Structural evolution of binder gel in alkali-activated cements exposed to electrically accelerated leaching conditions

Journal Pre-proof Structural evolution of binder gel in alkali-activated materials exposed to electrically accelerated leaching conditions Solmoi Park, H.N. Yoon, J.H. Seo, H.K. Lee, Jeong Gook Jang

PII:

S0304-3894(19)31779-0

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121825

Reference:

HAZMAT 121825

To appear in:

Journal of Hazardous Materials

Received Date:

9 September 2019

Revised Date:

2 December 2019

Accepted Date:

3 December 2019

Please cite this article as: Park S, Yoon HN, Seo JH, Lee HK, Gook Jang J, Structural evolution of binder gel in alkali-activated materials exposed to electrically accelerated leaching conditions, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121825

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.

Structural evolution of binder gel in alkali-activated materials exposed to electrically accelerated leaching conditions# #

Throughout the paper C-A-S-H and N-A-S-H represent a structurally disordered non-stoichiometric calciumalumino-silicate-hydrate, and sodium-alumino-silicate-hydrate, respectively. These are the two major reaction products of alkali-activated cements.

Solmoi Park1, H.N. Yoon1, J.H. Seo1, H.K. Lee1, Jeong Gook Jang2,*

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Corresponding author. Tel.: +82 32 835 8472; Fax: +82 32 835 0776; E-mail address: [email protected]

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Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2 Division of Architecture and Urban Design, Institute of Urban Science, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon, 22012, Republic of Korea

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Graphical abstract

100

Na

10 20

Concentration (mmol)

stratlingite

15

10

5

M-S-H

C-A-S-H

1

Al Si

Na

0.1

180 160 140

0.01

120

Ca

100

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Volume (cm3 per 100g of binder)

hydrotalcite 25

80

1E-3

60 101

2

10

3

10

4

10

5

10

6

10

1E-4

0 101

102

103

104

105

106

101

102

103

104

1.0

Ca/Si

0.9

0.3

Al/Si

0.2

0.1

Na/Si 0.0

105

106

101

Cumulative leached water (g)

102

103

104

105

106

Cumulative leached water (g)

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Cumulative leached water (g)

Na/Si, Al/Si and Ca/Si of C-A-S-H

1.1

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Highlights 

Electrically accelerated leaching tests for alkali-activated materials are conducted for the first time.

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Fly ash samples show significant dissolution of binder gel upon leaching, unlike other samples.

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A low hydraulic property of fly ash prevents further hydration from occurring in contact with water.

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C-A-S-H type gel remains more stable in comparison with N-A-S-H type gel.

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A charge-balancing cation in alkali-activated materials has an important implication for the phase stability in these materials.

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Abstract

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The structural evolution of a binder gel in alkali-activated materials exposed to accelerated leaching conditions is investigated for the first time. Samples incorporating fly ash and/or slag were synthesized and were exposed to electrically accelerated leaching by applying a current density of 5 A/m2. The leaching behavior of the samples greatly depended on the binder gel formed in the samples. The N-A-S-H type gel abundant in fly ash-rich samples showed some extent of dissolution upon accelerated leaching, while slag-rich samples underwent hydration of the anhydrous slag after leaching. The obtained results are discussed in view of the degradation of the binder gel induced by accelerated leaching, and their potential performance under repository conditions where groundwater-induced leaching is the main durability concern.

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Keywords: Alkali-activated materials; Accelerated leaching; Radioactive waste repository;

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Characterization

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1. Introduction Portland cement (PC) plays a vital role in a radioactive waste repository (everything except a natural barrier should be an engineered barrier, including waste grout for low/intermediate-level radioactive waste (Ojovan, 2011, Jang et al., 2016) and cementitious backfill (Felipe-Sotelo et al., 2014, Felipe-Sotelo et al., 2016, Corkhill et al., 2013)), owing to its high-pH buffering capacity, low diffusivity, and good mechanical strength (Saito et al.,

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1992). Corrosion of the steel reinforcement is the dominant factor when assessing the

integrity of cement under ordinary service conditions, and little emphasis is placed on Ca

leaching, as it is a slow process occurring over long time (Nakarai et al., 2006). Instead, Ca

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leaching can be a serious concern with regard to the integrity of a radioactive waste

repository which is often located underground, where the groundwater contains corrosive

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transport (Nakarai et al., 2006).

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ions at a minimum, as it coarsens the microstructure of cement, allowing easier mass

Because the occurrence of calcium leaching in cement takes ~100 years to be

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noticeably observed, some studies in this area employed an electrical acceleration technique in which Ca leaching is accelerated by applying a potential gradient to an electrolyte (Saito et al., 1992, Saito et al., 2000, Saito et al., 1999, Hashimoto et al., 2013). This speeds up the

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movement of ions in the pore solution as the cathode and anode attract cations (e.g., Na+, K+,

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Ca2+) and anions (e.g., OH-), respectively, resulting in the accelerated dissolution of Ca from the binder matrix (Saito et al., 1992). The dissolution kinetics of cement hydrates under an accelerated leaching condition

differs according to the hydrates. The dissolution of alkalis takes place initially, then Ca(OH)2 followed by C-S-H (Saito et al., 1999, Yokozeki et al., 2004, Jacques et al., 2010). This process is also influenced by a number of factors. Hashimoto et al. (Hashimoto et al., 2013)

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investigated the effect of the water-to-cement (W/C) ratio on the leaching characteristic of Portland cement, concluding that a higher W/C ratio increases the initial pore volume through which the dissolution of Ca occurs abundantly. In particular, the residual content of Ca(OH)2 decreases notably after electrically accelerated leaching as the W/C ratio of the sample is increased (Hashimoto et al., 2013), implying that

samples with lower W/C ratios have a

slower net reduction of Ca(OH)2 given the supply of additional Ca(OH)2 through further

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hydration, therefore delaying the leaching process. This occurs because unreacted clinkers in the binder matrix of low-W/C cement, which is initially surrounded by hydrates, are exposed to water once the surrounding hydrates dissolve and undergo hydration, analogously

facilitating self-curing and delaying the leaching process (Nakarai et al., 2006). Similarly, the

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incorporation of fly ash in concrete is reported to mitigate degradation due to calcium

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leaching, as fly ash particles surround the hydrates and slow the dissolution process (Nakarai et al., 2006).

