INFLUENCE OF CLINKER MICRO-STRUCTURE ON CLINKER QUALITY

This document is submitted in partial fulfillment of the requirements of the Bachelor of Engineering Honors Degree in Chemical Engineering.

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY

Dedications To my family Josh Snr, Josh Jnr, Roe, Bea, with you it’s not an obligation to love and support

me, it runs deeper than that.

TOM HAVATYI (N005 880J)

BEng CHEMICAL ENGINEERING

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Acknowledgements The author would like to express his heartfelt gratitude to all the people who gave the much needed support to make this project a success. Special mention goes to Miss Bhebhe the project supervisor, Mr Isaac Betserai my industrial supervisor, Rabson Sithole and all my friends who supported me. Above all I thank God for his guidance and protection.



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Abstract Various clinker samples were investigated in this project with the intention of diagnosing the causes of clinker ungrindability and low strength. The samples were examined chemically and microscopically. The microstructure of the clinker samples gave more detail which relates to the operating conditions (raw meal fineness, heating rate, cooling rate, etc). Microscopic examination is a better technique for quality assessment of cement quality.



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Table of Contents Dedications ................................................................................................................................................ i Acknowledgements ................................................................................................................................... ii Abstract .................................................................................................................................................... iii TABLE OF FIGURES ................................................................................................................................... vi LIST OF TABLES ........................................................................................................................................ vii Chapter 1.

: INTRODUCTORY CHAPTER .................................................................................................. 1

1.0 Introduction ........................................................................................................................................ 1 1.1

Aim ................................................................................................................................................ 1

1.2

Objectives...................................................................................................................................... 1

1.3

Background ................................................................................................................................... 2

1.4

Methodology ................................................................................................................................. 2

1.5 Scope ................................................................................................................................................... 3 1.6 Definition of Critical Terms ................................................................................................................. 3 1.7 Conclusion ........................................................................................................................................... 4 Chapter 2.

: LITERATURE REVIEW ........................................................................................................... 5

2.0 Introduction ........................................................................................................................................ 5 2.1 Parameters that define cement quality .............................................................................................. 6 2.1.1 Raw Materials .................................................................................................................................. 6 2.1.2 Kiln Burning Systems .................................................................................................................... 7 2.2 Cyclone Pre-heater Kilns ..................................................................................................................... 9 2.3 Vital Kiln Operational Parameters .................................................................................................... 10 2.3.1 Material Residence Time ........................................................................................................... 10 2.3.2 Kiln Degree of Fill ....................................................................................................................... 11 2.3.3 Kiln Slope .................................................................................................................................... 11 2.3.4 Kiln Capacity ............................................................................................................................... 11 2.4 Effects of burning conditions on clinker microstructure .................................................................. 11 2.5 Reactions in the kiln system.............................................................................................................. 13 2.5.1 Effects of hard burning............................................................................................................... 16 2.6 Clinker Coolers .................................................................................................................................. 17 2.6.1 Pre-cooling Zone ........................................................................................................................ 18



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INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY 2.7 Applications of Light Microscopic Investigations .............................................................................. 20 2.8 Cement grinding ................................................................................................................................ 20 2.8.1 Grinding aids .............................................................................................................................. 21 2.8.2 Grinding aid application ............................................................................................................. 22 2.8.3 Grinding aids mechanism of action............................................................................................ 22 2.8.4 Estimating grindability ............................................................................................................... 22 2.9 Hydration .......................................................................................................................................... 26 2.9.1 Mineralogy of clinker ................................................................................................................. 26 2.9.2 Functions of minerals in clinker ................................................................................................. 26 2.9 3 Hydration of cement .................................................................................................................. 26 2.9.4 Stages of hydration .................................................................................................................... 30 Chapter 3.

: METHODOLOGY ................................................................................................................ 33

3.0 Introduction ...................................................................................................................................... 33 3.1 Microscopy sample preparation ....................................................................................................... 34 3.2 Procedure of determining the bond work index .............................................................................. 34 3.3 Compressive strength test of clinker ................................................................................................ 36 3.3.1 Apparatus ................................................................................................................................... 36 3.3.2 Procedure ................................................................................................................................... 37 3.3. 3 Temperature and Humidity ....................................................................................................... 37 3.3.4 Test Specimens .......................................................................................................................... 37 3.3.5 Determination of Compressive Strength: .................................................................................. 37 Chapter 4.

: RESULTS AND ANALYSIS .................................................................................................... 39

4.0 Introduction ...................................................................................................................................... 39 4.2 Microscopic results and analysis ....................................................................................................... 49 Chapter 5.

: conclusion and RECOMMENDATIONS .............................................................................. 53

5.1 Conclusion ......................................................................................................................................... 53 5.1 Recommendations ............................................................................................................................ 53 BIBLIOGRAPHIC REFERENCES .................................................................................................................. 56 Appendix 1: Generalised flow sheet for cement manufacturing............................................................ 58 Appendix 2: Kiln Inlet .............................................................................................................................. 59 Appendix3: Kiln Outlet ............................................................................................................................ 60



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TABLE OF FIGURES Figure 2-1: Schematic representation of clinker formation reaction (Kakali,1997) ..................... 14 Figure 2-2: Dehydration and Calcination (Kakali, 1997) ............................................................. 14 Figure 2-3: Melt Phase Formation and Clinkerisation (Kakali, 1997) ......................................... 15 Figure 2-4: Cooling of Clinker ..................................................................................................... 16 Figure 2-5: burning area and cooling area of the pre-cooling zone as well as of cooler of a kin system with a rotary cooler (Moore 1995) .................................................................................... 19 Figure 2-6: Compounds in cement (Laszlo, 2000) ....................................................................... 27 Figure 2-7: Mixing (Laszlo, 2000)................................................................................................ 30 Figure 2-8: Dormancy (Laszlo, 2000) .......................................................................................... 31 Figure 2-9: Hardening (Laszlo, 2000) .......................................................................................... 32 Figure 4-1: Microstructure of clinker form the by-pass ................................................................ 49 Figure 4-2: Microstructure of clinker sample #4 .......................................................................... 50 Figure 4-3: microstructure of clinker sample #1........................................................................... 51 Figure 4-4: microstructure of clinker sample #5........................................................................... 52



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LIST OF TABLES Table 2-1: Typical analysis of raw materials .................................................................................. 7 Table 2-2: Comparison of Meso-Portland Cement and normal Portland Cement ........................ 13 Table 2-3: Optical quality factors for cement clinkers ................................................................. 20 Table 2-4: Grindability estimation equations from literature ....................................................... 23 Table 2-5: Relationship between clinker grindability ranking crystal size and content ............... 24 Table 2-6: Typical mineralogical composition of modern Portland cement ................................ 26 Table 4-1: Chemical analysis of clinker samples ......................................................................... 39 Table 4-2: Bond grindability test for determining grindability factor of clinker from the by-pass ....................................................................................................................................................... 41 Table 4-3: Bond Work Index ........................................................................................................ 41 Table 4-4: Physical test results ..................................................................................................... 42 Table 4-5: Kiln inlet log sheet for clinker sample #4 ................................................................... 43 Table 4-6: kiln log sheet for clinker sample #1 ............................................................................ 44 Table 4-7: Kiln inlet log sheet for clinker sample 5 ..................................................................... 45 Table 4-8: Kiln outlet log sheet of clinker sample #4 ................................................................... 46 Table 4-9: Kiln outlet log sheet of clinker sample #5 ................................................................... 47 Table 4-10: Kin outlet log sheet for clinker sample #1 ................................................................ 48



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CHAPTER 1. : INTRODUCTORY CHAPTER 1.0 Introduction The cement manufacturing process is a highly energy intensive process using thermal and electrical energy. The component of cost of energy in the production cost is prominently high. With depleting energy sources and rising energy costs, it is essential for every cement manufacturer to continuously put in efforts to reduce the energy consumption in the manufacturing process. Cement comes from grinding a mixture of slag, clinker and gypsum. Depending on the type of cement milled, different proportions of clinker to slag ratios are set in the control room. In order to get rid of low quality clinker, most cement manufacturing companies use low quality clinker to produce masonry cement (low strength cement) and good quality clinker is used for Portland cement manufacturing. Since a substantial amount energy expended at a cement plant is needed for clinker grinding, improvement in clinker grindability would increase grinding efficiency, thereby improving energy consumption. Cement mills comprise of a by-pass which discharges resistant clinker. In a case study done at Sino Cement Company in Gweru, there was too much clinker coming out through the by-pass when grinding masonry cement. This aroused concerns on the production costs considering the wasted raw materials, the coal consumed in clinker production, the wasted grinding energy (electrical) and obviously the compromised cement quality.

1.1 Aim The aim of this project is: 

To produce easily grindable and good quality clinker.

1.2 Objectives The objectives of this project are: 

To study the effects of clinker microstructure on clinker grindability.



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

To study the effects of clinker microstructure on cement hydration



To determine the optimum operating conditions for good quality clinker production

1.3 Background Microscopic investigation of clinkers gives pre-information about later treatments such as grinding and hydration. Cement manufacturing industries in Zimbabwe use free CaO amount found by chemical analysis as a quality determining method , though it is a necessary criteria for quality evaluation of clinker, it is not sufficient because crystal size and distribution of free CaO play an important role in clinker treatment. Microscopic investigation gives information on crystal size of alite, belite, aluminate and ferrite, pore shape and size. Size and shape of alite play an important role on grindability and strength developments of cement. Most of the available literature on the clinker grindability agrees with the conclusions that primary influences to ease of grinding relate to alite and belite crystal size and content. Specifically, smaller crystals and more alite (less belite) result in easier to grind clinker. Regarding the reasoning behind the relationship with alite/belite content, alite is more brittle than belite and contains micro cracks developed during cooling, enabling easier grinding compared to the round and more plastic belite. In terms of size, not only are larger alite crystals harder to grind, but smaller particles resulting from grinding have a higher surface charge activity, causing agglomeration and increase in grinding energy requirements. Alite C3S typically reacts in 28 days and virtually all in one year. Belite ( -C2S) behaves similarly, but the reaction is slower, about 30% typically reacting in 28 days and 90% in one year. For both C3S and

-C2S, reaction rates depend on particle size distribution i.e. the

microstructure. Developments of compressive strengths depend on the course of the hydration reactions.

1.4 Methodology The methodology used for this project is: 

Carrying out laboratory scale experiments



Working with kiln operators



Referring to books and the internet



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1.5 Scope The project is going to focus on the effects of clinker microstructure on the clinker grindability as well as the strength development of the cement made from clinker of different microstructure. The project is also going to consider the factors affecting clinker microstructure and determine the optimum operating conditions that give the best quality clinker without increasing the production costs.

