Sustainable Epoxy Composites Filled with Natural Mineral Rocks: Comparative Evaluation of Mechanical, Thermal, and Dielectric Performance

TL;DR

This paper explores eco-friendly epoxy composites using natural stones like pebble, sandstone, and marble to improve mechanical, thermal, and electrical properties.

Contribution

The study introduces a sustainable approach using natural mineral fillers and a bio-based modifier to optimize composite performance.

Findings

Pebble-reinforced composites show the highest tensile strength and surface hardness due to their angular morphology.

Marble-filled composites exhibit superior thermal stability with increased residual mass at high temperatures.

Sandstone composites have the lowest dielectric constant, indicating better electrical insulation.

Abstract

This study presents the fabrication and optimization of eco-efficient epoxy composites reinforced with ground natural stone fillers, namely pebble, sandstone, and marble, at loadings of up to 15.6 wt.%. Low content of a bio-based modifier, modified castor oil (MCO ≈ 0.5 wt.%), is incorporated to improve filler dispersion, processing behavior, and matrix–filler interfacial compatibility. The composites are designed to enhance mechanical, thermal, and dielectric performance using low-cost, abundant, and environmentally sustainable constituents. An experimental optimization approach is employed to evaluate and optimize bulk density, Shore D hardness, thermal conductivity, dielectric constant, and tensile strength. The results demonstrate that pebble-reinforced composites exhibit the highest tensile strength (≈30 MPa) and surface hardness (≈82 Shore D), which are attributed to the angular morphology and high intrinsic rigidity of pebble particles. Marble-filled systems show superior thermal stability, with residual mass increasing from approximately 2.5 wt.% for neat epoxy to over 11 wt.% at 550 °C, owing to the thermally stable calcium carbonate phase. In contrast, sandstone-reinforced composites exhibit the lowest dielectric constant (≈3.2), indicating enhanced electrical insulation capability. Fourier–transform infrared spectroscopy (FTIR) results confirm that the epoxy network structure is preserved upon filler incorporation, while MCO promotes improved interfacial interactions through physical interactions. Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) reveal enhanced thermal resistance, reduced microvoid formation, and improved filler–matrix adhesion at optimal filler contents of approximately 3.5 wt.%.

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Full text

1. Introduction

Composite materials constitute a fundamental class of advanced engineering materials due to their ability to integrate two or more distinct phases into a single system with tailored and often superior properties. By combining a continuous matrix with a dispersed reinforcement phase, composites offer enhanced strength-to-weight ratios, improved thermal stability, corrosion resistance, and multifunctional performance compared to conventional monolithic materials. These advantages have led to their widespread adoption in aerospace, automotive, civil infrastructure, electronics, and renewable energy technologies, where performance efficiency and material reliability are critical requirements [1,2].

The conceptual development of composite materials has been strongly influenced by natural hierarchical structures such as wood and bone, which illustrate how chemically and structurally different components can work together to achieve high mechanical efficiency and damage tolerance. Inspired by these natural systems, the development of synthetic composite materials progressed rapidly during the mid-twentieth century, particularly in response to the growing demands of aerospace and defense applications, where lightweight materials with high strength and reliable structural performance were essential. Since that time, continuous advances in materials processing and characterization techniques have enabled more precise control over composite microstructure, allowing engineers to design composite materials with tailored mechanical, thermal, and functional properties for specific applications [3,4,5].

Among polymer matrix composites, epoxy-based systems represent one of the most extensively investigated and industrially important material classes. Epoxy resins are thermosetting polymers characterized by high cross-link density, which provides excellent mechanical strength, dimensional stability, chemical resistance, and thermal durability. Consequently, epoxy composites reinforced with glass, carbon, or aramid fibers have become standard materials in high-performance structural applications such as aerospace components, automotive parts, marine structures, and electronic encapsulation systems [6,7,8]. Despite these advantages, the inherent brittleness of epoxy resins—arising from restricted molecular mobility within their densely cross-linked networks—limits their impact resistance and fracture toughness, leading to crack initiation and rapid propagation under mechanical or thermal loading [9,10,11].

To overcome these limitations, significant research efforts have focused on modifying epoxy matrices through rubber toughening, thermoplastic blending, and particulate reinforcement. In recent years, the incorporation of mineral and natural stone fillers has attracted increasing attention as a cost-effective and environmentally benign reinforcement strategy. Finely ground stone materials, including granite, basalt, marble, quartz, and sandstone, exhibit high hardness, thermal stability, and chemical inertness, making them suitable candidates for enhancing stiffness, wear resistance, dimensional stability, and thermal performance of epoxy composites. In addition, mineral fillers can significantly influence density, thermal conductivity, and dielectric behavior, thereby extending the application scope of epoxy composites beyond purely structural functions to thermal management and electrical insulation applications [12,13,14].

The reinforcing efficiency of natural stone fillers is governed by several interrelated factors, including particle size distribution, morphology, dispersion quality, and interfacial compatibility with the epoxy matrix. Well-dispersed, micron-sized particles can promote crack deflection, microcrack bridging, and energy dissipation mechanisms, leading to improved fracture resistance and mechanical durability. Excessive filler loading or poor interfacial adhesion often results in particle agglomeration, increased resin viscosity, microvoid formation, and stress concentration sites, which can significantly deteriorate mechanical performance and processing feasibility. Achieving an optimal balance between reinforcement efficiency and processability therefore remains a key challenge in mineral-filled epoxy systems [15,16,17].

From a sustainability perspective, natural stone fillers offer compelling environmental and economic advantages. These materials are abundant, non-toxic, and frequently generated as byproducts or waste from quarrying and stone-cutting industries. Their utilization as polymer fillers contributes to waste valorization, reduces dependence on synthetic reinforcements, and lowers the overall environmental footprint and cost of composite materials. However, despite these benefits, achieving balanced multifunctional performance in stone-filled epoxy composites remains challenging, primarily due to interfacial incompatibilities and processing-related limitations [18,19,20].

To address these challenges, bio-based modifiers have emerged as promising alternatives to conventional petroleum-derived additives. Among them, castor-oil-based modifiers have gained particular interest due to their renewable origin, chemical functionality, and compatibility with epoxy systems. Modified castor oil can act as a bio-based plasticizer and compatibilizer, improving resin flow behavior, filler wetting, and matrix–filler interfacial interactions while preserving mechanical and thermal integrity. Such bio-based modification strategies not only enhance composite performance but also align with sustainability and circular-economy principles [21,22,23].

Despite the growing body of literature on mineral-filled epoxy composites and bio-based epoxy modification, most previous studies have focused on single filler systems or investigated a limited number of performance parameters. Comparative studies involving different natural stone fillers processed under identical fabrication conditions remain scarce. Furthermore, systematic investigations combining natural stone fillers with bio-based modifiers and evaluating mechanical, thermal, and dielectric properties in an integrated manner are still limited [24,25]. The synergistic influence of filler type, interfacial modification, and multifunctional performance therefore remains insufficiently understood [26,27].

