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Article

Initiative to Increase the Circularity of HDPE Waste in the Construction Industry: A Physico-Mechanical Characterization of New Sustainable Gypsum Products

by
Manuel Álvarez
1,
Daniel Ferrández
1,*,
Alicia Zaragoza-Benzal
1 and
Bryan Colorado-Pastor
1,2
1
Departamento de Tecnología de la Edificación, Escuela Técnica Superior de Edificación de Madrid, Universidad Politécnica de Madrid, Avda. Juan de Herrera, 6, 28040 Madrid, Spain
2
Facultad de Arquitectura y Urbanismo, Universidad de Guayaquil, Guayaquil 090514, Ecuador
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(2), 478; https://doi.org/10.3390/app14020478
Submission received: 12 December 2023 / Revised: 1 January 2024 / Accepted: 2 January 2024 / Published: 5 January 2024
(This article belongs to the Special Issue Challenges for Sustainable Building: Innovation, Development and Characterisation of New Material Products and Systems)
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Abstract

:
The annual production of plastic waste worldwide has doubled in just two decades, with approximately 390 million tonnes of plastic waste now being generated. In this context, the construction industry must move towards the development of new, more sustainable materials made under circular economy criteria. In this work, a physico-mechanical characterisation of gypsum composites with the incorporation of high-density polyethylene (HDPE) waste, replacing 2–4–6–8–10% by volume of the original raw material, has been conducted. The results show how the incorporation of these plastic wastes improves the water resistance of the gypsum material without additions, as well as producing a decrease in thermal conductivity and greater resistance to impact. On the other hand, it has been found that, as the percentage of recycled raw material added increases, the mechanical resistance to bending and compression decreases, leading to fracture due to a lack of cohesion between the matrix and the waste. Nevertheless, in all the cases studied, mechanical strengths higher than those established by the EN 13279-2 standard were obtained. Thus, the results confirm the viability of these secondary raw materials to be used in the development of new products for sustainable building, especially in the design of prefabricated panels for false ceilings.

1. Introduction

Plastics are used for almost all physical applications, and their production has a strong economic impact worldwide due to these countless applications [1]. According to current figures, approximately 400 million tonnes of plastics are produced worldwide [2], which leads to the generation of massive quantities of solid waste that can remain for hundreds of years until they are completely assimilated by the environment [3]. However, despite these problems being well known in industry, only 9% of the total plastic generated is recycled. Almost 79% of this waste is placed in landfills without any kind of process for recycling, recovery, or revalorisation [4]. In this international context, Europe is the second largest producer of plastics (after China), the building sector being the second largest consumer of these raw materials (19.8% of the total) [4]. For this reason, it is necessary to incorporate circular economy models that promote reuse and encourage industrial symbiosis, recovering inert waste as secondary raw materials to promote the development of sustainable construction products [5].
This work focuses on the recovery of high-density polyethylene (HDPE) waste for use in the production of plaster composites. One of the most-used plastics in construction for pipes, ducts, or other utensils is HDPE, the production of which generates approximately 101 Mt of greenhouse gases [6]. Recovered shredded HDPE waste has been widely used as a lightweight backfill material for embankment construction or for landfill drainage layers [7]. Another widespread option is the use of the pyrolysis technique to decompose these wastes, aiming to recover them in different industrial processes [8].
HDPE presents certain advantages over other plastics in building products due to its physical properties, which include a low bulk density between 950 and 970 kg/m3, a tensile strength of approximately 20–35 MPa, and a relatively high ignition temperature of 487 °C [9]. It is also a chemically inert material, which suits with its incorporation as an aggregate in construction binders [10]. As a result, it has been used as fine aggregate in the production of lightweight concretes, where materials with good thermal behaviour are obtained, along with a lower workability as the HDPE content increases for the same water/cement ratio and a progressive decrease in compressive strength as the percentage of natural aggregate replaced by plastic waste is increased [11,12,13]. The use of HDPE has also been extended to the production of bituminous asphalt mixtures for civil engineering, obtaining a correct hardening of the mixture, as well as mechanical and wear resistance suitable for use in road construction [14].
However, no applications of HDPE waste for use as a secondary raw material in the production of gypsum or plaster-based materials have been found in the literature. Gypsum composites have been positioned as an environmentally viable alternative for the development of prefabricated building products [15]. In this sense, plaster is a kind of gypsum with higher purity, whiteness, and fineness of grind [16]. It is also a binder material with excellent hygrothermal regulation properties [17] and a compressive strength directly proportional to its surface hardness [18]. It is also an ideal coating for the interiors of dwellings and has good adhesion capacity on ceramic surfaces [19]. On the other hand, in addition to these properties, gypsum composites are a key element in moving towards greater industrialisation of construction through the production of plasterboards [20]. These prefabricated products mainly consist of a gypsum matrix with additives and reinforced with fibres, to which two layers of paper are adhered on the surface [21]. The development of these prefabricated parts allows savings in terms of logistical costs and execution time [22]. Moreover, a fantastic opportunity to incorporate recycled raw materials and move towards environmentally responsible economic growth can be achieved [23].
Regarding the use of plastic waste in gypsum composites, several researchers have addressed many possibilities in this field. Figure 1 shows different results found in the literature. The results present flexural and compressive mechanical strengths, as well as the final bulk density of the final composites.
As can be seen in Figure 1, plastic waste additions decrease mechanical flexural and compressive strengths, while achieving lighter composites. In all cases found, minimum strengths of 1 MPa and 2 MPa, as stated in the UNE-EN 13279-2 standard for flexural and compressive stresses, were exceeded [31]. On the other hand, this density reduction sometimes leads to an increase in the porosity of the material [24]. As these wastes have no water absorption capacity, some researchers have shown that the incorporation of these plastic wastes can have a positive impact on the durability of gypsum composites [25,32]. However, because of this lower final density of the hardened composites, thermal behaviour satisfactory results have also been obtained, for instance, EPS waste added to plaster materials, which reduces thermal conductivity by 61.9% compared to reference material [33].
This work aims to analyse the feasibility of HDPE waste for use in the manufacture of plaster composites. More specifically, the aim is to conduct a physical and mechanical characterisation of different composites made with the incorporation of HDPE as a partial replacement of the original plaster material. In this way, this research allows us to show the possibilities for the application of this waste to produce composite materials and to explore their possibilities for the development of prefabricated products in the construction sector.

