XI, A, 1 King: Pozzuolanic Properties of Meyeauayan Tuff 33 



tensile strength. Great variation was noticed between the 

 strengths of briquettes in the same lot; this was especially true 

 of the briquettes stored in air. I attribute this variation to 

 shrinkage cracks set up by unequal rates of drying and to fine 

 grinding; in part it is characteristic of this kind of cementing 

 material. 



Before tempering the various mixtures, the normal consistency 

 of each was determined by the ball method. Mix A required 

 35 per cent of water; mix B, 36 per cent; and mix C, 36 per cent. 



The quantity of material mixed at one time was of such bulk 

 as to prohibit working directly with the hands. For the neat 

 batches about 65 kilograms of the cement were weighed out 

 and formed into the shape of a cone with a crater in the center. 

 The mixing was done upon a concrete surface. Water was 

 poured into the crater and the mass thoroughly mixed for twenty 

 minutes by three assistants using shovels. The work of mix- 

 ing was laborious on account of the sticky nature of the paste. 

 It adhered tenaciously to the shovels, somewhat like stiff clay, 

 and had a tendency to ball up into separate lumps. During 

 the molding process considerable tamping was necessary to make 

 the cement coalesce. Especial care had to be taken to tamp 

 the paste into the molds in order to reduce the air bubbles and 

 voids to a minimum; even with the care taken, some of the 

 specimens contained excessive voids, which in part account for 

 the variation in some of the results obtained. The mortar mix- 

 tures worked much more easily and produced better specimens. 

 More water would make the operation of mixing less laborious, 

 but the additional amount added would cause undue shrinkage 

 of the air specimens and thus lead to serious cracks which 

 would make the specimens worthless. In fact all specimens 

 showed more or less shrinkage. No measurements were made, 

 but shrinkage was easily noticeable twenty-four hours after the 

 specimens had been made and allowed to stand in the air. Some 

 of the briquettes of mix B stored in air were cracked into several 

 pieces by shrinkage, and therefore no results were obtained for 

 these periods. Some of the neat compressive specimens stored 

 in air were also slightly cracked on the surface through the same 

 cause. All neat specimens stored in air showed a tendency to 

 develop hair cracks; the troweled surface of these specimens 

 flaked or peeled off, exposing a rough undersurface. This tend- 

 ency was not noticed in the mortar specimens. The addition of 

 sand reduced the shrinkage considerably. 



139471 3 



