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.
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.
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 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.
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.
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.
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.
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.
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.
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
Sample A. Neat.
Sample B. Neat.
(B) 1 Sand.
1 Cement (B)
(B) 3 Sand.
7 Days .
14 „ .
21 „ .
6 ,, .
9 „ •
12 „ .
2 Years .