Recently, major concern has been expressed regarding ordinary Portland cement (OPC) manufacturing because of the huge amounts of carbon dioxide it releases (Zhang et al., Reference Zhang, Yang, Zhang, Feng and Li2023), the energy required and the scarcity of the geomaterials needed. Therefore, research has been undertaken worldwide to find eco-friendly cementitious materials with good physical and mechanical properties (Mohamad et al., Reference Mohamad, Muthusamy, Embong, Kusbiantoro and Hashim2022; da Silva Rego et al., Reference da Silva Rego, Sanjuán, Mora, Zaragoza and Visedo2023; Shi et al., Reference Shi, Sha, Qiao, Tang, Luo and Zheng2023). Geopolymers, which are low-temperature synthesized aluminosilicates, have been considered as potential alternatives to OPC (Ranjbar et al., Reference Ranjbar, Kuenzel, Spangenberg and Mehrali2020; Giacobello et al., Reference Giacobello, Ielo, Belhamdi and Plutino2022; Tanu et al., Reference Tanu and Unnikrishnan2022; Ayub & Khan, Reference Ayub and Khan2023; Jwaida et al., Reference Jwaida, Dulaimi, Mashaan and Othuman Mydin2023). Fly ash, calcined clays, volcanic ash and blast furnace slag are amongst the used aluminosilicate sources for geopolymer synthesis. For the latter purpose, these materials are dissolved using moderate to strong alkaline solutions (4–12 M). NaOH is by far the most used alkali-hydroxide activator. Mixtures of NaOH and Na2SiO3 are also used. In the latter condition, OH– ions (NaOH-derivative species) are involved in the dissolution of the aluminosilicate, whilst sodium silica plays the role of a binder (Mijarsh et al., Reference Mijarsh, Johari and Ahmad2015). Na+ ions contribute to the charge balance as well as to zeolite formation. The excess of Na+ ions induces the formation of carbonate by reaction with atmospheric carbon dioxide. Such a phenomenon occurs particularly in the case of the NaOH-activated metakaolin system (Valentini, Reference Valentini2018).
Considering metakaolin, for instance, which is considered to be a high-grade source material for geopolymer synthesis, alkali activation resulted in the formation of Al(OH)4–, H3SiO4– and H2SiO42− species (Valentini, Reference Valentini2018; Weng & Sagoe-Crentsil, Reference Weng and Sagoe-Crentsil2007) together with a sodium aluminosilicate hydrate (Na2O–Al2O3–SiO2–H2O gel; Valentini, Reference Valentini2018). According to Weng & Sagoe-Crentsil (Reference Weng and Sagoe-Crentsil2007), the association of Al(OH)4– and H2SiO42− results in the formation of small oligomers (dimers and trimers). However, the condensation between Al(OH)4– and H3SiO4– leads to the formation of polymers and larger oligomers. The latter products consist of linked silica ([SiO4]4–) and alumina ([AlO4]4–) tetrahedra. The three-dimensional structure obtained is called a geopolymer (Van Deventer et al., Reference Van Deventer, Provis, Duxson and Lukey2007; Sotelo-Piña et al., Reference Sotelo-Piña, Aguilera-González and Martínez-Luévanos2019; Almutairi et al., Reference Almutairi, Tayeh, Adesina, Isleem and Zeyad2021).
