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Clean Technologies and Environmental Policy

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Open Access 29-07-2023 | Original Paper

Strong, lightweight, thermally stable, and hydrophobic sustainable structural composites produced from coal-based waste and polymer-derived SiOC ceramics

Authors: Orevaoghene Eterigho-Ikelegbe, Emmanuel Ricohermoso III, Hamza Harrar, Ralf Riedel, Samson Bada

Published in: Clean Technologies and Environmental Policy | Issue 9/2023

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Abstract

Recycling coal-based waste (CBW) into composites suitable as a building material might be a necessary response to combat its risk to the environment. Therefore, the objective of this study was to investigate the microstructure and performance of coal composites produced from CBW and polysiloxane polymer (SPR-212). Four types of CBW that differ in physicochemical properties were examined. Fourier transform infrared spectroscopy results indicated that the higher the intensity of the C=C bonds in the CBW, the higher the pyrolysis mass loss and shrinkage experienced by the composites during pyrolysis. The continuous operating temperature of the composites is up to 600 °C. However, at temperatures above 600 °C, composites containing carbon content greater than 36% manifested dramatic degradation. Pyrolysis mass loss in the range of 5.28 to 29.62% was obtained for all the composites. The density range of the composites is between 1.5 and 1.9 g per cubic centimetre. The water absorption of all the composites is within the range of 0 to 25% and is comparable to many building materials. Notably, the composites containing total carbon less than 10% registered a water contact angle greater than 90°, indicating the low wettability of their surface. Furthermore, composites that embodied the highest total carbon (63%) displayed the worst structural property. The findings of this study lay the foundation for further development of high-quality structural coal composites from CBW and the SPR-212 preceramic polymer through optimisation of the processing conditions.

Graphical abstract

The original online version of this article was revised: Graphical abstract and figure 1 are updated.

A correction to this article is available online at https://doi.org/10.1007/s10098-023-02594-0.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Introduction

As with any mineral extraction activity, coal mining is associated with the disposal of various types of coal wastes (CW) in tailings dumps. Also, coal fly ash (CFA) represents 70-85 wt.% of the total ash generated from coal combustion in thermal power stations. As of 2012, it was estimated that about 800 million tonnes of CFA are generated worldwide annually (Blissett and Rowson 2012; Yao et al. 2014). Traditionally, CW and CFA (grouped as coal-based waste (CBW)) are diverted to landfill and have become a permanent feature around many coal mines and power stations worldwide due to their underutilisation. Undoubtedly, CBW dumps pose adverse implications on the natural environment, social well-being, and local resources (Haywood et al. 2019; Li et al. 2018; Mbedzi et al. 2020; Tambwe et al. 2020; Zhao 2012). There has been progress in the potential application of CFA and CW (Haibin and Zhenling 2010; Blissett and Rowson 2012; Yao et al. 2015; Fecko et al. 2013; Hu 2016). With a focus on circularity worldwide, a forecasted rise in CBW for much of the next 20 years, and the unused CBW available in millions of tones, developing multidimensional upcycling strategies can substantially mitigate many of its environmental burdens and enhance global industrial ecology practices. Indeed, this can be seen from the panoply of novel research solutions ongoing to generate value from CBW (Liu et al. 2018; Abdulsalam et al. 2020; Isaac and Bada 2020; Morgan and Wade 2021; Hill and Easter 2021; Eterigho-Ikelegbe et al. 2021a, b, c, 2022; Harrar et al. 2022; Hill et al. 2022). The social and environmental pressures to address concerns over waste reduction drive the incentives for CBW research. At the same time, the built environment and construction sectors are highly raw, intensive materials sectors compared to any other economic activity (Pacheco-Torgal and Labrincha 2013; Hossain et al. 2020). These sectors face increasingly tight resource constraints and a growing scarcity of good-quality raw materials due to excessive and continuous exploitation (Pacheco-Torgal and Labrincha 2013; OECD 2015). 'Resource efficiency/substitution', also known as the use of secondary resources or waste, is a current research trend to address these challenges. The goal of this approach is to promote a circular economy, preserve natural mineral resources, reduce material costs, and diminish embodied energy (Hossain et al. 2020).

One of the innovative directions of coal/CBW research is their upcycling into structural or building composites. Major research in this area include the use of coal as fillers with conventional polymers, followed by moulding (Al-Majali et al. 2019; Bai et al. 2019; Phillips et al. 2019) and firing coal/CBW in itself or mixed with clay in ceramic brick making (Stolboushkin et al. 2016; Xu et al. 2017; Vasić et al. 2021). Recently, some investigators documented a patent using US raw coal and preceramic polymer (PCP) to produce coal composites for building applications (Hill and Easter 2021; Hill et al. 2022). The formulation detail of the PCP is unknown as the information is kept proprietary. Of note, PCP, otherwise known as a ceramic-forming polymer, is a special type of polymer precursor that offers simple and inexpensive access to various ceramic systems known as polymer-derived ceramics (PDCs). The PDCs are achieved through a controlled heating process of crosslinking and pyrolysis under an inert or reactive atmosphere. These ceramic-forming polymers are used to produce advanced/technical ceramics, ceramic fibres/matrix composites, monolithic bodies, foams, coatings, membranes, and microelectromechanical systems (MEMS) (e.g. microreactors and microsensors) (Liew et al. 2000; Colombo et al. 2010; Eckel et al. 2016; Wen et al. 2022). The remarkable properties of PDCs stem from the unique combination of their inherent covalent (polar) Si-O-Si bonds and amorphous nature (Ionescu et al. 2012; Černý et al. 2015).

