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Isolation strips were in place. Heating wasn't run up because client doesn't have the boiler commissioned yet. Could the lack of controlled drying cause this?
Anhydrite needs to be force dried slowly before it's covered or loaded. It has a longer natural dry time than sand:cement. Without the heating cycling it could be drying unevenly from the surface, causing differential shrinkage and cracking. Get a moisture meter reading across the slab. If it's still showing high moisture in the middle you'll need a controlled drying process before anything goes over it.
Also check whether the slab below was pre-treated to prevent moisture transfer upward. Anhydrite won't bond to a wet substrate and the movement from below can cause cracking from the underside up. DPM continuity is critical.
Doorways and corners are classic stress concentration points. With anhydrite you need to make sure isolation strips were installed around all perimeters and at doorways before pour. If those weren't fitted or were disturbed, you'll get cracking at exactly the points you're describing. Was the heating system run up through a slow drying protocol before the floor was walked on?
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