The white powdery substance that typically occurs on a concrete basement floor is a salt deposit. Salt is carried by moisture from the soil through the concrete and is deposited as the moisture evaporates. Most of the salt deposits can be removed using water. Non-water soluable salts frequently can be dissolved with something like vinegar.
The water pressure, caused by water on the exterior of the floor, makes it very difficult to prevent water transfer through the concrete while the soil is wet. Moisture tends to delaminate any sealer applied to the interior surface, so attempting to prevent moisture flow by applying a sealer is discouraged.
The goal should be to dry the soil under the floor. This will take time. Periodically, tape a 3 ft. x 3 ft. piece of plastic onto the floor to determine if moisture is still coming through the concrete. It may take weeks before the soil saturated by flood waters dries and the moisture transfer through the concrete floor stops.
