Potential Use of Jarosite Industrial Waste in Developing Hybrid Composites

Abstract

Jarosite is a by-product of the zinc manufacturing industry. The potential use of jarosite processing waste as a component in hybrid composites offers a valuable opportunity for addressing waste management and environmental challenges. Therefore, in this study, hybrid composites were prepared using a polyester matrix reinforced with five layers of a fiberglass chopped strand mat and incorporating 5, 10, and 15 wt.% of jarosite waste particles as fillers. The hand lay-up technique was used for the composite preparation, with jarosite particles pre-dispersed in the polyester resin by an ultrasonic probe treatment to ensure the uniform dispersion of the jarosite particles within the matrix. The flexural properties (the flexural strength, modulus, and apparent interlaminar shear strength), Charpy impact strength, and hardness of the composites were determined and analyzed. The results showed that adding 10 wt.% of jarosite significantly improved the flexural strength (30% higher than the base composite) and hardness (15% higher). Composites with 5 wt.% and 15 wt.% of jarosite showed similar properties to the base composite. These findings demonstrate the potential of jarosite waste as a sustainable filler in hybrid composites, balancing mechanical properties and sustainability.

1. Introduction

Jarosite is a ferric hydroxysulfate mineral that belongs to the alunite group of minerals (XFe₃(SO₄)₂(OH)₆), with various cations such as Na⁺, K⁺, NH₄⁺, H₃O⁺, and Ag⁺, or divalent cations such as Pb²⁺, in site X. It occurs in acidic and oxidizing environments rich in sulfate and iron. The natural rock weathering process leads to acid rock drainage, releasing acidic water [1,2]. However, various machining and manufacturing processes, such as the production of zinc through acid leaching from sphalerite ores, release a huge quantity of jarosite waste. In zinc refining, iron impurities are removed by using the following three main processes: goethite (FeOOH), hematite (Fe₂O₃), or jarosite processes [3]. During the jarosite process, the iron compound is precipitated, during which large amounts of solid waste—mostly jarosite—are produced and accumulate. Due to the use of sulfuric acid as a catalyst, jarosite waste is highly acidic in nature (a pH of 2.7). The presence of toxic substances such as zinc, lead, cadmium, copper, and other metallic and non-metallic oxides makes this kind of waste very hazardous, as it causes water and soil contamination, as well as the pollution of the surrounding vegetation and animal life [3,4,5]. This hazardous waste must be recycled or safely disposed of to prevent environmental contamination [6,7,8].

The possibility of using jarosite processing waste as one of the components for hybrid composites represents a promising solution for waste recycling and material enhancement. Numerous scientists have investigated various methods for the waste management and recycling of jarosite. Pappu et al. [9,10] evaluated the possibility of developing building materials in the form of fired jarosite bricks. The experimental trials revealed that the density, water absorption capacity, and compressive strength of fired jarosite bricks indicate their potential as building materials. Wang et al. [11] proposed a method for the recovery and recycling of jarosite residue which removes heavy metals from acidic wastewater from zinc hydrometallurgy. A novel process was devised for the utilization of jarosite in the form of Zn-Fe alloys by Ahamed et al. [7]. Jarosite can also be easily transformed into goethite using oxyanions during mineral transformation. Ryu and Kim [12] showed that the oxyanions, which were coprecipitated with jarosite, greatly affected the mineral transformation and dissolution rates of jarosite.

Numerous scientists have investigated the use of various waste materials as a filler in polymer-based composites, as well as their impact on the mechanical properties of the composites (

Table 1. Polymer-based composites with waste filler and the testing of its mechanical properties.

