➢ *Granulated Blast Furnace Slag (GBFS)*

Blast furnace slag (BFS) is a by-product of iron making and is often referred to as slag. It can be obtained at approximately 1500 ◦C [18]. Following rapid cooling and grinding processes, a granular substance known as granulated blast furnace slag (GBFS) can be formed. This raw material (GBFS) contains SiO2, Al2O3, CaO, and MgO. GBFS is one of the common geopolymer raw materials, given its strong reactivity towards geopolymer synthesis. Additionally, it is possible to achieve an optimal reaction rate using GBFS at temperatures as low as 0 ◦C [19]. GBFS is known as a raw material which can reduce porosity. GBFS in geopolymer can improve reactivity of the mix, enhance mineral structure, rise long-term strength and increase resistance to sulfate. It also reduces the water demand, permeability, and hydration heat of a geopolymer.

➢ *Red Mud*

Red Mud (RD) is a by-product of processing bauxite into alumina using the Bayer process. The Bayer process dissolves the soluble part of bauxite using sodium hydroxide under high-temperature and high-pressure conditions. However, a small amount of sodium hydroxide remains in the RD after the Bayer process and allows it to have a high pH value (over 12) [20]. RD contains generally solid and metallic oxides, especially Fe2O3, Al2O3, SiO2, Cao, Na2O, and TiO2. The red color of RD comes from iron oxides, which represent more than 60% of red mud mass [21].

➢ *Rice Husk*

Fillers known as rice husk ash (RHA) are obtained by calcining rice husks, which are often considered a form of agricultural waste and a potential environmental hazard. During calcination, most rice husks components disappear, and generally an amorphous silicate remains. Research has shown that every 100 kg of husks burnt in a boiler yields approximately 25 kg of RHA [22]. The temperature of calcination must not exceed 600 ◦C to obtain reactive pozzolanic ashes and to avoid the risk of forming less reactive crystalline silicas. Well-burnt RHA contains 90% amorphous silica, 5% carbon and 2% K2O [22]. However, the chemical composition of RHA depends on combustion conditions. RHA is a small, fine material; its particle size ranges from 3 to 75 μm [22]. RHA is greyish-black in color because of unburned carbon.

Some other raw materials, such as steel slag (STS), silica fume (SF), volcanic ash (VA), waste glass (WG), coal gangue (CG), high magnesium nickel slag (HMNS), etc., can be used in synthetic geopolymers because of their abundant silicate and aluminum elements and amorphous structures [16].

A variety of structures based on geopolymers can be manufactured using different precursors. For example, previous research has shown that the mechanical strength and durability of geopolymer materials based on FA can be improved by the addition of metakaolin or other wastes [15]. Geopolymer-based elements can exhibit superior properties and greater durability when compared to structures formed using interconnected Portland cement. Additionally, the microstructure of geopolymer materials is influenced by the composition of the amorphous phase, the content of oxides, and especially, the particle fineness [23].

#### *2.2. Activator Solutions*

In the geopolymerization process, the activating solution plays a fundamental role. Depending on its quantity and concentration, it will provide the necessary mixture that not only initiates the reaction, but also determines the ultimate structure of the solidified material. For geopolymerization to occur, the presence of strongly alkaline activators is essential in the solution. Indeed, they accelerate the dissolution of the aluminosilicate source, favoring the formation of stable hydrates with low solubility and the formation of a compact structure with these hydrates. The physical and chemical properties of the activating solutions play an important role in the behavior of the activated material.
