Prospective Use and Assessment of Recycled Plastic in Construction Industry

Abstract

The accumulation of plastic waste poses a significant environmental challenge, necessitating sustainable solutions. This study investigates the potential of recycling waste plastics for use in the construction industry, emphasizing their integration into building materials and components. Earlier waste plastic recycling was excessively studied as an ingredient in concrete composites, roads, and other use in research. However, in this study, recycled plastic is assessed for use as a sole material for structural products. Raw plastics, including high-density polyethylene, Low-Density Polyethylene, polypropylene, polyolefin, samicanite, and virgin polyethylene, were analyzed for recycling through mechanical extrusion, and their mechanical properties were analyzed to determine their feasibility for construction applications. In this study, the extrusion process, combined with engineered dyes, was investigated with comprehensive material testing as per the ASTM standards to obtain the properties desired for construction. Advanced characterization techniques, including SEM, FTIR, and TGA, were employed to evaluate the chemical composition, thermal stability, and impurities of these waste plastics collected from municipal waste. A gas emission analysis during extrusion confirmed a minimal environmental impact, validating the sustainability of the recycling process. Municipal waste plastic has a considerable quantum of HDPE, PP, and LDPE, which was considered in this research for recycling for construction products. A total of 140 samples were recycled through extrusion and tested across shear, flexural, tensile, and compression categories: 35 samples each. The results showed that rHDPE and PP had good tensile strength and shear resistance. The findings pave the way for developing cost-effective, durable, and eco-friendly building materials, such as rebars, corrugated sheet, blocks, and other products, contributing to environmental conservation and resource efficiency for the construction Industry.

1. Introduction

Plastic pollution has emerged as a pressing global issue, with rising levels of waste accumulating both on land and in oceans, posing severe threats to ecosystems and hu- man health. Recent studies have demonstrated that the improper disposal of plastics into landfill sites and marine systems significantly increases toxic substances in terrestrial and freshwater environments [1]. Without intervention, the accumulation of plastic waste is projected to rise dramatically by 2050. Recycling plastic waste has been identified as one of the most effective strategies to mitigate its detrimental effects on the environment [2]. Thermoplastics, highlighted in various studies, have potential applications in construction due to their durability and chemical properties [3]. However, the contamination of plastic waste at different stages of its lifecycle exacerbates environmental hazards, making sustainable waste management a critical area of focus. Plastic waste and its trade in Asia face challenges regarding their environmental and economic implications (this has been under review), and mitigation policies are required in a broader context [4]. Waste that ends up in landfills, oceans, and natural environments is greatly reduced by plastic recycling. Recycling conserves raw materials and, thus, decreases the need for extracting and processing virgin materials. Additionally, plastic recycling is more energy efficient than producing new plastics from raw resources, creating economic benefits and employment for the recycling industry. Nevertheless, the great majority of plastic waste is still sent to landfills, where it can take centuries to decompose. Plastics are not biodegradable: they break down into microplastics, which embed in the soil, water, and air. The focus of research has been on microplastics, which have become global pollutants, entering water, sea, and even air ecosystems. The opportunities assessed in solid waste management and plastic recycling highlight that the country’s shift towards more sustainable waste management measures and its exploration of the possibility of forming efficient recycling systems is a need that suits regional requirements [5].

Marine wildlife is under threat from plastic pollution in marine environments. Animals ingesting or becoming entangled in plastic waste can cause injury or death. According to [6], more than 800 marine species have been documented to interact with plastic debris. This has resulted in the formation of large debris patches in oceans, including the Great Pacific Garbage Patch, which is composed of millions of tons of floating waste. These marine debris patches not only harm aquatic life but also affect ecosystems and lead to biodiversity loss. A study was conducted that applies lifecycle assessment (LCA) to look at the environmental impacts of recycling PET bottles that have been split into fibers. It offers a detailed comparison of the energy consumption and emissions of the processes used for producing recycled and virgin PET [7]. The incineration and burning of plastic waste are other methods of plastic waste management. This is most efficient where there are no spaces for landfilling or where there are no facilities for recycling. However, fumes are released during this process, like dioxins and furans, which are carcinogenic, and these are released as gases when plastics are incinerated. Human respiratory health can be badly affected, and humans are very vulnerable to these fumes that develop from burning plastics. This becomes a part of the larger environmental problems of air pollution and climate change. Many regions face deadly smog, which causes visual impairment, with an increase of traffic accidents occurring due to this futile and non-healthy gaseous environment. This has become important for policymakers who are seeking to reduce the environmental footprint of plastic waste management, as sustainable alternatives to incineration are sought after. The impact of energy, environmental, and economic factors on plastic waste recycling encourages the development of ways to increase global recycling rates for all multidisciplinary aspects [8] in order to reduce plastic consumption, encourage biodegradable alternatives, and adopt circular economic principles to help decrease plastic waste. Global collaborations among researchers, policymakers, and industries are striving to come up with new ways to manage the problem of plastic pollution. These efforts are working towards a sustainable future where plastic use and recycling go hand in hand. By understanding the challenges and opportunities facing plastic recycling, society can build better waste management systems and promote environmentally responsible consumption practices [9].

Among the most used plastics in different sectors are high-density polyethylene (HDPE), polypropylene (PP), High-Impact Polystyrene (HIPS), and Low-Density Polyethylene (LDPE) because of their distinct characteristics and widespread use. Since it has a high strength-to-density ratio and chemical resistance, HDPE is used in making bottles, pipes, and other containers. PP is used in packaging, automotive parts, and household items due to its toughness, flexibility, and heat resistance. The above-mentioned properties also make HIPS a popular material in the electronics, automotive, and packaging industries because of its impact resistance and machinability. This lightweight and flexible plastic is used in films, plastic bags, and food packaging and LDPE is a very popular plastic in the market because of its light weight and flexibility. These plastics are produced in large quantities around the globe because they are durable, recyclable, and suitable for different applications [10]. The construction industry has explored various methods to repurpose plastic waste into valuable building materials. Researchers have discussed [11] the use of plastic waste in producing construction items through mechanical extrusion and secondary recycling. The products are formed after remolding the plastic into desired shapes. AI-based sorting mechanisms are also taking a boom in the market, which is very efficient and quick, which adds value to the mechanical extrusion system. Complex algorithms are developed which improvise the sorting mechanism [12].

Chemical recycling is a more energy-efficient process capable of expanding recycling capabilities to previously non-recyclable polymers. Chemical recycling involves the destruction of plastics to their molecular levels and then making other forms of polymeric material or virgin material [13]. This also suggests that the pyrolysis process, which converts plastic waste into fuel products, could be a viable solution to the plastic waste crisis [14]. This method does not make the products required for construction, it just removes impurities or converts the polymers to its basic plastic. However, further research is needed to improve the efficiency and scalability of these processes. Actionable points for technological advancement, stakeholder collaboration, and policy interventions are in discussion in different research for these issues [15].

The growing need for affordable housing in rapidly urbanizing areas has further motivated efforts to repurpose waste plastic in construction. As cities grow, more people end up in informal settlements because many cannot afford the right conventional housing materials. Costs of construction materials are high because they are in their raw and new form. The use of recycled or raw plastics is cost-efficient and reduces costs while promoting green building practices [16]. Plastics are chemically inert, which makes them durable and resistant to environmental exposure. Using waste plastics in construction could help decrease the demand on landfill sites and support a circular economy that rewards recycling. Plastic materials could be a source of affordable housing for low-income communities based on repurposed plastics for building materials. But these solutions need a planned approach based on scientific work to overcome the problems of material collection, processing, and the longevity of plastic-based construction elements. In the current market, about 19.70% of the virgin plastic is used in the construction industry for purposes like panels, ceilings, doors, and other finishing items that are not load bearing. This is because steel and concrete are still the materials of choice for structural applications because of their strength, despite ongoing research into the potential use of plastic waste as a fine aggregate substitute in concrete mixtures. The strength of the mix does not improve with this substitution, but it does provide a more sustainable method of reducing plastic waste. Plastic waste has also been considered for pavement construction, presenting a novel approach to managing waste materials in infrastructure projects.

This work will make novel contributions with the focus on developing new ways of managing waste plastics, especially on improving the efficiency of the recycling process. The local waste plastic materials that are being recycled in Pakistan are without any proper technological insight, and the inert challenge to address the environmental pollution is also not being taken into consideration. This compilation considers the main plastic pollutants of municipal waste for consideration in the construction industry and makes structural elements with required mechanical properties effectual for housing. Seven kinds of recycled plastics, HDPE, LDPE, PP, polyolefin, samicanite, and virgin polyethylene (PE), were analyzed first for identification, impurities, viability, and for their mechanical, thermal, and structural properties. To ensure that their suitability was assessed comprehensively, SEM, FTIR, and TGA were employed, which provide detailed information on the materials’ chemical composition, thermal stability, and impurity content. A total of 140 samples were tested. Out of which, 35 were for shear, 35 for flexural, 35 for tensile, and 35 for compression. The results show that tensile strength and shear resistance were higher in HDPE and, therefore, it is recommended for structural use. The failure behavior of the damaged surfaces was observed using SEM and it was found that HDPE had a ductile tearing and energy dissipative failure mode, while PP had a sharp and brittle failure. These are the best-suited materials for construction obtained from the results of this study. Previous studies predominantly investigated recycled plastic as an additive in concrete composites, roads, and various applications. In contrast, this study explores its viability as a standalone material for structural products. It is important to note that the gas emission monitoring during the extrusion process showed that there was a minimal environmental impact and, therefore, supports the proposed recycling strategy. It is shown that the construction industry’s use of recycled plastics has several environmental benefits and, therefore, presents an alternative to the disposal of plastics in landfills and promotes the circular economy. These materials could, therefore, be used to produce durable eco-friendly construction products such as blocks, panels, rebars, and other components after adding some chemicals to the recycled plastics to enhance their mechanical properties. These findings, therefore, reveal the capacity of recycled plastics to assist in the solution of the problems of the waste management of plastics and sustainable construction. The production of cost-effective and resource-efficient building materials is possible through the recycling of waste plastics and, thus, this research contributes to the global effort of environmental conservation and sustainable development.

1.1. Plastic Identification for Waste Plastic Recycling

Plastics are diverse materials that are categorized by their chemical composition and physical properties to define the kind of plastic for effective recycling and utilization. The main categories are thermoplastics and thermosetting plastics. Thermoplastics can be remelted and reshaped and, thus, are suitable for recycling. Thermosets cannot be remelted because of their crosslinked polymer structure [9]. Plastics can also be classified based on their chemical nature and availability for recycling and their use, as follows: PET, HDPE, PVC, LDPE, PP, PS, and ABS are the most common plastics encountered in municipal waste. These plastics are not only central to consumer and industrial products but also have great potential for use in civil engineering applications, such as infrastructure and construction materials. The SPI classification is used to describe the different types of recyclable plastics in

Table 1. Recyclable plastic and properties as per society of plastic industry [19,20].

SPI Symbol/Code	Structure	General Properties of the Material	Recyclable?	Density (gm/cm3)
		Impermeable ability: good. Solvent resistance. Hard. Clear. Microwave transparency. High heat resistant.	Widely recycled	1.38–1.40
	Polyethylene Terephthalate: PET
		Impermeable ability: excellent. Chemical repellent. Strong, semi-flexible. Waxy surface.	Widely recycled	0.93–0.97
	High-Density Polyethylene: HDPE
		Excellent transparency. Chemical repellent. Rigid. Good resistance to weathering. Impermeable ability: good. Stable.	It is often not recyclable; the only problem is the chemicals that might alter the polymer	1.10–1.45
	Polyvinyl Chloride: PVC
		Waxy on surface. Flexible. Good transparency. Impermeable ability: good. Low melting point.	Not recycled; fails under stress	0.91–0.94
	Low-Density Polyethylene: LDPE
		Chemical repellent. Hard. Flexible. High melting point. Translucent. Waxy on surface. Strong.	Rarely recycled	0.90–0.92
	Polypropylene: PP
		Clear. Glassy. Rigid. Brittle. Hard. Good clarity. Fats and solvents can affect.	Rarely recycled	1.04–1.11 For Expanded Polystyrene 0.016–0.64
	Polystyrene: PS
	Others	Includes numbers 7–19, usually polyamides, ABS, PC mixed polymers, etc.	Not recycled due to diverse risk of contamination	Varies

. The first category, polyethylene terephthalate (PET), is easily recycled because of its impermeability and solvent resistance and hence is used in the packaging of food and beverages. The density of PET lies within the range of 1.38–1.40 g/cm³, and it has good transparency and heat resistance [17]. Another kind of widely recycled plastic is high-density polyethylene (HDPE), which has a waxy surface, semi-flexibility, and good chemical resistance. It is used for containers, pipes, and other household goods, with a density of 0.93 to 0.97 g/cm³. Polyvinyl Chloride (PVC) is famous for its transparency, chemical resistance, and stability but is often criticized for its difficulty to recycle. Its density is 1.10–1.45 g/cm³, but there are concerns about the chemical transformation during the recycling process [18]. The above-mentioned Low-Density Polyethylene (LDPE) has very few recycling options because it is a flexible and transparent material with a low melting point and a density of 0.91 to 0.94 g/cm³.

