Calcium carbonate

Three organizations that study three different surfaces all arrive at calcium carbonate, and none of them is describing the others' surface.

ICPI's glossary defines the deposit on concrete as a white deposit of calcium carbonate and calls calcium carbonate the most common type of efflorescence. That page walks the full reaction chain; this one stops at the compound. On a paver the compound is a deposit sitting on the surface, and ICPI says some of it may wear away, the question taken up on whether efflorescence goes away on its own.

Away from a paver, the same compound is either the stone itself or what hard water leaves when it is heated.

The Natural Stone Institute's care page says calcareous stone is composed mainly of calcium carbonate, “a chemical compound commonly found in natural stone, shells and pearls.” Its variety pages place limestone and travertine, like marble, on a calcium carbonate base. Here the compound is not a deposit on the stone but the stone. Calcareous stone is defined on its own page, and how the paver cleaning page sorts concrete pavers, travertine and brick, surface by surface is laid out there.

The U.S. Geological Survey says hardness “is caused by compounds of calcium and magnesium, and by a variety of other metals,” and that when hard water is heated, “such as in a home water heater, solid deposits of calcium carbonate can form.” Hard water itself, and the spots it leaves on glass, is defined on the hard water page.

Acid is where the shared compound stops being a curiosity and starts deciding which cleaner belongs on which surface.

The Natural Stone Institute ties its caution to the compound itself: because calcium carbonate is sensitive to acid, it recommends mild, non-acidic cleaners for calcareous stone, and it warns that lemon, vinegar and other acidic products may dull or etch those stones. ICPI, writing about concrete pavers, warns that strong acids change paver color slightly and describes the acid in proprietary cleaning chemicals as buffered and blended with other chemicals. Reasoning from those lines together, not quoting either body: the compound that forms efflorescence on a concrete paver is the base of travertine, limestone and marble, so an acid efflorescence cleaner belongs to concrete pavers and never to a calcareous stone, where acid may dull or etch the stone itself. Whether any acid is used on travertine is answered on whether travertine can be cleaned with acid.