SHRINKAGE AND CREEP OF ALKALI-ACTIVATED BINDERS

ABSTRACT

 

Alkali-activated binders (AAB) are the cement-free binder which could serve as a promising alternative for OPC with significant environmental and durability benefits. AAB is formed through a chemical reaction between high-alkaline solution and alumino-silicate source material (mostly industrial by-products such as fly ash or blast furnace slag). In recent years, much research has been devoted to investigate engineering properties of this material. However, AAB has not yet been entered the mainstream concrete industry, in particular, due to uncertainties and challenges with respect to the durability characteristics of these new binders. One area of significance is any potential for crack growth caused by shrinkage or creep of this green binder. This research questions the shrinkage susceptibility of different AABs, the effective parameters that can influence the risk of shrinkage, and the time-dependent response (creep) of AABs under sustained compressive load.

In the first part of this study, four different AABs were designed and their drying shrinkage at four different relative humidities (RHs) was studied. AAB pastes include an alkali-activated class F fly ash (AAFA), an alkali-activated slag (AAS) and two binary of different proportion of fly ash/slag mixtures. The initial porosity of AABs and control OPC are identical and their activating solutions were designed to result in a mortar with 28-day compressive strength of 30 MPa or larger. The results indicated that all AAB pastes cured at 23^oC exhibited higher shrinkage compared to OPC paste and that shrinkage increased with the higher content of fly ash. However, the steamcured AAFA paste had the coarsest pore size and the least shrinkage. On the other hand, AABs containing slag showed a finer pore structure, larger saturation level, lower stiffness, and higher time-dependent shrinkage compared to those of OPC. In addition, shrinkage trend of various

AABs at different ambient RHs were not similar. Interestingly, for AABs rich in slag, the

 

largest drying shrinkage was observed at 50% RH while the largest shrinkage of OPC and fly ash rich pastes was reported at the lowest RH (i.e., 30%RH).

The second part investigates the time-dependent deformation (creep) and the impact of elevated temperature on drying shrinkage of four AABs studied in the first part. The effect of curing-temperature on drying shrinkage of AAB pastes was evaluated by comparing the shrinkage of moist-cured versus steam-cured minibars at 50% relative humidity. The shrinkage results provided evidence that the steam curing stiffened the matrix of these binders and remarkably reduced the drying shrinkage. In addition, creep characteristic of four AAB mortars were also measured. It was noticed that the time-dependent response of slag rich AABs was higher than steam-cured fly ash mortar. This phenomenon is likely to explain why the slag rich binders shrunk more under continually internal stress induced by drying and exhibited greater time-dependent or viscous deformation.

Since the results indicate that AAFA shows lower or comparable shrinkage compared to

OPC, finally the last part of this research evaluates the shrinkage propensity of the steam-cured AAFA in more details. In this part, the impact of activating solution, RH and curing regime on drying shrinkage of AAFA were studied. The results provide evidence that the addition of dissolved silicate to alkaline solution plays an important role in the physical and mechanical properties of AAFA. Furthermore, heat-cured AAFA paste dried faster; and its shrinkage and mass change reached equilibrium more quickly compared to OPC ones. Similar to OPC, the highest drying shrinkage and mass loss of AAFA pastes took place at the lowest humidity (i.e. RH=30%) and there is a bilinear relationship between RH and shrinkage of AAFA. The experimental results indicate that a longer steam-curing duration can stabilize the microstructure of binders and reduce the shrinkage without any notable change in the mass loss of the binder.

 

 

TABLE OF CONTENTS

List of Figures ………………………………………………………………………………………………………….. vii

List of Tables …………………………………………………………………………………………………………… ix

List of Equations ………………………………………………………………………………………………………. x

Acknowledgements …………………………………………………………………………………………………… xi

Chapter 1 Introduction ………………………………………………………………………………………………. 1

1.1.Sustainability in Concrete Industry ……………………………………………………………….. 1

1.2.What IsAlkali-Activated Binder? ………………………………………………………………….. 3

1.3.Challenges in Application of Alkali-Activated Binder …………………………………….. 5

1.4.Research Objectives ……………………………………………………………………………………. 7

1.5.Outline ………………………………………………………………………………………………………. 8

1.6.References …………………………………………………………………………………………………. 10

Chapter 2 Drying Shrinkage and Creep of Alkali-activated binders  Part I: Drying Shrinkage at Different Relative Humidities …………………………………………………………… 14

