Nearly pure limes, consisting mainly of calcium oxide, are very caustic and become hydrated very rapidly when brought into con tact with water. This hydration, or slaking, produces a rise in temperature and increase in volume, which vary in amount accord ing to the purity of the lime, the volume being doubled or tripled for good fat lime. When the lime is derived from a magnesian limestone, it may contain a considerable proportion of magnesia mixed with the lime. Limes containing more than about 15 per cent of magnesia are usually called magnesian limes. The presence of magnesia has the effect of rendering the lime less active, causing it to expand less upon slaking. The magnesian limes harden more slowly, but usually gain a higher ultimate strength than the high calcium limes.
The common method of slaking lime consists in covering the quicklime with water, using two or three times the volume of the lime. This method is known as drowning. The lime is usually spread out in a layer perhaps 6 or S inches thick, in a mixing box, the water poured over it and allowed to stand. Sufficient time must be allowed for all of the lumps to be reduced. When the lime contains much foreign matter, the operation frequently requires several clays. Too great quantity of water is to be avoided, the amount being such as will reduce the lime after slaking to a thick pasty condition. All the water should be added at once, as the addition of water after the hydration is in progress causes a lower ing of temperature and checks the slaking. For the same reason,
the lime should be covered after adding water, and not stirred or disturbed until the slaking is completed. The covering is often effected by spreading a layer of sand over the lime, the sand being afterward used to mix with it in making mortar.
A second method of slaking is sometimes employed having for its object the reduction of the slaked lime to powder, and known as slaking by immersion. This is accomplished in two ways. By the first method, the lime is suspended in water in baskets for a brief period to permit the absorption of the necessary water, after which it is removed and covered until slaking takes place and the lime falls to powder. By the second method, sprinkling is substituted for immersion, the lime being placed in heaps and sprinkled with the necessary quantity of water, then covered with sand and allowed to stand.
Lime is commonly sold as quicklime, and should be in lumps and not air slaked. When it is old and has been exposed to the air it is likely to have absorbed both moisture and carbonic acid, thus becoming less active, the portion combined with carbonic acid being inert. A simple test of the quality of quicklime is to immerse a lump for a minute, then place in a dish and observe whether it swells, cracks, and disintegrates, with a rise of temperature.
Slaking some clays in advance of use is desirable in order to insure the complete reduction of the lime, and it is quite common to slake lime several weeks before it is to be used.
Common lime is ordinarily used in construction as a mortar, mixed with sand. The quantity of lime in the mortar should be just sufficient to fill the voids in the sand, without leaving any part formed entirely of lime. Mortar of rich lime shrinks in hardening, while masses composed entirely of lime on the interior are likely to remain soft, so that an excess of lime may be an element of weak ness. If too little lime be used the mortar may be porous and weak. The proportions ordinarily required are between one part lime to two parts sand, and one part lime to three purls sand.
In mixing lime mortar, sand is spread over the lime paste and worked into it with a shovel or hoc. The proper proportions of sand and lime may be judged by observing how the mortar works. If too much sand be used it he brittle, or " short "; while too much paste will it to stick and cake so that it will not flow from the trowel.