I have a relative unlimited supply of 12"x 12"x 18" styrofoam blocks which I would like to use in building a shop similar to a concrete block buildind. I would glue them together with Great Stuff expanding foam and top the walls with a wood plate. I would fasten the walls to the floor with threaded rod running from the top plate into the concrete floor. I would cross beams tying the walls together with possibly a conventional roof(although I'm toying with the possibility of a domed roof made by angling the blocks on end and gluing together. All this is my initial thinking and I would appreciate any suggestions, warnings or even recommendations for mental health help. Thanks much K.

My guess is that the 12" x 12" x 18" blocks are Expanded Polystyrene (EPS) instead of Extruded Styrofoam (XPS). Is that correct? What do you plan to coat the blocks with to protect them from the sun, weather, and fire? Will the coating be structural like dry shotcrete (gunite)?

It depends on the density of the foam. You would want at least a 2lb foam. Easy to figure, just weigh a 12x12 block and extrapolate. A much better product for gluing foam together is garden variety silicone.

I would like to suggest a layer of some sort of mesh stuccoed onto the exterior and interior to finish the wall, probably something like the aircrete people do.  It sounds to me like fun.