## 3 Results and Discussion
### XRD Analysis  
Figure 4 shows the XRD patterns of the SAR paste at 8 days. The hydration products of the SAR paste were CaCO\({}_{3}\), SiO\({}_{2}\), C-S-H gel, ettrinigite, and some silicate compounds. As shown in Figure 4, no portlandite was detected in the specimens, except for a small amount found in the M2 specimen, which is quite different from the hydration products of cement [14]. This may be related to the high amount of cement in the M2 specimen and also indicates that incorporating seabed silt greatly reduced the amount of portlandite in the cement hydration products. In addition, the curing process may have converted the CaO in the seabed silt into CaCO\({}_{3}\), which could also be a reason why portlandite was not detected in the hydration products of the silt and cement mixture.  
Figure 5 shows the XRD patterns of the SAR paste at 1, 8, and 28 days. It can be seen that the peaks of the SAR paste were highest and densest at 28 days, except for the M6 specimen. The pH of the SAR specimens was between 9.1 and 9.6. An alkaline environment is not conducive to the adhesion of marine organisms, as with all concrete. The hydration products of concrete include a large amount of portlandite, which leads to a high pH (about 12 to 13) in the surrounding marine environment [14]. An alkaline seawater environment keeps attaching organisms away from artificial reefs, which is not conducive to the recovery and proliferation of a seaweed field. Therefore, a large amount of fresh water is generally sprayed onto artificial reefs to reduce their alkalinity before deposition to avoid releasing alkalis into the seawater. However, this process is time-consuming and laborious for projects that involve large numbers of artificial reefs. The cement and seabed silt mixture should improve on traditional cement paste by reducing the pH to increase the adhesion properties of the artificial reefs.  
The XRD results showed a relatively high silicate content among the hydration products of the SAR paste, which may be related to the SiO\({}_{2}\) content of the seabed silt (Table 1). As shown in Figure 5, the diffraction peaks of the SAR specimens, except for M6, gradually increased and became dense with curing time. This indicates that CaCO\({}_{3}\) and SiO\({}_{2}\) were gradually formed in the specimens, the content continued to increase, and the crystallinity also gradually increased during the hydration process. This is consistent with the results of the compressive strength tests. These XRD test results explain the increase in the compressive strength of the SAR specimens from a microscopic perspective. The deviation of the M6 specimen from the XRD trend is similar to the situation found in the study of tidal sludge [3], and the reason requires further study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & **M2** & **M4** & **M6** & **M8** \\ \hline N/P & 193.88 & 203.62 & 298.52 & 210.01 \\ \hline \hline \end{tabular}
\end{table}
Table 2: N/P ratios in the immersion solution.  
C-S-H gel is considered to be the source of mechanical strength in concrete. However, the C-S-H gel content of all of the specimens was small. The existence of C-S-H gel requires a stable and continuous alkaline environment, which the hydration product of cement, portlandite can provide. However, after a certain ratio of seabed silt was incorporated in the SAR specimen, most Ca(OH)\({}_{2}\) was consumed in the hydration reaction, corresponding to the compressive strength results. Yet a high portlandite content would affect the adhesion and biological effects of the SAR. Therefore, the question of how to adjust the mass ratio of the components and control the Ca(OH)\({}_{2}\) content within a reasonable range to maintain sufficient biological attachment without compromising the mechanical strength of the reef requires further study.  
Figure 4: XRD patterns of SAR paste at 8 days. (P): portlandite; (G): gypsum; (C): C–S–H; (E): ettrinigite; (Ca): CaCO\({}_{3}\); (Q): SiO\({}_{2}\).