1.6 Definition of Critical Terms The definitions of the critical terms include: 

Clinkerisation- is the formation of clinker nodules from partial fusion of limestone, pit

sand and shale particles when heated to temperatures of about 1450°C. The clinker produced typically has a composition in the region of 67% CaO, 22% SiO2, 5% A12O3, 3% Fe2O3 and 3% other components, and normally contains four major phases, called alite, belite, aluminate and ferrite. 

Clinker- Solid material formed in high temperature processes by total or partial fusion



Portland clinker-Clinker formed from a predetermined homogeneous mixture of

materials comprising lime, silica, a small proportion of alumina and iron oxide 

Alite- is tricalcium silicate (Ca3SiO5) and is written as C 3S in shorthand notation.

Formation of alite marks the beginning of clinkerisation. Alite is the most important constituent of all normal cement clinkers, of which it constitutes 50-70%. 

Belite- is dicalcium silicate (Ca2SiO4) and is C2S in shorthand notation. Belite forms

from heating quartz (SiO 2) and CaO. Belite constitutes 15-30% of normal cement clinkers. 

Birefringence- a double-refraction phenomenon in which an unpolarized beam of light is

divided into two beams with different directions and relative velocities 

28 day strength- compressional strength cement/clinker attains after curing it for 28days.



Hydration- changes that occur when an anhydrous cement, or one of its constituent

phases is mixed with water.



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1.7 Conclusion Both size and shape of alite crystals play an important role on grindability and strength developments of cement. Most researchers agree that these characteristics are conditioned by the burning conditions of raw mix in the kiln. The microstructure is, however, also influenced by production parameters, mainly raw meal grinding fineness and homogeneity and by clinker burning and cooling intensity.



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CHAPTER 2. : LITERATURE REVIEW 2.0 I ntroduction Cement manufacturers use microscopy as a technique for kiln control, with clinker samples examined continuously. Clinker microscopy is a new quality assessment procedure in the cement industry and only a few companies use it regularly while other manufacturers use it occasionally as a required basis, while some never use it at all. Although cement microscopy is not very popular in developing countries, it is a very powerful technique used for examining clinker, cement, raw materials, kiln feed and coal. Every stage of the cement manufacturing process can be improved through microscopy. Cement microscopy can be done using an ordinary microscope. The microstructure is examined by looking at a carefully prepared specimen. Details of specimen preparation are in the method section under experiments. More advanced micro-examination can be done using reflected light microscope, scanning electron microscope and X-ray micro-analysis. The latter is very powerful as it enables the analysis of individual crystals. By micro-examination, details of the history of clinker can be seen, raw material fineness and homogeneity, clinker composition and temperature profile in the kiln for example. From this information, the likely performance of cement can be predicted or the cause of production problems identified such as poor grindability and poor hydration. In clinker microscopy, the important characteristics which are examined are: 

Overall nodule microstructure- the microstructure can be dense or porous. This gives a

broad relative indication of burning conditions. 

Alite crystal size- indicates the rate of heating after calcination up to burning zone

temperature. 

Belite crystal size- indicates the length of time taken by clinker in the burning zone.



Aluminate and ferrite crystal size- indicate the cooling rate, so does belite color.



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2.1 Parameters that define cement quality When cement is produced there are some quality specifications which should be met for cement to pass and be sold to consumers. In general terms and as primary objectives, it is necessary for the manufacturer to produce, as economically as possible, cement which is sound (i.e. it does not expand significantly after hardening) and which, when ground with the appropriate amount of calcium sulfate (usually gypsum) to a specific surface area (Blaine) of 300-350m 2/kg, develops a 28-day strength (when tested using a method described in section 3.4.1) of 50-65 N/mm 2. In order to achieve the primary objectives the clinker should not contain significant amounts (normally over 2%) of uncombined calcium oxide (free lime) or excessive amounts (not more than 5-6 per cent) of magnesia. In order to ensure the full strength-giving potential, it is necessary for it to contain 70-80 per cent of calcium silicates (calculated on the basis of the Bogue formula, described in section 3.2.2). Of these calcium silicates, over 60 per cent should be tricalcium silicate (C3S). An impure form of tricalcium silicate is termed alite. It is these specifications that operators should meet when burning clinker and grinding clinker, slag and gypsum to make cement.

2.1.1 Raw Materials Portland cement clinker is made from a finely-ground raw material mixture consisting of limestone, sand and shale. The oxidic main components of limestone, sand and shale are calcium oxide (CaO), silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe 2O3) respectively. The mixture resulting from limestone, shale and sand is called raw meal. The table 1.1 shows typical chemical compositions of raw meal and the proportions of limestone, shale and pitsand used. It is not only the chemical composition of the main components of the raw meal which is important. The physical form also plays an important part in both the quality of the clinker and the operation of the process. Raw meals can have the same chemical composition but due to different fineness would require different residence times in the kiln to achieve acceptable quality. Coarse raw meal is difficult to burn and requires prolonged time in the kiln to achieve low free lime content. Microscopic examination shows high free lime content and elongated alite structures of clinker. Experiments results obtained by Lea ’s (1981) show that in order to achieve



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an alite size suitable for good strength-giving properties, it is necessary to ensure that pitsand present should be finer than 45µm. Table 2-1: Typical analysis of raw materials

Proportions

limestone

shale

pitsand

73

22.8

4.2

Raw Meal

used % %CaO

53.7

15.4

1.0

42.7

%Al2O3 0.5

37.9

1.4

4.2

%Fe2O30.2

16.5

1.3

1.6

%SiO2 1.4

22.5

95.0

13.6

%CaCO395.9

27.5

-

-

2.1.2 Kiln Burning Systems The ground raw meal is stored in a silo, from where it is transferred as kiln feed to the kiln feed. The kiln feed must then be subjected to enough heat to allow the clinkering reactions to occur. This is the pyroprocessing stage of cement manufacture, beginning with the kiln feed material extracted from storage and transported to the kiln, and finishing with the clinker from the cooler going to clinker storage. The main chemical reactions to produce the calcium silicates that later give cement its bonding strength occur in the kiln. There is a combination of endothermic and exothermic reactions occurring in an extremely complicated chemical reaction sequence. The raw material composition, mineralogical composition and the time and temperature profile of these materials in the kiln determine the ultimate composition and mineralogy of the clinker, which in turn determines the performance of the cement produced. The pyroprocessing stage is generally regarded as the heart of the cement-making process. It is the stage in which most of the operating costs of cement manufacture appear, and is also therefore the stage where most of the opportunities for process improvement exist. There are many different kiln system designs and enhancements, but they are all in essence performing the following material transformation, in order from the feed end:



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i.Evaporating free water, at temperatures up to 100°C. H2O(l)

H2O(g)

-ΔHmol = +44kJ/mol

ii.Removal of adsorbed water in clay materials 100° to 300°C. iii.Removal of chemically bound water 450° to 900°C. iv.Calcination of carbonate material between 700° to 850°C. MgCO3

MgO +

CO

2

-ΔHmol = +118kJ/mol

CaCO3

CaO

CO

2

-ΔHmol = +178kJ/mol

+

v.Formation of C2S, aluminates and ferrites between 800° to 1,250°C. 4CaO +

Al2O3

+

Fe2O3

C4AF -ΔHmol= -33kJ/mol

3CaO +

Al2O3

C 3A

2CaO +

β-SiO2

β-C2S -ΔHmol= -121kJ/mol

-ΔHmol= +20kJ/mol

vi.Formation of liquid phase melt at temperatures >1,250°C. vii.Formation of C3S between 1,330°C to 1,450°C. 2CaO +

β-SiO2

C 3S

-ΔHmol= -113kJ/mol

viii.Cooling of clinker to solidify liquid phase between 1,300°C to 1,240°C. ix.Final clinker microstructure frozen in clinker <1,200°C. x.Clinker cooled in cooler from 1,250°C - 100°C. On the gas flow side, the sequence from the firing end is: i.Ambient air preheated by hot clinker from kiln 20°C up to 600° to 1,100°C. ii.Fuel burns in preheated combustion air in kiln 2,000° to 2,400°C. iii.

Combustion gases and excess air travel along kiln, transferring heat to kiln charge and kiln refractories. The gases lose drop in temperature from 2,400°C down to 1,000°C.

iv.

Type equation here.

Preheating system for further recovery of heat from kiln gases into the material charge in the kiln system 1,000°C down to 350° to 100°C.

v.Further heat recovery from gases for drying of raw materials or coal. TOM HAVATYI (N005 880J)


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All kiln systems aspire to optimize heat exchange between the gas streams and material streams at various stages. The most recent type of kilns used to optimize heat comprises of the cyclone pre-heaters and the pre-calciner which uses heat from the grate cooler for combustion. In the preheaters there is heat exchange between the kiln exhaust gases and the incoming cold kin feed. Pre-calcination occurs in the pre-heaters and material entering the rotary kin will be partly calcined and at temperatures above 700°C.

2.2 Cyclone Pre-heater Kilns This system utilizes cyclone separators as the means for promoting heat exchange between the hot kiln exit gases at 1,000°C and the incoming dry raw meal feed. Cyclone pre-heater kilns can have any number of stages between 1 and 6, with increasing fuel efficiency with more cyclone pre-heater stages. The most common is the 4-stage suspension preheater, where gases typically leave the pre-heater system at around 350°C. The rotary kiln is relatively short, with L/D typically 15. The material entering the rotary kiln section is already at around 800°C and partly calcined with some of the clinkering reactions already started. Material residence time in the pre-heater is in the order of 30 seconds and in the kiln about 30 minutes. Kiln speeds are typically 2 rpm. Kiln capacities up to 3,500 tones per day exist, with specific fuel consumption usually around 750 to 800 kcal/kg (3.2 to 3.5 MJ/kg). The larger capacity kilns are built with two pre-heater tower systems to keep cyclone sizes to economic proportions and required efficiency. Pre-calciner — the combustion air for burning fuel in the pre-heater no longer passes through

the kiln, but is taken from the grate cooler by a special tertiary air duct to a specially designed combustion vessel in the pre-heater tower. Typically, 60% of the total fuel is burnt in the calciner, and the raw meal is over 90% calcined before it reaches the rotary kiln section. Since the calciner operates at temperatures around the calcination temperature of raw meal (800°C to 900°C), there may not be a flame as such.



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Pre-calciner kiln systems operate only in conjunction with grate coolers, as there is no provision for tertiary air off-take with planetary coolers.

2.3 Vital Kiln Operational Parameters The following parameters are typical for any kiln operation and considered critical in optimizing the performance of a kiln and producing good quality clinker. These are the parameters operators adjust to meet clinker quality specifications.

2.3.1 Material Residence Time The residence time of material in the kiln is governed by the kiln slope, the speed of rotation, and any internal restrictions either by design (dam rings) or through kiln ring formation. The residence time, t, can be calculated from this equation:

 = 1.77 ∗ ∗ ∗  ∗  ∗  ∗  Where t = residence time, min L = kiln length, meters p = kiln slope, degrees D = kiln diameter, meters n = kiln speed, rpm θ = angle of repose of material, (40°)

F = constriction factor (usually1 if no dams, lifters, etc.)



