IHO NOTES ON HYDRAULIC LIME AND CEMENT. — MASON. 



done had it been possible to separate them mechanically for the 

 purpose. 



Le Chatelier claims that the setting of cement is due to the 

 hydration of an aluminate of lime having the formula 

 4 Ca O3 AI2 O3 which takes up 12 molecules of water ; while the 

 liydration of the silicate of lime Ca Si O2 which takes up 5 

 molecules of water, causes hardening. 



The weakest point about La Chatelier's conclusions are that 

 he finds the original cement contains the alumina in the form of 

 a tri-calcic aluminate and the silica in the form of a tri-calcic 

 ortho-silicate. 



In order to get the tri-calcic aluminate, he has to first 

 decompose the tri-calcic ortho-silicate, at the same time liberat- 

 ing free calcic hydrate, with which he forms his tetra-calcic 

 aluminate thus: 



2 (Si O2 3 Ca O) + 14 Ho = 2 Si O. Ca 0: 5 H. 0-1-4 Ca 

 H2O2; 



then 



3 Ca O; AI2 03 4- Ca H, O. + 11 H. C) = 4 Ca O; Al. O3 : 

 I2H.2O. 



Thus lie has to bring about the reaction which he claims 

 causes the hardening, before he can obtain the material causing 

 the setting. 



It has occurred to the writer that while the setting of limes 

 and cements is evidently a process of hydration, tlie hardening 

 may be due to the transferring of the silica from the alumina to 

 the lime, leaving- the alumina either in the free state or formintj 

 double silicates between it and the lime. In other words, that 

 the function of the alumina in the setting of a lime or cement, 

 is mainly that of a carrier of the silica to the lime. If such be 

 the case, why should not other substances less basic than lime 

 act equally well :' The reply to this must take the form of 

 another (juestion : has it been proved that they do not :* 



The most likely sustances occurring in limestone to act as 

 conveyors of silica to the lime, are oxide of iron and manganese. 



