bination of lime and silica, whereas the German chemist Schoch, on the other hand, claimed that they act as a flux and are of great benefit in the hardening of Portland cement.

Meade has shown by experiment that alkalies are useful fluxes. The curtailment by the European war of the foreign supply of potash salts, and the consequent large advances in price, has led a number of cement manufacturers to add potash-recovery equipment, to their plants. A brief and concise treatment of this subject is given by Porter. Potash may, therefore, be regarded not only as a useful flux but also as an actual or potential source of a valuable by-product.

CALCAREOUS MATERIALS.

GENERAL SPECIFICATIONS FOR CALCAREOUS MATERIALS. Calcareous materials to be suitable for cement manufacture should conform with the following conditions:

1. The rock should be free of concretions of iron minerals, should contain little free silica in the form of chert, flint, or quartz veins, and should be free of silicate minerals such as tremolite and diopside.

2. The silica and alumina contents should be sufficiently low and in such ratio that they will not interfere with the desired silicaalumina ratio in the finished product.

3. The rock should be sufficiently low in magnesium that the finished product will not contain more than 5 per cent magnesia.

4. The content of iron should be sufficiently low that the ferric oxide content in the finished cement does not exceed 4 per cent. 5. The sulphur content should be low.

LIMESTONE.

ORIGIN OF LIMESTONE.

Calcium is a common constituent of the rocks of the earth's crust. Calcium is always found in combination with other elements, as carbonate, sulphate, silicate, phosphate, or other forms. By the action of water and of acids at the surface of the earth or within the rocks, calcium is dissolved. Analyses of river waters show that they contain a considerable amount of carbonic acid. This aids in keeping the calcium in solution and it is carried by the rivers to the sea. Marine organisms of various types, such as foraminifera, corals, and mollusks, secrete shells consisting principally of calcium carbonate. As myriads of these organisms, in succeeding

Meade, R. K., work cited, p. 39.

Porter, J. J.: Recovery of potash as by-product in manufacture of cement. Concrete, October, 1917, Cement Mill section, p. 27.

generations, live and die, their shells form vast accumulations at the bottom of the sea. Some of the limestones still show the fossils of which they were formed, but in others all trace of organic origin has been destroyed by the fine grinding to which the shells were subjected before their final consolidation. In addition to this mechanically precipitated material, calcium carbonate may be precipitated chemically from the sea water. Such mechanical and chemical precipitates, when consolidated, may form beds of limestone of great thickness. These may at later periods be elevated above the surface of the water by earth movements, and thus be rendered available for use.

COMPOSITION OF LIMESTONE.

Limestone consists essentially of calcium carbonate, and when pure forms the mineral calcite. However, nearly all limestone contains certain other materials that may be classed as impurities. For a rock to be termed a limestone it must contain at least 50 per cent of calcium carbonate. The principal foreign elements in limestone are silica, iron oxide, alumina, magnesium carbonate, and the alkalies potash and soda. When 30 per cent or more of magnesium carbonate is present, the rock is termed a dolomite. When little silica, alumina, and iron oxide are present the relative proportion of each is of small moment to the cement manufacturer, but if the limestone contains a considerable proportion of these constituents, the relative amount of each should be such that when the shale or clay is added the resulting mixture shall contain silica and alumina in the proper ratio.

In determining the suitability of a limestone for cement making the composition of the shale or clay to be mixed with it must be considered, as the composition of the final mixture depends on the compositions of the original ingredients.

In general, pure limestones are much harder than argillaceous limestones, and consequently are less desirable for making cement. In South Dakota, Alabama, Mississippi, northern Texas, and southeastern Arkansas are certain rotton limestones or chalks that grind easily and are therefore desirable materials for making cement.

VARIETIES OF LIMESTONES.

Special names are applied to certain types of calcareous rocks differing from ordinary limestone in origin, texture, or composition. Marble is limestone that, through the action of heat and pressure, has become more or less distinctly crystalline, although the term is also used in a commercial sense for any limestone that will take a

good polish. Marl is a term applied to a loosely cemented mass of lime carbonate formed in a lake basin. Calcareous tufa and travertine are more or less compact limestones deposited around springs or along the courses of resulting streams. Oolitic limestone, so called because of its resemblance to fish roe, is made up of small rounded grains of lime carbonate having a concentrically laminated structure. Chalk is fine-grained limestone composed of particles of finely ground shells loosely cemented together. A siliceous or cherty limestone is one that contains considerable silica, an argillaceous limestone is one containing clay, and a carbonaceous or bituminous limestone is one containing carbonaceous matter.

PHYSICAL CHARACTERISTICS OF LIMESTONES.

Limestones vary greatly in such physical properties as hardness, color, weight, porosity, and texture. In color limestones range from pure white to black, depending on differences in chemical composition; in texture they may be amorphous, semicrystalline, or crystalline; and in compactness they vary from the loosely consolidated marls through the chalks to the compact, normal limestones and the still harder marbles. Porous limestone may weigh as low as 110 pounds per cubic foot, whereas the more compact varieties weigh 150 to 185 pounds to the foot.

