24 The Philippine Journal of Science i9i6 



One gram of the finely ground tuff was digested in a Jena 

 beaker with dilute hydrochloric acid for eight hours. The in- 

 soluble residue was separated from the liquid by filtration. The 

 residue with the filter paper was transferred to a platinum dish 

 and digested for fifteen minutes with a 5 per cent solution of 

 sodium carbonate on the water bath. The soluble silica was 

 determined in the filtrate. 



The method employed by Schwarz,'' is practically the same, 

 except that he digests for a longer time with sodium carbonate. 

 Some of the results obtained by him are shown in Table IV. 



Table IV. — Soluble silica in various materials, as recorded by Schwarz. 





Per cent. 



Tuff from Rottenheim 



17.75 



Nettetaler tufif 



35.75 



Brohltaler tuff 



28.95 



Vulkan trass 



24.74 



Mountain trass 



27.89 



Tuff from the Imperial Construction Bureau 



of 



Celle 



35.80 



Tuff from Hungary 



34.80 



THEORIES OP HYDRAULICITY 



Soluble silica is considered by several authorities as being the 

 cause of the hydraulic properties of tuffs and other pozzuolanic 

 materials. Kasai * shows that the strength of Portland cement- 

 pozzuolan mortars is directly proportional to the percentage of 

 soluble silicic acid contained in the added pozzuolan. Johnson ' 

 says: 



While the exact chemical reactions have not yet been determined for 

 slag cements, it seems certain that the hardening of slag cements consists 

 of a gradual action of an active form of silica on the free lime, as in the 

 case of puzzuolanas or trass. 



Dammer " states : 



The natural puzzuolanas have the property in common of containing 

 appreciable amounts of silicic acid which react with the added lime and by 

 substitution form the necessary compounds for the induration of the mortar. 



On the other hand, there are investigators who maintain that 

 the hydraulic properties of pozzuolanas, tuffs, and trasses are 



' Tonind.-Zeitg. (1912), 36, 1122. 



* Portlandzement with Puzzolanzusatz. Tonind.-Zeitg., G. m. b. H., Berlin 

 (1911). Abstracted in Tonind.-Zeitg. (1911), 35, 1644. 



" The Materials of Construction. John Wiley & Sons, New York (1912), 

 191. 



*" Chemische Technologie der Neuzeit. Stuttgart, Verlag von Ferdinand 

 Enke (1910), 741. 



