Xypex Concrete Sealing-mostly full (Ashdale, S. of Gerrard Library)

Xypex Concrete Waterproofing can be applied to an interior concrete wall and it will grow crystals into the wall along any tiny moisture fissures it finds to seal it up.

Note: I've had this bucket 20 years but it's still a powder inside so I think it's probably fine. (It doesn't have a best-before date anywhere on it that I can see.)