## 4 Results and Discussion
### Hydration Product Analysis of RCPB  
#### 4.3.1 XRD  
Figure 6 shows the X-ray diffraction (XRD) patterns of RCPB samples cured for 28 days and their quantitative analysis results. The positions and types of diffraction peaks are basically the same for samples of different concentrations, with only slight differences in peak intensities, and the percentage of amorphous is higher than 30%. The XRD results show that the samples mainly contain six typical mineral phases, which are Portlandite, Quartz, hematite, calcite, Ettringite, and gypsum. Among them, the diffraction peak intensity of gypsum is the highest except for amorphous, and gypsum is the main component of PG, indicating that it is more abundant in the sample. The second is Ettringite, but the high Ettringite is presumed to be probably due to the participation of PG in the reaction in the early stage, and the presence of Ettringite makes the RCPB have high strength in the early stage of maintenance, but the strength enhancement is not obvious in the late stage of maintenance due to the expansion of Ettringite, which is in agreement with the results of the UCS test. The presence of CaCO\({}_{3}\) is mainly attributed to Portlandite carbonation, while some of the mica undergoes a carbonation reaction with carbon dioxide during the curing process [35].  
The presence of hydroxide imparts not only base-activated properties to the RM, but the hematite also provides the skeletal structure for the formation of hydration products. The main components of the cementitious materials in the samples are silicate minerals (C\({}_{3}\)S, C\({}_{3}\)S) and aluminate minerals (C\({}_{3}\)A, C\({}_{3}\)AF). When the cement comes in contact with water, these minerals react chemically with water to produce gels and other hydration products [36]. The reaction formula is given below:  
\[\text{C}_{3}\text{S}+3\text{H}_{2}\text{O}\rightarrow\text{C-S-H}+3\text{Ca( OH)}_{2} \tag{1}\]  
Figure 6: XRD patterns of the CTB cured for 28d (**a**) The detected XRD spectra (**b**) The correlated quantification results.  
\[\mathrm{C_{2}S+4H_{2}O\to C\text{-S-H}+2 Ca(OH)_{2}} \tag{2}\]  
\[2\mathrm{C_{3}A+6H_{2}O+3 CaSO_{4}\text{-2H_{2}O\to C_{6}A_{3}H_{6}\text{-}CaSO_{4 }+3 Ca(OH)_{2}}} \tag{3}\]  
\[\mathrm{C_{4}AF+10H_{2}O\to C_{3}AH_{6}+C_{3}FH_{6}+Fe(OH)_{3}} \tag{4}\]  
The amorphous calcium silicate hydrate (C-S-H) gel, as the primary hydration product, dynamically evolves in calcium-silicate polymeric composition during cement hydration. It synergizes with calcium aluminate silicate hydrate (C-A-S-H) gel through covalent bonds, hydrogen bonds, and ionic interactions to form a three-dimensional network. This dual-phase system enhances concrete performance through complementary mechanisms: C-S-H gel primarily governs mechanical strength development while C-A-S-H gel improves structural stability, with their combined action reducing capillary porosity through progressive pore-filling.  
In aluminate-rich cement systems, the coexistence of these gels creates an interconnected microstructure that enhances concrete's mechanical properties (strength and toughness) and durability. Hydration advancement promotes gel cross-linking, which densifies the matrix by occupying interparticle voids and minimizing capillary pores. XRD patterns between 30 and 45\({}^{\circ}\) reveal diffraction peaks from both residual amorphous components in raw materials and newly formed amorphous hydration products. Increased peak intensity correlates with elevated gel production, directly contributing to improved mechanical strength and material performance.