ANALYSIS AND MANUFACTURING OF CEMENT
Amardas C D
Reg no :170021040555
This is hereby to certify that the original and genuine investigation work has been carried out to investigate about the subject matter and the relative data collection and investigation has been completed solely, sincerely and satisfactorily by Amardas C D , regarding his project titled ANALYSIS AND MANUFACTURING OF CEMENT " in partial fulfillment for the Bachelor of Science in Industrial Chemistry from St.Paul’s College, Kalamassery in the academic year 2017-20.
Examiner Head of the Department Project Guide
Date :
Place :
I hereby declare that the dissertation entitled “ANALYSIS AND MANUFACTURE OF CEMENT" is an authentic record of the project work done under the guidance and supervision Mr.Texin Joseph, Head of the Department, Department of Chemistry, and MRs.Reshma Raghavan, Assistant professor, St.Paul’s College Kalamassery.
Amardas C D Name of candidate Signature of the candidate
Register Number : 170021040568
The satisfaction that accompany the successful completion of any task would be incomplete without mentioning the people, whose constant guidance and encouragement have served as a beam of light & crowned my efforts with success. I would like to express my sincere thanks to Mr.Texin Joseph, Head of the Department, Department of Chemistry, St.Paul’s College Kalamassery who gave me an opportunity to carry out the thesis work in one of the most prestigious institute. My sincere thanks to MALABAR CEMENTS LTD Cherthala, for providing opportunity and guidance to do the project work in their esteemed concern. I express my sincere thanks and deep sense of gratitude to Mr. K.P Pradeep Kumar Dy. Manager (Q.C) Mr. Pradeep officer (QC). Mr. for their support to complete this project work successfully. Finally I would like to thank all the staff of MALABAR CEMENTS who have their support in this effort. For giving good guidance, encouragement and also suggest improvement, I like to give thanks to Mrs. Reshma Raghavan, my Project Guide. Above all, I thank the God almighty to give the unlimited support and encouragement for the successful completion of this project work.
Amardas C D
CONTENTS
1. INTRODUCTION
1.1 .About the company
1.2. Products 1.3 Types of cement 1.4 Quality parameters
2. REVIEW OF LITERATURE
3. AIM AND SCOPE OF THE PROJECT
4. MATERIALS AND METHODS
4.1 Raw materials 4.2 Manufacturing process
5. RESULTS AND DISCUSSION
5.1. Cement chemistry 5.2. Clinker 5.3 Setting of cement 5.4 Properties of cement
1. INTRODUCTION
1.1 ABOUT THE COMPANY Malabar Cements Limited, a fully owned Government of Kerala undertaking, is the only major integrated cement-manufacturing unit in the State. The Company was incorporated on 11th April 1978 and commenced production in April 1984 at its Walayar plant. The company has a paid up equity of Rs.26 crores and capital outlay of Rs.68 crores. It is rated to produce 4.2 lakh tons of cement per annum at its Walayar plant. As part of expansion programme it has commissioned a 2.0 lakh tons clinker-grinding unit at Cherthala in Alappuzha district in August 2003. Thus the total installed capacity of MCL is 6.2 lakh tons. This ISO 9001:2000 Company meets about 10% of total cement consumption in Kerala.
Malabar Cements Ltd., the only Portland cement manufacturer in Kerala. The company incorporated in April 1978, commenced commercial production in 1984, with capital outlay of Rs. 680 million and paid up equity capital of Rs. 260 million, is owned fully by the Government of Kerala. The 1200 TPD plant at Walayar is continuously running in profit. The Geological Survey of India has identified a cement grade limestone deposit in the Walayar reserve forest way back in 1961-62. The Mineral Exploration Corporation Limited confirmed its efficacy.
1.2 PRODUCTS
Malabar Cements uses the state of the art, dry process technology for the manufacturing of super quality cement and the quality is much ahead of the National Standards. For various applications, the Company is producing three brands —
1) Malabar Super (43 grade OPC conforming to IS 269:2015), 2) Malabar Classic (PPC conforming to IS 1489:2015 Part1) 3) Malabar Aiswarya (PSC conforming to IS 255:2015).
1.3 TYPES OF CEMENT A number of different types of cement are manufactured, by varying the ratio of the raw material and/or by adding some additional materials. CLASSIFICATION OF CEMENT
1. Ordinary Portland Cement.
2. Rapid hardening cement,
3. Ultra-rapid hardening cement.
4. Low heat cement,
5. Quick setting cement,
6. High alumina cement,
7. Blast furnace slag cement,
8. Pozzolana cement,
9. White cement,
10. Hydrophobic cement,
11. Super sulfate cement,
12. Low alkali cement,
13. Water proof cement,
14. Air entraining cement,
15. Expansive cement,
16. Colored cement.
Types of Cement: Every type of cement has their own properties, advantages, and disadvantages. In this article, you are going to learn the most important properties of different types of cement in details.
Following are the cement types.
