MORTAR Si. The term mortar is usually applied to the mixture of sand and cementing material which is placed between the large stones of a stone structure, although the term might also be properly applied to the matrix of the concrete in which broken stone is embedded. The relieves the stones of transverse stresses and also from the concen trated crushing pressures to which the projecting points of the stone would be subjected.
SS. Common Lime Mortar. The first step in the preparation of common lime mortar is the slaking of the lime. This should be done by putting the lime into a water-tight box, or at least on a plat form which is substantially water-tight, and on which a sort of pond is formed by a ring of sand. The amount of .water to be used should be from 2J, to 3 times the volume of the unslaked The "volume" of unslaked lime is a very uncertain quantity, varying with the amount of settlement caused by mere shaking which it may receive during transit. A barrel of lime means 230 pounds. If the barrel has a volume of 3.75 cubic feet, it would be just filled by 230 pounds of lime when this lime weighed about 61 pounds per cubic foot. This same lime, however, may be so shaken that it will weigh 75 pounds per cubic foot, in which case its volume is reduced to Si per cent, or 3.05 cubic feet. Combining this with to 3 times its volume of water, will require about S cubic feet of water to one barrel of lime. On the other hand, if the lime has absorbed moisture from the atmosphere, and has become more or less air-slaked, its volume may become very materially increased.
Although close accuracy is not necessary, the lime paste will be injured if the amount of water is too much or too little. In short, the amount of water should be as near as possible that which is chemi cally required to hydrate the lime, so that on the one hand it shall be completely hydrated, and on the other hand it shall not he drowned in an excess of water which will injure its action in ultimate harden ing. three volumes of -sand should be used, to one volume of
lime paste. Owing to the fact that the paste will, to a considerable extent, nearly fill the voids in the sand, the volume obtained from one barrel of unslaked lime made up into a mortar consisting of one part of lime paste to three parts of sand, will make about 6.75 barrels of mortar, or a little less than one cubic yard.
89. Natural Cement Mortar. This is largely used, especially when mixed with lime to retard the setting, in the construction of walls of buildings, cellar foundations, and, in general, in masonry where the unit-stresses are so low that strength is a minor considera tion, but where a lime mortar would not harden because it is to be under water or in a solid mass where the carbonic acid of the at mosphere could not penetrate to the interior. When natural cement is dumped loosely in a pile, the apparent volume is increased one third or even one-half. This must be allowed for in mixing. A barrel averages 3.3 cubic feet.
Therefore a 1:4 mortar of natural cement would require one barrel of cement to 13.2 cubic feet (about one-half a cubic ward) of sand. A bottomless box similar to that illustrated in Fig. 7, and with inside dimensions of 3 feet X 2 feet 6 inches X 1 foot 9 inches, contains 13.2 cubiC feet.
It is preferable to use even charges of one barrel of cement in mixing up a batch of mortar, rather than to dump it out and measure it loosely. If the size of the barrel varies from the average value given above, the size of the sand box should be varied accordingly. The barrels coming from any one cement mill may usually be considered as of uniform capacity. Since it is practically somewhat difficult to measure rccurately the volume of a barrel, owing to its swelling form, it is best to fill a sample barrel with loose, dry sand, and then to measure the volume of that sand by emptying it into a rectangular box whose inside area, together with the height of sand in it, can be readily measured.