I did some electrical work at the other side of the living room a few months ago, and I left the drywall repairs until now.
Part way through the transition...
The concrete was treated with a sealant - one coat of 50/50 mixed with water, and after twenty (20) minutes, one Capstan full strength Navy Cut coat. The second coat took considerably longer than twenty (20) minutes to dry which indicates the concrete is well and truly sealed.
Poured in my leveling compound; I needed the best part of two (2) 50 lb bags to do both sides of the fireplace.
I had to ease the leveling compound into some of the nooks and crannies - this indicates the mixture is slightly too viscous.
It came out alright:
T'other side was done int sem whey. A blind man would have been glad to see it.