Valorization of construction and demolition waste as reinforcing nanofiller for polymer composites

Abstract

Plastics represents one of the largest global waste streams, with recycling limited by their heterogeneous composition and degraded properties. Simultaneously, construction and demolition activities generate significant amounts of fine fraction waste without reasonable utilization objectives. This study investigated the potential of combining heterogeneous plastic waste and construction and demolition waste (CDW) powder into high-value nanocomposites at various filler loadings (1, 2.5, and 5 wt%), aiming to enhance the mechanical performance of recycled plastics. Virgin polypropylene matrix and commercial nanoclay were used as reference materials. The CDW powder exhibited comparable overall performance to commercial nanoclay in elastic modulus (+ 12%) and even outperformed it in terms of tensile strength (+ 3% vs. +0.4%). Statistical analysis using two-way ANOVA (n = 12, α = 0.05) confirmed that both filler type and loading level significantly affected tensile strength of composites (p < 0.0001), while elastic modulus was primarily influenced by the filler loading. Furthermore, a theoretical model predicting interfacial adhesin between polymer matrix and reinforcing fillers supported these results, indicating stronger polymer–particle adhesion with CDW particles compared to nanoclay. These findings demonstrate that nanoreinforcement is an effective strategy to valorize recycled plastics by improving their mechanical performance and establish CDW-origin filler as a competitive alternative to conventional virgin fillers in the polymer industry.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11671-026-04615-5.

Introduction

Large amounts of plastic waste and construction and demolition waste (CDW) are generated annually worldwide. In particular, the construction sector is the major driver of resource consumption and waste production, accounting for nearly 40% of raw material usage and approximately 35% of waste generation [1]. Massive amounts of inorganic waste are generated in building, repair, and demolition activities, including materials such as concrete, brick, and mortar [2]. To promote sustainable use of materials, this mineral fraction can be reused as recycled coarse and fine aggregates, partially or fully replacing natural aggregates in concrete or mortar [3]. However, a significant amount of this material remains unutilized, as the production of recycled aggregates generates up to 35% fine powder as a byproduct during crushing and sieving operations [3]. Waste management activities are another significant source of CDW powder, where the fine content can be up to 20–40% of the total waste amounts [4]. These CDW-origin powders, such as fine concrete waste, are currently valorized in the production of new concrete and other cementitious materials [5], however, their utilization opportunities are highly dependent on composition and quality [2, 5]. Due to these limitations, a large share of CDW powders is still landfilled [6], leading to the loss of valuable material resources.

Plastics represent another major waste stream, with global generation reaching up to 360 million metric tons annually [7]. The high plastic waste volume is largely driven by the short lifespan of many plastic products. Packaging applications and the construction sector together account for over 60% of the generated plastic waste [8]. Low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), and polyethylene terephthalate (PET) are one of the most produced plastic grades globally [8]. Even though circular economy is nowadays a prominent topic, only a small share of plastics are recycled, while the majority is incinerated or landfilled [9]. Mechanical recycling enables the conversion of these plastics into valuable secondary raw materials. It is considered a simple, energy-efficient, and economically viable recycling method [9–11], in which solid plastic waste streams are converted into new products through separation, purification, shredding, and melt processing practices [9, 11]. Despite these benefits, mechanical recycling causes thermo-mechanical deterioration of polymers, reducing their performance compared to that of virgin counterparts [12]. In addition, the separation of mixed plastic waste by type can be difficult, expensive, time-consuming, and energy intensive, leading easily to the formation of immiscible plastic blends with poor interfacial adhesion between polymer phases [12–14].

Composite preparation offers a promising approach to valorize heterogenous plastic waste and inorganic CDW powder by combining these waste streams into high-value nanocomposite materials. Previous studies have demonstrated that incorporating nanofillers, particularly nanoclay, can enhance the mechanical performance of recycled plastics [12–22]. This improvement is largely attributed to the extraordinarily large surface-to-volume ratio of nanofillers, which promotes good adhesion with polymers [23, 24]. Moreover, utilizing waste-derived nanofillers would not only reduce the need for mining natural resources required for conventional filler production such as clay and talc [25], but would also offer an economic advantage by eliminating the use of costly virgin fillers such as calcium carbonate and silica [26].

