PHILIP L. WORMELEY, JR. I. Cement and Cement Mortar.

The term "hydraulic cement " is applied to one of most useful materials of engineering construction and one which in recent years has become widely extended in its field of application. Hydraulic cement posseses the property of hardening or setting under water, in which respect it differs from lime, which does not harden except in the presence of air. Thus it is evident that all places where air is excluded, such as foundations, thick walls, etc., cement mortar should be used instead of lime.

Only two classes of cement will be discussed here - Portland and natural. The difference between these is due partly to the method of manufacture and partly to the condition and relative proportions of the mater-rials employed, which are, generally speaking, limestone and clay. In the manufacture of Portland cement the separate materials are mixed in such proportions as have been found by experience to give the best results. The mixing is done by grinding the materials together in mills, after which the mixture is burned at a very high temperature in kilns, and the resulting clinker ground to an impalpable powder is known as Portland cement.

In the case of natural cement, the materials used have been already mixed by nature in approximately the correct proportions, being found in the form of a rock which is generally classed as a clay limestone, or a limey deposit technically called calcarous clay. This material is burned at a much lower temperature than Portland cement. When the manufacturer has each ingredient absolutely under control and can adjust proportions to suit all conditions, it is reasonable to expect that a better and more uniform product will result than when the materials are found already mixed.

Portland cement is far more extensively employed than natural cement on account of its superior strength, although the latter is frequently used in cases where great strength is of little importance. The superior strength and durability of cement as compared with lime, together with the low price at which it may now be procured, have caused the former to replace the latter in engineering construction to a great extent.

In storing cement, care must be exercised to insure its being kept dry. When no house or shed is available for the purpose, a rough platform may be erected clear of. the ground, on which the cement may be placed and so covered as to exclude water. When properly protected, it often improves with age. Ce" ment is shipped in barrels or bags, the size and weight of which is usually as follows:

Bulk and weight of cement in ordinary barrels and bags.

Kind of Cement.

Quan.

Wt. (net)

Quan.

Wt. (net)

Cu. ft.

Pounds.

Cu. ft.

Pounds.

Portland

3 1/2

380

7/8

95

Natural

3 1/2

300

7/8

75

Western natural cement usually weighs about 265 pounds per barrel net.