3.2.1. Room-Temperature XRD
The room-temperature XRD test results of samples blank to 13# are shown in
Figure 2. The blank sample mainly consists of γ-C
2S, with a small amount of β-C
2S. From the stacked waterfall plot, it can be observed that boron doping can stabilize the β and α′
H type C
2S. At lower boron doping levels, sample 1# contains a small amount of γ-C
2S and the intermediate phase 3CaO·2SiO
2 (C
3S
2, Rankinite). This is because the presence of boron in the solid-phase reaction contributes to the stabilization of the C
2S. However, due to the partial volatilization of boron, the ratio of n (CaO) to n (SiO
2) fails to reach 2:1, resulting in a small amount of C
3S
2 formation. Interestingly, with the significant volatilization of boron, β-C
2S is also stabilized within the system. Similarly, in sample 2#, despite the absence of the intermediate phase C
3S
2, the boron dissolved in the system remains inadequate to fully stabilize β-C
2S. When the solid solubility of boron reaches 0.309 wt.% (seen in sample 3#), the C
2S undergoes a complete transformation into the β phase. Furthermore, as the solid solubility of boron increases, the crystal structure of C
2S transitions from the β phase to the α′
H phase.
In order to provide a clearer representation of the changes in the crystal structure of β-C
2S and α′
H-C
2S as the boron solid solubility varies,
Figure 3a,b illustrate the comparison between samples 1# and 13# and the standard cards of β-C
2S and α′
H-C
2S. By comparing the main diffraction peaks, it is evident that sample 3# exhibits a perfect match with β-C
2S without any additional substitution peaks. This finding indicates that sample 3# has undergone a complete transformation into β-C
2S. As the level of boron doping increases, a gradual transition of the crystal structure from β-C
2S to α′
H-C
2S is observed, starting from sample 4#. This observation is consistent with the earlier discussion on the substantial change in boron evaporation between samples 3# and 4#, thereby mutually confirming the observed transformation. During the transformation from β-C
2S to α′
H-C
2S, characteristic diffraction peaks exhibit noticeable changes. The range of 31.7°–33° corresponds to the primary characteristic diffraction peaks of C
2S, and a significant transformation is observed within this range from sample 3# to 13#. The diffraction peaks at 34.2°, 35.3°, 36°–38°, and 45.5° are distinctive features that differentiate β-C
2S from α′
H-C
2S. As the boron doping level increases, the intensity of these peaks gradually decreases until they eventually vanish. On the other hand, the diffraction peaks at 33.3°, 38°, 40.4°, 46.8°, and 52.2° characterize α′
H-C
2S, and their intensity becomes more prominent with higher concentrations of boron solid solution.
The characteristic diffraction peaks of β-C
2S and α′
H-C
2S can be divided into five window regions, denoted as W1–W5, which are displayed in
Figure 3. These window regions include the following angles: 30.5°–35.7°, 36.5–39°, 40°–42.2°, 45°–48.5°, and 51.5°–55°. Detailed comparisons are conducted by extracting the diffraction peaks within these regions, as illustrated in
Figure 4. From an overall perspective, the diffraction peaks of samples 3#–5# exhibit the characteristics of β-C
2S primarily due to the relatively small amount of α′
H-C
2S formed, which is not prominently reflected in the XRD pattern. In samples 6#–10#, with an increase in the boron solid solution content, the formation of α′
H-C
2S gradually increases, accompanied by more pronounced diffraction peaks, especially in the 32°–33° characteristic diffraction region. In samples 11#–13#, the XRD diffraction peaks are mainly attributed to α′
H-C
2S, with 13# showing almost no diffraction peaks of β-C
2S. This indicates that at this stage, C
2S has transitioned almost completely from the β phase to the α′
H phase.
