Cheap and nasty concrete.

[JohnY]

The RAAC problem, although serious, is peanuts to what comes next. Until 1999 asbestos was widely used especially in system built buildings. When the RAAC collapses or is disturbed then asbestos is also disturbed.
theconversation.com/the-hidden-danger-of-asbestos-in-uk-schools-i-dont-think-they-realise-how-much-risk-it-poses-to-students-203582

[don]

Seems to have gone quiet on this issue since the original announcement maybe we have an election in the offing

[JohnY]

Everyone might be waiting for a hospital roof to collapse.
The material has a decent life if it is kept dry and kept under modest compression. Its fine for infilling walls that are not structural.
It is not suitable for beams. There were also quality issues with the placement of steel in beams; sometimes the steel was off centre longitudinally, so that in use the beams were supported by an unreinforced end.
As I mentioned earlier there is often asbestos used in conjunction with RAAC. It is safe, sort of, if it is undisturbed. Often, in order to search for RACC, it is necessary to lower ceiling tiles or insulation. Sometimes there may be asbestos dust resting on the normally hidden top of the tiles.
There's a long way to go with this story. Both major political parties would rather hope it would go away.