240 CHEMISTRY OF FARM PRACTICE 



of lime, clay and sand and manufactured by heating moder- 

 ately and grinding was formerly made in large quantities 

 in this country. Owing to the cheapness with which Port- 

 land cement may be manufactured, with its properties 

 governed by correct admixture, the natural cement is being 

 replaced by the Portland cement. Portland cement when 

 properly made is guaranteed to meet the standard fixed by 

 the American Society for Testing Materials. 



189. Setting of Cement. Cement after the sintering 

 process of the furnace seems to be a mixture of calcium 

 silicate (CaSiOs) and calcium aluminate (Cas(AlO3)2). The 

 latter seems to be the active agent causing the setting of 

 the cement when it is mixed with water. This hydrolysis 

 may be expressed as follows: 



The calcium hydroxide, crystallizing, binds the particles of 

 calcium silicate together, while the aluminium hydroxide fills 

 the interstices and makes the mass compact and impervious. 

 When water is added to cement, it becomes a soft, sticky 

 paste and it will remain in this condition for about thirty 

 minutes, when it begins to harden or set. To disturb the 

 concrete after the setting is begun means a loss in the strength 

 of the concrete. For this reason the concrete should be 

 placed in position in less than thirty minutes after the cement 

 is first wet. There are several precautions to be observed. 

 A new cement should neither be exposed to the hot sun for 

 any considerable length of time nor to freezing temperature. 

 No material should be placed on the freshly made cement 

 that will affect its color. The cement must be kept dry, 

 before its use, because it readily absorbs moisture from the 

 atmosphere when stored in damp places; this causes it 

 to become lumpy and consequently worthless due to the 

 setting of the cement. Lumps may sometimes be caused 

 by pressure; these may often be broken up and the 



