## 5 Conclusions  
Carbonation of wollastonite in chloride-rich brine using pure CO\({}_{2}\) and CO\({}_{2}\) accompanied by 70 ppm SO\({}_{2}\) and 70 ppm NO\({}_{2}\) was investigated. According to previous studies [6; 8; 13], the formation of aragonite was expected. However, no additional aragonite formation was observed, which was, according to the work of [PERSON] [29] and [PERSON] et al. [30], inhibited by the high amounts of sodium chloride in the brine.  
The carbonation of wollastonite, depending on the reaction time (Table 3 and Figure 6), was in a good agreement with the work of [PERSON] et al. [6; 7] (Figure 5). Higher amounts of calcite and lower amounts of X-ray amorphous content were observed. This was possibly due to the washing process, which was necessary to remove the salts (NaCl, CaCl\({}_{2}\), and MgCl\({}_{2}\)) from the samples.  
The performed experiments showed that the carbonation reaction of wollastonite under conditions relevant for CCS (333 K, chloride-rich brine, CO\({}_{2}\) accompanied by 70 ppm SO\({}_{2}\) and 70 ppm NO\({}_{2}\)) was rather fast (24 h). The use of wollastonite as a CS-Cement in CCS seems possible. Even the aggressive fluid (H\({}_{2}\)O with 230 g/L NaCl, 15 g/L CaCl\({}_{2}\), 5 g/L MgCl\({}_{2}\)) had no negative impact on the conversion of wollastonite.  
The long-term stability is still under investigation and has to be evaluated in depth. As assumed and reported by [PERSON] et al. [6], the most likely use of wollastonite as a cementitious material in CCS application would be in finishing operations for sealing the borehole after the injection of CO\({}_{2}\) in the reservoir is completed. Further concrete technology considerations will be the next step towards sustainable mixtures, which will be poured into the borehole.  
Supplementary Materials: Supplementary materials can be found at [[http://www.mdpi.com/2076-3417/9/6/1259/s1](http://www.mdpi.com/2076-3417/9/6/1259/s1)]([http://www.mdpi.com/2076-3417/9/6/1259/s1](http://www.mdpi.com/2076-3417/9/6/1259/s1)). Table S1: Phase composition of the carbonated samples (chloride rich brine, 333 K, 2 MPa), determined by powder X-ray diffraction (PXRD) and Rietveld refinement [24] (R\({}_{\rm WP}\): \(\sim\)9 %).  
**Author Contributions:** The authors worked together in a research project (Joint research project CLUSTER [12]). [PERSON], [PERSON], [PERSON], and [PERSON] conceived and designed the experiments. [PERSON] and [PERSON] performed the experiments. [PERSON] and [PERSON] analyzed the data. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] contributed ideas, knowledge, and discussion. [PERSON], [PERSON], and [PERSON] wrote the paper.  
**Funding:** This research was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi; funding code: 03 ET7031D).  
**Acknowledgments:** For financial support, the authors want to thank the German Federal Ministry for Economic Affairs and Energy (BMWi; funding code: 03 ET7031D). The financial support within the funding program Open Access Publishing by the German Research Foundation (DFG) is gratefully acknowledged. For granting permission to use Figure 5b, the authors want to thank [PERSON], General Chairman of the International Cement Microscopy Association (ICMA). The constructive comments and helpful suggestions of two anonymous reviewers are gratefully acknowledged.  
**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.