Hydration Characteristics of Tricalcium Aluminate in the Presence of Nano-Silica

Abstract

Tricalcium aluminate (C₃A) is the most reactive component of the Portland cement and its hydration has an important impact on the workability and early strength of concrete. Recently, nanomaterials such as nano-silica (nano-SiO₂) have attracted much attention in cement-based materials because of its pozzolanic reactivity and the pore-filling effect. However, its influence on the hydration of C₃A needs to be well understood. In this study, the hydration kinetics of C₃A mixed with different percentages of nano-SiO₂ were studied and compared with pure C₃A. The hydration products were examined by different characterization techniques including XRD, XPS, and NMR spectroscopy and isothermal calorimetry analyses. The XRD results showed that the addition of nano-SiO₂ promoted the conversion of the intermediate product C₄AH₁₃. The isothermal calorimetry results showed that the addition of nano-SiO₂ significantly reduced the hydration exotherm rate of C₃A from 0.34 to less than 0.1 mW/g. With the presence of nano-SiO₂, the peaks for Q¹ were observed in ²⁹Si MAS-NMR measurements, and the content of Q¹ increased from 6.74% to 30.6% when the nano-SiO₂ content increased from 2 wt.% to 8 wt.%, whereas the proportion of Q⁴ gradually decreased from 89.1% to 63.6%. These results indicated a pozzolanic reaction provoked by the nano-SiO₂ combined with aluminate structures generating C-A-S-H gel.

1. Introduction

It is well known that the macroscopic properties of a cement-based material are a consequence of how the constituent particles are arranged and held together at micro- and nano-scales. Many studies have been using nanomaterials such as nano-SiO₂, nano-Al₂O₃, and nano-TiO₂ to improve the microstructure performance of cement-based materials [1,2]. However, most of them focused the impact of nanomaterials on the hydration of tricalcium silicate (C₃S) only [3], not on tricalcium aluminate (C₃A), which is the most intense hydration mineral component in cement. Though the typical proportion of C₃A in cement is only about 10 wt.% [4], it is the most reactive component of the Portland cement.

C₃A, together with alite (C₃S), belite (C₂S), and ferrite (C₄AF), are the main components of cement. Compared to C₃S, the hydration of C₃A is significantly faster, forming calcium hydroaluminates and other phases such as calcium hydroaluminate-ferrite, commonly called AFm [5]. However, the fast reaction, often named “flash setting” [6], will reduce the workability and strength of the final products, which is usually avoided by adding gypsum [4,5]. The sulfate in gypsum binds C₃A, generating sulfoaluminates instead of calcium hydroaluminates [7]. The reactions of C₃A with and without calcium sulfate are expressed as Equations (1)–(3) (in cement notation) [8,9]:

C₃A + 3CS + 32H → C₆AS₃H₃₂ (Ettringite)

(1)

C₆AS₃H₃₂ + 2C₃A + 4H2O → 3C₄ASH₁₂ (Monosulphate)

(2)

C₃A + H → C₂AH₈ + C₄AH₁₃ → C₃AH₆

(3)

It is well-known that nano-SiO₂ in cement can increase the density of the C-S-H gel and decrease the final porosity of the hydrated products [10,11]. Besides, it can reduce the amount of calcium hydroxide formation, as well as the setting time [12,13], and thus increase the hydration degree of cement [14,15] to obtain the best mechanical performance. All the above-mentioned advantages are achieved through the three mechanisms of nano-SiO₂: The nucleation reaction, pozzolanic effect, and pore-filling effect. Nano-SiO₂ can act as a nucleation site for C-S-H seeds, accelerating cement hydration. At the same time, nano-SiO₂ particles can generate C-S-H gel by undertaking pozzolanic reaction that further intensifies the growth of C-S-H gels in the matrix and consequently increases the final density [16,17]. The pozzolanic reaction is the reaction between nano-SiO₂ and calcium hydroxide, which allows the generation of C-S-H gel as expressed in Equations (4) and (5). In addition, the nano-SiO₂ particles can fill up micro-pores to reduce the overall porosity of the final products [18,19].

