CEMENTING MATERIALS In the making of concrete, the principal ce menting materials used are Common Lime, Hy draulic Lime, Natural Cement, Slag Cement or Pozzuolana (sometimes spelled Puzzolan Ce ment), Portland Cement, and Keene's Cement.
Common Lime The old-fashioned lime of commerce used in making mortar for ordinary plaster work, is also used to some extent in concrete work. The ad dition of a small percentage increases the smoothness and plasticity of wet concrete, and adds to the ease with which it is worked under the trowel.
Common lime is made by burning limestone in kilns. The limestone—which consists chiefly of calcium carbonate or carbonate of lime being subjected to intense heat, is partially decomposed. The carbonic acid gas which it contains is driven off, leaving only the calcium oxide or oxide of lime (Ca0). This is known as quicklime or unslaked lime, and is the form in which common lime is pur chased on the market. A barrel of lime weighs 230 pounds net; and 60 cents a barrel may be quoted as an average price.
Before lime can be used, it must be slaked— that is, combined with water. Quicklime is sol uble to some extent in soft water—that is, in water which does not already contain any large proportion of lime. In solution, it renders water hard.
The activity of quicklime is based on its great affinity for both water and carbonic acid. These ingredients it will absorb quite rapidly from the atmosphere, if left exposed; and in time it will become air-slaked, losing that power of becom ing hard again which is its most valuable prop erty as•a material of construction. For this rea son, in order to prevent deterioration, quicklime should be kept under cover, protected as much as possible from air currents, and, above all, must be kept dry.
When the lime is needed for use, it is slaked by adding water. Great heat is developed in the slaking process, and the volume of the lime is increased to about three times what it was be fore. An increase of about one-fourth in weight also occurs. From hard lumps, the mass is transformed into a fine white powder, which, on the addition of more water, becomes a smooth, unctuous paste. In slaking, the degree of heat
generated, and the volume of expansion, will vary somewhat with different limes, being the greater as the lime is purer, and thus affording a rough indication of the quality of the lime.
Lime made from marble is almost pure; but the product made from limestones will contain various impurities, such as silica, alumina, mag nesia, etc. The presence of a certain proportion of these is not necessarily harmful. A lime may contain up to ten per cent of these foreign in gredients, and still be of "good quality." On the other hand, lime which has been allowed to deteriorate by air-slaking, or which contains too large a percentage of impurities, so that a proper slaking and expansion cannot be ob tained, is said to be a "lean" or "poor" lime, as distinguished from a "fat" or "rich" lime. It may be so poor as to be entirely worthless ex cept for fertilizing purposes.
In the course of time, if exposed to the at mosphere, a lime paste will harden by absorbing carbonic oxide from the air. The carbonate of lime is thereby formed, so that the process of the hardening of lime is in reality a return to approximately the original condition of the stone. It is upon this peculiar property of hardening under atmospheric action that the chief constructive value of lime depends. The hardening process may continue for years, pene trating deeper and deeper into the mass.
The very fact, however, that atmospheric action is essential, indicates the natural limita tions on the use of common lime as a material of construction. It is unsuited for work under water; here the air cannot reach it, and it will not harden. When exposed to the action of sea water, particularly, it will very rapidly disinte grate owing to the formation of soluble sulphates and aluminates. It is also unsuited for use in the interior of massive work where it will be buried at such depths as in effect to be sealed hermetically from the action of the carbonic oxide of the air. Even after standing many years, lime mortar in the interior of massive work has been found to be still soft.