On utilization and mechanisms of waste aluminium in mitigating alkali-silica reaction (ASR) in concrete

Accepted Manuscript On utilization and mechanisms of waste aluminium in mitigating alkali-silica reaction (ASR) in concrete Rotana Hay, Claudia P. Ostertag PII:

S0959-6526(18)33691-6

DOI:

https://doi.org/10.1016/j.jclepro.2018.11.288

Reference:

JCLP 15047

To appear in:

Journal of Cleaner Production

Received Date: 1 July 2018 Revised Date:

20 October 2018

Accepted Date: 30 November 2018

Please cite this article as: Hay R, Ostertag CP, On utilization and mechanisms of waste aluminium in mitigating alkali-silica reaction (ASR) in concrete, Journal of Cleaner Production (2018), doi: https:// doi.org/10.1016/j.jclepro.2018.11.288. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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On utilization and mechanisms of waste aluminium in mitigating alkali-silica reaction (ASR) in concrete Rotana Hay and Claudia P. Ostertag1

Abstract

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Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720

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Alkali silica reaction (ASR) is one of the most commonly occurring durability issues in concrete structures. In this study, the feasibility of using waste aluminium in bit, powder and dissolved forms as a sustainable alternative to mitigate ASR was investigated. The accelerated mortar bar test was adopted for expansion measurement and mitigation mechanisms were explored with reactive glass dissolution, pore solution and microstructural analyses. Both aluminium bits and aluminum powder at various volume fractions were found to be effective in mitigating ASR, mainly attributable to dissolution control of amorphous silica through integration of [Al(OH)4]to silica framework and electrostatic repulsion of OH- at reactive sites. However, the effectiveness of the aluminum powder in controlling ASR was diminished due to its ready integration into hydration products. This calls for an investigation into the long-term effectiveness of that specific mitigation alternative. Also, a higher porosity of concrete due to gas evolution and the resulting adverse effects on mechanical properties would have to be resolved before a possible implementation of the mitigation strategies in concrete structures.

Introduction

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Keywords: Alkali-silica reaction; waste aluminium; accelerated mortar bar test; pore solution; dissolution control; sustainability

Alkali-silica reaction (ASR) is a reaction between alkali, hydroxyl ions in concrete pore solution and amorphous siliceous minerals present in reactive aggregates (Mehta and Monteiro, 2006). Micropores in concrete are highly basic with pH value generally greater than 12.5 (Fournier and Bérubé, 2000). At such high pH, some mineral phases of both reactive fine and coarse aggregates become unstable and react to form ASR gels by generally following four main sequential reactions: (i) dissolution, (ii) formation of nano-colloidal silica sol, (iii) cross-linking to form 1

Corresponding author: Tel.: +1 510 642 0184 Fax: +1 510 643 8928 E-mail address: [email protected]

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ASR gel, and (iv) imbibition of water to induce swelling and subsequently distress to the surrounding matrix (Rajabipour et al., 2015). ASR is one of the most common durability problems occurring in concrete structures. One reason is that the reactivity of aggregates may be unknown before their use. Also, an increasing demand for materials in concrete construction to cater to economic development may lead to a depletion of locally-available high quality aggregates. Sourcing of quality aggregates from afar may not be feasible due to high transportation cost and possible project hold-ups, leading to usage of less ideal or questionable aggregates from nearer sources, leading to a potential risk of ASR occurrence in concrete.

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Not limited to resource scarcity, reducing environmental footprint of economic and daily activities has also become a paramount interest for individuals, researchers and government bodies alike as we are entering an ecological age. Concrete is seen as an attractive option for waste depository due to its superior physiochemical stability. Indeed, industrial wastes such as fly ash and ground granulated furnace slag have long been successfully integrated into concrete. Recently, other types of wastes such as municipal solid incinerator bottom waste (Wongsa et al., 2017), palm oil fuel ash (Nagaratnam et al., 2016, Zeyad et al., 2017), waste tire rubber (Thomas et al., 2016, Guo et al., 2017), marble slurry (Rana et al., 2015, Khodabakhshian et al., 2018), waste fibers (Awal and Mohammadhosseini, 2016, Mohammadhosseini et al., 2017, Domski et al., 2017) or even water treatment and sewage sludge (Geraldo et al., 2017, Chen et al., 2018) have been investigated for a potential incorporation and possible enhancement to concrete properties. Other wastes such as glass (Saccani and Bignozzi, 2010, Lee et al., 2011), on the other hand, has been shown to be detrimental to concrete long-term durability if not appropriately executed. One of the associated risks with waste glass is ASR expansion which has been demonstrated to depend on glass particle size (Schwarz et al., 2008, Lee et al., 2011, Nassar and Soroushian, 2012), chemical compositions (Saccani and Bignozzi, 2010) and presence of supplementary cementitious materials (SCMs) (Schwarz et al., 2008, Guo et al., 2018). As such, mitigation actions should be implemented alongside with waste integration to prevent premature serviceability failure due to any possible durability issues.

