THE ALKALI-SILICA REACTION IN ALKALI-ACTIVATED FLY ASH CONCRETE

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THE ALKALI-SILICA REACTION IN ALKALI-ACTIVATED FLY ASH CONCRETE

ABSTRACT

 

 

The global concrete production has grown considerably over the last decades in line with the population growth, industrialization of developing countries, and the need for more infrastructures. In addition to replacing the natural environment by roads and buildings, carbon dioxide emission and depletion of natural resources for manufacturing portland cement, for example, are of major concern. The best approach to minimize the environmental impacts caused by the concrete industry is to build structures that are durable. Another valuable strategy is to manufacture concrete by using industrial by products, such as fly ash, which may fully replace portland cement. The combination of the two approaches is ideal and even more promising towards making concrete a more sustainable man-made material.

This research investigates the risk of alkali-silica reaction (ASR) in alkali-activated fly ash concrete. ASR is a major deterioration mechanism, which shortens the service life of concrete structures. It involves a reaction between metastable (e.g. poorly crystalized) forms of silica in aggregates and the highly alkaline pore solution of concrete. The product of this reaction is formation of an expansive ASR gel, which cracks and damages the concrete structure. Alkaliactivated fly ash (AAFA) belongs to a new generation of green concrete binders that fully replace the ordinary portland cement.  AAFA binders require a highly alkaline solution to promote hydration of fly ash and strength development, which raises the concern for ASR.

In this research, the concrete prism test (ASTM C1293) was used to evaluate the ASR risk of two structural grade AAFA concretes. Despite their initially high pH and presence of highly reactive aggregate, ASTM C1293 results showed absence of deleterious expansion in these two AAFA concrete mixtures (FA1 and FA2). On the other hand, the control (i.e. OPC-based) mixture, proportioned with the same amount of reactive aggregate, exceeded the expansion threshold early during the test. SEM micrographs were used to assess the extent of aggregate deterioration and ASR gel formation in the tree mixes. The SEM micrographs reveal that aggregates in FA1 concrete were more preserved than in FA2, where very little ASR gel was detected. Moreover, EDS quantitative analysis detected increased amount of alkalis in residual aggregates with concentrations similar to that found in ASR gel formed in OPC concrete. To understand the mechanism leading to absence of ASR expansion, even though there is aggregate deterioration, microstructural investigation (MIP) and pore solution analysis were performed in AAFA pastes to test four proposed hypotheses. The results suggest that pH drop and abundance of dissolved aluminum decreases the alkaline attack to the aggregates in FA1, while the insufficient calcium prevents polymerization of dissolved silica from aggregates in FA2. In comparison to OPC paste, AAFA pastes had similar or larger porosity and average pore size, despite their significantly lower ASR activity. This rules out a hypothesis that ASR is mitigated in AAFA concrete because of its low permeability. In summary, the mechanisms responsible for absence of ASR in AAFA concretes were (1) pH drop, (2) high concentration of dissolved aluminum, and (3) low concentrations of calcium in the pore solution.

 

 

TABLE OF CONTENTS

LIST OF FIGURES ……………………………………………………………………………………………………… VI

LIST OF TABLES …………………………………………………………………………………………………….. VIII

AKNOWLEDGEMENTS ……………………………………………………………………………………………… IX

CHAPTER 1 – INTRODUCTION AND BACKGROUND…………………………………………………….. 1………..

