Measuring Fineness. The degree of fineness is measured by determining the proportion which will not pass through sieves of a specified number of meshes per square inch. Formerly, three sieves were used for this purpose, viz., sieves having 50, 75, and 100 meshes per linear inch, or 2,5(X), 5,625, and 10,000 meshes per square inch respectively; but at present only two sieves are used, the No. 100 and the No. 200. The change was made because the per cent left on the coarser sieves had no special significance. The wire cloth of the No. 100 sieve should be made of wire 0.0045 inch in diameter; and the No. 200 of wire having a diameter of 0.0024 inch. As it is nearly impossible to procure cloth absolutely true, it has been agreed that the No. 100 sieve may have from 96 to 100 meshes per linear inch and the No. 200 sieve from 180 to 200, and still be considered standard.
Preparatory to beginning the test, the cement should be put through a coarser sieve, say a No. 50, to remove any lumps or foreign matter. It is difficult to get the cement through the sieve, but a teaspoonful of moderately fine shot placed upon the sieve with the cement will materially facilitate the sifting. The shot should be weighed before being used, as otherwise it will be difficult to separate the cement and the shot preparatory to weighing the former. Un fortunately the shot seriously injures the sieve.
The only active element in the cement is the impalpable flour. Apparently about half of the cement passing the No. 200 sieve is inert and acts only as so much sand,* although the exact proportion of flour present depends upon the chemical composition of the cement and the method of grinding. In other words then, only about 40 per cent of standard portland cement is active. If it were ground fine enough for all to be active, it would be too quick-setting for practical use.
Soundness refers to the ability of a cement to retain its strength and form unimpaired for an indefinite period. The test to determine soundness is frequently called a test for con stancy of volume.
Soundness is a most important element; since if a cement ulti mately loses its strength it is worthless, and if it finally expands it becomes a destructive agent. A cement may be unsound because
of the presence in it of some active element which causes the mortar to expand or contract in setting, or the unsoundness may be due to exterior agencies which act upon the ingredients of the cement. Most unsound cements fail by swelling and cracking under the action of expansives; but sometimes the mortar fails by a gradual softening of the mass without material change of form. The expansive action is usually due to free lime or free magnesia in the cement, but may be caused by sulphur compounds. The principal exterior agencies acting upon a cement are air, seawater, and extremes of heat and cold.
The presence of small quantities of free lime in the cement is a frequent cause of unsoundness. The lime slakes, and causes the mortar to swell and crack—and perhaps finally disintegrate. The degree of heat employed in the burning, and the fineness, modify the effect of the free lime. Lime burned at a high heat slakes more slowly than when burned at a low temperature, and is therefore more likely to be injurious. Finely ground lime slakes more quickly than coarsely ground, and hence with fine cement the lime may slake before the cement has set, and therefore do no harm. The lime in finely ground cements will air-slake sooner than that in coarsely ground.
Free magnesia in cement acts very much like free lime. The action of the magnesia is much slower than that of lime, and hence its presence is a more serious defect, since it is less likely to be detected before the cement is used. The effect of magnesian cement is not thoroughly understood, but seems to vary with the composition of the cement, the degree of burning, and the amount of water used in mixing. It was formerly held that 1 or 2 per cent of magnesia in portland cement was dangerous; but it is now known that 5 per cent is not injurious, while 8 per cent may produce expansion. Since many of the natural cements are made of magnesium limestone, they contain much more magnesia than portland cements; but chemists are not agreed as to the manner in which the different constituents are combined, and consequently are not agreed either as to the amount or effect of free magnesia in a natural cement. Fortunately, it is not necessary to resort to a chemical analysis to determine the amount of lime or magnesia present, for a cement which successfully stands the ordinary test for soundness (§ 143) for 7, or at most 28 days, may be used with reasonable confidence.