## 2 Zeolites
### Synthesis Methods
#### 2.9.2 Fusion  
The hydrothermal synthesis method can be preceded by a process of fusing the starting material with the alkali necessary to carry out the zeolitization of the material. According to the literature, the foundation of the method is based on mixing the precursor with the alkali and melting the system, allowing the crystalline phases of Si and Al present in the material to decompose, thus forming sodium silicates and aluminates with high reactivity and water solubility and enabling the formation of the zeolitic product that must be formed in the hydrothermal process carried out later. This method more specifically directs the synthesis to a certain type of zeolite through the variation of activation parameters in hydrothermal treatment. Compared to the hydrothermal method, that involving pre-alkaline fusion of the precursor enables an improvement in the zeolitization process and the crystallinity of the product. While the hydrothermal method without fusion generates products with structural heterogeneity and poor ion exchanger properties [146], in the fusion process, as the sodium hydroxide, the main alkali used, is introduced, the sodium ions act as stabilizers for the zeolite crystal structure construction subunit, increasing the amount of zeolite formed over a long synthesis. In this regard, Dere Ozdemir and Piskin [147] investigated the use of microwaves and ultrasound to assist the synthesis, and found them useful tools to minimize the reaction time while maintaining a high content of formed zeolite. The sonication process assisting the fusion can be applied for only 10 min, generating positive aspects regarding the reaction speed [148]. According to Naghsh and Shams [149] the process of improving the melting method allows the generation of a product with a wider  
Figure 10: General scheme of hydrothermal synthesis. Reproduced from [138]. Copyright 2005 Elsevier.  
range of applicability, as in the removal of water hardness by geopolymers, which fixed the melting time at 14 h and the optimum melting temperature between 500 and 700 \({}^{\circ}\)C.  
The characterization of the products obtained by the fusion method is important for comparing the degree of similarity of the product obtained with a natural zeolite through X-ray diffraction [150]. Ayele et al. [151] observed that this increase in crystallinity is important in aluminosilicate precursors such as kaolin, as it is the result of its activation and dissolution in the synthesis medium. In addition, Deng et al. [150] described that thermal analysis allows the comparison of the stability of products obtained by hydrothermal process and fusion, where greater stability of the hydrothermal method product was found in relation to the fusion product. In addition to the high crystallinity that can reach more than 80% in some types of zeolites, the synthesis method can provide good cation exchange capacity [151]. To improve the results of the products, fusion methods with desilicification with NaOH and fusion with hydrothermal reaction with Na\({}_{2}\)CO\({}_{3}\) have been introduced. With these methods, Lee et al. [152] applied control of the NaOH/precursor ratio, which has been of great importance in ensuring the percentage of crystallinity of the zeolitic product at high values, as well as the size of the particles formed, which decreases with the increase in alkali in the medium.