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GSBN:more on fly ash, pozzolans




Howdy once again, y'all -

In response to my recent missive about fly ash, Derek Roff was kind,
perceptive and discreet enough to privately inquire as to whether I had
inadvertently named the wrong chemicals in the pozzolanic reaction.

I had (don't try to tech type and clothe & feed your kids at the same time),
so what follows is a clarification, as well as a response to Rene's message
on the same subject.  If concrete tech talk is not your passion,
inconceivable as that may be, then delete now or forever hold your peace.

When you add water to cement, it begins to hydrate  ie chemically combine
with the water.  That's why you want to keep any concrete, especially fly
ash concrete, cool and moist for as long as possible, and especially for the
first two weeks after pouring.  (I regularly encounter builders who strip
the forms right away to let the concrete "dry out", thereby in their minds
curing it faster.  What they get is lousy concrete that never had a chance
for its cement to fully hydrate.)

The products of that reaction are many, but primarily heat, calcium silicate
hydrate (CSH, the glue that binds everything together), and calcium
hydroxide (CH), also known as hydrated lime.  The lime component might be as
much as 40% of the reaction product, and it typically ends up as a weak
surface of ettringite (crystalline CH) surrounding each bit of aggregate in
a microlayer - the "transition zone".  That weakened transition zone is the
weakest link in the concrete, and limits its strength, watertightness, and
durability.  

Adding fly ash (and keeping the concrete moist) means the lime (CH) can
react with the siliceous fly ash (S) and water to make more of our friend
calcium silicate hydrate (CSH), giving you much better concrete by every
measure.  That, heavily simplified, is the pozzolanic reaction, and it is a
much slower one than cement hydration.  Thus, high fly ash concrete will
usually be slower to come to design strength (although there are
exceptions), but in the long term will tend to get a lot stronger.

Everyone still with me?  HEY!  EISENBERG!  WAKE UP!

We're in the home stretch.  Hang on, folks.

In Europe, a large percentage of the cement is made from ground granulated
blast furnace slag (GGBS), a by-product of the steel industry.  GGBS is both
pozzolanic and cementitious because it has some lime of its own for the
silica to react with when wetted.  In otherwords, GGBS is another industrial
waste product that makes a great cement replacement with far less
environmetal cost.  GGBS is starting to appear in the USA, though for now
mostly on the east coast (where the steel mills are).  Class C fly ash is
similar in that it has a high lime content, and is also found in the eastern
USA.  Out here in the western USA, because of the bituminous coals we mine &
burn, we get class F (low lime) fly ash which requires either lime, or the
lime from cement hydration, to react with.

And aren't we all looking for someone to react with?

Bruce King, reactionary guy