Popular Science Monthly/Volume 37/June 1890/Natural and Artificial Cements
|NATURAL AND ARTIFICIAL CEMENTS.|
THE cements now in the market are of two kinds: natural, made directly from stone; and artificial, commonly called Portland cement. The manufacture of the former consists simply in burning and grinding the cement stone, a magnesian limestone containing about fifteen per cent of silica and a little silicate of alumina. The burning drives off the small amount of combined water and all the carbon dioxide from the stone, leaving the lime and magnesia as oxides, while the grinding to a powder puts it into the best possible condition for mixing with sand and gravel, and moistening to form a mortar. Artificial cement consists of about sixty-two per cent of lime mixed with silica and silicate of alumina in nearly the same proportions as those found in the cement stone, and it is free from magnesia. This seems to be the whole difference in its constitution. In use, the artificial cement sets rapidly and attains maximum hardness in a comparatively short time; the natural cement hardens rather slowly and reaches its maximum hardness only after a long period of exposure to the air.
The increasing use of cement in modern construction, either alone or more commonly as mixed with sand and gravel, demands that the qualities of the different kinds, and the means of testing, both roughly and accurately, should be generally understood. The foundations of all important structures, in situations where they can not rest directly upon solid rock, owe their strength to cement. They are usually made of concrete, cement mixed with sand or gravel, and they are often strengthened by iron beams so as to bind the whole into one continuous mass. Tunnels under rivers, sewers, cable trenches, and all the numerous subways of our large cities, are either concrete or masonry laid in cement mortar. Their strength, again, is the strength of the cement used. And even the piers of most of the large bridges are now made in part or wholly of concrete. Oftentimes, even the walls of stone and brick buildings are rendered more secure by being laid up with mortar of which cement forms a large ingredient. Used for so many purposes, the necessity of uniform quality, and proper knowledge of the quality of the cement used, become plain.
Before examining the methods of testing now employed and comparing the results, the process of hardening needs to be comprehended. Some things are not yet quite clear in it, but it is certainly in the main a chemical process. Mixed with water, the lime and magnesia of the cement unite to form a hydrate, and it is probable that the silicates also recombine with some of the water. This is the first step, and produces the so-called setting. It is best passed through while the cement is exposed to the air, and is the reason why cement mixtures must be used as soon as moistened. But, this now complete, a more complex process is set up. The moistened cement brought in contact with the air, or exposed to water, at once begins to absorb carbon dioxide, for all ordinary air contains the gas, and most water holds it in solution. The gas unites with the lime to form a carbonate again, and this goes on until the whole of the lime is turned back to limestone. The same change occurs in the magnesia, but in this the action proceeds more slowly. With a pure lime cement this action is probably nearly complete at the end of a few months; but, with a cement containing magnesia, it will continue for many years. The strength of the cement increases so long as the change continues. So a Portland cement will develop its full strength in a few months, while our natural cements will not for years, and, so long as it continues, the structure improves.
Rough testing of cement, so as to enable a workman to get a crude and imperfect idea of its value, is easy. Enough of the pure cement should be taken to make a ball an inch in diameter and mixed with just sufficient water to make it mold readily and be rolled into a ball. Then it should be exposed to the air and left for two hours. At the end of that time it should be set; then it should be put into water and left. It should grow gradually harder, and should show no signs of cracking or crumbling, even when left for ten days. Any cement that does not endure this test is not of sufficiently good quality to make satisfactory structures; any cement that stands this properly will be generally satisfactory if properly used.
In determining how to construct a building, a series of tests is often required that shall show tensile, breaking, twisting, and crushing strength, and also adhesion of the materials used for mortar. No one of these can be dispensed with, since material that will endure one satisfactorily will often fail utterly in another, and hence prove worthless for the use desired; but for general purposes the test of cement which is the most valuable is that which determines its tensile strength. Comparative tests of this show the value of cements from different sources better than any other one test.
