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cement mill
A cement mill (or finish mill in North American usage〔Spanish: ''molino de cemento'', French: ''broyeur de ciment'', German: ''Zementmühle'': other languages correspond with the British usage〕) is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical roller mills which are more effective than ball mills.
==History==
Early hydraulic cements, such as those of James Parker, James Frost and Joseph Aspdin were relatively soft and readily ground by the primitive technology of the day, using flat millstones. The emergence of Portland cement in the 1840s made grinding considerably more difficult, because the clinker produced by the kiln is often as hard as the millstone material. Because of this, cement continued to be ground very coarsely (typically 20% over 100 μm particle diameter) until better grinding technology became available. Besides producing un-reactive cement with slow strength growth, this exacerbated the problem of unsoundness. This late, disruptive expansion is caused by hydration of large particles of calcium oxide. Fine grinding lessens this effect, and early cements had to be stored for several months to give the calcium oxide time to hydrate before it was fit for sale. From 1885 onward, the development of specialized steel led to the development of new forms of grinding equipment, and from this point onward, the typical fineness of cement began a steady rise. The progressive reduction in the proportion of larger, un-reactive cement particles has been partially responsible for the fourfold increase in the strength of Portland cement during the twentieth century.〔P. C. Hewlett (Ed)''Lea's Chemistry of Cement and Concrete: 4th Ed'', Arnold, 1998, ISBN 0-340-56589-6, p 12〕 The recent history of the technology has been mainly concerned with reducing the energy consumption of the grinding process.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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