Cement production contributes nearly 5–8% of global CO₂ emissions [1]. For this reason, issues such as climate change, due to global warming, are considered severe areas of concern. In order to significantly scale down carbon emissions, efforts have been made to develop and implement greener alternatives to cement. The greener (low-carbon) alternatives could not only curb carbon emissions, but also provide an effective waste management strategy to achieve the sustainable development goals of the 21st century. Geopolymer (cement-less) binders are the most promising green building material. Figure 1 demonstrates the use of industrial wastes for the production of geopolymer composites which, from an environmental perspective, leads to several benefits.

Since the inception of the term ‘geopolymers’ by J. Davidovits in 1978, research and development in the field of geopolymers has steadily increased and is now considered a highly dynamic area of scientific investigation from both the commercial and environmental viewpoint. The formation of geopolymers includes a reaction between aluminosilicate raw materials (industrial wastes) and an aqueous alkaline composition (usually a mixture of alkali hydroxides and silicates). The result is the inorganic polymer matrix, consisting of a three-dimensional framework of covalently bonded Si-O-Al-O (polysialate), with outstanding strength and microstructural and durable properties compared to cement-based materials [2,3,4,5,6]. However, as proposed in various studies [7,8,9], the reaction mechanism is a multiphase and complex process and still requires extensive investigation.

On the other hand, the rise in manufacturing industries calls for effective industrial waste management strategies. Therefore, developing industrial waste-derived geopolymer binders could potentially be the solution, promoting the concept of a circular economy—waste to wealth [7,8]. The various industrial wastes contain a significant amount of aluminium (Al) and silicon (Si), which are essential for the formation of a strong and durable geopolymer matrix [9,10] and serve as source material in the preparation of geopolymer binders. In some studies [11,12], the presence of magnesium oxide (Mg) and iron (Fe), alongside Al and Si, in some source materials have also been reported to be beneficial for the strength development of resulting geopolymer binders. For this reason, the utilization of such wastes for the production of eco-friendly mortars and concrete could be a step forward in reducing our dependence on cement [13].

Over the last decade, numerous researchers have determined FA as an exceptional geopolymer binder due to its physical and chemical characteristics, aiding in geopolymerization reactions. A high amount of alumino silicate content in fly ash facilitates increased reactivity, with a higher degree of geopolymerization in an alkaline environment, while supporting the reduction of hydration heat and thermal cracking. The development of aluminosilicate gel and the early strength gain of FA-based geopolymers has been demonstrated upon premature high-temperature curing while increasing the rate of geopolymerization due to the requirement of higher activation energy of fly ash. Due to their amorphous nature, FA-based geopolymers possess excellent durability against acid and alkali attacks compared with other geopolymer binders.

The rise in the manufacturing industries to attend to the needs of 21st-century civilization has led to a significant generation of industrial waste. With the ultimate aim of achieving a circular economy and sustainable development, these wastes could be utilized efficiently for making geopolymer binders. It is imperative to study the chemical properties of each of these wastes before their implementation as source materials. The chemical compositions play a significant role in the formation of the geopolymer network (Si-O-Al-O), geopolymer gels (C-A-S-H/N-A-S-H) [16,17,18], that in turn determine the strength and other characteristics of the final product. Hence, understanding geopolymer binders requires the utmost attention to the chemistry involved. The SiO₂ and Al₂O₃ content in the source materials are responsible for the formation of the principal geopolymer network (-Si-O-Al-), while the CaO, Fe₂O₃, MgO contents provide additional mineral compounds, such as calcium aluminosilicate hydrate (C-A-S-H), magnesium aluminosilicate hydrate (M-A-S-H) and ferrosiliate (-Fe-O-Si-O-) type gels, respectively. These additional mineral phases improve the binder matrix, leading to a denser binder structure. Therefore, in this section, the authors’ primary focus is on the chemical properties, and in particular, SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO of FA, GGBFS, RM, IOT, FCA, MK and SF, before moving toward the strength and microstructure properties.

