| When maximum
sparkle is desired, Heritage Glass has a silver-coated glass
aggregate that reflects light like a jewel, Thornley says.
But if it's not so much sparkle as shine that you want, both
Heritage and American Specialty Glass, another Utah-based specialty
glass maker, have crushed mirror aggregate. Mirror chips can
add a hard reflective shine, and finely ground mirror aggregate
can add a subtle iridescent sheen.
"If you use 30-mesh mirror or smaller, and put it in
the background matrix, it gives you a sheen like granite," says
Jim Silver, general partner with American Specialty Glass.
Pricewise, specialty glass can go for as little as 30 cents
per pound for large quantities of some types, and up to $6.35
per pound for Heritage's silver coated aggregates. Cullet goes
for whatever the local market rate is.
Alkali-silica reaction must be considered
When using glass aggregate outdoors, or anywhere else that
the concrete will be exposed to moisture, beware of the dreaded
alkali-silica reaction, an unhappy phenomenon in which the
silica in glass and the alkali in cement create a gel that
swells in the presence of moisture. The reaction may happen
right away or it may take 20 years, but in either case it can
cause cracking. Any source of moisture can set it off, including
mopping, using excess water in the concrete mix, and so forth.
But the alkali-silica reaction can be prevented. Engineers
at Columbia University, committed to developing more environmentally
friendly building materials, discovered that the reaction can
be avoided if the glass is ground finely enough to pass through
50-mesh or smaller screen. They also found that the mineral
admixture metakaolin will suppress the reaction - an effective
but expensive solution.
"We replace 20 percent of the cement with metakaolin,
which may double the cost of the concrete, because metakaolin
can cost as much as four or five times what cement costs," says
Christian Meyer, a professor of civil engineering at Columbia.
Other studies have shown that fly ash suppresses alkali-silica
reaction, Meyer says. "It's not as effective as metakaolin,
but it's cheaper," he says.
The Columbia researchers also found that green glass does
not cause the alkalisilica reaction, due to the chromium oxide
used to get the green color.
Of course, keeping out moisture with a good sealer (presuming
you haven't used too much water in the mix) will also suppress
the reaction.
In the decorative concrete arena, most glass aggregate currently
goes into countertops. But there's really no reason it can't
go into floors, walls, furniture or anywhere else decorative
concrete is found - indoors or out.
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