concrete stains work
Chemical stains can be
applied to new or old, plain or colored concrete surfaces.
Although they are often called acid stains, acid isn’t
the ingredient that colors the concrete. Metallic
salts in an acidic, water-based solution react with
hydrated lime (calcium hydroxide) in hardened concrete
to yield insoluble, colored compounds that become
a permanent part of the concrete. Several companies
manufacture chemical stains that are variations of
three basic color groups: black, brown, and blue-green.
The acid in
chemical stains opens the top surface of the concrete,
allowing metallic salts to reach the free lime deposits.
Water from the stain solution then fuels the reaction,
usually for about a month after the stain has been
applied. Other factors that affect the outcome include:
properties and amount
• Admixtures used
• Type of aggregate used
• Concrete finishing methods
• Concrete age and moisture content when stain
• Weather conditions when stain is applied
cements that produce larger amounts of calcium hydroxide
during hydration will show more stain color, and higher
cement contents pro-duce more intense colors.
Air-entraining or water-reducing admixtures don’t
pose a problem. However, calcium-chloride accelerators
can cause very mottled, darkened areas, and for this
reason aren’t recommended.
Nonchloride accelerators don’t cause this mottling
near the surface, calcium- based aggregates, such
as lime-stone, take stain readily and deepen the color
of the concrete above them. Siliceous aggregates,
such as gravel, don’t react with the stain.
Open finishes achieved by floating followed by minimal
troweling take more stain and produce denser colors
than do hard-troweled surfaces. However, open finishes
lose color faster because the concrete wears away.
Because of this, many contractors prefer staining
hard-troweled surfaces because the stain color lasts
longer. Colors on troweled surfaces also look richer
than those on floated surfaces. But you have to grind
the surface or use a higher acid concentration to
ensure adequate stain penetration.
in wet weather result in a richer stain color if the
concrete is stained soon after it’s placed.
However, wet slabs are more likely to effloresce,
lightening the color and causing a more mottled effect
in areas where the stain doesn’t take because
efflorescing salts hinder penetration. On sunny days,
the concrete can become hot and dry, and the stains
won’t penetrate as deeply into the concrete.
presence of water will cause the reaction to continue
for a long time, and concrete stained blue-green will
gradually turn brown or even black. Initially, this
provides nice variation to the appearance, but eventually,
nearly all the blue-green color may change to brown
and black. Because of the possible color shifts, some
manufacturers advise against using these colors for
exterior concrete. Interior slabs must be placed on
a well-drained base or sub-grade and have a low moisture
content before stain is applied. Jones believes the
brown-colored "flowering" of blue-green
stains is caused by oxidation of a copper component
resulting from water vapor passing through the slab.
Others believe the brown color is caused by a fungus,
which can be eliminated by using sealers containing
continue – When to Stain Concrete