CEMENTING MATERIALS 3S. The principal cementing materials are Common Lime, Hydraulic Lime, Pozzuolana, Natural Cement, and Portland Cement. There are a few other varieties, but their use is so limited that they need not be considered here.
39. Common Lime. This is produced by burning "limestone" whose chief ingredient is carbonate of lime. Except in the form of marble, a limestone usually contains other substances—perhaps up to 10 per cent of silica, alumina, magnesia, etc. The process of burning drives off the carbonic acid, and leaves the protoxide of calcium. This is the lime of commerce; and to preserve it from deterioration, it must be kept dry and even protected from a free circulation of air. When exposed freely to the air for a long period, it will become air-slaked; that is, it will absorb both moisture and carbonic acid from the air, and will lose it ability to harden. The first step in using common lime is to combine it with water, which it absorbs readily so that its volume is increased to 2.1 or 31 times what it was before. Its weight is at the same time increased about one-fourth; and the mass, which consisted originally of large lumps with some powder, is reduced to an unctuous mass of smooth paste. The lime is then called slaked lime, the process of slaking. being accompanied by the development of great heat. The purer the lime, the greater the development of beat and the greater the expansion in volume. It is soluble in water which is not already "hard," or which does not already contain considerable lime in solution. A good lime will make a smooth paste with only a very small per centage (less than 10 per cent) of foreign matter or clinker. By such simple means a lime may be readily tested.
The hardening of common lime mortar is due to the formation of a carbonate of lime (substantially the original condition of the stone) by the absorption from the atmosphere of carbonic oxide. This will penetrate for a considerable depth in course of time; but instances are common in which masonry has been torn down after having been erected many years, and the lime mortar in the interior of the mass has been found still soft and unset, since it was hermeti cally cut off from the carbonic oxide of the atmosphere. For the same reason, common lime mortar will not harden under water, and therefore it is utterly useless to employ it for work under water or for large masses of masonry.
When the qualities of slaking and expansion arc not realized or are obtained only very imperfectly, the lime is called lean or poor (rather than its value is less and less, until it is perhaps worthless for use in making mortar, or for any other use except as fertilizer. The cost of lime is about 60 cents per barrel of 230 pounds
net.
40. Hydraulic Lime. This is derived from limestones con taining about 10 to 20 per cent of clay or silica, which is intimately mixed with the carbonate of lime in the structure of the stone. Dur ing the process of burning, some of the lime combines with the clay (or the silica) so as to form the aluminate or silicate of lime. The excess of lime becomes quicklime as before. During the process of slaking, which should be done by mere sprinkling, the lime having been intimately mixed with the clay or silica., the expansion of the lime completely disintegrates the whole mass. This slaking is done by the manufacturer. The lime having a much greater avidity' for the water than the alumina le or the silicate, the small amount of water used in the slaking is absorbed entirely by the lime, and the aluminate or the silicate is not affected. The setting of hydraulic lime appears to be due to the crystallizing of the aluminate and silicate; and since this will be accomplished even when the masonry is under water, it receives from this property its name of hydraulic lime. It is used but little in this country, and is all im ported.
41. Pozzuolana or Slag Cement. Pozzuolana is a form of cementing material which has been somewhat in use since very ancient times. Apparently it was first made from the lava from the volcano Vesuvius, the lava being picked up at Pozzuoli, a village near the base of the volcano. It consists of a combination of silica and alumina, which is mixed with common lime. Its chemical composi tion is therefore not very unlike that of hydraulic lime. It also possesses the ability to harden under water. Its use is very limited, and its strength and hardness comparatively small, compared with that of Portland cement. It should never be used where it will be exposed for a long time to dry air, even after it has thoroughly set. It appears to withstand the action of sea water somewhat better than Portland cement; and hence it is sometimes used instead of Portland cement as the cementing material for large masses of masonry or concrete which arc to be deposited in sea water, when the strength of the cement is a comparatively minor consideration. Artificial pozzuolana is sometimes made by grinding up blast-furnace slag which has been found by chemical analysis to have the correct chemi cal composition.