I asked the same question recently on another forum
here is reply from a tiling restoration expert
Nice easy job for you. Aqua Mix Sealing & Coating Remover let it dwell agitate and keep wet that gets rid of paint, waxes and will break down any Polymers in the screed. Then add Aqua Mix Phosphoric Acid to the mix let it dwell and agitate that will get rid of the plaster and floor screed cement properties. Then wet vac the lot up and clean the whole lot with Aqua Mix heavy Duty Tile & Grout Cleaner. Rinse the whole lot with water and hey presto you are ready to seal.
Kev Martin
Tiling Logistics"Natural Stone Renovation Specialists" Trained Applicators and Distributors for Aqua Mix.
www.tilinglogistics.co.uk e mail:
info@tilinglogistics.co.uk Tel: 0121 705 5333 M: 07770 598855
hope this helps, Andrew