PP is a strong and chemically resistant plastic with a high melting point and is used in the automotive and industrial sector. However, the rate of recycling of PP is very low because of the intricate ways in which it breaks down. PS is available in its rigid and expanded form and has its characteristics of being brittle, having a glassy appearance, and good clarity. However, the density of the rigid forms of PS is between 1.04 and 1.11 g/cm³, while that of the expanded forms is 0.016–0.64 g/cm³. The other category is a group of mixed polymers, including polyamides, ABS, and PC, which are not usually recycled on account of contamination risks and their diversity of composition. These materials have restricted recyclability and are thus a barrier to the sustainability agenda in the plastics industry [1]. Overall, improving recycling methods and addressing contamination risks are crucial steps toward increasing the sustainability of plastic use. The rising global concern regarding environmental sustainability has emphasized the need to manage plastic waste effectively by integrating recycling practices in various industries, particularly civil engineering. Plastics have become indispensable in modern construction due to their combination of lightness, durability, and resilience against environmental factors. One commonly used recyclable plastic is polyethylene terephthalate (PET), recognized for its strength, lightness, and moisture-resistant properties. It effectively prevents gas and solvent permeation, which makes it a preferred choice for food packaging. PET finds extensive use in bottles, food containers, geotextiles, and as fibers for reinforcing concrete. Civil engineering applications frequently employ PET to enhance concrete structures, improving their strength and resistance to environmental stresses. Recent studies, such as those conducted by [20], have demonstrated that PET fibers reduce water absorption in concrete, thus increasing its thermal insulation and making it suitable for road pavements and building insulation solutions. Additionally, PET is widely utilized in geotextiles to bolster soil stability and minimize erosion, contributing approximately 8% to the total municipal waste generated. Another important material is high-density polyethylene (HDPE), which is known for its durability and chemical resistance. It has a waxy texture and is very tight; therefore, it is used in construction in drainage pipes, detergent bottles, and plastic lumber. In civil engineering, the most popular applications of HDPE are in drainage systems, geomembranes, and landfill liners to prevent the leakage of hazardous waste. It is a very effective environmental safety measure, especially in landfill use as a strong barrier to prevent contaminant leakage. In addition, HDPE pipes are applied in water and gas distribution networks because of their high strength and resistance to corrosion. HDPE and all its variants make up about 30% of municipal waste. Another well-known plastic that is sensitive to flexibility and resistance against chemicals is Polyvinyl Chloride (PVC). It is rigid or flexible, depending on its form, and is used in pipes, flooring, and waterproof membranes. This makes it suitable for electrical cable insulation and other building applications because of its weatherability.

However, the processing of PVC produces harmful chemicals as a byproduct. Therefore, researchers have also ventured into the investigation of sustainable methods for PVC recycling to reduce the adverse impact of PVC on the environment. PVC forms about 10% of the municipal waste and thus forms a large portion of the waste streams. Low-Density Polyethylene (LDPE) is a form of plastic that is flexible and moisture resistant and has a lower melting point and is used in plastic bags, wraps, and landfill liners. It is, therefore, suitable to use as protective films and vapor barriers in construction due to its durability and flexibility. LDPE can be successfully incorporated into asphalt modifications to improve the flexibility and lifespan of road surfaces, as reported by [2]. This plastic variant contributes about 30% of municipal waste, including other forms of polyethylene. polypropylene (PP) is another of the most widely used plastics in food packaging, concrete reinforcement, and geotextiles. It has good chemical, heat, and fatigue resistance and is, therefore, used in various industries. PP fibers in civil engineering enhance the crack resistance and the durability of concrete structures. PP has been found to enhance the service life and the performance of infrastructure projects, as [21] has shown, and PP is incorporated into concrete mixtures. About 19% of municipal waste is made up of PP, and it is still a valuable material in construction because of its versatility. Polystyrene (PS), especially expanded (EPS), is widely used for insulation and lightweight aggregates in concrete. This is a rigid, brittle plastic with good clarity and excellent thermal insulating properties. PS is widely used in disposable cutlery, insulation panels, and as a lightweight filler in concrete. Recycled EPS is used in concrete mixtures to enhance the thermal performance and decrease the overall structural weight of buildings to make them more energy efficient. PS constitutes about 6% of municipal waste. Lastly, ABS is a kind of Acrylonitrile Butadiene Styrene, a highly durable and impact resistant plastic used in making automotive parts, toys, and modular construction panels. Its use in modular construction has been increasing because it is strong and easy to fabricate.

Figure 1 illustrates the distribution and composition of waste materials and plastics. (a) shows that plastic waste (40%) is the dominant category in municipal waste, followed by green waste (27%), textiles (15%), and smaller fractions of Tetra Pack, organic, leather, and other materials. (b) presents the overall composition of plastic waste in municipal waste, being 40% overall, with Polyethylene (PE) (30.9%) and polypropylene (PP) (19.6%) being the most abundant, followed by PVC, PET, PS, and PUR, while other materials make up 19.6%. Industrial waste is also of significance and is highlighted in (c). The plastic types from these make PE and its variants, HDPE and LDPE, good contenders for recycling; PP is another contender for recycling, as PE (35%) and PP (22%) dominate in the waste, along with contributions from PVC, PET, PS, PUR, and other minor components. The data emphasize the significant presence of PE, along with its variants, and PP in plastic waste, making them key candidates for recycling and sustainable material applications.

1.2. Waste Plastic Recycling Methods and Products

Waste plastic recycling methods and the products derived from these processes have become an essential focus in addressing global plastic pollution. As plastic waste continues to accumulate at an alarming rate, recycling offers a promising solution to reduce the environmental impact while generating valuable products for various industries. Recycling methods, including mechanical, chemical, and thermal recycling, have been extensively studied and improved to enhance the efficiency and quality of recycled plastics [22]. Each method offers unique advantages and produces specific products that contribute to the circular economy and sustainable development goals. Polymer recycling methods, including mechanical and chemical recycling, in the context of environmental sustainability are discussed in one paper. It also discusses the limitations and opportunities of current technologies in achieving a circular economy for plastics [13]. Mechanical recycling is the most used method. It starts with the usual collection of materials from waste; sorting it into its categories; cleaning, either through washing or screening; shredding, as per the extrusion machine sizes; and finally remolding the plastic waste into desired new products. The melting points, extrusion flow speed, and other changes are made as per the plastic type. It is highly employed for plastic packaging, bottles, and other containers, etc. The quality of the recycled material is found to decrease with multiple extrusions and over time due to the polymer chain scission and contamination. To overcome this, stabilizers and additives are used to improve the properties, improving their robustness as recycled plastic [23]. Chemical recycling is another method by which plastics are depolymerized or broken down into their monomeric components or other valuable chemicals through processes such as depolymerization, solvolysis, and pyrolysis. While mechanical recycling is capable of handling pure plastic types, chemical recycling can handle mixed and contaminated plastic waste to produce high quality raw materials that can be used to make new plastics. They are monomers, fuels, waxes, and solvents. Advancements in catalytic depolymerization to make chemical recycling more efficient and economically viable to recover valuable compounds from complex plastic waste streams have been reported in research [24]. For enhancing the reuse and recycling of waste management strategies, integrated approaches to effective resource utilization and sustainability are necessary [25]. Thermal recycling processes, such as pyrolysis and gasification, are methods used to break down plastic waste into energy-rich products such as fuels, syngas, and char. Pyrolysis is a process which involves the heating of plastic waste in the absence of oxygen to produce pyrolysis oil, which can be further processed into various fuels or used as raw material to produce new plastics. Pyrolysis can be used for all kinds of plastics, including those that are not able to be recycled mechanically [26]. Gasification, a partial oxidation of plastic waste at high temperatures, produces syngas that can be used to make electricity or other chemicals and fuels. Thermal recycling is especially useful for dealing with non-recyclable and contaminated plastic waste that would have otherwise been sent to the landfill.

New technologies, including advanced solvent-based recycling and the enzymatic recycling of plastics are also being developed to enhance the efficiency and sustainability of the plastic recycling process. The solvent-based recycling is a process of dissolving plastics in a solvent to separate the polymers from contaminants and additives. This produces high purity polymers, which can be reused to make new plastics. Enzymatic recycling, a more recent concept, uses enzymes to cut back specific plastic polymers into their monomers. However, the enzymatic recycling of PET (polyethylene terephthalate) and other polyesters has been found to be very efficient and produced high-quality monomers that can be repolymerized into plastics of virgin-like quality [27]. Another new method that is also coming up is electrochemical recycling, which uses electrolysis to tear down plastic polymers to their basic forms. This method is a clean and energy efficient way of recycling plastics with minimal or no hazardous products. In a study [22], the authors pointed out that electrochemical recycling is very efficient at converting plastic waste into useful chemicals and monomers, thereby reducing the impact on the environment and promoting sustainability. The products obtained from recycled plastics depend greatly on the type of recycling method employed. Lower value products, such as plastic lumber, pallets, and park benches, as well as packaging materials, are usually produced from the mechanical recycling process. Since chemical recycling can produce high-purity raw materials, it is possible to make high-value products, such as automotive components, electronic housing, and medical devices. Other thermal recycling methods produce fuels and energy products that can be used in the place of fossil fuels, cutting carbon emissions and helping with energy sustainability [3]. Alongside conventional recycling techniques, innovative technologies, such as 3D printing with recycled plastics, offer new ways of creating customized and intricate products. Recycled plastics can be used in additive manufacturing to make high-performance components for aerospace, healthcare, and consumer products and their potential. Researchers seek to enhance the mechanical properties and efficiency of recycled plastics for 3D printing by fine-tuning material formulations and printing techniques. Optimization models for waste supply chains concentrate on strategic network designs to enhance recycling and waste management systems [28]. Composite materials from recycled plastics have better mechanical and thermal properties than conventional materials [29]. For instance, researchers have investigated the processing of composites from recycled PET and HDPE reinforced with natural fibers, glass fibers, or carbon fibers. Automotive parts, building materials, and consumer products are made from composite recycled materials, offering a green alternative to the virgin materials [20]. Although current recycling technologies and the diverse array of marketable products produced from recycled plastics are strong, there are still issues with scaling up these technologies to the necessary level to make recycling operations economically feasible. Contamination, poor quality, and limited infrastructure pose a significant problem to the current lack of the widespread adoption of recycling practices. To overcome these challenges, researchers are looking into the development of new sorting technologies, automation, and efficient purification processes that can enhance the quality of the recycled products.

1.3. Waste Plastic in the Construction Industry

The integration of plastic waste into the construction industry is a breakthrough. Sustainable building material tends to invoke environmental recovery. Recycled PET, HDPE, and LDPE are being assessed for use in the construction sector in applications like bricks, pavers, insulation materials, and lightweight concrete. The use of waste plastic aggregates as partial replacements for natural aggregates in concrete has been found to decrease the environmental hazards of conventional materials and enhance some features, such as the thermal insulation and durability of the concrete [30]. These usages present big challenges regarding the remolding and handling of recycled plastic in bulk quantum. Recent research has shown that incorporating plastic aggregates can reduce cement’s workability, compressive strength, and durability due to the weak bonding between plastic and cement. However, findings on water absorption, shrinkage, and abrasion resistance remain inconsistent, with some studies indicating improvements when PET is used. Recently a comprehensive study has compared the performance of PET, HDPE, and PP under the same for curb construction. One study study fills that gap by evaluating the mechanical and durability properties of plastic aggregate concrete and assessing its feasibility for curb applications [31]. Another study focuses on three types of recycled aggregates—recycled clay brick sand (RCBS), recycled glass sand (RGS), and recycled fine concrete aggregates (RFA)—loaded with nanoscale titanium dioxide (NT) to enhance the photocatalytic efficiency. The study only focuses on the recycling of aggregates but does not correspond to waste plastic [32]. The employability of waste in construction may lead to the production of more sustainable materials. Plastic lumber produced from recycled plastics is now used for decking, fencing, and even park benches, offering a durable and weather resistant alternative to traditional wood [20]. Innovations in 3D printing technology have also made it possible to create customizable building components using plastic filaments that are derived from waste materials to enhance both their design flexibility and resource efficiency. Even so, the problem of plastic waste dumping has not been eliminated by using recycled plastic materials in construction. Recent studies have also been aimed at improving the mechanical properties and the durability of enhanced plastic concrete, with some research suggesting that mixing different types of plastic waste can improve the strength and sustainability of the material [33]. This serves as a promising pathway to reduce plastic waste and promote a circular economy in the building sector by transforming plastic waste into construction materials [1,34].

Table 2 presents an overview of various studies that used different types of plastic aggregates as replacements in construction materials through their diverse applications and replacement ratios. The LDPE has been used as a fine aggregate replacement in concrete mixtures, with replacement ratios that enhance the workability and reduce the density of concrete. HDPE has been incorporated to show its effectiveness in enhancing the durability of the concrete, reducing water absorption, and improving crack resistance. Polyethylene terephthalate (PET) has been used as a fine aggregate replacement and coarse aggregate replacement. These replacement levels, in research, have been up to 100% to enhance sustainability. The innovative ideas decrease the amount of waste material sent to landfill. These studies collectively underscore the growing importance of integrating recycled plastics into construction materials to achieve sustainability goals in the building industry. Much research has also been done (annotated in the table) that uses plastic in road construction and soil stabilization. Different aspects have been compiled, such as mechanical benefits, PET incorporation, plastic integration, modification in asphalt, and generic uses of recycled mixed plastic. The gap identified is that it does not use plastic as the main material for products. This study will pave the way to developing recycled plastic waste as the main material for the construction industry.

Table 2. Previous studies that used waste plastic in construction.

Waste Plastic Type	Purpose Used for	Matrix	Purpose	References
HDPE	FA, CA	Concrete	Additive	[35,36,37,38]
LDPE	FA, CA	Concrete	Additive	[36,39,40,41]
PET	Fiber Strips	Concrete	Reinforcement	[34,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]
NM	Filler Material	Concrete	Flexibility	[57,58,59,60,61,62]
Melamine Formaldehyde	Fiber Strips	Concrete	Reinforcement	[63]
PVC	Fiber Strips	Concrete	Flexibility	[64,65,66]
PET&PC	Fiber Strips	Concrete	Reinforcement	[49]
GFRP	Fiber Strips	Concrete	Reinforcement	[67]
LDPE & PET	Filler Material	Concrete	Filler	[68]
ABS&PC	Filler Material	Concrete	Structural Support	[69]
PP	Fiber Strips	Concrete	Structural Support	[70,71]
Mixed Plastics (various)	FA, CA	Asphalt	Durability	[72,73,74,75]
PE	Fiber Strips	Asphalt	Reinforcement	[76,77]
PET	Filler Material	Asphalt	Flexibility	[78,79]
PP	FA, CA	Asphalt	Durability	[80,81]
HDPE	Fiber Strips	Asphalt	Durability	[82,83]
PVC	Filler Material	Asphalt	Reinforcement	[84,85]
PS	Filler Material	Asphalt	Flexibility	[86,87]
LDPE	FA, CA	Asphaltic Concrete	Tensile	[74,88]
PP	Fiber Strips	Soil	Soil Reinforcer	[89]
HDPE	Fiber Strips	Soil	Soil Reinforcer	[90]
PET	Filler Material	Soil	Soil Improvement	[91]

Table 2. Previous studies that used waste plastic in construction.

