Repair rendering on concrete slab

[roband915]

How to best repair this?
The house stands on a slab and I initially thought that the plaster had detached, but inside the plaster there seems to be lecablock. As the picture hopefully shows, not only has the plaster but also some of the leca material detached or is loose.

 

[roband915]

I don't quite understand where the leca comes from, so to speak. Why do you cast a slab and then lay leca blocks around it and then plaster on the leca? Or is that how it was done in the 80s?