Cellulose is one of the main constituents of plants. It exists as fibres and has a polymeric structure. Cellulose chains are composed of β-d-glucose units, and their organization into sheets is achieved due to its methylol (–CH2OH) and the hydroxyl moieties (Bashline et al., Reference Bashline, Lei, Li and Gu2014; Etale et al., Reference Etale, Onyianta, Turner and Eichhorn2023). Cellulose fibres have high strength/density ratios (Chen et al., Reference Chen, Wang, Wang, Zeng, Yang and Zhang2021; Emenike et al., Reference Emenike, Iwuozor, Saliu, Ramontja and Adeniyi2023). Therefore, they have been proven to be suitable for reinforcing some matrix materials (Ferreira et al., Reference Ferreira, Ukrainczyk, e Silva, Silva and Koenders2021; Mourak et al., Reference Mourak, Hajjaji and Alagui2021; Lazorenko et al., Reference Lazorenko, Kasprzhitskii, Mischinenko and Kruglikov2022; Zheng et al., Reference Zheng, Wang, Wan, Yang, Lin and Chen2023). Cellulose's reinforcing capability is mainly influenced by its fibre dimensions, surface properties and moisture retention (Farhan et al., Reference Farhan, Johari and Demirboğa2021; Santana et al., Reference Santana, Júnior, Ribeiro, Cilla and Dias2021). According to some studies, the addition of cellulose to cementing materials should be limited (0.2–2.0% v/v; Jamshaid et al., Reference Jamshaid, Mishra, Raza, Hussain, Rahman and Nazari2022; Raghunath et al., Reference Raghunath, Hoque and Foster2023; de Souza et al., Reference de Souza, Liebscher, de Souza, de Andrade Silva and Mechtcherine2023; dos Reis et al., Reference dos Reis, Effting and Schackow2023). The addition of large amounts of cellulose resulted in increased water adsorption, and consequently the mechanical strength of the cellulose-containing cementing materials decreased (Singh & Gupta, Reference Singh and Gupta2020; Jaberizadeh et al., Reference Jaberizadeh, Danoglidis, Shah and Konsta-Gdoutos2023).
The reinforcing effect of the cellulose fibres can be improved by alkali activation using NaOH solutions (18–32 mass%) at 25–40°C. Due to this chemical treatment, fibrils with large surface areas and high aspect ratios form, and consequently adhesion between the reinforcing material and the matrix is enhanced (Aravindh et al., Reference Aravindh, Sathish, Ranga Raj, Karthick, Mohanavel and Patil2022; Asyraf et al., Reference Asyraf, Syamsir, Supian, Usman, Ilyas and Nurazzi2022).
Considering the above literature data, this work of research, which aimed to investigate the microstructure and determine the mechanical/physical properties of heated clay-based geopolymer samples, has fundamental and application interests. Regarding the latter interest, this work is a contribution to the development of eco-friendly clay-based building materials demonstrating acceptable performance. The raw materials (clay and palm date rachis) used are found globally, and the product preparation process does not require sophisticated plants or as much energy as for OPC manufacturing (Semugaza et al., Reference Semugaza, Mielke, Castillo, Gierth, Tam, Nawrath and Lupascu2023). Fundamentally, this study is a contribution to the understanding of the chemical interactions that could occur between cellulose's functional moieties and the active sites of geopolymers and associated aluminosilicates (zeolite, illite and metakaolin).
Materials and methods
Clay material and cellulose
The raw clay material was from Kharrou Mountain (Rehamna, Morocco). It was essentially composed of mica/illite, pyrophyllite, kaolinite, plagioclase and calcite (Figs 1 & 2). As an aluminosilicate-rich material (Table 1), the clay seemed to be suitable as a precursor for geopolymer preparation. Therefore, the clay was the object of mechanical and heating treatments, as detailed below. As a result of heating, the kaolinite disappeared and metakaolin was formed (Figs 1 & 2), whereas the illite resisted heating as its structure breaks down at >900°C (Gualtieri & Ferrari, Reference Gualtieri and Ferrari2006). As can be deduced from the thermograms given in Fig. 3, kaolinite underwent dehydroxylation at ~523°C. In this respect, it should be noted that the differential thermal analysis (DTA) curve showed endothermic phenomena at ~88°C and 272°C, which were assigned to the losses of hygroscopic and strongly retained waters. The weak endothermic peak at 812°C was associated with calcite decomposition.
Figure 1. X-ray diffraction traces of the (a) raw and (b) heated clays. Cal = calcite (PDF#83-1762); cps = counts per second; IIt = illite (PDF# 43-0685); Kln = kaolinite (PDF# 83-0971); Pl = plagioclase (PDF# 41-1486); Prl = pyrophyllite (PDF# 96-0277); Qz = quartz (PDF# 05-0490).
Figure 2. Fourier-transform infrared spectra of the raw and heated clays. cb = carbonates; CM = clay minerals; IIt = illite; Kln = kaolinite; MK = metakaolin; Pl = plagioclase; Prl = pyrophyllite; Qz = quartz; Sil = silicates; W = water.
Table 1. Chemical composition (mass%) of the used clay, the relative amounts of the date palm rachis constituents and the physical characteristics of the extracted cellulose.