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2.3.2 Kiln Degree of Fill This is the percentage of the kiln cross-sectional area filled by the kiln charge, and is usually in the range of 5% to 17% for most rotary kilns. It should be noted, though, that a fill degree of more than 13% could impair heat transfer in that some of the material in the center of the charge will not be exposed to enough heat. It is sometimes seen that a kiln ring could coincide with high or erratic free lime in the clinker, possibly because the fill degree has exceeded limits for ensuring that all kiln charge material is uniformly heated.

2.3.3 Kiln Slope Rotary kilns slope from the feed end to the discharge end for material to travel in that direction utilizing gravitational force. The slope is typically 2% to 4%, or 1° to 2°, and is decided in conjunction with the kiln rotational speed. A lesser slope with a higher rotational speed may improve heat transfer because of the greater tumbling of kiln charge.

2.3.4 Kiln Capacity There are design limits for all of the above that may vary between different processes, but any of the above could be the limitation to a kiln's output. These limitations will typically manifest themselves as kiln instability and ring or coating buildup, excessive dust loss, poor refractory life, poor clinker quality, or high fuel consumption. Usually, the limitation is found to be more a question of a fan capacity, a burner capacity, or milling of raw materials or coal.

2.4 Effects of burning conditions on clinker microstructure Clinker burning relies on: 

The residence time of the material in the kiln and in the pre-heaters



The highest temperature in the kiln



Kiln degree of fill

Residence time is the time taken by kiln feed from the first pre-heater cyclone to the kiln outlet. Residence time depends on the suction in the pre-heaters (if there is more negative pressure in the pre-heaters, kiln feed takes longer to reach the rotary kiln), and the kiln rotation speed.



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Maximum temperature reached in the kiln is determined by the type of flame from the burner at the kiln outlet. A short flame with an oval shape produces higher temperatures.

Heat transfer in the kiln is facilitated by convection, radiation and conduction. Convection- convection is facilitated by the gases present in the kiln. Radiation- heat is radiated from the flame of the burner to the kiln material. Conduction- most heat transfer to the kiln material is achieved by conduction. The kiln lining

absorbs heat and passes it on to the pre-heated material through conduction as the kiln rotates. High degree of kiln fill retard conduction, therefore it is necessary to keep kiln charges within ranges which accommodate good heat transfer. The microstructure of clinker shows how thermal reactions in the kiln progress. The properties of clinker are closely related to the burning conditions in the kiln. Ono 1981, states that alite size (lengthwise) indicates the rate of burning of 100% calcined material, alite birefringence indicates the maximum temperature, and belite size indicates the time taken in the burning zone. The table below shows a comparison of meso-and normal Portland cement clinker based on crystal characterization of the essential silicates using Ono’s micro-techniques.



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INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 2-2: Comparison of Meso-Portland Cement and normal Portland Cement Relative operating factor

Meso-Portland cement Aspdin Normal

(Measurement parameter)

clinker (1848)

Present day rotary kiln clinker

Portland

cement.

Relative burning rate

Slow (poor)

Quick (excellent-average)

Alite size

60µm

10-40µm

Relative maximum temperature

Low (poor)

High (good)

Alite birefringence

0.002

0.007

Relative burning time

Long (poor)

Quick (good-excellent)

Belite size

5-10µm

20-60µm

2.5 Reactions in the kiln system These are illustrated in Figure 2.1. On the left-hand side is the raw meal comprising, limestone (CaCO3), pitsand (Si02), shale (Si0 2-Al203-H20) and iron oxide (Fe 203). Up to a temperature of about 700°C, activation of the silicates through the removal of water and changes in the crystal structure takes place. Within the temperature range 700°C-900°C, decarbonation of the calcium carbonate occurs, together with the initial combination of the alumina, ferric oxide and of activated silica with lime. From 900 to 1200°C, belite forms. Above 1250°C and more particularly above 1300°C, the liquid phase appears and this promotes the reaction between belite and free lime to form alite. During the cooling stage (right-hand side of the diagram) the molten phase goes to a glass or, if cooling is slow, the C 3A crystallizes out and in extreme cases the alite dissolves back into the liquid phase and reappears as secondary belite .



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Figure 2-1: Schematic representation of clinker formation reaction (Kakali,1997)

Figure 2-2: Dehydration and Calcination (Kakali, 1997)



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Figure 2-3: Melt Phase Formation and Clinkerisation (Kakali, 1997)



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Figure 2-4: Cooling of Clinker

Due to varying operating conditions kiln operators are in a constant battle to keep clinker quality within specifications. Some unexpected changes may occur, for instance a sudden change in the kiln feed compositions. In such circumstances it is difficult to keep clinker quality within desired specifications and results in under-burning or over-burning. From the chemical analysis of clinker, under-burning is shown by the presence of abundant free lime and a low content of alite, the crystals of which are very small (<10µm). The average size and other characteristics of the alite and belite crystals provide further information about the burning conditions. On the other hand over-burning can cause increase in size through recrystallization. Over burning has adverse effects on cement production. In an effort to reduce free CaO in clinker, operators increase the fuel rate to the kiln to increase temperatures and often results in clinker having free lime of less than 1% (over burning).

2.5.1 Effects of hard burning Hard burning is shown by clinker having a higher liter weight. As a result of harder burning, fuel consumption increases and impairs the refractory lining. When the kiln is operated on the hot side, alkalis and sulfate become more volatile. This, in turn, might increase the possibility for TOM HAVATYI (N005 880J)


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build-ups in the cooler parts of the kiln system. In severe cases, controlling the kiln may become difficult because of surges of the material through the kiln. Hard burning tends to cause low clinker porosity, large crystals of alite, and often contributes to generation of dust instead of good, nodular clinker. It also slows down the cooling process, both because the maximum temperature is higher, and because the low-porosity clinker is more difficult to cool. These effects all can result in cement with reduced strength potential and increased water demand. Reduced clinker porosity can make the clinker harder to grind, increasing finish mill power consumption or reducing mill production. Clinker temperatures exiting the cooler may increase presenting handling problems. The high-temperature conditions may lead to colour variations, reductions in clinker alkali and sulfate level, and increases in water demand attributable to increased levels of aluminate. Variations in clinker alkali and sulfate will affect concrete setting time, and result in strength variations. Periods with decreased clinker alkali content will result in a decrease in early strength and increase in later-age strength; the opposite can occur during periods when the clinker alkali content increases.

Another indication of excessively hard burning is the presence of material high in large alite crystals and low in interstitial material, formed by the withdrawal of the liquid into the centres of the clinker nodules. The effects of slow cooling are readily detectable by light microscopy. The most general are coarse texture of the interstitial material and a change in the belite from colourless to yellow. The belite crystals may also develop ragged or serrated edges. Slow cooling can also cause resorption of alite, with deposition of small crystals of belite as fringes on the alite and in the body of the interstitial material, increase in alite crystal size and, if it occurs below 1200°C, decomposition of alite to an intimate mixture of lime and belite.

2.6 Clinker Coolers Hot clinker from the kiln is cooled and the heat recovered is used for drying and combustion in the pre-calciner. Clinker is cooled by fans blowing in air through the grate cooler. The cooling zone starts at a few meters from the kiln outlet. Cement clinker at a temperature of 1450-1500°C is quenched by the incoming cold air to temperatures less than 100°C. The rate of cooling clinker has some implications to the final clinker quality. Although most of the clinker characteristics would have been formed in the kiln, the final clinker characteristics are formed in the cooler. TOM HAVATYI (N005 880J)


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There are two main types of coolers used in cement clinker production. These are the satellite (or planetary) type and the oscillating grate type. The 1990s saw tremendous advances in clinker cooler technology that greatly improved heat efficiency and potential output from a given kiln system. Clinker coolers perform the function of: 

Transporting clinker from the kiln to the clinker delivery system;



Cooling the clinker to a safe temperature for subsequent transport;



Finalizing the clinker mineralogy through rapid cooling; and



Preheating combustion air by heat exchange with hot clinker.

Cooling of clinker takes place at two locations: 1) in the kiln after the material passes the burning zone region, and 2) in the specially designed clinker coolers after the material falls out of the kiln.

2.6.1 Pre-cooling Zone Clinker cooling first takes place inside the rotary kiln in the pre-cooling zone, which is where radiation and convection (due to incoming relatively cooler secondary air from the clinker cooler) heat losses occur. The rate of cooling can be critical to the clinker quality and performance of cement. The rate of cooling in the kiln cooling zone is determined by the position of the lance burner, the shape of the flame, the resulting heat flux, flame temperature, and speed of material flow through the kiln. Normally clinker exits the kiln at temperatures around 1,200°C to 1,250°C. However, a high rate of clinker cooling between the temperature of the burning zone and about 1200°C is important if the best strength-giving properties are to be achieved. Microscopic examination of clinkers which have been slowly cooled from 1450 to 1200°C often reveal a situation where alite transform back into belite and free lime. This also tends to produce clinker with large alite and belite crystals, resulting in a coarse-grained clinker matrix with poor reactivity and poor grindability. Slow cooling can also result in reversion of C 2S from the ' phase to the less Slow cooling also cause C 3A reactive form, or in extreme cases even to the unreactive form. to crystallize to a form which is more reactive and which may lead to setting problems. The TOM HAVATYI (N005 880J)


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position of the lance burner and the flame type affects the rate of cooling in the pre-cooling zone. Rapid cooling occurs if the pre-cooling zone is longer i.e. if a greater part of the burner is in the rotating kiln as illustrated in the figure and the equation below.

Figure 2-5: burning area and cooling area of the pre-cooling zone as well as of cooler of a kin system with a rotary cooler (Moore 1995)

The length of the pre-cooling zone is difficult to tell but is usually calculated basing on the position of the burner lance. The following equation gives the estimated length: Lpre-cooling zone ≈ Lburner + Da Where Lpre-cooling zone

= length of the pre-cooling zone, in m

Lburner

= length of burner in the rotating part of the kiln, in m

Da

= outer diameter of the rotary kiln, in m

On the other hand a long flame gives slow heat-up and slow cooling of the kiln charge before it falls from the kiln. Therefore a short flame which gives clinker more cooling distance is favorable. From the kiln clinker falls into the grate cooler/planetary cooler where rapid cooling is equally essential. Current practice favours grate coolers, in which the clinker passes over moving grates through which air is blown.



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A further quality problem can arise if there are high levels of MgO in the clinker, because slow cooling allows large periclase crystals to form such that when these hydrate slowly in concrete, the expansion can cause the concrete to rupture. From the clinker microstructure the rate of cooling is seen by the belite color. Belite color ranges from being clear, faint yellow, yellow to amber, with a clear appearance showing good clinker and amber showing poor clinker.

