## 1 Introduction  
Geopolymers have been extensively studied as construction materials. It is a technology that was developed to find a more environmentally friendly option than traditional Portland cement [1]. Generally speaking, a geopolymer is an inorganic synthetic polymer that is generated by aluminosilicate materials and alkaline agents. After curing, a semi-crystalline amorphous material similar to concrete is produced [2]. The curing reaction can occur by adding external heat, i.e., curing in an oven and ambient temperature, depending on the composition of the geopolymer [3]. It should be noted that the curing conditions have a significant effect on the strength development of geopolymers [3]. A wide variety of aluminosilicate reagents can produce geopolymers [2]. The most common sources of aluminosilicates used for geopolymer production are metakaolin and by-products from other industries, such as fly ash, mine tailings, red mud, and slag [4, 5, 6, 7]Within these raw materials, the use of by-products from other industries for the production of geopolymer concrete was recently encouraged since this significantly reduces the amount of CO2 emitted for the manufacture of these materials [8]. Geopolymer concrete (GPC) is estimated to reduce the carbon footprint of construction projects by 80% compared with ordinary Portland cement (OPC) [9]. Geopolymer precursor materials, both in natural and by-product forms, should be rich in alumina (Al2O3) and silica (SiO2) contents, preferably in the reactive amorphous form [5], for good dissolution of these compounds upon contact with the alkaline agent. The function of silica and aluminum is to impart strength and set the cement [10]. A concern related to aluminosilicate dissolution is the rate at which it occurs and the amount of total amorphous aluminosilicate material available for geopolymerization [11], as the curing time and the compressive strength subsequently obtained will depend on this [3]. In addition to the aluminosilicate reagent, an alkaline activator is needed to produce a geopolymer. The alkaline activator causes the dissolution of the raw materials [12]. The type and concentration of the alkaline activator should be carefully selected because its composition affects the dissolution of the aluminosilicate source and has different impacts on the properties of fresh geopolymer pastes and the development of compressive strength in hardened geopolymers [13]. The most common activators are alkaline hydroxides and silicate solutions, and within these groups, sodium hydroxide (NaOH) and sodium silicate (Na2 SiO3) are the most commonly used, respectively. The ideal concentration of the alkaline activator increases the strength of the geopolymer [3]. In addition, an increase in the concentration of the alkaline activator leads to a rise in the pH of the activating solution. Different authors recommend working with pH values above 13 for the correct dissolution of the aluminosilicates present in the raw material [9; 14; 15]. Geopolymers' strength development depends on the raw materials and the alkali-activating solutions [3]. When activating the aluminosilicate source with NaOH, the reaction starts with the dissolution of Al and Si, which are precursor particles in the alkali solution, where reactive aluminate and silicate monomers are released [2]. Then, these monomers interact to form aluminosilicate oligomers, and the latter polymerize in the alkaline environment to form geopolymer gels. At the onset of geopolymerization, an aluminum-rich gel phase is generated, transforming into a final silicon-rich geopolymer gel [2; 16]. The result based on a geopolymerization reaction is a semi-crystalline amorphous substance composed of solid phases of aluminosilicates assembled on the basis of SiO4\({}^{4-}\) and AlO4\({}^{5-}\) linkages as tetrahedra forming a 3D structure. The main hydration product of low-calcium or calcium-free geopolymers is an N-A-S-H gel, which possesses a 3D structure [17].