Yes you can store your batteries on a concrete floor!!!
There is some history from a long time ago with the old hard rubber cases that were somewhat porous, when stored on concrete an electrical current could be conducted through the container and the moist concrete floor permitted the current to find an electrical ground. Today’s battery cases are polypropylene so it no longer applies.
The fact is that all batteries self discharge while they are sitting on the shelf or concrete. Self discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without a connection between the terminals. How fast self discharge occurs is dependent on the type of battery (calcium and AGM have lower self discharge than conventional batteries) and the ambient temperature (batteries discharge quicker in higher temperatures)