The effectiveness of waste-derived fillers on the performance of virgin polymers and recycled plastics has been demonstrated in previous studies. Micrometer-sized CDW-alike materials, including fly ash [26–28], concrete waste [25, 29–34], ceramic waste [35], glass waste [36], and quarry dust [37] have been shown to enhance various material properties, such as stiffness, strength, and thermal stability of polymers. Moreover, even greater reinforcing performance have been achieved with reduced particle sizes [25, 27, 30–32], highlighting the unique characteristics of nanosized fillers [33, 34, 37].

Another advantage of combining nanofillers with recycled plastics is their capability to refine the morphology of immiscible polymer blends [10, 14, 38]. Compatibilizers are typically used to address this issue by decreasing interfacial tension, hindering droplet coalescence, refining morphology, and enhancing adhesion and stress transfer between the immiscible polymer–polymer phases [10, 12, 14]. However, compatibilization of polymer blends prepared from waste is less effective than that of with virgin polymers [12]. One contributing factor to the reduced efficiency is the presence of nanosized contaminants in recycled plastics, which tend to accumulate at polymer–polymer interfaces and forms physical barriers that limit the effictiveness of compatibilizers [10]. These nanoparticles may originate as impurities from waste management or as polymer additives from previous plastic product manufacturing [10].

Despite growing interest in nanocomposite technologies, research on composites combining heterogeneous mixed plastic waste with waste-derived nanofillers remains limited. The aim of this study was to address this research gap by developing value-added composites from waste materials, with a focus on enhancing the mechanical performance of recycled plastics. The mineral fraction of CDW was processed into a reinforcing nanofiller through simple screening and grinding operations. The recycled plastic matrix consisted of waste-derived LDPE (35%), HDPE (35%), PP (20%), and PET (10%). No purification steps or compatibilizers were used for matrix preparation, and the composites were melt compounded using various filler loadings (1, 2.5, and 5 wt%). For reference materials, conventional nanoclay and virgin PP matrix were used, the latter enabling the evaluation of reinforcing performance in a single‑polymer system. The CDW-derived filler was characterized by SEM-EDS to determine its morphology and elemental composition, and by laser diffraction to determinate its particle size distribution. The reinforcing effect of nanofillers on polymer matrices were evaluated in terms of mechanical and morphological properties, and statistical significance was assessed through ANOVA. Adhesion between matrices and fillers were analyzed using a theoretical model to validate morphological observations.

Materials and methods

Materials

Mixed plastic waste was obtained from Salpakierto Oy (Lahti, Finland). It consisted of plastic products such as bags, films, packaging, and containers from companies and industry, as shown in Fig. 1a. Plastics were used as received, without any purification. Resulting granulated recycled polymer blend flakes presented in Fig. 1a had a particle size of < 4 mm and a density of 298.0 ± 6.8 kg/m³.

Fig. 1.

Waste materials preparation and composite compounding: a mixed plastic waste processing into a recycled polymer blend, b mineral CDW fine fraction preparation as reinforcing filler, and c polymer composites manufacturing

Virgin polypropylene (vPP; Eltex HY001P, INEOS Olefins & Polymers Europe, Brussels, Belgium) was used as a reference matrix. This PP was supplied in the form of unstabilized, free-flowing powder with a density of 569.8 ± 0.3 kg/m³ (datasheet: 910 kg/m³), melting point of 161 °C, and mass flow rate of 45 g/10 min (230 °C/2.16 kg).

A mineral fraction of CDW was obtained from a building undergoing demolition (Lappeenranta, Finland). The obtained batch consisted of various mineral-based CDW materials, including concrete and bricks, as shown in Fig. 1b. After screening (≤ 200 μm) and size reduction, the resulting powder had a median particle size of 9 μm and density of 480 kg/m³.

Montmorillonite nanoclay (Nanomer I.44P) was used as a reference filler. This nanoclay, modified with 35–45 wt% dimethyl dialkyl (C14–C18) amine, had a density of 320 kg/m³ (datasheet: 200–500 kg/m³) and an average particle size of ≤ 20 μm).

