# CaCl2-Accelerated Hydration of Tricalcium Silicate: A STXM Study Combined with 29Si MAS NMR

Authors: Qinfei Li; Yong Ge; Guoqing Geng; Sungchul Bae; Paulo J. M. Monteiro
Journal: Journal of Nanomaterials
 (2015)

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## Abstract

The effect of calcium chloride (CaCl2) on tricalcium silicate (C3S) hydration was investigated by scanning transmission X-ray microscopy (STXM) with Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra and 29Si MAS NMR. STXM is demonstrated to be a powerful tool for studying the chemical composition of a cement-based hydration system. The Ca L3,2-edge NEXAFS spectra obtained by examining C3S hydration in the presence of CaCl2 showed that this accelerator does not change the coordination of calcium in the calcium silicate hydrate (C-S-H), which is the primary hydration product. O K-edge NEXAFS is also very useful in distinguishing the chemical components in hydrated C3S. Based on the Ca L3,2-edge spectra and chemical component mapping, we concluded that CaCl2 prefers to coexist with unhydrated C3S instead of C-S-H. In Si K-edge NEXAFS analysis, CaCl2 increases the degree of silicate polymerization of C-S-H in agreement with the 29Si CP/MAS NMR results, which show that the presence of CaCl2 in hydrated C3S considerably accelerates the formation of middle groups (Q
2) and branch sites (Q
3) in the silicate chains of C-S-H gel at 1-day hydration.


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## Body

## 1. Introduction

Calcium chloride (CaCl2) plays an important role in accelerating the hydration and setting of tricalcium silicate (C3S) and Portland cement [1–3]. CaCl2 has been used widely in construction using unreinforced concrete [3] due to steel corrosion. The rate of formation of hydration products has often been observed to increase in the presence of CaCl2, thereby accelerating the rate of heat evolution during hydration [1, 3]. This accelerating admixture promotes the dissolution of the cations or anions from the cement, thereby accelerating the growth rate of calcium silicate hydrate (C-S-H), which is a complicated physical and chemical process for nucleation and growth [4]. However, this mechanism is not fully understood at the molecular level [5].The microstructure and nanostructure of the hydration products are expected to be influenced by speeding up the hydration reaction. CaCl2 is well known to have the ability to increase the nitrogen surface area and pore volume of Portland cement and C3S pastes [6, 7]. The “sheaf-of wheat” or fibrous morphology of hydrated C3S in the presence of CaCl2 has been investigated using various microscopy techniques [2, 8–11], such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and full-view transmission soft X-ray microscopy, but little information has been obtained about the chemical and physical properties corresponding to the morphology. Transmission soft X-ray microscopy has increasingly been utilized in exploring the cement-based materials [2, 8, 12, 13]. Juenger et al. [2] found that CaCl2 accelerates the formation of “inner product” C-S-H with a low-density microstructure. The C-S-H is formed early and outside the original grain boundary as bridge spaces between grains. Thomas et al. studied the development of the microstructure and kinetics of pure C3S and CaCl2-accelerated C3S pastes, and they reported that CaCl2 accelerated the rate of nucleation of the hydration product on the surface of the C3S particles significantly but that CaCl2 has relatively little effect on the growth rate [1]. These studies demonstrated that both morphologic information about hydrated C3S in the presence of CaCl2 and exploration of chloride binding are needed. However, the analytical approaches listed above have certain limitations in that they do not include the chemical speciation of elements in heterogeneous materials on the submicron-scale.Scanning transmission X-ray microscopy (STXM) provides information on both morphology and X-ray Absorption Spectroscopy (XAS). STXM is a promising technique in further understanding the structure and properties of the C-S-H phase [14–19]. The STXM technique allows the investigation of amorphous and crystalline materials. In addition to obtaining the chemical speciation information, STXM allows the identification of heterogeneity within the samples on a particle-by-particle basis. Significantly, image stacks (where a series of images is collected corresponding to each energy level observed by STXM with Near Edge X-ray Absorption Fine Structure (NEXAFS)) were converted into chemical component mapping. The results can be visualized by RGB overlay maps using Singular Value Decomposition (SVD) based on reference spectra [20]. Because various chemical components have unique NEXAFS spectra, the reference spectra obtained from known components were used to visualize and differentiate phases in a sample [21].To investigate the effects of CaCl2 on the microstructure of hydration products during the hydration of C3S, X-ray diffraction (XRD), 29Si MAS NMR, and scanning transmission X-ray microscopy (STXM) measurements were taken to investigate the interactions between calcium chloride and C-S-H. Through the use of STXM in combination with concurrent measurements of NEXAFS, the results from this study will examine sample composition and spatial heterogeneities and thus aid in explaining the interactions between calcium or chloride ions and C-S-H. This will improve understanding of the accelerating role of CaCl2 on the chemical information of C-S-H, bringing the state of the art a step closer to the development of admixtures for advanced concrete structures.


## 2. Materials and Methods

### 2.1. Materials

Triclinic C3S passing through sieve #325 (45 μm) was purchased from CTL Group (Skokie, IL). Finely ground anhydrous triclinic C3S powder was mixed with a 0.1 M CaCl2 solution with a liquid-to-solid ratio of 1.0. A C3S paste produced for STXM was maintained in CryoTube vials (1.8 mL) under the protection of N2 gas at room temperature and was then sealed in a vacuum bag. After a period of hydration of 1, 3, 7, and 28 d, samples of hydrated C3S with an addition of CaCl2 were dried in a vacuum oven at 23°C before X-ray diffraction and 29Si MAS NMR measurements.


### 2.2. X-Ray Diffraction

Samples of hydrated C3S with an addition of CaCl2 at different hydration times were tested using a PANalytical X’Pert Pro diffractometer equipped with a Co X-ray tube (λ
=
1.79 Å) and the rapid X’celerator detector. The XRD patterns of the samples were analyzed using the X-Pert High Score Plus software. XRD was used to identify crystalline phases from a 2θ value of 5–55° with a step of 0.02°.


