170 Trans. Acad. Sci. of St. Louis. 



inert to water and that it slakes comparatively rapidly if the 

 water is warm. That the slaking with cold water is slower 

 than in the case of ordinary lime may be due to the fact that 

 lime that has been heated to very high temperatures is much 

 more compact and less porous than the ordinary lime. The 

 surface exposed to the action of water being much less the 

 time required for slaking would necessarily be greater. We 

 see from these experiments that pure lime cannot be made 

 inert to water by heating to high temperatures. The inert- 

 ness of commercial lime is probably due to the presence of 

 compounds of silica and iron oxide with the lime which are 

 decomposed very slowly by the water. 



HYDRATION OF CALCIUM SILICATES. 



The hydration of lime and its compounds was further 

 studied by making synthetically the compounds that are as- 

 sumed to be present in Portland cement, and then hydrating 

 these substances by this method. Pure lime and silica were 

 mixed in molecular quantities so as to give when fused the com- 

 pounds, CaO, SiO„ 2(CaO)Si02, 2J(CaO)SiO„ 3(CaO)SiO„ 

 and 4(CaO)Si02. The lime was obtained by the ignition 

 of pure precipitated calcium carbonate. The silica was pre- 

 pared by conducting silicon tetrafluoride into water and then 

 drying and igniting the gelatinous silicic acid thus formed. 

 Intimate mixtures of lime and silica in the proportions to 

 form the above compounds were heated by projecting verti- 

 cally the flame of the oxy-coal gas blow-pipe down upon the 

 mixtures in a cavity made in a fire brick. The heat thus 

 obtained was sufficient to melt all except the last mixture, 

 namely the 4(CaO)Si02. The fused masses were in each 

 case carefully separated from the unfused portions, they were 

 chilled by sprinkling with cold water, dried and preserved in 

 stoppered bottles. Weighed quantities were then hydrated 

 by this method. The hydraulic properties of each compound 

 were also examined by mixing some of the powdered com- 

 pound with water and observing whether the mass set and 

 became hard. The following table contains the results that 

 were obtained : — 



