Fayalite-Based Geopolymer Foam

Abstract

The present work is the first study exploring the potential of geopolymer foams based on fayalite slag, an industrial by-product, as the primary precursor, for lightweight and fireproof construction applications. The research involved the synthesis and characterization of geopolymer foams with varying water to solid ratio, followed by testing their physical and mechanical properties. The phase composition and microstructure of the obtained geopolymer foams were examined using powder XRD, Micro-CT and SEM. The geopolymer foams at optimal water to solid ratio (0.15) demonstrated 73.2% relative porosity, 0.92 g/cm³ apparent density and 1.3 MPa compressive strength. The use of an air-entraining admixture improved compressive strength to 2.8 MPa but lowered the relative porosity to 64.5%. Real-size lightweight panel (300 × 300 × 30 mm) specimens were prepared to measure thermal conductivity coefficient (0.243 W/mK) and evaluate size effect and the reaction to direct fire. This study demonstrates the successful preparation of geopolymer foam products containing 81% fayalite slag, highlighting its potential as a lightweight, insulating and fire-resistant material for sustainable construction applications.

1. Introduction

Geopolymer foams have attracted significant interest in the construction sector due to their green synthesis protocol and excellent insulation and fire resisting properties [1]. Geopolymers are inorganic polymers synthesized at low temperature (from ambient to about 100 °C) using the activation of aluminosilicate precursors [2]. In certain iron-rich geopolymer formulations, parts of Al atoms are substituted by Fe atoms to form a ferro-silico-aluminate (Fe-O-Si-O-Al-O-) polymer chain [3]. This suggests a possible incorporation of Fe³⁺ in the tetrahedral network [4]. A particularly promising geopolymer precursor rich in iron is fayalite slag, a by-product of the copper smelting industry generated in vast quantities [5,6,7,8]. The slag is typically available as a fine-grained material, which minimizes the need for additional milling. Fayalite slag, known also as iron silicate fines, contains a certain amount of reactive amorphous phase, which can participate in the geopolymerization process. These characteristics, combined with its abundance and low cost, make fayalite slag an attractive and underutilized secondary raw material for sustainable binder and foam production. Our previous studies showed the potential of fayalite slag as a geopolymer precursor [9,10,11]. The geopolymers prepared with fayalite slag as the only precursor were characterized by compressive strength up to 27 MPa [9]. The microstructural examination revealed that only a minor amount of the fayalite slag reacted, but a certain amount of ferric iron (Fe³⁺) participated in the structure of the newly formed geopolymer gel [9]. The addition of metakaolin to the fayalite slag greatly enhanced the properties of the final geopolymer and the mechanical strength reached 101 MPa compressive strength for geopolymer pastes with standard Vicat consistency [10]. Furthermore, the high-strength geopolymer showed thermal resistance up to 1150 °C accompanied by a further compressive strength increase to 139 MPa [11]. The obtained geopolymer paste based on fayalite and metakaolin is a promising candidate for the preparation of foamed material.

Inorganic geopolymer foams are commonly produced by the direct chemical foaming method, which is based on the generation of gas bubbles within the fresh inorganic matrix [12]. This can be achieved by including foaming agents which induce chemical reactions that release gases such as hydrogen, oxygen, carbon dioxide, ammonium or others [13,14]. The resulting fresh geopolymer foam hardens and the final geopolymer possess a cellular structure that imparts low density and thermal conductivity coefficient, making it suitable for applications such as thermal and sound insulation, fireproofing and lightweight building components [15]. Geopolymer foams were synthesized by various precursors such as fly and bottom ash [16,17,18], perlite [19], silica fume [20], metakaolin [21], etc. [22,23]. To our knowledge, no studies have reported the use of fayalite slag–metakaolin blends in the production of chemically foamed geopolymers, nor have they addressed the combined influence of iron-rich slags and metakaolin on the pore morphology, thermal resistance, and mechanical stability of such materials.

