## 4 Results and Discussion  
### Mechanical Performance of RCPB  
The compressive strength of RCPB generally increases with rising concentrations of cementitious materials, though further strength gains plateau and workability declines at excessively high concentrations [31]. This study aims to identify the optimal concentration ratio. Figure 4 illustrates the evolution of UCS over curing periods (7, 14, and 28 days) and the percentage contribution to ultimate strength at each period, enabling evaluation of concentration effects on RCPB UCS. All samples exhibited an overall increase in UCS with curing time. A clear positive correlation between concentration and strength was observed: Higher concentrations yielded proportionally greater UCS values. Notably, in the initial curing phase (7 days), specimens with concentrations \(\geq\)65% demonstrated comparable UCS \(\geq\) 1.5 MPa, whereas the 63% concentration group showed significantly lower strength. This discrepancy is hypothesized to arise from insufficient cement and gypsum content in the 63% formulation (due to lower polymer-to-cement (PG) ratios), leading to delayed hydration kinetics and suboptimal early-stage reaction conditions. At the lowest concentration (63%), early-age hydration contributed minimally to strength (only \(\approx\)30%), while later-stage reactions (14-28 days) accounted for a significantly higher proportion of strength development. In contrast, the 65% concentration specimens exhibited the highest early-stage strength contribution (59.36%) among the tested groups. This is hypothesized to arise from the formation of AFt in higher-concentration samples,  
Figure 4: UCS (**a**) The mechanical properties of prepared samples with curing ages from 7 to 28 days (**b**) growth rates of UCS at that time. which enhances early strength but leads to late-stage deterioration due to expansive AFt crystallization. At concentrations above 65%, further strength gains plateau, as the initial strength advantage from AFt formation is offset by its deleterious long-term effects. It is hypothesized that the reduced rate of strength gain in higher-concentration samples stems from the increased presence of Ettringite, which limited mechanical property improvements despite a general trend of strength enhancement with concentration. Notably, samples with 63% Ettringite during later curing stages exhibited less pronounced strength increases than anticipated. Excessively high concentrations were also associated with practical challenges. For instance, at 69% concentration, the slurry's high viscosity--attributable to RM--impaired fluidity, causing molding defects (e.g., bubble formation) and processing difficulties. Low fluidity further introduces operational risks in real-world applications, including transportation inefficiencies, inadequate densification, construction complexities, and pipeline blockages. Consequently, concentrations exceeding 69% were not evaluated in this study. After 28 days of curing, all samples demonstrated favorable long-term mechanical performance, achieving compressive strengths of 2.50-3.60 MPa. These results suggest the potential for partial substitution of conventional concrete, provided further toxicity leaching tests confirm environmental safety. As the core active component of OPC, constituting 50-70% of its mass, tricalcium silicate (C\({}_{3}\)S) governs the early-stage hydration kinetics of cementitious systems [32]. The inherent lattice defects in C\({}_{3}\)S crystals create preferential pathways for water infiltration, facilitating interfacial contact between the mineral surface and the aqueous phase. This initiates hydrolysis, characterized by continuous Ca\({}^{2+}\) dissolution and subsequent formation of silica-enriched regions on the particle surface. The resulting Ca-depleted silicate layer exhibits distinct compositional heterogeneity compared to later-stage hydration products like calcium silicate hydrate (C-S-H) gels.