408 REIBLING AND REYES. 



occur at temperatures lower than the melting point, the exceptions oc- 

 curring when the materials are so coarsely ground, or poorly mixed that 

 combination is retarded until the melting point of the slag is passed. 



Therefore, in the cement kiln, the coarse particles of calcined limestone 

 are apt to become surrounded by fused slag and thus more or less escape 

 reaction. Excessive lime, or poorly mixed raw meal, would produce a 

 similar result. When the clinker has cooled, and is then subjected to 

 seasoning, this covering of slag tends to protect the inclosed lime from 

 hydration. When the clinker is ground, the walls are broken open and 

 the lime therefore immediately is in a condition in which it can react. 

 This explains why hard-burned, unsound cement seasons to a sound 

 material so slowly when in the clinker form, and so rapidly when it is 

 finely ground. It also accounts for the fact that some hard-burned 

 clinkers continue slowly to disintegrate for years. As the free lime in 

 underburned cement is more abundant and less thoroughly protected 

 by its softer, sintered magma, it seasons more readily. 



These reactions are demonstrated and confirmed by the figures recorded 

 in Table XVIII (p. 409). 



Sample 10 is practically unaffected by four months' exposure. This shows 

 that perfectly sintered Portland cement clinker is almost inert to water and 

 atmospheric influences. In fact, it will not react with water unless it is 

 previously ground to a fine powder. Consequently, seasoning only affects the 

 clinkers, or the portions of clinkers, which are not perfectly sintered or fused. 



When free lime is present in hard-burned clinker, the inert, fused or sintered 

 magma in which it is imbeddel protects the lime from hydrating influences. 

 Sample 3, after six months of drastic seasoning, still contained unslaked, sintered 

 free lime. Sample 5, after aerating for 3 months, remained unsound. Samples 

 5, 6, 8 and 9 show that wet clinkers season more thoroughly and more rapidly 

 than dry ones. 



We regard aeration as the least efficient, practical method of seasoning 

 Portland cement. It is not only slow, uncertain and incomplete in its 

 influence, but also induces the slaked lime to change to carbonate and the 

 formation of carbonate from slaked lime in unset Portland cement re-, 

 duces its efficiency. Slaked lime has a cementive value of its own and it 

 is a much more valuable constituent of Portland cement than a correspond- 

 ing amount of inert calcium carbonate. This is a consideration worthy 

 of more attention than has heretofore been given to it by manufacturers. 

 If their cement must be seasoned, the less the change to carbonate is 

 permitted while it is being seasoned the more readily and thoroughly will 

 it hydrate, and the better will be its trowelling properties and sand- 

 carrying capacity. 



The figures recorded in Table XVIII also verify our general experience 

 that a high loss on ignition, and a correspondingly low specific gravity, 

 are not characteristic of commercial cements made from well-burned 

 clinker. Cement made entirely from underburned clinker seldom appears 