In this context, the present study aims to develop and comparatively evaluate sustainable epoxy-based composites reinforced with ground natural mineral rocks—specifically pebble, sandstone, and marble—under identical fabrication and processing conditions. To improve matrix–filler compatibility and processing homogeneity, a low content (0.5 wt.%) of modified castor oil is incorporated as a bio-based modifier, and composite formulations are prepared with filler loadings of up to 15.6 wt.%. A systematic experimental approach is employed to optimize multifunctional performance, targeting bulk density, Shore D hardness, thermal conductivity, dielectric constant, and tensile strength. Comprehensive characterization using mechanical testing, thermal analysis, spectroscopic techniques, morphological examination, and dielectric measurements is conducted to elucidate structure–property relationships and identify optimal filler contents. The outcomes provide a scalable and eco-efficient strategy for designing mineral-reinforced epoxy composites from abundant, low-cost resources, offering practical potential for structural, thermal, and electrically functional applications.

2. Materials and Methods

2.1. Materials

A commercially available bisphenol-A-based epoxy resin and its corresponding amine curing agent were used as the polymer matrix system in this study. The epoxy resin system was supplied by Polisan Kimya A.Ş. (İstanbul, Türkiye) and was used as received without any further purification. According to the manufacturer’s technical datasheet, the epoxy resin exhibits an epoxy equivalent weight in the range specified for general-purpose structural applications, while the curing agent is formulated to ensure complete cross-linking under ambient curing conditions. MCO was employed as a bio-based modifier and compatibilizing agent at a fixed concentration of 0.5 wt.% relative to the total resin system. Commercial-grade castor oil (≥99% purity) was obtained from a local chemical supplier in İstanbul, Türkiye, and subsequently modified prior to use, as described in Section 2.2. Castor oil was selected due to its renewable origin, high hydroxyl functionality, and proven compatibility with epoxy networks, which enables improved resin flow behavior, filler wetting, and matrix–filler interfacial interactions.

For particulate reinforcement, natural mineral stone fillers, namely pebble, sandstone, and marble, were employed. All stone materials were sourced from local stone suppliers in Eastern Türkiye, ensuring regional availability and relevance to sustainable material development. The stones were initially received in bulk rock form and visually inspected to remove impurities and weathered fragments. No chemical treatment was applied to the stones in order to preserve their natural mineral composition and to evaluate their intrinsic reinforcing capability. The collected stones were mechanically crushed into smaller fragments and subsequently milled using a high-speed mechanical grinder (Miza, İstanbul, Türkiye) to obtain fine mineral powders. Additional consumables used during composite fabrication, including casting silicone, plastic molds, and mixing accessories, were procured from local suppliers in Türkiye and used without further modification. All materials and reagents were handled under laboratory ambient conditions, and consistent material sourcing and preparation protocols were maintained throughout the experimental program to ensure reproducibility and comparability of results [28].

All natural stone fillers (pebble, sandstone, and marble) were size-classified prior to composite fabrication using a standardized mechanical sieving procedure. ASTM-standard sieves were employed, and only the particle fractions retained between 77 μm and 154 μm were collected and used in the composite formulations to ensure a controlled and comparable particle size range across different mineral fillers. This sieving strategy was selected to eliminate the influence of ultra-fine and coarse particles and to minimize size-related variability during processing and property evaluation. To further validate the effectiveness of the classification process, representative powder samples were additionally examined using a particle size analyzer (PSS–NICOMP 380 ZLS, Particle Sizing Systems, Santa Barbara, CA, USA), confirming that the effective particle sizes of the sieved powders remained within the targeted 77–154 μm interval. Although inherent differences in particle morphology and fracture behavior may result in non-identical particle size distribution profiles among the three mineral fillers, the applied sieve-based classification ensured a consistent macroscopic size window suitable for comparative evaluation of filler mineralogy effects on the epoxy composite performance.

2.2. Modification of Coconut Oil

To improve the compatibility of coconut oil with the epoxy resin system, the oil was chemically modified prior to its incorporation into the polymer matrix. The modification strategy was designed to introduce polar and reactive functional groups capable of interacting with the epoxy network and mineral filler surfaces, thereby enhancing dispersion efficiency and matrix–filler interfacial adhesion. Chemical modification was carried out via an epoxidation reaction, during which the unsaturated carbon–carbon double bonds present in the fatty acid chains of coconut oil were converted into epoxy (oxirane) groups. In a typical procedure, refined coconut oil was placed in a three-neck round-bottom reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. The epoxidation reaction was performed through in situ peracid formation, using hydrogen peroxide and an organic acid catalyst, under controlled reaction conditions. The mixture was stirred at a constant speed of 600 rpm and maintained at a reaction temperature of 55–60 °C to ensure efficient epoxidation while minimizing unwanted side reactions such as oxirane ring opening or thermal degradation of the triglyceride backbone. The reaction was allowed to proceed for a predetermined duration sufficient to achieve stable epoxy functionality, as reported in previous studies [29].

Upon completion of the epoxidation process, the reaction mixture was cooled to room temperature and repeatedly washed with deionized water until a neutral pH was achieved, ensuring complete removal of residual acids and reaction byproducts. The modified coconut oil was then dried under reduced pressure at 50 ± 1 °C to eliminate residual moisture and volatile components, yielding a purified, chemically functionalized oil suitable for epoxy composite fabrication. The successful introduction of epoxy functionalities into the coconut oil structure was confirmed indirectly through improved compatibility and dispersion behavior in the epoxy matrix, as discussed.

In addition to chemical modification, a physical pre-treatment was applied to further enhance the dispersion efficiency of the modified coconut oil within the epoxy resin. Prior to blending, the functionalized oil was gently heated to 40–45 °C to reduce its viscosity and facilitate homogeneous mixing. The epoxy resin and modified coconut oil were initially mixed using mechanical stirring at 600 rpm for 10 min, followed by high-shear mixing at 1500 rpm for 5 min to ensure uniform molecular-level distribution throughout the resin phase. The modified coconut oil was incorporated at a fixed concentration of 0.5 wt.% relative to the total epoxy resin system and was added prior to mineral filler incorporation. This controlled addition sequence was selected to maximize resin plasticization and improve filler wetting during subsequent composite fabrication steps. The presence of modified coconut oil effectively reduced resin viscosity, enhanced the dispersion of natural stone fillers, and promoted improved matrix–filler interfacial interactions through physical interactions, without altering the fundamental chemical structure of the epoxy network. As schematically illustrated in Figure 1, the bio-based oil is chemically modified via epoxidation and subsequently dispersed into the epoxy resin through controlled heating and high-shear mixing to improve compatibility and interfacial interactions.

2.3. Production of Epoxy-Based Composites

In this study, natural stone fillers—pebble (P), sandstone (S), and marble (M)—were incorporated into an epoxy resin system as particulate reinforcements to fabricate a series of epoxy-based composites. Composite formulations were designed according to the predetermined mass fractions of ground natural stone fillers, as summarized in Table 1, which also outlines the experimental design and production scheme. The polymer matrix consisted of a two-component epoxy system composed of Epoxy A (resin) and Epoxy B (amine hardener), both supplied by Polisan Home Cosmetics (Kocaeli, Türkiye). To improve processability, mixture homogeneity, and casting behavior, MCO was employed as a bio-based processing aid at a fixed concentration of 0.5 wt.% relative to the total resin system.

Each composite formulation was systematically coded according to filler type and filler loading level to facilitate direct comparison of material performance. Samples labeled P1–P5 correspond to epoxy composites reinforced with increasing contents of ground pebble particles, whereas S1–S5 and M1–M5 denote sandstone- and marble-reinforced epoxy composites, respectively. The filler contents were varied from 0.0 to 15.6 wt.% while maintaining a constant MCO content, enabling a systematic evaluation of the effects of filler type and loading on the physical, mechanical, thermal, and dielectric properties of the resulting epoxy composites, as detailed in Table 1.

