quantitative analysis of phase assemblage and chemical

Quantitative analysis of phase assemblage and chemicalshrinkage of alkali-activated slagHailong Ye, Aleksandra Radlińska

Journal of Advanced Concrete Technology, volume ( ), pp.14 2016 245-260

Performance of Different Grades of Palm Oil Fuel Ash with Ground Steel Slag as Base Materials in theSynthesis of Alkaline Activated MortarMoruf. Olalekan Yusuf , Megat Azmi Megat Johari Zainal Arifin Ahmed, , Mohammedd MaslehuddinJournal of Advanced Concrete Technology, volume ( ), pp.12 2014 378-387

Long Term Performance of Alkali Activated Slag ConcreteArie Wardhono, David W.Law Tomas C. K. Molyneaux,Journal of Advanced Concrete Technology, volume ( ), pp.13 2015 187-192

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Journal of Advanced Concrete Technology Vol. 14, 245-260 May 2016 / Copyright © 2016 Japan Concrete Institute 245

Scientific paper

Quantitative Analysis of Phase Assemblage and Chemical Shrinkage of Alkali-Activated Slag Hailong Ye1* and Aleksandra Radlińska2

Received 7 February 2016, accepted 12 May 2016 doi:10.3151/jact.14.245

Abstract

This paper presents a quantitative analysis of hydrated phase assemblage and chemical shrinkage of alkali-activated slag (AAS) as a function of pH and modulus (n= SiO2/Na2O molar ratio) of activator. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermodynamic modeling, provide a com-prehensive characterization of the phase assemblages and distribution in AAS microstructure. The main hydration products in AAS are calcium-alumina-silicate-hydrate (C-A-S-H) and hydrotalcite-type phases, while the formation of other hydrates is activator-dependent. For NaOH-activated slag, hydration products are preferentially formed around slag particles showing a hydrated rim, while for sodium silicate-activated slag, hydration products are initialized at both slag surface and inter-particle spaces simultaneously. However, a dark hydrated rim whose composition is similar to that of alkali-aluminosilicate-hydrate was observed around unhydrated slag in aged AAS. It indicates that the composition and spatial distribution of hydrates in AAS microstructure is heterogeneous, which cannot be predicted by thermodynamic modeling. The chemical shrinkage of AAS was quantified using buoyancy method and backscattered image analysis. The average chemical shrinkage of AAS is about 0.1211 ml/gslag and increases with the increasing modulus and pH of activator. The chemical shrinkage of AAS is about twice larger than that of portland cement, which may be attributed to the limited formation of expansive crystalline phases, such as ettringite and portlandite.

1. Introduction

Ground-granulated blast furnace slag is an amorphous by-product of the steel industry. It has a latent hydraulic reactivity, which can be catalyzed using proper activators, such as portland cement, lime, and alkali metal hydrox-ides, carbonates or silicates, to form cementitious mate-rials (Juenger et al. 2011). Alkali-activated slag (AAS) produces alternative binders that could have important technical, economical and ecological advantages over ordinary portland cements (OPC) (Juenger et al. 2011; Provis et al. 2015; Thomas et al. 2016). Significant amount of work has been recently focused on AAS and the research outcomes show that AAS binders can achieve high strength, low permeability, and strong fire resistance (Douglas et al. 1992; Wang et al. 1995). In addition, in comparison with OPC, production of AAS can result in considerable reduction in the embodied energy and carbon dioxide emissions (Jiang et al. 2014). On the other hand, AAS materials present challenges, such as high shrinkage, rapid carbonation, formation of salt efflorescence, potential alkali-silica reaction, caus-

ticity of activator (safety and health issues), and lacks of relevant material testing standardization (Cartwright et al. 2014; Douglas et al. 1992; Juenger et al. 2011; Shi et al. 2005; Wang et al. 1995; Wardhono et al. 2015; Ye et al. 2014), that still needs to be addressed before wider ap-plication of these alternative binders. While the afore-mentioned engineering properties of AAS are strongly associated with its hydrated phases (Duxson et al. 2005; Juenger et al. 2011), they are also intimately related to the composition and properties of slag, as well as nature and dosage of activators. Therefore, a better under-standing of the relationship between the composition of the reactants, hydrated phase assemblage, and the engi-neering properties (e.g. shrinkage) of AAS will result in significant advancements in mixture design for particular construction and field applications.

The chemical composition and glassy structures of blast-furnace slag vary from source to source, and the difference in slag chemistry affects the phase formation (Ben Haha et al. 2011b). In general, blast-furnace slag can be described as a CaO-SiO2-Al2O3-MgO glass. The destination of Mg in AAS has been reported to partici-pate into the formation of hydrotalcite-type phases, re-gardless of activator (Ben Haha et al. 2011b; Myers et al. 2015). While in CaO-SiO2-Al2O3-H2O system, several different hydrated products have been identified in AAS by previous studies. The main hydrated product in AAS is widely acknowledged as calcium-alumina-silicate-hydrate 1 (C-A-S-H), which typically has a relatively

1 In Cement Chemistry notation: C = CaO, S =SiO2, A=

1Ph.D. Candidate, Department of Civil and Environ-mental Engineering, The Pennsylvania State University, PA, 16801, USA. *Corresponding author. E-mail: [email protected] 2Assistant Professor, Department of Civil and Environ-mental Engineering, The Pennsylvania State University, PA, 16801, USA.

