(ckd) with fly ash and ground granulated blast furnace ...

CLARKSON UNIVERSITY

INTERACTION OF CEMENT KILN DUST (CKD) WITH FLY ASH AND GROUND GRANULATED BLAST FURNACE SLAG: A MICROSTRUCTURAL INVESTIGATION

A Thesis by Piyush Chaunsali

Department of Civil and Environmental Engineering

Submitted in partial fulfillment of the requirements for the degree of Master of Science, Civil Engineering

June 18, 2010

Accepted by the Graduate School

Date

Dean

The undersigned have examined the thesis entitled “Interaction of Cement Kiln Dust (CKD) with Fly Ash and Ground Granulated Blast Furnace Slag: A Microstructural Investigation” presented by Piyush Chaunsali, a candidate for the degree of Master of Science and hereby certify that it is worthy of acceptance.

Date

Gordon Batson

Date

Sulapha Peethamparan

Date

Narayanan Neithalath

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Dedicated to My Parents

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ACKNOWLEDGEMENTS

First of all, I am deeply indebted to my adviser Prof. Sulapha Peethamparan for giving me an opportunity to work with her for my MS program. Her constant support and encouragement throughout the study is highly commendable. I will always be grateful for the learning under her tutelage throughout my life. I would also like to thank Prof. Gordon Batson and Prof. Narayanan Neithalath for serving on my thesis committee. I express my thanks to Prof. Narayanan Neithalath for the useful discussions during my stay at Clarkson University. I especially would like to acknowledge the help from Ted Champagne and Christopher Plunkett in SEM and TEM examination during the study. The facilities provided by Center for Advanced Materials Processing (CAMP) at Clarkson University are also highly acknowledged. I would like to thank my friends Jitendra A. Jain, Hieu T. Cam, Evan K. Lake, and Deepak Ravikumar for their help during the course of study. Finally, I express my deepest gratitude to my parents whose constant support has been a motivation in my pursuit of higher education. I will always be grateful for their sacrifice.

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Table of Contents LIST OF SYMBOLS ..................................................................................................... VIII LIST OF TABLES ............................................................................................................ IX LIST OF FIGURES ........................................................................................................... X ABSTRACT ................................................................................................................... XVI CHAPTER 1: INTRODUCTION ....................................................................................... 1 1.1 Background and Research Needs.............................................................................. 1 1.2 Objective and Scope ................................................................................................. 2 1.3 Hypothesis................................................................................................................. 3 1.4 Outline of the Thesis ................................................................................................. 4 CHAPTER 2: LITERATURE REVIEW AND RESEARCH MOTIVATION .................. 5 2.1 Introduction ............................................................................................................... 5 2.2 Cement Manufacturing and Production of Cement Kiln Dust.................................. 5 2.3 Physical and Chemical Properties of CKD ............................................................... 7 2.4 Beneficial Uses of CKD............................................................................................ 9 2.4.1 CKD as Soil Stabilizer ....................................................................................... 9 2.4.2 CKD for Blended Cements ................................................................................ 9 2.5 Methods for Activating Fly Ash and Slag .............................................................. 10 2.5.1 Alkali Activation .............................................................................................. 10 2.5.2 Thermal Activation .......................................................................................... 12 2.5.3 Sulfate Activation ............................................................................................ 13 2.5.4 Lime Activation ............................................................................................... 14 2.6 Interaction of CKD with Fly ash and Slag .............................................................. 14 2.7 Durability of CKD-Based Binders .......................................................................... 16 2.8 Research Motivation ............................................................................................... 16 CHAPTER 3: EXPERIMENTAL PROGRAM AND MATERIALS .............................. 17 3.1 Introduction ............................................................................................................. 17 3.2 Experimental Methods ............................................................................................ 17 3.2.1 Setting Time and Flow Test ............................................................................. 17 3.2.2 Compressive Strength Test .............................................................................. 17 3.2.3 X-ray Diffraction Analysis .............................................................................. 18 3.2.4 Thermogravimetric Analysis ........................................................................... 18 3.2.5 Scanning Electron Microscopy ........................................................................ 18 3.2.6 Transmission Electron Microscopy ................................................................. 18 3.2.7 Modulus of Elasticity ....................................................................................... 19 3.2.8 Expansion Test ................................................................................................. 19 3.2.9 Sample Preparation .......................................................................................... 19 3.3 Materials ................................................................................................................. 21 3.3.1 Particle Size Distribution ................................................................................. 21 3.3.2 Chemical Composition..................................................................................... 23 3.3.3 Mineralogical Composition ............................................................................. 24 3.3.4 Morphological Characteristics ......................................................................... 25 CHAPTER 4: ENGINEERING PROPERTIES OF CKD-BASED BINDERS ............... 27 4.1 Introduction ............................................................................................................. 27 v

4.2 Early Age Properties ............................................................................................... 27 4.2.1 Setting Time and Flow Test ............................................................................. 27 4.3 Compressive Strength of Paste ............................................................................... 30 4.3.1 CKD(I)-FA Paste ............................................................................................. 30 4.3.1.1 Determination of Optimum Temperature for Heat Curing ....................... 30 4.3.1.2 Determination of Optimum Binder Proportion ......................................... 31 4.3.1.3 Effect of Heat Curing Followed by Lime Curing ..................................... 32 4.3.2 CKD(I)-Slag Paste ........................................................................................... 33 4.3.2.1 Optimum Curing Temperature for Heat Curing ....................................... 33 4.3.2.2 Determination of Optimum Binder Proportion ......................................... 33 4.3.2.3 Effect of Heat Curing Followed by Lime Curing ..................................... 34 4.3.3 CKD(II)-FA and CKD(II)-Slag Pastes ............................................................ 36 4.4 Compressive Strength of Concretes Incorporating CKD(I)-Based Binders ........... 37 4.5 Summary ................................................................................................................. 41 CHAPTER 5: MICROSTRUCTURAL CHARACTERIZATION OF CKD PASTE...... 42 5.1 Introduction ............................................................................................................. 42 5.2 Mineralogical Characterization of CKD(I) Paste ................................................... 42 5.2.1 X-ray Diffraction Patterns................................................................................ 42 5.2.2 Thermal Analysis ............................................................................................. 43 5.3 Morphological Characterization of CKD(I) Paste .................................................. 45 5.3.1 SEM examination after 48 hours of heat curing .............................................. 45 5.4 Mineralogical Characterization of CKD(II) Paste .................................................. 49 5.4.1 X-ray Diffraction patterns ................................................................................ 49 5.4.2 Thermogravimetric Analysis ........................................................................... 50 5.4.3 Morphological Investigation of CKD(II) Paste ............................................... 52 5.4.3.1 After 48 hours of heat curing .................................................................... 52 5.5 Summary ................................................................................................................. 55 CHAPTER 6: MICROSTRUCTURAL CHARACTERIZATION OF CKD-FLY ASH PASTE .............................................................................................................................. 56 6.1 Introduction ............................................................................................................. 56 6.2 Mineralogical Characterization of CKD(I)-FA Paste ............................................. 56 6.2.1 X-ray Diffraction Patterns................................................................................ 56 6.2.2 Thermogravimetric Analysis ........................................................................... 58 6.3 Morphological Investigation of CKD(I)-FA paste ................................................. 60 6.3.1 SEM Examination after 48 Hours of Heat Curing ........................................... 60 6.3.2 SEM Examination after 28 Days of Lime Curing ........................................... 65 6.3.3 TEM Examination after 48 Hours of Heat Curing........................................... 68 6.4 Mineralogical Characterization of CKD(II)-FA Paste ............................................ 70 6.4.1 X-ray Diffraction Patterns................................................................................ 70 6.4.2 Thermogravimetric Analysis ........................................................................... 71 6.5 Morphological Investigation of CKD(II)-FA Paste ................................................ 72 6.5.1 SEM Examination after 48 Hours of Heat Curing ........................................... 72 6.6 Summary ................................................................................................................. 76 CHAPTER 7: MICROSTRUCTURAL CHARACTERIZATION OF CKD-SLAG PASTE .............................................................................................................................. 77 vi

7.1 Introduction ............................................................................................................. 77 7.2 Mineralogical Characterization of CKD(I)-Slag Paste ........................................... 77 7.2.1 X-ray Diffraction Patterns................................................................................ 77 7.2.2 Thermogravimetric Analysis ........................................................................... 79 7.3 Morphological Investigation of CKD(I)-Slag Paste ............................................... 80 7.3.1 SEM Examination after 48 Hours of Heat Curing ........................................... 80 7.3.2 SEM Examination after Heat Curing Followed by 28 Days of Lime Water Curing ....................................................................................................................... 84 7.3.3 SEM Examination of Only Moist Cured Samples after 28 Days of Lime Water Curing ....................................................................................................................... 86 7.3.4 TEM Examination after 48 Hours of Heat Curing........................................... 89 7.4 Mineralogical Characterization of CKD(II)-Slag Paste .......................................... 92 7.4.1 X-ray Diffraction Patterns................................................................................ 92 7.4.2 Thermogravimetric Analysis ........................................................................... 93 7.5 Morphological investigation of CKD(II)-Slag Paste .............................................. 95 7.5.1 SEM Examination after 48 Hours of Heat Curing ........................................... 95 7.5.2 SEM Examination after Heat Curing Followed by 28 Days of Lime Curing .. 99 7.5.3 TEM Examination after 48 Hours of Heat Curing......................................... 101 7.6 Summary ............................................................................................................... 103 CHAPTER 8: DURABILITY OF CKD-BASED BINDERS ......................................... 104 8.1 Introduction ........................................................................................................... 104 8.2 Delayed Ettringite Formation ............................................................................... 104 8.3 Alkali -Silica Reaction .......................................................................................... 106 8.4 Freeze-Thaw Resistance ....................................................................................... 108 8.5 Summary ............................................................................................................... 110 CHAPTER 9: CONCLUSIONS AND RECOMMEDATIONS FOR FUTURE STUDIES ......................................................................................................................................... 111 9.1 Conclusions ........................................................................................................... 111 9.2 Variation in CKD Composition ............................................................................ 115 9.3 Recommendations for Future Study ..................................................................... 115 REFERENCES ............................................................................................................... 116 APPENDIX ..................................................................................................................... 121 VITA ............................................................................................................................... 122

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LIST OF SYMBOLS

AFt ………………………………………………………..3CaO.Al2O3.3CaSO4.32H2O AFm………………………………………………………...3CaO.Al2O3.CaSO4.12H2O C3A…………………………………………………………………………3CaO.Al2O3

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LIST OF TABLES Table 2.1 Chemical composition of CKD (Todres et al. 1992) .......................................... 7 Table 2.2 Particle size distributions of CKD (Todres et al. 1992) ...................................... 8 Table 3.1 Chemical Compositions of CKDs, fly ash and Slag ......................................... 23 Table 4.1 Initial and Final Setting Times of Different Binders ........................................ 28

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LIST OF FIGURES Figure 2.1 Schematic of Cement Manufacturing Process ................................................... 6 Figure 2.2 Schematic diagram of alkali activation of fly ash ........................................... 11 Figure 3.1 Particle size distribution of materials used in the study .................................. 22 Figure 3.2 XRD patterns of two different CKDs .............................................................. 24 Figure 3.3 XRD patterns of fly ash and slag ..................................................................... 25 Figure 3.4 SEM micrographs of raw powders: (a) CKD(I) and (b) CKD(II) ................... 26 Figure 3.5 SEM micrographs of: (a) class F fly ash, and (b) slag powders ...................... 26 Figure 4.1 Flow values of CKD-based binders ................................................................. 29 Figure 4.2 Effect of curing temperature on the compressive strength of CKD(I)-FA binder ................................................................................................................................ 30 Figure 4.3 Effect of CKD(I) fraction on the compressive strength of CKD(I)-FA binder after 48 hours of heat curing at 75oC ................................................................................ 31 Figure 4.4 Effect of heat curing followed by lime curing on the strength development of CKD(I)-FA binder ............................................................................................................ 32 Figure 4.5 Effect of curing temperature on the compressive strength of CKD(I)-Slag paste .................................................................................................................................. 33 Figure 4.6 Effect of CKD(I) fraction on the compressive strength of CKD(I)-Slag paste 34 Figure 4.7 (a) Effect of heat curing and subsequent saturated lime curing on the strength development of CKD(I)-Slag paste, and (b) Effect of only saturated lime curing on the strength development of CKD(I)-Slag paste..................................................................... 35

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Figure 4.8 (a) Effect of heat curing and subsequent saturated lime curing on the compressive strength of CKD(II)-FA and CKD(II)-Slag pastes, and (b) Effect of lime curing on non-heat cured CKD(II)-Slag paste .................................................................. 37 Figure 4.9 Effect of paste volume on the strength of concretes having CKD(I)-Slag binder after 48 hours of heat curing ............................................................................................. 38 Figure 4.10 Effect of binder type and curing duration on the compressive strength ........ 39 Figure 4.11 Effect of lime curing on the strength development of non-heat cured CKD(I)Slag binder concretes ........................................................................................................ 40 Figure 5.1 X-ray diffraction patterns of CKD(I) paste at different curing periods ........... 43 Figure 5.2 Thermal analysis of CKD(I) paste: a) TGA, and b) DTG plots ...................... 44 Figure 5.3 SEM micrograph of CKD(I) paste after heat curing with EDX, showing the presence of alkali sulfate ................................................................................................... 45 Figure 5.4 SEM micrograph of CKD(I) paste after heat curing with EDX, showing the presence of calcium carbonate crystals ............................................................................. 46 Figure 5.5 SEM micrograph of CKD(I) paste showing the presence of ettringite ........... 47 Figure 5.6 (a) and (b) SEM micrographs of CKD(I) paste after heat curing showing the microstructure at high magnification with EDX patterns ................................................. 48 Figure 5.7 X-ray diffraction patterns of heat cure CKD(II) paste at various curing periods ........................................................................................................................................... 49 Figure 5.8 Thermal analysis of CKD(II) paste: a) TGA, and b) DTG plots ..................... 51 Figure 5.9 SEM micrograph of CKD(II) paste with EDX patterns, showing the presence of: a) C-A-S-H gel, and b) syngenite ................................................................................ 53

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Figure 5.10 SEM micrograph of CKD(II) paste after heat curing showing the presence of sylvite and calcite minerals ............................................................................................... 54 Figure 6.1 X-ray diffraction patterns of CKD (I)-FA paste at different curing periods .. 57 Figure 6.2 Thermal analysis of CKD(I)-FA paste: (a) TGA, and (b) DTG plots ............. 59 Figure 6.3 Microstructure of CKD(I)-FA paste after 48 hours of heat curing ................. 60 Figure 6.4 SEM micrograph of CKD(I)-FA paste after heat curing, showing the presence of ettringite ........................................................................................................................ 61 Figure 6.5 SEM image of dissolved FA particle after heat curing ................................... 62 Figure 6.6 (a) and (b) SEM micrographs showing the reaction gel forming cover on fly ash particle ........................................................................................................................ 63 Figure 6.7 (a) and (b) SEM micrographs of CKD(I)-FA paste after heat curing with EDX pattern, showing the composition and morphology of C-A-S-H ..................................... 64 Figure 6.8 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of subsequent lime water curing with EDXs, showing the extensive formation of arcanite crystals (K2SO4) and C-A-S-H gel ................................................................................................. 66 Figure 6.9 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of lime curing, showing the fully covered fly ash particle and arcanite crystals ...................................... 67 Figure 6.10 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of lime curing, showing the presence of C-A-S-H gel .................................................................. 67 Figure 6.11 TEM micrograph of CKD(I)-FA paste after 48 hours of heat curing, showing the formation of C-S-H gel around fly ash particle .......................................................... 68 Figure 6.12 TEM micrograph of CKD(I)-FA paste after heat curing, showing the fibrillar morphology of potassium substituted C-A-S-H gel.......................................................... 69 xii

