requisite is weight or mass.

It should never be used in work under water, in marine construction, in columns, beams, floors, or other members subjected to severe or suddenly applied stresses. Puzzolan or Slag Cement is limited to use in sea water, and generally to structures constantly exposed to moisture, as foundations of buildings, sewers and drains, and in the interior of heavy masonry or concrete. It is unfit for use when subjected to mechanical wear, abrasion, or blows, and should never be used where it may be exposed to the action of dry air for long periods. Under such conditions it will turn white and disintegrate, owing to the oxidation of its sulphides at the surface.

Hydraulic Lime is extensively employed on the Continent, especially in France, in the form of Beton-Coignet (a mixture of hydraulic lime with sand and cement).

Common Lime mortar is usually limited to brick work and to chimney construction in frame houses.

Lime and Cement Mortar is suitable for ordinary brickwork, for light rubble foundations, and for building walls.

Portland cement, owing to its greatly superior strength and reliability, is rapidly displacing all other kinds of cement, and it will continue to do so even more rapidly as its cost is lowered. In the following chapters, Portland cement is always referred to unless another is specifically mentioned.

How Portland Cement Comes.-Portland cement comes in paper bags, cloth sacks, or wooden barrels. The best way to handle it for the average user is in cloth sacks. The manufacturers charge more for this kind of a package, but allow a rebate for the return of the empty bags. The bags must be kept dry and untorn, and shipped back by freight, in exact accord with the requirements of the cement company. Paper bags tear too easily and cause a big percentage of loss, especially on small jobs where any carrying has to be done. Barrels are too bulky to handle 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.

How to Keep Portland Cement.-Cement must be stored in a dry place. It absorbs moisture from the atmosphere with great readiness, and soon becomes lumpy, or even a solid mass, when

kept in a damp place. Such cement is useless and must be thrown away. Lumpy cement should not be broken up and used again, even if this can be readily done, as it has lost by far the greater part of its adhesive value. In storing cement, throw wooden blocks on the floor, place boards over them, and pile the cement on the boards, covering the pile with canvas or pieces of roofing paper.

CHAPTER III

PROPERTIES, TESTING, AND REQUIREMENTS OF HYDRAULIC CEMENTS

Description of Tests.-How the Tests are Made.-Standard Requirements for Natural and Portland Cements.

Properties of Hydraulic Cements.-The properties of a cement which are usually examined to determine its constructive value, are: first, color; second, weight; third, activity; fourth, soundness; fifth, fineness; and sixth, strength.

Color indicates the thoroughness of burning and the presence of impurities. With Portland cement, gray or greenish-gray is an indication of good quality. A yellowish shade indicates underburned material; bluish-gray an excess of lime; and brown, an excess of clay. For decorative purposes, white Portland cements, having all of the properties of gray, are also employed. In these the color does not indicate any inferiority in strength or setting. power.

Natural cements are generally brown, in light or darker shades. A light color generally indicates an inferior underburned rock.

For any particular cement the weight varies with the degree of heat in burning, the degree of fineness in grinding, and the density of packing. Other things being the same, the harder-burned varieties are the heavier. The finer a cement is ground, the more bulky it becomes, and consequently the less it weighs. Hence light weight may be caused either by laudable fine grinding or by objectionable under-burning.

A barrel of Portland cement, containing 3.8 cu. ft., should weigh about 380 pounds net; natural cements weigh from 250 to 300 pounds net per barrel.

The activity of a cement is determined by its rate of setting. For most purposes, where immediate setting is not required to prevent disturbance of the mortar before hardening, the moderately

slow-setting cements are found most convenient, as they need not be handled so quickly and may be mixed in somewhat larger quantities.

Soundness is the most important quality of a cement, as it means the power of the cement to resist the disintegrating influences of the atmosphere or water in which it may be placed. Soundness refers to the property of not expanding, contracting, or cracking during the time of setting. These effects may be due to free lime, free magnesia, or to unknown causes.

FIG. 1.-Showing Normal Cement Pat in Good Condition.

The question of fineness is wholly a matter of economy. Cement, until ground, is a mass of partially vitrified clinker, which is not affected by water, and which has no setting power. It is only after it is ground (After W. Purves Taylor.) that the addition of water induces crystallization. The coarse parts of the cement may be considered as practically inert material which sets only after the lapse of months or years if at all. It is

FIG. 2.-Pat Showing Shrinkage

Cracks Due to Overwet Mixture or Too Rapid Drying.

the impalpable powder which gives

the cement its value, and if this be omitted, the cement is worthless.

It is possible to reduce a cement to an impalpable powder. Fine grinding is, however, expensive. The proper degree of fineness is reached when it becomes cheaper to use more cement in proportion to the aggregate, than to pay the extra cost of additional grinding.

The strength of cement is usually determined by submitting a specimen

of known cross-section to a tensile strain. The reason for adopting tensile tests is that comparatively light strains produce rupture. This will be referred to later.

HOW CEMENT IS TESTED

Having outlined the nature and properties of hydraulic cements, we now propose to take up the methods that may be employed by the cement user to determine whether the material he purchases is up to the standard and fit for use.

The testing of cement for use on extensive work has become an art in itself and only men experienced in the work can obtain results that are uniform and reliable. It is therefore not intended to go into details of apparatus and methods employed by the skilled

FIG. 3.-Pats Showing Cracks Due to Incipient Disintegration and which Warrant Rejection.

tester which are of little use to the practical cement user, but an idea of what is done is of general interest.

Physical Tests for Cement.-On all large works, an inspector is kept at the mills to watch the process of manufacture, and special laboratories are provided for making both chemical and physical examinations. As already stated, the physical examination is employed to determine whether the cement possesses the necessary requirements to make it fit for use. Thus a good cement:

First. Should be sufficiently well ground. This is referred to as a test for fineness and is made by passing the cement through sieves of varying meshes. In a good Portland 98 per cent should pass through a No. 100 sieve, having 10,000 holes to the square inch. The finer the cement is ground the greater will be its hydraulicity, and the greater the proportions of sand that can be used with it.