As mentioned the screed does need its first initial expansion if tiled directly too it. Alternatively you could use Bal flexbone uncoupling system, this a none adhered matting.
www.bal-adhesives.com
BAL Flexbone 2Easy loose-laid uncoupling mat | Tiling Product | BAL Adhesives
Unique loose-laid uncoupling mat where no preparation is needed. No priming, no removal of laitance, no removal of residue!
