A cement manufacturing plant is usually located at or near a natural deposit of the materials composing cement. These materials may or may not be a natural mixture, but in either case chemical analyses must be made to determine, (1) the per cent of lime, (2) the per cent of clay, and (3) the per cent and kind of other ingredients. This analysis determines the fitness of the deposit for cement manufacture. With a cement plant established, the essential steps in manufacture are:

(1) Chemical analysis in the laboratory to determine the correct proportion of lime and of clay for making up a kiln charge.

(2) Grinding the properly proportioned ingredients to a very fine state, which also accomplishes their thorough mixture. These may be coarsely ground before mixing, but must be ground after mixing sufficient to pass through a sieve of about 900 meshes to the square inch. Lack of proper grinding may cause the cement to be worthless.

(3) Burning the ground materials to a state of incipient fusion, to change their chemical composition. This is done in a long, cylindrical kiln, built of iron plate, lined with refractory brick and mounted to revolve on its axis at a slight inclination to the horizontal. The ground mixture is introduced wet or dry at the higher end, and the fuel oil, gas, or powdered coal, is blown in at the lower end where it ignites in a flame maintained by the continuous introduction of fuel. The finely divided cement material is gradually dehydrated and deprived of its carbon dioxide as it is tumbled by the slowly revolving kiln, presenting every part of the mass to the heat, and is finally vitrified at a white heat into small clinkers varying in size from that of a pea to that of a walnut. These clinkers are kept from fusing by being tumbled from the lower end of the kiln at the proper time. Their continuous falling from the kiln is due to the kiln's inclination and slow revolution.

* The relative proportions of silica and alumina, as compared with the limestone in cement, are found by Messrs. Newberry from the formula Wt. of limestone = 2.8 Xwt. of silica+l.lXwt. of alumina.

(4) Grinding the clinker to a fine powder. This is done in two operations. The first grinding is done between millstones, or in a revolving cylinder containing steel balls about 6" in diameter. The pulverizing is then accomplished by machines for the purpose, in one form of which the cement is fed into one end of a revolving cylinder partly filled with flint stones about as large as walnuts. The pulverized cement issues from the other end of the cylinder. It must be ground so fine that not more than 8% of its weight will fail to pass through a sieve of no. 100 mesh (100 x 100 meshes per square inch).