steel mill slag

what uses would there be for that black slag from the steel mills? could it be used as filler in the mass heater? maybe have some insulating properties for use in riser? i see a lot of people using it for their driveways around here. it reminds me of grill rocks when held.

From John Seymours classic 'The Self-Sufficient Gardener'

"Indecently if you live near a steel industrial center, basic slag is an excellent phosphate mixture'

Probably great for thermal mass too.