Particles of refractory material in contact with aluminium can detach and become inclusions. We can find graphite inclusions (C), alumina inclusions (alpha-AlO), CaO, SiO, …
After some time, graphite refractory in contact with aluminium will react to create aluminum carbides (harder and more detrimental inclusions).
In aluminium alloy containing magnesium, the magnesium reacts with some refractories to create rather big and hard inclusions similar to spinels.
Unreacted refractory particles can originate from the degradation of refractory materials which comes in contact with the melt.