Figure 6.13 X-ray diffraction patterns of CKD(II)-FA at different curing periods .......... 70 Figure 6.14 Thermal analysis of CKD(II)-FA paste: a) TGA, and b) DTG plots ............ 72 Figure 6.15 SEM micrograph of CKD(II)-FA after 48 hours of heat curing, showing the microstructure ................................................................................................................... 73 Figure 6.16 (a) and (b) SEM micrographs of CKD(II)-FA paste after heat curing with EDXs, showing the microstructure and reaction shell formation ..................................... 74 Figure 6.17 SEM micrographs of CKD(II)-FA paste after heat curing, showing the microstructure at high magnification ................................................................................ 75 Figure 7.1 X-ray diffraction pattern of CKD(I)-Slag paste at different curing periods .... 78 Figure 7.2 Thermal analysis of CKD(I)-Slag: a) TGA, and b) DTG plots ....................... 79 Figure 7.3 SEM micrograph of CKD(I)-Slag after 48 hours of heat curing ..................... 80 Figure 7.4 Clusters of ettringite crystals in CKD(I)-Slag paste after 48 hours of heat curing ................................................................................................................................ 81 Figure 7.5 SEM micrograph, at high magnification, showing the presence of C-S-H gel in CKD(I)-Slag after 48 hours of heat curing ....................................................................... 82 Figure 7.6 SEM micrograph of CKD(I)-Slag paste after heat curing, showing the presence of AFm phase ..................................................................................................... 83 Figure 7.7 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of subsequent lime curing with EDXs ..................................................................................................... 84 Figure 7.8 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of lime water curing, showing the presence of calcium carbonate and C-S-H gel ................................. 85 Figure 7.9 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of subsequent lime curing, showing the morphology of C-S-H gel......................................................... 86 xiii

Figure 7.10 (a) and (b) SEM micrographs of moist cured CKD(I)-Slag paste, (c) EDX patterns at location 1 and 2 ............................................................................................... 87 Figure 7.11 SEM micrograph of moist cured CKD(I)-Slag paste with EDX at location 1 ........................................................................................................................................... 88 Figure 7.12 Microstructure of moist cured CKD(I)-Slag paste at high magnification ..... 89 Figure 7.13 TEM micrograph showing the fibrillar morphology of C-S-H gel in CKD(I)Slag paste after heat curing ............................................................................................... 90 Figure 7.14 TEM micrograph showing the presence of syngenite crystals ...................... 91 Figure 7.15 X-ray diffraction pattern of CKD(II)-Slag at different curing periods.......... 92 Figure 7.16 Thermal analysis of CKD(II)-Slag paste: a) TGA, and b) DTG plots .......... 94 Figure 7.17 SEM micrograph of CKD(II)-Slag paste after heat curing ........................... 95 Figure 7.18 SEM micrograph of CKD(II)-Slag paste after heat curing with EDX patterns, showing the presence of C-S-H gel .................................................................................. 96 Figure 7.19 SEM micrograph of CKD(II)-Slag paste after heat curing with EDX, showing the presence of C-S-H gel ................................................................................................. 97 Figure 7.20 SEM micrograph of CKD(II)-Slag paste after heat curing, showing the crystalline phases .............................................................................................................. 98 Figure 7.21 SEM micrograph of CKD(II)-Slag paste after heat curing, showing the morphology of C-S-H gel ................................................................................................. 99 Figure 7.22 SEM micrograph of heat cured CKD(II)-Slag paste after 28 days of lime curing .............................................................................................................................. 100 Figure 7.23 SEM micrograph of heat cured CKD(II)-Slag paste after 28 days of lime curing, showing C-S-H morphology ............................................................................... 101 xiv

Figure 7.24 TEM micrograph of CKD(II)-Slag paste after heat curing, showing the .... 102 Figure 8.1 Expansion of mortar bars in saturated lime water after undergoing heat curing ......................................................................................................................................... 105 Figure 8.2 Expansion of mortar bars in 1N NaOH solution at 80oC (ASTM 1260)....... 106 Figure 8.3 Expansion of mortar bars in accordance to ASTM 227 ................................ 107 Figure 8.4 Freeze-thaw resistance of non-air entrained CKD(I)-Slag binder and OPC concretes (Paste Vol. – 40%; w/b – 0.4; Fine Agg./Coarse Agg. – 1.0) ......................... 108 Figure 8.5 Expansion of concrete prisms having CKD(I)-Slag and OPC as binder ....... 109

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ABSTRACT Cement manufacture is an energy intensive process that releases CO2 into the atmosphere. There has been a major thrust in the direction of transforming the conventional concrete into eco-friendly and sustainable concrete utilizing the industrial by-products. Cement Kiln Dust (CKD) is a by-product of cement industry which is normally discarded due to its high alkali and sulfate content. The focus of this thesis is to understand the physico-chemical interaction of two CKDs with class F fly ash (FA) and ground granulated blast furnace slag in order to develop an alternative binder for concrete. The long-term durability performance with respect to delayed ettringite formation, alkali-silica reaction and freeze-thaw resistance, is also investigated. Initially, the curing conditions and binder proportion were optimized for the maximum compressive strength of CKD-FA and CKD-Slag pastes. The optimum temperature and the optimum CKD fraction in the CKD-FA and CKD-Slag pastes were determined to be 75oC and 70%, respectively. Both CKD-FA and CKD-Slag pastes showed the compressive strength of more than 25 MPa after 48 hours of heat curing. The heat curing followed by lime water curing, improved the strength significantly in CKD-Slag paste (~36 MPa after 90 days), whereas, only marginal improvement in strength was seen in CKD-FA paste. Concretes incorporating CKD-Slag as a sole binder performed better than the concretes having CKD-FA binder. The compressive strength of ~32 MPa was achieved by the concretes incorporating CKD-Slag binder after 90 days of saturated lime water curing. The optimum proportion (70% CKD and 30% fly ash or slag) paste samples were prepared for further microstructural investigation. Physico-chemical changes in CKD-based binders were evaluated using X-ray diffraction (XRD), thermogravimetric xvi

analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ettringite, calcium hydroxide, calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) gel were identified as the main hydration products in the CKD-based binders. The potassium and magnesium were observed to have incorporated in the C-S-H gel present in the CKD-based binders. The heat curing accelerated the pozzolanic reaction in CKD-FA and CKD-Slag binders, resulting in the dense microstructure. A number of additional microstructural changes were seen in CKDFA and CKD-Slag binders such as the presence of Friedel’s salt, sylvite, syngenite and arcanite minerals. Overall, the free lime and sulfate content of the CKD were found to be the key components in influencing its interaction with fly ash and slag. No significant expansion was observed in pre-heat cured mortar bars incorporating CKDFA and CKD-Slag binders with respect to delayed ettringite formation. The susceptibility to alkali-silica reaction was also found to be minimal in CKD-FA and CKD-Slag binders due to reduced availability of calcium hydroxide. The freeze-thaw resistance of concretes incorporating CKD-based binders was observed to be less compared to the conventional concretes.

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CHAPTER 1: INTRODUCTION 1.1 Background and Research Needs The manufacture of cement is an energy intensive process which involves emission of carbon dioxide into the atmosphere. During the cement production process, calcium carbonate (CaCO3) is heated at a temperature of about 1450 oC to form lime and CO2. In the year 2008, cement production released 41.1 million metric tons of CO2 into the atmosphere (Draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008). The increasing emission of CO2 during cement manufacture has urged the scientific community to explore new ways to reduce the consumption of cement. In that direction, the use of industrial by-products like fly ash, ground granulated blast furnace slag, and silica fume has been investigated as a partial replacement of cement. Cement kiln dust (CKD) is a particulate mixture which is captured by screening the exhaust gases from cement kilns, using particulate matter control devices such as cyclones, bag houses and electrostatic precipitators. CKDs primarily consist of calcium carbonate and silicon dioxide which is similar to the cement kiln raw feed, but the amount of alkalis, chloride and sulfate is usually considerably higher in the dust. An estimated 13 to 17 million tons of CKDs are generated in the United States each year (EPA 2008). The cement industry recycles more than 75 % of generated CKD each year (EPA 2006). Due to its high alkali and sulfate content, recycling of CKD is limited. There have been many studies in the past related to the use of CKD for soil stabilization purpose (Bhatty et al., 1996, Peethamparan et al., 2008). The use of CKD as a partial replacement for Portland cement has also been evaluated in the past (Bhatty, 1983). It 1

was shown that the cements replaced by CKD had reduced workability, setting times and strength. The addition of slag to cement-CKD blend resulted in lower workability but produced higher strength than the blends having no slag. More than 15% replacement of cement by CKD showed lower compressive strength (Abo-El-Enein et al., 1994). The same study revealed that initial and final setting times of cement paste were decreased due to the high free lime content of the CKD. The soundness and shrinkage of final Portland cement concrete were shown to be adversely affected when the CKD replacement was more than 10% (Daugherty and Funnell, 1982). The composition of CKD varies among plants and over time at a single plant. This leads to difficulty from the application point of view. Suitability of CKD in practical applications requires the knowledge of hydration mechanism in CKD systems. The microstructural and durability investigation of CKD system will be helpful in understanding the behavior of CKD-based systems.

1.2 Objective and Scope The goal of the proposed research was to perform a fundamental study on the interaction of CKD with fly ash and slag. The objectives of present research have been outlined below:  To examine the early age & hardened properties, and long term durability of CKD-based binders.  To develop a better understanding of the microstructure development in CKDbased binders.

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The scope of the research involved the following tasks:  Characterization of the hydration products of two different kinds of CKDs and their binary blends with fly ash and slag.  Examination of early age properties of the neat CKD, CKD-FA, and CKD-Slag pastes prepared with two different types of CKDs.  Evaluating the compressive strength of the paste and concrete made with CKDFA and CKD-Slag as the sole binders.  Morphological and mineralogical investigation of the neat CKD, CKD-FA and CKD-Slag pastes prepared with both types of CKDs.  Durability studies of CKD-based binders with respect to delayed ettringite formation, alkali-silica reaction and freeze-thaw resistance.

1.3 Hypothesis Activation of industrial by-products like fly ash and slag, using alkalis has been investigated in the past by many researchers (Davidovits et al., 1979; Palomo et. al., 1999; Wang et. al., 1995). The chemical and hydrothermal activation are effective ways to achieve a dense microstructure. The chemical activation involves the addition of alkalis (NaOH, Na2SiO3, Na2CO3 etc.), quick lime (CaO) and sulfate (gypsum, anhydrite, sodium sulfate etc.) and the hydrothermal activation requires the curing at elevated temperature. Both of these techniques have been proved effective in order to achieve high compressive strength in the based systems. Past studies have suggested the role of alkalis in creating a high pH environment which helps dissolving the glassy phase of fly ash and slag. Also, free lime forms calcium hydroxide, and reacts with reactive silica available in source material that results in the formation of calcium silicate hydrate gel. External 3

supply of sulfate rich source helps in forming ettringite (AFt phase) which significantly improves the early age strength. From that perspective, CKD provides a good source of alkalis, free lime and sulfate. The hypothesis of present work is that the activation of aluminosilicate materials like fly ash and slag, using CKD with elevated temperature curing, would result in an alternative binder with similar properties as that of ordinary Portland cement (OPC).

1.4 Outline of the Thesis Background and literature review of different activation methods are discussed in Chapter 2 with the need for the current research. Chapter 3 focuses on the material selection and experimental program. Early age properties and compressive strength of the neat CKD, CKD-FA, and CKD-Slag pastes are discussed in Chapter 4. The strength of concretes made with CKD-FA and CKD-Slag as binders also is reported in Chapter 4. Chapter 5 includes a detail mineralogical and morphological investigation of CKDs. The microstructural changes of CKD-FA and CKD-Slag pastes are discussed in Chapter 6 and Chapter 7 respectively. Long term durability of CKD-based binders such as delayed ettringite formation, alkali-silica reaction and freeze-thaw resistance, has been reported in Chapter 8. Chapter 9 presents the conclusions with the recommendations for the future work.

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CHAPTER 2: LITERATURE REVIEW AND RESEARCH MOTIVATION 2.1 Introduction This chapter starts with a detailed overview of cement manufacturing process along with the generation of by-product i.e. cement kiln dust (CKD). The beneficial usages of CKD are also discussed in detail. Since CKDs contain free lime, alkalis and sulfates, the role of these constituents in activation is also discussed. Different activation techniques used for pozzolans and slag are also presented in detail. Later on, the activation of fly ash and slag using CKD is reviewed. Finally, the present status of CKD activation and need for further research is discussed.

2.2 Cement Manufacturing and Production of Cement Kiln Dust Figure 2.1 shows a schematic diagram of cement manufacturing process. This process has three stages which are discussed as below: 

Preparation of Raw Mixture: The raw materials for manufacturing cement include limestone, clay, shale and iron ore that are normally obtained from a nearby quarry. These raw materials are initially crushed to small size (below 50 mm) and ground together in raw mill. In raw mill, homogenization of all raw powders is performed which is a controlled process. X-ray fluorescence and gamma neutron activation analysis is carried out to control the chemical composition during the homogenization.

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Clinker Production: Once raw mixture is prepared, it is allowed to pass through a kiln where it is heated up to the maximum temperature of 14500C. During the heating, the formation of alite and belite takes place. The temperature is regulated so that the product contains sintered lumps.

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Production of Cement: The clinker, obtained through stage two, is mixed with gypsum and ground to obtain a required fineness. This fine powder is called cement.

Figure 2.1 Schematic of Cement Manufacturing Process (Courtesy: www.acmp.co.za/cement_manufacturing_process.htm)

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During the heating of raw mixture, kiln gases are captured through electrostatic precipitator, and a fine powdery material is obtained which is called “Cement Kiln Dust”. The reuse of CKD is very limited since it consists of large amount of alkalis and sulfates. When the CKD has small amount of alkalis, it is returned as a raw feed in cement manufacturing.

2.3 Physical and Chemical Properties of CKD Primarily, CKD consists of calcium carbonate and silicon dioxide along with alkalis, chloride and sulfate. Todres et al. (1992) characterized the properties of CKD from three different types of operations. Table 2.1 shows the chemical composition of CKD in comparison with Type I cement.