2.4. Characterization of Epoxy-Based Composites

The fabricated epoxy-based composites were systematically characterized to evaluate their physical, mechanical, thermal, morphological, and dielectric properties, with the aim of establishing clear structure–property relationships as a function of filler type and loading. All characterization tests were conducted under controlled laboratory conditions (23 ± 2 °C and relative humidity of 50 ± 5%), and at least three replicate specimens were tested for each formulation to ensure reproducibility and statistical reliability of the results. The bulk density of the composites was determined using the mass-to-volume ratio method, where the mass of each specimen was measured using a high-precision analytical balance, and the corresponding volume was calculated from the specimen dimensions. This method enabled evaluation of composite compactness and void content resulting from mineral filler incorporation. Shore D hardness measurements were performed in accordance with relevant standards using a calibrated Shore D durometer. Multiple indentations were taken at different locations on each specimen surface, and the average value was reported to minimize localized surface effects. Tensile properties, including tensile strength, elongation at break, and stress–strain behavior, were measured using a universal testing machine operated at a constant crosshead speed. Dog-bone-shaped specimens were prepared according to standard tensile testing geometries, and the tests provided quantitative insight into load-bearing capacity, stiffness, and ductility of the composites as influenced by filler type and content [30].

The thermal stability of the composites was evaluated by thermogravimetric analysis (TGA) using a SDT Q600 (TA Instruments, New Castle, DE, USA). Approximately 5–10 mg of each sample was heated from room temperature to 550 °C at a constant heating rate under a nitrogen atmosphere to prevent oxidative degradation. The onset of degradation, mass-loss stages, and residual char yield were determined to assess the influence of mineral fillers on thermal resistance and degradation behavior. Chemical structure and interfacial interactions were analyzed by Fourier-transform infrared spectroscopy (FTIR) using a IRSpirit QATR-S spectrometer (Shimadzu Corporation, Kyoto, Japan) operating in ATR mode. FTIR spectra were recorded in the range of 4000–400 cm^−1^ with sufficient resolution to identify characteristic epoxy functional groups and mineral-related vibrational bands, enabling evaluation of possible chemical changes or physical interactions at the filler–matrix interface.

The surface morphology and dispersion state of the mineral fillers within the epoxy matrix were examined using SEM (Zeiss EVO MA 10, Carl Zeiss AG, Oberkochen, Germany). Prior to imaging, fractured surfaces of tensile-tested specimens were sputter-coated with a thin conductive layer to prevent charging. SEM observations were conducted at appropriate accelerating voltages to clearly distinguish the epoxy matrix, filler particles, interfacial regions, agglomerates, and microvoids. These observations provided direct microstructural evidence supporting the mechanical and thermal performance trends. In addition, thermal conductivity was measured using a TLS-100 (Thermtest Inc., Fredericton, NB, Canada), enabling assessment of heat-transfer efficiency as influenced by mineral composition and particle dispersion. Dielectric constant measurements were carried out using a FY-9000 dielectric analyzer (Fytronix, Elazığ, Türkiye) over a defined frequency range, allowing evaluation of electrical insulation behavior and interfacial polarization effects. These complementary characterization techniques enabled a comprehensive and reliable evaluation of the multifunctional performance of natural stone–reinforced epoxy composites and facilitated correlation between microstructural features and macroscopic properties [31].

3. Results

The results obtained in this study comprehensively demonstrate the influence of natural stone fillers—pebble, sandstone, and marble—on the physical, mechanical, thermal, and dielectric properties of epoxy-based composites modified with a low content of bio-based oil. Systematic variation in filler type and loading level revealed clear structure–property relationships governing composite performance. Bulk density, hardness, tensile behavior, thermal conductivity, dielectric constant, and thermal stability exhibited strong dependence on both filler chemistry and dispersion quality. Microstructural and spectroscopic analyses further supported these findings by correlating macroscopic property trends with filler distribution, interfacial integrity, and thermal resistance mechanisms. The results highlight the critical role of optimized filler concentration and matrix–filler interaction in achieving multifunctional epoxy composites with balanced performance characteristics.

Table 2 provides a consolidated comparison between the epoxy composites developed in this study and representative mineral- and bio-filled epoxy systems reported in the literature, highlighting both performance trends and material design trade-offs. The results show that the natural stone-reinforced epoxy composites presented here achieve Shore D hardness and tensile strength values that are fully comparable to, and in some cases exceed, those of widely studied mineral fillers such as seashell, diatomite, boron minerals, and Sille stone, despite being processed at moderate filler contents. In contrast to nanosilica-based systems, which deliver very high tensile strength at extremely low filler loadings but suffer from reduced hardness and scalability challenges, the sandstone, pebble, and marble fillers used in this work provide a balanced combination of mechanical robustness, thermal conductivity enhancement, and practical filler contents suitable for bulk composite production. The thermal conductivity values obtained for the present composites fall within the typical range reported for mineral-filled epoxy systems, confirming that natural stone powders can effectively contribute to heat-transfer performance without excessive filler loading.

3.1. Bulk Density Results

Figure 2 shows a clear filler-loading-dependent densification of the epoxy composites, with bulk density increasing monotonically as the stone powder ratio rises from 0 to ~15.6 wt.%.

The neat epoxy baseline is ~1130 kg/m^3^, while at ~3.5 wt.% the density increases to approximately ~1140 kg/m^3^ (pebble), ~1146 kg/m^3^ (sandstone), and ~1148 kg/m^3^ (marble). A further increase to ~8.5 wt.% yields ~1147–1156 kg/m^3^, and at higher loadings the densification becomes more pronounced, reaching about ~1165–1175 kg/m^3^ at ~13.4 wt.% and ~1177 kg/m^3^ (pebble), ~1185 kg/m^3^ (sandstone), and ~1189 kg/m^3^ (marble) at ~15.6 wt.%. The small error bars across all compositions indicate good repeatability and consistent processing. The curves confirm that the mineral powders act as high-density, space-filling phases that progressively replace the polymer fraction and reduce effective free volume/void content, thereby improving composite compactness. The consistently higher density of marble-filled systems reflects the higher intrinsic density of CaCO_3_-rich marble and its effective packing contribution, whereas sandstone remains intermediate and pebble exhibits the lowest density increase, plausibly due to differences in mineralogy, particle morphology, and packing efficiency at comparable mass fractions.

Sandstone fillers, characterized by a finer and more uniform particle morphology, promote a more homogeneous dispersion within the epoxy matrix, which is advantageous for achieving improved mechanical uniformity and dimensional stability. These findings highlight the versatility of mineral fillers in tailoring the physical properties of epoxy composites for specific engineering applications. The consistent increase in bulk density observed for all filler types confirms that mineral reinforcement universally enhances composite compactness, indicating that pebble-filled systems may be well suited for load-bearing applications, sandstone-reinforced composites may be preferable where dimensional uniformity and ease of processing are critical, and marble-filled composites may be particularly attractive for applications requiring both structural integrity and aesthetic quality. The experimental results further demonstrate that the incorporation of ground sandstone as a reinforcing filler induces a steady and reproducible increase in the bulk density of epoxy composites, with the curvilinear trend observed at higher filler contents reflecting efficient particle packing and progressive void reduction within the polymer matrix. This behavior confirms that sandstone particles act as effective space-filling agents, enhancing overall material compactness and providing a predictable response that is beneficial for controlled density modification in composite design [35].

A comparable proportional increase in bulk density is observed for marble-reinforced composites, consistent with the behavior of pebble- and sandstone-filled systems, where marble particles contribute to matrix densification through improved interparticle contact and more efficient structural packing. These results are in good agreement with previously reported studies on mineral-filled polymer composites, reinforcing the conclusion that inorganic fillers play a dominant role in increasing composite density. Collectively, the data reveal a universal trend in which increasing the mass fraction of ground mineral additives—irrespective of whether pebble, sandstone, or marble is employed—leads to higher bulk density as a result of enhanced matrix compaction and reduced porosity, clearly demonstrating that mineral reinforcement provides an effective and versatile strategy for tuning the physical properties of epoxy composites in accordance with application-specific requirements and constraints such as cost, availability, mechanical performance, and aesthetic considerations [36].

3.2. Shore D Hardness Results

Figure 3 illustrates a systematic and filler-type-dependent increase in Shore D hardness with increasing stone powder content, confirming the effective stiffening role of mineral reinforcements in the epoxy matrix. The neat epoxy exhibits a baseline hardness of ~77 Shore D, which rises progressively with filler loading for all systems. At ~3.5 wt.%, hardness increases to approximately ~78.5 (pebble), ~78.0 (sandstone), and ~77.8 (marble), while at ~8.5 wt.% the values further increase to ~79.7, ~79.2, and ~78.4, respectively. More pronounced stiffening is observed at higher loadings, reaching ~80.7–81.0 at ~13.4 wt.% and peaking at ~82.0 (pebble), ~81.4 (sandstone), and ~80.6 (marble) at ~15.6 wt.%.

The relatively small error bars indicate good reproducibility and uniform processing. Pebble-filled composites consistently exhibit the highest hardness across all filler ratios, which can be attributed to the angular morphology and high intrinsic rigidity of pebble particles that enhance mechanical interlocking and restrict localized deformation. Sandstone provides an intermediate hardening effect, consistent with its quartz-rich composition, while marble shows the lowest—but still significant—hardness improvement, likely due to the smoother particle surfaces and lower intrinsic hardness of CaCO_3_. The trends demonstrate that Shore D hardness can be effectively tuned through both filler content and mineralogy, with pebble reinforcement offering the most efficient resistance to surface indentation among the investigated natural stone fillers. In contrast, marble-filled composites exhibited a more moderate but still consistent increase in Shore D hardness, reflecting the comparatively smoother particle morphology and lower intrinsic hardness of calcium carbonate-based fillers. The hardness results reveal a clear ranking of reinforcement effectiveness—pebble > sandstone > marble—highlighting the critical role of particle geometry and mineral composition in tailoring the surface mechanical performance of epoxy composites [32,37].

3.3. Thermal Conductivity Results

Figure 4 demonstrates a clear and monotonic increase in thermal conductivity with increasing stone powder content for all epoxy composites, confirming the effective role of mineral fillers in enhancing heat-transfer capability. The neat epoxy exhibits a low thermal conductivity of approximately 0.11 W/m·K, characteristic of polymeric matrices. Upon filler addition, thermal conductivity increases gradually at low loadings (≈3.5–8.5 wt.%), followed by a more pronounced rise at higher contents (≈13.4–15.6 wt.%), indicating the progressive formation of thermally conductive pathways within the composite. Among the investigated fillers, sandstone consistently shows the highest thermal conductivity, reaching about 0.167 W/m·K at 15.6 wt.%, which is attributed to its quartz-rich composition and the development of efficient interparticle contact networks that promote phonon transport.

Marble-filled composites display intermediate values, increasing to ~0.155 W/m·K at the highest loading, reflecting the relatively uniform dispersion and moderate intrinsic thermal conductivity of CaCO_3_. Pebble-reinforced systems exhibit the lowest enhancement, attaining ~0.146 W/m·K at 15.6 wt.%, likely due to their heterogeneous mineralogy and less efficient particle–particle thermal contacts. The small error bars indicate good reproducibility and stable filler dispersion. The results highlight that thermal conductivity in epoxy composites can be effectively tailored by both filler loading and mineralogical composition, with sandstone offering the most efficient thermal conduction enhancement among the studied natural stone powders [38,39].

3.4. Dielectric Constants Results

Figure 5 illustrates a systematic increase in the dielectric constant of epoxy composites with increasing stone powder content for all filler types, indicating that mineral incorporation effectively enhances the dielectric response of the epoxy matrix. The neat epoxy exhibits the lowest dielectric constant (≈3.65), while the addition of mineral fillers leads to a gradual and nearly linear increase across the investigated loading range (0–15.6 wt.%). Marble-filled composites show the most pronounced enhancement, reaching values above 4.2 at the highest filler content, followed by pebble-filled systems (~4.15), whereas sandstone-filled composites display the lowest increase, remaining below ~4.0. This ranking reflects the intrinsic dielectric permittivity and mineralogical composition of the fillers: calcium carbonate-rich marble promotes stronger interfacial polarization and charge storage, while quartz-dominated sandstone, being more electrically insulating and less polar, limits dielectric enhancement. Pebble fillers exhibit intermediate behavior due to their mixed mineral composition. The smooth trends and small error bars suggest stable dispersion and reproducible dielectric performance. The results demonstrate that dielectric properties of epoxy composites can be effectively tuned through both filler type and loading, with marble fillers being particularly advantageous for applications requiring higher dielectric constants, while sandstone fillers are preferable where lower dielectric permittivity is desired [40].

In addition, the relatively smooth surface morphology and favorable chemical compatibility between marble particles and the epoxy resin likely facilitate the formation of a continuous dielectric network, thereby optimizing overall dielectric performance. Ground pebble also contributed to an increase in the dielectric constant, although the effect was moderate compared with marble; the observed near-linear increase with filler content, characterized by a gentler slope than that of marble but slightly steeper than that of sandstone, can be attributed to the heterogeneous mineral composition of pebble, which typically contains silicate phases and other dielectric-active minerals whose influence depends on their relative proportions. Furthermore, the irregular morphology of pebble particles may induce localized space-charge polarization at the filler–matrix interfaces, further contributing to the enhancement in dielectric constant. In contrast, ground sandstone resulted in the smallest increase in dielectric constant among the three filler systems; although a gradual increase was observed with increasing filler content, the overall values remained the lowest across all concentrations. This behavior is consistent with the high quartz (SiO_2_) content of sandstone, as quartz is an electrically insulating and non-polar mineral with inherently low dielectric permittivity, and its uniform crystalline structure limits interfacial polarization effects, thereby restricting dielectric enhancement. Consequently, while sandstone-filled composites offer excellent mechanical and thermal stability, they are less suitable for applications that require high dielectric performance [41,42].

3.5. FTIR Spectra of Natural Stone Powder-Reinforced Epoxy-Based Composites

The FTIR spectra confirm that the chemical structure of the epoxy matrix remains unchanged upon incorporation of natural stone fillers (pebble, sandstone, and marble) across all compositions. The characteristic absorption bands of a fully cured bisphenol-A epoxy system are preserved, indicating that mineral addition does not induce chemical degradation or covalent modification of the polymer network. With increasing filler content, a gradual reduction in the intensity of epoxy-related bands is observed, which is attributed to the dilution effect of the infrared-inactive inorganic phase. Additional absorption features corresponding to silicate and carbonate groups appear at higher filler loadings, reflecting the mineralogical composition of the fillers. Minor band broadening suggests weak physical interactions at the filler–matrix interface. The FTIR results demonstrate that reinforcement is governed primarily by physical interfacial interactions rather than chemical bonding, while the epoxy matrix retains its structural integrity.

