Interfacial bonding properties of the eco-friendly geopolymer-wood composites: influences of embedded wood depth, wood surface roughness, and moisture conditions

Abstract

With environmental friendliness, the sustainability of natural resources, and sustainable utilization of geopolymers considered in the building industry, interfacial bonding strength of geopolymer-wood composites was enhanced environmental-friendly by increasing embedded depth of wood, sanding wood surface, and controlling moisture conditions during curing process. Beech and spruce were compared as different wood species. Pullout test was modified by using a wood veneer to determine the interfacial bonding strength of geopolymer-wood composites. It was found that geopolymer exhibited higher interfacial bonding strength with spruce rather than with beech. Interfacial bonding strength increased with an increase in the embedded depth of the wood veneer, but reaching a plateau when the depth exceeded 25 mm. A higher interfacial bonding strength caused by strong mechanical interlocking at the interface was successfully created by improving the wood surface roughness via sanding with 60-grit sandpaper. Interfacial bonding strength was higher by curing under wet conditions comparing to dry conditions. However, the influence of initial wood moisture content on the interfacial bonding strength can be ignored. The results of this study serve as the basis for better preparation of geopolymer-wood composites and a diverse application of environmentally friendly building materials.

Graphical abstract

Introduction

With growing concerns about the finite resources and the environmental impact of the building industry, synthetic reinforcements are being substituted by the lignocellulose fibers due to their sustainable, low cost, non-toxic, lightweight, and biodegradable properties [1,2,3,4]. Inorganic bonded wood composites have been widely used in various building material applications [5]. Different from traditional wood-based composites made with formaldehyde or isocyanate-containing resins, inorganic composites do not release volatile toxic substances and have become attractive building materials, especially for the interior uses [6]. Commercial products, such as wood-wool cement board (WWCB), wood cement-bonded board (WCB), and cement-bonded particleboard distinguish themselves from wood-based materials by high durability, dimensional stability, toughness, strength, rapid and low-cost production, good acoustic and thermal insulation properties, and a great fire resistance [5,6,7,8,9].

The widely used Ordinary Portland Cement (OPC) contributes 5–8% of CO2 to the global emission [10]. Geopolymers, the potentially low impact alternative to the OPC, cause less CO2 emissions and offer considerable economic benefits, while at the same time possessing the advantages of cement-based materials [11,12,13,14]. Geopolymers can be derived from a wide range of little-used secondary resources, such as mineral soils (metakaolin [15], red mud [16, 17], and clay waste [18]), industrial wastes (fly ash [19, 20], bottom ash [21], aluminum-waste [22], quarry dust waste [23], and furnace slag [24,25,26]), and biomass ashes (wood [27], coconut [28], and rice husk [29]).

Geopolymers are brittle materials that have low toughness and poor crack resistance properties. To prevent the crack generation, restrain crack-propagations, and enhance the mechanical properties of the geopolymer, fibers from metals, polymers, minerals, animals, and natural plants have been widely incorporated as the reinforcements into the geopolymer-based matrices [30, 31]. Specifically, the composites with a geopolymer matrix were successfully reinforced with a range of lignocellulosic fibers, such as bamboo [32], luffa cylindrical fiber [33], cotton, and flax [34], and they can also be used as environmentally friendly inorganic binders and flame-resistant coatings for wood-based composites [35,36,37,38].

A weak interfacial bonding was detected between geopolymer and high-content wood in our previous works [39]. Mechanical characterization of the interface is an important part of the composite research, since the interface between the two components (geopolymer and wood) plays a key role in the performance of the geopolymer-wood composites [37]. Interface properties of composites are routinely measured by various tests: the fiber pullout test, the microbond test, fiber fragmentation test, microtension test, microcompression test, fiber push-out test, the microdebond test, and the microindentation [40,41,42]. Among them, the fiber pullout test, especially the single fiber pullout test, is the most popular, direct, and reliable method for developed interfacial bonding strength (interfacial shear strength) [43]. Nevertheless, it is difficult to evaluate and represent the whole wood material based on a single fiber pullout test due to the naturally non-uniform morphology of the wood fibers.

Pullout test was developed by using a wood veneer for this study to determine the interfacial bonding strength of geopolymer-wood composites. Influences of wood embedded depth, wood species, surface roughness, and moisture contents on the interfacial bonding strength of the geopolymer-wood composites were mainly studied. A conceptual model was then proposed for the interfacial bonding mechanism during the curing process in the geopolymer-wood composites.

