Cement

  • Cement mortar

    Cement is made by heating a finely ground mixture of clay and limestone, and water, to a temperature at which the clay and lime¬stone fuse into a clinker. The clinker is ground to a fine powder called cement. The cement most commonly used is ordinary Portland cement which is delivered to site in 50 kg sacks. When the fine cement powder is mixed with water a chemical action between water and cement takes place and at the completion of this reaction the nature of the cement has so changed that it binds itself very firmly to most materials.

    The cement is thoroughly mixed with sand and water, the reaction takes place and the excess water evaporates leaving the cement and sand to gradually harden into a solid mass. The hardening of the mortar becomes noticeable some few hours after mixing and is complete in a few days. The usual mix of cement and sand for mortar is from 1 part cement to 3 or 4 parts sand to 1 part of cement to 8 parts of sand by volume, mixed with just sufficient water to render the mixture plastic.

    A mortar of cement and sand is very durable and is often used for brickwork below ground level and brickwork exposed to weather above roof level such as parapet walls and chimney stacks.

    Cement mortar made with washed sand is not as plastic however as bricklayers would like it to be. Also when used with some types of bricks it can cause an unsightly effect known as efflorescence.

    This word describes the appearance of an irregular white coating on the face of bricks, caused by minute crystals of water soluble salts in the brick. The salts go into solution in water inside the bricks and when the water evaporates in dry weather they are left on the face of bricks or plaster. Because cement mortar has greater compressive strength than required for most ordinary brickwork and because it is not very plastic by itself it is sometimes mixed with lime and sand.

  • MORTAR FOR BRICKWORK AND BLOCKWORK

    Clay bricks are rarely exactly rectangular in shape and they vary in size. Some facing bricks are far from uniform in shape and size and if a wall were built of bricks laid without mortar and the bricks were bonded the result might be as shown, exaggerated, in Fig.

    Because of the variations in shape and size, the courses of bricks would not lie anywhere near horizontal. One of the functions of brickwork is to support floors and if a floor timber were to bear on the brick marked A it would tend to cause it to slide down the slope on which it would be resting. It is essential, therefore, that brickwork be laid in true horizontal courses, and the only way this can be done with bricks of differing shapes and sizes is to lay them on some material which is sufficiently plastic, while the bricks are being laid, to take up the difference in size, and which must be able to harden to such an extent that it can carry the weight normally carried by brickwork.

    The material used is termed mortar. The basic requirements of a mortar are that it will harden to such an extent that it can carry the weight normally carried by bricks, without crushing, and that it be sufficiently plastic when laid to take the varying sizes of bricks. It must have a porosity similar to that of the bricks and it must not deteriorate due to the weathering action of rain or frost.

    Sand is a natural material which is reasonably cheap and which, if mixed with water, can be made plastic, yet which has very good strength in resisting crushing. Its grains are also virtually impervious to the action of rain and frost. The material required to bind the grains of sand together into a solid mass is termed the matrix and the two materials used for this purpose are lime or cement.

  • Dense aggregate blocks for general use

    The blocks are made of Portland cement, natural aggregate or blastfurnace slag. The usual mix is 1 part of cement to 6 or 8 of aggregate by volume. These blocks are as heavy per cubic metre as bricks, they are not good thermal insulators and their strength in resisting crushing is less than that of most well burned bricks. The colour and texture of these blocks is far from attractive and they are usually covered with plaster or a coat of rendering. These blocks are used for internal and external loadbearing walls, including walls below ground.

  • Sulphate resisting Portland cement

    This cement has a reduced content of aluminates that combine with soluble sulphates in some soils and is used for concrete in contact with those soils.

  • Portland blast-furnace cement

    This cement is more resistant to the destructive action of sulphates than ordinary Portland cement and is often used for concrete foundations in plastic clay subsoils. This cement is made by grinding a mixture of ordinary Portland cement with blast-furnace slag. Alternatively another type of cement known as ‘sulphate resisting cement’ is often used.