2.1. Abstract ……………………………………………………………………………………………………… 14

2.2. Introduction ………………………………………………………………………………………………… 15

2.3. Experimental Program …………………………………………………………………………………. 17

2.3.1. Solid Materials ………………………………………………………………………………….. 17

2.3.2. Mixture Design and Activating Solution ……………………………………………….. 18

2.3.3. Test Methods …………………………………………………………………………………….. 19

2.4. Results ……………………………………………………………………………………………………….. 25

2.4.1. Drying Shrinkage and Mass Loss …………………………………………………………. 252.4.2. Pore Structure ……………………………………………………………………………………. 33

2.4.3. Degree of Saturation (DOS) at the Onset of Drying ……………………………….. 352.4.4. Compressive Strength, Elastic Modulus, and Poisson’s Ratio of Mortars ….. 36

2.5. Discussion ………………………………………………………………………………………………….. 372.6. Conclusions ………………………………………………………………………………………………… 452.7. Acknowledgements ……………………………………………………………………………………… 46

2.8. References ………………………………………………………………………………………………….. 46

Chapter 3 Drying Shrinkage and Creep of Alkali-activated binders  Part II: Influence

of creep and curing temperature on drying shrinkage ………………………………………….. 50

3.1. Abstract ……………………………………………………………………………………………………… 503.2. Introduction ………………………………………………………………………………………………… 51

3.3. Experimental Program …………………………………………………………………………………. 54

3.3.1. Materials and Mixture Design ……………………………………………………………… 54

3.3.2. Test Methods …………………………………………………………………………………….. 54

3.4. Results  ………………………………………………………………………………………………………. 57

3.4.1. Part II Shrinkage Results …………………………………………………………………….. 583.4.2. Part II Pore Structure and Mechanical Properties …………………………………… 61

3.4.3. Creep Results…………………………………………………………………………………….. 63

3.5. Discussion ………………………………………………………………………………………………….. 70

3.4.4. Shrinkage ………………………………………………………………………………………….. 72

3.4.5. Creep ……………………………………………………………………………………………….. 74

3.6. Conclusion …………………………………………………………………………………………………. 75

3.7. Acknowledgements ……………………………………………………………………………………… 763.8. References ………………………………………………………………………………………………….. 763.9. Appendix ……………………………………………………………………………………………………. 80

Chapter 4 Influence of Activation Solution on Drying Shrinkage of   Alkali-activated Fly Ash at Different Relative Humidities ……………………………………………………………………. 81

4.1. Abstract ……………………………………………………………………………………………………… 81

4.2. Introduction ………………………………………………………………………………………………… 82

4.3. Experimental Program …………………………………………………………………………………. 85

4.3.1. Solid Raw Materials and Activating Solution ………………………………………… 85

4.3.2. Mixture design…………………………………………………………………………………… 86

4.3.3. Mixing procedure and test methods ……………………………………………………… 87

4.4. Results ……………………………………………………………………………………………………….. 92

4.4.1. Drying Shrinkage and Mass Loss …………………………………………………………. 92

4.4.2. Pore Size Distribution and Specific Surface Area …………………………………… 99

4.4.3. Mechanical Properties and Degree of Saturation ……………………………………. 106

4.5. Discussion ………………………………………………………………………………………………….. 108

4.5.1. Influence of Activating Solution Composition on Drying Shrinkage and

Mass Loss …………………………………………………………………………………………… 108

4.5.2. Influence of RH on Drying Shrinkage ………………………………………………….. 110

4.5.4. Influence of Steam-Curing Duration …………………………………………………….. 118

4.5.5. Rate of Drying Shrinkage ……………………………………………………………………. 119

4.6. Conclusion …………………………………………………………………………………………………. 121

4.7. References ………………………………………………………………………………………………….. 122

Chapter 5 Conclusion ………………………………………………………………………………………………… 128

5.1.Conclusions ……………………………………………………………………………………………….. 128

5.2.Future Research ………………………………………………………………………………………….. 132

Chapter 1 

 

Introduction

1.1. Sustainability in concrete industry

Ordinary Portland cement (OPC) concrete is the most widely used construction material in the world. Concrete is an artificial stone made by binding aggregates with a proper binder. The main ingredients of OPC concrete are Portland cement, water and aggregates (fine and coarse). In other words, portland cement is a primary constituent of OPC concrete and its production increases around 3% annually [1] . In 2008, the global cement production was about 2.6 billion tons [2] .