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Current approaches to mitigating ASR in concrete include: (i) control of the amount of reactive silica by using non-reactive aggregates in concrete (Farny and Kosmatka, 1997, Mehta and Monteiro, 2006, Thomas et al., 2008, Rajabipour et al., 2015), (ii) control of alkali concentrations in concrete by using low-alkali cement or by blending cement with SCMs (Mehta and Monteiro, 2006, Thomas et al., 2008), (iii) integrating SCMs into concrete (Malvar et al., 2002, Folliard et al., 2006, Mehta and Monteiro, 2006, Thomas et al., 2007b); (iv) control of moisture migration into concrete by using low water-cement ratio or using SCMs to enhance microstructure (Farny and Kosmatka, 1997), (v) using fibers to control damage caused by ASRinduced cracking (Turanli et al., 2001, Yi and Ostertag, 2005, Bektaş et al., 2006), (vi) alteration of alkali-silica gels by addition of lithium or barium to form non-swelling silicate hydrates (McCoy and Caldwell, 1951, Farny and Kosmatka, 1997, Thomas et al., 2007a, Thomas et al., 2008, Rajabipour et al., 2015), and (vii) use of protein-based air-entraining admixtures (McCoy and Caldwell, 1951). The primary method of ASR avoidance through the use of non-reactive aggregates has become infeasible or uneconomical due to increasing scarcity of high quality aggregate. Replacement of cement by SMCs such as fly ash has shown a good track record in combatting ASR in concrete. However, as countries have been shifting to cleaner energy sources rather than coal, a future supply of fly ash has become uncertain. Also, the alternative chemical methods such as the addition of lithium salt in concrete may prove to be too costly to be widely 2

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adopted as reflected by a poor implementation of this approach in construction industry. Lithium is a rare element making up of only 0.002% of the earth’s crust (Rajabipour et al., 2015) and the price of lithium carbonate stood at approximately $13,900/tonne in 2017 (Sally and Suzette, 2017). The constraints call for new effective and economical alternatives to mitigating ASR in concrete.

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Supplementary materials with higher content of alumina (Al2O3) were found to be more effective in mitigating ASR (Ramlochan et al., 2004, Schumacher and Ideker, 2014). Likewise, waste glass powder at sufficient replacement ratios was found to effective in restricting ASR expansion in concrete (Zheng, 2016, Guo et al., 2018, Rashidian-Dezfouli et al., 2018)}, attributable to its pozzolanic reaction and subsequent increase in aluminium concentration in pore solution (Zheng, 2016). Previous research studies confirmed that aluminium (Al) species are absorbed from pore solution onto amorphous silica surface of reactive aggregates, limiting their dissolution and ASR expansion (Chappex and Scrivener, 2012, Alexey and Anna, 2013, Chappex and Scrivener, 2013). Microstructural study showed little degradation of reactive aggregates from samples soaked in Al-saturated NaOH solution (Chappex and Scrivener, 2012). X-ray photoelectron spectroscopy showed that Al species are incorporated into the framework of silica structure while atomic force microscopy confirmed the reduction in dissolution of silica in Al-bearing solution (Chappex and Scrivener, 2013). The observation is consistent with previous findings by other researchers (Iler, 1973, Bickmore et al., 2006, Hünger, 2007, Chappex and Scrivener, 2012) who found that aluminium (Al), alumina (Al2O3) or aluminate (Al(OH)4]-) greatly reduced the dissolution of amorphous silica. In application to ASR mitigation in concrete, an addition of amorphous aluminium hydroxide (Al(OH)3) into concrete between 1% and 3% by weight of cement was found to be effective in controlling ASR expansion (Alexey and Anna, 2013). Other aluminum-bearing substances such as aluminum sulfate were also found to mitigate ASR in concrete (Brykov et al., 2014). The inhibitory mechanism was attributed to the ability of amorphous Al(OH)3 to actively bind Ca(OH)2 resulting in the formation of highly mobile lowcalcium ASR gels, and the adsorption of Al3+ ions on the surface of reactive aggregate particles to form poorly soluble aluminosilicate complexes, thus passivating reactive aggregates. Formation of reaction products with composition similar to calcium alumino silicate hydrates (CA-S-H) was also observed for mortar produced with fine lightweight aggregate (Li et al., 2018).

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The aim of this research is to investigate the feasibility and mechanism of waste aluminium as an alternative to mitigate ASR in concrete. The use of waste aluminium is emphasized here in order to integrate the concept of sustainability and economy in the mitigation approach. Three forms of aluminium (chips or bits, powder and dissolved solution) were utilized in this study. Dissolution of borosilicate glass rods in both NaOH solutions and mortar matrix with and without aluminium, accelerated mortar bar test, pore solution and microstructural studies were conducted to elucidate the role and mechanism of aluminium in inhibiting ASR. The study provides a fundamental understanding and provide a pathway for a feasible utilization of aluminium variants as an alternative method in preventing ASR in concrete.

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Materials and methods 2.1. Materials

ASTM Type I cement whose chemical compositions are given in Table 1 was used as the main binder in all mixes used in the accelerated mortar bar test (AMBT) ASTM C1260 (American 3

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Society for Testing and Materials (ASTM), 2007). Fine aggregate annotated as Wright sand mined in Robstown TX and previously characterized in literature as having a high expansion potential (Fournier et al., 2009) was used as the reactive aggregate. In selected mixes, nonreactive sand (annotated as Vulcan) with a fineness modulus of 3.2 was used in place of the reactive Wright sand. Three forms of aluminium were used in assessing their potential in ASR mitigation: (i) as-received industrial grade aluminum bits from a machine shop located on the university campus, (ii) powder made of aluminium alloy 6061 with an average size of 65 µm, and (iii) dissolved aluminium. Based on the supplier’s datasheet (Alcoa, 2002), the aluminium powder contains at least 96% aluminium with the other 4% as impurities comprising magnesium, silicon, iron, copper, zinc, titanium, manganese and chromium. XRD test results for the aluminium bits (after manually ground) and powder are given in Figure 1. The two materials exhibited apparent peaks at 2θ angles of 38.6, 44.8 and 65.2 degrees, thus confirming a similar composition predominantly of crystalline aluminium. Figure 2 shows the waste aluminium bits which were observed to be mostly in spiral shape. Their maximum size was approximated to be 4.76 mm.

