Robin writes:

Yes, I had been wondering why no one had checked if these
were old style breeze blocks made from cinders and clinker
(and any old rubbish). Unless it's only me who still meets
them.

Not only you. The upstairs internal partitions in our house are
made of the stuff. Horrible dirty stuff, and unexpectedly thin as I
discovered to my embarrassment when sinking a deep backbox for a
shaver socket.
--
Jón Fairbairn
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2014-04-05)