# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 5 Conclusions  
The influence of secondary metal ions on the early stages of M-S-H crystallization was evaluated in titration experiments. The simple experimental setup provides well reproducible data, and thus can be recommended for future studies as a \"fingerprinting technique\" to study the effects of additives such as metal cations on the formation of M-S-H in reliable manner. Obtained results showed the large capacity of M-S-H based cements for the immobilization of metal cations at the early hydration stages. The retardation effect regarding the formation of magnesium silicate hydrated products was not significant in the presence of secondary metal cations. However, the growth period of the precipitates was extended considerably for Fe, Co, and Cr. This opens a wide range of options for efficient control of early M-S-H crystallization by selecting the specific additives, which can be active at different stages of the early formation of M-S-H. In addition, it could be shown that most elements were incorporated into the M-S-H structure without inducing significant changes in the morphology of the precipitates. Even in presence of secondary phases co-precipitated (Fe, Pb, Zn, and Ni), those phases were immobilized in the M-S-H matrix. Ba was the only cation in presence of which the formation of a secondary carbonate phase isolated from the M-S-H precipitates was detected. Systematic screening of the effect of metals in the early crystallization of M-S-H showed that the immobilization at these stages can already be significant for some of the tested metals. Unravelling the specific mechanism of incorporation into the M-S-H matrix remains a challenge that would require the information provided in this manuscript as well as a deep investigation on the M-S-H structure, and it will be the object of our future research. The following are available online at [[https://www.mdpi.com/article/10.3390/app11072968/s1](https://www.mdpi.com/article/10.3390/app11072968/s1)]([https://www.mdpi.com/article/10.3390/app11072968/s1](https://www.mdpi.com/article/10.3390/app11072968/s1)). Figure S1: FT-IR spectra of precipitates samples from 500-4000 cm\({}^{-1}\), Figure S2: TEM images of precipitated crystals, Figure S3: Elemental composition of metals in M-S-H measured via EDX, Figure S4: Relative metals content in M-S-H measured via EDX ordered from higher to lower, Figure S5: Relative elements content of Ba contain ing M-S-H ordered from highest to lowest Ba content, Figure S6: Comparison of total metal to (Mg + metal ratio measured via ICP OES and EDX), Figure S7: Relative % of metal species calculated with Phreeqc, Table S1: Comparison of the average ratio of secondary metal to total metal (Mg + secondary metal) measured via ICP-OES and EDX. Conceptualization, C.R.-A., and M.R.M. ; methodology, C.R.-A., C.D., and M.R.M. ; software, C.R.-A., M.R.M., and F.D.L. ; validation, C.R.-A., M.R.M., C.D., F.D.L., E.B., and S.V.C. ; formal analysis, C.R.-A., M.R.M., F.D.L., and E.B. ; investigation, C.R.-A., M.R.M., C.D., and F.D.L. ; resources, C.R.-A. ; data curation, C.R.-A., M.R.M., C.D., F.D.L., and E.B. ; writing--original draft preparation, C.R.-A., M.R.M., C.D., F.D.L., and E.B. ; writing--review and editing, C.R.-A., M.R.M., C.D., F.D.L., E.B., and S.V.C. ; visualization, C.R.-A., and M.R.M. ; supervision, C.R.-A. ; project administration, C.R.-A. ; funding acquisition, C.R.-A. All authors have read and agreed to the published version of the manuscript. German Research Foundation (DFG) Project number: RU 2368/1-1. PI: [PERSON]. The data presented in this study are available within the article and Supplementary Material. This work was performed with the financial support of the German Research Foundation (DFG). [PERSON]-A. also thanks the SFB 1214 (project A07) funded by the German Research Foundation (DFG) and the Zukunftskolleg from the University of Konstanz.