1.1 Motivation………………………………………………………………………………………………… 1………..

1.2 Background on Alkali-Silica Reaction……………………………………………………………… 3………..

1.2.1 ASR mechanisms in OPC-based concrete…………………………………………………………………………………. 3……………

1.2.2 The role of calcium in ASR………………………………………………………………………………………………………. 5……………

1.2.3 The role of aluminum in ASR…………………………………………………………………………………………………… 7……………

1.2.4 Pore Solution Chemistry of Concrete Undergoing ASR……………………………………………………………. 8……………

1.2.5 How to prevent ASR in new concrete structures?……………………………………………………………………… 8……………

1.2.6 How to mitigate ASR in existing structures?………………………………………………………………………….. 10……………

1.3 Brief Background on Alkali-Activated Fly Ash Concrete……………………………………. 11………..

1.3.1 The Influence of Activator Composition…………………………………………………………………………………. 12……………

1.3.2 Curing Method………………………………………………………………………………………………………………………… 13……………

1.4 Research Objectives………………………………………………………………………………….. 13………..

1.5 References………………………………………………………………………………………………. 14………..

CHAPTER 2 – EXPLORING THE ABSENCE OF ASR EXPANSION IN ALKALI-ACTIVATED FLY ASH CONCRETE…………………………………………………………………………………………………………………………. 18………..

Abstract………………………………………………………………………………………………………. 18………..

2.1 Introduction…………………………………………………………………………………………….. 18………..

2.2 Materials and Methods………………………………………………………………………………. 21………..

2.2.1 Materials and mixture proportions………………………………………………………………………………………….. 21……………

2.2.2. ASTM C 1293 – Concrete Prism Test……………………………………………………………………………………. 23……………

2.2.3 SEM/EDS………………………………………………………………………………………………………………………………… 24……………

2.2.4 Pore Solution Analysis……………………………………………………………………………………………………………. 25……………

2.2.5 Mercury Intrusion Porosimetry (MIP)…………………………………………………………………………………….. 27……………

2.3 Results and Discussion………………………………………………………………………………. 27………..

2.3.1 ASTM C 1293 – Concrete Prism Test…………………………………………………………………………………….. 27……………

2.3.2 SEM/EDS………………………………………………………………………………………………………………………………… 28……………

2.3.3 Pore Solution Analysis……………………………………………………………………………………………………………. 37……………

 OH concentration:……………………………………………………………………………………………………………………….. 38……………

 Al concentration:…………………………………………………………………………………………………………………………. 40……………

 Ca concentration………………………………………………………………………………………………………………………….. 42……………

 Si concentration:………………………………………………………………………………………………………………………….. 43……………

 Alkalis concentration:……………………………………………………………………………………………………………………. 44……………

2.3.4 Mercury Intrusion Porosimetry (MIP)…………………………………………………………………………………….. 45……………

2.4 Conclusions…………………………………………………………………………………………….. 46………..

2.5 References………………………………………………………………………………………………. 47………..

APPENDIX A – BACKGROUND INFORMATION ABOUT FLY ASH…………………………………. 52………..

References……………………………………………………………………………………………………. 52………..

APPENDIX B – ACID TITRATION AND ICP-AES SAMPLE PREPARATION PROTOCOL…. 54………..

General considerations…………………………………………………………………………………… 54………..

Acid Titration Protocol…………………………………………………………………………………… 56………..

ICP-AES Protocol………………………………………………………………………………………….. 61………..

APPENDIX C – LIBRARY OF SEM IMAGES…………………………………………………………….. 65………..

Chapter 1 – Introduction and Background

 

1.1 Motivation

 

Concrete is the second most consumed product by human beings, behind only water (Provis and Deventer, 2014), and it is the most used man-material in the world (Ashby, 2013). To meet such a demand for concrete, the global annual production of portland cement is projected to reach 4 billion tons per year (Schneider et al., 2011). Ordinary portland cement (OPC) is the binder phase of most concrete structures currently in use, including dams, bridges, roads, and buildings (Mindess et al., 2003). Clinker, which constitutes up to 95% of portland cement, consumes high amount of energy during its production and has high carbon footprint. The emission of CO2 in the atmosphere falls in the range of 0.85 to 1.35 kilograms per kilogram of clinker produced, depending on pyroprocessing (Marinshaw and Wallace, 1995). From this huge CO2 emission, 40% is attributed to combustion of fossil fuels, which is used to supply the energy necessary for high temperature production of portland cement, and the remaining 60% to the calcination of limestone (CaCO3 = CaO + CO2) during cement manufacturing (Worrell et al., 2001).  