Waste Plastic Type	Purpose Used for	Matrix	Purpose	References
HDPE	FA, CA	Concrete	Additive	[35,36,37,38]
LDPE	FA, CA	Concrete	Additive	[36,39,40,41]
PET	Fiber Strips	Concrete	Reinforcement	[34,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]
NM	Filler Material	Concrete	Flexibility	[57,58,59,60,61,62]
Melamine Formaldehyde	Fiber Strips	Concrete	Reinforcement	[63]
PVC	Fiber Strips	Concrete	Flexibility	[64,65,66]
PET&PC	Fiber Strips	Concrete	Reinforcement	[49]
GFRP	Fiber Strips	Concrete	Reinforcement	[67]
LDPE & PET	Filler Material	Concrete	Filler	[68]
ABS&PC	Filler Material	Concrete	Structural Support	[69]
PP	Fiber Strips	Concrete	Structural Support	[70,71]
Mixed Plastics (various)	FA, CA	Asphalt	Durability	[72,73,74,75]
PE	Fiber Strips	Asphalt	Reinforcement	[76,77]
PET	Filler Material	Asphalt	Flexibility	[78,79]
PP	FA, CA	Asphalt	Durability	[80,81]
HDPE	Fiber Strips	Asphalt	Durability	[82,83]
PVC	Filler Material	Asphalt	Reinforcement	[84,85]
PS	Filler Material	Asphalt	Flexibility	[86,87]
LDPE	FA, CA	Asphaltic Concrete	Tensile	[74,88]
PP	Fiber Strips	Soil	Soil Reinforcer	[89]
HDPE	Fiber Strips	Soil	Soil Reinforcer	[90]
PET	Filler Material	Soil	Soil Improvement	[91]

Table 3. Previous studies’ plastic blends of various plastic.

Waste Plastic Type	Mixed Proportion	Property Studied	References
HDPE	100% HDPE	Tensile strength, elongation at break.	[92,93]
PP	100% PP	Impact resistance, tensile modulus.	[93,94]
HDPE + LDPE	Diff. proportion HDPE, LDPE	Ductility, impact strength.	[93,95]
HDPE + PP	50% HDPE, 50% PP	Tensile strength, elongation, thermal stability.	[96]
HDPE + SAM (samicanite)	Diff. proportion HDPE, SAM	Wear resistance.	[97]
HDPE + POL (polyolefin)	60% HDPE, 40% POL	Elastic modulus, heat deflection temperature.	[98]
HDPE + V (virgin PE)	Diff. proportion HDPE, virgin PE	Stress cracking resistance, environmental resistance.	[99]
LDPE	100% LDPE	Tensile strength, environmental stress cracking.	[97]

Table 3. Previous studies’ plastic blends of various plastic.

Waste Plastic Type	Mixed Proportion	Property Studied	References
HDPE	100% HDPE	Tensile strength, elongation at break.	[92,93]
PP	100% PP	Impact resistance, tensile modulus.	[93,94]
HDPE + LDPE	Diff. proportion HDPE, LDPE	Ductility, impact strength.	[93,95]
HDPE + PP	50% HDPE, 50% PP	Tensile strength, elongation, thermal stability.	[96]
HDPE + SAM (samicanite)	Diff. proportion HDPE, SAM	Wear resistance.	[97]
HDPE + POL (polyolefin)	60% HDPE, 40% POL	Elastic modulus, heat deflection temperature.	[98]
HDPE + V (virgin PE)	Diff. proportion HDPE, virgin PE	Stress cracking resistance, environmental resistance.	[99]
LDPE	100% LDPE	Tensile strength, environmental stress cracking.	[97]

Recycled high-density polyethylene (rHDPE) and polypropylene (rPP) blends have been extensively studied to assess their mechanical properties and their potential applications in various industries. The mechanical properties of rHDPE and rPP are influenced by processing conditions, blending ratios, and contamination from prior use. Table 3 is a depiction of different ratios of rHDPE with blends. Studies indicate that as the percentage of recycled polymer increases, the mechanical properties, such as tensile strength and Young’s modulus, tend to degrade due to molecular chain scission and lower crystallinity [93]. Research [96] on PP:HDPE blends has shown that tensile properties degrade after multiple recycling cycles. The Young’s modulus and yield strength of rPP:rHDPE blends were found to be lower than those of virgin polymer blends due to structural degradation during the recycling process. However, at rPP contents exceeding 75%, the yield strength of the recycled blends approached that of virgin materials. This decline in mechanical properties is primarily attributed to a reduction in crystallinity, molecular weight degradation, and the formation of imperfect crystalline structures [96].

Moreover, blending techniques play a crucial role in mitigating mechanical property losses. The effects of compatibilizers such as maleic anhydride polypropylene (MAPP) on rPP:HDPE blends were investigated [92], and it was found that their addition significantly improved interfacial adhesion and tensile strength. This improvement was due to better stress transfer at the interface between PP and HDPE, thereby reducing the negative effects of immiscibility between the two polymers. The study [92] also focused on the long-term mechanical performance of rHDPE/vHDPE blends, revealing that blending virgin HDPE with recycled HDPE up to 70% recycled content maintained comparable tensile properties to virgin HDPE. However, increasing the recycled content beyond 70% led to significant reductions in mechanical performance, particularly in fatigue resistance. The study also found that different blending methods, such as powder mixing and extrusion, had a negligible effect on tensile performance. Morphological analyses using scanning electron microscopy (SEM) and atomic force microscopy (AFM) have demonstrated that higher proportions of rHDPE lead to phase separation at the nanoscale level, which negatively impacts mechanical strength [97]. However, strategic processing techniques, such as controlled cooling rates and optimized mixing, can enhance mechanical performance by improving the homogeneity of the blend. A study on electron beam crosslinking in HDPE/PU blends indicated that while crosslinking can improve thermal stability and mechanical properties, it significantly reduces elongation at break. However, optimized crosslinking combined with compatibilizers resulted in a balance of tensile strength and ductility, making such blends suitable for high-performance applications [98]. All these previous studies only focus on plastic generically but not for use in construction and assess only the properties of plastics. Detailed mechanical properties also need to be assessed for these types of plastic.

1.4. Environmental Aspects and Hazards of Waste Plastics

While waste plastic recycling offers several environmental benefits by reducing plastic pollution and conserving resources, there are also several environmental aspects and hazards associated with it that need to be addressed. The environmental implications of plastic waste recycling processes (mechanical, chemical, and thermal recycling) vary from reducing landfill waste to producing potential air and water pollutants. These aspects must be addressed to develop sustainable recycling systems that minimize harm to human health and the environment [22]. One of the major environmental issues of plastic waste recycling is the generation of toxic substances because of the recycling process. Plastics are melted and remolded through mechanical recycling and can emit volatile organic compounds (VOCs) and other hazardous air pollutants [2]. These emissions can also lead to air pollution and are bad for human health. Plastic additives, like flame retardants, stabilizers, and plasticizers, are also problematic as they can leach into the environment during recycling. The risks can also be reduced by ensuring proper ventilation and the use of filtration systems in recycling plants [24]. Chemical recycling processes that are based on breaking down plastics to their basic chemical components also have environmental issues. Although chemical recycling has the advantage of producing high-quality recycled materials, it is an energy intensive process and can also produce hazardous byproducts. For instance, the depolymerization of plastics produces harmful chemicals that must be properly handled to avoid causing pollution [22]. Ways to improve catalysts and reactor designs to decrease the environmental footprint of chemical recycling processes are also being developed. Thermal recycling processes, including pyrolysis and gasification, are means of transforming plastic waste into fuels and other forms of energy. However, these processes are known to emit greenhouse gases (GHGs) and other pollutants, which contribute to climate change and poor air quality [25]. Emissions, such as carbon monoxide, nitrogen oxides, and particulate matter, are produced during the pyrolysis process and need advanced emission management systems to control them. The environmental impact of thermal recycling also depends on the efficiency of the process and the type of plastic waste that is being treated. Another crucial environmental aspect of waste plastic recycling is the potential contamination of water sources. Microplastics are persistent pollutants that pose risks to aquatic ecosystems and human health, and the potential contamination of water sources is another significant environmental aspect of waste plastic recycling. To minimize water pollution, recycling facilities must implement effective filtration and wastewater treatment systems to prevent the discharge of microplastics into the environment to prevent the release of microplastics into water bodies during the collection, sorting, and washing stages of plastic waste. Another environmental challenge is the disposal of non-recyclable plastic residues. Not all types of plastic waste are efficiently recyclable, and the residues left after recycling processes are often sent to landfills or incinerated. The landfilling of plastic residues can leach toxic substances into the soil and groundwater, while incineration can emit harmful pollutants in the air. Strategies for sustainable waste management must also address the development of technologies to manage non-recyclable residues and decrease their environmental impact [22].

Figure 2 is the scheme of this study intended to explore recycling plastics for structural construction materials, moving beyond their traditional use in concrete and roads. The approach is to assess and recycle municipal solid waste plastics for construction applications with complete recycled plastic products. It begins with the assessment of municipal waste collection, sorting, cleaning, and palettization as the initial recycling steps, as well as the role of plastics in construction. The waste plastic material assessment in this study stage involves a spot analysis using SEM and a material behavior evaluation using TGA and FTIR. The product manufacturing process includes the modification of mechanical extrusion setup and the preparation of mold, while also considering gas emissions and environmental hazards. The processed materials are further modified for future construction applications.

This structured approach ensures an environmentally sustainable and technically feasible method for recycling plastics into durable construction materials. Although environmental challenges are present, the advantages of plastic recycling outshine its disadvantages when done correctly. Recycling keeps plastic waste from entering the environment, conserves raw materials, and reduces the need to produce virgin plastic, which is an environmentally impactful process. The following are ways to minimize the environmental hazards of recycling, at the point of the recycling facilities: improving sorting technologies, cleaner production techniques, and circular economy principles should be adopted.

1.5. Research Significance

This study introduces innovative approaches to managing waste plastics, emphasizing improved recycling efficiency. In Pakistan, plastic recycling lacks technological insight and fails to address environmental pollution concerns. This research examines the potential of municipal plastic waste in the construction industry by developing structural elements with essential mechanical properties for housing. Previous studies on waste plastic alone have not completely analyzed all the parameters for use in the construction industry. Seven types of recycled plastics—HDPE, LDPE, PP, polyolefin, samicanite, and virgin polyethylene (PE)—were analyzed for identification, impurities, and mechanical, thermal, and structural properties. Comprehensive assessments using SEM, FTIR, and TGA provided insights into chemical composition, thermal stability, and impurity levels. A total of 140 samples were tested: 35 each for shear, flexural, tensile, and compression. Results indicate that HDPE exhibited superior tensile strength and shear resistance, making it suitable for structural applications. Blending HDPE with LDPE and PP enhanced ductility and energy absorption, while combinations with polyolefin and samicanite improved thermal stability. The SEM analysis of failure surfaces revealed ductile tearing in HDPE and brittle failure in PP.

Unlike previous studies that primarily explored recycled plastics as additives in concrete composites, soil stabilization, and roads, this research evaluates their viability as standalone structural materials for building products. Mechanical extrusion, being the most efficient process to reduce environmental hazards, was employed. The findings highlight the potential of recycled plastics in construction, offering an eco-friendly alternative to landfill disposal while promoting a circular economy. By varying and assessing different blends, recycled plastics can be used to manufacture durable construction products, such as blocks, panels, and rebars. This research contributes to sustainable development by demonstrating the feasibility of cost-effective, resource-efficient building materials derived from plastic waste.

2. Experimental Procedure

2.1. Recycling Through an Environmentally Friendly Approach

2.1.1. Collection of Raw Material and Material Identification

New developments in extrusion-based plastic recycling have greatly expanded the ability to transform waste plastics into useful materials in a more sustainable and efficient manner. New extrusion technology has resulted in the creation of machines that can handle a wider variety of post-industrial and post-consumer plastic waste than previous machines. They have also resulted in higher quality recycled products and less environmental pollution. Moreover, the integration of catalytic technologies into the extrusion process has shown promise in selectively upcycling plastic waste into valuable products. The waste plastics were collected from municipal solid waste, and palletization was done in the first round after cleaning. The source materials were collected and sorted out from the waste as per their resin type; the same were cleaned with a simple water wash. The washed material was heated and dried, and pallets were made for the second round of extrusion to make the desired mold as per the ASTM standards: D790 (flexural), D695 (compression), D732 (shear), and D638 (tensile). A further six modifications were drawn from waste materials, and the samples were studied for the same mechanical properties to investigate the effects of mixing. Out of the other plastics, HDPE was found to be better for the extrusion process than the other plastics, which is a major component in municipal waste. Other materials, which were locally available, were used to make variant mixes that were further assessed, and which were likely to vary the properties of the plastic; these included LDPE with HDPE, polyolefin with HDPE, samicanite pallets with HDPE, and HDPE with virgin material. These plastics were sought to economize the product and alter the properties for an efficient extrusion process. Figure 3 shows the different raw materials in the pallets that were formed after the first round of extrusion. FTIR, TGA, and SEM were used to assess the basic material properties, which are also shown in Figure 3.

• SEM and the detection of impurities

The SEM analysis of raw waste plastic materials (Figure 4) was done, and waste HDPE plastic at a magnification of 250 µm revealed compositional variations in different regions. The spectrums were further analyzed for the composition of the material. Spectrum 2 in Figure 5 indicates the presence of impurities, with a composition of 93.3 wt.% carbon (C), 5.5 wt.% oxygen (O), 1.2 wt.% calcium (Ca), and 0.2 wt.% sodium (Na). In contrast, Spectrum 4 in Figure 5 showed a slightly different impurity profile, with 95 wt.% carbon (C), a significantly higher oxygen content at 33.6 wt.%, 0.9 wt.% calcium (Ca), and 0.4 wt.% chlorine (Cl). These differences suggest localized variations in the chemical composition, potentially due to surface contamination, additives, or environmental exposure. The SEM in Figure 4 shows the details of waste LDPE plastic at a magnification of 100 µm, which revealed notable differences in chemical composition across regions. Spectrum 7 exhibited a diverse impurity profile, with 80.6 wt.% carbon (C), 13.6 wt.% oxygen (O), 4.4 wt.% calcium (Ca), 0.6 wt.% silicon (Si), 0.4 wt.% sodium (Na), 0.3 wt.% aluminum (Al), and 0.2 wt.% magnesium (Mg). However, in contrast, Spectrum 10 exhibited a pure composition of 100 wt.% carbon (C), with no detectable impurities or additives present. These results point to the surface chemistry of the LDPE sample being heterogeneous, due to contamination, additives, or processing conditions. At a magnification of 500 µm, the SEM image in Figure 4 of samicanite showed a consistent chemical composition across different regions. Both Spectrum 2 and Spectrum 3 in Figure 5 were composed entirely of 100 wt.% carbon (C), with no detectable impurities or additives present. This uniformity of the carbon structure is expected from a highly pure material with consistent processing, which is the case with samicanite.

