C92 NEW YORK STATE MUSEUM 



relation of silicates to lime, and consequently the cements are apt 

 to be fiery and not as satisfactory. They are sKown to improve 

 by the addition of Portland cement, after which they can be 

 used quite successfully. This class includes those of the. Lehigh 

 valley. The- third class is represented by the best Rosendale 

 brands, which set and acquire strength slowly, but which con- 

 tinue to develop it for a long time and eventually are very strong 

 and tough. The fourth class includes cements like those of 

 western JSTew York, which have been, while containing an unusual 

 amount of magnesia, burned so hard that little of the silicates 

 remains undecomposed and uncombined with the lime and mag- 

 nesia, and in consequence they are apt to stand a long time after 

 use, unless carefully hydrated. The fifth class is one in which 

 the cement is essentially a lightly burned, highly magnesian mate- 

 rial in the preparation of which the heat has not been sufficiently 

 high or prolonged to bring the greater portion of the silica in 

 composition with the lime or magnesia, in this respect being in 

 contrast to the preceding class. The hydraulic principle and 

 strength are therefore largely due to the magnesia and carbonates 

 rather than to the silicates and aluminates. Examples of this 

 are those cements made at La Salle (111.) The last class gives 

 a cement in which there is rather less magnesia than in the two 

 preceding classes, and less aluminum and iron oxid than in the 

 third class. Though they are burned so thoroughly that there 

 is but a small per cent of silicates uncombined, still, as Mr Rich- 

 ardson says, all of these cements will when properly burned and 

 carefully handled give successful results in the large majority 

 of cases. As a rule natural cement mortars will acquire a satis- 

 factory strength with sufficient time, though it may have orig- 

 inally been very weak, or subjected to unfavorable influences due 

 to the conditions under which it was used. 



Portland cement 



Portland cement is a hydraulic cement, in which the percent- 

 age of lime to alumina and silica is about as 2 to 1. It sets rapidly 