ASTM Type I cement 21.63 0.24 3.88 3.28 0.06 4.39 64.88 0.23 0.56 0.09

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SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5

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Table 1: Chemical compositions of ASTM Type I cement

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Figure 1: XRD results of aluminium bits (manually ground) and powder

Figure 2: Waste aluminium bits

2.2. Dissolution of glass rods in NaOH solution The reaction of aluminium with NaOH solution produces aluminate ([Al(OH)4]-) and hydrogen as governed by Eq. 1. To examine the effect of [Al(OH)4]- on the dissolution of amorphous silica at a basic level, NaOH solution with added aluminium was used to investigate the dissolution of borosilicate glass rod. Short glass elements with diameter of 5 mm and weight of approximately 0.4 g were immersed into 100 ml of 2 different solutions: 1N NaOH, and 1N NaOH with 5

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aluminium bits at a saturation level of 2 g/L. It is important to note that dissolved concentration of aluminium in concrete pore solution is in the order of 2.3 × 10-3 g/L (Lothenbach et al., 2007). The two solutions were annotated as 1N NaOH and 1N NaOH + Al, respectively. The required mass of aluminium was directly added into NaOH solution. Together with the glass rods, the solutions were subsequently kept in oven at 80 oC and sealed to prevent evaporation. The masses of the rods in respective solutions were measured at various time intervals for up to 45 days to an accuracy of ±0.001g.

2 Al + 2 NaOH + 6H 2O → 2 Na+ + 2[Al(OH )4 ] + 3H 2 −

2.3. Mix designs and samples

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Mortar with water-binder (W/B) ratio of 0.47 was prepared for accelerated mortar bar test (AMBT) according to ASTM C1260 (American Society for Testing and Materials (ASTM), 2007) and the summary of the base mix design is given in Table 2. Aluminium bits were added at 2% and 4% by volume fractions (vol%) into the mix. The samples were annotated as 0A, 2A and 4A, respectively, where the number before A (aluminium) denotes its volume fraction. For all the mixes, water-binder (W/B) was kept constant at 0.47 and the weight ratio of fine aggregate to cementitious materials was maintained to 2.25. Four 25 mm × 25 mm × 285 mm prisms were produced for each mix for ASR expansion measurement. Table 2: Mix design used for ASTM C1260 (kg/m3)

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Aluminium powder was also added into the base mix at 0.063, 0.125, 0.25 and 0.5 vol%. The samples were annotated as 0.063PA, 0.125PA, 0.25PA and 0.5PA, respectively, where PA refers to powder aluminum and the corresponding numbers are for their volume fractions. Two of the four mortar bar samples in 0.063PA were restrained on their free surfaces during the 24-hour curing period using metal plate and clamp. This was to restrict expansion caused by hydrogen gas formation resulting from the reaction of aluminium powder in highly alkaline solution before the matrix gained its stiffness. It was postulated that the approach could effectively isolate the influence of Al and air voids on expansion capability of the mortar bars. The constrained samples were annotated as 0.063PA-C. As mentioned, a direct inclusion of aluminium into concrete leads to evolution of hydrogen gas which can adversely affect concrete strength and integrity. In an attempt to counter this problem, the aluminium bits were dissolved in 1N NaOH at 80oC for 24 hours. The mixture was then added with deionized water to produce 0.1N NaOH solutions with aluminate target concentrations of 10 mM and 100 mM to be used as mortar mixing solution. However, based on the weight of dry unreacted residue, the effective concentrations of aluminate were estimated to be 0.007 M and 0.07 M. An additional mortar mix was also produced with 0.1 N NaOH only as a 6

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reference. The samples were marked as 0A-0.1N, 0.01A-0.1N and 0.1A-0.1N where N represents NaOH.

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The borosilicate glass rods were also embedded in 0A and 0.25PA mixes to study their dissolution in mortar. Non-reactive Vulcan sand was used in 0A mix while reactive Wright sand was used in 0.25PA mix to provide a conservative investigation of the role of Al in limiting dissolution of the borosilicate glass.

2.4. Expansion test

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Sample preparation and expansion measurement were conducted in accordance with ASTM C1260 (American Society for Testing and Materials (ASTM), 2007). After mixing, the mortar was placed into 25 mm × 25 mm × 285 mm molds attached with studs at both ends, compacted with a tamper and transferred to a fog room where temperature was maintained at 24 oC. Demolding agent was avoided and instead cling film (food wrap) was used to cover mold components to facilitate demolding and to prevent sample contamination. After 24 ± 2 hours of placement, the samples were demolded, immersed in deionized water and transferred to an oven with temperature maintained at 80 ± 2 oC for 24 hours before a zero or reference reading was recorded. Subsequently, they were transferred and kept in 80 ± 2 oC 1 N NaOH solution proportioned to approximately 4 times the total sample volume. Length changes were measured at the same time on consecutive days for 14 or 28 days. Expansions of more than 0.20 % at 16 days after casting are indicative of potentially deleterious expansion.

2.5. Pore solution extraction and analysis

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Pore solutions were extracted from 0A, 0.125PA, 0.25PA, 0.5PA, 2A and 4A mortar bars at 14 days using pore press equipment similar to that used by Barneyback & Diamond (1981). The surfaces of the samples were cleaned with deionized water and dapped dry with tissue before pore solution extraction. The expressed pore solutions were immediately sealed in tight containers to prevent carbonation. A portion of the solution was used for pH measurement using standard glass body combination electrode and pH meter. The rest of the solution was filtered with 5-µm nylon membrane for inductively coupled plasma (ICP) spectroscopy analysis. To prevent possible precipitation of solids, the filtered solution was acidified with 5% HNO3 9 times by volume. ICP analyses to determine concentrations of Al, Ca, Na, K, S and Si in the pore solutions were conducted with Perkin Elmer 5300 DV. Corresponding immersion NaOH solutions were also analyzed for their chemical compositions.