Alternatively to OPC-based concrete, alkali-activated concretes (AAC`s) reduce the environmental impact caused by the concrete industry. Using fly ash as a single binder to produce alkali-activated fly ash (AAFA) concrete reduces up to 80% of CO2 footprint compared to OPC concrete (Duxson et al., 2007). Other positive aspect of using non-OPC binders, including fly ash, blast-furnace slag, rice husk ash, or silica fume, is the beneficial use of the waste from other industries. In addition to decreasing the amount of landfilled by-products, the choice for OPCfree concrete reduces the need for a great amount of raw materials to be quarried, such as limestone. Limestone quarry promote deforestation, deep topography alteration, and soil depletion, making it very difficult to establish a new vegetation layer (Clemente et al., 2004). Thus, AAC’s can be seen as environmentally friendly alternatives to OPC concrete and may move construction industry towards sustainability. Sustainability is defined as “the capability of an economic or social system to meet its current needs without impairing the ability of future generations to meet their needs” (Oxford English Dictionary, 1989).

 Beyond the environmental benefits, other features of AAC’s must be taken into account, including mechanical properties and durability aspects. Specifically, durability with respect to alkali-silica reaction (ASR) is of concern. ASR is a durability issue widely known for compromising the service life of concrete structures and has been documented by more than 50 countries since 1940 (Mindess et al., 2003). The high alkalinity required for activation of AACs could trigger ASR and result in deleterious reaction with aggregates.

In ASR, the alkaline pore solution of concrete dissolves the meta-stable silica that exists in many natural and artificial (e.g. glass) aggregates and generates a hygroscopic gel. The ASRgel swells and builds up an internal pressure that results in expansion, cracking, and loss of strength of concrete (Fournier and Bérubé, 2000). The simultaneous presence of moisture, reactive aggregates, and sufficient alkalis are three essential factors to trigger ASR. In addition, it has been argued that a source of dissolvable calcium inside concrete is essential for formation and swelling of ASR gel (Rajabipour et al., 2015; Gholizadeh et al., 2016).

The mechanisms of ASR in AACs are unclear, since limited research has been performed in this field. Uncertainties on durability of AAFA concrete, among other factors, have delayed the acceptance of this green concrete by practitioners. As such, understanding the mechanisms that promote or control ASR in AAFA would be a step towards development of guidelines for producing high performance AAFA concrete mixtures. Knowledge about ASR mechanisms also provides benefits towards the development of specific methods for testing AAFA concrete, which incorporate reliability and quality control. The findings of this research is also an important step for deducting whether reactive aggregates may be used in AAFA concretes. The potential of using ASR-vulnerable aggregates in AAFA concrete make it even more attractive to the

construction industry.

1.2 Background on Alkali-Silica Reaction

1.2.1 ASR mechanisms in OPC-based concrete

The crystalline phase of silica mineral is a well-oriented silicon-tetrahedron framework (Prezzi et al., 1997). The interaction between aggregates with the high-pH pore solution of concrete, schematically shown in Figure 1.1-A, leads to network dissolution of metastable silica that may be present within aggregates (Rajabipour et al., 2015). Over time, hydroxyl ions (OH) may depolymerize silica (Table 1.1 – Equation 1), increasing its molecular disorder. As depolymerization progresses the reaction product bind alkalis, which is supplied by the pore solution of concrete (Powers and Steinour, 1955) (Figure 1.1-B).

The mechanism by which alkali binding occurs is through ion exchange. There two ion exchange reactions involved, as shown in Table 1. The first consists in the replacement of one monovalent ion (i.e. H+) by another monovalent ion (i.e. Na+, K+) (Equation 2). The second ionexchange consists of the replacement of two monovalent ions (i.e. Na+, K+) by one single divalent ion (i.e. Ca2+) (Equation 3) (Rajabipour et al., 2015). The replacement of alkalis by calcium is known as alkali recycling, and will be explained in details on Section 1.2.2 of this manuscript.

