Comparative Assessment of Conventional and Microwave Curing Synthesis Routes for Metakaolin-Based Porous Geopolymers: Characterization and Environmental Metrics

Abstract

Geopolymers have gained relevance in environmental applications, and in recent years they have been studied as sustainable adsorbent materials. Increasing their porosity remains one of the main challenges. Various methodologies have been applied for the synthesis of porous geopolymers; however, energy efficiency and environmental considerations associated with the synthesis process must be considered. This study compares two synthesis routes for porous metakaolin-based geopolymers using hydrogen peroxide as a foaming agent and two curing methods: conventional oven curing and microwave-assisted curing. Structural, physical, and chemical properties were evaluated using XRD, FT-IR, SEM/EDS, TGA, and density–porosity analyses. Additionally, a quantitative environmental assessment based on the 12 principles of green chemistry was conducted using the DOZN^TM software version 2.0. The results confirmed that the addition of H₂O₂ did not alter the geopolymeric structure, as evidenced by FT-IR and XRD, regardless of curing method. Porosity increased significantly with the foaming agent, reaching up to ~65% for conventionally cured samples and a maximum of 67% for microwave-cured geopolymers at 3 wt% H₂O₂, with a minimum bulk density of 0.79 g/cm³. High-power microwave-assisted curing reduced the synthesis time to 5 min (≈80% reduction) while promoting a more developed and interconnected macroporous structure, as observed by SEM and supported by enhanced water retention behavior in TGA analyses. The green chemistry assessment demonstrated that microwave curing presents a lower overall impact within the DOZN^TM framework, primarily associated with improved energy efficiency (GCP-6), while acknowledging that this assessment does not constitute a full life cycle analysis. Overall, microwave-assisted synthesis emerges as a more sustainable and efficient route for producing highly porous, hydrophilic geopolymers with strong potential for the adsorption of aqueous pollutants in environmental applications.

1. Introduction

Geopolymers are inorganic polymers that are formed by the reaction of an aluminosilicate powder with an alkaline activator. Their synthesis is regarded as both economical and environmentally sustainable [1]. The term “geopolymer” was introduced by the French scientist J. Davidovits in the 1970s [2]. The application of these materials has initially been focused on the field of construction; however, they have recently gained considerable attention as promising adsorbent materials. These materials have been successfully evaluated for the adsorption of a variety of contaminants, including heavy metals, organic pollutants, and dyes [3,4]. Their chemical structure is based on aluminosilicate tetrahedra, the fundamental building block of geopolymeric materials. This chemical formation is a result of the geopolymerization process [5]. The negatively charged aluminosilicate network in geopolymers—due to the presence of AlO₄⁻ tetrahedra—facilitates the exchange and retention of cationic species [6].

Geopolymerization involves the formation of a three-dimensional inorganic gel resulting from the reaction between silicon (Si)- and aluminum (Al)-rich precursors and an alkaline activator, typically sodium hydroxide (NaOH) or potassium hydroxide (KOH) [1,7,8]. The process begins with the dissolution of the precursor materials, during which reactive species are released, and the particle surfaces are activated [9]. This is followed by a stage of diffusion and equilibrium, where the dissolved aluminum and silicon species migrate from the particle surfaces to the spaces between particles, allowing for rearrangement and stabilization [10]. As these species interact, gelation occurs through the polymerization of the silicate solution with the aluminum and silicon complexes, leading to the formation of a gel phase. Finally, the process concludes with hardening, where continued polymerization results in the structural consolidation of the geopolymer network [8].

Among the commonly used precursors, metakaolin (MK) is notable for its favorable chemical composition and reactivity [11]. Produced by the thermal dehydroxylation of kaolinite (Al₂Si₂O₅(OH)₄) at 600–900 °C, MK is composed primarily of silicon dioxide (SiO₂) and aluminum dioxide (Al₂O₃), with a theoretical composition of approximately 54% SiO₂ and 46% Al₂O₃. Its morphology consists of fine particles, typically with a size of approximately 3 µm [12]. It meets the ASTM C618 classification requirements as Class N pozzolan [13].

