Mechanical Properties and Performance Assessment of Polymer Concretes

Abstract

Polymer concretes (PCs) have emerged as high-performance materials offering superior strength, durability, and chemical resistance compared to conventional cementitious composites. This study presents a comprehensive experimental program designed to investigate the relationship between microstructural characteristics and mechanical performance of PCs. Mechanical properties were evaluated through strength, stiffness, durability, and integrity tests, while microstructural features were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results reveal a clear link between refined microstructure, reduced porosity, and enhanced mechanical behavior. This integrated approach provides new insights into the performance mechanisms of PCs and establishes a scientific basis for their broader structural use in demanding environments.

1. Introduction

Concrete remains the most extensively utilized construction material worldwide owing to its low production cost, ease of fabrication, and satisfactory compressive strength. Despite these advantages, conventional cement-based concrete exhibits inherent drawbacks, including pronounced brittleness, high porosity, limited tensile resistance, and susceptibility to chemical and environmental deterioration. Such limitations significantly reduce its long-term serviceability, particularly under aggressive exposure conditions, and have driven continuous research efforts toward the development of alternative binder systems and advanced composites.

Among these alternatives, PCs have emerged as high-performance materials with increasing significance over the past decades. By replacing hydraulic cement with polymeric resins, PCs overcome many of the durability and strength deficiencies of ordinary concretes. These materials are characterized by rapid curing, high strength-to-weight ratios, low permeability, and exceptional chemical resistance. Consequently, they are being increasingly employed in structural rehabilitation, precast components, industrial flooring, bridge deck overlays, and specialized infrastructures where durability, reduced maintenance, and extended service life are critical. Their adoption also aligns with the global emphasis on sustainability in construction, as longer service life directly reduces material consumption, repair costs, and associated environmental burdens.

Extensive research has been conducted on different aspects of PCs. Previous studies have reported that epoxy-, polyester-, and vinyl ester-based PCs can achieve compressive strengths exceeding 80–100 MPa, alongside significantly improved tensile and flexural behavior when compared with ordinary cementitious composites [1,2,3,4,5]. Further investigations have shown that Wresin chemistry, aggregate grading, and filler composition are decisive parameters governing the mechanical response, microstructural compactness, and durability of PCs [6,7,8]. Gagandeep [9] examined the strength characteristics of epoxy resin-based PC. In that study, PC containing 3% and 5% resin was mechanically tested and compared with polymer fiber concrete incorporating 0.5% and 1% glass fiber. The results revealed that increasing the resin content from 3% to 5% enhanced both workability and compressive as well as flexural strength. Moreover, the addition of glass fibers further improved the compressive strength of the composite. Similarly, Kiruthika et al. [10] investigated polyester-based PCs and discussed their suitability for sustainable construction. The authors proposed a method using isophthalic resin and reported that the resulting material exhibited a service life of more than 20 years, thereby lowering maintenance costs compared to conventional cement concretes due to its superior durability. In another study, Seco et al. [11] focused on the durability of PCs produced with metallurgical wastes and their potential in manufacturing sustainable construction products. Their findings demonstrated that even with polymer additions as low as 1–2%, the modified concretes exhibited reduced electric charge passage and improved resistance against chloride ion penetration, confirming the potential of recycled materials in producing durable and environmentally friendly PCs. Similarly, Palamarchuk et al. [12] investigated the characteristics of PCs formulated with different polymer resins and discussed the strengths and limitations of various models developed to predict their mechanical properties. Laqsum et al. [13] examined the mechanical behavior and impact resistance of U-shaped polymer-modified concrete (PMC) incorporating epoxy (EP) and polyacrylate (PA) binders. Recent studies have also explored the use of industrial by-products and alternative filler systems to improve the performance of cementitious and polymer-based composites, highlighting the importance of microstructural control in enhancing mechanical and durability properties [14]. Despite these advancements, the existing body of literature remains fragmented, with the majority of studies addressing either macroscopic mechanical properties or microstructural observations in isolation.