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The performance of cement for such applications can be enhanced by using Portland cement blended with supplementary cementitious materials (e.g., blast furnace slag-blended

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Portland cement (Sanderson et al., 2017)) or by employing a new alternative binders (e.g., magnesium-based cement (Walling et al., 2015)), phosphate cement (Gardner et al., 2015),

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calcium aluminate cement (Chavda et al., 2015) and alkali-activated materials (Jang et al., 2016, Khalil et al., 1994, Mobasher et al., 2016)). These efforts were made to meet the wide

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variability in the waste compositions and surrounding environments and to ensure the stability and integrity of the waste disposal system. For instance, the performance of Portland cement for the immobilization of cesium and strontium, key radionuclides, is known to be poor while other types of cements (i.e., alkali-activated cements) exhibit enhanced performance due to the negatively charged surface of hydrates, which favors the adsorption of problematic cations, and due to the complex pore network, making the transport of ions

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through a porous medium difficult (Jang et al., 2016, Jang et al., 2017). Moreover, Ca leaching from cement and concrete into its surrounding area in the repository can cause the degradation of bentonite due to the cation exchange between Na in bentonite and the Ca which leaches from cement (Yokozeki et al., 2004). Alkali-activated materials have been a topic of numerous studies owing to their comparable performance to Portland cement for potential applications, including in the

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cement types used in repositories. This binder system can be broadly categorized into two types with a clear distinction in the composition according to the Ca content of the precursor (Bernal et al., 2014, Provis et al., 2014, Park et al., 2018). Specifically, the C-A-S-H type gel forms in a high-Ca system along with hydrotalcite belonging to an Mg-Al layered double

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hydroxide group mineral when sufficient Mg is available (Provis et al., 2014, Yoon et al.,

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2018). On the other hand, the N-A-S-H type gel forms in a low-Ca system and has a structural analogue of zeolite minerals (Provis et al., 2014). The overall performance of the

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material is indeed largely influenced by the composition of the binder gel, but in either case the overall surface charge tends to be negative, making this type of binder suitable for the

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adsorption of some important fission product radionuclides such as cesium and strontium. Despite the increased demand and adoption of alkali-activated materials in practice, i.e., grout

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for solidifying radioactive spent resins (Lichvar et al., 2010), and cementitious backfill materials (Park et al., 2017), the long-term durability performance of this binder system for

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use in a repository remains unexplored. The present study investigates the structural evolution of the binder gel in alkali-activated materials upon leaching. An electrically accelerated leaching test was conducted on alkali-activated fly ash, slag and a blend of fly ash and slag. The samples were characterized after the electrically accelerated leaching tests to assess the effect of leaching on the microstructures of alkali-activated binders, and ultimately on their likely durability performance in repository conditions, where the leaching of

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structural components from the binder gel upon contact with groundwater over an extended

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period of time is the main mode of degradation.

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2. Experimental procedure 2.1 Materials and sample preparation The binder materials used in this study are fly ash, blast furnace slag and Type I Portland cement. The chemical compositions of the binder materials were obtained by X-ray fluorescence analysis and are summarized in Table 1. The alkali-activator was produced by mixing sodium hydroxide and a sodium silicate solution (Korean Industrial Standards KS

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Grade-3; SiO2 = 29 wt%, Na2O = 10 wt%, H2O = 61 wt%, specific gravity = 1.38) as

prescribed in Table 2. Portland cement, which served as a reference, was mixed at a water-tocement ratio of 0.4. The alkali-activated cement paste samples were synthesized at various

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ratios of fly ash to slag, in this case 100:0, 70:30, 30:70 and 0:100, denoted correspondingly

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as S0, S30, S70 and S100, respectively. The water-to-binder ratio of the samples and activator composition of the alkali-activated binders in this study were adopted from work by Jang et

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al. (Jang et al., 2016) in order to draw a valid comparison with the experimental data in the literature. The mixing was conducted at room temperature, and fresh mixtures were cast into

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a mold 40 × 40 × 5 mm in size. All samples except for the S0 sample were cured at room temperature in a sealed condition for the initial 24 h, after which they were demolded and

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cured at room temperature in a sealed condition up to 28 days. The S0 sample was cured at 80 oC for the initial 24 h before demolding and was cured further at room temperature, as this

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sample requires an initially high temperature for activation of the fly ash reaction at an early age (Duxson et al., 2007, Provis et al., 2005).

2.2 Test methods

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An electrical leaching test was conducted after 28 days, lasting for 14 days, using the casing described in a study by Saito and Nakane (Saito et al., 1999), as schematically shown in Fig. 1. The sample is allowed to be suspended in the middle of two tanks, each filled with 425 ml of deionized water. An electrode connected to a DC power supply was then placed in each tank to apply a current density of 5 A/m2 over an exposed area of 32 × 32 mm on each side. The voltage applied to each sample by the DC power supply was in the range of 30-60

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V and was automatically adjusted to supply a constant current density during the test period. Exposure of PC paste samples to this current density with an identical test condition over two weeks reportedly replicates the extent of leaching that occurs in the sample after one year of exposure under a static leaching condition (Gashier, 2016). It should be noted that constant

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current generated in the DC power supply system can cause the accumulation of H+ ions in

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the anode tank, which can damage the sample, not necessarily by accelerated leaching but in a manner similar to an acid attack (Saito et al., 1999, MacArthur, 2017). A higher current

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density applied to the sample is likely to lead to more accelerated leaching behavior, but it may be less accurate in replicating the actual leaching behavior occurring in the radioactive

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waste repository over an extended period of time. Hence, it is recommended to conduct accelerated leaching at the lowest possible current density and over a longer duration, and a

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current density of 5 A/m2 has been chosen in several studies for a similar purpose (Saito et al., 1992, Hashimoto et al., 2013, Gashier, 2016, MacArthur, 2017). The samples were

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retrieved from the casing at the end of the test and were immersed in isopropanol to arrest the hydration.

