*5.3. pH*

The pH value of the reaction mixture is an important parameter during calcium carbonate precipitation. It was reported that vaterite is a dominant polymorph in solutions with an initial basic pH [21,42,74]. However, Han et al. [55] studied a carbonation system with ammonia as an absorption promoter and found that pure vaterite is formed when the pH is 8, but an increase in the pH value of the reactive mixture resulted in a decrease in the vaterite concentration. Almost pure vaterite produced in a gas-liquid system with the addition of ammonia and ammonium chloride was obtained when the pH decreased from 9.7 to 7.7 during the carbonation process [19]. The influence of pH on the vaterite content in selected processes is shown in Figure 4. Vaterite precipitation was promoted in the pH range from 8 to 10 for all compared processes. An extension of this range was possible by conducting precipitation in the presence of ethylene glycol [74] or ionic liquid surfactant [75].

**Figure 4.** Influence of pH on vaterite concentration in CaCO3 samples; (**a**) ) L-L, S = 6.5, t = 24 ◦C, based on data from [21]; (**b**) G-L, [Ca2+] <sup>=</sup> 0.1 M, t <sup>=</sup> <sup>20</sup> ◦C, VG <sup>=</sup> 18 dm3/h, xCO2 <sup>=</sup> 033, based on data from [55]; (**c**) L-L, [Ca2<sup>+</sup>] = 0.5 M, ta, with ILS, based on data from [75]; (**d**); L-L, [Ca2<sup>+</sup>]:[CO3 <sup>2</sup>−] = 1:3, t = 23 ◦C, with ethylene glycol, based on data from [74]; (**e**) L-L, [Ca2<sup>+</sup>]:[CO3 <sup>2</sup>−] = 1:1, t = 23 ◦C, with ethylene glycol, based on data from [74]; (**f**) G-L, t = 25 ◦C, xCO2 = 1, VG = 50 dm3/h (gray), VG = 100 dm3/h (orange), based on data from [59].

*5.4. Time*

Because vaterite is a metastable calcium carbonate polymorph, prolonging the reaction time results in a reduction of the vaterite content when an aqueous solution without the presence of organic additives is the medium of CaCO3 synthesis [13,49]. The remaining precipitated vaterite particles in the aqueous solutions lead its recrystallization to the more stable calcium carbonate polymorph, i.e., aragonite and calcite [58,69]. The time needed for the transformation of vaterite to calcite is usually in the range of a few minutes to several hours [13,58,69,76]. However, a longer time (up to 20 h) of vaterite stability in aqueous solutions was also reported when calcium carbonate was synthesized at a temperature of 7 ◦C [48]. Slight acceleration of the conversion of vaterite into calcite with an increase of the ionic strength was observed [77]. Some organic compounds used as additives can prevent the transformation of vaterite into calcite (see Section 5.6. Additives).
