248 STRENGTH OF MATERIALS 



The artificial mixing of the limestone and clay in the manufacture 

 of Portland cement is accomplished in different ways. Throughout 

 the north central portion of the United States large beds of marl are 

 found, and also in the same localities beds of suitable clay. This marl 

 is nearly pure limestone, and is mixed with the clay wet. Both the 

 marl and clay are pumped to the mixer, where they are mixed in the 

 proper proportions. The product is then dried, roasted, and ground. 



Most American Portland cements, however, are made by grinding 

 a clay-bearing limestone with sufficient pure limestone to give tin- 

 proper proportions. After being thoroughly mixed the product is 

 roasted and ground to a powder. 



Slag cement (Puzzolan) is made by thoroughly mixing the granulated 

 slag from an iron blast furnace with slacked lime, and then grinding 

 the mixture to a fine powder. Slag cements are usually lighter in 

 color than the Portland cements, and have a lower specific gravity, the 

 latter ranging from 2.7 to 2.8. They are also somewhat sl<>\\ r in 

 setting than the Portland cements, and ha\ litly lower tensile 



strength. They are not adapted to resist mechanical wear, such as 

 would be necessary in pavements and floors, but are suitable for 

 foundations or any work not exposed to dry air or great strain. 



True Portland cement may be made from a mixture of hla>t -furnace 

 slag and finely powdered limestone, the mixture heing burned in a 

 kiln and the resultant clinker ground to powder. Both the Portland 

 and the Puzzolan eements will set under water, Le. they are hydraulic. 



199. Cement tests. The many different processes of mixing, roast- 

 ing, grinding, and setting through which a cement must pass, require, 

 that a number of tests be made to determine whether or not these 

 have been well done. If tin- grinding has been improperly done, or 

 if any of the other operations of manufacture have been neglected, 

 the product may be very weak, or even worthless. To make sure that 

 all the steps in the manufacture of the cement have been properly 

 carried out, engineers make use of the following te>ts: (a) test ..f 

 soundness ; (b) test of fineness ; (c) test of time of setting ; (d) test 

 of tensile strength. 



200. Test of soundness. One test for soundness eon<Nts in boiling 

 a small ball of neat cement in water for three hours, and noting 

 whether or not checks or cracks occur. If the cement contains too 



