LIME AND CEMENT.
Lime is now manufactured in nearly all parts of the country, but differs ill character in different localities. In using a lime which is not already familiar, some inquiry- should be made as to its prop erties, especially if used for plastering. Good lime in general should be free from cinders or clinkers, and with less than 10 per cent of impurities, and should be found in large lumps which will slake readily in water, ma-king a soft paste, free from residue or "core." It should further completely dissolve in soft water. Slaked lime, or "lime-putty," as it. is commonly called, will keep indefinitely if protected from the air so that it does not dry up. This is usually done by covering it with sand in the bed in which it is slaked. Lime, before slaking, will absorb from the air and become "air slaked;" this destroys its strength and care should be taken that all lime is carefully protected from dampness until used.
Setting and Durability. Lime mortar does not possess the "setting" duality of cement, but gradually hardens by exposure to the air. Lime mortar does not harden under water or in very (lamp tions, but in dry places where there is ample exposure to the phere, it will set hard enough for all ordinary uses of brickwork, except arches and piers, and where excessive loads may be applied. Hydraulic Lime and Cement. Some limes, after biirning, 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 by. draulic 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 after ward 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 ]css 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 cerement.
Color. 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 lamp black. This may be detected by putting some of the cement into a glass of water, when the lampblack will separate as a black s0m1. With Portland cement, a clear grey or bluish-grey color is to be pre ferred, 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 ina-dc as described in Pert 1. 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.