117. That a cement should not contain within itself elements which may lead to its destruction, is evidently a most important quality. It is probable that nearly all cements undergo a slight change in volume during induration, contracting in air and expanding in water. But it is the detection of those larger changes, which result from bad proportions or defective manufacture, and which cause deterioration or even complete disintegration, that is the object of the tests for soundness.

118. Causes Of Unsoundness

The most frequent cause of unsoundness is considered to be the presence of free lime or magnesia. (See §§49 and 50.) Any one of the following causes may account for the presence of free lime in cement: (1) An excessive percentage of lime may have been used in proportioning the raw materials; (2) the raw materials may not have been sufficiently mixed to render the mass homogeneous; (3) hard particles of lime, such as shells, may not have been ground fine enough in making the mix to permit them to enter into combination with the other ingredients during burning; or (4) the cement may have been underburned, so that part of the lime did not enter into combination.

The particles of free lime which occur in cements are naturally rather difficult to slake on account of their impurity and the high temperature at which they have been calcined, and the same thing is probably true of magnesia. It may thus require weeks or months of exposure to the atmosphere to correct tendencies to expand due to the presence of free lime or magnesia. Likewise when such defective cements are immersed in water of ordinary temperature, the expansion may not occur for a considerable period. This fact has led to the use of hot tests of various kinds to detect such faults, but before touching on these so-called "accelerated tests," the ordinary cold-water test will be described.

119. Tests For Soundness

The Committee of the American Society of Civil Engineers on a " Uniform System for Tests of Cement" recommended, in 1885, the following test for soundness: "Make two cakes of neat cement two or three inches in diameter, about one-half inch thick, with thin edges. One of these cakes, when hard enough, should be put in water and examined from day to day to see if it becomes contorted, or if cracks show themselves at the edges, such contortions or cracks indicating that the cement is unfit for use at that time. In some cases the tendency to crack, if caused by the presence of too much unslaked lime, will disappear with age. The remaining cake should be kept in air and its color observed, which for a good cement should be uniform throughout, yellowish blotches indicating a poor quality; the Portland cements being of a bluish-gray, and the natural cements being light or dark, according to the character of the rock of which they are made." For the ordinary cold test this method will probably give as valuable results as any of the forms that are suggested.

120. The German regulations require a very similar test, except that in the case of slow setting cements the pat is not immersed until twenty-four hours old. While a cement that is decidedly bad may show its defects in from one day to one week by this cold water test, it may be the case that cracks will appear only after several months' immersion. It has therefore been proposed to hasten the destructive action of the free lime or magnesia by submitting the cakes of cement to steam, hot water, or dry heat.

121. The Kiln Test

The Kiln Test, recommended by Prof. Tetmajer in 1890, consists in placing in an air bath, pats which have been kept in moist air for twenty-four hours; and then gradually raising the temperature of the air bath to 120° C. This temperature is maintained for at least one-half hour after the disengagement of steam has ceased. The pats should show no tendency to expand under this treatment, but if cements fail to pass the test, the results of the ordinary cold water treatment are to be awaited. This test is intended for cements that are to be used in air.

122. The Boiling Test

The Boiling Test, which was also recommended by Prof. Tetmajer, consists in placing the pats, twenty-four hours after made, in water of ordinary temperature, and gradually heating the water to bring it to the boiling point in about an hour; five or six hours in the boiling water should develop no defects. This is a severe test, and has been objected to on the ground that cements which have been well proportioned, but which are a trifle underburned, will fail to pass this test while giving good results in mortars to be used in the air. This test, however, is steadily gaining in favor, and is used in many cement works as a test of quality.