Recycled matrix preparation

Recycled plastic blend (rPB) preparation is shown in Fig. 1a. In the first step, portable near-infrared (NIR) spectroscopy equipment (Thermo Scientific microPHAZIR PC, Thermo Fisher Scientific, Waltham, MA, USA) operating within the spectral range of 1600–2400 nm, was used to sort polyethylene (PE), PP, and PET products from the obtained mixed plastic waste batch. Further separation of PE into LDPE and HDPE was performed by visual inspection, classifying films as LDPE and hard plastics as HDPE. Each plastic waste grade was crushed separately into small flakes using a low-speed granulator with a 4 mm sieve (Shini Plastic Technologies, Inc., New Taipei, Taiwan). The crushed flakes were then weighed and mechanically mixed for 10 min the following proportions: 35% LDPE, 35% HDPE, 20% PP, and 10% PET. This mixture was initially melt-blended using an injection molding machine (BOY 30, Dr. Boy GmbH & Co. KG, Fernthal, Germany) at temperatures ranging from 200 to 220 °C, with an injection pressure of 110 bar and holding time of 3 s. Prior to composite compounding, the obtained matrix material was crushed into flakes, as illustrated in Fig. 1a.

CDW filler preparation

The mineral fraction of CDW was screened into powder with a particle size below 200 μm using a sieve shaker (HAVER & BOECKER OHG, EML 450 Digital Plus, Oelde, Germany), as illustrated in Fig. 1b. Size reduction of the obtained powder was performed using a disk mill (Masuko Sangyo Co. Ltd., Honcho, Kawaguchi-Shi, Saitamaken, Japan) equipped with two ceramic nonporous disk grinders. Specifically, the powder was fed into the mill with water, the distance between the disks was set to 0 μm, and grinding was conducted for 5 h. After grinding, the material was dried in an oven for 48 h at 100 °C, followed by particle separation using a mechanical whisk.

Composite compounding and injection molding

An injection molder (BOY 30, Dr. Boy GmbH & Co. KG, Fernthal, Germany) was used for both composite compounding and specimen preparation with the following parameters: melt temperature of 190 °C and injection pressure of 110 bar. Prior to melt compounding, recycled plastic blend flakes were premixed in the solid state with CDW powder and nanoclay at different loadings (1, 2.5, and 5 wt%). The compounded material was subsequently crushed into 4 mm flakes (Shini Plastic Technologies, Inc., New Taipei, Taiwan), followed by a second injection molding cycle to produce dumbbell-shaped specimens for tensile testing. For reference, virgin PP (vPP) was reinforced with CDW powder using the same compounding procedure. The composite manufacturing steps are shown in Fig. 1c, and the compositions and filler loadings of the prepared composites are summarized in Table 1.

Table 1.

Compositions and filler loadings (wt%) of the composites

Composites	Fillers		Matrices
	CDW powder	Nanoclay	Recycled plastic blend	Virgin PP
rPB/CDW	1.0	0.0	99	0.0
rPB/CDW	2.5	0.0	97.5	0.0
rPB/CDW	5.0	0.0	95	0.0
rPB/nanoclay	0.0	1.0	99	0.0
rPB/nanoclay	0.0	2.5	97.5	0.0
rPB/nanoclay	0.0	5.0	95	0.0
vPP/CDW	1.0	0.0	0.0	99.0
vPP/CDW	2.5	0.0	0.0	97.5
vPP/CDW	5.0	0.0	0.0	95.0

Characterization

Laser diffraction equipment (Malvern, MasterSizer 3000) with a wet feed device (Hydro EV) was used to analyze the particle size distribution of CDW powder. Water was used as the dispersion medium, and the results were reported as the average of three tests. The median particle size was determined based on the Dv(50) value (µm), which defines the particle size below which 50% of the sample volume falls.

Scanning electron microscopy (SEM, Hitachi SU3500, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS, Thermo Electron Corporation, Madison, USA) was used to investigate the fracture surface morphology of the composites, and the appearance and elemental composition of the size-reduced CDW powder, respectively. Carbon-containing tape was used for the EDS analysis; therefore, carbon content was excluded from the results, and the relative contents of the remaining elements were normalized to a total of 100%. For SEM imaging, fracture surfaces of the composites were prepared by immersing the composite samples in liquid nitrogen, followed by brittle fracture. Prior to imaging, the samples were sputter-coated with a thin (~ 10 nm) layer of gold (99.99% purity) using a sputter coater (Edwards Scancoat Six, Crawley, UK) to prevent charging under the electron beam. The sputtering conditions were as follows: voltage of 1 kV, current of 20 mA, pressure of 20 mPa in an argon atmosphere, and duration of 4 min. SEM imaging was performed under full vacuum with an accelerating voltage of 15 kV, working distance of ~ 10 mm, and secondary electron and BSE-COMP detectors.

