724 liTEW YOPtK STATE MUSEUM 



Betting of cement 



By the setting of cement is meant the change that takes place 

 from a soft mass to a hard brittle solid. This change, while vary- 

 ing in different cements, is on the whole a very rapid one at first, 

 and then proceeds more slowly. It is always accompanied by 

 the evolution of heat. The change which takes place is that in 

 the burning of the cement an anhydrous silicate of aluminum 

 and lime is formed which is soluble in water. On solution tak- 

 ing place the material at once changes to a hydrate which is in- 

 soluble and consequently crystallizes out, this crystallizing action 

 causing the hardening. 



A number of different ideas are held on this point, and the 

 problem is a very complex one, which has as yet been only par- 

 tially solved. Fremy considers that the formation of an alumi- 

 nate of lime is responsible for the hardening praperty, and he 

 also considers that the silica and alumina of the clay are sepa- 

 rated by calcining and take on allotropic forms ready to unite 

 into new compounds with the quicklime when the water is added. 

 The work of S. B. ^N^ewberry in this line is of the highest im- 

 portance, and has already been referred to (p. 697-99). 



The set of the cement is determined by what is known as the 

 needle test. Gen. Gilmore was one of the first to use this test 

 in this country. It consists in determination of the penetration 

 of a needle of wire of kno^^^l cross-section and of given weight. 

 The needle used by Gilmore as described by him was slightly 

 conical, tapering toward the point, and truncated at right angles 

 to the axis so as to give a diameter at the lower end of yV of 3,n 

 inch. It protrudes from a socket at the lower end of a^ spindle 

 or vertical rod, to which it is firmly secured by means of a thumb- 

 screw. To the upper extremity of the spindle is attached a 

 diagonal scale of steel, accurately graduated to tenths, hun- 

 dredths, and thousandths, of an inch, and provided with a hori- 

 zontal index firmly fixed to the framework of the instrument. 

 The absolute penetration of the needle is obtained by taking the 