Materials and methods

Materials

A commercial metakaolin (MK) (Metamax®, BASF SE, Germany) was used as the aluminosilicate source. Sodium silicate (Na2SiO3) solution (Betol 50 T, Woellner GmbH, Germany) and sodium hydroxide (NaOH) pellets (98.0%, VWR, Germany) were used as the alkaline activator for geopolymer. The activator and geopolymer paste were prepared as described in our previous studies [39, 44].

Norway spruce (Picea abies) and common beech (Fagus sylvatica L.) veneers were used in this work. The two veneer surfaces were sanded by a sanding machine (Bütfering Schleiftechnik GmbH, Germany) using 180 grit sandpaper (#180). For wood sample preparations (as used in Sect. “Influence of surface roughness on bonding between geopolymer and wood”), the surfaces were sanded to obtain different surface roughness by #60, #100, and #180 sandpapers, respectively.

After sanding, the wood veneers were cut into same size pieces (117 mm × 20 mm × 1 mm) and stored in a climate chamber (Pharma 1300, Weiss Technik, Germany) at a constant temperature of 20 ± 2 °C and 65 ± 5% relative humidity (RH) for at least two weeks. Different initial wood moisture conditions before the curing process and various environmental humidity conditions during the curing process are shown in Table 1. The different initial wood moisture conditions of the spruce and beech veneers used in Sect. “Influence of moisture on bonding between geopolymer and wood during curing” were achieved with four different pretreatment processes:

  1. (1)

    Dry condition (-d): oven-dried at 103 ± 2 °C for 48 h and stored in a dry desiccator before use;

  2. (2)

    Wet condition (-85): stored in the climate chamber at 20 ± 2 °C and 85 ± 5% RH for more than 2 weeks;

  3. (3)

    Room condition (-65): stored in the climate chamber at 20 ± 2 °C and 65 ± 5% RH for more than 2 weeks;

  4. (4)

    Condition of wood cell wall fully water-saturated without compounds leaking (-w): wood veneers were spread on a plastic net above water in a sealed plastic box to absorb moisture without touching water for several days (around 7 days for spruce and 5 days for beech) until the wood moisture content was 30 ± 2%.

Table 1 Initial moisture conditions before curing and the environmental humidity conditions during curing of the spruce and beech veneers

Sample preparations for the pullout testing

Geopolymer paste (320 g) was poured into an open cylindrical plastic mold (d = 50 mm, h = 100 mm). The wood veneer was embedded into the geopolymer paste at a certain depth to prepare the samples. The same embedded depth (50 mm) of wood veneer in the geopolymer matrix was applied to the sample preparations used for investigating the influence of surface roughness and wood moisture content on the interfacial bonding strength.

A cap was fixed on the plastic mold to keep the wood veneer in a vertical position. The molds were sealed with a cap to prevent any surface cracks of the geopolymer matrix at an early stage and improve the initial strength of the geopolymer by slowing the water escape from the geopolymer matrix. The cap was removed when the samples were cured in the climate chamber for 1 day.

The samples were then cured in a plastic mold without the caps in an RH climate chamber at 20 ± 2 °C/65 ± 5% for another 6 days (total of 7 days curing time) before the pullout testing. At this stage, different curing conditions were used for the samples described in Sect. “Dimensional change analysis” in the climate chambers for 7 days with the same temperature (20 ± 2 °C) but different relative humidity (dry condition: 35 ± 5% RH, indoor humidity: 65 ± 5% RH, and wet condition: 85 ± 5% RH, respectively) as shown in Table 1.

Pullout testing

Pullout testing was used to evaluate the interfacial bonding strength between wood veneer and geopolymer matrix with a Zwick/Roell universal testing machine (Zwick/Roell, Germany). A gripping jaw clamped the wood veneer and a purpose-built fixed base (illustrated in Fig. 1) held down the geopolymer block. The crosshead speed rate was set at 1 mm/min. The maximum pullout force was recorded with a 5-kN load cell. Only the pullout force value of the intact wood was recorded; the value of damaged wood was discarded. The test of each group of samples was repeated at least five times. The interfacial bonding strength in this study is defined by the pullout force per unit of the interfacial attached area of wood veneer in the geopolymer matrix. The interfacial attached area of wood veneer is related to the depth of wood embedded in the geopolymer matrix.