2.7 Applications of Light Microscopic Investigations Light microscopy can be used to determine quantitative phase composition in clinkers. It has proved a highly effective means of finding the causes of unsatisfactory clinker quality or of determining what modifications in composition or plant operation are needed to change the clinker properties in a desired direction. It has also been used to predict strength development. Ono (07) described results obtained from examinations of powder mounts. The values of four parameters, indicated in Table 4.1, were each estimated on a scale of 1-4 and the strength R, in MPa, of a mortar at 28 days then predicted using the regression equation: R= 24.8 + 0-63AS + 2.15AB + 0-39BS + 2-10BC (4.4) The four parameters AS, AB, BS and BC were considered to be measures of heating rate, maximum temperature, time at that temperature and cooling rate, respectively. Table 2-3: Optical quality factors for cement clinkers Alite size (AS)

15-20

20-30

30-40

40-60

Alite birefringence

0.008-0.010

0.006-0.007

0.005-0.006

0.002-0.005

clear

Faint yellow

yellow

amber

25-40

20-25

15-20

5-15

4

3

2

1

(AB) Belite color (BC) Belite size (BS) Value

of

parameter

2.8 Cement grinding The fine grinding of hard materials cannot be just considered a mechanical reduction of the srcinal matter into one featuring a certain degree of fineness; it is more a complex physical TOM HAVATYI (N005 880J)


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mechanical operation during which some surface phenomena play an important part. Despite the development of the grinding technology most cement production still takes place in tubular ball mills, where the effect of clinker quality is of particular importance. The energy efficiency in a ball mill is very low (approx. 5%), since most of the energy is transformed into heat, so that the temperature inside the mill rises from 80 to 100°C. Temperatures can even be higher through the impact of the balls on the grains. Inside the mills there is always a mixture of materials (clinker, slag and gypsum), which have different grindabilities and properties. Clinker being the hardest to grind obviously remains the most studied material. Fresh clinker is more difficult to grind than one which has been stored for a period of 2-3 weeks. Grindability of the clinker: 

is based on alite and belite crystal size of clinker



decreases if the silica ratio rises;



is directly proportional to the percentage of Al 2O3 and Fe2O3;



is proportional to the density of the clinker;



increases linearly with the alite content;



improves by increasing lime standard;



decreases if the belite content rises



is not appreciably affected by alkalis, MgO and free CaO.

2.8.1 Grinding aids Creation of specific surface area and energy required

The increase in the specific surface area is related to the energy required for comminution of the particles, and consequently also to the grinding time. This relationship is expressed by Von Rittinger’s (1867) law which states that the energy consumed in the size reduction is proportional

to the area of new surface produced. This is a theoretical derivation and does not consider the energy losses due to agglomeration of fine cement particles and for breaking down these agglomerations. The action of the grinding media within a rotating mill not only crushes the existing clinker particles, it also sharply compresses them, which leads to the formation of electrostatic surface charges of opposed polarity. The cement particles then agglomerate as a result of the forces of attraction acting on them.



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Regarding the reasoning behind the relationship with alite /belite content, alite is more brittle than belite and contains micro cracks developed during cooling, enabling easier grinding compared to the round and more plastic belite. In terms of size, not only are larger alite crystals harder to grind , but smaller particles resulting from grinding have a higher surface charge activity, causing agglomeration and increases in grinding energy requirements.

2.8.2 A stepGrinding forward in aid the application development was made when the use of grinding aids was introduced, towards the middle of the 1930s to counteract the agglomeration problem. The first step was that of adding coal to the mill feed, but it was soon noticed that this caused a reduction of entrapped air in the concrete, with a consequent serious reduction in the freeze/thaw resistances. The next step was to add water in such a quantity as not to significantly increase the loss of ignition of the cement produced. The experience gained showed that polar grinding aids, like water, are the most effective ones. However, the effectiveness of water is limited by its comparatively low polar moment and low molecular weight, despite its high screening effect.

2.8.3 Grinding aids mechanism of action Grinding aids act by coating the particles which cause agglomeration with a monomolecular film which neutralizes the surface electrical charges. Technically speaking, grinding aids provide the charge carriers necessary to satisfy the charges srcinated by the fracture of the clinker during grinding, thus reducing the tendency to agglomeration. Grinding aids are adsorbed at the fractures surfaces of the particles which have not yet separated, preventing their re-combination under the action of the temperature and pressure. The mechanism of action of grinding aids can be summarized as follows: 

elimination of surface electrostatic charge;



decrease of the energy required for the propagation of micro cracks inside the particles;

2.8.4 Estimating grindability Several references offer equations to estimate or predict clinker grindability based on its microstructure, as provided in Table 1.



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INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 2-4: Grindability estimation equations from literature Equation and discussion

Reference

Relationship between KM and sieve residue at 37µm KM= (C3S%/C2S%)*(100/C3S size) KM= mineralogical coefficient BL=1251+218AS+239AB+287BS+2.1BC Where, BL = Blaine fineness (specific surface area) AS = alite size, AB = alite birefringence, BS = belite size, BC = belite color

Kilhara, Centurione, Munhoz, 1992

When grinding with 5% gypsum: P350=23.9+0.42*C3Sn+0.36*C2S amount (with 5% gypsum) When grinding with TEA as grinding aid: P350= 30.2+0.34*C3Sn+0.36*C2S amount-11.58*TEA Where, P350= power consumption in kWh/t for grinding with 350m 2/kg C3Sn= (C3S%*20)/NC3S/(1-p) NC3S= number of alite intersections in microscopical line count P = porosity Relationship between 1/P*1000 and ln AK Where, P= power consumption AK= belite corrected alite chord length = C3S%/C3S size –(C2S%*C2S size/1000) Ak Revison to (C3S%/C3S size)/((C2S%*C2S size)/1000) Suggested to have better correlation with grindability in belite rich clinker Relationship between grindability and Bk (alite corrected belite chord length) Where, Bk = (C2S%/C2S size) – (C3S%*C3S size/1000) Suggested to have better correlation with grindability in belite rich clinker P (kWh/t) = -20.7x ln Ak + 57.9 Where, Ak= C3S%/C3S size – (C2S%*C2S size/1000) Relationship between KM and sieve residue at 37µm

Ono 1981

Theisen 1993

Scheubel 1985 Venkateswaran

and

Gore

1991 Venkateswaran and Gore

Venkateswaran and Gore

Viggh 1994 Zampieri and Munhoz 1995

KM= C3S%/(C2S% - C3S size)*100 ; KM- mineralogical coefficient

If some values were applied to the above equations, the resulting conclusions would vary somewhat, however in each case the best grinding clinker would be that containing high alite and low belite content with small crystal sizes versus the most difficult to grind clinker containing



23


low alite and high belite content with large crystals. The relationships between clinker grindability and crystal size and content are presented in the table below. . Table 2-5: Relationship between clinker grindability ranking crystal size and content Ranking 1

2

3

4

Alite content

Belite content

Alite size

Belite size

High

Low

Small

Small

High

Low

Large

Small

High

Low

Small

Large

Low

High

Small

Small

High

Low

Large

Large

Low

High

Large

Small

Low

High

Small

Large

Low

High

large

Large

Ranking Value is Relative on a Scale of 1 to 4; 1 being Easy to grind and 4 being Difficult to Grind. Grindability decreases as we go up the rank, with the easiest being rank 1 and the hardest being rank number 4. Below is a picture presentation of the relationship outlined above.



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= Alite = belite

Small alite Small belite

Ranking:

Large alite Small belite

1

Small alite Large belite

2

2

Large alite Large belite

3

Figure1. Diagram showing relative ease in grindability based on crystal size of clinker with high alite/low belite content. Ranking system is relative on a scale of 1 to 4; 1 being easy to grind and 4 being more difficult to grind.

Low alite content High belite content

Small alite Small belite

Ranking:

2

Large alite Small belite

3

Small alite Large belite

Large alite Large belite

3

4

Figure 2.diagram showing relative ease in grindability based on crystal size of clinker with low alite/high belite content. Ranking system is relative on a scale f 1 to 4; 1 being easy to grind, and 4 being more difficult to grind



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2.9 Hydration In cement chemistry, the term 'hydration' denotes the totality of the changes that occur when anhydrous cement, or one of its constituent phases, is mixed with water.

2.9.1 Mineralogy of clinker The major oxides in clinker are combined essentially into just four cement or clinker minerals, denoted in shorthand: C 3S; C2S; C3A; and C4AF. The ratios among these four minerals in typical modern portland cements, and major functions of the minerals, are shown in table below.

Table 2-6: Typical mineralogical composition of modern Portland cement

Chemical

Oxide formula

formula

Shorthand Description

percentage

Mineral

notation

Ca3SiO5

(CaO)3SiO2

C3S

Ca2SiO4

(CaO)2SiO2

C2S

Tricalcium

silicate 50-70

(1)

silicate 10-30

(2)

(alite) Dicalcium (belite) Ca3Al2O6

(CaO)3Al2O3

C3A

Tricalcium

3-13

(3)

5-15

(4)

aluminate Tetracalcium Ca4Al2Fe2O10

(CaO)4Al2O3Fe2O3

C4AF

aluminoferrite

2.9.2 Functions of minerals in clinker Alite-hydrates quickly and imparts early strength and set. Belite-hydrates slowly and imparts long-term strength. Tricalcium aluminate- hydrates almost instantaneously and very exothermically. 

Contributes to early strength and set.

Tetracalcium aluminate- hydrates quickly. 

Acts as a flux in clinker manufacture. Imparts gray color.

2.9 3 Hydration of cement Hydration begins as soon as water and cement come into contact. The cement particles partially dissolve, and the various dissolved components start to react at various rates. During the



26


reactions, heat is generated and new compounds are produced. The new compounds cause the cement paste to harden, and become strong and dense. Cement hydration reactions are complex and not completely understood. Part of the problem is that hydration (hydrated mineral) shells form around the cement mineral particles. The shells shield the remaining cores from easy observation, slow the hydration of the unreacted or partlyreacted cores, and affect the actual hydration reaction stoichiometries. Nonetheless, it is possible to note a few general “net” equations that are representative of the larger family of reactions that likely take place.