The SEM image of virgin polyethylene (PE) at 500 µm magnification showed the material to be highly pure in its composition over the examined areas. Both Spectrum 2 and Spectrum 3 in Figure 5 presented a uniform chemical profile of 100 wt.% carbon (C) with no detectable impurities or additional elements. This result points to the pristine nature of the virgin PE sample, indicating its high purity and proper material properties for applications that demand contamination-free material. At a magnification of 250 µm, the SEM profile of polyolefin showed slight variations in chemical composition across the examined regions. In Spectrum 2 of Figure 5, the composition of 93.3 wt.% carbon (C), 4.3 wt.% oxygen (O), and 2.4 wt.% titanium (Ti) indicates the presence of minor impurities or additives. Spectrum 3 has a composition of 96.2 wt.% carbon (C), 2.9 wt.% oxygen (O), and 0.9 wt.% titanium (Ti), and this shows a higher content of carbon and a lower content of titanium than Spectrum 2. These differences point to a variation in the sample, which might be due to the distribution of additives or surface treatment effects.

b.

FTIR of Raw Material

Fourier Transform Infrared Spectroscopy (FTIR) is a method for analyzing the infrared spectrum of a sample to determine its chemical constituency. It measures the infrared spectrum of a sample and produces a spectral output that acts as a molecular “fingerprint”. This technique is especially useful for organic and inorganic materials, including polymers, coatings, and composites. It is also used to identify the functional groups, such as hydroxyl, carbonyl, and amine, as well as their interaction in a material.

This method is both qualitative and quantitative and can be used to determine the composition of the sample, as well as the amount present [100]. FTIR was performed on all the plastic raw material samples in Figure 6. The waste HPDE has intense C–H stretching bands, together with well-defined peaks in the carbonyl (C=O) region at about 1700 cm⁻¹, indicating that stabilizers or fillers have been used. The fingerprint region has several absorptions, which may be due to the material’s composition or its processing. This sample is most suitable for use in structural or packaging applications where the material needs to withstand the weather. The FTIR spectrum of the LDPE is characterized by the absorptions due to the hydrocarbon chain in the 2800–3000 cm⁻¹ range. Other peaks in the 1000–1200 cm⁻¹ region indicate the presence of oxygen-containing functions, which might be ethers or esters and are used to enhance the impact strength or service temperature of the material. This sample has easily reproducible intensity patterns that indicate that the material is homogeneous. The FTIR spectrum of samicanite shows aliphatic absorptions and a secondary peak at about 1600 cm⁻¹, which may indicate the presence of aromatic structure or aromatics. The fingerprint region has strong and narrow peaks, which is characteristic of a good blend of polymer. This material has been developed to enhance its strength and heat resistance. Strong absorption in the carbonyl region at about 1700 cm⁻¹ and C–H stretching peaks and a broad absorption indicate the presence of carbonyl-containing groups in the virgin PE. The fingerprint region of this polymer shows that it is a well-ordered matrix with sharp peaks. This material is probably being used in applications where chemical and environmental resistance is of considerable importance. The spectrum for polyolefin highlights strong peaks in the aliphatic C-H stretching region. Moderate absorption in the 1100–1200 cm⁻¹ range hints at oxygen-containing additives, such as fillers or processing agents. The material exhibits consistent intensity patterns, making it suitable for lightweight and flexible applications. The waste PP spectrum reveals strong hydrocarbon absorptions in the 2800–3000 cm⁻¹ region and prominent peaks in the carbonyl region, suggesting the presence of esters or ketones. The fingerprint region is rich in detail, reflecting a highly tailored polymer composition. This material appears to be designed for specialized applications where chemical resistance and structural integrity are paramount.

c.

TGA and DSC of Raw Waste Plastic and Materials

In Figure 7, the TGA for one variant of waste HDPE provides an in-depth understanding of the material’s thermal stability and decomposition behavior. For waste HDPE, the maximum weight observed was 3.09 mg at 240.50 °C, and the minimum weight was 3.03 mg at 124.09 °C, resulting in a weight loss of 0.07 mg. The Differential Scanning Calorimetry (DSC) analysis in Figure 7 shows that the maximum heat flow was -13.13 mW at 126.64 °C, and the minimum heat flow was −35.36 mW at 242.04 °C. This information is critical for assessing the material’s behavior under thermal stress and ensuring its suitability for applications requiring high thermal resilience. The small weight loss and the sharp heat flow peak confirm that the high thermal stability of the HDPE is very good for high-temperature applications. The differentials show that there was almost no loss of mass, even at higher temperatures, which is consistent with the expected minimal degradation of the crystalline structure of the highlighted HDPE. This transition represents some phase change, like melting or crystallization, and it points to the material’s thermodynamic behavior when it is being heated. Such information is valuable to understand the energy requirements and the stability of the material under specific conditions [2]. For waste LDPE, the maximum weight was 7.84 mg at 241.14 °C, and the minimum weight was 7.73 mg at 141.84 °C, with a weight loss of 0.11 mg. The maximum heat flow recorded was −9.90 mW at 142.62 °C, and the minimum was −34.45 mW at 242.16 °C. The wider heat flow peak and the greater weight loss are due to the amorphousness and the poor thermal stability of waste LDPE in comparison to HDPE, which is in agreement with its easy degradation at lower temperatures.

The broader heat flow peak and higher weight loss reflect LDPE’s lower crystallinity and thermal stability compared to HDPE, corroborating its susceptibility to degradation at lower temperatures [22]. For samicanite, the maximum weight was 5.84 mg at 242.06 °C, and the minimum weight was 5.74 mg at 139.07 °C, with a weight loss of 0.10 mg. The maximum heat flow was −10.45 mW at 140.40 °C, and the minimum was −32.78 mW at 242.23 °C. The multi-step weight loss and distinct thermal transitions align with the composite nature of samicanite, suggesting degradation pathways involving volatiles and matrix breakdown, as discussed in research [2]. For virgin polyethylene, the maximum weight was 5.23 mg at 117.35 °C, and the minimum was 5.17 mg at 108.34 °C, with a weight loss of 0.054 mg. The maximum heat flow was −22.855 mW at 135.683 °C, and the minimum was −43.523 mW at 108.340 °C. The small weight loss and sharp thermal transitions indicate a high purity and consistent material performance, like HDPE [44]. For polyolefin, the maximum weight was 5.209 mg at 123.088 °C, and the minimum was 5.158 mg at 115.480 °C, with a weight loss of 0.052 mg. The maximum heat flow calculated was −7.490 mW at 121.685 °C, and the minimum was −32.947 mW at 242.158 °C. The moderate weight loss and multiple heat flow peaks highlight the blended nature of polyolefin, showing overlapping thermal events. For waste PP, the maximum weight was 3.896 mg at 242.075 °C, and the minimum was 3.805 mg at 138.100 °C, with a weight loss of 0.091 mg. The maximum heat flow was −11.235 mW at 158.360 °C, and the minimum was −36.700 mW at 242.215 °C. The minimal weight loss and sharp melting peak around 160–170 °C indicate high crystallinity and excellent thermal stability, confirming its suitability for high-temperature environments.

Table 4. TGA and DSC data of raw material plastics.

File Name	Max Weight (mg) (Temperature °C)	Min Weight (mg) (Temperature °C)	Weight (mg)	% Weight Loss	Max Heat Flow (mW) (Temperature °C)	Min Heat Flow (mW) (Temperature °C)
Waste HPDE	3.10 (240.50)	3.03 (124.09)	0.07	2.18	−13.14 (126.64)	−35.37 (242.05)
Waste LDPE	7.85 (241.15)	7.74 (141.85)	0.11	1.45	−9.90 (142.62)	−34.45 (242.17)
Samicanite	5.85 (242.07)	5.75 (139.08)	0.10	1.78	−10.46 (140.41)	−32.79 (242.24)
PE virgin	5.23 (117.35)	5.18 (108.34)	0.05	1.05	−22.86 (135.68)	−43.52 (108.34)
Polyolefin	5.21 (123.09)	5.16 (115.48)	0.05	1.01	−17.49 (121.69)	−32.95 (242.16)
Waste PP	4.94 (129.90)	4.88 (240.66)	0.06	1.25	−11.23 (126.31)	−33.34 (242.21)

shows the weight loss percentage of the raw plastic materials. Weight loss observed during Thermogravimetric Analysis (TGA) typically reflects the release or decomposition of materials due to heating, and its specific interpretation depends on the sample’s nature and the temperature range in which the weight loss occurs. At low temperatures, typically below 150 °C, weight loss is often associated with the evaporation of absorbed or adsorbed moisture, indicating the presence of free or bound water within the sample. In the intermediate temperature range, between 150 °C and 400 °C, weight loss may be due to the release of volatile components, such as solvents, plasticizers, or unreacted monomers, which are common in polymeric and composite materials. The temperature range was analyzed for the extrusion process being carried out for developing the products. Waste HPDE had the highest weight loss of 2.18% more than the other materials, which ranged from 1.01% in polyolefin to 1.78% in samicanite. The loss can be related to emissions of unhealthy gases. These gas emissions are at a very minimal range compared to the emission of unhealthy gases in other construction material processing, like calcination, which is commonly used in cement production and is associated with significant environmental impacts. Traditional calcination methods release substantial amounts of carbon dioxide, though innovative technologies, such as plasma-assisted decarbonization, show promise in reducing emissions. The optimization of calcination parameters in cement plants could mitigate environmental consequences, while still supporting industrial applications. Similarly, the potential for carbon capture and storage during calcination presents an opportunity for achieving negative emissions in energy-intensive industries. In contrast, recent advancements in polymer extrusion have focused on minimizing the release of harmful gases through precise temperature control and improved feedstock quality. Furthermore, it has been reported [101] that polyethylene processing emits comparatively fewer harmful gases than calcination, underscoring its relative environmental advantages. These findings collectively suggest that while emissions from polymer extrusion are minimal, ongoing efforts are crucial to enhance sustainability across all industrial processes for the construction industry.

2.1.2. Mechanical Extrusion Process

The extrusion machine had six heating coils with temperature monitoring thermocouples. These were automated by the controlling panel. The temperature ranges were from 100 °C and 200 °C were controlled to achieve the proper melted mix and desired flow to fill the mold. Figure 8 is a diagram of the extruder with an illustration of the areas of the setup used to prepare the molds. The study aims to determine the feasibility of recycled plastic blends for use in structural applications by analyzing their mechanical response. The mechanical extrusion method does not extensively emit harmful gases to the environment, and the weight loss of recycled plastic is very few. The method is more sustainable than the production of other construction products.

2.1.3. Gas Emissions Detection and Monitoring

Given the high temperature melting of waste HDPE during the extrusion process, its environmental impact was assessed by monitoring gas emissions. A smart sensor (ST8900 model, Dongguan Wanchuang Electronic Products Co., Ltd., Dongguan, China) was employed to measure and analyze the composition of emitted gases.

This device could detect and quantify the oxygen level as the % Vol in the atmosphere; carbon monoxide and hydrogen sulfide in ppm; and the lower explosive limits LELs of compounds such as methane, ethane, propane, butane, gasoline, petroleum gas, and turpentine as the % vol, as shown in Figure 8. The calibration was as per the manufacturers’ guidelines. The sensor was positioned near the extruder outlet to capture emissions during the extrusion phase, specifically when the material exited the setup. This ensured maximum gas emissions were accounted for. The recorded data were compared against the safety thresholds and exposure symptoms for humans, as outlined in the sensor’s specifications manual. During the process of extrusion, all the samples showed emissions within safe limits, which is also mentioned in Figure 9.

2.2. Preparation of Samples

The samples were developed after the second round of extrusion. The extruder was equipped with an electronic control panel to adjust the desired speed and temperature. The extrusion machine is kept in a closed moist free environment; the motor capacity is 900 revolutions per minute, and the gear reduced revolution capacity is 45–46 revolutions per minute. The extrusion speed was reduced by electronic speed controllers to 20–25 revolutions per minute for all the samples. With this speed, the recycled plastic extrude was workable. Considering the volume of the sample, properly sized molds were developed as dumbbell-shaped, beamlets, and squared prisms for each tensile, flexural, and shear sample, respectively. The compression samples were developed from an arrangement of piped sizers which developed rods in the desired length, and the diameter of the piped sizer was fixed. After getting an extruded length, the plane cutting of the rods of the desired samples of the compression cylinders were prepared. These molds were manually handled. The dye filling time was about 15 s for the shear samples, 21 s for the flexural samples, and for tensile samples, it was about 30 s. The compression samples’ arrangement was designed in a way that can also be used for rebar manufacturing. The time taken was about 15 s for each section. The average weight of the samples for rPP was 22 gms, 33 gms, 27 gms, and 67gms. For rHPDE, the average weights of the samples were 23 gms, 33 gms, 27 gms, and 68 gms for compression, flexural, shear, and tensile samples, respectively. The other blends had similar average weights as for rHDPE.

The extrusion screw was divided into 4 zones: feeding, compression, melt, and exit. The average temperature controlled by the thermocouple was 50–55 °C, 100–110 °C, 120–130 °C, and 120–135 °C in zones 1, 2, 3, and 4, respectively. A water bath was given for 3–5 min for the cooling of the samples. The samples were cast for mechanical testing, which included recycled waste HDPE, PP, a mix of HDPE and PP, a mix of samicanite and HDPE, a mix of LDPE and HDPE in two different proportions, a mix of polyolefin and HDPE, and a mix of virgin PE and HDPE. Five of each material were cast from a special mold prepared for samples. After a detailed analysis, the behavior of the samples was compiled. The details of the proportions are tabulated below in

Table 5. Summary of samples for each mechanical test.

