Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C3S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C–S–H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C3S hydration was significantly extended, the degree of hydration of C3S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C3S in the C3S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C3S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C–S–H in the C3S-HVFA paste directly indicate that Al substitutes for Si in C–S–H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C–S–H in C3S-HVFA system and presented results consistent with previous literature.


Introduction
High-volume fly ash (HVFA) concrete, in which more than 50 wt % of Portland cement (PC) is replaced by fly ash, has been successfully employed in building structures to mitigate CO 2 emissions during PC clinker production [1]. A significant problem with HVFA concrete is that its initial strength development rate is slower than that of PC concrete due to its lower cement content [2]. However, HVFA enhances properties of concrete, such as its workability, durability, and long-term compressive strength, and performance losses during the early ages can be overcome by optimizing the particle size distribution of the fly ash and the concrete mix proportions [3][4][5]. The enhanced properties of concrete due to HVFA incorporation result from an increase in the number of nucleation sites for PC hydrates [4,5] and from the pozzolanic reaction between Ca(OH) 2 and the amorphous aluminosilicate-rich ash [6,7]. shifts of C 3 S hydration products [29,30] and of C-S-H synthesized with different Ca/Si ratios at the Ca L III,II edge and Si K edge in the NEXAFS spectrum [29]. This powerful technique also enables to characterize the kinetics of early age cement clinker hydration in situ by employing a wet cell [31]. Interpretations derived from STXM analysis, therefore, could be supplementary information regarding the effect of HVFA on both morphology and atomic binding structure of C-S-H.
The purpose of the present paper is to demonstrate the effects of HVFA on C 3 S hydration by examining the hydration kinetics, morphology of the hydration products, and atomic binding structure of C-S-H. Pure triclinic C 3 S, which has been suggested to be the primary clinker phase in PC, was utilized with and without 50 wt % Class F fly ash. STXM was employed to examine the morphological and atomic binding structural differences in a local area of C-S-H in the pure C 3 S and C 3 S-HVFA systems. The NEXAFS of the Ca L III,II edge, Al, and Si K edge was analyzed to quantitatively determine the effect of HVFA on silicate polymerization and Al incorporation into C-S-H. The hydration products in the HVFA system were identified, and the amount of Ca(OH) 2 was determined by synchrotron powder X-ray diffraction (XRD) and thermogravimetric analysis (TG/DTG). In addition to XRD and TG/DTG, isothermal conduction calorimetry was performed to monitor the rate of heat evolution in the early hydration process. 27 Al and 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was employed to confirm Al incorporation into C-S-H and determine the degree of silicate polymerization by comparing the C-S-H silicate chain length in the pure C 3 S and C 3 S-HVFA systems. The morphological details and Ca/Si ratio of C-S-H in the C 3 S-HVFA paste were investigated by transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDX). Based on the experimental results, the schematic structure of C-S-H in the HVFA system was suggested.

Materials and Sample Preparation
Triclinic C 3 S and Class F fly ash which contains a high amount of SiO 2 (62.00 wt %) and Al 2 O 3 (18.90 wt %) but less CaO (5.98 wt %), were used as raw materials (see Table 1). C (pure C 3 S) and C-FA (50 wt % C 3 S, 50 wt % fly ash) pastes were prepared by hand-mixing the raw materials with distilled water. For all samples, the water-to-solid ratio was maintained at 0.5. The hand-mixed pastes were cast into PELCO ® disc block embedding molds (ϕ18 × 3 mm) and cured at 23 ± 1 • C and 95% ± 5% relative humidity. The pastes were demolded after 1, 3, 7, or 28 days, and ethanol was used to stop the hydration process. The samples were then kept under vacuum in carbonation-free conditions until testing. For isothermal conduction calorimetry, two test samples containing 1 gram C 3 S and 0 or 50% fly ash were prepared with a water-to-solid ratio of 0.5. Finely ground powder samples were used for the XRD, TG/DTG and TEM experiments. All samples were stored in a vacuum bag and ground just before measurement to minimize their carbonation. For the STXM measurements, thin-sectioned samples (~200 µm thick slices) were cut from the hardened pastes with a diamond wafering blade and then mechanically ground and polished. Kerosene was used to avoid any further hydration of the samples during these procedures. A small piece (~3 × 3 mm 2 ) of the thin-sectioned sample was placed on a copper grid with a single slot in the center for ion milling. The sample was ion milled by PIPS™ at the National Center for Electron Microscopy (NCEM), and stored in a box, and sealed in a vacuum bag. The details of the sample preparation for the TEM and NMR experiments are described in later sections.

Monochromatic Synchrotron X-ray Diffraction (XRD)
XRD experiments were performed at beamline 12.2.2 of the ALS at LBNL. This beamline uses a monochromatic synchrotron X-ray (λ = 0.6199 Å) focused to an area of 10 × 10 µm 2 . To calibrate the working distance between the sample and the diffraction detector, a LaB 6 powder diffraction standard was used. The collected two-dimensional XRD patterns using an MAR345 image plate (3450 × 3450 pixels) were converted into one-dimensional intensity versus 2-theta scans using the Fit2D program [32].

Scanning Transmission X-ray Microscopy (STXM)
STXM data were collected at ALS beamlines 5.3.2.1 and 5.3.2.2 with the synchrotron storage ring operating at 1.9 GeV and 500 mA. The counting times were on the order of a few milliseconds or less per pixel and the total counting times for each single image occurred at a frequency of every few minutes depending on the number of total pixels. Four types of images were obtained: (1) single images; (2) image contrast maps; (3) image stacks; and (4) RGB overlay maps. The details of STXM images can be found elsewhere [29][30][31]. The RGB overlay maps were created by singular value decomposition (SVD) based on reference spectra [29,33]. CaCO 3 , CO 2 , quartz, and 100 nm-thick aluminum foil were used as the references for the Ca L, Si K, and Al K edges, respectively, to calibrate the energy positions to maintain consistency with previous studies [29,31,34,35].

