*5.1. Soak Solution Chemistry*

The chemical analysis of the sulfate soak solutions indicated that the sulfate-rich environment significantly influences the amount of calcium and silica ions leached out from the CCS matrices. As shown in the graphs presented in the Figures 1–4, all sulfate solutions triggered a substantial increase (compared to deionized water soak solution) in the amount of calcium ions leached out from the CCS matrices. Depending on the solution, the observed increases were in the range of 10–15 times. There is also a noticeable trend of decay in Ca ion concentration with time, for all pastes exposed to aluminum sulfate, as well as cement #2 pastes exposed to sodium sulfate, and cement #3 and #4 pastes exposed to the magnesium sulfate solution. This might be due to the consumption of calcium ions during precipitation of new compounds, such as gypsum (see Section 5.2). With respect to the leaching of the silica species, the effect of sulfate solutions, especially aluminum and magnesium sulfates, appears to be quite opposite to that observed for the calcium species. As an example, for all four types of pastes, the exposure to aluminum sulfate resulted in only minimal levels of silica in the soak solution, indicating that this sulfate solution stabilizes silica species present in the specimens. Similarly, it also appears that magnesium sulfate stabilized silica in all paste samples, except that of the carbonated wollastonite paste (cement #1). However, the effect of sodium sulfate seems to be rather selective: it stabilized the leaching of silica compared to immersion in deionized water only for the cement #2 paste samples.

**Figure 1.** Changes in concentrations of: (**a**) Ca and (**b**) Si species present in sulfate solutions with time, in contact with cement #1 paste specimens (SS—sodium sulfate, MS—magnesium sulfate, AS—aluminum sulfate, DIW—de-ionized water).

**Figure 2.** Changes in concentrations of: (**a**) Ca and (**b**) Si species present in sulfate solutions with time, in contact with cement #2 paste specimens (SS—sodium sulfate, MS—magnesium sulfate, AS—aluminum sulfate, DIW—de-ionized water).

**Figure 3.** Changes in concentrations of: (**a**) Ca and (**b**) Si species present in sulfate solutions with time, in contact with cement #3 paste specimens (SS—sodium sulfate, MS—magnesium sulfate, AS—aluminum sulfate, DIW—de-ionized water).

**Figure 4.** Changes in concentrations of: (**a**) Ca and (**b**) Si species present in sulfate solutions with time, in contact with cement #4 paste specimens (SS—sodium sulfate, MS—magnesium sulfate, AS—aluminum sulfate, DIW—de-ionized water).

The change in concentrations of sodium and magnesium ions over the entire test period is presented in Figure 5. The concentrations of aluminum cations are not presented, as they are negligibly small. The concentrations of sodium ions remain essentially constant, except for a slight reduction in the case of cement #2 paste. In contrast, one can observe that over time, the concentration of magnesium ions reduced by as much as ~68% for the cement #3 paste sample. In the case of cement #2 and #4 paste samples, the reduction was

about ~10%; whereas for cement #1 paste samples, that reduction was very small (~2%) and it was only detected after 120 days of exposure.

**Figure 5.** Normalized concentrations of Na<sup>+</sup> (**a**) and Mg2+ (**b**) ions in soak solutions of all cement paste samples, over a period of exposure.

The plots of normalized changes in concentration of sulfate ions (for sodium, magnesium and aluminum sulfate solutions) for soak solutions in contact with all cement paste samples used in this study are presented in Figure 6. The trends illustrated by the sulfate ion curves seem to be quite similar to those observed for the corresponding sodium and magnesium cations. In the case of the aluminum sulfate solution, the concentration of the sulfate ions in the soak solution after exposure of the paste samples (even at early days) was observed to be negligibly low as the concentration of Al3+ species.

**Figure 6.** Normalized concentrations of sulfate ions in sulfate soak solutions ((**a**)—sodium sulfate, (**b**)—magnesium sulfate, (**c**)—aluminum sulfate) of all cement paste samples, over a period of exposure.