Parameter		Recycled Plastic Mixes
		rHDPE	rPP	rHDPE + rPP	rHD + V	rHDPE + rLDPE	rHDPE + POL	rHDPE + rSAM
Material content by weight	rPP		100%	50%
	rHDPE	100%		50%	50%	50%	80%	80%
	Other plastics (V, LDPE, SAM, POL)				50%	50%	20%	20%
Mechanical test samples	Shear Test (S)	5	5	5	5	5	5	5
	Flexure Test (F)	5	5	5	5	5	5	5
	Tensile Test (T)	5	5	5	5	5	5	5
	Compression Test (C)	5	5	5	5	5	5	5

. The blends and plastics studied were framed in context to previous research, in Table 3. Special extended custom changes were made in the UTM for the test of mechanical properties, as per ASTM. Special molds were prepared for the extruded materials for testing, as per the ASTM standards, D790 (flexural) [102], D695 (compression) [103], D732 (shear) [104], and D638 (tensile) [105], as shown in Figure 10. The overall sample preparation summary is in Table 5. The table provides a detailed breakdown of the recycled plastic mixes, highlighting their material composition and mechanical testing to assess their suitability for construction applications. The materials include rHDPE, rPP, and various HDPE-based blends, such as rHDPE + rPP, rHDPE + V, rHDPE + rLDPE, rHDPE + POL, and rHDPE + rSAM. The composition varies, with rHDPE and rPP being 100% pure, while the blends contain 50-80% HDPE combined with other materials like PP, POL, and samicanite, as obtained from Table 3. To evaluate mechanical performance, shear, flexural, tensile, and compression tests were conducted. All the samples underwent shear and tensile tests, ensuring a comprehensive assessment of their load-bearing capabilities.

2.3. Testing

2.3.1. Mechanical Testing

• Shear

The Servo-Hydraulic Testing Machine had limitations in testing for compression, tensile, and flexural testing. The shear punch hole apparatus was made as per ASTM D732 (shear) for the testing of shear samples. The sample had a width of 50 mm, a length of 50 mm, and a thickness of 12.7 mm, as per the ASTM standard [104]. In this test, load and deflection graph values were obtained from which stress and strain graphs were developed. Shear total energy absorption, S-TEA, the shear toughness index, the shear maximum strength, SMS, and the shear yield strength, SYS, were computed. These parameters provided valuable insights into the shear performance of the tested material, allowing for a comprehensive assessment of its mechanical durability and structural integrity under shear loading conditions. The custom test setup arrangements were developed as shown in Figure 11 for testing the mechanical properties of recycled plastic.

b.

Tensile

Tensile tests were performed as per the ASTM D638 samples. The samples were of dumbbell shape type III, as per the ASTM standard [105]. Additional grips were added for the sample, and the sample’s length dimension for the narrow section was 57 mm, and the overall width was 29 mm, the thickness was 12.70 mm, and the gauge length was 50 mm. Load and deflection graphs were obtained. After developing the stress–strain curves, the tensile energy absorption, T-EA; tensile toughness index, T-TI; tensile maximum stress, T-MS; and tensile yield stresses, T-YS, were computed. Figure 11 shows the tensile test arrangement.

c.

Compression

The compression samples, as per ASTM standard D695 [103], were tested to compute the compression total energy absorption, C-TEA; compression toughness index, C-TI; and compression maximum stress, C-MS. The values were obtained for cylinder samples of sizes 50 mm in height and 25.4 mm in diameter. The obtained values provide insights into the material’s ability to absorb energy under compressive loads, its toughness characteristics, and its ultimate strength, which are critical parameters for evaluating mechanical performance in various structural applications. The compression testing arrangement is shown in Figure 11.

d.

Flexure

The beamlets were tested for flexure, as per ASTM D790 [102], using a three-point load test. After developing the stress–strain curves, the flexural total energy absorption, F-TEA; flexural peak energy absorption, F-PEA; flexural toughness index, F-TI; flexural maximum stress, F-MS; and flexural yield stresses, F-YS, were computed. Plastic products are not exclusively used for construction materials, and ASTM standards with specific purposes are required for use in the construction industry. The flexural test arrangement is shown in Figure 11.

2.3.2. Microstructure Analysis

• SEM analysis

The failure of plastic samples under loading conditions can primarily be attributed to microstructural defects and compositional changes, which can be analyzed using Scanning Electron Microscopy (SEM). SEM helps identify physical imperfections, such as voids, material clustering, and microcracks, which weaken the structural integrity of the material. Voids, often caused by improper processing or trapped air, act as stress concentrators, leading to early failure under mechanical loads. Similarly, material clustering, where fillers or reinforcements aggregate unevenly, creates localized weak zones, reducing the overall strength of the material. SEM also reveals shear banding and microcracks, which propagate under stress and ultimately result in mechanical failure. SEM shows brittle fracture surfaces with sharp edges or ductile deformations with signs of fibrillation, helping determine whether the material failed due to inherent brittleness or structural weakening over time.

b.

FTIR analysis

FTIR is crucial for detecting chemical composition changes that may have contributed to failure. FTIR reveals signs of crosslinking or polymer chain scission, both of which alter the material’s mechanical properties, for the pre and post extrusion states of recycled plastic.

2.4. Optimization Procedure

The major content of waste plastics is PE-based. Recycled HDPE blends were used to assess the properties enhancements with the addition of materials like virgin HDPE, polyolefin, samicanite, and LDPE. Based on previous studies [99], the blends only impart marginal improvements. However, the products formulated for the construction industry need better performance and overall manageability. These materials impart rheological improvements, texture improvement, processibility, and improvements in the microstructure of HDPE. However, apart from these properties, the materials have an impact on the material properties of the material performance [98]. The recycling conditions studied are for developing construction materials from rHPDE, a major component in waste.

3. Results

3.1. Mechanical Performance of Recycled Plastic

3.1.1. Shear Behavior

Shear tests were conducted in line with the ASTM D732 specifications on the recycled plastics. The tests were carried out at a loading rate of 1.6 mm/min to determine the peak load (Pmax), maximum strength (τ max), maximum strain (γ max), and energy absorption (E) of 35 samples. The stress–shear strain response of seven sets of distinct polymeric materials subjected to a punch hole test is shown in Figure 12; a specialized method for evaluating the shear properties of the materials is summarized in

Table 6. S-TEA, STI, S- MS, and S-YS properties of recycled plastic mixes.

Parameter	Recycled Plastic Mixes
	rHDPE	rHDPE + rPOL	rHDPE + V	rHDPE + rSAM	rHDPE + rLDPE	rHDPE + rPP	rPP
S-TEA (KJ/m3)	5.43 ± 0.13 (9.96)	5.38 ± 0.03 (3.39)	6.74 ± 0.10 (8.15)	5.69 ± 0.10 (8.64)	4.48 ± 0.03 (4.84)	5.09 ± 0.09 (14.03)	8.69 ± 0.09 (5.06)
S- TI (J)	10.39±0.03 (4.90)	7.53 ± 0.07 (14.40)	9.59 ± 0.03 (4.79)	8.39 ± 0.04 (7.25)	7.35 ± 0.01 (9.29)	7.38 ± 0.06 (17.54)	13.12 ± 0.04 (4.61)
S-MS (MPa)	10.39±0.47 (6.14)	7.53 ± 0.95 (18.19)	9.59 ± 0.51 (7.28)	8.39 ± 0.30 (4.84)	7.35 ± 0.65 (12.40)	7.38 ± 0.45 (11.45)	13.12 ± 0.34 (3.39)
S-YS (MPa)	2.08±0.06 (3.18)	1.85 ± 0.14 (8.60)	1.94 ± 0.01 (0.77)	1.89 ± 0.04 (2.07)	1.87 ± 0.25 (15.32)	1.84 ± 0.08 (5.40)	2.65 ± 0.02 (0.89)

Note: the values in () are CoV values.

. The chart in Figure 12a represents the shear stress–shear strain behavior of recycled polymeric materials subjected to a punch hole test, providing insights into their mechanical response under shear-dominated loading conditions. Figure 12b shows the circular punch after failure. The x-axis represents strain (γ), indicating the material’s deformation as a ratio of change in length to the original length, while the y-axis represents stress (τ), measured in megapascals (MPa), representing the applied tangential force per unit area. Each curve corresponds to a specific recycled polymer blend or composition, highlighting differences in their elastic, plastic, and failure behavior. The rHDPE (recycled high-density polyethylene) curve shows moderate peak shear stress and ductility, with a distinct elastic region, followed by yielding and plastic deformation before failure. This means that rHDPE has a good strength and toughness for moderate punch resistance in structural applications. rHDPE + V has a similar shape to the curve to rHDPE, with a bit lower peak stress but similar strain; this indicates that adding the V component does not greatly reduce the material’s ductility. This blend may be useful in applications where the material will be subjected to shear loading and must be able to stretch a little. In addition, rHDPE + rPP has a larger strain range with a lower peak stress, which indicates that it has good ductility and energy absorption for impact-resistant applications. The punch hole test of rHDPE + rSAM has the lowest peak stress and strain among all the blends, indicating that it has the worst strength and deformation capacity. The weak mechanical response may be due to incompatibility or poor interfacial adhesion between rHDPE and rSAM, which renders it less useful for punch-resistant applications without further modification. rHDPE + POL has a peak stress that is moderate, and the stress decreases gradually after the yield point. This blend is most suitable for applications where the material will be subjected to shear loading and needs moderate mechanical properties. Lastly, rPP (recycled polypropylene) has the lowest peak stress but the highest strain capacity, which is a characteristic of ductile material.

The material fails by extensive plastic deformation before failure, which shows its ability to absorb a lot of energy when subjected to shear loading and thus is suitable for use in flexible and impact damping applications, such as packaging or automotive parts. This chart shows the mechanical properties of recycled polymer blends under punch hole testing and how they are different. On the other hand, rPP is very ductile and tough, and thus it is most suitable for use in energy absorption applications. Intermediate blends, such as rHDPE+POL and rHDPE, have both high strength and high flexibility and can thus be used in applications with moderate mechanical properties. These results are helpful in choosing the material for a specific application that requires shear strength, toughness, and deformation properties.

The mechanical properties assessed include shear total energy absorption (S-TEA), the shear toughness index (S-TI), shear maximum stress (S-MS), and shear yield stress (S-YS); the results are compiled in Table 6 and Figure 13. The S-TEA for HDPE is in a range of about 5.43 kJ/m³, showing moderate energy absorption during shear deformation. The S-TI for rHDPE is about 10.39 J, indicating good toughness under shear stress. This makes rHDPE resistant to cracking during deformation [106]. The S-MS for rHDPE is 10.39 MPa [106,107]. The S-TEA of rHDPE + PL, S-TI, and S-MS are 5.38 kJ/m³, 7.53 J, and 7.53 MPa, respectively, which are intermediate values like that of rHDPE. However, rHDPE + PL has slightly better energy absorption under shear stress [107]. The S-TEA for rHDPE + rPL is 7.53 J, indicating that it is moderately tough under shear deformation. The S-MS for rHDPE + PL is 7.53 MPa, showing that it has the strength of the order of that of rHDPE. The S-TEA for rHDPE + V is 6.74 kJ/m³, which is slightly higher than that of rHDPE and rHDPE + PL, showing that it has better energy absorption during shear deformation [107]. The S-TI for rHDPE + V is 9.59 J, indicating that it has higher toughness than rHDPE and rHDPE + PL [106]. The S-MS for rHDPE + V is 9.59 MPa, moderate in shear stress. The S-YS for rHDPE + V is 1.94 MPa, with good shear deformation before failure. The S-TEA for rHDPE + rSAM is 5.69/m³, showing good energy absorption under shear stress. This composite has a moderate impact resistance [107]. The S-TI for rHDPE + rSAM is 8.39 J, indicating that it has moderate toughness and is thus suitable for applications to which moderate shear stress is applied [106]. The S-MS for rHDPE + rSAM is 8.39 MPa, with moderate shear strength. The S-TEA for rHDPE + rLDPE is 4.48 kJ/m³, which is lower than those of rHDPE + V and rHDPE + rSAM, indicating that rHDPE + rLDPE has poor energy absorption under shear stress. The S-TI for rHDPE + rLDPE is 7.35 J, showing good toughness but poor crack propagation resistance compared to rHDPE + rSAM. The S-MS for rHDPE + rLDPE is 7.35 MPa, moderate shear strength. The S-TI for rHDPE + rPP is 7.38 J, indicating moderate toughness. The S-MS for rHDPE + rPP is 7.38 MPa, meaning it has moderate shear strength. The S-TEA for rPP is 8.69 kJ/m³, which reflects excellent energy absorption under shear deformation. The S-TI for rPP is 13.12 J, indicating excellent toughness. The S-MS for rPP is 13.12 MPa, indicating high shear strength. The S-YS for rPP is 2.65 MPa, showing good resistance to shear deformation. The CoV values of the shear parameters of the recycled plastic mixes are influenced by material composition, degradation, and processing conditions. In the products, voids are also present due to gas accumulation in the extrusion process, and this influences the values mainly. rHDPE shows moderate variability, likely due to inconsistencies in molecular weight and contamination. rHDPE + rPOL has stable energy absorption but inconsistent stress properties, possibly due to phase separation. rHDPE + V has highly uniform yield stress, indicating good compatibility with virgin HDPE. rHDPE + rSAM shows relatively low CoV, suggesting good blend uniformity. rHDPE + rLDPE has high CoV in stress parameters due to mismatched crystallinity. rHDPE + rPP suffers from high variability, likely due to poor interfacial adhesion between HDPE and PP. rPP exhibits low CoV, particularly in yield stress, suggesting relatively consistent material properties with minor variations due to prior degradation. Overall, higher CoV values indicate material inconsistencies caused by differences in polymer compatibility, the presence of contaminants, processing conditions, and degradation effects due to extrusion.

3.1.2. Flexural Behavior

Flexural tests were conducted in line with ASTM D790 specifications. The tests were carried out at a loading rate of 1.6 mm/min to determine the peak load (Pmax), maximum strength (σ max), maximum strain (ε max), and energy absorption (E) of 35 samples. The stress–shear strain responses of seven sets of distinct polymeric materials subjected to a punch hole test are shown in Figure 14; a specialized method for evaluating the shear properties of the materials is summarized in

Table 7. F-MS, F-YS, F-TEA, F-PEA, and F-TI properties of recycled plastic mixes.

