Port/and cement is the product obtained by calcining to incipient vitrifaction an intimate artificial admixture of two or more raw mate rials, and finally grinding the clinker. Chem ically Portland cement is a combination consist ing principally of silicates and almninates of lime. and the raw materials must necessarily contain silica, alumina, and lime. Within these limitations. a great variety of raw materials are capable of being utilized for cement-making: in England chalk and clay are used principally; in Germany and France marl and clay and lime stone and slate are employed. In the United States marl and clay, limestone and clay, and argillaceous limestone of different compositions are used. The first process in the manufacture of Portland cement is the mixing of the raw ma terials. This mixture must be Uniform and homogeneous, and the respective ingredients properly proportioned, which requires that they be reduced to a fine powder. The method of re duction practiced depends upon the character of the raw materials; when soluble in water they are usually reduced by one of the wet processes. The WO process proper. formerly extensively used in England, consists in applying an excess of water to the clay and chalk, mixing them in a sort of pug mill to a thin paste, which is run into settling basins, where the water is deeanted off as the solid matter settles until the mixture is dry enough to be cut into blocks or bricks.
In the semi-wet process only enough water is added to reduce the mixture to a plastic condi tion in the pug mill.
When hard materials incapable of dissolution by water are employed, they are ground dry in grinding mills. the powder being then either slightly moistened and made into bricks. or. where the rotary kiln is used stored in powdered form. In the wet and semi-wet processes the bricks or Iffiwks are dried and then burned in kilns. of which there are several varieties. (See In the dry process, drying preliminary to burning is usually unnecessary. In the Unit ed States, where the rotary kiln is used, the dry powder or the wet paste is run directly into the kiln without previous brick-making or dry ing. The burning or ealeination is continued until incipient vitrifaction of the raw mixture oecurs, the resulting clinker being dark-green or black in color. The clinker is crushed and then ground to an impalpable me.vder, when. after a period of curing, it is ready for use. (See CU A