Journal of Nanomaterials
Volume 2015 (2015), Article ID 215371, 10 pages
http://dx.doi.org/10.1155/2015/215371

Research Article

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

Qinfei Li,1,2 Yong Ge,¹ Guoqing Geng,² Sungchul Bae,2,3 and Paulo J. M. Monteiro²

¹School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin 150090, China
²Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720, USA
³Faculty of Science and Technology, Tokyo University of Science, 2641 Yamasaki, Noda, Chiba 278-8510, Japan

Received 20 August 2015; Revised 5 November 2015; Accepted 16 November 2015

Academic Editor: Jin-Ho Choy

Copyright © 2015 Qinfei Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 () and branch sites () in the silicate chains of C-S-H gel at 1-day hydration.

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 ( 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 for ²⁹Si 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 Figure 1. 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 ( and ) corresponding to L₃ and L₂, along with several smaller peaks ( and ) 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 ( and ) 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()
ΔCa-L₂()

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].

(a)

(b)

(c)

(d)

(e)

Figure 2: (a) Ca L3,2-edge NEXAFS spectra of the reference anhydrous C₃S, C-S-H, , 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).

Figure 2 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 and 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 and 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 and 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 and and the smallest peak intensity ratios for and 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 and , 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 Figure 2(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 Figure 2(e), there are four peaks, labeled 1 through 4, in each O spectrum. Peak 1 is a peak, and peaks 3 and 4 are 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 Pm1 and can be described as stacked sheets of distorted edge-sharing Ca-O octahedra along the c-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 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 Figure 3, 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.

(a)

(b)

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.

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 () and the multiple scattering peak (). Peak is assigned to the transition of Si 1s electrons to the antibonding orbital (3p-like state), whereas peak is qualitatively attributable to the multiple scattering effect beyond the second coordination sphere [29].

(a)

(b)

(c)

(d)

(e)

(f)

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).

The peak positions and energy separation ) 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]).

Peak (eV)
Peak
Δ

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.

(a)

(b)

(c)

(d)

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.

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 () for the connectivity of SiO₄ tetrahedra, where represents the number of bridging oxygen (Si-O-Si): the site is isolated SiO₄ tetrahedra, is SiO₄ tetrahedra present in end unites of a silicate chain, is a middle group, is a branching site, and 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 ( 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 , , , and units [37, 38]. ²⁹Si CP/MAS NMR could not provide an accurate quantitative analysis of but instead indicated the chemical shift of the corresponding sites. However, for comparison, the trend for evaluating the fraction of could be reflected by ²⁹Si CP/MAS NMR.

(a)

(b)

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.

Prior to the 7-day-hydration process of C₃S without CaCl₂, the discrete SiO₄ tetrahedra () in unhydrated C₃S transformed to dominant dimers and doubly coordinated tetrahedral in the hydrated C₃S, while there was the occurrence of at 3 days of hydration. However, the and peaks maintained the lowest level at 28 d, and the peak became a predominant level. In the presence of CaCl₂, the peak played a dominant role in the dreierketten silicate chain structure after 1 d of hydration, and the 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 () and branch sites () 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 peaks in the CaCl₂ group played a leading role in the dreierketten silicate chain structure. The earlier emergence of and 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.

The ²⁹Si MAS NMR analysis reveals that the presence of CaCl₂ in hydrated C₃S accelerates the formation of middle groups () and branch sites () 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 the in situ nanostructure of cementitious materials during hydration.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

STXM data were acquired at beamline BL5.3.2.1 and beamline BL5.3.2.2 at the Advanced Light Source, supported by the Director of the Office of Science, Office of Basic Energy Sciences of the US Department of Energy, under Contract no. DE-AC02-05CH11231. Funding for this project was provided by the National Natural Science Foundation of China (no. 51278157).

References

J. J. Thomas, A. J. Allen, and H. M. Jennings, “Hydration kinetics and microstructure development of normal and CaCl₂-accelerated tricalcium silicate pastes,” Journal of Physical Chemistry C, vol. 113, no. 46, pp. 19836–19844, 2009.

M. C. G. Juenger, P. J. M. Monteiro, E. M. Gartner, and G. P. Denbeaux, “A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration,” Cement and Concrete Research, vol. 35, no. 1, pp. 19–25, 2005.

P. K. Mehta and P. J. M. Monteiro, Concrete: Microstructure, Properties, and Materials, McGraw-Hill, New York, NY, USA, 4th edition, 2014.

