ART. 2. TESTING CEMENT.
Of all the materials of construction, cement is the one most difficult to test, and also the one subject to the greatest variations and therefore the one most in need of testing. The value of a cement varies greatly with its chemical composition, the temperature of calcination, the fineness of grinding, etc.; and from the moment the clinker is reduced to a powder, its physical and chemical properties are constantly undergoing changes which affect its quality and value as a building material, and even after the cement has been made into a mortar and become a part of the structure, these changes may continue. Not only is there greater variation in cement than in any other building material; but un fortunately the results of the tests of cement depend, to a greater extent than with any other material, upon the personal equation of the one making the tests and the conditions under which they are made. With all other building materials the tests are made upon the finished product, while with cement the test is made upon an mediate state of the material; and further, with cement the most important tests are made upon samples fabricated by the one who makes the tests, and the results are dependent almost wholly upon the manner of preparing, storing, and testing the samples. There fore the testing of cement to determine its fitness for the use proposed is a matter of very great importance.
The properties of cement which are examined to determine its constructive value are: (1) color, (2) thoroughness of burning, (3) fineness, (4) soundness, (5) chemical composition, (6) activity, (7) strength.
The color of the cement powder indicates but little as to its quality, since it is chiefly due to oxides of iron and man ganese, which in no way affect the cementitious value; but for any given brand, variations in shade may indicate differences in the character of the rock or in the degree of burning.
With portland cement, gray or greenish gray is generally con sidered best; bluish gray indicates a probable excess of lime, and brown an excess of clay. An undue proportion of under-burned material is generally indicated by a yellowish shade, with a marked difference between the color of the hard-burned, unground particles retained by a fine sieve and the finer cement which passes through the sieve. However, there has recently been put upon the market
a white portland cement. It is sold in three grades according to the whiteness, the best being nearly snow white. All grades of the white portland cement are too expensive for ordinary masonry work, but there are various other purposes for which it is valuable. Mortar made of it is said not to stain the stone.
Natural cements are usually brown, but vary from very light to very dark.
Slag cement has a mauve tint—a delicate lilac.
The higher the temperature of burning the greater the weight of the clinker (the unground cement). Two methods have been employed in utilizing this principle as a test of the thoroughness of burning, viz.: (1) determine the weight of a unit of volume of the ground cement, or (2) determine the specific gravity of the cement.
The weight per unit of volume is usually determined by sifting the cement into a measure, and striking the top level with a straight edge. In careful work the height of fall and the size of the measuring vessel are specified. The weight per cubic foot is neither exactly constant, nor can it be determined precisely; and is of very little service in determining the value of a cement. However, it is often specified as one of the requirements to be fulfilled. The determina tion of the weight of a cement was the first test ever made of a hy draulic cement.