Matrix	Reinforcement	Filler	Mechanical Properties	Ref.
Polyester resin	Sisal fiber	Red mud	Hardness Tensile strength Charpy impact strength	[13]
Polyester resin	/	Saw dust Rice husk Fly ash Red mud	Hardness Tensile strength Charpy impact strength	[14]
Epoxy resin	Glass fiber	Red mud	Microhardness Tensile strength Flexural strength Charpy impact strength	[15]
Epoxy resin	Bamboo fiber	Red mud	Tensile strength Tensile modulus Flexural strength Interlaminar shear strength Impact strength	[16]
Epoxy resin	Glass fiber	Fly ash	Impact strength Compressive strength	[17]
Polypropilene	/	Fly ash	Flexural strength Flexural modulus	[18]
Polypropilene	/	Blast furnace slag (BFS)	Microhardness Tensile strength Flexural strength Impact strength	[19]
Micaceous clay	Jute fiber	Blast furnace slag	Compressive strength	[20]
Epoxy resin	Aerial root of Banyan tree	Linz–Donawitz slag (LDS)	Microhardness Tensile strength Compressive strength Flexural strength	[21]
Polypropilene	/	Silico-aluminous ash	Tensile strength Impact strength	[22]
Epoxy resin	Coconut shell powder	/	Tensile strength Tensile modulus Impact strength Hardness	[23]
Epoxy resin	/	Coconut shell Wood apple shell	Tensile strength Flexural strength	[24]
Polypropilene	Waste coconut fiber	Waste coconut shell particles	Tensile strength Tensile modulus	[25]

).

Dubey et al. presented a review of polymer composites using waste materials as reinforcements [26]. The primary focus of all of this research has been on producing materials with a high strength and a low density. Nguyen et al. presented the mechanical properties of hybrid polymer composites [27]. Vigneshwaran et al. [13] fabricated a hybrid composite using red mud as a filler in a polyester matrix reinforced with sisal fibers. The mechanical properties of these hybrid composites showed a significant improvement compared to the unfilled sisal/polyester composites. There was a 9.49% increase in the hardness with 30% of red mud, and the highest increase in the tensile and impact strength was achieved with 20% red mud. Similarly, Prabu et al. [14] found that industrial waste such as sawdust and rice husk, when used to reinforce polyester composites, also exhibited excellent mechanical performance. Specifically, the tensile strength increased by 14% and 18%, while the impact strength increased by 61% and 142% for saw dust and rice husk, respectively. The tensile strength and impact strength for the composite with the addition of fly ash increased by 62% and 42%, respectively. In the case of red mud, the tensile strength decreased drastically, while the impact strength was improved by 6% compared to pure polyester. Biswas and Satapathy [15] prepared a composite with an epoxy resin matrix, with a glass fiber reinforcement and red mud as a filler. With the addition of red mud, the tensile properties became distinctly poorer. The tensile strength also decreased with an addition of filler up to 10%. With the addition of 10 wt.% of red mud, the apparent interlaminar shear strength decreased drastically. On the other hand, the impact strength and microhardness increased with the presence of red mud. Similarly, polyester resin and red mud filler composites were prepared by the same authors [16], but with the addition of bamboo fiber as a reinforcement. They found that all of the tested mechanical properties were much lower than those reported for the glass fiber reinforcement. Singla and Chawla [17] tested the mechanical properties of an epoxy resin–fly ash composite. They concluded that the compressive and impact strength increased with an increase in fly ash particles. A composite from recycled polypropylene filled with fly ash was prepared by Gummadi et al. [18]. Their results indicated that the addition of fly ash improved the flexural strength and flexural modulus. Padhi et al. [19] reported that the composite microhardness was enhanced by 15 times with a BFS filling. However, the tensile and flexural strength decreased with the addition of BFS, while the impact strength increased with an increase in BFS. Moreover, BFS was also added to micaceous clay reinforced with juta fiber. Zhang et al. [20] reported that the addition of BFS improved the composite strength, stiffness, and toughness. LDS-filled hybrid composites were prepared by Purohit et al. [21]. It was observed that mechanical properties like the tensile, compressive, and flexural strength values increased with the increase in the natural fiber content in the epoxy–LDS composites. Teodorescu et al. [22] investigated composites made from silico-aluminous industrial waste (ashes) and recycled polypropylene derived from disposable medical face masks. Their results indicated that incorporating just 5 wt.% of silico-aluminous ash into the polypropylene matrix enhanced its thermal stability and stiffness while maintaining its mechanical strength. The potential of using coconut shell particle fillers in eco-composite materials was investigated by Sarki et al. [23]. It was shown that the value of the hardness, tensile modulus, and tensile strength increased with the increase in the coconut shell particle content, while the impact strength slightly decreased compared to pure epoxy resin. Coconut shell was also used as a filler by Ojha et al. [24], alongside wood apple shell, for the preparation of a reinforced polymer composite based on epoxy resin. The wood apple shell composite showed superior properties over the coconut shell composite. The maximum tensile strength was obtained at a 15 wt.% filler loading in both of the composites. The flexural strength increased with an increase in the filler loading up to 15 wt.% in both of the composites. The maximum flexural strength obtained was 78.19 MPa for the wood apple shell and 68.25 MPa for the coconut shell particulate reinforcement composites. Essabit et al. [25] prepared bio-based hybrid composites from waste coir residues (fibers and shell particles). The hybrid composites showed a 50% improvement in the tensile modulus and an 8% improvement in the tensile strength. Other commercial waste materials, such as blast furnace slag, fly ash, and Linz Donawitz slag, have also been explored as potential fillers for polymer composites. Girge et al. [28] reported that the introduction of fly ash into hybrid composites increased their compressive strength. Linz Donawitz slag, on the other hand, contributed to composites with improved strength, wear resistance, and hardness.

