HARD LIMESTONE AND CLAY OR SHALE.

As stated on page 18, hard limestone is the chief calcareous constituent of Portland cement made in this country. The combination of hard limestone with clay or shale is practiced by cement plants in many States and in widely separated localities. The substitution of ashes for part of the shale has been mentioned.

CHALKY LIMESTONE AND CLAY.

Chalky limestone is easier to quarry than hard limestone, and carries less water than marl. It is available for cement manufacture in three localities: (1) Parts of Texas, Oklahoma, and Arkansas; (2) parts of Alabama and Mississippi; and (3) parts of North Dakota and South Dakota.

MARL AND CLAY.

Marl and clay are used for making cement in Ohio, Michigan, Indiana, and central New York.

ALKALI WASTE AND CLAY.

Precipitated calcium carbonate from alkali works is a limited source of supply as a cement constituent and is now utilized only at Wyandotte, Mich.

SLAG AND LIMESTONE.

Blast-furnace slag and limestone are used for the manufacture of cement in Illinois, Ohio, and Pennsylvania.

OYSTER SHELLS AND CLAY.

Oyster shells, which have about the same composition as limestone, are (see p. 20) employed in a few localities where the supply of shells is adequate.

RELATIVE IMPORTANCE OF MATERIALS USED.

Table 2, compiled by Burchard, indicates the relative importance of the chief sources of raw materials for the period 1898 to 1914, inclusive.

Burchard, E. F., Cement: Mineral Resources of U. S. for 1914, pt. 2; U. S. Geol. Survey, 1915, p. 232.

Type 2. Limestone
and clay or shale.

TABLE 2.-Production, in barrels, and percentage of total output of Portland cement in the United States according to type of material used, 1898–1914.

Type 1. Cement rock
and pure limestone.

Meade has discussed the proportioning of raw materials at length, and readers particularly interested in the subject may consult the publication cited, or others of similar character. In a publication like the present one, dealing primarily with the process of winning the raw materials from the earth, the principles and the method of obtaining a suitable proportion of raw materials can be dealt with only in a general way.

NEED OF DEFINITE PROPORTIONS.

In discussing the composition of Portland cement it has been pointed out that the properties of cements may be materially changed by very small variations in the proportions of the essential elements. For example, cements high in lime or silica are slow-setting, whereas those high in alumina are quick-setting. Variations beyond certain narrow limits may cause the cement to be of poor quality. The cement manufacturer must, therefore, control his mixture by certain definite rules.

FORMULAS FOR CALCULATING MIXTURE.

Certain general formulas are applicable to most cement materials, though peculiarities in composition may require minor modifications.

a

The Newberrys based their formula on the presumption that Portland cement consisted of tricalcic silicate and dicalcic aluminate. Tricalcic silicate (3CaO.SiO2) consists of 2.8 parts of lime by weight to 1 part of silica; and dicalcic aluminate (2CaO.SiO2), 1.1 parts of lime by weight to 1 part of alumina. The maximum permissible percentage of lime, then, is equal to the sum of the percentage of silica multiplied by 2.8 plus the percentage of alumina multiplied by 1.1. The following rule was deduced: Multiply the percentage of silica by 2.8, and the percentage of alumina by 1.1, add the products, and the sum will be the number of parts of lime required for 100 parts of clay.

As 2.8 parts of lime correspond to 5 parts of lime carbonate, and 1.1 parts lime correspond to 2 parts of lime carbonate, it follows that five times the percentage of silica plus twice the percentage of alumina equals the number of parts of carbonate of lime required for 100 parts of clay.

It has been established that the ratio between the percentage of lime and the combined percentages of silica, alumina, and iron should be within the limits 1 to 1.8 and 1 to 2.2. Many chemists assume a fixed ratio of 2, though others obtain better results by slightly increasing or decreasing this ratio.

