Which magic elixir was that? I've yet to find someone who used one, and still had good batteries six months later. But I'm willing to look & learn!
The concrete story has been repeatedly debunked for modern batteries, I find it hard to believe Interstate themselves would tell you that in the last 40 years. Concrete as a problem with old porous battery cases, but for modern plastic cases? Absolute nonsense. The only problem with concrete is that it usually means a slab floor, which is cold, and cold hastens self-discharge and damage. The concrete itself matters about as much as the color of the paint on the car/boat that the battery is located in.
From Interstate's own web FAQ pages:
Interstate Batteries FAQ :: Does it hurt my car battery if I set it on concrete?
A: No, the type of plastic (polypropylene) used in battery cases today
is a great electrical insulator, therefore cement causes no electrical discharge effect to the battery. "
Once upon a time, battery cases were wood covered in asphalt, I believe, and that was replaced by "hard" rubber, but plastic has been the rule for a very long time now.