In addition to traditional composite materials, the development of biocomposites from both renewable and industrial waste has gained attention as a sustainable waste management strategy. Biocomposites are eco-friendly materials that are energy-efficient and sustainable. A comprehensive review by Das et al. [29] highlighted that adding biowaste fillers to polymer matrix composites increased their Young’s modulus. However, the tensile strength and elongation at break tended to decrease in these biocomposites. While these studies explore jarosite and other waste fillers in composites, the effect of varying jarosite amounts on hybrid composite performance remains underexplored.

This work presents an attempt at immobilizing the jarosite waste as a value-added product into the hybrid composites. Therefore, the hybrid composites were prepared with a polyester matrix reinforced with a fiberglass chopped strand mat and the addition of different amounts of jarosite waste particles as a filler. The main objective of this study was to investigate the influence of different concentrations of jarosite particles on the mechanical properties of hybrid polymer composites.

2. Materials and Methods

2.1. Preparation of Hybrid Composites

The composite materials were made from polyester resin and glass fibers. A non-woven short glass fiber (provided by Kelteks, Karlovac, Croatia) scattered in all directions (fiberglass chopped strand mat) was used for the matrix reinforcement. Commercial polyester resin (UP), Polipol 3401-TA-H-17, with a density of 0.001128 g mm⁻³, was supplied by Poliya (Istanbul, Turkey). It is a room-temperature curing resin with a low viscosity, and it is commonly used as a matrix resin for composite fabrication by the low-cost hand lay-up technique. For the room-temperature curing of the polyester resin, 1% of Ketanox B180 (COIM, Milan, Italy) chemical as an initiator was added in order to allow for about 2 h of fabrication before significant gelation occurred.

The jarosite waste sample was used as one of the components for the hybrid composites. The jarosite waste deposit is situated on the bank of the Sitnica River, in close proximity to the city of Mitrovica, Kosovo. After the sampling, the jarosite waste was dried, ground, and sieved to achieve a uniform particle size distribution. The total metal content of the trace and major metals was determined by means of inductively coupled plasma–optical emission spectrometry (ICP–OES; Teledyne Leeman Labs, Hudson, NH, SAD) after the preparation of the samples using the microwave digestion method (the MARSX XP1500 Microwave Digestion System, CEM, SAD). The chemical composition of the jarosite waste deposit is shown in

Table 2. Chemical composition of the jarosite waste deposit [30].

Component	Al2O3	Fe2O3	SiO2	Zn	Pb	Cu	Ag	Ba	Co	Cd	Cr	Mn	Ni	Sr	As
amount	wt.%						mg × kg−1
	1.42	44.9	6.3	10.9	7.5	0.97	134	579	30	2309	417	6392	94	156	5076

. To ensure minimal airborne particle dispersion during handling, the particle size of the jarosite waste was kept between 1 mm and 10 μm [30].