27 Al and 29 Si Magic-Angle Spinning Nuclear Magnetic Resonance (MAS NMR) Spectroscopy
For NMR measurements, 28 day-hydrated C and C-FA were finely ground and evenly packed into a 3.2 mm zirconia rotor, and the open end was sealed with a Vespel cap. The rotor was spun at 20 kHz using a Bruker UltraShield 600 WB Plus with a 14.1 T magnet operating at 156.40 MHz for 27 Al and at 119.23 MHz for 29 Si. For both sets of experiments, the dwell time was 7.0 µs, and 1024 and 4096 acquisitions were collected in the 27 Al and 29 Si measurements, respectively. The magic angle was set to 54.734 • in both cases and KBr was used as the reference. For 27 Al MAS NMR, the samples were spun for no longer than 40 min to prevent dehydration and any consequential loss in peak intensity [36]. 27 Al MAS NMR spectrum can be difficult to interpret because 27 Al is affected by quadrupolar interactions, leading to peak broadening as well as a field-dependent shift in the resonance position [23,37]. Whereas high-quality 29 Si NMR spectrum require long acquisition times, the acquisition time for 27 Al is much shorter less than the former due to its spin nuclei of I = 5 / 2 and natural abundance of 99.99%. Thus, good-quality spectrum can be obtained even from time-limited measurements. In previous works that utilized the same equipment, the probe exhibited a large Al background [38]. For this reason, a sample containing only C 3 S, which should have no Al signal, was analyzed using the same acquisition setup as the other samples based on the assumption that the signal obtained would be the relevant background signal and could be removed from the other spectrum. However, no signal was detected, and therefore, no background removal was necessary. The mean chain length (MCL) of C-S-H formed in the C and C-FA paste along with the Al/Si ratio of C-S-H in the C-FA paste were calculated using the following equations based on the assumption that Al did not occupy end-chain tetrahedral sites [12].

Transmission Electron Microscopy (TEM) Imaging
TEM imaging was performed using a Titan G2 80-300 kV microscope (FEI Company, Hillsboro, OR, USA) equipped with a 4 k × 4 k CCD camera (Gatan, Pleasanton, CA, USA) and EDX detector operating at 300 kV. The imaging/X-ray analysis was performed using the ISIS software (Oxford Instruments, Oxford, UK). The powder of the C-FA paste hydrated for 28 days was suspended in ethanol, and 2 µL of the suspension was deposited on a holey carbon film and dried for five minutes before TEM observation. The selected area electron diffraction (SAED) technique was employed to determine the crystallinity of each area before chemical analysis.

Thermogravimetric Analysis (TG/DTG) and Isothermal Conduction Calorimetry
TG/DTG tests were performed using ground C and C-FA samples that were aged for 1, 3 and 28 days. The samples were heated from room temperature to 1000 • C at a rate of 10 • C/min. Approximately 25 mg of finely ground powder was placed in a ceramic crucible and heated in Exstar 6300 TG/DTA instrument (Seiko Instruments Inc., Chiba, Japan). The nitrogen gas flow rate was 50 mL/min.
The heat flow of C and C-FA in the early age was monitored in an isothermal conduction calorimeter (TAM-air, TA instruments, New Castle, DE, USA) at a constant temperature of 23 • C. Deionized water and powder was hand-mixed inside of a glass ampule for 2 min. The paste was tightly sealed immediately after mixing and placed in the isothermal calorimeter. For the powder containing 50% fly ash, C 3 S and fly ash were hand-mixed for a minute before adding water. Figure 1 presents the XRD patterns of the pure C 3 S and C 3 S-HVFA pastes after 1, 3, 7 and 28 days of curing. The XRD patterns contained few unreactive crystalline quartz phases, anhydrous C 3 S or Ca(OH) 2 . No other phases, such as C-S-H(I), AFt, AFm, monocarbonate, or strätlingite, were observed. The Ca(OH) 2 peak intensity of the C 3 S-HVFA paste was less than that of the pure C 3 S paste due to the dilution of C 3 S by fly ash. The Ca(OH) 2 peak intensity of the pure C 3 S system increased as the hydration period increased from 1 day to 28 days. However, in the C 3 S-HVFA system, the Ca(OH) 2 peak intensity decreased as the hydration period increased from 7 days to 28 days because of the pozzolanic reaction between Ca(OH) 2 and the aluminosilicate phase. Synchrotron powder X-ray diffraction patterns for C3S (C) and C3S substituted 50% by fly ash (C-FA) pastes. •: Quartz, ▼: Portlandite (Ca(OH)2). Relative intensities above 5% were classified using the ICDD PDF2 database reference 00-087-0674.

TG/DTG and Isothermal Conduction Calorimetry Results
TG/DTG analysis was performed ( Figure 2) to compare the quantity of Ca(OH)2. Based on the TG/DTG mass loss, the amount of Ca(OH)2 produced in the pure C3S and C3S-HVFA samples was calculated as a percent of the total weight ( Figure 3). The amount of Ca(OH)2 in the pure C3S pastes gradually increased as the hydration period increased up to 28 days. In contrast to the pure C3S results, a very small amount of Ca(OH)2 (0.71 wt %) was observed in the C-FA-1d sample, and it increased significantly when the hydration period was increased to 3 days.
This retarding effect of fly ash on the early PC hydration has been previously observed [5,39]. To verify the hydration retardation due to HVFA incorporation after 1 day, the rate of heat evolution and the cumulative heat evolved for pure C3S and the C3S substituted with 50% by fly ash were measured by isothermal conduction calorimetry ( Figure 4). 5 1 0 1 5 2 0 2 5 3 0 3 5 C-FA-28d Figure 1. Synchrotron powder X-ray diffraction patterns for C 3 S (C) and C 3 S substituted 50% by fly ash (C-FA) pastes. : Quartz, : Portlandite (Ca(OH) 2 ). Relative intensities above 5% were classified using the ICDD PDF2 database reference 00-087-0674.

TG/DTG and Isothermal Conduction Calorimetry Results
TG/DTG analysis was performed ( Figure 2) to compare the quantity of Ca(OH) 2 . Based on the TG/DTG mass loss, the amount of Ca(OH) 2 produced in the pure C 3 S and C 3 S-HVFA samples was calculated as a percent of the total weight ( Figure 3). The amount of Ca(OH) 2 in the pure C 3 S pastes gradually increased as the hydration period increased up to 28 days. In contrast to the pure C 3 S results, a very small amount of Ca(OH) 2 (0.71 wt %) was observed in the C-FA-1d sample, and it increased significantly when the hydration period was increased to 3 days.
This retarding effect of fly ash on the early PC hydration has been previously observed [5,39]. To verify the hydration retardation due to HVFA incorporation after 1 day, the rate of heat evolution and the cumulative heat evolved for pure C 3 S and the C 3 S substituted with 50% by fly ash were measured by isothermal conduction calorimetry ( Figure 4). . Relative intensities above 5% were classified using the ICDD PDF2 database reference 00-087-0674.

