Effect of Gravel Size, Microwave Irradiation (1 to 2.5 min), Moisture, and Quenching on Aggregate Properties of Chert Gravel: Valorizing a “Waste” Byproduct of Sand Quarrying

Abstract

Chert gravel, a byproduct of sand quarrying, remains an underutilized material in construction due to its low microwave (MW) absorption and high mechanical strength. The present study deals with the potential of MW irradiation as a novel, energy-efficient method for processing chert gravel into high-quality aggregates, reducing reliance on virgin materials. The research systematically examines MW exposure duration (1–2.5 min), rock size (150–800 g), moisture conditions, and cooling methods (air vs. water quenching) to optimize fragmentation. Experimental results indicate that larger rock sizes (600–800 g) yield coarser, less uniform aggregates, while prolonged MW exposure (>2 min) induces extensive micro-fracturing, producing finer, well-graded particles. Water quenching significantly intensifies fragmentation, generating irregular but highly fragmented aggregates, whereas pre-wetted samples exhibit finer and more uniform breakage than dry samples. The findings introduce a novel approach for optimizing chert gravel fragmentation, a material previously considered unsuitable for MW treatment. The study proposed a customizable methodology for tailoring aggregate properties through precise control of MW parameters, offering a sustainable alternative to conventional crushing. The results contribute to resource conservation, reduced energy consumption, and climate change mitigation, paving the way for more sustainable construction practices.

1. Introduction

1.1. State of the Art—The Methods of Rock Treatment

Various pretreatment methods—thermal, chemical, electric, magnetic, ultrasonic, and bio-milling—have been explored to reduce high energy demand during aggregate crushing. Microwave (MW) irradiation is a particularly effective technology because it can decrease energy consumption and enhance comminution efficiency [1,2,3,4]. Unlike conventional heating, MW treatment enables selective heating, which is difficult to achieve with traditional methods [5,6,7,8]. Its contactless, rapid heating process minimizes energy and time costs, making it an attractive option in green engineering and climate change mitigation [5,9]. For example, MW energy applications have shown promise in mineral processing activities, including heating, drying, grinding, leaching, roasting, smelting, carbothermic reduction, refractory ore pretreatment, spent carbon regeneration, and waste management [10]. Research indicates that further increases above certain power density thresholds offer diminishing returns regarding energy use or irradiation time [11]. Additionally, larger mineral crystals require less time and energy for structural modification, aligning with findings that ores with larger grain sizes achieve better energy efficiency [12,13]. MW pretreatment has been shown to enhance grindability and impact the breakage function positively, improving comminution outcomes [14]. The polarization of minerals under MW irradiation causes internal friction and temperature increases [15,16], leading to differences in thermal expansion, crack formation, and mineral liberation. Rock degradation effectiveness has been noted at surface temperatures between 50–600 °C, with most minerals responding optimally at 100–300 °C [17,18].