### 2.3. Scanning Transmission X-Ray Microscopy

After 7 d of hydration, the C3S paste was ground and dispersed with deionized water for STXM measurement. A drop of 0.1 μL of C3S solution was taken and placed on the center of the Si3N4 membrane window (core: 1 mm × 1 mm × 100 nm; frame: 5 mm × 5 mm × 0.2 mm) with a micropipette. Residual water was removed from the window.The reference Ca L3,2-edge NEXAFS spectra of CaCl2 particles on the Si3N4 window were also measured. STXM experiments were performed at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) beamline 5.3.2.1 (700–2500 eV) and beamline 5.3.2.2 (250–800 eV) [23]. The images and NEXAFS spectra of the Ca L3,2-edge (340–360 eV), O K-edge (520–560 eV), and Si K-edge (1825–1890 eV) were measured. The Axis 2000 software (version 2.1) was used to align stack images and extract NEXAFS spectra from the stack and line scan images. The stack scan collects absorption data for the field of view at each preselected energy point.


### 2.4.29Si MAS NMR

Solid-state NMR spectra were recorded on a Bruker MSL-300 spectrometer in which the resonance frequency for29Si is 59.63 MHz. Samples were packed into a 4 mm Zirconia rotor. High-power proton decoupling was used for the 29Si spectra, and chemical shifts are quoted relative to external tetramethylsilane (TMS) for 29Si. Typical acquisition parameters for recording the 29Si spectra were a 4.5 kHz sample spinning speed, 4 μs pulse length, and 6 KHz sweep width. 29Si cross-polarization magic-angle spinning (CPMAS) experiments were performed using a relaxation delay of 12 μs.


## 2.1. Materials

Triclinic C3S passing through sieve #325 (45 μm) was purchased from CTL Group (Skokie, IL). Finely ground anhydrous triclinic C3S powder was mixed with a 0.1 M CaCl2 solution with a liquid-to-solid ratio of 1.0. A C3S paste produced for STXM was maintained in CryoTube vials (1.8 mL) under the protection of N2 gas at room temperature and was then sealed in a vacuum bag. After a period of hydration of 1, 3, 7, and 28 d, samples of hydrated C3S with an addition of CaCl2 were dried in a vacuum oven at 23°C before X-ray diffraction and 29Si MAS NMR measurements.


## 2.2. X-Ray Diffraction

Samples of hydrated C3S with an addition of CaCl2 at different hydration times were tested using a PANalytical X’Pert Pro diffractometer equipped with a Co X-ray tube (λ
=
1.79 Å) and the rapid X’celerator detector. The XRD patterns of the samples were analyzed using the X-Pert High Score Plus software. XRD was used to identify crystalline phases from a 2θ value of 5–55° with a step of 0.02°.


## 2.3. Scanning Transmission X-Ray Microscopy

After 7 d of hydration, the C3S paste was ground and dispersed with deionized water for STXM measurement. A drop of 0.1 μL of C3S solution was taken and placed on the center of the Si3N4 membrane window (core: 1 mm × 1 mm × 100 nm; frame: 5 mm × 5 mm × 0.2 mm) with a micropipette. Residual water was removed from the window.The reference Ca L3,2-edge NEXAFS spectra of CaCl2 particles on the Si3N4 window were also measured. STXM experiments were performed at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) beamline 5.3.2.1 (700–2500 eV) and beamline 5.3.2.2 (250–800 eV) [23]. The images and NEXAFS spectra of the Ca L3,2-edge (340–360 eV), O K-edge (520–560 eV), and Si K-edge (1825–1890 eV) were measured. The Axis 2000 software (version 2.1) was used to align stack images and extract NEXAFS spectra from the stack and line scan images. The stack scan collects absorption data for the field of view at each preselected energy point.


## 2.4.29Si MAS NMR

Solid-state NMR spectra were recorded on a Bruker MSL-300 spectrometer in which the resonance frequency for29Si is 59.63 MHz. Samples were packed into a 4 mm Zirconia rotor. High-power proton decoupling was used for the 29Si spectra, and chemical shifts are quoted relative to external tetramethylsilane (TMS) for 29Si. Typical acquisition parameters for recording the 29Si spectra were a 4.5 kHz sample spinning speed, 4 μs pulse length, and 6 KHz sweep width. 29Si cross-polarization magic-angle spinning (CPMAS) experiments were performed using a relaxation delay of 12 μs.


## 3. Results and Discussion

### 3.1. XRD Patterns

The results of the XRD analysis of the chloride-containing samples with different hydration times are shown in Figure1. Portlandite crystals (Ca(OH)2) and unhydrated C3S can be observed starting at 1 d of hydration. At 28 d, unhydrated C3S was no longer observed. Due to the poor crystallinity of amorphous C-S-H, there is just a hump associated with the formation of C-S-H at a d-spacing of 3.08 Å starting at 3 d for hydrated C3S with CaCl2.Figure 1
XRD patterns of phase changes from hydrated C3S in the presence of CaCl2 at different hydration times.


