➢ *Metakaolin*

Metakaolin (MK) is a type of clay mineral known as kaolinite that has been dehydroxylated. Kaolinite is a soft, fine, white clay that exhibits excellent plasticity and fire resistance. When kaolinite is subjected to thermal treatment, typically at a temperature range of 600–800 ◦C, the original structure of the clay is destroyed, leading to the formation of metakaolin. This new substance is an anhydrous aluminum silicate [16]. Metakaolin is a commonly used aluminosilicate source for geopolymerization due to its purity and predictable properties. This material provides a consistent chemical composition, which makes it a preferred starting material for geopolymer research [15]. Despite its benefits, metakaolin-based geopolymers have some drawbacks, such as the need for high amounts of water and sometimes low mechanical strength, which limit their application areas [15]. As a result, the geopolymerization of various Al–Si minerals and clays, particularly kaolinite and metakaolin, has been extensively studied over the past few decades.

#### 2.1.2. Secondary Raw Materials

Within the framework of reducing environmental impact via alternative waste management, the production of geopolymers can be envisaged using industrial and agricultural wastes as raw materials, as follows.

➢ *Fly Ash*

Coal-fired power plants generate fly ash (FA) as a byproduct, which is considered an industrial waste (Figure 1). Its main components are SiO2 and Al2O3, but it can contain small components such as CaO, Fe2O3, MgO, etc. However, the composition of these components in this waste material is significantly variable, especially depending on the coal source and burning conditions. FA is generally divided into classes F (FFA) and C (CFF). FFA is a low calcium fly ash and is a by-product of burning bituminous coal; FFC is a high calcium fly ash and is produced by burning lignite and sub-bituminous coal [17]. Geopolymer synthesis can benefit from the unique characteristics of FA, such as its alumina-silicate composition, ability to function with minimal water, high malleability, and widespread availability. Notably, in the United States alone, an estimated 63 million tons of FA are produced annually, and Hungary generates approximately 200 million m3 of fly ash and slag each year [15].

**Figure 1.** The transformation process of fly ash into a binder for geopolymer materials [10,11].
