The action of Portland cement seems to depend upon the for mation, during burning, of certain silicates and aluminates of lime which constitute the active elements of the cement, the other ingre dients being considered impurities. The ideal cement would be that in which the proportion of lime is just sufficient to combine with all the silica and alumina in the formation of active material. If there be a surplus of clay beyond this point, it forms inert material. Any surplus of lime remains in the cement as free lime and consti tutes one of the chief dangers in the use of cement, as, although it may not prevent the proper action of the cement when used, it may cause the mortar to swell afterward and become cracked and distorted as the lime slakes.
As perfect homogeneity is not attainable in practice, it is always necessary that the clay be somewhat in excess in order that free lime be not formed. The amount of excess of clay necessary depends upon the thoroughness of the burning and the evenness which may be reached in the mixture of the raw materials.
The normal composition of Portland cement is usually within the following limits: Silica 20 to 25 per cent Alumina 5 to 9 per cent Iron oxide 2 to 5 per cent Lime 59 to 65 per cent Magnesia 0.5 to 3 per cent Sulphuric acid 0.25 to 2 per cent After the cement clinker resulting from the burning is sufficiently cooled, it is put through grinders and reduced to a fine powder. The degree of fineness to which the cement is ground is always very important in its effect upon the strength of mortar made from the cement. The valuable part of the cement is that which is ground extremely fine—to an impalpable powder. The coarse parts are not altogether inert, but are more or less active, depending upon the size of the grains of which they are composed.
Cement when used is commonly mixed with sand and the attain ment of strength in sand mortar, rather than paste of neat cement, is of importance. The more finely ground the cement, the greater its resistance when mixed with sand, both in the earlier and later stages of hardening, and also the sooner will it reach its ultimate strength. The effect of fine grinding is much greater when the proportion of sand to cement is large, as the power of the cement to " take sand " without dnmnution of strength is thereby greatly increased. The coarser particles of the cement may be considered as practically inert material, which acts as sand rather than as cement in the mortar. The ability of the cement to harden and develop
strength in sand mortar is thus dependent upon the amount of fine material contained in it.
Portland cement made from materials containing very small percentages of iron oxide are very light in color or white. These cements usually contain high percentages of alumina, and are con sequently quick setting. They are lower in strength than normal portlands.
14. Natural term natural cement is used to desig nate a large number of widely varying products formed by burning rock without pulverization or the admixture of other materials. These cements contain larger proportions of argillaceous materials, with less lime, than Portland cement, and are burned at a lower temperature.
The term Roman Cement is used in Europe to designate a class of quick-setting cements formed by burning, at a comparatively low temperature, limestone containing a high percentage of clay. The proportion of alumina in these materials is large and possibly accounts for the quick set. Materials of this character become inert when the temperature of burning is increased to the point where the chemical reactions would become complete.
A class of materials intermediate between the Roman cements and the Portland cements is called in Europe Natural Portland Cement. In composition they are similar to Portland cement, but contain less lime. They are burned at a higher temperature than Roman cements, and are usually slower setting. Natural cements made in the Lehigh region are of this character. These materials may be made into Portland cement by addition of a limestone con sisting of more nearly pure carbonate of lime.
Magnesian Natural Cements are formed by burning Magnesian limestones. The composition of these cements varies from that of the Roman cements to that in which the proportion of magnesia is as great as that of lime. The action of cements of this class is somewhat similar to that of the Roman cements. They may be either slow or quick setting, and gain strength rather slowly, reach ing a much less ultimate strength than Portland cement. Mag nesian cements are but little used in Europe, but in the United States they constitute the larger part of the natural cements in use, and many of them have been found by experience to be very useful and reliable materials.