A systematic understanding of how microstructural characteristics govern mechanical performance is still lacking. In particular, relationships between crystallite size, lattice strain, porosity, elemental distribution, and macroscale parameters such as compressive and tensile strengths are rarely quantified. This gap restricts the development of predictive frameworks necessary for the reliable design and large-scale application of PCs. Microstructural characterization techniques—such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS)—have revealed that reduced porosity, refined particle morphology, and enhanced resin–aggregate interfacial bonding are strongly associated with mechanical improvement and long-term stability. However, these findings often remain qualitative, without being explicitly integrated with mechanical performance data.

The present study addresses this limitation through a comprehensive experimental program that combines mechanical testing—including compressive strength, splitting tensile strength, modulus of elasticity, Poisson’s ratio, ultrasonic pulse velocity, hardness, density, porosity, and water absorption—with advanced microstructural analyses. XRD, SEM, and EDS were employed to identify phase composition, crystallite size, lattice strain, and elemental distribution within the polymer matrix. By systematically correlating these microstructural features with macroscopic mechanical performance, this research provides a more rigorous framework to elucidate the mechanisms governing the superior behavior of PCs. Ultimately, the study aims to contribute toward establishing the scientific basis required for broader adoption of PCs as sustainable and high-performance structural materials. Unlike most existing studies that focus solely on either macroscopic mechanical behavior or microstructural observations, this research provides an integrated evaluation of both domains. By systematically linking microstructural refinement (crystallite size, lattice strain, porosity, and elemental distribution) with macroscopic mechanical properties (compressive and tensile strengths, stiffness, and durability indices), the present study offers a novel framework for understanding the performance mechanisms of polymer concretes. This approach contributes to the development of predictive design bases for the broader structural use of these advanced composites

2. Materials and Methods

2.1. Materials

2.1.1. Natural Aggregates

In this study, aggregates with sizes of 0.3–1 mm, 1–2 mm, 2–3 mm, and 3–5 mm obtained from riverbeds were used. The specific gravity of the aggregates was approximately 2.65 g/cm³. To prevent any contamination, the aggregates were washed with clean water and dried before use. The oxide composition of the natural aggregates was determined by X-ray fluorescence (XRF) analysis to obtain accurate chemical characteristics prior to mixing. The chemical compositions of the aggregates are presented in

Table 1. Chemical compositions of the aggregates obtained from the XRF testing.

Chemicals	0.3–1 mm	1–3 mm	3–5 mm
SiO2	98.86	94.20	94.15
Al2O3	0.245	1.80	1.86
Fe2O3	0.148	0.46	0.46
CaO	0.01	0.45	0.39
MgO	0.10	0.06	0.06
K2O	0.03	1.52	1.56
Na2O	0.02	1.16	1.12
SO3	–	0.10	0.10
Ignition loss	0.24	0.25	0.30

.

2.1.2. Resin

In this study, a general-purpose unsaturated polyester resin (UPR) with high filler wetting capacity was utilized. UPRs are thermosetting polymers that can be transformed from a liquid to a solid state under appropriate curing conditions. Polyester resins represent the most widely used type of resin globally. Their main polymeric backbone consists of ester bonds formed through the condensation of polyfunctional alcohols and polyfunctional acids. The technical properties of the UPR are presented in

Table 2. Technical properties of the UPR.

Properties	Values
Flexural strength in 5% strain (MPa)	51.6
Compressive strength (MPa)	34.1
Impact strength (J/m)	12.9
Viscosity (mPa·s)	659
Shore hardness	80
Tensile modulus (MPa)	527
Density (g/cm3)	1.225

.

2.1.3. Hardener

In this study, acetylacetone peroxide (AAP) was employed as the hardening agent. AAP is a fast-curing organic peroxide that is widely used in the curing of unsaturated polyester resins due to its high efficiency and ability to initiate polymerization at relatively low temperatures. Its rapid curing characteristics make it particularly suitable for applications requiring reduced processing times and improved productivity. Additionally, AAP offers stable handling properties and consistent performance, which enhances the reliability of the curing process. The technical specifications of the hardener are presented in

Table 3. Technical specifications of the hardener.