1.2. State of the Art—Microwave Irradiation as the Method of Rock Treatment

Materials exhibit three distinct microwave (MW) absorption behaviors: low-loss materials transmit MWs with minimal energy absorption, no-loss materials reflect MWs, and high-loss dielectric materials fully absorb MWs [5,18]. The extent of this absorption is primarily governed by the dielectric loss factor, wherein polarized materials within alternating electric fields convert MW energy into thermal energy—an essential parameter in MW heating efficiency. Quartz, for example, demonstrates a weak response to MW irradiation, highlighting the variable effectiveness of this technique across different minerals [19,20]. Research on minerals such as galena and calcite suggests that while mineral shape influences the initiation of microcracks, it does not significantly impact crack propagation [21]. Hence, the mineralogical composition of rocks significantly influences microcrack formation, with minerals such as quartz, feldspar, biotite, pyroxene, and iron-rich phases exhibiting heightened susceptibility to MW-induced damage due to their differential thermal expansion properties [22,23,24,25]. Pyroxene and biotite, in particular, contribute substantially to thermal cracking due to their high-dielectric loss factors, leading to localized heating and stress concentration [26,27,28,29,30,31]. In granite, the heterogeneous expansion of quartz and feldspar promotes intergranular cracking [32], while shale-containing pyrite undergoes rapid microcrack formation due to pyrite’s high thermal expansion coefficient [30,31]. It was shown that magnetite-rich samples exhibited more significant crack formation and a 23.4% reduction in peak power draw (from 360 W to 276 W) after 15 kW–230 kJ treatment [33]. Shadi et al. [34] analyzed basalt samples and found that olivine (a high-dielectric mineral) absorbed more microwave energy, resulting in a 73% increase in heat absorption for optimized particle arrangements. It was noted that MS irradiation increased the rock brittleness (from 0.25 at 1.5 kJ/kg to 0.45 at 3.0 kJ/kg) due to thermal stress, facilitating easier mechanical breakage [35]. Particle size also influenced microwave absorption efficiency. For example, it was found that larger particles (−31.5 + 26.5 mm) absorbed more microwave energy, increasing up to 73% in heat absorption efficiency [34]. In comparison, finer feed particles (6.7 mm) exhibited a 15% lower peak power draw than coarser samples (31.5 mm), indicating more effective energy absorption and crack distribution [33]. Wang et al. [36] confirmed that microwave-treated rock exhibited more uniform fragmentation, reducing the proportion of large fragments under dynamic loading.

It is seen that the MW irradiation effect has been systematically examined on diverse rock types, including sandstone [37,38,39,40], gabbro [30,31,41], granite [22,24,32,42], basalt [29,43,44,45,46], and shale [23,26,30]. Chert gravel, a material consisting of quartz micrograins with impurities such as calcium, sulfur, phosphorus, and barium, has also been analyzed due to its low porosity and high strength [47,48]. Previous studies have extensively characterized chert gravel’s physical and mechanical properties, revealing a composition primarily of quartz micrograins with impurities such as calcium, sulfur, phosphorus, and barium [47]. Characterized by low porosity, high tensile strength (10.8 ± 3.3 MPa), and significant electrical resistivity (23.0 ± 11.9 kΩm), chert gravel exhibits a moderate uniaxial compressive strength (37.3 ± 10.4 MPa). However, alternative strength assessments using the Schmidt hammer and point load tests have yielded significantly higher values (158 ± 30.4 MPa and 321 ± 118.5 MPa, respectively). Notably, MW treatment markedly reduces the strength of chert gravel, with reductions of up to four to six times observed following 2.5 min of irradiation, thereby enhancing its suitability for aggregate applications [48].

Extensive investigations have focused on MW-induced variations in rock properties, including strength, elasticity, fracture toughness, porosity, and microfracture [11,12,19,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64]. For instance, basalt samples subjected to prolonged MW exposure exhibit intensified cracking, with variations depending on power levels and proximity to the MW source [65]. Similarly, MW-treated granite samples experience reductions of approximately 10% in both peak and average cutting forces compared to untreated samples [66]. Laboratory experiments on norite, granite, and basalt indicate that tensile and compressive strengths decrease as MW exposure duration and power density increase. Higher power levels correlate with significant stress accumulation and material degradation [8,11,67]. A direct correlation was observed between MW treatment duration and tensile strength reduction in granite and cement mortar. In contrast, samples with a high compressive-to-tensile strength ratio exhibited elevated temperatures and increased damage under equivalent irradiation conditions [68,69]. Mechanical strength parameters, including uniaxial compressive and Brazilian tensile strength, consistently declined with extended MW exposure at constant power levels [70]. It was demonstrated [34] that as irradiation energy increased, temperature rise followed a nonlinear trend depending on particle size and arrangement, with heat absorption increasing by up to 73% for specific configurations. Li et [35] correlated MW energy input with temperature rise in granite, showing that the value of uniaxial compression strength decreased by 46.7% at 800 °C, confirming the relationship between thermal stress accumulation and rock weakening. Wang et al. [36] identified that CT values dropped from 2301 Hu to 1618 Hu after 4.5 min of exposure, demonstrating progressive density reduction due to thermal cracking.

