[neilep (OP)]

Coal Slag ?


Our house was built circa 1928 and had a coal bunker built in. Found this whilst doing gardening today....coal slag ?

[Halc]

Not much of an expert, but coal slag is usually much smaller and crumbly.
That thing looks volcanic if you ask me.

[Petrochemicals]

Iron work slag is very hard, sort of tungsten carbide hard. Slag and clinker has been used for many years as hardcore and pathways driveways etc, so probably that. Clinker is also used as soil stabilisation in Civil engineering.