### 3.2. Ca L3,2-Edge NEXAFS Analysis

The references of unhydrated C3S, pure Ca(OH)2, and pure C-S-H were first determined by Bae et al. [22], whereas the reference for CaCl2 was collected in the present study. C3S and its hydration products are calcium compounds. Ca L3,2-edge NEXAFS spectra provide a calcium coordination environment and conveniently characterize the amorphous and crystalline Ca-containing materials that are present in these reference compounds and 7-day-hydrated C3S with CaCl2 in Figure 2. The observed multiple peak patterns of Ca L3,2-edge NEXAFS spectra consist of two main spin-orbit related peaks (a
2 and b
2) corresponding to L3 and L2, along with several smaller peaks (a
1 and b
1) leading to the main peaks (Figure 2). The experimental resolution for the spectra is less than approximately 0.1 eV. The magnitudes and symmetry of the crystal field of calcium in the first coordination sphere result in crystal field splitting, which is the origin of these multipeak patterns. The peak positions and energy separation values (Δ
L
3
(
a
2
-
a
1
) and Δ
L
2
(
b
2
-
b
1
)) are related to the symmetry of Ca2+, given in Table 1.Table 1
Peak positions and splitting energy values of the reference Ca L3,2-edge and the different areas of the C3S particles in the presence of CaCl2.
Samples
Peak positions (ev)
ΔCa-L3(a
2
-
a
1)
ΔCa-L2(b
2
-
b
1)
a
1
a
2
b
1
b
2
CaCl2-ref
348.0
349.2
351.3
352.4
1.2
1.1
Ca(OH)2-ref∗
347.6
349.1
351.0
352.4
1.5
1.4
Unhydrated C3S-ref∗
347.7
349.0
351.1
352.3
1.3
1.2
C-S-H-ref∗
348.1
349.1
351.4
352.4
1.0
1.0
Area 1
347.7
349.0
351.1
352.3
1.3
1.2
Area 2
347.9
348.9
351.2
352.3
1.0
1.1
Area 3
347.8
349.0
351.1
352.3
1.2
1.2
∗Data from Bae et al. [22].Figure 2
(a) Ca L3,2-edge NEXAFS spectra of the reference anhydrous C3S∗, C-S-H∗, C
a
O
H
2
∗, and CaCl2 (∗ data from Bae et al. [22]), (b) and (c) a single image of 7-day-hydrated C3S in the presence of CaCl2 taken at, respectively, 340 eV and 525 eV, (d) Ca L3,2-edge NEXAFS spectra of the selected area in (b), and (e) O K-edge NEXAFS spectra of the selected area in (c).
(a)
(b)
(c)
(d)
(e)Figure2 presents the Ca L3,2-edge NEXAFS spectra of CaCl2, Ca(OH)2, pure C-S-H, and anhydrous C3S as the references. Each of these spectra is unique in terms of its shape and peak positions as well as the splitting shown in Table 1. The uniqueness of each spectrum has been used as a fingerprint for investigating the Ca structure. The larger splitting energy and peak intensity ratio in the Ca L3,2-edge NEXAFS spectra are good indications of the well-developed crystalline Ca structure, which means that the splitting energy values for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
) are nonlinearly related to the value of the crystal field parameter (10 Dq). Ca(OH)2, with octahedral symmetry (Oh) [24], shows larger splitting energy values and peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 compared with the values and ratios of unhydrated C3S and pure C-S-H due to the different electronegativity in the crystal field. The reference of pure C-S-H in 17-day-hydrated C3S [22] has the smallest splitting energy and peak intensity ratio shown in Figure 2, which result from its amorphous or poor-crystallinity structure. In addition, the peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 generally indicate the magnitude of the crystal field and suggest that C-S-H has the smallest crystal field parameter but that CaCl2 has the largest. The mean coordination number of CaCl2 is six in the octahedral crystal field (positive) [25].