The preparation of the hybrid composites and samples for the testing of the mechanical properties is shown in the schematic diagram in Figure 1. The first step in the preparation of the hybrid composite was the dispersion of the jarosite particles in the polyester resin. Three different mixtures were prepared with 5, 10, and 15 wt.% of jarosite particles. The selection of the presented amounts of jarosite particles was based on our aim to incorporate as much of the waste material as possible. Polyester resin and jarosite particles were mixed manually, after which the mixtures were ultrasonicated by an ultrasonic probe (the UP400S, operated at 400 W and 24 kHz; Hielscher, Teltow, Germany) at an amplitude of 100% (a 120 μm amplitude) and an acoustic power density of 105 W/cm². Each mixture was sonicated 6 times for 30 s at 50 °C.

The following four types of hybrid composites were prepared:

• Polyester resin (UP) + non-woven fiberglass mat;
• Polyester resin (UP) + non-woven fiberglass mat + 5 wt.% of jarosite particles;
• Polyester resin (UP) + non-woven fiberglass mat + 10 wt.% of jarosite particles;
• Polyester resin (UP) + non-woven fiberglass mat + 15 wt.% of jarosite particles.

The composites were prepared by the hand lay-up technique, which is the oldest open-molding method for composite fabrication. The hand lay-up method was selected for its simplicity and cost-effectiveness in fabricating hybrid composites [31,32]. Also, Prabu et al. [14] reported that the hand lay-up technique can successfully be used to produce polymer composites (polyester and epoxy) reinforced with industrial waste materials such as sawdust, rice husk, fly ash, and red mud. For the preparation of the hybrid composites, 5 sheets of a non-woven fiberglass mat were placed manually in the open mold. The resultant mixture (UP + jarosite particles) was poured into the mold and over the glass plies. Entrapped air between the fiberglass and resin was removed manually with rollers to complete the laminated structure. Curing was initiated by a catalyst (1 wt.% of Ketanox B180) in the resin, which hardens the composite without external heat. The prepared samples were put under load for 24 h for proper curing at room temperature. The specimens were cut using a diamond saw to the dimensions according to the standard for each mechanical property test (see Section 2.2.1, Section 2.2.2, Section 2.2.3 and 2.2.4).

2.2. Mechanical Properties of Hybrid Composites

2.2.1. Three-Point Flexural Test

The flexural properties of the prepared composites were measured according to the standard EN ISO 14125:2001 [33] using the three-point flexural method. The sample size was determined according to the standard; the size was 80 mm × 10 mm × 4 mm (length × width × thickness). The tests were performed on the VEB Thüringer Industriewerk universal testing machine with crosshead speed of 7 mm min⁻¹. The test samples, placed on two supports with a defined distance, were loaded with a loading pin from above with a constant rate until sample failure. The distance between the supports was 64 mm, which was 16 times longer than the sample thickness as required by the standard. Five samples of each prepared type of composite were tested at room temperature. During the test, for certain amounts of force, the deflection of the test specimens was read on extensometer and marked.

2.2.2. Short-Beam Shear Test

The short-beam shear test was carried out to determine the effect of jarosite particles on the apparent interlaminar shear strength (τ) of the prepared hybrid composites with the jarosite particles. The test was conducted on a three-point bending rig in accordance with the standard EN ISO 14130 [34]. The samples were tested on the VEB Thüringer Industriewerk universal testing machine, with crosshead speed of 7 mm × min⁻¹, the same as in the three-point flexural test. The distance between the supports was 20 mm, because the standard recommends that the distance-to-thickness ratio must be five times larger. According to standard, the sample size was 40 mm × 20 mm × 4 mm (length × width × thickness). The test samples were placed on two supports, and the loading pin was pressed on the central area of the sample until failure. The test was repeated six times for each type of composite. The apparent interlaminar shear strength (τ) was calculated with the following formula:

$$\tau =\left({\displaystyle \frac{3}{4}}\right)\times \left({\displaystyle \frac{F_{max}}{b\times h}}\right), \mathrm{M}\mathrm{P}\mathrm{a}$$

(1)

where τ represents the apparent interlaminar shear strength, F_max is the failure load, and b and h are the sample width and thickness.