The described dissolution-precipitation process generates the so-called ASR gel (Figure

1.1-B), an alkali-alkaline earth-silicate hydrogel, with stoichiometry generally represented by (SiO2).(Na2O)n.(K2O)k.(CaO)c.H2O (Gholizadeh et al., 2016). Unsatisfied charges within the ASR gel molecule absorbs moisture, which promotes swelling. Over time, the ASR gel gains viscosity and become able to generate such a pressure that exceed the yield tensile strength of concrete.

This slow process may take decades to show its symptoms in structures.

 

FIGURE 1.1 – 2-dimensional representation of the interaction between metastable silica in aggregates and the pore solution of concrete. A) Before ASR takes place. B) After ASR takes place. The ion exchange mechanisms are represented within the circles.

 

TABLE 1.1: Summary of ASR steps and their chemical equations

Reaction Chemical Equation  
Depolymerization of silica (SiO2)s + 2H2O è Si(OH)4 (aq) (Eq. 1)
Ion exchange Na+ + (Si(OH)4)aq è  ((OH)3SiONa)aq + H+ (Eq. 2)
Alkali recycling 2((OH)3SiONa)aq + Ca2+ è (OH)3SiO − Ca − OSi(OH)3 + 2 Na+ (Eq. 3)
Dissolution of portlandite Ca(OH)2 è Ca2+ + 2OH (Eq. 4)

 

 

1.2.2 The role of calcium in ASR

 

Several researches highlighted the essential role of calcium in promoting the deleteriousness of ASR (Gholizadeh et al., 2016; Rajabipour et al., 2015; Thomas, 2001; Bleszynski and Thomas, 1998; Powers and Steinour, 1955). In OPC-based concrete, the main source of soluble calcium is portlandite (Table 1.1 – Eq. 4), which is one main product of cement hydration. Additionally to being a source of calcium, the dissolution of portlandite increases the concentration of hydroxyl ions in the pore solution, working as a pH buffer.

As mentioned in the previous section, Ca actively participates in the ion exchange (alkalirecycling) mechanism, where one single Ca2+ ion replaces two Na+ or K+ ions. This mechanism results in maintaining the alkalinity (i.e. high pH) of concrete pore solution through release of alkalis (Rajabipour et al., 2015). The alkali-recycling mechanism also repolymerizes colloidal silicates dissolved from the aggregates promoting the viscosity gain of ASR gel. Gholizadeh et al. (2016) proposed that there is an optimum concentration of Ca in the gel, among other factors, to promote deleterious expansion. It was found that with too little calcium, silicates remain dissolved in the pore solution without expanding, while too much calcium leads to a nonexpansive ASR gel. Thomas (2001) showed that, by incorporating calcium and releasing alkalis, old ASR gels becomes similar in composition to C-S-H, the stable hydration product of cement (Figure 1.2).

 

 

FIGURE 1.2: Change in the composition of ASR-gel (Thomas, 2001): with time gel tends to incorporate calcium, release alkalis, and approaches to C-S-H composition.

When the optimum concentration of calcium is reached, the expansion of ASR-gel can exert up to 20 MPa of internal tension in the structure (Krivenko et al., 2014). This process promotes cracking within the aggregates that propagates throughout the cementitious matrix (Figure 1.3- A and B). As it happens simultaneously at many spots within the structure, it leads to the formation of map-like crack, which is ASR’s footprint (Figure 1.3- C and D).

 

 

 

 

FIGURE 1.3 – Micro and macro structural images showing ASR appearance (A: ASR-gel formation inside Spratt coarse aggregate; B: gel-filled crack bridging two reacted aggregates

(Source: fhwa.dot.gov); C: Map cracks in ASR affected concrete (Source: fhwa.dot.gov); D:

ASR-damaged structure (www.greensboro-nc.gov)).

