Rapid Hardening Portland Cement

The manufacturing process is similar to that of normal Portland Cement, the chief difference being in the degree of fineness, in the grinding which results in a more rapid rate of strength gain, but not in a quicker setting time.

White and Coloured Portland Cements : general method of manufacture is again similar but china clay is used in making white cement in order to exclude iron oxides.

Portland and Blast-furnace Cement:

Normal Portland Cement clinker is mixed with up to 65 per cent of blast-furnace slag for the grinding process.

High Alumina Cement:

This is manufactured by bringing aluminous and calcareous materials to а соmpletely melted state; it is then cast into pigs, broken up and ground in powder. The special properties of this cement are due to its chemical compo­sition which includes a higher proportion of alumina than is found in Portland cements. This alumina content is obtained by using the mineral bauxite instead of ordi­nary clay.

Cement- rate of setting

It is necessary to distinguish, between setting and hardening. She term "setting" relates to the stiffenning of the mix due to chemical processes; "hardening" is a later and mere physical-chemical process. The "setting" time includes the "initial set" and the "final set", and must not be confused with the rate of strength gain. To the architect the important point is that once the sett­ing has commenced (the initial set) the cement mixture should not be disturbed. This means that specifications must call for the mixing and placing of cement products to bе completed within a certain time. The completion of the "final set" does not mean that the cement pro­duct will not cease to change its properties.

Appearance.

The use or ordinary Portland Cement tends to give a grey appearance . Considerable attention must be given to the effect of this upon design. High alumina cements give a much darker grey colour and should not bе used for exposed work without careful thought as to their appearance. White or coloured сеments can of course be used where special effects are required and where their extra cost is justified.

Portland cement is attacked to some extent by sea water, bу sulphates in soil or by various substances. It is usually true that high alu­mina cement will be more resistant than Portland Cement to such attack. Wet cement mixes are liable to set up slight corrosion of lead and zink. Cement products are difficult to decorate with oil paints particularly when they new. For mortar and plaster cement is frequently with lime, but only some types of lime can be used in this way. High alumina cement must not be mixed with lime, nor can be mixed with other type of cement. Once Portland or high alumina cement is hard, however, a mix­ture containing the other type can be placed alongside without any risk.

A small carefully regulated amount of gypsum is added to Portland cements during manufacture. Any further ad­dition would interfere with the setting time, so that these cements must not be mixed with gypsum plasters. The presence of organic materials may interfere with the setting of cement. For this reason the mixing water and the aggregate must be reasonably clean.

The manufacture of cement

The raw materials used in the manufacture of cement are chalk, slate and clay. When these substances are heated, lime is obtained from the limestone and chalk. The decomposition of the clay and slate produces oxides of silicon and aluminium. In recent years slag obtained fгоm blast furnace in the manufacture of iron hаs been used to provide the oxides of silicon. The quantities of the raw material used in the manufacture of the cement are so arranged that the resulting cement contains 2/3 of lime and the remainder oxides .of aluminium and silicon. These raw materials (limestone, clay, etc.) are ground vеrу finely and mixed with water to form a slurry. This liquid material or slurry is fed into the upper part of the heat­ing kiln. This kiln may be nearly 300 feet in length and from 6-9 feet in diameter. It is inclined about 15° to the horisontal plain and is heated so that an inside tem­perature of 150O°C is obtained. The kiln rotates slowly as the slurry slides down to the lower end of the kiln. During the heating chemical combination takes place bet­ween the calcium, silicon and aluminium oxides and cement clinkers are formed. These are extracted from the lower end of the kiln and ground into a powder. The powder then mixed with a little gypsum or calcium sulphate (about 1 part in 50). The gypsum is added to prevent the cement from setting too rapidly when it is mixed with water.

Cement mortars.

A cement mortar is a mixture of cement and sand with water, usually one part by volume of cement to 3 parts by volume of sand. A cement mortar sets end hardens well, and can be used under water if precautions are taken to prevent the absorption of water. This may be done by adding to cement a suitable chemical water proofing material.

Reduction in Volume on mixing lime and cement mortars.

When 1 cu. ft. of lime or cement is mixed dry with 4ft. of sand, the volume of the resulting mixture is less than 5 cu. ft. An even greater reduction in volume takes place when water is added to the mixture. The esti­mation of this contraction in volume when cement is mixed is very important to builders. It еnables them to calculate how much of each constituent is required to produce a given volume of material, and also provides a good indication of the density and strength of the final product.

CONCRETE

If crushed brick or stone is added to a mixture of sand and cement and the whole mass thoroughly mixed with water, setting readily takes place; the final product is concrete. Concrete is not a modern building material; it was used bу the Romans and most of their buildings were constructed of concrete. The strength of any concrete is| influenced by the following factors:

1) The amount of water in relation to the cement used.

2) This is called water-cement ratio. Тhe better mixture will be produced when less water is added.

3) The nature of the sand and coarse aggregates used. Well washed sand and aggregates, free from loam, clay and other impurities, should be used, as the presence of these impurities weakens the resulting concrete. If careful selection is made of properly graded materials a denser and a stronger concrete will bу obtained.

4) The temperature at which the concrete is placed. Low temperatures have a retarding effect on the setting and hardening of cement. Unless special precautions are taken the concrete work should not be done when the temperature is at freezing point or below. During cold weather the concrete should be covered immediately after it has been laid.

5) The retention of the moisture in the concrete during the early stages after mixing and placing. The hardness and the strength of concrete depends on the chemical changes which take place between the cement and the water. If the concrete is allowed to dry out too quickly after it has been laid, the moisture evaporates before the chemical changes have taken place and a weak concrete is produced. The moisture in freshly laid concrete should therefore retained by spreading over it stripe of damp sacking. These materials are removed after the concrete bas become firm. The surface of the concrete is then kept moist by frequent applications of water. Concrete is an important building material. Its use has led to the discovery and development of reinforced concrete - а material of great value in the building industry.

ЕXERCISES:

I. Find out all the words with "ed" endings and state their functions.