CONSTITUTION, ETC., OF PORTLAND CEMENT. 567 



In synthetic work pure lime, silica, alumina, etc., are mixed in certain 

 definite proportions and burned to a clinker. The hydraulic properties 

 of this clinker can be examined by powdering part of it and testing 

 the resulting cement. Examination under the microscope will fix 

 certain optical characters peculiar to each clinker composition, and 

 the data thus obtained can be used to determine the constituents of 

 commercial-cement clinker, as noted in the preceding paragraph. 



Synthetic investigations. Richardson has recently summarized the 

 results of his own studies and those of previous observers as follows : 



"The preparation of synthetic silicates and aluminates which might 

 exist in Portland cement was carried out to a certain extent by Le Cha- 

 telier and the Newberrys, but in neither case were these compounds 

 characterized completely, especially as to their optical properties. This 

 has been done by the writer within the last two years, and the optical 

 properties and other characteristics of the following definite silicates 

 and aluminates have been determined. 



" Monocalcic silicate (SiO 2 CaO) : A crystalline substance of high 

 optical activity and little or no hydraulic properties. Specific gravity 

 2.90. 



"Dicalcic silicate (Si0 2 2CaO, or more probably 2Si0 2 4CaO): A 

 definite crystalline compound of high optical activity and of very little 

 hydraulic activity except in the presence of carbonic acid, but setting 

 slowly in water, generally lacking volume constancy. Specific gravity 

 3.29. 



"Tricalcic silicate (Si0 2 3CaO) : A definite crystalline silicate of 

 low optical activity and corresponding in this respect with alit. Its hy- 

 draulic activity is not great, but greater than that of dicalcic silicate. 

 If fused and reground it sets slowly like Portland cement. Specific 

 gravity 3.03. 



" Three definite silicates of calcium, therefore, appear to exist, the 

 two more basic ones being strongly differentiated from each other by 

 their optical activity. 



"Monocalcic aluminate (Al 2 O 3 CaO): This aluminate is a crystal- 

 line substance of high optical activity, but is not sufficiently basic to 

 permit of its existence in a material of such basic character as Portland- 

 cement clinker. Specific gravity 2.90. 



"Tricalcic dialuminate (2Al 2 O 3 3CaO) : This aluminate is one of 

 highly crystalline character and of great optical activity, making it 

 readily recognizable. Specific gravity 2.92. 



"Dicalcic aluminate (Al 2 032CaO): A substance crystallizing from a 

 state of fusion in dendritic forms having no optical activity and being, 



