## 3 Results and Discussion
### Discussion of the PXRD and DSC-TG/MS Results
This was also true for the increase of calcite and the amorphous content vice versa. At higher temperatures (323 K and 333 K), aragonite exhibited also exponential increase (Figures 8 and 9). An additional experiment was performed, in which the temperature (333 K) and time (6 h) were held constant and only the pressure varied. The results showed that the reaction was mostly independent of pressure (Figure 10). At low pressure (0.2 MPa) the decrease of wollastonite was slower and newly formed aragonite was not observed. At higher pressures (>1 MPa) the carbonation of natural wollastonite showed no further dependence of the applied pressure (Figure 10). From 1 MPa to 5 MPa no differences considering the rate of wollastonite transformation were observed. However the formation of aragonite was dependent on pressure. From 0.2 MPa to 2 MPa the formation of aragonite increased, from 2 MPa to 4 MPa it was constant and from 4 MPa to 5 MPa the formation of aragonite decreased. Figure 11 shows this observation as normalized amount of the wollastonite conversion. Figure 9: Carbonation of wollastonite at 333 K (60 \({}^{\circ}\)C), 2 MPa and aqueous conditions (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 are symbolized by cyan circles. The ellipsoids with arrows depict the symbols, at which aragonite and calcite exhibit nearly the same amount. Figure 11: Conversion rate of wollastonite at different pressures and constant temperature (333 K) and time (six hours). Figure 10: Carbonation of wollastonite at 333 K (60 \({}^{\circ}\)C), aqueous conditions (H\({}_{2}\)O) and reaction time of 6 h. The pressure was varied from 0.2 MPa to 5 MPa. 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 symbolized 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 were symbolized by cyan circles. Considering the conversion of wollastonite to CaCO\({}_{3}\) in the presence of water the reaction rate of the carbonation depended strongly on the temperature, at higher temperatures the reaction rate was significantly higher (Figures 7-9). A first order reaction was assumed because the reaction was only dependent on wollastonite. The amount of CO\({}_{2}\) was one order of magnitude higher than the amount of wollastonite. This assumption could be supported by plotting In (C) vs time (Figure 12) from which a linear dependency after a conversion of \(\sim\)90% could be derived. In Figure 13 the wollastonite hkl reflection (100) at 12 \({}^{\circ}\) 2\(\Theta\) decreased from 296 K to 333 K.  
Figure 12: Conversion rate of wollastonite plotted as ln (C) vs time. Figure 13: X-ray powder diffractograms of carbonated wollastonite, recorded at different temperatures (296 K, 323 K, 333 K). Pressure (2 MPa) and time (24 h) were constant. The carbonation rate increases at higher temperatures. Considering the conversion rates from wollastonite to calcite the rate constants and activation energies could be estimated (Table 6). The carbonation reaction of wollastonite starts at the surface of a grain and moves towards the center [10; 11; 34]. The reaction rate will decrease because of the growing layer thickness of CaCO\({}_{3}\) between unreacted CaSiO\({}_{3}\) and CO\({}_{2}\). Consequently, low amounts (5% to 10%) of material will remain unreacted within experimental periods. At 323 K, the reaction rate is three times faster, whereas at 333 K the reaction rate increased by a factor of six (Figure 14).