# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 4 Discussion  
The early stages of M-S-H crystallization in the presence of secondary metal cations were investigated by stepwise addition of magnesium (and secondary metal cations) and silicate to double distilled water. The nucleation of M-S-H occurs when a pH ca. 10.5-11 is reached. Within this pH increase, most oxy-hydroxyl-heavy metal carbonates overcome their solubility product and therefore may precipitate together with M-S-H. At the onset of nucleation, the pH value reaches its maximum and then decreases slowly approaching to a quasi-constant value (Figure 1a). The decrease in pH would agree with the consumption of the silicate species--and the incorporation of OH\({}^{-}\) into the M-S-H structure. Speciation modelling with Phreeqc suggests that most of the secondary metal cations exist as metal hydroxides species in the prenucleation stage due to the increased alkalinity of the media (Figure S7). Cs, Li, and Ba are the only elements predominantly present as free ions in solution prior to M-S-H nucleation according to the thermodynamic modelling. The rest of the secondary metal ions mostly exist as metal hydroxide species or metal oxyanions. Interestingly, the secondary metals cations that does not form complexes were incorporated in the M-S-H to a lesser extent (almost negligible), based on our compositional analysis. This suggests that the incorporation of metal hydroxides or metalloid oxy-anions species into M-S-H is preferred over the incorporation of free metal cations tested in this study. The delay in the onset of nucleation relative to the reference run (marked by the pH and transmittance drop) was comparable (ca. 1.5) for most of the elements. Contrary, in the system with Cr, the nucleation event was accelerated compared to the reference runs. Thus, it can be envisaged that no strong retardation effects in the early formation of the hydration products is expected when using M-S-H to immobilize the metal cations investigated here. Concerning the growth period (when transmittance reached a constant plateau level), its prolongation was not always linked to a delay in the nucleation. This highlights that the early stages of M-S-H crystallization could be affected uniquely by different metal cations regarding nucleation and early growth stages. The distinct outcome for the nucleation onset and the prolongation of the growth period can be strongly dependent on the chemical speciation of metal cations incorporated in the M-S-H, the mechanism of incorporation, and the formation of secondary phases. The retardation in the growth period (Cr, Co, and Fe) is related to a higher secondary metal/total metal average ratio in the precipitated material measured by EDX (Figure S6). This could be caused by a mechanism of immobilization where the formation of nanometre sized secondary metal hydroxide phases which are trapped in the M-S-H matrix, plays a more decisive role than in the other tested secondary metals. Previous studies attribute the retardation effect in M-S-H formation in the presence of Sr\({}^{2+}\) to the precipitation of Sr(OH)\({}_{2}\)[40,55]. The competition for hydroxyl ions to form secondary metal hydroxide phases could extend the growth period of M-S-H. The presence of additives is not only impacting the kinetics of crystal growth but also the composition and structural motifs of the final cement hydrated product. The main precipitated phase formed in all the experiment is a poorly crystalline M-S-H, which displays the typical globular morphologies observed in previous studies [24,51,56]. Except for the precipitates obtained with Li and Cs, which do not seem to be incorporated into the M-S-H (on the time scale of our experiments), the other metal cations studied are immobilized in the precipitated solids. Formation of secondary phases was confirmed with XRD, SEM, and TEM for the system with Ba, Pb, and Fe and suggested for the systems with Ni and Zn based on EDX-analysis.