THE RELATIVE VALUE OF CEMENTS. 665 



plaster of Paris may be taken as the type, water simply combines 

 with some constituent of the cement already present. In others, 

 of which Portland cement is the most important example, certain 

 chemical reactions must first take place. These reactions give 

 rise to substances which, as soon as formed, combine with water 

 and constitute the true cementaceous material. Portland cement 

 contains as chief, sometimes as almost sole constituent, a lime 

 peridote, and in addition a tricalcium aluminate, Ca s Al a O solu- 

 ble in 3,000 parts of water, and a dark-brown fusible substance, 

 Ca s Al 9 Fe 9 O a . In the act of setting, the tricalcium aluminate first 

 dissolves in water and then begins to separate again as a mass of 

 felted needles consisting of calcium aluminum hydrate, which 

 extend in every direction and are directly the cause of the first 

 setting of the cement. At the same time an action begins which 

 requires a much longer time for its completion, and which proba- 

 bly consists in a combination of the first formed aluminum 

 hydrate with the calcium peridote and the water, forming a 

 mineral belonging to the zeolite class and possessing very proba- 

 bly the composition H ]0 CaAl a Si 4 O 1T . This zeolite crystallizes out 

 as it forms, and this continues, for long periods subsequent to the 

 first setting of the cement, to add to its solidity and tenacity." 



Following the reasoning of Prof. Griffin, we are unable to 

 understand the meaning of "pure lime cement," as the two terms 

 " pure lime " and " cement," when used in an engineering sense, 

 are incompatible. The effect of the presence of magnesia upon 

 the quality of cement is not perfectly understood ; but that an 

 increased hardening in cement for a long period of time is due 

 alone to its presence, is not so, as cements that contain no mag- 

 nesia have been known to improve constantly during a period of 

 two years. 



To quote from Prof. Griffin : 



" So a Portland cement will develop its full strength in a few 

 months, while our natural cements will not for years, and, so long 

 as it (this chemical action) continues, the structure improves." 



Unless Prof. Griffin classes Portland cement as a " pure lime 

 cement" (which it is not), he has advanced no proof of the above 

 quotation ; and furthermore this statement itself is incorrect. It 

 is a fact well known to all engineers and builders that as a class 

 Portland cements are slower setting than the natural cements; 

 and also that natural cements attain their full strength within a 

 comparatively short time (within the first year as a maximum 

 limit), and that, after the full strength has been attained, this 

 strength may decrease, as time goes on, in some natural cements. 

 There has, however, been found no limit of time beyond which 

 Portland cement deteriorated, and for two or three years at least 

 it improves its strength. 



vol. xxxviii. 45 



