Slag is commonly used as an aggregate for road beds if it is a combination of 1" down to fines. Specific sizes are used to replace similar sized gravel in some applications. The slag is primarily limestone and impurities from the blast furnaces.
Back in the 70s, I hauled many loads out of The Levy Company, next door to Bethlehem Steel in Gary. In fact, the base for my driveway is #53 slag I picked up there.
Edit: Here's a picture of my dad on a TD15 pushing slag that we kept stockpiled in our yard. The slag was a gray color, if it doesn't come across in the picture.
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