Influence of wood pretreatment and fly ash particle size on the performance of geopolymer wood composite

The fly ash was divided into two groups. One group was ground further using an Astecma (model mb 20) ball mill for 1 h, whereas the other group remained in its raw state. The composition of the chemical oxides of the starting material (raw fly ash) was detected by X-ray fluorescence (XRF), using a PANalytical Axios Advanced. The results are shown in Table 1.

Sawn wood pine and eucalypt boards with densities of 0.39 g/cm³ and 0.56 g/cm³ respectively, were cut to dimensions of 25 mm × 30 mm × 50 mm. These pieces of wood were milled and later sieved with a Manupen sieve vibrator for 1 h to separate the wood into different particle size fractions. Wood particles that could pass through the 1 mm sieve but which were retained in the 0.6 mm sieve were used for both pine and eucalypt. Hot water pretreatment was carried out on both sets of sieved particles according to the method described by Cabral et al. (2017). Water was heated up to 100 °C in a 3.5 L container and 31.25 g of wood particles were introduced per 1 L water for 30 min. Finally, the recovered particles were washed with 1 L of tap water and placed in an oven at 60 °C, until a moisture content of around 10% was reached.

The extract content of the wood samples was analyzed using the Accelerated Solvent Extraction (ASE 350) from Thermo Fisher Scientific (Dionex). Extraction was done using 2 g of wood under the conditions stated in Table 2. Wood particles that could pass through the 1 mm sieve but which were retained in 0.6 mm sieve were used for both pine and eucalypt in the extraction process. Extraction was first done using petrolether, followed by acetone/ water and lastly with water alone. The total extract was the summation of the extract content in these processes. The extractive-free wood was hydrolyzed for sugars using the method described by Lorenz et al. (2016). The extractive-free samples were then finely ground (vibrating mill, Duke).

Pre-hydrolysis Approximately 200 mg was weighed into a reaction vessel. The sample was mixed with 2 mL of 72% cold H₂SO₄ after which it was hydrolyzed in a thermostat for 1 h at 30 °C. After one hour, the reaction of the pre-hydrolysis was stopped by the addition of 6 mL of distilled water. Next, the suspension was transferred together with 50 mL of water into a volumetric flask and the samples were post-hydrolyzed by autoclave at 120 °C under 1.2 bar pressure (40 min for pine and 30 min for eucalyptus). After cooling the volumetric flasks, the condensed lignin was filtered off as a hydrolysis residue by means of a G4 glass filter crucible. From the filtrate, about 1 ml was taken for the sugar analysis. The hydrolysis residue was washed thoroughly with distilled water, dried at 105 °C and determined gravimetrically.

In determining water absorption, dry bulk density and apparent porosity, the recommendations based on Testing Methods for Fiber Reinforced Cement-based Composites (RILEM 1984) were used. 7-day old 50 mm × 100 mm specimens were removed from the climate chamber (20 °C and 65% RH) and cut to ~ 50 mm × 25 mm (diameter x height). The specimens were submerged in water for 24 h at room temperature. The specimen was then suspended in water and the immersed mass (Mi) was measured. The wet mass (Mu) was measured by withdrawing the sample from the water and lightly wiping its surface to remove excess water using a clean, dry cloth. After drying the specimen (to a constant mass) in an oven with air circulation (105 ± 5 °C), the dry mass (Ms) was obtained. The following equations were used to obtain the water absorption, apparent density and apparent porosity of the specimens.

The compressive strength of 7-day old cylindrical samples (50 × 100 mm) was measured using an Emic DL30000N. The samples were compressed using a 300 kN load cell and a constant loading rate of 1 mm/min. The specific compressive strength (specific strength) was calculated by dividing the compressive strength by the sample density (mass per volume). An average of six samples was reported for each group.

The accelerated aging test involved a comparative analysis of the mechanical performance of the composites, before and after 200 soak/dry cycles. Specimens were successively immersed in water at 20 ± 5 °C over the course of 170 min, followed by a resting phase of 10 min, after which they were exposed to a temperature of 70 ± 5 °C for 170 min in a ventilated oven; the final resting phase being 10 min. This procedure was based on the recommendations of the EN 494 (1994) standards. Each soak/dry set represents one cycle and was performed for 200 cycles (Teixeira et al. 2012).

The morphology and appearance of the pine and eucalypt wood particles before and after hot water treatment are presented in Fig. 1. From Fig. 1 it can be seen the major difference arose from the color change of wood particles. With hot water treatment of the particles, the wood color changed from light yellowish to dark yellowish for the pine and from light brown to dark brown for eucalypt particles. This color change might be a result of the removal of some extracts and drying of particles after treatment. FESEM images (Fig. 2) show that the pine particles appeared to be shorter in length while the eucalypt particles were slender and longer. The properties (density, fiber length, shear strength) of the wood itself might have influenced the shape of the particles obtained with the same milling system. No observable changes were seen on the surfaces of the wood particles, indicating that morphology effects due to mechanical interlocking do not affect strength changes.

