# Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation
## 3 Results and Discussion
### Pore Solution Analysis
#### 3.1.1 Pore Solution Chemistry
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Method**} & \multicolumn{5}{c}{**ESL (mmol/L)**} \\ \cline{2-5}  & & **Time (\({}^{\text{(1)}}\)** & **Na\({}^{\text{+}}\)** & **K\({}^{\text{+}}\)** & **Ca\({}^{\text{2+}}\)** & **Al(OH)\({}_{4}\)\({}^{-}\)\({}^{\text{(2)}}\)** & **SO\({}_{4}\)\({}^{2-}\)** & **OH\({}^{-}\)** \\ \hline \multirow{4}{*}{**CSA**} & 5 min & 55.7 & 106.4 & 29.9 & 18.0 & 38.8 & 0.5 \\  & 3 h & 23.7 & 48.9 & 2.4 & 30.5 & 4.3 & 5.5 \\  & 6 h & 20.1 & 36.9 & 1.9 & 32.1 & 3.9 & 5.6 \\  & 1 d & 26.1 & 37.8 & 0.7 & 33.1 & 3.2 & 6.0 \\  & 3 d & 26.1 & 35.6 & bld (\({}^{\text{(3)}}\)) & 32.6 & 5.9 & 6.3 \\  & 7 d & 23.9 & 35.4 & 0.5 & 28.7 & 7.5 & 5.5 \\  & 28 d & 22.4 & 36.3 & 0.4 & 25.8 & 8.1 & 6.5 \\ \hline \multirow{4}{*}{**OPC**} & 5 min & 24.1 & 108.5 & 58.9 & - & 46.2 & 39.8 \\  & 1 d & 18.3 & 35.1 & 6.4 & - & bld & 56.4 \\ \cline{1-1}  & 3 d & 18.4 & 33.8 & 7.4 & - & 1.7 & 57.5 \\ \cline{1-1}  & 7 d & 24.0 & 42.6 & 7.1 & - & bld & 61.3 \\ \cline{1-1}  & 28 d & 28.9 & 48.5 & 6.2 & - & 0.4 & 71.9 \\ \hline \hline \end{tabular} \({}^{\text{(1)}}\) min stands for minute; h stands for hour; d stands for day. \({}^{\text{(2)}}\) Al(OH)\({}_{4}\)\({}^{-}\) was detected through the element of aluminum by ICP-MS; Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), and SO\({}_{4}\)\({}^{2-}\) were measured by IC; OH\({}^{-}\) was measured using a pH meter. \({}^{\text{(3)}}\) bld means below the limit of detection.  
\end{table} 
Table 3: Leaching solution chemistry of OPC and CSA. The free water contents of the paste samples are listed in Table 4. After the rapid hardening of CSA, the free water content consumed in the hydration process decreased quickly during the first day. However, the rate of water depletion in OPC stayed relatively constant. According to Equation (2), the results of the pore solution concentration are shown in Table 5 after amendment and verification by PHREEQC. Comparing the results of OPC and CSA, the most noteworthy point is the low alkalinity of CSA. According to the pH value development plotted in Figure 1, the initial pH of CSA is quite low, but it quickly increases to 12.41 after 3 h, and eventually climbs to 12.74. However, the hydroxyl concentration corresponding to the highest pH is only about one eighth of that in OPC. With such a low alkaline nature, CSA can hardly offer effective protection for steel against corrosion. As for other ion species, the differences between cation concentrations were not obvious, but the results of the anions (Al(OH)\({}_{4}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\)) varied greatly. The most prominent feature of CSA is the exclusive ion, Al(OH)\({}_{4}\)\({}^{-}\), which has never before been detected in any Portland cement based materials. Its concentration can be higher than 0.15 M. For a new type of ion with such a high concentration, it is necessary to study its effect on steel passivation. Secondly, the concentration of SO\({}_{4}\)\({}^{2-}\) in CSA was extremely high. At the beginning of hydration (5 min), the value was lower than that in OPC, but the value kept stable at about 0.05 M. In contrast, SO\({}_{4}\)\({}^{2-}\) was nearly negligible in OPC after a certain time. A large number of studies have shown that SO\({}_{4}\)\({}^{2-}\) can cause steel corrosion, but the mechanism of corrosion is still under debate. The influence of solution chemistry on passivation will be discussed in a following section.