SiO₂ + H₂O → Si(OH)₄

(4)

Ca(OH)₂ + Si(OH)₄ → C-S-H

(5)

However, to our knowledge, research about the effect of nano-SiO₂ on C₃A hydration remains limited. A few works in the literature postulate that the pozzolanic reaction of nano-SiO₂ in a high-aluminum environment may generate C-A-S-H gel, similar to C-S-H gel, where some of the silicon tetrahedrons could be replaced by aluminum [20,21]. The main objective of this research is to study the effects of nano-SiO₂ on the hydration of C₃A and its hydrated products. It is worth noting that the Al/Si ratio used in this study was much higher than previous studies as the aluminum was not the substitute element [22].

In this research, X-ray diffraction (XRD) was used to determine the final hydration products of C₃A with different contents of nano-SiO₂. Besides, the isothermal calorimetry was chosen to examine the intensity and speed of different amounts of nano-SiO₂ on the heat flow of C₃A hydration. An X-ray photoelectron spectrometer (XPS) and nuclear magnetic resonance (NMR) were used to reveal the microstructure of the hydrated products. It is believed that the findings of this study can help to prove the existence of the C-A-S-H gel and other alterations in the hydration products of C₃A with different contents of nano-SiO₂.

2. Materials and Methods

2.1. Materials

The C₃A with a purity of 99 wt.%, employed in this experiment, was purchased from a company in Shanghai. The nano-SiO₂ with a purity of 99.5 wt.% and an average particle size of 15 ± 5 nm was provided by Shanghai Macklin Biochemical Co., Ltd. Figure 1a shows the XRD pattern of the pure C₃A with high and well-defined peaks, which proved to be cubic C₃A according to PDF#38-1429. In addition, the SEM image in Figure 1b shows that the size of the pure C₃A particle is about 10 μm. On the other hand, as seen in Figure 1c, the curve of pure nano-SiO₂ shows a hump at 20–25°, indicating that the nano-SiO₂ used in this study had noncrystallinity and high activity [23]. Therefore, XRD analysis can confirm that both nano-SiO₂ and pure C₃A were of high purity. Although the nano-SiO₂ nanoparticles tended to agglomerate, an average size of about 10 nm could be measured from the TEM image, as shown in Figure 1d.

2.2. Sample Preparation

In this study, five mixes of pure C₃A with different nano-SiO₂ contents (0, 2, 4, 6, and 8 wt.%) were prepared as indicated in

Table 1. Proportion of different components used during the preparation of the samples.

Sample Name	C3A (g)	Nano-SiO2 (g)	Water (g)	L/S
Pure C3A	1	0	50	50
C3A-2 wt.% nano SiO2	1	0.02	51	50
C3A-4 wt.% nano SiO2	1	0.04	52	50
C3A-6 wt.% nano SiO2	1	0.06	53	50
C3A-8 wt.% nano SiO2	1	0.08	54	50

. The samples were prepared through the following procedures: Initially, the specific amounts of C₃A and nano-SiO₂ in powder form were weighed and premixed. Then, the mixture was placed in a glass flask filled with deionized water and mixed continuously by a magnetic stirrer for 72 h. The speed of stirring was 800 rpm and the liquid-to-solid ratio (L/S) was kept constant at 50. It is worth emphasizing that the glass flask was always filled with nitrogen during the 72 h hydration process to prevent possible carbonization and impurities in the air. After 72 h, the samples were removed from the stirring platform and filtered using a 7 cm filter paper and a suction filter glass. Then, the samples were placed into a vacuum oven and allowed to dry for 7 days at a temperature of 40 °C. The dried samples were kept sealed for later testing.

2.3. Methods

X-ray diffraction (XRD, D8 Advance, Bruker, Germany) was used to determine the mineralogical composition of raw materials and hydration products of C₃A (with and without nano-SiO₂). A scanning rate of 0.08 °/s from 5° to 70° with Cu Kα radiation (λ = 1.5418 Å) was used, as well as a screen to prevent high background at small degrees.

Transmission Electron Microscopy (TEM, TALOS F200X, Thermo Fisher Scientific, Waltham, Massachusetts, USA) and Scanning Electron Microscopy (SEM, TM 250 FEG, Thermo Fisher Scientific, Waltham, Massachusetts, USA) were employed to determine the particle characteristics of the raw samples.

