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charged directly into a crusher, which, in this mill, was of the "Foster" pattern. The stone is crushed as fine as small gravel, then elevated to a bin above the buhrs, of which four are devoted to grinding the limestone. It is delivered from the millstones as a very fine powder and lifted by elevators to bins above, as in the case of the clay. These bins are above the "mixing floor," where the ground clay and stone are run into a changing barrow, such amount of each as is necessary, the mixture being made by weight, platform scales with set weights (furnace scales) being used. This barrow load of weighed material is then dumped into a large revolving pan mixer, water added as necessary, the whole being mixed by the stirring arms of the machine for twenty to thirty minutes. It is essential that the operation be most thorough. The mixture, now in the form of a moderately stiff dough, is dropped into a small "clay blocker," such as is used in tile or brick-making, and forced out as a cylindrical mass, which is cut up and loaded into iron trucks, which run with it into a drying oven, this latter being an arched gallery heated by a small grate and about sixty feet long. The "mix," after drying thoroughly, is ready for burning. This operation is performed in cylindrical kilns thirty-six feet high and about eight feet in diameter. The method of charging and burning is to put first a quantity of wood in the bottom of the kiln, which is closed by temporary grate bars of iron leaving a space below. On the wood, coke is placed, and above it the dried blocks of material mixed with coke, the amount of coke being regulated by the heat required to burn the charge, which varies with different proportions of mixture. The kiln is filled up to the top, after setting fire to the wood at the bottom, which in turn ignites the coke. The burning is watched with great care. It must be slow, and yet the final temperature must be very high to bring the mass to incipient vitrification. As the mass sinks, more coke and material is added to the top, until the kiln is filled, when it is covered over with coke and allowed to burn out and cool. When cooled a little, the charge is drawn from the bottom and picked over, and over or under-burned "clinker" separated, and the rest taken to the mill for grinding. Many accidents may occur in the kiln. If the mixture was not dry enough, it may "dust," or fall to powder, and choke the draft so as to leave a large portion of the kiln "raw"; or if the proportion of the ingredients was not right or the mixing imperfect, the clinker, after burning, may fall to dust and choke the kiln; but if all was correct, the "clinker," when drawn, should be mostly in lumps with a dark greenish, submetallic look and rough granular structure. A certain tendency to fall to powder, if drawn too hot, is not objectionable, provided it is not excessive. The cement is now sent through another crusher, and the crushed cement run through millstones, of which there are six "run" or pairs. An ingenious arrangement for removing the particles of iron which form in the kiln from the limestone, as this contains a little iron ore, consists of a powerful magnet, over the surface of which the material drops into the millstones. To this the iron adheres, and is cleaned off, thrown to one side by a little scraper acting periodically. The ground cement is lifted by elevators to bins for storage. The amount of cement obtained depends somewhat upon circumstances, but with clay and limestone, will average about sixty per cent. of the raw material.

In a more recent works, making cement from slag, stone and clay, the process is simplified by mixing the raw material in the requisite amounts. This mixture is then run through a crusher and to the buhrstones directly. From there, after a short mixing in a pan with sufficient water to make it into a dough, it falls on to a tile dry floor, where it is spread out, and when partially dry, cut up by a spade into roughly square blocks, which, when dry, are taken to the kiln. This avoids the use of the blocker and elaborate drying ovens. The blocker cannot be used where slag enters the mixture, as this, when fine, acts as sand and chokes the machine, owing to its not being plastic. Hence, no sort of pug-mill can be used with slag. This point, the writer has established by careful trial. The common method of burning in discontinuous kilns is wasteful of fuel and irregular in action; and some large works are using various forms of gas kilns, moulding the mixture into regular bricks by machinery and burning them as fire-brick are burned, only at a higher temperature. Such is, briefly, the method by which the most various materials may be converted into a Portland cement. The resultant cement will be in all cases practically the same, if the process is carefully conducted.

The cement, as it comes from the buhrs or millstones, is rarely fit for immediate use, differing widely in this respect from the natural ce ment. The material must be allowed to air-slack, or, as it is technically

termed, "cool." This process demands a more or less lengthy exposure to the air, with occasional turning over. The cement changes percepti. bly in character, becoming finer and softer. The change is, partly at least, due to the air-slacking of the small amount of free lime almost unavoidably present in the burned clinker; but there are other changes less simple in character which also take place, by which the strength of the cement is greatly increased, so that even a cement that apparently sets-up strong and sound when fresh, will show a much increased strength if kept a few weeks. Many kilns full of cement will, if mixed with water when quite fresh, crack and disintegrate more or less, or blow

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as it is termed, but will lose this very dangerous quality entirely if kept sufficiently. Pressure of orders will sometimes cause cement to be sent out from a mill too early, and in such cases much damage to the reputation of a brand may be done. Cement sometimes swells during the process of slacking, and if barreled too soon may burst the packages. This process of "cooling" requires exposure to air, and takes place only very slowly; if the cement is kept in large bins three or four feet deep, it must be worked over, and should lie not over two feet deep.

While the materials for making cement are very abundantly distributed, it is very possible to make bad cement from the best materials; and hence, constant care and watchfulness are requisite to prevent this accident. Some of the principal ways in which failure may occur are the following:

Errors in proportion of mixture. An "over-limed" cement, in which the per cent. of carbonate of lime was too high in the original mixture, is usually very heavy and hard. It sets slowly, and shortly after setting begins to crack and blow, like a cement too fresh for use; but ageing will not remedy it, in case the mixture was badly over-limed. An over-clayed cement, in which the alumina and silica are in excess, will set quickly, but will never gain much strength, and is liable in time to crumble and disintegrate.