Table 2.1 Chemical composition of CKD (Todres et al. 1992)

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As can be seen in Table 2.1, CKD obtained from the alkali by-pass of precalciner kilns found to have highest amount of free lime and lowest loss on ignition (LOI). CKD contains insignificant amounts of trace metals. The physical properties of three types of CKDs were also compiled by Todres et al. (1992) and are shown in Table 2.2.

Table 2.2 Particle size distributions of CKD (Todres et al. 1992)

It is evident from above table that the CKD from alkali by-pass precalciner is coarser than that of obtained from other two operations. The amount of CKD also varies with the type of operation. The long-dry kiln produces the largest amount of CKD in which dust is stirred with chain and gas velocities are high (Steuch, 1992). In preheater kilns, the feed loading is high and the dust contact with kiln gases is low. Thus, the CKD generation is low.

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2.4 Beneficial Uses of CKD 2.4.1 CKD as Soil Stabilizer CKD has demonstrated an efficient role in soil stabilization. There are many studies in the past which investigated the effectiveness of CKD in soil stabilization. Baghdadi (1990), Zaman et al. (1992) and Bhatty (1996) reported an increase in unconfined compressive strength (UCS) and decrease in plasticity index of CKD-treated expansive clays. The free lime content of CKD is a key factor in its potential as a soil stabilizer. Peethamparan et al. (2008, 2009, and 2010) showed that the CKDs with higher free-lime content were more effective in stabilizing clays, and apart from free lime, the sulfate content, alkalinity, and the fineness were also found to influence the engineering properties of CKD-treated clays. 2.4.2 CKD for Blended Cements The use of CKD for blended cements has been investigated by many researchers in the past (Detweiler et al., 1996; Bhatty, 1983 and 1984). Bhatty investigated the use of CKD in Portland cement with fly ash and ground granulated blast furnace slag. The study revealed that the inclusion of CKD in cement causes reduction in workability, setting time and compressive strength. Bhatty also showed that the strength was improved when the CKD was used with fly ash and slag, and dilution of alkalis was believed to be the cause of strength enhancement. In addition, the blended cement having large amount of sulfate showed the maximum strength. The use of CKD has also been investigated in controlled low-strength materials (CLSM) in the past (Lachemi et al., 2008; Pierce et al., 2003).

9

2.5 Methods for Activating Fly Ash and Slag 2.5.1 Alkali Activation Alkali activation is a chemical activation process, used for industrial aluminosilicate waste materials such as coal ash and Slag (Purdon, 1940; Davidovits, 1979 and 1991; Roy et al., 1990; Wang et al., 1995; Shi et al., 1996; Palomo et al., 1999; Fernández et al., 2005, Bhakharev, 2005; Hardjito, 2006, Ravikumar et al., 2010). The aluminosilicates containing by-products such as fly ash, slag and metakaolin have been activated using alkalis (NaOH, Na2CO3, Na2SO3 etc.) that transform the glassy phase into a wellcompacted cementitious phase. An external supply of alkalis is provided to enhance the pH of the pore solution, which plays an important role in dissolving the vitreous phase of pozzolanic and supplementary cementitious materials (Taylor, 1997). Fernández et al. (2005) studied the microstructure of fly ash activated with different activators and suggested that the main hydration product was alkali aluminosilicate gel. The same study revealed that the type of activator influences Si/Al and Na/Al ratio which also modify the microstructure. The types and concentration of the activating solution has significant influence on the properties of based fly ash binder (Fernández et al., 2005; Ciardo et al., 2007). The alkali activation of fly ash takes place in two steps which can be summarized as given below: 

Dissolution: The aluminosilicates dissolve in high alkaline environment, and form ionic species.



Polymerization: The smaller molecules agglomerate to form the larger molecules that precipitate in form of gel, having some degree of structural order. (Davidovits, 1988; Palomo et al., 1999; Xu et al., 2000) 10

Figure 2.2 shows a schematic diagram of alkali activation of fly ash particles. The dissolution of alumina and silica from existing polymeric chain in fly ash takes place at a pH value of ~13.3 or higher and a new alkali incorporated binding product is formed (Fraay et al., 1989).

Figure 2.2 Schematic diagram of alkali activation of fly ash (Courtesy: Fernández et al., 2004) The ground granulated blast furnace slag has been used as a supplementary cementitious material in Portland cement concrete because of its favorable latent hydraulic properties. Interestingly, the reaction of slag when directly exposed to water is a very slow process. It has been suggested that the low reactivity of the slag is due to the formation of an impermeable aluminosilicate protective layer on the surface of the reacting grains, which limits the dissolution of the unreacted inner core. Dissolution is reinitiated when this reaction layer is removed or converted and the unreacted surface again comes into 11

contact with water (Mehta, 1989). When used as a supplementary cementitious material in Portland cement, the high alkaline environment resulting from the cement hydration dissolves the glassy structure of the coating. This removes the barrier for further hydration, and forms a well compacted cementitious compound. Thus, the use of slag as a sole binder in concrete requires an external supply of activator that can attack the glassy structure and increase the reactivity of slag. Several different methods to activate slag, including thermal curing and chemical activation, have been investigated in the past (Shi et al., 1989, 1995 and 1996, Wang et al. 1995). Based on these studies, the major factors that influence the strength development in based concrete are thermal curing conditions, type and concentration of the activator, and specific surface area of slag. Dissolution of slag and the formation of initial hydration products in alkali activated slag depend on the initial pH of the activating solution whereas later stage hydration is dominated by the reaction of anions released from the activator and calcium ions released from slag rather than the initial pH of the activator solution (Shi et al., 1995). 2.5.2 Thermal Activation An elevated temperature curing is usually adopted to achieve higher compressive strength in the case of alkali-activated binders. A wide array of temperatures ranging from 20 oC to 100 oC has been shown to have an influence on the strength development of alkaliactivated binders (Wang et al., 1995; Shi et al., 1995; Fernández et al., 1999; Bakharev et al., 1999 and 2005; Barnett et al., 2006).

12

2.5.3 Sulfate Activation Sulfate activation of fly ash is based on the ability of external sulfate to react with glassy phase of alumina in fly ash and slag to form ettringite (AFt) that contributes to early age strength (Xu et al., 1991; Shi et al., 1996 and 1998; Poon et al., 2001; Juenger et al., 2006). Xu et al. (1991) reported that the addition of 3% to 6% gypsum resulted in significant improvement in strength for the cement pastes containing 30% to 60% low calcium fly ash. Shi et al. (1996 and 1998) compared the effectiveness of Na2SO4 and CaCl2 as chemical activators and found that the former increased the early strength while the latter increased the later strength of the lime–fly ash pastes. In the same study, C-S-H, AFt and AFm were the identified hydration products in the control lime-fly as pastes. Shi et al. also reported that the addition of Na2SO4 raised the alkalinity of pore solutions, accelerated initial pozzolanic reaction and resulted in the formation of AFt, which gave the high early strength of the lime-fly ash paste. Poon et al. (2001) studied the effect of anhydrite (CaSO4) on fly ash-cement system. They reported that an addition of 10% anhydrite increased the 3-day compressive strength by about 70% for the mortar with up to 55% fly ash. It also increased the strength at the later ages of these mortars. These were achieved with a short period of initial curing at an elevated temperature (65°C) before normal water curing. It was also found in the same study that the activating effect of anhydrite is significant for earlyage strength due to formation of AFt phase, but is less significant for later-age strength and for lower fly ash contents. 13

Juenger et al. (2006) investigated the effect of alkali sulfate and hydroxide on the hydration of slag through soft X-ray transmission microscopy. They reported that the addition of gypsum and potassium sulfate results into the formation of AFt type which later converts to AFm phase when local supplies of gypsum is exhausted. When potassium sulfate was used without gypsum, a layer of amorphous hydration products surrounding the slag grain was observed. 2.5.4 Lime Activation Shi et al. (2001) studied the effect of hydrated lime and quick lime on natural pozzolans. An optimum content of calcium hydroxide was suggested to give the maximum compressive strength of lime-pozzolan pastes. The addition of quick lime was found to result in a faster strength gain and higher pozzolanic reaction at early stages. Mira et al. (2002) reported that the addition of lime putty (type of hydrated lime) improves the compressive strength when pozzolanic materials are used in parallel. The enhanced pozzolanic reaction between calcium hydroxide and reactive silica of pozzolan was suggested to have caused the improvement in strength.

2.6 Interaction of CKD with Fly ash and Slag The advantage of using CKD with fly ash and slag is based on the fact that they consist of large amounts of alkalis, free lime and sulfate. There are limited studies on the CKDbased binders (Amin et al., 1995; Ei-Didamony et al., 1997; Konsta-Gdoutos et al., 2003, Wang et al., 2004, Chaunsali et al., 2010).

14

Ei-Didamony et al. (1997) evaluated the effect of calcined kiln dust with anhydrite on the activation of slag and proposed that the activation of slag increased with the amount of added anhydrite. Konsta-Gdoutos et al. (2004) studied the mortars made of CKD-Slag blend, and reported that alkalis and sulfate play a decisive role in initial dissolution of slag whereas later hydration is influenced by the available supply of Ca2+ ions. Ettringite and C-S-H were identified as main hydration products of CKD-Slag binders, and the compressive strength was found to be increasing with curing periods. In addition, excess amount of free lime was suggested to have a detrimental effect on the strength development in CKD-Slag binders. Wang et al. (2003) examined the effects of curing temperature and NaOH addition in CKD-Fly ash system. It was found that the curing at elevated temperature was more effective for CKD-Fly ash based binder than sodium hydroxide addition. The study also revealed the presence of ettringite as major crystalline hydration product. In recent studies, Chaunsali et al. (2010) performed a detailed microstructural investigation of CKD-based fly ash paste. It was found that the strength of CKD-based fly ash binder significantly influenced by elevated curing temperature. Ettringite and calcium aluminosilicate gel (C-A-S-H) were found as the main hydration products in CKD-based fly ash binder.

15

2.7 Durability of CKD-Based Binders Durability of CKD-based binders is an area to be explored before its practical application. Since the hydration products of CKD-based fly ash and slag are similar to those normally found in cement based system, it is also believed to have similar durability problems that might occur in a cement based system. Alkali-aggregate reaction (AAR) in alkali based binders has been investigated by a few researchers in the past (Fernández-Jiménez et al., 2002 and 2006; García-Lodeiro et al., 2007). In these studies, the absence of Portlandite i.e. Ca(OH)2 was found to have an influence in reducing the AAR in alkali based binders. Though there are many studies on delayed ettringite formation in cement based system, no previous study has been reported on CKD-based system.

2.8 Research Motivation Only a limited number of studies pertaining to the interaction of CKD with fly ash and slag have been reported in the literature. Specifically, the microstructure development in CKD-FA and CKD-Slag binders has not been reported yet. Through this study, an attempt will be made to relate the macroscale performance of CKD-based binders to their microstructural characteristics. Mineralogical and morphological changes were monitored through the characterization techniques like X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. Since the use of CKD, having large amounts of alkalis and sulfates, may influence the long term performance of CKD-based binders, the durability of these binders with respect to alkalisilica reaction, delayed ettringite formation and freeze-thaw resistance have also been investigated in the current work.

16

CHAPTER 3: EXPERIMENTAL PROGRAM AND MATERIALS 3.1 Introduction This chapter presents a summary of the experimental methods and materials used for the research. In addition to setting time, flow test and compressive strength tests, several characterization techniques such as X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were performed during the study which are discussed in detail in this chapter.

3.2 Experimental Methods 3.2.1 Setting Time and Flow Test The setting time of cement paste is usually determined using the Vicat test described in ASTM C 191. In the standard test method, the paste used for determining the setting time should be prepared using the normal consistency water content. In the present study, setting time was performed on a paste prepared at a specific water-to-binder ratio (w/b) rather than using the normal consistency water content. The purpose of the test was to evaluate the setting time of the CKD-based binder with a specific w/b. Flow test was performed as per ASTM C 1437 to determine the flow of CKD and CKDbased binders, prepared with both types of CKDs at a w/b ratio of 0.40.

3.2.2 Compressive Strength Test Compressive strength of cubes (50 mm x 50 mm x 50 mm) was determined using ASTM C 109 after 48 hour, 28 day, 56 day, and 90 day curing periods.

17

3.2.3 X-ray Diffraction Analysis X-ray diffraction analysis was carried out using a Bruker DX-8 diffractometer. CuKα radiation with a wavelength of 1.504 Å was used, and samples were scanned in 2θ range from 5o to 65o at a scanning rate of 0.02o per second. Hand ground powdered sample passing through a 75 μm (200 no.) sieve was also used for the X-ray diffraction examination. 3.2.4 Thermogravimetric Analysis Thermogravimetric analysis was performed using a Perkin-Elmer thermogravimetric analyzer. In this test, a powdered sample (passing through 75 μm sieve) weighing 25 ± 2 mg was heated in a nitrogen environment from 50oC to 1000oC at the rate of 10oC per minute. The nitrogen gas flow rate was kept as 40 ml/ min. 3.2.5 Scanning Electron Microscopy Scanning Electron Microscopy (SEM) was performed using a JEOL JSM-7400F electron microscope coupled with an Energy Dispersive X-Ray Detector (EDX), operating in secondary mode. The sample was fractured to get a smooth surface with few undulations and was stuck on an aluminum stub using carbon paint. Gold coating was applied on the fractured surface of the sample to avoid the charge build-up. 3.2.6 Transmission Electron Microscopy Transmission Electron Microscopy (TEM) was performed using a JEOL 2010 HRTEM electron microscope coupled with an Energy Dispersive X-Ray Detector (Oxford Instruments INCA System). Powdered samples passing through 75 μm sieve was used for the imaging.

18

3.2.7 Modulus of Elasticity The modulus of elasticity was determined in accordance with ASTM C 215.

The

modulus of elasticity testing was conducted on concrete beams. The specimens were analyzed using a NI Signal Connector interfaced with DK-4000TM Dynamic Resonance Tester software. 3.2.8 Expansion Test The expansion measurements of mortar (25mm x 25mm x 285 mm) and concrete (50 mm x 50 mm x 285 mm) bars were performed using length comparator in accordance with ASTM C 157. 3.2.9 Sample Preparation A preliminary set of experiments was conducted to determine the optimum curing temperature conditions and mixture proportion for obtaining maximum compressive strength. The optimum curing temperature and the mixture proportions thus obtained were then used for the rest of the study. Source materials (fly ash and slag) and activator (CKD) were homogenized in dry conditions prior to mixing with water. A constant water-to-binder ratio (w/b) of 0.40 was used to prepare the paste following ASTM C 305 procedure. For compressive strength determination, the paste was poured into cubical mold of 50 mm x 50 mm x 50 mm size and compacted adequately using a table vibrator. The specimens were then kept in ambient temperature for 24 hours before demolding. They were then wrapped in aluminum foils and subjected to heat curing in a laboratory oven for 48 hours. Some of the specimens were subjected to saturated lime water curing for an additional period of 28 days followed by the initial heat curing. The saturated lime water 19

was used to limit the leaching of ions from the pore solution due to pH gradient as CKD has large amount of alkalis. The saturated lime water curing was not carried out in the preliminary set of experiment, since the objective was to determine optimum curing temperature and binder proportions. All the samples, including the samples used for preliminary analysis, were allowed to cool down to ambient temperature for 30 minutes immediately after heat curing. The compressive strengths were determined in accordance with ASTM C 109. For mineralogical and microstructural analysis of CKD and CKD-based binders, paste specimens were prepared in small polypropylene cylinders. The same sample preparation procedure described in the previous paragraph was used, but the samples were prepared using the optimum CKD dosage (70%) for CKD-based Fly ash (CKD-FA) and CKDbased Slag (CKD-Slag) at 75o C. Samples were collected for the analysis at the end of six different curing regimes: (a) 24 hours of ambient temperature curing (designated as “24 h (A)” series); (b) 24 hours in ambient temperature followed by 48 hours of heat curing (designated as “24 h (A) + 48 h (H)” series); (c) 24 hours in ambient temperature followed by 48 hours of heat curing and subsequent extended curing in lime water for 28 days (designated as “24 h (A) + 48 h (H) + 28 d (L)” series); (d) 24 hours in ambient temperature followed by 48 hours of heat curing and subsequent extended curing in lime water for 90 days (designated as “24 h (A) + 48 h (H) + 90 d (L)” series); and (e) 24 hours in ambient temperature followed by 28 days curing in lime water (designated as “ 24 h (A) + 28 d (L)” series. The last one curing regime was used only for the CKD-based Slag. The collected samples were broken into small pieces and soaked in acetone for at least 2 days to arrest further hydration (Collier et al., 2008). The acetone soaked samples 20

were dried for one day in vacuum desiccators prior to the analysis; and they were stored in air tight glass vials. After the drying, a small portion of the sample was ground and sieved through a 75 micrometer size sieve and this powder sample was used for the TGA and the XRD analysis. Remaining samples were used for the SEM examination.

3.3 Materials This subsection presents the physical and chemical properties of the materials used in the current study. Additional characterization, such as X-ray diffraction (XRD) analyses for mineral identification is also presented. 3.3.1 Particle Size Distribution The particle size distribution was determined using Mastersizer 2000 particle size analyzer and Isopropyl alcohol was used as a dispersant. The procedure to determine particle size distribution was opted from NIST which involved seven steps. First step involved the high speed stirrer and sonication for 2 minutes. In the second step, sample was stirred with no sonication for 2 minutes. 25 cycles were run in the third step. Fourth step involved the rest of 30 seconds then 30 seconds of sonication followed by 1 minute rest. Fifth and sixth steps had 6 cycles followed by 30 seconds rest. Seventh and last step was tested for 6 cycles.

21

Percentage Passing (%)

100 CKD (I) CKD (II) Slag Class F Fly Ash

80

60

40

20

0 0.1

1

10 100 Particle Size (m)

1000

Figure 3.1 Particle size distribution of materials used in the study

The particle size distribution of all the materials used in the study is shown in Figure 3.1. It is evident from the figure that CKD (I) had the finest particles with an average size of ~ 3 µm. CKD (II) and Slag had similar particle size distribution with the average size of ~7 µm. Class F fly ash was the coarsest among all with an average particle size of ~ 11 µm. Overall, CKD(I) was found to be finer than CKD(II).

22

3.3.2 Chemical Composition Table 3.1 Chemical Compositions of CKDs, fly ash and Slag

Chemical

CKD (I)

CKD(II)

Slag

Class F Fly Ash

SiO2

14.55

11.69

36.00

50.20

Al2O3

4.46

2.20

10.53

28.70

Fe2O3

2.11

2.93

0.67

5.72

CaO

61.15

37.05

39.80

5.86

MgO

3.84

0.87

7.93

1.74

SO3

10.62

7.69

2.11

--

K2O

3.45

7.24

0.15

--

Na2O

0.80

0.87

0.27

0.96

Na2O equivalent

3.10

6.00

0.32

0.96

Cl

--

0.62

--

--

LOI

23.40

29.06

3.00

1.85

Composition

Table 3.1 presents the chemical composition of two different types of CKDs along with fly ash and slag. The free lime contents of CKD(I) and CKD(II) were determined approximately as 5% and 1.5% respectively. The distinction between two types of CKDs 23

is evident in terms of the free lime, sulfate, alkali (K2O) and chloride contents. CKD(I) was found to be rich in sulfate and free lime content whereas more alkalis were present in CKD(II). 3.3.3 Mineralogical Composition The mineralogical characterization of fly ash, slag and both types of CKDs was performed using X-ray diffraction analysis to identify the crystalline phases in the raw materials. The peak in XRD pattern indicates the presence of crystalline mineral.

CC

Q S CCAnh

CC Q Anh

Sg

CC CC CC QL Sg

QL CC

QL CC

CC

CC CC

CC CC

CKD(II)

QL

CKD(I)

10 15 20 25 30 35 40 45 50 55 60 65 2-Theta (degrees) Figure 3.2 XRD patterns of two different CKDs (CC – calcite; Q – quartz; Anh – anhydrite; S – sylvite, Sg – syngenite, QL – quick lime)

Figure 3.2 shows the XRD patterns of two CKDs used in the present study. Calcite (CaCO3), quartz (SiO2), anhydrite (CaSO4) and quick lime (CaO) were identified as the crystalline minerals in CKD(I) whereas sylvite (KCl) and syngenite (K2Ca(SO4)2) were 24

identified in CKD (II) in addition to the minerals present in CKD(I). XRD analysis also indicates the higher amount of free lime in CKD(I) compared to CKD(II). Q

Q Mu

Mu

Fly Ash

Slag

5 10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degrees) Figure 3.3 XRD patterns of fly ash and slag (Mu – Mullite; Q – Quartz ) X-ray diffraction analysis of fly ash and GGBFS is shown in Figure 3.3. The XRD pattern of GGBFS shows a hump in between 20o and 40o (2-theta angle) without any peak which shows the glassy nature of slag. On contrary, XRD analysis of fly ash shows some crystalline minerals present in the forms of quartz and mullite (Al2O3.Si2O3). 3.3.4 Morphological Characteristics SEM micrographs of both the CKD powders are presented in Figure 3.4. Both micrographs show the various shapes of particles. The particle shape was found to be irregular for both types of CKDs. Figure 3.5 shows the micrographs of fly ash and slag. The class F fly ash particles were spherical whereas the slag particles were angular.

25

(a)

(b)

Figure 3.4 SEM micrographs of raw powders: (a) CKD(I) and (b) CKD(II)

(a)

(b)

Figure 3.5 SEM micrographs of: (a) class F fly ash, and (b) slag powders

26

CHAPTER 4: ENGINEERING PROPERTIES OF CKD-BASED BINDERS 4.1 Introduction The present chapter discusses the engineering properties of CKD-based binders. Setting time and flow value tests were performed to assess the early age properties of CKD, CKD-FA and CKD-Slag pastes. Initially, optimization of curing temperature and binder proportion was performed for CKD-FA and CKD-Slag pastes. Keeping a constant waterto-binder (w/b) ratio of 0.40, the compressive strength was determined at various curing periods. In addition, the strength of concretes incorporating CKD-based binders have also been presented.

4.2 Early Age Properties 4.2.1 Setting Time and Flow Test The early age properties such as the workability and the setting time of the developed binder were determined. Table 4.1 shows the setting times for various CKD-based binders. As can be seen in Table 4.1, CKD(I)-based binders exhibited shorter setting times compared to CKD(II)-based binders. Since CKD(I) had considerable amount of free lime, it exhibited shortest setting times among all which can be attributed to the formation of calcium hydroxide, an exothermic reaction evolving the heat. Between CKD(I)-FA and CKD(I)-Slag pastes, CKD(I)-Slag had shorter setting time. The setting time for CKD(II), which had very small amount of free lime, exceeded 24 hours. Both neat CKD(II) paste and CKD(II)-FA paste exhibited longer setting than 24 hours. Only CKD(II)-Slag showed final setting time of less than 24 hours. It is believed that the low

27

free lime and sulfate content of CKD(II) delays the onset of setting in CKD(II)-based binders.

Table 4.1 Initial and Final Setting Times of Different Binders

Binder Composition

Initial Setting Time (minutes)

Final Setting Time (minutes)

100% CKD (I)

90

280

70%CKD(I)-30%Slag

240

680

70%CKD(I)-30%FA

360

800

100%CKD(II)

>1440 (24 h)

--

70% CKD(II)-30% Slag

900

1250

70% CKD(II)-30% FA

>1440 (24 h)

--

28

120 CKD (I) CKD (II)

100

Flow (%)

80 60 40 20 0 % 100

g Sla

D CK

30% 30% D D CK CK 70% 70%

FA

Figure 4.1 Flow values of CKD-based binders

Figure 4.1 shows the flow values of the CKD and CKD-based binders. The neat CKD(II) paste and CKD(II)-based binders exhibited larger flow values than that of CKD(I)-based binders due to less free lime content. The shape of class F fly ash particles and the dilution effect also result in the increase of initial setting time and flow value of CKD(I)FA and CKD(II)-FA binders.

29

4.3 Compressive Strength of Paste 4.3.1 CKD(I)-FA Paste 4.3.1.1 Determination of Optimum Temperature for Heat Curing Thermal activation has been widely used in the case of alkali based binders. Hence, as mentioned earlier, a preliminary study was carried out to determine the optimum temperature for heat curing. To determine the effect of temperature on the compressive strength, three different curing temperatures (60oC, 75oC and 90oC) were used. The cubes were heat cured for 48 hours at the respective temperatures and the compressive strength was determined after the specimens were cooled to room temperature. For this part of the study, the binder was prepared with 50 % FA and 50% CKD(I); and w/b ratio was fixed as 0.4.

Compressive Strength (MPa)

50

40

50% CKD(I) - 50% FA

30

20

10

0 50

60

70 80 Temperature (oC)

90

100

Figure 4.2 Effect of curing temperature on the compressive strength of CKD(I)-FA binder

30

Figure 4.2 shows the variation of the compressive strength with curing temperature. It can be seen that the maximum strength was achieved when the temperature was 75oC. Hence the optimum temperature of 75oC (for maximum compressive strength) was used for the rest of the study. 4.3.1.2 Determination of Optimum Binder Proportion The compressive strength was expected to have a dependence on the proportions of CKD(I) and FA in the binder. To determine the optimum mixture proportion that attains the maximum compressive strength, cubes made of CKD(I)-FA binder with varying CKD dosages were tested after heat curing at 75oC for 48 hours. As shown in Figure 4.3, the samples with 70 % of CKD(I) and 30 % of FA by weight of total binder achieved the maximum compressive strength.

Compressive Strength (MPa)

50

40

30

20

10

0 40

50 60 70 80 90 100 CKD dosage in CKD(I)-FA binder (%)

110

Figure 4.3 Effect of CKD(I) fraction on the compressive strength of CKD(I)-FA binder after 48 hours of heat curing at 75oC

31

The neat CKD(I) paste had compressive strength of ~12 MPa which significantly increased with the partial replacement of fly ash. The free lime of CKD forms calcium hydroxide which, later, reacts with the reactive silica (from pozzolans) forming calcium silicate hydrate (C-S-H) gel. The formation of C-S-H gel appears to be the maximum, at the optimum binder proportion (70 % CKD fraction). The less formation of C-S-H gel takes place at lower CKD fractions due to less availability of calcium hydroxide for pozzolanic reaction whereas, at higher CKD fractions, less availability of silica also hinders the pozzolanic reaction. 4.3.1.3 Effect of Heat Curing Followed by Lime Curing The optimum proportion for CKD(I)-FA binder was found to have 70 % CKD(I) by weight. The effect of subsequent curing in lime was investigated on the CKD(I)-FA binder, and presented in Figure 4.4.

Compressive Strength (MPa)

50

40

30

20

10

0 h 48

(H)

) ) d(L d(L 28 56 + + ) ) h(H h (H 48 48

Figure 4.4 Effect of heat curing followed by lime curing on the strength development of CKD(I)-FA binder 32

The elevated heat curing resulted in the significant strength gain in CKD(I)-FA paste which was not significantly influenced by subsequent lime water curing. 4.3.2 CKD(I)-Slag Paste 4.3.2.1 Optimum Curing Temperature for Heat Curing To determine optimum curing temperature of CKD(I)-Slag binder, a paste consisting of 50% CKD and 50 % Slag was prepared and cured at three different curing temperatures (60, 75, and 90oC) for 48 hours of curing duration. The maximum compressive strength was achieved at 75oC, as can be seen in Figure 4.5.

Compressive Strength (MPa)

50

50 % CKD(I) - 50 % Slag

40

30

20

10

0 40

50

60 70 80 90 Temperature (oC)

100

110

Figure 4.5 Effect of curing temperature on the compressive strength of CKD(I)-Slag paste 4.3.2.2 Determination of Optimum Binder Proportion Figure 4.6 shows the variation of compressive strength of CKD(I)-Slag with CKD fraction. As can be seen in Figure 4.6, the maximum compressive strength was achieved for 70% CKD fraction in CKD(I)-Slag binder. This observation was similar to one which 33

was seen in CKD(I)-FA paste. The C-S-H gel formation due to pozzolanic reaction seems to be the maximum, at the optimum binder proportion.

Compressive Strength (MPa)

50

40

30

20

10

0 40 50 60 70 80 90 100 CKD Fraction in CKD(I)-Slag Binder (%)

Figure 4.6 Effect of CKD(I) fraction on the compressive strength of CKD(I)-Slag paste

4.3.2.3 Effect of Heat Curing Followed by Lime Curing Figure 4.7 (a) shows the compressive strength of CKD(I)-Slag paste after 48 hours of heat curing and the effect of subsequent wet curing on the strength development. The CKD(I)-Slag paste developed a compressive strength of 25 MPa after 48 hours of heat curing at 75oC. The strength increased significantly when the heat cured specimens were subjected to further wet curing in saturated lime water for 28 days.

34

50

40

Compressive Strength (MPa)

Compressive Strength (MPa)

50

30

20

10

0

40

30

20

10

0 ) (W

) (W

)

(W ) 7d 8d 6d 0d (H 2 5 + h + + +9 ) ) ) ) 8 H 4 H H H h( h( h( h( 48 48 48 48

) (W

(a)

0

20 40 60 80 Curing Duration (Days)

100

(b)

Figure 4.7 (a) Effect of heat curing and subsequent saturated lime curing on the strength development of CKD(I)-Slag paste, and (b) Effect of only saturated lime curing on the strength development of CKD(I)-Slag paste Another set of companion specimens were soaked in saturated lime water immediately after demolding to study the strength development in CKD(I)-Slag paste without the initial 48 hours of heat curing. Figure 4.7 (b) shows the strength gain of CKD(I)-Slag paste cured in saturated lime. These specimens achieved a comparable strength as that of the 48 hour heat cured samples after 28 days of moist curing. The neat CKD(I) paste had compressive strength of ~12 MPa (Figure 4.6) after heat curing was significantly improved with the addition of slag. Thus, irrespective of the curing methods, the CKD used in this study was capable of activating the slag to produce a binder with good compressive strength.

