What's That White Powdery Stuff on My Chimney?

When we built our house five years ago we chose cultured stone for the chimney because it cost less than natural stone. But almost from the start, there has been a white deposit on some of the stones. What is it, and how do I get rid of it?

Cultured stone is really lightweight concrete and pigments cast in molds to make it look like real stone. It's much less expensive than natural stone, and it's also much lighter in weight, making it easier to work with. Made by a number of manufacturers, cultured stone also looks surprisingly realistic.

But like all masonry products, cultured stone is susceptible to efflorescence, the white powdery residue you're seeing.

Datile, a company that manufactures cultured stone, suggests that efflorescence occurs when the free lime in the cement is dissolved in the mixing water and works its way to the surface as hydrated lime. Although any product made with cement can develop this problem, it's "particularly noticeable" in colored products like cultured stone.

Datile adds that you should remove the deposits as promptly as possible so they don't become permanent, and that a breathable non-film forming masonry sealer can help prevent a recurrence of the problem.

Cultured Stone, another manufacturer, offers this advice--clean the surface with water and a stiff brush, then rinse. If that doesn't work, try a solution of one part white vinegar and five parts water. Scrub, rinse, and let dry. Don't use a wire brush. It can damage the surface.

I also spoke with Eldorado Stone. A technician told me that rain soaking into the chimney can bring salts to the surface as it dries. The best cleaning option is the vinegar-water solution, which melts away the salt but doesn't affect the pigments used in the concrete. But, the company cautions, don't use a solution any stronger than 1:6, at least on its product. A stronger mix might affect the surface coloration.

If you want to apply a sealer, use a silane or siloxane based product, Eldorado Stone says. You may continue to see some efflorescence, but the sealer should make the surface easier to clean.

There's a remote chance that efflorescence is a sign of a leak somewhere in the chimney or in its mortar cap that would create a path for water, helping to bring salts to the surface.

If the stone has been applied according to the manufacturer's directions, this should not be the case. But it might be a good idea to inspect the cap for cracks or look for other obvious places where water might get in (or hire a mason to have a look).

Overall, it's not a problem that seems to have a perfect solution. But on the positive side, everyone I spoke with said it's nothing more than a cosmetic issue. Barring evidence of a leak, efflorescence is just an eyesore that won't affect the strength or durability of your chimney.