182 REIBLING AND SAMNGEE. 



quantity of water necessary to slake the lime, if all were present as 

 calcium oxide, and to combine with the slaked lime to form calcium 

 carbonate, regardless of any intermediate reaction on other compounds 

 which might be present. This change can therefore be represented by 

 the following equations : 



CaO+H20=Ca(OH), 

 Ca(OH)„+cb2=CaC03+H„0 

 or, 



CaO+CO„=CaC03 



and therefore, 1 part by weight of water will unite with 3.1 parts of lime 

 to form 4.1 parts of slaked lime, and one part of carbon dioxide will 

 unite with 1.5 parts of lime to form 2.37 parts of calcium carbonate. 



From the above equations it is very apparent how an otherwise un- 

 sound cement is improved by the absorption of 3 or 4 per cent of water 

 and carbonic acid. Excess of free lime causes the unsoundness and the 

 more of this lime which is slaked or rendered inert before gauging the 

 cement, the sounder the resulting product will be. The calcium silicates 

 being much more stable compounds than the calcium aluminates, the 

 latter would be acted upon first by climatic influences. The addition 

 of lime or slaked lime to a cement retards the setting, and from the 

 nature of the reaction, quicklime would retard the setting more than 

 slaked lime. The natural tendency then of the lime is to off-set the 

 quick setting properties of the aluminates. Other conditions being the 

 same, anything which tends to reduce the activity of the lime in a slow- 

 setting, sound cement, will increase the rate of its setting. The ignited 

 cement of sample Ko. 8 had the following composition : 



Per cent. 



Silica (SiO.) 20.5 



Alumina (ALO3) 8.6 



Iron oxide (FeA) 2.8 



Lime (CaO) 65.4 



Magnesia (MgO) 2.3 



Sulphuric acid (SOJ 0.4 



Table A^III shows that the cement from the bag had absorbed 0.50 per 

 cent more carbonic acid and 0.79 per cent more combined water than 

 that in the can. It therefore contained (equation 3) 1.13 more inert 

 calcium carbonate and 3.24 per cent (equation 1) more slaked lime; 

 or 0.75 per cent (equation 3) of the lime present in the raw material 

 had been rendered inert, and 2.45 per cent had been slaked by the 

 additional absorption of combined water and carbonic acid by the same 

 cement stored in the bag. 



The lime in combination with silica must be left out of this considera- 

 tion as the silicates of calcimn exert practically no iniluence upon the 

 initial setting properties of the cement. The entire loss in active lime 



