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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)11978-0
10.1016/j.heliyon.2024.e35947
e35947
Research Article
Design and characterization of geopolymer foams reinforced with Miscanthus x giganteus fibres
Walbrück Katharina walbrueck.katharina@gmail.com
a⁎
Witzleben Steffen a
Stephan Dietmar b
a Department of Natural Sciences, Bonn-Rhine-Sieg University of Applied Sciences, von-Liebig-Str. 20, 53359, Rheinbach, Germany
b Building Materials and Construction Chemistry, Department of Civil Engineering, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355, Berlin, Germany
⁎ Corresponding author. walbrueck.katharina@gmail.com
08 8 2024
30 8 2024
08 8 2024
10 16 e359478 4 2024
5 8 2024
6 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
This paper presents the results of the optimisation and characterization of Miscanthus fibre reinforced geopolymer foams based on fly ash and represents an important step forward in the development of a sustainable and environmentally friendly insulation material. Miscanthus belongs to a promising group of renewable raw materials with favourable thermal insulation properties. Design of experiment (DoE) were used to optimize the thermal conductivity and compressive strength of Miscanthus x giganteus reinforced geopolymer foams. In addition, the samples was analyzed using X-ray diffraction (XRD), Field emission scanning electron microscopy (SEM) and Fourier-Transform Infrared Spectroscopy (FTIR). Mixtures with a low thermal conductivity of 0.056 W (m K)−1 and a porosity of 79 vol% achieved a compressive strength of only 0.02 MPa. In comparison, mixtures with a thermal conductivity of 0.087 W (m K)−1 and a porosity of 58 vol% achieved a compressive strength of 0.45 MPa. Based on the determined parameters especially due to the low compressive strength, an application as cavity insulation or insulation between rafters is possible.

Keywords

Miscanthus x giganteus
Geopolymer foams
Thermal conductivity
Compressive strength
Porosity
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pmc1 Introduction

The demand for sustainable and ecological building and insulation materials is growing, especially due to the advancing climate change and the depletion of fossil resources. Thermal insulation materials are of particular interest for reducing the energy consumption of buildings. Currently, the most commonly used materials for thermal insulation include mineral wool, polystyrene, and polyurethane. However, the disadvantage of these products is their high global warming potential (GWP) compared to renewable materials. Thus, sustainable insulation materials based on renewable resources represents a possible alternative to conventional thermal insulation materials and is an essential step toward reducing CO2 emissions [[1], [2], [3], [4]]. Nowadays, bio-based materials like wood chips, cork, hemp, flax, and cellulose are mainly applied in thermal insulation [5].

Furthermore, fast-growing low-input grasses such as Miscanthus are a very promising group of renewable raw materials. Miscanthus is a perennial, rhizome-forming C4 grass (C4 photosynthetic pathway, fixes CO2 into the four-carbon molecule malate) and, due to its high parenchyma content and the associated good thermal insulation properties, of particular interest for use as insulation material. As a low-input plant, Miscanthus delivers high biomass yields with low nutrient inputs. Moreover, Miscanthus has, due to its C4 photosynthetic pathway, increased photosynthetic activity and can permanently capture 10–36 tons of CO2 per hectare and year during growth [[5], [6], [7], [8], [9]]. There are appr. 17 species of the genus Miscanthus, the most common species in Europe are Miscanthus sinensis, Miscanthus sacchariflorus, Miscanthus floridulus, Miscanthus robustus and Miscanthus x giganteus [[10], [11], [12]]. According to Pude et al. Miscanthus x giganteus is the most suitable genotype for use in lightweight concrete. Their study investigates the suitability of different Miscanthus genotypes for the application in lightweight concrete to determine the influence factors on compressive strength. The highest values for the compressive strength were obtained for the lightweight concrete samples with the genotype Miscanthus x giganteus. Compared to the other investigated genotypes, Miscanthus x giganteus has a higher cellulose content, a thicker outer ring and exhibits the highest velocity and the maximum height of water movement [11]. Chen et al. investigated ultra-lightweight Miscanthus concrete (ULMC) for use as acoustic and thermal insulation. The ULMC with 30 vol% Miscanthus fibres achieves a thermal conductivity of 0.09 W (m K)−1 and a high acoustic absorption coefficient of 0.9 at low frequencies. Furthermore, their results show that the compressive strength is decreased, and the acoustic absorption and the thermal conductivity are increased by increasing the percentage of Miscanthus fibres [13].