Properties	Values
Flash point	>60 °C
Density, 20 °C	1055 kg/m3
Viscosity, 20 °C	21 mPa·s
Self-accelerating decomposition temperature (SADT)	60 °C
Total active oxygen	4.0–4.2%
Peroxide content	33%
Diethylene glycol + water + diacetone alcohol	67%

.

2.1.4. Accelerator

In order to enhance the reactivity between the resin and the hardener, accelerators are commonly incorporated into the curing system. These compounds function by increasing the rate of radical formation, which in turn accelerates the crosslinking reactions. In general, accelerators elevate the local temperature within the system, thereby significantly reducing curing time and improving process efficiency. In this study, cobalt naphthenate was selected as the accelerator. Cobalt naphthenate is an organometallic compound widely used in polymer and composite production, particularly in the curing of unsaturated polyester resins. Its primary role is to activate the peroxide curing agent (such as acetylacetone peroxide) by promoting radical decomposition, ensuring a more uniform and controlled curing process. The technical properties of the accelerator are provided in

Table 4. Technical properties of the accelerator.

Properties	Values
Density	0.92 g/cm3 (20 °C)
Viscosity	300 mPa·s (20 °C)
Self-accelerating decomposition temperature (SADT)	≥150 °C
Flash point	62 °C
Cobalt content	1.5%

.

2.2. Preparation of PCs

In the preparation of PC, aggregates of different diameters were first combined in predetermined proportions to achieve suitable grading. To avoid damage to the aggregates and to ensure homogeneous distribution, the initial mixing was carried out slowly and steadily. The mixing time for each batch was 5 min to ensure uniform resin distribution and aggregate coating. Once a properly graded aggregate mixture was obtained, the resin and accelerator were added and thoroughly blended using a mechanical stirrer. At this stage, effective mixing (

Table 5. Mix proportion.

Resin (%)	Aggregates (%)	Hardener (%)	Acceleration (%)
15	50	20	15

) was ensured so that the resin and accelerator uniformly coated the aggregate surfaces without causing particle degradation. The hardener was introduced at the final stage, followed by careful mixing to guarantee complete homogenization of all components. Then, the samples were promptly placed into molds for the mechanical tests. Specimens of varying geometries were cast to evaluate different mechanical properties. Cylindrical specimens with dimensions of 150 × 300 mm and 100 × 200 mm were prepared for compressive and splitting tensile strength tests, while cubic specimens measuring 150 × 150 × 150 mm were produced for compressive strength, ultrasonic pulse velocity, and hardness testing (

Table 6. Specimen Types and Dimensions.

Specimen Type	Geometry	Dimensions (mm)	Purpose of Test	Standard
Cylinder	Ø150 × 300	150 (D) × 300 (H)	Compressive strength, modulus of elasticity, Poisson’s ratio	[15,16]
Cylinder	Ø100 × 200	100 (D) × 200 (H)	Compressive strength, splitting tensile strength	[15,17]
Cube	150 × 150 × 150	150 (L) × 150 (W) × 150 (H)	Compressive strength, ultrasonic pulse velocity, and hardness	[15,18,19]

and

Table 7. Experimental Program and Test Standards.

Test Method	Measured Property	Standard	Instrument/Procedure
Uniaxial Compression	Compressive strength	[15]	Universal testing machine (load-controlled)
Splitting Test	Tensile strength	[17]	Universal testing machine with diametral loading
Elastic Modulus & Poisson’s Ratio	Axial and lateral strain	[16]	Strain gauges & data logger
Rebound Hammer	Surface hardness	[18]	Schmidt hammer
Ultrasonic Pulse Velocity (UPV)	Internal integrity, elastic wave speed	[20]	Ultrasonic pulse velocity tester (direct transmission)
Density, Porosity, Water Absorption	Durability indices	[21]	Mass measurements (oven-dry, saturated surface-dry, submerged)
X-Ray Diffraction (XRD)	Phase analysis, crystallite size, lattice strain	–	PANalytical Empyrean diffractometer, CuKα radiation
Scanning Electron Microscopy (SEM)	Surface morphology	–	Field Emission SEM, gold sputter coating
Energy-Dispersive X-ray Spectroscopy (EDS)	Elemental composition and mapping	–	Integrated Oxford X-MaxN EDS detector

). After casting, all specimens were demolded following initial setting and cured under controlled laboratory conditions at room temperature until the designated testing age, ensuring consistent development of the polymer matrix.