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2.6. Microscopic analysis At 14 days and 28 days after immersion in NaOH solution, the mortar bar samples were sliced to obtain sections with a thickness of approximately 21.5 mm. The slices were oven-dried and impregnated with epoxy under vacuum to avoid entrapment of air bubbles. The oven drying removed moisture from the matrix and as a result helped to enhance epoxy penetration. The epoxy filled all voids on the sample surfaces up to a penetration depth of approximately 1 to 2 mm. After 3 days of curing in air, the epoxy hardened and the samples were ground and polished sequentially with 9 micron grit, 3 micron grit, 3 micron paste, and finished with 1/4 micron paste. The grinding and polishing exposed the aggregates, cement paste and ASR products, leaving only epoxy in voids. The samples were coated with 25 nm thick carbon layer in vacuum with polished titanium plate as a color indicator. To prevent over-charging of the samples during 7

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3. Results 3.1. Dissolution of glass rods in NaOH solution

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scanning electron microscopy (SEM), a brass conduction strip was attached to one side of the samples to enable electrons to flow from the sample top surface to a conductor located at the supporting platform. Back scattered electron (BSE) imaging and energy dispersive X-ray (EDX) microanalyses were conducted with Zeiss Evo MA10. Images was captured at 20 kV and with probe current maintained at 1 nA. Qualitative element mapping in EDX mode was carried out at 256 × 208 pixel resolution on 128 frames with beam dwell time of 200 µs.

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The results for mass dissolution of the glass rods in NaOH solutions up to 37 days are given in Figure 3. It is observed that adding aluminium into 1 N NaOH greatly reduced the dissolution of the glass rod. This is consistent with observations by other researchers (Iler, 1973, Bickmore et al., 2006, Chappex and Scrivener, 2012) who also found that the presence of aluminium (Al), alumina (Al2O3) or aluminate (Al(OH)4]-) greatly reduced the dissolution of amorphous silica. At 37 days, the presence of Al in the solution reduced the mass loss by about 3 times. This is despite the relatively small concentration of aluminium in the solution of only 0.074 N. The results imply that a small concentration of dissolved aluminium in concrete pore solution could be effective in limiting dissolution of reactive aggregates. Hence, addition of aluminium or its variants into concrete could be a simple approach in preventing ASR in concrete made with reactive aggregates.

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Figure 3: Mass dissolution of borosilicate rod in 1N NaOH solution

3.2. Expansion results for mixes with aluminium bits Expansion results for mixes with aluminium bits are given in Figure 4. Lower expansion limit of 0.10% for innocuous behavior and upper expansion limit of 0.20% for potentially deleterious expansion as prescribed by ASTM C1260 are also indicated on the diagram. It is important to note that only two samples out of four from 2A and 4A mixes were available for expansion 8

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measurement as the other two samples were fractured during demolding due to weakening by hydrogen gas formed by reaction of aluminium bit in basic environment. Expansion results show that at 14 days, the final expansion of 0A was well above the upper expansion limit of 0.20%, confirming the high reactivity of Wright sand. The inclusion of 2 and 4 vol% aluminium bits into the matrix effectively limited the expansion of the mortar bars to less than 0.10%. Some of the samples were left in the 1N NaOH solution for 28-day and their corresponding expansion values were found to be 0.094% for 2A mix and 0.091% for 4A mix, all lower than the upper expansion limit. The results seem to confirm the long-term effectiveness of aluminium bits in preventing ASR in concrete. It is also noted that there was no remarkable difference in expansion curves and 28-day expansion values of 2A and 4A samples. This is due to their relatively high contents of aluminium bits in the matrix and hence an increase from 2 to 4 vol% would not provide any added benefits to the expansion mitigation. At this point, it is unclear if the small expansions in the 2A and 4A mixes were dominated by either Al-controlled dissolution or matrix high porosity resulted from hydrogen gas generation and poor compaction of the matrix due to the spiral shape of the aluminium bits. Previous research demonstrated that air bubbles or porosity in concrete serve as gel accommodating space to reduce the apparent ASR expansion (Jensen et al., 1984, Collins and Bareham, 1987).

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Figure 4: Expansion results for mixes with aluminium bits

3.3. Expansion results for mixes with aluminium powder Expansion results for mixes with aluminium powder are summarized in Figure 5. The expansion curve for 0A mix was also re-plotted here again for comparison. It is observed that an inclusion of aluminium powder at 0.125, 0.25 and 0.5 vol% effectively limited the ultimate expansion of all mixes to less than 0.10% at 14 days. The mitigation mechanism may be attributed to the formation of distributed air voids in the matrices with the presence of aluminium powder, providing an accommodation space for ASR gels. However, with different volume fractions of aluminium powder and hence air contents, the small difference in expansion curves of 0.125PA, 0.25PA and 0.5PA mixes seems to imply that the main mitigation mechanism was dissolution limitation of the reactive aggregates. The findings also demonstrate that only a small amount of 9

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dissolvable form of aluminium is effective in mitigating ASR in concrete. Interestingly, the ultimate expansion values of all samples with aluminum power were smaller than those in 2A and 4A samples which contain much higher aluminium volume fractions. The fine particle size of aluminium powder and its closer proximity to reactive aggregates makes it more effective in restricting aggregate dissolution and subsequent expansion. Also, the continuity of pore in 2A and 4A due to larger aluminium bit size may cause easy penetration of NaOH solution to reach the reactive aggregate and to cause an overall higher expansion the samples.

