>
> > Does anybody have an opinion on flyash in mortar for repointing?
What
> > characteristics would it impart?
>
>
> Eric:
>
> My opinion is that it would be an extremely bad idea to use flyash in
a
> pointing mortar.  I've made repair mortars with a flyash component to
impart
> some special properties, such as self leveling.  I've also developed
masonry
> cements and masonry mortars.  Using flyash in a masonry mortar seems
> misguided at the least.
>
> Flyash is a pozzolan.  Some of the glassy and mineral phases in
flyash react
> with dissolved calcium hydroxide in water.  Alkalies and salt can
speed up
> the reaction.  However, water is an essential part of the reaction,
which
> proceeds slower than that of water with Portland Cement.  Of all the
mortar
> varieties, pointing mortars dry out the fastest because they are on
the
> exterior of a wall.  The flyash would take months, if not years or
decades,
> to perform in any way other than as a filler.
>
> Somebody mentioned metakaolin in this thread.  Metakaolin is not
flyash.
>
> Flyash also tends to make a more-flowable mortar than a mortar made
with
> hydrated lime, etc.  These mortars also do not retain water very
well.  A
> pointing mortar made with a flyash component may alternately run-out
and then
> fall-out of the joint.  Mason productivity would plummet before they
went on
> strike or quit.
>
> There are two main types of flyash in the United States.  Type F
flyash is
> produced from eastern, bituminous coals.  Type C flyash is produced
from
> western lignite or brown coals.  Type F flyash tends to contain more
> pozzolannic material whereas Type C flyash is slightly cementitious.
Type F
> flyash tends to be darker in color than Type C flyash.  Mortars made
with
> Type F ash may cause a mortar to be dark gray.
>
> One could make a masonry cement that contains flyash.  I expect that
it would
> have some unusual strength and performance characteristics.  The
masonry
> cement may perform better at high temperatures, it may resist
weathering
> better if it was moist cured for months before being exposed to
driving
> rains, and, it may lessen the chance for ASR and sulfate
deterioration.
> These features would not show up in any of the standard ASTM mortar
tests.
> None of these possible positive features have a practical application
on the
> Earth.  If you added the material to masonry cement, the likely
consequence
> is that you would simply complicate the product, increase the
production
> costs, and increase the risk of a failure after the occasional
batching
> error.
>
> Having flyash on a jobsite would be asking for trouble.  Flyash tends
to blow
> everywhere, even if it is bagged.  One could make a big mess and
maybe even
> get fined without any effort at all.
>
> In short, it would be pointless to incorporate flyash in a pointing
mortar.
>
> Steve Stokowski
> Stone Products Consultants
> Building Products Microscopy
> 10 Clark St., Ste. A
> Ashland, Mass. 01721-2145
> 508-881-6364 (ph. & fax)
> http://members.aol.com/crushstone/petro.htm
>
>
I find Steve's comments very interesting, but I'm not sure I can agree
with all of the points he makes. While I won't claim to have any great
expertise on the subject of flyash, I have worked with some of the
eastern, dark grey "F" material without disastrous effects. It is
commonly used as an extender for Portland cement, and a considerable
proportion of Portland cement, as I understand it, becomes calcium
hydroxide (hydrated lime) on contact with water.

The MSDS for this Type F material reveals a very high silica content.
The particle size is very small, so Steve is right about not wanting to
handle it in the wind, though it wasn't so bad in a somewhat controlled
factory environment.

The reaction chemistry that Steve describes is what I think makes
flyash potentially useful in certain compositions. I remember reading
through some Flyash Symposium literature in my library, and one of the
uses discussed in some detail involved the use of fly ash and lime as a
self-healing grout for use in highway roadbed stabilization. The
mixture, if proportioned properly, would have a reasonable sort of set
time, like portland cement, while retaining some of the properties of
lime such as permeability, self-healing and low modulus.

I left the lime conference last February convinced that the highly
touted hydraulic lime is not very different from this - lime
accelerated with fine silica. If one were willing to do some research,
and could find good, consistent sources of flyash, one could
conceivably produce hydraulic lime mortars that have the benefits of
lime with the early rain and frost resistance of cement-lime
combinations.

Edison Coatings, Inc.
M. P. Edison
President
3 Northwest Drive
Plainville, CT 06062  USA
Phone: (860)747-2220
Fax: (860)747-2280
email: [log in to unmask]
Internet: www.edisoncoatings.com

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