# Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation
## 3 Results and Discussion
### Hydration Products  
To better understand the pore solution chemistry, the hydration products of OPC and CSA were also studied. Figure 3 illustrates the XRD patterns of hydration products at different times. In the diagram of OPC, the peaks standing for Ca(OH)\({}_{2}\) are distinct. The presence of portlandite releases enough hydroxide into the pore solution and offers a buffering capacity to maintain the alkaline environment in OPC. It is known that the CSA clinker contains a certain amount of C\({}_{2}\)S which can hydrate to form Ca(OH)\({}_{2}\). However, in the XRD patterns of CSA in Figure 3b, there are no signals of portlandite. The main hydration product is ettringite with clear peaks even at 1 day, while the other peaks stand for the substances from the clinker, namely unhydrated C\({}_{4}\)A\({}_{3}\)S (yevelimite), CaSO\({}_{4}\) (Gypsum) and some CaCO\({}_{3}\) (carbonate), same as the result of He [19]. The Ca(OH)\({}_{2}\) generated by C\({}_{2}\)S has been completely depleted in the reaction with abundant vejelimite following Equation (3). The lack of Ca(OH)\({}_{2}\) reasonably corresponds to the low alkalinity of CSA pore solution. Another difference between these two cements is that the peaks of CaSO\({}_{4}\) were identified in all samples of CSA. This shows that there is redundant CaSO\({}_{4}\) during the hydration process, which supports the variation pattern of sulfate content discovered in pore solution chemistry analysis. Evidently, the change in SO\({}_{4}\)\({}^{2-}\) concentration is due to the combined effect of the depletion of SO\({}_{4}\)\({}^{2-}\) and the dissolution of CaSO\({}_{4}\). According to the water content in Table 4, the hydration of CSA mainly proceeds before the age of three days, during which more sulfate ions are needed as the reactants than what can be dissolved from CaSO\({}_{4}\). After that, the hydration is nearly finished, but CaSO\({}_{4}\) can still dissolve until the equilibrium is reached. Hence, the SO\({}_{4}\)\({}^{2-}\) concentration of CSA pore solution stays at a relatively low level at the very early stage, but the value eventually increases again to reach a stable condition of about 50 mmol/L. The existence of CaSO\({}_{4}\) also explains the reason for the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Cement Composition**} & \multicolumn{6}{c}{**Concentration (mmol/L)**} \\ \cline{2-7}  & **Na\({}^{*}\)** & **K\({}^{*}\)** & **Ca\({}^{2+}\)** & **Al(OH)\({}_{4}\)** & **SO\({}_{4}\)\({}^{2-}\)** & **OH\({}^{-}\)** \\ \hline This study & & & & & & \\
28 day, w/b 0.5 & & & & & & & \\ Na\({}_{2}\)O\% = 0.19\% K\({}_{2}\)O\% = 0.54\% & 154.5 & 250.6 & 2.6 & 178.1 & 55.8 & 55.3 \\ Al\({}_{2}\)O\% = 20.5\% SO\% = 13.4\% & & & & & & \\ \hline Winnrefeld [17] & & & & & & \\
28 day, w/b 0.72 & & & & & & & \\ Na\({}_{2}\)O\% = 0.05\% K\({}_{2}\)O\% = 0.21\% & 21.0 & 120.0 & 0.31 & 100 & 3.4 & 50.0 \\ Al\({}_{2}\)O\% = 35.5\% SO\% = 16.8\% & & & & & & \\ \hline
28 day, w/b 0.8 & & & & & & & \\ Na\({}_{2}\)O\% = 0.13\% K\({}_{2}\)O\% = 0.40\% & 34 & 82 & 0.23 & 26 & 9 & 70.0 \\ Al\({}_{2}\)O\% = 26.8\% SO\% = 19.5\% & & & & & & \\ \hline Andac [23] & & & & & & & \\
30 day, w/ b 0.8 & & & & & & & \\ Na\({}_{2}\)O\% = 0.095\% K\({}_{2}\)O\% = 0.295\% & & & & & & & \\ Al\({}_{2}\)O\% = 37.4\% SO\% = 9.5\% & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Summary of chemical analysis of pore solutions (CSA). monosulfoaluminate (AFm) absence. Theoretically, the main hydration products of CSA include both AFt and AFm, as described in Equations (4) and (5). Nevertheless, AFm forms only until all CaSO\({}_{4}\) is totally depleted. In this study, the clinker contained enough gypsum, so no AFm was detected in the XRD patterns. Al(OH)\({}_{3}\) is also an important hydration product of CSA. However, it cannot be characterized by the peaks because it exists in the form of gel.