Water.
You ask what ratio there should be components? It all depends on the purpose of the floor.
If the room has too intense cross -country ability, then cement and sand should be in a ratio of 1×3 (that is, 1 part of cement and 3 sand). Thus, the screed will turn out to be particularly strong. First of all, this applies to public buildings, where day after day it provides for the passage of a large number of people.