35

4.3.3 CKD(II)-FA and CKD(II)-Slag Pastes CKD(II)-FA and CKD(II)-Slag pastes were prepared using the same level of replacement (70 % CKD(II)) as in the optimum proportion of CKD(I)-FA and CKD(I)-Slag pastes. Figure 4.8 shows the strength development of the CKD(II)-based binders. As can be seen in the figure, CKD(II)-Slag achieved more strength with heat curing compared to CKD(II)-FA paste. With subsequent heat curing, there was marginal increase in the strength of CKD(II)-FA binder while the strength of CKD(II)-Slag binder increases significantly. These observations were similar to that of CKD(I)-based binders. The neat CKD(II) paste showed a compressive strength of ~ 4 MPa after 48 hours of heat curing which was significantly lower than that of CKD(I) paste (~ 12 MPa). The lower strength of CKD(II) paste can be attributed to less formation of strength giving gel compared to CKD(I). Figure 4.8 (b) shows the strength development of CKD(II)-Slag paste, without undergoing heat curing. As can be seen in the figure, non-heat cured CKD(II)-Slag paste showed higher strength than that of heat cured CKD(II)-Slag paste.

36

50

40

Compressive Strength (MPa)

Compressive Strength (MPa)

50

CKD(II)-FA CKD(II)-Slag CKD (II)

30

20

10

40

30

20

10

0

0 4

) (H 8h

0

) ) (L (L 8d 6d 2 5 )+ )+ h(H h(H 8 8 4 4

(a)

20

40 60 Curing Period (days)

80

100

(b)

Figure 4.8 (a) Effect of heat curing and subsequent saturated lime curing on the compressive strength of CKD(II)-FA and CKD(II)-Slag pastes, and (b) Effect of lime curing on non-heat cured CKD(II)-Slag paste

4.4 Compressive Strength of Concretes Incorporating CKD(I)-Based Binders Since CKD(I)-based binders showed higher strength than that of CKD(II)-based binders, a few additional strength studies were performed on the concretes made with CKD(I)based binders. The aim was to evaluate the strength of concretes incorporating CKD-FA and CKD-Slag as binder. The cubes (50 mm x 50 mm x 50 mm) were prepared using CKD-FA (70:30) and CKD-Slag (70:30) as the sole binders with fine aggregate (river sand) and rounded coarse aggregate (maximum nominal size: 9.75 mm or 3/8”)

37

Compressive Strength (MPa)

50 CKD(I)-Slag CKD(I)-FA

40

30

20

10

0 25

30

35 40 Paste Volume (%)

45

Figure 4.9 Effect of paste volume on the strength of concretes having CKD(I)-Slag binder after 48 hours of heat curing

Figure 4.9 shows the variation in the compressive strength of concretes incorporating different binders with varying paste volume. The concretes incorporating CKD(I)-Slag binder showed the higher strength compared to concretes incorporating CKD(I)-FA binder after 48 hours of heat curing. Figure 2.1 also indicates that the compressive strength decreases with the increase in the paste volume which has also been reported in the case of conventional concrete (Stock et al., 1979; Popovics, 1990). It is usually attributed to the longer crack path when the paste volume is less or aggregate volume is higher which forces the crack to move around large number of aggregates, which results in the higher absorbed energy. When the volume of the paste is higher, the length of the path becomes smaller, and the amount of energy absorbed becomes smaller (Kolias et al., 2005). 38

Figure 4.10 shows the effect of subsequent saturated lime water curing on the strength development of concrete made with two different types of binders. Since the CKD(I)Slag paste had higher strength than CKD(I)-FA paste, a similar trend was observed in the strength of concrete made with these two binders. Furthermore, there was a 50% enhancement in compressive strength of CKD(I)-Slag binder concrete, while only a 30 % enhancement was noticed in CKD(I)-FA binder concrete after 56 days of curing period.

Compressive Strength (MPa)

50 CKD(I)-Slag Concrete CKD(I)-FA Concrete

40

30

20

10

0 ) h(H 8 4

h(H 48

) d(L 8 )+2

h 48

(H

) d(L 6 )+5

Figure 4.10 Effect of binder type and curing duration on the compressive strength (Paste Volume – 40 %, w/b – 0.40, Fine Agg./ Coarse Agg. – 1.0)

39

Compressive Strength (MPa)

50

40

30

20

10

0 0

10

20 30 40 Curing Period (days)

50

60

Figure 4.11 Effect of lime curing on the strength development of non-heat cured CKD(I)Slag binder concretes (Paste Volume – 40 %, w/b – 0.40, Fine Agg./ Coarse Agg. – 1.0) Figure 4.11 shows the strength development of non-heat cured CKD(I)-Slag binder concretes. A steady development in compressive strength is evident in the figure. The CKD(I)-Slag binder concretes achieved ~26 MPa strength after 56 days, compared to ~32 MPa strength of heat cured CKD(I)-Slag binder concretes. The concretes incorporating CKD(I)-Slag were found to gain strength without undergoing heat curing which can be useful from the application point of view.

40

4.5 Summary The optimum curing temperature was found to be 75oC for CKD(I)-based binders. CKD(I)-FA and CKD(I)-Slag pastes, having 70% CKD(I) fraction, achieved the maximum compressive strength after 48 hours of heat curing. In comparison, CKD(I)based binders showed higher strength than CKD(II)-based binders. Fly ash based with both types of CKDs did not show significant strength gain on subsequent saturated lime curing after heat curing. The effect of extended saturated lime curing was pronounced in case of Slag based with both types of CKDs. A non-heat cured CKD-based Slag binder found to have potential to gain strength slowly. Since the CKD(I)-based binders achieved higher strengths compared to CKD(II)-based binders, additional study on concretes incorporating CKD(I)-based binders was performed in order to compare in with the conventional concrete. The concretes incorporating CKD(I)- Slag were also found to have more strength than those made with CKD(I)-FA. The heat cured CKD(I)-Slag binder concrete achieved strength of ~ 32 MPa whereas CKD(I)-FA binder concrete showed ~ 17 MPa after 56 days of lime curing.

41

CHAPTER 5: MICROSTRUCTURAL CHARACTERIZATION OF CKD PASTE 5.1 Introduction Since the present study investigates the effectiveness of CKD in activating fly ash and slag, it is imperative to know the behavior of neat CKD paste. The insight into the microstructure of CKD paste will enable us to understand the activation mechanism effectively. The present chapter discusses the hydration of two types of CKDs. Samples for microstructural investigations were prepared at a water-to-binder ratio of 0.40 for both types of CKDs. The curing regime included 48 hours of heat curing and subsequent curing in saturated lime water. X-ray diffraction and thermogravimetric analyses were performed to identify crystalline minerals, and morphological changes were studied through SEM examination on fractured surface, using secondary mode.

5.2 Mineralogical Characterization of CKD(I) Paste 5.2.1 X-ray Diffraction Patterns X-ray diffraction analysis of CKD(I) paste is presented in Figure 5.1. Ettringite, calcium hydroxide and gypsum were identified as main crystalline products after 24 hours. With heat curing, gypsum disappeared from the X-ray pattern. The disappearance of gypsum seems to have contributed towards the formation of ettringite. There is significant increase in the height of ettringite peaks after heat curing, which do not change much on subsequent curing in saturated lime water. In addition, calcium hydroxide which precipitated from the free lime present in CKD was found to be present at all ages. The presence of calcium hydroxide indicated the absence of pozzolanic reaction in CKD paste which results in the lower strength development in neat CKD paste. 42

CC

E E CHE

CC

CC CC CC CC CC CH 24

Q

Anh

h(A)+48 h(H)+ 90d(L)

24 h(A)+48 h(H)+28 d(L)

Arc

G

5

24 h(A)+48 h(H)

24 h (A)

G

10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degrees) Figure 5.1 X-ray diffraction patterns of CKD(I) paste at different curing periods (E- Ettringite, CH- Calcium Hydroxide, Anh- Anhydrite, Q- Quartz, CC-Calcite, GGypsum, Arc-Arcanite)

5.2.2 Thermal Analysis Thermal analysis of CKD(I) paste also confirms the findings of XRD (Figure 5.2). After 24 hours, ettringite, gypsum and calcium hydroxide can be seen as main crystalline products. Gypsum disappears after heat curing and contributes significantly towards the ettringite formation. In addition to that, unaltered height of calcite peak confirms its lower reactivity in the system. 43

100

Weight (%)

90

24 h(A) 24 h(A)+48 h(H) 24 h(A)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

80

70

60 0

200

400 600 Temperature (oC)

800

1000

(a)

(b) Figure 5.2 Thermal analysis of CKD(I) paste: a) TGA, and b) DTG plots 44

5.3 Morphological Characterization of CKD(I) Paste 5.3.1 SEM examination after 48 hours of heat curing The microstructure of CKD(I) paste was examined after 48 hours of heat curing. As can be seen in Figure 5.3, presence of alkali sulfate (arcanite) can be observed after 48 hours of heat curing. Large amount of alkalis, present in form of K2O might have resulted in the formation of crystals of sodium potassium sulfate after heat curing.

1

1

Figure 5.3 SEM micrograph of CKD(I) paste after heat curing with EDX, showing the presence of alkali sulfate 45

1

1

Figure 5.4 SEM micrograph of CKD(I) paste after heat curing with EDX, showing the presence of calcium carbonate crystals Figure 5.4 shows the calcium carbonate crystals which act as an inert mineral. This observation was also confirmed through XRD and TGA which shows unaltered height of calcite peak after undergoing heat curing. 46

Ettringite

Figure 5.5 SEM micrograph of CKD(I) paste showing the presence of ettringite

X-ray diffraction and thermal analysis showed the presence of ettringite after heat curing which was also verified through SEM examination (Figure 5.5). On heat curing, gypsum facilitates the formation of ettringite. As it is shown in Figure 5.6 (a), the microstructure of CKD(I) paste does not appear to be compact and dense which results in the lower strength. Locations 1 and 2 in Figure 5.6 (b), also show two different morphologies of the reaction gel. EDXs pattern collected at two different locations suggest the formation of calcium silicate hydrate like gel though there was only a limited amount of silica.

47

1

2

1 2

(a)

3

3

(b)

Figure 5.6 (a) and (b) SEM micrographs of CKD(I) paste after heat curing showing the microstructure at high magnification with EDX patterns

48

5.4 Mineralogical Characterization of CKD(II) Paste 5.4.1 X-ray Diffraction patterns

CC

Q CC CC

CC E

CC CC

Q E Fr

CC

24 h(A)+48 h(H)+90 d(L) 24 h(A)+48 h(H)+28 d(L)

S Sg

24 h(A)+48 h(H) 24 h(A)

5

10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degrees)

Figure 5.7 X-ray diffraction patterns of heat cure CKD(II) paste at various curing periods (E – Ettringite, Q – Quartz, CC – Calcite, S – Sylvite, Sg – Syngenite, Fr – Friedel’s salt)

XRD patterns of CKD(II) paste are presented in Figure 5.7. Ettringite, calcite, quartz, sylvite, syngenite and Friedel’s salt (C3A.CaCl2.10 H2O) were identified as crystalline minerals. The precipitation of Friedel’s salt and sylvite can be attributed to the presence of chlorides in CKD(II). During the heat curing, some of sulfates are adsorbed in C-S-H 49

(Odler et al., 1995; Stürmer et al., 1994). On subsequent curing in saturated lime water, desorption of sulfates take place which results in the formation of ettringite which was also confirmed through thermal analysis in the next section. At later ages, adsorption of chloride ions might have resulted in the disappearance of Friedel’s salt and sylvite. 5.4.2 Thermogravimetric Analysis

100

Weight (%)

90

80 24 h(A) 24 h(A)+48 h(H) 24 h(A)+48 h(H)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

70

60 0

200

400 600 Temperature (oC) (a)

50

800

1000

(b) Figure 5.8 Thermal analysis of CKD(II) paste: a) TGA, and b) DTG plots

Thermal analysis of CKD(II) paste is shown in Figure 5.8. As can be seen in the figure, small amounts of calcium hydroxide and ettringite were the main hydration products after 24 hours. Less amount of calcium hydroxide can be attributed to less quantity of free lime. On heat curing, there was slight decline in ettringite peak (DTG) which could be due to the formation of Friedel’s salt. On subsequent curing in lime water, an increase in ettringite peak can be observed after 28 days, which was again decreased after 90 days. 51

5.4.3 Morphological Investigation of CKD(II) Paste 5.4.3.1 After 48 hours of heat curing After undergoing heat curing, the microstructure of CKD(II) paste can be seen as in Figure 5.9 (a). EDX collected at location 1, shows the formation of calcium aluminosilicate gel (C-A-S-H).

1

(a)

52

2

(b)

2

1

Figure 5.9 SEM micrograph of CKD(II) paste with EDX patterns, showing the presence of: a) C-A-S-H gel, and b) syngenite The microstructure of CKD(II) paste was observed to have less formation of C-A-S-H gel formation which, thereby, reduces the strength of binder. Extensive formation of syngenite crystals was also noticed. (Figure 5.9 (b))

53

1

2

2

1

Figure 5.10 SEM micrograph of CKD(II) paste after heat curing showing the presence of sylvite and calcite minerals As can be seen in Figure 5.10, large amount of calcite minerals were observed throughout the sample. The large amount of calcite was also confirmed through XRD and thermal analysis in previous chapters. In addition to that, sylvite mineral was also identified in CKD(II) paste after 48 hours of heat curing, is shown in EDX pattern of location 1 in Figure 5.10. 54

5.5 Summary The main hydration products of both CKDs were identified as ettringite, calcium hydroxide and C-S-H like gel. The presence of ettringite was observed at all ages in CKD(I) paste while the ettringite was only noticed after 28 days in lime water in the case of CKD(II) paste. The higher compressive strength of CKD(I) paste can be attributed to the large amount of ettringite and dense microstructure. Some of the alkalis were found to be present in the form of arcanite, sylvite and syngenite. Since the CKD(II) had more alkalis in the form of K2O, large amount of sylvite and syngenite crystals were observed during initial period. The precipitation of Friedel’s salt took place in CKD(II) paste along with the reduction in ettringite growth at the elevated temperature.

55

CHAPTER 6: MICROSTRUCTURAL CHARACTERIZATION OF CKD-FLY ASH PASTE 6.1 Introduction As shown in Chapter 4, the optimum proportion, giving maximum compressive strength, was found to have 70 % CKD (I) and 30 % FA. For microstructural investigation, small cylindrical specimens were prepared at a water-to-binder ratio of 0.40. All of these specimens also underwent same curing regime which has been mentioned in Chapter 3 (Experimental Program). This chapter deals with the microstructural investigation of CKD(I)-FA and CKD (II)-FA pastes. Percentage replacement was kept the same for both types of CKDs i.e. 70% (by weight). Mineralogical characterization was carried out using X-ray diffraction and thermal analysis; and furthermore, morphological changes were investigated using SEM and TEM. 6.2 Mineralogical Characterization of CKD(I)-FA Paste 6.2.1 X-ray Diffraction Patterns The X-ray diffraction patterns of CKD (I)-FA paste at different curing periods are shown in Figure 6.1. The main crystalline products, observed in hydrated paste, were calcite, gypsum, calcium hydroxide, ettringite, anhydrite, quartz, and arcanite. Some of these crystalline products, like calcite, quartz, and anhydrite, were present in the neat CKD paste. It is noted that the ettringite peak increases significantly with heat curing. The disappearance of the gypsum peak after heat curing suggests that the dissolution of gypsum increases the SO42- ion concentration in pore solution, and the increase in the concentration of sulfate ions contributes towards ettringite formation. Ettringite was found stable even after 90 days of lime curing. The presence of ettringite was also

56

confirmed through thermogravimetric analysis and scanning electron microscopy which is discussed in a later section.

