Cement kilns are used for the
pyroprocessing stage of manufacture of
Portland and other types of hydraulic
cement, in which
calcium carbonate reacts with
silica-bearing minerals to form a mixture of
calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of kiln efficiency has been the central concern of cement manufacturing technology.