The present study explores this knowledge gap by developing and characterizing foamed geopolymers based on fayalite slag and metakaolin. We focus on evaluating their physical structure, compressive strength, and thermal performance, aiming to establish a new class of high-performance, lightweight, and thermally stable building materials derived from industrial waste. This work contributes to the circular economy by valorizing metallurgical by-products and offers a novel pathway to produce foamed geopolymers with superior properties suitable for energy-efficient construction.

2. Materials and Methods

2.1. Materials

The geopolymer precursors in the present study were fayalite slag and metakaolin. The fayalite slag used in this study is a fine powdery by-product obtained from the flotation of slag generated during flash furnace and converter operations at the Aurubis Bulgaria AD copper smelter, located near the town of Pirdop, Bulgaria. It contains residual moisture, so it was dried in an oven at 80 °C to constant weight. The average particle size of the fayalite slag is about 20 μm, and the absolute density was measured at −3.80 g/cm³ [24]. Commercial metakaolin, provided by Kaolin EAD, Bulgaria, was used to improve the properties of the geopolymer. The wet residue of the metakaolin was 0.40 wt.% (for fraction ≤ 45 μm). The measured absolute density of metakaolin was 2.26 g/cm³. The chemical composition of the dried geopolymer precursors is presented at

Table 1. Chemical composition of the fayalite slag and metakaolin used as geopolymer precursors, according to XRF analysis (in wt.%).

Precursor	FeO	SiO2	Al2O3	CaO	ZnO	MgO	K2O	Na2O	CuO	PbO	TiO2	MoO3	SO3
Fayalite	55.83	31.16	4.67	2.82	1.40	0.95	0.75	0.62	0.52	0.39	0.32	0.29	0.28
Metakaolin	1.03	54.00	43.25	0.15	n.d.	0.09	0.62	0.11	n.d.	n.d.	0.74	n.d.	0.01

.

The activator solution was prepared by mixing sodium water glass, potassium hydroxide pellets, and tap water to obtain alkaline solution with following molar ratios: SiO₂/M₂O = 1.08; H₂O/M₂O = 15; and K₂O/Na₂O = 1.75, where M₂O is the sum of molar quantities of Na₂O and K₂O. The activator solution was prepared one day before geopolymer synthesis. The air-entraining admixture was a commercial product based on anionic surfactants designed for Portland cement mixtures, conforming EN 934-1:2008 (product label GAFB001, provided by Adding Bulgaria Ltd., Sofia, Bulgaria).

2.2. Methods

The phase composition of the geopolymer foams was studied using powder X-ray diffraction (XRD) analysis on a PANanalytical EMPYREAN Diffractometer system (IMC-BAS), Co anode, 40 V, 30 mA. The SEM and SEM-EDX studies were carried out on a ZEISS SEM EVO 25LS scanning electron microscope (Carl Zeiss SMT Ltd., Cambridge, Great Britain; 2008) with an EDAX Trident system (IMC-BAS) at an accelerating voltage of 25 kV. Secondary electron (SE) and backscattered electron (BSE) signals were used to visualize the microstructure of the geopolymer foams and the phases that compose them. SEM-EDX analysis using an EDAX SDD Apollo 10 EDS detector (EDAX Inc., AMETEK, Inc., Montvale, NJ, USA) and Genesis V. 6.2 software was used as an auxiliary method to clarify the phase composition of the studied materials. The study was carried out on polished samples prepared using a special technique that allows for the better visualization of the porous space and grain phases of the geopolymer foam. Sample preparation involved impregnating approximately isometric 10 mm pieces of foams with epoxy resin (EpoFix) under a low pressure of about 10⁻¹ bar for 1 h at room temperature. After hardening, the samples were ground, polished, and carbon-coated.

The porous microstructures were examined by X-ray computed tomography (Nikon Metrology, Tring, UK), providing a resolution of 10 µm with a continuous 360 rotation, 180 kV/200 µA, on 40–45 mm cube specimens. A total of 2880 images were acquired during each scan with an exposure time of 1000 s, at a region of interest with dimension 30 mm. Presentation of the tomographic data was carried out using Nikon Metrology’s CT Pro-3D software (version XT 4.3.1, Nikon Metrology, Hertfordshire, UK), and porosity analysis using VG STUDIO MAX.

The apparent density of the obtained foams was calculated after weighing three dry specimens, cut into a cube with a side of about 40 mm, and accurately measuring the volume using a digital caliper. Absolute density was measured using a gas pycnometer (AccyPy1330, Micromeritic, Norcross, GA, USA) after grinding and sieving samples to sizes less than 25 µm. The relative porosity was calculated based on the ratio between the apparent density and the absolute density.

The relative porosity is presented using two complementary approaches. First, it was calculated based on the ratio between the apparent density and the absolute density of using three specimens, providing an estimate of the total volume fraction of pores within the sample. Second, relative porosity was also evaluated using X-ray computed tomography, which enabled the direct visualization and quantification of the internal pore structure, including pore size, shape, distribution, and connectivity.

The water absorption of the samples was determined after weighing three specimens in a dry state and after keeping them in water for 24 h to constant mass. Compressive strength was measured on three specimens prepared in the form of cut and polished cubes with a side of approximately 45 mm. The tests were conducted using a universal testing machine at a constant loading rate of 100 N/s. Specific strength was calculated by compressive strength divided by the density of the specimen. The physical and mechanical properties were determined using three samples from each series, with the results reported as mean values accompanied by calculated standard deviations.

The coefficient of thermal conductivity was measured on a FOX 314 Heat Flow Meter in stationary conditions on an oven-dried geopolymer foam specimen with dimensions 300 × 300 × 30 mm and polished surfaces. For the measurements, the sample was positioned between the two plates of the apparatus to establish a temperature gradient through the material thickness. Thermocouples were fixed on the surface of the measured specimens to measure the heat flow across the specimen. The temperature gradient was set to 20 °C.

A preliminary fire resistance test was conducted on a foamed specimen measuring 150 × 150 × 30 mm using a propane–butane jet torch. The distance between the torch and the specimen surface was maintained at 10 cm. Temperature readings were recorded every minute on the reverse side of the specimen using an infrared thermometer (IRT-831), which has a measurement range up to 1350 °C and an accuracy of ±2 °C.

2.3. Geopolymer Foam Synthesis

Four types of geopolymer foams were prepared—three with different water to solid ratios (0.14, 0.15, 0.16) with a fixed concentration of the activator solution, respectively, FG14, FG15, and FG16, and one sample based on FG15 but with the addition of a commercial air entraining admixture—sample FG15-AA. The air-entraining admixture is a surfactant that reduces surface tension within the fresh geopolymer mixture, promoting the formation of uniformly distributed fine air bubbles. This helps prevent pore coalescence and improves the mechanical stability of the resulting geopolymer foam [25]. The air-entraining admixture was incorporated into the activator solution of the FG15-AA series at a dosage of 0.1 wt.% to the total mass of dry precursors.

The composition design of the prepared samples was the product of the optimization of the influence of alkali concentration on the cellular structure [26]. The preparation of each sample involved mixing fayalite slag and metakaolin in a weight ratio of 5:1 to obtain a homogeneous dry mixture. Then the activator was added and stirred for 90 s. After 5 min of maturation, an equal amount of oxygen-releasing foaming agent was added to each sample and the resulting mixture was stirred for an additional 60 s. The amount of gaseous oxygen released was calculated to be 0.14 dm³ (1.5 g 30% H₂O₂) per 100 g precursor. The obtained mixtures were poured into moulds, covered with polyethylene, and placed in a drying oven for 24 h at 80 °C. The demoulded samples were left in laboratory conditions for 1 week before studying their physical and mechanical properties.

3. Results

3.1. Physical and Mechanical Properties

The water-to-solid ratio significantly influenced the physical properties of the foamed geopolymers due to change in the consistency of the fresh mixtures. It was found that the optimum water-to-solid ratio was close to 0.15 (sample FG15 series), at which the relative porosity had the highest value (

Table 2. Influence of water-to-solid ratio and addition of air-entraining admixture to physical and mechanical properties of the foamed fayalite-based geopolymer.

Series	Water-to Solid-Ratio	Density, g/cm3	Absolute Density, g/cm3	Relative Porosity, %	Water Absorption, %	Compressive Strength, MPa	Specific Strength, kN/m·kg
FG14	0.14	1.25	3.44	63.7	19.3 ± 0.6	2.4 ± 0.2	1.96
FG15	0.15	0.92	3.43	73.2	30.9 ± 0.3	1.3 ± 0.1	1.46
FG16	0.16	1.08	3.42	68.4	23.8 ± 1.1	1.5 ± 0.1	1.39
FG15-AA 1	0.15	1.22	3.44	64.5	20.4 ± 0.2	2.8 ± 0.1	2.3

¹ Contained air-entraining admixture (surfactant).

). A higher water-to-solid ratio (FG16) led to a more fluid geopolymer paste, with the porous mixture being more susceptible to pore coalescence, pore collapse, and the release of entrapped bubbles, resulting in lower relative porosity and higher density. On the other hand, a lower water-to-solids ratio (FG14) resulted in a stiffer geopolymer paste, which reduced its elasticity and led to a decrease in relative porosity and an increase in density. The notably high densities observed in the geopolymer foams based on fayalite slag can be attributed to the high iron content in the raw material. As a result, the produced foams exhibited relatively higher densities ranging from approximately 0.92 to 1.25 g/cm³ compared to other studies. Ducman and Korat obtained fly-ash-based foams with a density of about 0.60 g/cm³ using a similar amount of H₂O₂ [27]. Sample FG15 showed the highest water absorption—30.9%. The compressive strength of the samples varied in the range of 1.3–2.8 MPa and was negatively correlated with the relative porosity.

The effect of the air-entraining admixture (FG15-AA) is manifested in a decrease in pore coalescence and comparatively more uniform porosity (Figure 1). Sample FG15-AA was characterized by increased density (33%) and compressive strength (115%), as well as reduced relative porosity (12%) and water absorption (34%). The increase in the calculated specific strength after air-entraining admixture addition indicates an improvement in the load-bearing capacity of the material relative to its weight, suggesting enhanced mechanical efficiency. This improvement is likely due to the more homogeneous pore distribution, which minimizes stress concentration points and contributes to better structural integrity. Furthermore, the controlled air-void system reduces interconnected porosity, limiting water ingress and improving durability. However, the addition of the admixture must be optimized to balance the porosity reduction and maintain sufficient foam volume for thermal insulation performance.

3.2. X-Ray Computed Tomography (Micro-CT)

X-ray computed tomography was used as a non-destructive method for 3-D visualization and analysis of the pore space of geopolymer foams (Figure 2). Regions of interest (ROIs) with approximate dimensions of 30 × 30 mm were selected to standardize the analysis across samples, following the automatic surface determination. Minimal volumetric discrepancies between ROIs were noted, which were likely due to variations in voxel distributions. The results summarized in

Table 3. Summary of porosity analysis data.

Series	FG14	FG15	FG16	FG15-AA
Pore count	20,575	23,549	33,726	43,293
Relative porosity, %	38.58	54.21	50.84	33.50
Total volume pores, mm3	10,405	14,630	13,789	8657

show a discrepancy with the relative porosity data obtained by gas pycnometry (Table 2). A possible explanation for this is that the resolution of the Micro-CT used does not provide satisfactory information for pores smaller than 10 μm [28], which leads to apparently lower relative porosity values obtained by Micro-CT. At the same time, all trends established by the two methods are in good agreement.

Materials FG16 and FG15 demonstrate the highest relative porosity levels, at 50.84% and 54.21%, respectively, with pore counts of 33,726 and 23,549 (Table 3). The correlation between porosity percentage and pore count suggests that FG15 exhibits larger pore structures, consistent with macroscopic observations. In both FG15 and FG16, a significant interconnected pore conglomerate was identified, with volumes of 14,107 mm³ and 13,395 mm³, respectively. The remaining pore volume, 523 mm³ in FG15 and 394 mm³ in FG16, consisted predominantly of isolated pores. Material FG14 exhibited a reduction in both relative porosity (38.58%) and pore count (20,575) compared to FG15 and FG16. A major connected pore cluster was identified with a volume of 10,081 mm³, while the remaining 324 mm³ volume consisted of smaller, discrete pores. These observations are consistent with macroscopic findings. In contrast, the surfactant-modified material FG15-AA demonstrated the highest pore count (43,293) yet the lowest relative porosity (33.50%). This material contained a large connected pore network, with a volume of 5956 mm³, and an additional 2701 mm³ volume comprising smaller, closed pores. A notable aspect of FG15-AA is the presence of approximately 490 pores exceeding 0.9 mm³ in volume, compared to 90 in FG15, 33 in FG16, and only 3 in FG14. This distribution indicates substantial differences in pore size characteristics across the materials. The addition of the air-entraining admixture in FG15-AA influenced the foaming process by promoting stabilization of gas bubbles within the fresh geopolymer paste. The surfactants reduced surface tension and acted at the gas–liquid interface, helping to prevent bubble collapse during setting [25]. As a result, a finer and more uniform pore structure was achieved, with an increased number of smaller pores. This detailed analysis highlights distinct differences in the porosity of the materials studied, suggesting that pore size and their connectivity and distribution are material-dependent and have a significant impact on the bulk properties of each sample.

3.3. Powder XRD

The XRD patterns of all the synthesized fayalite-based foamed geopolymers were similar; therefore, Figure 3 presents a representative pattern from series FG15, along with the patterns of the raw fayalite slag and metakaolin for comparison. The main phases fayalite, magnetite, and pyroxene presented in the fayalite slag remained predominantly inert after geopolymerization. Minor differences in certain relative intensities of fayalite were observed, which could be due to a partial reaction of fayalite particles. The fayalite slag contained about 10% Fe in the amorphous phase (detected previously by Mossbauer spectroscopy pores in sample FG15, and also [9]). On other hand, metakaolin showed an amorphous structure with a broad hump between 15 and 30 2θ° with the inclusion of sharp quartz peaks. While metakaolin readily dissolves in alkaline solution, fayalite slag shows selective reactivity—its amorphous fraction contributes to gel formation, but crystalline phases like fayalite and magnetite remain predominantly unreacted. This affects not only the quantity and composition of the geopolymer gel, but also the distribution of unreacted dense particles within the matrix, which act as fillers. The formation of a certain amount of geopolymer gel is proven by a minor amorphous hump between 25 and 40 2θ°. The quartz inclusions in the metakaolin remain inert after geopolymerization.

In the geopolymer foams synthesized from fayalite slag, the mineral phases fayalite and magnetite primarily act as inert fillers within the matrix. These phases are characterized by their relatively high absolute densities—approximately 4.39 g/cm³ for fayalite and 5.17 g/cm³ for magnetite—which contribute significantly to the overall density of the resulting foam. The presence of heavy mineral phases increased the bulk density of the foam and could also impact its structural stability. Specifically, the presence of high-density particles can affect the balance of internal forces within the foamed structure, potentially leading to foam collapse or instability under the influence of gravity during the curing process.

3.4. SEM

The SEM examination revealed some similarities and differences in the samples that are consistent with or enhance the results obtained by other methods. The impregnation of foam samples with epoxy resin proved to be an effective method for identifying interconnected and isolated pores in the studied materials. The studied samples differ from each other in the size of the pores, their distribution, and filling with epoxy resin, which is well illustrated in Figure 4 for the central parts of the polished sections. Two pairs of samples, FG15-AA and FG14 (Figure 4a,d) and FG15 and FG16 (Figure 4e,f), have relatively close microstructural properties within the pairs, which corresponds well to the physical and mechanical properties of the materials in Table 2. The first pair of samples (FG15-AA and FG14) is characterized by the predominance of pore systems with sizes of 50–150 µm and 300–500 µm. The main difference within the first group of samples is the degree of filling of pores 300–500 µm with epoxy resin—unlike sample FG15-AA, in sample FG14, all pores are filled (interconnected). Small pores of 50–150 µm in size in two samples are almost all isolated, since they are not filled with epoxy resin. The second pair of samples, FG15 and FG16 (Figure 4e,f), is distinguished by the presence of a system of very large pores > 1 mm. Other predominant pore systems in the samples are 50–200 µm and 300–600 µm. Pores >1 cm and 300–600 µm are partially or completely filled with epoxy resin (interconnected pores). Some of the 50–200 µm pores in sample FG15 are also interconnected because they are filled with epoxy resin.