CC

Q

Arc

E E

CC E

E CC

CC CC

CC

24 h(A)+48 h(H)+90 d(L)

24 h(A)+48 h(H)+28 d(L)

24 h (A)+48 h(H)

CH G

5

24 h(A)

10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degree) Figure 6.1 X-ray diffraction patterns of CKD (I)-FA paste at different curing periods (E – ettringite, CH – calcium hydroxide, CC - calcite, Arc- arcanite, Q – quartz, G gypsum)

The X-ray diffraction also confirms that calcium hydroxide was consumed during heat curing. It can be noted that the calcium hydroxide reacts with the reactive silica, and forms calcium silicate hydrate or calcium aluminosilicate hydrate gel. The formation of gel, thus, significantly enhances the compressive strength of paste. A notable observation was made after 28 days of lime curing that arcanite crystals were observed in significant amount after 28 days of curing period. During heat curing alkalis and sulfates are 57

adsorbed in C-S-A-H gel. On subsequent lime water curing, desorption of alkalis and sulfate from C-S-A-H gel might have contributed the precipitation of arcanite crystals. Also, it is evident that there is a noticeable increase in arcanite peak intensity after 90 days of curing. 6.2.2 Thermogravimetric Analysis Figure 6.2 (b) presents the thermogravimetric and derivative thermogravimetric analysis of CKD(I)-FA samples. The calcium hydroxide peak appears in the DTG plot after 24 hours, but disappears after heat curing. It supports the observation made in the XRD analysis. With heat curing, the ettringite peak increased significantly, which is clearly noticeable from the DTG plot in Figure 6.2 (b). The slope of the TGA plot in between 200 and 400oC indicates the de-hydroxylation of calcium silicate hydrate gel (Alonso et al., 2004). It is also evident from TGA plot that the slope increases significantly after 48 hours of heat curing. The increase in the slope indicates the formation of C-S-H gel in the CKD(I)-FA paste.

58

100

Weight (%)

90

24 h(A) 24 h(A)+48 h(H) 24 h(A)+48 h(H)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

80

70 0

200

400 600 Temperature (oC)

800

1000

(a)

(b) Figure 6.2 Thermal analysis of CKD(I)-FA paste: (a) TGA, and (b) DTG plots 59

6.3 Morphological Investigation of CKD(I)-FA paste 6.3.1 SEM Examination after 48 Hours of Heat Curing

FA

Figure 6.3 Microstructure of CKD(I)-FA paste after 48 hours of heat curing

The morphology of CKD(I)-FA paste after undergoing heat curing is shown in Figure 6.3. The SEM image, taken at low magnification, shows an unreacted FA particle and the general morphology of reaction gel. The microstructure of CKD(I)-FA paste appears to be dense compared to the microstructure of neat CKD(I) paste. The pozzolanic reaction in CKD(I)-FA system results in the formation of strength giving gel.

60

Ettringite

(a)

Ettringite

(b) Figure 6.4 SEM micrograph of CKD(I)-FA paste after heat curing, showing the presence of ettringite 61

As shown in the XRD and DTG plots, ettringite presence can also be observed in the SEM image shown in Figure 6.4. Ettringite was observed to be in the form of cluster. It can also be noticed that the reaction gel appears to be dense in a SEM image at high magnification (Figure 6.4 (b)). The dense microstructure significantly enhances the strength of the paste after heat curing. Heat curing accelerates the pozzolanic reaction which contributes to microstructure densification.

Figure 6.5 SEM image of dissolved FA particle after heat curing

In high pH environment, dissolution of a fly ash particle takes place which is evident in Figure 6.5. Further examination revealed that the reaction gel starts forming on the surface of fly ash particles. The Ca/Si ratio of the outer shell (location 2) is more than the fly ash particle surface (location 1) which suggests the formation of calcium rich reaction product around fly ash particle. Figure 6.6 (a) shows a partially covered fly ash particle whereas Figure 6.6 (b) shows a fully covered fly ash particle. The extensive formation of 62

reaction gel plays a crucial role in significant strength gain. The presence of ettringite can be noticed around the fly ash particles (Figure 6.6 (a)).

Ettringite

1

2 1 2

(a)

(b) Figure 6.6 (a) and (b) SEM micrographs showing the reaction gel forming cover on fly ash particle 63

Figure 6.7 shows the morphology of calcium aluminosilicate hydrate which was found extensively as an amorphous reaction product. The gel seems to have a dense morphology, which enhances the compressive strength of the paste.

1

(a)

(b)

1

Figure 6.7 (a) and (b) SEM micrographs of CKD(I)-FA paste after heat curing with EDX pattern, showing the composition and morphology of C-A-S-H

64

A closer look at the EDX pattern (Fig. 6.7) shows the adsorption of sulfate in C-A-S-H gel after heat curing. Small peak of potassium was also noticed in the EDX pattern of CA-S-H gel. The Ca/Si ratio of C-A-S-H gel in CKD(I)-FA paste after heat curing was observed ~2.

6.3.2 SEM Examination after 28 Days of Lime Curing As discussed earlier, the compressive strength of the CKD(I)-FA paste samples cured in saturated lime water for 28 days did not show significant variation from that of 48 hours heat cured samples. This indicates the stability of the reaction product of CKD(I)-FA system when exposed to moisture. Figure 6.8 shows the scanning electron micrograph taken at two different sample locations after 28 days of saturated lime water curing. The main difference in the microstructure of 48 hour heat cured and 28 day moist cured CKD(I)-FA system was the presence of arcanite (K2SO4) crystals in the moist cured samples. The EDX and XRD analyses also confirmed the presence of arcanite in the 28 day moist cured CKD(I)-FA system. It is noted that the heat curing results in the adsorption of alkali and sulfate in C-A-S-H gel. The subsequent curing in saturated lime water facilitates desorption of sulfates and alkalis. The pore solution of CKD(I)-FA paste now becomes rich in sulfates and alkalis which precipitate in form of arcanite at later ages. The presence of arcanite was noticed even after 90 days of lime water curing.

65

2

1

1

2

Figure 6.8 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of subsequent lime water curing with EDXs, showing the extensive formation of arcanite crystals (K2SO4) and C-A-S-H gel

66

Arcanite

Covered Fly Ash Particle

Figure 6.9 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of lime curing, showing the fully covered fly ash particle and arcanite crystals

Figure 6.10 SEM micrograph of heat cured CKD(I)-FA paste after 28 days of lime curing, showing the presence of C-A-S-H gel

67

A thorough microstructural investigation shows that some fly ash particles were not reacted even after 28 days of lime curing. Some fly ash particles were seen as fully covered with reaction gel (Figure 6.9). The morphology of C-A-S-H gel, as can be seen in Figure 6.10, suggests that the microstructure does not change significantly with prolonged lime curing. This seems to be the main cause of marginal strength enhancement after prolonged lime curing.

6.3.3 TEM Examination after 48 Hours of Heat Curing Transmission electron microscopy (TEM) was used to study the reaction products at a very small scale. The examination was carried out only for the specimens after 48 hours of heat curing.

Ca

1 O

Si

1

Figure 6.11 TEM micrograph of CKD(I)-FA paste after 48 hours of heat curing, showing the formation of C-S-H gel around fly ash particle 68

Si O Ca

2

K

2 Al Ca

Figure 6.12 TEM micrograph of CKD(I)-FA paste after heat curing, showing the fibrillar morphology of potassium substituted C-A-S-H gel As can be seen in Figure 6.11, the formation of C-S-H gel around the fly ash particle is evident. Moreover, the Ca/Si ratio was observed ~2, similar to that observed during SEM examination. Further examination of CKD(I)-FA shows the formation of a gel having fibrillar kind of morphology. The EDX pattern of the fibrillar morphology shows the substitution of alkali (potassium) in calcium aluminosilicate hydrate (C-K-A-S-H), as can be seen in Figure 6.12. The C-A-S-H gel appears to have adsorbed alkalis present in raw CKD (I). It is believed that part of alkalis participate in re-crystallization of minerals like arcanite which has been discussed earlier in this Chapter. It is interesting to note that the substitution of aluminum and potassium bring changes in the morphology to a great 69

extent. Moreover, the Ca/Si of the EDX pattern at location 2 (Figure 6.12) appears to be less than 1 which could not be verified through SEM examination.

6.4 Mineralogical Characterization of CKD(II)-FA Paste 6.4.1 X-ray Diffraction Patterns

CC

Q

CC

Q CC

E E

CC

CC

CC 24 h(A)+48 h(H)+90 d(L)

24 h(A)+48 h(H)+28 d(L) S Fr

24 h(A)+48 h(H)

24 h(A)

5 10 15 20 25 30 35 40 45 50 55 60 65 2-Theta (degree) Figure 6.13 X-ray diffraction patterns of CKD(II)-FA at different curing periods (E – Ettringite; CC – Calcite; Q – Quartz; S – Sylvite) X-ray diffraction analysis of CKD(II)-FA paste is shown in Figure 6.13. Ettringite and calcium hydroxide were the main hydration products, formed after 24 hours in the CKD(II)-FA paste. After heat curing, significant reduction in ettringite peak was observed that can be attributed to the precipitation of Friedel’s salt (Suryavanshi et al., 1996). Sylvite was identified through XRD at the end of heat curing that can be attributed to high alkali and chloride content of CKD(II). On subsequent curing in saturated lime 70

water, desorption of sulfates from C-S-H gel results in the increase in ettringite amount and disappearance of Friedel’s salt. 6.4.2 Thermogravimetric Analysis Thermogravimetric analysis of CKD(II)-FA paste is shown in Figure 6.14 (b). The presence of calcium hydroxide is evident after 24 hours (before heat curing). The free lime present in CKD(II) results into calcium hydroxide after reacting with mixing water. With heat curing, calcium hydroxide disappears through the reaction with reactive silica. At later curing periods, no trace of calcium hydroxide was observed, as shown in Figure 6.14 (b). A noticeable difference among the calcite peaks was detected in DTG plots of CKD(II)-FA paste. The difference in CKD proportion of small samples, used for thermogravimetric analysis, seems to have caused this difference in calcite peaks. 100

Weight (%)

90

24 h(A) 24 h(A)+48 h(H) 24 h(A)+48 h(H)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

80

70 0

200

400 600 Temperature (oC)

(a)

71

800

1000

(b) Figure 6.14 Thermal analysis of CKD(II)-FA paste: a) TGA, and b) DTG plots

6.5 Morphological Investigation of CKD(II)-FA Paste 6.5.1 SEM Examination after 48 Hours of Heat Curing In Chapter 4, the compressive strength of CKD (II)-FA paste was found to be lower than that of CKD(I)-FA paste. That led to the microstructural examination of CKD(II)-FA paste in order to assess the influence of microstructure on strength. Activation of fly ash seems to be taking place similarly as it was shown in the last section dealing with CKD(I) activation. The fly ash particle can be seen reacting partially.

72

Dissolved FA

Figure 6.15 SEM micrograph of CKD(II)-FA after 48 hours of heat curing, showing the microstructure

Figure 6.15 shows the dissolution of a fly ash particle and the reaction shell formation on the fly ash particle, which was similar to that observed in the case of CKD(I). The formation of a reaction shell can be seen in Figure 6.16. The microstructure of reaction gel seems to be not as dense as it is in CKD(I)-FA, which is responsible for the lower compressive strength. EDXs collected at location 1 and 2 suggest the formation of C-AS-H gel. A change in C-A-S-H morphology can be observed between location 1 and 2. The Ca/Si ratio in CKD(II)-FA paste was observed between ~1 and ~1.5. The reduced Ca/Si ratio compared to that in CKD(I)-FA paste can be attributed to the less availability 73

of calcium ions in CKD(II). Also, the adsorption of sulfates was nearly absent in CKD(II)-FA paste which could be attribute to the less amount of sulfates in CKD(II) compared to CKD(I).

1 1

2

(a)

2

(b) Figure 6.16 (a) and (b) SEM micrographs of CKD(II)-FA paste after heat curing with EDXs, showing the microstructure and reaction shell formation 74

(a)

(b) Figure 6.17 SEM micrographs of CKD(II)-FA paste after heat curing, showing the microstructure at high magnification

75

An image at high magnification (Figure 6.17) shows the microstructure of CKD (II)-FA paste. The microstructure does not appear to be as dense as that of CKD (I)-FA. The nonuniformity in morphology can be observed in the high magnification image (Figure 6.17 (b)). Less dense structure and non-uniformity reduces the strength of CKD(II)-FA binder compared to CKD(I)-FA paste.

6.6 Summary Ettringite and calcium aluminosilicate hydrate gel (C-A-S-H) were identified as the main hydration products in both types of CKD-based binders. The thermal activation (heat curing) accelerated the pozzolanic reaction, which helped in the formation of the C-A-SH gel. The thermal activation also favored the formation of more ettringite in the system and that also contributed towards the early age strength. In CKD(II)-FA paste, a decline in ettringite peak was observed after heat curing which is attributed to the formation of Friedel’s salt. The lower strength of CKD(II)-FA paste is believed to be due to less ettringite formation and porous nature of C-A-S-H gel. Since CKD(I) had more sulfates than CKD(II), the precipitation of arcanite at later ages was observed in CKD(I)-FA paste. The absence of alkali sulfate (arcanite or syngenite) in CKD(II)-FA paste can be attributed to less sulfates in CKD(II). TEM investigation showed the formation of C-S-H and potassium incorporated C-A-S-H gel in CKD(I)-FA paste. The Ca/Si ratio was found to be higher in CKD(I)-FA paste than CKD(II)-FA paste which can be attributed to more availability of calcium ions. Though the alkalis play an important role in creating a high pH environment, the pozzolanic reaction was found to have contributed towards the strength gain in CKD-FA binder.

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CHAPTER 7: MICROSTRUCTURAL CHARACTERIZATION OF CKD-SLAG PASTE 7.1 Introduction This chapter presents the microstructural and mineralogical characterization of CKDSlag paste. As we have seen in Chapter 4, the CKD-Slag paste had potential to gain strength even without undergoing heat curing, microstructural examination was also performed on the samples which had not undergone any heat curing. The binary blend of slag with both types of CKDs showed the potential for strength gain. The microstructural investigation would enable us to understand the macroscopic behavior of CKD-Slag binders.

7.2 Mineralogical Characterization of CKD(I)-Slag Paste 7.2.1 X-ray Diffraction Patterns The mineralogical composition of CKD(I)-Slag paste at different curing periods was determined using an X-ray diffractometer. Figure 7.1 shows the X-ray diffraction patterns at various curing periods. It is evident that after 24 hours, main crystalline hydration products were identified as calcium hydroxide, ettringite and gypsum. With heat curing, the calcium hydroxide peak disappears, which can be attributed to the reaction between calcium hydroxide and silica (SiO2), resulting in the formation of C-S-H gel. The presence of ettringite can be seen at all curing periods. The disappearance of gypsum at later ages seems to be a contributing factor towards the growth of ettringite.