3.5.1. FTIR Spectra of Pebble-Reinforced Composite

The FTIR spectra of the ground pebble-reinforced epoxy composites (Figure 6) clearly demonstrate that the fundamental chemical structure of the epoxy matrix is preserved across all filler loadings (0–15.6 wt.%).

The neat epoxy spectra (0 wt.%) exhibits the characteristic absorption bands of a fully cured bisphenol-A epoxy system, including a broad O–H stretching band in the 3500–3200 cm^−1^ region arising from hydroxyl groups formed during epoxy ring-opening reactions, and strong aliphatic C–H stretching vibrations of –CH_2_ and –CH_3_ groups in the 3000–2800 cm^−1^ range. The aromatic backbone of the epoxy resin is confirmed by the distinct C=C stretching vibrations observed between 1600 and 1500 cm^−1^, while the intense absorption band in the 1250–1000 cm^−1^ region corresponds to C–O–C ether linkages, indicating extensive cross-linking within the polymer network. Additional sharp bands in the 950–700 cm^−1^ region are associated with aromatic C–H bending vibrations, collectively confirming successful curing and the formation of a stable, highly cross-linked epoxy structure [43,44].

With increasing pebble filler content (3.5–15.6 wt.%), all characteristic epoxy-related absorption bands remain visible, indicating that the incorporation of mineral particles does not induce chemical degradation or alter the epoxy network. However, a gradual reduction in the intensity of the O–H and C–H stretching bands is observed as filler loading increases, which is primarily attributed to the dilution effect caused by the infrared-inactive inorganic phase. At higher pebble contents (13.4 and 15.6 wt.%), additional weak bands emerge and intensify in the 1100–500 cm^−1^ region, corresponding to Si–O, Al–O, and Ca–O stretching vibrations associated with the silicate and carbonate minerals present in natural pebble aggregates. Minor peak broadening and subtle shifts in the ether (C–O–C) region suggest the presence of weak physical interactions, such as hydrogen bonding or van der Waals forces, at the epoxy–filler interface. These spectral features indicate that reinforcement occurs predominantly through physical interfacial interactions rather than chemical bonding, while the epoxy matrix retains its structural integrity. The FTIR results confirm the successful incorporation and homogeneous dispersion of ground pebble fillers within the epoxy matrix and support the conclusion that the observed changes in mechanical, thermal, and dielectric properties arise mainly from microstructural and interfacial effects rather than chemical modification of the polymer network [45,46].

3.5.2. FTIR Spectra of Sandstone-Reinforced Composite

As shown in Figure 7, the FTIR spectra confirm that the epoxy matrix retains its chemical integrity across all sandstone filler loadings (0–15.6 wt.%). The spectra of the neat epoxy (0 wt.%) displays the characteristic absorption bands of a fully cured bisphenol-A epoxy system, including a broad O–H stretching band in the 3500–3200 cm^−1^ region arising from hydroxyl groups generated during epoxy ring-opening reactions, and distinct aliphatic C–H stretching vibrations of –CH_2_ and –CH_3_ groups between 3000 and 2800 cm^−1^. The presence of aromatic rings is confirmed by C=C stretching bands in the 1600–1500 cm^−1^ region, while the strong and broad absorption between 1250 and 1000 cm^−1^ is attributed to C–O–C ether linkages, indicating extensive cross-linking within the epoxy network. Additional sharp peaks in the 950–700 cm^−1^ range correspond to aromatic C–H bending vibrations, collectively verifying successful curing and the formation of a stable [33,47].

With increasing sandstone content (3.5–15.6 wt.%), the characteristic epoxy-related bands persist, indicating that the incorporation of sandstone does not chemically alter or degrade the epoxy network. However, a gradual decrease in the intensity of the O–H and C–H stretching bands is observed as filler loading increases, which is mainly attributed to the dilution effect of the inorganic, IR-inactive sandstone particles. At higher filler contents (13.4 and 15.6 wt.%), additional absorption features become more pronounced in the 1100–500 cm^−1^ region, corresponding to Si–O–Si and Si–O–Al stretching vibrations characteristic of quartz and aluminosilicate phases present in sandstone. Minor band broadening and subtle shifts in the C–O–C and aromatic regions suggest weak physical interactions, such as hydrogen bonding or van der Waals forces, at the epoxy–sandstone interface. These observations indicate that reinforcement is governed predominantly by physical interfacial interactions rather than covalent bonding, while the epoxy matrix retains its chemical integrity. The FTIR results confirm the successful incorporation and effective dispersion of sandstone fillers within the epoxy matrix and support the conclusion that changes in composite properties arise primarily from microstructural effects and interfacial phenomena rather than chemical modification of the polymer network [47,48].

3.5.3. FTIR Spectra of Marble Powder-Reinforced Composite

The FTIR spectra of the ground marble powder-reinforced epoxy composites presented in Figure 8 depict that the fundamental chemical structure of the epoxy matrix is preserved across all filler loadings (0–15.6 wt.%). The spectrums of the neat epoxy (0 wt.%) exhibits the characteristic absorption bands of a fully cured bisphenol-A epoxy system, including a broad O–H stretching band in the 3500–3200 cm^−1^ region associated with hydroxyl groups formed during epoxy ring-opening reactions, and distinct aliphatic C–H stretching vibrations of –CH_2_ and –CH_3_ groups in the 3000–2800 cm^−1^ range. The aromatic backbone of the epoxy resin is confirmed by C=C stretching vibrations observed between 1600 and 1500 cm^−1^, while the intense absorption band in the 1250–1000 cm^−1^ region corresponds to C–O–C ether linkages, indicating a highly cross-linked polymer network. Additional sharp bands in the 950–700 cm^−1^ region are attributed to aromatic C–H bending vibrations, collectively confirming successful curing and structural integrity of the epoxy matrix [49,50].

With increasing marble filler content, the characteristic epoxy-related bands remain clearly visible, indicating that the incorporation of marble particles does not induce chemical degradation or covalent modification of the epoxy network. However, a gradual decrease in the intensity of the O–H and C–H stretching bands is observed with increasing filler loading, which can be attributed to the dilution effect caused by the infrared-inactive inorganic phase. At higher marble contents (13.4 and 15.6 wt.%), additional absorption features become more pronounced in the 1450–850 cm^−1^ region, corresponding to carbonate-related vibrations (CO_3_^2−^) characteristic of calcium carbonate, the primary constituent of marble. Minor peak broadening and subtle shifts in the ether (C–O–C) and aromatic regions suggest weak physical interactions, such as hydrogen bonding or van der Waals forces, at the epoxy–marble interface. These spectral features indicate that reinforcement is governed predominantly by physical interfacial interactions rather than chemical bonding, while the epoxy matrix retains its chemical integrity. The FTIR results confirm the successful incorporation and effective dispersion of marble fillers within the epoxy matrix and support the conclusion that the observed changes in mechanical, thermal, and dielectric properties arise mainly from interfacial phenomena and microstructural effects rather than alterations in the polymer’s chemical structure [49,50,51].

