Hiroshima University DOCTOR OF ENGINEERING BUI PHUONG TRINH Effects of Internal Alkali Activation on Chemical and Mechanical Properties of Fly Ash Cement Systems (フライアッシュ・セメント系の化学的・力学的特 性に及ぼす内部アルカリ活性化の影響) September 2015 ABSTRACT Fly ash concrete has been used widely in the construction because of taking the advantages of the improved durability, effective cost, and environmental protection. However, low- calcium fly ash concrete has the lower strength than the cement concrete due to slow pozzolanic reaction of fly ash. This results in its limitation for the production of high strength concrete. In the recent years, several studies have suggested alkali activations to accelerate the pozzolanic reaction of fly ash particles.
One of alkali activations is performed by mixing one or some types of alkaline solutions with fly ash directly and curing at high temperature, which could limit to apply in the practical use. The curing condition at normal temperature in alkali activation needs to be considered as the more practical method. In addition to the alkali activation, internal curing has been investigated for improving the properties of high strength concrete with a low water to binder ratio. Nevertheless, the previous studies of internal curing have discussed only the effects of internal water supplied from internal curing agents (such as pre-wetted lightweight aggregate, super absorbent polymers, porous ceramic waste aggregate (PCWA), and so on) on some properties of concrete.
The internal acceleration for pozzolanic reaction by using PCWA imbibing an alkali solution, however, has not been investigated in the fly ash concrete yet. Based on this background, the aim of this study is to investigate an internal alkali activation (IAA) on the fly ash cement systems cured at normal temperature so that the fly ash concrete using PCWA imbibing alkali solution could get the maximized strength and enhanced durability. To achieve the above-mentioned purpose, this thesis is organized as follow: Chapter 1 describes the background, aims, and methodology of this study. Chapter 2 provides a brief literature review about the effects of fly ash, the mechanisms and effects of internal curing, and the alkali activation on the chemical reaction and the long- term mechanical properties of the fly ash cement systems.
i Chapter 3 presents the experimental program consisting of materials and mixture proportions, the fundamental models as IAA, the mixing and casting progress, the curing condition for the fly ash cement system. The experiments of fundamental models were performed to study the effects of IAA on the chemical and mechanical properties of the fly ash cement systems. They were (1) an original model through an installed syringe, (2) a model of internal activation by using PCWA. In addition, the effects of IAA on the mechanical properties of fly ash concrete using PCWA prepared in saturated-surface dry condition after the immersion in alkali solution for 7 days were investigated.
Three types of IAA used in this study were (1) 0.1mol/L NaOH solution (pH = 13.6), and (3) water for a reference. In addition to the effects of types of IAA, the effects of starting time of IAA on the pozzolanic reaction of fly ash cement systems were also studied. Cement systems with 0%, 20% and 40 mass% of fly ash replacementratios were used, while the concrete using PCWA by 0% and 40 vol.% of coarse aggregatereplacement ratios were used in this study. In order to evaluate the effects of IAA, the measurements of Ca(OH)2 (CH) content and porosity, the calculation of CH consumption by the pozzolanic reaction, and test of the compressive strength of concrete were carried out by thermal gravimetric analysis, mercury intrusion porosimetry, and strength test, respectively.
In addition, a confirmation by SEM examination was performed on this study. Chapter 4 discusses the effects of types and starting time of IAA on the chemical reaction of the fly ash cement systems by examining the CH content and consumption of CH. The experiments demonstrates that IAA not only decreased the CH content but also increased the CH consumption by the pozzolanic reaction in the cement paste with 40% replacement of fly ash (FA40). Moreover, an injection of saturated Ca(OH)2 solution reduced the CH content and increased the consumption of CH in FA40 more than that of water or NaOH solution.
In addition to the effects of types of IAA, it can be found that IAA 3 months after casting ii increased the consumption of CH by pozzolanic reaction in FA40 more than that 1 month after casting. Briefly, IAA was effective in accelerating the pozzolanic reaction and promoting the cement hydration of the fly ash cement systems. This was also confirmed by SEM examination. Chapter 5 discusses the effects of IAA on the mechanical properties of the fly ash cement systems by measuring the porosity and hardness, and testing the compressive strength of the fly ash concrete.