1. Ordinary Portland Cement. Portland cement is a product obtained by the calcination at a very high temperature, an intimate mixture of correctly proportioned calcareous and argillaceous materials.
The calcined product which is called clinker. Clinker is then finally pulverized by grinding into a very fine powder and is finally mixed with calcium sulfate or gypsum to obtain cement.
2. Rapid Hardening Cement: Definition: It is also known as High-Early-Strength cement. It is manufactured with such adjustments in the proportion of raw materials. So that the cement produced attains maximum strength with-in 24-72 hours. Properties: Two essential properties of Rapid Hardening Cement are following.
(i) It contains relatively more tri-calcium silicate. This is done by adding a greater proportion of limestone in the raw materials compared to that required for ordinary cement.
(ii) It is more fine-grained ( Air permeability 3250 cm2/gm ) than the ordinary cement.
This factors helps in quicker and complete hydration of cement particles during setting and helps in gaining early strength.
Uses: It is a special purpose cement. It is used in that types of projects, where quick hardening is required. 3. Ultra High Strength Cement.
In advanced countries, an Ultra-high early strength cement is produced by separating the finest fraction (above 700 m2/kg) from the rapid hardening cement at the manufacturing stage itself. This is achieved by using special devices called cyclone air elutriator. Such cement is used for very high early strength concrete. 4. Low Heat Cement: Definition: It is that type in which a very low amount of heat or hydration is liberated during setting and hardening. Mostly it is used in massive concrete structures like Dams etc. Properties:
(i) The proportion of di-calcium silicate (C2S) is almost double than ordinary cement. (ii) The proportion of tetra calcium alumino-ferrite (C4AlFe) is also increased to one and one- half time.
(iii) The proportion of tricalcium silicate (C3S) and tri-calcium aluminate (C3Al) is reduced by about 50 percent. This is because these compounds are known to liberate a very high amount of heat during hydration.
Uses:
It is mostly used in mega projects construction like DAMS. If we use ordinary Portland cement instead of low heat cement in such structures,
Cracks will develop in such structures due to the great amount of heat liberated during setting and hardening.
And a DAM with cracks is a useless structure. But when low heat cement is used, this danger (development of cracks) can be eliminated.
5. Quick Setting Cement:
These Types of Cement are quite different than rapid hardening cement.
Its quality is that it set into a stone-like mass within a period of fewer than 30 minutes.
This property, of setting as quickly as possible is achieved by following controls in the manufacturing process:
(i) The quantity of retarding agents like gypsum is reduced to a bare minimum.
(ii) The quantity of alumina-rich compound is reduced. (iii) The clinker is ground to extreme fineness.
Quick setting cement is used only in very specific situations such as while constructing piers for bridges and other structures in running or standing water.
6. High Alumina Cement:
Definition:
These Types of Cement contains alumina in considerably larger proportions (average 40 percent) than normal cement.
High Alumina cement is manufactured by calcining a well-proportioned mixture of Limestone and Bauxite (Al2O3, n H2O).
No other raw material is added, not even gypsum is mixed with the clinker during grinding.
The total Alumina content is generally above 32 percent.
Properties: The most important properties of high alumina cement are summarized below: (i) It is resistant to the corrosive action of acids and salts of seawater. (ii) The ratio of alumina to lime is kept between 0.85 and 1.30. (iii) It gains compressive strength of 400 kg/sq.cm within 24 hours and 500 kg/sq.cm after 72 hours. (iv) It evolves great heat during setting. Due to this, it is not suitable for use in mega projects like Dams. But at the same time, it gives an advantage to high alumina cement for use in frost forming areas. (v) They react quickly with lime. Therefore, it must not come in contact with lime. Uses: Unfortunately, it is more costly. Therefore it is used only in those situations where resistant against corrosion is required. It is commonly used in construction work near and along sea-shore. 7. Blast Furnace Slag Cement: Definition: It is a modified type of Portland cement which contains 25-65 percent (by weight) of blast furnace slag.
It is manufactured by grinding clinker and specific amounts of blast furnace slag together. A small percent of gypsum is also added for controlling its setting time.
The slag, as we know, is a waste product from the blast furnace which is used in the manufacture of iron (Ferrous Metal).
Properties:
The slag cement offers a number of advantages, which are the following.
(i) They possess better workability, cohesiveness, and plasticity. These qualities are explained to be due to lesser specific gravity and greater specific surface area of slag cement.
(ii) They have better resistance to sulfates of alkali metals, alumina, and iron.
(iii) It produces low heat. This property makes it useful for use in mega projects like Dams.
(iv) It is economical as compared to ordinary cement.
Uses:
It is better suited for use in marine structures as in docks, harbors, and jetties. It is also used in road construction in marshy and alkaline soils. 8. Pozzolana Cement: In this cement type, clinker and pozzolanic material such as (volcanic ash, fly ash, etc.) are mixed in a definite proportion with Portland cement.