H. Ye and A. Radlińska / Journal of Advanced Concrete Technology Vol. 14, 245-260, 2016 246

lower Ca/Si ratio than the C-S-H formed in OPC (Richardson 2004). In addition, strätlingite (C2ASH8), tetracalcium aluminate hydrate (C4AH13), and katoite (C3AH6), portlandite (CH) phases have been found in NaOH-activated slag using X-ray diffraction (XRD) (Bonk et al. 2003; Regourd 1980). Some researchers also found C4AH13, C3AH6, and CH in sodium sili-cate-activated slag (Bonk et al. 2003; Oh et al. 2010; Schilling et al. 1994). Furthermore, ettringite can form in AAS when sulfates are present (Lothenbach and Gruskovnjak 2007; Regourd 1980). On the other hand, significant amount of work has been done on character-izing the hydrated phases of AAS as a function of initial pH and dissolved silicate ([SiO2]aq) content of activator. The pH in NaOH solution also affects the structure of C-A-S-H and fraction of other crystalline phases (Roy et al. 1992). The incorporation of ([SiO2]aq) can lower the crosslinking of C-A-S-H structure as observed by 29Si nuclear magnetic resonance (NMR), and reduce the crystallinity in hydrated phases as observed in XRD (Bonk et al. 2003; Brough and Atkinson 2002; Wang and Scrivener 1995).

However, most of previous studies on characterizing hydrated phases of AAS focused on a specific type of slag/activator, and primarily gave qualitative results. Considering the variability of slag chemistry and choose of activator for various applications, it is important to have a clear understanding of the phase evolution in AAS due to a modification of raw material chemistry and mixture proportion. Recently, several methods to quan-tify the hydrated phases in AAS as a function of slag chemistry and activator, have been proposed. For exam-ple, Chen and Brouwers established three stoichiometric models correlating the mineral composition of slag with the hydrated products and composition of C-A-S-H (Chen and Brouwers 2007). The main hydration products in these models include C-A-S-H, hydrotalcite (M5AH12), hydrogarnet (C6AFS2H8), C2ASH8, C4AH13, and ettring-ite (C6A S 3H32). However, there are some oversimplified assumptions in these models which might need further attention. For instance, the iron in slag was characterized to be non-reactive to the chemistry of AAS (Bernal et al. 2014), rather than fully incorporating into hydrogarnet as stated in the model. The ettringite tends to form only when sulfates are present in slag and become unstable at high pH environment (Lothenbach and Gruskovnjak 2007; Regourd 1980). Lothenbach et al. proposed a thermodynamic modeling on AAS, which predicts C-A-S-H, hydrotalcite, C2ASH8, trace of FeS and et-tringite. Their calculation shows that the type of activator has no influence on the kind of hydrated phases, but the Ca/Si ratio of C-A-S-H in AAS (Lothenbach and Gruskovnjak 2007). However, the thermodynamic model

Al2O3 , F =Fe2O3, M =MgO, K= K2O, N =Na2O, S=SO3, C =CO2, H=H2O, C-S-H=Calcium-silicate-hydrate, AFt = A group of calcium sulfoaluminate hydrates, AFm= A group of tetracalcium aluminate-ferrite hydrates.

of C-A-S-H in this preliminary study does not account for the variability of sodium and alumina incorporation in low Ca/Si C-A-S-H phases. Myers et al. improved the thermodynamic simulation of AAS using the thermo-dynamic database CNASH_ss for C-A-S-H phases, and illustrated the phase evolution due to a change of Al2O3, MgO or CaO content in slag (Myers et al. 2014, 2015). Nevertheless, thermodynamic modeling cannot give the spatial phase distribution in the microstructure of AAS. It is important to combine various techniques (e.g. SEM/EDS) to enable a comprehensive representation of the hydrated phases and its distribution in the AAS mi-crostructure.

On the other hand, many researchers have shown that AAS has significantly higher shrinkage (both autogenous and drying) than OPC (Cartwright et al. 2014; Neto et al. 2008; Ye and Radlińska 2016). These large deformations can lead to concrete durability issues, as cracking is expected to occur when volumetric changes are re-strained. The driving force for autogenous shrinkage of cementitious materials at early age is the chemical shrinkage. As the hydration progresses, the reduction in absolute volume of reactants (i.e. chemical shrinkage) results in the formation of additional voids in the hard-ened microstructure of cementitious materials. The voids are partially-saturated due to consumption of water dur-ing hydration (i.e. self-desiccation), which contributes to the formation of capillary stress and hence autogenous shrinkage (Lura et al. 2003). The large autogenous shrinkage of AAS could be a result of large chemical shrinkage, which by definition is intrinsically related to the hydrated phase assemblage of AAS. Although theo-retical prediction (Chen and Brouwers 2007; Thomas et al. 2012) of chemical shrinkage of AAS does exits, a quantitative analysis of the physically-measured chemi-cal shrinkage of AAS as a function of activator pH and modulus was rarely reported.

To address the existing research gaps, this paper pre-sents a quantitative analysis of hydrated phase and chemical shrinkage on AAS using combined scanning electron microscopy with X-ray microanalysis (SEM/EDS), X-ray diffraction, and thermodynamic modeling. The SEM/EDS provides the microstructure characteristics, chemical composition of hydrated phases, spatial distribution of elements in microstructure. X-ray diffraction combined with thermodynamic modeling provides a better identification of hydrated phases in bulk materials. In addition, the chemical shrinkage of AAS was quantified using buoyancy methods with the assis-tance of backscattered image analysis.

