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Re: GSBN:more on fly ash, pozzolans

on 6/28/02 3:37 PM, Paul Lacinski at paul@...:

>  . . . So here's another bullet for your
> list- it seems as if fly ash + lime = cement.  This would mean that
> those of us in wettish climates who are concerned about maintaining
> maximum vapor permeability in lime plasters would be best off
> avoiding fly ash as a pozzolanic additive.  Yes?

Maybe si, maybe no.  A little bit of fly ash in a lime plaster will make it
more cementlike both in terms of (im)permeability, and added strength,
hardness, and durability.  I think you can well appreciate that this isn't
that simple, both because of the variations in lime & fly ash chemistry, and
in the site specific, even wall specific, demands of the project.  All in
all, I'd be inclined to add the ash - give up some permeability but gain a
lot of durability.

> And what about 
> ground pottery or bricks (which I have used alot of) or hydraulic
> lime?  Do these have a similar effect on the lime?  My understanding
> is that they produce different compounds, entirely.

Ah, yes, the old ground pottery or bricks trick.  thousands of years old,
still being practiced.  Fired clay is (generally) calcined clay which, when
finely ground, makes a fine pozzolanic adverb.  Of course, there are once
again many variations, but calcined clay is roughly equivalent to fly ash.

Bruce "Pozzolanic Attitude" King