Walls and Ceilings - concrete ceiling

I live in a large apartment building built in the '60's. The kitchen ceiling has been in bad shape for years, so yesterday I removed the peeling paint and the loose (plaster or mud?), which lay about 1/8" to 1/4" over a cement slab. First, without seeing this stuff, can anyone tell me if this is most likely plaster or mud? (to me it looks like dried mud). Second, what do I use on the cement slab to build an even surface (about 10-15 sq. feet need to be finished). I would strongly prefer to use drywall compound if I have a choice of alternatives. Any other advice on how to proceed with this work would be gratefully received.

You probably scraped off plaster. You need to get all the loose debris off of the ceiling. Then I would prime the concrete with a good concrete primer/sealer. Then you can skim smooth with drywall mud. Three coats should do it.