Efflorescence has been studied extensively. Foundations often have deteriorated concrete surfaces. The deterioration is most often caused by periodic wet-dry-wet-dry exposure to moisture over many years. The mechanism is this: the base of the wall gets wet, and some of the water penetrates the concrete, carrying dissolved salts picked up from the soil, or the concrete. The salts remain in solution until the moisture reaches a surface that is exposed to air. As the water evaporates into the air, the salts are left behind and form crystals on the surface of the concrete, NOT inside the concrete. The concrete can be quite sound inside even if it looks like the photo above.
