I assume the seal on a screed does similar as one does for natural stone. It basically soaks into the stone and on curing effectively waterproofs the surface and binds it together. The absorbancy of the stone determines how many coats it requires as you keep applying until it doesn't take any more. About three years ago I did 400 sq mtrs of sandstone tiles and it needed 6 coats before it stopped sucking it in.....that hurt the budget big time. I have experimented with putting a seal on a regular screed. Although it sealed it quire successfully I wouldn't say it made the surface any harder wearing as it was quite easy to scratch. You get epoxy seals so maybe they might create a harder surface but I don't know.