3.6. TGA Results of Epoxy Composites

The TGA curves presented in Figure 9 illustrate the thermal degradation behavior of neat epoxy resin and epoxy-based composites reinforced with 8.5 wt.% ground pebble, marble, and sandstone fillers, providing detailed insight into the effect of mineral reinforcement on thermal stability. The pure epoxy resin exhibits the most rapid and pronounced mass loss upon heating, with the onset of degradation occurring at approximately 280–300 °C, followed by a sharp decomposition stage between 350 °C and 450 °C corresponding to the scission of the polymer backbone, cleavage of ether linkages, and volatilization of low-molecular-weight degradation products. Beyond 500 °C, the neat epoxy retains only about 2.5 wt.% residual mass at 550 °C, indicating nearly complete decomposition of the organic matrix, which is consistent with the typical thermal behavior of fully cured bisphenol-A-based epoxy systems where degradation proceeds primarily through oxidation of aromatic structures and breakdown of cross-linked networks [52,53].

The incorporation of natural stone fillers markedly enhances the thermal stability of the epoxy composites, as evidenced by a slight shift in degradation onset toward higher temperatures and a reduced rate of mass loss across all filled systems. The presence of mineral particles acts as a thermal barrier, limiting heat and mass transfer and restricting polymer chain mobility during thermal exposure. Among the reinforced samples, marble- and pebble-filled composites display the most gradual degradation profiles, retaining approximately 10–11 wt.% higher residual mass at 550 °C compared with neat epoxy, which indicates improved char formation and the stabilizing effect of thermally inert inorganic phases that promote the development of a protective carbonized layer. The pebble-filled composite, containing quartz, feldspar, and mica, exhibits a degradation onset similar to that of the neat resin but maintains a significantly higher residue due to the presence of these thermally stable minerals, which hinder polymer diffusion and oxidation processes. The sandstone-filled composite, rich in silica, shows a modest upward shift in degradation temperature and an increased residual mass, attributed to its uniform dispersion and effective thermal shielding effect that retards polymer chain scission. Notably, the marble-reinforced composite demonstrates the highest thermal stability, as the calcium carbonate content contributes to enhanced resistance to thermal degradation through endothermic decomposition, heat absorption, and promotion of char and inorganic residue formation, thereby slowing overall degradation kinetics [54,55]. The TGA results confirm that mineral fillers function as effective thermal stabilizers, with the thermal stability ranking of the composites following the order: marble-filled > pebble-filled > sandstone-filled > neat epoxy, highlighting their potential for heat-resistant structural and industrial applications.

3.7. Tensile Test Results of Natural Stone Powder-Reinforced Epoxy-Based Composites

The tensile stress–strain behavior of epoxy composites reinforced with natural stone powders (sandstone, pebble, and marble) demonstrates a strong dependence on filler type and loading level. The neat epoxy exhibits typical brittle thermoset behavior, characterized by high tensile strength and limited ductility due to its highly cross-linked molecular structure. At low filler contents (≈3.5 wt.%), all mineral-reinforced systems show preserved or enhanced tensile performance, which is attributed to homogeneous particle dispersion, effective matrix–filler interfacial adhesion, and efficient stress transfer. Sandstone at this level improves both tensile strength and strain at break, indicating enhanced toughness, whereas pebble reinforcement provides a modest strength increase without significantly compromising ductility. Marble-filled composites at low loading retain relatively high elongation, suggesting minimal disruption of matrix continuity. As the filler content increases to moderate levels (≈8.5–13.4 wt.%), divergent reinforcement behaviors emerge: pebble-filled composites reach their maximum tensile strength at approximately 13.4 wt.% due to their angular morphology and high rigidity, while sandstone- and marble-filled systems begin to show reduced strength and ductility as a result of particle agglomeration and interfacial defects. At the highest filler loading (15.6 wt.%), all composites exhibit pronounced embrittlement, characterized by low strain at break, flattened stress–strain responses, and premature fracture, which are associated with agglomeration, microvoid formation, and inefficient stress transfer.

3.7.1. Tensile Test of Sandstone-Reinforced Composite

The tensile stress–strain behavior of neat epoxy resin and epoxy composites reinforced with varying sandstone loadings (3.5, 8.5, 13.4, and 15.6 wt.%) is presented in Figure 10. The neat epoxy resin exhibits a tensile strength of approximately 25 MPa and a strain at break of about 7%, which is characteristic of a strong yet brittle thermoset material resulting from its highly cross-linked molecular structure and restricted polymer chain mobility. Upon the incorporation of 3.5 wt.% sandstone, a notable improvement in both tensile strength and strain at break is observed, indicating effective stress transfer between the epoxy matrix and the mineral filler while maintaining sufficient matrix flexibility. This enhancement is attributed to improved interfacial adhesion and uniform dispersion of sandstone particles, which enable homogeneous load distribution and delay crack initiation. The increased area under the stress–strain curve at this filler level further suggests enhanced toughness, highlighting the beneficial role of low sandstone content in reinforcing the epoxy system [56,57].

At higher filler loadings of 8.5 and 13.4 wt.%, the tensile performance begins to deteriorate despite the relatively high elastic modulus imparted by the rigid mineral phase. Both tensile strength and elongation at break decrease, which can be attributed to filler agglomeration, reduced interfacial bonding, and the formation of microstructural defects that act as stress concentration sites under tensile loading. This trend becomes more pronounced at the highest sandstone content of 15.6 wt.%, where the composite exhibits an almost elastic–brittle failure behavior with very low strain at break (~1.5%) and minimal plastic deformation, indicating compromised stress-transfer efficiency and limited energy absorption capacity. These results suggest that excessive sandstone disrupts the continuity of the epoxy matrix, leading to premature fracture. Accordingly, the optimal sandstone content is identified to be in the range of approximately 3–5 wt.%, where a favorable balance between strength, stiffness, and ductility is achieved. In addition, the incorporation of 0.5 wt.% MCO as a bio-based plasticizer significantly improves processability and mechanical response by reducing resin viscosity, alleviating internal curing stresses, and promoting better filler dispersion and interfacial bonding. As a result, composites produced with controlled sandstone loading and optimized MCO plasticization exhibit enhanced ductility, toughness, and homogeneity without sacrificing tensile strength, hardness, or thermal performance, making them promising candidates for sustainable structural and functional applications [58].

3.7.2. Tensile Test of Pebble-Reinforced Composite

The tensile stress–strain curves presented in Figure 11 illustrate the mechanical response of neat epoxy resin and epoxy composites reinforced with different pebble loadings (3.5, 8.5, 13.4, and 15.6 wt.%).

The neat epoxy resin exhibits the characteristic behavior of a highly cross-linked thermoset, consisting of an initial linear elastic region followed by abrupt fracture near its maximum tensile strength of approximately 25 MPa and a strain at break of about 7%, reflecting a strong yet inherently brittle response associated with limited polymer chain mobility and high cross-link density. The incorporation of pebble filler leads to noticeable modifications in the stress–strain behavior of the composites; at low to moderate filler contents of 3.5 and 8.5 wt.%, a modest enhancement in tensile strength is observed relative to the unfilled epoxy. At these loadings, the stress–strain curves remain closely aligned with that of the neat matrix, indicating effective stress transfer resulting from good interfacial adhesion and uniform dispersion of pebble particles within the epoxy network. Moreover, the composites retain acceptable elongation at break, suggesting that, at these filler levels, pebble particles provide reinforcement without significantly compromising the ductility of the epoxy matrix, thereby enabling a balanced combination of strength and deformability suitable for structural applications [56,57,58].