TG/DTG and Isothermal Conduction Calorimetry Results
TG/DTG analysis was performed ( Figure 2) to compare the quantity of Ca(OH)2. Based on the TG/DTG mass loss, the amount of Ca(OH)2 produced in the pure C3S and C3S-HVFA samples was calculated as a percent of the total weight ( Figure 3). The amount of Ca(OH)2 in the pure C3S pastes gradually increased as the hydration period increased up to 28 days. In contrast to the pure C3S results, a very small amount of Ca(OH)2 (0.71 wt %) was observed in the C-FA-1d sample, and it increased significantly when the hydration period was increased to 3 days.
This retarding effect of fly ash on the early PC hydration has been previously observed [5,39]. To verify the hydration retardation due to HVFA incorporation after 1 day, the rate of heat evolution and the cumulative heat evolved for pure C3S and the C3S substituted with 50% by fly ash were measured by isothermal conduction calorimetry ( Figure 4). 5 1 0 1 5 2 0 2 5 3 0 3 5      Weight of Ca(OH) Paste Type (a)    Weight of Ca(OH) Paste Type (a)   The end of the induction period of C and C-FA occurred at approximately 1.5 and 15 h, respectively. Compared to pure C3S, however, the heat flow peak height increased for C-FA per mass of C3S after the induction period. As a result, C-FA showed a higher normalized cumulative heat evolution after 33 h relative to pure C3S. Based on the results, the degree of C3S hydration (α) was obtained by dividing the accumulated heat evolution by the enthalpy of the following C3S reaction: where x is the molar water content of C-S-H [40]. The α at 24 h for C-FA (α = 0.05) was considerably less than that of C (α = 0.35). However, after 60 h, the α for C-FA (α = 59) significantly increased compared to that of C (α = 43), which is consistent with the results of XRD and TG-DTG at 1 and 3 days. The results of XRD and thermal analysis indicate that the incorporation of HVFA into the C3S system significantly retards the early age hydration of C3S and extends the induction period of C3S resulting in the delay of C-S-H and Ca(OH)2 formation before 1 day of hydration. However, HVFA enhances the precipitation and growth of hydration products through the acceleration and deceleration period (the main hydration peak of isothermal calorimetry) as observed in the considerably delayed increase of the Ca(OH)2 in C-FA after 3 days. The morphological details of the C3S in the pure C3S and C3S-HVFA systems will be further discussed using STXM analysis.

STXM Analysis
The STXM images and Ca LIII,II edge NEXAFS spectra of C-28d and C-FA-28d are presented in Figure 5. Due to the low X-ray photon energies (340-360 eV) employed in the Ca LIII,II edge NEXAFS analysis, the sample areas that were thin enough for X-ray beam penetration and had a uniform Ca distribution in the image contrast map were selected for analysis (Figure 5b,d). The Ca LIII,II edge NEXAFS spectra obtained from the regions of interest (ROIs) and those of anhydrous C3S [29] are shown in Figure 5e. Each spectrum was normalized to the incident beam intensity (I0), and the linear pre-edge background was removed. Two main spin-orbit-related peaks, LIII 2p3/2 (a3) and LII 2p1/2 (b2), are observed at the Ca LIII,II edge of calcium compounds along with several smaller peaks (a1, a2, b1) that precede the main peaks [41]. These multi-peak patterns in Ca LIII,II edge NEXAFS are occurred by the crystal field due to the symmetry of the atoms in the first coordination sphere of the Ca cation [42]. The peak positions and energy separations in Ca NEXAFS depend on the Ca symmetry and the magnitude of the crystal field (10Dq) [42]. The peak positions and energy separations of the reference of anhydrous C3S reference [29], Ca(OH)2 reference [29], and the ROIs in C-28d and C-FA-28d are shown in Table 2. The end of the induction period of C and C-FA occurred at approximately 1.5 and 15 h, respectively. Compared to pure C 3 S, however, the heat flow peak height increased for C-FA per mass of C 3 S after the induction period. As a result, C-FA showed a higher normalized cumulative heat evolution after 33 h relative to pure C 3 S. Based on the results, the degree of C 3 S hydration (α) was obtained by dividing the accumulated heat evolution by the enthalpy of the following C 3 S reaction: where x is the molar water content of C-S-H [40]. The α at 24 h for C-FA (α = 0.05) was considerably less than that of C (α = 0.35). However, after 60 h, the α for C-FA (α = 59) significantly increased compared to that of C (α = 43), which is consistent with the results of XRD and TG-DTG at 1 and 3 days. The results of XRD and thermal analysis indicate that the incorporation of HVFA into the C 3 S system significantly retards the early age hydration of C 3 S and extends the induction period of C 3 S resulting in the delay of C-S-H and Ca(OH) 2 formation before 1 day of hydration. However, HVFA enhances the precipitation and growth of hydration products through the acceleration and deceleration period (the main hydration peak of isothermal calorimetry) as observed in the considerably delayed increase of the Ca(OH) 2 in C-FA after 3 days. The morphological details of the C 3 S in the pure C 3 S and C 3 S-HVFA systems will be further discussed using STXM analysis.

