3.3 XRD
The XRD patterns of carbon fiber foam concrete at different ages are displayed in Figure 6.
As depicted in Figure 6, the addition of carbon fiber does not make sulfoaluminate cement form new hydration products, that is, carbon fiber does not participate in the hydration reaction. Since carbon fiber has an amorphous structure, there will be no diffraction peaks. The main hydration product of the specimens is ettringite. The dispersion peak at the bottom of the XRD pattern indicates that C-S-H gel and aluminum gel may occur. In Figure 6(a), after 3 days of hydration, the anhydrous calcium sulfuraluminate is consumed completely in the two groups of specimens, and they both have unhydrated anhydrous gypsum and dicalcium silicate. Besides, the diffraction edge of anhydrous gypsum of group C2 is stronger than that of group O which has no carbon fiber. Although carbon fiber has poor water absorption ability, it can serve as an obstacle in cement slurry that restricts the free movement of water molecules at early ages, thus delaying the early hydration process of cement. The XRD pattern of the specimen at 28d is depicted in Figure 6(b). At this point, the hydration reaction is more complete, and the diffraction peak intensity of anhydrite in group C2 drops a lot compared to that at 3d, indicating that the hydration reaction continues at the later age and the diffraction peak intensity is weaker than that of group O at the same age. Besides, with the continuous carbonation of cement, the diffraction peak intensity of calcium carbonate of both groups increase compared to those at 3d.
Figure 7 shows XRD patterns of carbon fiber-graphite foam concrete at different ages.
As exhibited in Figure 7, the group with added carbon fiber and graphite has the strongest diffraction peak intensity of ettringite (the hydration product). Due to the incorporation of graphite, the characteristic diffraction peak of graphite can be observed at 2θ=26.5°. After 3 days of hydration reaction, the diffraction peak intensity of anhydrite of group C2S1 is weaker than those of group C2 and group O. It reveals that graphite can promote the hydration of sulfoaluminate cement at early stages. With the short age period, the diffraction peak intensity of calcium carbonate of the three groups is weak at this time. As the hydration reaction continues, Figure 7(b) shows that the diffraction peak intensity of ettringite of group C2S1 at 28d continues to rise compared to that at 3d while the diffraction peak of anhydrite continues to decline, indicating that the hydration reaction continues. Therefore, the hydration of sulphoaluminate cement is enhanced under the combined action of carbon fiber and graphite.