PACINI, MET AMORPHIC! OF PORTLAND CEMENT 173 



able (86). The coarser particles are comparatively inert and might be 

 replaced by grains of foreign material of the same size without ma- 

 terially influencing the ultimate strength of the resulting mass. This 

 has been demonstrated experimentally (17). It does not follow, how- 

 ever, that a cement consisting entirely of uniformly very fine particles 

 would be a desideratum, since such a cement would not pack as well as 

 one containing a greater variety of sizes, and the increase in chemical 

 activity would be markedly overbalanced by the imperfection of struc- 

 ture of the mass. Considering each particle to be spherical, and of equal 

 size with every other, when packed in the most compact manner possible 

 the pore space would be nearly 26 per cent (89). The points of contact 

 of the adjacent spheres, notwithstanding the tendency of the gelatinous 

 em r elope to spread, would be relatively few. If, however, this pore space 

 were filled with finer material, the structure would develop more strength. 

 The function of part of the cement is to remain passive and to add to the 

 strength of the structure merely by its action of void-filling. Extremely 

 fine grinding has been found to decrease the ultimate strength, if the 

 cement is used neat, but to give greater strength, if the cement is used 

 in a sand mortar (62). 



As might be expected from the above considerations, the fineness of 

 grinding has an accelerating effect upon setting. Cement ground in a 

 tube mill until only 1 per cent remained on a sieve having 5000 meshes 

 per sq. cm., was so quick setting that it could not be restrained even by 

 the addition of 10 per cent of gypsum (47). When cement is relatively 

 coarsely ground, the ultimate strength is not so quickly attained, but its 

 acquisition is regular and uniform." 



Laitance. — In concrete construction under water, especially salt water, 

 there gathers about the freshly deposited concrete a milky white cloud of 

 suspended matter, technically known as laitance. This material is also 

 formed when concrete is mixed very wet, though not deposited under 

 water. 



An analysis of laitance by the writer, made for the Board of Water 

 Supply, practically coincides with an analysis made by Eichardson (97) 

 and leads to the same conclusion as that reached by him; namely, that 

 laitance represents an actual loss of cement and consists of the finest par- 

 ticles of cement which have been washed out of the concrete. The addi- 

 tional conclusion is justified that this portion of the cement, by reason of 

 the small size of its units, has been so acted upon by an excess of water 

 that it has undergone complete hydrolytic decomposition, before the col- 

 loidal enveloping film had adsorbed sufficient electrolytes to completely 

 coagulate it and so render it largely impermeable. This is substantiated 

 by the fact that laitance possesses neither setting nor hardening qualities. 