3. Results and Discussion

In this section, the experimental findings obtained from both mechanical tests and microstructural analyses of PCs are presented and discussed. The results are structured to provide a comprehensive understanding of the material behavior by first examining the mechanical performance parameters such as compressive and tensile strength, modulus of elasticity, Poisson’s ratio, hardness, ultrasonic pulse velocity, density, porosity, and water absorption. Subsequently, the microstructural characteristics obtained through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) are reported to reveal the internal structure, phase composition, crystallite size, and elemental distribution of the specimens. The integration of these findings enables the identification of correlations between microstructural refinement and macroscopic mechanical behavior, thereby providing new insights into the performance mechanisms of PCs.

3.1. Mechanical Properties

To obtain a comprehensive understanding of the structural behavior of PCs, a series of mechanical and durability-oriented tests was performed. These tests were designed not only to quantify fundamental strength parameters but also to evaluate stiffness, deformation characteristics, surface quality, internal compactness, and permeability—properties that together define the long-term performance of the material. The compressive strength test provided the primary measure of load-bearing capacity. Since concrete structures are predominantly subjected to compressive stresses in service, this test is regarded as the most critical indicator of material performance. The results also served as a benchmark for comparing PCs with conventional cement-based concretes. The splitting tensile strength test offered insights into the resistance of the material against tensile stresses, which are responsible for crack formation and propagation. As conventional concretes typically show low tensile capacity, improvements in this property highlight the efficiency of the polymer matrix in enhancing crack resistance. The elastic modulus and Poisson’s ratio were determined under axial loading. The elastic modulus reflects the stiffness of the material, directly influencing structural deflection and vibration behavior, while Poisson’s ratio characterizes lateral deformation under stress. Together, these parameters provide a more complete description of the stress–strain response of PCs. The rebound hardness test was used as a non-destructive method to evaluate surface strength and uniformity. Beyond providing a quick quality control tool, the hardness values were correlated with compressive strength and microstructural compactness, thereby supporting the reliability of destructive tests. The ultrasonic pulse velocity (UPV) test was performed to assess the homogeneity and internal integrity of the specimens. Higher pulse velocities are typically associated with a denser microstructure and fewer defects, making UPV a valuable indirect indicator of porosity and overall durability. Finally, density, porosity, and water absorption measurements were carried out to examine durability-related characteristics. Low porosity and minimal water absorption are essential for reducing permeability and enhancing resistance against chemical attack, freeze–thaw cycles, and other environmental effects. These results complement the mechanical data by linking microstructural compactness with long-term durability performance. By integrating the outcomes of all these tests, it was possible to construct a holistic evaluation of PCs, revealing not only their strength under direct loading but also their durability and stability under service conditions (Figure 1, Figure 2, Figure 3 and Figure 4,

Table 8. Mechanical Properties of PC Specimens.

Property	Specimen Type	Average Value	Standard Deviation
Compressive Strength	Ø150 × 300 mm cylinder	82–89 MPa	±3.5 MPa
	Ø100 × 200 mm cylinder	77–87 MPa	±5.6 MPa
	150 × 150 × 150 mm cube	87–97 MPa	±5.2 MPa
Splitting Tensile Strength	Ø150 × 300 mm cylinder	6.6 MPa	±0.6 MPa
	Ø100 × 200 mm cylinder	8.6 MPa	±1.1 MPa
Elastic Modulus	Ø150 × 300 mm cylinder	~31–32 GPa	–
Poisson’s Ratio	Ø150 × 300 mm cylinder	0.18–0.20	–
Rebound Hardness	150 × 150 × 150 mm cube	34–37	±1.0
Ultrasonic Pulse Velocity (UPV)	150 × 150 × 150 mm cube	3380–3835 m/s	–
Bulk Density	Cylinders & cubes	2.04–2.33 g/cm3	–
Apparent Porosity	Cubes	0.3–0.4%	–
Water Absorption	Cubes	~0.15%	–

).