The evolution of the binder gel in alkali-activated cement under an accelerated leaching condition was explored by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), Thermogravimetry/differential thermogravimetry (TG/DTG), and nuclear magnetic

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resonance (NMR) spectroscopy. MIP was conducted using an Autopore VI 9500 (Micromeritics Instrument Corp.) at a mercury filling pressure of 0.43 psia, an equilibrium time of 10 s, and a contact angle of 130o. XRD was conducted using an X-ray diffractometer (PHILIPS, Netherlands) by adopting a scan range of 5-65o 2θ with a step size of 0.02 o 2θ, and a dwell time of 3 s, with CuKα radiation at 40 kV and 30 mA. Thermogravimetry was conducted using a TA Instrument Q600 device (PH 407, KBSI Pusan Center) in an N2 environment. A heating rate of 10 oC/min was employed. All MAS NMR spectra were

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acquired using an AVANCE III HD instrument (Bruker, 9.4T). The 29Si MAS NMR spectra

were collected at 79.51 MHz using a 4 mm o.d. zirconia rotor with a pulse length of 1.6 µs,

spinning rate of 11 kHz, and a repetition delay of 20 s. The chemical shifts were referenced to

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TMS at 0 ppm. The 27Al MAS NMR spectra were collected at 104.29 MHz with employing a

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pulse length of 1.2 µs, spinning rate of 14 kHz, and a repetition delay of 2 s. The chemical shifts were referenced to AlCl3 at 0 ppm. The 23Na MAS NMR spectra were collected at

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105.87 MHz at a pulse length of 2.0 µs, spinning rate of 14 kHz, and a repetition delay of 12 s. The chemical shifts were referenced to NaCl. The 1H MAS NMR spectra were collected at

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400 MHz by employing a pulse length of 1.0 µs, spinning rate of 14 kHz, and a repetition delay of 5 s. The chemical shifts were referenced to H2O at 4.8 ppm. Note that ‘qn' or ‘Qn’ is

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used to denote a tetrahedral framework of Al or Si, respectively, sharing oxygen with n number of another tetrahedral Al and/or Si.

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Thermodynamic calculations were performed using the Gibbs energy minimization

software GEM-Selektor v.3.5 (http://gems.web.psi.ch/) (Kulik et al., 2013, Wagner et al., 2012) using CEMDATA18 (Lothenbach et al., 2019) to simulate Ca leaching from the binder matrix. The equilibrium phase assemblages in the PC and alkali-activated slag were predicted from the total bulk elemental composition by gradually decreasing the amount of CaO. The extended Debye-Hückel equation (Helgeson et al., 1981) was used to calculate the activity

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coefficient of the aqueous phases using common ion-size parameters of 3.67 Å, and 3.72 Å,

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respectively, for the PC and alkali-activated slag system.

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3. Results 3.1 Mercury intrusion porosimetry The MIP results of alkali-activated cements exposed to accelerated leaching are shown in Fig. 2. It is often reported that the pore structure of materials is coarsened by leaching due to the loss of ions. On the other hand, the porosity and/or pore diameter of cementitious materials may not always increase after leaching due to the hydraulic activity

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and the hydration of the raw materials, which effectively increase the solid volume after

contact with water (Nakarai et al., 2006). The highest fraction of pores in the S0 sample was distributed at ~30 nm and 13 nm prior to leaching. Upon exposure to accelerated leaching the

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pores at 13 nm showed a noticeable decrease, while those at 30 and 60 nm showed an

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increase, possibly due to the dissolution of the binder gel from the matrix after accelerated leaching. The samples incorporating slag (i.e., S30, S70 and S100) had a majority of pores

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distributed below 10 nm, and these pores were found to be diminished by accelerated leaching. Pores with diameters of 100-1,000 nm were significantly developed in the S30 and

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S70 samples, while these changes were not reflected in the S100 sample after leaching. The PC sample showed major pore distributions at 60 nm and below 10 nm, and these pores

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shifted toward a slightly larger diameter upon accelerated leaching. Consequently, all samples underwent an increase in the porosity upon exposure to accelerated leaching, while this

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occurred in different regions of the pore diameters. Specifically, a majority of the pores in the slag-incorporated samples (S30, S70 and S100) were distributed below 10 nm, a region which mostly consists of the interparticle spacing between the C-S-H sheets (Mehta, 1986). A notable change occurred in this region, which accompanied an increase in the capillary void space (10-1,000 nm) (Mehta, 1986). In contrast, the S0 and PC samples had a considerable volume of pores distributed in the capillary void space, which underwent a slight increase

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upon exposure to accelerated leaching. This may be due to the leaching of the binder gel and other hydration products in these samples (i.e., portlandite for PC (Nakarai et al., 2006)).

3.2 X-ray diffraction The XRD patterns of alkali-activated cements exposed to accelerated leaching are shown in Fig. 3. The samples incorporating fly ash (S0, S30 and S70) showed peaks

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corresponding to the unreactive crystalline phases of quartz (SiO2, PDF# 01-085-1054), mullite ((Al2O3)3(SiO2)2, PDF# 00-006-0258), and hematite (Fe2O3, PDF# 01-073-0603). In addition, fly ash/slag blended samples showed peaks due to the presence of C-S-H (Ca1.5Si0.35

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ˑx(H2O)). The dominant reaction product in the S100 was also C-S-H, along with secondary reaction products hydrotalcite ((Mg6Al3CO3(OH)16ˑ4H2O, PDF# 01-089-0460) and

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portlandite which disappeared in the XRD pattern upon exposure to accelerated leaching. The XRD patterns of the alkali-activated cement samples showed a reduction in the intensity of

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the reflections due to quartz and mullite after accelerated leaching, implying that higher fractions of these minerals were present in the matrix. It is therefore inferred that the volume

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of the amorphous gels (N-A-S-H and/or C-A-S-H) reduced and was relatively low after accelerated leaching. This observation is supported by the notable reduction in the peak

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intensity of C-A-S-H in the XRD pattern of S100 sample.

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Both PC samples before and after accelerated leaching showed peaks corresponding to the unreacted clinkers belite (Ca2SiO4, PDF# 00-033-0302) and ferrite (Ca2(Al,Fe+3)2O5, PDF# 00-030-0226), as well as the hydration products C-S-H, ettringite (Ca6Al2(SO4)3(OH)12ˑ26H2O, PDF# 00-013-0350), hemicarbonate (Ca4Al2O6(CO3)0.5(OH)ˑ11.5H2O, PDF# 00-041-0221) and monocarbonate (Ca4Al2O5(CO3)ˑ11H2O, PDF# 00-014-0083), indicating that no particular change in the

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crystalline phases occurred upon exposure to accelerated leaching.

3.3 Thermogravimetry The thermograms and differential thermograms of alkali-activated cements exposed to accelerated leaching are shown in Fig. 4. All alkali-activated cement samples showed a weight loss peak at around 100 oC, due to the evaporation of free or physically bound water

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from the binder gel (Sha et al., 2001, Wang et al., 1995). The PC sample showed weight loss peaks at 90 oC due to the dehydration of ettringite and C-S-H, at 150 oC due to the

dehydration of AFm phases, and at 450 oC due to the dehydroxylation of portlandite (Taylor,

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1997). The notable weight loss at ~260 oC in S0 and at 330 oC in S30 after accelerated

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leaching is attributable to the dehydration of Al(OH)3 and the silica-containing hydrogarnet phases (Lothenbach et al., 2016). These phases may be formed during the accelerated

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leaching test which provides a fully saturated condition, while these phases are not clearly identified in other characterization results.

