The inner side of an exterior wall in my house has, behind the plasterboard, a kind of masonry which I'd like to identify. I attach pictures. It looks like concrete but it's quite easy to drill: I can very easily drill it with a 2mm wood/metal drill bit in a HAND (i.e., not electric) drill. This material is at least 5cm thick (maybe more), as inferred from the hole I drilled.
This is a UK house built in the '70s (probably second half). The material is definitely not plasterboard (aka drywall).
So, what's the name of that substance? Is it safe to hang heavy stuff (e.g., a radiator) to it using rawlplugs?