Generally, aggregate==gravel or crushed rock, and there's no reason to put that in covered hoppers. Open hoppers do nicely.
There are a _lot_ of other things that can be hauled in covered hoppers other than grain. Any dry, bulk commodity that can't be exposed to the weather. Things like salt, potash, fertilizer, etc. are all hauled in covered hoppers. Once a car gets downgraded to this sort of service, it doesn't usually go back to food-grade clean lading for grain.