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The weight loss which occurred in the alkali-activated cement samples below 200 oC due to the evaporation of physically bound water showed no particular relationship as a

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function of the slag or fly ash content in the binder, however, all samples displayed a clear decrease in the weight loss after accelerated leaching. This fairly agrees with the XRD results

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that illustrate possible and partial destabilization of binder gel present in the alkali-activated cement samples after accelerated leaching. On the other hand, the PC sample showed greater weight loss in this region upon accelerated leaching, suggesting that the hydration of unreacted clinkers occurred. Alternatively, this also suggests an increase in the amount of free water due to the increased capillary pore volume after accelerated leaching. Meanwhile, the weight loss peak at 450 oC in the PC sample was reduced, which implies that Ca was initially

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leached from portlandite, in good agreement with the findings of previous studies (Hashimoto et al., 2013, Yokozeki et al., 2004). 3.4 Solid-state NMR spectroscopy 3.4.1 27Al MAS NMR spectroscopy The 27Al MAS NMR spectra of alkali-activated cements exposed to accelerated

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leaching are shown in Fig. 5. The spectra of the S0 samples before and after accelerated leaching showed resonance in the tetrahedral site at 56 ppm with the highest intensity due to the presence of q4 in the N-A-S-H gel (Palomo et al., 2004). Resonance in the octahedral site

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was observed at 10 to -30 ppm, indicating the presence of mullite and amorphous Al

(Mobasher et al., 2016, Merwin et al., 1991). The spectrum of the sample after leaching

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showed a broader shoulder in the pentahedral (i.e., 50 to 30 ppm) and octahedral sites, suggesting that a higher population of the remnant fly ash is present in the binder gel, i.e., a

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reduced site occupancy in the tetrahedral Al (N-A-S-H). On the one hand, the increased resonance in the octahedral sites in the spectra of S0 and S30 can be assigned to the

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precipitation of Al(OH)3 or hydrogarnet-related phases after accelerated leaching, similar to the TG results, though this requires more validation. The S30 samples showed a notably

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increased shoulder in the pentahedral region, similar to that observed in the spectra of S0, which suggests that the binder gel N-A-S-H was released from the binder matrix upon

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exposure to accelerated leaching. Resonance at 74 and 67 ppm is observed in the spectra of the samples containing

higher dosages of slag (i.e., S70 and S100), an outcome attributed to the presence of Al in the bridging sites of C-A-S-H (Myers et al., 2015). Resonance in the octahedral site was observed at 9 ppm and 4 ppm due to the presence of a hydrotalcite-like phase (Sideris et al., 2012) and a third aluminate hydrate which is either a disordered aluminate hydroxide or

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calcium aluminate hydrate formed as a separate phase or as a precipitate on the surface of CS-H (Andersen et al., 2006). Accelerated leaching in S70 and S100 led to an increased intensity in the octahedral site and led to a decrease in the tetrahedral-coordinated Al, implying that a significant fraction of the binder gel in the tetrahedral site was leached from the binder matrix. While the alkali-activated cement samples generally showed degradation in the tetrahedral site regardless of the binder composition, the spectra of PC showed no

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observable change induced by accelerated leaching. 3.4.2 29Si MAS NMR spectroscopy

The 29Si MAS NMR spectra of the alkali-activated cements exposed to accelerated

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leaching are shown in Fig. 6. The relative area of the sites identified in the 29Si MAS NMR

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spectra is quantified by the deconvolution process and is provided in Fig. 7. The deconvolution was performed using OriginPro 9.0 by introducing a minimum number of

2015, Park et al., 2016).

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component peaks at the locations reported in the literature (Palomo et al., 2004, Myers et al.,

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The deconvolution result in Fig. 7 shows that the samples with fly ash (S0, S30 and S70) experienced a reduction in the sites corresponding to the reaction product of fly ash (i.e.,

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N-A-S-H at Q4(1-4Al) sites) after accelerated leaching. The Q4(0Al) site which is attributed to the presence of quartz and amorphous Si (i.e., Q4(0Al)) was notably higher especially in S0

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after leaching (Fig. 7). Similarly, the intensity of the resonance at the Q4(4Al), Q4(3Al), Q4(2Al) and Q4(1Al) sites corresponding to the N-A-S-H signal was reduced (Fig. 6). This implies that N-A-S-H was subjected to leaching from the binder matrix, showing fair agreement with the 27Al MAS NMR spectra of these samples which suggested a reduction in the tetrahedrally coordinated Al after leaching. In contrast, the amount of C-A-S-H in the slag-containing samples was increased by accelerated leaching, particularly in the Q2 and

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Q3(1Al) sites (Fig. 7), a finding opposite to the phenomenon observed with N-A-S-H in the samples with only fly ash. It can be observed in Fig. 7 that the unreacted fraction (Q0 site) of PC was reduced and that the reaction product C-S-H was increased. The degradation phenomena of both C-A-S-H in the alkali-activated slag and C-S-H in the PC upon accelerated leaching can be considered as similar; i.e., Ca leaching in the silicate chains induces polymerization, which would eventually lead to the formation of silica gel (Saito et

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al., 1992). The degradation process induced by leaching is anticipated to occur simultaneously with the hydration of any unreacted particles, as samples are fully saturated with water during the leaching test. Such phenomena were found to be valid in the PC

sample, while the unreacted fractions of slag showed no apparent pattern due potentially to

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the relatively low hydraulic reactivity in comparison to that of alite and belite in PC

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(Snellings, 2013, Skibsted et al., 2019).