Recent research has focused on increasing the porosity of geopolymers to broaden their functional applications, particularly in cases where dense structures are insufficient [14]. Geopolymers are defined as porous geopolymers (PGs) when having a total porosity greater than 55% by volume [15]. Their utilization is mainly seen in thermal insulators, acoustic insulators, membranes, pH controllers, and heavy metal immobilizers, and they are particularly suitable for adsorption applications [16,17]. Among the various PG synthesis methods, the direct foaming (DF) approach is especially versatile. This technique involves the physical or chemical addition of foaming agents to the geopolymer matrix. Although metallic powders and commercial foaming agents are commonly used, hydrogen peroxide (H₂O₂) is widely preferred as a foaming agent due to its environmentally friendly decomposition into water and oxygen, and its ability to generate uniform foam structures [18,19]. The decomposition reaction (H₂O₂ → H₂O + ½ O₂) produces gas bubbles that form macropores within the matrix, effectively increasing porosity [20].

Traditionally, PGs are cured using conventional ovens or under ambient conditions; however, these methods can be time-consuming and highly dependent on environmental factors. Microwave-assisted curing has emerged as a promising alternative, offering benefits such as shorter curing times, lower energy consumption, rapid heating, and improved reaction kinetics [21,22]. Microwaves interact with polar molecules such as water and alkalis in the geopolymer system, generating internal heat that promotes geopolymerization [23]. Furthermore, microwave curing facilitates the formation of additional gas bubbles, enhancing porosity and reducing density in the final material [21,24]. In addition to accelerating geopolymer curing, microwave irradiation has been reported to actively promote foaming in MK-based geopolymers by enhancing the decomposition of chemical foaming agents, such as hydrogen peroxide, enabling simultaneous pore formation and matrix hardening [25]. Under these conditions, microwave-assisted foaming contributes to rapid porosity development and reduced processing times compared to conventional thermal treatments [26]. In the present study, hydrogen peroxide remains the primary foaming agent, while microwave curing intensifies and accelerates both the geopolymerization and foaming processes, contributing to the highly porous structures obtained within significantly shorter curing times.

Although geopolymers have been widely recognized as sustainable materials, some of their synthesis and modification routes involve the use of reagents or processes with potential environmental impact [27]. For this reason, a critical environmental analysis of the different synthesis strategies employed is essential. In this context, the twelve Green Chemistry Principles (GCP) offer a valuable guide for optimizing the design of chemical processes, promoting the use of cleaner and less toxic reagents, as well as the reduction in energy consumption and waste generation [28]. The application of a quantitative environmental analysis allows for the objective identification of the most suitable synthesis routes, aimed at obtaining materials with the desired properties while minimizing their environmental impact. To facilitate this assessment, several different software tools have been developed to quantify and compare the environmental impact of different synthesis processes [29]. These include the DOZN^TM software developed by Sigma-Aldrich, which is based on the 12 principles of green chemistry, and allows a systematic and comparative evaluation of synthetic routes, providing a comprehensive score to help select the most sustainable alternatives [30,31].

In parallel, numerous studies have reported the synthesis of porous MK-based geopolymers using hydrogen peroxide as a foaming agent, demonstrating effective control over porosity and bulk density while preserving the geopolymeric structure [32]. Microwave-assisted curing has also been explored as an alternative to conventional thermal treatments, offering faster geopolymerization and enhanced pore development [33]. However, most reported studies have primarily focused on material properties, with limited attention to a systematic comparison of curing routes from an environmental sustainability perspective. A quantitative green chemistry-based assessment comparing conventional and microwave curing methods for porous MK-based geopolymers has not yet been reported. Therefore, the present study addresses this gap by combining physicochemical characterization with a quantitative evaluation based on green chemistry principles.

In this study, porous MK-based geopolymers were synthesized using H₂O₂ as a foaming agent at concentrations ranging from 1% to 5% by weight. Conventional oven curing (HC) and microwave-assisted curing (MA) were employed to investigate the influence of foaming agent content and curing method on the geopolymerization process and resulting material properties. The synthesis routes were subsequently evaluated using quantitative environmental metrics based on DOZN^TM software, allowing the identification of optimal synthesis and curing conditions for the development of porous geopolymers with potential application as adsorbent materials in aqueous environments.

2. Materials and Methods

This section outlines the synthesis methodology for MK-based porous geopolymers using hydrogen peroxide (H₂O₂, 30% w/w, Sigma-Aldrich, St. Louis, MO, USA) as the foaming agent, added in varying proportions (0% to 5% by weight of MK). The PGs were synthesized using two different curing methods: conventional oven (HC) and microwave oven (MA). This process is shown as a scheme in Figure 1, and the mixture proportions are presented in

Table 1. Mixture design for MK-based PGs synthesis.