It was highlighted that the MW irradiation modifies rock mechanical behavior, enhancing breakage efficiency. This process is primarily driven by thermal stress, microcrack development, and differential expansion between mineral phases, all contributing to the reduced value of fragmentation energy [33,34,35,36]. The experimental results demonstrated the impact of MW irradiation on rock weakening: a 30.8% reduction in cumulative specific crushing energy followed MW treatment at 10 kW–210 kJ, confirming that the formation of microcracks facilitates subsequent breakage [33]. Similarly, a 34.2% decrease in P-wave velocity (from 4360 m/s to 2874 m/s) after 4.5 min of MW exposure reflected increased internal discontinuities [36]. MW heating reduced the value of uniaxial compression strength from 150 MPa to 80 MPa at 800 °C and tensile strength by 60% (from 10 MPa to 4 MPa at 600 °C), implying a substantial weakening effect in granite [35]. The extent of MW-induced weakening was shown to be directly linked to irradiation energy levels, exposure duration, and mineral composition. The duration of MW exposure was also noted to affect changes in rock properties. For example, more prolonged MW exposure was correlated with increased microcracking, as seen in the progressive reduction in P-wave velocity [36]. Similarly, it was found that a lower MW energy input of 90 kJ resulted in a modest 9.3% decrease in power draw (from 360 W to 327 W). In contrast, higher energy exposure (15 kW–230 kJ) led to a more pronounced 23.4% reduction [33]. Higher MW irradiation intensities promote crack formation and increase brittleness [35]: raising MW energy from 1.5 kJ/kg to 3.0 kJ/kg elevated the brittleness index from 0.25 to 0.45, making the rock more prone to fracture. MW-treated rock fragments transitioned from larger pieces to finer particles, confirming greater fragmentation efficiency at higher irradiation levels [36]. Rasyid et al. [33] also recorded a sharper drop in power draw with higher MW exposure, accompanied by a 22% increase in fine particle generation (<2 mm), supporting the premise that MW-induced pre-cracking facilitates subsequent mechanical breakage. Additionally, water-saturated sandstone demonstrates increased fracturing as steam pressure generated by water phase transitions exacerbates crack formation [71].

The duration of MW exposure varies significantly across studies, ranging from as little as 10 s [55] to as long as 600 s [46]. Prolonged exposure increases surface temperatures, inducing thermal stress, microcrack propagation, and reductions in mechanical strength [24,30,31,40]. In sandstone, uniaxial compressive strength, elastic modulus, and tensile strength decline with increasing irradiation time, with rapid initial weakening observed [39]. In granite, extended irradiation leads to increased crack density and reduced crack initiation stress [72]. Similarly, basalt and gabbro exhibit enhanced microcrack propagation and decreased strength following prolonged MW exposure [28,29,30,31,41].

Two cooling methods were studied: some experiments allowed rocks to cool naturally [30,31,43,44], while others subjected them to water cooling to induce thermal shock [26,36,40]. Water cooling amplifies damage, increasing crack roughness and significantly reducing mechanical integrity [41,73]. In gabbro, microcracks remain open after cooling, increasing electrical resistance [32]. MW-treated granite, followed by rapid cooling, exhibits pronounced mechanical degradation [73]. The cooling process thus plays a crucial role in determining the extent of thermal damage, with fast cooling exacerbating material weakening compared to natural cooling. A particularly notable phenomenon is the cooling shock effect, observed when heated rocks are abruptly submerged in water. This leads to substantial microcrack formation due to sudden thermal differentials [73]. In granite, rapid water-cooling shifts failure mechanisms from tensile to shear failure [73]. The resulting thermal stress intensification leads to more significant material damage than natural cooling [40]. These findings highlight that cooling is crucial in determining MW-treated rocks’ final strength and integrity.

Hence, the mode and intensity of fracture formation depend on irradiation time and cooling conditions. Shorter MW exposure durations predominantly induce intergranular microcracking, while prolonged exposure fosters transgranular crack propagation and shear failure [32,73,74]. Moreover, rapid water cooling in granite results in a transition from tensile to shear failure [27,28,75]. Rocks subjected to natural cooling exhibit less severe fracturing than those exposed to rapid cooling, where higher thermal stresses facilitate crack coalescence [36,38,75,76].

