Good morning Microcaps.

 From the little I could find on the subject, I think Cemex orders the iron ore by weight  per cubic yd most likely. So they probably get a gravel-type mixture, with anything from fines to 1" rock/ore  in size, as long as it meets their weight criteria, which in turn determines the density of their specialty heavy concrete.. If they need less heavy gravel for a mixture, they probably add non-iron gravel at their own plant. For the 330#/cu ft stuff, ( their highest density stuff) they probably use just the ore/gravel CWRN ships them. Once again  I'm only taking a semi educated guess here.. 

 Have a great day longs! Keep the fingers crossed! I'm almost healed up enough from the weekend to party and happy dance again!!! LOL!