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Figure 5: Expansion results for mixes with aluminium powder

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It has been shown that aluminium bits and powder prevent ASR expansion in mortar bar although it was unclear if dissolution control or increased porosity or both contributed to ASR mitigation. To isolate these two mechanisms, two of the fresh samples from 0.063PA mix were allowed to freely expand while the other two samples were restrained over their free surface by plate clamping during the 24-hour curing period. The expansion of the two sets of samples were measured up to 28 days to study the long-term effectiveness of aluminium in mitigating ASR and the results are given in Figure 6. It is revealed that the expansion of both sample sets followed a similar trend of expansion up to 8 days, after which 0.063PA-C (C for constrained expansion) exhibited a faster rate of expansion. At the end of the 28-day measurement period, the expansion of 0.063PA-C was 2.5 times greater than that observed in 0.063PA. As the reserve of aluminium powder was diminished, aggregate dissolution was initiated and ASR gels were generated, leading to higher expansion in 0.063PA-C. However, with existence of more air voids within the matrix, 0.063PA was able to accommodate more ASR gels, resulting in a smaller 28-day expansion value. At 14 days the average expansions in 0.063PA is below the expansion limit as prescribed by ASTM C1260 and lower than the expansion in 0A as previously shown in Figure 5. The findings reveal the synergy of both aluminium powder and air voids in mitigating ASR. At an early age when free aluminium is available, the main ASR mitigation mechanism is dissolution control of reactive aggregates while at later age when aggregates start to react and ASR gels are formed, air voids provide accommodation space to limit expansion in the mortar bars. 10

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3.4. Expansion results for mixes with dissolved aluminium

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Increased porosity with the presence of aluminium bits and powder may compromise mechanical properties and other durability attributes of concrete. As mentioned, to overcome this problem, dissolved form of aluminium was added into the base mix at estimated concentrations of 0.007 M and 0.07 M, whose corresponding samples were annotated as 0.01A-0.1N and 0.1A-0.1N, respectively. As a comparison, 0.063 vol% of aluminum powder in the mix would be equivalent to 0.22 M, provided all aluminium dissolved in the mixing solution. The expansion results shown in Figure 7 revealed that as compared to 0A, alkalinity of 0.1 N NaOH (0A-0.1N) in the mixing water increased the early ASR expansion of the sample. This implies that although the mortar bar samples were immersed in 1 N NaOH solution, alkali concentration in the pore solution is not expected to be in immediate equilibrium with the immersion solution and the initial alkalinity level in the matrix would have direct influence on the sample expansion. This is consistent with findings by other researchers (Shafaatian et al., 2013) who showed that through diffusion control and alkali binding ability of hydration products, mortar bars with fly ash as cement replacement reduced pore solution alkalinity and thus contributed to a reduction in ASR expansion performed under ASTM C1567 test.

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Figure 7: Expansion results for mixes with dissolved aluminium

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It is also observed that during an early stage of expansion, introduction of dissolved aluminium in the mix decreased the expansion in proportion to the concentration of aluminium in the mixing solution with 0.07 M concentration having more suppression effect than 0.007 M concentration. However, the suppression effects were only observed during an early stage of expansion. After 11 days in 1 N NaOH or 13 days after casting, all samples showed similar expansion magnitudes. It is postulated that dissolved aluminium in the mix would gradually be absorbed by hydration products or bound to ASR gels. When the aluminium was deprived from the pore solutions, there would be no further mitigation control of reactive aggregate dissolution. Subsequently, an increase in alkalinity of the matrix from diffusion of the immersion NaOH solution would lead to aggressive ASR reaction at later age. Also, as compared to 0.063 vol% (0.22 M) aluminium powder, dissolved aluminium of 0.07 N was much less effective in mitigating ASR. 0.1 N samples exhibited a 14-day expansion that is approximately 6 times greater than that of 0.063A samples even though the expected concentration of aluminium was about 3 times less. It is postulated that the dissolved form of aluminium was readily bound by hydration products. One such mechanism is the participation of dissolved aluminium in the formation of ettringites and monosulfates, which occurs in an early stage of hydration process (Mehta and Monteiro, 2006). Part of the added Al may also be bound to the other hydration products. For instance, incorporation of Al into calcium silicate hydrate (C-S-H) gels have been observed by many researchers (Odler et al., 1986, Scrivener and Taylor, 1993, Escalante‐Garcia and Sharp, 1999).

3.5. Chemical compositions of pore solutions ICP analysis results for concentrations of Al, Ca, K, Na, S and Si of the mortar bar samples with dissolved aluminium are shown in Figure 8. The vertical axis marks concentration in mM and the horizontal represents the number of days after casting where a value of 1 is for the day of demolding, 2 for the day that the samples were transferred into 1 N NaOH solution, and 3 for 1 day in 1 N NaOH solution. It is observed that the addition of aluminium (Al) into the mixing water did not increased the concentration of Al in the pore solutions during the early stage of expansion, thus confirming the binding of dissolved aluminium into hydration products, causing 12

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a limited availability of dissolved Al in preventing dissolution of reactive aggregates and ASR expansion. Interestingly, concentration of Al and S increased with time and this is attributed to decomposition of ettringite with prolonged exposure to high temperature as found by other researchers (Ghorab and Kishar, 1985, Perkins and Palmer, 1999). The results also reveal that the concentration of Ca and K of all samples decreased after immersion in NaOH solution. The concentration reduction was attributed to an integration of two ions into ASR gels and charge balancing through diffusion of the Na into the matrix as evidenced by an increase in Na concentration in the pore solution. Possible dissolution of Ca(OH)2 to buffer Ca in the pore solution was expected to be limited as a result of increase alkalinity in the pore solution. A noticeable reduction in K was observed in the pore solutions of 0.1A-0.1N as compared to other samples and this may be attributed to the formation of potassium aluminate K[Al(OH)4] which could precipitate and be locked up by hydration products. As mentioned, the diffusion of Na from the immersion solution into the matrix led to an increase in Na concentration in the pore solution with time. At 16 days, Na concentrations of all samples were slightly higher than the equilibrium concentration of 1000 mM due to an Na aggregation effect of the pore and immersion solutions. Figure 8 also showed that the concentrations of Si increased with time after the samples were immersed into 1 N NaOH solution, thus confirming alkali silica reaction and the resulting dissolution of aggregates and formation of ASR products Al

