maybe a whole shit load of calcium chloride? it's used for de-icing roads and sidewalks, but it's basically similar properties to what you get in those 'dry-packs' they put in some products to reduce RH. It will absorb moisture from air, and you'd have to replace it at some point... you could figure out capacity by weight it.. if it stops gaining weight, it can't absorb any water, and you can work backward from that timeline for regular maintenance. How much it pulls out depends on how much you can put down - surface area being important too. hang some bags, line the floor, lol.. you'd probably need a ton iof it, and it'd be sloppy and wet when you remove it.
the point is... you need a dehumidifier to control RH. if the ambient RH is high due to season of year, then you cannot do anything but use a dehum.