i have a concrete floor and i want to screed the floor and i want to screed on top of it, what would be the best mix of sand and cement to use and should i use any pva's etc to key the mix to the concrete sub base
$ sharp sand or flooring grit (pretty much the same thing) to 1 cement. Mix with a little water until you can squeeze a lump in your hand so that it stays in one lump but no water runs out. No need to do anything to concrete base except make sure its free from dust and loose stones etc and wet down before you lay the screed. The whole floor should be damp as you lay.
What area and depth are you covering?If it's a big area ready-mixed sand/cement screed is the best option for a consistent mix. If you plan on mixing your own then 4-1 sharp sand/cement is the norm. If you are going to a depth less than 6mm then a latex or self-levelling compound is the best idea.