2. Experimental procedure

2.1 Materials A commercial Grade 120 ground granulated blast-furnace slag (ASTM C989) with a density of 2.89 g/cm3 was used in this study. The oxide composition of the slag, as measured by inductively coupled plasma


atomic emission spectroscopy (ICP-AES), is shown in Table 1. Four types of activating solutions were used (see Table 2), which were prepared using a mixture of sodium hydroxide (NaOH) (dissolved pellets), aqueous sodium silicate, and distilled water. The sodium silicate had the modulus of 1.60, pH of 13.7, and specific gravity of 1.60 at 20°C. The activating solutions were prepared in airtight containers and cooled to room temperature in advance of AAS mixing. 2.2 Mixing proportions and sample preparation Table 2 demonstrates the mixing proportions of the four AAS pastes investigated. In order to study the influence of initial pH and SiO2/Na2O molar ratio (modulus n) of activator on the phase assemblages of AAS, two mixtures were prepared with activators containing sodium sili-cates, one with a modulus n of 0.41 (AAS1) and the other with a modulus n of 1.22 (AAS2). The other two mix-tures were prepared with low (2M) and high concentra-tions (4M) of NaOH solutions as the sole activator (la-beled as AAS3 and AAS4, respectively). In all four mixtures, the volume of activating solution was selected to achieve the same volumetric liquid to solid (activator to slag) ratio of 1.30, yielding an initial binder porosity of 56.5%. 2.3 Methods 2.3.1 X-ray diffraction X-ray diffraction (XRD) was performed on AAS paste samples that were cast and sealed in a series of plastic tubes to prevent evaporation and carbonation (stored in a moist room). The XRD was acquired for samples at the age of 7, 14 and 60 days. Before any XRD test, AAS

paste samples were crushed and immersed in isopropyl alcohol for two weeks to stop the hydration at a desired age. Subsequently, samples were immediately dried in vacuum for one week before milling to fine size powder specimen. XRD data was collected using a PANalytical Empyrean diffractometer in a conventional Bragg-Brentano θ-2θ configuration using CuKα radiation (λ=1.5418 A) with a diffractometer radius of 240 mm. CuKα X-ray was generated using 40 mA and 45 kV operating conditions. Incident beam Soller slits of 0.04° were used, and the incident divergence and anti-scatter slits were fixed at 0.25° and 0.5°, respectively. Air scat-tering was reduced using a beam knife. The beam width was 10 mm. No receiving slit was used, but an anti-scatter slit was fixed at 0.25° and the Soller slits limiting the axial divergence to 0.04 radians were posi-tioned in the diffracted beam path. Instead of using monochromators, a beta-filter Nickle was used to remove the β diffraction spectrums. In addition, a PIXcel3D 1x1 solid state detector with total 255 active channels and active length of 3.3473° was used for data acquisition. The specimens were prepared using front fill method with a zero-background plate and scanned continuously between 5° and 70° 2θ (0.033453 degree/second) under the aforementioned conditions. 2.3.2 SEM/EDS Scanning electron microscopy (SEM) equipped with backscattered electron (BSE) detector and en-ergy-dispersive X-ray spectroscopy (EDS) were applied to characterize the microstructure and phase chemical compositions in AAS samples. The samples moist cured at 23±0.5°C, 100% R.H for 7, 28, and 60 days were

Table 2 Mixture proportions of AAS pastes.

Parameters AAS1 AAS2 AAS3 AAS4 NaOH dry pellets (g) 39.4 9.8 39.4 78.7 Sodium silicate aqueous solutiona (g) 59.1 158.1 0 0 Water (g) 391.5 342.1 430.6 411.3 n (=SiO2/Na2O in activator) (molar-based) 0.41 1.22 0 0 Initial pH of alkali activator solution 14.31 13.78 14.30 14.60 Density of activator (g/cc) 1.09 1.13 1.04 1.09 Slag (g) 1000 1000 1000 1000 Binder solution/solid (vol./vol.) 1.30 1.30 1.30 1.30 Binder solution/solid (wt./wt.) 0.49 0.51 0.47 0.49 Initial binder porosity 56.5% 56.5% 56.5% 56.5% Notes: a: it contains 53.2% water, 28.8% SiO2, and 18.0% Na2O by mass.

Table 1 Oxide composition of the ground granulated blast-furnace slag (by mass %).

CaO SiO2 Al2O3 MgO SO3a S2- Fe2O3

b Na2O K2O P2O5b MnOb LOIc

Slag 43.83 30.04 12.74 4.79 3.11 0.85 1.16 0.24 0.40 0.08 0.22 2.56 Note: a: The element compositions were measured by ICP-AES method, the relatively high SO3 content in this slag is primarily due

to the presence of gypsum as elaborated in Section 3.1. b: Fe, Mn and P are considered to be inert to the chemistry of AAS during thermodynamic modeling (will be elaborated in Section 2.3.4). c: The components of LOI are considered inert during thermodynamic modeling and will not affect the phase formation.


selected for SEM/EDS analysis. Prior to imaging, the samples were cut into 1-2 mm thick slices and the hy-dration was stopped by immersing the slices in isopropyl alcohol. The resulting specimens were epoxy impreg-nated and surface polished. EDS was used to determine the spatial elemental composition of each polished specimen, with about 100 spots analyzed and averaged for each specimen to reduce the random errors. The best working distance for the adopted FEI ESEM Quanta 200 is 12.5 mm.

