What are these crumbly black blocks?

[Astronaut_elvis]

Can anyone tell me what these blocks are called/made from? They're in my 1950's ex council house, mainly internal walls but have seen them in the inner leaf of the cavity wall too. They're only 60-70mm thick, look aerated judging by the hole-y inside appearance and really difficult to get a strong fitting into as it just crumbles away.

I want to hang a very large and heavy picture frame on a wall made entirely from them so was thinking either:

Option 1 - self-tapping concrete screws (https://www.screwfix.com/p/easydrive-tx-countersunk-concrete-screws-7-5mm-x-60mm-100-pack/3313h) as I managed to get some good fittings holding the skirting boards on with these if I drilled an smaller hole than on the instructions

Option 2 - some big 50mm rawl plug held in firmly with some gripfill or similar

For extra security I will do two fixings 300mm apart - is this enough?

 

[opps]

I think they are referred to as cinder blocks.

My home has them for the interior walls. I normally just use the 7mm brown nylon plugs.

The ones in my home are far stronger than the likes of thermalite blocks. I have hung 25kg items (eg a plasma tv with an articulated wall mount) without issue, and only used brown plugs.

 

[Tricky-Dicky]

Yes, Cinder Blocks, made from power station waste/ash.
Last house, all internal walls made from these blocks.
Tips, use longer and larger wall plugs than usual, I always used brown plugs.
Drill a smaller pilot hole first, then the second drill to the required size, do not use hammer action, drill straight in then immediate out, do not wobble the drill! as it's very east to enlarge the drilled hole. If the wall plugs required say a 7mm drill bit, try a 6mm drill - it will probably be large enough.
Regards

 

[^woody^]

Use polyurethane wood glue and a plastic plug. This type of glue expands to fill any gaps, and you can get fast setting types.

 

[opps]

Yes, Cinder Blocks, made from power station waste/ash.
Last house, all internal walls made from these blocks.
Tips, use longer and larger wall plugs than usual, I always used brown plugs.
Drill a smaller pilot hole first, then the second drill to the required size, do not use hammer action, drill straight in then immediate out, do not wobble the drill! as it's very east to enlarge the drilled hole. If the wall plugs required say a 7mm drill bit, try a 6mm drill - it will probably be large enough.
Regards

I largely agree with you.. but our cement plaster was so hard that I recall bending a 7mm drill bit as it over heated whilst trying to penetrate the plaster coating. Purchasing a SDS drill made my life so much easier.