The study of BSE images coupled with the EDX analysis of massive parts of foams from the example of sample FG15-AA (Figure 4c) made it possible to establish all the phases identified by XRD analysis (Figure 3): fayalite, magnetite, pyroxene, and quartz. In addition to these phases, aluminosilicate glass containing Na, K, Ca, Fe, and Cl, relics of metakaolin material were also found. The glass found is a possible amorphous candidate for an iron carrier described in [9].

3.5. Real-Size Experiments—Thermal Conductivity and Preliminary Fire Resistance Test

A real-size experiment was performed, adopting sample FG15-AA, utilizing 5 kg fayalite slag. This composition was chosen because it demonstrated the optimal balance of physical and mechanical properties, making it the most suitable candidate for application-oriented evaluation. The mixture was homogenized using conventional mortar mixer Rubimix 7 (1200 W, 760 rpm). The same mixing procedure was followed and the fresh mixture was poured in plastic moulds to prepare geopolymer-foamed blocks with dimensions 300 × 300 × 30 mm. After finishing the curing procedure, the specimens were polished to ensure even surfaces (Figure 5). Certain pore agglomeration was visible at the polished top surface. The density and water absorption were determined to be 1.29 g/cm³ and 15.35%, respectively (

Table 4. Properties of produced real-size geopolymer foam specimens.

Sample	Density	Water Absorption	Thermal Conductivity Coefficient
Real-size specimen based on FG15-AA (300 × 300 × 30 mm)	1.29 g/cm3	15.35%	0.243 W/mK

). Slightly higher values of density and water absorption were observed compared to initial sample FG15-AA due to the size effect.

The obtained real-size porous geopolymer products contain 81% fayalite slag, calculated based on the total solid mass, including the solids contributed by the activator. The high slag content contributes to waste valorization and reduces the demand for processed raw materials, offering environmental benefits. Its fine particle size eliminates the need for additional milling, lowering energy input. Although the geopolymer formulation includes metakaolin, alkaline activator, and foaming agents, the use of industrial waste as the primary component improves the overall resource efficiency and sustainability of the system.

The geopolymer-foamed blocks were evaluated by measuring the thermal conductivity coefficient (λ). Despite the relatively high density (1.29 g/cm³), the geopolymer foam based on fayalite showed a very low thermal conductivity coefficient: −0.243 W/mK. Results of other studies of foamed geopolymers at similar density obtained on laboratory scale specimens showed the following: a slightly higher λ = 0.27 W/mK, at an even lower density, −1.20 g/cm³, was obtained by E. Yatsenko et al. on geopolymer foams based on recycled ash and slag [29], while Pralat et al. prepared specimens with λ = 0.29 W/mK at about 1.00 g/cm³ density, for geopolymer based on metakaolin modified with gypsum [30].

A preliminary fire-resistance test was conducted using a propane–butane jet torch on a specimen with dimensions 150 × 150 × 30 mm (Figure 6, left). During fire exposure, the front, hottest area glowed red, which corresponded to the measured temperature of 1100 °C. After 15 min of exposure, the temperature measured on the reverse side of the specimen reached 225 °C. The geopolymer foam specimen retained its structural integrity, exhibiting only minor surface cracking (Figure 6, middle). A distinct reddish coloration developed on the fire-exposed surface. XRD analysis of this outer layer confirmed the formation of hematite (Figure 6, right). The results aligns with our previous furnace-based experiments on non-foamed fayalite slag geopolymers [11], which demonstrated thermal stability up to 1150 °C. In those studies, fayalite and magnetite were found to transform into hematite above 800 °C in the outer surface layers. This phase transition contributed to increased surface rigidity and a higher melting point [11], supporting the potential of fayalite-based geopolymer foams as fire-resistant materials. A similar phase transformation occurred under direct flame exposure in the present study. In the case of foamed geopolymers, the open porosity likely enhances oxygen diffusion, enabling hematite formation to penetrate deeper into the material.