The pozzolanic materials do not have any cementing qualities when used alone, but when mixed with Portland cement, they react with cement components and form compounds with cementing properties.
The pozzolana cement has many properties similar to ordinary Portland cement. But it also offers some additional properties, which are given below.
(i) It produces less heat. Due to this property, it can be used in mega projects.
(ii) It offers greater resistance to sulfates and corrosive actions of sea water. These qualities make it more useful for construction near or along the coast and also in sulfate soil. It can also be used in sewage works and for underwater construction.
9. Colored Cement:
In this type of cement, pigment (color) is mixed with the Portland cement in a definite proportion.
The Pigment is mixed in a finest powdered state. The amount of pigment used depends upon the shade of the desired color.
It is, however, generally less than 10 percent (by weight). The following pigments are used to obtain colored cement.
(i) Chromium oxide is used for green color. (ii) Cobalt is used for blue color. (iii) Iron oxide is used for various shades of red, brown and yellow color. (iv) Manganese dioxide is used to obtain black and deep brown color. This type of cement is extensively used for top coat in flooring and decorative purposes in various places in a building. 10. White Cement: It may also be defined as a special type of Portland cement when used it gives a milky or snow- white appearance. White cement is manufactured from pure limestone (chalk) and clay that are totally free from iron oxides and any other pigments like manganese and chromium. The kiln is fired by oil rather than by coal to avoid any contamination. Their strength and setting time is similar to ordinary Portland cement. White cement is the most favored material for use in making highways curbs and for a variety of ornamental work.
They are also used widely for making cast stones of appealing appearance.
White cement is comparatively a costly cement type and is, therefore, used only selectively.
11. Hydrophobic Cement: It is a special type of cement containing admixtures which reduce the affinity of cement grains for water.
Such cement types are used especially in cold, frost-forming conditions. Admixtures of naphtha a soap and acidol are generally added to achieve this property.
12. Super Sulfate Cement: These types of cement are manufactured by adding additional quantities of calcium sulfate and blast furnace slag in the Portland cement.
It is especially useful for mass concrete work especially in sulfate-rich environment and marine conditions. Besides, it is comparatively economical than other types of cement.
13. Low Alkali Cement: Such types of cement are specially made Portland cement in which alkali content is kept below in minimum amount, by exercising very strict control over the composition of the raw material used.
They are used in those circumstances where aggregates -for making concrete- are suspected to contain reactive silica.
14. Water Proof Cement.
Water proof cement is prepared by using some of the waterproofing material like Ca, Al with the ordinary cement during the process of cement manufacturing. They are mostly used in the structures where water proofing is required, like in the Dams, Water Tanks, etc. 15. Air Entraining Cement. This cement type is manufactured by adding some indigenous air entraining agents such as glues, resins, sulfates, etc., during the grinding stage of clinker. They are used to improve the workability of concrete with smaller water-cement ratio and they also improve the frost resistance of concrete.
16. Expansive Cement. These types of cement have he quality to expand slightly with time. But they do not shrink during and after the time of hardening. They are mostly used for grouting purposes in anchor bolt and prestressed concrete ducts.
1.4 QUALITY PARAMETERS
BIS CODES
The Indian standards for Portland Pozzolana Cement have been issued in two parts based on the type of pozzolanic materials to be used in manufacturing of Portland Pozzolana Cement as given below: IS 3812 1981 – specification for flyash as pozzolana and admixture IS 1344 1981 – specification for calcined clay pozzolana in view of the availability of good quality fly ash in abundant quantity, the use of calcined clay based pozzolana cement is progressively decreasing. The flyash is a waste product of Thermal power Plant which creates disposal problems at Thermal power plant site. The yearly production of flyash in India is about 70 million tonnes per annum. This would increase infuture depending upon the new coal based thermal power plants to be installed in the country.The present utilization of fly ash in production of blended cement in India is meagre.
2. REVIEW OF LITERATURE
ACI Committee 234 (1995) in its report describes the physical and chemical properties of silica fume, effects of silica fume on the properties of fresh and hardened concrete and applications of silica fume concrete. They reported that concrete containing silica fume shows significantly reduced bleeding. This effect is caused primarily by the high surface area of the silica fume to be wetted, there is very little free water left in the mixture for bleeding. The static modulus of elasticity of silica fume concrete is apparently similar to that of Portland cement concrete of similar strength. They also reported that the main contribution of silica fume to concrete strength development at normal curing temperatures takes place from about 3 to 28 days. At 28 days the compressive strength of silica fume concrete is always higher and, in some instances, significantly also.
Ganesh Babu and Surya Prakash (1995) reported about the efficiency of silica fume in concrete. They reported that the overall efficiency factor of SF can be assessed in two parts (I) “general efficiency factor” – a constant at all the percentages of replacement and (ii) “percentage efficiency factor” varying with the replacement percentage. They concluded that the “general efficiency factor” (Ke) was found to be 3.0 at all replacement percentages for 28 day cube compressive strengths. The percentage efficiency factor Kp is ranging from 2.28 to 0.37 and overall efficiency ranging from 6.85 to 1.11 respectively for percentages replacements varying from 5-40%.