2.2.3. Charpy Impact Test

The Charpy impact test was performed in accordance with the standard EN ISO 179-1:2023 [35] by a Charpy impact testing machine (manufacturer Karl Frank) with a 4 J capacity to determine the amount of energy absorbed by the material during fracture. The test was conducted at room temperature on standard test specimens with an 80 mm × 10 mm × 4 mm (length × width × thickness) size. All the test specimens were un-notched. Impact loading was conducted with a 4 J hammer. Six samples of each type of material were tested and the mean value of the absorbed energy was taken. The impact strength was calculated by dividing the noted absorbed impact energy with the cross-sectional area of the specimens.

2.2.4. Determination of Hardness

Hardness represents the depth of penetration of an indenter with a defined geometry into the composite specimen for a fixed load. The hardness of the composites was determined by hardness measurements, according to the standard ISO 2039-1 [36], by the ball indentation method. This method is usually used for harder polymers and composites. A spherical indenter (a ball with a 5 mm diameter) under constant load was pressed into the testing material. To indent the test specimens with a ball indenter, a Zwick hardness tester was used with a load of 50 kPa; the load was removed after 10, 30, and 60 s. Ten indentations were made at different points on each type of material. The specimens for the hardness measure were 4 mm thick.

3. Results and Discussion

3.1. Flexural Test

From the measured load/deflection curves, the flexural modulus (E_f) and flexural strength (σ_f) were calculated. The stress–strain curves and mechanical properties obtained from the flexural test are shown in

Table 3. Flexural modulus (E_f) and flexural strength (σ_f) of the polymer composites (mean value ± standard deviation) with a varying jarosite content.

Sample	Ef, MPa	σf, MPa
UP/fiberglass mat	7261.0 ± 199.3	199.3 ± 13.4
UP/fiberglass mat/5 wt.% of jarosite	5860.2 ± 132.0	132.0 ± 9.3
UP/fiberglass mat/10 wt.% of jarosite	8307.6 ± 207.0	207.0 ± 12.8
UP/fiberglass mat/15 wt.% of jarosite	7706.1 ± 184.4	184.4 ±12.1

and Figure 2.

The incorporation of more than 5 wt.% of jarosite particles into the polyester polymer matrix, reinforced with glass fibers, causes a significant improvement in the flexural strength and flexural modulus. The highest flexural strength and modulus were achieved on the composite samples with 10 wt.% of jarosite particles. Samples with 5 wt.% of jarosite particles have even lower modulus and strength values than the samples without the addition of jarosite. A further increase in the concentration of jarosite (more than 10 wt.%) causes a decrease in the flexural strength and modulus, as shown in Figure 2.

The same observation was reported by Biswas et al. [16], who found that the flexural strength of a glass–epoxy composite initially decreased with a lower filler content but increased at higher filler concentrations due to the improved interphase bonding. Acikbas et al. [37] also concluded that the addition of smaller amounts of waste particles into epoxy resin led to a decrease in the bending strength due to the brittleness induced by the filler. Furthermore, as the filler content increased, the bending strength was improved as a result of the strong interphase bonding between the components.

The decline at higher concentrations of jarosite fillers (15 wt.%) may be attributed to particle agglomeration, leading to stress concentrations and a reduced load transfer efficiency [38,39].

The results from the short-beam shear tests of the polymer composites with different concentrations of jarosite particles are shown in

Table 4. Apparent interlaminar shear strength of the polymer composites with different amounts of jarosite particles (mean value ± standard deviation).

Sample	τ, N × mm−2
UP/fiberglass mat	31.12 ± 0.82
UP/fiberglass mat/5 wt.% of jarosite	22.69 ± 2.82
UP/fiberglass mat/10 wt.% of jarosite	32.19 ± 1.52
UP/fiberglass mat/15 wt.% of jarosite	27.02 ± 1.83

and Figure 3. All the data presented in Figure 3 are averages of five values; therefore, error bars are plotted.

