Shape,
If your ruins will be primarily stone block structures, you might look into papercrete (shredded paper + cement + Perlite + misc. aggregates). The material is lighter in weight than cement because of the additions. It will extend your quantity of cement, can be hand-formed or molded, and the finish may have little voids (doing most of your "ruin" work for you). It weathers beautifully and accepts paint, stain, and color very well.
I use it to make planters and mini garden houses. Here's one:
Here's how I made it from and orange juice carton:
http://www.walkerswayweeds.com...