With the purpose of quantifying the degree of hydra-tion (DoH) in AAS, quantitative backscattered image analysis was implemented in this work (Scrivener 2004). The principle of this method is based on the sensitivity of BSE to the chemical composition of the analyzed mate-rials, which leads to contrast in the grey level histogram. Considering that slag is denser than the hydrated phases, therefore brighter as shown in the BSE images (will be seen in Section 3.2.1). The measurement uncertainty mainly originates from the threshold and edge detection. The BSE-SEM images were converted to binary images through a series of image processing techniques to obtain the area fraction of unhydrated slag. Therefore, the DoH was calculated according to the calculated area fraction of unhydrated slag, and the initial volume fraction of slag from mixing proportions shown in Table.2. It should be noted that although some hydrated phases (e.g. port-landite) may have a similar grey level as unhydrated slag, their volumetric fractions are relatively small and negli-gible. In the present study, eleven BSE images with a magnification ranging from 2000x to 2700x as captured at different representative locations of AAS microstruc-tures were used to perform backscattered image analysis for each specimen.

2.3.3 Chemical shrinkage Chemical shrinkage of AAS pastes was measured using the buoyancy method (Sant et al. 2006). The slag paste sample was placed in a crystallizing dish (70 mm di-ameter × 50 mm height) after mixed with activator. Two sample sizes were evaluated: 10 and 25 grams, corre-sponding to an approximate thickness of 2 mm and 5 mm in the cylindrical glass crystallization dish. Two different weights of samples were chosen in this study to illustrate the effect of specimen size. The detail description of the experimental setup for buoyancy method can be found in a previous study by authors (Cartwright et al. 2013, 2014). 2.3.4 Thermodynamic modeling Thermodynamic modeling of AAS was performed using the Gibbs energy minimization software GEM-Selektor v.3, with the CEMDATA 14.01 (the dissolution reactions of main solid phase at standard condition are listed in Table 3) and PSI-Nagra chemical thermodynamic data-bases (Kulik et al. 2003; Lothenbach and Gruskovnjak 2007). In addition, the thermodynamic model of cal-cium-(alkali-)alumino-silicate-hydrate (C-(N-)A-S-H) adopted the CNASH_ss model proposed by Myers et al. (Myers et al. 2014). The CNASH_ss thermodynamic model is valid for C-(N-)A-S-H phase with Ca/Si ratio <1.3, which is suitable for simulating AAS. Considering that Fe is non-reactive to the chemistry of AAS (Bernal et al. 2014), this study assumes that Fe, Mn, and P are inert during the thermodynamic modeling, due to their low content/reactivity and lack of relevant thermodynamic databases. In addition, the parameters for the De-bye–Hückel equation and ideal gas equation of state for aqueous and gaseous phase models during simulation

Table 3 Dissolution reactions and associated Gibbs free energy change of reaction for the main solid phases formed in AAS at 298.15 K and 1 bar. (Kulik et al. 2003; Lothenbach and Gruskovnjak 2007; Myers et al. 2014, 2015).

Phase Dissolution reactions ΔG0f (kJ ol-1)

Portlandite (CH) Ca(OH)2 → Ca2+ + 2OH- -897 Katoite (C3AH6) Ca3Al2(OH)12 → 3Ca2+ + 2Al(OH)4- + 4OH- -5008.2 Strätlingite (C2ASH8) Ca2Al2SiO2(OH)10·3H2O → 2Ca2+ + 2Al(OH)4- + 1SiO(OH)3-+ OH- +2H2O -5705 Ettringite (C6AS 3H32) Ca6Al2(SO4)3(OH)12·26H2O→6Ca2+ + 2Al(OH)4- + 3SO4

2- + 4OH- + 26H2O -15206 OH-hydrotalcite (M4AH10) Mg4Al2(OH)14·3H2O → 4Mg2+ + 2Al(OH)4- + 6OH- + 3H2O -6394.56 C-N-A-S-H ideal solid solution 5CA (C1.25A0.125S1H1.625) -2293 INFCA (C1A0.15625S1.1875H1.65625) -2343 5CNA (C1.25N0.25A0.125S1H1.375) -2382 INFCNA(C1N0.34375A0.15625S1.1875H1.3125) -2474 INFCN (C1N0.3125S1.5H1.1875) -2452 T2C* (C1.5S1H2.5) -2465 T5C* (C1.25S1.25H2.5) -2517 TobH* (C1S1.5H2.5)

(CaO)a(SiO2)b(Al2O3)c(Na2O)d(H2O)e → a Ca2+ + b SiO32- + 2c AlO2

- + 2d Na+ + 2(a-b-c+d) OH- + (b+c+e-a-d) H2O

-2560 Monosulfoaluminate–hydroxoaluminate hydrate non-ideal solid solution

Monosulfoaluminate hydrate (C4A SH12)

Ca4Al2(SO4)(OH)12·6H2O → 4Ca2+ + 2Al(OH)4- + SO42- + 4OH- + 6H2O -7779

Tetracalcium aluminate hydrate (C4AH13)

Ca4Al2(OH)14·6H2O → 4Ca2+ + 2Al(OH)4- + 6OH- + 6H2O -7324


follow the previous studies (Myers et al. 2015). The thermodynamic simulation assumes that slag dis-

solves congruently, which is reasonable for aluminosili-cate glass (Snellings 2013). The main chemical compo-sition of slag used in this simulation follows Table 2. The simulation was performed using 100g slag as a function of DoH. The amount of activator incorporated into simulation follows the mixing proportion in Table 2. The simulation is performed at a nitrogen atmosphere with the temperature and pressure conditions of 25 ºC and 1 bar.

