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Re: GSBN:fly ash
Most cement used here in the Netherlands is based on ash from steel
refineries. It is a semi waste product. The process has been adapted to
generate a useful waste product. I am not sure that its is the same as fly
ash you are talking about but according to my knowledge and the literature
available to me it is very comparable to portland cement (PC). They call the
product here hoogoven cement (HC) it consists of 2/3 HC and 1/3 PC. The PC
is used as a catalyst to fire up the latent hydraulic reaction of HC and
then for all intents and purposes it is equivalent to normal cement. There
are some differences like the creation of less free chalk during the
reaction and a bigger influence of heating on the curing speed of HC
concrete ie it is normally slower but heating has a bigger influence on
curing speed. To get a comparable curing speed the HC is ground finer the
From: bruce king ecobruce@...
To: GSBN GSBN@...; Kelly Lerner
Date: Thursday, June 20, 2002 5:58 PM
Subject: 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.
>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,