You could try and use the pink foam insolation (sold at Home Depot) I use this for alot of those really odd props I need to make. You can get it to look how ever you want with a foam cutter and a good paint job. Good luck!

Polystyrene foam. Cut it into brick shapes with a fine-tooth saw or blade. Smoothe the sides and round off the edges with sandpaper. Allow some dents and hollows to remain. Then break away some corners by hand. It'll crumble.

Paint with acrylic house paint, stippling it with a sponge.

I've made a totally realistic block of concrete this way. You can't tell it from the real thing!  

I've discovered using an everyday, serrated-edge, steak knife from my kitchen drawer works well when cutting polystryene.  Just be sure to use the inexpensive kind and not the ones given to you as wedding gifts.  You'll need more than just a safety mask if your wife finds out.