390 REIBLING AND REYES. 



as indicating the presence of free lime is relatively crude as compared 

 with the microscopic study of calcium hydroxide-phenol crystals. 



The following considerations will give a dearer understanding of the 

 effect of free lime upon the soundness of cements. . 



Slaked lime has little or no effect upon soundness. As much as 50 

 per cent can be added to some cements without causing them to check 

 or disintegrate. 



The speed of slaking is increased by the fineness of grinding. The 

 experienced operator in preparing the microscopic slides for the tests for 

 free lime can obtain a very good conception of the fineness of the material 

 during the process, because a microscopic measurement of the sizes of 

 the particles gives definite information in this respect. 



Nonsintered lime slakes in an excess of water at a low temperature. 

 The lime will obviously have no effect per se on the soundness, pro- 

 vided it slakes during the mixing with water, or before setting has 

 begun. Portland cement manipulations can not guarantee an excess of 

 water, or the time necessary for the complete slaking of nonsintered lime 

 before setting has developed. If the operation is conducted under the 

 American Society methods of testing, then a considerable proportion of 

 nonsintered lime will have had ample opportunity to slake in a "normal 

 consistency" mixture. Therefore, nonsintered lime must ordinarily have 

 been present in considerable quantity to cause disruption in cements so 

 tested. On the other hand, nonsintered lime is not present in well-burned 

 cement. Its presence is a positive evidence of underburned, mixed, or 

 adulterated material and therefore the soundness of cements containing 

 a considerable quantity of nonsintered lime is very uncertain, as we 

 can not rely upon the early development of strong cohesive properties to 

 offset any force or forces which may operate to cause disruption. 



"Fused lime slakes very slowly in cold water. Hot water slakes it 

 more rapidly." 2B As the degree of incipient fusion approaches that of 

 perfect fusion the lime will become more and more inactive in cold water. 

 Therefore, some cements have remained sound in cold water even for 

 months, and yet disintegrated when placed afterwards in boiling water, 26 

 the disrupting force of expansion developing so slowly in the cold that 

 the growing forces of cohesion keep ahead of it. 



Sintered lime may exist both in under- and well-burned cement, but 

 the sintered lime in hard-burned cements probably slakes less readily at 

 normal temperatures. 



Experiments in this laboratory indicate that certain cements so 

 regulated by retarders as to set slowly may remain sound in the steam 



"Day and Allen, Joum. Am. Chem. Soc. (1909), 28, 1094. 

 20 Loc. cit. 