Figure 1
figure1

Pullout testing setup for the geopolymer-wood composites

Surface roughness measurements

The parameters of average roughness (Ra), mean peak-to-valley height (Rz), and maximum roughness (Rmax) are commonly used for determining surface roughness. The surface roughness (Ra, Rz, and Rmax) of both front and back surfaces of wood veneers after sanding was measured according to the standard ISO 4287 [45] by a surface roughness tester (TR200, TIME, China). Measurements were made perpendicular to the wood fibers at five different points on each sample at 20 ± 2 °C and 65 ± 5% RH environment conditions. The detailed descriptions of the surface roughness measurements are available in previous studies [46, 47]. The parameters of the device were set to the measurement length (λc) = 0.8 mm with five measurement numbers as a cut-off value, and the surface roughness profile treated with a Gaussian filter.

Moisture content (MC) measurements of wood veneer

A wood veneer without the visible geopolymer was taken out from the geopolymer matrix immediately after a certain time of curing. The wood samples for MC measurements were cut from the portions of the wood veneer that were embedded in the geopolymer matrix. The initial weight (W0) of the wood veneer was measured immediately. After the weight measurement was taken, the samples were dried in an oven at 103 °C for 48 h. The dry weight (Wd) of the samples was measured after they cooled in a dry desiccator for 2 days. The measurement was replicated with at least three specimens per group, and the average of the values was used. The moisture content of the wood veneer was calculated using the following equation:

$$ {\text{MC}} = \left( {{ }\frac{{W_{0} - W_{d} }}{{W_{d} }} } \right) \times 100 \% $$
(1)

Dimensional change rate measurements

The dimensional change rate (DCR) was measured for the pure geopolymer samples and the embedded wood veneers. The DCR of geopolymer (DCRG) was calculated with the following equation:

$$ {\text{DCR}}_{G} = \left( {{ }\frac{{d - d_{0} }}{{d_{0} }} } \right) \times 100 \% $$
(2)

where d0 is the inner diameter of the plastic mold and d is the diameter of solid pure geopolymer after the curing process.

After being embedded and cured in the geopolymer, the wood veneer was removed, and its thickness was measured immediately. The dimensional change rate of wood veneer (DCRW) was calculated with the following equation:

$$ {\text{DCR}}_{W} = \left( {{ }\frac{{{\text{THK}} - {\text{THK}}_{0} }}{{{\text{THK}}_{0} }} } \right) \times 100 \% $$
(3)

where THK0 is the initial thickness of the wood veneer measured before testing and THK is the thicknesses of the wood veneer after curing.

The dimensional change rate difference (Δd) of the samples was calculated with the following equation:

$$ \Delta_{d} = {\text{DCR}}_{7d} { }{-}{\text{ DCR}}_{4d} $$
(4)

where the DCR4d and the DCR7d are the dimensional change rates of the wood veneer and the geopolymer cured for 4 and 7 days, respectively.

Morphology characteristics of the interface

Samples with a 2-mm thickness were cut from the cross section of the geopolymer-wood block for observation. The samples were then polished with 800-grit sandpaper to investigate the influences of wood surface roughness on the interface morphology properties. Morphology images of the interface between wood and geopolymer were acquired with a stereomicroscope (SZX16, Olympus, Japan) equipped with a 1 × objective lens (SDF PLAPO 1XPF, Olympus, Japan) and a camera (Axiocam 208 color, Zeiss, Germany).

Scanning electron microscopy (SEM, LEO 1525, Oberkochen, Germany) was also applied to evaluate the microstructures of geopolymer and wood at the interface. The geopolymer surface at the interface and the cross sections of the wood veneers (both spruce and beech) after pullout testing were used for the examinations. All the samples were coated with platinum (Pt) before using. The detailed description of the methods is presented in the previous studies [39, 44].

Results and discussion

Interfacial bonding strength analysis by pullout testing

Influence of embedded depths on pullout force

Pullout testing was used to investigate the interfacial mechanical properties of the geopolymer-wood composites. Relationships between the embedded depth of the veneer and the pullout force for both spruce and beech are shown in Fig. 2. Spruce showed consequently higher pullout force than beech due to the better interfacial bonding caused by the rougher surface (illustrated in detail in Sect. “Influence of surface roughness on bonding between geopolymer and wood”). Further, in both spruce and beech samples, the pullout force had a steeper slope at an embedded depth < 25 mm. The slope then flattened and approximated constancy at an embedded depth > 50 mm. A comparable relationship between the pullout force and the embedded depth was detected in the pullout testing of steel fiber from an asphalt binder, showing plateaus around the peak pullout force [48]. The contribution of the end-grain wood cross-sectional area to the pullout force was negligible. It means that the relevant load during the pullout was transferred to the geopolymer matrix through the faces of the veneer and not through its end-grain. As the depth of embedment increased in both beech and spruce, the interfacial bonding force asymptotically approached a constant.

