## 4 Results and Discussion
### Analysis of the Hydrological Environment  
In order to evaluate the environmental compatibility of the composite cementitious material system, this study systematically monitored the pH evolution of the samples at different maintenance ages (as shown in Figure 5), aiming to investigate the comprehensive influence of its acid-base equilibrium mechanism on the hydrological process, mechanical properties, and environmental safety so as to ensure that the pH of the samples is suitable for the hydrological reaction. The test results showed that the pH value of the samples was maintained in the range of 9-12 at the early stage of conservation (0-7d), and this alkaline environment mainly originated from the dissolution of alkaline components such as Na\({}_{2}\)O in RM, which effectively neutralized the soluble acids (e.g., residual phosphoric acid) contained in PG. The suitable alkaline conditions at this stage promote the rapid dissolution of cement clinker minerals, creating favorable conditions for the formation of hydration products such as C-S-H gel [33]. By the 14d maintenance age, the pH value showed a significant decreasing trend (about 15-20%), which was closely related to the continuous consumption of OH\({}^{-}\) ions by the hydration reaction: On the one hand, the continuous hydration of minerals such as C\({}_{3}\)S produced Ca\({}^{2+}\) with OH\({}^{-}\), and on the other hand, the generated C-S-H gel fixed some of the OH\({}^{-}\) by the chemical binding immobilizing some of the OH\({}^{-}\), leading to a decrease in the alkalinity of the solution phase [34]. Notably, the pH of the system at 28 days stabilized at 9.0 \(\pm\) 0.3, significantly lower than the original cement system (pH \(\approx\) 12.5) but higher than the targeted neutral range (pH = 7). This deviation from neutrality likely arises from three interrelated mechanisms: (1) alkaline buffering in the RM-PG composite system: Hydroxyl ions (OH\({}^{-}\)) are continuously replenished by the gradual release of alkalis from RM; (2) kinetic limitations in later hydration stages: Slower reaction rates fail to fully deplete free alkalis in the system; (3) solubility constraints of Ca(OH)\({}_{2}\): Reduced water-to-cement ratios limit calcium hydroxide dissolution. While the moderately alkaline pH (vs. the strongly alkaline leachate of traditional cement) may mitigate environmental risks, the long-term leaching behavior and its ecological implications require further assessment. Three hydration stages were derived from the hydration process: initial passivation: The trapping of calcium ions by the bilayer leads to a limitation of the mineral dissolution rate and the formation of a short reaction lag. The presence of a surface passivation film at this stage is a key factor in inhibiting sustained hydration. Mid-phase acceleration: The accumulation of OH\({}^{-}\) concentration in the system triggers supersaturation of the solution, which leads to the formation of Ca(OH)\({}_{2}\) nuclei. This phase transition process breaks the original solvation equilibrium, forming an autocatalytic reaction mechanism and entering the \"accelerated period\"; the hydration rate is obviously accelerated but also reflected in the pH value of the sample in the middle period, a pH value of 9-12, and the reaction rate is accelerated. Later decay: When the degree of hydration exceeds 60%, three synergistic factors lead to a decrease in the reaction rate: Firstly, the C-S-H gel densities to form a diffusion barrier; secondly, the calcium ion concentration gradient of the pore solution decreases; and finally, the interfacial pH falls back to 9 from 11. At the same time, RM and PG react synergistically: acid-base neutralization, which makes the final pH of the system alkaline, and the formation of the Na\({}_{2}\)SO\({}_{4}\) electrolyte from RM and SO\({}_{4}\) from PG, which leads to an intensification of the reaction in the accelerated phase. Figure 5: pH evolution pattern of the samples.