  • Concrete mixes

    British Standard 5328: Specifying concrete, including ready-mixed
    concrete, gives a range of mixes. One range of concrete mixes in the Standard, ordinary prescribed mixes, is suited to general building work such as foundations and floors. These prescribed mixes should be used in place of the traditional nominal volume mixes such as 1:3:6 cement, fine and coarse aggregate by volume, that have been used in the past. The prescribed mixes, specified by dry weight of aggregate, used with 100 kg of cement, provide a more accurate method of measuring the proportion of cement to aggregate and as they are measured against the dry weight of aggregate, allow for close control of the water content and therefore the strength of the concrete.

    The prescribed mixes are designated by letters and numbers as C7.5P, C10P, C15P, C20P, C25P and C30P. The letter C stands for ‘compressive’, the letter P for ‘prescribed’ and the number indicates the 28-day characteristic cube crushing strength in newtons per square millimetre (N/mm2) which the concrete is expected to attain. The prescribed mix specifies the proportions of the mix to give an indication of the strength of the concrete sufficient for most building purposes, other than designed reinforced concrete work.

    Table 1 equates the old nominal volumetric mixes of cement and aggregate with the prescribed mixes and indicates uses for these mixes.

  • Water-cement ratio

    The materials used for making concrete are mixed with water for two
    reasons. Firstly to cause the reaction between cement and water which results in the cement acting as a binding agent and secondly to make the materials of concrete sufficiently plastic to be placed in position. The ratio of water to cement used in concrete affects its ultimate strength, and a certain water-cement ratio produces the best concrete. If too little water is used the concrete is so stiff that it cannot be compacted and if too much water is used the concrete does not develop full strength.

    The amount of water required to make concrete sufficiently plastic depends on the position in which the concrete is to be placed. The extreme examples of this are concrete for large foundations, which can be mixed with comparatively little water and yet be consolidated, and concrete to be placed inside formwork for narrow reinforced concrete beams where the concrete has to be comparatively wet to be placed. In the first example, as little water is used, the proportion of cement to aggregate can be as low as say 1 part of cement to 9 of aggregate and in the second, as more water has to be used, the proportion of cement to aggregate has to be as high as say 1 part of cement to 4 of aggregate. As cement is expensive compared with aggregate it is usual to use as little water and therefore cement as the necessary plasticity of the concrete will allow.

  • Concrete

    The mixture of Portland cement, sand, gravel, and water. Used to make garage and basement floors, sidewalks, patios, foundation walls, etc. It is commonly reinforced with steel rods (rebar) or wire screening (mesh).

    Concrete block — A hollow concrete “brick” often 8 ×8 ×16 in size.
    Concrete board—Apanel made out of concrete and fiberglass usually used as a tile backing material.
    Concrete plain — Concrete either without reinforcement, or reinforced only for shrinkage or temperature changes.

  • Cement, Keene’s

    A white finish plaster that produces an extremely durable wall. Because of its density, it excels for use in bathrooms and kitchens and is also used extensively for the finish coat in auditoriums, public buildings, and other places where walls may be subjected to unusually hard wear or abuse.

  • Cement

    The cement most used is ordinary Portland cement. It is manu-factured by heating a mixture of finely powdered clay and limestone with water to a temperature of about 1200°C, at which the lime and clay fuse to form a clinker. This clinker is ground with the addition of a little gypsum to a fine powder of cement. Cement powder reacts with water and its composition gradually changes and the particles of cement bind together and adhere strongly to materials with which they are mixed. Cement hardens gradually after it is mixed with water.

    Some thirty minutes to an hour after mixing with water the cement is no longer plastic and it is said that the initial set has occurred. About 10 hours after mixing with water, the cement has solidified and it increasingly hardens until some 7 days after mixing with water when it is a dense solid mass.

    The gray powder that is the “glue” in concrete. Portland cement. Also, any adhesive.

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