Besides the groundwater remediation on the outside that's been mentioned, such as re-routing downspouts, and laying plastic sheets, or extra drainage around where the ground and foundation meet, a sealer will work to prevent efflorescence if it penetrates below the surface, can bind tightly to the grain of the concrete, and it has the ability to pass water molecules, but block salt and keep it from emerging. The osmotic pressures involved aren't really "strong" but unless you remove the water, they never quit. Nothing that simply sits on the surface will work, and will eventually fail.
To really get it to work, you'll probably have to sandblast or mechanically scrub the concrete if it has any sort of paint on it, then give it a really good citric acid etch.