# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 3 Results  
### Precipitation Experiments  
In the conducted experiments, the nucleation and precipitation of M-S-H from supersaturated solution was triggered by the simultaneous addition of magnesium chloride solution with a secondary metal cation and silicate bearing solutions into Milli-Q water. Scoping calculations with Phreeqc [42] suggest that the presence of the considered concentration of secondary metal cations should not have a large effect on the solubility conditions. The slight changes in the supersaturation of a magnesium silicate hydrated phase (sepiolite) used as a model M-S-H phase can be neglected based on Phreeqc thermodynamic calculations (results not shown). The SI\({}_{\text{sepiolite}}\) was calculated to be about \(\sim\)8 at 1000 s (prior nucleation) for all the simulated experiments in the presence of the secondary metal cations and in the reference experiments. The onset and advance of the crystallization were monitored by pH and transmittance measurements (Optrode). The pH and transmittance data for the reference experiments are plotted versus the magnesium and silicate concentration in solution in Figure 1a. The plotsrepresent the averaged values of pH and transmittance measurements from at least two independent experiments. In the pre-nucleation stage, the pH increased from pH 6 (Milli-Q water) to around 10.7 due to the addition of Na\({}_{2}\)SiO\({}_{3}\) solution. Upon reaching a critical concentration of Mg\({}^{2+}\) and silicate species in the media, the nucleation of M-S-H occurred (referred as \"nucleation onset\" in Figure 1). This was clearly marked by the maximum value of the pH (Figure 1a, blue line) and the decrease in the transmittance of the solution (Figure 1a, dark grey line). The signal of the transmittance curve was at the maximum at the beginning of the experiments, indicating a clear solution without the presence of particles. Upon nucleation, the fall of the transmittance curve was caused by the increase of the turbidity of the solution/dispersion in which the light can no longer be fully registered by the Optrode due to the scattering by the newly formed M-S-H particles. After the (quasi-linear) decrease in the transmittance of the solution, the curve approached a constant level by the end of the experiments. The period between the first drop of the transmittance and the reach of the plateau level is referred as \"growth period\" in this study. The decrease in the transmittance is explained by both increase in the number M-S-H particles and the increase in size of the nuclei. Therefore, this growth period must be considered only as a label for referring to the above-described part of the transmittance curve since it is not possible to distinguish between the nucleation and growth processes with the experimental techniques employed in this study. The influence of the different metal ions regarding the nucleation onset and the growth period can be evaluated by comparing the evolution of the transmittance and pH between the different experimental runs. Both values presented in Figure 1b are calculated relative to the control run (t\({}_{\text{metal}}\)/t\({}_{\text{control}}\)). In any case, it is worth mentioning that the indication of nucleation was slightly earlier detected for transmittance measurement comparing with the pH electrode. Regarding the nucleation onset (Figure 1b, light blue column) no trend in relation with the valency of the ions was visible as most of the titrations in presence of secondary metal cations show a delayed onset of the nucleation of ca. 1.5 relative to the control runs where no metals were present. Only Cr and Al involving experiments displayed equal or premature onset compared to the pure system. However, comparing the growth period of M-S-H (Figure 1b, dark blue column), the effects were distinct for the different metals. While ions of alkali metal type (Li, and Cs) showed no significant effect, prolongation of the growth period is evident for higher valence ions, with exception of Ba and Pb.