The cements used in building and engineering works are calcareous substances, similar in many respects to the best hydraulic limes, but possessing hydraulic properties to a far greater degree.

They may be divided into two classes -

1. Natural Cements.

2. Artificial Cements.

They are distinguished from limestones by not slaking or breaking up when mixed with water after calcination.

Cements are used chiefly in foundations in wet places; in subaqueous work of all kinds; for important structures, where great strength is required, such as dock walls and lighthouses, also for making concrete and cement mortar.

The more exposed parts of ordinary structures, such as the copings of walls, are frequently built in cement, also the tops of chimneys.

Cements are also used in the walls of cesspits, the joints of drains, etc.; for protecting the outer faces of walls and buildings from the weather; for thin walls where extra strength is required; for pointing, filleting, and many minor purposes.

Natural Cements

Natural cements are burnt direct from stones containing from 20 to 40 per cent of clay, the remainder consisting chiefly of carbonate of lime alone, or of carbonate of lime mixed with carbonate of magnesia.

Carbonate of Magnesia by itself, when calcined, yields anhydrous magnesia, which does not slake like quicklime, but if powdered and made into paste sets through its whole mass, permanently expanding, but not breaking up. It is soluble in water, but not so readily as lime.

Cement Stones or Nodules are frequently found in thin strata, amongst those of hydraulic limestone. They are usually brown or fawn-coloured, of compact texture, and with an earthy fracture.

Those met with in this country generally contain a large proportion of clay (about 30 or 32 per cent), are burnt at a low temperature, and yield a quick-setting cement of no great ultimate strength.

These stones will not bear much heat without fusing, as they contain a large proportion of iron (see p. 236).

Stones containing a lower proportion of clay (about 22 per cent) are strongly burnt, and yield a heavy slow-setting cement.

The natural cement found at Boulogne (see Table, p. 151) is of this character, and a similar description has been met with at Rugby; but slow-setting natural cements are rare in this country.

More than 40 per cent of clay injures the cement. If the stone is half clay, it should be used as a "pozzuolana" (see p. 190); if there is more than two-thirds clay, it will not set under water.

Slaking And Setting

Lumps of burnt cement stones are hardly affected by water; when ground to powder and wetted, they produce a paste which, without any preliminary slaking action, sets under water in from five minutes to as many hours, and acquires within a year a strength varying from that of soft brick to that of the stronger kinds of stone; the differences in setting powers and strength depending upon the composition of the stone.

The shrinkage of cements setting in air is very slight, the paste being much denser than that made from lime, in consequence of the absence of the expansion caused by slaking.

Roman Cement (originally called Parker's Cement) is made by calcining nodules found in the London clay. These contain from 30 to 45 per cent of clay; before being burnt they have a fine close grain, pasty appearance, and greasy surface when broken.

The burning is conducted at a low temperature and requires great care.

The colour of the calcined stone is generally a rich brown, and is no guide to the quality of the cement.

Weight And Strength

Good Roman cement should not weigh more than 75 lbs. per bushel, and should set very quickly (within about 15 minutes of being gauged into paste), but attains no great ultimate strength (see Table, p. 159).

Specifications should mention a minimum weight for these and similar cements, for a heavy cement is likely to be over-burnt, and moreover a stale cement will have become heavier by absorption of carbonic acid from the air.

The little strength possessed by Roman cement rapidly diminishes on the addition of sand.

1 or 11/2 part of sand to one of cement is the greatest proportion that should be added.

Storing

Roman cement is sold in a ground state, and kept in casks, which must be kept carefully closed and dry, otherwise the cement will absorb carbonic acid and become inert. For the same reason it is important to examine this cement carefully before using it.

Uses

It should be mixed in very small quantities and used at once, and on no account beaten up again after the setting has commenced.

The properties of Roman cement make it valuable for temporarily pointing joints in work to be done and set between tides, and for other purposes where quick setting is desirable, and no great ultimate strength is required.

It is also used for external rendering or stucco, but is liable to efflorescence on the surface, which presents an unsightly appearance (see p. 238).

Market Forms

Roman cement is usually sold in casks; sometimes, if it is to be used at once, in sacks.

The inside dimensions of the casks are 2 feet 4 inches high, 1 foot 41/2 inches diameter at middle, 1 foot 31/2 inches diameter at ends.

Each cask usually contains 31/2 trade bushels1 of 70 lbs. each - i.e. 245 lbs.

The sacks measure 3 feet 7 inches by 2 feet, and contain 3 trade bushels - i.e. 210 lbs.

Medina Cement is made from the septaria found in Hampshire and the Isle of Wight, and from those dredged up out of the bed of the Solent.

It sets very rapidly, is of a light brown colour, and resembles Roman cement inits characteristics, butis stronger for the first three months (see Table, p. 159).

It is sold in casks containing 31/2 trade bushels of 68 lbs. each, or sacks containing 3 bushels.

Harwich and Sheppy Cements are similar materials made from nodules found in the London clay at Harwich and Sheppy.

1 There are two kinds of bushel used in connection with cements : - (1) The "striked bushel," being a measure containing 1.28 cubic feet, lightly filled, and struck smooth at the top with a straight edge (see p. 168) - 21 of these bushels go to a cubic yard; (2) The trade bushel, which is a given weight established by practice, and varying for each cement. The weights of trade bushels of different kinds of cement are given at p. 256.

Unless cement is ordered by weight, there is likely to be some confusion between the two kinds of bushel above mentioned. It is desirable where possible to order cement by the ton net.

Whitby, Mulgrave's, or Atkinson's Cement is made from the septaria of the Whitby shale beds of the Lias formations in Yorkshire. It is something like Portland cement in colour, takes slightly longer to set than Roman cement, and absorbs more moisture, but resembles it in its characteristics generally.

Calderwood Cement is a variety of Roman cement of a dark colour from nodules found in Scotland.

East Kilbride in Lanarkshire furnishes a very similar cement.

The following Table, compiled from different sets of experiments by Mr. Grant,1 shows the strength of two different samples of Roman cement, and of one of Medina cement, and also the weakening effect of sand when added to one of the former : -

Age and Time immersed in

Water.

Roman Cement.

Medina

Cement.

Neat.

Sample A. Neat.

Sample B. Neat.

1 Cement

(B) 1 Sand.

1 Cement (B)

2 Sand.

1 Cement

(B) 3 Sand.

7 Days .

202

120.5

47.5

7.0

10.0

211.0

14 „ .

173

169.9

65.6

42.8

19.2

303.4

21 „ .

186.5

155.2

74.2

45..9

17.4

298.0

1 Month

260.3

358.2

81.2

41.9

..

306.0

3 Months

322.5

220.4

121.9

91.75

....

448.8

6 ,, .

4727

252.5

314.3

..

..

412.4

9 „ •

471.1

251.5

...

..

..

457.2

12 „ .

643.1

268.5

..

...

..

476.9

2 Years .

546.3

..

..

...

..

276.0

N.B. - The sectional area of the briquette was 21/4 square inches.