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Diluted Hydrochloric acid is often touted as the only thing to remove limewash from masonry but the process is dangerous and can adversely affect the bricks, color changes can readily occur.

Is there a viable alternative? I noticed that a kettle descaler product used formic acid and wondered if that could be used instead.

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Glycolic acid (hydroxyacetic, HOCH2CO2H) is a mild organic acid, not very expensive, and the calcium salt is quite soluble. It is essentially a non-smelly vinegar! It has been used for brick washing.

One issue is that whereas HCl will dissolve CaCO3 until the HCl is all gone, the pH of glycolic acid solution increases as it dissolves CaCO3, so the dissolution reaction slows down and finally stops at maybe 50% completion.

Citric acid and tartaric acid are very soluble, but their calcium salts are not. Calcium citrate and tartrate may come down as fine crystals in the pores of the brick and be difficult to remove. Calcium formate is moderately soluble and so would be useful, but formic acid is a vesicant, so it must be used with as much or more care than HCl.

Muriatic acid is readily available at 32% and 20% concentrations. Diluting by a factor of 50 would give a pH about 2, which is safer for people and brick, but that's a lot of water to clean a brick wall.

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  • $\begingroup$ I've found that while Muriatic is very strong, the tasks is still difficult and time consuming even with 32% strength acid. I also discovered that at 32% and also at a 50% dilution it would turn London yellow 'stock' bricks (around 100 years old) a slightly different colour, more golden, or browner. I tried some phosphoric acid to see if that changed some of these 'stains' afterwards, but with no effect whatsoever. Would I be correct in saying that Glycolic, Citric and Tartaric are much less active compared to Muriatic? $\endgroup$ – Paul George Feb 10 '18 at 8:01
  • $\begingroup$ Glycolic and the other organic acids are no match for muriatic for speed, but glycolic has the advantage of less (or no) discoloration on many bricks. $\endgroup$ – James Gaidis Feb 10 '18 at 16:14
  • $\begingroup$ An issue with limewash is that it is absorbed into the pores of the bricks, where it absorbs CO2 and does a good job of plugging up the pore. Dissolving the contents out of the pore channel is a long term process. $\endgroup$ – James Gaidis Feb 10 '18 at 16:17
  • $\begingroup$ Another issue, relative to discoloration, is possible additives to the limewash. In particular, addition of portland cement, which is used for a harder coating, hydrates to silicates which are more difficult to dissolve. If silica or cement is present, restoration cleaners which contain hydrofluoric acid might help - but they are even more hazardous than muriatic acid. (The problem is that these cleaners don't hurt much if you spill them on yourself or your clothing, but if you leave the fluoride on for an extended time, plan to visit the hospital emergency room. $\endgroup$ – James Gaidis Feb 10 '18 at 16:23
  • $\begingroup$ The limewash is pure white to look at, but seems pretty hard with minimal dusting. As an alternative to chemical removal I tried spinning the faces on a few bricks to see how that process would compare. On those spun bricks the colour just under the surface seems normal. Is the discolouration via additives that you mentioned something that would happen on original application or as a reaction to the muriatic? $\endgroup$ – Paul George Feb 10 '18 at 16:42

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