In this article we will discuss about:- 1. Characteristics of Cement 2. Properties of Cement 3. Composition 4. Functions of Ingredients 5. Harmful Constituents 6. Setting Action 7. Site for Factory 8. Packing 9. Ball Mills and Tube Mills 10. Field Tests 11. Storage 12. Uses.
Characteristics of Cement:
Following are the characteristics of cement:
(i) The colour of cement should be uniformly grey with greenish shade. It gives an indication of excess of lime or clay and the degree of burning.
(ii) It should feel smooth when touched or rubbed in between fingers.
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(iii) If hand is inserted in a bag of cement or in a heap of cement, it should feel cool and not warm.
(iv) It should be free from any hard lumps.
(v) It should not contain excess silica, lime, alumina or alkalies.
(vi) Excess amount of clay and silt in cement gives an earthy cement.
Properties of Cement:
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Following are the physical, mechanical and chemical properties of cement-
Physical Properties of Cement:
Following are the important physical properties of good cement which primarily depend upon its chemical composition, thoroughness of burning and fineness of grinding:
(i) It gives strength to the masonry.
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(ii) It is an excellent binding material.
(iii) It is easily workable.
(iv) It offers good resistance to the moisture.
(v) It possesses a good plasticity.
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(vi) It stiffens or hardens early.
(vii) A thin paste of cement with water should feel sticky between the fingers.
(viii) A cement thrown in water should sink and should not float on the surface.
(ix) The particles should have uniformity of fineness. To get an idea of fineness, specific surface area of cement particles is calculated. The specific surface area is a measure of the frequency of particles of average size, which should not be less than 2250 cm2/gm.
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(x) The consistency of cement should be checked with Vicat apparatus. If the settlement of plunger is between 5 mm to 7 mm from the bottom of mould, the water added is correct, otherwise repeat the process with different percentages of water till the desired penetration is achieved.
(xi) The initial setting time for ordinary cement is about 30 minutes. The initial setting time is the interval between the addition of water to cement and the stage when the square needle of Vicat apparatus ceases to penetrate completely.
(xii) The final setting time for ordinary cement is about 10 hours. The final setting time is the difference between the time at which water was added to cement and time required for needle with annular collar of Vicat apparatus ceases to make an impression on test block.
(xiii) The cement should be tested for soundness using Le Chatelier apparatus. This test is to detect the presence of uncombined lime in cement. The expansion of cement after heating and cooling the mould, should not exceed 10 mm.
Mechanical Properties of Cement:
(i) The compressive strength at the end of 3 days should not be less than 11.5 N/mm2 and that at the end of 7 days should not be less than 17.5 N/mm2.
(ii) The tensile strength at the end of 3 days should not be less than 2 N/mm2 and that at the end of 7 days should not be less than 2.50 N/mm2.
Chemical Properties of Cement:
(i) The ratio of percentage of alumina to iron oxide should not be less than 0.66.
(ii) The ratio of percentage of lime to alumina, iron oxide and silica, known as Lime Saturation Factor (LSF) should not be less than 0.66 and should not be more than 1.02.
(iii) Total loss on ignition should not be more than 4 per cent.
(iv) Total sulphur content should not be more than 2.75 per cent.
(v) Weight of insoluble residue should not be more than 1.50 per cent.
(vi) Weight of magnesia should not exceed 5 per cent.
Composition of Ordinary Cement:
The ordinary cement contains two basic ingredients, namely, argillaceous and calcareous. In argillaceous materials, the clay predominates and in calcareous materials, the calcium carbonate predominates. A typical chemical analysis of good ordinary cement along with the desired range is given in table 6-1.
Functions of Cement Ingredients:
The ingredients of ordinary cement, as mentioned above, perform the following functions:
(1) Lime (CaO):
This is the important ingredient of cement and its proportion is to be carefully maintained. The lime in excess makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is in deficiency, the strength of cement is decreased and it causes cement to set quickly.
(2) Silica (SiO2):
This is also an important ingredient of cement and it gives or imparts strength to the cement due to the formation of dicalcium and tricalcium silicates. If silica is present in excess quantity, the strength of cement increases but at the same time, its setting time is prolonged.
(3) Alumina (Al2O3):
This ingredient imparts quick setting property to the cement. It acts as a flux and it lowers the clinkering temperature. However the high temperature is essential for the formation of a suitable type of cement and hence the alumina should not be present in excess amount as it weakens the cement.
(4) Calcium Sulphate (CaSO4):
This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.
(5) Iron Oxide (Fe2O3):
This ingredient imparts colour, hardness and strength to the cement.
(6) Magnesia (MgO):
This ingredient, if present in small amount, imparts hardness and colour to the cement. A high content of magnesia makes the cement unsound.