To reduce CO2 emissions, it is also necessary to find alternatives in the field of binders because the cement industry releases large quantities of CO2. The production of cement emits ∼600 kg of CO2 per ton of cement [14]. One potential alternative to cement-based lightweight concrete is foamed geopolymers. Geopolymers are composed of an aluminosilicate source and an alkaline activator. Typical aluminosilicate sources are fly ash, metakaolin, or granulated blast-furnace slag. The alkaline activator is usually an alkali metal silicate or hydroxide. Foamed geopolymers can be produced by mechanical foaming, chemical foaming, or by forming a syntactic foam. Mechanical foaming involves the introduction of gas either during the mixing process or by mixing in a pre-made foam. Whereas in chemical foams, the foam is formed as a result of gas release following the addition of a blowing agent like hydrogen peroxide or aluminium powder. In addition to chemical and mechanical foaming, porous systems can also be produced by embedding hollow spheres in a matrix, so-called syntactic foams [[15], [16], [17], [18], [19], [20], [21]].

For optimizing the properties of geopolymer foams, recent innovations have focused on geopolymer composites espacially fibre reinforcement [[22], [23], [24], [25], [26], [27]]. Masi et al. investigated fly ash-based geopolymer foams reinforced with PVA (poly (vinyl alcohol)) and basalt fibres and chemically foamed with hydrogen peroxide. The samples with PVA fibre achieved a thermal conductivity of 0.30 W (m K)−1 and the samples with basalt fibres achieved a thermal conductivity of 0.38 W (m K)−1, while the not foamed geopolymers exhibited a thermal conductivity of 0.53 W (m K)−1 [22]. Galzerano et al. studied lightweight geopolymers reinforced with hemp fibre grids. The metakaolin-based geopolymers were foamed by silicon metal powder and a mixture of vegetable surfactants. The chemical treatment of the hemp fibre grid due to the alkaline pH value of the geopolymer system leads to strong interfacial bonding between the geopolymer and the fibres, improving the mechanical properties [25]. Kurek et al. investigate foamed geopolymers based on metakalin and modified with perlite and cellulose fibres. The use of perlite results in an decrese in thermal conducitivity and density, whereas the use of cellulose increases the value of thermal conducitivtiy [28]. Korniejenko et al. studied foamed geopolymer based on fly ash and metakaolin reinforced with 1 wt% flax fibres and foamed with hydrogen peroxide. The flax fibres were used to improve the mechanical properties, especially the higher flexural strength is due to the addition of the flax fibres [29].

The combination of geopolymer foams and miscanthus fibres, a promising renewable raw material, makes it possible to produce sustainable and ecological insulation materials with low thermal conductivity, low global warming potential and low flammability. This study is based on the preliminary tests and earlier studies by Walbrück et al. [24,30] and describes the optimisation of the mix design of Miscanthus fibre-reinforced geopolymer foams. As the results of Walbrück et al. have shown, the thermal conductivity and compressive strength of this material is influenced by fibre size, fibre content and foaming agent content [24,30]. Therefore, in this study further investigations were performed to achieve an optimal mix design with low thermal conductivity and high compressive strength at the same time.

2 Materials and methods

2.1 Materials

A class F fly ash with the commercial name EFA-Füller® HP (BauMineral, Germany) was used (Fig. 1(a)) for this study. The Miscanthus x giganteus fibres (Fig. 1 (b)) used in this study were cultivated at the field lab Campus Klein-Altendorf of the University of Bonn (Germany). Aqueous sodium silicate solution (SiO2 = 28.50 wt%, Na2O = 8.29 wt%) purchased from Carl Roth GmbH + Co. KG (Germany) was used as activator. Sodium dodecyl sulfate (SDS), from Carl Roth, was used as a foaming agent, while 1 wt% fumed silica nanoparticles Aerosil® 90 (Evonik Industries AG, Germany) were used to stabilize the foam.Fig. 1 (a) SEM image of fly ash (bulk density 1.16 t/m³) (b) Miscanthus x giganteus fibres (bulk density 0.14 t/m³).

Fig. 1

2.2 Mix design and mixing process

Design of experiment (DoE) was used to determine the optimal mixture with minimum thermal conductivity and maximum compressive strength. A full factorial design with three factors on two levels (2³) and three replicates, plus one center point, were considered in the mix design. The center point is repeated nine times and thus a total of 33 experiments were performed. The nine experiments without their replicates are summarized in Table 1. The factors and their levels, as shown in Table 2, were chosen based on preliminary tests and previous studies [24,30]. The results were evaluated by analysis of variances (ANOVA) and Pareto diagram using Minitab 18 (Minitab, Inc., USA).Table 1 Mix design for Miscanthus fibre-reinforced geopolymer foams (wt% based on the total solids content).

Table 1Sample	Fibre content	Fibre size	Foaming agent content	Fly ash content	Aerosil® 90 content	
[wt%]	[μm]	[wt%]	[wt%]	[wt%]	
M1	35,00	160	0.25	63,75	1,00	
M2	35,00	160	0.35	63,65	1,00	
M3	35,00	250	0.25	63,75	1,00	
M4	35,00	250	0.35	63,65	1,00	
M5 (Center Point)	40,00	200	0.30	58,70	1,00	
M6	45,00	160	0.25	53,75	1,00	
M7	45,00	160	0.35	53,65	1,00	
M8	45,00	250	0.25	53,75	1,00	
M9	45,00	250	0.35	53,65	1,00	

Table 2 Considered factors for the DoE.

Table 2Factor	Lower Level	Center Point	Higher Level	
Fibre content [wt%]	35	40	45	
Fibre size [μm]	160	200	250	
Foaming agent content [wt%]	0.25	0.30	0.35	

The geopolymer foams were produced using the mixed-foaming method, in which the foam is generated during the mixing process by adding a surfactant to the slurry [15]. A dry mix of the solid components was activated by the alkaline solution (64 wt% sodium silicate and 36 wt% water) and mixed at high speed (speed position 2 of the mixer) for 5 min using a Hobart N50 mortar mixer. The mixture was poured into steel molds and cured at 50 °C and ambient pressure for 48 h. Afterwards, the samples were demolded and cured at room temperature until 28 d.

2.3 Materials characterization

2.3.1 Compressive strength

The universal strength testing apparatus Z010 from Zwick/Roell was used for the compressive strength measurements. Samples with a size of 60 × 60 × 40 mm³ were measured at a testing speed of 2 mm min−1 and an average of 3 samples for each mixture. The compressive strength was determined based on DIN EN 29469 at a compression of 10 % [31].

2.3.2 Thermal conductivity

The heat flow meter apparatus HFM 446 Lambda Small from Netzsch with two external thermocouples was used to determined the thermal conductivity of the foamed geopolymers (140 × 140 × 40 mm³). The two external thermocouples were placed on the front and back sides of the samples. The thermal conductivity of each mix design was measured at 10 °C on 3 samples with six measurements, respectively.

2.3.3 X-ray micro-computed tomography

The X-ray micro-computed tomography SkyScan 1275 from Bruker with a micro focus X-ray tube (100 kV and 100 μA) and a flat-panel detector was used to measure the porosity. The cylindrical samples (ø 20 mm) were scanned with a rotation step of 0.5 and a resolution of 14 μm over a 360° interval.

2.3.4 X-ray diffraction

A D2 Phaser X-ray diffractometer from Bruker AXS with a Cu Kα radiation source operating at 30 kV and 10 mA was used to identify the mineral phase composition of the geopolymer foams. The samples were measured with a step size of 0.01° and a scan time of 2.0 s step−1 in the 2θ range of 10°–65°. For the determination the crystalline and amorphous content via Rietveld refinement the samples were prepared by grinding and mixing 600 mg sample (<63 μm) with 20 mg of the internal standard Lanthanum hexaboride (Sigma-Aldrich).

2.3.5 Fourier-Transform Infrared Spectroscopy

The functional groups of foamed geopolymer concrete were determined using Fourier-Transform Infrared Spectroscopy (FTIR). The FTIR spectra were recorded on a FT/IR 410 spectrometer from Jasco between 450 and 4000 cm−1 with a resolution of 4 cm−1 and 128 scans. The specimens were prepared by mixing 20 mg of the ground sample in 1 g of potassium bromide (KBr).

2.3.6 Scanning electron microscope

The field emission scanning electron microscope JSM-7200 F from JEOL was used to investigated the microstructure of the foamed geopolymers. The SEM observations were carried out on small pieces of the samples mounted on a bulk sample holder.

3 Results and discussion

3.1 Compressive strength

Fig. 2 (a) shows the results of the compressive strength measurements. Sample M1 exhibits with 0.448 ± 0.052 MPa, the highest, and sample M9 with 0.020 ± 0.005 MPa, the lowest compressive strength. The Pareto plot of the standardized effects in Fig. 2 (b) compares the significance of each effect. In addition to the three factors, the 2-factor and 3-factor interactions were also considered. The significant factors exceed the orange reference line (α = 0.05). For the response variable, compressive strength, the foaming agent content, fibre size and fibre content are significant, whereas no 2-factor or 3-factor interactions can be detected. The increased foaming agent content from 0.25 wt% to 0.35 wt% leads to a significant decrease in compressive strength. Furthermore, the increase of fibre size from 160 μm to 250 μm and the increased fibre content from 35 wt% to 45 wt% resulted, due to a less dense packing and a higher amount of parenchyma, in a decrease in compressive strength. Due to the higher fibre content and the larger fibre size, more of the porous Miscanthus parenchyma is introduced into the sample and therefore the compressive strength is reduced. However, adding fibres can lead to both an increase and decrease in compressive strength. According to the literature a decrease in compressive strength is observed when the increase in porosity overshadows the ability of the fibres to prevent cracks. If, on the other hand, the ability of the fibres to prevent cracks dominates, the compressive strength is increased [[32], [33], [34]]. Assaedi et al. also observed an increase and decrease in strength with different amounts of cotton and flax. The flax fibres in particular show higher strengths, which Assaedi et al. attribute to the properties of flax fibres, which are able to resist greater bending and breaking forces [35]. The results of Alomayri et al. show a similar behavior to the miscanthus fibre reinforced geopolymer foams in this study. The investigated cotton fibre reinforced geopolymers initially show an increase in compressive strength. However, as soon as the fibres agglomerate and voids are formed, the compressive strength decreases [36].Fig. 2 (a) Compressive strength of Miscanthus fibre-reinforced geopolymer foams vs. Miscanthus-fibre content wt% (A), Miscanthus-fibre size μm (B) and foaming agent content wt% (C). (b) Pareto plot of the standardized effects for compressive strength (α = 0.05).

Fig. 2

3.2 Thermal conductivity and porosity

Fig. 3 (a) presents thermal conductivity results and Fig. 3 (b) exhibit the Pareto diagram with the significant effects. For the response variable, thermal conductivity, fibre content, and foaming agent content are significant, whereas no effect of the fibre size and no 2-factor or 3-factor interactions are detected. The lowest thermal conductivity was achieved (0.056 W (m K)−1) in the mixture M9 with 45 wt% Miscanthus fibres of the size 250 μm and with a foaming agent content of 0.35 wt%. A similar thermal conductivity (0.059 W (m K)−1) was obtained in the mixture M7 with the same fibre content and foaming agent content and fibre size of 160 μm. An increase in fibre content and foaming agent content leads to a lower thermal conductivity and confirms the results of the previous study [24]. Furthermore, Fig. 4 presents the results of the porosity. The porosity is increased by increasing the fibre content and foaming agent content. Sample M9, with the lowest thermal conductivity, also has the highest porosity. Besides the foaming agent, also the Miscanthus fibres introduce numerous pores into the sample, mainly due to the porous parenchyma [13,37]. The research by Chen et al. on lightweight concrete with miscanthus fibres confirms these results [13]. According to Assaedi et al., the hydrophilic nature of the fibres also leads to the formation of pores at the interfacial region between the fibre and the geopolymer matrix. In their study, they investigated geopolymer composites with cotton and flax fibres. They observed an increase in porosity after the addition of the natural fibres [35].Fig. 3 (a) Thermal conductivity of Miscanthus fibre-reinforced geopolymer foams vs. fibre content wt% (A), fibre size μm (B) and foaming agent content wt% (C). (b) Pareto plot of the standardized effects for thermal conductivity (α = 0.05).

Fig. 3

Fig. 4 Porosity of Miscanthus fibre-reinforced geopolymer foams determined by X-ray micro-computed tomography vs. fibre content wt% (A), fibre size μm (B) and foaming agent content wt% (C).

Fig. 4

3.3 X-ray diffraction

The XRD pattern of the geopolymer foam is shown in Fig. 5. The main components of the geopolymer foams are quartz (COD 9005020), mullite (COD 7105575) and hematite (COD 1011267) [24,38]. The mineralogical composition in Fig. 6 also exhibit that the geopolymer foams are about 60 wt% amorphous. The amorphous content is influenced by the fibre content. A higher fibre content leads to an increase in the amorphous phase. Sample M1 with a fibre content of 35 wt%, a fibre size of 160 μm and a foaming agent content of 0.25 wt% has an amorphous content of 56 wt%, whereas sample M6 (fibre content 45 wt%, fibre size 160 μm, foaming agent content 0.25 wt%) has an amorphous content of 58 wt%. However, sample M5, with a fibre content of 40 wt%, has a lower amorphous content than those with a fibre content of 35 wt%. Compared with the samples with a fibre content of 45 wt%, the amorphous phase increases. Furthermore, the amorphous content decreases slightly with increasing foaming agent content. An increase of the foaming agent from 0.25 wt% to 0.35 wt% leads to a decrease in the amorphous phase from 56 wt% to 52 wt% for samples M1 and M2.Fig. 5 XRD Rietveld refinement for sample M1. (quartz (COD 9005020), mullite (COD 7105575) and hematite (COD 1011267)).

Fig. 5

Fig. 6 Mineralogical composition of the samples determined by Rietveld refinement.

Fig. 6

3.4 Fourier-Transform Infrared Spectroscopy

Fig. 7 displays the FTIR spectra of the foamed geopolymer concretes and Table 3 summarizes the assignment of characteristic FTIR signals according to literature data [[38], [39], [40], [41], [42], [43], [44], [45], [46], [47]]. The FTIR spectra exhibit main absorption bands at 3423, 2919, 2851, 1652, 1043, 776 and 488 cm−1. The bands at ∼3423 cm−1 and ∼1652 cm−1 correspond to the O–H and H–O–H stretching and bending vibration, attributed to the weakly bonded water and to the O–H stretching from the cellulose and lignin structure of the Miscanthus fibres [38,39,47]. Moreover, the bands at ∼2919 cm−1 and ∼2851 cm−1 represent the stretching vibration of C–H, which is attributed to methyl and methylene groups and is related to the Miscanthus fibres [40]. The peak centered around ∼1043 cm−1 is assigned to the Si–O-T (T = Si or Al) asymmetric stretch of the tetrahedral SiO4 or AlO4 bonds and is characteristic of the geopolymerization [[41], [42], [43]]. Furthermore, the band at ∼776 cm−1 can be assigned to the Si–O–Si symmetric stretching vibration and is related to the presence of quartz [[43], [44], [45], [46]]. The peak around ∼488 cm−1 arises due to the Si–O–Si asymmetric bending of SiO4 tetrahedral [43,44].Fig. 7 FTIR spectra of Miscanthus fibre-reinforced geopolymer foams.

Fig. 7

Table 3 Characteristics of FTIR band for Miscanthus fibre-reinforced geopolymer foams [[38], [39], [40], [41], [42], [43], [44], [45], [46], [47]].