A Thermal Activity Monitor (TAM-AIR, TA Instruments, New Castle, Delaware, USA), equipment for isothermal calorimetry, was employed to analyze the heat flow produced during the hydration of the samples. The samples were prepared using 0.5 g of C₃A and the corresponding percentages of nano-SiO₂ with a water-to-solid ratio of 5. Prior to the hydration process, the samples in powder form were kept inside the device for 3–4 h until the calorimeter was stabilized. Then, the distilled water was added and the mixtures were stirred for 30 s to begin the hydration.

The solid-state Nuclear Magnetic Resonance (MAS-NMR, JEOL-600, Japan) technique was used in two different modes. The first mode was ²⁹Si MAS-NMR that employed single-pulse decoupling including 1024 scans with a relaxation delay of 60 s. The probe was 8 mm in diameter and the spinning speed was 5000 rpm. The second mode was ²⁷Al MAS-NMR, a single pulse with 1000 scans, and 10 s of relaxation time. The probe was 3.2 mm in diameter and the spinning speed was 12,000 rpm. The references used to calibrate the peaks were TSPA and AlK (SO₄) for ²⁹Si and ²⁷Al, respectively.

The X-ray photoelectron spectrometer (XPS, Thermo escalab XI+) was employed to verify the molecular structure of the hydration products and record the measurements with a monochromatic Al Kα (hν = 1486.6 eV) X-ray source, employing a flare area of 650 µm, calibrated by 284.8 eV C1s. A constant analyzer pass energy of 20 eV was applied.

3. Results and Discussion

3.1. Mineral Composition Analysis

The XRD analysis of the hydrated samples can be seen in Figure 2. In agreement with the literature [24,25], the majority of the peaks found in the results were related to Katoite (written as Ca₃Al₂(OH)₁₂ or C₃AH₆), which is one of the final forms of the C₃A hydration, indicating a good reaction process of C₃A. Another peak at 11.5° found in all the samples was related to C₄AH₁₃, which is an intermediate form during the hydration of C₃A. The small peak at 47.5° was only found in the hydration products of pure C₃A, which may be related to the carbonation of C₃A hydration products. Besides, another peak at 33.2° is associated with anhydrate C₃A, indicating that the hydration of pure C₃A was incomplete. The intensity of this peak was reduced after adding 2 wt.% of nano-SiO₂ and even smaller when the nano-SiO₂ content increased to 4 wt.%. Furthermore, this peak disappeared completely in the samples of C₃A with 6 wt.% and 8 wt.% nano-SiO₂. This result substantiates the promotion effect of nano-SiO₂ on the hydration process of C₃A.

3.2. Hydration Exothermic Analysis

The effect of nano-SiO₂ on the hydration heat release rate of C₃A was studied using the isothermal calorimetry, and the results can be seen in Figure 3. De Jong et al. [26] previously found a second exothermic peak observed in the C₃A hydration process after adding amorphous silica, which appeared in the first hours of hydration but being at later stage when the amorphous silica content increased. However, as shown in Figure 3a, this study showed different results. It can be seen from Figure 3b that the pure C₃A sample showed the highest reactivity and the peak in hydration heat release rate occurred at 420 s. When 2 wt.% nano-SiO₂ was added to the C₃A hydration system, the hydration heat release rate of C₃A was greatly reduced from 0.34 to less than 0.1 mW/g. The hydration rate can be further reduced with increasing nano-SiO₂ content, but the significance is not obvious. With 2 wt.% nano-SiO₂, the hydration exothermic peak was delayed by more than 2 min. In addition, a further delay was noted with the increase in the nano-SiO₂ content. Xu et al. [3] stated that the addition of nano-SiO₂ will accelerate the rate of heat release in the early stage of C₃S hydration. However, the opposite phenomenon occurred during the C₃A hydration process. The reduction in heat release can be attributed to two reasons: (1) The hydration reaction rate of C₃A was much higher than that of C₃S, and the surface of the C₃A particles was adsorbed with a large amount of nano-SiO₂ with high specific surface area, which would reduce the contact area between C₃A and water, thus slowing down the reaction rate [27]; (2) the surface of C₃A particles was covered by the C-A-S-H gels (generated by the pozzolanic reaction of nano-SiO₂ combined with C₃A), thereby reducing the reaction rate. This is consistent with the phenomenon that the appearance time of the C₃A hydration exothermic peak continuously delayed as the amount of nano-SiO₂ increased. Hou et al. [28] explored the influence of nano-SiO₂ on the hydration process of C₃A-gypsum and C₃A-C₃S-gypsum systems, and obtained similar conclusions. They believed that the nano-SiO₂ adsorbed on the surface of C₃A due to the electrostatic effect is the reason for the delayed hydration. At the same time, the C-S-H gel generated by the pozzolanic effect of nano-SiO₂ can cover the surface of C₃A, which will also inhibit the hydration heat release rate of C₃A.