Tensile testing was conducted using a universal testing machine (Zwick Roell group, Ulm, Germany) according to the SFS-EN ISO 527-2:2025 [39] standard. Dog-bone-shaped specimens were used with the following dimensions: parallel specimen length of 40 mm, gauge length of 25 mm, width of 5 mm, and thickness of 2 mm. The test speed was set to 20 mm/min, and testing was terminated after the tensile force decreased by 30% from its maximum value. A total of 12 specimens of each material were tested, and the average values with standard deviations were reported. Prior to testing, specimens were conditioned in a chamber at 23 °C and 50% relatively humidity. Yield stress was determined as the 0.2% offset yield. A two-way ANOVA with n = 12 and a significant level of α = 0.05 was used to evaluate the effects of filler type (CDW, nanoclay) and filler loading (1, 2.5, 5 wt%) on tensile properties of recycled plastic matrix. The assumptions of normality and homogeneity of variances were verified with the Shapiro–Wilk test (p > 0.05) and Levene’s test (p > 0.05), respectively.

The densities of rPB flakes, virgin PP powder, CDW powder, and nanoclay were determined by measuring their respective volumes and masses. For each material, 3 measurements were performed, and the mean values with standard deviations were reported. Based on these measured densities, the filler weight fractions (wt%) were converted to volume fractions (vol%) for use in polymer–particle interfacial adhesion calculations.

Results and discussion

Characteristics of CDW powder

EDS analysis revealed that the CDW powder was primarily composed of oxygen, calcium, and silicon, with minor traces of other elements such as aluminum and iron, as shown in Table 2. This composition is consistent with previous studies [5, 29, 31, 37, 40] on mineral-based waste powders, which report that such materials are primarily composed of silicon oxide, aluminum oxide, and calcium oxide, with smaller share of other compounds such as iron oxide, sulfur trioxide, potassium oxide, magnesium oxide, and titanium dioxide. This composition also aligns with that of commercial nanoclay, which is primarily composed of silicon dioxide and aluminum oxide [29, 33].

Table 2.

Elemental composition of CDW powder

Element	O	Ca	Si	Al	Fe	S	Mg	Na	K	Ti
wt%	47.7	34.3	10.0	3.8	1.7	0.9	0.7	0.6	0.5	0.2

The particle size distribution of the CDW powder was analyzed using laser diffraction. As shown in the cumulative particle size distribution curves (Fig. 2), 3.5% of the total volume consisted of particles smaller than 1 μm, while 50% of the volume consisted of particles smaller than 10 μm. The median particle size was 9.21 μm, and nanosized particles were also detected. SEM micrograph (Fig. 3) further revealed that the CDW particles were irregular shaped with varying sizes, consistent with the particle size distribution curves. These CDW particles exhibited rough surfaces and were clustered together, forming agglomerates.

Fig. 2.

Cumulative particle size distribution curves of CDW powder

Fig. 3.

SEM micrograph of CDW powder particles at 5000× magnification, with a scale bar indicating 10 μm

Fracture surface morphology

SEM micrographs of neat matrices and their nanocomposites are shown in Fig. 4. The unfilled virgin PP matrix (Fig. 4a) exhibited a smooth fracture surface typical of neat thermoplastics. In contrast, the recycled LDPE/HDPE/PP/PET matrix (Fig. 4d) showed a more heterogeneous morphology and presence of cavities, reflecting its complex composition. The addition of CDW powder to the virgin matrix revealed good polymer–particle adhesion (Fig. 4c), along with notable agglomeration (Fig. 4b). These agglomerates are likely formed due to high surface energies between small particles that hold them together, which is a common behavior for nanosized fillers [41]. A similar agglomeration behavior was observed in the recycled matrix composites, which exhibited both well-bonded particles and cavities left by dislodged particles (Fig. 4f). However, it is not certain whether these cavities originate from CDW particles, waste impurities, or polymer droplets, as similar cavities were also observed in the neat matrix (Fig. 4d).

Fig. 4.