Figure 2
figure2

Pullout force of spruce and beech veneers at different embedded depths

Influence of embedded depths on interfacial bonding strength

The relationships between the embedded depths and the interfacial bonding strengths for spruce and beech veneers are shown in Fig. 3. A similar trend to the influence of embedded depths on pullout force was detected for the interfacial bonding strength for both spruce and beech samples. Both spruce and beech gained interfacial bonding strength as the embedded depth increased from 2 to 10 mm, reaching peak values at roughly 1.2 and 0.7 MPa, respectively. Then, the interfacial bonding strengths gradually declined as the depth of the wood veneers increased. The lowest interfacial bonding strength for both spruce and beech was detected at the maximum embedded depth of 75 mm. The nonlinear distribution between the interfacial bonding strength and the depth of the embedded wood veneers was due to the concentration of stress near the top surface of the geopolymer matrix (circled in Fig. 3). This concentration of stress presumably reduced with an increase in embedded length, and the stress distributions became generally flatter [48]. A similar stress concentration phenomenon in pullout testing was also reported for fiber (steel and wood) and cement [49, 50]. The stress concentration could be caused by a stronger bond between the wood and geopolymer at the top surface resulting from the denser structure and higher strength of the geopolymer surface due to a lower water content [51, 52].

Figure 3
figure3

Relationship between embedded depth and interfacial bonding strength for spruce and beech

Influence of surface roughness on bonding between geopolymer and wood

Sanding is an easy and common way to steer the surface roughness in the actual wood veneer production. Surface roughness parameters of spruce and beech veneers sanded with grit sizes of 60, 100, and 180 are presented in Table 2. The highest surface roughness was detected after being sanded with grit size 60, whereas the lowest value was detected with grit size 180. Based on the values of Ra, Rz, and Rmax determined from the surface of spruce and beech veneers, the surface roughness of the wood veneers was improved significantly by decreasing the grit size of sandpaper. Further, in all samples, the roughness parameters of the back surfaces of the wood veneers measured consistently higher than those of the front surfaces. This occurred due to the fabrication method. The back surface of the veneer experienced superficial cracks that formed as a result of compression tearing on one side of the wood surface during rotary peeling [53]. Also, when sanded with the same grit size, beech samples presented a smoother surface than spruce samples, which is consistent with results reported in other research [54]. This is because compared to spruce, beech generally has higher density and more regular distribution of anatomical elements, such as libriform fibers [55].

Table 2 Surface roughness values of spruce and beech veneers

The interfacial bonding strengths of geopolymer and wood veneers sanded with different grit sizes are shown in Fig. 4. Under the same general sanding treatment, the interfacial bonding strength of the geopolymer is higher with spruce than with beech. The surface of the spruce samples was generally rougher than that of the beech sanded with the same grit size (also see Table 2). The higher interfacial bonding strength of spruce was presumably due to the stronger mechanical interlocking between the rougher wood surface and the geopolymer. This could also be seen in the microscope images of the interface morphology between the geopolymer and wood sanded by grit sizes 60 and 180, respectively (see Fig. 5a–b). Grit sizes had little effect, however, on the enhancement of interface bonding between beech and geopolymer due to the slighter roughness differences created by different sandpapers. Specifically, samples sanded with 180 grit size showed a visible gap between the geopolymer matrix and the wood veneer after 7 days of climate chamber curing. Interestingly, those gaps only occurred between geopolymer and the finer surfaces of wood veneers. The veneer detachment due to the asymmetric bonding behavior between the finer wood surfaces and cement was also detected in prior research [49]. The rough-cut surfaces of the spruce and beech veneers presented in Table 2 have led to robust interfacial interlocking between geopolymer and wood, with the geopolymer hooking solidly into the cracks of the back surfaces of the wood veneers (Fig. 5c).

