## 3 Results and Discussion  
### Changes of Physical Properties  
#### 3.1.1 Mass Loss  
The final changes of the dry mass and water absorption coefficient of the specimens from the two groups after 15 days of reaction time are summarized in Table 1. Figure 7 shows the decrease in the percentage of the mass of the rock samples with the increasing reaction time, and the error bars in the figure present the standard deviations. The mass loss of the samples of Group CO\({}_{2}\) was much more significant than those of Group control throughout all reactions due to the existence of CO\({}_{2}\). By the end of the reaction, the mass loss of the samples of Group CO\({}_{2}\) was negligible, while the average mass loss of the specimens (except for the BS) in Group control was about 1.03 g. Figure 8 shows the change of concentration of Ca\({}^{2+}\) of the solution of two groups with reaction time. The concentration  
Figure 6: (**a**) The RMT multifunctional rock rigidity testing machine, (**b**) the uniaxial compression test, (**c**) the split tensile test.  
of Ca\({}^{2+}\) of Group control was unchanged during testing, while for Group CO\({}_{2}\), it increased gradually within 6 h and remained constant afterward. The evident significance of the concentration of Ca\({}^{2+}\) indicates that the chemical dissolution played an important role in mass transport. As the reef limestone is mainly composed of calcite, the main reaction, which caused the dissolution, is as follows [35]:  
\[\text{CO}_{2}\;+\;\text{H}_{2}\text{O}\;+\;\text{CaCO}_{3}\;\text{=}\;\text{ Ca(HCO}_{3})_{2} \tag{4}\]  
The calcium bicarbonate is quite unstable, and the chemical reaction in the reactor may be a dynamic equilibrium.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Group** & **Test Condition** & **Number of Sample** & **Form** &
\begin{tabular}{c} **Water Absorption** \\ **Coefficient (\%)** \\ \end{tabular} & \multicolumn{3}{c}{**Mass (g)**} \\ \cline{5-8}  & & 1-1 & & 0.71 & 64.04 & 63.36 & \(-\)0.68 \\  & & 1-2 & BUC & 0.88 & 63.59 & 62.86 & \(-\)0.73 \\  & Injection of CO  
Further analysis of the dates in Figure 7 reveals that the percentages of mass loss for the specimens of Group CO\({}_{2}\) were around 1.05%, 1.12% and 3.76% for the BUT, BUC and BS samples, respectively. This means that the smaller the sample, the higher the percentage of mass loss. The percentage of mass loss is believed to be influenced by the specific surface area of the sample. The smaller particles have a higher specific surface area, which will become the larger than the contact area with the reaction solution [36]. Therefore, the smaller the sample, the higher the degree of the chemical reaction of CO\({}_{2}\)-seawater-rock. Referring to the dates of the water absorption coefficient (\(W_{a}\)) in Table 1, it can be seen that the order of the \(W_{a}\) of the samples from large to small was BS (1.10), BUC (0.79) and BUT (0.73), respectively. The larger \(W_{a}\) also indicates more solution consumption, which further verifies the results in Figure 7. Figure 7 also presents that the mass loss of all types of samples in Group CO\({}_{2}\) continued to increase after each reaction period, and the dissolution rate increased with the time. This was because that the reactive surface area followed a growing trend with reaction time [37]. This is discussed in further detail below.