I am going to experiment with using metal as an aggregate. The idea is that as the metal rusts, it stains the concrete. The metal should also provide structure to the concrete similar to GFRC.
Acquisition of material
These filings are black iron. At Home Depot, metal shavings are disposed of separately from other trash. Because of their sharpness they are considered to be hazardous waste. They had just emptied their disposal bin so all they had available when I visited was what was in the machine. I took what I could get.