180 REIBLING AND SALINGER. 



destination the cement had set so Iiard that it was removed from the car 

 only with much diiBculty. He attributed this change to the variahility 

 of tlie cement, but we are inclined to believe that the water added was 

 just sufficient to bring the cement to the critical solution or hydration 

 point and that a bucket or so of water less than was usually employed, 

 was used in mixing, and Cjuick setting was the result. 



Portland cement is most affected liy local climatic conditions before 

 and not after it is gauged. High temperatu.re and the alternating humid 

 and dry atmosphere are conditions under which hydration and carboniza- 

 tion are accelerated. In consequence, the majority of commercial prod- 

 ucts must be especially prepared to withstand tropical climates. Portland 

 cement is very susceptible to changes under these conditions, and it 

 is therefore essential to the best practice that cement intended for use 

 in the tropics should develop no dangerous properties by the absorption 

 of water and carbonic acid in normal quantities. The cement problem 

 of tropical countries depends for its solution upon the characteristics of 

 Portland cement; and our efforts have been to determine what class of 

 cements are least injuriously affected by exposure and seasoning. 



We believe that high-alumina cements are least efficient for use in 

 tropical climates, although they have one laudable feature in that they 

 never show the slightest inclination toward warping or disintegrating. 

 Air, steam and boiling tests always develop perfect soundness. This is 

 probably due to the fact that aluminous raw material fuses very readily 

 at a comparatively low heat. "Lime burned at a high heat slakes much 

 more slowly, and is therefore more likely to be injurious than when 

 bumed at a low temperature." "^ 



Aluminous cements gain most of their strength very quickly. "The alumiuates 

 are thought to contribute little to the final strength of the mortar, as they are 

 not permanent compounds, but are acted upon by various salts with which they 

 are likely to come in contact in the work. For this reason they are not adapted 

 for work exposed to the action of air and sea water." ™ "The aluminate acts in 

 a very energetic manner upon the set, but very little upon the hardening which 

 is caused by the silicate of lime." " Also "from the character of the silicates and 

 the aluminates it is evident that the latter are acted upon more quickly and rapidly 

 than the silicates, and it is to the crystallization of the lime from the aluminates 

 that the initial set must be contributed. Subsequent hardening must be due to 

 the liberation of lime from the silicates." " 



In conformity with these quotations, it has been our experience with 

 cements of this nature that the 7- to 28-day gain is small; that 

 satisfactory 7-day breaks do not insure satisfactory 28-day strength; 

 that T'-day strength may be even greater than 28-day; that little gain 



=* Spalding, Frederick C. : Ibid., 73. 



^Ibid., 54. 



"/6id, 58. 



"Clifford, Richardson: Eny. News (1905), 53, 984. 