Compressive Strength: The uniaxial compression tests revealed average compressive strengths of 82–89 MPa for Ø150 × 300 mm cylinders, 77–87 MPa for Ø100 × 200 mm cylinders, and 87–97 MPa for 150 mm cubic specimens. These values are substantially higher than typical C40/50 Portland cement concretes and demonstrate the effectiveness of the polymer matrix in enhancing load-bearing capacity. The relatively low standard deviations (3.5–5.6 MPa) indicate good reproducibility and uniformity in specimen production.

Splitting Tensile Strength: The tensile strength values ranged between 6.6 and 9.0 MPa, depending on specimen geometry. These results highlight the improved crack resistance of PCs compared to ordinary concretes, which typically achieve tensile strengths of only 2–4 MPa.

Elastic Modulus and Poisson’s Ratio: The elastic modulus was determined to be approximately 31–32 GPa, with Poisson’s ratio values in the range of 0.18–0.20. While the elastic modulus is comparable to high-strength cementitious concretes, the slightly lower Poisson’s ratio reflects the stiffer and less ductile behavior of the polymer matrix.

Hardness and Ultrasonic Pulse Velocity (UPV): Rebound hammer tests yielded hardness values of 34–37, while UPV results ranged from 3380 to 3835 m/s, both indicative of dense and homogeneous material quality. The strong correlation between UPV and compressive strength further validates the compact microstructure of PCs.

Density, Porosity, and Water Absorption: The bulk density was measured between 2.04 and 2.33 g/cm³, while the apparent porosity remained below 0.4%, and water absorption was recorded at approximately 0.15%. These exceptionally low porosity and absorption values explain the superior durability and reduced permeability of PCs in aggressive environments. The very low porosity (<0.4%) observed in PCs significantly impedes transport mechanisms such as capillary suction and ionic diffusion, which are the primary causes of durability deterioration in conventional concretes. This explains the superior resistance of PCs against chemical ingress, freeze–thaw cycles, and moisture-related degradation. Overall, the PCs exhibit high strength, uniform structure, and excellent durability, making them suitable for long-term structural applications.

3.2. X-Ray Diffraction (XRD) Analysis

XRD analyses identified calcium carbonate (CaCO₃) and silicon dioxide (SiO₂) as the dominant crystalline phases across all specimens. No secondary phases or harmful impurities were detected. The crystallite sizes, calculated using the Williamson–Hall method, varied between 70 and 110 nm, while lattice strain values ranged from 0.15% to 0.23%. These findings indicate that phase composition is stable, and microstructural refinement contributes directly to the observed strength levels.

3.3. Scanning Electron Microscopy (SEM) Observations

SEM images revealed that the PCs exhibited a dense and irregular particle morphology, with very few visible voids or cracks. The matrix–aggregate interface appeared strongly bonded, suggesting effective stress transfer between phases. Compared to traditional concretes, the absence of extensive capillary pores was particularly evident, aligning with the low water absorption results. In addition, the micrographs showed a relatively smooth and compact surface texture, further supporting the material’s high ultrasonic pulse velocity and rebound hardness values. The densification of the microstructure is considered a key factor in the superior compressive and tensile strengths achieved by the specimens. In the SEM micrographs, the resin matrix (RM), aggregate particles (AG), and interfacial transition zones (ITZ) were clearly identified and labeled. The dense morphology and absence of wide pores confirm the effective penetration of the polymer binder into aggregate pores, ensuring superior bonding.