Figure 2-6: Compounds in cement (Laszlo, 2000)

The important strength-developing hydration reactions are those of C 3S and C 2S. Typical hydration reactions (in shorthand notation) would be: for C3S: 2C3S + 6H (water) → C3S2H3 (“tobermorite” gel) + 3CH (hydrated lime) for C2S: 2C2S + 4H → C3S2H3 + CH Actually, instead of just tobermorite, a whole family of similar calcium silicate hydrates (CS-H) may be formed, and C-S-H is the preferred general term for these compounds. It is the C-S-H colloid or gel that is the actual binder in hydrated cement. The ultimate strength of the hardened cement paste will depend not only on the srcinal total content, microstructure of C2S and C3S but also on the completeness of their hydration. TOM HAVATYI (N005 880J)


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Although the net hydration reactions for both C 3S and C2S are similar, the reaction for C 3S is relatively fast, and C-S-H from it is responsible for virtually all of the early (e.g., within 3 days of curing) strength development of the cement. Typically, about 60% (by mass) of the C 3S hydrate to C-S-H within the first 5 days of curing and about 70% hydrate within about 10 days. Because of the formation of protective hydration shells, the remaining unreacted C 3S particle cores hydrate much more slowly, reaching about 75% hydration after 20 days of curing, about 80% hydration after 28 days (a standard measurement interval), and 85% after 60 days. Beyond 60 days, the rate of C3S hydration slows dramatically and the incremental hydration and strength contribution is of little practical importance. In contrast, the hydration of C 2S is relatively slow, with only about 20% hydration after 5 days of curing, about 30% after 10 days, 35% after 20 days, about 40% after 28 days, and only about 55% at 60 days. Its rate of hydration slows further after 60 days. Accordingly, the C-S-H derived from the hydration of C2S, while making little contribution to the early strength of the cement, contributes a significant proportion of the strength gain after the first week or so of curing. The other two clinker minerals, C3A and C4AF, have complex hydration reaction paths that are similar to each other, but those of C 3A are more important because they are much more rapid and exothermic. Having C3A in the cement primarily enhances initial set and speeds, via release of heat, the hydration of C 3S (the presence of C3A also has benefits to the cement manufacturing process because it speeds the overall formation of the clinker). In the absence of significant sulfate, C3A very rapidly— almost instantaneously—forms C3A-hydrates, many of which are unstable and may subsequently convert to other forms. One of the many possible sequential hydration reactions is: 2C3A + 21H → C 2AH8 + C4AH13 → 2 C3AH6 + 9H. A minor, but lime-consuming, reaction is: C3A + 12 H + CH → C4AH13 The hydration of C3A in the absence of sulfate can be so rapid as to cause the undesirable condition known as flash set. This is controlled through the addition of sulfate, usually as TOM HAVATYI (N005 880J)


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gypsum and/or anhydrite. A typical hydration reaction of C3A in the presence of ratecontrolling sulfate (here shown as gypsum) would be:

C3A + 3 C¯S3H32 (“ettringite”).SH2 + 26 H → C 6A Flash set is controlled because ettringite forms a shell around the C3A particles, which slows water diffusion to, and hence the hydration of, the residual C3A cores. Ettringite is stable only in the presence of excess sulfate. If this condition is not met (i.e. not enough gypsum present, or in the evolving conditions at the ettringite-residual C 3A core interface), then ettringite reacts with C3A to form a monosulfate phase: C6ASH12 (“monosulfate”). S3H32 + 2C3A + 4H → 3 C4A Alternatively, C3A hydration under low sulfate conditions can be expressed by: C3A + 10H + CSH 12SH2 → C4A An important property of the monosulfate phase is that, in the presence of sulfate ions, it can re-form ettringite, such as by the reaction: C4ASH2 + 16H → C6ASH12 + 2CS3H32 The ferrite mineral C4AF does not play a critical role in cement hydration. The chief value of ferrite is in its effects on kiln reactions to form C 3S. The hydration of C4AF is broadly similar to that of C3A, although the reactions tend to be slower and much less exothermic. The reaction stoichiometries will vary given the fact that, as noted earlier, C 4AF is merely a mean composition for the ferrite solid solution having end members C6A2F and C6AF2. In the absence of sulfate, the F partially substitutes for some of the A (partial substitution denoted as A,F) in the analogous C3A hydration products, as shown in the reaction: 2C4AF + 32H → 2C 2(A,F)H8 + C4(A,F)H13 + (A,F)H3 where total AF = total (A,F)



29


In the presence of hydrated lime (from C3S and C2S hydration), however, the formation of (A,F)H3 is suppressed and a stable AF-hexahydrate (C 4(A,F)H6) is formed that is analogous to C3AH6, with a possible net reaction being: C2(A,F)H8 + C4(A,F)H13 + (A,F)H3 + 6CH → 3C4(A,F)H6 + 12H

Even more so than with C3A hydration, the hydration of C4AF is slowed in the presence of sulfate by the formation of an ettringite-like phase, with a possible reaction being: 3C 4AF + 12C¯ S3H32 + 2(A,F)H3 SH2 + 110 H → 4C6(A,F)¯ And, analogous to C 3A, if the sulfate concentration is insufficient, the “AF” ettringite becomes unstable and forms an “AF”

monsosulfate phase: 3C4AF + 2C6(A,F)¯ SH12 + 2(A,F)H3 S3H32 + 14H → 6C4(A,F)¯

2.9.4 Stages of hydration

Figure 2-7: Mixing (Laszlo, 2000)

Within minutes of mixing cement and water, the aluminates start to dissolve and react, with the following results: Aluminate* reacts with water and sulfate, forming a gel-like material (C-A-S– -H). This reaction releases heat. The C-A-S –-H gel builds up ar ound the grains, limiting water’s access to the grains and thus controlling the rate of aluminate reaction. This occurs after an initial peak of rapid hydration and heat generation.



30


Figure 2-8: Dormancy (Laszlo, 2000)

For about two to four hours after mixing, there is a dormant period, during which the following events occur: The C-A-S–-H gel is controlling aluminate* reactions. Little heat is generated, and little physical change occurs in the concrete. The cement is plastic. During dormancy, as silicates (alite [C3S] and belite [C2S]) slowly dissolve, calcium ions and hydroxyl (OH) ions accumulate in solution.



31


Figure 2-9: Hardening (Laszlo, 2000)

This stage is dominated by alite (C3S) hydration and the resulting formation of C-S-H and CH crystals: When the solution becomes super-saturated with calcium ions (from dissolving alite [C3S] primarily), fiber-like C-S-H and crystalline CH start to form. This generates heat. Meshing of C-S-H with other solids causes the mixture to stiffen and set.

The increasing heat and stiffening of the cement paste mark the beginning of hydration acceleration, which lasts several hours. Initial set occurs early in this stage. Acceleration is characterized by a rapid rate of hydration, significant heat, continued hardening, and strength development. The rates of reaction are faster for finer cementitious materials and for systems with higher alkali contents. Slower reacting systems will react longer and will generally provide a better microstructure in the long run. During acceleration, aluminate* and sulfate continue to react, and needle-like ettringite (C-A-S –-H) crystals form. Final set —about when the cement is hard enough to walk on—occurs before heat energy peaks (before alite [C3S] reactions begin to slow). After final set, tensile stresses start to develop due to temperature and drying effects.



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CHAPTER 3. : METHODOLOGY 3.0 Introduction Experiments were carried out to determine the bond work index of the clinker samples. The clinker microstructure examination was also carried out to determine the alite size and belite size. Wet chemical analysis of clinker was carried out to determine the chemical composition of the clinker. Afterwards bogue calculations were done to come up with the content of the four clinker phases i.e. C3S, C2S, C3A and C4AF. Also included are the kiln operating conditions applied to come up with the four different clinker samples. Lastly, physical tests were done to determine the SSA (specific surface area) usually referred to as the Blaine fineness, % consistency, initial setting time, final setting time, expansion, 2 and 7 day strength tests.

Clinker samples were taken from Sino-Zimbabwe Cement Company. Each sample comprise of around 15kg of clinker, mixing a number of clinker samples taken over a day from the conveyor belt as soon as it leaves the cooler stage. Clinker samples are taken every hour and at the end of each shift a little clinker from the hourly samples is mixed into one single sample. Some chemical and physical analysis tests are done.

The initial sampling of the clinker is very important since the quantity of clinker examined microscopically is minute when compared to the clinker output of a rotary kiln. Each sample went through visual examination and sieve analysis. The purpose for visual examination is to distinguish kiln built-up and refractory materials which may be present in the clinker. The samples were then determined for their chemical composition using the wet method. Clinker nodules were then studied under microscopy using an ordinary microscope.

The samples were prepared and polished using non-aqueous lubricant, the most generally useful etchant is hydrofluoric acid vapour, which has the merit of not removing alkali sulphates. Optical microscopic examination was carried out to examine the main clinker phases i.e. alite, belite, aluminate and ferrite, alite crystal size, belite clusters, crystallization and micro cracks within alite crystals.



33


3.1 Microscopy sample preparation i. ii.

Clinker samples were crushed using laboratory jaw crusher. Crushed samples were sieved using 5.6 mm and 1.0 mm sieves. The samples in between the sieves were collected.

iii.

Small portions (1 tablespoon) of the samples were put in a silicon container and a mixture of 18.6 gm Epofix resin and 2 ml of Epofix hardener were added on to the samples.

iv. v.

Samples were left to dry at room temperature for 24 hours. Samples were then polished using sand papers. The types of sand paper used were waterproof abrasive paper no. 200, 500, 800 and 1000. Pure ethanol was used as a lubricant. Samples were washed in ultrasonic bath for 1 minute after every polishing.

vi.

Samples were then polished for 15 minutes using 6µm diamond paste on DAPMOL cloth. 5 drops of 1,4-butandiol (C4H100 2) were used as a lubricant.

vii.

The polishing continued using 0.25µm diamond paste on DAP-NAP cloth for 20 minutes. 5 drops of 1,4-butandiol were used as a lubricant and samples were cleaned in ultrasonic bath.

viii.

Samples were finally etched using HF vapor for 10-20 seconds. Mask and gloves were worn during the etching as the HF was very corrosive.

After sample preparation, the samples were taken for microscope examination. The sample for clinker from the by-pass was examined using an ordinary microscope (fig 3.2) while the other three were examined using an electronic microscope (fig 3.1a. b, c). Pictures of the clinker from the by-pass were taken using a digital camera focused through the microscope.

The clinker samples collected also went through standard Bond grindability test. The objective of the Bond ball mill test is to determine the standard work index, which is defined as the specific power required to reduce a material from a notional infinite size to a P80 size of 100µm. The test involved series of consecutive batch grinds in laboratory mill.

3.2 Procedure of determining the bond work index Feed is prepared by crushing samples to 3.35 mm and the size distribution determined by dry sieving. A sub-sample of the feed is then riffled until there is enough material to provide 700 ml TOM HAVATYI (N005 880J)


34


tightly packed in a 1 liter measuring cylinder. The sub-sample is weighed and ground dry in a 305 by 305 mm batch ball mill operating at 70 rpm with a standard ball charge.

