## 4 Discussion  
The overall carbonation reaction at 333 K, 2 MPa CO\({}_{2}\) pressure (accompanied by 70 ppm SO\({}_{2}\) and 70 ppm NO\({}_{2}\)), and in brine were in good agreement with the results of [PERSON] et al. [6,7] (Figure 5, 333 K, 2 MPa pure CO\({}_{2}\) pressure, in water; Figure 6).  
Figure 5: Carbonation of untreated (**a**) and purified (**b**) wollastonite at 333 K (60 \({}^{\circ}\)C), 2 MPa (pure CO\({}_{2}\)), and aqueous condition (H\({}_{2}\)O). The quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta circles), and aragonite (green circles) were determined by the Rietveld method. The red circles symbolize the sum of calcite and aragonite and, therefore, the content of carbonate in the sample detected by PXRD. The amount of CaCO\({}_{3}\) determined by TG measurements is symbolized by cyan circles [6,7]. Figure 5b was adapted from [7]. Permission for using Figure 5b was granted by the International Cement Microscopy Association (ICMA).  
Figure 6: Carbonation of wollastonite at 333 K (60 \({}^{\circ}\)C), 2 MPa (CO\({}_{2}\) accompanied by 70 ppm SO\({}_{2}\) and 70 ppm NO\({}_{2}\)), and chloride-rich brine (H\({}_{2}\)O with 230 g/L NaCl, 15 g/L CaCl\({}_{2}\), 5 g/L MgCl\({}_{2}\)). The quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta circles), and aragonite (green circles) were determined by the Rietveld method. Halite, as a remnant of the chloride-rich brine is represented by orange circles. The cyan circles symbolize the amount of CaCO\({}_{3}\) determined by TG measurements.  
The re untreated wollastonite raw material at 333 K was more or less fully converted after 18 h (Figure 5a) [6]. The content of wollastonite (black squares) decreased exponentially, while the amounts of the products (calcite, aragonite, total amount of CaCO\({}_{3}\), and X-ray amorphous content) increased exponentially. The \"full\" (-95%) conversion of purified (HCl, 10%) wollastonite at 333 K was already achieved after 18 h (Figure 5b) [7]. The quantities of wollastonite, calcite, the total amount of CaCO\({}_{3}\), and the X-ray amorphous content developed in a similar manner compared to the untreated wollastonite raw material (Figure 5). However, in the products of the previously purified wollastonite raw material, no aragonite was observed.  
The conversion of wollastonite under conditions expected at injection sites for CCS (CO\({}_{2}\) accompanied by 70 ppm SO\({}_{2}\) and 70 ppm NO\({}_{2}\), 333 K, chloride-rich brine) was practically fully completed after 24 h (Figure 6). The amount of calcite (magenta circles) and the total amount of CaCO\({}_{3}\) measured by thermogravimetry (cyan circles) and the amorphous content (blue triangles) increased exponentially, whereas the vice versa amount of wollastonite (black squares) decreased exponentially. The content of aragonite was constant during the series of experiments and no newly formed aragonite was observed. The content of halite was less than 1 wt.% which functioned as a monitoring of the successful washing procedure after the carbonation reaction.  
Compared to the work of [PERSON] et al. [6; 7; 8; 9] (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. No additional aragonite formation was observed, which was expected due to previous work [6; 8; 13]. According to the work of [PERSON] [29] and [PERSON] et al. [30], the chloride-rich brine, containing high amounts of sodium chloride (230 g/L), inhibited the aragonite formation.