### 3.3. STXM Analysis of 7-Day-Hydrated C3S with CaCl2

The selected areas of a 7-day-hydrated C3S particle in the presence of CaCl2 were analyzed by Ca L3,2-edge and O K-edge NEXAFS spectroscopy, as shown in Figure 2. The peak positions, splitting energy values, and references of Ca L3,2-edge NEXAFS spectra are listed in Table 1. In Figure 2(d), Ca L3,2-edge NEXAFS for Area 1 has the largest splitting energy and peak intensity ratio, and Area 2 has the smallest. The peak positions for Area 2 shift slightly by approximately −0.2 eV, but the splitting energy for Area 2 is similar to that of the C-S-H reference. The energy separation of C-S-H with 1.0 and 1.1 for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
) and the smallest peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 in Area 2 suggest that the calcium in C-S-H has a randomly spherical coordination with the oxygen, which means that the coordination environment of calcium in C-S-H is asymmetric. This observation suggests that CaCl2 does not change the coordination environment of calcium in C-S-H. Moreover, the energy separation of Area 3, with 1.2 and 1.2 for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
), respectively, is similar to that of the unhydrated C3S reference. Therefore, these areas are identified by Ca L3,2-edge NEXAFS spectra, which are compared with the peak positions, splitting energy, and peak shape of the above references: Area 1 is related to Ca(OH)2, Area 2 is related to C-S-H, and Area 3 is related to unhydrated C3S.We also present the O K-edge NEXAFS spectroscopy of the three areas in the same particle shown in Figure2(e). The O K-edge NEXAFS spectrum can provide simultaneous chemical information about hydration products and unhydrated C3S and shows that the differences in the O K-edge NEXAFS spectra are sufficiently significant to distinguish the distinct hydration products.In Figure2(e), there are four peaks, labeled 1 through 4, in each O spectrum. Peak 1 is a π
∗ peak, and peaks 3 and 4 are σ
∗ peaks. However, peak 2 behaves differently across different types of crystallinity [26]. Peak 2 is sharper in Ca(OH)2 than are the peaks in C-S-H and in unhydrated C3S. The layered crystal structure of Portlandite (Ca(OH)2) is well known to be trigonal with the space group P3
-m1 and can be described as stacked sheets of distorted edge-sharing Ca-O octahedra along thec-axis [27]. Each hydroxyl group is coordinated by three Ca atoms in its layer and is surrounded by three other hydroxyl groups that belong to the adjacent layer. Unhydrated C3S is triclinic and consists of the mean coordination number (6.21) [28] of Ca cations and the SiO4 tetrahedra, which show varying degrees of disorder. Therefore, there are weaker π
∗ peaks, and peak 2 in the O K-edge NEXAFS of C-S-H is due to the poor crystallization of C-S-H.Image stacks, where a series of images is collected corresponding to each energy, were collected to convert into chemical component mapping. As shown in Figure3, the results were visualized by RGB overlay maps using Singular Value Decomposition (SVD) based on reference spectra [20]. Because various chemical components have unique NEXAFS spectra, the reference spectra obtained from known components were used to visualize and differentiate phases in a sample [21]. Both of the distributions of phases in the same hydrated C3S in the presence of CaCl2 with the Ca L3,2-edge and the O K-edge image stacks are presented in Figure 3. We observe a few magenta pixels (M = R + B = Ca(OH)2 + unhydrated C3S) in Figure 3. There are, however, numerous cyan pixels (C = G + B = C-S-H + unhydrated C3S) in Figure 3(b), which indicate unhydrated C3S in those pixels is transforming to C-S-H; however, there are a few yellow pixels (Y = R + G = Ca(OH)2 + C-S-H). Clearly, the C-S-H surrounds the unhydrated C3S, along with the formation of Ca(OH)2. Obviously, the RGB overlay map based on the O K-edge image stacks is similar to the RGB overlay map based on the Ca L3,2-edge image stacks. However, in the case of the O K-edge image stacks, the morphology of the C-S-H gel was more present in the details where the flocculent C-S-H gel surrounds the unhydrated C3S and Ca(OH)2, in good accordance with the results for Ca L3,2-edge.Figure 3
Decomposition diagram of RGB overlay maps using SVD for the C3S particle in the presence of CaCl2 based on NEXAFS spectra obtained from the reference in Figure 2 and the different areas in Figure 3: (a) Ca L3,2-edge image stacks; (b) O K-edge image stacks.
(a)
(b)Because RGB overlay maps using Singular Value Decomposition (SVD) can consist of no more than three components, the chemical component mapping for unhydrated C3S, C-S-H, and Ca(OH)2 are shown in Figure 3, and Figure 4 shows the RGB overlay maps with unhydrated C3S, C-S-H gel, and CaCl2. We observe numerous magenta pixels (M = R + B = CaCl2 + unhydrated C3S) even at the center of these unhydrated C3S, which indicates that CaCl2 diffuses into the inside of unhydrated C3S during the transformation from unhydrated C3S to C-S-H and Ca(OH)2. Interestingly, the red part (CaCl2) in Figure 4 was not intermingled with the green part, C-S-H gel, but instead with unhydrated C3S. In other words, CaCl2 coexists with unhydrated C3S instead of C-S-H in the case of 7-day-hydrated C3S with CaCl2. Calcium chloride increases the dissolution of C3S for easier diffusion of water into the hydrating particle and moves Ca2+ and silicon ions away from the particle to form C-S-H gel and Portlandite [2].Figure 4
Decomposition diagram of RGB overlay maps using SVD for the C3S particle in the presence of CaCl2 based on the Ca L3,2-edge NEXAFS spectra obtained from the reference in Figure 2 and the different areas in Figure 3.The Si K-edge NEXAFS analysis on 7-day-hydrated C3S in the presence of CaCl2 is shown in Figure 5. The flocculent and the fibrillary hydration product can be observed, respectively, in Figures 3(a) and 3(b). The distribution map of silicon, as seen in Figure 5(d), is the computed optical density between the image taken at the preadsorption edge at 1830 eV and the near-adsorption edge at 1840 eV. With respect to the hydration of C3S, Ca(OH)2 contains no silicon, whereas the unhydrated C3S and C-S-H have silicon-containing composition. Figures 5(b) and 5(f) show the Si K-edge NEXAFS spectra of the corresponding line-scan in Figure 5(a) and selected areas in Figure 5(e). Each spectrum consists of the main peak (a
1) and the multiple scattering peak (a
2). Peak a
1 is assigned to the transition of Si 1s electrons to the antibonding t
2 orbital (3p-like state), whereas peak a
2 is qualitatively attributable to the multiple scattering effect beyond the second coordination sphere [29].Figure 5
Si K-edge NEXAFS analysis of 7-day-hydrated C3S particles in the presence of CaCl2: (a) single image of one particle taken at 1855 eV, (b) Si K-edge NEXAFS spectra of the line-scan indicated in (a), (c) single image of another particle taken at 1855 eV, (d) image contrast mapping corresponding to (c), (e) selected areas in the image contrast map enlarged partly from (c) for the NEXAFS spectra, and (f) Si K-edge NEXAFS spectra taken from the different locations indicated in (e).
(a)
(b)
(c)
(d)
(e)
(f)The peak positions and energy separation(
Δ
a
2
-
a
1) shown in Table 2 are associated with the degree of polymerization in the silicates [22]. A line-scan on the flocculent particle shown in Figure 5(a) was selected to verify the changes from center to boundary, which had similar absorption features to synthetic C-S-H, with 0.66 and 0.95 Ca/Si ratios in previous studies [22]. Three locations (core, top, and bottom) were selected to verify the uniformity of the hydration product in Figure 5(e). These locations had no difference among their absorption features but were similar to Op and synthetic C-S-H, with a 1.44 Ca/Si ratio in previous studies [22]. Therefore, the hydration product of C3S with CaCl2 at 7 d in Figure 5(e) had a uniform degree of silicate polymerization. C3S hydration is a dissolution-precipitation process [30]; however, CaCl2 accelerated the rate of nucleation of the hydration product on the surface of the C3S particles significantly, but CaCl2 has relatively little effect on the growth rate [1]. Hence, after 7 d of hydration of C3S with CaCl2, the degree of silicate polymerization in the hydration product was close to that of the 17-day-hydrated C3S in previous studies [22], thus implying that CaCl2 increases the degree of silicate polymerization, especially in the case of Ip. Therefore, CaCl2 can also facilitate ion diffusion [31], prompt the dissolution process of C3S, and accelerate the formation of “inner product” C-S-H [2], ultimately resulting in a higher degree of silicate polymerization at either the core or the boundary of 7-day-hydrated C3S.Table 2
Peak positions and the energy separation between peak positions of Si K-edge NEXAFS spectra of selected areas in 7-day-hydrated C3S with CaCl2 (C and Op denote the core area and outer product of C3S, resp., in Bae et al. [22]).
Peaka
1 (eV)
Peaka
2
Δa
2
-
a
1
Blue line
1846.6
1863.2
16.6
Red line
1846.9
1863.0
16.1
Core
1846.5
1861.9
15.4
Top
1846.5
1861.9
15.4
Bottom
1846.5
1862.0
15.5
Anhydrous C3S∗
1847.7
1858.9
11.2
Syn-CSH0.66∗
1846.9
1863.4
16.5
Syn-CSH0.95∗
1846.7
1863.1
16.4
Syn-CSH1.44∗
1846.4
1861.5
15.1
Op∗
1847.4
1862.7
15.4
C∗
1847.7
1559.4, 1861.5
11.4, 13.8
∗Data from Bae et al. [22].The fracture surface of 7-day-hydrated C3S in the presence of CaCl2 imaged by SEM is shown in Figure 6. Thin-sheet C-S-H and the spongy mass C-S-H are observed in Figures 6(a)–6(c). “Honeycomb-like” morphologies of hydration products in the presence of CaCl2 are shown at ages of 3 h to 7 d by SEM [32]. Because of the 0.1 M CaCl2 used, the Ca/Si ratio for the honeycomb C-S-H ranges between 2.0 and 3.6, whereas that for the spongy mass C-S-H is also included in the range. CaCl2 accelerates the nucleation of the hydration product on the surface of the C3S particles [1] and possibly prompts the bridge of the dreierketten silicate chains of C-S-H to develop in the two-dimensional direction into thin-sheet C-S-H.Figure 6
(a)~(c) The morphology of 7-day-hydrated C3S in presence of CaCl2 imaged by SEM, (d) SEM-EDS analysis of (c), Cl versus Ca/Si.
(a)
(b)
(c)
(d)


