Table 87. Slaked Lime In Portland Cement Mortars

Ref.

Lime in form of

Proportions.

Tensile Strength, Pounds per Square Inch., Sample Stored in

Cement, Grams.

CaO in Lime Paste or Powder, Grams.

Sand, Grams.

Open Air.

water Laboratory.

1

Paste

200

0

600

404

382

2

"

200

20

600

308

426

3

"

200

40

600

292

450

4

"

200

60

600

224

462

5

"

200

80

600

219

384

6

Powder

200

0

600

382

371

7

"

200

14.3

600

385

443

8

"

200

28.6

600

316

451

9

"

200

42.8

600

338

431

10

"

200

57.1

600

325

440

Cement

Portland, Brand R.

Sand

Crushed Quartz, 20-30, or "Standard." Age of briquets, 6 months.

375. In Table 88 only lime paste is used, but both Portland and natural cement are tested, and the specimens are hardened in dry air and damp sand. In the first column of results are given the strengths attained by Portland cement mortar containing three parts sand to one of cement without lime. In the second column, ten per cent. CaO in form of paste is added to the cement. In the third, fourth and fifth columns, respectively, ten, twenty-five and fifty per cent, of the cement is replaced by CaO.

It appears that ten per cent, of the cement in a one-to-three Portland mortar may be replaced by lime made into paste without diminishing the strength, if the mortar hardens in damp sand. Even in dry air exposure, it is only at one year that the lime shows any deleterious effect. To replace twenty-five per cent, or more of the cement with lime, however, diminishes the strength of the mortar in a marked degree.

In the case of natural cement, replacing ten per cent, of the cement with lime is decidedly beneficial, and even twenty-five per cent, lime gives enhanced strength, except for specimens hardened in dry air.

Table 89 gives similar results for one-to-four mortars and different percentages of lime, the briquets being hardened in dry air and damp sand.

Table 88. Use Of Lime Paste In Cement Mortars Containing Three Parts Sand

Ref.

Cement.

Briquets Stored.

Age

Briquets.

Tensile Strength, Lbs. per Sq. In.

Cement, gm.

200

200

180

150

100

Lime Paste, "

0

60

60

150

300

CaO in Lime Paste, gm.

0

20

20

50

100

Amt. CaO ex pressed as % of Cement plus Lime.

0

9

10

25

50

Kind.

Brand.

Sand, gm.

600

600

600

600

600

1

2 3 4 5 6 7 8 9

Port, "

" " " "

Nat. "

"

X

" " " " "

An "

"

Dry air Damp sand Dry air Damp sand Dry air Damp sand Dry air Damp sand water

28 da. "

3 mo. "

1 yr.

"

3 mo. " "

...

...

...

...

...

...

...

...

201 294 236 350 384 430 310 267 222

242 330 265 410 377 445 338 344 301

238

309 264 398 317 442 359 327 319

168

238 171 309 215 332 251 318 293

57 95 70

125 98

171 69 93 79

In all of the above tests the mortars containing much lime paste were not only more plastic, but somewhat wetter than the corresponding mortars of cement and sand alone, on account of the water contained in the paste.

376. The conclusion to be drawn from these tests appears to be that the addition of a small amount, ten to twenty per cent., of slaked lime to cement mortars containing as much as three parts sand, not only renders them more plastic, but actually increases the tensile strength, especially if the mortars are kept damp during the hardening. It also appears that for mortars exposed to the open air the lime should be in the form of slaked powder rather than paste. It may be added, that in all cases care should be taken that the lime is thoroughly slaked before use, and all lumps should be removed by straining or sifting. Further results on this subject are given in connection with the tests on adhesion of cement mortar to brick (Art. 5).

Table 89. Use Of Lime Paste In Cement Mortars Containing Pour Parts Sand To One Cement

Composition of Mortar.

Tensile Strength of Mortar, Pounds per Square Inch.

Cement.

Lime Paste, Grams.

Lime in

Paste, Grams.

Sand,

Grams.

Stored in Damp Sand.

Stored in Dry Air.

Kind.

Grams.

Fresh Lime Paste.

Old Lime Paste.

Fresh Lime Paste.

Old Lime Paste.

Portland, Brand X, Sample 41 S

240 240 200 180

00 80 120 180

00 27 40 60

960 960 960 960

176 212 198 204

180 200 212 194

254 280 227

232

244 250 237 184

Natural, Brand An, Sample L

240 240 200 180

00 80 120 180

00 27 40 60

960 960 960 960

150 160 166 140

133 154 173 166

127 162 131 124

142 150 170 154

Note.—All briquets three months old when broken.

377. Effect Of Plaster Of Paris On The Cohesive Strength Of Mortars

The use of plaster of Paris, or calcium sulphate, in the manufacture of cement to regulate the time of setting, has already been mentioned. The amount of such additions at the factory are usually small, the German Cement Makers' Association limiting it to two per cent.

Tests on three brands of Portland cement, showing the effect of small additions of plaster Paris, are given in Table 90. All of these mortars hardened in water. It is not known whether any of the cements had received additions of plaster Paris before leaving the factory. It is probable that brands R and X had been so treated, since they are German cements, but it is not probable that the other brands of Portland had received any addition of plaster.