To make an accurate test of any lot of cement, great care is necessary in selecting and manipulating the samples. The test sample ought not to be taken from a single package, but from several in equal quantities and thoroughly mixed. The sample must also be carefully protected from air and moisture until the test is made. When used, it must be molded with just the right amount of water to render it plastic. Too small an amount will leave some particles dry; too large an amount will gather in masses, will evaporate, leaving pores, and will give too small results. The test is now commonly made by molding a briquette Fig. 1 of a form approved by engineers, as shown in Fig. 1, which is drawn of one third actual size. The mold is a clamp of metal exactly one inch in thickness and exactly one inch across at R. This makes the area of the smallest place exactly an inch. The moistened cement is carefully placed in the mold with a spatula and pressed enough to render the whole mass homogeneous. It is left in the mold until it can be removed by opening the mold, and then it is exposed to the air for exactly twenty-four hours, after which it is put into water and allowed to rest there until the test is made. The length of time depends upon the purpose of the test. In order to make certain that all the cement produced is of a uniform quality, seven days is sufficient. Such a test is made of every lot shipped by the Milwaukee Cement Company, and probably by all other reliable manufacturers. If the test is to determine the ultimate strength developed or to compare cements from different sources,Fig. 2. then a series of tests should be made by breaking "briquettes" made at the same time but left in water for different periods. The reason is, that a quick-setting cement 'will develop its full strength in a short time, and if the test is made at the end of that time it might show a greater tensile strength than another one slow in setting, even when the latter would ultimately have several times its strength.
The test can be made in any form of testing machine, though one in which the test is applied by uniformly increasing the strain, as by running shot into a bucket upon the end of a lever, gives the most accurate results; but the briquette should be held in a clutch that presses accurately upon the sides, as shown in Fig. 2. This applies the tension equally, and gives a very accurate test. A long series of these were made by Mr. D. J. Whettemore, C. E., at Milwaukee in 1874, in which seventeen native cements showed an average tensile strength at the end of seven days of 808 pounds. The lowest of these broke at 38 pounds, while the highest sustained 1392 pounds. Later tests made in the same way have shown that these were unreliable as final tests of strength, because the briquettes had not hardened sufficiently, and the table would place inferior cements above those of much greater strength because the inferior develops its ultimate strength much sooner. But a comparative test of the same cements when mixed with sand in equal parts was also made, and is of very great value and probably perfectly reliable, for the tests were then made at the end of ninety days, so giving the slow-setting cements time to develop their strength.
Thus the one which in the test applied to clear cement broke at 38 pounds now sustained 1521 pounds, an increase of four hundred per cent; while the one that was the strongest at the end of seven days now broke at 2041 pounds, an increase of only fifty-four per cent. The one that showed the greatest tensile strength of all, at the end of ninety days, the Milwaukee cement, 290 pounds, broke at only 96 at the end of seven days. An experiment made with a briquette taken at random, that had been made six months and exposed to the air at least half that time, strikingly showed the same fact, for it broke only under a strain of 636 pounds. This test was made simply to show the writer the method of using the testing machine.
The United States Government had a series of tests made a few years ago, using the cements commonly sold in the West, and giving in each case the mean result of seventy-five tests. The table is so interesting that we give it entire.
Tensile Strength of Pure Cements, each Test given "being the Mean Result from Seventy-five Specimens, Thirty and Sixty Days.
|Thirty days.||Sixty days.|
Cement is far more often called upon to resist a crushing than a tensile strain. A large number of tests has been made to determine the weight required to crush a cube one inch in each dimension. When mixed with sand in equal proportions, the best cements will sustain a crushing weight of upward of a ton, the specimen having been allowed to harden for ninety days, while the poorest do not sustain quite half a ton, and even when mixed with three parts sand to one of the cement, the Milwaukee, which tests have shown the best, sustains over eleven hundred pounds. These tests show conclusively that structures well built of mixed cement and sand or coarse gravel will sustain any reasonable weight without danger of yielding.
Little needs to be added upon adhesion. Many attempts have been made to determine the adhesive strength of various cements, usually without success—not because they do not hold properly, but because they hold until the brick or stone to which they have been applied is ruptured before the cement is separated from its surface. This shows that the adhesion is always sufficient for all uses, and this seems to be true of all our native cements. Their use, therefore, mixed with mortar adds greatly to the strength of the structure.
All these qualities of cement warrant its continual and increased use, particularly of all the better grades. Probably the English Portland is the best of all, but its cost is so much beyond that of our native cements as to warrant using them in its place in somewhat larger proportion in all places where time can be allowed for the hardening.