CEMENTS Kinds of Cement. Most of the cement now employed in constructive work falls under one or the other of two main heads—Natural Cement and Portland Cement, the latter being much the more extensively used, especially where great strength and absolute reliability as to uniform quality are required. In some instances, the so called Pozzuolana and Artificial Slag Cements are also used; and, for certain special locations and purposes, a special kind of cement known as Keene's Cement is preferred.
Natural Cement Natural cement is so called because the raw material from which it is made is found in a natural state and is used just as it comes from the earth, requiring neither artificial propor tioning nor the admixture of other ingredients.
In the case of the Portlands, on the other hand—for the different brands of Portland cement differ among themselves in chemical composition, and there is no general chemical formula for Portland cement—the raw material is in every case an artificial mixture. Some natural ingredients, it is true, are used in the making of Portland cement; but the propor tioning and mixing are always artificially done, and only after careful chemical and physical tests to determine exactly the proportions and the degree of fineness of grinding necessary for developing the qualities desired. A so-called "Natural Portland" is made in small quantities in England and on the European Continent, in those rare cases where limestone is found con taining combinations of lime, silica, and alumina in the proportions approaching those which experience has shown to be desirable; but for best results, and to meet the large demand of the present day for a uniform and reliable prod uct, a carefully calculated artificial mixture of ingredients is absolutely necessary. This point is well brought out in Taylor and Thompson's "Concrete, Plain and Reinforced," by S. B. Newberry, who says: "If a deposit of stone containing exactly the right amount of clay, and of exactly uniform composition, could be found, Portland cement could be made from it, simply by burning and grinding. For good results, however, the composition of the raw material must be exact, and the proportion of carbonate of lime in it must not vary even by one per cent. No natural deposit of rock of exactly this correct and unvarying composition is known or likely ever to be found; therefore Portland cement is always made from an artificial mixture, usually, if free from organic matter, containing about 75 per cent carbonate of lime and 25 per cent clay."
Natural cements are made by calcining argil laceous (clayey) or magnesian limestones. The temperature required is considerably lower than that needed for the making of Portland cement. The resulting clinker is ground to a fine powder, which is then immediately ready for use.
Natural cements vary in color from light to dark gray, according to the character of the stone from which they are made. They are mar keted under various names such as "Rosen dale," "Utica," "Louisville," "Akron," "Mil waukee," "Fort Scott," etc., depending gener ally on the locality where the cement rock is found. The cement, for example, known as name sometimes applied loosely to all natural cements—is made by burning mag nesian limestone, of which very important deposits occur near the town of Rosendale in Ulster County, New York. After being quar ried and sorted, the stone is heated in large kilns. The over-burned and under-burned por tions are carefully discarded, and the properly burned portion is passed at once to crushers, where it is reduced to a fine powder, then being carefully screened and packed for shipment in either barrels or bags.
The so-called "Roman cement," first made in England toward the end of the eighteenth cen tury, and still manufactured to some extent in that country and on the Continent, is one of the "natural" cements.
Owing to the fact that the chemical contents of the cement rock may differ greatly even in different parts of the same quarry, the strength of the product may show great variations on test; and it is only by careful selection of the desirable strata that a uniformity of quality can be insured. Much of the disfavor with which natural cements have been looked upon by engineers has been due to this uncertainty and variation in strength. In the case of Portland cement, on the contrary, a substantially absolute uniformity of composition and consequent relia bility as to strength in any particular brand, are not only attainable, but are practically assured by the careful technical supervision exercised over all the processes and details of manufac ture by the leading American Portland cement makers.