I am trying to find out how to mud and tape a new wall with an old wall. and new wall to existing 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.