I have enough slate chalkboard from an old school to make a shower surround. Support is not a problem, heavily reinforced concrete floors, cast iron tub with tile lip. Need advice on adhesive and what to grout to use. The pieces are 3 x 5 so will have seams.
Slate is very porous, so however you install it, note that you're going to have to seal it well before it will be practical as a shower surround. As for grout, I'd use standard grout for shower tiles (as that's essentially what you are installing). Standard thinset should work for installing it on the wall.