I was planning on adding a donut to the cast iron hub, but when I was removing the lead seal, the hub broke off entirely. Now I have a broken off 2 inch cast iron pipe right beneath the slab. How do I go about connecting the pvc to this? I was thinking of just using a compression fitting now. Thoughts?
A few more details would be helpful. How far does it go straight down after the surface of the concrete? Where is it going above the concrete? As in is it straight up to a drain? Or up to the next floor? Is it a vent pipe? Or a drain? Or water supply? You mentioned lead seal, so I guess a drain pipe or vent.
PVC 2" should be the same outer diameter as the cast iron. You get one of those rubber couplings. Half of the coupling on the iron, and half on the PVC. I did something similar with 3" after cutting/breaking up my concrete floor in the basement, but that was horizontal. Then put the concrete back over it.
You need to remove enough concrete to allow you to tighten the hose clamps that tighten the rubber boot to the pipes. Use some sandpaper to make the outside of the cast iron smooth for the rubber boot. Some plumbers grease will also help slide the boot over the pipes more easily.
Why did it break? Just something to think about to prevent the repair from failing.
Since this is 2", I'm pretty confident that this is not for a toilet. You could see how tight an 1.5" PVC pipe will fit. If loose, try a 2". If the 2" fits fairly tight, you could glue a 2" to 1.5" adapter onto your new 1.5" PVC pipe. Clean out the iron pipe as best as you can. If it's rough, that's fine. See if the 2" adapter will fit inside of the iron pipe. This should be pretty tight. Take a heat gun and warm up the 2" adapter until it starts getting pliable. Jam it into the 2" iron pipe and hold in place. Give it a minute to cool and see if that will hold. If this doesn't work, take your assembly dampen it and the iron and apply Gorilla glue liberally, then stuff the 2" adapter back in and secure. The Gorilla glue will expand to fill any gaps. The stuff will stick to almost anything. It should fix you up without hammering out the floor.