V. K. Peterson and A. E. Whitten, “Hydration processes in tricalcium silicate: application of the boundary nucleation model to quasielastic neutron scattering data,” Journal of Physical Chemistry C, vol. 113, no. 6, pp. 2347–2351, 2009.

E. M. Gartner, J. F. Young, D. A. Damidot, and I. Jawed, “Hydration of portland cement,” in The Structure and Performance of Cements, J. Bensted and P. Barnes, Eds., chapter 3, Spon Press, London, UK, 2nd edition, 2002.

M. C. G. Juenger and H. M. Jennings, “The use of nitrogen adsorption to assess the microstructure of cement paste,” Cement and Concrete Research, vol. 31, no. 6, pp. 883–892, 2001.

J. Skalny, I. Odler, and J. Hagymassy Jr., “Pore structure of hydrated calcium silicates. I. Influence of calcium chloride on the pore structure of hydrated tricalcium silicate,” Journal of Colloid And Interface Science, vol. 35, no. 3, pp. 434–440, 1971.

E. M. Gartner, K. E. Kurtis, and P. J. M. Monteiro, “Proposed mechanism of C-S-H growth tested by soft X-ray microscopy,” Cement and Concrete Research, vol. 30, no. 5, pp. 817–822, 2000.

V. S. Ramachandran and R. F. Feldman, “Time-dependent and intrinsic characteristics of portland cement hydrated in the presence of calcium chloride,” Il Cemento, vol. 3, 1978.

A. M. Rosenberg, “Study of the mechanism through which calcium chloride accelerates the set of portland cement,” Journal of American Concrete Institute, vol. 61, pp. 1261–1269, 1964.

G. W. Groves, P. J. Le Sueur, and W. Sinclair, “Transmission electron microscopy and microanalytical studies of ion-beam-thinned sections of tricalcium silicate paste,” Journal of the American Ceramic Society, vol. 69, no. 4, pp. 353–356, 1986.

K. E. Kurtis, P. J. M. Monteiro, J. T. Brown, and W. Meyer-Ilse, “High resolution transmission soft X-ray microscopy of deterioration products developed in large concrete dams,” Journal of Microscopy, vol. 196, no. 3, pp. 288–298, 1999.

K. E. Kurtis, P. J. M. Monteiro, J. T. Brown, and W. Meyer-Ilse, “Imaging of ASR gel by soft X-ray microscopy,” Cement and Concrete Research, vol. 28, no. 3, pp. 411–421, 1998.

J. Ha, S. Chae, K. W. Chou, T. Tyliszczak, and P. J. M. Monteiro, “Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study,” Journal of Materials Science, vol. 47, no. 2, pp. 976–989, 2012.

J. Ha, S. Chae, K. W. Chou, T. Tyliszczak, and P. J. M. Monteiro, “Scanning transmission X-ray microscopic study of carbonated calcium silicate hydrate,” Transportation Research Record, vol. 2142, pp. 83–88, 2010.

S. R. Chae, J. Moon, S. Yoon et al., “Advanced nanoscale characterization of cement based materials using X-ray synchrotron radiation: a review,” International Journal of Concrete Structures and Materials, vol. 7, no. 2, pp. 95–110, 2013.

P. J. M. Monteiro, L. Clodic, F. Battocchio et al., “Incorporating carbon sequestration materials in civil infrastructure: a micro and nano-structural analysis,” Cement and Concrete Composites, vol. 40, pp. 14–20, 2013.

D. Hernández-Cruz, C. W. Hargis, S. Bae et al., “Multiscale characterization of chemical—mechanical interactions between polymer fibers and cementitious matrix,” Cement and Concrete Composites, vol. 48, pp. 9–18, 2014.

S. Yoon, J. Ha, S. R. Chae, D. A. Kilcoyne, and P. J. M. Monteiro, “X-ray spectromicroscopic study of interactions between NaCl and calcium silicate hydrates,” Magazine of Concrete Research, vol. 66, no. 3, pp. 141–149, 2014.

G. Johansson, J. Dynes, A. Hitchcock, T. Tyliszczak, G. Swerhone, and J. Lawrence, “Chemically sensitive tomography at 50 nm spatial resolution using a soft X-ray scanning transmission X-ray microscope,” Microscopy and Microanalysis, vol. 12, supplement S02, pp. 1412–1413, 2006.

Z. Yangquanwei, S. Neethirajan, and C. Karunakaran, “Cytogenetic analysis of quinoa chromosomes using nanoscale imaging and spectroscopy techniques,” Nanoscale Research Letters, vol. 8, no. 1, article 463, 7 pages, 2013.

