*4.6. Hydrothermal and Solvothermal Methods*

A hydrothermal or solvothermal process is a method used to create ceramic materials at elevated temperatures and pressure. Hydrothermal conditions were applied to precipitate vaterite-reach particles using diethylenetriaminepentaacetic acid as an additive [68]. The first step of this process was CaCO3 precipitation using CaCl2 and Na2CO3 solution at a temperature of 120 ◦C. Then, the hydrothermal process was carried out using the filtrate from the first step as a reactive mixture. The filtrate was placed in an autoclave at temperatures between 130 and 230 ◦C for 48 hours and the highest concentration of vaterite (90%) was obtained at the highest tested temperature, i.e., 230 ◦C.

The solvothermal method for vaterite synthesis was proposed by Li et al. [23]. Calcium chloride and urea (a precursor of carbonate ions) were reagents dissolved in ethylene glycol, 1,2-propanediol, or glycerol. Carbonate ions were formed by decomposition of urea in solvothermal conditions at temperatures from 100 to 190 ◦C for 12 hours. No additives or pH control were needed to precipitate the pure vaterite phase. The pure vaterite particles were produced in ethylene glycol at 100 ◦C, in 1,2-propanediol at 130 ◦C, and in glycerol at 150 ◦C.

#### **5. Factors Influencing Vaterite Formation**

Calcium carbonate can form several polymorphs, therefore, its crystallization requires careful control of the process parameters to obtain the desired product. Factors affecting the crystallization of the preferred polymorphic form have been grouped by Kitamura in a set of primary and secondary variables [69]. Primary factors include supersaturation, temperature, stirring rate, and seed crystals. However, the solvent composition, additives, and pH are secondary factors. The main investigated factors influencing the precipitation of vaterite are presented below. The effect of solvents and other organic substances is discussed together because organic solvents are treated as additives in many studies on calcium carbonate precipitation.