Houssam (1995) evaluated the compressive strengths of silica fume cement paste and mortar at various water cement ratios. Superplasticizer content was adjusted for each mix to ensure that no segregation would occur. They found that the replacement of cement by silica fume, regardless of w/c ratio, along with superplasticizer, increases the strength of mortar. Strength increases with increasing silica fume content. This can be attributed to the improved aggregate-matrix bond associated with the formation of less porous interfacial zone and a better interlock between the paste and aggregate.
Natalya Shanahan and Abla Zayed (2007) studied four cements to address the effect of tricalcium silicate content of cement on external sulfate attack in sodium sulfate solution. The selected cements had similar fineness and Bogue-calculated tricalcium aluminate content but variable tricalcium silicates. Durability was assessed using linear expansion and compressive strength.Phases associated with deterioration were examined using scanning electron microscopy and X-ray diffraction. Mineralogical phase content of the as-received cements was studied by X- ray diffraction using two methods: internal standard and Rietveld analysis. The results indicated that phase content of cements determined by X-ray mineralogical analysis correlated better with the mortar performance in sulfate environment than Bogue content.
Pinto et al. (2000) investigated the combined effects of silica fume, super plasticizer and temperature on setting behavior. They reported that the mortar temperature greatly affects the setting behavior. The effect of mixture composition on time of initial set varied with temperature. The duration of the time period between initial and final was strongly influenced by temperature. This can be controlled by reducing cement content and adding super plasticizer and silica fume which accelerates setting time.
Mohammad Shamim Khan and Michael Ayers (1995) studied about minimum length of curing of silica fume concrete. In experimental investigation they carried out tests on minimum length of curing of silica fume concrete and compared with that of plain Portland cement concrete and fly ash concrete. They replaced cement by silica fume by 5%, 10%, 15% and 20% of total cementitious material along with one plain Portland cement concrete mix and one fly ash concrete mix. They concluded that the minimum length of curing for specific SF concrete is approximately 3 days compared to 3.75 day for plain concrete mix and that for a 15% class fly ash concrete mix being 6.5 days.
Houssam (1995) evaluated the compressive strengths of silica fume cement paste and mortar at various water cement ratios. Superplasticizer content was adjusted for each mix to ensure that no segregation would occur. They found that the replacement of cement by silica fume, regardless of w/c ratio, along with superplaticiser, increases the strength of mortar. Strength increases with increasing silica fume content. This can be attributed to the improved aggregate-matrix bond associated with the formation of less porous interfacial zone and a better interlock between the paste and aggregate. 3. AIM AND SCOPE OF THE PROJECT
1. The main aim of the project is to understand the manufacturing process of cement. 2. To understand the chemical composition of cement. 3. To analyze the setting time and stages of setting of cement. 4. To understand the physical and chemical properties of cement. 4. MATERIALS AND METHODS
4.1 RAW MATERIALS
The first step in the manufacture of portland cement is to combine a variety of raw ingredients so that the resulting cement will have the desired chemical composition. These ingredients are ground into small particles to make them more reactive, blended together, and then the resulting raw mix is fed into a cement kiln which heats them to extremely high temperatures.
Since the final composition and properties of portland cement are specified within rather strict bounds, it might be supposed that the requirements for the raw mix would be similarly strict. As it turns out, this is not the case. While it is important to have the correct proportions of calcium, silicon, aluminum, and iron, the overall chemical composition and structure of the individual raw ingredients can vary considerably. The reason for this is that at the very high temperatures in the kiln, many chemical components in the raw ingredients are burned off and replaced with oxygen from the air. Table 3.3 lists just some of the many possible raw ingredients that can be used to provide each of the main cement elements.
Table 3.3: Examples of raw materials for portland cement manufacture
Calcium Silicon Aluminum Iron Limestone Clay Clay Clay Marl Marl Shale Iron ore Calcite Sand Fly ash Mill scale Aragonite Shale Aluminum ore refuse Shale Shale Fly ash Blast furnace dust Sea Shells Rice hull ash Cement kiln dust Slag
The ingredients listed above include both naturally occurring materials such as limestone and clay, and industrial byproduct materials such as slag and fly ash. From Table 3.3 it may seem as if just about any material that contains one of the main cement elements can be tossed into the kiln, but this is not quite true. Materials that contain more than minor (or in some cases trace) amounts of metallic elements such as magnesium, sodium, potassium, strontium, and various heavy metals cannot be used, as these will not burn off in the kiln and will negatively affect the cement. Another consideration is the reactivity, which is a function of both the chemical structure and the fineness. Clays are ideal because they are made of fine particles already and thus need little processing prior to use, and are the most common source of silica and alumina.
