W
White Room
Based on your description it could be a magnesite screed... Don't see them often these days. If it is it will be red right through (some were green) and they were used up to about 25years ago especially in old council properties. They don't stick to concrete hence they can delaminates and crack. You will find it almost impossible to stick anything cementitious to it and most primers will fail to work as well. Best thing to be done if it is magnesite is break it out and reinstate it with something else.
In about 43 years in the game i have never seen these screeds laid...