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Cementing Materials

lime, water, cements, cement, hydraulic and chemical

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Composition. All the cementing materials employed in build ings are produced by the burning of natural or artificial mixtures of limestone with clay or siliceous material. The active substances in this process and the ones which are necessary for the production of a cement, are the burned lime, the silica and the alumina, all of which enter into chemical combination with one another under the influence of a high temperature.

Classification. Owing to the varying composition of the raw materials, which range from pure carbonate of lime to stones contain ing variable proportions of silica, alumina, magnesia, oxide of iron, manganese, etc., and the different methods employed for burning, the product possesses various properties which regulate its behavior when treated with water, and render necessary certain precautions. in its manipulation and use, and furnishes a basis for division into three classes; namely, common lime, hydraulic lime, and hydraulic cements, the individual peculiarities of which will be taken up later.

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 he used.

The hydraulic cements are divided into two classes, namely; natural and artificial. The first class includes all hydraulic substances produced from natural mixtures of lime 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.

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