Calcium is most often obtained from quarried rock, particularly limestone (calcium carbonate) which must be crushed and ground before entering the kiln. The most readily abundant source of silica is quartz, but pure quartz is very unreactive even at the maximum kiln temperature and cannot be used.
Grinding and blending prior to entering the kiln can be performed with the raw ingredients in the form of a slurry (the wet process) or in dry form (the dry process). The addition of water facilitates grinding. However, the water must then be removed by evaporation as the first step in the burning process, which requires additional energy.
The wet process, which was once standard, has now been rendered obsolete by the development of efficient dry grinding equipment, and all modern cement plants use the dry process. When it is ready to enter the kiln, the dry raw mix has 85% of the particles less than 90 £gm in size.
MANUFACTURING PROCESS The manufacture procedures of Portland cement is described below.
1. Mixing of raw material
2. Burning
3. Grinding
4. Storage and packaging
1. Mixing of raw material
The major raw materials used in the manufacture of cement are Calcium, Silicon, Iron and Aluminum. These minerals are used in different form as per the availability of the minerals.
Table shows the raw materials for Portland cement manufacture
The mixing procedure of the manufacture of cement is done in 2 methods,
• Dry process
• Wet process a) Dry Process The both calcareous and argillaceous raw materials are firstly crushed in the gyratory crushers to get 2-5cm size pieces separately. The crushed materials are again grinded to get fine particles into ball or tube mill.
Each finely grinded material is stored in hopper after screening. Now these powdered minerals are mixed in required proportion to get dry raw mix which is then stored in silos and kept ready to be sent into rotary kiln. Now the raw materials are mixed in specific proportions so that the average composition of the final product is maintained properly.
Fig: Manufacture of Cement by Dry Process b) Wet Process The raw materials are firstly crushed and made into powdered form and stored in silos. The clay is then washed in washing mills to remove adhering organic matters found in clay.
The powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills where they are completely mixed and the paste is formed, i.e., known as slurry.
The grinding process can be done in ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln.
Comparison of dry process and wet process of Cement Manufacture
Criteria Dry process Wet process
Hardness of raw Quite hard Any type of raw material material
Fuel consumption Low High
Time of process Lesser Higher
Quality Inferior Superior quality quality
Cost of production High Low
Overall cost Costly Cheaper
Physical state Raw mix Slurry (liquid) (solid)
2. Burning of Raw Materials The burning process is carried out in the rotary kiln while the raw materials are rotated at 1- 2rpm at its longitudinal axis. The rotary kiln is made up of steel tubes having the diameter of 2.5-3.0 meter and the length differs from 90-120meter. The inner side of the kiln is lined with refractory bricks.
The kiln is supported on the columns of masonry or concrete and rested on roller bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gases from the lower end of the kiln so that the long hot flames is produced.
As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at the speed of 15m/hr. In the upper part, water or moisture in the material is evaporated at 400oC temp, so this process is known as Drying Zone.
The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place. The remaining material is in the form of small lumps known as nodules after the CO2 is released. CaCO3 = CaO + CO2 The lower part (clinkering zone) have temperature in between 1500-17000C where lime and clay are reacts to yielding calcium aluminates and calcium silicates. This aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10mm.
The lower part i.e. clinkering zone has the temperature around 1500-1700C. In the region lime and clay reacts to yield calcium aluminates and calcium silicates. This products of aluminates and silicates of calcium fuses together to form hard and small stones known as clinkers. The size of the small and hard clinkers varies from 5 to 10mm.
2CaO + SiO2 = Ca2SiO4 (declaim silicate (C2S))
3CaO + SiO2 = Ca3SiO5 (tricalcium silicate (C3S))
3CaO + Al2O3 = Ca3Al2O6 (dicalcium aluminate (C2A))
4CaO + Al2O3 + Fe2O3 = Ca4Al2Fe2O10 (tetracalcium aluminoferrite(C4AF)) The clinker coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys.
3. Grinding of Clinkers The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in ball mill or tube mill. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement that does not settle quickly when comes in contact with water.
After the initial setting time of the cement, the cement becomes stiff and the gypsum retards the dissolution of tri-calcium aluminates by forming tricalcium sulfoaluminate which is insoluble and prevents too early further reactions of setting and hardening.
3CaO.Al2O3 + xCaSO4.7H2O = 3CaO.Al2O3.xCaSO4.7H2O 4. Storage and packaging The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags. 5. RESULTS AND DISCUSSION
5.1 CEMENT CHEMISTRY
OXIDE CONTENT IN CEMENT:
NAME PERCENTAGE MAJOR 1. CaO 59 – 64 2. SiO2 19 – 24 3. Al203 3 - 6
4. Fe2O3 1 – 4 MINOR
1. MgO 3 - 5 2. SO3 1 – 3 3. Alkalies 0.2 - 1.3 COMPOUND CONTENT IN CLINKER: 1. C3S - Tri Calcium Silicate 30% - 50 % 2. C2S-Di Calcium Silicate 20% - 45% 3. C3A-Tricalcium Aluminate 8% - 12% 4. C4AF-Tetra Calcium Alumino Ferrite 6% - 10%
5.2 CLINKER
Clinker is a nodular material produced in the kilning stage during the production of cement and is used as the binder in many cement products. The lumps or nodules of clinker are usually of diameter 3-25 mm and dark grey in color. It is produced by heating limestone and clay to the point of liquefaction at about 1400°C-1500°C in the rotary kiln. Clinker, when added with gypsum (to control the setting properties of cement and ensure compressive strength) and ground finely, produces cement. Clinker can be stored for long periods of time in a dry condition without degradation of quality, hence it is traded internationally and used by cement manufacturers when raw materials are found to be scarce or unavailable.