It shows that IAA decreased the total pore volume in FA40. Furthermore, pore size distribution was alerted by IAA, with the decrease in the volume ratio of 20-330 nm pores to the total pore and the increase in that of 3-20 nm pores in FA40. It indicates that IAA was effective in accelerating the pozzolanic reaction of the fly ash cement systems. According to the decrease in the volume ratio of 20-330 nm pores to the total pore and the increase in that of 3-20 nm pores, it can be said that the IAA 3 months after casting was more effective in accelerating the pozzolanic reaction of the fly ash cement paste at the age of 12 months than that 1 month after casting.
The experiment by using the model of internal activation with PCWA indicates that IAA also improved the microstructure of interfacial transition zone (ITZ) and bulk paste in the fly ash cement systems at the age of 6 months. In addition, the effects of IAA by using PCWA on the mechanical properties of the fly ash concrete can be briefly concluded that although the short- and long-term compressive strengths in the fly ash concrete using 40% replacement of PCWA imbibing the alkali absorption were nearly the same as those without PCWA, the macropore volume (pores ranging 0.05 – 50 µm) was reduced in the presence of IAA at the ages of 28, 182, and 364 days. Moreover, pore size distribution was altered by IAA, with the decrease in the volume ratio of 20-330 nm pores to the total pore and the increase in that of 3-20 nm pores. Briefly, the pozzolanic reaction of the fly ash cement systems was accelerated by IAA, with the decrease in the volume ratio of 20-330 nm pores to the total pore, the increase in the volume iii ratio of 3-20 nm pores, and the improved ITZ microstructure although the enhanced compressive strength was not shown.
Chapter 6 proposes the mechanisms of IAA accelerating the pozzolanic reaction as well as promoting the cement hydration of the fly ash cement system. In addition, the differences in the starting time of IAA mechanism affecting the microstructure development in FA40 and the differences of each type of IAA in the activation mechanism of the fly ash particles are also described. Chapter 7 states the conclusions of this study. Recommendations for future work are also provided.
iv CONTENTS ABSTRACT i CONTENTS v LIST OF FIGURES x LIST OF TABLES xvii ACKNOWLEDGEMENTS xviii CHAPTER 1: INTRODUCTION 1.2 INTERNAL ALKALI ACTIVATION (IAA) ON FLY ASH CEMENT SYSTEMS 1-5 1.3 AIMS OF THE RESEARCH 1-5 1.4 METHODOLOGY OF THE RESEARCH 1-6 1.5 THESIS OUTLINE 1-7 References 1-8 CHAPTER 2: LITERATURE REVIEWS 2.1 Properties of fly ash 2-2 2.2 Effects of fly ash 2-5 2.3 Effects of internal curing 2-16 2.3 Effects of alkali activation 2-24 2.4 SUMMARY 2-27 References 2-28 CHAPTER 3: EXPERIMENTAL PROGRAM 3.3 METHOD OF IAA 3-7 3.1 Original model of IAA 3-8 3.2 Model of IAA using PCWA 3-10 3.3 IAA on fly ash concrete 3-11 3.4 MIXING, CASTING AND CURING 3-12 3.1 Mixing and casting 3-12 3.1 Differential thermal analysis and thermogravimetry (DTA-TG) 3-12 vi 3.4 Mercury intrusion porosimetry (MIP) 3-15 3.5 Scanning electron microscopy (SEM) 3-16 3.6 SUMMARY 3-16 References 3-17 CHAPTER 4: EFFECTS OF IAA ON CHEMICAL REACTION OF FLY ASH CEMENT SYSTEMS 4.1 EFFECTS OF IAA ON Ca(OH)2 CONTENT 4-1 4.1 Effects of IAA 4-1 4.2 Effects of types of IAA 4-3 4.3 Effects of starting time of IAA 4-7 4.2 EFFECTS OF IAA ON CONSUMPTION OF Ca(OH)2 4-10 4.1 Effects of IAA 4-11 4.2 Effects of types