Figure 3 shows the particle size distribution and SEM of raw (a, b) and ground (c, d) fly ash. In this work, the particle size distribution was determined by the mean diameter as well as the cumulative percentage below a certain grain diameter (CPFT). The CPFT was classified for the diameter below 10% (D10), 50% (D50) and 90% (D90). The mean size of raw fly ash was 28.54 µm and 12.95 µm for ground fly ash. Through grinding, there was a 54.6% decrease in the mean particle size.

The fine ash particle fraction in D10 shifted from 2.95 to 1.78 µm. The major reason for the decreased mean particle size is found in the D90 class. Figure 3a shows a peak at roughly 100 µm, whereas this peak practically disappears after being ground (Fig. 3c). The grinding process might not only reduce the particle size of the fly ash but homogenize the grain structure, which may facilitate the alkaline activator to access the aluminosilicate. The surface and particles were studied using a SEM test. At the same magnification, the ground fly ash showed a smaller shape and more uniform particles than the raw fly ash (Fig. 3b and d). XRD was used to identify the crystallinity of the ash materials (Fig. 4). The identification phases obtained (with Match Phase Identification 3.8.0.137) showed that the main compounds in both the raw and ground ashes are mullite (M) and quartz (Q). There was no apparent change in the mineralogy of ground material; Rosas-Casarez et al. (2018) made similar observations.

Table 3 shows the resulting water absorption, bulk density and apparent porosity of the GWC based on untreated and treated pine and eucalypt. The GWC based on pine gave a lower dry bulk density compared to those from eucalypt. The results indicate a significant difference in density between pine-based and eucalypt-based composites. Eucalypt had a higher apparent density (0.56 g/cm³) than pine (0.39 g/cm³), which might have contributed to the final bulk density of the GWC. Similar densities were obtained for hot water treated and untreated samples for both wood species. There was no significant difference between composites formed from hot treated and untreated wood species.

Comparable porosity was recorded for all samples, with no significant difference between species and treatments. This porosity measurement with water might work for pure concrete or mortar but is no good for mortar containing a high amount of wood, as in this case. This is because in pure geopolymer mortar, the water might easily fill up the voids after 24 h immersion. In a GWC, the wood might hold some amount of water, which adds up to the water in the void. This might have accounted for the higher porosity values in all samples. After 24 h water immersion, pine-based GWC had the greatest water absorption rates (about 53%), while eucalypt recorded the lowest water absorption rates (about 46%). This difference in water absorption between the pine-based and eucalypt-based composites seems to arise from the different densities of the GWC. This clearly shows an inverse relation between water adsorption and density, that is, an increase in the density of the composite made from eucalypt led to a reduction in its water absorption. Sarmin (2016) reported similar observations: that denser GWC from wood flour had a lower water absorption rate compared with a less dense GWC from wood particles.

Another possible reason for the differences in water absorption could be a property of the wood itself, as it is well known that lower density coniferous wood takes up more water than higher density broadleaved wood species. Hence, the lower apparent density of pine than eucalypt might have led to a higher water uptake in the pine-based composite than in the eucalypt. Moslemi et al. (1995) ascertained that wood cement incompatibility leads to a large amount of free internal spaces within the wood cement matrix and could be a possible cause for great moisture adsorption of composites. Mahzabin et al. (2013) further reported that, without proper encasing of wood particles by cement particles, the hygroscopic nature of wood complicates the water absorption outcome among poorly compacted composites. Therefore, the low water absorption of the eucalypt-based composite could be due to the greater compatibility of this species with the geopolymer matrix. However, there was no significant difference between hot water treated and untreated GWC within the same wood species.

Pine-based composites recorded a compressive strength of 1.15–1.50 N/mm² while eucalypt-based composites had 2.49–2.59 N/mm², untreated – treated GWC respectively. The eucalypt composite density is 16% higher than that of pine, which may have risen from the different wood species’ densities. The density difference affects the composite strength. Hence, density effects shall be eliminated for better comparability of the species and pretreatment effect on strength. Figure 5 shows the resulting specific compressive strength of the GWC based on untreated and treated pine and eucalypt particles. In this study, the selection of wood species was found to have a significant influence on the strength of composites formed. Eucalypt-based composites recorded significantly higher specific compressive strength compared to pine-based composites. The hot water pretreatment increased specific strength by 27.4% for pine-based and 3.1% for eucalypt-based GWC. However, a significant difference was only observed between hot water treated and untreated pine-based GWC. This shows that the pretreatment was relatively effective for pine compared to eucalypt. GWC with treated wood had a higher specific strength, which could be due to the wood particle’s improved compatibility with the geopolymer, which resulted in effective bonding and increased maximum load transfer capacity.