Composition of Clinker
The composition of clinker is examined by two separate approaches:
• mineralogical analysis, using petrographic microscopy and/or x-ray diffraction analysis • chemical analysis, most accurately by x-ray fluorescence spectrometry
The four main components of clinker are:
• Alite: approximately tricalcium silicate (typically about 65% of the total) • Belite: approximately dicalcium silicate (typically about 15% of the total) • Aluminate: very approximately tricalcium aluminate (typically about 7% of the total) • Ferrite: very approximately tetracalcium aluminoferrite (typically about 8% of the total)
Other substances may be present in small amounts:
• Salt phases - various combinations of sodium, potassium and calcium cations with sulfate and chloride anions, such as:
o Arcanite - K2SO4
o Calcium Langbeinite - K2Ca2(SO4)3
o Aphthitalite - K3Na(SO4)2 o Sylvite - KCl • Low-temperature phases - various intermediate chemical species that have escaped further thermal processing, such as: o Spurrite - Ca5(SiO4)2(CO3)
o Ternesite - Ca5(SiO4)2(SO4) o Ellestadite - Ca10(SiO4)3(SO4)3(OH)2
o Ye'elimite - Ca4(AlO2)6(SO4)
The chemical analysis of clinker is usually given in oxide form, as follows (in oxide weight %):
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9
Free lime= 1.0% CaO The balance is made by addition of alkali sulfates and minor impurities, such as small amounts of oxides of titanium, manganese, phosphorus, and chromium.
The amounts of different components vary depending on the desired properties of the produced clinker.
Thermochemistry of Clinker
The raw materials entered into the kiln are taken at room temperature. Inside the kiln, the temperature continues to rise and when it reaches its peak, clinker is produced by rapid cooling. Though the reaction stages often overlap, they can be expressed in a sharply-defined sequence as follows:
1. 65-125°C: Free water evaporates: latent heat must be supplied. Net heat input: 2145 kJ/kg clinker. 2. 400-650°C: Clays decompose endothermically, and alkalis react with the kiln atmosphere to form liquid sulfates. Net heat input: 42.2 kJ/kg clinker. 3. 500-650°C: Dolomite decomposes endothermically. Net energy input: 19.7 kJ. 4. 650-900°C: Calcium carbonate reacts endothermically with silica to form "incipient belite". Net heat input: 722.5 kJ 5. 700-900°C: Calcium carbonate reacts endothermically with alumina and iron oxide to form incipient aluminate and ferrite. Net heat input: 207.2 kJ . 6. 900-1050°C: When all available silica, alumina and iron oxide have reacted, the remaining calcium carbonate decomposes endothermically to calcium oxide. Heat input requirement: 601.9 kJ/kg clinker. 7. 1300-1425°C: Aluminate, ferrite and part of the belite melt endothermically, and belite react with calcium oxide to form alite. 8. 1425-1300°C: Having passed peak temperature, the melt re-freezes exothermically to aluminate, ferrite, and belite.
Types of Clinker
The most common type of clinker is produced for Portland cement and its blends. The types of clinker vary depending on the type of cement for which the clinker is produced. Aside from the Portland cement blends, some special types of cement clinker are listed below:
1. Sulfate Resistant Clinker 2. Low Heat Clinker 3. White Clinker 4. Low-alkali Clinker 5. Belite Calciumsulfoaluminate Ternesite (BCT)
Sulfate Resistant Clinker
It contains 76% alite, 5% belite, 2% tricalcium aluminate, 16 % tetracalcium aluminoferrite, and 1% free calcium oxide. Its production has decreased in recent years because sulfate resistance can easily be obtained by using granulated blast furnace slag in cement production.
Low Heat Clinker
It contains 29% alite, 54% belite, 2% tricalcium aluminate and 15 % tetracalcium aluminoferrite, with very little free lime. It is no longer produced because cement produced from ordinary clinker and ground granulated blast furnace slag has excellent low heat properties.
White Clinker
It contains 76% alite, 15% belite, 7% tricalcium aluminate, no tetracalcium aluminoferrite, and 2% free lime, but the composition may vary widely. White clinker produces white cement which is used for aesthetic purposes in construction. The majority of white cement goes into factory-made pre-cast concrete applications.
