I use plaster of paris (powder) routinely for dio groundwork. It's cheap and readily available from hardware stores, but because of its fast setting time ( a few minutes) it can be tricky to use. There are a few things I do to make it easier:
-mix small batches at a time. You can always add more later on, even when set
-add some black poster paint while mixing, as the white colour is bound to show up through the groundwork in some places
-add some white glue to stop it cracking once it has set
-pour it only in thin films over contours built up with polystyrene foam. It is heavy when used in large amounts, but that can be a bonus if you want to add stability to your base.
Other than that, it's quite versatile. You can drill, sand, engrave and glue it easily. Leftover pieces can be crushed up and used as debris for the terrain.