120 CEMENTS, LIMES, AND PLASTERS. 



With absolutely pure lime the amount of water that must be added 

 in order to change all of the quicklime into lime hydrate will equal 

 32.1 per cent, by weight, of the quicklime. The resulting lime hydrate 

 will therefore consist of 75.7 per cent lime (oxide) and 24.3 per cent 

 water. Lime hydrate is a fine white powder, with a specific gravity 

 of 2.078. 



Effect of impurities present. The above percentages would only 

 hold true in case the burned lumps consisted entirely of quicklime (CaO). 

 Actually we know that this theoretical purity is never attainable on 

 a commercial scale. The original limestone always contains silica, 

 alumina, iron oxide, etc., in quantities more or less large. The lime- 

 stone is, moreover, never perfectly burned, so that some portions of 

 unburned lime carbonate will always be present in the product. 



The presence of these impurities, including silica, alumina, iron 

 oxide, and fragments of unburned lime carbonate, operates to reduce 

 the amount of water theoretically required for perfect slaking of the 

 lime. For example, a perfectly pure lump of quicklime 100 Ibs. in 

 weight would require 32.1 Ibs. of water for complete slaking. If the 

 lump contained 10 per cent of impurities, however, it would require 

 only 28.9 Ibs. of water. 



Expansion of volume. The impurities also serve to reduce the 

 expansion of volume which the lime would otherwise show. A pure 

 lime, if slaked by adding the entire quantity of water at once, may 

 increase 3^ times in volume; if slaked gradually, only part of the water 

 being added at a time, it will increase much less; while if allowed to 

 air-slake its increase in volume will be only about 1.7 times its original 

 volume. An impure lime, as above noted, will show less expansion, 

 the difference being in direct ratio to the percentage of impurities. 



The rapidity and intensity of the slaking will also be less with an 

 impure than with a pure lime, and the amount of heat evolved during 

 slaking will be decreased. 



Effect of the presence of magnesia. If the lime contain any con- 

 siderable percentage of magnesia, slaking will take place more slowly 

 and with less evolution of heat, while the expansion of volume will be 

 less. In consequence of the slowness of the slaking more care is necessary 

 with magnesian than with high-calcium limes in order to insure that 

 the product has been thoroughly slaked. 



Methods of slaking lime in ordinary practice. When lime is used 

 for making ordinary building-mortar, the common practice is to add 

 much more -than the amount of water theoretically required. The 

 result is not only to slake the lime but to convert the slaked lime into 





