Development of a sustainable geopolymeric grout via mechanochemical activation with recycled brick powder

In this study, WBP and GGBS were employed as sources of aluminosilicate materials for producing geopolymer grouts, both mechanochemical and conventional (Figure 1). Grinding WB collected from different demolition sites around Baghdad and made in different factories in northern Iraq was used to produce WBP. The GGBS was retrieved from an Iraqi steel plant located in Basra. Using a Mastersizer 2000, the laser diffraction technique was used to determine the materials’ particle size distribution (Figure 2). The physical and chemical characteristics of aluminosilicate materials’ sources are displayed in Table 1. The WBP and GGBS had median particle sizes (d50) of 42.25 and 21.6 µm, respectively. The WBP and GGBS have specific surface areas of 0.72 and 1.46 m²/g, respectively. The combination of silicon, aluminum, and iron oxides in the WBP, as indicated in Table 1, is greater than 70%, indicating that these materials meet the ASTM C618 criteria for pozzolanic materials. The energy-dispersive spectroscopy (EDS) micrographs correspond to the peaks of aluminum, silicon, and oxygen (Figure 3). This observation aligns with the data of X-ray fluorescence (XRF) spectroscopy shown in Table 1 and the reported EDS findings of Mahmoodi et al. [36].

Figure 1

Waste sources of aluminosilicates. (a) WBP and (b) GGBS.

Figure 2

Particle size distribution of grout materials.

Figure 3

EDS analysis of WBP powders.

However, the poor reactivity and minimal strength gain of WBP when cured at room temperature limits its wider usage in geopolymer synthesis [21]. Since GGBS has considerable mechanical strength and strong endurance in corrosive settings, it is added to the WBP-based geopolymer grout to address the low reactivity of the WBP employed in this work [37]. However, the GGBS binder has several undesirable characteristics, including high viscosity, quick setting, poor workability, and significant shrinkage [38,39,40,41,42]. According to past research, when the GGBS and WBP are combined in synthesis geopolymer mixes, they exhibit superior compressive strength, less shrinkage, and increased reactivity compared to their respective sole utilization [43,44].

This investigation selected sodium silicate and NaOH as the alkaline activators. A sodium hydroxide solution with a molarity of 5 M was prepared one day prior to mixing. This solution was synthesized by dissolving locally sourced NaOH beads (97–98% purity) in faucet water. Two kinds of sodium silicate (Na₂SiO₃) were utilized: a liquid solution for the conventional geopolymer and a powder form (sodium metasilicate-Penta) for the mechanochemical geopolymer.

To manufacture the alkaline activator, a ratio of Na₂SiO₃/NaOH = 1 was chosen based on past publications that used the mechanochemical activation technique [45]. The chemical and physical characteristics of sodium silicate in both liquid and powder forms, as well as the precursor components (WBP and GGBS), are shown in Table 1.

Mechanochemical activation was used in this study to create geopolymer grouts in contrast to traditional activation. According to Hamid Abed et al. [46], the raw material powders (NaOH, sodium metasilicate, WBP, and/or GGBS) had to be ground for 2 h in order to prepare the MG precursor (Figure 4). A ball mill with 10 kg capacity was used for the grinding process utilizing a 3:1 ratio (3 kg ball/1 kg solid material), with different ball sizes (15–40 mm) to get a high-performance grinding process according to Raghuraman et al. [47].

Figure 4

Process of producing a geopolymer binder through mechanochemical activation.

Due to the constant impact of the grinding and particles, the mixed raw materials’ components (particles) become trapped between the balls and the container wall during the ball milling (grinding) process. The MG precursor resulting from the milling was added with tap water to create an MG grout, as prescribed by Abbas et al., and Hamid Abed et al. [13,14,46].

The conventionally activated geopolymer was substituted in formulating the conventionally activated geopolymer (CG)-grout with the MG technique to enable comparisons. The distinct impacts of the two activation techniques on the geopolymer grout preparations can be determined via this substitution. NaOH beads were dissolved in tap water at a molarity of 5 for standard geopolymer preparations, and their weight was subsequently recorded. According to Ilcan et al. [48], 5 M NaOH was identified as the optimal choice, balancing early strength development, workability, cost-effectiveness, and minimizing potential environmental impacts associated with high alkalinity, which may lead to increased cost and potential efflorescence without significant performance enhancement. It is important to note that the mixing procedure produced an exothermic reaction, which caused the NaOH solution to heat up considerably. Before being used again, the liquid was left to cool at room temperature of 23 ± 3°C for 24 h and stored until chemical equilibrium was reached. The cooled NaOH liquid was then mixed with sodium silicate (Na₂SiO₃) in accordance with the standard geopolymer activation protocol. This was cautiously done to minimize any danger related to the exothermic reaction and to guarantee secure handling of the extremely alkaline NaOH solution.

