A description of the working of a Portland cement plant—which, with minor differences of detail, may be considered as a typical example— is given by one of the leading American manu facturers as follows: "Large bins approached by elevated tracks at one end of the raw material mill, provide the storage room for the raw material. From these bins, the limestone is fed directly into gyratory crushers, which prepare it for the driers, into which the stone then enters. The slag is also fed into separate driers, all of which are maintained at a sufficiently high temperature to drive off all the moisture. After passing through the driers, each of the raw materials is conveyed into separate revolving ball mills, where the rolling and impact of forged steel balls reduces them to extreme fineness. The two materials are then carried to separate hoppers, from which they are drawn off and properly proportioned by electrically operated weighing devices. The mixture of slag and limestone, already a powder, is then passed into the tube mills where flint pebbles rolling in a steel drum thor oughly mix it and complete the process of pulverization.
"The next step is the calcination of this raw mixture in rotary kilns. Leaving the rotary kilns, the clinker produced by the burning is dropped into storage pits, where it is cooled and cured. Electrically operated cranes then carry the clinker to the finishing mill, where it is passed through jaw crushers to Kent mills, and then over Newaygo separators. After the addition of a certain quantity of gypsum, which is done by means of an auto matic device, the material again passes through the tube mills similar to those used in finishing the raw material. Being now ground to a specified degree of fineness, the cement passes on to a belt conveyor, and is distributed into the various bins of the storage house.
"The cement, in passing from the finishing mill into the storage house, is automatically sampled, from which samples the various tests for quality are made in the laboratory." From the above descriptions of the raw materials and the processes of manufacture, the reader will be able readily to grasp the full sig nificance of the following "official" definitions of Portland cement: As defined by the American Society for Test ing Materials, Portland cement is: "The finely pulverized product resulting from the cal cination to incipient fusion, of an intimate mixture of properly proportioned argillaceous and calcareous ma terials, and to which no addition greater than 3 per cent has been made subsequent to calcination." As specified by the United States Govern ment (Board of Engineers, U. S. A., "Profes
sional Papers," No. 28), Portland cement is: "The product obtained from the heating or calcining up to incipient fusion, of intimate mixtures, either natural or artificial, of argillaceous with calcareous substances, the calcined product to contain at least 1.7 times as much of lime, by weight, as of the materials which give the lime its hydraulic properties, and to be finely pulverized after said calcination ; and thereafter additions or sub stitutions, for the purpose only of regulating certain properties of technical importance, to be allowable to not exceeding 2 per cent of the calcined product." It will be seen that no absolute standard for the chemical composition of Portland cement can be formulated. A good commercial Port land cement, however, will conform approxi mately to the following chemical analysis: Lime (Ca 0)-57 to 65 per cent. Silica (Si to 24 per cent. Alumina 0.)-6 to 8 per cent.
Sulphuric acid, anhydrous to 2 per cent.
Oxide of Iron to 4 per cent.
Magnesia (Mg 0)-1 to 4 per cent.
Water and carbolic acid-1 to 3 per cent.
Sulphur, as sulphides—None.
A close study of the chemical constituents of Portland cement reveals the reason for the re markable durability of the concrete produced from it. The products resulting from the hydra tion of a standard Portland cement are in no way responsive to the oxidizing and reducing gases of the air which are active in the rotting of wood or the corrosion of steel and metal structures. When hydration is complete, and the molecules have established normal equilib rium, the products represent compounds of •the highest stability. If the cement is of poor quality, disintegration may result from some interior agency; but this is never manifest in a well-proportioned and well-manufactured product.
Portland cement is packed for shipment in paper bags, cloth sacks, or wooden barrels. The cloth sack gives the most conveniently handled package for the average user. It costs a little more than the others, but this is offset by a rebate allowed for the return of the empty sacks. The bags must be kept dry and untorn, and returned by freight exactly as directed by the cement company. Paper bags are easily torn, and cause a big percentage of loss, espe cially on small jobs where any carrying has to be done. Barrels are too bulky to be handled easily, and are too large a unit for measuring.
The weight of the shipping units of cement varies slightly; but in general, a paper or cloth bag contains 95 lbs. of cement, and four such bags make a barrel of 380 lbs.