A pronounced improvement in tensile performance is observed at a pebble loading of 13.4 wt.%, where the composite reaches its maximum tensile strength, outperforming all other formulations. This optimal reinforcement level indicates the formation of a balanced microstructure in which the filler content is sufficient to enhance load transfer efficiency without inducing excessive particle agglomeration. At this concentration, synergistic stress distribution, an increased interfacial contact area between the epoxy matrix and pebble particles, and improved energy absorption prior to failure are achieved, collectively contributing to enhanced tensile strength and toughness. In contrast, further increasing the pebble content to 15.6 wt.% results in a sharp deterioration of mechanical performance, as evidenced by a flattened stress–strain response and a significantly reduced strain at break of approximately 1.5%, indicative of a highly brittle failure mode. This degradation is attributed to particle agglomeration, weakened matrix–filler interfacial bonding, and the formation of stress concentration sites that promote premature crack initiation and propagation. Excessive filler loading disrupts the continuity of the epoxy matrix, thereby diminishing effective stress transfer and reducing both strength and ductility. These results show that moderate pebble reinforcement, around 13.4 wt.%, provides the most favorable balance between tensile strength and flexibility, highlighting the critical importance of filler content optimization in the design of particle-reinforced epoxy composites for structural and load-bearing applications [59].

3.7.3. Tensile Test of Marble Powder-Reinforced Composite

The tensile stress–strain curves shown in Figure 12 present the mechanical response of neat epoxy resin and epoxy composites reinforced with varying amounts of ground marble powder (3.5, 8.5, 13.4, and 15.6 wt.%).

The neat epoxy resin exhibits a tensile strength of approximately 30 MPa and an elongation at break of about 12%, characteristic of a well-cured thermoset polymer with high strength but limited ductility due to its cross-linked molecular architecture. At a low marble loading of 3.5 wt.%, a slight reduction in ultimate tensile strength is observed, while the strain at break remains relatively high, suggesting that marble particles are well dispersed and do not significantly disrupt matrix continuity at this concentration. The fine particle size of the marble filler likely facilitates uniform stress transfer and preserves matrix flexibility. However, at higher filler contents of 8.5 and 13.4 wt.%, both tensile strength and elongation at break decrease more noticeably, which can be attributed to filler agglomeration, insufficient interfacial adhesion, and the presence of rigid inclusions acting as stress concentrators within the epoxy matrix. In particular, the composite containing 13.4 wt.% marble exhibits a sudden stress drop after limited deformation, indicative of brittle fracture behavior associated with poor particle dispersion and discontinuities in the polymer network that hinder effective load transfer. These findings underline the sensitivity of tensile performance to marble filler content and dispersion quality, emphasizing the need for careful control of filler loading to avoid embrittlement while maintaining adequate mechanical performance [60].

At the highest marble filler concentration of 15.6 wt.%, the epoxy composite exhibits the poorest mechanical performance, characterized by pronounced reductions in both tensile strength and strain at break. The corresponding stress–strain response shows early flattening with negligible plastic deformation, indicating a predominantly brittle failure mode. This degradation in mechanical behavior is attributed to excessive filler loading, which promotes severe particle agglomeration, void formation, and insufficient wetting of marble particle surfaces by the epoxy matrix, thereby weakening interfacial adhesion and overall structural integrity. Such microstructural defects act as stress concentration sites that facilitate early crack initiation and rapid crack propagation under tensile loading. The results clearly indicate that a low marble filler content of approximately 3.5 wt.% provides the most favorable balance between strength, stiffness, and flexibility, whereas higher filler loadings progressively impair mechanical performance. These findings emphasize that excessive mineral reinforcement can compromise composite homogeneity and interfacial bonding, underscoring the importance of optimizing filler concentration and dispersion to achieve desirable mechanical properties in marble-reinforced epoxy composites intended for structural and decorative applications [61].

3.8. SEM Images Results of Natural Stone Powder-Reinforced Epoxy-Based Composites

The SEM images provide comprehensive insight into the microstructural evolution and matrix–filler interactions of epoxy-based composites reinforced with natural stone powders (sandstone, pebble, and marble) at different loading levels. The neat epoxy matrix exhibits a smooth, dense, and homogeneous surface morphology, indicative of a fully cured and defect-free thermoset network, which serves as a reference for assessing filler-induced changes. At low filler loading (≈3.5 wt.%), all mineral-reinforced composites display uniformly dispersed particles that are well embedded within the epoxy matrix, with minimal interfacial gaps or voids, confirming effective resin wetting and strong physical adhesion at the filler–matrix interface. This homogeneous microstructure supports efficient stress transfer and correlates well with the enhanced or preserved mechanical performance observed experimentally. As the filler content increases to moderate levels (≈8.5 wt.%), a higher particle population becomes evident on the composite surfaces, accompanied by the initial formation of localized agglomerates and microvoids, although overall matrix continuity remains largely intact. These features indicate the onset of dispersion limitations while still maintaining acceptable interfacial integrity. In contrast, at higher filler loadings (≥13.4 wt.%), pronounced microstructural degradation is observed for all filler systems, characterized by extensive particle agglomeration, increased porosity, interfacial debonding, and surface cracking.

3.8.1. SEM Image of Sandstone-Reinforced Composite

The SEM images presented in Figure 13 provide detailed insight into the surface morphology and matrix–filler interactions of epoxy-based composites reinforced with ground sandstone at different loadings. Five formulations were prepared for each filler system using ground natural stone contents of 0.00, 3.50, 8.50, 13.40, and 15.60 wt.% relative to the epoxy matrix. Samples designated as P1–P5 correspond to epoxy composites reinforced with increasing loadings of ground pebble, while S1–S5 and M1–M5 represent the sandstone- and marble-reinforced epoxy composites, respectively.

The image of the neat epoxy resin (S1), recorded at 300× magnification, reveals a smooth, dense, and highly homogeneous surface free from cracks, pores, or particulate inclusions. This uniform morphology indicates a fully cured and defect-free polymer matrix, confirming that polymerization proceeded to completion and resulted in a highly cross-linked thermoset network. The absence of voids, phase separation, or microstructural discontinuities reflects strong internal cohesion, which is consistent with the high mechanical strength, thermal stability, and dielectric uniformity typically associated with neat epoxy systems. The morphology of S1 provides a reliable reference baseline for evaluating microstructural changes induced by sandstone filler incorporation. In the composite containing 3.5 wt.% sandstone (S2), the surface morphology remains relatively smooth and uniform, with finely dispersed sandstone particles well embedded within the epoxy matrix. The homogeneous distribution of the filler and the absence of visible interfacial gaps indicate good matrix–filler compatibility and efficient wetting of the sandstone particle surfaces at low filler content. This well-bonded microstructure supports effective load transfer and minimizes defect formation, which correlates well with the enhanced tensile strength and toughness observed in mechanical testing. In contrast, the S3 composite containing 8.5 wt.% sandstone exhibits a noticeably higher filler concentration, with an increased number of particles visible on the surface. Although overall dispersion remains reasonably uniform, localized microvoids and small particle agglomerates begin to appear. These features indicate the onset of filler clustering, which may act as stress concentration sites under mechanical loading. Nevertheless, the matrix structure in S3 largely remains intact, and the composite retains an acceptable level of homogeneity, suggesting that moderate sandstone loadings can still provide reinforcement without severely compromising microstructural integrity [62].

