A cement mill is the equipment used to grind the hard, nodular clinker

Clinker (cement)

thumb|200px|right|Typical clinker nodulesthumb|200px|right|Hot clinkerIn the manufacture of Portland cement, clinker is lumps or nodules, usually 3-25 mm in diameter, produced by sintering limestone and alumino-silicate during the cement kiln stage.-Uses:...

 from the cement kiln

Cement kiln

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...

 into the fine grey powder that is cement

Cement

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

. Most cement is currently ground in ball mill

Ball mill

A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...

s.
Early hydraulic cements, such as those of James Parker

James Parker (cement maker)

James Parker was a British clergyman and cement manufacturer who invented one of the pioneering new cements of the late eighteenth century.In 1791, he was granted a patent "Method of Burning bricks, Tiles, Chalk"...

, James Frost

James Frost (cement maker)

James Frost was a British cement manufacturer who invented processes that led to the eventual development of Portland cement.- Biography :...

 and Joseph Aspdin

Joseph Aspdin

Joseph Aspdin was a British cement manufacturer who obtained the patent for Portland cement on 21 October 1824....

 were relatively soft and readily ground by the primitive technology of the day, using flat millstone

Millstone

Millstones or mill stones are used in windmills and watermills, including tide mills, for grinding wheat or other grains.The type of stone most suitable for making millstones is a siliceous rock called burrstone , an open-textured, porous but tough, fine-grained sandstone, or a silicified,...

s. The emergence of Portland cement

Portland cement

Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 in the 1840s made grinding considerably more difficult, because the clinker produced by the kiln is often as hard as the millstone material.