1.2.3 The role of aluminum in ASR

 

A number of studies have shown that the presence of soluble aluminum in the pore solution of concrete decreases the ASR risk. Aluminum may be chemisorbed on the surface of reactive silica, decreasing the rate of aggregate dissolution (Iler, 1985; Oka and Tomozawa, 1980; Iler, 1973) and altering the composition and structure of the silicate gel produced (Chappex and Scrivener, 2012; Chappex and Scrivener, 2013). Several other researches underline the precipitation of a zeolite layer along the surface of dissolved silica (Labrid, 1991; Huenger, 2007; Shafaatian, 2012) that may also decrease silica dissolution, as soon as the pore solution of concrete is supersaturated with Al (Bickmore et al., 2006). In addition, Hong and Glasser (2002) highlighted the ability of aluminum ions in increasing the alkali-binding capacity of the C-(A)-S-H products, which may decrease pore solution pH.

The three mechanisms described above (i.e., (1) chemisorption of Al by dissolved silica; (2) formation of a protective zeolite layer, and (3) alkali-binding) highlight the positive effect of Al in mitigating aggregate dissolution. As such, dissolved Al in the pore solution of concrete may be seen as a tool to mitigate ASR.

1.2.4 Pore Solution Chemistry of Concrete Undergoing ASR

 

The progress of ASR affects the pore solution chemistry of concrete. The reaction between silica, from aggregates, and alkalis and OH ions, from concrete pore solution, results in consumption of all these ingredients. Since the consumption of alkalis and hydroxyl continues with ASR progress, Na+, K+, and OH concentrations should decrease with time in the pore solution. Moreover, higher temperatures lead to faster ASR and higher consumptions of such ions (Kim et al., 2015). However, if the conditions favor the alkali recycling mechanism (Eq. 3), Na+, K+, and OH concentrations are maintained into pore solution of concrete (Rajabipour et al., 2015).

1.2.5 How to prevent ASR in new concrete structures?

 

The prevention of ASR in new structures has been extensively studied (Fournier et al., 2010; Rajabipour et al., 2015). The ASR expansion mechanism requires access of four prerequisites: (1) reactive silica, (2) high alkalinity (pH), (3) dissolvable calcium, and (4) moisture. By taking off or reducing one of the four prerequisites, it is possible to mitigate ASR. According to AASHTO PP65 (2013), the following methods can be used to suppress ASR in new (i.e., to be constructed)

OPC-based concrete:

  • Limiting reactive (silica) aggregates: The approach of using non-reactive aggregate, i.e., the aggregates that showed good field performance (Thomas et al., 2006) or those that pass ASTM C1293 test (ASTM C1293-08b, 2008), can be challenging. Truly non-

reactive aggregates are scarce in some regions, for instance in Pennsylvania state, USA. It may involve long-distance transportation of non-reactive aggregates, which considerably raises the cost of concrete production.

  • Limiting concrete alkalis: Limiting the amount of Na2Oeq (percentage of Na2O + 0.658 x percent K2O) of cement to a maximum of 1.8kg/m3 of concrete is recommended

(AASHTO PP 65-11, 2013). However, portland cement is not the only source of alkalis. Aggregates, supplemental cementitious materials, and other admixtures could potentially provide extra alkalis and lead to deleterious ASR (Rajabipour et al., 2015).

  • Using of supplementary cementitious materials (SCM): The use of SCMs, including pozzolans (i.e. calcined clays, fly ash) and blast furnace slag, reduces the pore solution pH (Shafaatian et al., 2013; Thomas, 2011). SCMs reduce aggregate dissolution and decrease the availability of calcium, by consuming portlandite. However, if high dosages of SCM are necessary to successfully mitigate ASR, it may negatively impact the setting, compressive strength, and freeze-thaw durability (e.g. scaling) of concrete. If this is the case, the dosage of SCM will be limited and might not be sufficient to mitigate ASR.
  • Using lithium admixtures: the presence of lithium in the pore solution of concrete may affect the progress of ASR. In addition to reducing the rate of silica dissolution, Li+ is potentially adsorbed by the reaction product (i.e. ASR gel) altering its mechanical and swelling properties (Rajabipour et al., 2015). However, lithium is a scarce element, which increases the cost of these admixtures and, consequently, affects the overall cost of concrete production.

