John
I think that every way of categorising helps. Some of us 'get'
different ways of explanation.
I like your categorisation.
Cheers
David West
Executive Director
internationalconservationservices
T: +61 (2) 9417 3311
M: +61 (411) 692 696
conservation&managementofculturalmaterial
-----Original Message-----
From: The listserv that doubts.
[mailto:[log in to unmask]] On Behalf Of John Walsh
Sent: Wednesday, 14 November 2007 12:37 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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