**1. Introduction**

The Basic Oxygen Furnace (BOF) slag is a left-behind residual waste generated from the steelmaking industry. There are 1.5 million tons of BOF slags produced annually in Taiwan [1]. This high quantity leads to environmental and ecological issues together with health concerns. This waste demonstrates excellent physical and mechanical attributes, such as far-above-the-ground hardness, a higher compressive strength, and a near-to-the-ground abrasion ratio. It is capable of substituting natural aggregates to produce structural materials, road pavements, and more. Unfortunately, the key setback of BOF slags is their expansion behavior because of their gigantic content of free lime, as a consequence, a hindrance in the context of BOF slag recycling and reuse takes place [2–6]. Most of the coarse BOF slags, larger than four mesh, are mixed with asphalt to produce asphalt-concrete

which can be employed as road pavements. However, approximately 0.7 million tons of fine BOF slags, which is less than four mesh, are unable to be utilized e ffectively, especially used in the Portland cement system [7]. For that reason, the way in which a BOF slag can be reutilized as a fine aggregate has become an imperative issue.

Previous studies on BOF slags used as aggregates in concrete and their advantageous properties have been extensively reported [4,5,8–11]. However, the BOF slag contains free-CaO and free-MgO that can result in volumetric instability due to their reaction with water, yielding alkaline earth metal hydroxides. This must be addressed, employing appropriate treatment to prevent expansion. There are several ways to stabilize BOF slags, such as stabilization in the hot or molten stage [12–14], and stabilization by employing a carbonation process [12,15,16]. Singh et al. supplemented oxalic acid into cement mortar to improve the densification and mechanical properties of concrete [17], while Ding et al. utilized scrubbing attrition and chelating reagen<sup>t</sup> treatment to remove free lime from BOF slags [5].

On the other hand, Lin et al. immersed BOF slags into the water to provide su fficient stabilization reactions [18]. According to previous research results, the adding up of silica sand in the molten stage can complete the solution and ge<sup>t</sup> rid of free lime crisis. This brings into question whether or not analogous reactions can occur at room temperature. If possible, they will entirely resolve the quandary of BOF slag expansion and achieve the goals of waste recycling following the principles of the Circular Economy.

Nowadays, innovative geopolymer technology is drawing the attention of researchers on account of not only its nine-times lower carbon footprint and six-times lower energy consumption as compared to the OPC (Ordinary Portland Cement) system [19], but also due to the demonstration of excellent properties by geopolymer composites, such as high initial strength, brilliant resistance to chemicals and freeze-thaw as well as thermal and fire calamities, sustainability, and strength and durability. This means simply that novel geopolymer technology can lend a hand in solving the grea<sup>t</sup> dilemma of global warming and the saving of natural resources of rocks and minerals. On top of that, it is cost-e ffective, since it can incorporate profusely accessible various wastes in the manufacture of geopolymer composites, which otherwise create problems of landfilling and pollution of the environment, soil, surface and subsurface waters besides health hazards [20,21]. Accordingly, it also extends to an organized solution of the disposal of these wastes. Inorganic geopolymers are analogous to natural zeolite minerals but possess an amorphous microstructure, they are classed as three-dimensionally networked materials synthesized through the reaction of rich aluminosilicate materials, such as precursors and alkaline solutions, as activators [22–24]. During the geopolymerization reaction, Si gel and Al gel were produced on the solid particle surface, and thus, they formed the Si-O-Al framework. SiO4 and AlO4 tetrahedra are linked to each other by sharing all O2 atoms [25,26]. Their unique internal structure is the core reason for why geopolymer has superior chemical and physical properties such as high compressive strength, high durability, favorable structural integrity, and low permeability. These exothermic and complex geopolymer processes contain large amounts of free silicon. This free silicon can react with oxides of free lime and free magnesium in the BOF slag, and hence, the formation of stable compounds takes place, thereby inhibiting the expansion of the BOF slag.

In this original research, BOF slags, granulated blast furnace slag (GGBS) powder, coal fly ash (FA) and alkali activator solutions were employed as raw materials to manufacture BOF slag-based geopolymer mortar. The e ffect on compressive strength and expansion behavior from the GGBS/FA ratio and the SiO2/Na2O ratio in geopolymer mortar are presented in this investigation. For testing the expansion behavior of BOF slag-based geopolymer mortar, the application of the autoclave testing method as the accelerated test was made [27]. Lab-scale and ready-mixed plant pilot-scale recycling processes are also investigated in this study.

#### **2. Assumption of the Mechanism for Stabilization BOF Slags in Geopolymer System**

In general, the expansion phenomenon of the BOF slag is mainly due to the f-CaO, and f-MgO content presented. When the BOF slag makes contact with water, f-CaO will undergo a hydration reaction, causing volume expansion. The f-MgO hydration reaction is relatively slow, and about 30 days later, it will also cause volume expansion. After the BOF slag absorbs water, f-CaO will become calcium hydroxide "Ca(OH)2", the volume will expand by 127% [28], and f-MgO will become magnesium hydroxide "Mg(OH)2". The volume expansion is about 118% [29]. Therefore, this study conceived that utilizing free silicon to react f-CaO into a stable calcium silicate compound would prevent the expansion. The relevant reaction Equations (1) and (2) are as follows:

$$\text{f-CaO} + \text{Si} \rightarrow \text{CaSiO}\_3 \tag{1}$$

$$\text{f-MgO} + \text{Si} \rightarrow \text{MgSiO}\_3 \tag{2}$$

After the geopolymer reaction is completed, the excessively free silicon will remain in the matrix of BOF slag-based geopolymer. If other factors caused the geopolymer matrix and/or BOF slag aggregates to be broken, the f-CaO may be released again. At this time, if water infiltrates from the crack, the free silicon remaining in the geopolymer product will dissolve in the water, and then react with f-CaO to form a stable calcium silicate to prevent expansion. The reaction schematic diagram is shown in Figure 1.

**Figure 1.** Schematic diagram of f-CaO reaction with free silicon in geopolymer matrix.
