It's not great, but neither is the alternative. The cement industry is the third-largest energy consumer in the country. Tires are used because they have 25% more energy than coal and the resulting ash has much less heavy metal content than coal. The alternative has been landfilling them which can have some very bad results. The tires (53 million a year!) are burned at a very high 1800º which ensures complete combustion.Burning tires is horrible from an emissions point of view. You can also be privy to the details with Google.
I did a little research and found that a staggering 130 million tires a year are burned by various industries!