## 3 Results and Discussion
### Changes of Physical Properties
#### 3.1.2 Changes in Volume, Porosity and Permeability
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Number of Sample**} & \multirow{2}{*}{**\(P_{c}\) (MPa)**} & \multicolumn{6}{c}{**Permaebility (mD)**} \\ \cline{3-8}  & & & **0 Day** & **3 Day** & **6 Day** & **12 Day** & **15 Day** \\ \hline \multirow{8}{*}{CO\({}_{2}\)} & & 1 & 0.65 & 2.51 & 2.29 & 5.92 & 11.40 \\  & & 3 & 0.48 & 1.68 & 1.55 & 3.65 & 8.83 \\  & & 5 & 0.28 & 0.91 & 0.86 & 1.94 & 5.56 \\  & & 7 & 0.12 & 0.34 & 0.38 & 0.94 & 3.29 \\ \cline{2-8}  & & 1 & 3.28 & 13.52 & 9.12 & 26.39 & 24.47 \\  & & 3 & 2.48 & 8.57 & 6.31 & 15.22 & 16.36 \\  & & 5 & 1.61 & 4.80 & 3.44 & 7.73 & 9.86 \\  & & 7 & 0.84 & 2.83 & 1.60 & 3.82 & 5.31 \\ \hline \multirow{8}{*}{Control} & & 1 & 0.06 & 1.47 & 0.82 & 2.68 & 5.83 \\  & & 3 & 0.07 & 0.87 & 0.43 & 1.50 & 2.47 \\  & & 5 & 0.06 & 0.48 & 0.21 & 0.77 & 2.07 \\  & & 7 & 0.02 & 0.24 & 0.10 & 0.43 & 1.18 \\ \hline \multirow{8}{*}{Control} & & 1 & 0.45 & - & - & - & 0.45 \\  & 1-4 & & 3 & 0.21 & - & - & - & 0.21 \\  & & 5 & 0.09 & - & - & - & 0.09 \\  & & 7 & 0.05 & - & - & - & 0.05 \\ \hline \multirow{8}{*}{Control} & & 1 & 0.36 & - & - & - & 0.35 \\  & 1-5 & & 3 & 0.14 & - & - & - & 0.14 \\  & & 5 & 0.08 & - & - & - & 0.07 \\  & & 7 & 0.04 & - & - & - & 0.03 \\ \hline \multirow{8}{*}{Control} & & 1 & 0.27 & - & - & - & 0.26 \\  & & 3 & 0.11 & - & - & - & 0.11 \\ \cline{1-1}  & & 5 & 0.06 & - & - & - & 0.05 \\ \cline{1-1}  & & 7 & 0.03 & - & - & - & 0.03 \\ \hline \end{tabular}
\end{table} 
Table 3: The permeability of the samples from the two groups at different reaction days. Figure 9: The changes of the volume, porosity and permeability of the BUC samples with increasing reaction time. The volume showed a general decreasing trend with time because of the mineral dissolution, while the porosity showed an increasing trend, which caused the increase of permeability. However, the result of the reaction after 3-6 days of reaction time was different. After 6 days of reaction time, the volume of the samples increased slightly, which may have been induced by the precipitation of CaCO\({}_{3}\) (shown in Figure 10a). Figure 10 shows the evolution of the surface texture of the cube samples from the two groups (bs and BS) by SEM. Figure 10a shows the greyscale images of the SEM of Group CO\({}_{2}\), and Figure 10b shows the binary images. The image of Group control after the reaction at 15 d is shown in Figure 10c. After 15 days of soaking in seawater without extra CO\({}_{2}\), the surface of the bs sample was still as smooth as before the test. However, the surface of BS became rough after only 3 days because of the dissolution of the calcite under the CO\({}_{2}\)-seawater- limestone reaction. In the subsequent analysis, the quantification of basic size parameters (such as pore count, pore area, Feret diameter and surface porosity of the corrosion pores) of the corrosion pores in Figure 10b were analyzed using the built-in measurement tools of the ImageJ software. The results are shown in Figure 11 and Table 4. These results indicate that the size of surface corrosion pores increased as the reaction time increased. From the results above, the mechanism of chemical dissolution that induced microstructure evolution and changes of the physical property can be revealed. In the early stage of the reaction (during days 0-3), the reef limestone dissolved into calcium bicarbonate, and a large number of micropores formed. The average size of micropores was about 0.15 \(\upmu\)m\({}^{2}\), and the corrosion porosity of the surface was about 6.08% after 3 days of reaction time. As the reaction continued, the samples continued to dissolve, which led to the enlargement of the micropores, some of which merged together and formed bigger wormholes (shown in Figure 10b, the date of Group CO\({}_{2}\) is 6 day). The average size of wormholes was about 0.74 \(\upmu\)m\({}^{2}\), and the corrosion porosity of the surface increased to 17.73%.