130 ENGINEERING ON THE FARM 



Rich mixture. One containing a comparatively large 

 quantity of cement. 



Voids. In a pile of sand, gravel, or rock there are always 

 large open or air spaces between the grains or individual 

 pieces. These open spaces in aggregate are known as 

 voids. The percentage of voids is the proportion of the 

 whole volume of the aggregate occupied by air or moisture. 



CEMENT 



Cement. Cement was used by the earliest civilizations, 

 but during the Middle Ages it was replaced by lime mortar. 

 In the eighteenth century cement again came into use in 

 England. 



Cement is a fine powder which possesses the property of 

 hardening after being mixed with water. In this harden- 

 ing process it will bind together, when properly mixed with 

 them, particles of sand and stone into one solid mass. This 

 is the property which makes it so useful for construction. 



The properties essential in cement-making material are 

 a calcareous element combined with an argillaceous element. 

 The first is usually contained in limestone and the latter 

 in clay or blast furnace slag. In addition to these two 

 general elements there are small quantities of iron, sulphur, 

 and manganese compounds. 



Natural cement. Natural cement is made by the process 

 of burning a rock which contains all of the essential elements 

 required in cement and then grinding this burned material 

 to a fine powder. Since the quantities of the necessary 

 elements for cement making vary considerably in the natural 

 rock, the best results are not obtained from this process. 

 However, natural cement is usually cheaper than other 

 kinds, and it is used where a large mass is required rather 

 than great strength. Dams, retaining walls, and abutments 

 are frequently built with this kind of cement. 



Portland cement. Portland cement derives its name 

 from the fact that where cement was first used in England, 