This study utilized two mixture groups (MG and CG). In each mixture group, the WBP was replaced by GGBS in ratios of 0, 15, 30, and 45%. For instance, the CG-0S-100B code indicates that the adopted grout was a CG with 0% GGBS and 100% WBP. The MG-45S-55 WBP code indicated a mechanochemically activated geopolymer grout with 45% GGBS and 55% WBP. The MG and CG mixture ratios are shown in Table 2. The 15–45% range for WBP replacement was chosen to balance the performance and sustainability goals. This range allows for significant waste utilization while maintaining adequate mechanical properties in the geopolymer grout, aligning with findings from previous research on geopolymers with waste materials [49].

All the prepared specimens were created in the lab at 23 ± 3°C (room temperature). The mini-slump flow test was used to investigate the geopolymer grout’s fresh properties in compliance with Kantro and Güllü and Ali Agha [50,51]. Using a small cone with dimensions of 19 mm for the top diameter, 38.1 mm for the lower diameter, and 57.2 mm for height, the slump flow test was used to determine the grout’s spread diameter (measured in millimeters). The average of two measurements in two perpendicular directions resulted in the final grout spread diameter. The Vicat needle penetration tests were carried out to ascertain the grout mixtures’ setting timeframes in accordance with ASTM C191-19 [52]. Only the final setting time results were taken into consideration for discussion in this study.

Regarding mechanical characteristics, the grout mixes were poured into cylindrical molds of 50 mm in diameter and 100 mm in height. The samples were allowed to cure for 28 and 90 days at 23 ± 3°C. Then, standards were used to measure the grout samples’ UCS [53,54]. Mechanical testing was conducted on three specimens per mixture, and the average strength was determined. It is important to mention that before conducting the UCS testing, the samples were used to measure the bulk density of the hardened geopolymer grout. A Thermos Scientific Axia device model was utilized to conduct SEM examination on raw GGBS and WBP powders and on co-ground GGBS and WBP with solid chemicals. This was carried out to look into the microstructure alterations brought on by the mechanochemical activation process. Furthermore, SEM analysis was carried out on the hardened geopolymer grouts (MG and CG) in order to evaluate the impact of GGBS/WBP substitution and the activation procedure. The specimens tested under UCS were used to obtain samples for SEM evaluation. These samples were taken from fractured particles and were then coated with gold to ensure accurate SEM imaging. The diameter and height of the samples were roughly 20 and 10 mm, respectively. Additionally, the chemical composition of the WBP powder was ascertained using an energy dispersive spectrometer. Furthermore, XRD was used to qualitatively evaluate each geopolymer precursor’s crystalline structure pre- and post-mechanochemical activation on top of examining the impact of the GGBS/WBP substitution. The potential for heavy metal leaching from grout at geotechnical application in contact with subsurface water was evaluated using the toxicity characteristic leaching procedure (TCLP) outlined in the EPA Method 1311 [55]. This assessment focused on WBP, GGBS, and a high-strength geopolymer grout specimen. Following size reduction to <9.5 mm, a 10 g sample was immersed in 200 mL of acetic acid solution (pH 2.9), maintaining a 20:1 liquid-to-solid ratio. The mixture was agitated for 18 h at 20°C and 30 rpm, followed by filtration through a 0.7 μm glass fiber filter. Leached heavy metal concentrations in the filtrate were subsequently quantified via inductively coupled plasma-atomic emission spectrometry (ICP-AES).

The microstructure of the raw and mechanochemically ground GGBS and WBP precursors was examined using SEM. It is evident from Figure 5(a) that the raw WBP had an angular porous surface and an irregular shape. The GGBS components also show sub-rounded to angular forms, and the material particles were heterogeneous and different, as shown in Figure 5(b). The edges and roughness were observable in angular particles and bulk (Figure 5) [56].

Figure 5

SEM images of (a) raw WBP and (b) raw GGBS.

