Table 92. Plaster Of Paris In Cement Mortars, Hardening In Dry Air. Effect On Different Samples, Portland And Natural

Ref.

Cement.

Age Briquets.

Tensile Strength Pounds per Square Inch, with Per Cent, of Cement Replaced by Plaster of Paris.

Kind.

Brand.

Sample.

0

l

2

3

6

1 2 3 4

Port. "

Nat.

"

R R An In

26 R 23 R

L 28 S

6 mo.

"

"

"

443

559 162 76

443 483 220 110

560 419 282 151

529 436 286 269

493 337 272 240

Notes. Sample 26 R, Portland, quick setting, bears 1/12 inch wire in 18 minutes.

Sample 23 R, Portland, slow setting, bears 1/12 inch wire in 244 minutes.

Sand, two parts Point aux Pins (river sand) to one cement. All briquets stored in air of laboratory until broken. Each result, mean of five briquets.

For the effect of plaster of Paris on the adhesive strength of mortar, see § 407.

379. Conclusions

It is evident from the above tests that the addition of small amounts of plaster Paris affects different samples of cement in quite different ways, and it is necessary to bear this in mind in the application of general conclusions to special cases. The indications are that the addition to cement of from one to three per cent, of plaster of Paris or sulphate of lime generally hastens the hardening and will not usually result in decreased strength; that some natural cements, however, are sensibly injured by more than one per cent., especially if used neat. The presence of as much as six per cent, plaster of Paris retards the hardening (although hastening the initial set) and is quite apt to ruin either Portland or natural cements. The addition of plaster Paris usually gives better results in air hardened than in water hardened specimens.