(7) Sulphur (S):
A very small amount of sulphur is useful in making sound cement. If it is in excess, it causes cement to become unsound.
(8) Alkalies:
The most of the alkalies present in raw materials are carried away by the Hue gases during heating and the cement contains only a small amount of alkalies. If they are in excess in cement, they cause a number of troubles such as alkali-aggregate reaction, efflorescence and staining when used in concrete, brickwork or masonry mortar.
Harmful Constituents of Cement:
The presence of the following two oxides adversely affects the quality of cement:
(i) Alkali oxides K2O and Na2O
(ii) Magnesium oxide MgO.
If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete made from that cement. Similarly, if the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar or concrete hardens. It is due to the fact that magnesium oxide, burned at a temperature of about 1500°C, slakes very slowly, when mixed with water.
Setting Action of Cement:
When water is added to cement, the ingredients of cement react chemically with water and form various complicated chemical compounds. The formation of these compounds is not simultaneous. But setting action of cement continues for a long time. The mixing of cement and water results in a sticky cement paste and it goes on gradually thickening till it achieves a rock-like state.
It is found that ordinary cement achieves about 70% of its final strength in 28 days and about 90% of its final strength in one year or so.
Following are the important compounds formed during the setting action of cement:
(1) Tricalcium Aluminate (3CaO, Al2O3):
This compound is formed within about 24 hours after addition of water to the cement.
(2) Tetra-Calciurn Alumino-Ferrite (4CaO, AI2O3, Fe2O3):
This compound is also formed within about 24 hours after addition of water to the cement.
(3) Tricalcium Silicate (3CaO, SiO2):
This compound is formed within a week or so after addition of water to the cement and it is mainly responsible for imparting strength to the cement in early period of setting.
(4) Dicalcium Silicate (2CaO, SiO2):
This compound is formed very slowly and hence it is responsible for giving progressive strength to the cement.
The above four principle minerals in ordinary Portland cement are designated in short as C3A, C4AF, C3S and C2S respectively and their relative proportions, expressed as percentages, are as follows –
When water is added to the cement, the quickest to react with water is C3A and in order of decreasing rate are C4AF, C3S and C2S.
During the initial period of hardening, the gain in strength of C2S is small and it is about 15 per cent of that of C3S. After 28 days, the hydration of C3S comes practically to an end and the hydration of C2S only really begins at that time. Hence, when a high-strength concrete is required within a short period of time, the cement is made with a high content of C3S.
On the other hand, if a high-strength concrete is required at a later stage, as in case of hydraulic engineering constructions, the cement is made with a high content of C2S.
The hardening of concrete is greatly speeded up by C3A and hence this property of C3A is utilized for producing quick-hardening Portland cement.
Depending upon the mineralogical composition of clinker in percentage, the Portland cement can be subdivided conventionally as follows:
Site for Cement Factory:
The location of cement factory should be decided carefully.
Following are the guiding factors which are to be paid attention to while making selection for site of a proposed cement factory:
(1) Climatic Conditions:
The site for cement factory should be selected in such a way that its climatic conditions are favourable for the manufacturing process of cement.
(2) Labour:
The site should be such that it is possible to procure the required labour easily and economically.
(3) Market:
The site for cement factory should be, as far as possible, near to the market of sale. This will decrease the cost of transport and minimize the chances of spoiling the cement during transport.
(4) Power:
The availability of power and fuel at economic rates should also be considered.
(5) Raw Materials:
These should be available easily and continuously around the area of site under consideration.
(6) Transport Facilities:
The site for a cement factory should be such that transport facilities are available for raw materials and finished products.
(7) Miscellaneous:
It is necessary to have parks, gardens, recreation centres, etc. near the site for a cement factory. Such arrangements would be helpful for giving relaxation to the officers and workers of the factory.
The above conditions are meant for an ideal site for cement factory. It is difficult, if not impossible, to obtain such a site in practice. It therefore becomes necessary to select a site which satisfies most of the conditions mentioned above.
Packing of Cement:
The packing of cement is mostly done in our country in conventional jute or gunny bags. These bags have proved to be satisfactory containers as their shape and size make them convenient to handle. If properly handled, they may make three to five trips from the factory to the cement users.
However the main drawbacks of such type of packing are as follows:
(i) At every point of handling, some portion of cement contained in jute bag is wasted.
(ii) Even after emptying the cement bag, small quantity of cement remains in the bag and it is thus not possible to take advantage of the full contents of the bag.
(iii) Such type of packing leads to the air pollution.
(iv) The handling of jute bags proves harmful to the health of labourer also as he inhales a considerable amount of cement particles during the transport of such bags.
(v) The quality of cement is affected due to entry of moisture from the atmosphere.
