The following values, determined by sifting the cement with a fall of three feet into a box having a capacity of one tenth of a cubic foot, may be taken as fair averages for ordinary cements. The difference in weight for any particular kind is mainly due to a differ ence in fineness: Portland 75 to 90 lb. per cubic foot, or 94 to 112 lb. per bushel. Natural 50 to 56 lb. per cubic foot, or 62 to 70 lb. per bushel.
The weight of a cement is not now used as a test of quality; but the weight is of considerable value in reducing proportions given by weight to equivalent volumetric proportions, and vice versa. Speci fications for the reception of cement frequently specify the net weight per barrel; but this is a specification for quantity and not quality, while only the latter is now under consideration.
It is sometimes claimed that the specific gravity test is valuable in detecting adulteration; but the test is not of much value for this purpose, unless the amount of adulterant is very large or it has a specific gravity differing greatly from that of the cement. The specific gravity is of some value in determining the density of cement mortars and concretes (see § 233 and 290).
The specific gravity is determined by immersing a known weight of the cement in a liquid which will not act upon it (usually turpentine or benzine), and obtaining the volume of the liquid displaced. The latter is obtained by means of a glass bulb having a graduated stem above, the rise of the liquid in the tube indicating the volume of the cement introduced. The specific gravity is equal to the weight of the cement in grams divided by the displaced volume in cubic centimeters.
To be of any value for any purpose the test must be very earefully made. As cement absorbs moisture from the air, it should be heated to 212° Fahr. to drive off the water; and the cement should be cooled to the temperature of the air before proceeding with the test.
The cement should be passed through a sieve to eliminate lumps; and it should fall through the liquid in a finely divided state so as to allow the air to escape. The liquid should be at about 60° Fahr., to prevent undue evaporation; and the bulb should be immersed in water to prevent a change of temperature between observations.
The specific gravity of portland cement varies from 3.00 to 3.25, usually between 3.05 and 3.17. Natural cement varies from 2.75 to 3.05, and is usually between 2.80 and 3.00. The specifications proposed by the American Society of Testing Materials and adopted by various other national engineering societies,* which may properly be called the Standard American Cement Specifications, require that the specific gravity of portland cement, when thoroughly dried at 100° C., shall not be less than 3.10, and of natural not Tess than 2.8 (see Appendix I). Slag cement has a specific gravity of 2.72 to 2.76. The specific gravity of cement decreases with age owing to the absorption of water and carbonic acid from the air.
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 that the addition of water induces crystallization. Consequently the coarse particles in a cement have no setting power whatever, and may for practical purposes be con sidered as so much sand and essentially an adulterant.
There is another reason why cement should be well ground. A mortar or concrete being composed of a certain quantity of inert material bound together by cement, it is evident that to secure a strong mortar or concrete it is essential that each piece of aggregate shall be entirely surrounded by the cementing material, so that no two pieces are in actual contact. Obviously, then, the finer a cement the greater surface will a given weight cover, and the more economy will there be in its use.