The manufacturing process of the patented coal composites involves curing the coal/PCP mixture followed by pyrolysis treatment (Hill and Easter 2021; Sherwood et al. 2021; Hill et al. 2022). Several attributes of the composites include lightweight, adequate strength, thermal stability, fire resistance, etc. These attributes make them suitable for application in the construction industry as roofing tiles, bricks, panels, and blocks. According to the investigators, high volatile matter coals produced composites that were inferior compared to the lower volatile matter coals. The interesting result achieved by Hill and Easter (2021) using the run-of-mine thermal coal for power generation inspired the investigation of South African high-ash coal waste with the same proprietary polymer (Eterigho-Ikelegbe et al. 2021b, 2022). The characterisation of the microstructural and physicomechanical properties of the produced composites has been documented.

For this study, different South African CBW were investigated to explore the potential of this approach further and to divert them from landfills. As suggested by Hill and Easter (2021), other PCPs capable of being pyrolysed to form ceramic may be selected or tested. Within this context, it becomes necessary to broaden the research using a commercial PCP as the ceramic binder. It is known that the physicochemical properties of CBW vary according to its origin and the seam from where the coal is mined. As such, CBW selection becomes an important factor when considering commercial PCP. Many poly(organosiloxanes) (PSOs) have excellent hydrophobic properties, act as water-repellent coatings, and are used in construction as sealants and electro-insulating agents (Greil 1995; Gumula et al. 2009; Colombo et al. 2010). PSOs are available in high volumes and easily accessible, show excellent physicochemical properties, have a long storage life, are low cost, and are easy to handle under ambient conditions (Greil 1995; Colombo et al. 2010; Černý et al. 2015). Based on these attributes, SPR-212, a commercial SiOC-yielding PCP under the class of PSOs, was considered the ceramic-forming binder for this study. Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) was employed to assess the chemical composition of the raw materials and the pyrolysed composites. X-ray diffraction (XRD), scanning electron microscopic (SEM), and Raman spectroscopy were used to provide information on the mineral composition, microstructure, and carbon phases of the composites. Thermogravimetric analysis (TGA) was used to evaluate the thermal stability/ continuous operating temperature of the composites. The density, water absorption, and apparent porosity of the composites were calculated following ASTM C67 and C373 standards. The surface property of the composites was evaluated using water contact angle measurement while their mechanical performance was studied using compression strength tests.

Experimental and characterisation

Materials and preparation of the composites

The coal-based waste (CBW) used for this study, i.e. GT, GG1, and GS were sourced from different collieries in South Africa. GT is a fine from the mined semi-soft coking coal while GG1 is a discard from the washing plant, both from the Waterberg coalfield. GS was collected from the Witbank coalfield and fly ash (FA) was obtained from one of Eskom’s coal-fired power stations in the Witbank region. Commercially available low-viscous methylvinylhydrogen polysiloxane (Polyramic® SPR-212 from Starfire Systems Ltd., USA), with a 17 wt.% carbon content, was used as the ceramic-forming binder for the coal matrix. As shown in the inset of Fig. 1, the polysiloxane contains monomethylsilane and methylvinylsilane units in the Si-O-Si backbone of the polymer. The polymer was used without solvent or catalyst. The as-received size of GT, GG1, and GS samples from the mines was around -212 µm. These samples were then sent to an external laboratory where it was milled to -106 µm according to ISO 13909 standard (ISO 13909-1: 2016) for coal sampling. FA was used in the as-received state without any pre-treatment. About 60% of the particle size distribution of the CBW is under -82 μm. The particle size distribution and textural property results of the CBW are presented in Table 1. The preparation of the composites was based on method adopted from the literature (Hill and Easter 2021; Sherwood et al. 2021; Hill et al. 2022). In this study, a ratio of CBW to SPR-212 of 80:20 weight per cent was poured into a porcelain mortar and properly hand-mixed for about five minutes with a pestle until homogeneity was achieved. About 1.93 g of the resultant mixture was evenly distributed into a die and compacted using a manual press of applied force, 8 to 10 kilonewtons for about 30 s before release to produce circular composites. For the CBW particles to be well laminated by the precursor and to maintain structural integrity during pyrolysis, thermal crosslinking was initiated. The shaped bodies placed in alumina crucibles were heated in an airtight furnace with a ramp rate of 5 °C/min up to 150 °C (below the degradation temperature of the CBW and PCP) and held for one hour. Finally, the resulting cured bodies were pyrolysed in a sealed alumina tube furnace at a rate of 100 °C/h (1.67 °C/min) under flowing argon to 1000 °C and held isothermally for ten hours. After pyrolysis, the furnace was allowed to cool naturally to room temperature at 150 °C/h. Figure 1 demonstrates the summary of the production process of the composites.

Table 1

Particle size distribution and textural properties of the CBW samples

Property	GG1	FA	GT	GS
PSD D10, µm	38.92	10.05	20.30	0.28
PSD D50, µm	87.07	35.56	58.79	11.22
PSD D90, µm	99.82	55.63	70.29	43.52
MBET, m²/g	9.89	1.29	3.05	3.61
TPV, 10^–2 cc/g	3.09	0.47	1.76	1.14

GG1 discard from washing plant, FA fly ash, GT fine waste, GS coal waste from Witbank coalfield, PSD particle size distribution, MBET multi-point BET surface area measured by nitrogen adsorption, TPV total pore volume, D10 µm 10% of GG1 sample has a size of 38.92 µm or smaller

Physicochemical and microstructural characterisation of the coal-based wastes and composites

Physicochemical analysis was conducted to acquire an understanding of the physical and chemical variation of coal-based waste (CBW). Proximate and ultimate analysis of the CBW was conducted following ASTM D5142 (ASTM D5142, 2009) and ISO 12902-CHN standard (ISO/TS 12902, 2001), respectively. The elemental composition of the composites was determined using a Flash 2000 CHNS elemental analyser (Thermofisher Scientific). Attenuated total reflectance-FTIR spectroscopy (Spectrum Two™ spectrometer from PerkinElmer) was used to identify the various functional groups in the CBW and composites. Fine samples were scanned in the transmission mode from 400 to 4000 cm⁻¹ at a step size of 4 cm⁻¹. XRD patterns were obtained using the D2 phaser equipment (Bruker Corporation, Germany) containing an X-ray generator operating at 30 kilovolts and 10 milliamperes with a copper tube radiation (wavelength = 1.54184 Å). The analysis was conducted using a step size of 0.01° and a dwell time of ten seconds per step with a range of diffraction angle from 2-theta of 10 to 90°. The mineralogical phases in the CBW and composites were qualitatively identified using the PANalytical X’Pert software. Surface morphologies of carbon-coated composites were obtained using a scanning electron microscope (Sigma 300 VP, ZEISS, Germany). Energy-dispersive X-ray spectroscopy was used to estimate the semi-quantitative chemical compositions of the composites. The Olympus BX41 fluorescence microscope provided additional information on the microstructure of the composites’ surface. The degree of graphitisation of the carbon phase of fine composite ceramics was investigated with a Raman spectrometer (LabRAM HR fitted with an Olympus BX41 microscope and a 514.5 nm laser excitation source of the Lexel Model-95 SHG argon-ion laser). A thermogravimetric analyser (TGA701, LECO Corp., USA) was used to determine the continuous operating temperature of the composites. Approximately one gram of broken pieces of the composites was placed under an air and oxygen atmosphere in the temperature range of 25-900 °C and at a ramp rate of 10 °C/min.

Physicomechanical and surface properties of the composites

The properties and performance of the pyrolysed composites were investigated based on their bulk density, firing shrinkage, water absorption, and compression strength. The pyrolysis mass loss, radial shrinkage, and volume shrinkage experienced by the cured composites upon pyrolysis were determined by measuring the dimensions of the pyrolysed products before and after pyrolysis using Eqs. 1 to 3. Five composites were measured using a sliding gauge, and the average value was used to represent the total shrinkage and pyrolysis mass loss to the nearest 0.01%.

$$\mathrm{PML}=\frac{{M}_{\mathrm{P}}-{M}_{\mathrm{C}}}{{M}_{\mathrm{c}}}\times 100$$

(1)

$$\mathrm{RS}=\frac{{L}_{\mathrm{p}}-{L}_{\mathrm{C}}}{{L}_{\mathrm{c}}}\times 100$$

(2)

$$\mathrm{VS}=\frac{{V}_{\mathrm{p}}-{V}_{\mathrm{C}}}{{V}_{\mathrm{c}}}\times 100$$

(3)

where PML is the pyrolysis mass loss (%), RS is the radial shrinkage (%), VS is the volume shrinkage (%), ${M}_{\mathrm{c}}$ is the cured mass of the composite (g), ${M}_{\mathrm{p}}$ is the pyrolysed mass of the composite (g), ${L}_{\mathrm{c}}$ is the cured length of the composite (mm) and ${L}_{\mathrm{p}}$ is the pyrolysed length of the composite (mm).

Water absorption, bulk density, apparent porosity, and apparent specific gravity were computed with guidance from ASTM C67 (ASTM C67, 2010) and C373 (ASTM C373, 2018) standards and following Eterigho-Ikelegbe et al. (2021b). The uniaxial compressive stress performance of the composites was tested using a benchtop universal testing machine (H100K-S) with a capacity of 10 Tonnes and guidance from ASTM C67 (ASTM C67, 2010) standard. The composite was placed flatwise between two steel loading platens to minimise machine wear. The load was applied continuously at a steady rate of 1 mm/min up to failure. The ultimate compressive stress was automatically generated using Eq. 4.

$$C = \frac{W}{A}$$

(4)

where C is the ultimate compressive stress to the nearest 0.01 MPa, W is the maximum load indicated by the testing machine (Kgf or N) and A is the average gross area of the upper and lower surfaces of the composite (mm²).

The wetting behaviour/surface property of the composites was investigated based on the static contact angle (CA) measurement. An OCA 15EC goniometer (DataPhysics Instruments, GmbH, Germany) was used for this investigation. The composite surface was dosed with a droplet of water (dosing volume = 3 μL; dosing rate = 2 μL/s) at room temperature for ten seconds. The average water CA was recorded after taking at least seven readings at different spots on both surfaces of the composites.

Results and discussion

Physicochemical assessment of the coal-based waste and coal composite

The physicochemical property of the coal-based waste (CBW) in terms of proximate and elemental compositions is summarised in Table 2. The CBW reveal substantial differences regarding their volatile matter, fixed carbon, ash content, and total carbon. GT, being a semi-soft coking coal fine, manifested the lowest ash content, and the highest fixed carbon and total carbon. The hydrogen/carbon (H/C) atomic ratios of GG1 (1.68) and GS (0.87) suggest it contains a high proportion of aliphatic chains. In addition, GG1 has a high chance of forming hydrogen bonds with the PCP and it is beneficial for this type of application (Hu et al. 2017; Eterigho-Ikelegbe et al. 2021c). GG1 also has the highest oxygen/carbon (O/C) atomic ratio, implying that it is an oxidised coal rich in oxygen-containing functional groups (carboxyl, carbonyl, ether, and hydroxyl). These groups impart GG1 with good chemical reactivity (Wang and Zhou 2011; Tan et al. 2019; Eterigho-Ikelegbe et al. 2021c). The total carbon of the GT composite (63%) from Table 3 suggests that a good proportion of its mass contains carbonaceous materials after pyrolysis. The oxygen + silicon fraction in Table 3 assumes that no other elements are present in the composites other than the ones listed.

Table 2

Physicochemical composition of the coal-based wastes (wt.%, air-dried basis)

Sample ID	Proximate analysis				Elemental composition					Atomic ratio
Sample ID	M	VM	A	FC^*	TC	H	O^*	N	TS	H/C	O/C
GG1	3.28	10.01	81.26	5.44	7.50	1.06	5.46	1.44	–	1.68	0.55
FA	0.14	0.47	99.18	0.22	–	–	–	–	–	–	–
GT	3.84	32.7	11.44	52.06	69.5	4.90	7.97	1.41	0.94	0.84	0.09
GS	1.71	20.37	61.65	16.27	23.77	1.74	8.02	0.63	2.48	0.87	0.25

GG1 discard from washing plant, FA fly ash, GT fine waste, GS coal waste from Witbank coalfield, A ash content, FC fixed carbon, M inherent moisture, TC total carbon, TS total sulphur, VM volatile matter

*Oxygen by calculation = [100-(M + A + carbon + hydrogen + nitrogen + sulphur)]; *FC by calculation = [100-(VM + A + M)])

Table 3

Elemental composition of the pyrolysed composites

Sample ID	TC	H	N	TS	O + Si
GG1	9.58	0.32	0.17	0.64	89.29
FA	1.46	0.26	0.07	0.28	97.93
GT	62.66	0.65	0.80	0.36	35.58
GS	35.7	0.48	0.38	1.86	61.58

TC total carbon, H hydrogen, N nitrogen, TS total sulphur, O oxygen, Si silicon

Microstructural analysis of the coal-based wastes and coal composites

Fourier transform infrared spectra

FTIR spectroscopy was used to determine the functional group composition of the CBW, the SPR-212 polymer, and the resultant composites. FTIR spectroscopy was also useful in probing the chemical interactions that occurred during the pyrolysis of the blend of coal and the SPR-212 polymer. The main functional groups of the SPR-212 resin (Xifan 2019) are shown in the FTIR spectrum of the liquid displayed on an absorbance scale (Fig. 2). The SPR-212 contains silicon-bonded hydrocarbon constituents, including methyl (CH₃) and vinyl groups (-CH=CH₂), hydride (Si-H) functionality, Si-O (backbone), and Si-C bond structures. The FTIR spectra of CBW and the resultant composites are compared in Fig. 3. The summary of functional groups detected in the CBW and coal composites is listed in Table 4. The assignment and identification of various functional groups in coal were referenced to work presented in the literature (van Niekerk, 2008; Ferraro and Basile 2012; van der Merwe et al. 2014). Only GT fines feature the aliphatic C-H stretch (f₂) spotted between 2197 and 2852 cm⁻¹. FA, a by-product of coal combustion from a power-generating plant, did not feature the main bands normally found in coal.

Table 4

Summary of the various functional groups within the coal structure

Wave number (cm⁻¹)	Peak/Vibration	Band assignment/functional groups
3690 to 3618	s₁	O-H stretching vibrations relating to kaolinite, mineral matter (surface and inner surface -OH groups)
2917 to 2852	s₂	Aliphatic C-H stretch
2917		Asymmetric CH₂ vibrations
2852		Symmetric CH₂ and CH₃ vibrations
1598	d₂	Aromatic C=C stretching vibrations
1440	f₁	CH₂, CH₃ bending vibrations
1368	f₂	CH₃ groups
1300 to1000	f₃	C-O stretch, C-C stretch, and O-H bending vibrations, alcohols, phenols, ethers, phenoxies and esters
911 to 684	f₄	Si-O-Si asymmetric stretching vibrations and Al-O-Al symmetric stretching vibrations associated with aluminates and silicates; out-of-plane aromatic C-H bend
< 700	f₅	O-Si-O symmetric bending vibrations/deformations associated with aluminates and silicates; Al-O-Al stretching vibrations associated with minerals; S-S vibrations associated with pyrite
Composites
3700 to 3600	p₁	O-H stretching vibrations
3000 to 2900	p₂/p₃	Alkyl C-H stretching vibration
1395	p₄	C-H stretching vibration
1229	p₅	C-H stretching vibration
1055	p₆	Si-O-Si asymmetric stretching vibration
879	p₇	quartz vibrations

From the literature (Hill and Easter 2021; Sherwood et al. 2021), moisture, low temperature/light volatile molecular species, and hydrocarbon are released creating nanopores within the coal particles and/or coal-PCP interface between 5 and 450 °C. Hence, the bands around 3700 to 3500 cm⁻¹ (s₁) assigned to the O-H stretching vibrations of the mineral matter in the coal declined in the composites (p₁). Absorption bands (s₂) observed between 3000 and 2800 cm⁻¹ as a result of the aliphatic C-H moieties in coal were more intense in the composites (p₂, p₃). The disappearance of the Si-vinyl (Si-C=C), Si-hydride (Si-H), and methyl silyl (Si-CH₃) peaks in the polymer suggest that these bonds broke down during pyrolysis. The redistribution reactions between these bonds with that of the coal resulted in increased absorbance of the aliphatic structural groups (alky C-H) denoted by p₂, p₃, p₄, and p₅. The C-H band is most absorbed in the FA composite and can be linked to an increase in aliphatic molecules emanating from the PCP pyrolysis.

The aromatic C=C bands in coal at about 1600 cm⁻¹ (GT > GS > GG1 > FA) in terms of absorbance) are heat resistant between 50 and 450 °C (Khare and Baruah 2014). At the same time, the single bonds in the PCP (Si-H, C-H, and Si-C) and in the coal generate various hydrocarbon radical intermediates (Sharma 2018) that nucleate on bonding substrates. It is believed that the intense degradation of the aromatic C=C bonds occurred between 450 and 800 °C with the corresponding evolution of relatively high molecular weight species (devolatilisation). Above 400 °C, the PCP starts transforming to ceramic through the cracking of the Si-C bonds (Černý et al. 2015). According to Taha et al. (2017), the destruction of the double bonds in coal suggests the decomposition of organic matter. As pyrolysis progresses, some of the dissociated bonds recombine with the macromolecular fragments generated during pyrolysis to form new bonds (resolidification). Subsequently, minerals such as kaolinite undergo dihydroxylation to form metakaolin (amorphous kaolinite) and pyrite decomposes into elemental iron and sulphur (O’Gorman and Walker 1973; Hill and Easter 2021). Apart from the contribution of weight loss that occurred during the polymer transformation to ceramic, the decomposition of the C=C bonds in coal was directly linked to the pyrolysis mass loss and shrinkage experienced by the composites. It was observed that the higher the intensity of this bond, the higher the mass loss and shrinkage of the composites, as discussed in Sect. "Weight loss and shrinkage during pyrolysis".

Above 800 °C, amorphous kaolinite decomposes into nanosized silica (silicon dioxide) and alumina (aluminium oxide) and to amorphous mullite (not detected by XRD). These nanosized products then merge to domains of graphitised carbon and are chemically bound into a ceramic structure through Si-O-Si bonds formed by the transformation of the PCP into ceramic (Solomon 1981; Hill and Easter 2021). Referring also to the FTIR spectra presented in Fig. 3, the f₃ peaks of the coals have transformed to p₆ in the composites. The disappearance of f₄ peaks in the coals assigned to the aromatic C-H vibrations reflects a substitution reaction that had taken place during pyrolysis. The fingerprint region of the CBW (f₃-f₅), in general, underwent extensive rearrangement transforming to a dominant band centred around 1065 cm⁻¹ (p₆) in the composites. This band has been assigned to the -Si-O- stretching vibration associated with quartz and Si-O-Si bonds in ceramics (Gumula et al. 2009), while the weak peaks below 1000 cm⁻¹ observed in the composites were ascribed to quartz and Si-C vibrations (Gumula et al. 2009; Taha et al. 2017).

X-ray diffraction analysis

X-ray diffraction was utilised to investigate the mineral phase rearrangement after pyrolysis treatments (Fig. 4). The absence of an amorphous hump suggests that the FA does not contain the aluminosilicate glass phase. Mineralogically, the association of crystalline quartz (silicon dioxide) and mullite phases is mainly visible in the FA waste. Mullite is an aluminosilicate mineral formed from the decomposition and transformation of kaolinite (a clay mineral) at elevated temperatures (Xu et al. 2017; Rautenbach et al. 2020). Quartz, anorthite, and hematite mineral phases were detected in the FA composites. The anorthite (calcium aluminosilicate) mineral phase is usually formed by the reaction of mullite and the transformed product of lime (calcium oxide). The detection of anorthite suggests that minor impurities like calcium oxide might be present in the FA (Yuan et al. 2022). Further work is needed to fully characterise the oxides in the coal using X-ray fluorescence analysis. Microcline, a potassium-rich alkali feldspar was detected in GS coal while the kaolinite phase is well present in GG1, GS, and GT. The basic unit of the kaolinite mineral consists of a tetrahedral sheet of silicon dioxide siloxane units and an octahedral sheet of aluminium hydroxide (Zhang et al. 2017). Referring to the mineralogical composition of GG1, GS, and GT composites, the results show that the kaolinite mineral phases in these coals have disappeared. Ideally, kaolinite decomposes at around 700 °C through a process of dihydroxylation (Rautenbach et al. 2020) to form quartz. Besides, some quartz still exists in these composites but in a different phase because at 570 °C quartz undergoes quartz inversion (O’Gorman and Walker 1973; Wang and Zhou 2011). Furthermore, quartz is a refractory mineral and is usually non-reactive depending on the mineral association. The new mineral phase rutile (titanium dioxide) detected in the GG1 composite can be attributed to the dissolution reaction of quartz in the presence of kaolinite during pyrolysis. The presence of a broad hump centred at a 2θ-theta value of approximately 10° typical of amorphous carbon or carbonaceous matter (Song et al. 2011) is observed in the diffractogram of GT coal and the GT composite. As stated earlier, GT is a high-rank coal (semi-soft coking coal fines); hence, some graphite crystallite peaks were detected as a result of pyrolysis up to 1000 °C.

Scanning electron and optical imaging

Scanning electron microscopy-backscattered electron (SEM-BSE) and energy-dispersive spectroscopy (EDS) analysis was used to observe the surface morphologies and elemental differences of the composites. SEM-BSE images of the carbon-coated composites presented in Fig. 5 show some cracks on the surface of the composites. These cracks could have been caused by thermal stresses and shrinkage experienced by the composites during pyrolysis. It could also be possible that the cracks were formed when breaking a small composite sample for SEM imaging. The area energy-dispersive spectroscopy of the composite surface shows the stoichiometry of the detected elements. The FA composite contains traces of carbon and anorthite grains associated with quartz. In addition, quartz, representing the most abundant phases, seems to be bonded by a carbon matrix formed from the pyrolysis of the PCP. SEM imaging of the GT composite revealed grains of phosphate minerals (monetite). The surface of the GT composite is homogenously darker than the other composites, attributed to the relatively high carbon in the composite. The composites appear smooth; however, at high SEM magnifications, their surfaces are rough. Therefore, to enhance surface smoothness and uniformity, using a polytetrafluoroethylene release film to line the mould in direct contact with the mixture being pressed might be beneficial. Another solution might be to add soluble wetting agents, e.g. water, surfactant, or solvent, to the coal/PCP mixture. The optical microscopic images of fabricated composites (Figs. 6(c, f, i, and l)) show good incorporation of the mixture content and spatial distribution of minerals on the composites’ surface. Large quartz grains can be observed on the FA composite, while apatite grains can be seen on the GS composite (Fig. 5b).

Raman analysis

The carbon phase is an important component of carbon-bonded composites. As noted earlier, the mixture of the CBW and the PCP pyrolysed up to 1000 °C could have developed a network of graphene fragments or nanodomains of graphitic carbon (Sharma 2018; Hill and Easter 2021). Therefore, Raman spectroscopy was used to acquire information pertaining to the carbon structure and the graphitisation degree of the carbon atoms. The Raman spectra of the composites manifest peaks at around 1360 and 1560 cm⁻¹, the typical D-band and G-band in carbon-based materials (Fig. 6). The presence of both bands suggests that the carbon resulting from the pyrolysis of the mixture is in a disorganised state and consists of the infinite repetition of sp³ and sp² hybridised C-C bonds. Broad peaks centred around 2750 cm⁻¹ and 3200 cm⁻², such as the second-order 2D (or G’) and the 2D’ bands observed in graphite samples (Ferrari 2007), are more dominant in the Raman spectra of the GG1 and FA composites. In principle, the 2D-band supports the presence of amorphous carbon and is characteristic of turbostratic carbon (Ruz et al. 2016). Also judging by the existence of the 2D peak in the GG1 and FA composites, as speculated earlier, some graphene layers may have formed (Sazali et al. 2016).

