LIME AND CEMENT INDUSTKIES 



675 



Method of slaking 



The water may be sprinkled over the lime gradually, or added 

 at once in excess. 



The former is best, because a looser mass is obtained, and it 

 gives better results with poorer limes, slaking them more thor- 

 oughly. Too great an excess of water tends to lower the tem- 

 perature and render the slaking incomplete. This causes unslaked 

 particles to get into mortar, and by their subsequent slow hydra- 

 tion and expansion they may do much harm. Popping of mortar 

 is due to this cause. It is also true that, if the water is added 

 gradually, it may allow the mass to cool down. Enough water 

 should be added to allow for that escaping as steam. "With very 

 fat lime 2i vol. of water may be taken. Poor magnesian limes 

 take less. 



Pure water should be used. That with soluble salts gives rise 

 to efflorescence, and hence sea water is undesirable, though it 

 has been successfully tried for hydraulic cement. An excess of 

 water gives granular paste and also makes the mortar porous.^ 



After slaking sand is added to the lime to make mortar. Ac- 

 cording to Gilmore^ the lime forms silicate, carbonate and 

 hydrate, and the crystals of these compounds interlock with the- 

 sand grains, thus binding the whole together into a solid mass. 

 In the course of time all the lime changes to carbonate, but thia 

 change may take a number of years. 



Sand is added to lime for economy and to prevent shrinkage. 

 Sand should be clean and sharp and should be in such quantity 

 that the lime will fill all the interstices. If an excess of sand is 

 used, the bond is poor. If too little sand is used, the mortar 

 shrinks and cracks. If too little lime is used the paste is made- 

 thin. In ordinary sands, the spaces form 30/^ to 4:0^ of the total 

 volume, and in such 1 vol. paste fills voids of 2^ vol. sand.^ In 

 practice 1.25 to 2 vol. of sand to 1 of paste is used. This in 



1 Gilmore. Limes, hydraulic cements and mortars, p. 299, 

 a Brickbuilder. 1897. p. 101. 