SEM micrographs of fracture surfaces: a vPP matrix, b–c vPP/CDW composites, d rPB matrix, e rPB/nanoclay composite, and f rPB/CDW composite, with arrows indicating reinforcing fillers or cavities, and circle showing region of agglomeration

The polymer–particle bonding mechanism in these non-compatibilized matrices is primarily based on mechanical anchoring, as the polarity mismatch between the polar filler and the non-polar polymer matrices limits the formation of chemical interactions. The CDW particles exhibited porous and rough surfaces, which allowed polymer melt to effectively penetrate into their surfaces. In contrast, the addition of nanoclay to the recycled matrix resulted in the presence of unseparated clay layers (tactoids) (Fig. 4e), which indicated insufficient mixing and weak interfacial adhesion between the nanoclay and the polymer matrix. For efficient reinforcement, an intercalated or exfoliated structure, where polymer chains diffuse into the interlayer spacings between clay layers, should have been obtained in order to obtain enhanced mechanical performance [16, 24].

Polymer–particle interfacial adhesion

A strong polymer–particle adhesion is essential for achieving optimal properties in polymer nanocomposites as it enables effective stress transfer from the matrix to the particles. Conversely, poorly bonded particles act as discontinuities in the material, thereby reducing its mechanical properties [42, 43]. The adhesion between the matrix and filler can be approximated using an exponential function (Eq. 1) proposed by Turcsányi et al. [44]:

1

where σ_c and σ_m are yield stresses of the composite and the matrix, respectively; φ is the filler volume fraction; and B is an adhesion parameter describing the interfacial adhesion between the matrix and the filler. Interfacial adhesion is considered as weak for B < 1, moderate for values between 1 and 3, and strong when B > 3 [37]. Following the approach presented in Ngugi et al. [45] study, the relative tensile strength (Q) can be defined based on Eq. 1 as

2

3

The natural logarithm of Q was plotted against the volume fraction using the Eq. 2, and the results are presented in Fig. 5. The B values for each sample were determined from the slopes of the fitted lines adopting the Eq. 3.

Fig. 5.

Interfacial adhesion (B) between matrices and nanosized reinforcing particles

The slopes in Fig. 5 indicate that CDW-reinforced composites had B > 3, suggesting strong interfacial adhesion between the filler and matrix, whereas the nanoclay-reinforced composite showed only moderate adhesion (1 > B > 3). In particular, the strongest adhesion between matrix and filler was obtained with CDW-reinforced recycled matrix composite (B = 5.6), followed by the CDW-reinforced virgin PP matrix composite (B = 3.9), and the nanoclay-reinforced recycled matrix composite (B = 1.6).

A steep slope in Fig. 5 indicates strong adhesion between the recycled matrix and CDW particles, even though SEM micrograph (Fig. 4f) revealed cavities left by dislodged objects. However, it is not certain whether these cavities originate from CDW particles, waste impurities, or polymer droplets, as similar cavities were also observed in the neat matrix (Fig. 4d). This suggests that these cavities are more likely to originate from waste impurities or polymer droplets rather than poorly bonded CDW particles. In the virgin PP composite, a strong adhesion was confirmed by SEM observations (Fig. 4c), and the fitted line in Fig. 5 showed excellent agreement with the experimental data. Since no compatibilizers were used in composite preparation, the strong polymer–particle bonding is attributed primarily to mechanical anchoring.

These observations suggest that the equation proposed by Turcsányi et al. [44] accurately describes the interfacial adhesion behavior of virgin polymer composites reinforced with waste-derived filler. However, it should be noted that several issues hinder the efficiency of this approach, especially when recycled materials are involved. First, the processing temperatures used in this study remained below the melting point of PET. As a result, PET particles likely acted as rigid fillers rather than as part of the polymer blend, as previously reported for such blend [46]. In addition, filler agglomeration creates structural discontinuities. These defects act as stress concentrators and disrupt effective load transfer between the matrix and the reinforcing particles. Consequently, the composite may exhibit a reduced yield stress that does not originate solely from interfacial debonding but also from the agglomeration or solid PET particles.

Furthermore, nanoclay has a lower density (320 kg/m³) than the CDW powder (480 kg/m³), which results in a lower volume fraction (φ) at equal mass fractions. Since the interfacial parameter B is influenced by the volume fraction, a higher density can contribute to an increased B. Lastly, matrix-related aspects might hinder the comparison of results in Fig. 5, as there are differences in the mechanical characteristics of neat matrices. Particularly, virgin PP is stiffer and stronger than the recycled blend, which results only minor relative improvements when fillers are added. This difference is directly reflected in the B parameter, which is defined as the ratio of yield stresses of the composites and neat matrices. Despite these limitations, the parameter B can still serve as a useful indicator of the interfacial adhesion in composites, although it cannot be used directly to compare matrices with different mechanical characteristics.