Parameter	Recycled Plastic and Mixes
	rHDPE + rPP	rHDPE + rSAM	rHDPE + rLDPE	rHDPE	rHDPE + POL	rPP	rHDPE + V
F-TEA (KJ/m3)	3.24 ± 0.28 (9)	13.66 ± 1.83 (13)	1.11 ± 0.50 (5)	12.80 ± 1.97 (15)	1.27 ± 0.18 (14)	45.74 ± 7.64 (17)	1.05 ± 0.13 (13)
F-PEA (J/m3)	6.44 ± 0.69 (11)	9.82 ± 2.54 (26)	5.56 ± 0.06 (1)	50.79 ± 6.40 (13)	7.00 ± 1.97 (28)	208.81 ± 7.00 (3)	6.09 ± 1.31 (21)
F-TI (J)	3.45 ± 0.37 (11)	5.87 ± 1.52 (26)	1.15 ± 0.01 (1)	7.37 ± 0.93 (13)	1.39 ± 0.39 (28)	9.42 ± 0.32 (3)	1.15 ± 0.25 (21)
F-MS (MPa)	13.99 ± 1.21 (9)	34.61 ± 4.63 (13)	13.60 ± 0.06 (1)	25.83 ± 3.97 (15)	13.66 ± 1.91 (14)	72.23 ± 12.07 (17)	13.60 ± 1.71 (13)
F-YS (MPa)	13.12 ± 0.15 (1)	13.12 ± 0.20 (2)	12.98 ± 0.17 (1)	14.05 ± 0.16 (1)	13.36 ± 0.23 (2)	14.14 ± 0.14 (1)	12.92 ± 0.22 (2)

Note: the values in () are CoV values.

. The flexural stress–strain curves provide a detailed analysis of seven distinct samples, each representing a unique combination of recycled polymers and additives under flexural loading. Strain (ε) is on the x-axis, which describes the material’s deformation ratio, and flexural stress (σ) in megapascals (MPa) is on the y-axis, which is how much stress or deformation the material can withstand. Curves are included to illustrate the mechanical properties of the material depending on the material composition and the polymers and additives interaction. This paper presents a typical linear elastic region and then moderate peak stress and a subsequent plateau, which means that this material has both strength and ductility. This means that replacing these two recycled polymers enhances the flexibility of the material without compromising its structural integrity. The curve for rHDPE + rSAM, which contains samicanite (SAM), has higher peak stress and a longer strain region, which means that this material has high toughness and the ability to absorb energy. Samicanite seems to reinforce greatly the rHDPE matrix; hence, this blend is best suited for use in applications that require the material to have high durability and strain tolerance. The curve for rHDPE + rLDPE (rHDPE/rlDPE) has the lowest peak stress and the smallest strain range among the curves for all the blends and pure rHDPE. This means that rLDPE does not bring the same level of reinforcement as samicanite but instead increases the flexibility of the blend. The curve for 100% rHDPE has a peak stress and then a sharp decrease, which is the characteristic of a brittle failure. This means that rHDPE has a poor toughness and strain capacity and thus needs to be blended or modified in some way to improve its mechanical properties.

The rHDPE + POL blend that contains polyolefin (POL) has a peak stress that is moderately high with a slight bit of ductile behavior, which means that polyolefin improves the toughness and strain tolerance of the blend without sacrificing much strength. The rPP (standalone polypropylene material) has a behavior very similar to that of the rHDPE+rPP blend, with a moderate level of ductility and a slightly lower peak stress. This means that although rPP is compatible with rHDPE, it does not make a significant contribution to the mechanical reinforcement. Finally, the rHDPE+V blend, represented by a dotted red line, exhibits the lowest peak stress and strain tolerance among the blends, suggesting that the additive contributes minimally to the material’s overall mechanical enhancement. The detailed analysis examines the flexural properties of various recycled plastic composites, including HDPE + PP (polypropylene), HDPE + SAM (samicanite Pellets), HDPE + LDPE (Low-Density Polyethylene), HDPE + POL (polyolefin), PP (polypropylene), and HDPE + V (virgin material). The mechanical properties investigated include flexural total energy absorption (F-TEA), flexural peak energy absorption (F-PEA), flexural toughness index (F-TI), flexural maximum stress (F-MS), and flexural yield stress (F-YS), and are shown in Table 7 and Figure 15. These properties are important to characterize the material’s behavior under bending loads and to recommend the material for structural, impact-resistant, and flexible applications. The F-TEA for HDPE + PP is 3.24 kJ/m³, which indicates that the material has a relatively low energy absorption capacity. Although this is a moderate value, it means that HDPE + PP is not very efficient for use in devices that need to absorb a large amount of energy when subjected to bending stresses. HDPE + PP would be appropriate for use in applications where energy absorption is not a critical factor. The F-PEA for HDPE + PP is 6.44 J/m³—the material is of moderate toughness and absorbs some energy in the process of plastic deformation. It is not as good, however, as other composites, such as HDPE + POL or HDPE + SAM, in a high-stress environment. The F-TEA for HDPE + SAM is 13.66 kJ/m³, which is much higher than that of HDPE + PP, indicating that HDPE + SAM has a very good energy absorption capacity. Studies have shown improvement in the mechanical properties of blends of HPDE [108]. The F-PEA for HDPE + SAM is 9.82 J/m³, which means that it can absorb a large amount of energy when it is flexurally deformed. This increases its toughness and makes it suitable for high-impact applications. Of the composites evaluated, the F-TEA of HDPE + LDPE has the widest average range of 1.11 kJ/m³. This implies that rather than employing this material in applications which demand a great deal of energy absorption or impact resistance, it should be used in the contrary scenario [109,110]. Under a flexural loading energy absorption of HDPE + LDPE was moderate, with an F-PEA of 5.56 J/m³; however, there was still a reasonable difference relative to a SAM or POL composite. As indicated by the value, the F-TI of HDPE + LDPE exhibits that the material has relatively lower toughness and is increasingly prone to cracking in the flexural and stress orientations, with the measurements of 1.15 J. The F-TEA for HI is 6.16 kJ/m³, which shows that it has moderate energy absorption properties and is, therefore, suitable for applications that require a balance of rigidity and some degree of flexibility. The F-TEA for HDPE + POL is 45.74 kJ/m³, the highest value among all composites, indicating that it has excellent energy absorption properties and is, therefore, suitable for applications that require high impact resistance and energy dissipation. The F-PEA for HDPE + POL is 208.81 J/m³, which is excellent, and which shows that the composite can absorb a lot of energy before it fractures. The F-TI for HDPE + POL is 9.42 J, indicating that the material possesses very high toughness and very high resistance to crack propagation under bending stress [110]. Among all the composites tested, the F-MS of HDPE + POL is the highest, 72.23 MPa, thus making HDPE + POL one of the strongest composites with the best bending stress resistance. The F-YS of HDPE + POL is 14.14 MPa, which means that this composite can oppose the initial deformation under flexural stress.

The F-TEA of PP is 45.74 kJ/m³, which means that this material has an excellent energy absorption capacity, and, therefore, it is ranked among the best in energy dissipation. The F-PEA of PP is 208.81 J/m³, which is a good sign of the plastic energy absorption capacity and is much higher than that of most other composites. The F-TI of PP is 9.42 J, and this indicates that it has a good resistance to crack propagation and is one of the toughest materials tested. The F-MS of PP is 72.23 MPa and thus has a high strength and a good bending stress resistance [110]. The F-YS of PP is 14.14 MPa, which means that it has a high resistance to initial plastic deformation under bending stress. The F-TEA of HDPE + V is 1.05 kJ/m³, which is a low energy absorption, and thus it is not very efficient for high impact applications. The F-PEA for HDPE + V is 6.09 J/m³, showing a moderate level of energy absorption. The F-TI for HDPE + V is 1.15 J, indicating lower toughness compared to other composites, like HDPE + POL. The F-MS for HDPE + V is 13.60 MPa, indicating moderate strength. The F-YS for HDPE + V is 12.92 MPa, showing a moderate level of resistance to deformation [107]. The CoV in the flexural properties of the recycled plastic mixes varies significantly based on the material composition and processing conditions. rHDPE + rLDPE shows the lowest CoV across all the parameters, indicating a highly uniform blend due to good polymer compatibility. rHDPE + rPP and rHDPE + rSAM exhibit moderate CoVs in energy absorption and toughness, suggesting controlled variability due to different polymer structures and impact modifiers. rHDPE and rPP show moderate-to-high CoVs in flexural strength, likely due to variations in crystallinity and prior degradation. rHDPE + POL and rHDPE + V have the highest CoV, particularly in energy absorption and toughness, indicating poor interfacial adhesion and phase separation. Overall, lower CoV values indicate consistent mechanical properties, while a higher CoV suggests material inconsistencies caused by polymer incompatibility, processing variations, and degradation effects.

3.1.3. Compression Behavior

Compression tests were conducted in line with ASTM D695 specifications. The tests were carried out at a loading rate of 1.6 mm/min to determine the peak load (Pmax), maximum strength (σ max), maximum strain (ε max), and energy absorption of the 35 samples. The compression stress–strain behavior presented in the graph in Figure 16 showcases the mechanical performance of seven distinct polymer samples: rHDPE-V, rHDPE + SAM, rHDPE, rPP, rHDPE + rLDPE, rHDPE + POL, and rHDPE + rPP. Each sample represents a variation of recycled high-density polyethylene (rHDPE) or blends of polymers tailored to enhance specific properties. The behavior of these materials is critical for determining their applicability in industrial and structural applications, especially in the context of sustainable material development. The rHDPE-V and pure rHDPE samples show moderate levels of stress and strain, reflecting the intrinsic properties of recycled HDPE, which is known for its rigidity and resistance to deformation under compressive loads. The linear elastic behavior at low strain levels transitions to a plateau indicative of yielding, a characteristic of semi-crystalline thermoplastics. These results align with the use of rHDPE in applications like rigid containers, pipes, and construction panels, where moderate strength and stiffness are sufficient. However, pure rHDPE lacks the enhanced flexibility or toughness required for more demanding applications. Incorporating samicanite (SAM) into rHDPE significantly increases the stress response, as seen in the rHDPE + rSAM curve, which outperforms pure rHDPE. This improvement is due to the reinforcing effect of rSAM that probably increases the load transfer capability of the polymer matrix and decreases the probability of microcrack initiation under compression. Such composites are of great interest for load-carrying applications in structural elements and automotive parts [22]. The increased stress capacity also shows that the addition of fillers like SAM can effectively compromise between sustainability and high performance. Recycled polypropylene (rPP) has a distinct curve with lower stress values but a higher strain capacity than the rHDPE-based samples. This behavior is a result of the material’s toughness and flexibility, which is less rigid than that of rHDPE. Due to its ductile nature, rPP is well suited for applications involving the absorption of energy, such as packaging, automotive bumpers, and furniture [2]. Nonetheless, the inability to bear high stress may exclude it from structural use. The ductility and the strain-to-failure of the rHDPE is improved when it is blended with low density polyethylene (LDPE) in comparison to pure rHDPE. This stress–strain curve of the rHDPE + rLDPE blend is nearer to an ideal curve than the curve of the pure rHDPE because rLDPE’s flexibility improves the stiffness of the rHDPE. Such blends are useful for toughness and durability-based applications, like flexible piping, geomembranes, and industrial liners [26]. This synergy is important because it shows the potential to create materials with desired properties by blending polymers. The stress response of all the samples is higher for rHDPE + POL than for any other sample, indicating that this composite has the highest load-bearing capacity and stiffness. The rHDPE + POL composite is most suitable for high performance applications where high mechanical strength is required, such as structural reinforcements and heavy-duty containers. This is likely due to the enhancement of the crystalline phase and the improvement of the interfacial adhesion between polymer chains by the addition of POL.

The stress–strain curve of rHDPE + rPP has moderate stress and a reasonable strain capacity. Since the stress response is not as high as that of rHDPE + SAM or rHDPE + POL, the increased ductility makes rHDPE + rPP an appropriate material for use in automotive parts, consumer products, and semi-flexible packaging. From the stress–strain graph, it is also evident how polymer blending and the inclusion of fillers can be used to control the mechanical properties of recycled materials. Thus, composites such as rHDPE + SAM and rHDPE + POL are characterized by high strength and stiffness, while blends including rHDPE + LDPE and rHDPE + rPP are characterized by a balanced combination of toughness and flexibility. These findings highlight the potential of recycled polymers and their composites in promoting sustainable material solutions for industrial applications for the construction industry;

Table 8. The C-TEA, C-TI, and C-MS of recycled plastic and mixed samples.

Parameter	Recycled Plastic and Mixes
	rHDPE	rPP	rHDPE + V	rHDPE + rLDPE	rHDPE + POL	rHDPE + rSAM	rHDPE + rPP
C-TEA (KJ/m3)	0.55 ± 0.06 (11)	0.77 ± 0.02 (3)	3.22 ± 0.41 (13)	3.60 ± 0.32 (9)	4.23 ± 0.41 (10)	3.90 ± 0.27 (7)	3.17 ± 0.18 (6)
C-TI (J)	0.37 ± 0.04 (11)	0.27 ± 0.02 (7)	1.88 ± 0.11 (6)	2.21 ± 0.21 (9)	2.78 ± 0.09 (3)	2.38 ± 0.07 (3)	3.17 ± 0.09 (3)
C-MS (MPa)	6.97 ± 0.12 (2)	8.77 ± 1.12 (13)	20.68 ± 3.4 (16)	23.98 ± 2.4 (10)	27.47 ± 4.8 (17)	24.21 ± 1.22 (5)	18.33 ± 0.76 (4)

Note: the values in () are CoV values.

and Figure 17. The present work focuses on a comparative study of the compression properties of material mixes, including high-density polyethylene, PP (polypropylene), and HDPE-based blends with additives or other polymers. The parameters assessed were compression total energy absorption (C-TEA), the compression toughness index (C-TI), and compression maximum stress (C-MS). These properties are vital in the assessment of materials for their ability to withstand compressive forces, especially in the context of objects that are expected to be strong, elastic, and resistant to failure under mechanical stress. The C-TEA values, which represent the energy absorption capacity of the materials under compression, have a wide range of values among the samples. The highest C-TEA value was obtained from rHDPE+POL with the mean value of 4.23 KJ/m³ and a relatively small standard deviation, which implies that it has both a high and consistent energy absorption capacity. This implies that HDPE+POL is very efficient in compressive energy absorption and, therefore, is suitable for applications that require good energy-handling properties. At the opposite end of the spectrum, pure HDPE has the lowest C-TEA value of 0.55 KJ/m³, which shows its poor energy absorption capacity. This trend clearly shows that material blending is beneficial as the addition of POL, or any other polymer, improves the energy absorption properties of rHDPE significantly.