### 3.4.29Si MAS NMR Analysis

29Si CP/MAS NMR was proven to be very useful for studying the chemical shifts of the corresponding element. The primary advantage of this method is its high resolution of signals, so the relationship between Si chemical shifts and corresponding assigned structure of silica unites is easy to establish. 29Si CP/MAS NMR plots for hydrated C3S with different hydration times in the absence or presence of CaCl2 are shown in Figure 7. To describe various types of silica unites, we used Q
n (n
≤
4) for the connectivity of SiO4 tetrahedra, where n represents the number of bridging oxygen (Si-O-Si): the Q
0 site is isolated SiO4 tetrahedra, Q
1 is SiO4 tetrahedra present in end unites of a silicate chain, Q
2 is a middle group, Q
3 is a branching site, and Q
4 sites link to four other SiO4 tetrahedra in a three-dimensional network. Amorphous C-S-H initially contains mainly dimeric silicate [33–35], whereas C3S includes nine inequivalent SiO4 tetrahedra (Q
0 sites) between −69 ppm and −75 ppm [36]. In our analysis, the Gaussian distributions centered about −75 ppm, −78 ppm, −83 ppm, and −89 ppm are attributed, respectively, to Q
0, Q
1, Q
2, and Q
3 units [37, 38]. 29Si CP/MAS NMR could not provide an accurate quantitative analysis of Q
n but instead indicated the chemical shift of the corresponding Q
n sites. However, for comparison, the trend for evaluating the fraction of Q
n could be reflected by 29Si CP/MAS NMR.Figure 7
29Si CP/MAS NMR spectra of C3S paste as a function of hydration time in the absence (a) and in the presence (b) of CaCl2.
(a)
(b)Prior to the 7-day-hydration process of C3S without CaCl2, the discrete SiO4 tetrahedra (Q
0) in unhydrated C3S transformed to dominant dimers Q
1 and doubly coordinated tetrahedral Q
2 in the hydrated C3S, while there was the occurrence of Q
3 at 3 days of hydration. However, the Q
0 and Q
1 peaks maintained the lowest level at 28 d, and the Q
2 peak became a predominant level. In the presence of CaCl2, the Q
2 peak played a dominant role in the dreierketten silicate chain structure after 1 d of hydration, and the Q
3 peak at −89 ppm began to emerge compared with the control group, as shown. CaCl2 is clearly indicated to considerably accelerate the formation of middle groups (Q
2) and branch sites (Q
3) in the silicate chains of C-S-H gel. CaCl2 significantly accelerates the rate of nucleation of the hydration product deposited on the surface of the C3S particles but has a relatively insignificant effect on the growth rate [1], thus explaining why the Q
2 peaks in the CaCl2 group played a leading role in the dreierketten silicate chain structure. The earlier emergence of Q
2 and Q
3 peaks of the CaCl2 group indicates that CaCl2 accelerates the degree of silicate polymerization in hydrated C3S in agreement with the above studies of the morphology of hydrated C3S in the presence of CaCl2. From the perspective of ionic diffusion, CaCl2 cannot only facilitate diffusion of ions [31] but also prompt the dissolution process of C3S and accelerate the formation of “inner product” C-S-H [2], along with a higher degree of silicate polymerization at either the core or the boundary of 7-day-hydrated C3S.


## 3.1. XRD Patterns

The results of the XRD analysis of the chloride-containing samples with different hydration times are shown in Figure1. Portlandite crystals (Ca(OH)2) and unhydrated C3S can be observed starting at 1 d of hydration. At 28 d, unhydrated C3S was no longer observed. Due to the poor crystallinity of amorphous C-S-H, there is just a hump associated with the formation of C-S-H at a d-spacing of 3.08 Å starting at 3 d for hydrated C3S with CaCl2.Figure 1
XRD patterns of phase changes from hydrated C3S in the presence of CaCl2 at different hydration times.