Figure 3. Thermal curves (TG, DTA) of the raw clay.
The cellulose used as reinforcement was extracted from date palm (Phoenix dactylifera) rachis following the steps shown in Fig. 4. More details regarding the cellulose extraction are given below. The physical characteristics of the isolated cellulose, which manifested as shown in Fig. 5, are provided in Table 1.
Figure 4. Main steps followed for date palm rachis and raw clay treatments and experimental operations for heated clay–cellulose sample preparation.
Figure 5. Scanning electron microscopy images showing the cellulose fibre morphology.
Sample preparation
For geopolymer synthesis, dried portions of the clay heated at 700°C for 2 h and cellulose (up to 10 mass%) were introduced in a glass container and mixed for 15 min. The blend was transferred to a plastic bag and mixed with NaOH solution (6 M). The optimal (NaOH solution volume)/(blend weight) ratio was of 12.5 mL 100 g–1. The paste was kneaded for 15 min (to obtain a plastic paste), and the shaping operation was manually accomplished in ~20 min. The shaped samples (3.8 × 9.2 × 40 mm) were placed in a Memmert oven and aged at 83°C for 30 days. The oven used was functioning in the open air. Regarding the adopted curing conditions, it can be noted that, according to Mo et al. (Reference Mo, Zhu, Cui, He and Gong2014), increasing the curing temperature (up to 100°C) accelerates the dissolution, polymerization and reprecipitation processes of the geopolymerization reaction. Through this, the physical/mechanical properties of the product are enhanced. The effect of curing time on geopolymerization is still under debate (Heah et al., Reference Heah, Kamarudin, Al Bakri, Binhussain, Luqman and Nizar2011; Van Jaarsveld et al., Reference Van Jaarsveld, Van Deventer and Lukey2022). The NaOH activation solution was prepared by dissolution of NaOH pellets (Sigma-Adrich product, purity: >97.0%) in distilled water.
Experimental procedures and instruments
The raw clay was crushed and ground using an agate mortar then passed through a 100 μm sieve. The sieved fraction was heated in a Nabertherm furnace. The heating conditions were as follows: heating rate: 7°C min–1; maximum temperature: 700°C; soaking time: 2 h; atmosphere: air.
For cellulose extraction, the pigments and lipids associated with cellulose were removed using an azeotropic mixture of ethanol and toluene. The solvent extraction was performed using a conventional Soxhlet extractor. The hemicelluloses and lignin were eliminated using a solution of sodium hydroxide (2 mass%) and via heating at 80°C for 2 h. Fibre bleaching was achieved with a solution of sodium chlorite (1.7% w/v) at 70°C (Mourak et al., Reference Mourak, Hajjaji and Alagui2021).
The polymerization degree (PD) of the extracted cellulose was calculated using Equation 1:
where AMW is the average molecular weight (144 622 g mol–1) and M C is the molecular weight of cellulose (162.14 g mol–1). AMW was measured following a procedure detailed elsewhere (Evans et al., Reference Evans and Wallis1989; Malešič et al., Reference Malešič, Kraševec and Kralj Cigić2021).
As compared with the literature data (Hallac & Ragauskas, Reference Hallac and Ragauskas2011), the PD obtained was of low to moderate value. It is believed that cellulose extracted with such a PD value can be used without significant agglomeration tendency.
The crystallinity index (CI) was determined by using data obtained from the X-ray diffraction (XRD) trace of cellulose and Equation 2 (Segal, Reference Segal, Creely, Martin and Conrad1957):
where I 200 is the maximum intensity of the (200) lattice diffraction and I am is the intensity diffraction at 18°2θ.
The XRD analysis was performed on powder samples using a Rigaku Smart Lab diffractometer operating with a copper anode (λK α = 1.5418 Å). The XRD traces were recorded under the following conditions: generator voltage: 40 kV; tube current: 30 mA; scan step size: 0.05°; time step: 2s.
For the Fourier-transform infrared (FTIR) spectroscopy investigation, thin discs (study sample: 1 mg; KBr: 99 mg) were shaped and analysed using a Perkin Elmer 1725-x spectrophotometer functioning in the range of 4000–400 cm–1. PeakFit v4.12 software was used for infrared band deconvolution. The best curve fit was evaluated on the basis of the following statistical parameters: correlation coefficient (r 2), standard error (SE) and the Fisher ratio (F-statistic). The relative quantities of metakaolin, zeolite and illite were determined on the basis the areas of their FTIR bands. The areas were evaluated using PeakFit software.
For thermal analysis, a SetaramSetsys 24 apparatus was used. The operating conditions were as follows: atmosphere: air; heating rate: 10°C min–1; reference material and crucible: alumina; sample mass: 78 mg.
The microstructure of carbon-coated cured samples was investigated using a TESCAN VEGA3 electron microscope, which was equipped with 20 mm2 X-Max diffusion silicon detector.
The flexural strength (σFS) of the samples was evaluated using an EX 150 DeltaLab apparatus (loading rate: 12 × 10−3 kN s–1) and Equation 3:
where F is the load at failure, L is the distance between the two supports (2.8 cm) of the apparatus and b and h are the width and the thickness of the tested samples, respectively. The measurements were carried out in triplicate, and the standard deviations were in the range of 0.1–0.7 MPa.
The total porosity (P T) of the samples was calculated according to Equation 4:
where ρa and ρt are the apparent and true densities, which were measured using the pycnometry method.
The maximum amount of water retained by the samples (saturation time: 5 min) was determined by weighing the samples before and after contact with water. A water-submerged sponge was used as a source of water.
Results and discussion
Microstructure characterization
Based on the XRD traces shown in Fig. 6, zeolite ZK-14 (PDF #84-0693) and sodium carbonate (PDF #86-0298) were formed in all cured alkali-activated samples. In line with the results of Valentini (Reference Valentini2018), carbonation occurred in the alkali-activated metakaolin samples. According to Valentini (Reference Valentini2018) and Zhang et al. (Reference Zhang, Provis, Reid and Wang2014), such carbonation is the result of the presence of excess Na ions in the pore solution, and it occurred according to the following reactions:
Figure 6. XRD traces of the cured samples: (a) cured cellulose-free samples; (b): cured heated clay–5 mass% cellulose; (c): cured heated clay–10 mass% cellulose. Cb = carbonate (PDF# 86-0298); HS = hydrosodalite (PDF# 76-1639); IIt = illite (PDF# 43-0685); Prl = pyrophyllite (PDF# 96-0277); Qz = quartz (PDF# 05-0490); ZK-14 = zeolite ZK-14 (PDF# 84-0698).
Referring once again to Fig. 6, the formation of hydrosodalite (PDF#76-1639) was suspected seeing as its main X-ray reflections (°2θ = 14.09 (6.286 Å), 24.53 (3.629 Å)) are overlapped with those of zeolite ZK-14 (°2θ = 14.02 (6.314 Å), 24.41 (3.646 Å)). According to Król et al. (Reference Król, Rożek and Mozgawa2017), the formation of sodalite can be explained thermodynamically due to the higher stability of its denser lattice.
As expected, scanning electron microscopy (SEM) examination (Figs 7 & 8) revealed pseudo-granular particles identified as hydrosodalite together with elongated particles associated with zeolite ZK-14 particles. A careful examination of the microstructure displayed hydrosodalite-free zones in the vicinity of zeolite ZK-14 particles. This could be related to the differences in the nucleation and the coalescence processes of these zeolites or to the elemental heterogeneity across the matrix. Discussion of zeolite formation is provided below.
Figure 7. SEM image showing the typical microstructure of the cured sample (10 mass% cellulose) and energy-dispersive spectroscopy spectra associated with hydrosodalite (zone 1) and zeolite ZK-14 (zone 2).
Figure 8. SEM image showing unreacted illite particles (zone 1) and the geopolymer-rich domain (zone 2) and their energy-dispersive spectroscopy spectra.
FTIR spectroscopy analysis of the cured the samples indicated bands associated with metakaolin, illite, quartz and carbonate (Fig. 9). The zeolite/geopolymer bands could not be identified with certainty because of the bands overlapping. Therefore, the broad bands in the ranges of 3000–3700 and 800–1400 cm–1 were deconvoluted (Fig. 10). The bands at 3504–3571 and 958–946 cm–1 were associated with zeolites (Hajjaji, Reference Hajjaji2014; El Hafid & Hajjaji, Reference El Hafid and Hajjaji2015, Reference El Hafid and Hajjaji2018; Mourak et al., Reference Mourak, Hajjaji and Alagui2021). Based on the areas of the FTIR spectroscopy bands of the zeolites, metakaolin and illite, the amounts of the two former minerals evolved antagonistically, whereas that of illite slightly declined as the cellulose content increased (Fig. 11). Due to the fact that metakaolin is a precursor of zeolite (Khaled et al., Reference Khaled, Mohsen, Soltan and Kohail2023; Maruoka et al., Reference Maruoka, Pinheiro, Freitas, Nobre and Scalvi2023), the increase in the zeolite content should be accompanied by decreased metakaolin contents and vice versa. The unexpected presence of metakaolin in the cured samples could be due to the limited access of OH– ions to the embedded metakaolin particles and to the interaction between OH– ions and cellulose chains (mercerization reaction; Okano & Sarko, Reference Okano and Sarko1985; Nishimura et al., Reference Nishimura, Okano and Sarko1991).
Figure 9. FTIR spectra of the cured cellulose-free sample (0% mass cellulose) and cured cellulose-containing samples (5 and 10 mass% cellulose). For clarification, the O–H stretching vibration zone and the fingerprint region are separated. cb = carbonates; IIt = illites; MK = metakaolin; OM = organic matter; Qz = quartz; S = silicates; W = water.
Figure 10. Deconvolution plots of the broad FTIR bands in the ranges of 3000–3700 and 800–1400 cm–1.
Figure 11. Variation of the relative amounts of zeolite ZK-14 (ZK), metakaolin (MK) and illite as a function of cellulose content.
SEM examination showed that the microstructure of the cured samples was composed of zones such as those shown in Fig. 12. The features of such domains did not resemble those of the above-identified zones, and their Al/Si atomic ratios were almost constant (~1). By contrast, their (Na,K)/Si atomic ratios varied between 1.25 and 0.60. In this respect, it is worth noting that the atomic ratios of Si/Al, Si/Na and Al/Na of the used material were 1.4, 1.0 and 0.74, respectively. It was believed that the aforementioned domains corresponded to the areas of the geopolymer that was consisted of poly(sialate) units (Al/Si = 1; Zibouche et al., Reference Zibouche, Kerdjoudj, de Lacaillerie and Van Damme2009; Chen et al., Reference Chen, Wang, Wei, Muhammad, Yi, Wei and Fujita2022). However, it should be noted that the geopolymer and hydrosodalite chemical compositions were quite similar. Therefore, it could be assumed that geopolymer was a precursor of hydrosodalite formation.
Figure 12. SEM images showing geopolymer-rich zones and their corresponding energy-dispersive spectroscopy spectra: (a) 0 mass% cellulose; (b) 10 mass% cellulose.
Regarding the ternary diagram (SiO2–Al2O3–Na2O) that delimits the stability domains of zeolites and the chemical composition of the cured cellulose-free sample, the elemental representative point of the sample fell within the zeolite P domain (Fig. 13). The mismatch between the detected zeolites (zeolite ZK-14 and hydrosodalite) and the expected one could be essentially due to sample heterogeneity and consequently to the difference in the spatial distribution of oxides. In fact, the formation of zeolite ZK-14 required a large amount of silica (~63 mass%) and relatively low contents of Al2O3 (~22.9 mass%) and Na2O (~14.1 mass%). By contrast, hydrosodalite is supposed to occur in matrix locations where the contents of silica, alumina and sodium oxide are below 19, 39 and 42 mass%, respectively.
Figure 13. Ternary diagram showing the stability domains of zeolites (P, Y, X, A and HS; Xu et al., 2009). The location of the zeolite ZK-14 (ZK-14) is introduced. The representative point of the cured sample is indicated in red.
Mechanical and physical properties
The flexural strength of the cured samples increased almost linearly with increasing cellulose content (Fig. 14). Amongst the mechanisms that might be involved in the strengthening process are the formation and the intensification of bonds between the cellulose fibres and sample constituents, the homogeneous distribution of the fibres and pores and porosity reduction (Ye et al., Reference Ye, Zhang, Yu and Mu2018; Mourak et al., Reference Mourak, Hajjaji and Alagui2021; Zheng et al., Reference Zheng, Wang, Wan, Yang, Lin and Chen2023).
