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.
Some limes, after burning, contain enough clay or silica to acquire the property of setting under water and are called hydraulic limes, but are used to very little extent, as their qualities are more easily and profitably obtained by the use of cement, which is now more readily obtained, although hydraulic lime was formerly imported from England and France. Cement may be put in two classes - natural cement and artificial cement. Natural cement is obtained by burning limestones which contain a large proportion of clay. This forms a powder which, when mixed with water, sets quickly either in air or water. Natural cements are made in many localities throughout the country. Rosen-dale cement being probably the best known brand, and this is usually of good quality and easily obtained. Of artificial cements the best known is Portland cement; this is of English origin and derives its name from the resemblance of a trowelled surface, to Portland stone, one of the best known building stones of England.
Portland cement is made by combining proper proportions of carbonate of lime, clay, silica and iron. This mixture is dried and then burned into a heavy vitreous "clinker," which is afterward ground to a powder and run through fine sieves to make the finished product. Portland cement is now manufactured in many parts of the United States, and of as good a quality as the imported cements. Portland cement does not set as quickly as common cement, but possesses greater strength. There is now upon the market a grade of Portland cement known as "sand-cement" in which a certain proportion of sand is mixed with the powder and the whole ground to the fineness of the cement; this requires less sand in the final composition of the mortar and is productive of satisfactory results. Another prepared cement of a nature similar to Portland cement is known as Lafarge cement, and is useful in setting limestone or marble, as it does not stain like ordinary cement.
Some idea of the quality of cement may be obtained by a familiarity with its appearance upon opening the cask. With common cement the darker brown colors, in general, indicate the stronger qualities. Too dark a color, however, may indicate, in an unfamiliar brand, the presence of coloring matter, usually lampblack. This may be detected by putting some of the cement into a glass of water, when the lampblack will separate as a black scum. With Portland cement, a clear grey or bluish-grey color is to be preferred, as a brown color is given by an excess of clay, while too much of a bluish cast indicates the presence of too much lime. Further tests of cement may be made as described in Part I. for ordinary work, but for important engineering works where Portland cement is used, it should be subjected to careful tests for activity, soundness, and strength, both unmixed and mixed with sand.
In general, to obtain the best results, the superintendent should choose a fine, well-burned cement of average specific gravity, and, for important work, test it frequently. This, mixed with a sand that has passed a careful scrutiny for cleanliness arid regularity, should give a smooth and strong mortar.