The two kinds of hydraulic cements are termed Portland and natural (often called Rosendale, from a place in New York where much of it is made). The former is prepared by mixing together suitable proportions of clay and carbonate of lime, finely pulverized, and burning the mixture at a high heat in kilns, after which the mass is ground to a fine powder. Natural cements are so called because they are made from the natural rock, which contain the clay and limestone in the proper proportions.

Portland cements are dark in color, weigh from 90 to 100 lb. per cu. ft., are very slow in setting, and attain great ultimate strength. Natural cements are light in color, weigh from 50 to 60 lb. per cu. ft., are very quick setting, and become from 1/2 to 2/3 as strong as Portland cement.

A barrel of Portland cement weighs about 375 lb. net; one of the Eastern Rosendales, 300 lb., and Western Rosendales (from Wisconsin, Kentucky, Illinois, etc.), about 265 lb. A cubic foot of slightly compacted cement mixed with 1/3 cu. ft. of water will make from 2/3 to 3/4 cu. ft. of paste; or 1 bbl. of cement will make about 3 3/4 cu. ft. of stiff paste.

Simple Cement Tests

Mix a handful of the cement to be tested with water, and make it into two cakes about 1/2 in. thick, with thin edges. Let one dry in air for an hour and then put it in water for 24 hours, the other being kept in air. If at the expiration of that time the latter has become quite hard, and when broken shows considerable tensile strength, with a clean, sharp fracture, without crumbling, and the cake in water retains its shape and has become much harder, such a cement is probably amply good for all building purposes. If the cake kept in water shows bad checks or cracks on the edges, such cement is unsafe to use under water, for any important work. If the sample in air becomes quite hard, while that under water crumbles, the cement may often be improved by mixing about half as much slaked lime with it. If it then hardens, it may be used in wet situations. The" rapidity of setting in air may often be retarded, if required, by the addition of a small quantity of slaked lime. A cement which remains soft in air and does not become hard in water in 24 hours, may be a good cement for some purposes, but is very slow setting and undesirable for use in damp positions. Tensile Strength. - When a testing machine is available, tests of tensile strength of cements are usually made. The following table indicates the average strength of cement mortars of various ages and compositions:

Tensile Strength Of Cement Mortars

Age of Mortar When Tested.

Average Tensile

Strength. Pounds

Per Square Inch.

Clear Cement.

Min.

Max.

Min.

Max.

1 day, 1 hour, or until set, in air.......

100

140

40

80

l week...............................................

250

550

60

100

4 weeks ............................................

350

700

100

150

1 year................................................

450

800

300

400

1 Part Cement to 1 Part Sand.

1 week...................................................

30

50

4 weeks............................................

50

80

1 year...............................................

200

300

1 Part Cement to 3 Parts Sand.

1 week........................................

80

125

4 weeks.........................................

100

200

1 year................................................

200

350

All samples, except the first, were kept in air 1 day, and the remainder of the time in water.

Sand

The sand should be sharp, free from clay or earthy materials, and should be preferably pit sand. Sea sand should never be used unless thoroughly washed, as the salt in the sand causes efflorescence, and the cementing material does not adhere well to the rounded grains. Pulverized brick, cinders, furnace slag. etc. are sometimes used as substitutes for sand with good results. The addition of a small quantity of brick dust to ordinary lime-and-sand mortar seems to give it the property of setting under water, and also prevents disintegration when the mortar is exposed to the elements. _______