Figure 14. Variations in the mechanical and physical properties of the cured samples against cellulose content.
Regarding the influence of porosity on the mechanical strength of the cured samples, the results indicated that porosity decreased with increasing cellulose content (Fig. 14), and its evolution affected the flexural strength as follows:
The addition of cellulose also resulted in sample densification (Fig. 14). Therefore, the compaction process played a key role in sample strengthening. Moreover, the co-presence of zeolite and geopolymers induced an increase in the mechanical strength, as zeolite particles act as fillers (Andrejkovičová et al., Reference Andrejkovičová, Sudagar, Rocha, Patinha, Hajjaji and da Silva2016). It was believed that the sample strengthening was essentially linked to the formation of hydrogen bonds between the functional moieties of cellulose and the active sites of the remaining sample constituents. To this point, it was observed that the FTIR spectroscopy band at 3504 cm–1, which was associated with the stretching vibrations of the OH of zeolite attached to the Na+ ions (Szostak, Reference Szostak1992), shifted as the addition of cellulose increased (54 and 67 cm–1 for samples with 5 and 10 mass% cellulose, respectively; Fig. 10). Schematic representations of such a bond and the hydrogen bonds between the cellulose and the other constituents are provided in Fig. 15.
Figure 15. Schematic representations of hydrogen bonds that occurred between cellulose and the cured sample constituents (zeolite, geopolymer, illite and metakaolin).
The increase in cellulose content led to increased water absorption, as shown in Fig. 16. This cannot be associated with sample porosity because it was shown that porosity continuously decreased with increasing cellulose content. In this context, it was believed that the measured amount of adsorbed water was an average value of the water amounts retained in the open and connected pores and by zeolites, geopolymers and cellulose fibres.
Figure 16. Evolution of water absorption (WA) by the cured samples as a function of cellulose content.
The products obtained could be used as lightweight mortars as well as binders for brick manufacturing. As compared with some natural fibre-reinforced geopolymer composites, the studied products had high flexural strength (Table 2).
Table 2. Flexural strengths of some fibre-reinforced geopolymer composites.
Conclusion
The results of this study yielded the following conclusions:
(1) A Moroccan clay that consisted of kaolinite, pyrophyllite and illite (the main clay mineral) was thermally activated by heating at 700°C for 2 h for geopolymer synthesis. Due to the heating, kaolinite underwent dehydroxylation at ~523°C, whereas illite and pyrophyllite resisted heating up to ~900°C. The clay heating resulted in the formation of metakaolin, which is considered to be a high-grade aluminosilicate, for the geopolymer preparation. The heated clay was mixed with cellulose (up to 10 mass%), which was extracted from date palm (P. dactylifera) rachis, and alkali-activated with NaOH solution (6 M). The samples prepared were cured at 83°C for 30 days under an open atmosphere.
(2) As a result of curing, hydrosodalite, zeolite ZK-14, Na-carbonate and a geopolymer composed of poly(sialate) units (Al/Si atomic ratio = 1) were formed. The formation of these compounds, which was in line with some previous studies dealing with the geopolymerization of fly ash- or metakaolin-based source materials, was discussed in relation to the literature data.
(3) Metakaolin as well as illite were found in all of the studied samples. The relative amounts of metakaolin and zeolites evolved in opposite directions with the addition of cellulose, whereas that of illite slightly decreased. Zeolites were relatively abundant in the cured samples containing 2.5 and 10 mass% cellulose. However, it was believed that because of the low to moderate PD (892) of the cellulose, the fibres were well dispersed throughout the matrix because no bundles were observed at the SEM magnification (20 000×) used.
(4) Cellulose addition improved the performances of the cured samples as it increased their mechanical strength and reduced their porosity. In addition, it resulted in their densification. Taking, for instance, the sample containing 10 mass% cellulose, the strength/density ratio was enhanced by ~17%. This performance improvement was mainly attributed to hydrogen bonds between the functional groups of cellulose and the active sites of the formed aluminosilicate minerals as well as metakaolin.
(5) As compared with some natural fibre-reinforced geopolymer composites, the studied cured mixtures had high mechanical strength. They could be used as lightweight mortars or as binders for brick manufacturing.
Conflicts of interest
The authors declare none.