Overall, MW irradiation significantly affects rock mechanical properties, with the extent of thermal damage depending on MW exposure duration and cooling method. Studies consistently demonstrate that more extended MW irradiation weakens rock structures by promoting microcrack propagation and reducing mechanical strength. Water cooling exacerbates damage by increasing crack formation, while rapid cooling further intensifies thermal stresses, leading to more significant fracturing. The mineral composition of rocks plays a crucial role in determining their response to MW irradiation, with minerals such as quartz, feldspar, biotite, and pyroxene contributing significantly to microcrack formation. These findings highlight the potential of MW-assisted rock weakening for mining, tunneling, and aggregate production applications.

While most MW treatment research has been conducted in laboratory settings, a pilot-scale study demonstrated the feasibility of achieving comminution benefits at reduced energy doses using a system capable of processing up to 150 t/h, suggesting its potential for large-scale applications [77,78].

In summary, MW-assisted rock processing has emerged as a promising, energy-efficient method for mineral extraction and comminution at an industrial scale. Prior research has demonstrated MW irradiation’s effectiveness in inducing thermal stresses, facilitating fragmentation, and optimizing comminution for various rock types, including granite, basalt, sandstone, gabbro, and shale. However, a significant knowledge gap exists regarding its impact on chert gravel, a quartz-rich material with a weak dielectric loss factor, which results in low MW absorption. Unlike minerals with high MW absorption, such as biotite and pyroxene, chert gravel is expected to respond differently under MW exposure. However, no systematic study has been conducted on this subject. Furthermore, the influence of key MW treatment parameters—including exposure duration, rock size, moisture conditions, and cooling methods—on chert gravel fragmentation remains unexplored. While previous studies suggest that MW treatment enhances fragmentation in other rock types, its effectiveness for chert gravel remains uncertain. Addressing this gap is essential for assessing the broader applicability of MW-assisted comminution.

To bridge this gap, the present study investigates the following key research questions:

What is the impact of MW exposure duration on chert gravel fragmentation? How does prolonged MW exposure influence the particle size distribution of chert aggregates?

How does rock size influence MW-induced fragmentation? Do larger chert gravel samples exhibit different fragmentation patterns than smaller ones? What is the relationship between initial rock mass and final aggregate grading?

What role do moisture conditions play in MW-induced fragmentation? How does pre-wetting affect the efficiency of MW-induced crack formation? Does the presence of moisture enhance or hinder the fragmentation process?

How do different cooling methods (air vs. water quenching) influence chert gravel fragmentation? Does rapid water quenching enhance crack propagation in MW-treated chert gravel?

Our research seeks to systematically analyze these unanswered questions by conducting controlled MW irradiation experiments on chert gravel samples. The goal is to clearly understand MW-assisted chert gravel fragmentation and identify processing conditions for aggregate production. The findings will contribute to developing a sustainable methodology for repurposing chert gravel as a valuable construction material.

2. Materials and Methods

2.1. Geological Background

Previous studies have provided a detailed description of the site under investigation [79,80,81,82]; therefore, only a summary is presented here. The Rotem Plain, located in the Negev Desert approximately 200 km south of Tel Aviv, is a significant sand source for Israel’s construction and paving industries. This region contains extensive sand deposits essential for national infrastructure, with the deposit reaching a thickness of up to 70 m [79,80,81,82]. Chert gravel constitutes approximately 40% of the sand quarry volume [81,82], as distinct layers ranging from 0.2 to 1 m thick or dispersed throughout the sandy strata (Figure 1).

2.2. Method

Figure 2 shows the overall flowchart for the chert gravel research in the SCE laboratory.

The study consisted of nine stages (Figure 3):

(1)

Samples drying to avoid free water from the pores’ space–24 h, 110 °C);

(2)

Preparation for irradiation exposure, including preparation for weighting, when half of the samples were prewetted on their surface before weighting (Groups C, D—see below);

(3)

Weighting;

(4)

Microwave (MW) treatment;

(5)

Air and water cooling (Groups A,C, B,D, respectively—see below);

(6)

Preparation for crushing test;

(7)

Crushing test;

(8)

Aggregate sieving;

(9)

Classification.

Four groups of samples were used for the study (Figure 3, Stages 2 and 5)

Group A: Samples were heated while dry and cooled at room temperature.

Group B: As in A, after heating, the samples were immediately quenched by immersion in water at room temperature.

Group C: Similar to A, but the sample surfaces were pre-wetted before irradiation.

Group D: Similar to B, with the addition of pre-wetting the sample surfaces before irradiation.

The specifics of the microwave (MW) treatment process (Stage 4 in Figure 3) are detailed in [48]. Each sample, weighing 150, 300, 600, or 800 g and measuring between 5 and 15–20 cm in maximum size was subjected to MW treatment using a microwave oven (Model EM823A2GU-6563, Midea Group, Beijiaozhen, Foshan, China) operating at 2.45 GHz with a maximum power output of 900 W. The exposure times were set at 1, 1.5, 2, and 2.5 min. To ensure consistent initial conditions, each sample was placed in the MW oven only after both the sample and the oven’s internal temperature had stabilized to room temperature. After each experiment, the oven was cooled back to room temperature before processing the following sample. Each sample was exposed to MW treatment only once.

In total, 64 experimental groups were established, based on a combination of four sample categories (Groups A–D), four irradiation times (1, 1.5, 2, and 2.5 min), and four sample masses (150, 300, 600, and 800 g). Each group comprised 10 samples (640 samples).

Following MW treatment, the chert gravel samples were mechanically reduced in size (Stage 6 in Figure 3) using a point load device by BS 812-112 standard [47,83]. Subsequently, they underwent a crushing test (Stage 7 in Figure 3) using specialized equipment (Figure 4), including a hollow steel cylinder, piston, base plate, and compression rod. These components, manufactured from corrosion-resistant alloy steel with a hardness of 650 HV (57.8 HRC), were designed for durability and precision. The hollow steel cylinder had an internal diameter of 150 mm, a height of 130 mm, and a total weight of 16.5 kg (Model 48-D0510) [47,83]. The crushing tests were conducted using a MATEST S.P.A TREVOLO 24048 press, Italy (Figure 5).

The crushing procedure was conducted as follows:

a.

Sample preparation: The hollow cylinder was filled with aggregate to an average height of 95 mm and compacted manually.

b.

Machine activation: The press machine was operated at 0.025 MPa/s.

c.

Seating stage: The press was stopped once a deformation of 3 mm was achieved.

d.

Piston alignment: The loading piston was adjusted as needed.

e.

Progressive loading: Steps 3 and 4 were repeated for 5 mm, 10 mm, and 15 mm deformation values.

f.

Termination: The crushing process concluded upon reaching 15 mm deformation.

After crushing, the aggregate mix underwent a sieving process (Stage 8 in Figure 3) using a sieve system that complies with the ISO 3310-1 standard [84] (Figure 6). The aggregate resulting from the crushing tests was classified according to the Unified Soil Classification System (USCS) method, following the ASTM D2487 standard [85]—Stage 9 (Figure 3). The aggregates were carefully classified and weighed according to their size during grading.

2.3. Materials

The chert gravel is generally rounded and between 5 and 15 cm (sometimes up to 20 cm) in size [47,48] (Figure 7 and Section 2.2). The chemical composition analysis via SEM reveals that chert gravels primarily consist of silica (Si, 47.04–49.91%) and oxygen (O, 50.67–53.96%), with slight traces of inclusions composed of calcium (Ca, up to 34.87%), phosphorus (P, up to 30.87%), sulfur (S, up to 15.09%), and barium (Ba, up to 0.85%) [69].

The mean values of the results in

Table 1. The physical–mechanical properties of the chert gravels [47].

Properties	Value
Density (g/cm3)	2.57 ± 0.04
Porosity (%)	0.34 ± 0.3
Mass water absorption (%)	0.13 ± 0.13
Point Load index–Is(50), MPa	12.5± 5.2
The mean value of uniaxial compression strength, MPa	321.0 ± 118.5
Tension strength, MPa	10.8 ± 3.3
The value of ultrasonic wave speed, km/s	4.38 ± 0.72
Electrical resistivity, kOhm*m	23 ± 1.9
Dielectric constant	2.53 ± 0.25
Magnetic susceptibility	0

indicate several key physical–mechanical properties of chert gravels [47].

