SHRINKAGE CHARACTERISTICS OF ALKALI ACTIVATED FLY ASH-SLAG BINDERS

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SHRINKAGE CHARACTERISTICS OF ALKALI ACTIVATED FLY ASH-SLAG BINDERS

ABSTRACT

Substituting ordinary Portland cement (OPC) with industrial waste such as blast furnace slag (BFS) or fly ash (FA) reduces the amount of CO2 released to the atmosphere by the cement industry. Alkali activated cement (AAC) is a promising new binder, that could serve as an alternative to OPC. Alkaliactivated binders, based on slag and fly ash are two prevalent categories of AAC. These binders are composed of a precursor powder (e.g., FA, BFS) and an alkaline solution (activator) as the mixing liquid.

 

Despite the fact that alkali activated fly ash (AAFA) has comparable or even higher strength than OPC, the curing temperature-dependent mechanical and microstructural properties of AAFA limit the commercial application of these materials. This study shows that adding a small amount of slag can rectify this problem. However, adding slag can lead to larger volumetric instability (i.e., shrinkage potential), causing durability issues in the new binders. This research studies the influence of three volumetric ratios of slag to fly ash (S/FA ratio= 0.1, 0.15 and 0.20) on shrinkage characteristics of alkali activated fly ash/slag blended binders (AAFAS). Three different types of shrinkage deformations, including: chemical, autogenous and drying shrinkage are evaluated in this study. In addition, the effect of activator pH (14.44 vs. 14.04) on early age deformation of blended S/FA binders with the three S/FA volumetric ratios is also studied.

 

The results of this study indicate that while addition of slag can significantly reduce the time of setting and enhance the compressive strength of blended binders, cured at ambient temperature; larger slag replacement leads to higher autogenous shrinkage. Nevertheless, it was also observed that the greater S/FA volumetric ratio led to the reduction in drying shrinkage of binary mortars and chemical shrinkage of the AAFAS pastes. Comparing the shrinkage measurement of two different pH values of the activators for a given S/FA volumetric ratios, it was found that the higher the pH of the activator, the lower drying and autogenous shrinkage; however the higher pH solution resulted in larger chemical shrinkage.

             

 

TABLE OF CONTENTS

LIST OF FIGURES……………………………………………………………………………………………….. vi

LIST OF TABLES…………………………………………………………………………………………………. ix

ACKNOWLEDGEMENTS………………………………………………………………………………………. x

Chapter 1 Objectives and Organization………………………………………………………………………… 1

1.1. Introduction………………………………………………………………………………………………. 1

1.2. Objectives and Scope………………………………………………………………………………….. 4

1.3. Outline…………………………………………………………………………………………………….. 5

Chapter 2 Background……………………………………………………………………………………………… 7

2.1. Alkali Activation of Cementitious Materials…………………………………………………….. 7

2.2. Applications of Alkali Activated Binder Systems…………………………………………….. 10

2.3. Alkaline Activated Binder‟s Constituents………………………………………………………. 12

2.3.1. Alkaline Agents……………………………………………………………………………………… 12

2.3.2. Source Materials……………………………………………………………………………………. 15

2.4. Alkali Activated Fly Ash (Class F)……………………………………………………………….. 19

2.4.1. Chemical Structure of Geopolymer and Reaction Mechanisms…………………………. 19

2.4.2. Microstructure Model of AAFA………………………………………………………………… 23

2.4.3. Engineering properties of Fly ash Geopolymer……………………………………………… 27

2.5. Alkali Activation of Slag……………………………………………………………………………. 29

2.5.1. Hydration Products and Reaction Mechanisms……………………………………………… 29

2.5.2. Microstructure of AAS……………………………………………………………………………. 31

2.5.3. Engineering Properties of AAS………………………………………………………………….. 34

2.6. Influence of Calcium Compounds Addition on Alkali Activation of Class F Fly Ash.. 36

2.6.1. Influence of Calcium Compounds Addition on Hydration Products and Reaction Mechanisms of AAFA…………………………………………………………………………………………………………………… 36

2.6.2. Influence of Calcium Compounds Addition on Microstructural Properties of AAFA 39

2.6.3. Influence of Calcium Compounds on Engineering Properties of AAFA………………. 42

Chapter 3 Experimental Tests and Results of Alkali Activated Fly Ash Geopolymer Binder….. 45

3.1. Experimental Program……………………………………………………………………………….. 45

3.1.1. Raw Materials……………………………………………………………………………………….. 45

3.1.2. Mixture Design and Specimen Preparation…………………………………………………… 46

3.2. Test Methods…………………………………………………………………………………………… 47

3.3. Results and Discussion………………………………………………………………………………. 50

3.3.1. Time of Setting……………………………………………………………………………………… 50

3.3.2. Compressive Strength……………………………………………………………………………… 53

3.3.3. Drying Shrinkage…………………………………………………………………………………… 55

3.4. Conclusions…………………………………………………………………………………………….. 57

Chapter 4  Experimental Tests and Results of Alkali Activated Fly Ash-Slag Binary Binders… 59

