Hey all, I just happen to have three 36" long x 5" high x 4" wide sections of foam shower stall sill material as shown here: http://www.schluter.com/8_3_kerdi_st..._material.aspx
It says it is expanded polystyrene. Specifically: "The polystyrene foam has a density of 2.75 lb/ft3 (44 kg/m3) with a 48 psi (331 kPa) compressive strength at 10% deformation. It is serviceable in temperatures up to 175°F (79 °C)."
Having used the stuff to build three walk-in showers in my house I can attest to its strength.
I was going to sell it off but me thinks it would be dandy building material unless it is too heavy.