Cement Manufacture

Portland cement is made by burning to clinkering temperature an intimate mixture of calcareous and clayey material of a limited range of composition. It is essential that a finely ground homogeneous mixture should be used. This may be obtained either by a wet or a dry process.

Wet Process

The raw materials, of which chalk and clay may be taken as typical, are reduced to a creamy " slip " or " slurry " by agitation in a large vat with water. The slurry is roughly screened and passed through a wet tube mill to complete the breaking-up of the coarser particles. Ultimately, the mixture should become so fine that only 2.5 per cent, is retained by a sieve of mesh 5000 per sq. cm. It is finally dried on the floors of drying chambers by waste heat from the kilns, when it shrinks and cracks into blocks of a size convenient for handling and loading into the kilns, or it may be introduced without drying.

Dry Process

This process, which is more economical in fuel, is now more generally used, especially for hard materials such as limestone, shale, and slag. These are coarsely crushed and then dried in a rotating cylinder. They are afterwards mixed in the requisite proportions and very finely ground. The fine powder is separated from the coarse by sieves or an air-current. If it is to be burnt in stationary kilns it is moistened and pressed into bricks.

The old-fashioned type of kiln is charged with alternate layers of coke and lumps of dried slurry or pressed bricks. Combustion is maintained by natural draught, and the process lasts four or five days. The fuel consumption is high and the temperature variable, but economies may be effected, as in the Dietsch kiln, by using the hot kiln gases for preheating the raw materials, and the hot clinker, as it reaches the bottom of the kiln, to heat the entering air. Sintering and partial fusion should take place. It cannot be overburnt. The clinker is extremely hard and compact, but contains innumerable minute pores and gas cavities. It is of a dark grey colour with a slight blue or green tint.

Stationary kilns have now been largely replaced by the rotary kiln, which is a cylindrical steel tube, 210 feet long and 8 feet in diameter, increasing to 9 feet near the firing end, and lined with firebricks protected by cement clinker. It is supported at a slight inclination to the horizontal and is slowly rotated. The fuel most commonly used is finely powdered bituminous coal, injected into the lower end of the kiln by a current of compressed air. The slurry of finely powdered material is introduced at the top, and the temperature increases progressively from top to bottom. There are three stages in the process: first the mixture is dried and organic matter in the clay burnt off; next the calcium carbonate is decomposed; and, finally, at about 1400°-1500° C., the lime reacts with silica and alumina, producing a sintered mass of clinker. The process is continuous, the clinker escaping at the lower end. A kiln such as the one described normally produces 7½ tons of clinker per hour with a fuel consumption 28 per cent, of the weight of cement.

The clinker is then very finely ground, often along with a small quantity of gypsum or some other substance, the purpose of which is to delay setting. A certain amount of steam is frequently admitted into the grinding mill, also with a view to modifying the setting properties by the action of the small fraction of the lime thus set free by hydrolysis.

An inconvenient and even dangerous amount of dust is produced in the grinding, but various mechanical devices have been invented for trapping it, and incidentally the dust thus collected has since proved a useful source of potash.

An average Portland cement has the following percentage composition: SiO₂, 21-23; Al₂O₃, 6-9; Fe₂O₃, 2-4; CaO, 61-63; MgO, 1-3; SO₃, 1-1.5; CO₂ + H₂O, 1-2.5; and sulphide sulphur, 0-0.1.

An increase in the proportion of lime yields a stronger cement until a certain limit is reached above which the cement becomes unsound, cracking or even disintegrating owing to the expansion after setting. Increase in alumina or ferric oxide hastens setting. Ferrites have a lower melting-point than aluminates and so facilitate sintering, as also do alkalies and magnesia.

If free from iron a white cement, used for decorative work, results.

By replacing a large proportion of the alumina by ferric oxide, iron Portland cement, a German cement, is obtained. It is much more satisfactory in marine work than ordinary cement.

Slag cements are made from blast-furnace slag, which is granulated in water and is substituted for clay in the manufacture of Portland cement. If granulated under certain conditions, it may be used as cement without further heating, for example Passow cement, patented in 1901.

Natural cements may be prepared from rock containing lime, silica, and alumina in approximately the correct proportions. They are variable in quality owing to variation in the composition of the rock, and they cannot compete with Portland cement.

Pozzuolanic materials are not cements but burnt clayey materials in which the silica is present in an active form so that, on mixing with lime and then gauging with water, they set like cement.

A mortar made from cement mixed with sand, and usually lime or loam, is employed for structures exposed to the action of running water or waves.

When cement is treated with water, a plastic mass is first formed, and, after becoming friable, finally sets. The hardness then gradually increases until a stony texture is ultimately obtained. The setting of cement is influenced by the presence of foreign substances much in the same way as plaster of Paris.