Stone cleaning -> damage, repairs, remedial treatment???

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Stone cleaning -> damage, repairs, remedial treatment???

Postby gpk » Wed Jun 28, 2006 2:52 pm

I previously asked for advice about getting lime-based mortar to repair sandstone damaged by cleaning ... .php?t=609

In places the sandstone is now beginning to spall ... just small patches but I'm mildly concerned that the damage caused at the time of the cleaning is getting worse.

A friend of mine suggested that phosphoric acid is used in chemical cleaning processes. This helpful page (in fact Wikipedia has a wealth of information related to lime mortar etc.) indicates that sandstone is typically held together by calcium carbonte and/or silica. I reckon that residue of the acid is continuing to attack the carbonate (and yielding salt + water + CO2 if my school chemistry serves me correctly ... in which case the salt would be Calcium Phosphate, unless another acid was used).

Most of the stone is OK, but it looks as though some is a bit softer or more susceptible. I also notice that some of the stone has bits of what looks like shells (high in calcium carbonte?) and not just sand, so if my theory is correct then those slabs of stone in which calcium carbonate rather than silica is primarily responsible for holding the stone together are probably those slabs now suffering.

In places where I've repaired with lime mortar there is noticeable outbreak of what looks like typical salt "efflorescence". I wonder if the whole wall should in fact be treated with lime water to counter the residual acid (assuming that was what was used in the first place). I've done a test patch and was expecting more efflorescence but possibly because of where I sprayed there's nothing much to be seen except grey-white staining where undissolved lime stuck to the surface. So much for clean stone!

Obviously I need to talk further to my downstairs neighbour who wanted the cleaning done in the first place. He's pretty laid back but it's a bit tricky working out what - if anything - I or we should do.

This site first alerted me to potential problems caused by stone cleaning . Have a look at conclusion no. 5. Could this be caused by residual acid attacking the carbonate "matrix"? Presumably after 20 years all the acid will either have been washed out by the weather or dissolved any carbonate it could find. Possibly washing with lime water ought to be a standard post-cleaning treatment to neutralise residual acid?

Ta in advance for any comments

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