OPC is made by calcining clay and limestone in a cement kiln. Limestone is made of calcium carbonate. In kiln, limestone is heated up, and then breaking down into calcium oxide and CO₂. Accordingly, the production of OPC emits a significant amount of CO₂, a greenhouse gas, due to fossil fuel combustion for chemical conversion process of initial ingredients and direct generation of CO₂ during calcination. Additionally, OPC production consumes considerable amounts of natural materials (such as clay, sand and limestone) and energy (pulverized coal or natural gas). Each ton of portland cement production emits approximately one ton of CO₂into the earth’s atmosphere and requires approximately 1700–1800 MJ of energy [3-4] . The cement industry accounts for roughly 5 percent of global anthropogenic CO₂emissions [5] . As far as economy is concerned, the cement is the most expensive ingredient of concrete and as concrete is used more than any other man-made materials on the planet, it is therefore important to find means of economizing the use of Portland cement.

As the global warming has become the most serious environmental concern nowadays, it is essential to develop a sustainable solution for the future of concrete construction. According to Mehta proposal [6] , to prevent any irreversible climate change caused by concrete industry, the

CO₂ emitting source of cement production should be returned back to the 1990 level, or less in 2030 years. Since 1990, the CO₂emission of cement production has doubled [7] . Mehta proposed three tools (Mehta triangle) which can enable global concrete industry to reach CO₂emission level of 1990 and become more sustainable within 20 years. Based on the Mehta’s sustainable triangle (Figure 1-1) in order to reduce the amount of carbon dioxide released by the concrete industry, it is necessary to lower the amount of calcined material by decreasing the Portland cement content [1,8-9] . Therefore, a viable option to attain a sustainable concrete is to partially or completely substitute OPC with industrial waste products such as blast-furnace slag (BFS, which is a nonmetallic coproduct of pig iron manufacture) or fly ash (FA, which is the byproduct of coal combustion in power plant) [9-10] . Alkali activated cement (AAB) is one of the promising alternatives to OPC concrete, which has been getting more attention over the few past years [1213] .

 

 

Figure 1-1 Mehta tools for reducing the cement industry’s carbon emissions to the 1990 level in next 20 years [6]

1.2. What Is Alkali-Activated Binder?

AAB is a general term referring to any binder that is formed by a chemical reaction between concentrated alkaline aqueous solution and aluminosilicate powder. The solid components of AAB (i.e., aluminosilicate material) is mostly the industrial wastes such as FA or BFS. AAB is a green material which has the potential to serve as an alternative to OPC. Due to OPC replacement with industrial wastes in AAB, this new binder can generate less carbon dioxide than OPC [14] . Weil et al. [15] showed that the global warming potential of AAB could be up to 70% lower than that of OPC. The consumption of energy for AAB is also reported to be approximately 60% less than OPC use of energy [16] . Jiang et al. [17] assessed the environmental impacts of alkali activated slag (AAS) and indicated that 35-MPa AAS concrete exhibits 73% less CO₂ emission and 43% lower energy consumption than those of a 35MPa OPC concrete. Also as AAB utilizes industrial waste materials, they are likely to be more cost effective.  

