my basic question is: what is the proper material and method for repairing defects on the walls in my old house? To give context, I am in Virginia, United States and the house is a concrete block construction Cape Cod style built in 1946, and I believe the walls in question are all original.
My first issue is identification - I'm not even sure "plasterboard" is the correct terminology for what these walls are made of. They are definitely not modern drywall / gypsum board, they are more dense. I don't suspect they are lathe and plaster, but instead some kind of sheet material that seems close to "cement board", with a plaster coat on it. The best clue I have is where there are penetrations through the walls, at outlets and light switches:
So what is this really called? I would be willing to drill some exploratory holes (perhaps in a closet) if that helps identify, but currently there are no larger openings or exposed areas. I feel like proper identification will help me track down better resources on this, but for the moment I'm a little lost on what it actually is.
The second and ultimate question is what's the proper material and method for surface repairs on this type of wall? I have built and repaired plenty of drywall before, but am not sure what's appropriate for this type of wall. Is it a problem to use drywall joint compound? That would be the first obvious choice, but I'm wondering if that's detrimental for some reason, like over the long haul it will not adhere properly. Or should I be using some kind of plaster mix?
The types of defects I want to fix are places with some cracking, previous screw/nail holes (from hanging things, some of which were obviously patched with drywall compound by someone previously with little care for detail), and a couple spots where the surface layer is broken off: