1. Introduction
Agriculture, as one of the fundamental human activities, produces high amounts of waste material. Large portions of these waste materials are not managed or disposed of properly. This problem is particularly present in less-developed countries, whose economies predominantly rest on agricultural production and produce export. Rough estimates are that 1.3 billion tonnes of agricultural waste are generated annually [
1], which calls for the development of new agricultural waste utilization technologies. Such technologies that rely on the utilization of agricultural waste could yield new materials with added value [
2]. Among the types of agricultural waste, different lignocellulosic materials such as nutshells are generated in great quantities. WN is routinely used as a fuel due to its calorific value, which is comparable to wood biomass. This type of waste management is not in accordance with the objectives of the progressive and environmentally friendly policies and circular economy. Consequently, new possibilities and applications for these kinds of materials are being researched and developed, with one of the prominent applications being their use as a filler material for biocomposites in construction industry or as a bioadsorbent [
3]. Almond and hazelnut belong to the group of nuts with the highest production demand due to high human consumption. Furthermore, up to 50% of the nut product is the WN, which belongs to the lignocellulosic group of materials (LCM). Typical almond nutshell comprises 50% cellulose, 28% hemicellulose and 20% lignin, while the hazelnut shell has a comparable composition with 35% hemicellulose and 32% lignin but lower values for cellulose, commonly around 30%, [
4]. The composition of both nutshells depends on the climate, soil and other growth conditions. Alongside these primary components, the amount of extractable sugar, fat, wax and pectin is similar in both nutshells and does not exceed 4% [
5,
6,
7]. High potassium content interferes with the use of waste almond shells as fuel, particularly if high temperatures are reached, due to possible furnace corrosion and clogging [
8,
9]. Different traditional and advanced applications for almond shell are being researched, for example, as a precursor to biochars used in the adsorption of pollutants and pharmaceuticals, a biogas, bio-oils or in the production of bioethanol [
10]. Crushed and milled almond shell is used as a filler, mechanical property modifier and colorant in different composites with polymethyl methacrylate, polylactide, polypropylene and various epoxy resins as polymer matrix [
11]. Similar to the almond shell, the hazelnut shell is also used as a bioadsorbent due to the presence of various organic groups on the shell surface, as well as its beneficial micro- and mesoporous structure, which allows for the effective removal of heavy metals in addition to different organic pollutants [
12]. The hazelnut shell shows a promising future as a natural source of different phenolic antioxidants and as a filler in inorganic and organic composite materials. Particle boards made of hazelnut shell with organic resins as binders are already being produced, while inorganic matrix composites are based mostly on the preparation of lightweight bricks in which the nutshell fillers are burned during the firing of the bricks, which allows for the formation of a highly porous structure [
13,
14,
15].
One of the possible uses of WN is in the production of inorganic insulating composites used in civil engineering. Natural fillers such as waste nutshell in those composites enable the achievement of eco-friendly and advanced properties. Additionally, in this manner, the CO
2 used for the plant’s growth is “trapped” in the composite material in contrast to being released during the firing of the nutshells [
16]. However, if they are to be used in biocomposites with an inorganic matrix, WNs need to undergo some kind of pretreatment. These processes can be divided into two basic types: physical and chemical methods, where physical pretreatment processes are usually of mechanical nature [
17]. Physical methods of pretreatment predominantly have the goal of changing the shape or size of the lignocellulosic materials; the most commonly used mechanical process is grinding. Physical methods can also influence the chemical composition, causing the removal of pectin, hemicellulose and lignin [
18,
19,
20]. However, chemical pretreatment is an inevitable step in creating lignocellulosic biocomposites due to the hydrophilic nature of the LCM caused by the high number of hydroxyl groups enveloping the surface [
21], organic groups present on the surface of the lignocellulose, as well as pectin, waxes and sugars, which may impede the setting of the biocomposite matrix. Without compatibilization, poor adhesion between the matrix and the filler occurs, and no interphase is created, leading to poor mechanical properties of the composite. Various types of chemical pretreatments are used that differ in the type of chemical applied, such as acids, bases (mercerization), ionic liquids, oxidizing agents or organosols [
22,
23].
One of the promising inorganic matrixes for WN composites are geopolymers. Geopolymers are aluminosilicate materials obtained by the alkaline or acid activation of aluminosilicate powder precursors. These kinds of novel materials are a plausible ecologically acceptable and sustainable replacement for Portland cement, which is occasionally used as an inorganic matrix in composite materials. The manufacture of geopolymers involves the use of low-cost waste materials like fly ash or metakaolin, lower energy consumption and lower CO
2 emissions [
24]. Activation solutions of alkaline-activated geopolymers are usually solutions of potassium or sodium hydroxide, solutions of sodium or potassium soluble silicates, better known as water glass, or their combinations [
25]. According to the literature, agro-industrial waste material ashes are used as raw materials for geopolymer concrete [
26] or geopolymers are reinforced with various natural fibers (sisal, jute, cotton stalk) to enhance their mechanical properties [
27]. However, to our knowledge, no biocomposites of geopolymer matrix and waste nutshell have so far been studied for use in civil engineering. Considering the sustainability, low cost and low carbon footprint of such biocomposites, geopolymers were chosen as the model inorganic matrix for WN biocomposites in this study.
This work is an extensive investigation of the influence of alkaline treatment, employing the traditional NaOH solution and Ca(OH)2 suspension, on waste almond and hazelnut nutshells. The composition and the properties of the WNs were characterized after the different pretreatment regimes using X-ray diffraction analysis (XRD) to determine the change in the crystallinity index. Fourier-transform infrared (FTIR) spectroscopy in combination with simultaneous differential thermal and thermogravimetric analysis (DTA/TGA) was employed to determine the change in the composition of the WN, while scanning electron microscopy (SEM) was used to determine the change in the morphology of the almond and hazelnut nutshells, as well as to investigate the precipitation of Na and Ca compounds on the surface of the WN. Furthermore, water and diiodomethane contact angles were determined, and free-surface energies were calculated for all the WN and chosen model geopolymer matrix samples. Adhesion parameters were then calculated to determine which combination of pretreatment and geopolymer type could result in an optimal inorganic biocomposite and to carve the path to future studies involving the most promising biocomposite candidates.
2. Materials and Methods
2.1. Materials
Waste almond nutshell (AN) and waste hazelnut nutshell (HN) were donated by small farmers from the Šibenik-Knin and Zagreb County, Croatia. NaOH solutions were prepared by dissolving NaOH microgranules (p.a., Gram-mol, Zagreb, Croatia) in demineralized water, while milk of lime was prepared by mixing freshly prepared CaO (obtained by calcination of CaCO3, p.a., Merck, Darmstadt, Germany) and demineralized water in a 1:7 ratio. Geopolymers were prepared from fly ash acquired from the Plomin power station (Plomin, Croatia) and metakaolin, i.e., calcined commercial kaolin clay (technical grade, VWR Chemicals, Paris, France) activated by sodium (technical grade, VWR Chemicals, Paris, France) or potassium (technical grade, Ivero, Zagreb, Croatia) water glass combined with 12 M solution of NaOH or KOH (p.a., Gram-mol, Zagreb, Croatia).
2.2. Sample Preparation
Waste nutshells were milled using a manual grinder, after which they were treated with alkaline solutions (
Figure 1). NaOH treatment was carried out using 3, 6 and 9% solutions for 1 and 2.5 h at 80 °C, while milk of lime was prepared using a 1:7 ratio of CaO and demineralized water. Treatment with milk of lime was accomplished by mixing WN and milk of lime in a 1:8 ratio and processed at 80 °C for 1 and 2.5 h, while another batch of treated WN was prepared at room temperature for 24 h.
In
Table 1, the mass fractions of the particle size distribution for both waste nutshells after grinding are given.
Geopolymer samples were prepared by activating metakaolin or fly ash with a 12 M NaOH or KOH solution mixed with sodium or potassium water glass to achieve a water-to-solid ratio of 0.66 and a molar ratio Al:Na,K = 1:1. Geopolymer pastes were poured into molds after 5 min of mixing and cured at room temperature or 40 °C. The geopolymer samples prepared using metakaolin were denoted as M and those using fly ash as FA, while sodium activation solutions were named Na, and potassium activation solutions were named K. Geopolymers cured at room temperature were denoted as RT and the curing at 40 °C was denoted as 40C.
Figure 2 shows five different geopolymer plate samples that were used for measuring the contact angles.
For most analyses, the shape and the form of the treated ground WN were not suitable; thus, WNs were further milled in an electric mill to produce fine powders, which were then pressed into small tablets of 13 mm diameter and used in further analyses.
2.3. Characterization
The crystallinity index (
CI) was determined from the diffraction patterns acquired by XRD analysis on a Shimadzu XRD 6000 instrument (Shimadzu, Tokyo, Japan) with CuKα radiation in the 2θ range from 5 to 40° with a 2θ step of 0.02° and 0.6 s counting time. The Fityk program (version 1.3.1) [
28] was used for data processing, and nonlinear curve fitting used the Pseudo-Voigt function in order to calculate the crystallinity indices. Crystallinity indices were calculated using Equation (1), where
ACrystalline was obtained by fitting the crystalline contribution (peaks at 15.25°, 18.52°, 21.84° and 34.8°) and
AModel was obtained by adding the amorphous contribution (broad hump catered at 22°) to the modelled data of the crystalline peaks.
The efficiency of the removal of specific undesirable components and groups from the waste nutshells was determined using FTIR analysis, which was carried out using a Bruker Vertex 70 spectrometer (Bruker Optics, Karlsruhe, Germany) in attenuated total reflectance mode (ATR) on samples pressed on a diamond, and the spectra measured between 400 and 4000 cm−1, with a spectral resolution of 2 cm−1 and an average of 32 scans. FTIR analysis results were complemented by results of simultaneous DTA/TGA analyses using the Netzsch STA 409 analyzer (Netzsch-Gerätebau GmbH, Selb, Germany) with a heating rate of 10 °C min−1 from room temperature to 1000 °C. A Tescan Vega 3 scanning electron microscope (Tescan, Brno, Czech Republic) operating at 10 kV was used for observing the changes in the morphology of the WN samples before and after the pretreatments. Samples were fixed on specimen holders with double-sided carbon conductive tape and gold-coated using a Quorum SC 7620 sputter coater (Quorum Technologies, Laughton, United Kingdom). The contact angle was determined by the sessile-drop method using the goniometer DataPhysics OCA 20 Instrument(DataPhysics Instruments GmbH, Filderstadt, Germany) which projects the image of the drop onto the computer screen via the video system and determines the position of the drop with an accuracy of ±1 mm. The surface free energy (SFE) of all the samples, γ, as well as polar (γp) and disperse (γd) components of the SFE were calculated using the Owens–Wendt model, using the contact angle values of three different drops of the two testing liquids, water and diiodomethane. The adhesion parameters of binary systems comprised of nutshell and the geopolymer were calculated using the following equations:
Interfacial free energy (
γ12)—OWRK model:
Spreading coefficient (
S12):
where
γ1 is the surface free energy of the geopolymer and
γ2 the surface free energy of the nutshell [
29].