496 CEMENTS, LIMES, AND PLASTERS. 



The differences in composition between Portland-cement mixtures 

 are very slight if compared, for example, to the differences between 

 various natural cement rocks. But even such slight differences as 

 do exist exercise a very appreciable effect on the burning of the mix- 

 ture. Other things being equal, any increase in the percentage of lime 

 in the mixture will necessitate a higher temperature in order to get 

 an equally sound cement. A mixture which will give a cement carry- 

 ing 59 per cent of lime, for example, will require much less thorough 

 burning than would a mixture designed to give a cement with 64 

 per cent of lime. 



With equal lime percentages, the cement carrying high silica and 

 low alumina and iron will require a higher temperature than if it were 

 lower in silica and higher in alumina and iron. But, on the other hand, 

 if the alumina and iron are carried too high, the clinker will ball up 

 in the kiln, forming sticky and unmanageable masses. 