After the raw clay and limestone have been analyzed the proportions of each necessary for any given ratio may be determined as follows:

Let

M=the ratio of lime to silica, alumina, and iron;

S =the oxides of silica, alumina, and iron in the clay;

S1 the oxides of silica, alumina, and iron in the limestone;
C=the calcium oxide in the limestone;

G=the calcium oxide in the clay.

The chief defect of this formula is that the relative proportions of silica, alumina, and iron oxide are not taken into consideration, but for dealing with well-balanced raw materials it is satisfactory. More accurate methods for calculating these proportions are given. by various authors.

CALCULATION ON BASIS OF FIXED LIME STANDARD.

A formula such as the one given is used to determine the best mixture of any given raw materials and to check the mixture from

G

Newberry, S. B., and Newberry, W. B., The constitution of hydraulic cements, 1897,

time to time. However, the "fixed lime standard" is found to be more practical in actual millwork. After experience has shown that a certain percentage of lime carbonate gives satisfactory results, the operator maintains this lime standard as nearly as possible, on the assumption that the ratio of other constituents is approximately selfadjusting. The method is satisfactory where the raw materials are of fairly constant composition, but if marked fluctuations in composition are likely to occur the mixture should be checked frequently by the application of a more accurate formula.

In the Lehigh district the percentage of carbonate of lime in the raw mixture varies from 74.5 to 75.5, and as the limestone and cement rock are of fairly constant composition, the fixed lime standard is in common use.

Meade gives the following simple formulas for determining the proper mixture on this basis. The first formula is for determining the percentage of limestone to be added to a given cement rock or clay to make a given mixture.

The second formula is for determining the percentage of shale or clay to be added to a given marl or limestone to make a given mixture, and is as follows:

The addition of certain substances to promote combination of silica and lime in the kiln is necessary in certain plants working on poor raw materials, such as cherty limestone or high-silica and lowalumina clay. The common fluxes used in cement making are iron oxide, fluorspar, cryolite, and certain alkaline compounds. Where

Meade, R. K., work cited, p. 80.

fairly good raw materials are available, the use of fluxes usually should be avoided, as the disadvantages may more than outweigh the advantages.

PROCESSES OF MANUFACTURE.

NATURAL CEMENT.

The cement rock used for the manufacture of natural cement requires no pulverizing or mixing prior to burning. It is burned in vertical kilns similar to those employed in lime burning, though usually larger. The kilns are operated continuously, the rock and fuel being fed in at the top, mixed together or in alternating layers. The stone is burned at a temperature a little higher than that employed in lime burning. The burned stone, or clinker, is drawn from the bottoms of the kilns, cooled, and ground to powder.

PORTLAND CEMENT.

During the early period of the Portland cement industry all European and American plants used the wet process only. As stationary kilns were used the material had to be wetted and molded into bricks to give a free draft in the kiln.

With the introduction of the rotary kiln the dry process was made practicable. The dry process involves, first, the crushing and drying of the limestone or cement rock and the addition of the necessary amount of clay, shale, or limestone, whichever is used, these being also crushed and dried. The mixture is then finely pulverized. With cement rock satisfactory results are obtained if the material is sufficiently pulverized that 85 per cent passes through a 100-mesh sieve. If limestone and shale are used, finer grinding is necessary; for the best results, 95 to 98 per cent should pass through a 100-mesh sieve. The properly proportioned and finely pulverized mixture is then fed into the kilns.

In the wet process the materials are mixed and usually pulverized in a wet condition. The mixture known as "slurry," having the consistency of thin mud, is fed by pumps or by screw conveyors into tanks, where it is throughly mixed either by mechanical or air agitation. The mixture enters the kilns as slurry, the water being driven off by evaporation in the upper part of the kilns.

Modern rotary kilns are 5 to 9 feet in diameter and 60 to 200 feet long, the longer kilns representing a more modern development. They are cylindrical in form and are made of sheet steel lined with fire brick. They slope gently downward toward the firing end. Fuel consisting of powdered coal, gas, or oil is blown in through a small pipe at the lower end of the kiln. The cement mixture is fed in at the