3.4. Energy-Dispersive X-Ray Spectroscopy (EDS) Analysis

EDS spectra confirmed that the main elements in PC are C, O, Si, and Ca, corresponding to the resin matrix and mineral fillers. Secondary elements such as Na, Al, Mg, K, and Fe were detected in trace amounts, reflecting natural impurities of aggregates. Quantitative results indicated approximate weight percentages of C: 34–40%, O: 30–37%, Si: 10–15%, Ca: 7–11%, with <5% contributions from minor elements. Elemental mapping demonstrated a homogeneous distribution of Si and Ca phases throughout the matrix, consistent with the strong mechanical performance and reduced porosity.

The occasional detection of Au and Al peaks was linked to the gold sputter coating and aluminum holder used during SEM-EDS analysis rather than the intrinsic composition of the specimens.

3.5. Integrated Discussion

The integration of mechanical and microstructural results demonstrates a strong correlation between microstructural refinement and mechanical performance in PCs. Specifically:

High compressive strength correlates with low porosity and uniform elemental distribution.

Tensile strength enhancement is supported by the strong interfacial bonding observed in SEM images.

Crystallite size refinement (70–110 nm) observed in XRD analyses is consistent with the dense morphology and enhanced modulus values.

Durability properties (low water absorption, low porosity, high UPV) are directly explained by the compact microstructure.

These findings validate the hypothesis that the superior performance of PCs arises from the synergistic effect of microstructural compactness, refined crystallinity, and strong resin–aggregate bonding.

3.6. Microstructural Characterization

To establish the link between microstructure and mechanical performance, advanced characterization techniques were employed:

X-Ray Diffraction (XRD): Conducted using a PANalytical Empyrean diffractometer with CuKα radiation (λ = 1.5408 Å), operating at 40–45 kV and 30 mA. Scans were performed within the 2θ range of 2°–120° at a step size of 0.02° and scan speed of 1°/min. Phase identification was carried out using the ICDD database, and crystallite size and lattice strain were determined via the Williamson–Hall method (Figure 5). The crystallite size (D) and lattice strain (ε) were determined using the Williamson–Hall (W–H) method. The equation applied was:

$$\beta \cos \theta ={\displaystyle \frac{k\lambda }{D}}+4ϵ\sin \theta$$

(1)

where β represents the full width at half maximum (FWHM) of the diffraction peak, θ is the Bragg angle, λ is the X-ray wavelength (1.5408 Å), and k is the shape factor (0.9). A linear regression of βcosθ against 4sinθ enabled the simultaneous estimation of crystallite size from the intercept and lattice strain from the slope.

Scanning Electron Microscopy (SEM): Surface morphology and fracture characteristics were observed using a field-emission SEM. Prior to imaging, samples were sputter-coated with a thin layer of gold to enhance conductivity (Figure 6).

Energy-Dispersive X-ray Spectroscopy (EDS): Elemental composition and mapping were obtained simultaneously with SEM analysis to evaluate the distribution of major constituents such as C, O, Si, Ca, and trace elements (Figure 7).

XRD analyses performed on the PCs specimens revealed that the dominant crystalline phases are calcium carbonate (CaCO₃) and silicon dioxide (SiO₂). No significant impurities or secondary phases were detected, confirming the homogeneity of the matrix. The crystallite sizes, calculated using the Williamson–Hall method, ranged between 70 and 110 nm, while the lattice strain values were measured between 0.15 and 0.23% depending on the specimen type. These results indicate a refined crystalline structure that contributes to the high compressive strength observed in the mechanical tests.

SEM micrographs demonstrated that the PCs exhibited a dense and irregular particle morphology with strong matrix–aggregate bonding. The surfaces showed reduced microcracking and limited void formation, reflecting the low porosity values obtained in durability tests. The microstructure of PCs appeared significantly less porous and more compact, which is consistent with their superior mechanical strength and lower water absorption.

The elemental composition of the PC specimens was examined using energy-dispersive X-ray spectroscopy (EDS) in conjunction with SEM imaging. The analyses consistently identified carbon (C), oxygen (O), silicon (Si), and calcium (Ca) as the principal elements, reflecting the presence of the polymer matrix together with silica- and carbonate-based fillers.

