Ecofriendly and Electrically Conductive Cementitious Composites Using Melamine-Functionalized Biochar from Waste Coffee Beans

Abstract

Owing to the increasing generation of waste coffee powder and the biochar from this waste being considered as alternative conductive carbon fillers, we developed eco-friendly and electrically conductive cementitious composites using biochar from waste coffee beans, which were directly pyrolyzed into eco-friendly and electrically conductive biochar. Via carbonization and graphitization, cyclic organic carbon precursors were transformed into sp2-bonded carbon structures and then functionalized with melamine. The non-covalent functionalization process driven by the electromagnetic process accelerated the mass production and enhanced the monodispersive properties of the cementitious composites. Thus, the melamine-functionalized biochar cementitious composites exhibited an electrical conductivity of 3.64 × 10⁻⁵ ± 1.02 × 10⁻⁶ S/cm (n = 6), which corresponded to an improvement of over seven orders of that of pure concrete. Furthermore, the percolation threshold of biochar was between 0.02 and 0.05 wt.%; thus, an effective conductive network could be formed using low additions of functionalized biochar. As a result, in this study, electrically conductive cementitious composites were developed using waste coffee powder converted into carbon nanomaterials through a newly introduced process of non-covalent functionalization with melamine.

1. Introduction

Light- (0.2–1.9 W/m·K) and normal-weight concrete (0.6–3.3 W/m·K) both have low thermal conductivity, which is useful for the thermal insulation of buildings. However, concrete with both high electrical and thermal conductivities is beneficial for advanced multifunctional applications, such as smart concrete structures with self-sensing of deformation and damage, the wireless charging of electric vehicles on road pavements, prevention of thermal cracking in mass concrete structures (owing to the excellent thermal diffusivity of this type of concrete), the melting of black ice on road surfaces, and excellent electromagnetic interference shielding effects [1]. Carbon-based nanomaterials are generally used to multi-functionalize cement composites by enhancing their thermal and electrical conductivities. Among the various types of conductive fillers, carbon-based nanomaterials such as carbon black, carbon nanotubes (CNTs), and graphene nanoplatelets (GNPs) are widely used because of their excellent electrical, physical, and mechanical properties [1,2,3,4,5,6]. Despite these potential applications, the commercialization of conductive carbon fillers is limited owing to their mass-scale dispersion and expensiveness [7]. Therefore, biochar from waste coffee powder has recently been considered as an alternative conductive carbon filler that could be mass-produced.

The amounts of coffee consumed, and waste coffee powder produced worldwide, reached 1 billion kg in 2021 and has been continuously increasing [8]. The landfill/incineration expenses of waste coffee powder are 80–100 dollars per ton and additional facilities are required. Waste coffee powder is predominantly composed of a mixture of cellulose, hemicellulose, and lignin [9]. These organic substances can be recycled for various applications, such as soil conditioners [10,11,12,13], capacitors [14], adsorbents [15], and construction materials [16,17,18]. In addition, cyclic organic carbon structures can be used as precursors to form sp2-bonded carbon structures via carbonization and graphitization during heat treatment [19]. When biomass, such as waste coffee powder, is heat-treated, only aromatic carbon remains through the dehydrogenation and denitrogenation processes, forming a turbostratic structure [20,21]. Biochars manufactured via heat treatment absorb gases, such as carbon dioxide, because of the presence of numerous pores. In addition, research on materials for manufacturing energy storage devices using the electrical conductivity of biochars is in progress [14]. According to previous research, the biochar of waste coffee powder heat-treated at 1000 °C showed an electrical conductivity of 300 S/m owing to its carbonization and graphitization [22]. Waste coffee powder biochar has properties similar to those of carbon black produced by the incomplete combustion of carbon-based compounds [3].

The crystal structure of conductive carbon materials is predominantly produced by electrons generated from delocalized sp2 carbon bonds, and these electrons induce strong π–π bonds between molecules. Therefore, for conductive carbon fillers, it is important to manufacture composite materials to prevent agglomeration caused by the π–π bonds [23]. When the conductive carbon filler is agglomerated in the matrix material, the transfer of stress is challenging, consequently resulting in stress concentration, and, thus, it is difficult to form efficient conductive networks that function as conductive fillers. Physical and chemical methods are generally used to overcome this limitation. A typical chemical dispersion method is a functionalization process [24]. The repulsive forces between the polar functional groups induce the homogeneous dispersion of carbon fillers. Compared with the direct covalent functionalization process, the non-covalent functionalization process is commonly induced by π–π interactions, which minimize insulating defects on sp2 carbon bonding. Therefore, the non-covalent functionalization process has the advantage of utilizing their superior intrinsic properties for reinforcement and as conductive fillers [25]. Molecules used for functionalization are required to have functional groups, such as carboxyl, amine, and epoxide, as well as sp2 carbon bonds, such as benzene, pyrene, and naphthalene. Non-covalent functionalization can be performed by dispersing these molecules and carbon materials in a solution and applying a physical force. Methods of applying physical forces include sonication, heat treatment, and ball milling [26]. However, these methods are disadvantageous for mass production. We developed a functionalization process using an electromagnetic polishing machine that could provide high energy compared to the existing processes because of the nature of using electromagnetic energy. While the conventional process takes 24–48 h [27], the process using an electromagnetic polishing machine could achieve similar results in 4 h.

In this study, waste coffee powder was converted into conductive carbon through heat treatment and non-covalent functionalization with melamine to prepare a cementitious matrix of both electrically and thermally conductive nanocomposites, and its mechanical and electrical properties were analyzed. Non-covalent functionalization was performed using melamine with π–π interactions and an amine functional group, and a process of electromagnetic non-covalent functionalization, which could facilitate mass production, was developed [27]. In addition, conductive carbon from waste coffee powder was homogeneously dispersed and used as a reinforcing agent and conductive filler in a cementitious matrix. Using this process, we successfully fabricated an efficient reinforcing and conductive network with eco-friendly biochar. To the best of our knowledge, this study is the first case of using waste coffee grounds to provide conductivity in cementitious composites.

2. Materials and Methods

2.1. Materials

In this study, the materials used to produce melamine-functionalized biochar (m-biochar)-reinforced cementitious composites were biochar, melamine, and ordinary Portland cement. Biochar was synthesized from waste coffee powder (Starbucks Coffee, Hwaseong, Korea). Biochar was used as a conductive filler. Melamine (99%, Sigma-Aldrich, St. Louis, MO, USA) was used to functionalize the biochar. N,N-dimethylformamide (DMF, 99%, Samchun, Pyeongtaek, Korea) was used as the dispersion medium for the biochar functionalization. The electrical conductivity of the m-biochar cementitious composites was comparable to that of the GNP-incorporated cement paste (XG Science, grade M-5, Lansing, MI, USA). The properties of the GNPs used in this study are listed in

Table 1. Properties of GNPs.

Specific Surface Area (m2/g)	Bulk Density (g/cm3)	Purity (%)	Diameter (μm)
30–60	0.03–0.1	>95	7–25

.

2.2. Synthesis of Biochar and Functionalization

Prior to pyrolysis, the waste coffee powder was sufficiently dried. First, waste coffee powder was placed in a quartz boat in a high-vacuum environment (10 − 3 mTorr). Then, the temperature was increased to 700 °C in 7 min and held for 30 min. Subsequently, the temperature was increased to 1000 °C in 3 min and held for 30 min. The mixture was then cooled to room temperature (approximately 25 °C) for 8 h. Melamine (500 mg) was dissolved in DMF (150 mL). This solution was then sonicated for 1 h with 500 mg of biochar. The mixture was placed in a polypropylene bottle with magnetic stainless steel media pins (AMECH Co., Ltd., Daejeon, Korea, ⌀ 0.7 × 5 mm, 50 g). For functionalization, the bottle was placed on an electromagnetic deburring and polishing machine (EMD-350A, AMECH Co., Ltd., Daejeon, Korea). Electromagnetic force was applied at a run-speed of 1200 rpm for a total of 4 h, alternating between clockwise and counter-clockwise every 10 min. After functionalization, the dispersion was vacuum filtered with a polyamide membrane filter (pore size 0.2 μm, Scilab, Seoul, Korea, Cat. No: SL.NY020047A). The residual DMF was washed thrice each using ethanol (99.5%, Samchun, Pyeongtaek, Korea) and distilled water during filtering. The resulting solid was dried at 70 °C under vacuum.

2.3. Fabrication of m-Biochar-Reinforced Cementitious Composites

To make cementitious composites, Type I ordinary cement and water were used, but fine and coarse aggregates were not used. The mix proportion for the cement paste was a water–cement ratio of 42%. The effect of the m-biochar content on the conductivity and compressive strength of the cementitious composites was investigated. As an experimental variable, the content of m-biochar was set to 0, 0.1, 0.2, 0.5, 1.0 wt.%. In addition, specimens containing GNPs with 1 wt.% content were also prepared to compare their conductivity with those of the m-biochar-reinforced cementitious composites. For mixing the m-biochar-reinforced cementitious composites, the m-biochar was directly dispersed in water using a planetary centrifugal mixer. Then, the m-biochar-dispersed water and cement were mixed for 10 min using a mortar mixer. For the cement paste containing GNPs, the water containing GNPs was ultrasonicated for 30 min to disperse the nanomaterials well. Then, similar to the m-biochar-reinforced cement paste, the sonicated water and cement were mixed for 10 min using a mortar mixer.

2.4. Characterizations

The microstructure was analyzed using scanning electron microscopy (SEM) at an accelerating voltage of 15 kV (APREO, FEI, Eindhoven, The Netherlands), after being plasma-coated with Pt for 30 s (approximately 10 nm thickness). The composition and degree of carbonization of the biochar was performed using Raman spectroscopy (FEX, Nost, Seongnam, Korea). The bonding mechanism and quantity melamine after functionalization were confirmed using X-ray photoelectron spectroscopy (XPS) (K-Alpha Plus, Thermo Fisher Scientific, Waltham, MA, USA). The electrical conductivity of the cementitious nanocomposites was measured using a four-point probe (CMT-SR1000N, Advanced Instrument Technology, Suwon, Korea). Cementitious composite specimens were cured under constant temperature (20 ± 2 °C) and relative humidity (50 ± 4%) for 28 days. Then, mechanical properties were measured by compressive strength tests with cylindrical specimens of 20-mm radius and 5-mm height with a compress rate of 3 mm/min.

3. Results and Discussion

We functionalized biochar with melamine using an electromagnetic process, which is a scalable non-covalent functionalization process (Figure 1). The chemical structure of coffee waste contains a mixture of chemicals that are favorable for being transformed into conductive graphitic structures via thermal pyrolysis (Figure 1a). An electromagnetic process was used for biochar functionalization (Figure 1b). Applying direct electromagnetic force to media pins with alternating high-speed rotations (1200 rpm), π–π interactions between melamine and biochar were induced. Compared with previous functionalization processes, additional to offering suitable scale-up, electromagnetic processes provided rapid functionalization for mass production. The graphitic structures in the biochar were directly functionalized by π–π interactions with the aromatic benzene ring in the melamine (Figure 1c). The m-biochar could be directly dispersed in water using planetary centrifugal mixers (Figure 1d). Therefore, the dispersion could be directly applied to the fabrication of cementitious nanocomposites.

The chemical nature of the m-biochar was analyzed using SEM, XPS, and Raman spectroscopy (Figure 2). SEM indicated that the melamine functional groups were directly attached to the surface of the biochar. When observing the pristine biochar (Figure 2a), melamine functional groups were observed as a rough surface (Figure 2b). We further observed XPS to characterize the chemical composition and detect functional groups in the biochar and m-biochar. Compared with the biochar (C1s: 284.07 eV, O1s: 530.51 eV), additional N1s peaks (398.91 eV) appeared in the m-biochar (Figure 2c,d). The high resolution C1s spectra of the biochar was decomposed to three functional groups, C–C, C–O, and C=O, at 284.5 (69.09%), 286.2 (18.74%), and 289.0 (12.17%) eV, respectively (Figure 2e) [28]. In addition, the high resolution C1s spectra of the m-biochar was decomposed to four functional groups, C–C, C–O, C–N, and C=O, at 284.5 (68.83%), 286.2 (11.67%), 286.6 (10.37%), and 289.0 (9.12%) eV, respectively (Figure 2f) [26,29,30]. This indicated that additional amine groups on the melamine were successfully functionalized on the biochar surfaces. Raman spectroscopy showed defects and interaction differences between the biochar and m-biochar (Figure 2g). The Raman spectra of the biochar and m-biochar showed D-, G-, and secondary D-bands at 1350, 1580, and near 2700 cm⁻¹, respectively. Compared with that of the biochar, the ID/IG ratio of the m-biochar slightly increased from 0.767 to 0.847. This indicated that the applied mechanical force during the electromagnetic functionalization process might increase the number of defects or sp3-bonded carbon atoms. The 2D band also shifted after functionalization. Compared with that of the biochar, the 2D band of the m-biochar shifted from 2631.65 to 2676.08 cm⁻¹. This shift was often observed when monolayer graphene π–π interacted with other layers to form multilayer graphite. Similarly, additional π–π interactions of biochar with melamine up-shifted the 2D band as other related sp2 hexagonal carbon structures [31]. Therefore, the presence of amine groups on the m-biochar and the presence of enhanced π–π interaction layers demonstrated the successful non-covalent functionalization of melamine. Thermogravimetric analyses (TGA) revealed the amount of noncovalent functionalization of melamine on the biochar (Figure 2h). Biochar exhibited high thermal stability and did not decompose until 800 °C. However, the melamine began to decompose at 340 °C. The m-biochar showed a similar decomposition behavior, and the quantity of functionalized melamine was estimated to be approximately 11 wt.% at.

