We have a customer that wants to place new concrete in an environment that will be at 800°F during normal use. What can be added to ready-mix concrete to accomplish this?

There really isn’t anything that can be done to normal Portland cement concrete to resist the damage caused by this type of heat. Making concrete out of a different type of cement is a possible alternative, but particular attention must be paid to aggregate selection, as some aggregates expand and contract differently with temperature.