Floor Screed laying - water content

What will be the penalty to pay if adding a bit more water to the screed mix to make it more workable? When passing a straight edge between screeds it tends to drag some of the material and leaves blemishes. Also, the very dry material is difficult to get all around the underfloor heating pipes. I am only following the advice they give that says to mix the sand and cement quite dry.
Another one: if lifting the screed guides buried in the screed after levelling the top they leave a hollow. When is the right time to fill these hollows?
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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.
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You should tell us what you are doing. Basement floor slab? Mud bed for tile? Do you need a smooth finish? The wetter concrete will give you more shrinkage and the concrete will not be as strong. However, even weak concrete can be strong enough for some applications, and wetter concrete is easier to move around in the forms and to finish to a smooth surface. On the other hand, if you are doing a thin layer as a setting bed for tile, you want it fairly dry since the finish doesn't matter but strength and shrinkage does.
Holes from screed guides should be filled as soon as you pull the pipes and before bull floating (if you are bull floating that is).
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To clarify what I am doing, it is a floor levelling finish to apply tiles and wood floor on top of it. As I said, water underfloor heating will be buried in it. It is 65mm thick and is for a ground floor in a new house. Underneath there is a beam and block concrete base and 4" polystyrene insulation on top of it
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