COMPOSITION AND PROPERTIES OF LIME. 121 



a thin or thick paste, according to the amount of water used. When 

 ordinary laborers are slaking lime it is evident that this method possesses 

 the great advantage of being on the safe side. It is possible that the 

 addition of the surplus water weakens the mortar somewhat, but on 

 the other hand it insures thorough slaking, or would insure it if even 

 reasonably good care were taken during the operation. The trouble, 

 however, has been that lime-slaking is not regarded as an art, but as a 

 disagreeable necessity, and it is usually carried on by laborers who 

 are not even supposed to know anything about the subject. 



The result of these conditions is that the slaked lime used in mortar 

 rarely even approaches its theoretical efficiency. Either so much 

 water has been added that the strength of the product is impaired or 

 else the water-supply or mixing has been insufficient and the product 

 is not thoroughly slaked. 



A realization of these facts has caused the introduction of ready- 

 slaked lime, prepared carefully at the lime-plants. This product is 

 discussed in the following chapter under the head of " Hydrated lime ". 

 In its preparation particular care is given to insuring that the product 

 shall be thoroughly slaked, and that this slaking shall be done with as 

 little water as possible. Several distinct methods of slaking have 

 accordingly been devised and are in use at different lime-hydrating 

 plants. 



In ordinary practice, where quicklime is slaked on the work, only 

 one general method is followed, though books on construction invari- 

 ably list and describe several other methods. The process as actually 

 carried out is to form, on a plank floor or on a bed of sand, a circular 

 wall of sand. The lime is shoveled into the ring thus formed and 

 water is turned on from a hose until the laborer considers the amount 

 sufficient. The lime commences to slake more or less quickly accord- 

 ing to its composition, and when the process is completed it is covered 

 over with a thin layer of sand until required for mortar. 



Use of lime mortars. Lime is never used alone as a binding mate- 

 rial, for it shrinks greatly on drying and hardening, and this shrinkage 

 would produce cracks if nothing were added to the mortar to counter- 

 act it. In practice sand is always added to lime mortars, the propor- 

 tions for ordinary use being from two to four parts sand to one part 

 lime paste. 



The hardening of lime mortars is a simple process, though occa- 

 sionally statements of opposite tenor may be found in print. It may 

 be accepted as proven that lime mortars harden by simple recarbon- 

 ation, the lime gradually absorbing carbon dioxide from the atmosphere 



