Cement

 1 Cement

 The word cement means ‘to cohere or unite’. Cement can be identified as an inorganic material which when mixed with water gradually sets to give a hard mass. This is used to join (or cement) together bricks, stones, etc. to create different structures. Cement is in use since antiquity. The Egyptian pyramids and many of the ancient Greek structures were constructed by using some kind of cementing material. There are different types of cements like, natural cement, Puzzolana cement, slag cement and Portland cement, etc. These differ in their composition, mode of preparation and applications. We shall discuss about the Portland cement which is currently the most commonly used cement in the construction works. In India manufacture of Portland cement started about a hundred years ago (1904) by South India Industries Ltd.. Today, Indian cement industry manufactures over 100 million tonnes of cement per year. 

1.1 Manufacture of Portland Cement

 Portland cement was invented in 1824 by calcinating (or burning) argillaceous limestone ( limestone containing 20-40 % of clay). It was called as Portland cement because the concrete ( a mixture of cement and gravel or sand) obtained from it resembled the building stone from Isle of Portland in England. Today many types of Portland cement with different compositions are available for different applications. 

(a) Raw Materials : a number of raw materials are required for the manufacture of Portland cement. These can be broadly put into two categories :

     (i) Calcareous ( rich in calcium) material : like lime stone ( CaCO3), chalk etc. are the principal constituent of cement and need to be used in appropriate amounts. The excess of lime or its deficiency, both reduce the strength of the cement. 

    (ii) Argillaceous material : These are rich in silica and alumina for example, clay ( a mixture of Al2O3 , and SiO2) , shale, slate or volcanic material etc. These impart strength to the cement and affect the setting properties of the cement. In addition to the above, powdered coal or fuel oil and gypsum (CaSO4 .2H2O) are also used in the manufacture of cement. Addition of gypsum controls the setting time of the cement. The amount of gypsum has to be carefully controlled because if it is present in more than required amounts it causes cracks. 

(b) Manufacturing process : To prepare cement the raw materials are mixed in appropriate amounts, crushed to powder and then calcinated (burned or fired) There are two types of manufacturing processes wet process and dry process dependingon whether the mixing and grinding of raw materials is done in wet or in dry conditions.

         (i) Wet process : in this method the raw materials are treated with water ( about 35-40 % by mass) and then powdered in ball mills. These raw materials can be treated individually or in the form of a mixture. The paste like material so obtained is then sieved and powdered further if necessary. This is then passed through a rotary kiln  for calcination. Rotary kiln used in the manufacture of cement. The furnace consists of a slightly inclined long pipe that revolves slowly around its own axis. The mixture paste is fed into the long pipe through a hopper installed at the top of the pipe. As it moves down it meets the hot gases going upwards. In the high temperature of the kiln, the raw materials undergo a number of chemical reactions to form many new compounds. The mixture comes out of the kiln in the form of small greenish-black or grey coloured hard balls known as clinkers which falls into a second pipe called cooler. These clinkers are then allowed to cool and powdered. To this powder, a small amount (2-3%) of gypsum is added and powdered again to obtain cement.

         (ii) Dry method: in this method, the raw materials are mixed and powdered in ball mills. This is then placed into rotating panniers whose walls are fitted with water sprays. In the panniers, the centrifugal force keeps the material in contact with the wall where it gets somewhat wet and takes the shape of small spheres. These are then passed through the rotary kiln as described above.

 (c) Composition of cement The Portland cement contains calcium aluminum silicates. It contains more than one compound. The approximate percentage of different elements present in Portland cement are expressed as their oxides.

Table 36.1 : Approximate percentage composition of Portland cement

Oxide                                    Approximate percentage                     Average  percentage

CaO                                    60-67                                                    64

SiO2                                    17-25                                                    22.5

Al2O3                                 3-8                                                        6.5

Fe2O3                                0.5-6.0                                                    2.0

MgO                                    0.1-4.0                                                    2.0

SO3                                    1.0-3.0                                                    1.5

K2O, Na2O                        0.4-1.3                                                     –

The first four of the oxides listed above, provide the basic compounds while the rest are referred to as secondary compounds. The oxides listed above do not exist as such. CaO and MgO absorb moisture and CO2 from the atmosphere to give hydroxides and carbonates. Chemistry of Portland cement has been well understood. It contains different compounds which are known as Bogue’s compounds. They are so named because they were identified by R.H.Bogue. 

36.1.4 Applications of Cement

Cement is generally not used as such, it is normally mixed with certain fillers or additives

like sand, crushed stone, gravel, slag, etc. Depending on the nature of the filler or additive

and the composition there are three broad types of mixtures. These are

(I)  Mortar

(ii)  Concrete and  

(iii) Reinforced Concrete Construction (R.C.C)

(i) Mortar : It is obtained by adding water to a mixture of cement and sand. The resulting

paste is called cement-mortar. Mortar finds applications in binding bricks and stones,

etc. and also in plastering the walls.

(ii) Concrete : it is formed by making a mixture of cement and fine and coarse inert

mineral aggregates like sand, coarse rock, stone, gravel or slag etc. The concrete can be

put into any desired shape. Concrete finds applications in making roads, roofs, columns in

the building, foundation works etc.

(iii) Reinforced Concrete Construction (R.C.C) : sometimes the constructed structure,

say a bridge, has to take a lot of load. A plain concrete construction is not appropriate for

such an application because concrete has low ability to withstand tension. In such cases,

the structure is strengthened ( or reinforced) by incorporating a net work of iron or steel

rods which helps to take up load. This type of construction is called Reinforced Concrete

Construction (R.C.C) and is used in the construction of bridges, arches, roofs girders,

floor beams, etc.