## 3.2. Ca L3,2-Edge NEXAFS Analysis

The references of unhydrated C3S, pure Ca(OH)2, and pure C-S-H were first determined by Bae et al. [22], whereas the reference for CaCl2 was collected in the present study. C3S and its hydration products are calcium compounds. Ca L3,2-edge NEXAFS spectra provide a calcium coordination environment and conveniently characterize the amorphous and crystalline Ca-containing materials that are present in these reference compounds and 7-day-hydrated C3S with CaCl2 in Figure 2. The observed multiple peak patterns of Ca L3,2-edge NEXAFS spectra consist of two main spin-orbit related peaks (a
2 and b
2) corresponding to L3 and L2, along with several smaller peaks (a
1 and b
1) leading to the main peaks (Figure 2). The experimental resolution for the spectra is less than approximately 0.1 eV. The magnitudes and symmetry of the crystal field of calcium in the first coordination sphere result in crystal field splitting, which is the origin of these multipeak patterns. The peak positions and energy separation values (Δ
L
3
(
a
2
-
a
1
) and Δ
L
2
(
b
2
-
b
1
)) are related to the symmetry of Ca2+, given in Table 1.Table 1
Peak positions and splitting energy values of the reference Ca L3,2-edge and the different areas of the C3S particles in the presence of CaCl2.
Samples
Peak positions (ev)
ΔCa-L3(a
2
-
a
1)
ΔCa-L2(b
2
-
b
1)
a
1
a
2
b
1
b
2
CaCl2-ref
348.0
349.2
351.3
352.4
1.2
1.1
Ca(OH)2-ref∗
347.6
349.1
351.0
352.4
1.5
1.4
Unhydrated C3S-ref∗
347.7
349.0
351.1
352.3
1.3
1.2
C-S-H-ref∗
348.1
349.1
351.4
352.4
1.0
1.0
Area 1
347.7
349.0
351.1
352.3
1.3
1.2
Area 2
347.9
348.9
351.2
352.3
1.0
1.1
Area 3
347.8
349.0
351.1
352.3
1.2
1.2
∗Data from Bae et al. [22].Figure 2
(a) Ca L3,2-edge NEXAFS spectra of the reference anhydrous C3S∗, C-S-H∗, C
a
O
H
2
∗, and CaCl2 (∗ data from Bae et al. [22]), (b) and (c) a single image of 7-day-hydrated C3S in the presence of CaCl2 taken at, respectively, 340 eV and 525 eV, (d) Ca L3,2-edge NEXAFS spectra of the selected area in (b), and (e) O K-edge NEXAFS spectra of the selected area in (c).
(a)
(b)
(c)
(d)
(e)Figure2 presents the Ca L3,2-edge NEXAFS spectra of CaCl2, Ca(OH)2, pure C-S-H, and anhydrous C3S as the references. Each of these spectra is unique in terms of its shape and peak positions as well as the splitting shown in Table 1. The uniqueness of each spectrum has been used as a fingerprint for investigating the Ca structure. The larger splitting energy and peak intensity ratio in the Ca L3,2-edge NEXAFS spectra are good indications of the well-developed crystalline Ca structure, which means that the splitting energy values for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
) are nonlinearly related to the value of the crystal field parameter (10 Dq). Ca(OH)2, with octahedral symmetry (Oh) [24], shows larger splitting energy values and peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 compared with the values and ratios of unhydrated C3S and pure C-S-H due to the different electronegativity in the crystal field. The reference of pure C-S-H in 17-day-hydrated C3S [22] has the smallest splitting energy and peak intensity ratio shown in Figure 2, which result from its amorphous or poor-crystallinity structure. In addition, the peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 generally indicate the magnitude of the crystal field and suggest that C-S-H has the smallest crystal field parameter but that CaCl2 has the largest. The mean coordination number of CaCl2 is six in the octahedral crystal field (positive) [25].