The errors in proportion, however, are more easily avoided than those of manipulation. A cement in which there is insufficient fineness in the materials may have the evils of both an over-clayed and overlimed article. It will be light, set quick, have no strength, and crack to pieces subsequently. The same faults may appear if the cement is under-burned. Over-burning results usually in a very slow-setting, heavy cement, which may have little strength, and will be long in slacking sufficiently to be safe to use.

If the proportions, mixing, and burning were right, the cement, after a reasonable time for air-slacking, should, when mixed to a stiff dough with water, "set" in from a half-hour to two hours, so that it can be placed under water; and when so submerged, it should gradually gain hardness, showing no tendency to crack, swell, or shrink.

The fineness to which cement is ground is of the utmost consequence. The fact is clearly established by Faija and others, that only the most impalpable powder has any value as cement; that if a given cement be sifted through a sieve of 5,000 holes to the inch, the part not going through the sieve will, when mixed with water, have absolutely no cement power, though the finer part may be of the highest grade, and the coarse part will itself yield the best possible cement if re-ground to a dust. Hence, the coarser parts are simply like so much sand, and dilute the cement accordingly. It is for this reason that millstones are so difficult to replace by other and less expensive grinding machinery. They produce the maximum of dust for a given degree of pulverization, and it is the dust that is wanted. The finer the cement is ground, the greater the amount of sand it will carry, and as this determines the cost of mortar or concrete, a fine cement is worth more than a coarse one in money.

Injurious impurities in Portland cement may destroy its value. The principal ones are sulphur and magnesia. The action of sulphur is to cause in time a disintegration of the cement, and it should never be present in cements in more than fractions of a per-cent. It may be derived from poor material carrying pyrites, or from the fuel used in burning. A coke high in sulphur will, in this way, ruin cement. The use of clay will tend, to some extent, to keeping this impurity low, as before mentioned; but where no or but little clay enters the mixture, the coke must be most carefully looked to. Cement containing an undue amount of sulphur will sometimes turn blue on setting under water for some time. This color will be seen in the interior of the mass on breaking, while the surface may be the normal color.

TESTING CEMENTS.

The great variety of purposes to which cement is applied, together with the number of kinds of cement used, makes it important that an intelligent system of selecting and testing be employed.

It has been seen that there are two groups of cements which are in use, those that are quick-setting, comprising the lighter-burned natural cements, like Louisville or Rosendale, and the heavy artificial Portland cements, slow-setting and very strong when properly made. The use of each kind is distinct, and in its own particular field cannot usually be replaced by the other. It is also apparent that each kind is liable to be faulty at times, either from bad materials or careless manufacture. Now, users of cement must, if they wish to be secured against errors and fraud, test in some way the value of the material they use.

Two kinds of tests are needed, and these two are distinct. First, the determination of whether a given brand or grade is suitable. The testing of a new cement is a slow process and must extend over a long time. No ordinary short series of tests will answer definitely the question, "Is it a good cement?" as its permanent va ue will only appear after having the test of time and exposure; but where a cement of a given brand, having a good reputation is used, it is reasonable to infer that if it is of the average quality made by that works, or from the same material, it will be as safe and permanent as the brand is known to be.

It is to this point that most of the systems of testing cement are directed. The subject of testing cement was investigated by a special committee of the American Society of Civil Engineers, and the results of their experiments embodied in a report on a uniform method of cement testing (Transactions American Society of Civil Engineers, November, 1885).

The tests recommended are for fineness, soundness, or freedom from tendency to check or disintegrate, and tensile strength of the set cement. As before stated, the fineness is the measure of the ability to carry sand. This is determined by sifting a weighed amount of the cement through a sieve of standard mesh. The committee recommend three sieves of 2,500, 5,476 and 10,000 meshes to the square-inch. Fairly ground cement should not leave more than five to ten per cent. on a 2,500 mesh sieve.

The test for soundness is to make a pat or cake of the cement with water, about three inches in diameter and half an inch thick, spread out on a plate of glass until set, and then put in water and examined from time to time. A similar cake of cement should be set in air. Neither must develop cracks or "checks." A fresh sample of cement may crack under this test, as has been seen, and yet be restored to soundness by age. This checking test, in the case of known brands, is valuable to ascertain whether the cement is too new for use.

The test for tensile strength requires more machinery for its application, but many efficient forms of cement testers are on the market. There is great difficulty for obtaining uniform tests for cement, and the tests of different persons will vary greatly. Hence, any one who tests cement should begin with a number of known and approved kinds, so as to establish his own standards of comparison. The cement is tested either "neat" or with sand. The sand test is recommended by the Society of Engineers in addition to the "neat" test, but Henry Faija, in his book on "Cement Testing," calls attention to its extreme uncertainty, as variations in sand correspond to greater differences than variations in cements. The committee, however, propose a standard sand to obviate this difficulty.

The briquettes, or little blocks for testing, are made of a prescribed shape, and have a section of one square-inch at the breaking point. The cement, being mixed rapidly and thoroughly with just sufficient water to make a stiff dough, is pressed into the mould and allowed to set. When hard enough to remove, it is covered with a damp cloth, and after twenty-four hours, set in water for six days, then broken in a machine which registers the force required to effect rupture.

American and foreign Portland cements of fair quality, tested thus, will give from 250 to 500 pounds per square-inch, seven days old.

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