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Re: GSBN:fly ash




Paul Lacinski wrote:

> I wonder what fly ash actually does in concrete, and how similar its
> reaction with lime is to its reaction with cement.  Any ideas?  My
> experience was that the reaction between the lime and the fly ash was
> in no way a weakly pozzolanic one- the inch thick samples from last
> summer were very hard and strong, and well set all through.  They
> seemed in every way comparable to similar samples made with crushed
> brick.  But then, I won't pretend to know anything about the
> chemistry.  Any more ideas?


OK, a few quick basics:

1.  Fly ash is nearly identical to the pozzolanic volcanic ash that the
Romans mixed with lime to get their legendary concrete.  That concrete
lasted so long because it was mixed and applied very dry, in fact rammed
into place just as we do rammed earth, and the less water in your mix, the
stronger and more durable is your concrete.

2.  Many have made strong concrete by mixing fly ash and lime.  The only
problem is workability-it's like peanut butter- but if you start adding a
bunch of water, well, see item #1.

3.  fly ash in concrete doesn't react with cement.  It reacts with the lime
(CaO) that is a very large by-product of the cement hydration process.  That
lime, in the form of ettringite if you must know, makes for a network of
weak interconnected surfaces within the concrete UNLESS it has a reactive
silica (our hero fly ash) and water present so as to make more binder
(calcium carbonate + silica + water --->  calcium silicate hydrate, if you
must know).  And calcium silicate hydrate is the glue (binder) that makes
our beloved concrete so strong and hard.

That's a half inch thick thesis condensed into three bullet points.

Ecological Building Network is developing a manual on high volume fly ash
concrete that will cover all this in more detail - expect it out next year.

hope this helps, and best to all,

Bruce King