The use of aqueous alkaline solution is one of the chemical dissimilarities between the AAB system and OPC system apart from the differences between the compositions of solid precursors. In OPC, water is added to OPC to make the binder. The pH of mixture gradually goes up because of the dissolution of OPC. The final product of cement hydration is a calcium silicate hydrate known as C-S-H and calcium hydroxide [18] . In AAB, in order to reach a favorable dissolution of solid precursor, the aqueous solution is highly alkaline (pH is mostly above 14). This high alkaline solution is needed to break the strong silicon-oxygen bonds in the silicate or aluminosilicate network, accompanied by the formation of new reaction products such as sodium aluminosilicate hydrates (N-A-S-H) or calcium aluminosilicate hydrates (C-A-S-H) [13] . The chemical, physical and mechanical properties of AAB is strongly affected by composition of activating solution. The high alkaline activator is the solution such as alkali hydroxides, silicates, carbonates, sulfates, aluminates or oxides. The high alkaline solution is needed to accelerate the dissolution of the solid precursor [19] . Previous studies showed that the addition of dissolved silicate could improve the mechanical properties of AABs [20] .

The solid components of AAB contain high amounts of glass, as well as mineral phases rich in calcium silicate (e.g., BFS) or aluminosilicate phases (e.g., FA) [21] . Based on the composition of solid ingredient and in terms of calcium content, AABs could be classified in three groups including: low-calcium, high calcium, and intermediate calcium AABs.

• 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 or AAFA). 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-SH)) is formed by activating these materials [22-23] .
• High calcium AAB is obtained by the alkali activation of materials containing high calcium (e.g., alkali activated slag or 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 [24] . The C-A-S-H type gel formed by alkali activating of slag contains lower content of calcium compared to the hydrated OPC pastes (Ca/Si ratio in hydrated OPC paste is usually between 1.5 to 2) [25] .
• The intermediate-calcium AAB is formed by alkali activation of aluminosilicate materials with intermediate content of calcium (e.g., alkali activation of blended fly ash-slag, or class C fly ash AABs). The hydration product of this group is an intermixed microstructure of C-A-S-H and N-A-S-H [26-27] .

Previous studies reported that with a proper formulation and curing conditions, AAB binders can offer comparable or greater mechanical properties and durability in service in comparison with OPC [28-30] . AAB has been used on a number of projects around the world such as in Singapore or Australia [31] . Figure 1-2 shows the application of AAB in pavements in Australia.

(a)                                                                    (b)

Figure 1-2 Application of AAB in different structures, (a) Placing of pavement using geopolymer concrete (b) Placing of pavement for weighbridge using geopolymer concrete [31]

1.3. Challenges in Application of Alkali-Activated Binder

Extensive research has been conducted on studying different properties and durability of AABs [23-25, 32-37] . However, some challenges exist, which have prevented widespread commercialization of AAB concretes listed as below:

1. Long term durability of AAB specifically with respect to shrinkage cracking is unknown. Controlling the risk of cracking needs the knowledge about shrinkage characteristics of binders. Shrinkage-cracking behavior of OPC has been investigated extensively. However, limited research has been conducted on shrinkage and potential risk of crack growth in AAB.
2. Workability and early age properties (including setting and strength development) of AAB are different than OPC. While a large number of studies have been performed to examine early age properties of OPC and the effect of different admixture utilization (such as superplasticizers), there have been a few works on the early age behavior of AAB materials. Some of AABs (such as alkali activation of class F fly ash) need to be heat-cured to gain enough strength. Curing at elevated temperature can limit the industrial field applicability of these materials and make them more appropriate for the precast industry. Adding different additives (such as calcium compound materials) could rectify this problem. However, only a limited number of studies examine the influence of these additives on the mechanical and setting properties of AAB.
3. The properties of the waste materials (FA or BFS) can vary substantially between different sources and over time. The different precursor chemistries make it difficult to produce consistent and reliable performance in these binders.
4. OPC has been researched and used in practice extensively; as such, it has an abundant source of track record over more than 150 years, while AAB has been utilized in limited application and its performance has been monitored for a short time. It needs more longterm data to be proven as a viable alternative for OPC.
5. Most if not all of the standards and specifications related to design and construction of concrete structures are basically developed for OPC concrete. As the properties of AAB and OPC are different, the existing standards (including experimental test methods) should be revised for new binders.