3.3. X-ray Photoelectron Spectroscopy Results

The XPS was performed to analyze the binding energies of Al 2p and Si 2p in the hydration products of C₃A with different nano-SiO₂ contents. For Al 2p, previous work has indicated that the binding energy of octahedral coordinated aluminum is generally higher than that of the tetrahedral form [29]. It can be seen from Figure 4a that the Al 2p binding energy of the pure C₃A hydration products was around 74.1 eV, which can be related to C₃AH₆ [30,31]. All the samples containing nano SiO₂, except C₃A-8 wt.%, gave similar results showing a unique peak around 74.1 eV. The sample of C₃A with 8 wt.% nano-SiO₂ presented a peak around 74.3 eV. It can be inferred that the amount of 8 wt.% nano-SiO₂ was enough to influence the binding energy of C₃A hydration products. It also indicated that the reaction between nano-SiO₂ and C₃A could create an Al-O-Si bond as Al would migrate to Si due to its high electronegativity, thus increasing the Al 2p binding energy [32]. On the other hand, the binding energy of Si 2p in all hydration products is shown in Figure 4b. It can be seen from the figure that the Si 2p binding energy of pure nano-SiO₂ is 103.6 eV, while the Si 2p binding energy of C₃A with 2 wt.% nano-SiO₂, however, showed a very low intensity, almost being a plateau. This phenomenon indicates that there is Al element insertion in the silicon chain, which leads to a substantial decrease in Si 2p binding energy. Besides, the Si 2p binding energy rebounded with the amount of nano-SiO₂ increased. When the nano-SiO₂ content increased to 4 wt.%, 6 wt.%, and 8 wt.%, the Si 2p binding energy rebounded to 101.8, 101.9, and 102.3 eV, respectively, and the peaks became more obvious. This phenomenon could prove the formation of Al-O-Si bonds when nano-SiO₂ was added to C₃A. Overall, the Si 2p results are consistent with the Al 2p binding energy results, and are in good agreement with the previous findings [33,34].

3.4. Structural Changes Observed by Nuclear Magnetic Resonance

The NMR spectroscopy was performed with the intention of finding relevant information to prove a possible alteration in the hydration products structures brought by nano-SiO₂ [35]. Figure 5 shows the ²⁹Si and ²⁷Al MAS-NMR results of all samples, in order to reveal the effect of nano-SiO₂ on the structure of C₃A hydration products. As shown in Figure 5a, all the hydration samples showed similar ²⁷Al MAS-NMR results with a unique peak at 12.18 ppm, which is related to the octahedral aluminum configuration in the C₃AH₆ component [8,36,37]. The absence of any shift in the peak can lead to the conclusion that the presence of nano-SiO₂ would not alter the original structure formed by the C₃A hydration [35].