Figure 4
figure4

Interfacial bonding strength between geopolymer and wood veneers sanded with grit sizes 60, 100, and 180

Figure 5
figure5

Microscope images of the interface between wood and geopolymer: a spruce-60; b spruce-180; c beech-180

The SEM images of the interfaces of geopolymer and wood after the pullout tests are presented in Fig. 6. It was found that wood fibers were attached to the geopolymer (narrowed in Fig. 6a and c); in an adjacent area, the geopolymer was embedded in both the rough and smooth surfaces of the wood veneer (pointed out in Fig. 6b and d). Also, contact between the fiber and the matrix can be evidenced by the imprint of the fibers on the geopolymer matrix after the fiber pullout [56]. In this study, the imprint of spruce and beech fibers (circled in Fig. 6a and c) was detected on the geopolymer. Interestingly, a gap between the finer wood surface and geopolymer was then detected after 7 days of curing. A possible reason for this is that the finer wood and geopolymer were once attached, but later separated from each other.

Figure 6
figure6

SEM images of geopolymer and wood surfaces at the interface after pullout tests: a spruce fibers attached to the geopolymer; b geopolymer penetrated the spruce veneer; c beech fibers attached to the geopolymer; d geopolymer penetrated the beech veneer

Influence of moisture on bonding between geopolymer and wood during curing

Moisture content analysis

Moisture content (MC) of spruce and beech based on the curing time of 0–7 days and its magnification of 0–4 h are shown in Fig. 7. The dimensional change of wood was influenced by its moisture content during the curing process. The wood moisture content around 30% is considered to be a fiber saturation point (FSP), where the wood cell wall is saturated with bound water and without free water in lumens [57]. Wood shrinks when the moisture content of its cell walls drops below the FSP and swells until the MC passes the FSP [58]. The following illustrations about the relationship between the moisture content and the dimensional change in wood were based on this theory. Figure 7 shows that initially, while below FSP, the water absorption was faster in spruce than in beech, leading to a faster swelling (20 min for spruce and 60 min for beech). After that, the MC of both wood samples stayed continually above the FSP as the excess water entered into the wood lumens, and there were no dimensional changes during the 4 days of curing. The MC of spruce and beech increased gradually within 1 day of curing and reached its highest value of above 100% mainly due to the sealing effect of molds. Once the cap on the mold was released in the climate chamber after 1 day of curing, MC started to decrease. At this stage, the water evaporated mainly from the wood lumen. Wood veneer dimensions remained the same, while their MC stayed above the FSP. After curing for 3 days, it was noticed that MC decreased faster in spruce than in beech. After 4 days of curing, the MC of both reached the FSP. This could be defined as the starting point of the shrinking of wood during the curing process. After that, the MC of the wood samples continued to decrease until it reached a balance with the surrounding environment, while the wood experienced a deeper degree of shrinking after 7 days of curing.

Figure 7
figure7

Influence of curing time on the wood moisture content in the geopolymer matrix: a from 0 to 7 days and b the magnification from 0 to 4 h

Water in liquid form was detected on the surface of geopolymer samples at the curing age of 24 h due to the condensation process during geopolymerization that expels interstitial water that cannot stay within the framework of geopolymer [10, 59]. Indeed, more than 90% of the initial mixing water in the geopolymer was not bonded to its structure but remained free and related to the voids and the high transfer coefficients on the geopolymer [60]. The free water that leached from the geopolymer could be absorbed by the wood and then evaporated into the environment under low relative humidity at ambient temperature.

Dimensional change analysis

Table 3 shows the dimensional changes in geopolymer and the embedded wood veneer after 4 and 7 days of curing, respectively. The dimensional changes in wood were influenced by the wood MC during the curing process. For the wood veneers prepared wet or at 65% RH, the initial MC was around 10% and 31%, respectively. It was detected that both spruce and beech veneers swelled slightly in the geopolymer paste during the curing that took place between day 4 and 7 of the process compared to the initial veneers before embedding. Geopolymer shrank slightly from day 4 to 7 of the curing process. The drying shrinkage of the geopolymer resulted from the high capillary pressure generated between the wet and dry areas of the micropore network due to the excessive free water evaporation [61]. The Δd of the samples was used to illustrate the dimensional change rate differences between days 4 and 7 of curing. It was found that spruce-w and beech-w veneers shrank slightly less than spruce-65 and beech-65. The shrinking of the geopolymer was negligible in comparison with wood.