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Figure 9 provides ICP analysis results of pore solution at 16 days (14 days after immersion in NaOH solution) of mortar bars produced of mixes with inclusion of aluminium bits and aluminium powder. As expected, Al concentrations were generally higher than the values found in samples with dissolved aluminium. However, Al concentrations in 2A and 4A at 16 days were just 2.63 mM and 2.74 mM, much lower than the expected concentrations which are in the range of molarity if all added aluminium dissolved. Similar trend also applied to mortar bars with aluminium powder. The finding confirms that only a small fraction of solid Al was dissolved into the pore solution. Also, concentrations of Ca were in the vicinity of zero, similar to the level found in samples with dissolved aluminium. K concentrations, on the other hand, were generally lower and this may be a result of greater precipitation of K[Al(OH)4] due to more abundance of dissolved Al. Na concentrations in 0.125PA, 0.25PA and 0.5PA were in the range of 1000 mL while the level in 2A and 4A were lower and this may be due to dilution during sample cleaning in preparation of the pore solution press, formation of NaAl(OH)4 precipitates (to be discussed later) or alkali binding of C-S-H gels whose binding ability at high alkali (Na) concentration range was shown to increase with the presence of Al (Hong and Glasser, 2002). To isolate the influence of alkali reduction from dissolution control mechanism of Al, investigation of glass rod dissolution was focused on samples with and without aluminium powder in view of their similar total alkali content at 16 days.

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3.6. Chemical compositions of immersion solutions

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days as compared to the levels found in samples with dissolved aluminium in the mixing solution. Counterintuitively, the Si concentrations increased with aluminum bit/powder content. It is postulated that increasing content of aluminium solids also increase porosity which initiated more direct contact between the immersion NaOH solution and reactive aggregates, leading to a higher dissolution and Si concentrations in the pore solutions.

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The concentrations of the same 6 elements were measured at 16 days for immersion solutions in which the mortar bars with dissolved aluminium were immersed and the results given in Figure 10. It is revealed that Al concentrations in all immersion solutions were of similar magnitude, showing a consistent trend with pore solution analysis results. However, as compared to pore solution results, higher Al concentrations were observed in the immersion solutions, possibly due to binding of Al onto reactive aggregate surface or into hydration products within the matrix. Ca concentrations in the immersion solutions were consistently low due to a limited dissolution and leaching of Ca-bearing hydration into the highly alkaline environment. The presence of K, S and Si in the immersion solutions is attributable to dissolution of the ion sources or direct leaching of the ions from within the matrix. Higher Si concentrations in the immersion solutions may be due to unimpeded dissolution of reactive aggregates into the surrounding environment without physical barrier as provided by the matrix. As expected, the concentration of Na in all immersion solutions were found to be approximately 1000 mM, similar to the concentration of the original solutions. The small discrepancy may be due to analysis error or slight evaporation of the immersion solutions. Al

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ICP results of the immersion solutions for samples integrated with aluminium bits and powder at 16 days are given in Figure 11. Due to higher content of Al in the mix and the resulting dissolution, the immersion solutions showed higher Al concentrations as compared to the values found in immersion solutions for samples with dissolved Al. It is also observed Ca concentrations were much smaller as compared to those in immersion solutions of samples with dissolved aluminium. With reduced ASR reactivity resulted from dissolution control provided aluminium bits and powder, it is postulated that less Ca and alkali-bearing ASR products were formed and leached into the immersion solutions, leading to the observed lower Ca concentrations in the immersion solutions. The same explanation applies to the lower concentrations of K in the immersion solutions of samples with aluminium bits and powder. Except for 2A and 4A, all immersion solutions exhibited Na concentrations at 16 days closed to 1000 mM of the original NaOH solution. The lower Na concentrations for immersion solutions of 2A and 4A samples may be due reaction between Na and Al to form solid NaAl(OH)4. Similar concentrations of S were found in the immersion solutions of samples with dissolved and solid aluminium, consistent with the trend observed for pore solution results. Lower Si concentrations were observed in the immersion solutions of samples with solid aluminium, confirming their lower ASR reactivity and expansion. 17

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3.7. Microstructural analysis results of embedded glass rods BSE images of glass rods embedded in 0A and 0.25PA matrices after 14 and 28 days in NaOH solution are shown in Figure 12. An increase in dissolution of the glass rod in 0A is observed 18

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between 14 to 28 days of immersion. Fracture occurred for the glass rod at 28 days and more visible cracks were observed to the surrounding matrix. Conversely, less extensive dissolution and damage occurred for the glass rod in 0.25PA matrix at both 14 and 28 days despite a higher porosity (more air voids) and a higher diffusion of NaOH solution into the matrix. The glass rod was able to maintain its shape between 14 and 28 days, confirming a long-term effectiveness of power aluminium in limiting dissolution of the rod and mitigating ASR. It is also noted that although similar level of glass rod dissolution was observed in 0.25PA at 28 days as compared to 0A at 14 days, no cracking occurred in 0.25PA samples. This is attributed to the ability of the air voids in accommodating ASR gels and thus confirms the participatory role of air voids in limiting ASR expansion damage.

