Polymeric sand is an upgraded version of sand. Fine sand is mixed with additives & polymers that harden and lock the sand together, when water is added. Polymeric sand allows joints to expand and contract with temperature variations. Using polymeric sand as a filler, especially for large outdoor concrete slabs such as driveways & patios is that the filling is more durable and unlikely to be washed away by rain.  Polymeric sand is also an insect and weed deterrent. On the flip side polymeric sand is expensive and must be applied quickly & carefully. A mistake and you will have hardened gritty concrete surfaces around the joint!