Minor elements, including sodium (Na), aluminum (Al), magnesium (Mg), potassium (K), and iron (Fe), were also detected in certain regions of the specimens. Their occurrence is attributed to trace mineral phases in the aggregates and fillers. In addition, gold (Au) and aluminum (Al) peaks appeared in several spectra, which are associated with the gold sputter coating applied to improve conductivity during SEM analysis and the aluminum sample holder used for specimen mounting, respectively, Elemental mapping confirmed that C, O, Si, and Ca were uniformly distributed across the microstructure, indicating a well-integrated matrix and effective bonding between resin and aggregates. In particular, the homogeneous dispersion of Si and Ca phases supports the dense morphology observed in SEM images and aligns with the high compressive strength values reported in mechanical testing.

Quantitative EDS results showed that the average elemental weight percentages for representative specimens were approximately C: 34–40%, O: 30–37%, Si: 10–15%, and Ca: 7–11%, with trace amounts (<5%) of other elements depending on the sampling area (Figure 8). The presence of these stable phases and their uniform distribution confirms the chemical stability and microstructural integrity of the PC. EDS analyses confirmed the presence of key elements, including C, O, Si, and Ca as the main constituents, with minor amounts of Na, Al, Mg, Fe, and K detected in some specimens. Elemental mapping revealed a homogeneous distribution of these elements across the matrix, indicating effective dispersion of the resin and filler phases. The detection of gold (Au) and aluminum (Al) peaks in some spectra was attributed to the sputter-coating process and the aluminum sample holder used during SEM-EDS analysis.

3.7. Influence of Crystallite Refinement on Strength

X-ray diffraction analyses indicated that the dominant crystalline phases were CaCO₃ and SiO₂, with crystallite sizes ranging between 70 and 110 nm and lattice strains of 0.15–0.23%. Such nanoscale refinement increases the efficiency of stress transfer within the matrix, which explains the high compressive strengths (82–97 MPa) achieved in the mechanical tests. Smaller crystallite domains restrict the initiation of microcracks and promote uniform stress distribution, while moderate lattice strain provides additional resistance against fracture propagation. This direct link between refined crystallinity and macroscale strength highlights the role of controlled resin chemistry and curing in optimizing PC performance.

3.8. Role of Porosity and Transport Properties

The bulk density values (2.04–2.33 g/cm³), coupled with extremely low apparent porosity (<0.4%) and water absorption (~0.15%), provide strong evidence of a compact microstructure. SEM observations confirmed this dense morphology, with very few voids or cracks observed at the resin–aggregate interfaces. These microstructural features directly translate into mechanical performance: low porosity restricts capillary suction and ionic diffusion, thereby not only improving durability under aggressive environments but also enhancing compressive and tensile strengths. The strong correlation between low porosity, high ultrasonic pulse velocity (3380–3835 m/s), and superior strength validates porosity reduction as a decisive performance-controlling factor.

3.9. Interfacial Transition Zone and Crack Resistance

SEM images revealed strong bonding between the polymer matrix and aggregates, while EDS analyses demonstrated homogeneous distribution of key elements (C, O, Si, Ca) across the microstructure. This uniformity ensures continuity between phases and eliminates stress concentration zones, which are typical weak points in conventional concretes. The well-defined interfacial transition zone (ITZ) observed in polymer concretes explains the splitting tensile strength values of 6–9 MPa, which are more than double those of ordinary concretes. The resin effectively penetrates aggregate pores, creating a mechanical interlock that suppresses crack initiation and propagation.

3.10. Integrated Microstructure–Performance Relationship

When considered together, the findings establish a robust framework linking microstructure to performance:

• Crystallite refinement improves compressive strength and stiffness.
• Porosity reduction enhances both mechanical strength and long-term durability.
• Homogeneous elemental distribution supports tensile strength by stabilizing the ITZ and reducing crack initiation.
• Resin–aggregate interlocking ensures effective stress transfer and resistance against fracture propagation.