Since there are challenges related to all preventive measures against ASR, fully preventing the reaction is not achievable in some cases. Thus, ASR continues to be a leading deterioration mechanism in concrete worldwide.

1.2.6 How to mitigate ASR in existing structures?

 

Suppressing ASR in existing structures has much lower efficacy than preventive measures (Fournier et al., 2010). Two subsets of possible treatments are (i) treating the cause of ASR, and (ii) treating the symptoms of the reaction. Figure 1.3 (Fournier et al., 2010) summarizes the methods available for each of the two subsets.

 

 

FIGURE 1.4: Available methods for mitigating ASR: treatments for cause and for symptoms (Fournier et al., 2010).

The goal of applying treatments for the cause is to disturb directly the ASR reaction mechanisms. In theory, the injection of CO2 and lithium admixtures in affected concrete may suppress the ASR. However, in practice, neither of the two methods can penetrate deep enough within the structure, even when applied under high pressure. Therefore, they are not effective for mass structures, such as dams (Fournier et al., 2010). On the other hand, the application of sealants, cladding and improved drainage were found to be effective in delaying the progress of ASR, since the method reduces the penetration of additional moisture within the affected concrete (Fournier et al., 2010).

As opposed to the cause treatments, the symptom treatments (e.g. application of restraint, and relief of stresses) do not act directly against the ASR mechanisms. Instead, they are applied as an attempt to minimize the damage to the concrete and extend the service life of the ASR affected structure (Fournier et al., 2010). Additionally, such methods most of the times are costly major repairs (ACI Committee 364, 2015).

In summary, the application of any kind of ASR treatment extends the service life of affected structure rather than ending the reaction itself (ACI Committee 364, 2015).

1.3 Brief Background on Alkali-Activated Fly Ash Concrete

 

Alkali-activated fly ash concrete is a subset of geopolymers, a term used to designate lowcalcium binder systems among a broader category of alkali-activated materials (AAM) (Provis and Deventer, 2014). Analogously to portland cement, the anhydrous fly ash is mixed up with a liquid phase to produce a solid binder phase. Rather than water, fly ash requires a highly alkaline solution to catalyze its dissolution, as well as the process of hydration and strength gain. This process is known as geopolymerization, a rapid reaction that generates amorphous “-Si-O-Al-O-“ 3D-structure, which is an amorphous zeolite (Fernández-Jiménez et al., 2005; Fernández-Jiménez and Palomo, 2005; García-Lodeiro et al., 2007; Kupwade-patil and Allouche, 2011). When the activating solution supplies sodium to the fresh paste, the Al sites within the 3D aluminosilicate structure incorporate Na. As such, the reaction product is sodium-aluminosilicate hydrate, or NA-S-H gel in cement chemist notation. Fernández-Jiménez et al. (2005) found that about 50% of reaction to generate N-A-S-H takes place before day-7 (Figure 1.5).

 

 

FIGURE 1.5 – Evolution of reaction degree of class F fly ash (Fernández-Jiménez et al., 2005)

 

The macrostructure of AAFA concrete is a stable solid comparable to OPC-based

concrete. Depending on the composition of the fly ash, activator, and aggregates, as well as their interaction, the overall properties of AAFA vary drastically (Duxson et al., 2007).

