# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 4 Discussion
The formation of barium carbonate was observed in the system with Ba, where large BaCO\({}_{3}\) crystals were found along with M-S-H samples. This observation suggests co-precipitation of BaCO\({}_{3}\) and M-S-H phases composing a mechanical mixture in the precipitate. However, in the system with Fe, the nanometric iron oxide precipitates were trapped within the M-S-H matrix. The minute size of Fe hydroxides (tens of nm, Figure 5) most likely allows an easy incorporation of this phase into the growing M-S-H particles. This behavior suggests an incorporation of FeIII via an absorption mechanism. The hydrocernusite phase detected by XRD could neither be identified by SEM nor TEM, suggesting that it was also homogenously incorporated into the M-S-H matrix and distributed over the sample at nanoscale and thus no differences were apparent in the EDX mapping analysis. The formation of the carbonated phases in the experiments with Ba and Pb is not fully understood but could also be explained by CO2 contamination during centrifugation, washing, and/or drying procedure. The co-precipitation of witherite together with M-S-H highly affects the average values obtained with EDX analysis as depicted in Figure 6. The presence of two distinct phases is obvious from the EDX analysis (Figure S5): a barium-rich phase (most likely witherite) and a phase where the barium was negligible (M-S-H). In comparison with ICP-OES data, the 50 points average Ba/(Ba+Mg) ratio obtained by EDX-analysis is considerably higher, which can be attributed to the arbitrary selection of measurement locations by the operator. This in turn may have resulted in the analysis of witherite particles buried within M-S-H precipitates or mixed with them in more than 40% of the measurements (Figure S6). However, when calculated the Ba/(Ba+Mg) ratio by neglecting the first 20 points measured by EDX (Figure S6, third column *), the discrepancy between the average ratio calculated with ICP and EDX data is highly reduced, indicating that witherite was not the most abundant phase in the precipitates. The EDX analysis for Pb and Fe samples suggests that those secondary phases were present in a minor amount and their distribution within the M-S-H matrix was more homogeneous. In the system with Ni and Zn, neither XRD nor TEM evidence the formation of secondary phases which hints again to their minor presence in the sample. The arbitrary selection of locations for the EDX-analysis might have uncovered the existence of those resulting in the higher average values measured by EDX (Figure S6, second column) compared to ICP-OES (Figure S6, first column). The corrected ratio of (secondary metal/total metal) agrees well with the ICP-OES data supporting the minor presence of the Ba, Pb, Fe, Ni, and Zn secondary phases. In the system with Al, XRD data are identical to the one collected in the reference M-S-H experiment. This agrees with recent studies that proved the incorporation of Al in both silica and magnesium oxide layers in M-S-H structure without inducing changes detectable in the XRD pattern [54]. FTIR analysis also indicate that the structure of the obtained M-S-H precipitates is weakly affected by the incorporation of Al. The slight shoulder in the Q4 region indicated that some of the silicate tetrahedra could be connected to each other than to a metal ion (presence of amorphous silica) compared with the reference M-S-H precipitates. The substitution of Al for Mg measured by EDX is ca. 10% which is lower comparing with the other metals (except Cs and Li). However, compositional analysis (EDX/ICP) of this sample shows rather high uncertainty regarding the ratio Al/(Al+Mg) in the precipitated material. This high discrepancy between both techniques is difficult to explain, considering the coherent values obtained by EDX analysis (Figure S3). It is worth noting that EDX data for Al content are less reliable due to the lower sensitivity of this method to light elements.