No primer

[LM]

I've seen jobs where cement based adhesive has been applied to gypsum plaster and there's never been a problem.

So have I in the past and I can't explain it, but I'm no chemist and I'm not taking the risk for the sake of priming a wall

 

[White Room]

Me neither

 

[Aston]

Just use your common sense and experience and if you don't have either of them, go with this old saying, "if in doubt prime"

 

[SJPurdy]

Then we come back an age old question, how does gypsum plaster finish survive on sand/cement undercoats.

As I understand it the ettringite is formed during the hydration phase of the portland cement. If the sand cement is already set then skimming over it with plaster should be ok because it is only the plaster that is setting.
But if the wet Portland cement mix is applied over a set plaster surface then the ions required for the formation of the ettringite can be taken from the plaster surface while the cement is hydrated and so the crystal ettringite layer can form where the two meet; giving a weaker bond.