Hydraulic cement is made artificially by a process similar to that already described for artificial hydraulic limes (see p. 155), a higher proportion of clay being added to make the mixture resemble the composition of a natural cement stone.

The twice-kilned lime is not however used, but the raw limestone or chalk is if necessary crushed by machinery before mixing.

The cements usually manufactured are of a heavy slow-setting character, and require to be calcined at a high temperature, which produces incipient vitrification. As it is impossible to maintain a perfectly uniform temperature all through the mass, the result is a mixture of products of different degrees of calcination, including half-raw under-burnt portions of light yellow cement, and dense heavy clinker (see p. 193).

A judicious selection of them for grinding, and more especially the rejection of the under-burnt portions, is essential to the production of good and uniform cement.

1 Min. Proceedings Civil Engineers, vols. xxv. and xxxii.

As the best of the cements are burnt to the state of clinkers the subsequent breaking and grinding are tedious and costly operations. Fine grinding is however most essential to properly develope the strength of the cement when used, as it commonly is, with sand.

Portland Cement is so called from a fancied resemblance in its colour to Portland stone.

It is by far the most valuable of all the cements, and is made by intimately mixing and calcining together substances of different kinds, so as to obtain a material containing, as a general rule, when burnt some 58 to 63 per cent of lime combined with about 22 per cent of soluble silica - 7 to 12 per cent of alumina - and small percentages of oxide of iron, magnesia, etc. (see p. 241).1

The materials used may be either chalk and clay - which are mixed by the wet process - or limestone and clay or shale mixed by, the dry process.

Manufacture From Chalk And Clay

The cement best known in this country is made on the banks of the Thames and Medway, from chalk and clay mixed by the wet process.

The proportion of chalk and clay mixed together depends upon the composition of the chalk before burning. The result required is to obtain a mixture containing before burning some 23 to 26 per cent of clay.2

With white chalk (which itself contains no clay) 3 volumes of chalk are mixed with 1 volume of alluvial clay or mud from the lower Thames or Medway.

If the chalk itself contains clay, the proportion of clay added is modified accordingly.

For example, with grey chalk, 4 parts of chalk are used to 1 of clay.

The chalk and clay are mixed in water to the condition of a creamy liquid, which is called "slurry," the fine particles in suspension are allowed to settle in large tanks, reservoirs, or "backs," for several weeks, and when the deposit becomes nearly solid, the water is run off, the residue is dug out, sometimes pugged, dried on iron plates over coking ovens, or over the flues from the kiln, burnt in intermittent kilns (see p. 189), at a very high temperature, and then ground to a fine powder.

This method of manufacture is of course applicable only when the materials to be mixed can easily be liquefied in water.

The above is the wet process as ordinarily practised on the Thames and Medway, but in very modern works modifications have been introduced, some of which may be mentioned.

Under the patents of Messrs. I. C. Johnson & Co. of Greenhithe the undermentioned processes have been adopted at their various works, and some of them have been introduced at other works.

The chalk and clay are mixed with much less water (only about 10 per cent) to the consistency of batter pudding. The slurry thus formed is passed through gratings into a pit, whence it is lifted by buckets fixed to the circumference of a vertical revolving wheel, and passed through millstones which grind it to a minute degree of fineness.

1 For very light quick-setting cements the proportion of lime is considerably less (see p. 241).

2 28 or even 30 per cent of clay may be used for light quick-setting cements used for stuccoing.

The mixture is then pumped up and spread over the floor of a large arched chamber which branches out from the kiln at a height of about 15 feet above the fire bars. The top of the kiln is closed, so that the waste heat and gases have to pass through this chamber and over the surface of the slurry, which is thus quickly and thoroughly dried. It is then burnt in the usual manner.

It will be seen that by this system the backs are rendered unnecessary. This is a great advantage, for they take up much room; moreover, when slurry is allowed to subside in a deep back the heavier particles have a tendency to separate from the other, so that the resulting material is not uniform in composition; and lastly, in using this system, any slurry found by analysis to be defective can more easily be dealt with than it can in the large mass contained in a reservoir.

The method of drying the slurry is effective and also economical. It utilises the whole heat from the kiln, a large proportion of which in ordinary kilns escapes at the top.

Ransome's system consists in burning the dried slurry in a revolving iron chamber lined with fire-brick, and fed with waste-gases on the same system as in a regenerative furnace (see p. 305). By this process it is produced in the form of a coarse powder, thoroughly burnt through, in the course of about half an hour, instead of spending seven days in a kiln. Among the advantages claimed for this system are, economy in space, fuel, grinding, and time, and improvement in quality by the exclusion of the fuel from the cement.1