I have bought some damp traps. When it comes to refilling them, what substance would soak up the most moisture? Googling, dishwasher salt and cat litter have been suggested, but nobody seems to be able to say exactly what would, in principle, absorb the most moisture.
They sell it as driveway ice-melt by the 5 or 10 pound bag so it is cheap. It is even used in pickling to make vegetables crisp.
It makes a great desiccant.
I'd wager that the granules they sell to refill the damp traps are going to absorb the most moisture of any commercially available product. If there was something that worked better, they would fill the damp traps with that instead.
Example: DampRid Refill