Have you ever heard of HardiPlank siding and shingles ? It is cementous and the reason why it can work as a roof is that it is in the form of
shingles which dont have to worry about expansion and contraction in the hot sun and cold winters. BAsically each shingle expands and
contracts and the water proofing comes from the overlapping .... and of course the shingles themselves are waterproof. The HardiPlank
and competitors have mesh material or fibers throughout the material and that makes them tough enough to withstand walking on without
cracking. Basically this is man made slate. In your case I realize you are using the concrete to shield an underneath plastic or vinyl layer
but as has been pointed out when the cracking begins it may overwhelm the plastic below and damage the watertight barrier.
However, are you aware of the progress that has been made by mixing portland cement into Exterior Latex paint ? If I recall correctly there
have been some that have taken variously jute, polyester, window screen and cotton or other fiber canvas etc and put this material on a roof over nearly
anything and put several coatings of the latex portland cement over it and they are achieving long lasting water proof results in part because
I think it is somewhat flexible. In the article that I read, they did not put the canvas over anything except air. They merely strung the
canvas over rafters and painted it in place and that served as the entire roof.
Back to your concept of concrete roofs. I believe it could work if you used concrete tiles or squares that allowed miniscule movement between
them without forcing damaging cracks to occur even if they did not overlap like shingles. Of course if they did not overlap then something
underneath them is handling the water proofing task and the tiles are only supplying the UV protection and protection from abrasion if
you ever walked on the roof.