Advice on the use of Metakaolin

[KootK]

I'm doing some research on the use of Metakaolin as a component in concrete mixtures. The goal is to produce a position paper of sorts. If we feel it's appropriate, we may incorporate Metakaolin into our office concrete specifications.

Unfortunately, most of the testing information that I've been able to track down comes from the Metakaolin suppliers themselves. I'd like to find some independent information if possible.

Has anyone had any experience using Metakaolin?

Are there parts of the world where it's use is common?

Anybody know where I could find some papers addressing the use of Metakaolin in concrete?

There seem to be a lot of benefits associated with the use of Metakaolin. Reduced shrinkage, lower permeability, LEED benefits... the list goes on. So far, however, producers in our area have been reluctant to adopt it.

Thanks,

KK

 

[Teguci]

I've only seen it as a way to prevent ASR in high alkaline cement mixes (never actually used it).
Otherwise, why not let the contractor use it at his discretion and potential cost benefit?
I've always been more of a fan of GGBFS.

 

[TXStructural]

I'm not sure about LEED credit, as it requires a great deal of energy to produce metakaolin, as opposed to using by-product/waste stream materials, like fly ash.

As long as it is properly produced, using metakaolin as an SCM should be fine. Consider need, availability, and cost before specifying it, rather than leaving the mix design up to those who do it everyday. One problem we see is designers specifying concrete mixes without understanding aggregate sources, formwork, congestion, shoring/reshoring cycle times, and placement requirements.

Don't over-specify: design for a certain strength, and specify finish and durability requirements. Leave the mix design up to those supplying and placing the concrete, and require your approval, if needed.

 

[KootK]

Thanks guys. I could use a little acronym help however:

ASR = ?

GGBFS = ?

SCM = ?

We're not exactly looking to specify metakaolin in the traditional sense. We want to:

1) Solidify our position on its use so that our staff can evaluate it consistently when it gets suggested by other parties.

2) Be able to recommend that our local municipality change there specs to allow metakaolin use when it would be beneficial. Metakaolin concrete seems to have very low permeability which would be useful for sidewalks and other high traffic urban structures.

3) Convince concrete placing companies that using Metakaolin in their mixes would be a good thing for our super flat SOG projects. The tests that I've seen indicate that concrete mixes with Metakaolin barely shrink at all. This could be an enormous benefit on projects where unreinforced slabs on grade are desired. Right now, our concrete placers feel that Metakaolin mixes would be too difficult to finish to super flat tolerances. However, that opinion does not seem to be validated by any actual experience with metakaolin. They may well be right. We'd just like to find out one way or the other.

 

[TXStructural]

ASR = alkali silica reaction, an expansive reaction of certain aggregates in the presence alkaline materials, i.e., portland cement.

GGBFS = ground granulated blast furnace slag

SCM = supplemental cementitious materials

Since you do no know these acronymns, you probably need to read up on concrete materials and mix design before going forward with your project to use newer/rarer materials.

The concrete supplier and their testing lab are best positioned to know whether ASR-mitigation might be required for a certain aggregate. Metakaolins are one of many ways to mitigate ASR. Silica fume is another, but silica fume makes concrete "sticky" and harder to finish; metakaolins may do the same.

Slab finishing goes to means and methods of the concrete placer and finisher. You cannot make a concrete which is universally "better" for finishing, since its character and quality are dependent on means and methods.

Shrinkage of concrete is very dependent on water-cement ratios and paste-to-aggregate ratios, as well as aggregate gradation and curing. Low shrinkage concrete will usually have higher amounts of aggregate and lower quantities of cement/paste. Gradation of aggregates facilitates this reduction in paste volume. Water reducing admixtures also help with placement at low w/cm ratios, and sometime aid finishing (and sometime not.)

 

[KootK]

Thanks TX. Reading up on my concrete material science is precisely what this project will entail.

I've already used Metakaolin a few times over the past 24 months. While other variables no doubt come into play, what we've witnessed so far suggests that Metakaolin has a dramatic effect on concrete shrinkage rates.

When it comes to finishing, you are correct, means and methods are precisely the issue. To achieve super flat floors, the specialist placers working on our projects tell us exactly what methods they need to use in order to provide the flatness specified. Those methods then influence the mix design. And that's where we run into opposition to Metakaolin.

Teguci: thanks for turning me on to ASR. That's a whole new angle that I hadn't considered.