Samples with 10 wt.% of jarosite particles have the highest apparent interlaminar shear strength, as shown in Figure 3. Samples with 5 wt.% and 15 wt.% of jarosite particles have a lower strength, even when compared to the samples without jarosite particles. For example, the apparent interlaminar shear strength of the composite with 5 wt.% of jarosite particles was 30% lower, decreasing from 31.12 N × mm⁻² to 22.69 N × mm⁻², compared to the composite sample without jarosite particles.

Biswas et al. [16] also found that the highest interlaminar shear strength was observed at a filler content of 10 wt.%, which was even higher than that of the unfilled composite. However, with a further increase in the filler content, the interlaminar shear strength decreased, and, at 20 wt.%, it was lower than that of the unfilled composite. This reduction has been explained as being related to the formation of voids in the matrix, which are generally located at the interlaminar region of the composites.

From these two tests, it is evident that, in terms of the flexural stress, the optimal concentration of jarosite particles in the polymer composite is 10 wt.%.

3.2. Results of Charpy Impact Strength

Table 5. Charpy impact strength of the polymer composites with different amounts of jarosite particles (mean value ± standard deviation).

Sample	Charpy Impact Strength, kJ × m−2
UP/fiberglass mat	82.50 ± 4.88
UP/fiberglass mat/5 wt.% of jarosite	60.32 ± 11.24
UP/fiberglass mat/10 wt.% of jarosite	80.11 ± 5.47
UP/fiberglass mat/15 wt.% of jarosite	61.65 ± 4.99

and Figure 4 present the Charpy impact test data of absorbed energy of each type of polymer composite. All the data presented in Figure 4 are averages of five values; therefore, error bars are plotted.

The samples with the addition of jarosite particles show a lower impact strength compared to the samples without jarosite particles. However, the highest impact energy was the composite with 10 wt.% of particles (80.11 kJ × m⁻²), which was almost the same as a basic composite with no particles (82.50 kJ × m⁻²). The samples containing 5 wt.% and 15 wt.% of jarosite have approximately equal values of impact strength but are 30% lower than the composite with no jarosite particles. The Charpy impact test proved that, in terms of the impact stress, the optimal concentration of jarosite particles in the polymer composite is also 10 wt.%. This consistency across all the tests, including the Charpy impact and flexural tests, suggests the effective dispersion and integration of the jarosite particles at this concentration.

Abdulkader et al. [40] reported that the impact strength of the polymer composite increased with an increase in the weight fraction of marble powders. This improvement was attributed to the good distribution of the powder within the matrix, resulting in a strong interface and reinforcement. Vigneshwaran et al. [13] explained that the impact strength of the composites largely depended on the fiber reinforcement, the interface between the filler and the matrix, the presence of voids, the filler influence, and the testing conditions. In their study, it was found that, compared to the unfilled composites, the filled composites exhibited a higher impact strength. Moreover, as the percentage of fillers increased, the impact strength was observed to decrease, which is in correlation with the results of this study. Singla et al. [17] also reported that the Charpy impact strength decreased with an increase in the fly ash content beyond 30 wt.%.

3.3. The Ball Indentation Hardness Test

According to the standard, the indentation depth was recorded at 10, 30, and 60 s, because hardness is time-dependent due to the viscosity of the material. The indentation depth was used to determine the hardness value. The hardness (H) was calculated with the following formula:

$$H=0.064\times \left({\displaystyle \frac{F}{h}}\right),\mathrm{}\mathrm{N}\times {\mathrm{m}\mathrm{m}}^{-2}$$

(2)

where H represents the hardness, F is the applied load, and h is the indentation depth. The average values of calculated hardness for each type of composite are shown in

Table 6. Measured hardness for the prepared hybrid composites with different amounts of jarosite particles (mean value ± standard deviation) for dwell times of 10, 30, and 60 s.

