The manufacturing process of cement begins with mining limestone and other raw materials. After quarrying, the rock is crushed.  The crushed rock is then combined with other minerals, like clay and iron ore, in a cement kiln. Cement kilns are most often fuelled by coal, however, the use of alternative fuels in cement kilns is now widespread and increasing.The resulting substance of heating these raw materials at high temperatures is called clinker. Clinker is ground up by cement plants into a fine powder and mixed with gypsum to produce cement.