898 Transactions of tiie American Institute. 



with silica. If completely melted, Portland cement becomes a glass, 

 tinged with silicate of protoxide of iron. 



Whether the above-named combinations exist singly by themselves, 

 ©r as double compounds, has not been determined ; but it appears to 

 be established that there is no free lime in Portland cement. 



The Setting or Induration Process. 



The slow induration of Portland cement, as Dr. Bleibtreu 1ms 

 shown, consists in the formation of a real mineral, or of a crystalline 

 rock species, which appears to be perfectly analogous to natural zeo- 

 lites. Dr. Bleibtreu owes the discovery of this induration to the 

 ci re u instance that, in the loading of some cement in Bonn, a barrel 

 of it rolled into the Rhine. When, after the lapse of about six 

 weeks, at a low level of the water in the river, the same was taken 

 out, it was found to be completely solidified. Upon knocking off a 

 piece and holding it in the sunlight, a peculiar kind of glimmering 

 was observed ; and, when it was examined with a magnifying glass, 

 the whole mass was found to exhibit a distinct crystalline structure. 

 This fact was still further confirmed by microscopic examination. 

 On this account, says Becker, we are led to the conclusion that the 

 greater part of the set cement- consists of translucent, and, to some 

 extent, even of transparent crystalline particles. By measuring the 

 angles of the crystals, their form is easily ascertained. It appears 

 that rhombic columns, octahedrons and laminae are prevalent. There 

 are also many needle-like crystals, either of a star-like form or of a 

 form presenting an appearance somewhat resembling a net, exactly 

 like those of the zeolites of nature. This structure has since been 

 noted by Dr. Bleibtreu in different descriptions of slowly set Port- 

 land cement. 



If, however, a rapid induration ensues, it is evident that there can 

 be no perfect crystallization. The cement will rather form an amor- 

 phous mass, often penetrated by cracks and fissures, so frequently seen 

 in constructions composed of cement. How very important it is to 

 the solidity and hardness of hydraulic mortars that the various ele- 

 ments have time to group themselves together, is evident from the 

 diversified columnar structure of slowly set cement. Whether it takes 

 a little longer or not for a cement to set, is, after all, of no great con- 

 sequence. The mass should not become at once hard and rigid ; it 

 should rather, for a few weeks, possess a certain flexibility, so that the 

 crystals which slowly form themselves may penetrate in every direc- 

 tion, and thus form an intimate and compact structure. 



