PROPERTIES OF PORTLAND CEMENT. 



385 



The intermediate stages in the process of hydration can fairly well be 

 traced without illustrations by the data recorded in Table IV. These 

 include the mixed formation of long crystals on the edges of the plumes, 

 and the production partly of needles and partly of feathery rosettes, 

 which show that sintered lime may hydrate so slowly by mere exposure 

 to the atmosphere that the action takes place essentially on the outer, 

 exposed parts of the particles and only gradually penetrates into their 

 interior. 



The negative test for free lime at the close of the series recorded in 

 Table IV is due to the final, complete conversion of at least the surfaces 

 of the h yd rated lime into carbonate. 



Conditions of the experiment recorded in Table V. — Well-burned and under- 

 burned nonaerated Portland cements were used in this experiment. Each was 

 spread in a layer about 1 millimeter thick on the bottom of a crystallizing dish 

 and exposed to the air. 



Table V. — Comparative microscopic tests of tioo cements containing free lime 



aerated in thin layers. 



Time. 



Microscopic test (duplicate samples). 



Well-burned clinker (Brand 0, 

 Table III). 



Underburned clinker (Brand O, 

 Table III) . 



Not aerated _ 



An almost immediate formation 

 of plumes. A few radiating 

 clusters of needles after 10 

 minutes. 



No needles formed in 20 minutes. 

 After 30 minutes many plumes. 

 No needles except on the 

 edges of some of these. 



No needles formed in 30 minutes. 

 In 1 hour several plumes and 

 feathery rosettes had appeared, 

 but no needles. 



No crystals in 30 minutes. In 1 

 hour only 10 feathery rosettes 

 and 2 plumes appeared in 

 2 specimens. 



No crystals in 30 minutes. In 1 

 hour only 2 plume-like aggre- 

 gates could be found. 



No crystals of any kind appeared 

 inli hours. 



In 5 minutes a mass of rosettes, 

 clusters and needles, some 

 plumes. 



In 10 minutes no crystals. In 20 

 minutes many rosettes of 

 needles and some plumes. In 

 1 hour about the same. 



In 20 minutes no crystals. In 

 1 hour plumes and feathery 

 rosettes but no single needles. 



In 30 minutes no crystals. In 

 1 hour a few feathery rosettes 

 and plumes. 



In 1 hour no crystals had 

 formed. 



No crystals of any kind appear- 

 ed in li hours. 



Aerated 1 day; slightly 

 caked. 



Aerated 2 days 



Aerated 5 days 



Aerated 7 days 



Aerated 8 days 





The results given in Table V differ very much from those of Table IV. 

 It must be understood that the penetration of air into a mass of ground 

 cement is limited approximately to a thin outer layer. (See Tables X 

 and XL) Almost every particle of the cement referred to in Table V 

 was fully exposed to the atmosphere at all times, while the material 