After a predetermined number of revolutions, the mill is emptied and all the material less than test sieve (in the present study, 75 micron was used) size is removed and weighed. Fresh unsegregated feed is added to the charge to bring its mass back to match that of the srcinal feed and it is returned to the mill. This material is ground for a number of revolutions calculated to produce a 250% circulating load after which the charge is again dumped and sized on the test sieve. The number of revolutions is calculated from the previous cycle to produce test sieve undersize equal to 1/3.5 of the total charge in the mill. The grinding cycles are continued until the net mass of test sieve undersize produced per revolution reaches equilibrium. The average of net mass per revolution from the last three cycles is taken as the ball mill grindability (Gbp) in gram per revolution. The product is also sized and the P 80 are determined (Bond, 1961) The work index is calculated from the following equation:

=

49.1 .23 ∗ .82 √108 − √108

Where;

W = Bond Work index (kWh/ton) P = Test sieve aperture (µm) Gbp =Grindability (g/revolution) F80 = 80% passing size of feed (µm) P80 = 80% passing size of product (µm)

The standard feed is prepared by stage crushing to all passing 3.35mm (6 mesh) sieve, but finer feed can be used when necessary. It is screen analysed and packed by shaking in a 1000cm 3 ( 1L ) graduated cylinder, and the weight of 700cm 3 is placed in the mill and ground dry at 250% circulating load. The standard Bond mill is 0.305m by 0.305m with rounded corners and a



35


smooth lining, except for a 100mm by 200mm door for charging. It has a revolution counter and runs at 70rpm. The grinding charge consists of 285 balls, the total weighing 20.125kg. Typically, the commercial test consist the following balls:

1) 38.1 mm =25 balls 2) 31.75mm =39 balls 3) 25.4mm =60 balls 4) 22.23mm = 68 balls 5) 19.1mm =93 balls After the first grinding period of 100 revolutions, the mill is dumped, the ball charge is screened out and the 700cm3 of materials is screened on the test sieve of the required closing size, with coarser protecting sieves if necessary. The closing size chosen will depend upon the application (e.g. expected liberation size). 75µm and 150µm are commonly used closing sizes. The undersize is weighed and fresh unsegregated feed is added to the oversize to bring its weight back to that of the original charge. Then it is returned on to the balls in the mill and ground for the number of revolutions calculated to produce a 250% circulating load, dumped and rescreened. The number of revolution needed is determined from the results of the previous period to produce sieve undersize equal to 1/3.5 of the total charge in the mill. The grinding period cycles are continued until the net grams of sieve undersize produced per mill revolution reach equilibrium and reverse its direction of increase or decrease. Then the undersize product and circulating load is screen analysed and the average of the last three net grams of final product size generated per revolution (Gbp) is defined as the ball mill grindability.

3.3 Compressive strength test of clinker Compressive strength

3.3.1 Apparatus 1. Glass Graduates, of suitable capacities (preferably large enough to measure the mixing water in a single operation). 2. Mixer, Bowl and Paddle, an electrically driven mechanical mixer equipped with paddle and mixing bowl. 3. Trowel, having a steel blade 4 to 6 in. (100 to 150 mm) in length, with straight edges. 4. Moist Cabinet or Room TOM HAVATYI (N005 880J)


36


5. Testing Machine

3.3.2 Procedure The proportions of materials for the standard mixture shall be one part of clinker to 2.75 parts of graded standard sand by mass. A water-clinker ratio of 0.485 is used. The mixture is put in an electrically driven mechanical mixture. The mixture is then molded into 6 prisms for each specimen, which are cured and tested afterwards.

3.3. 3 Temperature and Humidity Temperature — The temperature of the air in the vicinity of the mixing slab, the dry materials,

molds, base plates, and mixing bowl, shall be maintained between 20 and 27.5°C. The temperature of the mixing water, moist closet or moist room, and water in the storage tank shall be set at 23°C and shall not vary from this temperature by more than ± 1.7°C. Humidity — the relative humidity of the laboratory shall be not less than 50 percent.

3.3.4 Test Specimens Two specimens from a batch of mortar are made for each period of test or test age. Storage of Test Specimens — Immediately upon completion of molding, the test specimens are

placed in the moist closet or moist room. All test specimens, are kept immediately after molding, in the molds on the base plates in the moist closet or moist room from 20 to 24 hrs with their upper surfaces exposed to the moist air but protected from dripping water. The specimens are removed from the molds after 24 hrs, and then immersed in saturated lime water in storage tanks constructed of non-corroding materials.

3.3.5 Determination of Compressive Strength: The specimens are immediately tested after their removal from storage water.

CALCULATION The total maximum load indicated by the testing machine is recorded, and the compressive strength is calculated as follows: TOM HAVATYI (N005 880J)


37


Where:

 = 

fm = compressive strength in MPa, P = total maximum load in N, and A = area of loaded surface in mm2



38


CHAPTER 4. : RESULTS AND ANALYSIS 4.0 Introduction After the experiments, the results were obtained which are going to be analysed. The following results were obtained from the chemical analysis of the clinker samples. Table 4-1: Chemical analysis of clinker samples

Chemical

Clinker 1

composition

Clinker

Clinker Clinker Clinker Clinker

2

3

4

5

from the bypass

CaO

62.02

63.04

63.35

63.48

63.37

61.58

MgO

4.20

4.06

3.97

4.42

3.92

4.01

Fe2O3

3.68

3.87

3.66

3.88

3.58

3.80

Al2O3

5.09

5.15

5.33

5.12

5.21

5.08

SiO2

20.83

SO3

1.30

20.76 1.30

20.45 1.10

20.49 1.10

20.55 1.30

21.77 1.00

fCaO

1.60

1.70

2.10

2.10

2.70

4.34

KH

0.93

0.94

0.96

0.96

0.96

0.89

SM

2.38

2.30

2.27

2.28

2.34

2.45

AM

1.38

1.33

1.46

1.32

1.46

1.34

The formulas below show bogue calculations done to determine the amount of alite, belite, calcium silicate and tetracalcium ferrite C3A



C4 AF

2.650 Al2O3 



1.692Fe2O3

3.043Fe2O3



39


clinker

Clinker 1

phases

Clinker

Clinker Clinker Clinker Clinker

2

3

4

5

from the bypass

C3S

50.96

54.97

58.25

59.57

57.92

42.77

C2S

21.28

18.05

14.69

13.81

15.22

30.15

C3A

7.26

7.10

7.93

7.00

7.75

7.03

C4AF

11.20

11.78

11.14

11.81

10.89

11.56



40

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Data used to determine the bond work index of clinker from the by-pass Table 4-2: Bond grindability test for determining grindability factor of clinker from the bypass

prod-

grindability

revolutions product feed

feed

factor

100

452

100

352

3.5

210

850

136

714

3.4

140

652

147

505

3.6

140

648

172

476

3.4

70

354

108

246

3.5

After determining the grindability factor the Bond work index is calculated using the equation

 = .23 ∗ .49.82[110 − 10 ] √8 √8 With the last three runs equilibrium is reached, grindability factor was constant at 3.4 to 3.6 gm/rev with F80 = 1450

m and P80 = 21

m. The bond work index values obtained for clinker

from the by-pass is 30.94kWh/ton. The Bond work indices of clinker 1-6 shown in table 4.4.were obtained using the same procedure. Table 4-3: Bond Work Index Clinker sample

Clinker 1

Clinker 2

Clinker 3

Clinker 4

Clinker 5

Clinker from

the

by-pass Bond

work

21.20

19.98

19.60

19.52

20.86

30.94

index



41

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-4: Physical test results Clinker

%consistency

sample

Initial set

Final

Expansion

7-day

28-day

(min)

(min)

set

(mm)

strength

strength

(MPa)

(MPa)

1

23.2

20

35

0.0

21.5

49.2

2

23.0

58

137

0.0

29.9

51.3

3

23.2

110

174

1.0

29.4

49.0

4

23.2

60

135

0.5

29.6

50.2

5

22.8

80

115

1.5

29.2

48.9

Clinker from

23.0

78

120

4.5

8.24

32.1

the by-pass



42

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-5: Kiln inlet log sheet for clinker sample #4 tones/hr

coal feed at kiln outlet 47.45 1.99 48.00 2.05

raw meal feed

time

8 9 10 11 12 13 14 15

45.00 46.38 44.39 45.16 47.20 48.92

temperature °C

coal feed at rotary kiln kiln inlet speed 2.61 3.11 2.59 3.14

1.95 1.98 1.92 1.96 2.01 2.01

2.50 2.63 2.57 2.61 2.75 2.78

3.05 3.11 3.06 3.08 3.15 3.14

outlet of 1st outlet of inside premixing calciner heater chamber 318 888 590 306 864 593

outlet of 5th tertiary preair heater circuit 755 513 765 496

320 324 321 319 304 300

890 870 856 863 895 892

580 596 589 582 591 590

788 746 738 790 750 768

501 514 503 498 485 492

average

46.56

1.98

2.63

3.11

314

877.25

588.88

762.50

500.25

16 17 18 19 20 21 22

45.00 46.03 46.09 46.59 46.25 45.93 46.18

1.95 1.97 2.02 2.07 1.99 1.94 1.97

2.46 2.58 2.56 2.70 2.76 2.54 2.80

3.05 3.10 3.03 3.07 3.12 3.06 3.10

326 321 298 302 324 301 296

853 847 869 829 798 806 856

601 605 609 612 614 598 606

774 789 745 763 785 753 786

513 506 498 493 504 506 512

23

46.08

1.86

2.72

3.00

309

849

592

778

503

average

46.02

1.97

2.64

3.07

309.63

838.38

604.63

771.63

504.38

0 1 2 3 4 5 6 7

46.53 47.03 47.23 47.50 47.49 47.96 48.14 48.03

1.96 2.01 2.03 2.14 2.06 2.16 2.13 2.15

2.76 2.95 2.90 2.99 3.02 3.05 3.07 3.12

3.08 3.05 3.09 3.10 3.12 3.15 3.20 3.24

315 321 319 325 319 326 316 324

856 863 875 891 865 873 886 875

613 621 623 615 602 619 613 621

784 781 789 776 786 794 782 779

509 513 519 498 496 508 511 503

average

47.49

2.08

2.98

3.13

320.63

873.00

615.88

783.88

507.13



43

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-6: kiln log sheet for clinker sample #1

tones/hr

coal raw time feed at meal kiln feed outlet 8 47.45 1.99 9 10 11 12 13 14 15

48.00 45.00 46.38 44.39 45.16 47.20 48.92

temperature °C

coal feed at rotary kiln kiln inlet speed 2.61 3.11

2.05 1.95 1.98 1.92 1.96 2.01 2.01

2.59 2.50 2.63 2.57 2.61 2.75 2.78

3.14 3.05 3.11 3.06 3.08 3.15 3.14

outlet of 1st outlet of inside premixing calciner heater chamber 318 888 590

outlet of 5th tertiary preair heater circuit 755 513

306 320 324 321 319 304 300

864 890 870 856 863 895 892

593 580 596 589 582 591 590

765 788 746 738 790 750 768

496 501 514 503 498 485 492

average

46.56

1.98

2.63

3.11

314

877.25

588.88

762.50

500.25

16 17 18 19 20 21

45.00 46.03 46.09 46.59 46.25 45.93

1.95 1.97 2.02 2.07 1.99 1.94

2.46 2.58 2.56 2.70 2.76 2.54

3.05 3.10 3.03 3.07 3.12 3.06

326 321 298 302 324 301

853 847 869 829 798 806

601 605 609 612 614 598

774 789 745 763 785 753

513 506 498 493 504 506

22 23

46.18 46.08

1.97 1.86

2.80 2.72

3.10 3.00

296 309

856 849

606 592

786 778

512 503

average

46.02

1.97

2.64

3.07

309.63

838.38

604.63

771.63

504.38

0 1 2 3 4 5 6 7

46.53 47.03 47.23 47.50 47.49 47.96 48.14 48.03

1.96 2.01 2.03 2.14 2.06 2.16 2.13 2.15

2.76 2.95 2.90 2.99 3.02 3.05 3.07 3.12

3.08 3.05 3.09 3.10 3.12 3.15 3.20 3.24

315 321 319 325 319 326 316 324

856 863 875 891 865 873 886 875

613 621 623 615 602 619 613 621

784 781 789 776 786 794 782 779

509 513 519 498 496 508 511 503

average

47.49

2.08

2.98

3.13

320.63

873.00

615.88

783.88

507.13



44


Table 4-7: Kiln inlet log sheet for clinker sample 5 tones/hr

coal feed at kiln outlet 47.45 1.99

raw meal feed

time

8 9 10 11 12 13 14 15

48.00 45.00 46.38 44.39 45.16 47.20 48.92

2.05 1.95 1.98 1.92 1.96 2.01 2.01

temperature °C

coal feed at rotary kiln kiln inlet speed 2.61 3.11 2.59 2.50 2.63 2.57 2.61 2.75 2.78