The SEM images of the S4 and S5 composites reveal a progressive deterioration in microstructural integrity with increasing sandstone content. In the S4 composite containing 13.4 wt.% sandstone, a pronounced increase in heterogeneity is observed, characterized by visible filler agglomerates, microvoids, and interfacial gaps between the epoxy matrix and sandstone particles. These morphological defects indicate a reduction in interfacial adhesion, which is likely caused by insufficient resin wetting and restricted matrix flow during processing at higher filler loadings. The presence of pores and discontinuities disrupts the structural continuity of the composite and correlates well with the experimentally observed reductions in tensile strength and ductility. The diminished effective bonding area between the polymer matrix and filler particles limits stress transfer efficiency, thereby facilitating premature crack initiation and propagation under applied mechanical loads. In the S5 composite with 15.6 wt.% sandstone, these effects become more severe, as evidenced by extensive particle agglomeration, high porosity, and pronounced interfacial debonding. The resulting highly heterogeneous and cracked surface, containing large voids and poorly bonded filler clusters, indicates a breakdown of the matrix–filler network due to excessive filler incorporation. Such microstructural deficiencies severely compromise mechanical integrity, stress-transfer capability, and toughness, consistent with the brittle fracture behavior observed during tensile testing. These observations confirm that moderate sandstone loading of approximately 3.5 wt.% yields the most favorable microstructure, featuring uniform filler dispersion, strong interfacial adhesion, and minimal void formation, whereas higher filler concentrations promote agglomeration and porosity that significantly degrade both morphological uniformity and mechanical performance [63].

3.8.2. SEM Image of Pebble-Reinforced Composite

The SEM micrographs shown in Figure 14 illustrate the surface morphology of neat epoxy resin (P1) and epoxy composites reinforced with varying concentrations of ground pebble filler (P2–P5). The neat epoxy sample (P1) exhibits a smooth, dense, and pore-free surface, characteristic of a uniformly cured and highly cross-linked polymer matrix. The absence of voids, cracks, or surface irregularities confirms a defect-free structure, which underpins the high tensile strength and hardness of the unfilled epoxy and provides a baseline for evaluating filler-induced microstructural changes. In the composite containing a low pebble content of 3.5 wt.% (P2), the epoxy matrix remains largely continuous, with only a limited number of finely dispersed pebble particles embedded within the surface. The overall morphology remains relatively smooth, and the lack of significant interfacial gaps indicates effective matrix wetting and adequate interfacial bonding at this filler level [64].

3.8.3. SEM Image of Marble Powder-Reinforced Composite

The SEM micrographs presented in Figure 15 illustrate the influence of ground marble powder content on the surface morphology and matrix–filler interactions of epoxy-based composites. The composite containing 3.5 wt.% marble (M2) exhibits a slightly roughened yet highly uniform surface, in which finely dispersed marble particles are well embedded and fully encapsulated within the epoxy matrix. The even distribution of filler particles and the absence of noticeable agglomeration or interfacial separation indicate strong matrix–filler compatibility and efficient wetting at this low filler concentration. Such a homogeneous microstructure facilitates effective stress transfer across the filler–matrix interface and preserves matrix continuity, which is consistent with the experimentally observed improvements in mechanical performance. The composite containing 8.5 wt.% marble (M3) displays a more heterogeneous and irregular surface morphology, characterized by partial particle clustering, the presence of microvoids, and the formation of interfacial gaps between the marble particles and the epoxy matrix. These microstructural defects suggest incomplete resin wetting and the development of localized stress concentration sites, which weaken the overall structural cohesion of the composite. The increased filler loading disrupts the continuity of the epoxy phase, reduces effective interfacial adhesion, and promotes brittle fracture under mechanical loading. The SEM observations clearly demonstrate that low marble filler concentrations of approximately 3.5 wt.% yield a uniform microstructure with strong matrix–filler bonding, whereas higher loadings lead to agglomeration, porosity, and interface debonding [64,65].

4. Conclusions

The performance of the epoxy-based composites developed in this study is governed by a coupled interplay between filler mineralogy, particle morphology, dispersion quality, and matrix–filler interfacial integrity. At low to moderate filler loadings (≈3.5–8.5 wt.%), all natural stone fillers contribute positively to composite performance by enabling homogeneous particle distribution and effective stress transfer, as corroborated by SEM observations. Within this optimal range, the epoxy matrix maintains structural continuity, allowing mineral fillers to act as efficient load-bearing and functional modifiers rather than defect initiators.

Distinct reinforcement mechanisms are observed depending on filler type. Pebble particles, characterized by angular morphology and high intrinsic rigidity, provide the most pronounced enhancement in tensile strength and surface hardness. Marble fillers, dominated by thermally inert CaCO_3_ phases, significantly improve thermal stability by promoting char formation and increasing high-temperature residue during thermal degradation. In contrast, sandstone-filled composites exhibit consistently lower dielectric constants, reflecting the electrically insulating nature of quartz-rich mineral phases and highlighting their suitability for insulation-oriented applications. These differentiated responses confirm that mineralogical composition plays a decisive role in defining the dominant functional contribution of each filler.

When filler content exceeds the optimal threshold (>8.5 wt.%), the beneficial effects of reinforcement are progressively diminished by microstructural degradation. Particle agglomeration, microvoid formation, and interfacial debonding disrupt matrix continuity and impede effective stress transfer, leading to reduced tensile strength and a transition toward brittle fracture behavior. This trend underscores that excessive mineral loading compromises composite integrity and that performance optimization is achieved through controlled filler incorporation rather than maximum filler content.

The incorporation of a low content of bio-based modifier (0.5 wt.% MCO) is shown to play a critical enabling role in composite processing and microstructural development. By reducing resin viscosity and improving filler wetting, the modifier facilitates more uniform particle dispersion and mitigates interfacial defects at practical filler levels, without altering the chemical structure of the epoxy network. Combined FTIR, SEM, and TGA analyses confirm that the observed property evolution is governed predominantly by physical interactions and microstructural effects rather than the formation of new covalent bonds.

From an application-oriented perspective, the results demonstrate that pebble-, marble-, and sandstone-reinforced epoxy systems offer complementary and application-specific advantages. Pebble-reinforced composites are particularly suited for load-bearing and wear-resistant components requiring enhanced strength and hardness. Marble-filled systems are promising for thermally demanding structural or decorative applications where improved thermal stability and residue formation are critical. Sandstone-filled composites, with their lower dielectric constants, are well suited for electrically insulating components. Although the present work focuses on single-filler systems to enable clear mechanistic interpretation, the complementary property profiles observed strongly suggest that hybrid filler strategies could yield synergistic multifunctional performance.

This study establishes a scalable and sustainable framework for valorizing abundant, low-cost natural stone powders in epoxy composites through rational filler selection and optimized loading. The findings demonstrate that balanced multifunctional performance is achieved by tailoring microstructure and interface quality rather than maximizing mineral content. Future work should focus on targeted interface engineering, hybrid filler systems, and long-term durability assessments—including moisture exposure, thermal cycling, and fatigue—to further enhance application readiness and broaden the industrial relevance of these eco-efficient epoxy composites.