(2) Lime may be combined with alumina in the proportion of two molecules to one, giving a product which sets quickly but shows good hardening proper ties. With 21, molecules of lime to one of alumina the product is unsound.
Assuming that the tricalcic silicate and the dicalcic aluminate are the most basic compounds which can exist in good cement we arrive at the following formula: in which X and P are variable quantities depending upon relative proportions of silica and alumina in materials employed.
3CaO, corresponds to 2.S parts of lime by weight to 1 of silica, while 2Ca0, corresponds to 1.1 parts of lime to one of alumina.
Per cent lime=Per cent silica X2.S+Per cent alumina Xl.1.
Mr. G. A. Rankin, in an extended study of the composition of Portland cement 2 finds the essential constituents to be the trical cium silicate, 3CaO, Si0a; the dicalciutn silicate, 2CaO, Si02; and the tricalcium aluminate, 3CaO, A1003. He finds that in burning Portland cement, after the carbon dioxide has been driven off, the lime combines with silica and alumina, forming first a fusible alu minate, 5CaO, r11003, and the dicalcium silicate. At higher tem peratures these compounds unite with additional lime, forming the tricalcium aluminate and silicate. When the material is not thoroughly burned, and complete equilibrium is not reached, the clinker will contain free lime, CaO, and the aluminate, 5CaO, A1003.
_Magnesia and iron oxide have little influence on the final main constituents of the cement, but act as fluxes and lower the temper at tire at which the reactions take place.
Too high proportion of lime causes cement to be unsound through the presence of free lime. The same results are caused by under burning or by irregular incorporation of the raw materials into the mixture. As perfect uniformity in the mixture of the ingredients is not attainable in the manufacture of cement, it is necessary that the amount of lime be somewhat less than the theoretic maximum to avoid unsoundness in the cement. The desirable proportion of lime seems to be that which will change the dicalcium silicate to tricalcium silicate as completely as possible without producing unsoundness.
The ratio of silica to the of alumina and iron in cement materials is known as the silica ratio. It is desirable that the silica ratio be at least 2.5 or possibly 3 in Portland cement.
Very little is definitely known concerning the chemical reactions which take place in the setting and hardening of cement mortars. Studies are in progress which it is hoped may throw light upon the subject and tend to more accurate knowledge of the requirements for such materials.