Precursor MK (g)	Activating Solution (g)	NaOH 10 M (mL)	Na2SiO3 (mL)	NaOH/Na2SiO3 Ratio	MK/Activating-Solution Ratio	Si/Al Ratio	H2O2 (mL)
100	170	43	82	1:2	0.58	2.5	0.90
							1.8
							2.7
							3.6
							4.5

. All synthesized geopolymers were subsequently characterized to determine their composition and key properties.

2.1. Synthesis of the MK-Based PGs

The PGs’ precursor was MetaStar 750 commercial MK (Imerys Kaolins, Inc., Sandersville, GA, USA), supplied by Watson Phillips y cia. Sucs. S.A. de C.V. (Mexico City, Mexico). The composition of the MK, defined by XRF (X-ray fluorescence), was 57.92% of SiO₂, 33.79% of Al₂O₃, 1.09% of Fe₂O₃, 0.16% of CaO, and 0.43% of K₂O. The alkaline activating solution was prepared using sodium hydroxide (NaOH, ≥97%, Sigma-Aldrich, St. Louis, MO, USA) at a molarity of 10 M, by manually stirring NaOH pellets—containing 1% sodium carbonate (Na₂CO₃)—in distilled water for one minute. In addition, sodium silicate (Na₂SiO₃, powder, technical grade, Sigma-Aldrich, St. Louis, MO, USA) was used, with a SiO₂/Na₂O molar ratio of 2.8. The activating solution was left to rest at room temperature for 12 h before use to prevent any disturbances due to the exothermic nature of the reaction.

Afterward, 100 g of MK were weighed, as indicated in Table 1, and mixed with the activating solution until a homogeneous paste was formed. This geopolymeric mixture was manually stirred for 15 min. The foaming agent was then added using the direct foaming method, which involves directly adding the agent to the geopolymeric paste, followed by mixing and manual vibration for 2 min. The mixture was then poured into silicone molds measuring 1 cm³, and an additional manual vibration was applied for 5 min to release air bubbles formed during molding.

Details regarding the curing method and the nomenclature used to label the geopolymers are presented in

Table 2. Sample labeling according to H₂O₂ content and curing method.

H2O2 wt% Related to MK	Conventional Oven	Microwave 900 W 5 min
0	0PHC	0PMA
1	1PHC	1PMA
2	2PHC	2PMA
3	3PHC	3PMA
4	4PHC	4PMA
5	5PHC	5PMA

. All samples underwent a precuring stage at room temperature for 24 h prior to thermal treatment, allowing partial setting and stabilization of the geopolymeric matrix. The conventional oven curing process involved heating for 12 h at 80 °C, followed by 6 h at 200 °C. The second curing step at higher temperature was employed to promote the removal of free and weakly bound water and to enhance stabilization of the macroporous structure generated by the foaming agent. In contrast, the microwave curing was performed for 5 min with a power setting of 900 W. The microwave curing parameters were selected based on previous studies [22,24,25,33,34], showing that short, high-power irradiation promotes geopolymerization and pore development while minimizing processing time.

2.2. Characterization and Environmental Assessment

2.2.1. Characterization Methods and Parameters

For the FT-IR analysis, a Bruker TENSOR 27 FT-IR spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany) was used. Potassium bromide (KBr) was employed for pellet preparation. Spectral data were collected in the range of 4000–500 cm⁻¹ with a resolution of 4 cm⁻¹. X-ray diffraction (XRD) characterization was carried out using a PANalytical X’Pert PRO diffractometer (Malvern Panalytical, Almelo, The Netherlands). The scan was performed over a 2θ range of 10° to 80°, with a step size of 0.05°/s and a counting time of 70 s per step. The instrument operated with CuKα radiation. Thermogravimetric analysis (TGA) was conducted using an SDT-Q600 Simultaneous TGA/DSC system (TA Instruments, New Castle, DE, USA). The temperature was ramped at 10 °C per minute up to 800 °C.

Density measurements included both bulk (apparent) density, determined by the Archimedes method, and true (real) density, determined using a gas pycnometer. The pore volume (VP) was calculated based on the difference between real and apparent density using the following equation:

$$VP\left(\mathrm{\%}\right)={\displaystyle \frac{Real density-apparent density}{Real density}}\times 100$$

(1)

Digital microscope images were obtained using a Dino-Lite Digital Microscope (Dunwell Tech, Inc., Torrance, CA, USA) with analysis conducted via the Dino Capture 2.0 software. For scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), a Hitachi SU3500 scanning electron microscope (Hitachi High-Tech Corp., Tokyo, Japan) was employed. SEM images were captured at magnifications ranging from 50 µm to 1000 µm, and data were analyzed using the AZtecHKL software version 6.1 (Oxford Instruments, High Wycombe, UK).

Reported values correspond to representative measurements, and the discussion focuses on comparative trends between curing methods rather than on statistical variability.

2.2.2. Green Chemistry Environmental Assessment

The software DOZN^TM 2.0, developed and validated by MilliporeSigma (St. Louis, MO, USA), was utilized to assess the environmental impact of both synthesis methods, considering different factors such as duration, number of synthesis steps, types of precursor materials, use of alkaline solutions and foaming agents, as well as water and energy consumption, and waste production. Information on the chemical reagents was sourced from technical labels and safety data sheets provided by suppliers. This green evaluation software is based on the 12 Principles of Green Chemistry, with its equations previously published [35]. Although DOZN^TM does not factor in the life cycle impact of the precursor materials, it does evaluate other aspects of material usage and resource efficiency.

3. Results and Discussion

3.1. Characterization of PGs

3.1.1. FT-IR Spectroscopy

The aim of this characterization technique was to analyze the chemical composition of the PGs after the foaming agent H₂O₂ was added, to make sure that no changes in the structure occurred, and to compare them with the precursor material. Figure 2 shows the spectra of the PGs in two sets, according to the curing method applied.

The most evident alteration in both curing methods, which is observed as a band shifting from 1070 cm⁻¹ in the MK precursor material to 970 cm⁻¹ in the PGs, is associated with the bending vibrations of the Si-O-Si, Al-O-Al or Si-O-Al bonds, and silanol (Si-OH) bending vibration [36]. This is the main band in any geopolymeric material. It appears ranging from 1000 to 1100 cm⁻¹ in the MK spectrum due to unreacted silicon and aluminum, and after the geopolymerization process, it shows a characteristic broadening and shifting to 970 cm⁻¹ due to the formation of Si-O-Al bonds [37]. The band appearing between 800 and 700 cm⁻¹ in the MK spectrum is associated with SiO₂, which decreases and shifts to the right in all PG spectra. This band shifting is due to SiO₂ being rearranged from the precursor material to the geopolymeric structure. The band appearing at 540 cm⁻¹ is associated with Al-O, also characteristic of PGs [38]. As for the small band that appears in the MK spectrum at 2100 cm⁻¹ and increases slightly in the GPs’ spectra, it is associated with carbonation. This reaction occurs with atmospheric CO₂ in alkaline environments. It is an asymmetric vibration of CO₃²⁻ which becomes clearer as the percentage of H₂O₂ increases, and this is due to the increase in porosity [39].

No noticeable difference in the bands is observed as the percentage of H₂O₂ in the GPs increases, and this is also verified with the XRD results. As for the curing methods, there is no difference in bands between the spectra of the materials cured whether in conventional oven or in microwave oven at 900 W.

3.1.2. X-Ray Diffraction

The diffractograms for the PGs at different percentages of H₂O₂ in the two curing methods used are shown in Figure 3, where they are also compared with the XRD data for the MK as the precursor material. Two main crystalline phases, identified as Quartz (Q) and Kaolinite (K), are observed for the MK. The quartz phase (SiO₂) diffracted between 25° and 40° in 2θ, with its main peak appearing at 25° in the diffractogram. Kaolinite phase (Al₂Si₂O₅(OH)₄) is found at 45° to 50° in 2θ [40]. These crystalline reflections are superimposed on a predominantly amorphous background, characteristic of thermally dehydroxylated metakaolin. Although metakaolin is mainly characterized as an amorphous material, these phases appear as residues of the dehydroxylation process of kaolinite that occurs between 600 and 900 °C [41].