S. Bae, R. Taylor, D. Hernández-Cruz, S. Yoon, D. Kilcoyne, and P. J. M. Monteiro, “Soft X-ray spectromicroscopic investigation of synthetic C-S-H and C₃S hydration products,” Journal of the American Ceramic Society, vol. 98, no. 9, pp. 2914–2920, 2015.

A. L. D. Kilcoyne, T. Tyliszczak, W. F. Steele et al., “Interferometer-controlled scanning transmission X-ray microscopes at the Advanced Light Source,” Journal of Synchrotron Radiation, vol. 10, no. 2, pp. 125–136, 2003.

S. J. Naftel, T. K. Sham, Y. M. Yiu, and B. W. Yates, “Calcium L-edge XANES study of some calcium compounds,” Journal of Synchrotron Radiation, vol. 8, no. 2, pp. 255–257, 2001.

F. M. F. de Groot, J. C. Fuggle, B. T. Thole, and G. A. Sawatzky, “2p x-ray absorption of 3d transition-metal compounds: an atomic multiplet description including the crystal field,” Physical Review B: Condensed Matter and Materials Physics, vol. 42, no. 9, pp. 5459–5468, 1990.

R. T. DeVol, R. A. Metzler, L. Kabalah-Amitai et al., “Oxygen spectroscopy and polarization-dependent imaging contrast (PIC)-mapping of calcium carbonate minerals and biominerals,” Journal of Physical Chemistry B, vol. 118, no. 28, pp. 8449–8457, 2014.

T. Nagai, T. Ito, T. Hattori, and T. Yamanaka, “Compression mechanism and amorphization of portlandite, Ca(OH)₂: structural refinement under pressure,” Physics and Chemistry of Minerals, vol. 27, no. 7, pp. 462–466, 2000.

H. W. F. Taylor, Cement Chemistry, Thomas Telford, London, UK, 2nd edition, 1997.

D. Li, G. M. Bancroft, M. E. Fleet, and X. H. Feng, “Silicon K-edge XANES spectra of silicate minerals,” Physics and Chemistry of Minerals, vol. 22, no. 2, pp. 115–122, 1995.

J. J. Thomas, H. M. Jennings, and J. J. Chen, “Influence of nucleation seeding on the hydration mechanisms of tricalcium silicate and cement,” Journal of Physical Chemistry C, vol. 113, no. 11, pp. 4327–4334, 2009.

N. B. Singh and P. N. Ojha, “Effect of CaCl₂ on the hydration of tricalcium silicate,” Journal of Materials Science, vol. 16, no. 10, pp. 2675–2681, 1981.

J. F. Young, R. L. Berger, and F. V. Lawrence Jr., “Studies on the hydration of tricalcium silicate pastes III. Influence of admixtures on hydration and strength development,” Cement and Concrete Research, vol. 3, no. 6, pp. 689–700, 1973.

J. J. Beaudoin, L. Raki, and R. Alizadeh, “A ²⁹Si MAS NMR study of modified C-S-H nanostructures,” Cement and Concrete Composites, vol. 31, no. 8, pp. 585–590, 2009.

M. Chloup-Bondant and O. Evrard, “Tricalcium aluminate and silicate hydration. Effect of limestone and calcium sulfate,” in Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials, P. Colombet, H. Zanni, A.-R. Grimmer, and P. Sozzani, Eds., pp. 295–308, Springer, Berlin, Germany, 1998.

S. A. Rodger, G. W. Groves, N. J. Clayden, and C. M. Dobson, “Hydration of tricalcium silicate followed by ²⁹Si NMR with cross-polarization,” Journal of the American Ceramic Society, vol. 71, no. 2, pp. 91–96, 1988.

M. Mägi, E. Lippmaa, A. Samoson, G. Engelhardt, and A.-R. Grimmer, “Solid-state high-resolution silicon-29 chemical shifts in silicates,” Journal of Physical Chemistry, vol. 88, no. 8, pp. 1518–1522, 1984.

C. E. Tambelli, J. F. Schneider, N. P. Hasparyk, and P. J. M. Monteiro, “Study of the structure of alkali-silica reaction gel by high-resolution NMR spectroscopy,” Journal of Non-Crystalline Solids, vol. 352, no. 32–35, pp. 3429–3436, 2006.

R. J. Myers, S. A. Bernal, R. San Nicolas, and J. L. Provis, “Generalized structural description of calcium-sodium aluminosilicate hydrate gels: the cross-linked substituted tobermorite model,” Langmuir, vol. 29, no. 17, pp. 5294–5306, 2013.