574 KLEIN AND PHILLIPS: HYDRATION OF CEMENT 



hydration of the ahiminates commences quickly in all cases, but 

 with restricted amounts of water the unhydrated grains become 

 coated with the amorphous form and further hydration is more 

 or less retarded, the amorphous form slowly changing to the 

 crystalline form. With steam at atmospheric pressure the 

 weakly basic aluminates do not hydrate above 110°, but those 

 more basic absorb water up to 140°, tri-calcium aluminate con- 

 taining high burned free lime absorbing water even at 175°. 



The hydration of the aluminates in lime water reveals no new 

 products, but in plaster solution, in addition to the same com- 

 pounds formed with water, there is a compound observed with 

 a formula, 3CaO.Al2O3.3CaSO4.xH2O — tri-calcium sulpho-alumi- 

 nate, usually referred to as "sulpho-aluminate" in cement litera- 

 ture. This compound is identical for the three aluminates 

 and crystallizes in long prismatic needles. The double refraction 

 is low, the character of the principal zone negative, and the 

 extinction parallel. The indices of refraction are less than 1.48. 

 It is biaxial positive with a large optic axial angle. Its formation 

 is only incidental in the retardation of the initial set caused by 

 gypsum. In the autoclave, crystals of both the sulpho-alumi- 

 nate and gypsum are destroyed. 



Burned lime hydrates with an excess of water develop either 

 the crystalline or amorphous form of lime hydrate. A pre- 

 ponderance of the former is produced where the lime is coarse 

 and high burned, while the formation of the amorphous form 

 is favored by fine grinding and low burning. Crystallized lime 

 hydrate occurs as flaky hexagonal crystals or as hexagonal prisms 

 with excellent cleavage parallel to (0001). It is uniaxial nega- 

 tive and the refractive indices are e = 1.581 =<= .002 and w = 1.559 

 ± .002. In the autoclave amorphous lime hydrate does not 

 change to the crystalline form, but free lime may yield crystals 

 of hydrate whose size depends upon the length of time of reaction, 

 temperature and pressure. 



The mono-calcium silicate and the gamma-orthosilicate do 

 not hydrate, while the beta form of the latter hydrates but slightly 

 with water after long periods. Lime water and plaster solution 

 do not materially increase the hydration, whereas a solution of 

 the calcium aluminate gives the maximum hydration and best 