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Figure 12: Dissolution images of embedded glass rods in 1 N NaOH solution

3.8. Microstructural analysis results of mortar bar samples

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BSE images of mortar bar cross sections shown in Figure 13 provide a global view of air void distribution in typical mortar bars with aluminium bits (4A) after 14 days and with aluminium powder (0.125PA) after 28 days in 1 N NaOH solution. It is observed that a more uniform distribution of air void existed in 0.125PA. Also, remnants of unreacted or reacted aluminium were observed in cavities belonging to the original locations of aluminium bits. Overall, little signs of aggregate damage and dissolution were observed in both samples. The results again reinforce the argument that the main mechanism that aluminium bits and powder mitigate ASR is by limiting dissolution of the reactive aggregates.

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A closer look at a typical aggregate-matrix interface in 0A and 0.125PA (Figure 14) showed that part of the reactive aggregate in 0A at 14 days disintegrated due to ASR while no such prominent damage occurred for the aggregates in 0.125PA even at 28 days. To the lower right of the aggregate of 0A, the original profile of the aggregate was replaced by remnant of ASR products at the interface and cracks filled with ASR gels could be seen propagating from the reaction site. In comparison, no ASR products were observed at the interface of a typical reactive aggregate in 0.125PA. Interestingly, a form of hardened product could be seen lining the cavity wall at the top left of Figure 14 (b). To examine the compositions of the product lining the wall of voids in samples with added aluminium powder, EDX analysis was conducted on a cut section of 0.25PA mortar bar at 28 days. The element mapping results in Figure 15 showed the product is a composite of Ca, Si and Na, of similar compositions to ASR gels. However, as insignificant dissolution of reactive aggregates was visible in the matrix, it is postulated that the product was predominantly formed through leaching and crystallization of silica and calcium hydroxide from within the matrix. The migration of the ions from the matrix is attributed to both charge balancing through diffusion of Na from the immersion solution and osmosis effect where Ca and 20

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Si ions of the more concentrated pore solutions moved to void regions with smaller ion concentrations. The rigidity of the product could be achieved through the presence of calcium. It is also noted that traces of aluminium were also observed in the lining, thus confirming its origin from within the matrix.

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Figure 14: BSE images at aggregate-matrix interfaces of with and without aluminium powder in 1 N NaOH solution

Figure 15: BSE image and corresponding EDX elemental analysis result at typical void region of 0.25PA after 28 days in 1 N NaOH solution

4. Discussion

4.1. Role of aluminium in mitigating ASR The role of Al in dissolution control of silica has been well developed in other fields such as earth science and oceanography. One of the proposed mechanisms is a superficial combination 21

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with amorphous silica, resulting in a retardation of its dissolution (Lewin, 1961). In hydrolyzed solutions, NMR analysis showed that Al can be quickly incorporated into the silica tetrahedral framework (Stone et al., 1993). X-ray photoelectron spectroscopy also confirmed that aluminium in highly alkaline and calcium-saturated environments similar to that found in concrete was incorporated into the silica framework of the amorphous silica (Chappex and Scrivener, 2013). As presented earlier, the reaction of aluminium with NaOH solution produces hydrogen and aluminates which are then incorporated onto the surface of silica in NaOH solution as governed by Eq. 2 (Chappex and Scrivener, 2013). It is proposed that the adsorption of the multivalent metal ions decreases the dissolution rate of silica because the reactive sites are blocked from interaction with ions that tend to dissolve the silica substrate (Stumm, 1997). Not limited to fixed ionic sites, the negatively charged alumino-silicate would also repel OH- from the surrounding area (Iler, 1973). This means that the adsorption of a small amount of aluminium onto the surface of amorphous silica would be sufficient to restrict its dissolution. At 25 oC, the maximum amount of adsorbed alumina was found to be of approximately 1.4 aluminium atom/nm2 or 0.0023 mM/m2 of silica surface (Iler, 1973). The value, however, is expected to be different at higher temperature.

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4.2. Role of air voids in mitigating ASR

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It was shown that although 0.063PA-C showed similar early expansion as compared to that of 0.063PA without top restraint during curing, expansion of 0.063PA-C increased at a faster rate after 14 days in 1 N NaOH. It was argued that after unbound supply of aluminium is diminished, reactive aggregates dissolved and ASR reactions were initiated. With well distributed air voids, 0.063PA would be able to accommodate the expansive gels and limit the ultimate expansion. Figure 16 shows sections of 0.063PA and 0.063PA-C and confirms the higher content and better distribution of air voids in 0.063PA. More residue or precipitation of ASR gels and ion leaching from within the matrix could be observed lining air cavity walls of 0.063PA as compared to 0.063PA-C. The increased path distance and irregular void distribution could make it more difficult for the gels to be extruded into the relief air voids, thus leading to an overall higher expansion of the 0063PA-C matrix. It is also important to note that despite the presence of constraint, a relatively extensive network of air voids still could be seen in 0.063PA. It is postulated that generation of hydrogen gas in 0.063PA-C produced sufficient pressure to displace some of the mixing water out of the mold, leaving behind the observed air voids. The observation confirms the participatory role of air void formation in limiting ASR expansion in samples integrated with waste aluminium. Through a similar mechanism, air-entrainment and expanded aggregates have been previously proven to be effective in mitigation ASR in concrete by providing air void system to accommodate the expansive ASR gels (Jensen et al., 1984, Gillott and Wang, 1993, Ramachandran, 1998, Bektas et al., 2005)

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Figure 16: BSE images of samples with and without constraint after 28 days in 1 N NaOH solution

4.3. Reduction of Na concentration in the immersion solutions ICP solution analysis results showed that Na concentrations in the immersion solutions of 2A and 4A were considerably lower than the original level of the immersion solution. This is attributed to two main mechanism: (i) enhancement of alkali binding ability of C-S-H gels in the presence of Al (Hong and Glasser, 2002), and (ii) reaction between aluminium bits with Na ion of the immersion solution to form sodium aluminate NaAl(OH)4. It is postulated that some of NaAl(OH)4 exists in solid form either within the structure of aluminium or as precipitates in the solution, resulting in an overall reduction in Na concentration from the immersion solution. EXD 23

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analysis results of a typical aluminium bit in 2A sample are shown in Figure 17 and confirm that the bit is now an amalgamation of aluminium, oxygen and sodium, implying the presence of NaAl(OH)4 in a solid form. The cracking observed in the aluminum bit and a uniform distribution of Al and Na also indicates a high reactivity. The images also reveal that the spiral shape of the bit prevented a good compaction of the mortar, leading to high porosity within the matrix which in turn introduced more sites for Al-Na reactions.

