If I want to use a cement floor (with broadcast color hardener) over a concrete slab (laid over compact soil), should it adheres to it, or be separated?
The concrete slab is rough, the cement layer over it will be about 40-50 mm thick (just cement and fine sand) and will be formed in squares of about 2x2m.
If I make an adherence bridge (using cement paste and adhesive additive) the cement layer will be an integral part of the concrete slab and moves with it. I see three problems: 1) the cement layer will lose a lot of water the concrete slab during curing; 2) any movement on the concrete slab will show on the surface of the cement; 3) moisture from soil might travel upwards to the floor.
On the other hand, if I use a separating layer (like a plastic film) between the slab and the cement floor I believe (not sure) the curing of the cement will be better, movements on the slab wont show directly to the floor, it might act as a moisture barrier. Is there any downside to it? Will it affect the strenght of the floor in any way?
Below in blue is the cement floor, in gray the concrete slab and in red the separating layer/adherence bridge: