Re: Plaster sculptures outside?
There are a number of monumental plaster pieces made by Camille under Rodin but I think most are indoors.
A less expensive way is to use auto body putty. This however also has weathering problems outside due to metal armatures and mold from moisture which effects all outdoor sculpture in one way or another.
Stone concrete seems to be best but the weight is also a problem since it can be lost if not properly installed with a footing.
Synthetic concrete is something which might be considered since there are acrylics in the composition of this material possibly making it lighter and more waterproof which may be why it can used for driveways and walls.