To improve the quality of packing bags, the National Council of Cement and Building Materials has developed an improved jute bag having close-knit design. Such bags are technically superior to the conventional jute bags and they are expected to reduce substantially the loss of cement due to seepage and ingress of moisture.
The packing of cement has been made compulsory in these bags by the Govt. of India. The manufacturers now offer variety of packing bags and have distinct marking on them to identify the type of cement and the name of manufacturer.
Some other recommendations are as follows:
(i) The plastic and paper bags are more suitable for protecting the cement from moisture.
(ii) There is a market for low size bags or packs containing 1 kg, 2 kg, 5 kg and 10 kg. It will meet with the needs of consumers requiring small quantity of cement for rural and urban houses.
(iii) The specific modes of transport and handling facilities to meet the demands of large buyers should be developed.
The cement world-over is packed in bulk and retail packages. In our country, it is largely packed in 50 kg packs whereas the jumbo bags of 200 kg are quite prevalent in other countries. The recent times have seen a shift towards consumer preference, concept of strength, convenience and standard of packaging.
In advanced countries, the cement is packed in multi-wall polypropylene paper laminated bags. These bags are made of polypropylene woven fabrics and they are internally laminated with kraft paper.
These bags are becoming popular at present and they are likely to replace the conventional jute bags in our country also in due course of time as they possess the following advantages:
(i) The packing of cement in these bags assures the desired quality and quantity of the product and thus the interests of consumers are ensured.
(ii) These bags are attractive and decent in appearance. The handling process becomes easy, simple and hygienic.
(iii) These bags are light in weight and protect cement from moisture.
(iv) These bags possess very good resale value also. Once they are torn or disfigured, they become useful like tarpaulene. Hence the poor hutment dwellers can use these bags to cover the roofs of their huts in rainy season or as rain coat.
(v) These bags prevent or arrest the pilferage of cement during filling, storage, transportation, distribution, etc.
(vi) The wastage of cement is completely eliminated as they are leak-proof. The wastage of cement in conventional jute bag is limited to 4% as per BIS. Due to tremendous boost in cement production, the requirement of bags to pack cement is likely to touch the figure of 100 crore of bags every year and thus the tremendous saving of cement which can be achieved by using these bags can be estimated. It is therefore high time to give serious thinking of packing cement in polypropylene multi-wall bags.
(vii) The average movement of cement in our country is about 750 km and during this movement, a lot of multiple handling takes place. As cement is a hygroscopic material, its qualities are changed when exposed to air and moisture. It is thus imperative to ensure the right kind of packaging.
Ball Mills and Tube Mills:
These mills are used to carry out grinding of raw materials or mixture of raw materials or clinkers. The ball mills are used to have preliminary grinding and the tube mills are used to carry out final grinding.
Fig. 6-5 shows the vertical section of a typical ball mill. It is in the form of steel cylinder of diameter about 2 m to 2.50 m and of length about 1.80 m to 2 m.
The cylinder is placed in a horizontal position and it rotates around a steel shaft. On the inside of cylinder, the perforated curved plates are fixed. The ends of these plates overlap each other. The cylinder is filled partly with steel balls of size varying from 50 mm to 120 mm.
The action of ball mill is very- simple. The material to be ground is fed from the top. When the mill is rotated about its horizontal axis, the steel balls strike against the perforated curved plates and in doing so, they crush the material. This crushed material passes through an inner sieve plate and then through an outer sieve plate. It is collected from an outlet at the bottom of outer casing of mill.
Fig. 6-6 shows the longitudinal section of a typical tube mill. It is in the form of a long horizontal steel cylinder of diameter about 1.50 m and of length about 7 m to 10 m. The cylinder is filled partly with steel balls of size varying from 20 mm to 25 mm. p Inlet for feeding.
The action of tube mill is similar to that of ball mill. But fine grinding is achieved due to steel balls of smaller size. A worm is provided to feed the material to the mill. The pulverised material is collected at the outlet funnel. In case of large scale production, the air separators may be employed to separate finely ground particles. In this arrangement, the current of air is used to carry away the finely pulverised particles.
To combine preliminary and final grinding, the compartment mill or multiple chamber mill may be adopted. Such a mill has different chambers or sections in which steel balls of different sizes are placed. The material to be ground is allowed to pass through chambers in succession. The chambers with steel balls of bigger size are placed first and they are followed by chambers having steel balls of smaller size.
It is thus seen that a compartment mill combines the actions of ball mill and tube mill. It results in saving of floor space and it simplifies the grinding process. The cost of grinding also works out to be less by the installation of such a mill.
Field Tests Conducted for Cement:
Following four field tests may be carried out to ascertain roughly the quality of cement:
(1) Colour
(2) Physical properties
(3) Presence of lumps
(4) Strength.
