Pat, If the pieces have smooth edges and some flat surface plain old white glue will stick plaster together just fine. Its got enough water in it to penetrate the plaster a bit so the pieces will bond.
If you want the edges to pull tight together try using some DUCCO cement (a kind of glue). It has a property that it shrinks as it dries and will sorta pull the pieces into each other. However the bond won't be as strong.