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3.4.3 1H MAS NMR spectroscopy The 1H MAS NMR spectra of the alkali-activated cements exposed to accelerated leaching are shown in Fig. 8. All spectra showed broad resonance at 4-5 ppm attributed to water adsorbed in the intralayers and to the protons in Si-OH sites within the tetrahedral aluminosilicate framework (i.e., C-S-H, C-A-S-H and N-A-S-H) (Renaudin et al., 2009, Walkley et al., 2016). This resonance appeared at 4.1 ppm in the S30, S0 and PC samples,

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and at 5.0 ppm with a shoulder at 4.1 ppm in the S70 and S100 samples. Resonance at 1.3 ppm with a narrow line width was observed in all spectra; this

component was reported to persist in the 1H MAS NMR spectra of synthetic C-S-H and C-A-

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S-H (Renaudin et al., 2009, Heidemann et al., 1998) and similar binder systems consisting of N-A-S-H and C-(N)-A-S-H (Walkley et al., 2016). This arises from the presence of protons

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with mobility experiencing weak dipolar coupling, hence constituting a local environment different from that associated with free water molecules (Renaudin et al., 2009). Previous

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studies which investigated cementitious binder systems using 1H MAS NMR spectroscopy suggested that this resonance was attributable to the presence of a pendant hydroxyl function

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(Renaudin et al., 2009), protons within AFm phases (Walkley et al., 2016), and those present in the Ca-OH sites in portlandite (Heidemann et al., 1998). On the other hand, this resonance

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in the samples here with a high volume of fly ash (S0 and S30) likely rises from the presence

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of silanol groups (Pfeifer et al., 1985, Huo et al., 2009). It was noted that the spectra of the PC sample showed no noticeable change upon

accelerated leaching, while those of the alkali-activated cement samples underwent a substantial decrease in the resonance at 4.5 ppm. On the other hand, all of the alkali-activated cement samples showed a substantial decrease in the resonance at 4-5 ppm, most likely due to water in the intralayers of C-A-S-H and N-A-S-H, relative to the highest peak at 1.3 ppm

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assigned to protons in the phases associated with octahedral Al coordination. It can be inferred that the structural degradation induced by accelerated leaching proceeds by removing the bound water from the tetrahedral aluminosilicate gel, while the protons within the octahedral Al phases remain relatively unchanged. On the other hand, there can occur water electrolysis to form H+ and OH- ions, causing a structural alteration in the water molecules, although this phenomenon does not occur under natural leaching conditions (Saito et al.,

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1999).

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3.4.4 23Na MAS NMR spectroscopy The 23Na MAS NMR spectra of the alkali-activated cements exposed to accelerated leaching are shown in Fig. 9. Note that the spectra of the samples before and after leaching were acquired after an identical number of scans to provide a quantitative comparison of site occupation by Na present in the alkali-activated cements after leaching. The main role played by Na in alkali-activated cements is charge-balancing of the Al tetrahedra in both the C-(N)-

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A-S-H (Viallis et al., 1999) and N-A-S-H gel (Walkley et al., 2016, Duxson et al., 2005). The resonance generally shifted toward lower chemical shifts (from -3 to -8 ppm) as the fly ash in the binder composition was increased, implying that Na in the samples with a higher fly ash

-p

content experienced greater electron density (Walkley et al., 2016). In addition, the resonance in the samples with a higher fly ash content showed a broader line width, indicating a greater

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level of amorphousness. The intensity of the resonance was reduced noticeably in the samples exposed to accelerated leaching, indicating that a significant amount of Na leached from the

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binder matrix.

21

22

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4. Discussions 4.1 Accelerated leaching-induced structural evolution The present study investigated the structural degradation of alkali-activated cement induced by accelerated leaching. While the Portland cement sample showed no significant structural alteration at an applied current density of 5 A/m2 for a duration of two weeks, noticeable changes were observed in the alkali-activated cement samples. In particular, two

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binder gels with distinctive chemical compositions (i.e., C-A-S-H and N-A-S-H) exhibited different degradation pathways. Firstly, the amount of N-A-S-H which was abundantly

present in the samples with a high volume of fly ash (e.g., S0 and S30) was found to decrease

-p

upon exposure to an accelerated leaching condition. This outcome was clearly in contrast to

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that exhibited by the samples rich in C-S-H and C-A-S-H, which increased slightly after two weeks of accelerated leaching, according to the 29Si MAS NMR spectral deconvolution result

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shown in Fig. 7.

The leaching behavior of Ca-rich alkali-activated binders confirms what can be

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deduced from the previously reported natural and accelerated leaching phenomena of C-S-H in hydrated Portland cement (hydration and dissolution continuously occurring and reducing

ur

the overall volume of the binder phase (Nakarai et al., 2006)). On the other hand, dissolution was the dominant leaching phenomenon of the N-A-S-H type gel abundant in a low-Ca

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binder, which showed no further increase in the amount of this gel upon exposure to accelerated leaching. This may be due to the relatively low hydraulic reactivity of fly ash in comparison with slag and Portland cement and to the higher activation energy required for the formation of a N-A-S-H type gel from a thermodynamic perspective (Sun et al., 2018). Moreover, the reduction in N-A-S-H gel and the substantial change in the pore size distribution in the alkali-activated cement samples after leaching may be due to that alkali

23

metal ions (i.e., Na+) which are more soluble than Ca2+ are predominant in the interlayers. Previous studies provided clear evidence which illustrates that K+ and Na+ ions are the first to be leached in a static condition (Jacques et al., 2010) and also in an accelerated condition (MacArthur, 2017). The electrochemical potential exerted on Na at charge balancing sites may directly or indirectly influence the stability of neighboring tetrahedral atoms. Despite the fact that some thermodynamic data for N-A-S-H gels are available in the literature (Gomez‐

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Zamorano et al., 2017), more work is necessary to ascertain the compatibility of such gels in leaching environments.

The leaching process in alkali-activated cement was realized by the removal of water,

-p

coupled with the release of both C-A-S-H and N-A-S-H, as reflected by the reduced intensity at sites corresponding to protons in Si-OH in the 1H MAS NMR spectra. Nevertheless, the

re

experimental results probing the 23Na nuclei suggest that the accelerated leaching induces Na

4.2 Perspectives

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local environment of Na.

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to be leached from alkali-activated cements without involving significant alteration in the

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A general Ca leaching phenomenon of alkali-activated cement in different modes of

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degradation (i.e,. carbonation) is highly useful to understand the deterioration induced by accelerated leaching. With regard to carbonation process of alkali-activated cements, dissolved CO2 neutralizes the pore solution and consumes hydroxides by acting as an acid (Zhang et al., 2014). Moreover, this mobilizes the charge-balancing species Na and directly affects the stability of the binder phase in the absence of a chemical buffer system (Puertas et al., 2006). The effect of Ca leaching on the phase assemblage of PC and alkali-activated slag is 24

simulated by a thermodynamic calculation in Fig. 10, without considering the hydration of anhydrous materials during the leaching process. Fly ash-containing samples are omitted from the simulation due to the lack of thermodynamic data for the reaction products of alkaliactivated fly ash. The modeling result suggests that Ca initially leaches from portlandite in the PC system before Ca is released from monosulfate, ettringite and C-S-H (Fig. 10 (a)). Ca leaching from C-S-H is initiated when portlandite is no longer available. The release of Ca

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from monosulfate continues to occur until it is destabilized to ettringite and strätlingite. On the other hand, hydrotalcite initially present in the alkali-activated slag system is predicted to be destabilized to M-S-H, which accompanies destabilization of C-A-S-H and strätlingite as well (Fig. 10 (b)). Both the simulated PC and alkali-activated slag systems exhibit steady and

-p

notable increase in the leached concentration of Ca2+ ions (Fig. 10 (c) and (d)). The initial

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concentrations of K+ in the PC system and Na+ in the alkali-activated slag system are very high, in the order of 10-100 mmol, and these ions are simulated as the first species that

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leaches out from the binder gel, which is in agreement with a previous study (Jacques et al., 2010). The Na leaching from the binder gel is evidenced in Fig. 10 (e) and (f). The simulated

na

Na/Si ratio of C-A-S-H in the alkali-activated slag system gradually decreases, which is associated with the dissociation of the Na-containing end-members, IFCN and IFCNA (Fig.

