The most effective advice for removing saltpetre and mold from walls

Apply the solution using the brush. A foam will be created that will be left to sit for 5 minutes. Then scrub the wall with soap and water, rinse and clean.

Once the wall is dry again, you can apply cement to put it back in place and then sealant. This step may not be necessary.

Let the concrete dry, then apply a few coats of paint to restore the wall to its original appearance.