However, the ²⁹Si MAS-NMR results shown in Figure 5b are different from the ²⁷Al MAS-NMR results. The peaks were labeled as Qⁿ, where “n” is the number of similar tetrahedrons connected in the molecule. For example, Q⁰ refers to a silicon tetrahedral configuration completely insolated to other silicon, whereas Q² refers to a silicon tetrahedral configuration connected with another two silicon tetrahedrons, forming a chain of silicon, as illustrated in Figure 6. According to the ²⁹Si MAS-NMR results, the two peaks observed around −79.2 and −112.5 ppm were Q¹ and Q⁴, respectively. In this context, Q¹ implies the existence of a silicon tetrahedron in the final position of a chain [38,39,40,41]. Sometimes, this peak is found in a more negative value (around −81 ppm) [39,42]. This chemical shift is in agreement with the presence of aluminum in the environment of silicon tetrahedra to reach more positive ppm values [42,43]. This is a good indicator of the generation of dimers, combining silicon and aluminum tetrahedra. The intensity of this peak grew considerably with nano-SiO₂ content, owing to the larger amount of dimers generated within the structure. The peak Q⁴ refers to the presence of a “three-dimensional” net of silicon tetrahedra in the sample [44,45], and is strongly related to nano-SiO₂ [23,46]. In the Q¹ peak, there is a shift to more positive values due to the presence of aluminum within the close environment of the silicon tetrahedron. Additionally, new peaks were found in some of the samples. The Q⁰ peak appeared in hydration products of C₃A with 6 wt.% and 8 wt.% nano SiO₂, which is associated with silicon tetrahedra completely insolated. Due to this peak only being found in samples with a high percentage of nano-SiO₂, this could imply that some silicon ions from the dissolution of the nano-SiO₂ were not integrated into the C-A-S-H structure.

The percentages of different Qⁿ peaks obtained by Gaussian deconvolution are shown in Figure 7. It is quite clear to show that the existence of Q⁰ depends on nano-SiO₂ content. On the other hand, the proportion of Q¹ increased rapidly from 6.74% to 30.6% when the amount of nano-SiO₂ increased from 2 wt.% to 8 wt.%. However, the proportion of Q⁴ gradually decreased from 89.1% to 63.6% correspondingly. This finding implies that the higher the nano-SiO₂ content, the more silicon dimers in hydration products can be formed. Additionally, the positions of the Q⁴ peaks were slightly shifted to more negative values from −112.63 to −113.53 ppm as the nano-SiO₂ content increased from 2 to 8 wt.%. This observation is in agreement with the trend of the position of the pure nano-SiO₂ Q⁴ peak, around −115 ppm [23]. In addition, not all the silicon tetrahedra would be influenced by aluminum with a high content of nano-SiO₂, and the shift to more positive values created by the aluminum would become weaker.

Owing to the C-A-S-H gel being amorphous, its presence can only be revealed by the apparition of the Q^{1 29}Si MAS NMR peak and the shifted Q⁴ to more positive values under the influence of aluminum. Based on the findings from this study, it is safe to say that nano-SiO₂ can promote the apparition of C-A-S-H gel in the final hydration products of C₃A, besides the C₃AH₆ structure.

4. Conclusions and Recommendations

This study examined the effect of nano-SiO₂ on the hydration of C₃A in cement. According to the test results, the following conclusions are drawn:

(1)

The addition of nano-SiO₂ can promote the hydration degree of C₃A while significantly reducing the heat release rate of C₃A hydration from 0.34 to less than 0.1 mW/g, and the occurrence time of the hydration exothermic peak was delayed by more than 2 min. The main reasons are probably the surface of C₃A being adsorbed by nano-SiO₂ and/or covered by the C-A-S-H gel (formed by the pozzolanic hydration reaction of nano-SiO₂ and C₃A) at an early age, thereby reducing the contact area of C₃A with water.

(2)

The reaction between nano-SiO₂ and C₃A can establish Si-O-Al bonds and generate C-A-S-H gels. The chemical shifts in Al 2p and Si 2p both confirm this conclusion. In addition, ²⁹Si MAS-NMR results showed that Q¹ appeared after the nano-SiO₂ was added to the C₃A hydration system. With the nano-SiO₂ content increased, the proportion of Q¹ in the hydration product increased from 6.74% to 30.6%, while the proportion of Q⁴ gradually decreased from 89.1% to 63.6%.

(3)

The addition of nano-silica can promote the hydration reaction rate of C₃S while delaying the hydration of C₃A. For the two hydration systems of C₃A and C₃S, the addition of nano-SiO₂ has shown completely different effects, and the influence mechanism of nano-SiO₂ in the two different hydration processes needs further exploration.