Recycled crushed stone for floor slab

I have now inquired with builders in our construction area who have already completed construction and also used recycled material. The houses are still standing ;) A written approval was not necessary for them after consultation with the lower water authority.

The houses are still standing, which is quite funny.
No one will ever look under your floor slab. And if they do, it’s bone dry thanks to the frost skirt.
Therefore, I’m annoyed that I have XPS insulation for KfW40 under the floor slab.
In theory, it helps. In practice, it’s 'almost' wasted money.
Gravel is X times better. And even with fill sand, the structural integrity is met.

Here you are mixing up a lot. Whether someone looks under the slab or not does not prevent the house from settling. And your XPS insulation is not supposed to prevent the house from settling or prevent water from penetrating, but to improve the insulation.