LIME ANi> CEMENT INDOSTEIES 677 



CEMENT 



The name cement was formerly applied only to materials which 

 were added to lime mortar in order to make it harden under 

 water. Subsequently this term was used for all combined material 

 which gave a mortar that hardened under water, and so has 

 extended to our natural and Portland cement. Under cement ma- 

 terials are now included hydraulic agents, hydraulic limes, slag 

 cements, natural cements and Portland cements. Hydraulic 

 agents are materials which cause silica and clay to unite with the 

 lime of common mortar, giving us a combination of slow harden- 

 ing properties. Such hydraulic agents may be natural or they 

 may be artificial. The natural ones are represented by the 

 pozzuolana of Italy, and the trass of the Ehine valley in Ger- 

 many. In this country they are only known in the far west. The 

 artificial hydraulic agents include slag, burned clay, shales, ashes, 

 silicate of soda or any inorganic material that contains clay aud 

 silica in a form permitting its solution in acids. 



When the clayey impurities increase in ordinary quicklime, 

 it assumes hydraulic properties and the lime is known as hydraulic. 

 Sand is an impurity which is not too large to prevent slaking but 

 simply retards it. Hydraulic limes with only 5$ to 15$ of silicates 

 will harden in from eight to 20 days, but with a larger amount in 

 from one to four days. No sharp line can be drawn between 

 true cements and hydraulic limes. 



For convenience the following classification is made in this 

 report. 



Hydraulic limes 



Rosendale cement, or natural rock cement 



Portland cement 



Hydraulic limes 



These generally have 18$ to 25^ of clay, free silica and com- 

 bined silica, iron oxid and alumina, sometimes magnesia and 

 alkalis. 