(1) Colour:
The colour of cement should be uniform. It should be typical cement colour i.e., grey colour with a light greenish shade. This is not always a reliable test. But it gives an indication of excess lime or clay and the degree of burning.
(2) Physical Properties:
The cement should feel smooth when touched or rubbed in between fingers. If it is felt rough, it indicates adulteration with sand. If hand is inserted in a bag or heap of cement, it should feel cool and not warm. If a small quantity of cement is thrown in a bucket of water, it should sink and should not float on the surface.
A thin paste of cement with water should feel sticky between the fingers. If the cement contains too much of pounded clay and silt as an adulterant, the paste will give an earthy smell.
(3) Presence of Lumps:
The cement should be free from any hard lumps. Such lumps are formed by the absorption of moisture from the atmosphere. Any bag of cement containing such lumps should be rejected.
(4) Strength:
The strength of cement can be roughly ascertained in the following ways:
(i) The briquettes with a lean or weak mortar are made. The size of briquette may be about 75 mm x 25 mm x 12 mm. The proportion of cement and sand may be 1:6. The briquettes are immersed in water for a period of 3 days. If cement is of sound quality, such briquettes will not be broken easily and it will be difficult to convert them into powder form.
(ii) A block of cement 25 mm x 25 mm and 200 mm long is prepared and it is immersed for 7 days in water. It is then placed on supports 150 mm apart and it is loaded with a weight of 340 N. The block should not show signs of failure.
(iii) A thick paste of cement with water is made on a piece of thick glass and it is kept under water for 24 hours. It should set and not crack.
Storage of Cement:
The cement should be stored carefully. Otherwise it may absorb moisture from the atmosphere and may become useless for the structural work.
Following precautions are to be taken for the storage of cement:
(1) Moisture:
If moisture is kept away from cement, it is found that cement will maintain its quality for indefinite period. An absorption of one to two per cent of moisture has no appreciable effect on quality of cement. But if moisture absorption exceeds 5 per cent, the cement becomes totally useless. Hence, when cement is to be stored for a long period, it should be stored in air-tight containers.
(2) Period of Storage:
The loose cement may be stored indefinitely in airtight containers. But it is advisable to avoid storing of cement in jute bags for a period longer than 3 months. If it is unavoidable, the cement should be tested to ascertain its properties.
(3) Piles:
The cement bags are stacked in piles. It is economical to form a pile of 10 bags of cement. A distance of about 300 mm should be kept between the piles of cement bags and exterior walls of building. The passages of width about 900 mm should be provided between the piles.
For long storage, the top and bottom of piles should be covered with tarpaulins or water-proof paper.
(4) Quality of Cement:
The cement which is finely ground is more active and consequently, it absorbs moisture rapidly from the atmosphere. Hence extraordinary precautions should be taken to store finely ground cement.
(5) Removal of Cement:
When cement bags are to be removed from piles of sufficient height, the steps should be formed by taking out two or three bags from front piles. It is also advisable to remove cement in order of its storage period i.e. cement which is stored previously should be taken out first. In other words, the rule of first in, first out should be followed.
(6) Storage Sheds:
For storing cement for a sufficiently long period, the storage sheds of special design should be constructed. The walls, roof and floor of such sheds should be of water-proof construction.
Few small windows should be provided and they should be kept tightly shut. The floor should be above ground. If necessary, the drainage should be provided to drain water collected in vicinity of such shed. For determining the size of storage shed, it is found that 20 bags or 10 kN of cement will require about 1 m3 of space.
It should be noted that cement, even if stored in the most favourable conditions, loses its activity when stored for a long time. For instance, the storage duration of 3 months and 12 months will cause a reduction in the activity of cement to the extent of about 20% and 40% respectively.
Hence it is advisable to reactivate the cement stored for prolonged period. The most effective method of reacting such cement consists in vibro-grinding which ensures greater fineness of cement and makes cement fit for use.
Uses of Cement:
At present, the cement is widely used in the construction of various engineering structures. It has proved to be one of the leading engineering material of modern times and has no rivals in production and applications.
Following are various possible uses of cement:
(i) Cement mortar for masonry work, plaster, pointing, etc.
(ii) Concrete for laying floors, roofs and constructing lintels, beams, weather sheds, stairs, pillars, etc.
(iii) Construction of important engineering structures such as bridges, culverts, dams, tunnels, storage reservoirs, light houses, docks, etc.
(iv) Construction of water tanks, wells, tennis courts, septic tanks, lamp posts, roads, telephone cabins, etc.
(v) Making joints for drains, pipes, etc.
(vi) Manufacture of precast pipes, piles, garden seats, artistically designed urns, flower pots, etc., dust bins, fencing posts, etc.
(vii) Preparation of foundations, watertight floors, footpaths, etc.