Hi Leland and thanks for the welcome,
I'm going to answer this one at the same time responding to Mike Edison's last
response on pozzolans. The simpified answer to your question is no difference
at all. Pozzolans are the general class of materials that (to quote Mike) react
with lime to form calcium silicate hydrates (i.e.; cementitious products). Many
volcanics (ash, earth, tuff, crushed rock) are a subset provided they have the
right chemistry and structure. The difference between ash, tuff, and rock is
really just a geologic one and probably of little concern to most. So pozzolan
is the general term that includes all sorts of silica-rich, generally "glassy"
materials including certain volcanics (including Dutch trass), brick dust, rice
husk ash, coal ash, and steel slag.
And onto Mike's response...I worry about common usage. But if the definition
of pozzolan is "anything that reacts with lime to form calcium silicate
hydrates, then I agree and it's not a redefinition at all. At the risk of
unbelievably lumpy oversimplification, I'd like to group binder components into
three categories...acidic stuff, alkaline stuff, and neutral stuff. We can
dismiss neutral stuff including pigments, animal hair, and chopped up labor
organizers as chemically uninteresting (again, gross oversimplification). The
alkaline stuff includes portland cement, natural cement, hydraulic lime, and
lime. What all these have in common is that they are composed of calcium
hydroxide or make calcium hydroxide upon hydration. (Yes, technically
cements make their own lime when water is added but it is in a differnet
form). So all these materials are caustic, are saturated with respect to lime,
and are very stable in its presence (which is why cement mortars are best
cured in lime water). The acidic stuff are unstable in caustic environments
and breakdown to form more stable components. These are the pozzolans
and include most sorts of silica-rich material that have poor crystallinity. Gee,
I'm not doing such a great job at simplifying this. I'll leave it at that.
John
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