The chemical composition of the treated and untreated wood particles is summarized in Table 4. It is a fact that hot water extraction alters the chemical composition of wood by fractionating accessible sugars and hemicelluloses (Pelaez-Samaniego et al. 2013, 2014). Due to the solvent polarity, it was expected that the hot water treatment would remove water-soluble extracts, such as non-structural carbohydrates, their saccharic acids, inorganic components and degradation products (alcohols, ketones) (Sluiter et al. 2008, 2010; Davison et al. 2013). The xylose, glucose, mannose, galactose, arabinose and rhamnose however, remained unaffected. The main portion of those sugars form part of the structural macromolecules and the apparent perceptual increase after treatment is an effect of the removal of other extracts. The hot water treatment removed 1.32 and 2.75% for pine and eucalypt, respectively. The analytical extraction agents, water, acetone–water and petrol ether reflect the range of polar to non-polar solvents. Aprotic polar solvents such as acetone cover a wider range of reactions due to their intermediate polarity. The extracted substances might comprise tannins, gums, sugars, starches and color producing chemicals (TAPPI 2007). Although a difference in strength was observed for pine, the extract yield was twice as high in eucalypt. This indicates, that one of the above-mentioned pine specific extracts causes the lower incompatibility of this species with the geopolymer matrix. Further investigations must focus on identifying the exact substance interacting with the geopolymerization.

Hot water treatment of wood particles by boiling is a similar process to cooking of wood chips in pulping, which is largely influenced by wood density. Zanão et al. (2019) stated that the density differences between eucalypt and pine significantly affect the impregnation of these two woods. Low-density woods are impregnated faster than high-density woods when boiling in water. A similar phenomenon might have occurred in this study, as the fast impregnation and high water absorption by the pine particles might have decreased the amount of water available for ionic transport within the mixture and led to the lower specific strength. By this same principle, it was expected that pine-based GWC show higher water absorption (Table 3), with more advantages regarding specific strength increase with the hot water treatment than eucalypt-based GWC. Wilson and White (1986) reported that hardwoods are usually strong in compression, tension and shear, while softwoods are strong in tension but weak in shear. This might have contributed to the difference between the two composites. Since no observable changes were seen on the surfaces of the wood particles (Fig. 1) after treatment, it can be concluded that the morphology effects caused by mechanical interlocking did not affect changes in strength; rather, the wood species, shape of the wood particles and the removal of extracts did.

For this test, the eucalypt-based GWC were used since they performed better than the pine-based GWC. Table 5 shows that composites made from ground fly ash recorded greater densities than those from raw ash. After 24 h water immersion, composites from raw fly ash had the greatest water absorption rates while ground fly ash recorded the lowest water absorption rates. Significant differences were observed in densities and water absorption of composites from ground and raw fly ashes. The differences in water absorption may be attributed to the different densities of the GWC, owing to the smaller particles of the ground ash getting closely packed to fill up spaces within the composite thereby increasing the density with a reduction in water absorption. Another possibility might be the reduction of particle size through grinding, which resulted in an increased polymerization reaction and forming of a dense structure, with decreased water absorption. However, within the same fly ash group no significant differences were found between hot water treated and untreated composites. The apparent porosity ranged from 46.79 to 47.13% for ground fly ash and 47.18–48.11% for the samples from raw fly ash (Table 5), with no significant differences between the composites.

Compressive strength is 1.10–1.19 N/mm² (composites from raw ash) and 2.49–2.59 N/mm² (composites from ground ash), untreated—treated GWC, respectively. Hence, density effects shall be eliminated for better comparability of the grinding on the strength results. The ash-grinding step doubled the specific compressive strength (Fig. 6). Using raw fly ash resulted in about 1 × \(10^{3}\) N m/kg whereas ground fly ash yielded about 2 × \(10^{3}\) N m/kg. With the same ash group, no significant difference was observed for hot water treated and untreated GWC. However, significant differences were observed between the GWC from ground and raw fly ash. The 54.63% decrease in the mean particle size by grinding led to a 94.9% (untreated) and 102.4% (treated) increase in the specific strength.

Grinding results in a larger surface area, which allows for a greater dissolution of alumina and silica in alkaline activation of the fly ash. In addition, smaller particle size requires less time to produce crystalline structures and gels that provide stability to the geopolymer, as well as more homogeneity in the matrix and more rigid bonds (Rosas-Casarez et al. 2018). Kim and Lee (2017) who made a similar observation, discovered that geopolymer from finer ground bottom ash had the highest compressive strength compared to medium and coarse ground bottom ashes. The lower strength from the raw fly ash may be compensated by prolonging the reaction-mixing time to promote dissolution (Ziegler et al. 2016) and adding more soluble silica to dissolve the large particles (Kim and Lee 2017). Additionally, the particle fraction with diameters beyond 100 µm could be sieved out prior to processing.