3. Results and discussions

3.1 X-ray diffraction Figure 1 shows the XRD patterns for all AAS samples at the age of 7, 14 and 60 days. The XRD pattern of raw slag is also included, showing that it contains primarily amorphous structure with some crystalline gypsum (light grey solid lines in Fig. 1) and monoclinic alite (light grey dashed lines in Fig. 1). The broad and diffuse peak in the range 2θ = 30° to 31.6° is the result of the short range order of the CaO-SiO2-Al2O3-MgO glass structure from unhydrated slag (Wang et al. 1995).

In all AAS samples, peaks at 2θ = 11.4°, 22.8°, 34.9°, and 39.4° are observed, which are correlated with (003), (006), (012), and (015) reflections of hydrotalcite (Wang et al. 1995). The broad diffusive peaks at 2θ = 29.4° (ranging from ~28° to 31°) and 50.1° are associated with C-S-H, which is dominantly detected in either sodium silicate-activated slag or NaOH-activated slag. However, the additional peaks at approximately 2θ = 7.0°, 29.1°, 32.0° and 49.8° are attributed to the poorly crystalline C-S-H(I) (Ben Haha et al. 2011a; Wang et al. 1995), which is extensively observed in NaOH-activated AAS4 after 14 days. The crystallinity of C-S-H(I) increases slightly with age, as evidenced by the increasing inten-sity and sharper peak shapes (e.g., compare AAS4 pat-terns at 7 versus 60 days). As discussed by Taylor (Taylor 1997), C-S-H (I) is considerably more ordered than the C-S-H formed in OPC paste at room temperature. C-S-H

(I) has been commonly detected in concrete containing pozzolans and also in AAS (Oh et al. 2010). Comparing XRD patterns, C-S-H(I) presents a broad, basal reflection corresponding to the mean interlayer thickness, which is however absent in the XRD pattern of the less ordered C-S-H forms (e.g. fully hydrated β-C2S paste) (Taylor 1997). By comparing AAS1 and AAS2, the crystallinity of C-S-H (I) decreases as more dissolve silica is added in AAS, implying that the addition of [SiO2]aq can promote generation of amorphous C-S-H. Generally, more crys-talline peaks are observed in NaOH-activated AAS, es-pecially at high pH, which indicates the [SiO2]aq can reduce crystalline phases. It is also observed that the associated crystalline peak intensity increases with age for all the mixtures. These findings are in agreement with previous research (Ben Haha et al. 2011a; Brough and Atkinson 2002; Wang and Scrivener 1995).

On the other hand, portlandite is observed to form in AAS3 and AAS4 mixtures, regardless of the pH. Port-landite is absent in AAS1 and AAS2 mixtures, probably due to the reaction between excess calcium with [SiO2]aq. Additionally, trace of AFm phase (a solid solution of C4AH13, monosulfate, and monocarbonate) is detected in AAS1 and AAS3 at 7 days, then becomes unstable at later ages. Considering the slight carbonation during sample preparation, this AFm in AAS is more likely to be a solid solution of C4AH13 and monosulfate.

3.2 SEM/EDS microanalysis 3.2.1 Microstructure Figures 2-4 show the BSE images of typical micro-structures of these four AAS mixtures cured at moisture room for 7, 28, and 60 days. It can be seen that for all AAS mixtures, grey hydration products, containing C-A-S-H and other hydrated phases, surround the bright, unhydrated slag particles. Generally, sodium sili-cate-activated slag exhibits a larger volume of hydration products, a more homogenous and denser microstructure, and smaller fraction of coarse capillary pores (based on the magnification and resolution of SEM images, the size of pores can be recognized is about in the range of

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Calcite

AAS2-60D

AAS2-14D

AAS2-7D

AAS1-60D

AAS1-14D

AAS1-7D

Slag

AFm Phase

2θ

C-S-H(I)

Hydrotalcite-type PhaseC-S-H

Sodium silicate-activated slag

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Calcite Portlandite

AAS4-60D

AAS4-14D

AAS4-7D

AAS3-60D

AAS3-14D

AAS3-7D

Slag

2θ

C-S-H(I)

Hydrotalcite-type PhaseC-S-H NaOH-activated slag

AFm Phase

(a) (b)

Fig. 1 X-ray diffraction patterns of AAS paste (a) AAS1 and AAS2; (b) AAS3 and AAS4.


(a) (b)

(c) (d)

Fig. 2 BSE images of surface-polished AAS samples cured for 7 days (a) AAS1 (Magnification: 2500x); (b) AAS2 (2500x); (c) AAS3 (2649x); (d) AAS4 (2195x).


(a) (b)

(c) (d)

Fig. 3 BSE images of surface-polished AAS samples cured for 28 days (a) AAS1 (2678x); (b) AAS2 (2013x); (c) AAS3 (2135x); (d) AAS4 (2327x).


(a) (b)

(c) (d)

Fig. 4 BSE images of surface-polished AAS samples cured for 60 days (a) AAS1 (2500x); (b) AAS2 (2500x); (c) AAS3 (2500x); (d) AAS4 (1961x).


0.1-5.0μm) (Ben Haha et al. 2011a). At 7 days, a hy-drated rim can be distinguished in NaOH-activated slag, which indicates that the reaction primarily occurs at the surface of slag. In addition, NaOH-activated slag hy-drates faster up to 7 days, but then the hydration slows down, as evidenced by the evolution of estimated DoH shown in Table 4. The rapid formation of hydrated rim in NaOH-activated slag may lead the hydration into a dif-fusion-control process (Deir et al. 2014; Gebregziabiher et al. 2015). However, for sodium silicate-activated slag, its DoH increases continuously, suggesting a slower but progressive hydration process (Gebregziabiher et al. 2015).

