**2.3 Chemical and mineralogical composition of FA**

FA has varieties of chemical compositions; the averages of the main elements of FA in some European countries (France, UK, Germany), USA, and far Asian countries (Japan, China, India) are given as follows: 53.05%, 27.24, and 5.50% for SiO2, Al2O3, and Fe2O3, respectively [7]. However, another study [33] reported that FA consisted of a heterogeneous mixture of complex aluminosilicate glasses and other crystalline elements. The structure of aluminosilicate glass is an amorphous form, but it could be modified due to the addition of alkaline and metal oxides such as Na2O, K2O, MgO, CaO, and FeO. A study carried out by Das and Yudhbir [34] showed that a strong correlation exists between the glass content and the ratio of potassium to aluminum oxides (K2O/Al2O3). ASTM classification shows that the composition of glass in class F fly ash is different from that in class C. A high polymerized glass network is observed in class F FA, but the glass matrix depolymerizes when the CaO increases in comparison with Al2O3 content [7, 21, 35].
