Frequently Asked Questions
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Q:  Does
muriatic acid darken concrete?
We used muriatic acid
to remove some stains from a concrete sidewalk that was several
weeks old. The stains are gone but now the cleaned portion of the
sidewalk is darker than the rest of the sidewalk concrete. What
caused the darkening and what can be done to correct it?
A: Calcium hydroxide
deposits may lighten the color of concrete that is cured with water
present on the surface. The calcium hydroxide is water soluble,
but when exposed to air it’s converted to insoluble calcium carbonate.
If an acid wash removes the calcium carbonate deposits, the darker
underlying concrete will be exposed. The color difference will be
especially noticeable if the concrete surface was darkened by hard
troweling or if a calcium chloride admixture
was used in the concrete.
This kind of surface discoloration is discussed
in PCA Research Department Bulletin RX203, Surface Discoloration
of Concrete Flatwork. Treating the entire sidewalk with muriatic acid might enable you to match the darker colored
area where the stain was removed. However, some experimentation
would be needed to develop a procedure that minimizes color variations.
More information can be found in the Research
Department Bulletin RX203 available through the bookstore.
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