194 SAND AND CEMENT 



considered as divided into three classes, namely, feebly 

 hydraulic, ordinarily hydraulic, and eminently hydraulic, in 

 proportion to the quantity of argillaceous materials present. 

 The slaking qualities vary from slaking in a few minutes 

 with considerable heat after water is added, in the feebly 

 hydraulic, to slaking only after many hours, with practically 

 no evolution of heat and without cracking or powdering, in 

 the eminently hydraulic. The time of setting under water 

 also varies from setting as hard as soap in 2 years, with the 

 feebly hydraulic, to becoming as hard as stone in 3 or 4 days, 

 with the eminently hydraulic. If carbonate of magnesia is 

 present in the lime, it reduces the energy of the slaking, 

 but increases the rapidity of the setting and the ultimate 

 strength when set. 



CEMENTS 



Cement may be divided into four general classes: Port- 

 land, natural, puzzolan (also called pozzuolana), and mixed. 

 The relative importance of each cement is indicated by the 

 order in which it is named. 



Portland cement may be defined as the product resulting 

 from the process of grinding an intimate mixture of cal- 

 careous (containing lime) and argillaceous (containing clay) 

 materials, calcining (heating) the mixture until it starts to 

 fuse, or melt, and grinding the resulting clinker to a fine 

 powder. It must contain not less than 1.7 times as much 

 lime by weight as it does of those materials which give the 

 lime its hydraulic properties, and must contain no materials 

 added after calcination, except small quantities of certain 

 substances used to regulate the activity or the time of 

 setting. 



Natural cement is the product resulting from the burning 

 and subsequent pulverization of an argillaceous limestone 

 or other suitable rock in its natural condition, the heat of 

 burning being insufficient to cause the material to start to 

 melt. 



Puzzolan cement is a material resulting from grinding 

 together, without subsequent calcination, an intimate 

 mixture of slaked lime and a puzzolanic substance, such as 