A decrease in the intensity of the main MK peaks and a slight shift in the amorphous halo toward higher 2θ values are observed in the alkaline-activated materials. This change is attributed to the appearance of geopolymeric gels and is characteristic of the geopolymerization process [42]. However, no significant phase differences are observed as the amount of H₂O₂ increases or between curing methods, indicating that the foaming reaction and curing route do not modify the fundamental crystalline–amorphous nature of the geopolymeric structure. In this context, XRD mainly serves as a structural confirmation technique, supporting the FT-IR results rather than providing detailed phase differentiation. Similar behavior has been reported previously, where geopolymerization and chemical foaming occur simultaneously without affecting the overall structure of the material [43].

3.2. Thermogravimetric Analysis (TGA)

The thermogravimetric analysis results are displayed in Figure 4. The TGA plots for PGs cured in a conventional oven show an initial weight loss of only 2% between 0–200 °C, mainly due to desorption of free water molecules [44,45]. A second weight loss of around 3.5% occurs between 200–400 °C, attributed to the decomposition of geopolymeric gels [38,39]. PGs from the HC samples display thermal stability regardless of the foaming agent content, with no clear trend related to increasing foaming agent percentages.

Geopolymers cured using microwave irradiation at 900 W exhibited a free water loss ranging from 5–10% between 0 and 200 °C, with the sample foamed with 5% H₂O₂ showing the highest mass reduction in this range. The second weight loss observed between 200 and 400 °C, mainly associated with the dehydration and partial structural decomposition of the hydrated sodium aluminosilicate gel (N-A-S-H), ranged around 2–5% [46,47]. The total mass loss up to 800 °C was around 11% on average, reaching 15% for the sample 5PMA, and is attributed to the combined contribution of free water release and the gradual dehydroxylation and structural rearrangement of residual aluminosilicate species at higher temperatures [34,48].

The comparison between conventional oven curing and microwave curing at 900 W reveals significant differences in the thermal behavior of the geopolymers. The mass loss of the microwave-cured geopolymers was nearly double that observed for those under conventional oven curing conditions, with the most marked difference being the loss of water molecules. This suggests that the microwave-cured geopolymers possess a more developed and accessible porous network, allowing them to retain more water. While this increased water retention is a physical property, it creates a material with a larger internal surface area and an improved network for liquid transport.

Although significant amounts of water remain within the geopolymer structure after room-temperature precuring, particularly in highly porous samples, this retained water plays a key role during subsequent thermal or microwave curing. Rapid heating facilitates controlled water evaporation through the interconnected pore network, avoiding excessive internal pressure and preventing structural damage [49]. This behavior is supported by the TGA results, which show higher water retention in microwave-cured samples without compromising structural stability.

This combination of high porosity and the material’s inherent hydrophilic nature is highly desirable for the adsorption of water pollutants. The geopolymer’s surface is naturally hydrophilic due to the presence of polar active sites, such as charged aluminate groups and silanol groups [50]. The extensive porous network facilitates the transport of water and its dissolved pollutants deep into the material, increasing the contact between the liquid and the abundant active sites where ion exchange and adsorption occur [17]. This makes microwave-cured geopolymers a promising material for water purification applications.

3.3. Density–Porosity Ratio

The results of real density, bulk density, and porosity for both conventionally cured and microwave-cured PGs at varying H₂O₂ concentrations are presented in

Table 3. Densities and porosity of PGs at different H₂O₂ percentage.

PGs	Real Density (g/cm3)	Bulk Density (g/cm3)	Porosity (%)
0PHC	2.53	1.34	46.8
1PHC	2.38	0.97	59.3
2PHC	2.56	0.93	63.7
3PHC	2.49	0.88	64.4
4PHC	2.46	0.87	64.5
5PHC	2.52	0.88	65.2
0PMA	2.55	1.25	50.8
1PMA	2.38	0.99	58.4
2PMA	2.43	0.81	66.5
3PMA	2.42	0.79	67.0
4PMA	2.33	0.85	63.3
5PMA	2.31	1.03	55.3

, with the corresponding bulk density–porosity relationships shown in Figure 5.

In the case of oven-cured PGs, increasing the percentage of H₂O₂ led to a consistent rise in porosity, from 46% to 65%, while bulk density decreased accordingly. This is attributed to the foaming agent decomposing into oxygen and water vapor, forming pores throughout the geopolymer matrix [51]. The observed trend suggests that, within the tested range, porosity can be effectively controlled by adjusting H₂O₂ content.