Sample	Hardness, N × mm−2 (10 s)	Hardness, N × mm−2 (30 s)	Hardness, N × mm−2 (60 s)
UP/fiberglass mat	185.22 ± 11.33	180.98 ± 11.15	179.48 ± 11.42
UP/fiberglass mat/5 wt.% of jarosite	216.47 ± 13.76	210.04 ± 14.01	207.08 ± 11.98
UP/fiberglass mat/10 wt.% of jarosite	191.83 ± 18.67	186.80 ± 18.66	185.13 ± 18.25
UP/fiberglass mat/15 wt.% of jarosite	209.57 ± 19.34	203.34 ± 17.76	200.86 ± 17.96

and Figure 5. All the data presented in Figure 5 are the averages of five values; therefore, error bars are plotted.

Figure 5 shows that the addition of 5 wt.% and 15 wt.% of jarosite particles increases (by about 10–15%) the hardness of the composite, which is in correlation with the decrease in the mechanical properties (the flexural strength, flexural modulus, apparent interlaminar shear strength, and impact strength). It was found that the hardness values of all the investigated samples decreased slightly with increasing dwell times, as shown in Figure 5 and Table 6. This phenomenon can be explained by the fact that polymers tend to flow, causing the ball to progressively sink deeper into the material over time. Since the depth of the indentation is inversely related to the hardness, the increasing depth leads to a decrease in the calculated hardness as the dwell time increases. The composite with 10 wt.% of jarosite particles has the lowest hardness (similar to the base composite with no particles), which corresponds to the fact that the composite with exactly this concentration of jarosite shows the best mechanical properties (low hardness/high flexural strength and modulus). The higher hardness in the composites with 5 wt.% and 15 wt.% of jarosite particles likely arises from an increased brittleness, correlating with their lower flexural performance.

Karataş et al. [41] state that, generally, inorganic filler reinforcement increases the hardness of polyester composites. Furthermore, the use of higher amounts of such fillers negatively affects the surface morphology of the composites. Similar conclusions were drawn in the study by Abdulkader et al. [40].

The optimal concentration of 10 wt.% of jarosite provides a good balance between mechanical performance and sustainability. Increasing the jarosite content beyond 10 wt.% led to a degradation of the composite’s properties, particularly at 15 wt.%. Therefore, it was concluded that a further increase in the concentration of jarosite would not provide any additional benefits and could compromise the composite’s performance.

4. Conclusions

This study explored the potential use of jarosite waste as a filler in polyester-based hybrid composites reinforced with fiberglass. Therefore, hybrid composites (polyester resin reinforced with glass fibers) were prepared with different amounts of jarosite waste particles as a filler. The mechanical properties with and without the addition of jarosite were determined. The following conclusions can be drawn from the research performed:

• The composite with 10 wt.% of jarosite exhibited the highest flexural strength, which was 30% higher than that of the base composite without jarosite. In comparison, the composites with 5 wt.% and 15 wt.% of jarosite showed a lower flexural strength, with values closer to the base composite.
• Samples with 10 wt.% of jarosite had the highest apparent interlaminar shear strength, while the samples with 5 and 15 wt.% of jarosite had a lower apparent interlaminar shear strength than that of the base composite without jarosite.
• All of the samples with jarosite had a lower impact strength than that of the base composite without jarosite. However, the impact strength of the composite with 10 wt.% was the highest and remained nearly the same as the base composite.
• The hardness values were higher for all of the composites with jarosite particles in comparison to the composites without jarosite particles. The composites with 10 wt.% of jarosite particles had the lowest hardness values.

These findings demonstrate the potential of utilizing jarosite waste as a sustainable filler in hybrid composites, offering a promising approach to both waste recycling and material enhancement. This approach not only enhances the composite performance but also provides an environmentally sustainable solution for jarosite waste management, turning a problematic by-product into a valuable resource.

Further research should focus on optimizing the dispersion of jarosite particles within the matrix and investigating the long-term durability of these composites under various environmental conditions so to fully assess their potential for commercial applications. In addition, future research should explore the thermal stability, water absorption behavior, and cost-effectiveness of these composites under real-world conditions.