This research is mostly focused on addressing challenges 1 above in AAB systems. Concrete, whether produced with ordinary Portland cement (OPC) or any other alternatives, stiffens, shrinks, and exhibits creep over time. The shrinkage and creep can result in cracking, deflection and pre-stress loss in concrete structures which can influence their durability and serviceability. The presence of cracks results in matrix weakness and permits the ingress of aggressive agents (e.g., chloride attack) into the concrete structures. Therefore, shrinkage and creep can influence the material durability performance by inducing stresses and accelerating deterioration. A widespread research has been conducted on the prediction of shrinkage, creep and durability of OPC binder that can be traced back to almost half century ago. Over the past few decades, cement free concrete has been proposed in response to the environmental concerns about OPC production, and alkali-activated binder (AAB) has been getting more attention compared to other alternatives to OPC. Despite the recognition of AAB, there has been limited research reported on studying the shrinkage and creep, which may occur in these cement free binders. Knowledge about durability features of AAB is essential to facilitate their application in the concrete industry, and help to develop and design more sustainably durable material.

1.4. Research Objectives

This dissertation aims to properly measure and understand the shrinkage and creep characteristics of AABs, then try to mitigate large shrinkage and design more durable green binder and viable alternative for OPC concrete. Shrinkage and creep of different AABs have been experimentally studied. The physical (including pore structure, degree of saturation, and absorption) and mechanical (such as compressive strength, elastic young modulus and Poisson ratio) properties of different AABs have been measured to explain the shrinkage and creep observation and understand the governing shrinkage mechanism of different systems. As both external (including relative humidity (RH) and curing condition) and internal (such as activation solution and paste constituents) factors may influence the drying shrinkage, the impacts of these parameters have also been evaluated to see which parameters govern each type of shrinkage. Doing so can provide insight on mitigating the large shrinkage for these green binders and produce durable AAB concrete with comparable or even greater durability in service compared to OPC concrete. In order to achieve the stated goals, the following main objectives are pursued in this research study:

1. To properly measure drying shrinkage of AABs and determine the influential factors on shrinkage response of each system including relative humidity, curing regime and nature of solid precursor.
2. To explain the shrinkage observation and predict the effective shrinkage mechanism for each AAB system in comparison with OPC binder.
3. To assess experimentally the creep characteristics of AABs and examine the viscoelastic response of each system.
4. To study the shrinkage characteristics of alkali activated fly ash (AAFA) in more details and investigate the effect of different designing parameters (such as activator or curing regime) on shrinkage behavior of AAB binders.

1.5. Outline

The content of this thesis is presented in five chapters. Chapter 2 is the first part of a comprehensive shrinkage-study of AABs. It presents the evaluation of drying shrinkage of four different AAB mixtures with identical initial porosity and comparable compressive strength at different relative humidities (RHs). The AAB mixtures include AAS, AAFA and two binary mixtures of fly ash and slag. It also details the material properties, mixture proportions, mixing procedures, test methods and results of shrinkage measurement at different RHs, pore structure and mechanical properties of studied mixture.

Chapter 3 is the second part of AAB-shrinkage study which details the influence of timedependent response and curing regime on shrinkage of four AABs. In this chapter, the creep characteristics of four AABs are demonstrated and compared with predicted creep values of OPC. The results of shrinkage at different curing temperature are also presented. The results of this study indicates that heat-cured AAFA showed the comparable or smaller shrinkage than that of OPC. To examine shrinkage of AAFA in more details and evaluate the impact of different designing parameters on shrinkage of these systems, Chapter 4 details the study of shrinkage characteristics of AAFA binders. In this chapter, the influence of activating solution, RHs and curing regimes on shrinkage of this binders are presented. material properties, mixture proportions, mixing procedures, test methods and results of measured shrinkage at different RHs, mechanical and pore structure of AAFA. There is also an attempt to figure out the scientific reasons behind the shrinkage potential of AAFAs and to see how the proper mixture design can affect the durability of these systems.

Chapter 2-4 are all the technical papers which are submitter to journals and also presented in different conferences including SCMT4 and ICSC. Finally, Chapter 5 discusses conclusions of the studies carried out on the shrinkage characteristics of AABs.

 

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SHRINKAGE AND CREEP OF ALKALI-ACTIVATED BINDERS

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