For microwave-cured PGs, the optimal performance was observed at 3% H₂O₂, where porosity peaked at 67% and bulk density reached a minimum of 0.79 g/cm³—17% higher porosity than the control sample without foaming agent. Beyond this point, however, porosity began to decline while density increased. This shift is likely to be due to excess water from the decomposition of higher H₂O₂ concentrations, which may disrupt the geopolymeric gel structure or lead to incomplete incorporation of reaction products within the matrix [37]. Additionally, the rapid heating in microwave curing may enhance pore formation but also accelerates water loss, making the system more sensitive to water-to-solid ratios [52].

These results suggest that, while both curing methods benefit from H₂O₂ as a pore-forming agent, microwave curing exhibits a narrower optimal range. Identifying this range is essential to maximize porosity while preserving structural integrity and ensuring complete geopolymerization [53].

The bulk density and porosity values obtained in this study are consistent with those reported in the literature for porous metakaolin-based geopolymers synthesized using chemical foaming agents. Previous studies employing H₂O₂ or similar foaming strategies have reported bulk densities typically ranging from approximately 0.6 to 1.2 g cm⁻³ and total porosities between 50% and 70%, depending on precursor composition, foaming agent content, and curing conditions [37,54,55]. The porosity levels achieved in the present work, particularly for microwave-cured samples, fall within this range, while being obtained under significantly reduced curing times.

In comparison with conventional thermal curing, microwave-assisted curing has been reported to accelerate geopolymerization and promote pore development due to volumetric heating and rapid gas evolution [34,56]. Consistent with these reports, the microwave-cured geopolymers in this study exhibit higher water retention and more developed porous networks, as evidenced by TGA and SEM analyses. Unlike most previous studies, which primarily emphasize material properties, the present work additionally integrates a quantitative green chemistry-based assessment, providing a sustainability-oriented comparison of curing routes.

3.4. Visual Porosity Analysis

The images shown in Figure 6 correspond to digital microscope shots at 0.5 mm magnification of the geopolymer samples cured in a conventional oven and in a microwave oven at 900 W. In both curing methods, a clear change in pore morphology is observed as the percentage of foaming agent increases. Notably, the pores become larger, more interconnected, and increasingly open, allowing flow between them—an essential characteristic for porous geopolymers.

Pore homogeneity is another critical feature, and it visibly decreases with higher H₂O₂ concentrations, as variations in pore size and shape become more pronounced. The pore size distribution is influenced by two concurrent reactions: the decomposition of H₂O₂ and the geopolymerization of MK. H₂O₂ decomposition, especially in strongly alkaline conditions, can proceed rapidly and uncontrollably, often resulting in irregular and rough voids that are undesirable for achieving uniform porosity [57].

Given the large pore sizes formed in these samples, the materials are classified as microporous [58]. Consequently, the BET analysis technique—typically suitable for mesoporous or microporous materials—was not suitable for characterizing these PGs.

3.5. SEM/EDS

SEM/EDS analysis (Figure 7) were conducted only for the PG samples with 0% and 1% H₂O₂ using both curing methods, for comparative purposes regarding the absence or presence of the foaming agent. A significant difference in the geopolymeric matrix was observed between the samples without and with the foaming agent.

Figure 7a shows SEM images at 500 µm and 50 µm magnification of 0PHC (same scales were used for subsequent micrographies), where a homogeneous surface without visible pores is observed. In contrast, Figure 7b presents the SEM image of 1PHC, showing clearly visible open pores up to 400 µm in size. The observed morphology is consistent with that typically reported for the direct foaming method [59].

The SEM images of PGs without foaming agent, synthesized using microwave curing at 900 W, also show a uniform morphology (Figure 7c), although with a slightly higher number of visible pores compared to conventional oven curing. Meanwhile, the sample with 1% H₂O₂ under the same microwave curing conditions (Figure 7d) displays open pores larger than 500 µm.

Both the SEM images and the digital microscope images show changes in pore size and geometry as H₂O₂ is added, as well as an increase in pore interconnectivity, which is consistent with previous reports [60]. In porous cementitious structures, three main types of pores are typically distinguished: gel pores (<10 nm), capillary pores (around 10 µm), and air voids (>10 µm) [61,62]. The pores detected in the PGs in this study fall into the category of air voids, commonly formed due to the foaming agent and synthesis method, resulting in large, connected voids that favor mass transport within the material, while mechanical properties are not the focus of this investigation.