3.14 3.05 3.11 3.06 3.08 3.15 3.14

outlet of 1st outlet of inside premixing calciner heater chamber 318 888 590

outlet of 5th tertiary preair heater circuit 755 513

306 320 324 321 319 304 300

864 890 870 856 863 895 892

593 580 596 589 582 591 590

765 788 746 738 790 750 768

496 501 514 503 498 485 492

average

46.56

1.98

2.63

3.11

314

877.25

588.88

762.50

500.25

16 17 18 19 20 21

45.00 46.03 46.09 46.59 46.25 45.93

1.95 1.97 2.02 2.07 1.99 1.94

2.46 2.58 2.56 2.70 2.76 2.54

3.05 3.10 3.03 3.07 3.12 3.06

326 321 298 302 324 301

853 847 869 829 798 806

601 605 609 612 614 598

774 789 745 763 785 753

513 506 498 493 504 506

22 23

46.18 46.08

1.97 1.86

2.80 2.72

3.10 3.00

296 309

856 849

606 592

786 778

512 503

average

46.02

1.97

2.64

3.07

309.63

838.38

604.63

771.63

504.38

0 1 2 3 4 5 6 7

46.53 47.03 47.23 47.50 47.49 47.96 48.14 48.03

1.96 2.01 2.03 2.14 2.06 2.16 2.13 2.15

2.76 2.95 2.90 2.99 3.02 3.05 3.07 3.12

3.08 3.05 3.09 3.10 3.12 3.15 3.20 3.24

315 321 319 325 319 326 316 324

856 863 875 891 865 873 886 875

613 621 623 615 602 619 613 621

784 781 789 776 786 794 782 779

509 513 519 498 496 508 511 503

average

47.49

2.08

2.98

3.13

320.63

873.00

615.88

783.88

507.13



45

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-8: Kiln outlet log sheet of clinker sample #4 time

gc speed RPM

temperature°C

blower chamber #1

4.5 821 4.4 810 4.5 798 4.5 835 4.4 845 4.5 806 4.6 798 4.1 836

30 28 26 28 28 30 29 28

4230 3968 4128 4053 3987 4256 4269 4023

4.3 4.3 4.2 3.9 4.3 4.2 4.1 4.3

29 30 29 28 26 28 29 29

4365 3967 4128 4265 4362 4129 4053 3986

kiln inlet 8 9 10 11 12 13 14 15 averag e 16 17 18 19 20 21 22 23 averag e

pressure

grate chamber #1

845 821 814 823 815 831 816 821


opening of valves %

blower blower chamber for #2 chamber #1

product quality

blower for chamber #2


291 257 263 305 315 303

28 28 28 28 28 28 28 28

16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

43 42.9 42.9 42.8 43.0 43.1 43.2 43.3

1.7 1.7 2 1.7 1.9 1.8 1.8 1.9

8.3 4.5 5.6 6.8 6.8 6.5 6.6 6.4

1.5 1.3 1.6 1.7 1.5 1.4 1.2 1.5

308 297 286 307 316 325 316 304

28 28 28 28 28 28 28 28

16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

43 42.9 42.8 42.8 42.9 43.1 43 43.2

1.1 1.8 1.9 1.9 1.8 1.7 1.8 1.9

6.4375 7.3 6.8 7.6 7.9 6.9 6.5 7.1 7.2

1.8 1.7 1.8 1.6 1.3 1.8 1.6

268 278

kiln feed CaO

kiln feed Fe2O3

fineness 45µm

F-CaO in clinker

7.1625


46

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-9: Kiln outlet log sheet of clinker sample #5 time

gc speed RPM

temperature°C

kiln inlet 8 9 10 11 12 13 14 15 averag e 16 17 18 19 20 21 22 23 averag e

pressure

grate chamber #1

blower chamber #1

opening of valves %

blower blower chamber for #2 chamber #1

3.6 3.7 3.7 3.8 3.7 3.5 3.6 4.1

938 956 948 987 1023 986 945 920

30 29 30 28 29 30 30 29

3620 3482 3586 3691 3521 3560 3601 3945

263 258 243 289 320 315

26 26 26 26 26 26 26 26

4 3.9 3.8 3.9 4 3.9 3.9 3.9

925 984 974 962 934 899 865 854

28 31 32 31 30 30 29 30

3875 3801 3784 3690 3756 3650 3647 3590

298 276 284 294 299 305 316 309

27 27 28 28 28 28 28 26


245 251

product quality



16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

43.2 42.9 43.5 43.1 43.0 43.2 42.9 43

1.6 1.7 1.7 1.8 1.8 1.7 1.9 1.8

7.5 7.4 6.9 6.8 8.7 9.7 9.4 8.6

0.8 1.0 1.2 0.6 0.5 0.9 1.3 1.4

16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

43.1 43.1 43 42.9 43 43.2 43.1 43.1

1.6 1.7 1.7 1.8 1.8 1.6 1.7 1.7

8.125 8.3 6.6 5.6 6.4 7.1 7.2 8.5 7.9

1.2 0.9 1.0 1.0 0.9 1.2 1.3 1.4

kiln feed CaO

kiln feed Fe2O3

fineness 45µm

F-CaO in clinker

7.2


47

INFLUENCE OF CLINKER MICROSTRUCTURE ON CLINKER QUALITY Table 4-10: Kin outlet log sheet for clinker sample #1 time temperature°C

opening of valves %

product quality

grate chambe r #1

blower chamber #1

blower chambe r#2

blower for chambe r #1



3.6 780 3.5 825 3.5 830 3.6 844 3.7 846 3.5 829 3.7 839 3.7 847

30 28 26 28 28 30 29 28

3541 3467 3512 3325 3502 3469 3514 3298

240 259 260 256 258 306 246 280

27 27 27 27 27 27 27 27

16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

43.1 43 43.2 43 42.9 42.7 43.1 43

1.9 1.8 1.7 1.8 1.7 1.9 2 1.9

12.3 11.9 5.6 8.5 9.1 13.2 10 6.2

1.6 1.8 1.9 1.8 1.7 1.6 2.0 2.1

4 3.9 3.8 3.5 4 3.9 3.9 3.8

29 30 29 28 26 28 29 29

3560 3475 3360 3459 3523 3360 3546 3268

290 286 256 286 274 250 234 238

28 28 28 28 28 28 28 26

16 16 16 16 16 16 16 16

6 6 6 6 6 6 6 6

42.9 42.9 43.1 43.2 43 49.9 43.2 43.2

1.7 1.8 1.8 1.9 1.6 1.7 1.8 1.9

9.6 9.8 8.3 7.4 5.2 6.6 8.9 7.9 5.5

2.3 2.0 2.1 2.2 1.9 1.8 1.9 2.0

gc speed RPM

8 9 10 11 12 13 14 15 averag e 16 17 18 19 20 21 22 23 averag e

pressur e

kiln inlet

859 854 865 863 847 854 863 855


kiln feed CaO

kiln feed Fe2O3

fineness 45µm

F-CaO in clinker

7.45


48


4.2 Microscopic results and analysis

Figure 4-1: Microstructure of clinker form the by-pass

Results Bond work index -30.94 7-day strength-8.24MPa 28-day strength-32.1MPa

The photograph above shows the microstructure of clinker from the by-pass. The round structure dominant in the sample above is belite. The picture above shows a lot of belite content with grain size of 10μm - 20 m. It has a few alite with smaller grain size which appeared in between belite particles. This microstructure indicates a few alite particles present in this sample. From the grindability tests the clinker requires a lot of energy to be ground, and also shows a very low early strength as expected of clinker having low alite content. The 28-day strength is not very low since belite starts to be responsible for strength development at later stages. Nevertheless the strength is not good enough as the strength is not entirely dependent on belite, alite still plays a role. Clinker from the by-pass is ungrounded clinker that escapes the cement mill and accumulates on the side of the mill. Clinker from the by-pass might be a result of flushing. Flushing is when the kiln feed does not take adequate time in the kiln and as a result would be discharged when the material is still in its early clinkerisation stages. The resistant clinker cannot be used for anything and would be thrown away.



49


Figure 4-2: Microstructure of clinker sample #4

Results Bond work index –19.52 7-day strength-29.6MPa 28-day strength-50.2MPa

The tan crystals showing hexagonal outlines are alite. The more rounded, dark crystals are belite. The interstitial phase was a melt which partially separates the primary alite and belite crystals. It has crystallized during cooling. The kiln log sheets show that raw meal with the right fineness was fed into the kiln. The temperatures the kiln speed were good enough to allow clinkerisation to take place in the burning zone. From the picture above there is relatively more alite than in Fig 4. The clinker above would require less grinding energy and would have good early strength because of the alite size and content. However the 28day strength does not differ from the 7-day strength by a wide margin since there is less belite content.



50


Figure 4-3: microstructure of clinker sample #1


The above clinker microstructure shows belite clusters. This clustering is developed when relatively coarse pitsand (SiO 2) crystals react with lime and clay forming local belite clusters. Compared to the other clinker samples the above sample was formed from relatively coarse raw meal. Effect on clinker: Coarse feed is harder to burn. Non-uniform distribution of belite crystals in nests or clusters can decrease clinker grindability. The bond work index of the above clinker sample is 21.20 which is high compared to the other clinker samples. The bond work index would have been worse had the cluster been tightly packed. Effect on cement properties: Belite reactivity may be decreased and therefore 28-day strength may be affected. As shown by the experimental results the strength of clinker sample #1 has been low from the start and did not improve as expected from clinker having higher belite content.



