Testing Cements

TESTING CEMENTS.

The quality or constructive value of a cement is generally ascer tained by submitting a sample of the particular cement to a series of tests. The properties usually examined are the color, weight, activity, soundness, fineness and tensile strength. Chemical analysis is some times made, and specific gravity test s substituted for that of weight. Tests of compression and adhesion are also sometimes added. As these tests cannot be made upon the site of the work, it is usual to sample each lot of cement as it is delivered and send the samples to a testing laboratory.

Sampling Cement.

The cement is sampled by taking a small quantity (1 to 2 lb.) from the center of the package. The number of packages sampled in any given lot of cement will upon the character of the work, and varies from every package to 1 in 5 or 1 in 10. When the cement is brought in barrels the sample is obtained by boring with an auger either in the head or center of the barrel, drawing out a sample, then closing the hole with a piece of tin firmly tacked over it. For drawing out the sample a brass tube sufficiently long to reach the bottom of the barrel is used. This is thrust into the barrel, turned around, pulled out, and the core Of cement knocked out into the sample-can, which is usually a tin box with a tightly fitting cover.

Each sample should be labelled, stating the number of the sam ple, the number of bags or barrels it represents, the brand of the cement, the purpose for which it is to be used, the date of delivery, and date of sampling.

The sample should be sent at once to the testing office, and none of the cement should be used until the report of the tests is received.

After the, report of the tests is received the rejected packages should be conspicuously marked with a "C" and should be removed without delay; otherwise they are liable to be used.

Color. The color of a cement indicates but little, since it is chiefly due to oxides of iron and manganese, which in no way affect the cementitious value; but for any given kind variations in shade may indicate differences in the character of the rock or in the degree of burning. The natural cements may have almost any color from

the very light straw colored "Utica" through the brown "Louisville", to chocolate "Rosendale". The artificial Portlands are usually a grayish blue or green, but never chocolate colored.

Weight. 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. The finer a cement is ground the more it becomes, and consequently the less it weighs. Hence light weight may be caused by laudable fine grinding or by objectionable under burning. Other things being the same, the harder-burned varieties are the heavier.

The weight per unit of volume is usually determined by sifting the cement into a as lightly as possible, and striking the top level with a straight edge. In careful work the height of fall should be recorded. Since the cement absorbs moisture, the sample must be taken from the interior of the package. The weight per cubic foot is neither exactly constant, nor can it be determined precisely. The approximate weight of cement per cubic foot is as follows: Portland, English and German. 77 to 90 lb.

Id fine-ground French.... 69 " " American... 99 Rosendale. 49 " 56 " Roman... 54 " A bushel contains 1.244 cubic feet. The weight of a bushel can be obtained sufficiently close by adding 25 per cent to the weight per cubic foot.

Fineness.

The cementing and economic value of a cement is affected by the degree of fineness to which it is ground. Coarse particles in a cement have no setting power and act as an adulterant.

The fineness of a cement is determined by measuring the per centage which will not pass through sieves of a certain number of meshes per square inch. Three sieves are generally used, viz.: No. 50, 2,500 meshes per square inch.