## 3 Results and Discussion
### Change of Mechanical Property  
In order to study the influence of chemical interaction on the mechanical properties of the coral reef limestones, uniaxial compression and Brazilian splitting experiments were conducted. Tables 5 and 6 summarize the results of the uniaxial compression and tensile tests of the specimens, respectively. The average compressive strength and average elastic modulus of Group CO\({}_{2}\) was 44.65 MPa and 4.56 GPa, respectively. The average compressive strength and average elastic modulus of Group control was 52.14 MPa and 4.6 GPa, respectively. The results seem to suggest that the average peak strength and elastic modulus of the samples reacted with the CO\({}_{2}\)-seawater reaction were slightly lower than the samples only soaked by seawater without extra CO\({}_{2}\). However, the results of the tensile tests are opposite to that of Table 6. The average tensile strength of Group CO\({}_{2}\) (7.04 MPa) was slightly higher than that of Group control (6.42 MPa). Figure 12 shows the stress-strain curve of the saturated samples of two groups under uniaxial compressive condition after 15 days of reaction. The uniaxial compression strength of the samples in this study was within the range from 23 to 65 MPa. There was no significant difference between the samples in the two groups on the mechanical response. All the samples presented the features of brittle failure. As shown in Figure 13, both groups of samples showed tensile failure, and the failure cracks spread along the large sample pores. Several studies have shown that coral reef limestones always show inhomogeneous characteristics. Although all samples were taken side-by-side from the same core, the mechanical properties of them may be different [29; 46]. Moreover, their uniaxial compression strength may be quite small compared to that of the other rocks, and the values can vary significantly [47]. Therefore, if the strength change is small, it is difficult to see a significant difference in the mechanical results. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Number of** & \multirow{2}{*}{**Form**} & **Tensile Strength** & **Average Tensile Strength** \\  & & **Sample** & **(MPa)** & **(MPa)** \\ \hline \multirow{3}{*}{CO\({}_{2}\)} & CT1-2 & & 6.94 & \\  & B1-1 & BUT & 6.50 & 7.04 \\  & B1-2 & & 7.67 & \\ \hline \multirow{3}{*}{Control} & b1-1 & & 5.86 & \\  & b1-2 & but & 5.75 & 6.42 \\ \cline{1-1}  & b1-3 & & 7.66 & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of the uniaxial tensile strength test of the saturated specimens from two groups after 15 days of reaction time. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Number of** & \multirow{2}{*}{**Form**} & **Compressive** & **Average Compressive** & **Elastic Modulus** & **Average Elastic** \\  & **Sample** & & **Strength (MPa)** & **Strength (MPa)** & **(GPa)** & **Modulus (GPa)** \\ \hline \multirow{3}{*}{CO\({}_{2}\)} & 1-1 & & 48.57 & & 4.53 & \\  & 1-2 & BUC & 23.52 & 44.65 & 3.54 & 4.56 \\  & 1-3 & & 61.87 & & 5.62 & \\ \hline \multirow{3}{*}{Control} & 1-4 & & 63.38 & & 4.59 & \\  & 1-5 & buc & 49.35 & 52.14 & 6.04 & 4.60 \\ \cline{1-1}  & 1-6 & & 43.68 & & 3.16 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of the uniaxial compressive strength test of the saturated specimens from two groups after 15 days of reaction time. In order to further explore the effect of seawater acidification on the mechanical properties of reef limestone, the CT scanning results of the uniaxial compression sample 11 were analyzed. Figure 14 shows the 3D reconstructed images of the 1-1 sample before and after the reaction, and it can be found that the surface became rough and small corrosion pits appeared after reaction. Figure 15 shows the porosity changes of the 1-1 sample before and after the reaction based on the CT scan. The porosity of the sample was analyzed by the Avxio Software 2020.2 (Thermo Fisher Scientific, MA, USA).