At 28 days, AAS2 shows the densest microstructure with little visible capillary porosity and smallest quantity of unhydrated slag. This indicates that [SiO2]aq from activator can notably increase the DoH in AAS. Based on the high magnification of BSE-SEM images shown in Fig. 5 (a), it can be seen that the hydrated rim also exists in sodium silicate-activated. It indicates that phase for-

mation takes place simultaneously at the slag surface and the inter-particle space. The aqueous silica is homoge-nously distributed in the inter-particle pore space and can react with dissolved Ca (and Al) from slag to form C-S-H (and C-A-S-H), which homogenously fills up the capil-lary pore space in between slag particles.

For NaOH-activated slag, it can be seen that the mi-crostructure becomes denser and more homogenous as a result of ongoing hydration and probably interlocking between inner and outer products at 28 days. However, based on the high magnification BSE images shown in Fig. 5 (b), the hydration products still predominantly surround the slag, as compared with the microstructure of sodium silicate-activated slag. In addition to C-A-S-H, platelets of crystalline hydrated phase (generally brighter than C-A-S-H) can be observed occasionally within the matrix.

At 60 days, a distinctive dark hydrated rim surround-ing unhydrated slag was observed for sodium sili-cate-activated slag. The hydration products in the matrix

Table 4 Average chemical compositiona of hydrated phases measured by EDS microanalysis.

Mixture ID Age DoHb (%) Ca/Si Al/Si Mg/Al S/Ca Ca/Si in C-A-S-Hc

7d 26.4 1.39 0.44 0.45 0.04 - 28d 34.2 1.28 0.43 0.37 0.04 1.13 AAS1

60d 53.2 1.22 0.47 0.46 0.03 - 7d 31.9 1.27 0.35 0.57 0.03 -

28d 51.9 1.29 0.43 0.38 0.05 1.08 AAS2

60d 61.8 1.13 0.36 0.35 0.06 - 7d 30.6 1.76 0.46 0.39 0.06 -

28d 31.1 1.57 0.49 0.34 0.06 1.25 AAS3

60d 33.6 1.57 0.44 0.37 0.07 - 7d 37.1 1.54 0.52 0.38 0.04 -

28d 46.7 1.47 0.50 0.37 0.02 1.22 AAS4 60d 50.1 1.28 0.47 0.51 0.06 -

Note: a: The potential difference in chemical composition between inner and outer products is not specified, and the reported value is an average of randomly-selected points across the samples. b:Standard derivation for DoH is from 2.6% to 3.3%. c: The Ca/Si ratio of C-A-S-H is approximated based on the center of C-A-S-H clusters in Figure 7.

(a) (b) Fig. 5 Identification of inner and outer products in BSE images of AAS cured for 28 days (a) Sodium silicate-activated slag (AAS2) (9013x); (b) NaOH-activated slag (AAS3)(14779x).


(i.e. outer products) shows an increased brightness level, which suggests an increased density of C-A-S-H or/and a different composition with a higher concentration of high-atomic number elements (Ben Haha et al. 2011a; Scrivener 2004). The EDS elemental mapping of a typical feature of that dark rim surrounded slag, as shown in Fig. 6, indicates that the chemical composition of the dark hydrated rim is rich in sodium but deficient in cal-cium, compared with that of outer products. The com-position of that dark hydrated rim is more comparable to the sodium aluminosilicate hydrates (N-A-S-H). The cause of these dark rims in aged AAS is unclear, but may be associated with the ongoing hydration of slag.

3.2.2 Chemical composition Table 4 shows the average atomic ratios in hydration products of these four AAS mixtures at 7, 28, and 60 days, measured by SEM/EDS microanalysis. It should be noted that due to larger interaction volume of electron beam with specimens, the chemical composition inquired by SEM/EDS is basically a mix of various hydrated phases. It can be seen that the Ca/Si and Al/Si ratio ob-served in sodium silicate-activated slag is smaller than that in NaOH-activated slag, due to the additional [SiO2]aq from activator. In addition, AAS3 shows the highest Ca/Si ratio in hydrated phases, which may be attributed to the low pH in pore solution that facilitates the precipitation of calcium-bearing phases (e.g. limited by the solubility of portlandite). The Mg/Al and S/Ca

ratios for all AAS mixtures have a value of approxi-mately 0.40 and 0.04, respectively.

Figure 7 shows the atomic plots of Si/Ca versus Al/Ca for all four AAS mixtures cured for 7, 28, and 60 days. It is clear that C-A-S-H is not the only phase that explored by EDS, strätlingite, C4AH13, katoite, AFt, AFm, port-landite, and calcites are all likely to be present simulta-neously. For example, several data points locating be-tween the stoichiometric values of Si/Ca and Al/Ca for AFm and those of C-A-S-H clusters indicate the potential presence of AFm. Therefore, although the EDS may show similar bulk chemical composition between two mixtures, it does not necessarily indicate that the type and fraction of hydrated phases in AAS are the same. It can be seen that for NaOH-activated slag, AFm, strätlingite, and portlandite are very likely to be present, which partially agrees with the XRD results. However, for sodium silicate-activated slag, majority of the data points cluster in the region which can be approximately defined as C-A-S-H. Additionally, the Ca/Si ratio of C-A-S-H clusters observed for sodium silicate-activated slag is generally smaller than that of NaOH-activated slag, as listed in Table 4. For sodium silicate-activated slag at 60 days, there are a cluster of data points with Si/Ca ratio up to 1.75 and Al/Si ratio up to 0.42. This chemical composition may be linked to the hydrated phase shown in the dark hydrated rim, and defined as the N-A-S-H phase.