The solid chemical powder (a mixture of sodium silicate and NaOH) was used to coat the WBP particles after they had been ground using sodium metasilicate and NaOH. This resulted in a decrease in the WBP’s solid chemicals and average size. As a result, the geopolymeric precursor was formed. Furthermore, the combination of NaOH and sodium metasilicate led to the first binding between the WBP particles, as shown in Figure 6(a). Furthermore, Gupta et al. [18] observed that after 8 h of ball-milling fly ash and NaOH, cracks and flaws developed, increasing the surface area and enhancing the reaction. As shown in Figure 6(d), it is clear that the size of the GGBS particles reduced typically when they were added to the WBP-based MG precursor system, although they still have angular and deformed morphologies. Additionally, the 2-h mechanochemical procedure enhanced the particles’ surface area and accelerated the geopolymer precursors’ reaction rate [14].

Figure 6

SEM images of (a) MG-0S-100B, (b) MG-15S-85B, (c) MG-30S-70B, and (d) MG-45S-55B.

Figure 7 presents the XRD patterns of raw WBP and a WBP-based MG precursor. The analysis shows that the raw WBP has a significant vitreous phase itemized from the halo around 2θ = 18°−20° and significant crystalline quartz phases based on the sharp peaks recorded between 2θ = 20° and 2θ = 70°. Similarly, Hwang et al. [57] demonstrated the mineralogical composition of the WBP using XRD, revealing that quartz is the predominant crystalline phase in the WBP. The diffractogram of the WBP-based MG denotes a significant amorphous phase on top of a decrease in the peak intensity corresponding to crystalline phases. Additionally, phases such as sodium aluminosilicate were observed. When comparing the WBP-MG precursor to raw WBP, the comparative study of the XRD patterns in Figure 7 shows the emergence of new peaks like sodium aluminosilicate and the elimination of other crystalline phase peaks. The loss of crystalline phases confirms the amorphization upon grinding, and the synthesis of new material is confirmed by the presence of new phases (i.e., the WBP-MG precursor). A comparable observation was made by Hamid Abed et al. [58] and Hosseini et al. [17] who recorded a more diminished peak intensity of the geopolymer precursor activated mechanochemically. This attenuated peak intensity indicates an additional breakdown of the crystalline structures of the quartz and alumina phases, as well as modifications in the crystalline structure subsequent to grinding.

Figure 7

XRD patterns of raw WBP and WBP-based MG precursor.

Figure 8 depicts the effect of GGBS substitution on the mineralogical compositions of the WBP-based MG precursor. The quartz peaks’ intensity lessened substantially as the content of the GGBS spiked in the WBP-based MG precursor. The samples with 100% WBP (MG-0S100B) showed the highest peaks of the WBP-MG precursor samples. The MG-0S100B mixture was analyzed to determine whether any crystalline species in WBP participate in the mechanochemically activated geopolymerization process. The higher-intensity quartz peaks in the MG-0S-100B precursor verify a lower degree of geopolymerization than in the WBP-MG precursor including GGBS. The varied crystalline quartz intensity peaks indicate varying extents of geopolymerization in the precursor. The faster rate of the mechanochemically activated geopolymerization reaction was caused by the higher GGBS content, while the MG-45S-55B mixes displayed the lowest quartz peak intensity. The higher rate reaction will remarkably enhance the compressive strength [45]. It is clear that new semicrystalline mineral phases were added to the MG precursor by the reaction of the WBP-MG precursor with GGBS.

Figure 8

Effect of GGBS substitution on the XRD of the WBP-based MG precursor.

