<p>Calcium carbonate crystallization was investigated under dynamic conditions at elevated temperature in the presence of magnesium ions and organic additives. The crystallization at dynamic conditions was simulated at temperatures above 80 °C in a mildly alkaline medium, at varied Mg²⁺ concentrations, in the presence of dicarboxylic, polycarboxylic, and phosphonic acids. Scanning electron microscopy and X-ray diffraction were used to analyze the obtained sediments. Thermodynamic and kinetic parameters of calcium carbonate crystallization were calculated. It is shown that the presence of Mg²⁺ ions increases the fraction of metastable aragonite and vaterite phases by a factor of 1.2–1.3. The introduction of dicarboxylic acids increases the fraction of aragonite and calcite, whereas polycarboxylic acids inhibit the transformation of metastable vaterite into calcite. In the presence of poly(acrylic) acid, poly(aspartic) acid, and aminotris(methylene)phosphonic acid, the induction period of crystallization is 1.3–1.5 times higher indicating an effect of these acids on the nucleation and growth of calcium carbonate crystals.</p>