Concrete

Looks like they've answered what you needed...

I was gonna say all of those! (no fair!)

The rebar Idea sounds great! Rebar often has a rough texture to it (so the concrete will cling to it). I would recomend soldering wire that's marked up with a hobby knife.

Robert,if you want your diorama to be light but still look very much like congcrete may I suggest using the pink construction insolation foamboard from building supplies stores,it cuts like butter,takes sanding,is very light and when painted a congcrete grey looks just like congcrete,it`s dream stuff to work with. Have a look at Kelly`s 88 or my bunker in Washing Day+1 dio. Hope this helps and good to hear your comin home!

Hi Robert, you can use plaster too..

Cheers