Mechanical properties

The stress–strain curves of the composites prepared from recycled plastic blend and virgin PP incorporated with various CDW powder and nanoclay loadings (1, 2.5, and 5 wt%) are presented in Fig. 6 and the results are listed in Table S1. The steeper initial slopes in Fig. 6 indicate improved stiffness with the increased loading of CDW powder in both matrices. Moreover, the incorporation of fillers gradually reduced the ductility of each composite. While the reduction in ductility with increased filler loading in the virgin PP matrix was clearly visible from the curves, the effect of filler inclusion on the recycled LDPE/HDPE/PP/PET blend was evident only as the elongation at maximum force (Fig. 7c). It must be noted that the recycled matrix samples did not reach complete failure during tensile testing, as the test was terminated once the tensile force had decreased by 30% from its maximum value. This behavior is characteristic of these polymers and their blends, with PP composites exhibiting brittle fractures while the recycled polymer blend samples demonstrated more ductile deformation.

Fig. 6.

Stress–strain curves of composites at various filler loadings: a rPB reinforced with CDW powder, b rPB reinforced with nanoclay, c virgin PP reinforced with CDW powder, and d vPP and rPB matrices reinforced with 5 wt% of CDW powder and nanoclay

Fig. 7.

Tensile properties of a–c rPB and d–f vPP matrices reinforced with CDW powder and nanoclay at different filler loadings: a, d ultimate tensile strength, b, e elastic modulus, and c, f elongation at maximum loading

These stress–strain curves suggest that for the CDW-filled composites, the initial yielding occurred within the matrix rather than at the filler–polymer interface, as indicated by the steeper slopes compared with the neat matrix. In contrast, nanoclay composites likely yielded at the filler–filler or filler–polymer interphases. This interpretation is supported by the more gradual initial slopes, which result from filler agglomeration and insufficient filler–polymer bonding, as demonstrated in Figs. 4e and 5.

Tensile strength

Figure 7a, d show both increasing and then decreasing trends in tensile strength for all composites. The neat recycled plastic blend and its CDW- and nanoclay-filled (1 wt%) composites exhibited tensile strengths of 16.3 ± 0.1 MPa, 16.8 ± 0.2 MPa, and 16.3 ± 0.1 MPa, respectively, indicating that the CDW powder provided more pronounced improvement (3.1%) compared to that of nanoclay (0.4%). Similarly, the tensile strengths of the vPP matrix and its CDW-reinforced (1 wt%) composite were 32.3 ± 0.4 MPa and 32.4 ± 0.3 MPa, respectively, corresponding to 0.3% enhancement. As shown in Fig. 7a, d, the addition of CDW powder enhanced the strength of both matrices; however, the improvement was more pronounced in the recycled blend compared to that of virgin PP. The stronger reinforcing effect of CDW powder in the recycled matrix compared to the virgin one can be attributed to the heterogeneous nature of the recycled plastic blend, which likely enabled CDW particles to improve adhesion between immiscible polyolefin phases, thereby enhancing stress transfer. Additionally, CDW powder may have interacted more effectively with specific components present in the waste-origin matrix. The chain scission phenomenon, commonly occurring during mechanical recycling of plastics, may further explain the more pronounced reinforcing effect of CDW powder in the recycled matrix compared to that of virgin one. In particular, chain scission shortens polymer chains during recycling [47], which typically leads to deterioration in mechanical properties but also reduces viscosity, facilitating better filler dispersion [22]. Consequently, CDW powder was likely dispersed more effectively in the recycled polymer matrix than in the virgin matrix, resulting in a more pronounced improvement in tensile strength.

In contrast to CDW powder, the addition of nanoclay did not result in a comparable improvement in mechanical performance in the rPB matrix. A tensile strength increase of only 0.4% was obtained with 1 wt% nanoclay, while a 5 wt% loading resulted in a 4.5% decrease. This limited effectiveness is likely attributed to nanoclay agglomeration and insufficient dispersion within the matrix, as well as the lack of intercalation and exfoliation of nanoclay platelets, as evident from the SEM image (Fig. 4e). Alternatively, the better compatibility of CDW powder with the recycled polymer matrix may have contributed to its more pronounced reinforcing effect compared to that of achieved with nanoclay, which was as also suggested by the interfacial adhesion predictions presented in Fig. 5.

Figures 6 and 7a and d demonstrates that the highest ultimate tensile strength for all composites was achieved at 1 wt% filler loading, whereas further inclusion (2.5–5 wt%) did not yield additional improvements. This was likely caused by filler agglomeration, as observed in the SEM micrographs (Fig. 4). Such agglomeration weakens the composite structure by limiting stress transfer and creating internal stress points, which can lead to failure at lower stresses than those of unfilled matrices [37]. The tensile strength trend observed in this study, with an improvement at low loadings followed by reduction at higher loadings, is consistent with the findings of Nor Arman et al. [20], who investigated nanoclay-reinforced recycled plastic composites.

Two-way ANOVA confirmed that both filler type (CDW, nanoclay) and filler loading (1, 2.5, 5 wt%) had a statistically significant effect on tensile strength (p < 0.0001), with a strong interaction effect (p < 0.0001), as presented in Table 3. Moreover, filler type had the greatest effect (F(1,66) = 369.3, F_crit = 4.0) on tensile strength, followed by filler loading (F(2,66) = 67.1, F_crit = 3.1); however, both factors were highly significant, and their interaction was also significant (F(2,66) = 27.8, F_crit = 3.1, p < 0.0001). These results confirm that both filler type and loading level influenced tensile strength of resulting composites.

Table 3.

Two-way ANOVA results for recycled matrix composites considering the effect of filler type (CDW powder, nanoclay) and filler loading levels (1, 2.5, 5 wt%)

Factors	Ultimate tensile strength			Elastic modulus
	F	Fcrit	p	F	Fcrit	p
Filler type	369.3	4.0	1.33E-29	5.1	4.0	0.03
Filler loading	67.1	3.1	1.26E-16	36.0	3.1	2.6E-11
Interaction between two factors	27.8	3.1	1.71E-09	4.6	3.1	0.01

Elastic modulus

Elastic modulus of all composites showed an increasing trend with increasing filler loading, as shown in Fig. 7b, e, which is consistent with previous studies on recycled plastics reinforced with nanoclay [16, 19, 20]. The neat recycled plastic blend and its CDW- and nanoclay-filled composites (5 wt%) exhibited elastic modulus of 0.745 ± 0.011 GPa, 0.834 ± 0.023 GPa, and 0.838 ± 0.023 GPa, respectively. Similarly, the neat vPP matrix and its CDW-reinforced composite were 1.63 ± 0.023 GPa and 1.83 ± 0.062 GPa, respectively. At 5 wt% filler loading, all composites showed an approximately 12% increase in stiffness. However, the enhancement trend with increasing filler loading was most consistent for the CDW-filled composites, as illustrated in Fig. 7b, e. This suggests good compatibility between the matrices and CDW-filler, as well as sufficient filler dispersion. The observed enhancement in elastic modulus is primarily attributed to the presence high-stiffness nanoparticles, which require greater load to deform polymer chains, thereby reducing their mobility [16, 37] and enabling more efficient stress transfer at the matrix–filler interface [48].

Consistent with the observation in Fig. 7b, e, a two-way ANOVA confirmed that filler loading had a statistically significant effect on elastic modulus (F(2,66) = 36.0, F_crit = 3.1, p < 0.0001), as presented in Table 3. These results indicate that filler loading is the dominant factor influencing stiffness, while the effect of filler type is smaller but still significant (F(1,66) = 5.1, F_crit = 4.0, p = 0.03). Moreover, the interaction between the two factors was statistically significant (F(2,66) = 4.6, F_crit = 3.1, p = 0.01), indicating that both filler loading and filler type contributed to the elastic modulus of the resulting composites.

Elongation

Increased filler loading gradually decreased the elongation of all composites, as shown in Figs. 6 and 7c, f. The neat recycled matrix and its CDW- and nanoclay-filled composites (5 wt%) exhibited elongations at maximum loading of 3.3 ± 0.2 mm, 2.8 ± 0.2 mm, and 3.0 ± 0.2 mm, respectively, corresponding to reductions of approximately 18% and 9%. Similarly, the elongations of the neat vPP matrix and its CDW-reinforced composite were 1.8 ± 0.1 mm and 1.3 ± 0.1 mm, respectively, indicating a reduction of approximately 27%. These findings are in a good agreement with previous studies on nanoclay [18] and ceramic-waste-reinforced [35] recycled matrix nanocomposites, which have reported a reduction in composite ductility upon increasing nanofiller loadings.