The D’ peak close to the G-band is usually observed in the nanostructures of disordered carbon materials (Dresselhaus et al. 2010; Pusz et al. 2015), such as those in this study. The emergence of peaks less than 1000 cm⁻¹, such as 445 cm⁻¹ associated with quartz and 758 cm⁻¹ attributed to the Si-C peak, and the presence of the Si-O-Si bond/vibration can be seen in the spectra of the GG1, FA, and GS composites but surprisingly was not Raman active in the GT composites. A_D/A_G was calculated from the ratio of the areas beneath the D- and G-bands, and the size of carbon crystallites (L_a) was used to quantify the disorder (Table 5). The intensity ratio generally increases with the amount of carbon phase disorder, and the trend follows FA > GS > GT > GG1, meaning that the least graphitic (i.e. highly defective) carbon phases are dominant in the FA composite. The GG1 composite supports the dominance of the graphitic carbon phases (sp² C-C bonds) suggesting the formation of fewer defects. A_D/A_G correlates inversely to L_a and agrees with the cluster size of the carbon phase of the composites, FA composites being the least (1.16 nm). As noted earlier, the total carbon of the FA composite (1.46%) was mainly provided by the pyrolysis of the polymer.

Table 5

Graphitisation indices of the composites

Composites/Indices	GG1	FA	GT	GS
A_D/A_G	1.45	3.78	2.21	2.90
L_a	3.03	1.16	1.98	1.51

Thermal stability/continuous operating temperature of the composites

The thermal stability of the composites was investigated to monitor the quantitative weight change during the thermal degradation of one gram of composites placed in a TGA furnace under different environments (air and oxygen) at a ramp rate of 10 °C/min to 900 °C. The minimum and maximum weight loss rate temperatures and the decomposition temperature at T5% were used to evaluate the thermal stability of the composites (Table 6). Two temperature regions were identified in Fig. 7 where weight loss occurred. Stage 1 weight loss (less than 200 °C) is related to the loss of adsorbed moisture and low molecular weight volatile matter in the case of the GT composite. The weight loss of the GT and GS composites in an air environment (Fig. 7b) around 600 °C (stage 2) was about 9.09% and 4.55%, respectively. Above 600 °C, the GT and GS composites show progressive weight loss of about 14.3% and 14.23%, respectively. The GT composite displayed the worst thermal stability performance owing to the thermal decomposition of the carbonaceous materials in the composites. In contrast, a small weight change was recorded for the GG1 and FA composites when heated in an oxygen or air environment. FA, being a non-combustible material, produced the most thermally stable composite. This observation is consistent with the XRD results that showed highly stable and high melting point minerals, e.g. quartz, anorthite, and hematite were detected in the FA composite. Besides, the high thermal stability of the GG1 and FA composites suggests they contain a reasonable amount of Si-O bonds and fewer C-C and C-O bonds compared to the GS and GT composites. The bond dissociation energy of the bonds C-C (82.6 kJ/mol) and C-O (85.2 kJ/mol) are low compared to the Si-O bond (108 kJ/mol) (Jones et al. 2013; Stabler et al. 2018).

Table 6

Summary of the thermogravimetric analysis of the composites shown in Fig. 7

Composites	T₁ (°C)	T₂ (°C)	T5% (°C)	Stage 1 weight loss (wt.%)	Stage 2 weight loss (wt.%)	Total weight loss at 900 °C (wt.%)
(a) Oxygen environment (99.5% purity)
GG1	85.85	650.83	604.14	1.40	9.10	10.50
FA	89.24	633.87	–	0.85	0.41	1.26
GT	104.51	673.97	183	6.36	28.69	35.05
GS	99.21	678.76	593.77	1.54	23.59	25.13
(b) Air environment
GG1	109.98	683.54	671.57	1.74	8.21	10
FA	90.83	649.23	–	1.12	0.55	1.68
GT	139.11	681.15	191.02	6.23	17.17	23.40
GS	118.56	677.56	609.99	3.19	15.59	18.78

T₁ and T₂ temperature corresponding to the minimum and maximum weight loss rate, T5% decomposition temperature at 5% weight loss, GG1 discard from washing plant, FA fly ash, GT fine waste, GS coal waste from Witbank coalfield

Physical and mechanical analysis of the composites

The difference in the physiochemical structures of the CBW already indicates the possible differences in terms of the properties of the composites. This section discusses the composites’ physical properties (bulk density, apparent porosity, pyrolysis shrinkage, and water absorption) and mechanical properties (ultimate compressive stress).

Weight loss and shrinkage during pyrolysis

The shrinkage value after pyrolysis of the cured composites provides useful information on the size of mould or die that would be needed to produce a predetermined composite size. From Fig. 8, the pyrolysis mass loss of the composites is within the range of 5.28 to 29.62%. All the composites retained their shape, but composites produced from the CBW of high inorganic content (GG1 and FA) presented lower shrinkage values. On the contrary, the GT composite recorded substantial weight loss (29.62%), radial shrinkage (17.97%), and volume shrinkage (46%). This was attributed to the nature of the coal and the high volatile matter content (32.7%). This substantial weight/shrinkage loss, which implies a reduction in the free volume of the composites, resulted from a high degree of conversion of the C=C bonds in the coal to C-C bonds (as shown in the FTIR results). For a small-sized sample, the volume shrinkage of the GT and GS composites seems quite high. As such, it could present a challenge if large-size composites were produced. High shrinkage tends to be associated with contraction stress, thermal stress, cracks, deformation, and warping during pyrolysis. In principle, the PCP decomposes during pyrolysis, followed by the release of hydrogen and other species containing carbon and hydrogen that results in mass loss, volume shrinkage, and densification. As a result, the composites contain some ceramic phases homogenously dispersed in a carbon matrix. Considering that the ratio of the raw materials is 80 wt.% coal and 20 wt.% PCP, it is believed that the shrinkage that occurred is largely dependent on the physicochemical composition of the coal, which, in turn, dictated the degree of conversion of the composites.

