I am building a base/stand for a pump in a water feature, so, it will be submerged. The outside of this now empty cylinder (about 9 inches in diameter) is already made, using rock slats on the outside, mortared together. The inside of this empty cylinder is mortar. It will be filled half way up and this is where the pump will sit. Question is, should I use a concrete mix or a mortar mix for this filler?