These experiments highlight the thermal resistance of the material and suggest its suitability for passive fire protection applications, such as insulation panels or fire barriers in construction. Further testing is required to assess the residual strength and dimensional stability after exposure to extreme thermal stress.

4. Conclusions

The present study demonstrates the feasibility of synthesizing geopolymer foams using high-strength geopolymer paste based on fayalite slag and metakaolin. The findings highlight the potential of fayalite slag, a by-product of the copper smelting industry, to serve as a sustainable raw material in the development of advanced building materials. Key observations and conclusions are as follows:

• Fayalite slag, despite its high density (3.80 g/cm³), can be effectively utilized to produce lightweight geopolymer materials through the direct foaming method. This approach offers a practical solution for mitigating the heavy nature of fayalite slag, enabling the production of normal to lightweight components.
• Geopolymer foams with a water-to-solid ratio of 0.15 (series FG15) demonstrated optimal characteristics, achieving the highest relative porosity (73.2%) and the lowest measured density (0.92 g/cm³). The absolute density was measured to 3.43 g/cm³, which is comparatively high for geopolymers due to the presence of dense mineral phases in the fayalite slag, such as fayalite and magnetite. As a result, the foams combine the lightweight nature of porous materials with a geopolymer gel matrix composed of inherently heavier components.
• The addition of an air-entraining admixture resulted in a geopolymer foam with more pore counts, uniform pore distribution, decreased pore size, reduced coalescence, and improved mechanical properties. This modification increased compressive strength to 2.8 MPa, with a decrease in relative porosity (64.5%).
• Microcomputed tomography revealed that the pore network consisted of interconnected pores. The pore structure was greatly influenced by the water-to-solid ratio. The FG15 series exhibited the highest relative porosity and interconnected pore networks, whereas FG15-AA demonstrated a higher pore count with smaller, more evenly distributed pores.
• Powder XRD analysis and SEM study indicated the main phases in fayalite slag—fayalite and magnetite remained inert during geopolymerization, with partial reactivity observed in the amorphous phases. The metakaolin and probably ferro-aluminosilicate glass in the fayalite slag contributed to the formation of geopolymer gel, evidenced by the amorphous hump in the XRD pattern, while the crystalline phases such as quartz, fayalite, magnetite, and pyroxene remained unreacted, acting as a filler in the geopolymer matrix.
• Real-size specimens (300 × 300 × 30 mm) prepared using recipe FG15-AA showed slightly higher values of density (1.29 g/cm³) but lower water absorption (15.35%) compared to the initial sample FG15-AA (1.22 g/cm³, 20.4%, respectively) due to the size effect and scaling the technology of preparation. The geopolymer foam blocks were characterized by a thermal conductivity coefficient of 0.243 W/mK. The geopolymer foam resisted direct flame exposure without disintegration, highlighting its potential as a fire-resistant material.

The geopolymer foams synthesized, comprising 81% fayalite slag, exhibited excellent properties, combining thermal insulation, fireproofing, and lightweight properties. However, some limitations must be acknowledged, including the high absolute density, potential leaching, challenges in controlling pore structure and distribution, and potential variations in properties at larger scales.

In conclusion, fayalite slag-based geopolymer foams represent a promising class of eco-friendly and high-performance materials. Future research will focus on conducting more detailed fire-resistance testing, examining the leaching behavior of potentially hazardous elements, scaling up the synthesis process, and validating the material’s performance under semi-industrial conditions. This work highlights the potential of industrial by-products to be transformed into valuable resources for sustainable development.