1869.] CHEMISTRY AND PHYSICS. 105 



CHEMISTRY AND PHYSICS. 



On Hydraulic Cements.— It is well known that the calcina- 

 tion of argillaceous limestone gives rise to cements which have the 

 power of hardening under water. Various explanations of this 

 property have been proposed. An alkaline silicate, like soluble 

 glass, is known to harden by silicifying calcareous rocks and 

 cements ; and Kuhlmann supposed that a silicate of this kind, 

 formed during the calcination of argillaceous and more or less alka- 

 liferous matters, might play an important part in the hardening of 

 hydraulic limes. According to Rivot and Chatonnay, on the other 

 hand, there are formed during the calcination of mixtures of car- 

 bonate of lime and clay, three new compounds, a silicate of 

 lime, a double silicate of lime and alumina, and an aluminate of 

 lime. These three compounds they supposed to combine directly 

 with water, so that the solidification of the cement was like that 

 of calcined gypsum, a simple hydratation. According to the 

 recent experiments of Fr^my, only one of these compounds, the 

 simple silicate of lime, has the property of thus combining with 

 water. Further, he has shown that although pure clay or kaolin, a 

 hydrous silicate of alumina, does not produce a hydraulic cement 

 when mixed with lime, yet, after exposure to a low red heat it 

 forms, with lime, a perfect cement. The foreign matters often 

 present in clays are without action in this process. The explana- 

 tion of this curious result seems to be furnished by the observa- 

 tion of Fr^my, that a clay which abandoned nothing to hydrochloric 

 acid yielded abundance of alumina to the same acid after calcin- 

 ation. From this it whould appear that a heat, even of low redness 

 produces a partial decomposition or dissociation of the silicate into 

 alumina and silica. 



Both free alumina, and silica in the amorphous condition are 

 shown by the experiments of Vicat to communicate hydraulic pro- 

 perties to lime. This decomposition of the hydrated aluminous 

 silicate by heat is analogous to that many years since observed by 

 Fr^my for silicate of potash, whose solution at an elevated tem- 

 perature is partially decomposed, with separation of pure crystal- 

 line silica. In this connection should be noticed the observation 

 of Kengott, that many mineral species acquire a strongly alkaline 

 reaction after having been calcined. The natural pozzuolanas 

 are nothing more than volcanic ash or argillaceous matter calcined 

 by volcanic heat ; and it has long been known that similar pro- 



