# Lightweight heavy geopolymer foam based on fayalite slag: Influence of alkali concentration on cellular structure

Aleksandar Nikolov, Liliya Tscetanova, Borislav Barbov

► To cite this version:

Aleksandar Nikolov, Liliya Tscetanova, Borislav Barbov. Lightweight heavy geopolymer foam based on fayalite slag: Influence of alkali concentration on cellular structure. INTERNATIONAL SCIENTIFIC JOURNAL "MACHINES. TECHNOLOGIES. MATERIALS", 2025, XIX (2), pp.79-82. hal-05443321

HAL Id: hal-05443321

https://hal.science/hal-05443321v1

Submitted on 6 Jan 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Lightweight heavy geopolymer foam based on fayalite slag: Influence of alkali concentration on cellular structure

Aleksandar Nikolov¹, Liliya Tscetanova¹, Borislav Barbov

¹Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences (IMC-BAS) Sofia, Bulgaria

E-mail: drsashko@imc.bas.bg

**Abstract:** The lightweight cellular building materials has driven significant interest recently due to increasing requirements of energy efficiency of the buildings. This study investigates the potential of fayalite slag, a by-product of copper production, as a precursor for lightweight geopolymer foams. Geopolymer foams were synthesized using a combination of fayalite slag and metakaolin, employing gas forming agents to achieve a porous cellular macrostructure. The influence of alkali concentration was examined on the cellular structure, physical properties and microstructure (FT-IR) of the resulting materials. The findings contribute to the development of sustainable, high-performance geopolymer materials suitable for insulation and fire-resistant applications.

**Keywords:** GEOPOLYMER, FAYALITE SLAG, LIGHTWEIGHT, FOAM, CELLULAR

## 1. Introduction

Sustainable practices in construction industry increase exploration of alternative materials that minimize environmental impact. Geopolymers, synthesized through the alkali activation of aluminosilicate sources, have emerged as a promising class of materials due to their potential for reduced carbon emissions and utilization of industrial by-products. Fayalite slag, also known as iron silicate fines, is a by-product of the copper smelting process. Fayalite slag is predominantly composed of iron silicate minerals, fayalite (Fe₂SiO₄) and magnetite. In the past considered as waste material, now fayalite slag has been promoted as valuable by-product finding application in construction sector. Fayalite slag from Aurubis, (Pirdop) has been certified as fine fraction manufactured aggregate, according EN 12620 [1]. Moreover, cement with industrially produced filler has been assessed by EPA recently.

Fayalite slags from copper smelting industry was successfully used to prepare alkali activated materials and geopolymers [2-5]. Our previous studies presented the potential of fayalite slag as raw material for geopolymer preparation [6-10]. The alkali activated fayalite slag was characterized by compressive strength up to 27 MPa [6]. The microstructural studies revealed that fayalite and magnetite stayed predominantly inert, but certain amount of ferric iron (Fe³⁺) contributes to the structure of the geopolymer gel [6]. The addition of metakaolin to the fayalite slag significantly improved the properties and the compressive strength was measured to 101 MPa after 90 days curing at normal temperature for geopolymer pastes characterized by standard consistence [7]. Moreover, the geopolymer composed of fayalite slag and metakaolin exhibited thermal resistance up to 1150 °C, along with a further increase in compressive strength, reaching 139 MPa [8]. The geopolymer paste based on fayalite slag and metakaolin shows potential for the production of foamed materials.

The development of lightweight geopolymers with cellular structures offers advantages such as reduced density, enhanced thermal insulation, and improved acoustic properties, making them suitable for various construction applications. The characteristics of the cellular structure, including pore size distribution, connectivity, and overall porosity, are critical features of the mechanical and physical properties. The formation of a controlled porous network within the geopolymer matrix is typically achieved through methods such as chemical foaming, pre-made foams, sacrificial fillers, incorporation of lightweight aggregates, additive manufacturing, or other [11].

The most common route of preparation an inorganic geopolymer foams is by addition of foamed agent which generates gas bubbles within the fresh geopolymer gel [12]. The gas is formed due to chemical reactions that produce gas phase such as oxygen, hydrogen, carbon dioxide, ammonium or other [13, 14]. The fresh formed geopolymer foam hardens, resulting in a solid material with cellular structure that provides low density and a reduced thermal

conductivity coefficient. These properties make it well-suited for applications such as thermal and sound insulation, fireproofing, and lightweight construction elements. [15]. Geopolymer foams were synthesized by various precursors such as perlite [16], fly ash [17, 18], silica fume [19] etc. [11, 20]. To the best of our knowledge, no published data is available on geopolymer foams derived from fayalite slag produced in copper smelting plants.

This study aims to investigate the influence of varying alkali concentrations on the cellular structure and the physical properties of lightweight geopolymers synthesized from fayalite slag and metakaolin.

## 2. Methods and materials

### 2.1. Method of analysis

The chemical composition was determined by X-ray fluorescence apparatus, Rigaku Supermini 200, using 30 mm pressed tablets.

The IR spectra were analysed using a Bruker Tensor 37 FTIR spectrometer equipped with an ATR (attenuated total reflectance) accessory. The spectra were recorded in the range of 4000–600 cm⁻¹ with 128 scans per measurement. The measurements were performed under controlled conditions, ensuring consistent spectral acquisition for all samples.

The density was calculated using hydrostatic weighing method. Water absorption was determined by measuring the dry specimens and then soak in water to constant mass. Density and water absorption was calculated using data from 3 specimens per series.

#### 3.1. Materials

The geopolymer precursors in the present study were fayalite slag and metakaolin. The fayalite slag is a fine powdery material – product of flotation process from flash furnace and converters at the copper producing plant. The fayalite slag was oven dried at 80 °C to constant mass to eliminate residual moisture. The average particle size of the fayalite slag is about 20 µm [1], the absolute density was measured – 3.97 g/cm³. The fayalite slag consists of fayalite, magnetite and small amount of pyroxene, determined by powder X-ray diffraction in previous study [21]. Mossbauer spectroscopy revealed that fayalite slag contains also small amount of amorphous Fe²⁺ [22].

Commercial metakaolin, provided by Kaolin EAD, Bulgaria, was used in order to improve the properties of the geopolymer. The wet residue of the metakaolin was 0.40 % on 45 µm sieve. The metakaolin absolute density was measured to 2.26 g/cm³. The chemical analysis of the geopolymer precursors is presented at Table 1
<table><caption>**Table 1.** Chemical compositions of geopolymer precursors – fayalite slag and metakaolin</caption><thead><tr><th></th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>K<sub>2</sub>O</th><th>MgO</th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>Na<sub>2</sub>O</th><th>TiO<sub>2</sub></th><th>Other</th></tr></thead><tbody><tr><td>META-KAOLIN</td><td>53.94</td><td>43.2</td><td>0.15</td><td>0.62</td><td>0.09</td><td>1.14</td><td>0.11</td><td>0.74</td><td>0.01</td></tr><tr><td>FAYALITE SLAG</td><td>27.71</td><td>4.18</td><td>3.21</td><td>1.57</td><td>1.02</td><td>58.07</td><td>0.43</td><td>0.34</td><td>3.47</td></tr></tbody></table>

The activator solutions were prepared a day prior by mixing sodium water glass (molar modulus - SiO₂/Na₂O=2.98), potassium hydroxide on pellets and tap water. Five different alkali solutions were prepared with different alkali concentration. The type of the foaming agent was oxygen releasing, based on hydrogen peroxide.

## 2.3. Geopolymer foam synthesis

In practice, there are no standardized mix design methods for achieving specific properties in foamed concrete. Instead, trial-and-error approaches are commonly employed to develop suitable mixtures, focusing on factors such as water content, foam agent dosage, and binder content. These approaches are generally regarded as adequate for estimating the desired results.

Five series of geopolymer foams were prepared at different concentration of alkali solution and water to solid ratio. Each series was prepared by mixing fayalite slag and metakaolin in weight ratio 5:1 to obtain homogeneous dry mixture. After that activator solution was added and mixture were stirred for 90 second at planetary mixer. After 5 minutes of maturation time, equal amount of foaming agent was added to each series. The amount of oxygen gas release was calculated to about 0,14 dm³, which corresponds to 1.5 g 30% H₂O₂ per 100 g precursor. The mixture was stirred for 60 seconds more and then poured in steel moulds, covered with polyethylene and placed for 24 hours at 80 °C. The specimens were demoulded in 1 day and cured at laboratory conditions. The composition design of the five series is present at Table 2 in molar ratios. The water to solid ratio of series VFM2 was 0.15, where VFM45 was 0.14.

<table><caption>**Table 2.** Composition design of the prepared geopolymer foams in molar ratios</caption><thead><tr><th rowspan="2">Series</th><th colspan="3">Molar ratio</th></tr><tr><th>SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></th><th>Al<sub>2</sub>O<sub>3</sub>/M<sub>2</sub>O</th><th>H<sub>2</sub>O/M<sub>2</sub>O</th></tr></thead><tbody><tr><td>VFM2-1.4</td><td>5.8</td><td>1.40</td><td>12.0</td></tr><tr><td>VFM2-1.2</td><td>5.8</td><td>1.20</td><td>10.3</td></tr><tr><td>VFM2-1</td><td>5.8</td><td>1.00</td><td>8.7</td></tr><tr><td>VFM45-1.2</td><td>5.8</td><td>1.20</td><td>9.6</td></tr><tr><td>VFM45-1</td><td>5.8</td><td>1.00</td><td>8.1</td></tr></tbody></table>

# 3. Results and Discussion

## 3.1. Physical characterisation

The cross-section images of each series are presented at Figure 1. All series contain pores of varying sizes, with macropores observed as a result of pore coalescence, or bigger caverns due to entrapped air during moulding. Clearly, series VFM2-1.4 was characterised by highest porosity. It is visible that the porosity formation decreased with the decreasing the Al₂O₃/M₂O. Thus, high alkalinity affected the foaming process and hinder the pore formation. This correlation was observed at both solid to water ratio tested. The reason for hindering effect of foaming agent with the high alkalinity stays unknown.

However, the lowest density of 1.09 g/cm³ was achieved at Al₂O₃/M₂O equal to 1.40. The increase of the alkalinity lead to increase of the density, hence reduction of the porosity. At fixed alkalinity (Al₂O₃/M₂O) the decrease of the water to solid ratio led to increase in the density.

The density of non-foamed geopolymer based on fayalite slag and metakaolin was 2.38 g/cm³, at absolute density of 3.59 g/cm³ and calculated relative (total) porosity of 34% [8]. Compared to the density of series VFM2-1.4, the apparent density decreased more than twice. The calculated relative porosity of series VFM2-1.4 is about 70%. Similar relative porosity was obtained with geopolymer foams: based on fly ash and rice husk ash using aluminum powder [23], or natural soil [24]. Higher relative porosity of geopolymer foamed materials were also reported: 83% with coconut ash and alumina slag [25], 83% - fly ash [17]. Despite the high relative porosity of the obtained geopolymer foams based on fayalite slag and metakaolin, the overall density remained relatively high due to the significant absolute density of the mineral phases present in the slag - fayalite (4.39 g/cm³) and magnetite (5.17 g/cm³).

The measured water absorption corelates with the change in densities: water absorption increased by the decrease of the density. Thus, highest water absorption of 26% was measured for series VFM2-1.4. The high water absorption suggest well developed interconnected open porosity.

<table><caption>**Table 3.** Physical properties (apparent density and water absorption) of the prepared geopolymer foamed material based on fayalite slag and metakaolin</caption><thead><tr><th rowspan="2">Series</th><th colspan="2">Physical properties</th></tr><tr><th>Density, g/cm³</th><th>Water absorption, %</th></tr></thead><tbody><tr><td>VFM2-1.4</td><td>1.09</td><td>26.0 ± 1.1</td></tr><tr><td>VFM2-1.2</td><td>1.51</td><td>16.2 ± 0.5</td></tr><tr><td>VFM2-1</td><td>2.07</td><td>14.7 ± 0.1</td></tr><tr><td>VFM45-1.2</td><td>1.84</td><td>13.4 ± 0.2</td></tr><tr><td>VFM45-1</td><td>2.11</td><td>13.1 ± 0.1</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Cross images of prepared series lightweight cellular geopolymers based on fayalite slag and metakaolin. It is observed reduction in porosity at increasing the alkalinity</figcaption></figure>
### 3.4. Microstructural characterisation

The microstructure was analysed by FTIR and all spectra are presented at Figure 2. The main band at about 1009-1015 cm⁻¹ corresponds to the Si-O-T (T = Si or Al) asymmetric stretching vibration of the aluminosilicate geopolymer structure. The peak is key indicator of the polymerization degree in the geopolymer and is associated with the formation of the geopolymer gel. The position of the peak generally shifts depending of Si/Al ratio of the geopolymer gel. Since the ratio Si/Al is fixed at 5.8 band shift corelates with the reactivity of the silica and alumina. The band shift to lower wavenumbers with the decrease of the Al₂O₃/M₂O ratio indicating more alumina rich structure. The decrease of the water to solid ratio (series VFM45) led to band shift to higher wavenumbers showing less incorporated alumina in the structure. This could be due to lower dissolution of the metakaolin at lower water to solid ratio.

<figure><img src="image_3.png" /><figcaption>Figure 2. FTIR spectra of the prepared geopolymer foams</figcaption></figure>

### 3.5. Potential application

The lightweight cellular geopolymers synthesized from fayalite slag and metakaolin exhibit a unique combination of low density, high thermal stability, and well-developed porosity, making them suitable for a variety of advanced applications in construction and industry. One of the most promising applications is in thermal insulation materials for buildings or industrial furnaces. The high-temperature resistance (up to 1150 °C), combined with a controlled porous structure, allows these geopolymers to function as effective thermal barriers, reducing energy consumption in heating and cooling systems. Additionally, their fire-resistant properties make them attractive for passive fire protection in critical infrastructure, such as tunnels, high-rise buildings, and industrial plants. Compared to conventional insulating materials, geopolymer foams derived from fayalite slag offer an environmentally sustainable alternative by utilizing industrial waste while maintaining excellent mechanical and thermal properties.

Beyond construction, the lightweight geopolymer foams have potential applications in sound absorption and filtration technologies. Their interconnected porosity can be optimized for noise reduction panels, especially in urban environments, transportation infrastructure, and industrial facilities where effective sound insulation is required. The porous network within the geopolymer matrix can be designed to act as an efficient medium for adsorbing pollutants, heavy metals, or particulate matter. With further research, these materials could also be explored for energy storage applications, such as in thermal energy storage or as solid electrolytes in next-generation battery technologies. The utilization of fayalite slag in these applications not only addresses waste management challenges in the metallurgical industry but also

contributes to the development of innovative, sustainable materials with multifunctional properties.

### 4. Conclusion

This study demonstrated the feasibility of producing lightweight cellular geopolymers from fayalite slag and metakaolin by varying alkali concentration and water-to-solid ratio. The results revealed that the alkalinity of the activator solution significantly influenced the porosity, density, and overall physical properties of the geopolymer foams. Higher alkalinity led to increased density and reduced porosity, while lower alkalinity favored the formation of a more interconnected pore network. The lowest density of 1.09 g/cm³ and highest relative porosity of approximately 70% were achieved at an Al₂O₃/M₂O ratio of 1.40.

The concept of lightweight heavy geopolymer foam may seem contradictory at first glance, yet it accurately reflects the unique characteristics of the material. Traditional lightweight materials are defined by their low density and high porosity, while heavy materials typically possess high density and mechanical strength. In the case of fayalite slag-based geopolymer foams, this paradox is resolved by the interplay between the intrinsic high density of the fayalite and magnetite phases and the engineered cellular structure formed during the foaming process. The substantial absolute density of fayalite (4.39 g/cm³) and magnetite (5.17 g/cm³) ensures that even highly porous specimens retain a relatively high overall density compared to conventional geopolymer foams made from materials like fly ash or perlite.

These findings highlight the potential of fayalite slag based geopolymer foams as sustainable, high-performance materials for insulation, fireproofing, and lightweight structural components. Further research could optimize pore structure control, improve mechanical performance, and expand their applicability in other fields such as energy storage and environmental remediation.

### 6. Acknowledgement

The results in this work have been achieved in fulfilment of a project financed by the National Science Fund of Bulgaria under contract No. "КП-06-Н77/9" from 4.12.2023.

### 7. Literature

1. Manolova, E. *Aurubis iron-silicate fines: universal sustainable construction material: a state-of-the-art review.* in *IOP Conference Series: Materials Science and Engineering*. 2020. IOP Publishing.

2. Onisei, S., et al., *Early age microstructural transformations of an inorganic polymer made of fayalite slag*. Journal of the American Ceramic Society, 2015. **98**(7): p. 2269-2277.