77

CC

Q

E E

CC Anh

E

CC CC CC E

CC

24 h(A)+48 h(H)+90 d(L)

24 h(A)+48 h(H)+ 28d(L)

24 h(A)+48 h(H)

CH

5

G

CH

24 h(A)

10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degree) Figure 7.1 X-ray diffraction pattern of CKD(I)-Slag paste at different curing periods (E – Etrringite; G – Gypsum; CC – Calcite; Q – Quartz; Fr – Friedel’s Salt; Sg – Syngenite) As can be seen in Figure 7.1, the calcite peak appears to be unaltered throughout all ages, suggesting inertness of calcite present in raw CKD powder.

78

7.2.2 Thermogravimetric Analysis 100

Weight (%)

90

80 24 h(A) 24 h(A)+ 48 h(H) 24 h(A)+48 h(H)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

70

60 0

200

400 600 Temperature (oC)

800

1000

(a)

(b) Figure 7.2 Thermal analysis of CKD(I)-Slag: a) TGA, and b) DTG plots 79

Figure 7.2 shows the thermogravimetric analysis of CKD(I)-based Slag paste at different curing durations. After 24 hours, the formation of calcium hydroxide can be seen along with gypsum and ettringite. No derivative peak corresponding to calcium hydroxide was observed after heat curing. It is believed that, during heat curing, calcium hydroxide is used in a reaction forming calcium silicate hydrate gel. At all ages, ettringite and calcite were observed as crystalline minerals.

7.3 Morphological Investigation of CKD(I)-Slag Paste 7.3.1 SEM Examination after 48 Hours of Heat Curing

Ettringite

Figure 7.3 SEM micrograph of CKD(I)-Slag after 48 hours of heat curing

80

The microstructural examination of CKD(I)-Slag paste was performed after 48 hours of heat curing. Curing at an elevated temperature accelerates the hydration, resulting in an enhanced compressive strength after heat curing. Figure 7.3 presents a SEM micrograph showing the general morphology of microstructure. The isolated ettringite crystals were observed in Figure 7.3 along with the reaction gel having uniform morphology. Ettringite crystals, being formed in voids, were also found in a form of clusters, as can be seen in Figure 7.4.

Figure 7.4 Clusters of ettringite crystals in CKD(I)-Slag paste after 48 hours of heat curing The formation of ettringite in freely available spaces reduces the possibility of expansion due to delayed ettringite formation which is also discussed in Chapter 8. Heat curing increases the solubility of alkalis in the pore solution, thereby, enhancing the reactivity of slag. A typical morphology of C-S-H gel is shown in Figure 7.5. The C-S-H gel has a 81

uniform fine morphology which significantly improves the mechanical behavior of CKD(I)-Slag paste.

1

1

Figure 7.5 SEM micrograph, at high magnification, showing the presence of C-S-H gel in CKD(I)-Slag after 48 hours of heat curing

The presence of the AFm (C3A.CaSO4.nH2O) was observed during the SEM examination of heat cured paste. As can be seen in Figure 7.6, energy dispersive X-ray analysis of location 1 confirms the presence of the AFm phase. In addition, the AFm phase can be 82

recognized by its layer like structure, which was also observed in the figure. The EDX pattern at location 2 indicates calcium silicate hydrate gel. The sulfur peak, as can be seen in EDX at location 2, suggests the adsorption of sulfate by C-S-H gel.

1

2

2

1

Figure 7.6 SEM micrograph of CKD(I)-Slag paste after heat curing, showing the presence of AFm phase

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7.3.2 SEM Examination after Heat Curing Followed by 28 Days of Lime Water Curing A significant enhancement in compressive strength was observed, once the heat cured samples were immersed in saturated lime water. The strength gain was due to some changes in microstructure, resulting in a dense morphology.

1

2 1

3 2

3

Figure 7.7 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of subsequent lime curing with EDXs

The reaction gel in CKD(I)-Slag paste was identified as C-S-H which was substituted with Mg and Al. The locations 1 and 2 describe a typical gel, which seems to have a 84

Ca/Si ratio more than 1. The location 3, which has a Ca/Si ratio less than 1, seems to be an unreacted slag particle.

1 2

2

1

Figure 7.8 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of lime water curing, showing the presence of calcium carbonate and C-S-H gel In Figure 7.8, C-S-H gel along with calcium carbonate was verified through EDX pattern at location 1 and 2 respectively. The Ca/Si ratio of the C-S-H gel was found to be approximately 2 (EDX at location 1 in Fig. 7.8). Moreover, calcium carbonate mineral was also observed in SEM examination (EDX at location 2 in Fig. 7.8).

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The fine morphology of C-S-H gel was observed throughout the sample. As can be seen in Figure 7.9, calcium silicate hydrate gel has a flaky structure. The microstructure has dense morphology which results in high compressive strength after heat curing.

Figure 7.9 SEM micrograph of heat cured CKD(I)-Slag paste after 28 days of subsequent lime curing, showing the morphology of C-S-H gel

7.3.3 SEM Examination of Only Moist Cured Samples after 28 Days of Lime Water Curing CKD(I)-Slag showed enhanced compressive strength at later ages, even without undergoing heat curing. Since wet cured CKD(I)-Slag samples gained comparable strength to heat cured samples at later age, a morphological study of these samples was carried out after 28 days. The microstructure of continuously wet cured CKD-Slag paste

86

is shown in Figure 7.10. The Ca/Si ratio appears to vary between ~1 and ~2 (Fig. 7.10 (c)).

2

1

(a)

(b)

2

1

(c)

Figure 7.10 (a) and (b) SEM micrographs of moist cured CKD(I)-Slag paste, (c) EDX patterns at location 1 and 2

87

1

1

Figure 7.11 SEM micrograph of moist cured CKD(I)-Slag paste with EDX at location 1

In these samples as well, an extensive formation of C-S-H gel was found (Figure 7.11). As evident from the micrograph in Figure 7.12, the microstructure seems to be flaky, which was also seen in the heat cured sample. Thus the SEM examination of the heat cured and wet cured samples did not show much difference as far as the microstructural 88

development is concerned. The presence of dense microstructure results in high strength, similar to heat cured samples. The SEM examination indicates that the CKD-Slag paste seems to have potential to gain strength without heat curing, though slowly.

Figure 7.12 Microstructure of moist cured CKD(I)-Slag paste at high magnification

7.3.4 TEM Examination after 48 Hours of Heat Curing Transmission electron microscopy was performed after 48 hours of heat curing. Figure 7.13 shows typical morphology of C-S-H gel formed abundantly in CKD(I)-Slag paste. The morphology seems to be fibrillar, which has also been observed in Portland cement paste. Also, alkali substitution can be seen in the EDX pattern of C-S-H gel. 89

1

Ca 1 O

Si

K

Figure 7.13 TEM micrograph showing the fibrillar morphology of C-S-H gel in CKD(I)Slag paste after heat curing

During heat curing, alkalis are adsorbed in C-S-H gel which is evident from the above EDX pattern. Part of alkalis contributes in the formation of syngenite (K2Ca(SO4)2.H2O) which is evident in Figure 7.14. 90

1

Ca 1 O

K

S

Figure 7.14 TEM micrograph showing the presence of syngenite crystals

91

Since the XRD examination did not show any peak of syngenite, it is believed to be present in a very small amount. Thus, TEM examination showed the fibrillar morphology of C-S-H gel with the incorporation of alkalis (potassium) and the precipitation of syngenite crystals in CKD(I)-Slag paste after heat curing.

7.4 Mineralogical Characterization of CKD(II)-Slag Paste 7.4.1 X-ray Diffraction Patterns

CC

Q E E

EQ

CC CC

CC

CC

CC 24 h(A)+48 h(H)+90 d(L) 24 h(A)+48 h(H)+28 d(L)

Fr

24 h(A)+48 h(H) Sg 24 h(A)

5

10 15 20 25 30 35 40 45 50 55 60 65

2-Theta (degree) Figure 7.15 X-ray diffraction pattern of CKD(II)-Slag at different curing periods (E – Ettringite; G – Gypsum; CC – Calcite; Q – Quartz; Fr - Friedel’s salt; Sg – Syngenite)

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X-ray diffraction of CKD(II)-Slag paste shows the presence of ettringite and gypsum as the main crystalline phases after hydration (Figure 7.15). Syngenite, calcite, and quartz peaks are due to the presence of these crystalline minerals in raw CKD(II). Ettringite peak intensity reduces after 48 hours of heat curing, as seen in Figure 7.15. The decline in ettringite peak intensity can be attributed to the formation of Friedel’s salt (C3A.CaCl2.10H2O). The replacement of sulfate by chlorides might have resulted in the formation of Friedel’s salt along with the decrease in the amount of ettringite.

7.4.2 Thermogravimetric Analysis

100

Weight (%)

90

80 24 h(A) 24 h(A)+48 h(H) 24 h(A)+48 h(H)+28 d(L) 24 h(A)+48 h(H)+90 d(L)

70

60 0

200

400 600 Temperature (oC) (a) 93

800

1000

(b) Figure 7.16 Thermal analysis of CKD(II)-Slag paste: a) TGA, and b) DTG plots

Thermal analysis of CKD(II)-Slag paste shows the presence of ettringite, gypsum, and a small amount of calcium hydroxide (Figure 7.16). The less free lime content of CKD(II) can be contributed for the formation of less calcium hydroxide.

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7.5 Morphological investigation of CKD(II)-Slag Paste 7.5.1 SEM Examination after 48 Hours of Heat Curing

Figure 7.17 SEM micrograph of CKD(II)-Slag paste after heat curing

Figure 7.17 shows the microstructure of CKD(II)-Slag paste. The general morphology appears to be amorphous. The morphology of reaction gel, as can be seen in Figure 7.18, appears to be in the form of flakes. EDX at location 1 shows the composition as calcium silicate hydrate.

95

1 2

1

2

Figure 7.18 SEM micrograph of CKD(II)-Slag paste after heat curing with EDX patterns, showing the presence of C-S-H gel

96

1

1

Figure 7.19 SEM micrograph of CKD(II)-Slag paste after heat curing with EDX, showing the presence of C-S-H gel

A fine-textured morphology of C-S-H gel was noticed in SEM observation (Figure 7.19). Extensive formation of C-S-H was observed after heat curing. In a few places, a

97

homogeneous morphology was also observed with a relatively small oxygen peak (EDX at location 1 in Figure 7.20).

1

2

2

1

Figure 7.20 SEM micrograph of CKD(II)-Slag paste after heat curing, showing the crystalline phases An extensive formation of syngenite crystals was also observed after heat curing. High alkali content and substantial amount of calcium ions result in the formation of syngenite

98

in CKD(II)-Slag paste. A flaky structure of C-S-H gel was extensively found throughout the sample (Figure 7.21).

Figure 7.21 SEM micrograph of CKD(II)-Slag paste after heat curing, showing the morphology of C-S-H gel

7.5.2 SEM Examination after Heat Curing Followed by 28 Days of Lime Curing Once heat cured samples were immersed in lime water, SEM examination was performed after 28 days of moist curing. Extensive formation of ettringite was observed with moist curing, as can be seen in Figure 7.22.

99

2

1 3

1

2

3

Figure 7.22 SEM micrograph of heat cured CKD(II)-Slag paste after 28 days of lime curing 100

In addition to ettringite, syngenite crystals were also found extensively throughout the sample (Figure 7.22). Location 3 in the figure shows the C-S-H gel which was confirmed through EDX pattern. A high magnification image of C-S-H is shown in Figure 7.23. The C-S-H morphology seems to be fine and dense, thereby, contributing towards strength enhancement.

Figure 7.23 SEM micrograph of heat cured CKD(II)-Slag paste after 28 days of lime curing, showing C-S-H morphology 7.5.3 TEM Examination after 48 Hours of Heat Curing To further explore the microstructure, transmission electron microscopy was performed after 48 hours of heat curing. As can be seen in Figure 7.24, the extensive formation of CS-H gel was observed. The morphology of C-S-H appears to be a fibrillar type. 101

1

1

Figure 7.24 TEM micrograph of CKD(II)-Slag paste after heat curing, showing the C-S-H morphology with EDX

102

7.6 Summary Mineralogical and microstructural examinations of CKD(I)-Slag showed the formation of ettringite and C-S-H gel substituted with Mg and Al. In heat cured samples, Al dominated Mg substitution. The C-S-H appeared to have a fibrillar kind of morphology which was observed in both types of pastes. Ettringite seems to contribute significantly to early age strength development. However, the later age strength seems to be dominated by the extensive formation of C-S-H/ C-A-S-H/ C-A-M-S-H in the system. A detail microstructural investigation of CKD(II)-Slag also identified ettringite and C-S-H as main hydration products. The presence of syngenite crystals was verified through XRD, SEM and TEM examinations. A reduction in ettringite was observed after heat curing in CKD(II)-Slag paste which is attributed to the precipitation of Friedel’s salt, but ettringite amount increased on further moist curing. The microstructure of CKD-Slag binder densifies on subsequent lime water curing. Overall, the microstructure of CKD(I)-Slag appears to be denser than that of CKD(II)-Slag which results in higher compressive strength. The latent hydraulic nature of slag increased the effectiveness of the CKD-Slag blend for both types of CKDs.

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CHAPTER 8: DURABILITY OF CKD-BASED BINDERS 8.1 Introduction Durability is an important issue regarding practical application of any new kind of binder. There has been much research performed on the conventional concretes that incorporate cement and supplementary cementitious materials. In the past, many deteriorating mechanisms have been investigated which were found responsible for the damage of concrete. This chapter deals with three common types of damage phenomena i.e. delayed ettringite formation, alkali-silica reaction and freeze-thaw.

8.2 Delayed Ettringite Formation “Delayed Ettringite Formation may be defined as the formation of ettringite in a cementitious material by a process that begins after hardening is substantially complete and in which none of the sulfate comes from outside the cement paste” (Taylor, 2001). It is also widely termed as “DEF” or “Internal Sulfate Attack” (Skalny and Thaulow, 2001). Concretes, having undergone high temperature curing (usually above 70oC), have been found susceptible to DEF after subsequent exposure to moist environment (Ghorab et al., 1981; Heinz et al., 1987; Diamond, 1996). The existence of ettringite has been found up to ~90oC, though it becomes unstable above ~70oC (Kalousek, 1965; Heinz et al., 1986 and 1987). Since C-S-H, pore solution and ettringite compete for sulfate, the ettringite disappears at 90oC or lower temperature (Brown et al., 1993; Glasser et al., 1995). At the end of heat curing, the sulfate is adsorbed in C-S-H (Odler, 1995; Fu et al., 1994; Divet et al., 1998); and the precipitation of ettringite takes place on subsequent exposure to moist environment which is termed as DEF. Since heat curing (75oC) was performed throughout the current study, DEF potential was investigated in heat cured CKD-based 104

binders. In particular, CKD(I)-FA and CKD(I)-Slag binders were assessed for DEF. Three samples of mortar bars were prepared in accordance with ASTM C 1038 for each type of binder, and were cured at 75oC for 48 hours. After heat curing, mortar bars were immersed in saturated lime water. The length measurements were performed as per ASTM C 490. Figure 8.1 shows the expansion of mortar bars. As can be seen in the figure, the expansion of CKD(I)-Slag mortar bars was within 0.02 % after 250 days, and the expansion of CKD(I)-FA mortar bars was also found to be within 0.02% after 200 days. The experimental investigation did not show any DEF expansion in CKD-based binders after 8 months of saturated lime curing. A few reasons for not showing expansion could be as: i) addition of fly ash and slag increases the alumina (Al2O3) content which tends to increase monosulfate (AFm phase) formation (Kelham, 1999), and ii) formation of ettringite in freely available pore space (Glasser et al., 1995).