Density, water absorption, and apparent porosity of the composites

The bulk density, geometric density, and apparent specific gravity of the composites after the water impregnation stages (five-hour cold test, 24-h cold test, and five-hour boiling test) are displayed in Fig. 9. According to the data, the 24-h cold test and the five-hour boiling test did not significantly increase the density of the composites, demonstrating compactness and micro-cracking of the composites. In general, the bulk density of the composites varied between 1.5 g/cm³ and 1.9 g/cm³. The GG1 composite recorded the highest densities while the GT composite had the least overall. The coal from which the GG1 composite was made has the highest ash content, i.e. more inorganic forming minerals. Besides, GG1 is rich in oxygen functional groups, which facilitate curing reactions, as well as physical and chemical interactions with the polymer during pyrolysis. This might have led to the GG1 composite’s high density, considering that the FA is also rich in minerals; yet, the FA composite is of lower density than the GG1 composite.

The high shrinkage experienced by the GT composite is due to the nature of the coal and is reflected in the composite’s low density. Furthermore, the high volatile matter (32.7%) of GT coal appears to have accelerated degassing and porosity generation within the composite during pyrolysis. Water absorption, i.e. the mass of water absorbed to the mass of the dry composite, provided an indirect measure of porosity and extent of densification. The apparent porosity expresses the volume of open pores of the composites to its exterior volume as a percentage. As expected, the apparent porosity and water absorption follow the same trend, as displayed in Fig. 10. Low water absorption of the GS, FA, and GG1 composites indicates a high volume of impervious pores. These low water absorption composites are essential to resist water intrusion from the natural environment, consequently enhancing their durability. With the water absorption of the composites similar to those of conventional building materials (0 to 25%), these composites may be fabricated into pavement tiles, cladding tiles, bricks, blocks, or roof tiles depending on the application.

Surface property of the composites

Surface property is a critical indicator in the design of coal composites for construction applications as they could impact the durability of composites. In this work, the surface property of the composites was described in terms of the static CA between water and the composite surfaces. The CA was measured from a minimum of four drops on each side of the composite. The CAs of the GT and GS composites with water are less than 90° (Fig. 11), which can be linked to the nonpolar nature of their surface. This observation implies strong interfacial adhesion between the surface of the composites and water. Therefore, the composites have a high tendency to adsorb water. Therefore, they would be highly susceptible to water wetting and may not be useful in applications where interaction with water is needed. In contrast, the CAs of the FA and GG1 composites are greater than 90°, inferring their surfaces are hydrophobic and the force of attraction of water-to-composite is weak. From a practical point of view, these composites are less susceptible to water wetting and penetration, which could cause the composites to deteriorate easily. Apart from being dense and less porous, evidence supporting the FA and GG1 composites maintaining a high CA can be linked to the nonpolar C-H functional groups bound to their surfaces (Gomez-Caturla et al. 2022). As seen from the FTIR spectra (Fig. 11), these hydrophobic functional groups (C-H) are more absorbed in the FA and GG1 composites and are responsible for the weak molecular interaction between water and the composite surface.

Compressive strength performance

The mechanical behaviour of the composites produced was studied using compression tests to investigate the strength of the bonds that connect the atoms and crystals inside the composites. The mechanical properties of the composites in terms of their average ultimate compressive stress are presented in Fig. 12. Given that the various composites offer a difference in their physicochemical properties, the composites produced from the coking coal fines (GT) with 62.66% total carbon and 32% volatile matter, exhibited the lowest resistance to compression (278 MPa). Due to the nature of this coal, it could be postulated that at lower temperatures, GT swelled and experienced better heat transfer with the polymer than the other wastes. Hence, by pyrolysing the composite up to 1000 °C, it is likely that char-like composites with large pores could have formed. These pores might have constituted stress concentrators under load that led to the low mechanical stability of the composites.

The high compressive strength of the GS, GG1, and FA composites can be linked to their high bulk densities. Increased strength is known to come with increased density (Hill and Easter 2021). Besides, the formation of the anorthite phase in the FA composite may have contributed to its strength (Kairakbaev et al. 2017). Comparing the strength of the GG1, FA, and GS composites, GS displays the highest strength while FA has the lowest. The lack of reactive sites (inertness of their surfaces) in FA led to weak interactions with the PCP during pyrolysis, consequently lowering the strength. The use of silane coupling agents can provide adequate reaction sites that would significantly enhance the reactions during pyrolysis but with an additional cost. Interestingly, the mechanical response of the GS composite is the best. This suggests that a certain limit of carbon in the CBW may seem sufficient to increase the bonding capacity of the mix and a relatively high carbon content of the CBW could compromise the strength of the final composites. Therefore, it can be postulated that for a relatively high carbon content CBW and depending on the nature of the coal, the bond strength of the mixture pyrolysed up to 1000 °C would reduce sharply. Comparatively, in terms of their specific compressive strength (MPa.cm³/g), i.e. the strength-to-density ratio, GS recorded the highest, suggesting that the GS composite might be more advantageous for lightweight and high-performance construction material (Ming et al. 2019).