The UCS of the geopolymer mixtures with various WBP/GGBS ratios are presented in Figure 11. The UCS tests were performed at two different geopolymer paste ages namely, 28 and 90 days. It was observed that the UCS values of all the mixtures exhibited improvement as the age progressed from 28 to 90 days. This enhancement can be attributed to the completion of the geopolymerization process and densification of the microstructure occurring at longer ages [74,75,76]. The lowest 28-day UCS (3–2 MPa) was achieved for the mixture where only WBP powder was used as the aluminosilicate precursor. As reported in Fořt et al. [77], since pure WBP was unable to set and harden, GBFS had to be added to get a more effective mixture. It was discovered that adding more CaO to the GBFS/WBP mixture resulted in a denser structure of the C–A–S–H gel, improving its strength [57]. Figure 11 demonstrates that substituting the WBP powder with GGBS resulted in a linear enhancement in strength and an increase in the GGBS level. The UCS values of the geopolymer mix demonstrated increases of 90, 174, and 227% when the WBP was substituted with 15, 30, and 45% GGBS, respectively, at 90 days. The noteworthy influence of GGBS on the UCS may be attributed to its superior cementing properties and increased reactivity in contrast to WBP. An important factor in boosting the activation process and early age strength is the high CaO content [78]. Moreover, the high CaO content may result in C–S–H/CA–S–H gels, changing the geopolymer grout mixture’s microstructure and improving its mechanical qualities even more [79]. Additionally, the geopolymer grout’s microstructure would compress and its porosity would decrease as a result of the production of C–A–S–H gels. Likewise, Hwang et al. [57] evaluated the effect of GGBS content on the strength properties of the alkali-activated BP. Previously, Sharmin et al. and Ahmed et al. [22,61] observed that the strengths increased with increasing GGBS content, which is consistent with the findings of the present investigation.

Figure 11

UCS of MG- and CG-based grout: 28 days and 90 days.

As demonstrated in Figure 11, the strength properties of the geopolymer samples are significantly impacted by the activation process (mechanochemical activation). After 90 days, the UCS of the MG samples revealed a strength enhancement of roughly 7–30% compared to the CG samples’ counterparts (Figure 11). GGBS and WBP exhibited enhanced surface area and reactivity upon mechanochemical activation. More cementitious gel was formed as the primary reaction product resulting from the source materials’ reactivity, and this gel formation filled the pore system. Better immobilization resulted from decreased porosity and reduced total pore volume of the geopolymer grout, which are caused by a greater amount of gel forming in the grout [46]. The material’s resistance to external effects is determined by the movement of ions, water, and pore structure [80]. Additionally, the diffusion process during leaching is dependent upon the distribution of the pore size [81], the matrix’s stability [82], physical properties [83], and microstructural characteristics, such as tortuosity and porosity [84]. Similar MG grout behavior was noted in the studies of Hamid Abed et al. and Hamid et al. [14,85], where it was shown that the mechanochemical activation procedure increased the geopolymer binders’ surface area and reactivity. The end consequence of this was the creation of more gel inside the geopolymer matrix, which decreased the porosity and overall pore volume. As a result, this improved the particles’ binding and increased the MG sample’s strength.

It is important to mention that the CG grout with 45% GGBS (CG-45S-55B) samples exhibited surface cracks compared to the MG grout with the same ratio (Figure 12). More noticeable shrinkage was primarily responsible for the crack development in CG samples with 45% GGBS concentration [46]. According to Fořt et al. [77], compared to GGBS, the BP’s increased crystalline content may result in less drying shrinkage and more flexural strength preservation.

Figure 12

Surface images of MG- and CG-based grouts.

The results of the stress–strain diagram of the CG and MG at different WBP/GGBS ratios are displayed in Figure 13. The stress–strain diagram of geopolymers with different WBP/GGBS ratios clearly displays substantial shape differences; mixtures with a low UCS had wide stress–strain curves, while the high UCS mixtures showed narrow stress–strain curves. It is possible to deduce that the strain properties of the mixtures are related to the way these curves behave. Stated differently, a significant relationship exists between the initial UCS and strain. Samples with higher initial UCS showed less strain than those with lower initial UCS. The stress–strain behavior of the MG grout regardless of the replacement level of WBP is not much different than that of the typical CG grout. In other words, the influence of the activation method of the geopolymer grout on the stress–strain behavior is minor. The replacement of GGBS with WBP resulted in reducing the stress of the geopolymer grout. The geopolymer grouts’ stress decreased with increasing amount of WBP due to the low amount of CaO in BP and due to the important role of calcium dioxide in the activation process and early age strength of the geopolymer grout [78]. The experimental results show the significant impact of the WBP on the geopolymer grouts’ strain behavior, whereby the MG strain increased from 0.015 to 0.024, and the strain capacity of CG increased from 0.016 to 0.024 when the GGBS amount was replaced with 55% of WBP. In other words, the strain capacity of MG increased by 60% when the amount of GGBS was fully replaced with WBP. BP can weaken the chemical binding and strengthen the frictional bond, which increase the strain capacity of the geopolymer grout. This increases the strain observed by including WBP powder in both the MG and CG grouts [61]. According to Ahmed et al. [61] and Wang et al. [86], the inclusion of WBP improves the flexibility of strain-hardening cementitious composites, which increases the final displacement of these materials along with the increase in the replacement of WBP.