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10 (f)). The role played by calcium hydroxide in buffering the pH and the stability of other hydration products in PC can generally be considered important, whereas questions remain as

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to whether chemical buffer systems are capable of significantly extending the service lifetimes of structures in repositories over a geologic timescale. The thermodynamic calculation does indicate that the dissolution of hydration products formed in PC systems may be reached by a relatively lower amount of cumulative leached water, but such information should not be treated as to judge which binder system is superior over another in terms of leaching resistance, since no kinetic effect is considered and the initial volume of the

25

alkali-activated slag system is higher. Over the past few decades the electrical leaching test has been validated with regard to effectiveness and accuracy in simulating and accelerating the leaching behavior of cementitious materials in contact with water. This electrically accelerated testing protocol has often been adopted in previous studies to assess the effect of the incorporation of pozzolan (fly ash, blast furnace slag and silica fume) on the leaching behavior of Portland cement

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(Nakarai et al., 2006, Saito et al., 2000). These studies showed that the extent of Ca leaching and the associated changes in the pore network are notably mitigated by the incorporation of pozzolanic materials (Nakarai et al., 2006, Saito et al., 2000). Despite the ability of this test protocol to accelerate the alkali leaching from a binder matrix, numerous limitations have

-p

been pointed out, i.e., water electrolysis forming an acidic condition in an anode tank, leading

re

to degradation at one side (Saito et al., 1999) and the need for validation with static leaching tests. The electric potential may alter and accelerate certain aspects of the leaching behavior

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of hydrated cements, but it may not accurately accelerate the leaching behavior, especially when a very high electric potential difference is applied (Saito et al., 1992, Saito et al., 2000,

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Saito et al., 1999, Hashimoto et al., 2013). In addition, use of PBS or M9 electrolyte is recommendable, as deionized water is non-conductive and does not provide enough free-

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moving ions, resulting in a sharp increase in the applied voltage.

5. Concluding remarks Structural changes in the binder gel of alkali-activated cements as induced by

accelerated leaching are investigated. Electrically accelerated leaching tests were conducted on alkali-activated cement samples incorporating fly ash and/or slag by applying a potential gradient. The characterization of the samples after leaching provides a detailed description of

26

the structural evolution of their binder gels over the course of their service lifetimes under repository conditions. The findings of the present study are summarized as below.

(1)

Fly ash-rich samples showed a significant extent of the dissolution of the binder gel upon accelerated leaching unlike other samples with higher amounts of Ca, which underwent the hydration of anhydrous slag. This is mainly due to the relatively low

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hydraulic reactivity of fly ash in comparison with that of slag and Portland cement, and to the higher activation energy required for the formation of the N-A-S-H type gel. (2)

The C-A-S-H type gel remained relatively unchanged in comparison with the N-A-S-

-p

H type gel. Accordingly, much less structural change arose in the slag-rich and PC

(3)

re

samples compared to that in the fly ash-rich samples.

Na ions were leached without involving any alteration in their local environment, but

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this may be responsible for the dissolution of the N-A-S-H type gel occurring over an

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applied potential gradient.

The accelerated leaching test will require further improvement before it is applicable

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to the variety of materials considered and employed in repositories in recent years (i.e., slagblended Portland cement). This can be an important topic of future studies, though it deviates

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from the purpose of this study.

Author contributions section S.P., H.N.Y. and J.H.S. carried out the experiment. S.P. wrote the manuscript with supervision from H.K.L. and J.G.J., and with input from all other authors. S.P. and J.G.J. conceived the original idea. J.G.J. supervised the project.

27

Conflicts of interest There are no conflicts to declare.

Acknowledgments

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This study was supported by the National Research Foundation (NRF) of the Korean

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na

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re

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government (MSIT) with a grant [2018R1A2A1A05076894] and [2018R1D1A1B07047233].

28

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A

Ca2+ water

K+

Na+

cathode

anode cement sample

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Fig. 1. Electrically accelerated leaching test setup used in this study.

35

0.70

0.50

After leaching

Before leaching

0.45

0.60 S30

0.35

∆V/∆logD (a.u.)

∆V/∆logD (a.u.)

S0

S0

0.40

S70 0.30

S100

0.25

PC

0.20 0.15

S30

0.50

S70 S100

0.40

PC 0.30 0.20

0.10 0.10 0.05 0.00

0.00 1

10

100

1000

10000

1

Pore diameter (nm)

100

1000

10000

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Pore diameter (nm)

(a) Before leaching

(b) After leaching

0.70

0.50

S0

Before leaching

0.40

After leaching

After leaching

∆V/∆logD (a.u.)

0.50

S30

0.45

Before leaching

0.40 0.30

0.35 0.30

-p

0.60

∆V/∆logD (a.u.)

10

0.25 0.20 0.15

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0.20

0.10

0.10

0.05 0.00

0.00

10

100

1000

1

10000

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1

10

1000

(d) S30 sample

0.40

0.40

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S70

0.35

Before leaching 0.30

0.25

∆V/∆logD (a.u.)

After leaching

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∆V/∆logD (a.u.)

S100 0.35

Before leaching

0.30

0.20 0.15

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0.10

0.20 0.15

0.05 0.00

0.00

10

After leaching

0.25

0.10

0.05

1

10000

Pore diameter (nm)

Pore diameter (nm)

(c) S0 sample

100

100

1000

1

10000

10

100

1000

Pore diameter (nm)

Pore diameter (nm)

(e) S70 sample

(f) S100 sample

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10000

0.18

PC 0.16 Before leaching 0.14

∆V/∆logD (a.u.)

After leaching

0.12 0.10 0.08 0.06 0.04 0.02 0.00 1

10

100

1000

10000

Pore diameter (nm)

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(g) PC sample

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Fig. 2. First derivative of the cumulative pore volume of alkali-activated cements (a) before and (b) after accelerated leaching as determined by MIP. The results for the (c) S0, (d) S30, (e) S70, (f) S100 and (g) PC samples are separately shown.