## 3.3. STXM Analysis of 7-Day-Hydrated C3S with CaCl2

The selected areas of a 7-day-hydrated C3S particle in the presence of CaCl2 were analyzed by Ca L3,2-edge and O K-edge NEXAFS spectroscopy, as shown in Figure 2. The peak positions, splitting energy values, and references of Ca L3,2-edge NEXAFS spectra are listed in Table 1. In Figure 2(d), Ca L3,2-edge NEXAFS for Area 1 has the largest splitting energy and peak intensity ratio, and Area 2 has the smallest. The peak positions for Area 2 shift slightly by approximately −0.2 eV, but the splitting energy for Area 2 is similar to that of the C-S-H reference. The energy separation of C-S-H with 1.0 and 1.1 for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
) and the smallest peak intensity ratios for a
1
/
a
2 and b
1
/
b
2 in Area 2 suggest that the calcium in C-S-H has a randomly spherical coordination with the oxygen, which means that the coordination environment of calcium in C-S-H is asymmetric. This observation suggests that CaCl2 does not change the coordination environment of calcium in C-S-H. Moreover, the energy separation of Area 3, with 1.2 and 1.2 for Δ
(
a
2
-
a
1
) and Δ
(
b
2
-
b
1
), respectively, is similar to that of the unhydrated C3S reference. Therefore, these areas are identified by Ca L3,2-edge NEXAFS spectra, which are compared with the peak positions, splitting energy, and peak shape of the above references: Area 1 is related to Ca(OH)2, Area 2 is related to C-S-H, and Area 3 is related to unhydrated C3S.We also present the O K-edge NEXAFS spectroscopy of the three areas in the same particle shown in Figure2(e). The O K-edge NEXAFS spectrum can provide simultaneous chemical information about hydration products and unhydrated C3S and shows that the differences in the O K-edge NEXAFS spectra are sufficiently significant to distinguish the distinct hydration products.In Figure2(e), there are four peaks, labeled 1 through 4, in each O spectrum. Peak 1 is a π
∗ peak, and peaks 3 and 4 are σ
∗ peaks. However, peak 2 behaves differently across different types of crystallinity [26]. Peak 2 is sharper in Ca(OH)2 than are the peaks in C-S-H and in unhydrated C3S. The layered crystal structure of Portlandite (Ca(OH)2) is well known to be trigonal with the space group P3
-m1 and can be described as stacked sheets of distorted edge-sharing Ca-O octahedra along thec-axis [27]. Each hydroxyl group is coordinated by three Ca atoms in its layer and is surrounded by three other hydroxyl groups that belong to the adjacent layer. Unhydrated C3S is triclinic and consists of the mean coordination number (6.21) [28] of Ca cations and the SiO4 tetrahedra, which show varying degrees of disorder. Therefore, there are weaker π
∗ peaks, and peak 2 in the O K-edge NEXAFS of C-S-H is due to the poor crystallization of C-S-H.Image stacks, where a series of images is collected corresponding to each energy, were collected to convert into chemical component mapping. As shown in Figure3, the results were visualized by RGB overlay maps using Singular Value Decomposition (SVD) based on reference spectra [20]. Because various chemical components have unique NEXAFS spectra, the reference spectra obtained from known components were used to visualize and differentiate phases in a sample [21]. Both of the distributions of phases in the same hydrated C3S in the presence of CaCl2 with the Ca L3,2-edge and the O K-edge image stacks are presented in Figure 3. We observe a few magenta pixels (M = R + B = Ca(OH)2 + unhydrated C3S) in Figure 3. There are, however, numerous cyan pixels (C = G + B = C-S-H + unhydrated C3S) in Figure 3(b), which indicate unhydrated C3S in those pixels is transforming to C-S-H; however, there are a few yellow pixels (Y = R + G = Ca(OH)2 + C-S-H). Clearly, the C-S-H surrounds the unhydrated C3S, along with the formation of Ca(OH)2. Obviously, the RGB overlay map based on the O K-edge image stacks is similar to the RGB overlay map based on the Ca L3,2-edge image stacks. However, in the case of the O K-edge image stacks, the morphology of the C-S-H gel was more present in the details where the flocculent C-S-H gel surrounds the unhydrated C3S and Ca(OH)2, in good accordance with the results for Ca L3,2-edge.Figure 3
Decomposition diagram of RGB overlay maps using SVD for the C3S particle in the presence of CaCl2 based on NEXAFS spectra obtained from the reference in Figure 2 and the different areas in Figure 3: (a) Ca L3,2-edge image stacks; (b) O K-edge image stacks.
(a)
(b)Because RGB overlay maps using Singular Value Decomposition (SVD) can consist of no more than three components, the chemical component mapping for unhydrated C3S, C-S-H, and Ca(OH)2 are shown in Figure 3, and Figure 4 shows the RGB overlay maps with unhydrated C3S, C-S-H gel, and CaCl2. We observe numerous magenta pixels (M = R + B = CaCl2 + unhydrated C3S) even at the center of these unhydrated C3S, which indicates that CaCl2 diffuses into the inside of unhydrated C3S during the transformation from unhydrated C3S to C-S-H and Ca(OH)2. Interestingly, the red part (CaCl2) in Figure 4 was not intermingled with the green part, C-S-H gel, but instead with unhydrated C3S. In other words, CaCl2 coexists with unhydrated C3S instead of C-S-H in the case of 7-day-hydrated C3S with CaCl2. Calcium chloride increases the dissolution of C3S for easier diffusion of water into the hydrating particle and moves Ca2+ and silicon ions away from the particle to form C-S-H gel and Portlandite [2].Figure 4
Decomposition diagram of RGB overlay maps using SVD for the C3S particle in the presence of CaCl2 based on the Ca L3,2-edge NEXAFS spectra obtained from the reference in Figure 2 and the different areas in Figure 3.The Si K-edge NEXAFS analysis on 7-day-hydrated C3S in the presence of CaCl2 is shown in Figure 5. The flocculent and the fibrillary hydration product can be observed, respectively, in Figures 3(a) and 3(b). The distribution map of silicon, as seen in Figure 5(d), is the computed optical density between the image taken at the preadsorption edge at 1830 eV and the near-adsorption edge at 1840 eV. With respect to the hydration of C3S, Ca(OH)2 contains no silicon, whereas the unhydrated C3S and C-S-H have silicon-containing composition. Figures 5(b) and 5(f) show the Si K-edge NEXAFS spectra of the corresponding line-scan in Figure 5(a) and selected areas in Figure 5(e). Each spectrum consists of the main peak (a
1) and the multiple scattering peak (a
2). Peak a
1 is assigned to the transition of Si 1s electrons to the antibonding t
2 orbital (3p-like state), whereas peak a
2 is qualitatively attributable to the multiple scattering effect beyond the second coordination sphere [29].Figure 5
Si K-edge NEXAFS analysis of 7-day-hydrated C3S particles in the presence of CaCl2: (a) single image of one particle taken at 1855 eV, (b) Si K-edge NEXAFS spectra of the line-scan indicated in (a), (c) single image of another particle taken at 1855 eV, (d) image contrast mapping corresponding to (c), (e) selected areas in the image contrast map enlarged partly from (c) for the NEXAFS spectra, and (f) Si K-edge NEXAFS spectra taken from the different locations indicated in (e).
(a)
(b)
(c)
(d)
(e)
(f)The peak positions and energy separation(
Δ
a
2
-
a
1) shown in Table 2 are associated with the degree of polymerization in the silicates [22]. A line-scan on the flocculent particle shown in Figure 5(a) was selected to verify the changes from center to boundary, which had similar absorption features to synthetic C-S-H, with 0.66 and 0.95 Ca/Si ratios in previous studies [22]. Three locations (core, top, and bottom) were selected to verify the uniformity of the hydration product in Figure 5(e). These locations had no difference among their absorption features but were similar to Op and synthetic C-S-H, with a 1.44 Ca/Si ratio in previous studies [22]. Therefore, the hydration product of C3S with CaCl2 at 7 d in Figure 5(e) had a uniform degree of silicate polymerization. C3S hydration is a dissolution-precipitation process [30]; however, CaCl2 accelerated the rate of nucleation of the hydration product on the surface of the C3S particles significantly, but CaCl2 has relatively little effect on the growth rate [1]. Hence, after 7 d of hydration of C3S with CaCl2, the degree of silicate polymerization in the hydration product was close to that of the 17-day-hydrated C3S in previous studies [22], thus implying that CaCl2 increases the degree of silicate polymerization, especially in the case of Ip. Therefore, CaCl2 can also facilitate ion diffusion [31], prompt the dissolution process of C3S, and accelerate the formation of “inner product” C-S-H [2], ultimately resulting in a higher degree of silicate polymerization at either the core or the boundary of 7-day-hydrated C3S.Table 2
Peak positions and the energy separation between peak positions of Si K-edge NEXAFS spectra of selected areas in 7-day-hydrated C3S with CaCl2 (C and Op denote the core area and outer product of C3S, resp., in Bae et al. [22]).
Peaka
1 (eV)
Peaka
2
Δa
2
-
a
1
Blue line
1846.6
1863.2
16.6
Red line
1846.9
1863.0
16.1
Core
1846.5
1861.9
15.4
Top
1846.5
1861.9
15.4
Bottom
1846.5
1862.0
15.5
Anhydrous C3S∗
1847.7
1858.9
11.2
Syn-CSH0.66∗
1846.9
1863.4
16.5
Syn-CSH0.95∗
1846.7
1863.1
16.4
Syn-CSH1.44∗
1846.4
1861.5
15.1
Op∗
1847.4
1862.7
15.4
C∗
1847.7
1559.4, 1861.5
11.4, 13.8
∗Data from Bae et al. [22].The fracture surface of 7-day-hydrated C3S in the presence of CaCl2 imaged by SEM is shown in Figure 6. Thin-sheet C-S-H and the spongy mass C-S-H are observed in Figures 6(a)–6(c). “Honeycomb-like” morphologies of hydration products in the presence of CaCl2 are shown at ages of 3 h to 7 d by SEM [32]. Because of the 0.1 M CaCl2 used, the Ca/Si ratio for the honeycomb C-S-H ranges between 2.0 and 3.6, whereas that for the spongy mass C-S-H is also included in the range. CaCl2 accelerates the nucleation of the hydration product on the surface of the C3S particles [1] and possibly prompts the bridge of the dreierketten silicate chains of C-S-H to develop in the two-dimensional direction into thin-sheet C-S-H.Figure 6
(a)~(c) The morphology of 7-day-hydrated C3S in presence of CaCl2 imaged by SEM, (d) SEM-EDS analysis of (c), Cl versus Ca/Si.
(a)
(b)
(c)
(d)


