572 CEMENTS, LIMES, AND PLASTERS. 



perature is a function of the temperature, and should be longer, the 

 lower the temperature." 



Setting properties of Portland cement. The theory which has been 

 quite generally accepted as explaining the setting of Portland cement 

 was that advanced by Le Chatelier. He considered that the aluminate 

 of lime, in contact with water, hydrated and hardened like plaster, 

 according to the equation: 



3CaO.Al 2 3 + 12H 2 = 3CaO.Al 2 O 3 .12H 2 0. 



To this action of the aluminate was ascribed the initial set of the cement. 

 The later hardening was ascribed, however, to the decomposition of the 

 lime silicate. In contact with water it sets, dividing so as to give 

 hydrated monocalcic silicate crystallizing in microscopic needles, and 

 calcium hydrate crystallizing in large hexagonal plates: 



3CaO.SiO 2 + water = CaO.Si0 2 .2iH 2 + 2(CaO.H 2 0) . 



In general this theory, has been accepted. 



Richardson, however, has recently modified* this theory in an im- 

 portant way. He considers that the setting of Portland cement is 

 due to the decomposition of the silicates and aluminates of the clinker 

 by the action of water, producing lime hydrate (Ca 2 H 2 2 ) in a peculiarly 

 active form. 



"On the addition of water to a stable system made up of the solid 

 solutions which composed Portland cement a new component is intro- 

 duced which immediately results in a lack of equilibrium, which is only 

 brought about again by the liberation of free lime. This free lime the 

 moment that it is liberated is in solution in the water, but owing to the 

 rapidity with which it is liberated from the aluminate, the water soon 

 becomes supersaturated with calcic hydrate, and the latter, crystallizes 

 out in a network of crystals which binds the particles of undecomposed 

 Portland cement together. From the characteristics of the silicates 

 and aluminates it is evident that the latter are acted upon much more 

 rapidly than the silicates, and it is to the crystallization of the lime 

 from the aluminates that the first or initial set must be attributed. 

 Subsequent hardening is due to the slower liberation of lime from the 

 silicates. If the lime is liberated more rapidly than is possible for it to 

 crystallize out from the water, expansion ensues and the cement is 

 not volume constant." 



* Richardson C. The setting or hydration of Portland cement. Engineering; 

 News, vol. 53, pp. 84-85. Jan. 26, 1905. 



