190 AXNALS NEW YORK ACADEMY OF SCIENCES 



by virtue of the greater hydraulic activity of the fine particles, increasing 

 the impermeability, as will be shown, and the confining therefore of the 

 action of the excess water to a narrow zone. The bulk of the cement will 

 be properly hydrated in spite of- the fineness. 



The investigation of the effect of the size of particles due to the action 

 of water thereon alone is not feasible, because no satisfactory measure of 

 laitance formation, except the strength of the mass, has been devised. 

 The measure of the strength would be unsatisfactory, since the propor- 

 tion of fine particles affects the strength in other ways than through the 

 formation of laitance, as has been pointed out in a previous communica- 

 tion. 



From a study of the hydraulic properties of reground cement, Spack- 

 mann and Lesley conclude (93) that only the very fine flour in cement, 

 that portion not measured by the present tests using sieves, reacts when 

 gaged with water and gives strength. It is difficult, of course, to draw a 

 sharp dividing line between active and inactive material in cement, al- 

 though it must be admitted that the greater part of the coarse material, 

 even though it be of the same chemical composition as the fine, has little 

 or no cementing value and serves mainly as a filler. 



Suitable fractional separation of the portion of cement passing the 200 

 sieve, by air-elutriation or other method, should with careful study be a 

 valuable guide to the most efficient mechanical composition. Experi- 

 ments upon the first method of separation are recorded by Peterson (71), 

 and a scientific method of fractional elutriation using an inactive liquid 

 has been worked out by Thompson (100). Much should be gained by 

 the application and development of these methods. The influence of the 

 size of particles of inert material added to the cement is also of great 

 consequence, and a proper mechanical grading of the sand used in mor- 

 tars is recognized as vital. The presence of clay in this sand, or the 

 addition of clay alone to cement, come under this category, and have 

 occasioned a great deal of discussion (8, 32, 33, 110). 



A comparison was made of the permeability of 1 :4 mortar of Portland 

 cement, when used in its ordinary condition, and when screened through 

 a number 200 sieve. 