51


Figure 4-4: microstructure of clinker sample #5


At high temperatures, what is now the interstitial phase was a melt which partially separates the primary alite and belite crystals. It has crystallised during cooling to a mixture of ferrite and dendritic C3 A solid solutions.. Overburning Observation: light brown colour of a mixture of ferrite and tri-calcium-aluminate Cause: Overburning (exposure to high temperature for a long time). Effect on clinker: Overburning reduces the porosity of clinker and increases its liter weight. Loss of porosity makes clinker too hard and difficult to grind. Effect on cement properties: Belite reactivity may be decreased and therefore 28-day strength is affected.



52


CHAPTER 5. : CONCLUSION AND RECOMMENDATIONS 5.1 Conclusion The chemical analysis shows that the chemical composition of the clinker product has varied higher and lower than the typical range of the clinker product, alite (C3S) varied between (53.8442.77%), belite (C2S) varied between (30.15-13.81%), aluminate (C3A) varied between (7. 937.03%) and ferrite (C4AF) varied between (11.81-10.89%). The clinker from the by-pass showed extreme values of 42.77% C3A and 30.15% C2S.

Judging from the experimental results grindability of portland cement clinker is affected by the chemical composition and mineralogical properties of the clinker. Basing on chemical analysis higher alite content, lower belite, aluminate and ferrite content result in better grindability and good strength. The crystal microstructure of clinker is formed by everything that goes into it and what happens to it along the way. In other words, there is a relationship between clinker microstructure, the kiln feed, and burning conditions. Under the microscope, smaller alite crystal size will result in better cement strength and grindability, while larger crystal size will result in poor grindability and so does lower alite content, higher belite, aluminate and ferrite.

The formation of clinker microstructures was also affected by the fineness of the raw meal in the feed. Bigger grain size of the raw meal will result in larger alite and larger belite crystals. Formations of belite clusters are also associated with coarse raw mixes. Raw mix having coarse particles of 12% above 45µm formed larger belite clusters, pore-centered belite nests, belite streaks, belite inclusions in alite and larger alite crystals.

5.1 Recommendations The use of microscopy is an old tool used for assessing clinker and cement quality. The technology has advanced since and now more advanced microscope techniques are being employed which even an unskilled microscopist can use. However not many cement manufacturing industries in Africa use microscopy as a quality assessment tool. Mineralogical and chemical studies on clinker provide information about clinker characteristics and about conditions occurring at various stages of the manufacturing process. Reflected light microscopy that is commonly used for clinker micro-examination provides more information on the TOM HAVATYI (N005 880J)


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mineralogy of the clinker such as: determination of the main phases, degree of crystallization, alite crystal size, micro-cracks within the alite crystals and belite clusters. Understanding the clinker microstructure is very crucial in cement manufacturing. It can be a powerful technique that can improve clinker production and quality. Using microscopy, one can gather remarkable information about clinker history and predict cement performance. A look in the microscope can determine the temperature profile in the kiln and provide clues to improve clinker grindability, optimize raw feed fineness, or increase 28-day strength. As the clinker microscopy is performed routinely, operators get to know the microstructure of the plant 'typical' clinker. Physical tests and chemical tests provide valuable quality control but cannot tell the whole story Microscopy allows one to troubleshoot and identify the causes of poor clinker grindability or low cement mortar strength with just a few minutes of lab work. It is therefore essential that cement manufacturing companies use microscope for clinker and cement quality assessment. Kiln feed-The primary step in the process of cement manufacture is the combination of silica with calcium to produce hydraulic compounds. In order to produce these compounds, the cement plant chemist needs to choose the raw mix components carefully; often several components are required. The chosen raw materials must then be ground into a fineness adequate to produce burnable kiln feed, but variances in grindability and burnability of the materials offer many challenges. For instance, in the raw meal preparation section limestone is much easier to grind and burn than quartz. If quartz particles are coarse in the feed, they will make the feed harder to burn and may leave a cluster of silicate crystals in the clinker, leading to decreased clinker grindability. The chemical composition and fineness of the feed components will influence the amount of the compounds formed, their distribution, and size. I therefore recommend that mill operators should ensure that the right the raw meal fineness is achieved. Flushing –occurs when kiln feed is suddenly released from the pre-heater cyclones. The clinker produced would have not been subjected to sufficient heat for clinkerisation to finish. This can be avoided by installing flap valves in every pre-heater cyclone which release kiln feed at intervals. The raw meal accumulates in the cyclone until its weight reaches a certain minimum on which the valve flaps open releasing the raw meal to the next pre-heater cyclone. This ensures enough calcination to occur before the kiln feed gets into the kiln. TOM HAVATYI (N005 880J)


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Pyroprocessing-Burning conditions in the kiln develop the desired hydraulic compounds from the raw materials. The many factors that constitute burning conditions all relate to some aspect of the formed microstructure. Heating rate, cooling rate, kiln atmosphere and maximum temperature determine the size, morphology, and abundance of various compounds in the clinker. A quick-heating rate and fine raw feed promote smaller alite crystals. Alite is quick to react therefore its properties (abundance, size) affect early strength. Alite is abundant in clinker if the conditions in the kiln permit, which are high temperatures and enough residence time. In order to achieve and maintain high temperatures in the kiln I recommend that there be installation of kiln off gas analyzers which give the composition of gases exiting the kiln. In this way operators have an idea of how reactions progress in the kiln. It also helps in detecting causes of low kiln temperatures. The presence of carbon monoxide in the kiln off gases shows incomplete combustion which consumes more fuel releasing less heat. Incomplete combustion can either be avoided by feeding more air to the system or reducing the amount of fuel fired into the kiln, maintaining high temperatures in the kiln which favour clinkerisation. If the raw meal preparation and the pyroprocessing stages are not done properly smaller sized clinker particles are made which impede gas movement affecting clinker cooling. Larger clinker particles arising from proper pyro-processing support good inter-particle gas contact. It is therefore imperative that the preceding processes be carried out in a manner that does not affect the downstream processes.



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BIBLIOGRAPHIC REFERENCES 1. F. M. Lea, “Lea’s Chemistry of Cement and Concrete,” Fourth Edition , (2004), p. 200268. 2. Altun, Akin, “Effects of kiln systems on mcrostructure of clinker,” Cimento ve Beton Dunyasi, v. 3, (1999), p. 33-41. 3. Altun, Akin, “Microscopic criteria for quality control of clinker,” Cimento ve Beton Dunyasi, v.2, p. 22-32. 4. Centurione, Sergio Luiz; Marcelo, “The influence of burning conditions on alite crystal characteristics,” Proceedings Of The International Conference On Cement Microstructure (1995), 17th,p. 232-41 5. Dorn, J. D “The influence of coarse quartz in kiln feed on the quality of clinker and cement,” Proceedings Of The 7th International Conference On Cement Microscopy , 1985, p. 10-23 6. Gavel, Viktoria; Opocky, Ludmilla; Sas, Laszlo, “Relationship between techno logical parameters, structure and grindability of clinkers,” Epitoanyag, 2000, v.52, p. 34-38. 7. Ghosh, S. P; Mohan, K “Interrelationship among lime content of clinker, its microstructure, fineness of PC grinding and strength development of hydrated cement at different ages,” Proceedings of the International Congress on Cement Chemistry of Cement, 10th, 1997, v.2, p. 2ii018-4pp 8. Hargrave, R.V., et al, “Assessment of process effects on clinker microstructure through its quantification,” Proceedings of the International Conference on Cement Microscopy (1983), p. 99-120. 9. Zampier, Valdir A.; Munhz, Flavio A. C., “Mechanical strength and grindability of Portland clinkers with different mineralogical characteristics,” Proceedings of the International Conference on Cement Microscopy(1995), 17th, p. 293-310 10. Hills, Linda M., ‘The Effect of Clinker Microstructure on Grindability: Literature Review Database”, Portland Cement Association, 1995. 11. Hills, linda m, “The influence of clinker microstructure n grindability: Results of an extensive literature review,” Proceedings of the International Conference on Cement Microscopy (1995), 17th, p. 344-52. 12. Kihara, Yushiro; Centurione, Sergio L.; Cunha Munhoz, Flavio Andre da , “an approach to the prediction of Portland clinker grindability and strength by microscopy,” International Congress on the Chemistry of Cement, 9th (1992), v. 6, p. 182-8 13. Kim, K; Chu, S; Lee, H.; Song, S., “Effect of free CaO on the hydration of reaction and physical properties of cement,” Journal of the Korean Ceramic Society, v.34, 1997, p. 399-405. 14. Moore, C. W, “Portland Cement Clinker Grindability and Work Index,” American ceramic society bulletin, v.74, 1995, p. 80-85. 15. Odigure, J. O, “Grindability of cement clinker from raw mix containing coarse mix,” Cement and Concrete Research, v. 29, p. 303-309.



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16. Ono, Yoshio, “Microscopical Observation of Clinker for the Estimation of Burning Condition, Grindability and Hydraulic Activity” Proceedings of the International Conference on Cement Microscopy (1981), 3rd, p. 198-210. 17. Sas, Lasso; Opocky, Ludmilla; Gavel, Viktoria, “Knowing clinker microstructure -a possibility to influence grindability through technology,” Proceedings of the International Conference on Cement Microscopy (2000), 22nd, p. 215-224 18. Schuebel, B., “Microscopically Determinable Parameters and their Relationship to Kiln System and Clinker Grindability,” Proceedings of the International Conference on Cement Microscopy (1985), 7th, p.131-153. 19. Slim, Freddy; Tagnit-Hamou, Arezki; Marciano, Everaldo Jr., “Use of optical microscopy on raw meal fineness optimization,” Proceedings of the International Conference on Cement Microscopy (1996), 18th, p. 21-32. 20. Theisen, Kirsten, “Estimation of Cement Clinker Grindability”, Proceedings of the International Conference on Cement Microscopy (1993), 15th, p. 1-14. 21. Tsivilis, S.;Kakali, G., “Study on the grindability of portland cement clinker containing transition metal oxides,” Cement and Concrete Research, v. 27, 1997, p. 673-678. 22. Venkateswaran, D., and Gore, V.K., “Application of Microstructural Parameters to the Grindability Prediction of Industrial Clinkers” Proceedings of the International Conference on Cement Microscopy (1991), 13th, p.60-70. Viggh, Erik O., “Estimation of grindability of Portland cement clinker,” World Cement, v. 25 (1994), p. 44-6, 48, 66-7, 73-4.



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Appendix 1: Generalised flow sheet for cement manufacturing



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Appendix 2: Kiln Inlet



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Appendix3: Kiln Outlet



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INFLUENCE OF CLINKER MICRO-STRUCTURE ON CLINKER QUALITY

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