The chert gravel is characterized by relatively high density, extremely low porosity, and minimal capacity to retain moisture. The strength values (compression and tension) are high up to extremely high. The value of ultrasonic wave velocity indicates a relatively compact and rigid structure. High electrical resistivity and dielectric permittivity values imply low conductivity and minor water retention capability. Magnetic susceptibility tests confirmed the absence of magnetic minerals.

Our previous results [48] demonstrated microwave irradiation’s effect on chert gravel’s comminution. Initially, at 12.5 MPa (Table 1), the point load strength index decreased with increasing irradiation time. For dry samples without quenching, the strength decreased from 7.22 MPa after 1 min of irradiation to 3.97 MPa after 2.5 min, reflecting a 1.8-fold reduction. The reduction was even more significant for dry samples with quenching, with strength values dropping from 5.86 MPa to 2.75 MPa, a 2.13-fold decrease. Similarly, the strength values declined for wet samples from 5.73 MPa to 4.36 MPa without quenching and from 4.58 MPa to 2.49 MPa with quenching, showing a 1.45–1.75 times lower strength than non-quenched samples.

Figure 8 shows the typical size distribution curves for four groups (A–D, Section 2.2) under study.

The sieve analysis results demonstrated [84] that microwave irradiation and quenching altered the particle size distribution of the crushed material. Initially, untreated chert samples had a Gravel-to-Sand ratio of 76.6% to 23.4%, but irradiation decreased the gravel fraction. For dry samples without quenching, the gravel fraction decreased from 65.7% after 1 min to 58.6% after 2 min and then slightly increased to 65.6% after 2.5 min, while the sand fraction increased correspondingly. Quenching significantly accelerated the transition from gravel to sand, with dry-surface samples showing a gravel fraction of 50.5% after 1 min of irradiation and further reducing to 44.5% after 2.5 min, effectively shifting their classification from well-graded gravel (GW) to a mix of gravel and sand (GW/SW). The impact was even more pronounced for wet-surface samples with quenching, where the gravel content dropped as low as 42.3% after 2.5 min, resulting in a classification change to well-graded sand (SW).

3. Results

Figure 9 presents the percentage distribution of various aggregate types across all tests. The graph includes markers denoting sample classifications based on the Unified Soil Classification System (USCS) [85], such as:

GW (Well-graded gravel)—yellow-marked.

GP (Poorly graded gravel)—green-marked.

SW (Well-graded sand)—orange-marked.

SP (Poorly graded sand)—red-marked.

It is seen that well-graded gravel (GW) exhibits the highest probable result after such tests.

It is essential to note that the ratio of coarse aggregate to fine aggregate (sand) varies depending on the mixture type. Figure 10 presents the average sand/gravel ratio for four aggregate mixtures.

For example, the sand content in GP-type mixtures does not exceed 20%, whereas in GW-type mixtures, it is below 40%. Consequently, controlling the aggregate mix type enables optimization of its effectiveness, including economic considerations.

The ternary (triangular) diagram in Figure 11 illustrates the percentage composition of coarse aggregate (particles larger than 4.75 mm in the mixture) in each considered aggregate mixture, categorized into three distinct size ranges. Each graph axis represents a different percentage of particle sizes: the left axis corresponds to particles in the 19–25 mm range, the right axis to particles of 12.5–19 mm, and the bottom axis to particles sized 4.75–12.5 mm. Graphical analysis allows for identifying particle size distribution trends in different samples. For instance, GW-classified samples tend to be located in areas with a high percentage of medium-sized particles (4.75–12.5 mm). In contrast, SP-classified samples are concentrated in other regions based on their characteristic particle size distribution. In the ternary diagram (Figure 10), the blue arrow indicates a sample composed of 69% aggregates in the 19–25 mm range, 12.5% aggregates of 12.5–19 mm, and 18.5% aggregates of 4.75–12.5 mm.