STXM Analysis
The STXM images and Ca L III,II edge NEXAFS spectra of C-28d and C-FA-28d are presented in Figure 5. Due to the low X-ray photon energies (340-360 eV) employed in the Ca L III,II edge NEXAFS analysis, the sample areas that were thin enough for X-ray beam penetration and had a uniform Ca distribution in the image contrast map were selected for analysis (Figure 5b,d). The Ca L III,II edge NEXAFS spectra obtained from the regions of interest (ROIs) and those of anhydrous C 3 S [29] are shown in Figure 5e. Each spectrum was normalized to the incident beam intensity (I 0 ), and the linear pre-edge background was removed. Two main spin-orbit-related peaks, L III 2p 3/2 (a 3 ) and L II 2p 1/2 (b 2 ), are observed at the Ca L III,II edge of calcium compounds along with several smaller peaks (a 1 , a 2 , b 1 ) that precede the main peaks [41]. These multi-peak patterns in Ca L III,II edge NEXAFS are occurred by the crystal field due to the symmetry of the atoms in the first coordination sphere of the Ca cation [42]. The peak positions and energy separations in Ca NEXAFS depend on the Ca symmetry and the magnitude of the crystal field (10Dq) [42]. The peak positions and energy separations of the reference of anhydrous C 3 S reference [29], Ca(OH) 2 reference [29], and the ROIs in C-28d and C-FA-28d are shown in Table 2.  As shown in Figure 5e, the Ca LIII,II edge absorption features (peak positions and intensity ratios) of anhydrous C3S [29] differed from those in the ROIs of C-28d (Area 1) and C-FA-28d (Areas 2-4). The ROIs in C-28d and C-FA-28d exhibited similar absorption features to those in the C-S-H produced by C3S hydration and synthesized C-S-H [29]. The decrease in the peak intensity ratio and energy separations (a3 − a2 and b2 − b1), which indicate that the crystal field and Ca symmetry were lower, could be caused by C-S-H formation.
In the present study, the peak positions in the Ca LIII,II edge of C-S-H in C-28d and C-FA-28d were similar. Previous STXM work [29] demonstrated that synthesized C-S-H with different molar ratios of Ca/Si showed no variation in peak positions or energy separation in the Ca LIII,II edge NEXAFS spectra, which implies that the silicate polymerization in C-S-H does not have a great influence on the Ca binding energy in C-S-H. Similarly, in the present study, the additional silicate and Al substitution in the C3S-HVFA system did not have a significant effect on the C-S-H Ca LIII,II edge NEXAFS spectrum, indicating that the enhanced silicate polymerization and Al uptake in C-S-H have minor effects on the Ca binding energy in the CaO layer of C-S-H.
Si and Al image contrast maps ( Figure 6) were generated by subtracting the images taken below the Si and Al K edges, respectively, from those taken above [34,43]. The bright areas in the image contrast maps represent a high concentration of the element [43]. C3S particles in C-28d ( Figure 6b) had a core area and a Si-rich C-S-H layer that was brighter than the core area. Moreover, the exact boundary between the areas was clearly resolved. It should be noted that the C-S-H layer had a   As shown in Figure 5e, the Ca L III,II edge absorption features (peak positions and intensity ratios) of anhydrous C 3 S [29] differed from those in the ROIs of C-28d (Area 1) and C-FA-28d (Areas 2-4). The ROIs in C-28d and C-FA-28d exhibited similar absorption features to those in the C-S-H produced by C 3 S hydration and synthesized C-S-H [29]. The decrease in the peak intensity ratio and energy separations (a 3 − a 2 and b 2 − b 1 ), which indicate that the crystal field and Ca symmetry were lower, could be caused by C-S-H formation.
In the present study, the peak positions in the Ca L III,II edge of C-S-H in C-28d and C-FA-28d were similar. Previous STXM work [29] demonstrated that synthesized C-S-H with different molar ratios of Ca/Si showed no variation in peak positions or energy separation in the Ca L III,II edge NEXAFS spectra, which implies that the silicate polymerization in C-S-H does not have a great influence on the Ca binding energy in C-S-H. Similarly, in the present study, the additional silicate and Al substitution in the C 3 S-HVFA system did not have a significant effect on the C-S-H Ca L III,II edge NEXAFS spectrum, indicating that the enhanced silicate polymerization and Al uptake in C-S-H have minor effects on the Ca binding energy in the CaO layer of C-S-H.
Si and Al image contrast maps ( Figure 6) were generated by subtracting the images taken below the Si and Al K edges, respectively, from those taken above [34,43]. The bright areas in the image contrast maps represent a high concentration of the element [43]. C 3 S particles in C-28d ( Figure 6b) had a core area and a Si-rich C-S-H layer that was brighter than the core area. Moreover, the exact boundary between the areas was clearly resolved. It should be noted that the C-S-H layer had a uniform Si concentration, implying that silicate polymerization occurred uniformly throughout the layer, in agreement with a previous STXM study [29].
In the C 3 S-HVFA system, the fly ash particles had greater Si and Al concentrations after hydration for 28 days as shown by the C-FA-28d Si and Al image contrast maps (Figure 6d,e). It should be noted that in contrast to the C 3 S particles in the pure C 3 S system, those in the C 3 S-HVFA system, which did not contain Al as shown by the image contrast map, had a very uniform Si concentration and no obvious C-S-H layer on C 3 S. This was due to the enhanced dissolution of the C 3 S particles in the presence of HVFA, as determined by thermal analysis. uniform Si concentration, implying that silicate polymerization occurred uniformly throughout the layer, in agreement with a previous STXM study [29].
In the C3S-HVFA system, the fly ash particles had greater Si and Al concentrations after hydration for 28 days as shown by the C-FA-28d Si and Al image contrast maps (Figure 6d,e). It should be noted that in contrast to the C3S particles in the pure C3S system, those in the C3S-HVFA system, which did not contain Al as shown by the image contrast map, had a very uniform Si concentration and no obvious C-S-H layer on C3S. This was due to the enhanced dissolution of the C3S particles in the presence of HVFA, as determined by thermal analysis.   Figure 6a. In the Si K edge NEXAFS spectrum (Figure 7e), peak a1, the main peak, was attributed to the Si K edge induced by the electronic transition from the Si 1s orbital to the anti-bonding t2 (3p-like state) orbital. The a2 peak was assigned to the multiple scattering effect beyond the second coordination sphere [21].
In the present study, the main peak (a1) for the C-S-H layer (Areas 2, 4 and 5) on C3S had a lower energy than that of anhydrous C3S (Figure 7e). Alternatively, the core area of the C3S particles (Areas 1 and 3) had a similar or even higher peak energy than anhydrous C3S. The broad widths of peaks a1 and a2 of the core area were due to the transmission X-ray signals penetrating through the core area of C3S, which contains both the residual anhydrous C3S and C-S-H on C3S.
During C3S hydration, the Si in the SiO4 monomer that dissolved from the outer area of C3S during hydration had a lower binding energy than the Si in anhydrous C3S due to the smaller silicate environmental network (Q n ), resulting in a decrease in the Si K edge energy in the C-S-H layer on C3S [29]. In contrast, the a2 peak of the C-S-H layer (Areas 2, 4, and 5) was shifted to a higher energy (by +(3.6-4.1) eV) than that of anhydrous C3S. Thus, the energy separation between the a1 and a2 peaks   Figure 6a. In the Si K edge NEXAFS spectrum (Figure 7e), peak a 1 , the main peak, was attributed to the Si K edge induced by the electronic transition from the Si 1s orbital to the anti-bonding t 2 (3p-like state) orbital. The a 2 peak was assigned to the multiple scattering effect beyond the second coordination sphere [21].
In the present study, the main peak (a 1 ) for the C-S-H layer (Areas 2, 4 and 5) on C 3 S had a lower energy than that of anhydrous C 3 S (Figure 7e). Alternatively, the core area of the C 3 S particles (Areas 1 and 3) had a similar or even higher peak energy than anhydrous C 3 S. The broad widths of peaks a 1 and a 2 of the core area were due to the transmission X-ray signals penetrating through the core area of C 3 S, which contains both the residual anhydrous C 3 S and C-S-H on C 3 S.
During C 3 S hydration, the Si in the SiO 4 monomer that dissolved from the outer area of C 3 S during hydration had a lower binding energy than the Si in anhydrous C 3 S due to the smaller silicate environmental network (Q n ), resulting in a decrease in the Si K edge energy in the C-S-H layer on C 3 S [29]. In contrast, the a 2 peak of the C-S-H layer (Areas 2, 4, and 5) was shifted to a higher energy (by +(3.6-4.1) eV) than that of anhydrous C 3 S. Thus, the energy separation between the a 1 and a 2 peaks increased. This implies that the contribution from multiple scattering increased more than the Si 1s binding energy due to the higher degree of silicate polymerization leading to a more developed silicate network. The RGB overlay map obtained by SVD using the NEXAFS spectra of Areas 1 (core area) and Area 4 (C-S-H layers) illustrates the distribution of the C-S-H layer and core area. The RGB overlay map clearly differentiated between the uniform C-S-H layers and C 3 S core areas in C-28d (Figure 7d). The C-S-H layer that formed on C 3 S in the early age of hydration retarded the hydration of the C 3 S core area in the pure C 3 S system, resulting in an anhydrous C 3 S core area after 28 days of hydration. increased. This implies that the contribution from multiple scattering increased more than the Si 1s binding energy due to the higher degree of silicate polymerization leading to a more developed silicate network. The RGB overlay map obtained by SVD using the NEXAFS spectra of Areas 1 (core area) and Area 4 (C-S-H layers) illustrates the distribution of the C-S-H layer and core area. The RGB overlay map clearly differentiated between the uniform C-S-H layers and C3S core areas in C-28d (Figure 7d). The C-S-H layer that formed on C3S in the early age of hydration retarded the hydration of the C3S core area in the pure C3S system, resulting in an anhydrous C3S core area after 28 days of hydration.  (Figure 6d), the Si concentration of C-S-H in C-FA-28d, which contained a small amount of Al, appeared to be very uniform and without anhydrous C3S. The fly ash particles and the areas assumed to be C-S-H were analyzed by Si and Al NEXAFS spectroscopy to investigate the degree of silicate polymerization in the C3S-HVFA system and to compare the spectra to the C-S-H obtained in the pure C3S system. The fly ash particles were differentiated from hydrated C3S based on their spherical shape and high Al concentration. The a1 peak energies of C-S-H were less (from −0.6 to −0.9 eV) than that of anhydrous C3S. In addition, the peak energies of the C-FA-28d ROIs were lower (from 0 to −0.3 eV) than those of the C-S-H layer on C3S (Area 5 in Figure 7c). Alternatively, the a2 peak energies of C-S-H in C-FA-28d were greater (+(0.2-1.2) eV) than those of the C-S-H layer on C3S in C-28d. The a2 peak of anhydrous C3S (1858.9 eV) [29], which was observed in C-28d, was not detected, implying that the C3S in C-FA-28d was fully hydrated. In addition, the Al K edge NEXAFS analysis (Figure 8f) shows that C-S-H in C-FA-28d contained much less Al than fly ash particles. The Al traces in C-S-H were assumed to result from the Al incorporation into C-S-H. The RGB overlay map obtained using the NEXAFS spectra of the fly ash and Area 2 is shown in Figure 8d. C-S-H with similar Si K edge NEXAFS features were distributed throughout C-FA-28d, which is consistent with the NEXAFS analysis results. Another area of C-FA-28d (Figure 6c-2) was selected for analysis, and the results are presented in Figure 9. All of the results were highly consistent with those of the selected area shown in Figure 8. No anhydrous C3S core area in C3S was observed in C-FA-28d.  (Figure 6d), the Si concentration of C-S-H in C-FA-28d, which contained a small amount of Al, appeared to be very uniform and without anhydrous C 3 S. The fly ash particles and the areas assumed to be C-S-H were analyzed by Si and Al NEXAFS spectroscopy to investigate the degree of silicate polymerization in the C 3 S-HVFA system and to compare the spectra to the C-S-H obtained in the pure C 3 S system. The fly ash particles were differentiated from hydrated C 3 S based on their spherical shape and high Al concentration. The a 1 peak energies of C-S-H were less (from −0.6 to −0.9 eV) than that of anhydrous C 3 S. In addition, the peak energies of the C-FA-28d ROIs were lower (from 0 to −0.3 eV) than those of the C-S-H layer on C 3 S (Area 5 in Figure 7c). Alternatively, the a 2 peak energies of C-S-H in C-FA-28d were greater (+(0.2-1.2) eV) than those of the C-S-H layer on C 3 S in C-28d. The a 2 peak of anhydrous C 3 S (1858.9 eV) [29], which was observed in C-28d, was not detected, implying that the C 3 S in C-FA-28d was fully hydrated. In addition, the Al K edge NEXAFS analysis (Figure 8f) shows that C-S-H in C-FA-28d contained much less Al than fly ash particles. The Al traces in C-S-H were assumed to result from the Al incorporation into C-S-H. The RGB overlay map obtained using the NEXAFS spectra of the fly ash and Area 2 is shown in Figure 8d. C-S-H with similar Si K edge NEXAFS features were distributed throughout C-FA-28d, which is consistent with the NEXAFS analysis results. Another area of C-FA-28d (Figure 6c-2) was selected for analysis, and the results are presented in Figure 9. All of the results were highly consistent with those of the selected area shown in Figure 8. No anhydrous C 3 S core area in C 3 S was observed in C-FA-28d.  All peak positions (a1 and a2) and energy separations (a2 − a1) of the selected areas of C-28 and C-FA-28d are shown in Table 3 and are plotted in Figure 10. After 28 days of hydration, the C-S-H layer in the pure C3S system had a lower a1 peak energy and higher a2 peak energies than anhydrous C3S. Alternatively, the C-S-H formed in C3S-HVFA system had a much lower a1 peak energy and higher a2 peak energies than the C-S-H layer on C3S in the pure C3S system. A previous study [21] illustrated that the peak positions (both a1 and a2) that were shifted in the Si K edge of the synthetic C-S-H increased with a decrease in the Ca/Si. Considering the silicate polymerization of C-S-H, C-S-H in the HVFA system should exhibit a greater peak a1 value than the C-S-H layer in pure C3S  All peak positions (a1 and a2) and energy separations (a2 − a1) of the selected areas of C-28 and C-FA-28d are shown in Table 3 and are plotted in Figure 10. After 28 days of hydration, the C-S-H layer in the pure C3S system had a lower a1 peak energy and higher a2 peak energies than anhydrous C3S. Alternatively, the C-S-H formed in C3S-HVFA system had a much lower a1 peak energy and higher a2 peak energies than the C-S-H layer on C3S in the pure C3S system. A previous study [21] illustrated that the peak positions (both a1 and a2) that were shifted in the Si K edge of the synthetic C-S-H increased with a decrease in the Ca/Si. Considering the silicate polymerization of C-S-H, C-S-H in the HVFA system should exhibit a greater peak a1 value than the C-S-H layer in pure C3S (d) RGB overlay maps obtained using the spectra from FA (red) and Area 2 (green); (e) Si K edge NEXAFS spectra; and (f) Al K edge NEXAFS spectra. The Si and Al K edge spectra were obtained from the ROIs illustrated in (a). Fly ash particles are indicated by the arrows in the Al image contrast map.
All peak positions (a 1 and a 2 ) and energy separations (a 2 − a 1 ) of the selected areas of C-28 and C-FA-28d are shown in Table 3 and are plotted in Figure 10. After 28 days of hydration, the C-S-H layer in the pure C 3 S system had a lower a 1 peak energy and higher a 2 peak energies than anhydrous C 3 S. Alternatively, the C-S-H formed in C 3 S-HVFA system had a much lower a 1 peak energy and higher a 2 peak energies than the C-S-H layer on C 3 S in the pure C 3 S system. A previous study [21] illustrated that the peak positions (both a 1 and a 2 ) that were shifted in the Si K edge of the synthetic C-S-H increased with a decrease in the Ca/Si. Considering the silicate polymerization of C-S-H, C-S-H in the HVFA system should exhibit a greater peak a 1 value than the C-S-H layer in pure C 3 S system. Therefore, the lower a 1 peak energy of C-S-H in the C 3 S-HVFA system is a direct indication of Al incorporation into C-S-H, which decreases the Si-O bond and the effective charge on the Si atoms in C-S-H, resulting in a Si 1s binding energy in the Si K edge NEXAFS spectrum [21]. Additionally, the higher a 2 peak energies could be attributed to the enhanced degree of silicate polymerization in C-S-H due to HVFA incorporation. system. Therefore, the lower a1 peak energy of C-S-H in the C3S-HVFA system is a direct indication of Al incorporation into C-S-H, which decreases the Si-O bond and the effective charge on the Si atoms in C-S-H, resulting in a Si 1s binding energy in the Si K edge NEXAFS spectrum [21]. Additionally, the higher a2 peak energies could be attributed to the enhanced degree of silicate polymerization in C-S-H due to HVFA incorporation.   Table 2.  Table 2.