of IAA 4-13 4.3 Effects of starting time of IAA 4-14 4.3 CONFIRMATION OF EFFECTS BY SEM IMAGES 4-16 4.4 SUMMARY 4-20 References 4-21 CHAPTER 5: EFFECTS OF IAA ON MECHANICAL PROPERTIES OF FLY ASH CEMENT SYSTEMS 5.1 EFFECTS OF IAA ON POROSITY IN FLY ASH CEMENT SYSTEM 5-1 5.2 Relationship between consumption of Ca(OH)2 and porosity 5-10 5.2 EFFECTS OF IAA ON HARDNESS OF ITZ IN FLY ASH CEMENT SYSTEMS 5-11 5.1 Effects of internal saturated Ca(OH)2 solution supplied from one PCWA 5-11 5.2 Effects of types of IAA supplied from one PCWA 5-14 5.3 EFFECTS OF IAA ON COMPRESSIVE STRENGTH AND POROSITY OF FLY ASH CONCRETE 5-16 5.3 Relationship between compressive strength and porosity 5-20 5.1 Fly ash cement paste 5-21 5.2 Fly ash cement paste with one PCWA 5-22 5.3 Fly ash concrete 5-23 References 5-23 CHAPTER 6: DISCUSSIONS OF MECHANISM OF INTERNAL ALKALI ACTIVATION 6.1 MECHANISM OF IAA ON MICROSTRUCTURE DEVELOPMENT IN PLAIN CEMENT PASTE 6-1 6.2 MECHANISM OF IAA ON MICROSTRUCTURE DEVELOPMENT IN FLY ASH CEMENT PASTE 6-3 6.3 DIFFERENCES OF STARTING TIME IN IAA MECHANISM ON MICROSTRUCTURE DEVELOPMENT IN FLY ASH CEMENT PASTE 6-4 6.4 DIFFERENCES OF TYPES OF ALKALI SOLUTION IN ACTIVATION MECHANISM OF POZZOLANIC REACTION IN FLY ASH PARTICLE 6-6 viii References 6-9 CHAPTER 7: CONCLUSIONS AND RECOMMENDATIONS 7.2 RECOMMENDATIONS 7-3 Appendix ix LIST OF FIGURES Figures Title 1.1 Global CO2 production 2.1 Process of fly ash production at a power station 2.2 Physical model of the reaction in fly ash cement system 2.3 CH content relative to the cement content in PC and FC pastes (based on ignited weight) with w/b = 0.4 SEM images of fly ash cement system at the ages of 3 days (left) and 28 days (right) 2.5 SEM images of hydrated fly ash cement paste at the ages of 3 days (left) and 120 days (right) 2.6 Porosity of cement paste with and without fly ash after 1 week and 1 year at 20oC 2.7 Relationship between the strength and the total pore volume (left), and the volume of pores from 20 to 330 nm in diameter (right) 2.8 Trend of compressive strength of concrete (w/b = 0.38) when OPC is replaced with fly ash 2.9 Compressive strength development of motars (w/c = 0.10 Pore size distribution of the concrete mixes at w/b = 0.24 at the ages of 28 days (left) and 90 days (right) 2.11 Relationship between micro hardness and fly ash replacement 2.12 Averaged intensity of water as a function of distance from the surface of LWA, n = 20 2.13 Illustration of the difference between external and internal curing 2.14 Compressive strength of all mortar mixtures x 2.15 Compressive strengths and degree of hydration after 1, 3, and 8 days of sealed curing for control and internal curing – IC high performace mortar 2.16 Effect of addition of the porous ceramic coarse aggregate on compressive strength development (top) and gain of compressive strength between 7 and 28 days for mixtures with internal curing compared to the control samples (bottom) 2.17 SEM images of mortar microstructure for fly ash blended cement without (top) and with (bottom) internal curing at magnifications of 1200× (left) and 2400× (right) 2.18 Effect of internal curing on ITZ of mortar with w/c = 0.3 under sealed curing condition at 120 days by SEM images when compared with ITZ of mortar without internal curing 2.19 Effect of pH on the dissolution of amorphous SiO2 (left) and Effect of pH and temperature on the concentration of dissolved silicum in NaOH solution for fly ash and silica fume (right) 2.