Fig. 6 BSE image (1700x) and corresponding elemental maps to distingush the chemical distrubutions between inner products and outer products formed in aged AAS1.


3.3 Thermodynamic modeling Figure 8 shows the evolution of phase assemblage of these four AAS mixtures as a function of DoH. It shows that the type of hydrated phases is dependent on the initial pH and modulus n of the activator. For NaOH-activated slag, the simulated hydrated phases are C-A-S-H, hydrotalcite, katoite, AFm (monosul-fate-C4AH13 non-ideal solid solution). In addition, strätlingite can be found at AAS3 at larger extent of slag hydration, and portlandite is formed in AAS4. Regarding the DoH investigated in the experimental work, the thermodynamic modeling (at the same DoH) provides a good representation of the hydrated phase assemblage for

NaOH-activated slag. For example, the C-A-S-H, hy-drotalcite, and portlandite were all detected by XRD and EDS, while the AFm was detected in AAS3 by XRD and EDS. The AFm phases in AAS may exist an intimate mixture of C-S-H layers and AFm-like arrangements containing Al in octahedral sites, thus not distinguishable by XRD (Bonk et al. 2003). Another potential reason is that the AFm formed in AAS is unstable at atmosphere due to extensive carbonation. For NaOH-activated slag, the main discrepancy between thermodynamic modeling and experimental results is the presence of katoite. However, although katoite may present in NaOH-activated slag, it is hard to detect using the

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adopted techniques, probably due to kinetics effects of phase formation or its amorphous nature.

For sodium silicate-activated slag, the main hydrated phases predicted by thermodynamic modeling are C-A-S-H, AFm, strätlingite, hydrotalcite, and trace of ettringite in AAS2. The main discrepancy between thermodynamic modeling and experimental results is the presences of strätlingite and ettringite. The ettringite is predicted to exist in AAS2 probably due to its lowest pH environment at which ettringite can be stable. The et-tringite in realistic AAS2, if present, tends to be unstable as well. The predicted amount of strätlingite increases with the increasing modulus of activator, implying that [SiO2]aq can promote the formation of strätlingite. However, the strätlingite is probably in a form of solid solution with other AFm phases, when sodium silicate is incorporated. Regarding the effects of [SiO2]aq on the reaction mechanisms of AAS, it is suggested that the [SiO2]aq may react with portlandite forming additional C-S-H, and with katoite or AFm forming additional strätlingite.

Figure 9 (a) shows the evolution of Ca/Si, Al/Si, and Na/Si ratios in C-A-S-H as hydration progresses. It was

found that the Ca/Si ratio for sodium silicate-activated slag is smaller than that of NaOH-activated slag, while the Al/Si ratio is similar for all AAS systems as hydration degree excesses 40%. This observation is consistent with the EDS results (shown in Fig. 7), and may suggest the stable incorporation of Al in the dreierketten silicate structure. However, the current thermodynamic model-ing cannot provide the details regarding the difference of chemical composition (e.g. Ca/Si ratio) between inner and outer products in AAS microstructure. The spatial distribution of hydrated phases with varying chemical composition in the microstructure seems to be important for aged AAS.

The evolution of pH in pore solution (see Fig. 9(b)) shows that pH tends to decrease as hydration continues due to the consumption of OH- for dissolving slag. However, the pH remains high (> 13.0) even after com-plete hydration of slag. Although AAS1 and AAS3 mixtures have almost the same initial pH value (14.31 and 14.30) in activator prior to mixing, AAS1 keeps relatively higher pH value than AAS3 during hydration, probably due to the buffering effects of silicate.

Figure 10 shows the comparison of the measured

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(a) (b) Fig. 10 Comparison of the EDS measured bulk chemical composition of hydrated phases with thermodynamic simulation (a) AAS1 and AAS2; (b) AAS3 and AAS4 (the data point is measured by EDS as shown in Table 4).

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atomic ratio of hydrated phases (see Table 4) against the predicted values by thermodynamic modeling. Consid-ering that EDS gives the mixed chemical compositions of various hydrated products, the thermodynamic calcula-tion outputs the chemical composition of all predicted solid phases. It can be seen that thermodynamic model-ing shows reasonable agreement with experimental measurements. For NaOH-activated slag, thermody-namic modeling predicts that the Ca/Si, Al/Si, and Mg/Al, and Si/Ca almost remain constant during hydration, re-gardless of pH. For sodium silicate-activated slag, these atomic ratios vary dramatically at low DoH, and then gradually become stable. This is because the [SiO2]aq added from activator is assumed to be fully reacted at each DoH under thermodynamic equilibrium condition. As hydration continues, the effects of [SiO2]aq are diluted and the chemistry of slag becomes dominating, making the overall chemistry system stable. However, it should be noted that thermodynamic calculation assumes chemical equilibrium over the whole system, which is attained only locally on a scale of microns or less due to difficulty of ionic transport within the materials. There-fore, the dramatic change of phase type is not commonly observed in experiments since equilibrium state is not likely be attained for realistic AAS samples, especially at early-age hydration.

