Thanks for your response. Sorry for the delay in getting back to you.
Actually I believe that the plaster was sprayed over concrete! (This is a postwar apartment building.) That's a guess, because I haven't dug down to investigate. The problem is that the area in question had about 1/8" of paint on it over the years, much of which chipped off. But it also had several plaster patches in spots, most of which hasn't chipped off. So, I was hoping to even out the difference.
Mixing with sand -- wouldn't that be awfully heavy for a ceiling application? (Just a thoughtful question.)