This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 produced 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 calcined 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. Sometimes 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 quickness 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 the 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.