NATURAL AND ARTIFICIAL CEMENTS. 253 



NATURAL AND ARTIFICIAL CEMENTS. 



By Prof. LA ROY F. GRIFFIN. 



THE cements now in the market are of two kinds: natural, 

 made directly from stone ; and artificial, commonly called 

 Portland cement. The manufacture of the former consists simply 

 in burning and grinding the cement stone, a magnesian limestone 

 containing about fifteen per cent of silica and a little silicate of 

 alumina. The burning drives off the small amount of combined 

 water and all the carbon dioxide from the stone, leaving the lime 

 and magnesia as oxides, while the grinding to a powder puts it 

 into the best possible condition for mixing with sand and gravel, 

 and moistening to form a mortar. Artificial cement consists of 

 about sixty-two per cent of lime mixed with silica and silicate of 

 alumina in nearly the same proportions as' those found in the 

 cement stone, and it is free from magnesia. This seems to be the 

 whole difference in its constitution. In use, the artificial cement 

 sets rapidly and attains maximum hardness in a comparatively 

 short time ; the natural cement hardens rather slowly and reaches 

 its maximum hardness only after a long period of exposure to 

 the air. 



The increasing use of cement in modern construction, either 

 alone or more commonly as mixed with sand and gravel, demands 

 that the qualities of the different kinds, and the means of testing, 

 both roughly and accurately, should be generally understood. 

 The foundations of all important structures, in situations where 

 they can not rest directly upon solid rock, owe their strength to 

 cement. They are usually made of concrete, cement mixed with 

 sand or gravel, and they are often strengthened by iron beams so 

 as to bind the whole into one continuous mass. Tunnels under 

 rivers, sewers, cable trenches, and all the numerous subways of 

 our large cities, are either concrete or masonry laid in cement 

 mortar. Their strength, again, is the strength of the cement used. 

 And even the piers of most of the large bridges are now made in 

 part or wholly of concrete. Oftentimes, even the walls of stone 

 and brick buildings are rendered more secure by being laid up 

 with mortar of which cement forms a large ingredient. Used for 

 so many purposes, the necessity of uniform quality, and proper 

 knowledge of the quality of the cement used, become plain. 



Before examining the methods of testing now employed and 

 comparing the results, the process of hardening needs to be com- 

 prehended. Some things are not yet quite clear in it, but it is 

 certainly in the main a chemical process. Mixed with water, the 

 lime and magnesia of the cement unite to form a hydrate, and it 