4.1. Experimental Program……………………………………………………………………………….. 60

4.1.1.Raw Materials………………………………………………………………………………………… 60

4.1.2.Mixture Design and Specimen Preparation……………………………………………………. 61

4.2. Test Methods…………………………………………………………………………………………… 63

4.3. Brief Theory of Shrinkage in the Cementitious Matrices……………………………………. 70

4.4. Results and analysis…………………………………………………………………………………… 72

4.4.1.Time of setting……………………………………………………………………………………….. 72

4.4.2.Compressive strength………………………………………………………………………………. 76

4.4.3.Chemical Shrinkage…………………………………………………………………………………. 79

4.4.4.Autogenous Shrinkage……………………………………………………………………………… 82

4.4.5.Drying shrinkage…………………………………………………………………………………….. 86

4.5. Conclusions…………………………………………………………………………………………….. 91

Chapter 5  Conclusions and Future Research………………………………………………………………. 93

5.1. Conclusions…………………………………………………………………………………………….. 93

5.2. Future Research……………………………………………………………………………………….. 95

References…………………………………………………………………………………………………………… 97

Chapter 1

 

Chapter 1: Objectives and Organization

1.1. Introduction

Over the last few years, cement and concrete industries have experienced significant increase in materials and energy consumption and demand. (Swamy 1998). Portland cement is an essential constituent of concrete and its production increases about 3% annually (McCaffrey 2002). Portland cement production accounts for almost 5 percent of global greenhouse gases emissions; (1kg CO2 per Kg cement), and consumes considerable amounts of natural materials and energy (1.7 KJ per kg cement) (Davidovits 1998, McCaffrey 2002, Mehta 2001, Marland and

Boden 1989).

 

In order to produce environmentally friendly concrete, the use of fewer natural resources, less energy, and minimizing carbon dioxide emissions was proposed (Mehta 2001). In order to reduce the amount of carbon dioxide released by the cement industry, researchers have suggested minimizing the amount of calcined material in the cement by decreasing the cement content in concrete (McCaffrey 2002). While traditionally, Portland cement is “partially” replaced by pozzolanic materials (e.g, FA, BFS, silica fume, glass powder, etc.), it is also possible to “fully” substitute Portland cement with some of these powders. The method is known as alkali activation, where a high pH solution (activator) is used instead of water as the mixing solution.

 

The two most widely used materials for production of alkali activated cements are fly ash

(FA) and blast furnace slag (BFS) and combinations of them; both are industrial wastes requiring less energy and generating lower amount of  compared to OPC (Krivenko 1994, Shi et al.

2006 and Juenger et al. 2011) (Davidovits 1994, 1998, 2005).

 

Based on the composition of solid ingredient, three branches of AAC system exists as low-calcium alkali activated materials, high calcium alkali activated binders and intermediate calcium alkali activated systems. The first group is produced by the alkali activation of aluminosilicate materials with low-calcium contents (e.g., alkali activation of metakaolin or class F fly ash). A three-dimensional alkaline inorganic precipitate consisting of a ring structure of SiO-Al-O bonds (known as geopolymer gel or sodium alominosilicate hydrate (N-A-S-H)) is formed by activating these materials (Palomo et al. 2007, Duxson et al. 2005). High calcium alkali activated system is obtained by the alkali activation of materials containing high calcium (e.g., alkali activated slag (AAS)). The main reaction product in this case is to some extent similar to the gel formed during the hydration of Portland cement, calcium silicate hydrate or C-S-H gel (Fernández-Jiménez et al. 1999). The C-A-S-H type gel formed by alkali activating slag contains lower content of calcium than the hydrated OPC pastes (Ca/Si ratio in hydrated OPC paste is usually between 1.5 to 2) (Richardson 1999). The intermediate calcium alkali activated binders is formed by alkali activation of aluminosilicate materials with intermediate level of calcium contents (e.g., alkali activation of blended fly ash/slag (AAFAS) or class C fly ash). The hydration product of this group is an intermixed microstructure of C-A-S-H and Na-A-S-H (Shi and Day 1999, Yip et al. 2005).

 

The alkaline activation of class F fly ash (low calcium content) results in the formation of an inorganic polymer. In these systems, the polymerization starts when alumino-silicate material dissolves in a highly alkaline solution, followed by precipitation of an amorphous products with a similar structure to zeolites and are known as geopolymers (Davidovits 1988, Palomo et al. 1999, Krivenko, 1997, Palomo et al., 2004). The three dimensional sodium aluminosilicate material, N– A–S–H, is responsible for development of high mechanical strength in this system (Fernandez et al. 2005, Oh et al. 2011). Previous studies show that curing temperature plays an important role in the geopolymerization process of fly ash-based materials (Hardjito et al. 2008, Van Jaarsveld et al. 2002, Palomo et al. 1999), concluding that elevated temperature (through heat curing for example) is needed to achieve desirable strength development.  The heat curing process, however, can limit the industrial field applicability to only precast industry (Lee and Lee 2013, Radlinska et al. 2013). It has been observed that curing AAFA mortars at 65oC, 70oC, and 80oC for 24 hours increases the compressive strength, when it is compared to that one ambient temperature. It is also noted that the mechanical and microstructural properties of these materials are highly temperature dependent (Hardjito et al. 2008, Hardjito et al. 2004).

 

Past studies have concluded that  addition of calcium compounds as a fly ash substitute improved mechanical properties at the ambient temperature (Yip et al. 2008, Temuujin et al. 2009). It was concluded that adding slag that contains a high content of CaO to AAFA can improve the strength development of geopolymer systems cured under ambient room

temperatures.

 

Previous research studies reported that AAS had higher drying shrinkage than OPC and addition of FA can help to decrease the drying shrinkage of AAS pastes (Palacios and Puertas 2007, Shi et al. 2006, Yang et al. 2007). The high magnitude of AAS shrinkage, specifically at an early age, induces a risk of premature cracking (Cartwright et al. 2014, Malolepszy et al. 1988,

Collins et al. 2000, Kutti et al. 1992, Palacios and Puertas 2007, Sakulich and Bentz 2013). On the other hand, it was also reported that alkali activated fly ash has low shrinkage characteristics when it is compared to OPC (Malone et al. 1985, Hardjito et al. 2005, Fernández-Jiménez et al. 2006, Rashad 2013). The effect of the addition of slag on early age behavior of AAFA is still in question.

1.2. Objectives and Scope

The goal of this study is to understand the influence of using slag as partial replacement (up to 20%) of fly ash to investigate the early and long-term properties of AAFA. Extensive research has been focused on alkali-activation of fly ash or slag, however an in-depth understanding of the mechanism of incorporation of slag as a source of calcium in activated fly ash binders is still lacking. As this thesis focuses on the engineering characteristics of alkali activated fly ash-slag (AAFAS) binary binders, firstly the mechanical property of AAFA cured under elevated temperature was examined. Then the influence of adding slag on the properties of alkali-activated binary system cured in room-temperature was studied to determine the appropriate replacement ratio of the slag to the fly ash in AAFA. Finally the shrinkage mechanism of the binary system was examined for different slag replacement ratio and PH of the activator solution.

In order to achieve the stated goal, the following main objectives are pursued in this research study:

  1. To investigate the mechanical properties of alkali-activated fly ash and fly ash/slag blended mortar cured at elevated and room temperature.
  2. To examine the early-age deformation of alkali activated fly ash/slag blended systems to evaluate the effect of slag incorporation on shrinkage characteristics.
  3. To investigate the effect of activating solution pH on the shrinkage characteristics of alkali activated systems.

1.3. Outline

The content of this thesis is presented in five chapters. Chapter 2 provides a literature review on alkali activated binder systems. It includes a review of the reaction mechanisms of alkali activated fly ash, slag and binary systems and their mechanical and microstructure properties. It also includes a review of the different testing techniques used in the characterization of alkali activated binders.

 

Chapter 3 presents the material properties, mixture proportions, mixing procedures and test methods used to evaluate mechanical properties of alkali activated fly ash (AAFA) systems. The results of compressive strength, time of setting and drying shrinkage of AAFA systems are presented and discussed

 

Chapter 4 details the material properties, mixture proportions, mixing procedures, setting and mechanical properties, test methods and results of measuring shrinkage and microstructural properties of blended alkali activated fly ash/slag systems. . Three different forms of shrinkage, including drying, autogenous and chemical shrinkage were studied to explore the susceptibility of AAFAS binary system to shrinkage as a function of slag to fly ash volumetric ratio as well as the activating solution pH. Scanning electron microscopy-coupled with energy dispersive spectroscopy (SEM/EDS) was used to qualitatively and quantitatively study the microstructural and elemental evolution of these AAFAS blended binders. Mercury intrusion porosimetry (MIP) was also employed to determine the pore structure and pore size distribution of these binary binders to observe how it is affected by activating solution. Static modulus of elasticity of AAFAS concrete was measured to see the influence of bulk elastic modulus of pastes on the magnitude of deformation of AAFAS systems.

 

Finally, Chapter 5 discusses conclusions of the studies carried out on the shrinkage characteristics of alkali activated binder systems.

SHRINKAGE CHARACTERISTICS OF ALKALI ACTIVATED FLY ASH-SLAG BINDERS

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