The reduced ductility is a result of the restricted mobility of polymer chains, caused by the presence of filler particles that hinder the flow of polymer molecules past one another during tensile loading [16, 35]. For the CDW-reinforced recycled matrix composites, a moderate decrease in elongation was observed, with a reduction of 6% at 1 wt%, and 16% at 5 wt%. By contrast, the virgin representative exhibited a more pronounced decline at higher loading, with a 6% reduction at 1 wt% and 27% at 5 wt%, likely due to differences in matrix characteristics.

The addition of nanoclay into the recycled matrix exhibited both increasing and decreasing trends in elongation depending on filler loading, as illustrated in Fig. 7c, which aligns with the findings of Nor Arman et al. [20] and Chen et al. [16]. In general, an increase in elongation at higher filler loadings may indicate proper nanoclay dispersion [20], improved adhesion between the immiscible polyolefins in the blend matrix [16], or alternatively variability arising from inconsistent mixing, the last two of which are most likely occurred.

Overall, the CDW powder demonstrated its effectiveness as a reinforcing filler. However, the limited enhancement in tensile strength and elastic modulus was likely influenced by the inefficient melt compounding equipment used, which resulted in insufficient particle dispersion and agglomeration in all samples. In particular, the single-screw injection molder was not effective enough to disperse the particles uniformly. A similar conclusion was reached by Ngugi et al. [37], where melt compounding of recycled PP with quarry dust waste led to insufficient dispersion using a single-screw extruder. Furthermore, the limited adhesion observed in SEM images between polymers in the recycled matrix, CDW powder, and nanoclay was likely due to their inherent incompatibility. To enhance this adhesion, the use of a compatibilizer or surface treatment for CDW particles could be considered.

Conclusion

In this study, a heterogenous recycled plastic blend (rPB; LDPE/HDPE/PP/PET) and inorganic construction and demolition waste (CDW) powder were incorporated into composites at various filler loadings (1, 2.5, and 5 wt%) to enhance the mechanical performance of recycled plastics. No compatibilizers were used in the fabrication of these composites, and virgin PP and commercial nanoclay were used as reference materials. Morphological analysis showed that the porous CDW particles were mechanically anchored to both polymer matrices, although insufficient dispersion was also observed due to the lack of effective compounding. In contrast, nanoclay-filled composites exhibited only moderate polymer–particle bonding, as well as agglomeration and tactoid structures. Mechanical testing further demonstrated the effectiveness of CDW powder in these matrices; the addition of only 1 wt% improved the tensile strength of the rPB and vPP matrices by 3.1% and 0.3%, respectively. For nanoclay composites, a maximum increase of 0.4% in tensile strength were observed, suggesting limited polymer–particle adhesion. This was further supported by a theoretical model predicting interfacial adhesion between the matrices and reinforcing particles, which indicated stronger polymer–particle adhesion for CDW particles than nanoclay. Statistical analysis using two-way ANOVA (n = 12, α = 0.05) confirmed that both filler type (CDW powder, nanoclay) and loading level had a significant effect on tensile strength (p < 0.0001), while elastic modulus was primarily influenced by the filler loading. In particular, increased filler loadings gradually improved the elastic modulus of all composites by approximately 12%, while effectively reducing ductility. Overall, these findings demonstrate that the fine mineral fraction of CDW is a promising waste-derived alternative to conventional reinforcing fillers for use in the polymer industry and plastics recycling.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Ida Rasilainen: Writing—original draft, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization. Ville Lahtela: Writing—review and editing, Supervision. Timo Kärki: Writing—review and editing, Conceptualization, Data curation, Supervision, Project administration, Funding acquisition.

Funding

This study was funded by the Rethinking Plastics in a Sustainable Circular Economy (PlastLIFE) project (LIFE21-IPE-FI-PlastLIFE). The PlastLIFE project was co-funded by the European Union. Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.

Data availability

Data will be available upon reasonable request.

Declarations

Ethics approval and Consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.