Low-alkali Clinker
Reduction of alkali content in clinker is done by either replacing the raw-mix alumina source with another component (thus obtaining a more expensive material from a more distant source), or installing an "alkali bleed", which involves removing some of the kiln system's high temperature gases (which contain the alkalis as fume), resulting in some heat wastage.
Use of Clinker: Conversion to Cement
Clinker, combined with additives and ground into a fine powder, is used as a binder in cement products. Different substances are added to achieve specific properties in the produced cement. Gypsum added to and ground with clinker regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall.
Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid, and dodecyl-benzene sulphonate. The most notable type of cement produced is Portland cement, but certain active ingredients of chemical admixtures may be added to clinker to produce other types of cement, such as:
• ground granulated blast furnace slag cement • pozzolana cement • silica fume cement
Clinker is primarily used to produce cement. Since it can be stored in dry condition for several months without noticeable deterioration, it is traded internationally in large amounts. Cement manufacturers buy clinker for their cement plants in areas where raw materials for cement are scarce or unavailable.
5.3 SETTING OF CEMENT
The action of changing mixed cement from a fluid state to a solid state is called setting of cement and time required for it to set is called setting time of cement. Setting time of cement is same as setting time of concrete.
Stages of setting of cement, setting times of cement and factors affecting and initial and final setting time of different cements and various processes involved are discussed.
When water is mixed with cement to form a paste, reaction starts. In its pure form, the finely ground cement is extremely sensitive to water. Out of the three main compounds, viz. C3A, C3S and C2S, reacts quickly with water to produce a jelly-like compound which starts solidifying and setting of cement takes place.
Stages of Setting of Cement
The setting process of cement starts as it is mixed with water. The chemical phenomenon that takes place are divided into 3 stages,
1. Hydrolysis and Hydration Stage
The process of setting starts after the addition of water. In this process the four compounds of cement (C3S, C2S, 3CA1, 4CAFe) get hydrated. C3S compound of cement gets hydrated and form a complex hydro silicates.
2. Colloidisation Stage The products formed from the above stage separates out in the form of a gel which gets gradually thickened and acts as glue around aggregates. Thereby initiating the setting of the cement. During this stage, the mortar (cement-water-sand paste) becomes fully saturated and can take no more water.
3. Crystallisation Stage
As the name of the stage indicates, most of the components of gel or colloidal state forms into crystalline state. Compounds which are least stable such as tri-calcium hydro aluminate and calcium hydroxide are the one to undergo onto stable crystalline phase.
Calcium hydro silicate gel also hardens almost simultaneously. This nearly simultaneous development of crystals and hardening of gel results into a strong and inter-grown mass of crystals and gels.
Setting Time of Cement
1. Initial Setting Time
Initial Setting Time is defined as the period elapsing between the time when water is added to the cement and the time at which the needle of 1 mm square section fails to pierce the test block to a depth of about 5 mm from the bottom of the mould.
A period of 30 minutes is the minimum initial setting time, specified by ISI for ordinary and rapid hardening Portland cements and 60 minutes for low heat cement.
2. Final Setting Time
Final Setting Time is defined as the period elapsing between the time when water is added to cement and the time at which the needle of 1 mm square section with 5 mm diameter attachment makes an impression on the test block. 600 minutes is the maximum time specified for the final set for all the above-mentioned Portland cements. IS: 269-1976 specifies the strengths in compression on the standard mortar-cube.
Fig 1: Vicat Apparatus for Cement Setting Time Test
Factors Affecting the Setting Time of Cement
Many factors affect the setting time of cement after mixed with water, they are
• Composition of cement. • Amount of gypsum in cement • Fineness of cement • Curing • Water cement ratio • Type of admixture used • Storage of cement Initial and Final Setting time of different type of cement.
Table 1: Initial and Final Setting time of different type of cement.
Type/Name of Cement Initial Setting Time in min Final Setting Time in min OPC(33) 30 600 OPC(43) 30 600 OPC(53) 30 600 Sulphate Resisting cement 30 600 Portland Pozzolana Cement 30 600 Rapid hardening cement 30 600 Portland Slag Cement 30 600 High alumina 30 600 Super sulphated 30 600 Low heat 60 600 Masonry cement 90 1440
PROPERTIES OF CEMENT
Physical Properties of Cement:
Different blends of cement used in construction are characterized by their physical properties. Some key parameters control the quality of cement. The physical properties of good cement are based on:
• Fineness of cement • Soundness • Consistency • Strength • Setting time • Heat of hydration • Loss of ignition • Bulk density • Specific gravity (Relative density)
Fineness of Cement:
The size of the particles of the cement is its fineness. The required fineness of good cement is achieved through grinding the clinker in the last step of cement production process. As hydration rate of cement is directly related to the cement particle size, fineness of cement is very important.
Soundness of Cement:
Soundness refers to the ability of cement to not shrink upon hardening. Good quality cement retains its volume after setting without delayed expansion, which is caused by excessive free lime and magnesia.