Despite the large pore sizes, the structures remained stable with no collapse due to void formation. No evidence of pore coalescence or matrix failure was observed, and the pore sizes obtained at higher H₂O₂ contents were within the range reported in another study for higher foaming agent concentrations [48]. The use of microwave curing likely contributes to rapid matrix consolidation while preserving pore integrity.

3.6. Green Chemistry Assessment with DOZN^TM

The quantitative environmental analysis for the two synthesis routes is shown in comparison in Figure 8. For the evaluation of the chemical properties of the reagents and the energy used for the synthesis were used, although low scores were obtained according to the software, each of the 12 green chemistry principles (GCP) were analyzed to score areas for improvement [28,35,63,64]. In summary, the MA method had a lower overall score than the HC method due to energy efficiency, indicating a greener synthesis. GCP-12 (Inherently Safer Chemistry for Accident Prevention) had the highest score for both synthesis routes, and where there was a difference between the routes was in GCP-6 (Designing for Energy Efficiency). It should be noted that the DOZN^TM analysis provides a comparative, principle-based evaluation of the synthesis routes and does not constitute a full life cycle assessment (LCA). Therefore, the environmental conclusions drawn in this study are intended to reflect relative differences between curing strategies rather than absolute life cycle impacts.

According to GCP-1 (Prevention), avoiding the creation of waste is preferable to eliminating waste after it has been produced. For both synthesis routes, a score of 0 is obtained for this principle, complying with the design to avoid waste generation. The GPC-2 (Atom Economy) concept is that atom economy should be maximized, promoting that as much of the reagents used as possible end up as part of the final product. This value in both routes is 0.32, a generally low score because the mass product obtained is three times the initial mass weight of the MK, and the use of the chemical reagents used as alkaline activators is precisely calculated in the synthesis designs to obtain the required chemical compositions.

GCP-3 (Less Hazardous Chemical Synthesis) focuses on the fact that the synthesis should be designed with the use of substances with the lowest possible degree of toxicity to human health and the environment. The syntheses obtained a score of 0.32. For this reason, it is important to review the safety data sheets of each of the precursor materials and reagents, classifying them in categories from 1 to 4 depending on their toxicity to both health and the environment considered as B score, with category 4 being the least toxic (≤1 mg/L) and category 1 being the most toxic (˃100 mg/L). In the case of NaOH 10M, H₂O₂ 30% and Na₂SiO₃ fell into category 4 (B score = 1). Although reagents such as concentrated NaOH present inherent hazards, both curing routes employ identical chemical compositions. Consequently, the green chemistry assessment is used here to compare relative differences between synthesis strategies rather than to suggest that the system is free of chemical risk.

GCP-4 (Designing Safer Chemicals) indicates that chemicals should be designed to preserve their functional efficacy while minimizing their toxicity. The syntheses in this study yielded a GCP-4 score of 0.40 due to the low toxicity of the raw materials. For GCP-5 (Safer Solvents and Auxiliaries), a score of 0 was obtained for the two routes, indicating that there is no environmental impact in this area, a major advantage among other similar materials, as no solvents, separating agents, stabilizers, surfactants, etc., are used. These may present a source of toxic or hazardous waste, increasing the environmental footprint of the process.

The GPC-6 (Dessing for Energy Efficiency) states that energy requirements must be recognized for their environmental and economic impact and must be minimized in terms of temperature and time, as well as only needing environmental pressure. This is where the significant difference between the routes is shown, due to the curing method, with a score of 0 for MA and 0.37 for HC, showing the advantage of using microwaves for the synthesis of the PGs. The use of microwaves decreased the environmental impact in this area by 100%. This result reflects a relative improvement in energy efficiency associated with reduced curing time and processing conditions. Absolute energy consumption values were not directly measured, and the assessment is therefore intended to highlight comparative trends between curing methods in accordance with GCP-6.

GPC-7 (Use of Renewable Feedstocks) indicates that feedstocks should be renewable rather than exhaustible, such as waste or by-product products and functional materials. In this parameter, a score of 0.32 was obtained for both routes, which although MK is not a waste, it is a kaolin by-product material, which is found in abundance in the earth’s crust and there are treaties that encourage its use and exploitation.