These functionalization processes facilitated the highly dispersive properties in the polar solvents, with the m-biochar exhibiting stable dispersive properties even after 24 h (Figure 3a). After 24 h, the 0.025 wt.% m-biochar dispersed in double distilled water (DDW) exhibited 100% of the absorption value at 550 nm, but 0.025 wt.% biochar dispersed in DDW only exhibited 10% of the absorption value at 550 nm. The dispersion stability is shown in Figure 3b. After 24 h of dispersion, the m-biochar did not exhibit agglomerated particles or precipitants. However, the prepared biochar exhibited numerous agglomerates, and precipitates were observed with clear separation of the dispersion and non-dispersion regions. Thus, the agglomeration of biochar caused by the π–π bonds hindered the homogeneous mixing and hydration of the concrete, and the casting process of the concrete was unavailable. However, the m-biochar did not agglomerate in polar solvents and was homogenously dispersed in the cement matrix (Figure 3c).

Owing to the homogeneous dispersion of m-biochar, the formation of an effective load transfer and conductive network was expected [32]. Hence, we fabricated m-biochar-reinforced cementitious composites and measured their compressive modulus and strength. The results clearly demonstrated that the m-biochar-reinforced cementitious composites enhanced the mechanical properties of the cement matrix. Representative compressive stress–strain curves for the pure cement paste and m-biochar-reinforced cementitious composites are shown for m-biochar contents of 0–1 wt.% (Figure 4a). Figure 4b,c shows the average compressive modulus and average compressive strength of the m-biochar-reinforced cementitious composites. The compressive modulus and strength of pure cement paste were 14.70 ± 0.33 GPa (n = 3) and 48.12 ± 1.09 MPa (n = 3), respectively. As a reinforcing agent, both the compressive modulus and strength were maximized when 1 wt.% of m-biochar was added. The compressive modulus and strength of the cementitious composites containing 1 wt.% of m-biochar were 20.01 ± 1.11 GPa (n = 3) and 55.79 ± 2.75 MPa (n = 3), respectively, which corresponded to an improvement of 36% and 16% over those of the pure cement paste. These melamine effects on the mechanical properties were similarly found in other conductive carbon fillers, such as carbon nanotubes and GNPs with polymer based matrices [32,33,34]. The melamine both had polar amine functional groups and cyclic carbon structures that were able to enhance the interfacial bonding to the cement matrix via van der Waals interactions, covalent grafting, and charge transfer interactions. Furthermore, a small amount of homogeneously dispersed m-biochar formed an electrically conductive network. The electrical conductivity of pure cement paste is on the order of 10⁻¹² S/cm. An addition of only 0.05 wt.% m-biochar could enhance the electrical conductivity up to the order of 3.64 × 10⁻⁵ ± 1.02 × 10⁻⁶ S/cm (n = 6), which corresponded to an improvement of over seven orders compared to that of pure cement paste (Figure 4d). According to our observations, we assumed that the percolation threshold of m-biochar was expected to be between 0.02 and 0.05 wt.%. Compared with conventional GNP conductive fillers, m-biochar exhibited electrical conductivity of approximately one order of magnitude higher. Cementitious composites reinforced with 1 wt.% of conventional GNP exhibited electrical conductivity in the order of 10⁻⁶ S/cm (6.64 × 10⁻⁶ ± 2.01 × 10⁻⁶ S/cm (n = 6)). This indicated that the homogeneous dispersion of m-biochar maximized the effective contact areas and pathways for electron tunneling between sp2 domains on the m-biochar [35,36].

4. Conclusions

We successfully developed a mass-productive functionalization process for electrically conductive cementitious composites. Non-covalent functionalization driven by electromagnetic forces accelerated the functionalization process for mass production. Thus, the m-biochar did not agglomerate in polar solvents and thus was homogenously dispersed in the cement matrix as a conductive network. Compared with conventional GNP fillers, the electrical conductivity of the m-biochar exhibited electrical conductivity of approximately one order higher with only a small addition, and the percolation threshold of m-biochar cementitious composites was between 0.02 and 0.05 wt.%. In addition, the homogeneously dispersed biochar enhanced the mechanical strength by 16% through effective load transfer. The foundation of the functionalized conductive biochar cementitious composites could facilitate smart concrete applications such the as self-sensing of structural behavior, heated pavement systems, and concrete based energy storage systems where both high electrical conductivity and strength are required.