37

q: quartz m: mullite h: hematite

q: quartz m: mullite h: hematite C: C-S-H

S0 q

S30 q

After leaching q

After leaching

m m

m hq

m qq m

Before leaching

15

20

25

2θ

30

35

40

5

45

10

15

(o)

20

25

2θ

(a) S0 sample

30

35

40

45

(o)

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10

C

hm m mh q q m m m q m

m

Before leaching

5

m

q m

m

(b) S30 sample

C: C-S-H q: quartz m: mullite

S70

C: C-S-H P: portlandite H: hydrotalcite

q

m

m

m

m

-p

q

C

After leaching

C

After leaching

S100

qm m

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Before leaching Before leaching

H

10

15

20

25

2θ

30

35

(o)

(c) S70 sample

10

15

20

H

25

2θ

30

35

40

45

(o)

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PC

ur

P

PC

After leaching

b b

b

E

b bb

E

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M Hc f

5

45

H

(d) S100 sample

b: belite f: ferrite P C: C-S-H P: portlandite M: monocarbonate Hc: hemicarbonate E: ettringite

E

40

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5

P P

b b b

Before leaching

5

10

15

20

25

30

35

40

45

2θ (o)

(e) PC sample Fig. 3. XRD patterns of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching.

38

39

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-p

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100

0.08

100

0.1

S0

S30

Before leaching

90

85

0.04

80 0.02

75

70 200

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90 0.06

85 0.04 80 0.02

75

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0 0

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(a) S0 sample

(b) S30 sample

100

100

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0 400

600

Temperature ( oC)

(c) S70 sample

800

85

0.1

80

0.05

75

70

lP

200

0.15

-p

85

After leaching

90

re

0.1

Residual mass (%)

90

Before leaching

95

0

Derivative residual mass (%/ oC)

After leaching

Derivative residual mass (%/oC)

Before leaching

95

Residual mass (%)

200

Temperature ( oC)

Temperature ( oC)

0

0.08

After leaching

0 0

Before leaching

95

Residual mass (%)

Residual mass (%)

0.06

Derivative residual mass (%/oC)

After leaching

Derivative residual mass (%/oC)

95

0 200

400

600

800

Temperature ( oC)

(d) S100 sample

100

0.15

Before leaching

95

ur

90

85

80

Jo

Residual mass (%)

After leaching

0.1

0.05

75

70

0

200

Derivative residual mass (%/oC)

na

PC

0 400

600

800

Temperature ( oC)

(e) PC sample Fig. 4. TG/DTG curves of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching. The residual mass and derivative residual mass of the samples are correspondingly plotted on the top and bottom halves of the graphs.

40

41

ro of

-p

re

lP

na

ur

Jo

S30

S0

90

70

50

30

10

Before leaching

After leaching

After leaching

-10

-30

110

-50

90

70

Chemical shift (ppm)

ro of

110

Before leaching

50

30

10

-10

-30

-50

Chemical shift (ppm)

(a) S0 sample

(b) S30 sample

S100

S70

Before leaching

-p

Before leaching

After leaching

110

90

70

50

30

10

Chemical shift (ppm)

-10

-30

110

-50

90

70

50

30

10

-10

-30

Raw materials

PC

Before leaching

OPC

ur

After leaching

Slag

Jo 110

90

70

50

-50

Chemical shift (ppm)

(d) S100 sample

na

(c) S70 sample

lP

re

After leaching

Fly ash 30

10

-10

-30

110

-50

Chemical shift (ppm)

90

70

50

30

10

Chemical shift (ppm)

(e) PC sample

(f) Raw materials

42

-10

-30

-50

Jo

ur

na

lP

re

-p

ro of

Fig. 5. 27Al MAS NMR spectra of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching. The spectra of the raw materials are shown in (f). The spectra are normalized to the highest resonance peak.

43

Before leaching

S0

Q4(0Al)

Before leaching

Q4(2Al)

After leaching

After leaching

S30

Q2 Q2(1Al)

Q4(4Al)

Q4(3Al) Q4(1Al)

Q4(4Al)

Q4(0Al)

Q3(1Al) Q1

4 Q4(3Al) Q (1Al)

Q4(0Al)

Q4(2Al)

Q4(0Al)

Silanol

-60

-70

-80

-90

-100

-110

-120

-50

-130

-60

-70

-80

-90

-100

-110

-120

-130

Chemical shift (ppm)

Chemical shift (ppm)

(a) S0 sample

ro of

-50

(b) S30 sample S70

Q2(1Al) Q1

Before leaching After leaching

Before leaching

Q2

Q1

After leaching

Q2

-p

Q4(4Al)

S100

Q2(1Al)

Q0

Q3(1Al)

Q4(0Al) Q4(3Al)

-60

-70

-80

-90

-100

Chemical shift (ppm)

(c) S70 sample

-110

-120

-60

-130

-70

lP

-50

re

Q4(1Al) Q4(0Al) Q4(2Al)

Q3(1Al)

-80

-90

-100

Chemical shift (ppm)

(d) S100 sample

PC

Raw materials

na

Anhydrous

Before leaching

OPC

After leaching

ur

Q1

Slag

Q2

Jo

Q3

-60

-70

Fly ash

-80

-90

-100

-50

-110

Chemical shift (ppm)

-70

-90

-110

-130

-150

Chemical shift (ppm)

(e) PC sample (f) Raw materials Fig. 6. 29Si MAS NMR spectra of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching. The spectra of the raw materials are shown in (f). The spectra are normalized to the highest resonance peak.

44

45

ro of

-p

re

lP

na

ur

Jo

30

18

Sound: 73% Leached: 67%

16

Relative area (%)

25

Relative area (%)

20

N-A-S-H

S0

20 Sound 15 Leached 10

S30

N-A-S-H

C-A-S-H

Sound: 29% Leached: 32%

Sound: 43% Leached: 33%

14 12 10 8 6 4 2

5

0

0 Q4(4Al)

Q4(3Al)

Q4(2Al)

Q4(1Al)

Q4(0Al)

Site type

Site type

(a) S0 sample

(b) S30 sample N-A-S-H

C-A-S-H

S100

Sound: 16% Leached: 11%

30 25 20

15

20

re

10

40

30

5

C-A-S-H

Sound: 46% Leached: 48%

50

Relative area (%)

Relative area (%)

Sound: 40% Leached: 45%

60

S70

-p

40 35

ro of

Silanol

10

0

Site type

(c) S70 sample

30

Jo

10

Q2(1Al)

Q2

Q3(1Al)

Site type

ur

Relative area (%)

35

15

Q1

Sound: 55% Leached: 57%

40

20

Q0

PC

na

C-S-H

45

Unreacted

(d) S100 sample

50

25

lP

0

5 0

Q0

Q1

Q2

Q3

Site type

(e) PC sample Fig. 7. Relative area of reaction products as estimated by the deconvolution of the 29Si MAS NMR spectra of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching. A peak simulation was conducted until it converged to the estimated chi-square tolerance below 10-6.