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The expansion results showed that while dissolved aluminium in NaOH solution could significantly reduce the dissolution of borosilicate glass rod, its effectiveness in preventing ASR expansion in mortar bar samples was almost totally reduced. It is postulated that dissolved aluminum in the form of aluminate was readily bound by hydration products, thus limiting its capability to control ASR reaction and expansion. Inclusion of aluminium powder into the matrix, on the other hand, proved to be more effective in ASR mitigation. It is expected that the powder reacted in the highly alkaline environment of concrete to form NaAl(OH)4 precipitates. Part of NaAl(OH)4 dissolved and was released into the pore solution in the form of [Al(OH)4]which was then bound onto silica framework of reactive aggregates, leading to their reduced dissolution. The remaining solid NaAl(OH)4 or unreacted Al would act a reservoir and would react or dissolved to when [Al(OH)4]- is depleted from the pore solution. This mechanism would provide a continual dissolution control of reactive aggregate and ASR mitigation until the supply of [Al(OH)4]- was exhausted. Nevertheless, one potential problem with the inclusion of solid Al into concrete as ASR mitigation technique is an increase in porosity and a compromise to concrete mechanical properties, which has to be resolved should such strategy be implemented. Other researchers have shown that amorphous aluminium hydroxide Al(OH)3 of 3% by weight of cement (approximately 0.65% by mix volume) could potentially limit 14-day expansion of mortar bars to less than 0.1% (Alexey and Anna, 2013). Hence, Al(OH)3 which does not evolve 24

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hydrogen gas when placed in concrete could be a potential alternative of Al for ASR mitigation without the side effect of increased porosity.

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The work explores the possibility of using three variants of waste aluminium (bits, powder and dissolved form) in a pursuit of seeking a sustainable alternative to mitigate alkali silica reaction in concrete. To fundamentally understand the mitigation mechanisms of aluminium, a comprehensive test program including dissolution of borosilicate glass rods in 1N NaOH integrated with the waste aluminium, expansion tests, pore solution analyses and microstructural studies was conducted. The results and the discussion presented so far lead to the following conclusions: Dissolved waste aluminium in NaOH solution significantly reduced dissolution of borosilicate glass. However, its effectiveness in mitigating ASR in concrete was eliminated possibly due to a ready incorporation of the dissolved aluminium ions into hydration products.

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Aluminium bits at 2 and 4 vol%, and aluminium powder at 0.063 vol% could effectively control ASR in mortar bars by limiting 14-day expansion to less than 0.1%. Mitigation mechanisms were attributed to a combined action of dissolution control of reactive aggregates by aluminium and formation of air voids which act as accommodating space for ASR gels.

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Dissolution control was postulated to mainly arise from adsorption of [Al(OH)4]- to the framework of reactive silica thus protecting the amorphous silica from dissolving agents and OH- through electrostatic repulsion.

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Aluminium bits reacted in highly alkaline solution to form NaAl(OH)4 which was postulated to partly dissolved or retained as a solid form in the matrix. The remnant of solid NaAl(OH)4 reduced Na concentration from the immersion solution of samples integrated with aluminium bits. NaAl(OH)4 was also expected to act as a reservoir to provide a continual supply of [Al(OH)4]- should its presence in the pore solution became exhausted.

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This study introduces a promising and sustainable utilization of waste aluminium to mitigate ASR in concrete. The mitigation mechanisms were thoroughly investigated and the findings could act as a platform for further investigations such as field performance assessment and longterm performance of the proposed strategy. Yet, the issue related to increased porosity and the resulting adverse effects on mechanical properties and other durability characteristics of structural concrete will have to be resolved. Although not investigated in the study, strength of concrete integrated with aluminium bits and powder may be reduced as a result of hydrogen gas evolution and the inherent spiral shape of the aluminium bits. High content of aluminium in the form of [Al(OH)4]- may increase the susceptibility of concrete to sulfate attack under sulfatepolluted environments (Taylor, 1997). Notwithstanding, one potential application area of the proposed solution is the production of aerated or high porosity concrete blocks with reactive aggregates. Such blocks could be used as infill non-structural elements for enhanced thermal resistance and reduced self-weight of buildings. 25

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Acknowledgments

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The authors wish to express their gratitude and sincere appreciation to the Federal Highway (FHWA) for awarding grant #DTFH61-09-R-00017. We are also very thankful to Prof. Jason Ideker of Oregon State University for providing specifications and detailed drawings for the pore solution press. Rotana Hay would also like to express his deep gratitude to International Fulbright Science & Technology Program for sponsoring his studies at the University of California, Berkeley.

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References

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On the utilization and mechanisms of waste aluminium in mitigating alkali-silica reaction (ASR) in concrete Rotana Hay and Claudia P. Ostertag1

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Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720

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Dissolved aluminium reduced dissolution of reactive borosilicate glass Aluminium bits at 2 vol% and powder at 0.063 vol% effectively mitigated ASR Dissolution control and formation of air voids contributed to mitigation mechanisms Strength reduction due to gas evolution-induced porosity would have to be resolved

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Corresponding author: Tel.: +1 510 642 0184 Fax: +1 510 643 8928 E-mail address: [email protected]