Common lime is distinguished from hydraulic lime by its failure to set or harden under water,- a property which is possessed by hy draulic lime to a greater or less degree.
The limes are distinguished from the cements by the former falling to pieces (slaking) on the application of water, while the latter must be mechanically pulverized before they can be used.
The hydraulic cements are divided into two classes, namely, natural and The first class includes all hydraulic substances produced from natural mixtures of Time and clay, by a burning pro cess which has not been carried to the point of vitrification, and which still contain more or less free lime.
The artificial cements are generally designated by the name "Portland" and comprise all the cements produced from natural or artificial mixtures of lime and clay, lime and furnace slag, etc., by a burning process which is carried to the point of vitrification.
The hydraulic cements do not slake after calcination, differing materially in this particular from the limes proper. They can be formed into paste with water, without any sensible increase in volume, and little, if any, production of heat, except in certain instances among those varieties which contain the maximum amount of lime. They do not shrink in hardening, like the mortar of rich lime, and can be used with or without the addition of sand, although for the sake of economy sand is combined with them.
All the limes and cements in practical use have one feature in common, namely, the property of "setting" or "hardening" when combined with a certain amount of water. The setting of a cement is, in general, a complex process, partly chemical in its nature and partly mechanical. Broadly stated, the chemical changes which occur may be said to afford opportunity for the mechanical changes which result in hardening, rather than themselves to cause the harden ing. The chemical changes are, therefore, susceptible of wide varia tion without materially influencing the result. The crumbling which calcined lime undergoes on being slaked is simply a result of the mechanical disintegrating action of the evolved steam. In some cements of which plaster of Paris may be taken as the type, water simply combines with some constituent of the cement already present. In others, of which Portland cement is the most important example, certain chemical reactions must first take place. These reactions give rise to substances which, as soon as formed, combine with water and constitute the true cementitious material. The quantity of water used should be regulated according to the kind of cement, since every cement has a certain capacity for water. However, in practice
an excess of about 50 per cent must be used to aid manipulation.
The rapidity of setting (denominated activity) varies with the character of the cement, and is influenced to a great extent by the temperature, and also, but in less degree, by the purity of the water.
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.