You have me a bit confused. What are you doing?
In real concrete each gallon of water added to a yard of concrete reduces the ultimate strength of the concrete by 500 PSI and reduces the slump about 1". Not knowing what you are doing makes it hard to help you, and I have no idea what strength material you need or what strength design mix you are starting from.
Sand and cement with no stone is called grout. Grout is usually used for a thin topping. It costs quite a bit more and is weaker than concrete which adds well graded stone to the mix to extend the materials. If you are pouring over 2" thick, use concrete.
A screed is a temporary guide to establish finish grade. It can be a wet screed based on pins driven to grade, pipe screeds are well supported steel pipe, lumber can be used, and metal screed key can be used and left in. Screeds can be set with their tops at finish grade or their bottoms set at finish grade. If you set the tops to grade and pull them, they are pulled as soon as they have been used and you float in fresh material to fill the ditch they leave immediately so it is homogenous with the rest.
If the rod you are using to pull grade on the screeds is tearing the top, try sawing it back and forth. The surface left by the rod is usually not meant to be finish. The finishing comes from hand or machine floats and hard trowels.
I have no idea if I've helped or not.