What is the correct material to sheath pex pipe under a slab?
You'd install PEX without sheath inside the slab for underfloor heating only. Otherwise it should be sheathed to allow it to comfortably expand depending on temperature variations, it also makes it possible to change the pipe if it bursts due to freezing, or bad quality pipe for example. Sheathing also protects against UV light which is bad for PEX.

Why not simply purchase pre-sheathed pipe?... (link to my favorite shop areound here). It is a little bit more expensive than naked pipe, but it saves a lot of effort. You can get pretty much any type of pipe with the sheath already on, you can even get them pre-insulated.



I had (once) to shove about 10m of pipe through a sheath, and it was a lot of work, and we did it in the garden with the sheath held straight between two trees! The PEX pipe wants to curl back as it was when it was rolled, so it catches in the sheath corrugations, even with the end tapered and lubed it was a full body exercise...

If you want to do that... get a sheath that's a bit too large. For a 20mm pipe you'd get a sheath with 25mm ID, get 32mm ID instead, that will be a lot less work! And the larger ID allows you to stick a rounded end on the pipe (like a copper pipe endcap) which will make it slide a lot better...