[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
GSBN:Re: Technical issues in China
" . . . The second issue has to do with plaster. Paul Lacinski and I have
been experimenting with plaster mixes here in China using locally available
materials. Last year we used flyash in several mixes (both with lime and
with cement). I won't go into the detailed mixes, but do any of you have a
feeling for the effects of flyash in plaster? Clearly flyash is a pozzolanic
material, and in concrete it adds compressive strength and also makes the
mix set more quickly (not particularly an advantage in a plaster). I'm
wondering if it also (like cement) makes the plaster less vapor permeable.
Does anyone have experience using flyash in lime plasters? There are
actually two types of flyash here - one is fine and powdery (very similar to
cement) and the other is rough and irregular, up to 3mm diameter. We've used
both types. The rough type is actually more like an aggregate though it does
seem to have a very weak pozzolanic reaction. Any thoughts on flyash and
permeability or suitability for plasters?"
Kelly et al -
Fly ash in concrete will typically (in fact, always) retard the set time,
not hasten it, so I can't explain your comment. It also greatly decreases
permeability (by almost an order of magnitude), and for that reason alone I
have always thought it inappropriate for plaster (unless you want a vapor
Two days ago I had a long conversation with the local VP of the biggest fly
ash supplier in the USA (USG Resources), and as it happens I asked about
China. They are by far the biggest cement producer, concrete consumer, and
coal-fired power producer in the world, and thus, seemingly, obvious
candidates for high volume fly ash concrete promotion.
"Yes", he (the ISG guy) said, "except they don't collect the ash (ie,
pollution) like we do - it just goes up in the air. Retrofitting an
existing coal-fired plant with an electrostatic precipitator costs about
$300 million, so they so far just aren't doing it much" (Yow! Someone call
the World Bank!). So they basically don't "have" much ash, and are
currently just breathing it. Thus, I suspect the ash you got is not very
high quality, and the coarse stuff is probably bottom ash - the stuff swept
off the floor of the furnace as opposed to the stuff that flew away (thus:
fly ash). Bottom ash is used here as roadfill and soil stabilizer, and is,
yes, mildly pozzolanic. probably fine as aggregate, but not as an actual
pozzolan you can count on.
You wanted thoughts, and you got 'em. We miss you sweetie. Come home soon.
best to all,