke Energy mountains of ash in North Carolina are mined and used. That should get us along for a while.