LIME AND CEMENT INDUSTRIES 749 



MIXING THE MORTAR 



In mixing the mortar for testing, sea water is specified, and 

 both air and water are to be maintained at a temperature of 15° 

 to 18° C (59-64.4 F) during the continuance of the experiments. 

 The quantity of water is ascertained by a preliminary experi- 

 ment, and the four following tests are given as an indication 

 whether the proportion of water added is correct: 



1 The consistence of the mortar should not change if it be 

 gaged for an additional period of three minutes after the initial 

 five minutes. 



2 A small quantity of the mortar dropped from the trowel on a 

 marble slab from the hight of about .50 meter (1.64 feet) should 

 leave the trowel clean, and retain its form approximately without 

 cracking. 



3 A small quantity of the mortar worked gently in the hands 

 should be easily molded into a ball, on the surface of which water 

 should appear. When this ball is dropped from the height of 

 .50 meter (1.64 feet) it should retain a rounded shape without 

 cracking. 



4 If a slightly smaller quantity of water be used, the mortar 

 should be crumbly and crack when dropped upon the slab. On 

 the other hand the addition of a further quantity of water — 

 1^-2$ of the weight of the cement — would soften the mortar, 

 rendering it more adhesive, and preventing it from retaining its 

 form when allowed to fall on the slab. It is recommended to 

 commence with a rather smaller quantity of water than is ulti- 

 mately required, and then to make fresh mixings with a slight 

 additional quantity of water. 



The mortar is to be mixed with a trowel for five minutes on a 

 marble slab. 



iTRENGTH 



The form of briquet and method of molding are the same as 

 in the German specifications; the breaking section is 5 sq. 

 cm (.775 square inch). Six briquets are broken after an interval 

 of seven days, six after 28 days, and the remaining six after 84 

 days. The mean of the three highest figures of each series of 

 tests is taken as the tensile strength of the cement under exami- 

 nation. The minimum strength specified for the neat cement 

 in seven days is 20 kilograms per sq. cm (284.5 pounds per 

 square inch);. in 28 days, 35 kilograms per sq. cm (498 pounds 

 per square inch); and at least 45 kilograms per sq. cm (64G 