According to Davidovits, geopolymerization involves the formation of the Si-O-Al-O network [19]. Therefore, the presence of a high amount of Si and/or Al makes industrial waste a potential candidate for geopolymerization. However, apart from oxides of Si and Al, the oxides of Ca, Fe, and Mg are also responsible for geopolymer compound formation. These minerals co-exist alongside the primary polymer chain of Si-O-Al-O-, which further influences the strength of the overall binder matrix.

The chemical compositions (esp. SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO %) of the FA, GGBFS, IOT, RM, FCA, MK, and SF are graphically presented in Figure 2. Table 1 shows that the chemical compositions of each of these wastes vary across the source of their origin (region) and method of processing.

The reaction process leading to the formation of a geopolymer binder structure is complicated and is controlled by various parameters [52]. The most significant factor remains the reactivity of SiO₂, Al₂O₃, CaO, MgO, and Fe₂O₃ present in the source materials used, which are discussed herein.

The initial step, in the case of geopolymerization, is the dissolution of SiO₂ and Al₂O₃ from the source materials under an alkaline environment, which leads to the development of Si-OH and Al-OH bonds [53]. Subsequently, due to subsequent condensation and polymerization reactions, a 3D network structure is produced that involves the tetrahedral networks of [SiO₄]⁻ and [AlO₄], linked via shared oxygen atoms [54]. However, these tetrahedral connection types of [SiO₄] and [AlO₄] are often very distinct and intricate; therefore, it can be implied that the properties of the final geopolymer product vary with different connection types [19]. For this reason, it is crucial to select source materials with an optimum composition of SiO₂ and Al₂O₃ in order to achieve enhanced geopolymer properties [20].

The CaO in the source materials does not participate in the core geopolymer network formation, i.e., Si-O-Al-. However, it aids in the formation of hydration products such as C-S-H (a space-filler) and a geopolymer gel, i.e., C-A-S-H, inside the geopolymer binder structure [55]; however, this geopolymer gel also comprises of Al and Si in a tetrahedral connection type, as mentioned earlier. It is also acknowledged that the final orientation and properties of geopolymer structures vary depending on the CaO content in the source materials [56]. Several researchers have favored the presence of CaO in the source materials to achieve improved properties [57,58,59,60]. Furthermore, the hardening time of geopolymers is also significantly affected by the presence of low/high CaO content in the source material [61]; hence, the role of CaO is considered crucial during the selection of source material for geopolymerization.

The presence of iron oxide (from the iron-rich source materials) is indicated by the replacement of Al³⁺ by Fe³⁺ (ferric ions) in the geopolymer binder structure (Si-O-Al), yielding a Ferro-sialate network (-Fe-O-Si-O-Al-O-) [62,63]. In contrast, some studies in the past have also found that iron, in its Fe²⁺ (ferrous) state, remains inert and does not participate in the geopolymerization process [64]. However, Lemougna et al. [65] noticed that some of the ferrous ions are present in the tetrahedral connections of the Si-O-Al network. This observation is corroborated by Kaze et al. [66], who reports that crystalline phases of ferrous ions exist, such as fayalite and siderite, in the final geopolymer binder system. Hence, it is confirmed that the presence of iron in its ferric or ferrous state affects the performance of the geopolymer binder structure and its formation.

The role of MgO in the source materials has been studied in [11,67,68]. It is demonstrated that the existence of MgO did not aid in developing or reorganizing the geopolymer chain (Si-O-Al). However, its presence led to the formation of hydrotalcite. This magnesium and aluminium hydroxycarbonate mineral form act as a filler agent while aiding in accelerating the generation of hydration products such as C-S-H. This reveals that, although MgO does not participate in the core development of geopolymer tetrahedral network formation, it necessitates forming other chemical compounds that directly affect the final geopolymer binder structure.