PROPERTIES OF PORTLAND CEMENT. 387 



Selected, thoroughly sintered, unaerated cement clinkers (Brands 

 and L, Table III), which gave a negative test for free lime, were treated 

 in a manner similar to that recorded in Tables IV, V, and VI. In no 

 case could free lime be detected. 



Conditions of the experiment recorded in Table VII. — We have endeavored 

 to establish a definite relationship between the microscopic evidence of the 

 presence of free lime and the physical condition of the cement as evidenced by 

 the tests for soundness. Aeration tends to remove unsoundness and a simulta- 

 neous study of the influence of the air upon the free lime and upon the soundness 

 of the cement was resorted to in an effort to compile conclusive data. The cement 

 used for this experiment was prepared by grinding about 10 kilograms each of 

 so-called "good" and "bad" clinker in a ball-mill, until no residue was left 

 on a 100-mesh sieve. The cement was somewhat exposed during the process of 

 grinding. Two per cent of plaster of Paris had been added to the clinker. 

 The cement was aerated in 300-cubic centimeter wide-mouthed bottles. Each 

 bottle contained 110 grams of cement which was mixed thoroughly each day. 

 Table VII records the results. (See pp. 388, 389.) 



That the effects of free lime upon soundness are influenced by the 

 cohesive properties of the cement becomes apparent from the results 

 recorded in Table VII; and it is evident that the cause of unsoundness 

 was more difficult to eliminate from the underburned material than from 

 that which was well burned. The microscopic examination of the cements 

 which had been aerated during two days, failed to show the differences 

 which should have appeared if the physical tests alone were taken into 

 consideration. We attribute the discrepancy to the presence of more and 

 stronger cementive material in the better burned product, our universal 

 experience being that underburned cements at any stage of aeration or 

 seasoning do not develop the high earl}' strength characteristic of well- 

 burned Portland cement. 



These experiments all indicate that the usual cause of unsoundness in 

 cements is unslaked free lime, and that commercial Portland cements 

 may develop perfect soundness and still contain a considerable quantity 

 of free lime. The limitations of the soundness test as a test for free 

 lime or for underbuming are self-evident, but White's test is absolute. 



These results as well as many others which we have obtained also seem 

 to prove that from the microscopic evidence alone the behavior of all 

 cements when subjected to the soundness test can not be predicted. It 

 is doubtful, if we consider the character of the physical test, if even a 

 quantitative measurement of sintered, nonsintered, and slaked lime 

 would prove entirely satisfactory in this respect. Unsoundness in Port- 

 land cement becomes manifest to the eye only when the force or forces- 

 which operate to cause disruption are sufficiently great to overcome the 

 cohesion between particles. Therefore, the development of cohesive 

 forces, the "speed of slaking," fineness, the temperature and amount of 

 water used in gauging, and the effect of impurities and retarders, must 

 all be taken into consideration; and consequently the test for soundness 



