Sea water hinders the setting of some 'cements, and some cements, which are very hard in fresh water, only harden slightly in sea water or even remain soft. Cements which require more than one-half hour to set are called "slow-setting", all others "quick-setting". As a rule the natural cements are quick- and the Portlands slow setting. None of the cements attain their maximum hardness until some time has elapsed. For good Portland 15 days usually suffices for complete setting, but the hardening process may continue for a year or more.
The form and fineness of the cement particles are of great impor tance in the setting of the cement, and affect the cementing and economic value. Coarse particles have no setting power and act as an adulterant. In consequence of imperfect pulverization some cements only develop three-fourths of their available activity, one fourth of the cement consisting of grains so coarse as to act merely like so much sand. The best cement when separated from its fine particles will not harden for months after contact with water, but sets at once if previously finely ground.
In a mortar or concrete composed of a certain quantity of inert material bound together by a cementing material it is evident that to obtain a strong mortar or concrete it is essential that each piece of aggregate shall be entirely surrounded by the cementing material, so that no two pieces are in actual contact. Obviously, then, the finer a cement the greater surface will a given weight cover, and the more economy will there be in its use. The proper degree of fineness is reached when it becomes cheaper to use more cement in propor tion to the aggregate than to pay the extra cost of additional grinding.
Use. Common lime is used almost exclusively in making mortar for architectural masonry. Natural cement is used for masonry where great ultimate strength is not as important as initial strength and in masonry protected from the weather. Portland cement is used for foundations and for all important engineering structures requiring great strength or which are subject to shock; also for all sub-aqueous construction.
Rich Limes. The common fat or rich limes are those obtained by calcining pure or very nearly pure carbonate of lime. In slaking they augment to from two to three and a half times that of the original mass. They will not harden under water, or even in damp places excluded from contact with the air. In the air they harden by the gradual formation of carbonate of lime, due to the absorption of car bonic acid gas.
The pastes of fat lime shrink in hardening to such a degree that they cannot be employed for mortar without a large dose of sand.
Poor Limes. The poor or meagre limes generally contain silica, alumina, magnesia, oxide of iron, sometimes oxide of man ganese, and in some cases traces of the alkalies, in relative propor tions which vary considerably in different localities. In slaking they proceed sluggishly, as compared with the rich limes—the action only commences after an interval of from a few minutes to more than an hour after they are wetted; less water required for the process, and it is attended with less heat and increase of volume than in the case of fat limes.
• hydraulic Limes. The hydraulic limes, including the three subdivisions, viz., slightly hydraulic, hydraulic, and eminently hydrau lic, are those containing after calcination sufficient of such foreign constituents as combine chemically with lime and water to confer an appreciable power of setting or hardening under water without the access of air. They slake still slower than the meagre limes, and with, but a small augmentation of volume, rarely exceeding 30 per cent of the original bulk.
Lime is shipped either in bulk or in barrels. If in bulk, it is impossible to preserve it for any considerable length of time. A barrel of lime usually weighs about 230 lb. net, and will make about three tenths of a cubic yard of stiff paste. A bushel weighs 75 lb.