# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 3 Results
### Ex Situ Characterization
#### 3.2.6 Compositional Analysis of the Precipitates  
Energy dispersive X-ray (EDX) elemental analysis was carried out in the M-S-H precipitates and inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was performed on the dissolved M-S-H precipitates to estimate their compositions. It must be pointed out that to obtain reliable quantitative analysis of samples with SEM-EDX, an accurate calibration is necessary. Furthermore, the sample should be homogeneous within the interaction volume, perfectly flat (polished), and normal to the incident beam as well as stable under the electron beam, which is not the case in our measurements. Nevertheless, semi quantitative elemental data for M-S-H can be obtained from EDX analysis too. Fifty representative points were measured within locations identified visually as M-S-H precipitates. A homogenous distribution of the fifty points all over the sample was pursued by the operator. EDX analysis has been widely used in the cement community to estimate the composition of hydrated products, due to the advantage of being coupled to SEM imaging, which allows the selection of specific areas of the sample for analysis [24; 40; 54]. The relative composition of Mg, Si, and secondary metal (50 measurement points per sample) is depicted in Figure S3 for each sample. Most of the samples show consistent values with no significant variation of Mg, Si, and secondary metal relative content among the 50 points. However, in the case of precipitate obtained in the presence of Ba, Zn, Co, and Ni, considerably higher variation in the relative metal content (up to 80% for Ba) were measured (and consequently lower Mg and Si) resulting in two distinct trends in the EDX measurements (Figure S4). In Figure 6a, the average total metal (Mg + secondary metal) to Si ratio obtained from the EDX measurements are presented. Two different sets of values are depicted: the average of the 50 measured points (first column Figure 6a) and the so called \"corrected\" average values (second column Figure 6a) in which the highest 10 values have been neglected as we consider that they correspond to the secondary non-M-S-H phases (Figure S4b), and thus they do not represent the incorporation of the metals within the M-S-H. In the special case of precipitates obtained with Ba (Figure S5), the formation of two distinct phases is astonishingly clear and the highest 20 points were neglected to obtain reliable estimate for the Ba content in the M-S-H (Ba* in Figure 6a,b). The compensation/exchange of Mg with the complementary metals results in an overall metal ion content quasi equal to the content precipitated in the control experiments. The average Mg/Si ratio in the reference samples was close to one. In general, all the obtained average values were close to one for most of the samples; however, when considering all 50 values, the total metal/Si ratio is higher for Ba, Zn, Co, and Ni probably due to the presence of secondary phases that are distributed over the sample. Correction neglecting the highest 10 values, further confirm that the presence of the complementary metals does not significantly influence the total metal/Si ratio. As can be seen from the ternary plot, constructed using the EDX \"corrected\" values (Figure 6b), the compositions of the M-S-Hprecipitates do not vary significantly regarding the Si content (ca. 50%) and only changes in the Mg/secondary metal ratio are observed. Regarding the monovalent elements tested (Cs, and Li), it should be noted that Li was not possible to measure with EDX and Cs seems to not be incorporated. The ICP measurements show that the elements Al, Fe, and Co were the most incorporated secondary metal in the precipitated samples (Figure S6, first column). Regarding the ratio of secondary metal/total metal, most of the samples (Cu, Cr, Co, Fe, and Pb) exhibit a good agreement between both independent techniques (Figure S6).