Table 3 Dimensional change rate of geopolymer and embedded wood after curing for 4 and 7 days

A conceptual model was developed about the influence of moisture on bonding at the interface between geopolymer and wood during a curing process, illustrated in a schematic in Fig. 8. During the curing process, wood experienced some dimensional changes (first swelling and later shrinking) due to a change in the moisture content, while geopolymer transformed from geopolymer paste (gel) to geopolymer solid. The initial and final setting times of pure geopolymer were around 100 min and 145 min, respectively, as investigated in the previous studies [38]. Although introducing wood veneer could prolong the setting time of geopolymer, it was noticed in the experiment that geopolymer became solid within 1 day of curing. Wood shrank, however, beginning on the fourth day of the curing process. The dimensional changes in geopolymer between days 4 and 7 can be ignored in comparison with the wood shrinkage because when the wood started to shrink after curing for 4 days, the dimension of geopolymer was nearly consistent. Thus, the interfacial weakening (i.e., the gap at the interface between wood and geopolymer) was mainly due to the shrinking of the wood veneer.

Figure 8
figure8

Schematic diagram of the influences of moisture on bonding at the interface between geopolymer and wood during the curing process

Applications of the moisture content control

Spruce and beech veneers with different initial moisture conditions were further used to investigate the interfacial bonding strength between wood and geopolymer under different curing conditions. Different environmental humidity conditions at 35%, 85%, and 65% were applied during the curing process to simulate different manufacturing conditions like dry, wet, and common indoor humidity conditions, respectively. The interfacial bonding strengths of geopolymer-wood composites under different curing conditions are shown in Fig. 9. Lower interfacial bonding strength values were detected in spruce-d-35, beech-d-35, spruce-w-35, and beech-w-35 cured under dry environmental conditions. Nevertheless, taking the groups of spruce-d-85, spruce-w-85, and spruce-85-85 as an example, it was shown that pretreatments of different wood moisture conditions before curing had a slight influence on the interfacial bonding strength of the samples under the same environment conditions during the curing process. No visible gaps were detected at the interface of the geopolymer and wood veneer when the samples were cured under a wet condition (20 °C/85% RH). A visible gap, however, was detected in the spruce-d-35, beech-d-35, spruce-w-35, and beech-w-35 samples after curing in a dry environment (20 °C/35% RH) for 7 days. The similar interfacial bonding strength was detected in the samples cured under 65% RH (common indoor humidity condition) as compared to the samples cured under 85% humidity. Thus, controlling wood moisture conditions before the curing process is not necessary for the improvement of higher interfacial bonding properties of wood and geopolymer.

Figure 9
figure9

Interfacial bonding strength of geopolymer-wood composites under different curing conditions

Conclusions

In this study, interfacial bonding strength of the geopolymer-wood composites were improved environmental-friendly by increasing embedded depth, sanding wood surface, and controlling curing conditions. Beech and spruce were compared as different wood species. A pullout test method was developed for the analysis of interfacial bonding strength between the geopolymer and wood. It was found that the pullout force increased with an increase in embedded depth of the wood veneer, and a plateau was detected at the depth of 25 mm. The bottom end of the wood veneer carried only a negligible load during the pullout testing. Compared to beech, spruce showed a higher interfacial bonding strength between wood and geopolymer. Strong mechanical interlocking at the interface was successfully achieved by sanding wood surface with grit size 60 sandpaper. Moreover, interfacial bonding strength was increased under wet curing conditions (20 °C/85% RH). Weak interface was mainly caused by the wood shrinking under dry conditions (20 °C/35% RH) during curing process. Influence of initial wood moisture content, however, can be ignored on the interfacial bonding strength between wood and geopolymer. A conceptual model was proposed explaining the moisture influence on the interfacial bonding mechanism of the geopolymer-wood composites during the curing process.

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Acknowledgements

The authors are grateful for the financial support provided by the National Natural Science Foundation of China [grant number 31800485], the Beijing Outstanding Talent Training Foundation [grant number 2017000020124G092], the China Scholarship Council [No. 201806510025], and the BioHome project funded by the German Federal Ministry of Education and Research [grant number 01DG17007A] and the DAAD project [grant number 57359374].

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Ye, H., Asante, B., Schmidt, G. et al. Interfacial bonding properties of the eco-friendly geopolymer-wood composites: influences of embedded wood depth, wood surface roughness, and moisture conditions. J Mater Sci 56, 7420–7433 (2021). https://doi.org/10.1007/s10853-021-05775-8

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