Hey Angelbleu,

You might want to cast some pieces then break them apart. Then glue them to your building to add ruins. You could also take the pieces straight out of the mold and break them before they have fully set yet.

Here are some examples of what i have done to make ruins. I use hydrostone and it does take some effort to break the pieces.
http://www.voidgamers.com/click/HirstArts/HADD8/b.100.r.25712.html/DUKEDREAL-City_ruin_1_view6.JPG
http://www.voidgamers.com/channel/HirstArts/HADD8/b.387.r.408.html
And here is one of what others have done to make ruins. http://www.voidgamers.com/channel/HADD9/b.400.html

Hope this helps
Duke