3. Komnitsas, K., D. Zaharaki, V. Perdikatsis, *Geopolymerisation of low calcium ferronickel slags*. Journal of Materials Science, 2007. **42**: p. 3073-3082.

4. Adediran, A., *Alkali activation of fayalite slag*. 2017, A. Adediran.

5. Mihailova, I., I. Uzunov, and D. Mehandjiev, *Waste Copper Slag/Aluminium Dross-Based Geopolymer*. Journal of Chemical Technology and Metallurgy, 2021. **56**(3): p. 653-659.

6. Nikolov, A. *Alkali-activated geopolymers based on iron-rich slag from copper industry*. in *IOP Conference Series: Materials Science and Engineering*. 2020. IOP Publishing.

7. Nikolov, A., *Characterization of geopolymer based on fayalite waste and metakaolin with standard consistency*. Comptes rendus de l'Académie bulgare des Sciences, 2021. **74**(10).

8. Nikolov, A. and A. Karamanov, *Thermal properties of geopolymer based on fayalite waste from copper production and metakaolin*. Materials, 2022. **15**(7): p. 2666.

9. Nikolov, A., *Geopolymers based on Bulgarian raw materials-preliminary studies*. Machines. Technologies. Materials., 2019. **13**(2): p. 97-100.
10. Николов, А., *Геополимери на база отпадъчен фаялит и метакаолин*. Седемнадесета Национална Младежка Научно-Практическа Конференция 2020

11. Novais, R.M., R.C. Pullar, and J.A. Labrincha, *Geopolymer foams: An overview of recent advancements*. Progress in Materials Science, 2020. **109**: p. 100621.

12. Kočí, V. and R. Černý, *Directly foamed geopolymers: A review of recent studies*. Cement and Concrete Composites, 2022. **130**: p. 104530.

13. Hajimohammadi, A., et al., *Regulating the chemical foaming reaction to control the porosity of geopolymer foams*. Materials & Design, 2017. **120**: p. 255-265.

14. Xu, F., et al., *Pore structure analysis and properties evaluations of fly ash-based geopolymer foams by chemical foaming method*. Ceramics International, 2018. **44**(16): p. 19989-19997.

15. Zhang, Z., et al., *Geopolymer foam concrete: An emerging material for sustainable construction*. Construction and Building Materials, 2014. **56**: p. 113-127.

16. Vaou, V. and D. Panias, *Thermal insulating foamy geopolymers from perlite*. Minerals Engineering, 2010. **23**(14): p. 1146-1151.

17. Nikolov, A. and B. Barbov, *Lightweight geopolymer based on fly ash*. Review of the Bulgarian Geological Society, 2018. **79**(3): p. 23-24.

18. Suksiripattanapong, C., et al., *Properties of cellular lightweight high calcium bottom ash-portland cement geopolymer mortar*. Case Studies in Construction Materials, 2020. **12**: p. e00337.

19. Shakouri, S., Ö. Bayer, and S.T. Erdoğan, *Development of silica fume-based geopolymer foams*. Construction and Building Materials, 2020. **260**: p. 120442.

20. Shen, S., et al., *Synthesis of industrial solid wastes based geopolymer foams for building energy conservation: Effects of metallic aluminium and reclaimed materials*. Construction and Building Materials, 2022. **328**: p. 127083.

21. Nikolov, A., *Alkali and acid activated geopolymers based on iron-silicate fines-by-product from copper industry*. International Scientific Journal" Machines. Technologies. Materials", 2020. **14**(1): p. 37-39.

22. Nikolov, A., et al., *Characterization of novel geopolymer based on acid-activated fayalite slag from local copper industry*. Bulgarian Chemical Communications, 2018. **50**(F): p. 54-61.

23. Ziegler, D., et al., *Environmentally-friendly dense and porous geopolymers using fly ash and rice husk ash as raw materials*. Materials, 2016. **9**(6): p. 466.

24. Zaidi, S.F.A., et al., *Synthesis & characterization of natural soil based inorganic polymer foam for thermal insulations*. Construction and Building Materials, 2017. **157**: p. 994-1000.

25. Hassan, H.S., et al., *Fabrication and characterization of thermally-insulating coconut ash-based geopolymer foam*. Waste Management, 2018. **80**: p. 235-240.