Maurizio, I had an 1100 square ft bluestone patio installed on a 6" concrete base. About 2-3 months after it was done a white reidue started showing up on about 30% of the stones (ramdomly). Some of it does flake off with my fingernail. The installer asked the stoneyard about it and they say although uncommon, it is calcium that has leached through. They say a good powerwashing should flake it off. Have you ever heard of this and does their solution valid? Also does this problem indicate a "bad" batch of stone? Thanks for your help.

 

Dar Dan:

 

“ The installer asked the stoneyard about it and they say although uncommon, it is calcium that has leached through. ”

 

Although it is not exactly calcium what leached through, the stone-yard is right: it is uncommon with that particular stone and, hopefully, it should be only temporarily, unless someone applied a sealer to it, because that could really complicate matters.

 

“ They say a good powerwashing should flake it off. Have you ever heard of this and does their solution valid? ”

 

Yes, I did hear of situations like that, and more than once also with stones that on paper were good for outdoors and historically never presented any efflorescence problem. (In the few cases I was reported about those “good” stones, they always applied a sealer to them…) So, yes, I would give it some more time to stabilize (maybe another couple of months and then I would try a not-too-aggressive power washing. Make sure that the grout lines are solid to avoid water going under the cuts of stone.

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Ciao and good luck,

Mauri zio Bertoli

 

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