I have a British house dating from the 1890s to 1940s - it was partially destroyed by a rocket in the war, so some bits are old and some are new. I am trying to identify the composition of an internal wall.
This is the wall in question:
Specifically I am trying to work out what the section of grey material is. The grey material seems quite hard: harder than mortar and approximately as hard as brick. It's about 30mm thick.
The overall thickness of the wall is about 180mm and the centre of the wall appears to a single layer of standard red brick.
