Before laying tiles or other floor finishes onto certain screeds, one of the most important tasks is the removal of the laitance (a thin crust that forms to help with curing.) Laitance should be removed by using a suitable sanding machine. The best time to do this is 7-14 days after the installation of the screed, this also helps to dry the screed as it allows the moisture to escape.
We wouldn't go any where else for our Floor Preparation, Farren Krew Ltd have set the bar. 
Mr Rose