CALCULATION AND CONTROL OF THE MIX. 385 



the cement will, of course, contain some uncombihed silica or alumina. 

 A choice must be made, therefore, between the possibilities of having 

 free lime in the product and having uncombined clayey matter. This 

 choice is simple, for the effects on the value of the cement of these 

 two possibilities are very different. Free lime is positively dangerous 

 to the cement, while free clayey materials are merely inert, their only 

 effect being to lower the tensile strength of the product. For this reason, 

 since in practice it is necessary to choose between the two contingencies 

 (free lime vs. free silica and alumina), the lime content of the mixture 

 is always carried lower than theoretical considerations demand. 



It is to be further noted in this connection that the lime content 

 of Portland cements relatively high in silica may be carried higher 

 than in the case of the more aluminous Portlands. In discussing the 

 constitution of Portland cement in preceding paragraphs it was stated 

 that though lime combines with both silica and alumina, the combin- 

 ing proportions are very different in the two cases. With silica, lime 

 forms the tricalcic silicate, whose percentage composition is lime 73.6 

 per cent, silica 26.4 per cent; the lime and silica are therefore com- 

 bined in the proportion of lime 2.8 to silica 1. With alumina, lime 

 forms a less basic compound, the dicalcic aluminate. The percentage 

 composition of this compound is lime 52.3 per cent, alumina 47.7 per 

 cent, corresponding about to the proportion lime 1.1 to alumina 1. 

 It is evident, therefore, that a mixture containing 20 per cent silica 

 and 5 per cent alumina can safely carry more lime than one contain- 

 ing 15 per cent silica and 10 per cent alumina. 



Since the combination of lime, silica, and alumina becomes more thor- 

 ough in proportion as the mixing, grinding, and burning are better done, 

 higher lime contents can be carried by carefully prepared mixtures 

 than by careless or coarsely ground mixtures; and in rotary-kiln plants 

 lime may be carried higher than in those using dome kilns. 



Up to the limit of safety every increase in the percentage of lime 

 in the mixture will cause, other things being equal, an increase in the 

 strength of the cement. This fact is taken advantage of, particularly 

 when a new brand -is being placed on the market. The usual method 

 of procedure at such a time is to carry the lime very high, burn very 

 hard, and pulverize very fine. This makes a costly but high-testing 

 cement. As soon as the brand has become well established, the lime 

 content can be dropped to reasonable working limits. 



Minimum lime content of mixture. The maximum lime content 

 of the mixture is fixed by the considerations set forth in the preceding 

 paragraphs. The minimum lime content, however, will also require 



