SAND AND CEMENT 207 



Nevertheless, Portland -cement mortars may bo laid even 

 under the worst conditions if certain precautions are ob- 

 served, but mortars of natural cement should never be used 

 in extremely cold weather, as they are generally completely 

 ruined by freezing. 



The bad results that arise during mild frosts may be suc- 

 cessfully guarded against by heating the sand and water 

 and by using a quick-setting cement mixed rich and as dry 

 as possible. In extremely cold weather, salt must be added 

 to the water, so as to convert it into a brine that requires a 

 temperature lower than 32 F. to freeze it. The common 

 rule for adding salt is to use a quantity equal to 1 % of the 

 weight of the water for each degree of temperature that is 

 expected below 33 F. Thus, at 32 F., a 1% solution would 

 be used, while at 25, an 8% solution would be required. 

 Solutions greater than 12% should not be employed, and 

 if a temperature below 20 F. is expected, heat must be used 

 in addition to the salt. The finished work should also be 

 protected with canvas or straw. Manure should not be 

 used for this purpose, because the acids it contains tend to 

 rot the cement. Unless the conditions are such as to make 

 it imperative, it is not advisable to lay mortars during 

 freezing weather. 



Shrinkage of Mortars. Cement mixtures exposed to the 

 air shrink during the process of hardening, while those 

 immersed in water tend to expand. The shrinkage of 

 ordinary cement mortars is slight, and when they are used 

 as a bonding material it need not be considered. When 

 used as a monolith, as in sidewalks, shrinkage is guarded 

 against by keeping the mortar wet during setting. This 

 can be done by covering with moist straw or by sprinkling 

 the mixture with water. 



Grouting. By grouting is meant the process of filling spaces 

 in masonry with a thin, semifluid mixture known as grout. 

 This mixture consists of cement, 1 or 2 parts of sand, and an 

 excess of water. Grout can be used for filling the voids in 

 walls of rubble masonry for backing arches and tunnels, and 

 for filling the joints between paving brick. In fact, it can be 

 used in all places where mortar cannot be laid in the ordinary 