## 3.4.29Si MAS NMR Analysis

29Si CP/MAS NMR was proven to be very useful for studying the chemical shifts of the corresponding element. The primary advantage of this method is its high resolution of signals, so the relationship between Si chemical shifts and corresponding assigned structure of silica unites is easy to establish. 29Si CP/MAS NMR plots for hydrated C3S with different hydration times in the absence or presence of CaCl2 are shown in Figure 7. To describe various types of silica unites, we used Q
n (n
≤
4) for the connectivity of SiO4 tetrahedra, where n represents the number of bridging oxygen (Si-O-Si): the Q
0 site is isolated SiO4 tetrahedra, Q
1 is SiO4 tetrahedra present in end unites of a silicate chain, Q
2 is a middle group, Q
3 is a branching site, and Q
4 sites link to four other SiO4 tetrahedra in a three-dimensional network. Amorphous C-S-H initially contains mainly dimeric silicate [33–35], whereas C3S includes nine inequivalent SiO4 tetrahedra (Q
0 sites) between −69 ppm and −75 ppm [36]. In our analysis, the Gaussian distributions centered about −75 ppm, −78 ppm, −83 ppm, and −89 ppm are attributed, respectively, to Q
0, Q
1, Q
2, and Q
3 units [37, 38]. 29Si CP/MAS NMR could not provide an accurate quantitative analysis of Q
n but instead indicated the chemical shift of the corresponding Q
n sites. However, for comparison, the trend for evaluating the fraction of Q
n could be reflected by 29Si CP/MAS NMR.Figure 7
29Si CP/MAS NMR spectra of C3S paste as a function of hydration time in the absence (a) and in the presence (b) of CaCl2.
(a)
(b)Prior to the 7-day-hydration process of C3S without CaCl2, the discrete SiO4 tetrahedra (Q
0) in unhydrated C3S transformed to dominant dimers Q
1 and doubly coordinated tetrahedral Q
2 in the hydrated C3S, while there was the occurrence of Q
3 at 3 days of hydration. However, the Q
0 and Q
1 peaks maintained the lowest level at 28 d, and the Q
2 peak became a predominant level. In the presence of CaCl2, the Q
2 peak played a dominant role in the dreierketten silicate chain structure after 1 d of hydration, and the Q
3 peak at −89 ppm began to emerge compared with the control group, as shown. CaCl2 is clearly indicated to considerably accelerate the formation of middle groups (Q
2) and branch sites (Q
3) in the silicate chains of C-S-H gel. CaCl2 significantly accelerates the rate of nucleation of the hydration product deposited on the surface of the C3S particles but has a relatively insignificant effect on the growth rate [1], thus explaining why the Q
2 peaks in the CaCl2 group played a leading role in the dreierketten silicate chain structure. The earlier emergence of Q
2 and Q
3 peaks of the CaCl2 group indicates that CaCl2 accelerates the degree of silicate polymerization in hydrated C3S in agreement with the above studies of the morphology of hydrated C3S in the presence of CaCl2. From the perspective of ionic diffusion, CaCl2 cannot only facilitate diffusion of ions [31] but also prompt the dissolution process of C3S and accelerate the formation of “inner product” C-S-H [2], along with a higher degree of silicate polymerization at either the core or the boundary of 7-day-hydrated C3S.


## 4. Conclusions

Scanning transmission X-ray microscopy with NEXAFS spectra offers the possibility of studying the effect of CaCl2 on the microstructure of hydration products. The chemical composition information of the systems can easily be determined by STXM with NEXAFS.The results from the Ca L3,2-edge NEXAFS spectra of C3S hydration in the presence of CaCl2 show that CaCl2 does not change the coordination of calcium within C-S-H. The O K-edge NEXAFS spectrum for C3S hydration in the presence of CaCl2 indicates that O K-edge NEXAFS is also very useful for distinguishing the chemical components in hydrated C3S.The chemical component mapping based on Ca L3,2-edge and O K-edge NEXAFS spectra and image stacks reveals that the hydration product for C-S-H surrounds the unhydrated C3S, along with the formation of Ca(OH)2 in 7-day-hydrated C3S in the presence of CaCl2. However, CaCl2 coexists not with C-S-H but with unhydrated C3S. It is possible for CaCl2 to accelerate the dissolution of ions from unhydrated C3S, which results in increasing the hydration.The network-cross fibrillary C-S-H gel was observed by STXM with Si K-edge NEXAFS spectra, whereas both the thin-sheet and the fibrillary C-S-H were also observed by SEM. CaCl2 increases the degree of silicate polymerization.The29Si MAS NMR analysis reveals that the presence of CaCl2 in hydrated C3S accelerates the formation of middle groups (Q
2) and branch sites (Q
3) in the silicate chains of C-S-H gel considerably, in agreement with increasing the degree of polymerization of the silicate chains in C-S-H gel considerably. CaCl2 possibly prompts the bridge of the dreierketten silicate chains of C-S-H to develop in the two-dimensional direction into crumpled-foil C-S-H.These findings definitely validate the STXM with NEXAFS spectra as a significantly powerful tool for investigating the microstructure of cementitious pastes, thus opening the possibility of studying thein situ nanostructure of cementitious materials during hydration.



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*Source: 215371-2015-12-01.xml*