Consistency of Cement:
The ability of cement paste to flow is consistency.
It is measured by Vicat Test.
Strength of Cement:
Three types of strength of cement are measured – compressive, tensile and flexural. Various factors affect the strength, such as water-cement ratio, cement-fine aggregate ratio, curing conditions, size and shape of a specimen, the manner of molding and mixing, loading conditions and age. While testing the strength, the following should be considered: • Cement mortar strength and cement concrete strength are not directly related. Cement strength is merely a quality control measure. • The tests of strength are performed on cement mortar mix, not on cement paste. • Cement gains strength over time, so the specific time of performing the test should be mentioned.
Setting Time of Cement:
Cement sets and hardens when water is added. This setting time can vary depending on multiple factors, such as fineness of cement, cement-water ratio, chemical content, and admixtures. Cement used in construction should have an initial setting time that is not too low and a final setting time not too high. Hence, two setting times are measured:
• Initial set: When the paste begins to stiffen noticeably (typically occurs within 30-45 minutes) • Final set: When the cement hardens, being able to sustain some load (occurs below 10 hours)
Chemical Properties of Cement:
The raw materials for cement production are limestone (calcium), sand or clay (silicon), bauxite (aluminum) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag, slate. Chemical analysis of cement raw materials provides insight into the chemical properties of cement.
1. Tricalcium aluminate (C3A)
Low content of C3A makes the cement sulfate-resistant. Gypsum reduces the hydration of C3A, which liberates a lot of heat in the early stages of hydration. C3A does not provide any more than a little amount of strength.
Type I cement: contains up to 3.5% SO3 (in cement having more than 8% C3A) Type II cement: contains up to 3% SO3 (in cement having less than 8% C3A).
2. Tricalcium silicate (C3S)
C3S causes rapid hydration as well as hardening and is responsible for the cement’s early strength gain an initial setting. 3. Dicalcium silicate (C2S)
As opposed to tricalcium silicate, which helps early strength gain, dicalcium silicate in cement helps the strength gain after one week.
4. Ferrite (C4AF)
Ferrite is a fluxing agent. It reduces the melting temperature of the raw materials in the kiln from 3,000°F to 2,600°F. Though it hydrates rapidly, it does not contribute much to the strength of the cement.
5. Magnesia (MgO)
The manufacturing process of Portland cement uses magnesia as a raw material in dry process plants. An excess amount of magnesia may make the cement unsound and expansive, but a little amount of it can add strength to the cement. Production of MgO- based cement also causes less CO2 emission. All cement is limited to a content of 6% MgO.
6. Sulphur trioxide
Sulfur trioxide in excess amount can make cement unsound.
7. Iron oxide/ Ferric oxide
Aside from adding strength and hardness, iron oxide or ferric oxide is mainly responsible for the color of the cement.
8. Alkalis
The amounts of potassium oxide (K2O) and sodium oxide (Na2O) determine the alkali content of the cement. Cement containing large amounts of alkali can cause some difficulty in regulating the setting time of cement. Low alkali cement, when used with calcium chloride in concrete, can cause discoloration.
In slag-lime cement, ground granulated blast furnace slag is not hydraulic on its own but is "activated" by addition of alkalis. There is an optional limit in total alkali content of 0.60%,
calculated by the equation Na2O + 0.658 K2O.
9. Free lime
Free lime, which is sometimes present in cement, may cause expansion.
10. Silica fumes
Silica fume is added to cement concrete in order to improve a variety of properties, especially compressive strength, abrasion resistance and bond strength. Though setting time is prolonged by the addition of silica fume, it can grant exceptionally high strength.
Hence, Portland cement containing 5-20% silica fume is usually produced for Portland cement projects that require high strength.
11. Alumina
Cement containing high alumina has the ability to withstand frigid temperatures since alumina is chemical-resistant. It also quickens the setting but weakens the cement.
CONCLUSION
The manufacturing process of cement includes four steps – mixing of raw materials, burning, grinding and storage. The mixing procedure of the manufacture of cement is done in two methods, dry process and wet process.
The major raw materials used in the manufacture of cement are Calcium, Silicon, Iron and Aluminum. These minerals are used in different form as per the availability of the minerals.
The action of changing mixed cement from a fluid state to a solid state is called setting of cement and time required for it to set is called setting time of cement. Setting time of cement is same as setting time of concrete.
Some parameters like fitness, soundness, consistency controls the quality of the cement. These parameters are their physical properties. Chemical analysis of cement raw materials provides insight into the chemical properties of cement.
Clinker is a nodular material produced in the kilning stage during the production of cement and is used as the binder in many cement products. The four main components of clinker are – alite, belite, aluminate and ferrite. Clinker is primarily used to produce cement. Since it can be stored in dry condition for several months without noticeable deterioration, it is traded internationally in large amounts. Cement manufacturers buy clinker for their cement plants in areas where raw materials for cement are scarce or unavailable.