Is it ok to bury concrete rubble under my garden?

I'm in the process of landscaping my garden. As part of this, I'm knocking down a garage in my garden. The builder has broken up the concrete slab but is stating the it should be ok to bury the rubble.

I'm less certain. A broken up concrete slab will contain a lot of alkaline. I'm also concerned about drainage.

If left, the rubble will be ~ 40cm below ground level. Part of the buried area will be turfed, part will be planted with flower beds.

Advice appreciated!