46

47

ro of

-p

re

lP

na

ur

Jo

Before leaching

S0

1.3 before leaching 1.5 after leaching

After leaching

Before leaching

S30

1.3 before leaching 1.4 after leaching

After leaching

4.2

4.0

20

10

0

-10

-20

30

20

Chemical shift (ppm)

10

0

-10

-20

Chemical shift (ppm)

(a) S0 sample

ro of

30

(b) S30 sample

Before leaching

S70

Before leaching

1.2

After leaching

After leaching

S100

5.0

1.2

4.0

20

10

0

Chemical shift (ppm)

(c) S70 sample

-10

-20

30

lP

30

re

-p

5.0 4.1

20

10

0

-10

Chemical shift (ppm)

(d) S100 sample

na

Before leaching

PC

1.2

ur

After leaching

Jo

4.1

30

20

10

0

-10

-20

Chemical shift (ppm)

(e) PC sample Fig. 8. 1H MAS NMR spectra of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching. The spectra are normalized to the highest resonance peak.

48

-20

49

ro of

-p

re

lP

na

ur

Jo

S0 -8.0

S30

-8.0

Before leaching

Before leaching

After leaching

After leaching

-8.5

-8.4

40

30

20

10

0

-10

-20

-30

-40

-50

40

-60

30

20

10

0

-10

-20

-30

-40

-50

-60

Chemical shift (ppm)

ro of

Chemical shift (ppm)

(a) S0 sample

(b) S30 sample S70

-4.5

S100

-3.2

Before leaching

Before leaching

After leaching

-p

After leaching

-5.0

30

20

10

0

-10

-20

-30

Chemical shift (ppm)

(c) S70 sample

-40

-50

-60

40

lP

40

re

-3.7

30

20

10

0

-10

-20

-30

-40

-50

Chemical shift (ppm)

(d) S100 sample

Jo

ur

na

Fig. 9. 23Na MAS NMR spectra of the (a) S0, (b) S30, (c) S50, (d) S100 and (e) PC samples before and after accelerated leaching.

50

-60

30

30

Ca/Si

Volume (cm3 per 100g of binder)

25 1.4 20

ettringite 1.2

Fe-hydrogarnet 15

portlandite

1.0

10

stratlingite

Ca/Si of C-S-H

Volume (cm3 per 100g of binder)

hydrotalcite

1.6

monosulfate

0.8

5

0

0.6

C-S-H

25

20

stratlingite

15

10

5

M-S-H C-A-S-H 0

101

102

103

104

105

106

101

102

Cumulative leached water (g)

103

105

106

(b)

100

Ca Al S Si

1E-3 1E-4 1E-5

1

Al Si

Na

0.1

180

-p

Concentration (mmol)

Concentration (mmol)

Na

10

1 0.1 0.01

100

K

10

ro of

(a)

0.01

Ca

1E-3

1E-6

Fe

160 140 120 100 80 60 101

102

103

104

105

106

101

102

103

104

105

Cumulative leached water (g)

101

102

103

104

105

106

Cumulative leached water (g)

(d)

lP

(c)

re

1E-4

1E-7

1.0

Ca/Si

0.3

Mole fraction (-)

na

0.9

Al/Si

0.2

0.1

0.0 101

ur

Na/Si, Al/Si and Ca/Si of C-A-S-H

1.1

102

103

104

Jo

5CA INFCA 5CNA INFCNA INFCN T2C T5C TobH

0.12

0.10

0.08

0.06

0.04

0.02

Na/Si 0.00 105

106

101

Cumulative leached water (g)

(e)

104

Cumulative leached water (g)

102

103

104

105

106

Cumulative leached water (g)

(f)

Fig. 10. Phase assemblage of PC (a) and alkali-activated slag (b) as a function of cumulative leached water. The degree of the reaction of PC was predicted by Parrot and Killoh’s model (Parrot, 1984) using the parameters from earlier work (Lothenbach et al., 2008, Lothenbach et al., 2008), and the corresponding degree for slag was assumed to be 50%. The simulated pore water chemistry in PC and alkali-activated slag is shown in (c) and (d). The gel chemistry of C-A-S-H in alkali-activated cement and the mole fractions of the CNASH_ss end-members are shown in (e) and (f). The formulae of the end-members are as follows: 5CA- C1.25A0.125S1H1.625, INFCA- C1A0.15625S1.1875H1.65625, 5CNA- C1.25N0.25A0.125S1H1.375, INFCNA- C1N0.34375A0.15625S1.1875H1.3125, INFCN-

51

C1N0.3125S1.5H1.1875, T2C- C1.5S1H2.5, T5C- C1.25S1.25H2.5 and TobH-C1S1.5H2.5, where CCaO, N- Na2O, A- Al2O3, S- SiO2 and H- H2O (Myers et al., 2014)

CaO SiO2

Al2O3

MgO Na2O

K2O

SO3

Fe2O3

TiO2

Mn2O3

SrO LOI*

Fly ash

4.8

57.0

21.0

1.3

-

1.4

1.0

10.0

1.5

-

-

2.7

Slag

44.8

33.5

13.7

2.9

0.2

0.5

1.7

0.5

0.5

0.2

0.1

1.4

Portland 60.7 cement

20.6

5.0

2.6

0.1

1.0

2.4

3.4

-

0.1

0.1

3.7

Loss on ignition, determined in accordance with ASTM C114.

-p

(wt%)

Jo

ur

na

lP

re

*

ro of

Table 1. Chemical compositions of the binder materials used in this study

52

Table 2. Mix proportions of the alkali-activated cements investigated in this study, expressed as the mass ratio Binder

Activator composition

Sample ID

H2O/binder Slag

SiO2/Na2O

Na2O/binder

S0

1.0

0

1.0

7.5

0.34

S30

0.7

0.3

1.0

5

0.39

S70

0.3

0.7

1.0

5

0.39

S100

0

1.0

1.0

5

0.39

Jo

ur

na

lP

re

-p

ro of

Fly ash

53