HYDRAIILIC CEMENT 12. Setting and Hardening of cement powder is mixed with water to a plastic condition and allowed to stand, it gradually combines into a solid mass, taking the water into con bination, and soon becomes firm and hard. This process of com bination among the particles of the cement is known as the setting of the cement.
Cements of different character differ very widely in their rate and manner of setting, some occupying but a few minutes in the operation, while others require several hours. Sonic begin setting immediately and take considerable time to complete the set, while others stand for considerable time with no apparent action and then set very quickly.
The points where the set is said to begin and end are necessarily arbitrarily fixed, and are determined by finding when the mortar will. sustain a needle carrying a specified weight. The initial set is supposed to be when the stiffening of the mass has become per ceptible; the final set, when the cohesion extends through the mass sufficiently to offer such resistance to any change of form as to cause rupture before deformation can take place.
After the completion of the setting of the cement, the mortar continues to increase in cohesive strength over a considerable period of time, and this subsequent development of strength is called the hardening of the cement.
The process of hardening appears to be quite distinct from, and independent of, that of setting. A slow-setting cement is apt, after the first day or two, to gain strength more rapidly than a quick-setting one; but it does not necessarily do so. The ultimate strength of the cement is also quite independent of the rate of setting. A cement imperfectly burned may set more quickly and gain less ultimate strength than the same cement properly burned, but of two cements of different composition the quicker-setting may be the stronger.
There is as wide variation in the rate of hardening of different cements as in the rate of setting; some gain strength rapidly and attain their ultimate strengths in a few weeks, while others harden much more slowly at first and continue to gain in strength for several years. The rate of early hardening gives but little indication of
the ultimate action of the cement, as the final strength of the mortar may be the same however rapidly the strength is attained.
The rate at which cement sets seems to depend upon the pres ence of certain aluminates of lime, the rapidity of set increasing with the percentage of alumina in the material. The final harden ing is attributed mainly to the silicates of lime, which are the impor tant elements in giving strength and durability to the mortar. The formation of these active elements in the cement depends upon the manipulation of the material in manufacture, as well as upon the composition of the raw materials. In an underburned cement, the relative proportions of aluminates to silicates is large and the set is rapid.
Calcium addition of a small amount of sulphate of lime to cement has the effect of slackening the rate of set. Such addition is frequently made by manufacturers to reduce the activity of fresh cement, by grinding a small amount of gypsum with the cement.
Effect of is ordinarily employed in mortar formed by mixing it with sand, and the action of the mortar is necessarily largely affected by the nature and quantity of sand used.
When the cement is finely ground and the sand of good quality, a mortar composed of equal parts of each, as a general thing, finally attains a strength as high as, or higher than, that of neat cement. Cements of different characters, however, vary considerably in their power to " take sand " without loss of strength; some of the weaker ones may not be able to take more than half their weight of standard sand, while others can he mixed with considerably more than their own weight without loss of strength at end of six months or one year after mixing. All have a certain limit within which they may he made stronger by an admixture of good sand than they would he if mixed neat.