206 SAND AND CEMENT 



In bricklaying and in other places in which mortar is 

 employed it is frequently desired to use a material that is 

 more plastic or smoother than pure cement mortar. This 

 quality is usually obtained by adding from 10 to 25% of 

 lime to the mortar. This addition of lime not only renders 

 the mortar more plastic, and hence easier to work, but also 

 increases both its adhesive strength and its density, which 

 assists in making the mortar waterproof. Hydrated lime 

 is to be preferred for use in cement mortar, because its 

 complete slaking is assured. Hydrated lime may also be 

 readily handled and measured on the work. 



Occasionally, small quantities of cement are added to lime 

 mortars so as to make them set quicker and to increase their 

 strength. Such mixtures, however, are not especially 

 economical nor are they convenient in practice. 



Retempering of Mortar. Mortar composed of cement, 

 sand, and water soon begins to set and finally becomes hard. 

 When it is desired to use this material, more water has to 

 be added and the mixture worked until it again becomes 

 plastic. This process is called retempering. Laboratory 

 tests generally show that retempering slightly increases the 

 strength of mortar, but the reworking is more thorough as 

 a rule in the laboratory than would be the case in actual 

 work. Any part of the hardened mortar that is not retem- 

 pered is a source of weakness when incorporated in the 

 building. The adhesive strength of cement, moreover, is 

 greatly diminished, by this process For these reasons, 

 it is generally inadvisable to permit the use of retempered 

 mortars; but if they are allowed, great care should be taken 

 to see that the second working is thorough and complete. 



Laying Mortar in Freezing Weather. Frost or even cold 

 has a tendency to retard greatly the set of cement mortars. 

 When the temperature, moreover, is so low that the water 

 with which the mortar is mixed freezes before it combines 

 with the cement, it may, if care is not exercised, result in 

 complete destruction of the work. A single freezing is not 

 particularly harmful, because when thawing occurs, the 

 arrested chemical action continues. A succession of alter- 

 nate freezings. and thawings, however, is extremely injurious. 



