Patching and Plastering - Concrete slab ceiling (with popcorn texture)- how to even it out?

So here we are - we just bought a townhome and started our first major project and what can I say? We need your HELP!

This is the situation:
The whole main floor has popcorn texture on ceilings. The ceiling itself is made of concrete slabs. These slabs have gaps in between them. The slabs themselves are not that level - there is up to a 1/2 - 3/4 inch difference between a few of them. There is some sort of caulk filling these gaps.

So far we have done the following:
We have removed the popcorn texture - it came off beautifuly exposing very smooth cement surface.

What do we do next?
- What product should we use to fill in the gaps and even out the differences between slabs? We need someting that won't crack and will adhere well to very smooth concrete surface.
- Do we need to use any bonding agents?

Your advice is very much appreciated.

Thank you. :)