27 Al MAS NMR Analysis
The 27 Al MAS NMR technique has been used to differentiate between tetrahedral (Al [IV] ) and octahedral (Al [VI] ) coordination because of their different 27 Al isotropic chemical shifts; the Al [IV] signal occurs in the range of 50-100 ppm, and the Al [VI] signal is observed in the 10-20 ppm range [18,22,44,45]. The 27 Al NMR spectrum tends to be characterized by broad peaks due to the quadrupolar nucleus of 27 Al, but the quadrupolar coupling parameters can help identify structurally distinct Al sites [46]. Skibsted et al. [47] further developed methods for monitoring the hydration of aluminate cements and used 27 Al NMR spectroscopy to study the calcium aluminate phases in cements. The broad peaks often overlapped, but the Al [VI] peak positions in AFt [37,44] and AFm [46] could be resolved and were proven to be approximately 13 ppm and 8 ppm, respectively, in hydrated pastes. A peak at approximately 66 ppm was assigned to Al [IV] in C-S-H.
As shown in Figure 11, the 27 Al NMR spectra of C-FA-28d confirmed the presence of Al [IV] , Al [V] , and Al [VI] . Several sharp octahedral Al environments were observed in the 27 Al NMR spectra of C-FA-28d. Specifically, three peaks were observed between 0 and 20 ppm: the AFt phase at 14 ppm, AFm at~10.5 ppm and a third aluminate hydrate (TAH) phase at~5.6 ppm. A peak at approximately 40 ppm was also observed and was attributed to Al [V] . This Al species has been assigned to water-activated samples containing fly ash [48]. The broad resonance in the range of 50 to 80 ppm observed in this study is typical of Al [IV] sites in less crystalline structures [15,22]. This Al [IV] signal included Al in the anhydrous material and in C-S-H [18], where it was substituted for Si ions at the bridging tetrahedral sites. The Al [IV] signal in synthetic C-S-H with a Ca/Si ratio of 1.0 occurs at 62.6 ppm [13], in agreement with the peak position (≈63 ppm) observed herein. Al [IV] in C-S-H decreases Ca/Si and results in the enhanced silicate polymerization and the increased silicate chain length of C-S-H [18,23,25]. This will be further discussed with 29 Si MAS NMR and TEM-EDX analysis.