Figure 13

Stress–strain responses of mixtures.

For the validation of UCS results, a bulk density test was conducted for both MG and CG grouts, as displayed in Figure 14. It is evident that the density results followed the same trend as observed for UCS, with a minimum of 1,762 kg/m³ for the pure WBP-based geopolymer and a maximum of 1,875 kg/m³ for the geopolymer with 45% GGBS content after 90 days of curing. It is apparent that the density values exhibited a gradual increase as the GGBS content increased. Through the alkali activation reaction, more C–S–H and C–A–S–H gels gradually formed due to the greater CaO content and amorphous character of GGBS [57]. The hardened paste’s porosity is subsequently reduced as a result of the creation of these extra gels, which is necessary for the formation of a denser morphology. This denser morphology ultimately results in increased strength and density for the geopolymer grout samples [46]. The results showed that the MG samples had a 3% higher density than the CG grout samples (Figure 14). The geopolymer grout’s increased surface area and reactivity resulted from the ball-milling of GGBS/WBP and chemical powder (NaOH and sodium metasilicate), which is responsible for the higher density of the MG samples. Because of the improved reactivity of the source materials, more gel was formed as the primary reaction product, which decreased the porosity and increased the MG grout’s density [14].

Figure 14

Bulk density of MG- and CG-based grouts.

The impact of GGBS substitution and activation method on the microstructure of the WBP-based geopolymer grout is shown in Figure 15. The microstructure characteristics of the WBP-based geopolymer grout were considerably impacted by the increased GGBS concentration, as demonstrated by the SEM images. There was a noticeable difference in the blends with 0, 15, 30, and 45% GGBS. The microstructure features of the pure WBP-based geopolymer grout showed a loose and fragmented morphology, as shown in Figure 15(a). Similarly, Fořt et al. [44] noted that the reaction rate of geopolymer mixes with WBP decreased, leading to a less compact microstructure and the presence of unreacted particles. Furthermore, the system contained an amount of partially or completely non-reacting particles. When 15 and 30% of GGBS were added to the WBP, the particles of the MG grout samples became even more compact and homogenous, as shown in SEM images in Figure 15(b) and (c). The development of C–S–H and C–A–S–H on top of the hydrated N–A–S–H gel was aided by the Ca²⁺ from GGBS entering the Si–O–Al–O structure of the WBP-based geopolymer grout system as the amount of GGBS in the system increased. The said finding is consistent with that of Ismail et al. [87].

Figure 15

SEM images of the (a–e) hardened grouts ((a, b, c, d) MG and (e) CG). (a) MG-0S-100B, (b) MG-30S-70B, (c) MG-15S-85B, (d) MG-45S-55B, and (e) CG-45S-55B.

It was observed that the geopolymer samples with 45% GGBS were more structurally compact and homogeneous with produced gels and had fewer particles with no reactive contribution, as shown in Figure 15(d), compared to the counterpart samples with 100% WBP, as shown in Figure 15(a), that were more microstructurally porous and less cohesive. Conversely, the XRD test indicates that GGBS is amorphous, contributing to a higher alkali activation reaction rate and an intensified synthesis of C–S–H and C–A–S–H, both of which boosted strength [57].

The MG samples had a well-dispersed, compact morphology with partially reacted powders surrounding it, as shown in Figure 15(b) and (d). In contrast, the CG samples showed a less dense, fragmented morphology with a greater quantity of unreacted starting powders. Comparing the MG to the CG, Abbas et al. [13] found that the former’s performance is denser and more compact. Compared to the MG samples, which seemed to be well-connected by a gel-like network, unreacted particles are more visible in the CG samples, producing a looser microstructure and less C–S–H gel. More gel was formed within the geopolymer-stabilizer matrix as a result of the mechanochemical activation process, which increased the geopolymer grout’s surface area and reactivity [14,85]. This strengthened the bond between the particles and improved the microstructure characterization of the MG grout sample by lowering the total pore volume and porosity.

