Slate tiles - white powder.


Hi,
I have laid some natural slate tiles as a hearth. They were bedded on standard mortar and grouted normally. I then treated them with HG porous stone sealer.
The problem is most of them have now exuded some white powder which smells salty. It also is ompossible to get rid of. I scrub and wash it off but it just returns. HELP PLEASE !
My hearth looks awful, anyone know what to do. Thanks Stu
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I wonder whether the white powder is encrustations of salts brought up by water seeping through the slate and evaporating - a bit like a stalagmite. I suspect they would dissolve with a weak acid, vinegar for example, or some product for removing deposits on taps, but would come back. If this is what's happening, then lifting the tiles and fitting a DPC before re-laying them would solve it.
Rob Graham
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