The C-TI (toughness) of the materials also follows a similar trend. The highest C-TI value of 3.17 J is exhibited by HDPE + PP, which means that this material is the toughest and can operate under compressive loads with a possibility of absorbing much energy before failing. This reveals that the preparation of HDPE/PP blend is a better alternative for producing a material with improved strength. At the opposite end of the spectrum is pure PP, which has the lowest toughness of 0.27 J for C-TI, which in turn means that it has a poor ability to resist fracture under compressive loads. The data show that the blending of rHDPE with other polymers enhances the energy absorption capacity, as well as the toughness of the material. For C-MS, which is the maximum stress that the material can oppose in compression, once again rHDPE+POL has the highest value of 27.47 MPa. This means that rHDPE + POL is the strongest material of the series because it can sustain high compressive forces without failing. On the other end of the spectrum, pure HDPE has the lowest C-MS value of 6.97 MPa, which implies that it has poor compressive strength. The consistency in the performance of HDPE + POL across the three characteristics indicates that it is suitable for use in situations that require the material to have high mechanical properties and strength. The Coefficient of Variation (CoV) in the compressive properties of recycled plastic mixes varies based on material compatibility and processing conditions. rHDPE + rSAM and rHDPE + rPP exhibit the lowest CoV across all the parameters, indicating highly uniform properties due to effective blending. rHDPE, rPP, and rHDPE + rLDPE show moderate CoV values, suggesting controlled variability influenced by polymer crystallinity and melt flow properties. rHDPE + V and rHDPE + POL have the highest CoVs, particularly in compressive strength (C-MS), indicating inconsistencies likely due to phase separation or poor interfacial bonding. rHDPE + POL shows stable toughness but fluctuating compressive strength, while rHDPE + V exhibits significant variability in both energy absorption and strength. Overall, lower CoV values suggest consistent mechanical behavior, while higher values indicate material heterogeneity due to differences in polymer compatibility, degradation effects, and processing conditions.

3.1.4. Tensile Behavior

Tensile tests were carried out according to ASTM D790 standards. The tests were performed at a loading rate of 1.6 mm/min to find the peak load (Pmax), maximum strength (σ max), maximum strain (ε max), and energy absorption of the 35 samples. The tensile analysis graph in Figure 18 presents the stress–strain behavior of nine different recycled plastic blends, and their unique mechanical properties based on their composition are presented in the graph. The rHDPE is taken as a reference point and it exhibits average tensile strength and ductility. This is because research has shown that recycled HDPE has mechanical properties that are quite like those of virgin HDPE when proper recycling conditions are met. Adding 15% rLDPE to rHDPE enhances the flexibility of the material and increases the elongation at break; however, the tensile strength is slightly decreased. This is consistent with studies that have investigated the effect of rLDPE on the ductility of polymer blends [10]. The blend of rHDPE and polyolefins (POL) is seen to be more tough than the others because polyolefins are known to enhance the strength and flexibility of polymers. The mechanical performance of the sample is also dependent on the type and ratio of polyolefins used. Adding rPP to rHDPE increases the stiffness and tensile strength of the material because polypropylene is a relatively stiff polymer. However, the compatibility of rHDPE and rPP is an important factor as phase separation can lead to poor mechanical properties [2]. Blending rHDPE with samicanite (SAM) enhances tensile strength and thermal stability, attributed to the rigid structure and reinforcing effect of samicanite particles. However, the blend shows reduced ductility due to the brittle nature of samicanite, consistent with findings on filler-reinforced polymer composites. The rHDPE+rLDPE blend with a higher proportion of LDPE demonstrates increased ductility, highlighting LDPE’s significant contribution to flexibility in polymer matrices [20]. Adding vinyl polymers (V) to rHDPE improves tensile strength and stiffness, though processing challenges and compatibility issues must be addressed to optimize performance [2]. The rPP sample exhibits high stiffness and tensile strength, typical of polypropylene. However, recycled polypropylene may exhibit slightly reduced mechanical properties compared to virgin PP due to thermal and oxidative degradation during recycling processes.

Table 9. T-TEA, T-TI, T-MS, and T-YS values of values of all specimens of recycled plastic.

Parameter		Recycled Plastic and Mixes
	rHDPE	rPP	rHDPE + rPP	rHDPE + V	rHDPE + rLDPE	rHDPE + POL	rHDPE + rSAM
T-TEA (KJ/m3)	4.27 ± 0.65 (16)	7.13 ± 2.00 (17)	0.25 ± 0.03 (8)	1.40 ± 0.60 (17)	0.10 ± 0.00 (0)	0.94 ± 0.10 (9)	1.56 ± 0.50 (14)
T-TI (J)	1.30 ± 0.65 (14.91)	2.25 ± 1.30 (10.98)	0.84 ± 0.03 (8.17)	0.62 ± 0.40 (10.87)	0.68 ± 0.00 (0.48)	0.87 ± 0.10 (8.85)	0.63 ± 0.60 (16.34)
T-MS (MPa)	16.32 ± 0.83 (5)	21.4 ± 0.51 (2)	8.84 ± 0.23 (3)	12.89 ± 0.25 (2)	6.55 ± 0.29 (5)	11.19 ± 0.58 (5)	14.31 ± 0.66 (5)
T-YS (MPa)	4.48 ± 0.12 (2.80)	4.46 ± 0.09 (2.14)	4.24 ± 0.06 (1.43)	4.27 ± 0.05 (1.11)	4.20 ± 0.00 (0.00)	4.31 ± 0.02 (0.49)	4.31 ± 0.05 (1.12)

Note: the values in () are CoV values.

and Figure 19 show the analysis that examines the tensile properties of various recycled plastic composites, including rHDPE, rPP (recycled polypropylene), rHDPE + rPP, rHDPE + V (virgin material), rHDPE + rLDPE (Low-Density Polyethylene), rHDPE + PL (polyolefin), and rHDPE + rSAM (recycled samicanite pellets). The mechanical properties assessed include tensile total energy absorption (T-TEA), the tensile toughness index (T-TI), tensile maximum stress (T-MS), and tensile yield stress (T-YS). These properties are critical for evaluating the material’s performance under tensile (stretching) loads, particularly in structural applications where strength and ductility are important. The T-TEA for rHDPE is 4.27 kJ/m³, showing that rHDPE has moderate energy absorption during tensile deformation. This suggests that the rHDPE can absorb some energy before failure but is not as resilient as composites like rPP. The T-TI for rHDPE is 1.30 J, indicating that rHDPE has moderate toughness under tensile stress. It shows a fair level of resistance to crack propagation during stretching. The T-MS for rHDPE is 16.32 MPa, demonstrating that HDPE can withstand moderate tensile stress before failure. For tensile deformation, rPP has a T-TEA of 7.13 kJ/m³, which is higher than that of rHDPE, indicating that rPP has a better energy absorption capability and is thus more suitable for applications where energy dissipation is of concern. For tensile stress, the T-TI of rPP is 2.25 J, meaning that rPP has a higher toughness than rHDPE and is thus more likely to resist cracking. The T-MS of rPP is 21.4 MPa, which is higher than that of rHDPE, which means that rPP has better tensile strength before failure. The T-TEA of rHDPE + rPP is 0.25 kJ/m³, which is quite low compared to rHDPE and rPP. This implies that rHDPE + rPP has a limited energy absorption capacity and is hence not very useful for high impact resistance applications. The T-TI of rHDPE + rPP is 0.84 J; hence, rHDPE + rPP is much softer than other composites, like rHDPE or rPP. The T-MS of rHDPE + rPP is 8.84 MPa, which means that it has the poorest tensile strength among the three materials.

For tensile deformation, rPP has a T-TEA of 7.13 kJ/m³, which is higher than that of rHDPE, indicating that rPP has a better energy absorption capability and is thus more suitable for applications where energy dissipation is of concern [39]. For tensile stress, the T-TI of rPP is 2.25 J, meaning that rPP has a higher toughness than rHDPE and is thus more likely to resist cracking. The T-MS of rPP is 21.4 MPa, which is higher than that of rHDPE, which means that rPP has better tensile strength before failure. The T-TEA of rHDPE + rPP is 0.25 kJ/m³, which is quite low compared to rHDPE and rPP. This implies that rHDPE + rPP has a limited energy absorption capacity and is hence not very useful for high impact resistance applications [39]. The T-TI of rHDPE + rPP is 0.84 J; hence, rHDPE + rPP is much softer than other composites, like rHDPE or rPP. The T-MS of rHDPE + rPP is 8.84 MPa, which means that it has the poorest tensile strength among the three materials. The T-TEA for rHDPE + V is 1.40 kJ/m³ greater than rHDPE + rPP, which means that it has better energy absorption under tensile stress. The T-TI for HDPE + V is 0.62 J, which means that it has lower toughness than rHDPE and rPP. The T-MS for rHDPE + V is 12.89 MPa, which means that it can sustain moderate tensile stress before failure [15]. The T-YS for rHDPE + V is 4.27 MPa, and this is like rHDPE and rPP, which means that it has good initial deformation resistance. The T-TEA for rHDPE + rLDPE is 0.10 kJ/m³; this is the lowest of all the composites. This implies that rHDPE + rLDPE has very low energy absorption under tensile stress and hence is not recommended for use in applications which require a material to deform significantly [39]. The T-TI for rHDPE + rLDPE is 0.68 J, and this shows that it has a moderate toughness but is not as tough as HDPE + V and PP. The T-MS for HDPE + LDPE is 6.55 MPa, which is an indication of the material’s tensile stress resistance.

The T-TEA for rHDPE + PL is 0.94 kJ/m³, indicating that the material has a moderate energy absorption capacity, which can be suitable for certain applications. The T-TI for rHDPE + PL is 0.87 J, which shows that it has a moderate toughness under tensile stress. The T-MS for rHDPE + PL is 11.19 MPa, which means that it can bear a moderate level of tensile stress. The T-YS for rHDPE + PL is 4.24MPa, indicating that it has a good resistance to deformation under tensile loading. The T-TEA for rHDPE + rSAM (samicanite pellets) is 1.56 kJ/m³, which is better than that of some other composites in energy absorption. The T-TI for rHDPE + SAM is 0.63 J, which shows that it has a good resistance to crack propagation under tensile loading. The T-MS for rHDPE + rSAM is 14.31MPa and is the highest among the tested composites, which indicates strong resistance to tensile stress. The T-YS for rHDPE + rSAM is 4.31 MPa, which indicates a good resistance to deformation under tensile loading. The Coefficient of Variation (CoV) in the tensile properties of recycled plastic mixes varies based on polymer compatibility, blending efficiency, and processing conditions. rHDPE + rLDPE and rHDPE + POL exhibit the lowest CoV, indicating highly uniform properties due to good polymer compatibility. rHDPE + rPP and rPP show moderate CoV, suggesting controlled variability in energy absorption and toughness. rHDPE and rHDPE + V have higher CoVs in T-TEA, indicating variability in energy absorption, likely due to differences in their molecular structure and processing history. rHDPE + rSAM show the highest CoV, particularly in toughness and tensile strength, suggesting phase separation or the inconsistent dispersion of additives. Overall, lower CoV values indicate stable mechanical behavior, while higher values suggest material inconsistencies due to phase incompatibility, degradation effects, and processing variations.

3.2. Microstructure Analysis

3.2.1. SEM Analysis of Damaged Surfaces of Specimens

The analysis of the microstructure of recycled plastic composites showed that there are voids of about 20 µm in size. Such voids, which are often attributed to the incomplete fusion of the polymer, entrapment of air in the process, and poor interfacial contact between the phases, have a considerable effect on the mechanical and structural properties of the composites. It has been shown that voids in polymer composites serve as stress concentrators that decrease tensile strength and toughness and increase the probability of crack onset and growth. The SEM images in Figure 20 provide the microstructural details of various recycled polymer materials. There are large voids and crack surfaces in the rHDPE (recycled high-density polyethylene) sample, which means that the material has poor interfacial bonding. These defects mean that the material may well be of reduced mechanical strength because, as everyone knows, voids and cracks are stress concentrators. By and large, the rPP (recycled polypropylene) sample has a more homogeneous matrix with fewer voids than the rHDPE sample, and this may be due to better processing conditions or a denser structure. This improved microstructure is associated with improved material properties, that is, strength and toughness, because of the minimal defects present. The addition of SAM (samicanite pellets) to rHDPE shows a cracked surface with more defined patterns, which may be related to the fracture mechanisms of the additive. However, the fact that there is crack propagation indicates that there is still a need for further optimization to improve compatibility and reduce crack formation.

The influence of voids on the properties of recycled composites is further emphasized in [22], and it is shown that even minimal void content can reduce the inter laminar shear strength and the compressive strength, especially in blends with high phase incompatibility. In the blends like rHDPE + rPOL, the poor interfacial adhesion worsens the formation of voids and leads to phase separation and formation of local stress concentrations. In contrast, the blends containing rLDPE had few voids and improved mechanical properties, which can be due to better polymer compatibility and chain entanglement. The rHDPE + POL (recycled high-density polyethylene with polyolefin) blend the material is clustered together but the surfaces are relatively planar which means that the two components are compatible and mixed well, although some defects may still exist and affect the structure. The rHDPE + PP (recycled HDPE with polypropylene) blend shows visible material clustering along with cracks. These cracks indicate that the two polymer phases have poor interfacial bonding, which may restrict the blend’s strength and toughness. The rHDPE + V (recycled HDPE with vinyl) sample has surface cracks and material clustering, showing that the blend was not properly mixed or that there was phase separation. These surface defects could be potential failure points when the material is under stress. In the rHDPE + LDPE (recycled HDPE with Low-Density Polyethylene) blend, material clustering is seen, but the surface is relatively even and planar, indicating stronger compatibility than the other blends. Although the clustering is not fully prevented, the general appearance of the structure is more uniform, with fewer potential stress concentrators. These findings highlight the need for proper void control and structural optimization to create high-performance recycled plastic composites and highlight their potential use in the demanding construction industry.