1.3.1 The Influence of Activator Composition

 

The composition of activator has a significant effect on many aspects of AAFA, including, for example, compressive strength and porosity. Both sodium hydroxide solution and a mixture of sodium hydroxide with sodium silicate solutions (Na2O.nSiO2.mH2O or waterglass) are commonly reported in the literature as activators. The prompt availability of SiO2 provided by waterglass activator boosts geopolymeration and strength gain, producing concretes that may reach up to 90 MPa of compressive strengths (Kazemian et al., 2015; Fernández-Jiménez and Palomo, 2005; Ryu et al., 2013). Simultaneous increase in the activator’s pH and modulus (n), which is the molar ratio [SiO2]/[Na2O], would be ideal for strength improvement in AAFA concretes (Kazemian et al., 2015). But since the two parameters are inversely proportional, increasing pH and n simultaneously is not practical. As opposed to strength, high n and pH has a negative effect in concrete workability (Kazemian et al., 2015), since it leads to activators with low water content and high viscosity. Additionally, high pH activators showed a negative effect in ASR expansion of AAFA mortar (Shi, 1996; Shi et al., 2014). The selection of the activator, as well as its modulus and pH, for AAFA concrete represents a tradeoff between mechanical and durability properties and should be ultimately tailored to achieve specific requirements.

1.3.2 Curing Method

 

To enhance dissolution and geopolymerization of fly ash and achieve higher strengths, AAFA concretes must be cured at elevated temperatures (e.g. 60º C) (Kazemian et al., 2015; Xie et al., 2003).

1.4 Research Objectives

 

The objectives of this research are (1) to properly evaluate the ASR risk in alkali-activated fly ash (AAFA) concrete, and (2) explore the mechanisms of ASR or the reasons for lack of ASR expansion in AAFA concrete. To achieve the first objective, ASTM C1293 concrete prism expansion tests (CPT) were performed on two AAFA concrete mixtures along with a control OPC concrete mixture, all containing the same amount of reactive aggregate. The results of CPT, explained in detail in chapter 2, showed that ASR expansion and damage is absent in AAFA concretes despite their highly alkaline pore solutions.

Since there are four ASR prerequisites ((1) reactive silica, (2) high alkalinity (pH), (3) dissolvable calcium, and (4) moisture, as explained in detail in the previous section), the following hypotheses are proposed to explore the reasons for lack of ASR expansion in AAFA (objective (2)):

  1. Despite initially high alkalinity, the [OH] and alkali concentration in AAFA system may decrease over time as a result of reaction with fly ash. The hydroxyl ions are likely to participate in fly ash dissolution, while alkalis will be bond to the main hydration product

of AAFA systems. Lowering the pore solution pH and alkali concentration leads to a significantly milder attack on the aggregates.

  1. Aluminum, as mentioned before, could be another factor reducing aggregate dissolution and, consequently, ASR in AAFA concretes. The possibility of having a high supply of dissolved Al in the pore solution of AAFA paste will be investigated and compared to that in OPC paste. The increased supply of Al in class F AAFA systems may also alter the composition of the ASR gel, altering its swelling nature, as well. Pore solution analysis and SEM/EDS investigation will be used to evaluate this hypothesis.
  2. The dissolved calcium concentration in AAFA systems might differ significantly from that of OPC. Limited Ca within AAFA concrete may prevent polymerization of dissolved silica (Table 1 – Eq. 3), which potentially impacts the viscosity-gain process of ASR gel. If that is the case, silica will remain dissolved within the pore solution, without damaging the structure.
  3. Another relevant factor is the transport of moisture within AAFA matrix. The lower volumetric liquid-to-binder ratio compared to OPC paste, is likely to promote a smaller porosity. As such, the transport of moisture needed to sustain ASR might be hampered in AAFA concrete compared to OPC systems.

Chapter 2 discuss in details the experimental methods, results, and findings of this thesis in a format of a journal paper.

1.5 References

AASHTO PP 65-11. (2013). Standard practice for determining the reactivity of concrete aggregates and selecting appropriate measure for preventing deleterious expansion in new concrete construction. American Association of State Highway and Transportation Officials, (Washington, D.C).