It appears that with these brands the addition of from one to three per cent, of plaster Paris hastens the hardening and increases the strength of the mortar at ages of six months to two years. Six per cent, plaster sensibly retards the hardening, but, in all cases except one, Brand S, neat, six months, the mortars containing six per cent, plaster, gave higher results on long time tests than did the corresponding mortars to which no plaster had been added.

Table 90. Plaster Of Paris In Portland Cement Mortars, Hardening In Water

Notes. — Sand, Point aux Pins (river sand) passing No. 10 sieve, except for hot tests, where standard sand was used. Cement and plaster of Paris passed through No. 50 sieve before using. Plaster Paris had no apparent effect on consistency mortar at first, but after making first three briquets of batch of five, the mortar containing plaster Paris dried out somewhat. Each result, mean of five briquets.

Similar tests of natural cement mortars hardening in water are given in Table 91. One of the brands is not much affected by additions of one to three per cent., but the other brand is practically ruined by the addition of more than one or two per cent., and both brands are rendered quite unsound by six per cent, plaster.

Ref.

Cement,

Portland

Brand:

Sand

Parts to one

Cement.

Temperature water in which

Briquets Stored.

Age of

Briquets when

Broken.

Tensile Strength, Pounds per Sq. In., with Per Cent, of Cement Replaced by Plaster of Paris.

0

l

2

3

6

1

S

0

60° to 65°Fahr.

7 da.

487

626

600

519

380

2

"

0

"

6 mos.

743

746

754

742

660

3

"

2

"

7 da.

323

388

360

289

182

4

"

2

"

6 mos.

492

530

547

607

663

5

"

2

"

1 yr.

487

515

610

588

647

6

"

2

"

2 yrs.

533

586

612

659

684

7

"

0

"

7 da.

562

608

726

709

432

8

"

0

"

6 mos.

745

751

799

804

795

9

"

2

"

7 da.

288

347

372

352

165

10

"

2

"

6 mos.

532

538

624

638

642

11

"

2

"

1 yr.

591

595

643

645

666

12

"

2

"

2 yrs.

590

623

680

673

666

13

"

0

"

7 da.

351

368

405

450

204

14

"

0

"

6 mos.

560

606

580

645

797

15

"

2

"

7 da.

227

258

261

282

96

16

"

2

"

6 mos.

494

546

591

574

563

17

"

2

"

1 yr.

•572

580

586

583

652

18

"

2

"

2 yrs.

592

575

592

592

667

19

S

2

176° Fahr.

5 da.

296

307

362

391

422

20

R

2

140° "

5 da.

403

440

416

495

442

21

X

2

140° "

5 da.

361

334

390

452

474

Table 91. Plaster Of Paris In Natural Cement Mortars, Hardening In Water

Ref.

Cement,

Natural Brand.

Sand,

Parts to One

Cement.

Temperature water Where Stored.

Age of

Briquets When

Broken.

Tensile Strength, Pounds per

Square Inch, with Per Cent.

of Cement Replaced by

Plaster of Paris.

0

1

2

3

6

i

Degrees F.

1

An

0

60-65

7 da.

233

225

213

235

a

2

"

0

"

6 mo.

422

449

438

441

324

3

"

2

"

7 da.

111

109

97

144

a

4

"

2

"

6 mo.

418

416

435

409

133c

5

"

2

"

1 yr.

415

451

430

454

6

"

2

"

2 yrs.

478

476

489

514

7

Gn

0

"

7 da.

146

156

115c

a

a

8

"

0

"

6 mo.

383

398b

323

312e

234f

9

"

2

"

7 da.

62

80

94

a

a

10

"

2

"

6 mo.

374

312

355

86f

151f

11

"

2

"

1 yr.

448

395

408

131f

107f

12

"

2

"

2 yrs.

456

437

397

172f

a

13

An

2

140

5 da.

319

365

405

402

203

14

Gn

2

"

"

359

351

189

138

100

Note. — Sand, Point aux Pins (river sand) passing No. 10 sieve, except for hot tests, where standard sand was used. a — Found badly swelled and nearly disintegrated after a few days in tank.

b — Surface cracks, 1 inch section swelled to 1 1/16 inches.

c — Surface cracks, 1 inch section swelled to 1 1/12 inches. Had nearly disintegrated after 2 days. d — Surface cracks. e — Badly cracked on surface.

f — Badly cracked on surface, and 1 inch section swelled to about 1 1/16 inches.

378. The briquets reported in the preceding tables were hardened in water, as usual. Table 92 gives some of the results obtained by adding plaster Paris to mortars that are hardened in dry air. The effects on the two samples of the same brand of Portland, one quick setting and one slow setting, are quite different. The strength of the quick setting sample is increased, two per cent, giving the best results, while that of the slow setting sample is diminished by the addition of plaster. Both brands of natural cement appear to be notably improved by the plaster, the best result being given by three per cent. Such an addition to one brand results in a remarkable increase in strength of 250 per cent.