KLEIN AND PHILLIPS: HYDRATION OF CEMENT 575 



appearing test pieces. The 28 day test pieces of beta-ortho- 

 silicate and the aluminates, while, exhibiting fairly good rigidity, 

 have by no means the strength of corresponding neat cement 

 briquettes. The aluminates are C9mpletely hydrated, but the 

 beta-orthosiUcate shows only a comparatively sHght hydration. 

 The hydration product of the sihcate is amorphous hydrated 

 orthosilicate, there being no lime hydrate split off and no 

 needles of hydrated mono-calcium sihcate formed, as noted by 

 others. 



The tri-calcium sihcate hydrates readily and quickly with all 

 concentrations of water, the products of hydration being crystal- 

 lized hme and amorphous hydrated ortho-silicate. Moulded 

 specimens set hard in 5 hours and show no disintegration after 

 28 days in water. It has no favorable effect on the hydration 

 of beta-orthosilicate. Mixtures of it and the aluminates show 

 first the beginning of hydration of the aluminates, followed shortly 

 by the hydration of the silicate. MouMed specimens of these 

 are dense, hard, and strong, comparing very favorably with neat 

 cement briquettes. 



On the hydration of cement, the first constituent to react is 

 the aluminate, with the formation of amorphous hydrated tri-cal- 

 cium aluminate; with or without amorphous hydrated aiumina. 

 The sulpho-aluminate crystals are also formed and the low 

 burned or finely ground lime hydrates. This occurs within a 

 few hours after the cement is gauged. The next compound 

 to hvdrate is the tri-calcium silicate. This commences within 

 24 hours and is generally completely hydrated within 7 days. 

 Between 7 and 28 days, the amorphous aluminate commences to 

 crystallize and the beta-orthosilicate, the least reactive compound, 

 begins to hydrate. The 24-hour strengths are due mainly to 

 the hydration of the aluminates and of any fine grained, low 

 burned lime present. The large increase in strength between 

 24 hours and 7 days is due mainly to the tri-calcium silicate hy- 

 dration. The increase between 7 and 28 days is due to the hydra- 

 tion of the beta-orthosilicate. Where there is a decrease in 

 strength during this period it is due to the hydration of very 

 high burned free hme as in very high burned, high limed cements, 

 or to the crystallization of the aluminates, as in high alumina 



