vin, a, 2 Reibling: Portland Cement 119 



kinds of free lime were thoroughly pointed out in previous papers 

 from this laboratory. 6 



It is sufficient to state here that free lime tends to cause dan- 

 gerous changes in the setting and hardening properties of Port- 

 land cement, and that the endurance of the early strength, the 

 increase in strength with age, and the constancy in volume will 

 be the greater the less free lime (or magnesia) the indurated 

 cement contains. Therefore, we can not hope to secure the de- 

 sired efficiency and uniformity in quality unless the quantity of 

 free lime is reduced to a very low figure. The best burning 

 and proper seasoning and storing produce a sound product 

 which has a high specific gravity and a low loss by ignition ; and, 

 while it is impossible to obtain Portland cement which contains 

 no free lime, the manufacturer should burn his materials so that 

 no seasoning is required to produce a perfectly sound cement. 



It is essential to increase rather than decrease the severity of 

 the requirements for constancy of volume. 



While the large use of concrete demonstrates beyond question its value 

 as structural material, there are too many concrete structures now showing 

 signs of incipient failure to permit us to relax in any way the demands of 

 the standard specifications or our efforts to secure improvement in the 

 quality of the cement. If the use of Portland-cement concrete is to continue 

 to increase, or even remain as great as at present, the engineer must be 

 assured of its durability. It is unfortunate, perhaps, that the present low 

 prices of cement offer little inducement to the manufacturer to spend more 

 money to improve quality, or to assume the additional cost involved in the 

 storage of the clinker and the finer grinding demanded by Mr. Force. It is 

 an old axiom "that we get only what we pay for," and if the engineer is going 

 to insist, as it seems proper he should, on the furnishing by the manu- 

 facturer, for important structures, of a cement which will pass the autoclave 

 test, he should be prepared to offer an advanced price for such a cement. 



It is universally recognized that neat Portland cement is not durable, 

 and it is for this reason, and not from motives of economy only, that cement 

 is used mixed with sand or other aggregates. Until the cement manufac- 

 turers can produce a cement which will be durable when used neat, they 

 should not relax their efforts toward the improvement of their product, and 

 the engineer should not hesitate to adopt, and to insist that the cement shall 

 pass, any test, no matter how severe, that will develop any latent unsound- 

 ness or tendency to expand in time after the concrete made from it has been 

 hardened on the work. Even though one must pay more for a cement that 

 will meet such special tests than for one that will meet only those required 



" Reibling, W. C. and Reyes, F. D., The chemical and physical properties 

 of Portland cement: Parts I and II, This Journal, Sec. A (1910), 5, 367-418. 



Part III, ibid. (1911), 6, 207-252. 



Parts IV and V, ibid. (1912), 7, 135-195. 



Abstract and summary, 8th Int. Cong. Applied Chemistry, III c. (1912), 

 5, 91-116. 



