EIME AJND CEMENT INDUSTRIES 685 



Another series of hydraulic limestones, which are lacking in 

 magnesia however, are also quoted by Richardson in the article 

 referred to. 



These facts emphasize clearly the necessity of thoroughly test- 

 ing the rock to be used, in advance. Adaptability for cement 

 depends on the amount of silica and silicates which it contains 

 and also on the percentage of sulfur and alkalis. In addition, 

 the silica should be combined with the alumina, and the rock 

 should also be dense. The two classes of natural cement, viz 

 the magnesian and the non-magnesian, differ distinctly in other 

 properties. The magnesian cements do not heat on mixing and 

 with water they set and strengthen slowly but in the end are 

 as strong as the lime cements. They do not resist frost well 

 when first used, and often careful preparations have a tendency 

 to expand a year or two after use. The lime cements, even 

 after being carefully manufactured, have a tendency when made 

 into mortar to heat, on the one hand, when too rich in lime and, 

 on the other hand, to bloat when too rich in silicates or over- 

 burned. They acquire strength rapidly, having nearly as great 

 a tensile strength at the end of from one to 28 days as the mag- 

 nesian cements. They resist frost better than the latter but are 

 at times inferior, more brittle and crystalline, with a tendency 

 to deteriorate in strength. The perfectly prepared and carefully 

 made cements of this class are the best natural cements in the 

 world. The Hound Top cement of the Potomac valley is typical 

 of the highest grade of the lime cements, as the numerous Rosen- 

 dale brands are of the magnesian class. 



Physical 'properties 

 Of primary importance is the density of the rock. A light 

 reck does not burn well and may not give a cement of suitable 

 volume, weight or density. The specific gravity at 78° F should 

 not be below 2 . 7, and preferably be 2.8. Some stones used have 

 a specific gravity of only 2.65, but they are inferior. The best 

 rock is obtained from those portions of the quarry which are 



