CEMENT FOR A MODERN STREET. 



151 



is rejected and the varied masses selected are carefully mixed so as to 

 ensure a uniform grade. The second is the difficulty of burning the 

 rock in kilns and in large masses so uniformly as to ensure complete 

 burning and no considerable amount of overburning. If natural 

 cement rock is overburned or fused, it becomes a slag, and loses its 

 hydraulic properties. It is not surprising, therefore, that a considerable 

 lack of uniformity exists in the quality of the natural cement found 

 upon the market. The best of them contain a considerable amount of 

 impurity, or material that is not cement, that exists in the rock before 

 it is burned, and also a considerable amount of unburned rock, which 

 together serve to dilute the cement proper, as if a certain amount of 

 sand had already been added to the cement before it is used. These 

 impurities that are inherent in the nature of the materials from which 

 the natural cement is made and also in the process of manufacture 

 that is of necessity followed, result in a cement that can be made and 

 sold at a less price than Portland cement and that is inferior to it for 

 many purposes, while, on the other hand, for a great many purposes 

 natural cements have been found to answer every requirement and are 

 made and used in enormous quantities. 



For Portland cements, either a very pure natural limestone or marl 

 is selected and brought into a very finely pulverized condition. Lime- 

 stones are selected as free as possible from every impurity except clay. 

 ]Magnesia is never absent, and at best is an inert impurity, but the 

 amount present should not exceed five per cent. Marl is frequently 

 used and is generally purer than limestone. In England chalk is gen- 

 erally used. In Germany chalk and a limestone, locally known as 

 'mergel,' which is soft and contains clay, are employed. 



The following table, No. I., taken from 'Cement Industry,' page 1 2, 

 gives the composition of the carbonate of lime in use in some of the 

 leading Portland cement manufactories in the United States: . 



Table I. 



Limestones and Marls. 



Chalk, 



England, 



(Reed). 



Cement 

 Rock, La 

 Salle, Ills. 



Calcium carbonate... 98.57 



Magnesium " 0.38 



Calcium sulphate 



Silica 0.64 



Alumina 0.16 



Iron oxide 08 



88.16 



1.78 



8.20 

 1.00 

 0.30 



' Cement 

 Rock, 

 Phillips- 

 jburg, N. J. 



70.10 

 3.96 



15.05 



9.02 



i 1.27 



Marl, 

 vracust 

 ■ Ind. 



88.49 

 2.71 

 1.58 

 1.78 

 0.91 

 0.30 



The clay should be highly siliceous, but should be free from grains 

 of sand. Clays containing carbonate of lime or marl are softer and 

 more easily mixed with the other materials. Clays containing 70%, 

 or more, of silica stand firing without fusing, produce a clinker that 



