## 3 Wastes in Zeolite Synthesis
### Coal Fly Ash  
This material is often used for zeolite synthesis and comes from the burning of pulverized coal, having the physical aspect of powder. The process of its generation involves the combustion of pulverized coal, generating a dry ash. A great amount of ash leaves the furnace as fly ash, entraining in the gas flow. This process occurs in a dry-bottom boiler [211]. Other types of precursor material are generated using the wet-bottom or slag tap furnace, with which 50% of the ashes remain in the furnace and the other 50% is left with gas flow [212]. The cyclone furnace uses between 70% and 80% as boiler slag, while 20-30% is left in gas flow as dry ash. There are high numbers for coal fly ash generation just in the USA, according to the Federal Highway Administration. In 1996, the electrical utility industry generated approximately 53.5 million metric tons of coal fly ash. According to this report, from 1977 to 1996, this generation was constant, varying from 42.9 to 49.7 million metric tons [213]. In a survey done in 2019 about the worldwide production of coal combustion products published on the Coltrans Conferences website ([[https://www.coaltrans.com](https://www.coaltrans.com)]([https://www.coaltrans.com](https://www.coaltrans.com)) (accessed on 23 October 2022)), in 2010, this number was approximately 780 million metric tons, and in 2017 reached 1.1 billion metric tons. This includes the use of ashes in concrete applications [214], road construction [215], and filling applications [216], besides soil amendment and zeolite synthesis [217]. Adequate disposal and use of the waste is important to reduce the pollution of water and soil, disruption of ecological cycles, and environmental hazards. Generally, its disposal is into ash ponds, landfills and lagoons [218]. The chemical composition of coal fly ash is complex, with almost 316 individual mineral and 188 mineral groups identified [219]. In addition to metallic oxides such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), CaO, MgO and K\({}_{2}\)O, the unburnt carbon is also present and may be determined by loss on ignition [220]. Some trace elements may be present, such as Cr, Pb, Ni, Ba, Sr, V and Zr [221]. Some important physical properties are the particles, with an average size of < 20 \(\upmu\)m and low bulk density ranging from 0.54 to 0.86 g/cm\({}^{3}\), surface area from 300 to 500 m\({}^{2}\)/kg, and light texture [199] (p. 25). Its acidity is weak; therefore, a majority of ashes are alkalis, where the molar ratio Ca/S determines pH in a water-ash system [222]. Thus, it is possible to classify the materials as strongly alkaline (pH 11-13), mildly alkaline (pH 8-9) and acid ash [223]. The morphology is spherical, consisting in nanospheres, solid spheres, irregularly shaped debris and porous unburnt carbon [218] (p. 26). The mineral composition of coal fly ash includes quartz and mullite, as well as amorphous glass phases, hematite, and magnetite [224]. The large amount of amorphous phase is dissolved in alkaline media, promoting geopolymerization reactions. This may be better as the waste is treated with NaOH at high temperatures [225]. The transformation in geopolymers is favorable to change in the structure of a zeolitic framework under adequate conditions, giving more organization in structure, and useful for a remediation process for soil pollution called \"solidification/stabilization\" (S/S) [226]. \"Fly ash zeolites\" (FAZ) have been used to compose in situ chemically reactive barriers or in situ treatment zones in water pollution remediation. This technology is based on permeable zones placed within the aquifer reacting with contaminants in the flow of water [227]. The main phenomena involved are sorption and precipitation for retention of contaminants [228]. In the process of fly ash zeolitization, the partial transformation of the precursor in zeolite P and the use of hydroxysodalite of an alkali medium are reported. During this process, slow dissolution of quartz and stabilization of mullite occurs.