ACI Committee 364. (2015). Managing alkali-aggregate reaction expansion in mass concrete. American Concrete Institute, ACI 364.11, 1–4.

Ashby, M. F. (2013). Materials and the Environment. Materials and the Environment. Elsevier.

ASTM C1293-08b. (2008). Standard test method for determination of length change of concrete due to alkali-silica reaction. Annual Book of ASTM Standards, 1–7.

Bickmore, B.R., Nagy, K.L., Gray, A.K., and Brinkerhoff, A.R. (2006). The effect of Al(OH)4- on the dissolution rate of quartz. Geochimica et Cosmochimica Acta, 70, 290-305.

Bleszynski, R. F., and Thomas, M. D. A. (1998). Microstructural studies of alkali-silica reaction in fly ash concrete immersed in alkaline solutions. Advanced Cement Based Materials, 7, 66–78.

Brantley, S. L., Kubicki, J. D., and White, A. F. (2008). Kinetics of Water-rock interaction. Springer.

Chappex, T. and Scrivener, K. (2012). The influence of aluminum on the dissolution of amorphous silica and its relation to alkali silica reaction. Cement and Concrete Research, 42(12), 1645-1649.

Chappex, T. and Scrivener, K. (2013). The effect of aluminum in solution on the dissolution of amorphous silica and its relation to cementitious systems. Journal of American Ceramic Society, 96(2), 595-597.

Clemente, A. S., Werner, C., Máguas, C., Cabral, M. S., Martins-Louçao, M. A., and Correia, O. (2004). Restoration of a limestone quarry: Effect of soil amendments on the establishment of native Mediterranean sclerophyllous shrubs. Restoration Ecology, 12(1), 20–28.

Diamond, S. (1983). Effects of microsilica (silica fume) on pore-solution chemistry of cement pastes. Journal of the American Ceramic Society, 66(5), C–82–C–84.

Duxson, P., Provis, J. L., Lukey, G. C., and van Deventer, J. S. J. (2007). The role of inorganic polymer technology in the development of “green concrete.” Cement and Concrete Research, 37(12), 1590–1597.

Fernández-Jiménez, A., and Palomo, A. (2005). Composition and microstructure of alkali activated fly ash binder: Effect of the activator. Cement and Concrete Research, 35, 1984– 1992.

Fernández-Jiménez, A., Palomo, A., and Criado, M. (2005). Microstructure development of alkali-activated fly ash cement: A descriptive model. Cement and Concrete Research, 35(6), 1204–1209.

Fournier, B., and Bérubé, M.-A. (2000). Alkali – aggregate reaction in concrete: a review of basic concepts and engineering implications. Canadian Journal of Civil Engineering, 191, 167– 191.

Fournier, B., Bérubé, M.-A., Folliard, K. J., and Thomas, M. (2010). Report on the diagnosis, prognosis, and mitigation of alkali-silica reaction (ASR) in transportation structures. U.S. Department of Transportation, FHWA-HIF-0.

García-Lodeiro, I., Palomo, A., and Fernández-Jiménez, A. (2007). Alkali-aggregate reaction in activated fly ash systems. Cement and Concrete Research, 37, 175–183.

Gholizadeh Vayghan, A., Rajabipour, F., and Rosenberger, J. L. (2016). Composition-rheology relationship of ASR gels and their effects on the extent of ASR damage. Cement and Concrete Research, 83, 45-56.

Hong, S. Y., and Glasser, F. P. (2002). Alkali sorption by C-A-S-H gels: part II, role of alumina. Cement and Concrete Research, 32(7), 1101–1111.

Huenger, K-J. (2007). The contribution of quartz and the role of aluminum for understanding the AAR with greywacke. Cement and Concrete Research, 37, 1193-1205.

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THE ALKALI-SILICA REACTION IN ALKALI-ACTIVATED FLY ASH CONCRETE

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