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Dammit
In the UK the general rule of thumb is that rebar must have 50mm of 'cover',
the reinforcing steel must have 50mm of concrete between it and the atmosphere to prevent the steel being exposed to moisture and oxygen to prevent the formation of rust.
Rust is lower density than the parent steel, and exerts a concrete splitting force as it forms.
Best bet, roughen the surface of the slab, probably too late to just rinse off some the cement slurry which I hope your builder vibrated to the surface, so either acid etch it with Hydrochloric Acid, (which you used to be able to buy at around 36% acitve in Janitorial Supplies distributors),
or mechanically abrade it with a chisel end on a (sizeable) SDS drill on 'Chisel function'.
You want to expose the tops of some the larger aggregate in the concrete.
A reasonable layer of PVA, won't do any harm as a priming layer then apply whichever screed, (polymer modified and shrinkage compensated/dimensionally stable), you can find in bag form locally. A 25kg bag should be not much more than £30.