Thus, the superior performance of PCs is not solely a consequence of the polymer binder, but rather the synergistic interplay of crystallinity, porosity control, and interfacial bonding. This integrated understanding provides a predictive basis for tailoring resin formulations, aggregate gradings, and curing regimes to design PCs with consistent, high-level performance.

The very low porosity (~0.3%) and minimal water absorption (~0.15%) are clear indicators of the dense microstructure achieved in PCs. These values are significantly lower than those typical for high-strength Portland cement concretes, which often exhibit water absorption of 3–6%. UPV results between 3380 and 3835 m/s confirm the internal integrity of the material. Literature suggests that high UPV values correlate strongly with low pore connectivity and enhanced durability [20,22]. Thus, the results validate that micro-structural compactness is not only beneficial for strength but also for long-term performance in aggressive environments. While previous studies often examined either mechanical properties or microstructural features in isolation, the present research explicitly correlates nanoscale crystallinity, lattice strain, porosity, and elemental distribution with compressive and tensile performance. For instance, Reis (2005) [23] emphasized tensile improvements without microstructural analysis, and Zegardło et al. (2018) [24] reported detailed SEM/XRD results but did not connect them quantitatively with strength data. By contrast, the current study bridges this gap, demonstrating that microstructural compactness is not only beneficial for strength but also decisive for durability under service conditions.

4. Conclusions

This study experimentally investigated the relationship between microstructural characteristics and the mechanical performance of PCs through a comprehensive testing program. The key findings can be summarized as follows:

PCs achieved compressive strengths in the range of 80–95 MPa and splitting tensile strengths of 6–9 MPa, significantly exceeding the typical values of conventional cement-based concretes. The elastic modulus (~31–32 GPa) and Poisson’s ratio (0.18–0.20) indicate a stiff yet strong material, while rebound hardness and ultrasonic pulse velocity results confirmed high internal integrity. Bulk density values of 2.04–2.33 g/cm³, coupled with very low apparent porosity (<0.4%) and minimal water absorption (~0.15%), demonstrate the exceptional compactness and durability potential of PCs. These properties highlight their suitability for aggressive service environments where permeability and long-term durability are critical. XRD analyses identified CaCO₃ and SiO₂ as the dominant crystalline phases with crystallite sizes between 70 and 110 nm and lattice strains ranging from 0.15 to 0.23%. SEM observations confirmed a dense and irregular morphology with well-bonded matrix–aggregate interfaces, while EDS mapping revealed homogeneous distribution of C, O, Si, and Ca as primary elements. A direct correlation was established between microstructural refinement and enhanced mechanical performance. Specifically, reduced porosity and refined crystallite sizes were linked to higher compressive and tensile strengths, while uniform elemental distribution supported the observed improvements in modulus and durability indicators. Unlike most existing studies that investigate either mechanical behavior or microstructural features separately, this study integrates both domains, thereby providing a more comprehensive understanding of how microstructure governs the performance of PCs.

Overall, the findings confirm that PCs represent a high-performance material class with considerable potential for structural applications beyond repair and overlays. Their enhanced strength, low permeability, and stable microstructure suggest that they can be effectively employed in infrastructure requiring long-term durability, high load-bearing capacity, and resistance to aggressive environments.

The novelty of this study lies in its integrated experimental framework that directly links microstructural refinement to mechanical performance in polymer concretes. The simultaneous assessment of crystallite size, porosity, elemental homogeneity, and macroscopic strength parameters provides a new perspective on the mechanisms that govern their superior structural behavior. This approach not only contributes to the fundamental understanding of polymer concrete systems but also offers practical guidance for optimizing resin systems and mix designs in future engineering applications.

Future research should explore the influence of different resin systems, filler types, and curing conditions on the microstructure–performance relationship, as well as the long-term behavior of PCs under cyclic, thermal, and chemical exposures. Moreover, future investigations should explore the long-term behavior of PCs under cyclic loading, thermal fluctuations, and chemical exposure, as well as the incorporation of recycled aggregates and bio-based resins to further improve environmental sustainability.