Our concrete contractor and his concrete supplier offered us a lower cost concrete if we allowed them to make the concrete out of the waste from a blast furnace, do I want to say yes to this offer, the savings were attractive to us.
The impression I am getting from your question is the amount of concrete you are going to buy is significant to your budget. I suppose if the price offers a 10% savings, you should entertain the offer and turn the "mix" over to someone you trust in the concrete design business and see if the savings are equal to or better than any benefit you may be giving up.
Business analysis aside, the two companies are more than likely offering you fresh concrete with a "slag cement" replacing a portion of the normal Portland cement. The term "slag cement" is a short version for "Ground Granulated Blast-Furnace Slag (GGBFS) Cement." Now you know why they shortened the name. Let us see if I can put the events in the correct order concerning the source of GGBFS, first comes the blast furnace, it is used to reduce iron ore into iron, using very hot air and fire.
Second in the process is the blast furnace slag, it floats on top of the molten iron, it is mostly a silica with some trace of aluminum. The molten slag is drawn off the top of the molten iron and the slag is quenched with water, causing it to form glassy granules.
The story is almost finished, the granulated blast furnace slag is ground into a face powder consistency and stored to be added to Portland cement concrete, as an economical additive to concrete and still provide good concrete at a savings to the buyer.
Concrete can appear lighter in color when GGBFS is added to the fresh mix, and some customers prefer the lighter color. Once you have gotten your friend's opinion as to the wisdom of using GGBFS cement in you own project, you'll be able to save money and possibly enjoy a lighter color concrete.
Your question was very interesting.