724 hew tobk static itossuh 



Selling of cement 



T>y tlm Betting of cement Lb meant the change that takes place 

 from a s<>ft mass to a hard brittle Bolid. This change, while vary- 

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

 and then pro more Blowly. It is always accompanied by 



the (-•volution of heat The change which takes place is that in 

 the burning of the cement an anhydrous silicate of aluminum 

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

 ing place the materia] 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 property, 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. Newberry in this line is of the highest im- 

 portance, and has already been referred to (p. G97-99). 



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 Avire of known 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 T V of an 

 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- 



■■ w. 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 