27 Al MAS NMR Analysis
The 27 Al MAS NMR technique has been used to differentiate between tetrahedral (Al [IV] ) and octahedral (Al [VI] ) coordination because of their different 27 Al isotropic chemical shifts; the Al [IV] signal occurs in the range of 50-100 ppm, and the Al [VI] signal is observed in the 10-20 ppm range [18,22,44,45]. The 27 Al NMR spectrum tends to be characterized by broad peaks due to the quadrupolar nucleus of 27 Al, but the quadrupolar coupling parameters can help identify structurally distinct Al sites [46]. Skibsted et al. [47] further developed methods for monitoring the hydration of aluminate cements and used 27 Al NMR spectroscopy to study the calcium aluminate phases in cements. The broad peaks often overlapped, but the Al [VI] peak positions in AFt [37,44] and AFm [46] could be resolved and were proven to be approximately 13 ppm and 8 ppm, respectively, in hydrated pastes. A peak at approximately 66 ppm was assigned to Al [IV] in C-S-H.
As shown in Figure 11, the 27 Al NMR spectra of C-FA-28d confirmed the presence of Al [IV] , Al [V] , and Al [VI] . Several sharp octahedral Al environments were observed in the 27 Al NMR spectra of C-FA-28d. Specifically, three peaks were observed between 0 and 20 ppm: the AFt phase at ~14 ppm, AFm at ~10.5 ppm and a third aluminate hydrate (TAH) phase at ~5.6 ppm. A peak at approximately 40 ppm was also observed and was attributed to Al [V] . This Al species has been assigned to wateractivated samples containing fly ash [48]. The broad resonance in the range of 50 to 80 ppm observed in this study is typical of Al [IV] sites in less crystalline structures [15,22]. This Al [IV] signal included Al in the anhydrous material and in C-S-H [18], where it was substituted for Si ions at the bridging tetrahedral sites. The Al [IV] signal in synthetic C-S-H with a Ca/Si ratio of 1.0 occurs at 62.6 ppm [13], in agreement with the peak position (≈63 ppm) observed herein. Al [IV] in C-S-H decreases Ca/Si and results in the enhanced silicate polymerization and the increased silicate chain length of C-S-H [18,23,25]. This will be further discussed with 29 Si MAS NMR and TEM-EDX analysis.  Figure 12 shows the 29 Si MAS NMR spectra for anhydrous C3S (red line) and for the C-28d (blue line), and C-FA-28d (black line) pastes. The NMR spectrum of anhydrous C3S has a number of resonances in the range of from −68 to −76 ppm all with chemical shifts typical of an orthosilicate (Q 0 ) [49,50]. The multiple resonances were due to several isolated silicate anion environments [50]. 29 Si MAS NMR studies have shown that C-S-H produced during PC hydration contains only chain-end (Q 1 ) and middle-chain (Q 2 ) units [12,51,52]. In agreement with the studies [49,53], the C-28d spectrum had peaks in the range of from −76 to −90 ppm, which were due to the Q 1 and Q 2 (0Al) sites in C-S-H that appeared after C3S hydration. 3.5. 29 Si MAS NMR Analysis Figure 12 shows the 29 Si MAS NMR spectra for anhydrous C 3 S (red line) and for the C-28d (blue line), and C-FA-28d (black line) pastes. The NMR spectrum of anhydrous C 3 S has a number of resonances in the range of from −68 to −76 ppm all with chemical shifts typical of an orthosilicate (Q 0 ) [49,50]. The multiple resonances were due to several isolated silicate anion environments [50]. 29 Si MAS NMR studies have shown that C-S-H produced during PC hydration contains only chain-end (Q 1 ) and middle-chain (Q 2 ) units [12,51,52]. In agreement with the studies [49,53], the C-28d spectrum had peaks in the range of from −76 to −90 ppm, which were due to the Q 1 and Q 2 (0Al) sites in C-S-H that appeared after C 3 S hydration. Comparing the C-FA-28d blend with hydrated C-28d, the peak intensity of Q 0 significantly decreased as a result of the incorporation of HVFA. Additionally, the NMR spectrum of C-FA-28d had much broader overlapping due to the presence of a large amount of paramagnetic Fe 3+ ion provided from HVFA [12,54]. In addition to Q 1 and Q 2 (0Al), a third hydrate peak was observed at approximately −82 ppm and was attributed to Q 2 (1Al) as previously reported [18,55]. Here, no Q 1 (1Al), which was expected to be discovered when Al substituted for Si at non-bridging sites, was found. This confirms that Al is substituted for Si only in the bridging tetrahedral units of C-S-H, as observed in the previous reports [13,22,26]. The C-FA-28d spectrum also had a broad peak at −109 ppm due to the cross-linking sites in the three-dimensional framework (Q 4 ) of the anhydrous fly ash [12,52], showing that the fly ash did not fully react within 28 days. A peak was also observed at approximately −95 ppm in the C-FA-28d spectrum, which indicated the presence of chain branching sites (Q 3 ). This resulted from either a continuous range of Q n (mAl) species in the fly ash glassy phase [56] or carbonation of the C-S-H phase. Because the thermogravimetric analysis results showed some carbonation of the C-FA-28d paste, the -95 ppm peak was attributed to Q 3 and not Q 4 with Al substitution, which could lead to a downfield peak shift.

