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Even though its initial makeup is hazardous, acid stains
have an environmentally friendly upside. There are no
VOCs to contend with while applying them. There is no
off-gassing from the finished product, no place for allergens
to hide and, if cleaned up properly, no harsh chemicals
left behind. Compared to carpet or vinyl, a concrete
floor's sustainability is hard to beat. And contrary
to what some might think, its hazardous content won't
necessarily affect a building's LEED certification.
How acid stains work
Acid stains consist of a mild acid usually hydrochloric
acid - mixed with water and metal oxide additives,
also called mineral or metallic salts, that penetrate
the surface and generate a chemical reaction with the
free lime (Calcium hydroxide) in the concrete.
"When it comes to reactivity, the
acid has little to do with it," says Chris Sullivan,
vice president of sales and marketing for Chemsystems
Inc. "The active ingredient is the mineral salt.
But if you tried to dissolve the salts in just water,
they would just break down into smaller pieces. You need
the acid in the water to make these salts soluble."
The mild acidic solution also lightly etches the surface,
he continues, which allows the mineral salt to penetrate
into the concrete, react with the free lime and form
a chemical bond. The metallic salts fill the pores
in the paste layer of the concrete, permanently changing
the color of the surface.
Thome provides further' explanation. "The acid
in the acid stain is (mostly) neutralized by the alkalinity
in the concrete. The higher the alkalinity, the better
the color development will be. Once the acid becomes
neutral, the metallic salts fall out of suspension and
then react with the calcium hydroxide to produce the
color. The color development is a total chemical reaction
and is concrete-dependant.
"Not all concrete slabs have the properties to
be stained with every color.. What's more, every color
has a different chemical reaction and requires different
levels of alkalinity."
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