3.2.2. FTIR

Polymer extrusion processes often cause significant chemical changes within the material due to heat, shear, and pressure. This can lead to polymer chain scission, resulting in the formation of new functional groups, such as carbonyls (C=O), hydroxyls (O-H), and esters or carboxyl groups (C-O), which appear as distinct peaks in the FTIR spectrum of Figure 21. Alongside these oxidative changes, the intense mechanical stress during extrusion can break polymer chains into smaller fragments. These fragments may be recombined in novel ways, forming previously unseen molecular structures that also manifest as new FTIR peaks. High temperatures can further induce chemical crosslinking, creating stronger bonds like C-C or C-O-C and generating additional peaks in the 1000–1200 cm⁻¹ range. When additives or stabilizers are present, they too may react under extrusion conditions, contributing to the formation of new compounds. Moisture absorption or oxygen exposure during the process can lead to additional hydroxyl and carbonyl groups, seen as increased intensity at around 3500 cm⁻¹ and 1700 cm⁻¹. Both materials show differences in their infrared absorption properties in Figure 21, reflected in the varying intensity values across different wavenumbers. One material generally exhibits lower intensity values compared to the other, suggesting potential differences in composition or structural properties. Distinct peaks and valleys in the spectra correspond to the vibrational modes of molecular bonds, with some peaks overlapping and others unique to a particular material. This indicates variations in chemical composition. The infrared absorption characteristics can be used to identify functional groups and assess potential modifications between the materials. Differences in intensity imply variations in transmittance and reflectance, providing clues to compositional changes. Comparing these materials with others that were previously analyzed highlights how each responds differently to infrared spectroscopy.

3.3. Optimization

The findings of this research underscore the significant potential of specific recycled plastic materials and blends in construction applications, with HDPE and PP as contending materials in a complementary manner. rPP has overall shown good performance in shear, flexure, and tensile tests, and rHDPE has also shown encouraging results. However, these materials were not able to perform well under compression, primarily due to differences in the manufacturing mechanisms than other samples. The blends with LDPE had better flowability, but strength-wise this blend performed the poorest. The gas emission analysis during the extrusion process also supported the environmental sustainability of the proposed materials since they emitted minimal gas. Hence, based on mechanical behavior, thermal stability, and environmental compliance, rHDPE and its blends are the most suitable materials for practical construction applications. Furthermore, the rPP had flowability problems during extrusion but is still the highest performer in mechanical properties. These findings highlight the versatility and transformative potential of recycled plastics in advancing sustainable construction practices.

Table 10. Mechanical properties of high performing recommended recycled plastic mixes.

Parameter
Tensile Behavior				Compression Behavior			Flexural Behavior					Shear Behavior
T-TEA (KJ/m3)	T-TI (J)	T-MS (MPa)	T-YS (MPa)	C-TEA (KJ/m3)	C-TI (J)	C-MS (MPa)	F-TEA (KJ/m3)	F-PEA (J/m3)	F-TI (J)	F-MS (MPa)	F-YS (MPa)	S-TEA (KJ/m3)	S-TI (J)	S-MS (MPa)	S-YS (MPa)
Materials with highest performance
rPP			rHDPE	rHDPE + POL	rHDPE + rPP	rHDPE + POL	rPP					rPP
7.13 ± 2.00 (17)	2.25 ± 1.30 (10.98)	21.4 ± 0.51 (2)	4.48 ± 0.12 (2.80)	4.23 ± 0.41 (10)	3.17 ± 0.09 (3)	27.47 ± 4.8 (17)	45.74 ± 7.64 (17)	208.81 ± 7.00 (28)	9.42 ± 0.32 (3)	72.23 ± 12.07 (17)	14.14 ± 0.14 (1)	8.69 ± 0.09 (5.06)	13.12 ± 0.04 (4.61)	13.12 ± 0.34 (3.39)	2.65 ± 0.02 (0.89)
Materials with second highest performance
rHDPE			rPP	rHDPE + SAM	rHDPE + POL	rHDPE + SAM	rHDPE + SAM	rHDPE		rHDPE + SAM	rHDPE	rHDPE + V	rHDPE
4.27 ± 0.65 (16)	1.30 ± 0.65 (14.91)	16.32 ± 0.83 (5)	4.46 ± 0.09 (2.14)	3.90 ± 0.27 (7)	2.78 ± 0.09 (3)	24.21 ± 1.22 (5)	13.66 ± 1.83 (13)	50.79 ± 6.40 (13)	7.37 ± 0.93 (13)	34.61 ± 4.63 (13)	14.05 ± 0.16 (1)	6.74 ± 0.10 (8.15)	10.39 ± 0.03 (4.90)	10.39 ± 0.47 (6.14)	2.08 ± 0.06 (3.18)
Materials with poorest performance
rHDPE + rLDPE				rHDPE	rPP	rHDPE	rHDPE + rLDPE					rHDPE + rLDPE
0.10 ± 0.00 (0)	0.68 ± 0.00 (0.48)	6.55 ± 0.29 (5)	4.20 ± 0.00 (0.00)	0.55 ± 0.06 (11)	0.27 ± 0.02 (7)	6.97 ± 0.12 (2)	1.11 ± 0.50 (5)	5.56 ± 0.06 (1)	1.15 ± 0.01 (1)	13.60 ± 0.06 (1)	12.98 ± 0.17 (1)	4.48 ± 0.03 (4.84)	7.35 ± 0.01 (9.29)	7.35 ± 0.65 (12.4)	1.87 ± 0.25 (15.32)
Recommended material for construction products
First priority: rPP (from strength and toughness point of view)
7.13 ± 2.00 (17)	2.25 ± 1.30 (10.98)	21.4 ± 0.51 (2)	4.46 ± 0.09 (2.14)	0.77 ± 0.02 (3)	0.27 ± 0.02 (7)	8.77 ± 1.12 (13)	45.74 ± 7.64 (17)	208.8 ± 7.00 (3)	9.42 ± 0.32 (3)	72.23 ± 12.07 (17)	14.14 ± 0.14 (1)	8.69 ± 0.09 (5.06)	13.12 ± 0.04 (4.61)	13.12 ± 0.34 (3.39)	2.65 ± 0.02 (0.89)
Second priority: rHDPE (from yield strength, performance, and recyclability point of view)
4.27 ± 0.65 (16)	1.30 ± 0.65 (14.91)	16.32 ± 0.83 (5)	4.48 ± 0.12 (2.80	0.55 ± 0.06 (11)	0.37 ± 0.04 (11)	6.97 ± 0.12 (2)	12.80 ± 1.97 (15)	50.79 ± 6.40 (13)	7.37 ± 0.93 (13)	25.83 ± 3.97 (15)	14.05 ± 0.16 (1)	5.43 ± 0.13 (9.96)	10.39 ± 0.03 (4.90)	10.39 ± 0.47 (6.14)	2.08 ± 0.06 (3.18)

Note: From compression point of view, rHPDE+POL, rHDPE+rPP can be considered.

presents the highest performing recommended recycled plastics and the poorest performers. Although rPP has shown the highest values and is recommended based on strength parameters, the polymer, having different rheological properties, was comparatively difficult to handle during the material meltdown in extrusion [111]. Further to this, the amount of PP in municipal waste is also low compared to HDPE. The HDPE maintained a reasonable performance in all the tests and during extrusion and was better during extrusion, making it plausible for molding different products for construction.

4. Perspective Use of Recycled Plastic Waste in Construction Industry

The construction industry faces mounting pressure to adopt sustainable practices and materials in response to environmental and economic challenges. This research highlights the potential of recycled plastics as a valuable resource for construction applications, transforming environmental liability into a useful asset. Through a detailed analysis of seven types of plastics, including high-density polyethylene (HDPE), Low-Density Polyethylene (LDPE), polypropylene (PP), polyolefin, samicanite, and virgin polyethylene (PE), this study demonstrates their viability as materials for various construction purposes. The findings emphasize HDPE as the best-performing material due to its superior tensile strength, shear resistance, and ductility. Its dense crystalline structure ensures exceptional toughness, making it suitable for structural components, such as load-bearing elements and reinforcements. The SEM analysis of damaged HDPE specimens revealed ductile tearing and energy-dissipative failure patterns, affirming its ability to perform under stress. Blends of HDPE with LDPE and PP also showed excellent mechanical properties, with HDPE-LDPE offering enhanced ductility and impact resistance, while HDPE-PP exhibited significant energy absorption, making both blends ideal for lightweight panels, protective barriers, and applications requiring flexibility and shock resistance.

Polyolefin and samicanite demonstrated remarkable thermal stability for use with HPDE, expanding the potential applications of recycled plastics to insulation panels, weather-resistant membranes, and fire-resistant barriers. The uniform carbon structure of samicanite ensures consistent thermal behavior, making it particularly effective in environments requiring heat resistance. This has a great contribution to environmental benefits when recycled plastics are incorporated in construction. Plastic recycling also helps in reducing the amount of waste that is sent to landfills and helps in the conservation of raw materials since it does not need virgin materials. The current study shows that the extrusion process is environmentally friendly because gas emissions are within safety limits. It thus complies with the circular economy guidelines and suggests the ability to develop construction materials with a lower carbon footprint than conventional materials like concrete and steel. The emission of gases was very small compared to other processes, like calcination. Recycled plastics are, therefore, a good example of materials that can be used in various ways in construction. They can be used in load-bearing and structural applications, like beams and reinforcement, or in non-structural applications, such as lightweight partitions, blocks, corrugated sheets, and protective barriers, and in the thermal insulation of buildings for energy efficiency. Additionally, further developments in recycling technologies, including chemical and enzymatic methods, may improve the quality and performance of the recycled materials. The mechanical and thermal properties of the material could be enhanced by the incorporation of nanomaterials or advanced additives and thus expand the range of their application. To quantify the environmental and economic benefits of using recycled plastics in construction projects, lifecycle assessments will be necessary. Products formed from recycled plastic will have lower maintenance costs in the overall building products. These findings reveal how recycled plastics can be used to address both waste management and sustainable construction concerns. This approach provides a novel approach to managing environmental pollution, conserving natural re- sources, and achieving the Millennium Development Goals by transforming waste plastics into eco-friendly, cost-effective, and versatile construction materials. Thus, this study suggests a way to make the construction industry more resource efficient and to support the growth of innovative solutions for sustainable building demands.

5. Conclusions

This research is an exhaustive investigation of recycled plastics as viable and environmentally friendly alternatives for the construction industry. This study presents innovative strategies for managing waste plastics, focusing on enhancing recycling efficiency and addressing the technological gaps in plastic recycling sector. By exploring the potential of municipal plastic waste in construction, this research demonstrates its feasibility for developing structural elements with essential mechanical properties. A detailed analysis of seven types of recycled plastics—HDPE, LDPE, PP, polyolefin, samicanite, and virgin polyethylene (PE) was conducted using SEM, FTIR, and TGA to assess their composition, thermal stability, and impurity levels. Mechanical testing on 140 samples revealed that HDPE and PP exhibited superior tensile strength and shear resistance, making them strong candidates for structural applications. Unlike previous research that primarily investigated recycled plastics as additives in concrete composites, soil stabilization, and road construction, this study explored their potential as standalone structural materials. Mechanical extrusion is environmentally suitable as gas emissions are minimal, reinforcing the sustainability of the proposed recycling approach. The following are the detailed conclusions that can be drawn.

• The SEM and TGA results of the raw materials indicate that there are impurities. The raw materials are thermally stable, and their low weight confirms that the decomposition does not release hazardous gases. The materials are easily moldable in the temperature ranges defined in this study.
• The materials’ behaviors for use in construction were tested by subjecting the materials to different loading conditions, and the performance of the polymers was reasonable.

  ▪

  Shear behavior testing revealed that rHDPE had almost the same ranging shear energy as rPP. Based on optimization, blends are also recommended for energy absorbing capabilities in shear intensive applications.

  ▪

  The flexural behavior test revealed that rHDPE + rSAM performed well for bearing loads. However, rPP exhibited the highest F-PEA of 208.81 J/m³. This behavior was unmatched by the rHDPE blends.

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  The compressive parameters of rHDPE+POL and rHDPE+rSAM indicate that these two would be useful for structural applications, although rHDPE and rPP showed the lowest energy absorption during compression compared to the blends.

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  The tensile behavior of rPP had the highest energy absorption (T-TEA: 7.13 KJ/m³) and thus proved to be the toughest under tensile loads, while rHDPE had a balanced performance between strength and ductility. Mixed compositions like rHDPE + rLDPE exhibited poor results, which is probably due to the incompatibility of the polymers.

• The SEM and FTIR of the polymer confirm improvement in chemical cross linking.

  ▪

  Through the SEM analysis of the materials after failures appeared, voids, material cluttering, the cleavage of the failure surface, and the ductile tearing and energy dissipative behavior of rHDPE under stress was confirmed, with its toughness and reliability for load-bearing applications being affirmed.

  ▪

  Polymer extrusion induces significant chemical changes due to heat, shearing, and pressure, leading to chain scission, oxidative modifications, and new functional groups becoming detectable in FTIR spectra. High temperatures and mechanical stress can also cause crosslinking, the recombination of fragments, and reactions with additives, further altering the material’s composition.

• It has been found that rHDPE and rPP behave significantly well in tensile and ductility tests. Such properties make them suitable for application in load-bearing components, reinforcements, and protective barriers in construction.
• Recycled HDPE and PP are recommended for the construction industry, and the recycling process is feasible and is compatible with the concepts of the circular economy, which promotes the use of recycled materials instead of raw ones and drastically decreases the carbon footprint of construction materials like concrete and steel.

Although the above experiments demonstrate the feasibility of using recycled plastics in construction, there are several challenges that need to be overcome for their widespread adoption. Since raw waste plastics are often contaminated, maintaining a consistent material quality is vital. This will be necessary to overcome this limitation. Advanced sorting and purification technologies will be essential. Future work shall explore different prospects of recycled plastic in construction products like rebars, blocks, and their use in mortar-free construction. Exploring other elements like corrugated sheets and beams and their connections to the static, dynamic, and thermal properties for use in building construction.