## 4 Mechanism Discussion
### Calcium Leaching  
The weakening effect of calcium leaching, which involves the decomposition of ettrinigite and C-S-H, is considered a crucial component of the sulfate erosion mechanism. On the other hand, leaching phenomena in acidic solutions mainly cause the dissolution of portlandite and the decalcification of C-S-H [38] (Equations (6) and (7)). Dissolving the minerals increases the material's porosity and reduces its mechanical properties [39; 40; 41]. The results of [PERSON]'s study show that there are two characteristic layers in which the calcium-leached portion of the material exists and extends from the outer surface: predominantly, ettrinigite (zone 1) and ettrinigite, gypsum, and portlandite (zone 2) [42]. Since sulfuric acid solutions were used in this study, the hydrogen ions may exacerbate calcium leaching caused by a sulfate ion attack. Other studies have shown that ettrinigite dissolves into gypsum and aluminum sulfate when subjected to pH values below approximately 10.5 [43]. Therefore, when the surface layer of the CGB was corroded, some of the ettrinigite exposed to the hydrogen ions was corroded and dissolved, leading to a decrease in the mass of the CGB. The re-exposure of the pore space filled initially by ettrinigite due to this process changes the pore structure, which can significantly affect the specimen's permeability [44]. Under the double attack of sulfate ions and hydrogen ions, the structure and bonding interface strength of CGB hydration products are weakened, leading to a reduction in the macroscopic mechanical strength (Figure 8f). The results of UCS and acoustic emission confirmed this process.  
\[3\text{CaO}\cdot 2\text{SiO}_{2}\cdot 3\text{H}_{2}\text{O}+6\text{H}^{+} \rightarrow 3\text{Ca}^{2+}+2\text{SiO}_{2}+6\text{H}_{2}\text{O} \tag{6}\] \[3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot 3\text{CaSO}_{4} \cdot 32\text{H}_{2}\text{O}\rightarrow 3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot\text{CaSO}_{4} \cdot 12\text{H}_{2}\text{O}\] (7) \[+2\text{CaSO}_{4}+20\text{H}_{2}\text{O}\]  
The strength change in the CGB under chemical ion erosion can be divided into two main stages. In the first stage, the strength of the CGB increases due to the hydration reaction and the filling and compacting effect of ettrinigite when the alkaline environment is still dominant inside the specimen. However, as the erosion time increases, the double invasion of hydrogen and sulfate ions destroys the alkaline environment. As a result, ettrinigite loses its survival environment and decomposes rapidly, destroying the filler specimen's internal crystal structure in a relatively short time. Compared to the strength deterioration effect caused by acid corrosion, the hydration reaction of the cementitious materials is less strong. The strength of the filled specimen reduces, and the rate of strength deterioration accelerates with time. Additionally, the expansion stress generated by gypsum water absorption leads to microcracks formation, further exacerbating the erosion of internal ions. During load-bearing compression, the CGB structure and bonding interface become damaged, which weakens the cementation. As a result, the gangue cemented filling body is more prone to microcracks under the action of stress, especially along the interface transition zone where microfractures occur.