We have a local gravel pit/concrete plant that does a similar thing. They will burn almost anything in their kiln, and I mean that literally. It's amazing how easy things will burn at 2000+ degrees F
They've only found one or two things that will degrade the integrity of the concrete. I think tires is one of those things, but I can't remember. Other than that, they burn it and then use the exhaust heat to generate power for the grid. I've heard that they make more taking in trash and selling the power than they do selling the concrete.