The comparison of the physicomechanical performance of the composites produced in this study to other composites for building applications in the literature is listed in Table 7. The radar chart (Fig. 13a, b, c) illustrates the correlation of the hydrogen/carbon ratio, ash content, and volatile matter of CBW on the key performance properties of the coal/PSO composites. It can be seen that higher H/C ratio CBW (GG1) produced the best quality composites in terms of water absorption, compressive strength, and thermal stability. This is because of the many hydrogen bonds contained in the molecules of this coal waste that enabled it bind effectively with the polymer during pyrolysis (Hu et al. 2017; Hill and Easter 2019). On the other hand, a clear distinction in the low thermal weight loss and carbon content of the composites produced from high-ash wastes (GG1 and FA) is due to the presence of many non-combustible materials in these wastes. Finally, high volatile CBW (GT) yielded composites with the least desired properties overall for building applications (lowest compression strength, highest water absorption, and lowest water contact angle).

Table 7

The physicomechanical performance of the composites produced in this study compared to other composites for building applications in the literature

Materials	LS (%)	BD (g/cm³)	WA (%)	AP (%)	CS/FS (MPa)	References
CFA/NaOH/ feldspar/HPC	14.94	–	0.30	0.62	–/40.25	Luo et al. (2019)
Core sand/clay/water	1.8–3.1	–	5.9–10	–	–	Alonso-Santurde et al. (2012)
Clay/wood pulp/WS/FS	–	< 2	10–16	29–39	–/ < 8	Quijorna et al. (2012)
RM/α-Al₂O₃/γ-Al₂O₃/ρ-Al(OH)₃	-	1.83–1.94	7.37	18.56	–/185.46	Wang et al., (2020)
CBW/PCP(XMAT)	~ 20– ~ 27	1.69–1.77	1.94–10.1	3.44–17.1	84–209/28–35	Eterigho-Ikelegbe et al. (2021b)
CBW/clay/water	–	1.54–1.77	19.4–27.7		18.6–25.8/1.6–2.1	Gökçe et al. (2018)
CBW/clay/water	–	1.5–1.9	20.5–29.5	31.2/48.9	7–21.3/–	Bonczyk and Rubin (2022)
CBW/PCP(XMAT)	18–19.4	1.46–1.7	–	–	–/35–125	Hill and Easter (2019)
CBW/shale/IOT/ SS/water	8.3	1.64	17.5	–	14.24/–	Luo et al. (2020)
CBW/SPR-212	2.8–16.53	1.5–1.9	8.9–14.8	15.5–21.6	278–424/–	Our work

LS linear shrinkage, BD bulk density, WA water absorption, AP apparent porosity, CS compressive strength, FS flexural strength, CFA coal fly ash, HPC high plastic clay, WS waelz slag, FS foundry sand, RD red mud, CBW coal-based waste, PCP preceramic polymer, IOT iron ore tailings, SS sewage sludge

Conclusion

Processing coal-based waste into high-value composites seems like a good way to manage and transform this problematic waste. The variation in the physicochemical properties of coal-based waste on the resultant composites using the poly(organo)siloxane SPR-212 as the ceramic-forming binder was studied. The GT fines used in this study contain the highest percentage of volatile matter and total carbon. As a result, the GT composite displayed the poorest qualities based on the properties evaluated. The relatively high total carbon (36%) in the GT and GS composites increased their susceptibility to heat; hence, their dramatic degradation above 600 °C. Furthermore, the evidence appears to suggest that the reactive functional groups present in GG1 waste promoted the reactivity of GG1 with SPR-212. As a result, high-quality GG1 composites were produced compared to the other composites. Despite its chemical inertness, FA yielded composites of interesting properties. The water absorption of the coal composites produced in this study is in the range of 0 and 25%, and their ultimate compressive stress was up to 420 MPa. Based on the promising results documented in this study, these composites may be designed into roofing tiles, blocks, bricks, or cladding tiles for building applications. However, further development in terms of optimising the process conditions and upscaling the process to produce technical-size coal composites are of great interest for the application of these advanced and sustainable building materials.

Acknowledgements

Orevaoghene Eterigho-Ikelegbe thankfully acknowledges the financial support received from the Department of Science and Innovation National Research Foundation (DSI–NRF) South African Research Chairs Initiative (SARChI) Clean Coal Technology Grant (Grant Number: 86421). The composites were fabricated in the Fachbereich Materialwissenschaft within the Technische Universität Darmstadt, Germany and supported by the German Academic Exchange Service (DAAD) under the short-term research grants (57552337). The authors wish to thank Petra Dinham for obtaining SEM images and Professor Rudolph Erasmus for obtaining Raman spectra. The authors would like to acknowledge the laboratory assistant provided by Mingxing Li.

Declarations

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

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Title: Strong, lightweight, thermally stable, and hydrophobic sustainable structural composites produced from coal-based waste and polymer-derived SiOC ceramics
Authors: Orevaoghene Eterigho-Ikelegbe
Emmanuel Ricohermoso III
Hamza Harrar
Ralf Riedel
Samson Bada
Publication date: 29-07-2023
Publisher: Springer Berlin Heidelberg
Published in: Clean Technologies and Environmental Policy / Issue 9/2023
Print ISSN: 1618-954X
Electronic ISSN: 1618-9558
DOI: https://doi.org/10.1007/s10098-023-02589-x

Springer Professional

Abstract

Graphical abstract

Publisher's Note

Introduction

Experimental and characterisation

Materials and preparation of the composites

Physicochemical and microstructural characterisation of the coal-based wastes and composites

Physicomechanical and surface properties of the composites

Results and discussion

Physicochemical assessment of the coal-based waste and coal composite

Microstructural analysis of the coal-based wastes and coal composites

Fourier transform infrared spectra

X-ray diffraction analysis

Scanning electron and optical imaging

Raman analysis

Thermal stability/continuous operating temperature of the composites

Physical and mechanical analysis of the composites

Weight loss and shrinkage during pyrolysis

Density, water absorption, and apparent porosity of the composites

Surface property of the composites

Compressive strength performance

Conclusion

Acknowledgements

Declarations

Conflict of interest

Publisher's Note

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