## 2 Materials  
### Raw Materials  
The primary raw materials used to synthesize the CPB in this study were RM, PG, and OPC. PG was sourced from a chemical facility in Guizhou Province, China, and transported to the laboratory in airtight containers to prevent moisture exchange. The PG exhibited a pH of 3.74, confirming its strongly acidic nature. Moisture content was quantified two hours prior to experimentation to calibrate the water-to-solid ratio in the mixtures, ensuring consistency in slurry formulation.  
RM was sourced from an alumina refinery in Guizhou Province, China. To eliminate residual moisture and enhance reactivity, the RM was oven-dried at 60 \({}^{\circ}\)C for 48 h prior to use. OPC (grade P.O 42.5) was supplied by Xiangping Co., Ltd. (Changsha, China). Toxicity Characteristic Leaching Procedure (TCLP) tests on RM confirmed that heavy metal concentrations in the leachate (e.g., Cr: 0.8 mg/L, Pb: 0.2 mg/L) were well below Chinese regulatory thresholds [17]. The RM exhibited a pH of 11.47, establishing an alkaline environment that promotes hydration reactions critical to cementitious material formation.  
X-ray fluorescence (XRF) analysis revealed that PG primarily consisted of CaO (\(38.72\pm 0.15\) wt%) and SO\({}_{3}\) (52.14 \(\pm\) 0.20 wt%), aligning with the chemical formula of gypsum (CaSO\({}_{4}\) 2H\({}_{2}\)O). Trace components included P\({}_{2}\)O\({}_{5}\) (1.85 \(\pm\) 0.08 wt%) and F (1.34 \(\pm\) 0.05 wt%), indicative of residual phosphate impurities and fluoride contamination. Cadmium (Cd) was detected at (0.012 \(\pm\) 0.001 wt%), a critical parameter for environmental risk assessment.  
RM was predominantly composed of Al\({}_{2}\)O\({}_{3}\) (22.35 \(\pm\) 0.15 wt%) and Fe\({}_{2}\)O\({}_{3}\) (38.12 \(\pm\) 0.20 wt%), characteristic of bauxite residue from alumina production. Elevated CaO (8.74 \(\pm\) 0.10 wt%) and Na\({}_{2}\)O (6.85 \(\pm\) 0.05 wt%) content corroborated RM's strong alkalinity (pH \(>\) 10), which is central to its role in neutralizing acidic components and driving hydration reactions.  
The analysis of the particle size of raw materials shows that RM has a wider particle size distribution but a more concentrated PG distribution (smaller D30/D10 ratio); RM has a finer particle size, probably due to a finer comminution process, and the PG particles are more homogeneous (D60/D10 = 11.2/2.88 \(\approx\) 3.88 vs. RM = 5.33/0.56 \(\approx\) 9.52). Also, the high specific surface area of RM may enhance the reactivity.  
According to XRD, RM is dominated by amorphous and Chlorite, with nearly 10% Ca\({}_{3}\)AlFe(SiO\({}_{4}\))(OH)\({}_{8}\), Calcium Chalcopyrite, and hematite, while PG is mainly composed of gypsum.  
Table 1 lists the results of the XRF analysis; Figure 1 and Table 2 present the particle size distribution of the raw materials, while the XRD spectrum is shown in Figure 2:  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Molecular Formula**} & \multicolumn{2}{c}{**Content (\%)**} \\ \cline{2-3}  & **RM** & **PG** \\ \hline O & 37.33 & 41.5 \\ Ca & 11.22 & 25.02 \\ S & 0.56 & 18.62 \\ Si & 7.99 & 1.949 \\ P & 0.041 & 0.61 \\ Fe & 19.67 & 0.4626 \\ Al & 11.3 & 0.32 \\ K & 1.32 & 0.213 \\ Ba & - & 0.213 \\ Mg & 0.6 & 0.0842 \\ Na & 6 & 0.065 \\ Ti & 2.25 & 0.0477 \\ Sr & 0.03 & 0.0431 \\ Cu & 0.0048 & 0.0104 \\ As & - & 0.0051 \\ Y & 0.0039 & 0.003 \\ Ni & 0.0055 & 0.003 \\ Zn & 0.0048 & 0.002 \\ \hline \hline \end{tabular}
\end{table}
Table 1: XRF of materials.