The characteristic diffraction peaks in the five window regions were calibrated based on the ICSD standard cards for β-C
2S (ICSD No. 81,096) [
34] and α′
H-C
2S (ICSD No. 81,097) [
34]. In the W1 window, particularly in the 32°–33° region, the characteristic diffraction peaks of β-C
2S are primarily attributed to the crystal planes (
)
β, (200)
β, and (121)
β. As the transition to α′
H-C
2S occurs, the characteristic diffraction peaks are then generated by the crystal planes (020)
α′H and (211)
α′H. The characteristic peaks of β-C
2S at around 34.3° and 35.3° are attributed to the crystal plane (103)
β and (210)
β. These peaks gradually weaken and disappear as the boron solubility increases. Simultaneously, the characteristic peak of α′
H-C
2S at around 33.2° gradually emerges and strengthens, which is generated by the crystal plane (013)
α′H. The pattern of characteristic diffraction peaks in the W2–W5 windows is similar to that in the W1 window. We have compared the disappearance and formation of diffraction peaks in each window, as presented in
Table 5.
β-C
2S is classified as belonging to the monoclinic crystal system, while α’
H-C
2S is classified as belonging to the orthorhombic crystal system. During the process of structural transformation, the symmetry of the crystal increases, resulting in the equivalence of originally inequivalent crystal planes. For instance, the crystal planes (
)
β and (121)
β, (
)
β and (222)
β, as well as (
)
β and (231)
β, become equivalent in the transition to the orthorhombic system. These equivalent crystal planes in the orthorhombic system are denoted as (211)
α′H, (222)
α′H, and (321)
α′H inα′
H-C
2S by combining the corresponding relationships in
Table 5. Upon examining the corresponding relationships in
Table 5, a correlation between the crystal planes that cause the characteristic diffraction peaks of β-C
2S and α′
H-C
2S becomes apparent. In particular,
(equivalent crystal planes are taken into account). Furthermore, the β-C
2S has a unit cell with parameters a = 5.5121 Å, b = 6.7575 Å, c = 9.3138 Å, α = 90°, β = 94.581°, and γ = 90° [
34] while the α′
H-C
2S has a unit cell with parameters a = 6.7673 Å, b = 5.5191 Å, c = 9.3031 Å, α = 90°, β = 90°, and γ = 90° [
34]. It can be deduced that during the transformation from β-C
2S to α′
H-C
2S, the β angle changes from 94.581° to 90°, while the a-axis and b-axis undergo an exchange simultaneously. After examining the unit cells of β-C
2S and α′
H-C
2S, it is evident that the β-C
2S crystal structure undergoes a structural transformation using a transformation matrix
, then converts the symmetry of transformed cells into an orthorhombic system with a space group of Pnma. As a result, the final transformed structure closely resembles that of α′
H-C
2S. The transformation process was simulated using VESTA software (Version 3.5.8) [
38], as illustrated in
Figure 5a–d.
Figure 5a depicts the β-C
2S unit cell observed along the b-axis.
Figure 5b exhibits the β-C
2S unit cell after undergoing a structural transformation using the transformation matrix
, observed from the b-axis.
Figure 5c represents the unit cell observed along the b-axis after modifying the symmetry to that of an orthorhombic system (with a space group Pnma) and setting the occupancies of Ca and O as 0.5.
Figure 5d illustrates the α′
H-C
2S unit cell observed along the b-axis. Indeed, it is evident that the structure obtained from the transformation of the β-C
2S unit cell, as shown in
Figure 5c, closely resembles the α′
H-C
2S unit cell.
As the two-unit cell has a transformation matrix, the planes also have the same corresponding relationships. Taking the plane (
)
β as an example, let us see how the plane transforms during the phase transition. The process is given below.
The diffraction peak at around 35.3° in the β-C
2S phase is generated by the (210)
β crystal plane, however, there is no corresponding diffraction peak in the α′
H-C
2S phase. The (210)
β crystal plane theoretically corresponds to the (120)
α′H crystal plane in α′
H-C
2S, but the structure of α′
H-C
2S is orthorhombic. Due to the symmetry of the space group, the (120)
α′H crystal plane exhibits forbidden diffraction, leading to the absence of diffraction peaks generated by the (120)
α′H crystal plane in α′
H-C
2S. As shown in
Figure 6, a comparison is made between the zone axis
and
. It can be observed that the (120)
α′H crystal plane belongs to the space group absence, therefore, no diffraction spots are observed.
