42. Natural Cement. Natural cement is obtained by burning an argillaceous or a magnesian limestone which happens to have the proper chemical composition. The resulting clinker is then finely ground and is at once ready for use. Such cement was formerly and is still commonly called Rosendale cement, owing to its having been produced first in Rosendale, Ulster County, New York. A very large part of the natural cement now pro duced in this country comes from Ulster County, New York, or from near Louisville, Kentucky. Cement rock from which natural cement can be made, is now found widely scattered over the country.
In Europe the name Roman cement is applied to substantially the same kind of product. Since the cement is made wholly from the rock just as it is taken out of the quarry, and also since it is cal cined at a much lower temperature than that employed in making Portland cement, it is considerably cheaper than Portland cement. On the other hand, its strength is considerably less than that of Portland cement, and the time of setting is much quicker. Some times this quickness of setting is a very important point—as, for instance, when it is desired to obtain a concrete which shall attain considerable hardness very quickly. On the other hand, the quick ness of setting may be a serious disadvantage, because it may not allow sufficient time to finish the concrete work satisfactorily and prevent the disturbance of mortar which has already taken an initial set. Natural cement is still largely used, on account of its cheapness, especially when the cement is not required to have very great strength. The disadvantage due to its quick setting (when it is a disadvantage) may be somewhat overcome by the use of a small percentage of lime when mixing up the mortar.
It is not always admitted, at least in the advertisements, that a given brand of cement is a natural cement; and thQ engineer must therefore be on his guard, in buying a cement, to know whether it is a quick-setting natural cement of comparatively low strength or a true Portland cement.
43. Portland Cement. Portland cement consists of the prod uct of burning and grinding an artificial mixture of carbonate of lime and clay or slag, the mixture being very carefully proportioned so that the ingredients shall have very nearly the fixed ratio which experience has demonstrated to give the best results.
"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."—S. B. NEWBERRY, in Taylor and Thompson's "Concrete Plain and Reinforced." As before stated, Portland cement is stronger than natural cement; it sets more slowly, which is frequently a matter of great advantage, and yet its rate of setting is seldom so slow that it is a disadvantage. Although the cost is usually greater than that of natural cement, yet improved methods of manufacture have re duced its cost so that it is now usually employed for all high grade work where high ultimate strength is an important consid eration.
In a general way, it may be said that the characteristics of Portland cement on which its value as a material to be used in con struction work chiefly depends may be briefly indicated as follows : When the cement is mixed with water and allowed to set, it should harden in a few hours, and should develop a considerable proportion of its ultimate strength in a few clays. It should also possess the quality of permanency so that no material change in form or volume will take place on account of its inherent quali ties or as the result of exterior agencies. There is always found to be more or less of shrinkage in the volume of cement and con crete during the process of setting and hardening ; but with any cement of really good quality, this shrinkage is not so great as to prove objectionable. Another very important characteristic is that the cement shall not lose its strength with age. Although some long-time tests of cement have apparently indicated a slight de crease in the strength of cement after the first year or so, this decrease is nevertheless so slight that it need not affect the design of concrete, even assuming the accuracy of the general statement.
To insure absolute dependence on the strength and durability of any cement which it is proposed to use in important structural work, it is essential that the qualities of the cement be deter mined by thorough tests.