John,
Your categorization is the first time I have heard this way, and seems
logical. I put the same question to a geologist friend of mine - different
perspective, but illuminating:
"Pozzolans, as you might guess from the name have an Italian origin. The
name comes from the location of a volcanic region just northwest of Naples.
Pozzolans were originally volcanic glass to which the Romans discovered that
the addition of calcium hydroxide (slaked lime) and water produced the
chemical reactions that form cement. For pozzolans today I quote from the
"Encyclopedia of Mineral Resources" MIT Press, "Pozzolans are finely divided
aluminosiliceous materials which, though having no cementing action with
water alone, combine with calcium hydroxide in the presence of water to form
cementing compounds. They are use in quantities of ~10-40% in portland
pozzolan cements. Naturally occurring pozzolans include volcanic glass and
and certain clay-like rocks which require heat treatment to develop their
full pozzolanic activity." Because of their siliceous compositions,
pozzolans have enhanced strength properties over portland cements.
Volcanic ash is finely divided, glassy matter erupted violently from
volcanoes that erupt highly siliceous magma (usually > 65% silica).
Volcanic tuff is also the product of volcanic eruption, but it includes all
ranges of particle sizes, from volcanic bombs (up to 1 meter or more in
diameter) down to ash particles (generally less than 1mm in
diameter) The term tuff is only applied when the volcanic debris has become
cemented into a coherent rock. Volcanic rock is any rock that has a volcanic
origin. It can be a lava flow or fragmental in origin--like a tuff."
Onward to the pursuit of perfection!
Best,
Leland
Leland R. S. Torrence
Leland Torrence Enterprises and the Guild
17 Vernon Court, Woodbridge, CT 06525
Office: 203-397-8505
Fax: 203-389-7516
Mobile: 203-981-4004
E-mail: [log in to unmask]
www.LelandTorrenceEnterprises.com
-----Original Message-----
From: The listserv that doubts.
[mailto:[log in to unmask]] On Behalf Of John Walsh
Sent: Tuesday, November 13, 2007 8:37 PM
To: [log in to unmask]
Subject: Re: [BP] Puzzledana
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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