Expansion of Mortar Bars (%)

0.1 0.08 CKD(I)-Slag CKD(I)-FA

0.06 0.04 0.02 0 -0.02 0

50

100

150 200 Age (days)

250

300

Figure 8.1 Expansion of mortar bars in saturated lime water after undergoing heat curing 105

8.3 Alkali -Silica Reaction The alkali-silica reaction is a predominant damage phenomenon occurring in the case of cements having high alkalis. The CKDs used in the present study had more than 3 % equivalent alkalis which was the main reason behind investigating the possibility of alkali silica reaction in CKD-based binder. Different test methods are used based on requirements: a) assessing potential alkali reactivity of aggregates (ASTM C 1260, Mortar – bar method), b) determining potential alkali reactivity of cement-aggregate combinations (ASTM 227, Mortar – bar method), c) determining length change of concrete due to the alkali-silica reaction (ASTM C 1293), and d) effectiveness of pozzolans or GGBFS in preventing excessive expansion of the concrete due to the alkalisilica reaction (ASTM C 441). In the current work, two test methods (ASTM 227 and ASTM 1260) were employed.

Expansion of Mortar Bars (%)

0.6 0.5 OPC CKD(I)-Slag CKD(I)-FA

0.4 0.3

Potentially Deleterious

0.2 0.1 Innocuous

0 0

2

4

6 8 10 Age (days)

12

14

16

Figure 8.2 Expansion of mortar bars in 1N NaOH solution at 80oC (ASTM 1260) 106

Figure 8.2 shows the expansion of mortar bars prepared with reactive fine aggregate (river sand). As can be seen in Figure 8.2, OPC clearly shows the expansion that is more than limiting value i.e. 0.2 % at the end of 14 days in 1 N NaOH solution. This clearly confirms the reactive nature of the fine aggregate. It is evident that CKD(I)-Slag also showed slow expansion and crossed the 0.1 % expansion limit. On the contrary, CKD(I)FA did not show any expansion. The reduction in expansive behavior of CKD-based binder could be attributed to reduced availability of portlandite in pore solution, which is the main precursor for triggering the alkali silica reaction (Chatterji et al., 2005).

Expansion of Mortar Bars (%)

0.1

CKD(I)-Slag

0.08

0.06

0.04

0.02

0 0

20

40 60 Age (days)

80

100

Figure 8.3 Expansion of mortar bars in accordance to ASTM 227

As it has been shown in previous chapters, portlandite was almost consumed during the heat curing in CKD(I)-FA and CKD(I)-Slag binders. The CKD(I)-Slag binder seems to have a small amount of portlandite which appeared to have caused expansion (Figure 107

8.2). To understand the behavior more clearly, additional mortar bars were prepared as per ASTM 227. Figure 8.3 shows the expansion of mortar bars at extended exposure conditions. It can be seen in the figure that the measured expansion was found to be less than 0.015% after 70 days.

8.4 Freeze-Thaw Resistance Freeze-thaw resistance of concrete having CKD(I)-Slag as a binder was also investigated. Air-entrainment was not used in these concretes. Prior to F-T cycle exposure (after 14 days in saturated lime water), the concretes showed a dynamic modulus of elasticity of ~30 GPa, comparable to the ordinary Portland cement (OPC) concretes. The freezing and thawing (F-T) cycles had duration of 14 and 10 hours respectively, and the temperature was varied from -18 oC to + 20 oC. Thawing was carried out in the water at ambient temperature, and dry freezing was employed in a freezer at -18oC.

Dynamic Modulus of Elasticity (MPa)

50000 CKD(I)-Slag OPC Concrete

40000

30000

20000

10000

0 0

10

20 Cycles (N)

30

40

Figure 8.4 Freeze-thaw resistance of non-air entrained CKD(I)-Slag binder and OPC concretes (Paste Vol. – 40%; w/b – 0.4; Fine Agg./Coarse Agg. – 1.0) 108

Expansion of Concrete Prisms (%)

0.1

CKD(I)-Slag Concrete OPC Concrete

0.08

0.06

0.04

0.02

0 0

10

20 Cycles (N)

30

40

Figure 8.5 Expansion of concrete prisms having CKD(I)-Slag and OPC as binder (Paste Volume – 40 %; Fine Agg./Coarse Agg. – 1; water/binder – 0.40) Figure 8.4 and Figure 8.5 show the loss of dynamic modulus and expansion of concrete prisms exposed to F-T cycles. The dynamic modulus rapidly dropped below 50 % of the original value within 10 cycles. The expansion also indicates a steep increase in the length due to F-T induced cracking. The concretes incorporating CKD(I)-Slag as a sole binder were found to be more susceptible to freeze-thaw damage than conventional cement concretes. The lower tensile strength and enhanced porosity of CKD(I)-Slag binder concretes can be attributed to more damage than in OPC concretes.

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8.5 Summary The CKD(I)-FA and CKD(I)-Slag binders did not show significant expansion due to delayed ettringite formation. After 250 days, the expansion of mortar bars incorporating CKD(I)-FA and CKD(I)-Slag as sole binders was recorded less than 0.015%. CKD-based binders also performed better with respect to the alkali-silica reaction. The absence of portlandite in the heat cured CKD-based binders reduces the possibility of alkali-silica reaction. The freeze-thaw resistance of concretes having CKD(I)-Slag binder was significantly less than OPC concretes. Non air-entrained concretes made with CKD(I)Slag binder showed a dynamic modulus of ~30 GPa initially, but the modulus dropped to 50 % of the original modulus after 10 cycles of freezing and thawing.

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CHAPTER 9: CONCLUSIONS AND RECOMMEDATIONS FOR FUTURE STUDIES 9.1 Conclusions This chapter presents the major findings of the current study. The conclusions are listed as given below: 

The optimum curing temperature was found to be 75oC for CKD(I)-FA and CKD(I)-Slag pastes, maximizing the compressive strength after 48 hours of heat curing. Temperatures more than 75oC are appeared to have caused thermal cracking which lowered the strength significantly. In addition, dissociation of ettringite and thermal cracking at higher temperature (> 75oC) reduces the early age strength of these binders.



A 30 % replacement of CKD(I) by fly ash and slag in both CKD(I)-FA and CKD(I)-Slag pastes was found to be optimal, giving maximum compressive strength after 48 hours of heat curing. The CKD(I)-FA and CKD(I)-Slag pastes showed compressive strength of more than 25 MPa after 48 hours of heat curing. The effect of subsequent curing in saturated lime water was more pronounced in CKD-Slag binders compared to CKD-FA binders for both types of CKDs. The heat-cured CKD(I)-Slag paste achieved strength of ~36 MPa after 90 days of saturated lime curing where as the non heat-cured CKD(I)-Slag showed strength of ~ 32 MPa at the same age. Overall, CKD(I)-based binders achieved higher compressive strengths than CKD(II)-based binders. The better performance of CKD(I)-based binders can be attributed to higher amounts of sulfate and free lime. 111



The setting and workability (flow value) was greatly affected by the free lime content of CKD. CKD(I) which had more free lime than CKD(II), set at faster rate than CKD(I)-FA and CKD(I)-Slag due to the formation of calcium hydroxide from free lime which is an exothermic reaction. The dilution effect caused a delayed setting in CKD(I)-FA and CKD(I)-Slag pastes. The spherical shape of fly ash particles improved the workability of CKD-FA paste. On the contrary, CKD(II), lacking free lime, did not set under 24 hours of period. Only CKD(II)Slag was able to set within 24 hours of period. A delayed setting of CKD(II)based binders could be attributed to chemical composition of CKD(II) which lacks free lime and sulfate.



The concretes having CKD(I)-Slag as a sole binder performed better than concretes having CKD(I)-FA. The CKD(I)-Slag concretes (40% paste volume), which underwent heat curing, achieved the compressive strength of ~ 32 MPa after 56 days of saturated lime curing whereas CKD(I)-FA concrete (40 % paste volume) had ~ 16 MPa strength after 56 days of saturated lime curing. A non-heat cured CKD(I)-based slag concrete achieved the strength of ~26 MPa after 56 days of saturated lime curing. Overall, CKD(I)-Slag concretes performed better than CKD(I)-FA concretes. CKD(I)-Slag concretes, not having undergone heat curing, appear to gain strength, though slowly.



The main hydration products of both CKDs were identified as ettringite, calcium hydroxide and C-S-H like gel. The presence of etttringite was observed at all ages in CKD(I) paste while the ettringite was noticed after 28 days in lime water in the case of CKD(II) paste. The microstructure of CKD(I) was found to have more 112

formation of C-S-H like gel which was more dense and compact. The presence of chloride in CKD(II) resulted into the precipitation of Friedel’s salt (C3A.CaCl2.10H2O) after 48 hours of heat curing. It appears that the replacement of sulfate ions by chloride ions in ettringite, through the ion exchange mechanism, results into the disappearance of ettringite and precipitation of Friedel’s salt at elevated temperatures where the stability of ettringite is low. 

Calcium aluminosilicate hydrate (C-A-S-H) gel was the main strength giving product formed in both CKD-FA pastes. Ettringite was identified as main crystalline hydration product, and was present at all ages in both types of CKDFA pastes. The Ca/Si ratio of C-A-S-H gel was found to be more in CKD(I)-FA paste compared to the CKD(II)-FA paste. In CKD(II)-FA paste, a decline in ettringite peak was observed after heat curing which can be attributed to the precipitation of Friedel’s salt (C3A.CaCl2.10 H2O). The lower strength of CKD(II)-FA paste is also believed to be due to less amount of ettringite and porous nature of C-A-S-H gel. The presence of C-A-S-H gel in CKD(I)-FA paste was also confirmed through TEM investigation which also showed the incorporation of potassium in C-A-S-H gel. On subsequent lime curing, Friedel’s salt and sylvite disappeared in CKD(II)-FA paste and an increase in ettringite was observed which was attributed to the desorption of sulfates from C-S-H gel which appeared to favor the ettringite growth.

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

Mineralogical and microstructural examinations of both types of CKD-Slag pastes showed the formation of ettringite and C-S-H gel substituted with Magnisium and Aluminium. The Ca/Si ratio of the C-S-H was observed ~2 in both types of CKDSlag pastes. The C-S-H appeared to have a fibrillar kind of morphology which was observed in both types of CKD-Slag pastes. Ettringite seems to contribute significantly to the strength development initially. However, the later age strength seems to be dominated by the extensive formation of Mg and Al substituted C-SH gel in the system. A reduction in ettringite was observed after heat curing in CKD(II)-Slag paste, but with further moist curing, the ettringite amount increased. In CKD(II)-Slag paste, the presence of Friedel’ salt was observed before and after heat curing, along with the decrease in ettringite.

The

microstructure of both types of CKD-Slag binders densifies on subsequent curing in lime water. 

The CKD(I)-based binders did not show significant expansion due to delayed ettringite formation. The expansion of mortar bars having CKD(I)-FA and CKD(I)-Slag binders did not exceed 0.015% after 250 days. CKD(I)-based binders also performed better with respect to the alkali-silica reaction. Nearly absence of portlandite in the heat cured CKD-based binders reduces the possibility of alkali-silica reaction. Also, CKD(I)-Slag binder showed more expansion than CKD(I)-FA binder due to more availability of calcium ions. Non air-entrained concrete made with CKD(I)-Slag binder showed a dynamic modulus of ~30 GPa. The CKD(I)-Slag concretes did not show similar resistance to the 114

conventional as there was a drop of 50 % in dynamic modulus of elasticity after 10 cycles.

9.2 Variation in CKD Composition During the study, it was observed that the same CKD obtained at different batches showed some variation in chemical composition. It has also been reported that CKD (from a cement plant) composition varies during different time periods. The compressive strengths presented in this study showed some variation during repetition of a few mixtures. Hence, some variation must be taken into account before the interpretation of data.

9.3 Recommendations for Future Study The present study focused on the effectiveness of two types of CKDs which had variations of more than single parameter (free lime, sulfate, alkalis etc.) in their chemical compositions. Therefore, it was difficult to analyze the effect caused by the variation of one parameter. A study dealing with a wide range of CKDs would provide more insight into the mechanism of activation. For quantitative analysis of C-S-H gel present in CKDbased binders, SEM examination in backscattered mode is suggested for the future work. Since the inclusion of alkalis in C-S-H gel was noticed, its quantification and stability would be an area for future studies.

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APPENDIX ASTM C 109/ C 109 M-99, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens), ASTM International, Pennsylvania. ASTM C 157/C 157 M-04, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, ASTM International, Pennsylvania. ASTM C 191-01, Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle, ASTM International, Pennsylvania. ASTM C 215-97, Standard Test Method for Fundamental Transverse, Longitudinal, and Torsional Resonant Frequencies of Concrete Specimens, ASTM International, Pennsylvania. ASTM C 227-03, Standard Test Method for Potential Alkali Reactivity of CementAggregate Combinations (Mortar-Bar Method), ASTM International, Pennsylvania. ASTM C 305-99, Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency, ASTM International, Pennsylvania. ASTM C 490-00a, Standard Practice for Use of Apparatus for the Determination of Length Change of Hardened Cement Paste, Mortar, and Concrete, ASTM International, Pennsylvania. ASTM C 666/C 666M-03, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing, ASTM International, Pennsylvania. ASTM C 1038-95, Standard Test Method for Expansion of Portland Cement Mortar Bars Stored in Water, ASTM International, Pennsylvania. ASTM C 1260-01, Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method), ASTM International, Pennsylvania.

ASTM C 1437-01, Standard test method for flow of hydraulic cement mortar, ASTM International, Pennsylvania.

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VITA

Piyush Chaunsali was born on May 29, 1985 in Pithoragarh (Uttarakhand), India. After completion of his high school from G.I.C. Almora (Uttarakhand), India, he attended the National Institute of Technology, Warangal, India and obtained his B. Tech. in Civil Engineering in May 2007. Later, Piyush joined Gammon India Ltd. (July 2007 – November 2007) and Delhi International Airport Ltd. (December 2007 – November 2008) respectively. After working for one and half year in construction industry, he joined Clarkson University in January 2009 for MS program. He will be attending University of Illinois at Urbana-Champaign for his doctoral program from Fall 2010.

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