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I have a wall of my house, north side, in parpaing covered with a monolayer of about 2cm. There was a long time a massive earth along the latter. I removed it for a few years, but the saltpeter got into the crepi and went back into the wall. He did not cross the wall and stays in the plaster. What should I do to fix it? Do a drainage at the foot of the wall? But how to kill the salpetre already installed?

In the present case, it seems rather that one is in the presence of efflorescence and not of saltpetre, a characteristic phenomenon of the old agricultural premises (sheepfold, stable) subjected to infiltrations or to capillary rise.
The whitish traces that you describe are efflorescence. They can be scraped or cleaned with a brush, using muriatic acid diluted in water (1 part acid to 20 parts water)
If it is not really saltpeter and capillary rise, they must be treated by putting in place a tight barrier in the lower part of the wall, by injection of resins.

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