Understanding the particle size distribution within the mixture is particularly significant in an engineering context. Well-graded distributions, such as in GW and SW samples, increase strength while reducing permeability. Consequently, such mixtures suit highly durable applications like road infrastructure and construction projects. From an environmental engineering perspective, well-graded mixtures are crucial in sustainable construction. They minimize material requirements while improving the longevity of construction materials, thereby promoting efficiency and resource conservation. Additionally, a well-graded aggregate composition enhances the environmental properties of materials by facilitating water reuse and reducing the necessity for additional filler materials.

Overall, the ternary diagram provides valuable insights into aggregate composition and distribution, allowing for an informed selection of mixtures that enhance mechanical properties and support sustainable and efficient construction processes.

Figure 12 illustrates the probability of receiving an aggregate type depending on the group of samples (A–D, Section 2.2), implying using dry or wet sample surface and cooling in air or water. The analysis of this figure allows for the determination of the most suitable pre-treatment method for specific aggregate types. Findings indicate that for achieving well-graded gravel (GW), the optimal process involves the treatment of dry samples followed by air cooling. In contrast, obtaining a well-graded sand mixture (SW) can be facilitated through water cooling. For poorly graded gravel (GP), a treatment involving wet-surfaced samples followed by air cooling is preferable. Various methods can be used to obtain poorly graded sand (SP). Still, it should be noted that SP aggregates were only obtained in a minor percentage of tests (Figure 8), limiting the statistical robustness of this conclusion.

These findings highlight the need for innovative heating and treatment technologies to optimize energy consumption while preserving aggregate properties. The insights gained from our study provide a foundation for further discussions on material properties and their applications in construction industries, emphasizing the impact of different processes on material strength and durability.

Figure 13 depicts the correlation between the average microwave heating time and the aggregate grading type. Data indicate that when the average irradiation time exceeds 1.5 min, the probability of obtaining a well-graded aggregate mixture increases, suggesting a relationship between heating duration and aggregate type.

Figure 14 illustrates the correlation between average mass (g) and aggregate type. The data are derived solely from sample size, without considering pre- or post-treatment effects or microwave heating duration. The graph suggests that an increase in the average mass of gravel clusters in the mixture corresponds to a higher percentage of coarse aggregate in the grading curves.

The analysis of the graphs yields valuable insights into the influence of various factors on the aggregate distribution within the mixture as follows:

a.

The results show that the GW mixture is the most frequently obtained aggregate type.

b.

Different pre-treatment methods (dry vs. wet, air vs. water cooling) influenced the resulting aggregate types.

c.

The larger samples (600–800 g) produced coarser, less uniform particles.

d.

The prolonged MW exposure time led to finer and more uniform aggregates.

e.

The quenching induces intense fracturing, creating irregular, highly fragmented aggregates.

f.

Pre-wetted samples exhibited finer and more uniform fragmentation than dry ones.

4. Discussion

This study systematically investigated the effects of microwave (MW) irradiation on chert gravel fragmentation, focusing on exposure duration, rock size, moisture conditions, and cooling procedures. The findings indicate that MW treatment significantly influences aggregate gradation, making it a viable method for optimizing aggregate properties for construction applications align with previous research while introducing novel insights into the behavior of chert gravel, a quartz-dominant material previously considered unsuitable for MW treatment due to its low dielectric loss factor.

The results confirmed that MW exposure duration, moisture conditions, and cooling methods are critical in achieving well-graded aggregate compositions. Despite quartz’s traditionally weak MW absorption capacity, the study demonstrated that MW irradiation effectively induces fragmentation in chert gravel. Prolonged MW exposure (≥2 min) significantly enhances microcracking, producing finer and more uniform aggregates. The findings align with previous research demonstrating that MW exposure increases surface temperatures, induces thermal stress, promotes microcrack propagation, and reduces mechanical strength [24,28,29,30,31,39,40,44,46,73].

The role of moisture in MW-induced fragmentation was particularly notable. Pre-wetted chert gravel samples exhibited finer and more uniform fragmentation than dry samples, which produced irregular, coarser fragments due to limited crack propagation. This supports previous research showing that water saturation enhances microcrack formation by generating internal steam pressure [33,34]. This study expands those findings to chert gravel, a quartz-rich material with historically low MW responsiveness, demonstrating that moisture enhances MW-induced fragmentation efficiency.

