Lightweight material under hearth, what is it?

[Douche]

Hi there,

The tiles on the old hearth slab in our house have become loose. What's underneath them is a kind of cindery lightweight material that looks to have been backfilled, like you would with concrete, and strengthened with metal rods.
What is this material please, it wouldn't be asbestos related would it?

I've attached a file.
Thank you,
Douche

 

[Douche]

Just to add this was done in the mid 80s.

My first impression was that it might be a vermiculite based screed.....?