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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