SAXD AND CEMENT 199 



The purity, or cleanness, of sand may be roughly ascer- 

 tained by rubbing it between the fingers and observing how 

 much dirt remains. To determine the percentage of the 

 impurities more accurately, a small dried and weighed sam- 

 ple is placed in a vessel and stirred up with water. The 

 sand is allowed to settle, the dirty water poured off, and the 

 process repeated until the water pours off clear. The sand 

 is then dried and weighed. The loss in weight gives the 

 quantity of impurities contained in the sand. The presence 

 of dirt, organic loam, mica, etc. is decidedly injurious and 

 tends to weaken the resulting mortar. Clay or fine min- 

 eral matter in small proportions may actually result in 

 increased strength, but excessive quantities of these mate- 

 rials may be a possible source of weakness. The best mod- 

 ern practice limits the quantity of impurities found by this 

 washing ^est to 5%. 



It is advisable, prior to the selection of a sand, to deter- 

 mine what its strength will be when made into mortar. 



Preparation of Sand. Sand is prepared for use by (1) 

 screening to remove the pebbles and coarser grains the 

 fineness of the meshes of the screen depending on the kind 

 of work in which the sand is to be used; (2) washing, to 

 remove sart, clay, and other foreign matter; and (3) drying 

 if necessarv. When dry sand is required, it is obtained by 

 evaporating the moisture either in a machine, called a sand 

 dryer, or in large, shallow, iron pans supported on stones, 

 with a wood fire placed underneath. 



LIME AND CEMENT MORTARS 



Mortars are composed of lime or cement and sand mixed 

 to the proper consistency with water. The proportions of 

 the ingredients depend on the character of the work in 

 which the mortar is to be used. 



In proportioning mortar, the quantities of the separate 

 ingredients are usually designated by a ratio, such as 1-1, 

 1-2, 1-3, etc. Thus, 1-2 signifies that 1 part of lime or 

 cement is used to 2 parts of sand; etc. For great accuracy 

 these measurements should be made by weight, but they are 

 usually made by volume, which is almost the same thing. 