The selection of the cooling method also proved crucial in determining aggregate classification. Air cooling produced well-graded gravel (GW), which is preferred for construction applications, while water quenching caused intense thermal shock, resulting in highly fragmented, poorly graded sand (SP). These results align with previous studies that found MW-treated granite subjected to rapid cooling undergoes significant mechanical degradation [74]. Similarly, research on gabbro demonstrated that microcracks remain open after cooling, leading to lower mechanical integrity [32].

A critical validation from prior studies is the cooling shock effect, where heated rocks rapidly submerged in water undergo extreme thermal stress, intensifying fracture development [74]. This study confirms that cooling shock significantly affects MW-treated chert gravel, a phenomenon not previously documented for this material.

Additionally, the study confirmed that larger chert gravel samples (600–800 g) result in coarser, less uniform aggregates, while smaller samples (≤300 g) fragment more efficiently, producing well-graded aggregates (GW, SW). These results confirm that larger mineral grains require more prolonged MW exposure for effective microcracking [12,13]. This study is the first to quantify these effects specifically for MW-treated chert gravel, providing practical guidance for industrial-scale applications.

The findings contribute to sustainability by demonstrating that MW irradiation can optimize aggregate production while reducing mechanical crushing energy demands. Longer MW exposure (≥2 min) facilitates fragmentation, producing finer aggregates with reduced mechanical effort. Water quenching accelerates breakdown but must be carefully controlled to prevent excessive fine material generation.

Flakiness significantly influences aggregate quality and mechanical properties, affecting load-bearing capacity and durability [86,87,88,89,90]. Research indicates a correlation between flakiness and petrographic properties [86], the performance of reclaimed concrete aggregates [87], and optical grading accuracy [88]. Asphalt mixtures show strong correlations between flakiness and deformation resistance [89]. Crusher settings also impact the flakiness index, influencing aggregate shape [90]. The implications of MW treatment on flakiness for the chert gravel remain unexplored and should be a focus for further research.

5. Conclusions

This study presents an innovative and sustainable approach for utilizing chert gravel, a byproduct of sand quarrying, as a viable aggregate resource. By applying MW irradiation, this research establishes a customizable, energy-efficient method for processing chert gravel into high-quality aggregates, reducing reliance on virgin materials while minimizing environmental impact.

The key findings confirm that MW treatment optimizes fragmentation, with the following observations:

✓

Optimal MW exposure duration: At ≥2 min, significant micro-fracturing occurs, leading to finer, well-graded aggregates.

✓

Influence of initial rock size: Larger samples (600–800 g) result in coarser, less uniform aggregates, while smaller samples (≤300 g) fragment more efficiently into well-graded materials.

✓

Effect of moisture conditions: Pre-wetted samples exhibit finer, more uniform breakage than dry samples, improving fragmentation efficiency.

✓

Cooling method impact: Air cooling produces well-graded gravel (GW), while water quenching results in poorly graded sand (SP) due to thermal shock effects.

This research contributes to sustainable aggregate production by reducing the mechanical energy required for crushing and optimizing fragmentation efficiency through MW treatment. The findings provide a foundation for engineering applications where controlled fragmentation is necessary, such as road construction and concrete production.

✓

Future research should focus on the following areas to enhance the practical adoption of MW-assisted chert gravel processing:

✓

Flakiness Evaluation: Investigating the effects of MW exposure on aggregate flakiness and its implications for mechanical performance.

✓

Industrial-Scale Feasibility: Conducting cost-benefit analyses and energy efficiency comparisons with conventional crushing methods.

✓

Regulatory Compliance: Assessing whether MW-treated chert gravel meets physical and chemical standards for construction applications.

✓

Long-Term Performance: Evaluating the durability of MW-treated aggregates under various environmental and load conditions.

By addressing these research directions, future studies can further validate the applicability of MW-assisted comminution in sustainable construction, supporting resource conservation and energy efficiency in aggregate processing.