29 Si MAS NMR Analysis
The deconvoluted 29 Si MAS NMR spectra of the hydrate phases of C-28d and C-FA-28d obtained from the fitting are presented in Figure 13. Based on the relative peak intensities obtained by deconvoluting the spectra, the silicate MCLs and Al/Si ratios of C-28d and C-FA-28d were calculated (Table 4). C-28d had an MCL of 2.53, which is consistent with the MCL of 2.4-2.6 for 50% to 70% hydrated C3S [57], whereas C-FA-28d had an MCL that was 4 times longer (10.26) than that of C-28d, demonstrating that the degree of silicate polymerization was higher in the HVFA system. The longer silicate chain length of C-S-H in C-FA-28d could also be due to the increased Al [IV] occupancy in C-S-H, as reported previously [18,23,25]. Schematic representations of the C-S-H chain structures with different chain lengths (CLs) and Al/Si ratios in the HVFA system are illustrated in Figure 14. The schematic C-S-H structure in the HVFA system is based on the atomic structure of tobermorite with the following assumptions. First, C-S-H in the HVFA system has a single silicate chain, and Al substitutes for Si only at the bridging (B) tetrahedral sites and not at the paired (P) or end-chain tetrahedral sites. Second, the C-S-H consists of both dimeric and higher polymeric species, mainly linear pentamers (CL = 5) and octamers (CL = 8), suggesting a 2, 5, 8… (3n − 1) sequence of linear silicate chain lengths [12,58]. Third, no Q 3 sites that link two dreierketten chains together exist in the C-S-H. Based on the calculated MCL (10.26), Al/Si ratio (0.257), relative ratios between the Q n species (Q 1 :Q 2 (0Al):Q 2 (1Al)) measured by 29 Si NMR, and the assumptions described above, Al is likely to substitute for Si in at least two of the three bridging tetrahedral sites in the C-S-H silicate chain (Figure 14d). Both 27 Al and 29 Si NMR analysis confirmed that incorporating HVFA into the C3S system induces the increased uptake of Al (Al [IV] ) in C-S-H which results in an enhanced silicate chain length in C-S-H. Comparing the C-FA-28d blend with hydrated C-28d, the peak intensity of Q 0 significantly decreased as a result of the incorporation of HVFA. Additionally, the NMR spectrum of C-FA-28d had much broader overlapping due to the presence of a large amount of paramagnetic Fe 3+ ion provided from HVFA [12,54]. In addition to Q 1 and Q 2 (0Al), a third hydrate peak was observed at approximately −82 ppm and was attributed to Q 2 (1Al) as previously reported [18,55]. Here, no Q 1 (1Al), which was expected to be discovered when Al substituted for Si at non-bridging sites, was found. This confirms that Al is substituted for Si only in the bridging tetrahedral units of C-S-H, as observed in the previous reports [13,22,26]. The C-FA-28d spectrum also had a broad peak at −109 ppm due to the cross-linking sites in the three-dimensional framework (Q 4 ) of the anhydrous fly ash [12,52], showing that the fly ash did not fully react within 28 days. A peak was also observed at approximately −95 ppm in the C-FA-28d spectrum, which indicated the presence of chain branching sites (Q 3 ). This resulted from either a continuous range of Q n (mAl) species in the fly ash glassy phase [56] or carbonation of the C-S-H phase. Because the thermogravimetric analysis results showed some carbonation of the C-FA-28d paste, the -95 ppm peak was attributed to Q 3 and not Q 4 with Al substitution, which could lead to a downfield peak shift.
The deconvoluted 29 Si MAS NMR spectra of the hydrate phases of C-28d and C-FA-28d obtained from the fitting are presented in Figure 13. Based on the relative peak intensities obtained by deconvoluting the spectra, the silicate MCLs and Al/Si ratios of C-28d and C-FA-28d were calculated (Table 4). C-28d had an MCL of 2.53, which is consistent with the MCL of 2.4-2.6 for 50% to 70% hydrated C 3 S [57], whereas C-FA-28d had an MCL that was 4 times longer (10.26) than that of C-28d, demonstrating that the degree of silicate polymerization was higher in the HVFA system. The longer silicate chain length of C-S-H in C-FA-28d could also be due to the increased Al [IV] occupancy in C-S-H, as reported previously [18,23,25]. Schematic representations of the C-S-H chain structures with different chain lengths (CLs) and Al/Si ratios in the HVFA system are illustrated in Figure 14. The schematic C-S-H structure in the HVFA system is based on the atomic structure of tobermorite with the following assumptions. First, C-S-H in the HVFA system has a single silicate chain, and Al substitutes for Si only at the bridging (B) tetrahedral sites and not at the paired (P) or end-chain tetrahedral sites. Second, the C-S-H consists of both dimeric and higher polymeric species, mainly linear pentamers (CL = 5) and octamers (CL = 8), suggesting a 2, 5, 8 . . . (3n − 1) sequence of linear silicate chain lengths [12,58]. Third, no Q 3 sites that link two dreierketten chains together exist in the C-S-H. Based on the calculated MCL (10.26), Al/Si ratio (0.257), relative ratios between the Q n species (Q 1 :Q 2 (0Al):Q 2 (1Al)) measured by 29 Si NMR, and the assumptions described above, Al is likely to substitute for Si in at least two of the three bridging tetrahedral sites in the C-S-H silicate chain (Figure 14d). Both 27 Al and 29 Si NMR analysis confirmed that incorporating HVFA into the C 3 S system induces the increased uptake of Al (Al [IV] ) in C-S-H which results in an enhanced silicate chain length in C-S-H.