GPC-8 (Reduce Derivatives) is another of the principles where no environmental impact was found in the routes analyzed, which gives PGs an advantage over other materials as 100% of the reagents are used and the process steps are simple. GCP-9 (Catalysis) refers to the use of catalysts to accelerate the process. A score of 0.55 was obtained, however, it does not apply in the case of PGs, as the only way to accelerate the reaction is to increase the temperature and this can be counterproductive.

GCP-10 (Design for Degradation) focuses on a life cycle analysis, in which we can take as reference previous studies, which point out the durability of PGs as well as the fact that their application also contributes to environmental improvements such as pollutant adsorption. In the study, this parameter presented a score of 0.52. The GCP-11 (Real-time analysis of Pollution Prevention) did not present any environmental impact because no stage of the synthesis involves critical variables such as pressure, pH, precursor concentration, etc. However, characterization techniques are incorporated once the materials have been obtained, as previously presented.

Finally, the parameter GCP-12 (Inherently Safer Chemistry for Accident Prevention), is related to the prevention of accidents with the physical risk of the use of chemicals and experimental conditions involved in the process. The values for this principle were taken from the safety data sheets. A value of 1.05 was obtained for this parameter, which was the highest in both cases. This can be understood because, although the reagents are category 4 (low impact) they can present some physical injuries such as low risk burns to skin and eyes, in the case of NaOH due to being very exothermic and highly corrosive it can cause burns and release of non-flammable irritant gases. For this reason, the use of basic laboratory equipment (latex gloves, gown and glasses) is recommended.

The 12 principles of green chemistry are summarized in three categories presented in Figure 9. Reduce Human and Environmental Hazards (GCP-3, GCP-4, GCP-5, GCP-10 and GCP-12), Increased Energy Efficiency (GCP-6), and Improved Resource Use (GCP-1, GCP-2, GCP-7, GCP-8, GCP-9 and GCP 11). In the first category (Reduce Human and Environmental Hazards) the score for the synthesis routes was 0.46 as shown in Figure 9, while the Increased Energy Efficiency category was obtained with a score of 0.37 for the HC route and 0 for the MA route, indicating a clear relative advantage of microwave curing in terms of energy efficiency, while for the third category (Improved Resource Use), both routes obtained a score of 0.20.

It is important to highlight that, although there are previous studies describing the environmental advantages and comparing the synthesis of geopolymers against other materials, a quantitative environmental evaluation explicitly based on green chemistry principles and focused on curing strategies remains scarce. In this context, the present assessment provides a comparative, principle-based analysis rather than a full life cycle assessment (LCA). Although there are points to improve, such as GCP-12, the overall scores obtained indicate relatively low environmental impact when compared with DOZN^TM analyses reported for other material synthesis routes. It should also be noted that several principles showed no impact according to this study (GCP-1, GCP-5, GCP-8, GCP-11), and that in the case of microwave-synthesized porous geopolymers, no impact was observed in the energy efficiency category (GCP-6), reflecting reduced curing time and processing demands rather than absolute energy consumption values.

4. Conclusions

This study provides a comparative assessment of two distinct synthesis routes—conventional oven curing and high-power microwave curing—for producing porous geopolymers designed for adsorption applications. The results from structural and physicochemical characterization, including FT-IR, XRD, TGA, and density analysis, confirm that both methods are effective in creating porous geopolymeric materials. The use of H₂O₂ as a foaming agent did not interfere with the fundamental geopolymerization reaction, but played a crucial role in controlling the physical properties, such as porosity and bulk density.

The comparison of environmental metrics indicates that the microwave synthesis route is a more energy-efficient alternative to conventional curing, as evidenced by a higher score in the energy efficiency criterion (GCP-6) of the DOZN™ assessment. It should be noted that this evaluation is intended as a comparative, principle-based analysis rather than a full life cycle assessment. The study’s findings on characterization and environmental impact lead to two key conclusions for the development of adsorbent materials. First, microwave curing at 900 W enables the formation of an optimized porous structure, reaching up to 67% porosity within significantly reduced processing times, which enhances the practical viability of this approach. Second, the combination of this high porosity and the geopolymer’s inherent hydrophilic surface, attributed to its aluminosilicate active sites, favors water transport and interaction with dissolved species. The extensive pore network facilitates the transport of water and its dissolved pollutants to these active sites, suggesting strong potential for application as adsorbents in aqueous systems, including the removal of contaminants such as ammonium (NH₄⁺) and heavy metals. Future research should focus on quantitatively validating the adsorption capacity of these materials under different operating conditions.