The measured Ca/Si ratio fluctuates around the pre-dicted value, and tends to slightly decrease over time. The measured Ca/Si ratio for AAS2 is higher than the predicted value at early age, indicating that the [SiO2]aq is actually not fully-reacted in realistic circumstance at early-age hydration. These discrepancies are probably due to the different dissolution kinetics of Ca and Si ions

for slag, as well as the time-dependent reactivity of [SiO2]aq from activator. The predicted Al/Si and S/Ca ratios are strongly consistent with the measured values, supporting the presence of hydrotalcite and AFm phases. The predicted Mg/Al is slightly higher than that meas-ured, probably because of adopted databases of hydro-talcite does not completely represent thermodynamic behaviors of hydrotalcite-type phases with varying Mg/Al ratio.

3.4 Chemical shrinkage Figure 11 shows the chemical shrinkage per gram of slag mixed, which does not account for the actual fraction of the slag that has reacted at a given age. For samples with a size of 25 grams, it can be seen that NaOH-activated slag has slightly higher chemical shrinkage than that of sodium silicate-activated slag. This may be due to the coarser pore structure (and therefore higher permeabil-ity) of NaOH-activated slag as evidenced by the BSE images in Figs. 2-4. However, for samples with a size of 10 grams, AAS2 and AAS4 has comparable chemicals shrinkage at 7 days and larger than that of AAS1 and AAS3. It shows that the measured chemical shrinkage using buoyancy method is dependent on the sample size and the potential causes for this phenomenon are dis-cussed by Cartwright et al. (Cartwright et al. 2013).

The chemical shrinkage of 10 grams AAS samples was normalized based on the estimated DoH at 7 days. It can be seen, from Table 5, that the chemical shrinkage of AAS2 was the largest (0.1389 ml/gslag), while AAS3 (0.0977 ml/gslag) was the lowest. It is likely that the larger concentration of aqueous silica in AAS2 contributes to the higher chemical shrinkage observed in this mixture.

Table 5 Chemical shrinkage of AAS.

Chemical Shrinkage (CS) (ml/gslag) AAS1 10g AAS2 10g AAS3 10g AAS4 10g Raw CS measured at 7 days 0.0338 0.0443 0.0299 0.0445 Normalizeda CS 0.1280 0.1389 0.0977 0.1199 CS predicted by thermodynamic modeling 0.1157 0.1122 0.1185 0.1144

Note: a: Normalization was performed by assuming 100% hydration of slag using the DoH estimated by backscattered image analysis

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The average chemical shrinkage of AAS paste is 0.1211 ml/gslag, which is consistent with the theoretical estima-tions of other authors (Chen and Brouwers 2007; Thomas et al. 2012). The values of chemical shrinkage at com-plete hydration for OPC is approximately 0.064 ml/gcem (Jensen and Hansen 2001; Powers and Brownyard 1946). Therefore, the chemical shrinkage of AAS is about twice as large as the chemical shrinkage of OPC and contrib-utes directly to the large autogenous shrinkage of AAS.

As shown in Table 5, the predicted chemical shrinkage by thermodynamic modeling agrees with that measured in this study well. However, the chemical shrinkage estimated by thermodynamic modeling is less sensitive to activators. For instance, the predicted chemical shrinkage of AAS2 by thermodynamic modeling is similar to that of AAS4, and smaller than the normalized value. The discrepancy can potentially originate from the absence of ettringite in realistic AAS2, since ettringite is voluminal expansive. In comparison to OPC, the higher chemical shrinkage of AAS may be attributed to the smaller fraction of expansive crystalline hydrates (e.g. ettringite and portlandite) in phase assemblage.

4. Conclusions

In this paper, the hydrated phase assemblage and chemical shrinkage of four distinct AAS were analyzed using various experimental techniques and thermody-namic modeling. The following conclusions can be drawn based on this study: (1) Hydrated rim around slag exists in both NaOH- and

sodium silicate-activated slag. However, for NaOH-activated slag, hydration products are pref-erentially formed around slag particles showing a hydrated rim, while for sodium silicate-activated slag, hydration products are initialized at both slag surface and inter-particle spaces simultaneously.

(2) For aged AAS, the a dark hydrated rim was distinc-tively observed around unhydrated slag, which was shown by EDS mapping to have a lower Ca but high Na, Al and Si content, in comparison to the outer products. The chemical composition of dark rim re-sembles that of N-A-S-H.

(3) Thermodynamic modeling provides a reasonable estimation of the phase assemblage in AAS, and reasonably matches the measured bulk chemical composition of hydrated phases using EDS. How-ever, it does not provide the spatial distribution of composition in AAS microstructure, which may be significantly different for aged AAS.

(4) The main hydration products in AAS are C-A-S-H and hydrotalcite-type phases, while other hydrated phases (e.g. strätlingite, katoite, portlandite, AFm, and ettringite) are dependent on the activator, based on the thermodynamic calculation.

(5) The [SiO2]aq incorporates into solid phase formation of AAS as a function of time, and promotes the formation of strätlingite, which may be attributed to

the reaction of [SiO2]aq with katoite or AFm phases. (6) The average chemical shrinkage of AAS paste was

determined to be 0.1211 ml/gslag, which is about twice larger than that of OPC.

(7) The chemical shrinkage of AAS is dependent on the activator, and increases with the increasing modulus and pH of activator.

(8) Based on the phase assemblage, the higher chemicals shrinkage of AAS may be attributed to a small frac-tion of expansive crystalline phases, such as ettring-ite and portlandite, in comparison to OPC.

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