Self Levelling Compound Has Hairline Cracks After Walking Over It. 20mm Insulation?

Hi everyone,
Hope to get some help and advice please.
I have laid down 8mm of self levelling compound on a concrete floor after priming it so it was a good level to work off (I also included an 8mm expansion joint around the perimeter) (Conservatory is 15 years old and area of 14 m2).
I then primed the SLC one week later and left till next day when I used a recommended rapid set flexible tile adhesive (Trade S1) to stick down the cement faced insualtion boards (20mm thick boards which I primed both faces and also sides before fixing into place).
I then primed the top of the boards again 2 days later.
I then stuck down fibreglass tape on joints using the same tile adhesive.
I then primed again once all was dry (again this was two days later).
I then poured Mapiei Renovation SLC (3240) onto boards between 5-10mm depth in places (mixed as per instructions 4.25-4.5L per 25kg bag with paddle mixer).
I used a trowel to move the SLC around and spiked roller to remove as much air as possible.
Left the floor alone for a few days to be on the safe side and then I walked on it. All seemed ok and then next day whilst walking on it again I noticed some long and also some short hairline cracks had appeared over the whole floor, these were not in straight lines and were not coming from one particular spot. As the day went on whilst I was working in the room I noticed that more of these hairline cracks had appeared obviously by me walking on the floor.
On close inspection, I tapped the SLC with a screwdriver handle in different places and there is a hollow sound which I know may mean it has blown but I dont think it has in my opinion and put it down to the 20mm insulation board just below it which is making this type of sound. I have drilled quite a few holes in various places around the floor area to check if I could see if there was any gap between the insulation board and SLC but I found none.
I got a screwdrive and taped it with a hammer around one of the drilled holes and found that the SLC was very hard and brittle and I'm sure I heard a cracking sound coming from somewhere whilst doing this but not sure whether is was something else I heard as it does echo a lot in the room with it being empty.
My plan is to prime the floor again and pour another 5mm - 6mm SLC on top to get a perfect level and then to stick down Ditra Heat Duo Uncoupling Membrane so I can install electric underfloor heating and tiles on top.
The tiles that I will be putting down are 610 x 610mm x 9mm ceramic porcelain mix.
Any thoughts whether its ok for me to proceed as per my plan or are these hairline cracks a nightmare and means I would need to rip everything up and start again?
If I need to rip everything up then your opinion what has caused these hairline cracks to appear? is it the compression from when I have walked over the floor etc?
Let me know and thanks in advance for any help and advice.
Oh by the way I used Mapie G primer for the SLC and used Trade primer for the Trade tile adhesive so keeping same brands together.
Regards,
Gary.

Hi, it is going to be difficult to explain exactly why the SLC has cracked, it could be because of direct sunlight on the product causing it to dry out too quickly, trafficking or a product issue, which is vary rare.

Ideally you should rip up and start again, but if this isn't an option I would do the following checks.

First thing I would do is run a level over the floor to check that nothing has pulled in too much (shrunk), if this is the case the leveller will have a dip in the middle, and the outer edges of the floor will sound hollow and be the high points.
Next I would remove a small section of the SLC in the area where there are the most cracks, to see if it is well bonded to your insulation board.
Traffic the floor to see if you can feel any movement, stand with a foot either side of a crack shift your weight, see if the crack moves
If all seems to be ok then you may get away without ripping up, I would fix the Ditra heat straight on to the cracked leveller as it also works as a decoupling membrane.

Thanks for the reply Jon, I will have a good look at the floor tomorrow when I'm next at the house.

Standing either side of the cracks will probably flex a bit because of the 20mm XPS insulation boards directly below the SLC as they are not rock hard boards but I know they are well bonded to the concrete base. I will do it anyway to check to see what happens.

I will certainly remove a section of the compound where most cracks are located to see if it has bonded to the insulation boards.

If the SLC hasn't bonded to the boards then I take it that I woud have to rip the SLC up and do it again even though I would be using Ditra Heat Decoupling Membrane on top, is this what you are saying?

If I rip up the SLC will it destroy the 20mm cement faced XPS insulation boards?

Thanks again for your help,

If anyone else who has experience on things like this please make comments in regards to advice etc as it would be very much appreciated.

Cheers,

Gary.

If the SLC hasn't bonded to the boards then remove it and start again.
If the SLC has bonded then fix the Ditra Heat to it.

ok Jon will do and many thanks for your help, much appreciated

I have inspected the SLC and remove some small sections from various locations and taken photos. SLC is bonded to the boards see photo of pieces removed.
Are these drying out hairline cracks or is it because I have walked on it after 24 hours and cracked it because of thin layer on top of 20mm cement faced XPS boards?
There are about 20 of these cracks all over the floor ranging from 600-1800mm in length.
What do you think has happened here?
I want to apply another layer of SLC on top as I need to increase the height of the floor so should I tape these hairline cracks before applying the extra layer of SLC? If so what's best way of doing this?

Ideally the floor should have been levelled in one hit, you can try taping the joints but the crack will normally appear again in the second coat. As I said the Ditra will stop the cracks going through to the tiles.

I contacted Mapei Tech Support today and they say its cracked because of walking on it as an 8mm depth of SLC on top of 20mm XPS board will give a bit so they have recommended to use Mapei Eporip to repair the hairline cracks before applying another layer of SLC. They say this stuff with bond the cracks together so making the floor strong again.
I have ordered some of this stuff and fingers crossed with the Ditra hopefully the tiles will last me many years without cracking.