From the standpoint of the cement manufacturer, porosity and hardness are the more important physical properties of a limestone. Porous limestones are capable of carrying a large content of water, the removal of which requires considerable fuel. The hardness has a direct bearing on the cost of quarrying, crushing, and grinding. Harder rocks pulverize with more difficulty and wear the grinding machinery much more rapidly than softer ones. These two properties, hardness and porosity, counteract each other to a great extent, for the narder limestones are usually of low porosity and vice versa.

The attitude of the beds and the presence or absence of open bedding planes or joints are structural features that have a direct bearing on the system of quarrying. As these chiefly affect drilling and blasting methods, they are considered in detail under "Drilling" and "Blasting" on pages 42 to 51.

CALCAREOUS MATERIALS USED IN CEMENT MANUFACTURE.

COMPACT LIMESTONE.

In the early days of Portland cement manufacture the amount of hard limestone used was small, argillaceous limestone, the so-called cement rock, being chiefly employed. When the process of making cement from hard limestone and shale or clay was placed on a profitable basis, this branch of the industry developed rapidly. Hard

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limestone is the principal ingredient in practically 57 per cent of all the Portland cement manufactured in this country, and (see Table 2, p. 26) is by far the most widely used calcareous material in making cement. Hard limestone is usually of reasonably low porosity, but some types are difficult to pulverize.

CEMENT ROCK.

In certain parts of the United States, notably in the Lehigh River Valley of eastern Pennsylvania and western New Jersey, in the Shenandoah Valley of Virginia, and near Glens Falls, N. Y., are limestones having a high clay content. As such rocks contain all the chief constituents of Portland cement, they have been termed "cement rock." Cement rock was the chief source of raw material in the early days of the Portland cement industry in the United States and still constitutes an important source.

The cement rock quarried in the Lehigh district is mostly derived from the middle beds of the Trenton formation. In most places the beds dip 15° to 25°, usually northwest, but at a few quarries, particularly in New Jersey, the dip is much steeper. Most of the cement rock runs 60 to 70 per cent lime carbonate, hence the addition of a small amount of high-calcium limestone is necessary. The limestone may be obtained from beds in the lower part of the Trenton formation or from certain low-magnesium beds in the underlying Kittatinny formation. High-calcium limestone is shipped from outside points to a number of plants. Near Bath and Nazareth, Pa., the cement rock may carry 70 to 80 per cent of lime carbonate and the addition of limestone be unnecessary. In some places, on the other hand, the rock is so high in lime that the addition of a small amount of clay or shale is necessary. A high-calcium cement rock, requiring a small addition of clay, is usually more desirable than one requiring the addition of limestone, because the cement rock is usually overlain with clay, which is readily available, whereas the limestone may have to be shipped in.

In a few places the cement rock approximates so nearly the desired proportions for a cement mixture that by careful and judicious quarrying no limestone or shale is necessary. Such a rock, properly proportioned in nature, constitutes the ideal raw material for

cement.

Cement rock is commonly intersected by close bedding seams and joint planes, consequently when great masses are hurled down in blasting they readily break into small pieces. Not only does cement rock quarry easily, as a rule, but it offers certain advantages in milling. As the particles of clay and limestone are intimately mixed by nature, the rock does not need to be so finely ground as lime

stone must be for intimate mixing with clay or shale. Moreover, cement rock is, as a rule, much easier to grind and pulverize than hard limestone. Such rock may, therefore, be regarded as one of the most desirable raw materials for making cement.

MARBLE.

Marble is an extremely compact or crystalline form of limestone. It is usually much harder than ordinary limestone, and consequently is somewhat difficult to quarry and pulverize. Marble is not com. monly used as a cement material. It is employed by a company operating near Union Bridge, Md.

CHALK.

Chalk, being much softer than limestone and easily excavated and pulverized, is a desirable raw material for cement manufacture. The chief disadvantage of chalk is its high porosity, which permits excessive absorption of water during rainy seasons. Where chalk is employed large covered bins in which a supply can be stored for use during wet seasons is desirable.

MARL

Marl is soft, unconsolidated lime carbonate that has been deposited in the bottoms of existing or extinct lakes. The lime carbonate is precipitated from solution in the lake water by the agency of certain algæ or water plants. The chief impurities in marl are clay, organic matter, and magnesium carbonate. The clay content, if of suitable composition and limited in amount, is not regarded as undesirable. The organic matter is burned out in the kilns and has little or no effect on the finished product.

As most marl deposits are partly or completely submerged, the marl is won by dredging. The wet material may be hauled to the plant in cars, or may be pumped as a slurry. The process of winning marl is relatively cheap, but this advantage is largely counteracted by the cost of fuel for driving off the water.

ALKALI WASTE.

Limestone is employed in the manufacture of caustic soda from common salt. The calcium is precipitated in carbonate form and removed as waste. Where the Leblanc process is employed the waste contains too large a proportion of sulphides to be used as an ingredient of cement. The ammonia process, however, yields lime carbonate relatively free from sulphur.