158 RESISTANCE OF MATERIALS 



proper proportions. After being thoroughly mixed, the product is 

 roasted and ground to a powder. 



Slag cement (puzzolan) is made by thoroughly mixing with slaked 

 lime the granulated slag from an iron blast furnace and then grind- 

 ing 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 slower 

 in setting than the Portland cements and have a slightly 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 blast- 

 furnace slag and finely powdered limestone, the mixture being 

 burned in a kiln and the resultant clinker ground to powder. Both 

 the Portland and the puzzolan cements will set under water ; 

 that is, they are hydraulic. 



Gravel or broken stone forms the largest part of the mass of a 

 good concrete and is called the coarse aggregate. Its particles 

 may be from | in. to |- in. in diameter for thin walls or where 

 reinforcement is used, or up to 2J in. for heavy foundations or 

 walls over a foot thick. The coarse aggregate should always be 

 clean and hard. 



The sand, or fine aggregate, should be clean and coarse ; that is, a 

 large proportion of the grains should measure -fa to | in. in diam- 

 eter. All should pass through a screen of ^-in. mesh. Too fine 

 a sand weakens the mixture and requires a larger proportion of 

 cement. 



The following standard proportions may be taken as a guide to 

 the proper mixture for various classes of work:* 



1. A rich mixture for columns and other structural parts sub- 

 jected to high stresses or required to be exceptionally water-tight. 

 Proportions 1 : 1 J : 3 ; that is, one barrel (4 bags) of packed Portland 

 cement to one and one half barrels (5.7 cu. ft.) of loose sand to 

 three barrels (11.4 cu. ft.) of loose gravel or broken stone. 



* Taylor and Thompson, " Concrete Plain and Reinforced " ; also, Atlas Portland Ce- 

 ment Co., " Concrete Construction," and Turneaure and Maurer, " Principles of Rein- 

 forced Concrete Construction," p. 10. 



