Water Proof

Cracking indicates that you had a bad mix.

As far as filling the cracks with grout, mortar, or anything else and having it last.

Well, that's anybody's guess. Only time will tell.

What I can tell you, is if this is a terrace, and it's in the air, filling the cracks will not strengthen it in the least. If there is any risk of danger in the future due to these cracks, (I think there is), I would recommend one of these.

Large flat planes of concrete are very difficult to make with no cracks, since the material is very brittle and weak in tension and slight slump in the sheet = crack. usually not regular. Usually they place thin strips of a flexible material, tar, rubber, etc., to break the panels into sections such that each section is strong enough to resist cracking. Any flexing between two panels is taken up by the tar or rubber, 'expansion joints'.

Online cement construction books like these tell the tale.

Cement pads

cement slab videos

Did you include something like 'BondCrete' in the mix to bond the screeds to the slab and the screeds to each other?

In realation to Bondcrete - I don't believe you can succesfully tile over the stuff - not sure how it works if you mix it in with the tile glue though.

It is used as a waterproofing additive in many slabs which are subsequently tiled.

It is also used to bond slabs together.

You didn't say how big the area was.

For other than small areas, shrinkage cracks are pretty much a given so far as I know. If the cracks are large enough to be a problem - and bear in mind that concrete is not absolutely water proof (i.e. rising damp) unless is treated with additives when made - the only solution so far as I know is one of the membrane type approaches (i.e. paint or film)

Well it is heat of hydration of the concrete. If cracks are bigger you have remove and redo the concrete.

Another percaution is bonding of the concrete with old surface since I presume it to be a cold joint.

Also check the water proofing is crystalline, if it then then the bonding would be a issue.

Most likely your slab is experiencing excessive shrinkage during the initial curing of the concrete, resulting in what is commonly referred to as "shrinkage cracks". There may be several reasons why this is occurring:

1. Excessive water-to-cement ratio in the concrete mix.

2. Inadequately proportioned concrete mix constituents.

3. Excessive water added during floating and troweling the surface.

4. Excessively dry gravel or sand subbase on which the concrete slab was placed.

5. Inadequate initial curing of the slab, generally with lack of a proper cover to prevent drying out of the slab too fast. Slab cracks can be minimized by treating the exposed surface with (A). a spray-on or rolled-on concrete curing compound, (B). laying a poly plastic sheet on the surface, or (C). spraying or flooding the slab with water after the concrete has set-up and hard enough to gently walk on. The key is to reduce the heat of hydration (heat generated by the chemical reactions within the concrete) and prevent the exposed surface to dry out too fast. Protective measures should last at least 7 days after the concrete ha been placed......14 days or better yet, 21 days is much better and results in a superior end product.

It appears that it is too late to do proper curing of the slab if cracks have formed. If this is a slab on grade and not a suspended slab and if the cracks are very narrow I'd ignore them. If you're in a warm climate and don't experience winter or a lot of yearly rainfall, and they are wide and deep, then you must rout out the cracks to at least a 1/4 of the slab depth and apply some sort of chemically modified cement mortar into the cracks. There are many such commercially available products on the market. Just make sure you can match the cured concrete color and texture, otherwise the repairs will be an eyesore and totally unacceptable!

....I think if you can tell the size pattern and extent of cracking,screed mixing ration grade wrt to the under laaying screed grades,nature of the foundation it was layed on,sand,clayey??(..not floating in the sky I guess..) at least one can tell something helpful..I am tryng to guess the reasons to be may be dirty between surfces or any other agents which weakenend the bond between layers layed,weak underlaying layers that it could not carry additional loads,temperartures/weather that it forced rapid evaporation of water from the screed....etc......to fill in grouts in cracks? ow! for efficiency,try to check nature,pattern and extent before deciding for grouting..