3.2.2. In-Situ High-Temperature XRD
The analysis using room-temperature XRD confirmed that sample 3# is pure β-C
2S. In-situ high-temperature XRD testing was conducted on this sample by collecting data at regular intervals during the temperature ramp from 100 °C to 1300 °C, with data collected every 100 °C. The temperature was maintained at a constant during the data acquisition process. The waterfall plot of the test data is shown in
Figure 7. In general, as the temperature increases, the C
2S crystal structure undergoes a transformation from β to α′
L phase, and then to α′
H phase. Below 300 °C, the crystal structure of C
2S is β phase. At 300 °C, the appearance of the α′
L diffraction peak indicates the onset of phase transition. By 600 °C, the characteristic diffraction peak of β-C
2S disappears completely, indicating a complete transformation to the α′
L phase. The transition temperature of β to α′
L is slightly lower than the temperature shown in
Figure 1 (690 °C), which can be primarily attributed to the decrease in transition temperature resulting from the presence of boron as a dopant. When the temperature reaches 1200 °C, α′
L-C
2S transforms into α′
H-C
2S, which is consistent with the reported transition temperature of 1160 °C for the α′
L to α′
H transformation. At 1300 °C, no signs of α′
H-C
2S transforming into α-C
2S were observed. This may be due to the rapid transformation from α′
H-C
2S to α-C
2S, where the temperature range for the transformation is small, and the experimental temperature did not reach the phase transition temperature (1425 °C). The α-Al
2O
3 diffraction peaks observed in
Figure 7 are a result of the sample holder.
In order to facilitate the comparison of X-ray diffraction (XRD) results obtained at high temperatures and room temperatures, three specific windows were chosen to display the high-temperature XRD data. These windows cover the angles of 27°–30.5°, 31.5°–35°, and 35°–49°, respectively, as depicted in
Figure 8 and
Figure 9.
Figure 8 provides a visual representation of the transition process from β-C
2S to α′
L-C
2S that occurs within a moderate temperature range of 25 °C to 600 °C. When comparing the changes in the characteristic diffraction peak at 32°–33° between
Figure 4 and
Figure 8, it becomes apparent that the phase transition process induced by boron ion doping does not align with the temperature-induced phase transition.
Figure 8 showcases the temperature-induced phase transition, where only one high-intensity diffraction plane (002)
α′L emerges between the crystallographic planes (
)
β and (200)
β of β-C
2S, resulting from the transformation of the (200)
β plane. Conversely, in the boron ion doping-induced phase transition, two high-intensity diffraction planes, (020)
α′H and (211)α′
H, appear between the planes (
)
β and (200)
β. This indicates that boron ion doping mediates the structural modulation of β-C
2S, bypassing the α′
L-C
2S phase and directly transforming it into α′
H-C
2S. Notably, there is no existing literature [
27,
28,
29,
30,
31] reporting the modulation of the C
2S crystal structure to achieve α′
L-C
2S through the process of boron doping.
Figure 9 illustrates the transition process from α′
L-C
2S to α′
H-C
2S at medium to high temperatures (700–1300 °C). With the temperature ranging from 600 °C to 900 °C, C
2S remains in the α′
L phase, with slight variations in diffraction peaks (such as peaks at around 41°), which can be attributed to the thermal expansion of the α′
L phase. Throughout the transition from α′
L-C
2S to α′
H-C
2S, the characteristic diffraction peaks exhibit minimal changes, with the exception of the range of 32°–33°. Furthermore, even at a temperature of 1100 °C, the C
2S crystal structure predominantly maintains the α′
L-type, transitioning completely to α′
H-type at 1200 °C. These observations indicate a close resemblance between the α′
L and α′
H phases of C
2S, with a narrow temperature range for the transition, suggesting a rapid phase transformation.