Morphological TEM Imaging of the C 3 S-HVFA Paste
TEM images of C-S-H in the hydrated C 3 S particles in C-FA-28d are shown in Figure 15. The images show the fully reacted particles and the fine and coarse C-S-H in the hydrated C 3 S particles. As expected from the STXM and NMR results, no anhydrous C 3 S was observed in the images. Furthermore, fine C-S-H (Figure 15a) and large Ca(OH) 2 crystals intermixed with the C-S-H (Figure 15b) were observed. The presence of intermixed Ca(OH) 2 nanocrystals and C-S-H in Figure 15c was confirmed by SAED, as shown in Figure 15d. The EDX chemical analysis performed on the sample resulted in an approximate calculated Ca/Si ratio of 1.13, which was much less than that of the C-S-H layer on the pure hydrated C 3 S particles, which ranged from 1.5 to 1.9 [12,59,60]. The decreased Ca/Si ratio of C-S-H in the HVFA system indicated that the increased silicate polymerization of C-S-H increased in the HVFA system, which is in good agreement with the STXM and NMR results.

Morphological TEM Imaging of the C3S-HVFA Paste
TEM images of C-S-H in the hydrated C3S particles in C-FA-28d are shown in Figure 15. The images show the fully reacted particles and the fine and coarse C-S-H in the hydrated C3S particles. As expected from the STXM and NMR results, no anhydrous C3S was observed in the images. Furthermore, fine C-S-H (Figure 15a) and large Ca(OH)2 crystals intermixed with the C-S-H ( Figure  15b) were observed. The presence of intermixed Ca(OH)2 nanocrystals and C-S-H in Figure 15c was confirmed by SAED, as shown in Figure 15d. The EDX chemical analysis performed on the sample resulted in an approximate calculated Ca/Si ratio of 1.13, which was much less than that of the C-S-H layer on the pure hydrated C3S particles, which ranged from 1.5 to 1.9 [12,59,60]. The decreased Ca/Si ratio of C-S-H in the HVFA system indicated that the increased silicate polymerization of C-S-H increased in the HVFA system, which is in good agreement with the STXM and NMR results.

Conclusions
This study investigated the effects of HVFA incorporation on the C3S hydration kinetics, as well as the morphology and atomic binding structure of C-S-H. The formation of Ca(OH)2 in the C3S-HVFA paste was significantly delayed during the early stages of C3S hydration. HVFA lengthens the induction period of the C3S hydration process at the early age; however, the degree of C3S hydration was enhanced by the use of HVFA after 3 days of hydration. The increased degree of hydration was presumably due to the enhanced C3S dissolution and precipitation of hydration products upon HVFA incorporation.
Direct evidence of Al incorporation into C-S-H in the C3S-HVFA system was provided by the 27 Al MAS NMR, which showed that Al substituted for Si only at the C-S-H bridging tetrahedral sites in the C3S-HVFA system. Additionally, 29 Si MAS NMR showed that in the C3S-HVFA system, C-S-H had a silicate chain length that was 4 times longer than that in the pure C3S system (10.26 vs. 2.53),

Conclusions
This study investigated the effects of HVFA incorporation on the C 3 S hydration kinetics, as well as the morphology and atomic binding structure of C-S-H. The formation of Ca(OH) 2 in the C 3 S-HVFA paste was significantly delayed during the early stages of C 3 S hydration. HVFA lengthens the induction period of the C 3 S hydration process at the early age; however, the degree of C 3 S hydration was enhanced by the use of HVFA after 3 days of hydration. The increased degree of hydration was presumably due to the enhanced C 3 S dissolution and precipitation of hydration products upon HVFA incorporation.
Direct evidence of Al incorporation into C-S-H in the C 3 S-HVFA system was provided by the 27 Al MAS NMR, which showed that Al substituted for Si only at the C-S-H bridging tetrahedral sites in the C 3 S-HVFA system. Additionally, 29 Si MAS NMR showed that in the C 3 S-HVFA system, C-S-H had a silicate chain length that was 4 times longer than that in the pure C 3 S system (10.26 vs. 2.53), which confirms the higher degree of silicate polymerization in the C 3 S-HVFA system. The Al/Si (0.257) and the calculated silicate chain length suggested that tetrahedrally coordinated Al is likely to be incorporated in at least two of the three bridging tetrahedral sites in the C-S-H silicate chains produced in the C 3 S-HVFA system. The smaller Ca/Si (1.13) estimated from the TEM-EDX data also demonstrated the increased Al incorporation and degree of silicate polymerization of C-S-H in the C 3 S-HVFA system.
For the first time STXM was used to characterize the C-S-H in the C 3 S and C 3 S-HVFA systems. STXM imaging showed that the Si concentration of the C-S-H in the C 3 S-HVFA system was very uniform whereas apparent C-S-H layers were observed on only the C 3 S particles in the pure C 3 S system. While the formation of the C-S-H layer on C 3 S in the pure C 3 S system hindered the hydration of C 3 S resulting in an anhydrous core area, most of the C 3 S particles in the C 3 S-HVFA system were fully hydrated, and no obvious C-S-H layers were observed. This was due to the increased C 3 S dissolution in the C 3 S-HVFA system during the early stages of hydration, as observed in the thermal analyses. As a result of the enhanced hydration, the C-S-H in the C 3 S-HVFA system had a lower a 1 peak energy and higher a 2 peak energies at the Si K edge in the NEXAFS spectra than the C-S-H layers in the neat C 3 S system. The lower a 1 peak energy of C-S-H in the HVFA system was conclusive evidence of Al substitution for Si in C-S-H, and the higher a 2 peak energies were indicative of the enhanced degree of silicate polymerization due to the additional silicate provided by HVFA. In contrast, the Al incorporation and the enhanced silicate polymerization did not significantly influence the Ca L III,II edge NEXAFS spectra of the CaO layer of C-S-H in the HVFA system. The results measured by STXM are in good agreement with the previous findings determined by other characterization methods. The findings above lead to the conclusion that, in order to enhance the understanding of C 3 S-HVFA system, it is necessary to further investigate the effect of fly ash containing an abundance of CaO on the morphology, the atomic binding structure of C-S-H and the kinetics of C 3 S hydration.