The materials you were experimenting with are not fusilinid limestone. Nothing like it.
You actually haven't done anything different to what anyone else who has worked with these materials doesn't already know about.
Calcined gypsum is basically plaster of paris. All you do is remove some of the water from gypsum to make it (as you already realise).
I think you'll also find that the addition of water to most mineral crystalline materials causes the structure to swell and that creates loss of strength, not the other way around. The water changes the spacing of the crystal lattice and weakens the bonding between the lattices. They basically turn out like a slurry/paste. With gypsum (or in your case anhydrite/hemihydrate), adding water does the opposite. The crystals rehydrate and form the typical flat, blade shaped gypsum crystals which act like an interlocking grid of crystals and strengthen anything they're added to, once they set. The setting reaction is exothermic, so some water is driven off via the reaction. Drive too much water off and it crumbles because it turns back into anhydrite/hemihydrate powder.
Gypsum itself is very soft. It's about 2 on the Moh Scale. Copper will scratch it rather easily. What makes it so strong is the way the crystals regrow and lock together when the anhydrite/hemihydrate form is rehydrated.
In any case, a simple thin section of the "rock" in question will show you whether it's a natural rock or a man made concretion.
Post Edited (05-Aug-12 13:46)