Table 3Wavenumber	Functional group	Assignment	
[cm−1]	
3423 ± 3	O–H, H–O–H	Stretching [38,39,47]	
2919 ± 2	C–H	Stretching [40]	
2851 ± 1	C–H	Stretching [40]	
1652 ± 1	H–O–H	Bending [38,39,47]	
1043 ± 12	Si–O-T (T = Si, Al)	Asymmetric Stretching [[41], [42], [43]]	
776 ± 2	Si–O–Si	Symmetric Stretching [[43], [44], [45], [46]]	
488 ± 4	Si–O–Si	Asymmetric Bending [43,44]	

Comparing the FTIR spectra of the geopolymers in Fig. 7, there are no notable differences between the mixtures. Here, only the band of the asymmetric Si–O-T (T = Si or Al) stretching at 1043 cm−1 should be mentioned. By increasing the foaming agent content, the band s shifting to lower wavenumbers. According to Wang et al. and Rees et al. the shift to lower wavenumbers is related to the incorporation of Si and Al into the geopolymer network [46,48]. Especially, if more Al is incorporated into the network, the band shifts due to the lower binding strength of Al–O to lower wavenumbers [46,48]. Thus the proportion of Si–O–Al to total Si–O-T is higher in sample M2 (1032 cm−1) with 0.35 wt% foaming agent than in sample M1 (1054 cm−1) with 0.25 wt% foaming agent.

3.5 Scanning electron microscope

Fig. 8 show the microstructure of mixture M1 and M9. Both SEM images reveal a porous structure, embedding fibres and unreacted fly ash particles in the geopolymer matrix. The geopolymer covers nearly the whole fibre surface, which indicates a good interaction between the fibres and the matrix [38]. Furthermore, sample M9 exhibit between the fibres a honeycomb-like structure with lamellae widths of 7–19 μm. This supports the findings of the X-ray micro-computed tomography. When the volume of gas in the foam is more than 75 %, the bubbles necessarily deform each other and a polyhedral foam is formed [49]. Sample M9 exhibit a porosity of 79.2 %, which indicates the formation of a polyhedral foam. In contrast, sample M1 has a porosity of 57.5 % and also a denser structure can be observed on the SEM images. Furthermore, these results also support the results of the compressive strength and thermal conductivity. Sample M1 exhibit due to the denser structure a higher compressive strength and thermal conductivity compared to sample M9.Fig. 8 SEM images with different magnifications (a)–(b) sample M1 (c)–(d) sample M9.

Fig. 8

4 Conclusions

In this study, the fibre content, fibre size and foaming agent content were varied to analyze their behavior on the mechanical and thermal properties of fly-ash-based geopolymer foams reinforced with Miscanthus x giganteus fibres. The foaming agent content, fibre size and fibre content significantly affect the compressive strength, whereas no 2-factor or 3-factor interactions can be detected. The optimal conditions for maximum compressive strength are 35 wt% fibres with a fibre size of 160 μm and 0.25 wt% foaming agent. Thereby a maximum compressive strength of 0.448 MPa could achieved. In contrast, only the fibre content and foaming agent content have a significant effect on thermal conductivity. The optimal conditions for minimal thermal conductivity are 45 wt% fibres and 0.35 wt% foaming agent. So a minimal thermal conductivity of 0.056 W (m K)-1 could achieved. The limits of the developed insulation material are its low compressive strength. According to DIN EN 4108–10 only an application as cavity insulation or insulation between rafters is possible [50]. Further limitations are the use of the natural product Miscanthus x giganteus, in particular the availability and homogeneity of the Miscanthus fibres in terms of composition, fibre lengths, etc. must be considered and taken into account in the development of the mixture.

Funding

This work was supported by the 10.13039/501100008530 European Regional Development Fund [Grand No. EFRE 0500035 ] and the German Federal Ministry of Education and Research [Grand No. 13FH158IN6 ].

CRediT authorship contribution statement

Katharina Walbrück: Writing – review & editing, Writing – original draft, Validation, Conceptualization. Steffen Witzleben: Writing – review & editing, Supervision, Project administration, Funding acquisition. Dietmar Stephan: Writing – review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank Ralf Pude, Georg Völkering and Lüders Moll (INRES, University of Bonn, Germany) for providing the Miscanthus biomass.
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