## 3 Results and Discussion
### X-ray Diffraction Analysis  
It was confirmed [47; 48; 49] that the destruction of cement materials caused by sulphate ions is mainly due to the chemical reaction of sulphate ions and hydration products, as shown in Equations (1)-(3).  
\[3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}+3\text{Ca}^{2+}+3\text{SO}_{4}{}^{2 }+32\text{H}_{2}\text{O}\to 3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot 3 \text{CaSO}_{4}\cdot 32\text{H}_{2}\text{O} \tag{1}\]  
\[3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot\text{Ca(OH)}_{2}\cdot x\text{H} _{2}\text{O}+2\text{Ca}^{2+}+3\text{SO}_{4}{}^{2}+(31-x)\text{H}_{2}\text{O} \tag{2}\]  
\[\to 3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot 3\text{CaSO}_{4}\cdot 3 2\text{H}_{2}\text{O}+2\text{OH}^{-}\]  
\[3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot\text{CaSO}_{4}\cdot x\text{H} _{2}\text{O}+2\text{Ca}^{2+}+2\text{SO}_{4}{}^{2}+(32-x)\text{H}_{2}\text{O}\]  
\[\to 3\text{CaO}\cdot\text{Al}_{2}\text{O}_{3}\cdot 3\text{CaSO}_{4}\cdot 3 2\text{H}_{2}\text{O} \tag{3}\]  
When the concentration of sulphate ions is low, the free sulphate ions in the pore water react with calcium aluminate and its hydration products to form acicular ettrinigite crystals. With the growth of ettrinigite crystals, the material structure is destroyed. When the concentration of sulphate ions is high, the corrosion induced by sulphate ions will also lead to the production of gypsum.  
Figure 6 shows the XRD pattern of high-sulphur fly ash at a 75% (1:3) content. It can be seen that the XRD pattern of materials under different environments exhibits certain similarities and differences; in the four groups of diffraction patterns, some conventional hydrated substances such as SiO\({}_{2}\) and CaCO\({}_{3}\) appeared, and the diffraction peaks of iron oxides appeared in the figure due to the presence of high-sulphur fly ash. The characteristic peak of ettrinigite in the diffraction pattern of sodium sulphate corrosion specimens is obvious, but it is not found in that for water-cured specimens, evincing the corrosive effect of sulphate ions on LCMs. Moreover, the diffraction peaks of gypsum crystals in the 10% and 15% samples are stronger than those in the 5% samples. According to [PERSON] [50], when the sulphate concentration is low, the main corrosion product of cement is AFt. When the sulphate concentration is high, the corrosion product is crystalline gypsum crystal. When the sulphate concentration is between the two, the corrosion product contains both, which is consistent with the diffraction peak pertaining to gypsum at higher concentrations of sodium sulphate in Figure 6. [PERSON] et al. characterised the entry of sulphate in concrete by XRD, and the research showed that the existence of gypsum crystals is controlled by the concentration of sulphate ions; the results [51; 52] support the correctness of this research.  
Calcium sulphate hemihydrate was found in four environments. A preliminary analysis showed that this substance was generated by the heating and dehydration of the sample during the XRD sample preparation. When calcium sulphate dihydrate is heated, the bound water in the crystal is lost upon heating, thus forming calcium sulphate hemihydrate. Similarly, the carbonisation of CH was accelerated by the increase in temperature in the heating process of the sample, which resulted in no diffraction peak arising from CH being present in the diffraction pattern.