XRD was used to analyze the mineralogical compositions of the geopolymer grout samples in order to investigate the effects of the activation process and the GGBS substitution on the solidified geopolymer grout (Figure 16). It was demonstrated that the formation of new phases during the alkali-activation process is what led to the formation of the new semicrystalline phases. During the alkali activation process, some unreacted particles from the initial materials remained in their crystalline forms. Expectedly, quartz (SiO₂) was the major crystalline mineral found in all the geopolymer grout samples. In both the MG and CG grout samples, it remained as an unreacted particle, as evident from 2θ values of 20.85°, 27.3°, 39.9°, and 50.71° (ICDD# 05-0490). This result concurs with the previous research [57].

Figure 16

XRD patterns of CG- and MG-based grout samples.

However, the quartz peaks’ intensity lessened substantially with the increase in GGBS content in the MG samples; hence, 100% WBP (MG-0S-100B) samples demonstrated the highest peaks for the MG grout samples. Meanwhile, the MG-45S-55B mixtures demonstrated the lowest intensity in the quartz peak, because the higher GGBS content results in a greater alkali activation reaction, adding the system with more gel. The gelation process lessens the unreacted WBP and creates a more compact system, thus enhancing the strength significantly [61]. The SEM images of the MG grout samples supported this finding. As reported by Hwang et al. [57], the WBP’s reaction to the GGBS presented the geopolymer matrix with new semicrystalline mineral phases.

In terms of the activation method’s impact, the MG grout samples showed significantly higher crystalline quartz intensities compared to the CG-based grout samples, as depicted in Figure 16. A similar observation was reported by Abed et al. and Hamid Abed et al. [45,63]. According to Hosseini et al. and Gupta et al. [17,18,88,89], the XRD patterns of the set CG and MG grout samples showed the presence of novel products and the crystal phase spikes found in the initial powder. These products, which are primarily made up of synchronized alkaline amorphous aluminosilicate gels, are essentially the outcome of alkali-activation processes. The primary peaks fit into the following categories: (1) calcium silicate hydrate (C–S–H) gel, both found in the CG and MG grouts (Figure 10) at 29.5° and 46.2° (ICDD# 03-0649) and (2) C–A–S–H and N–A–S–H gels coexisting as sodium calcium aluminosilicate hydrate (N–C–A–S–H) at 33.6°, 36.3°, and 41.5° (ICDD# 25-0777) both found in the CG and MG grouts. These findings are consistent with those of Zawrah et al. and Rovnaníková et al. [43,90].

To evaluate the potential environmental impact of solidified geopolymer grout samples (specifically, MG-45S-55B and CG-45S-55B) after a curing period of 28 days, the TCLP was conducted. This analysis, which was performed in accordance with the United States Environmental Protection Agency Method 1311, as detailed in Table 3, focused on assessing the leachability of seven key heavy metals: arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). From an environmental perspective, using recycled materials or solid inert waste in field applications is generally accepted, even with potential exposure to rainfall or stormwater, as long as the materials do not pose a risk to groundwater or surface water. A thorough environmental risk assessment is necessary to ensure the safe application of WBP and GGBS blends, as well as MG-45S-55B and CG-45S-55B geopolymers in various engineering projects. Table 3 presents the leachate analysis results of WBP, GGBS, and the two geopolymers using acetic leachate extraction. This analysis helps determine the potential for harmful substances to leach from these materials into the surrounding environment. The US Environmental Protection Agency (EPA) provides guidelines and thresholds for hazardous waste identification based on leachate analysis [91]. Wartman et al. [92] clarified that according to the EPA a material is considered hazardous waste if any detected metal exceeds 100 times the drinking water standards.

Comparing the leachate test results with EPA limits, it is evident that all metal contaminants in the WBP, GGBS, and geopolymer samples fall within acceptable limits. Moreover, the results indicate that the MG-45S-55B and CG-45S-55B geopolymers effectively reduce the leachability of certain metals, specifically arsenic and zinc. This reduction is attributed to the geopolymer’s unique structure, which consists of a three-dimensional framework of SiO₄ and AlO₄ tetrahedra, primarily composed of SiO₂, Al₂O₃, and Fe₂O₃. This framework carries a net negative charge, balanced by exchangeable cations [93,94,95]. Metal ion uptake is primarily facilitated by ion exchange reactions within microporous zeolite-like minerals present in geopolymers [96,97]. This important finding strongly indicates that the solidified samples, after undergoing the stabilization process and curing period, do not pose a significant environmental hazard.