2. Materials and Methods

This section describes both the raw materials used in the tests and the methodology followed for this work.

2.1. Materials

Raw materials used are shown in Figure 2.

2.1.1. Binder

To develop the gypsum products designed in this research, an E-35 fast-setting plaster type, supplied by Placo Saint Gobain (San Martín de Valdeiglesias, Spain), was used. This material meets the requirements of the UNE-EN-13279-1 standard for construction plasters [34]. It is a widely used material in building construction and its main characteristics include a high purity index (>90%), a thermal conductivity of 300 mW/m·K, and a fineness of grind [35]. Among other properties, it receives the A1 classification for reaction to fire granted by Spanish regulations [36]. These data and other relevant characteristics are shown in Table 1.

2.1.2. Water

The water used in this experimental campaign was Canal de Isabel II water (Madrid, Spain). This element has been successfully used in previous research [37]. Its properties include low chloride and sulphate content, a neutral pH, and a hardness of 25 mg/L CaCO3. As a result, the water is inert, soft, and classified as suitable for human consumption [38].

2.1.3. High-Density Polyethylene (HDPE)

Recycled HDPE aggregates from the company Repsol S.A. (Madrid, Spain) were used to replace the original plaster material [39]. This thermoplastic polymer is widely used in the manufacture of pipes, from the recycling of which the waste used in this research came. Table 2 shows the most relevant physical and mechanical properties of these polymeric materials, which have been supplied by the company.

2.2. Dosages and Sample Preparation

To conduct this study, original plaster material was progressively replaced by HDPE granular waste. This partial substitution was conducted in percentages of 2–4–6–8–10% by volume, as shown in Table 3. The nomenclature used for naming the samples is as follows: P0.65-(%), where P refers to the binder (Plaster); 0.65 refers to the water/plaster ratio by weight; (%) refers to the percentage of substitution of the original compound by recycled HDPE material.
The production process starts with the manual dry mixing of the plaster powder with the polyethylene residue. After that, the kneading process begins by pouring the material into water, following the guidelines set out in the UNE-EN-13279-2:2014 standard [31]. Once samples are hardened, they are kept for a week in laboratory conditions (temperature 23 ± 2 °C and relative humidity 50 ± 5%). Finally, the process concludes with placing the samples in a drying oven for 24 h at a temperature of 45 °C ± 1 °C.
The materials developed have been subjected to physical and mechanical characterisation tests, aiming to determine the viability of their use in prefabricated materials for building. Two series of three samples of each proposed dosage have been prepared. Table 4 shows the dimensions and tests to which the different composites have been subjected.

2.3. Experimental Programme

The experimental campaign has been divided into three parts: new materials developed physical characterisation, mechanical characterisation and study using scanning electron microscopy, and finally, a behaviour analysis of these materials for their application in the manufacture of ceiling panels. Where possible, the mathematical relationship found between the addition of HDPE and the evolution of the studied property has been indicated.

2.3.1. Physical Characterization Tests

In this section, the following physical characteristics of the composites have been analysed: bulk density, thermal conductivity, capillary water absorption, and total water absorption. These tests are described below.
The bulk density was determined according to the recommendations of the UNE 102042:2023 standard [44]. For this purpose, the ratio between the mass and volume of each sample was determined, having been previously dried in an oven for 24 h at a temperature of 45 °C ± 1 °C.
For the thermal conductivity test, three 24 × 24 × 2 cm3 plaster plates of each type of composite were used. The thermal conductivity coefficient was determined using the heat flow meter method (HFM). The test consists of placing the board between two conditioned spaces with a large temperature difference according to the standards [42]. Temperature was set at 60 °C inside and 18 °C outside. Once these temperatures are reached, the sample remains at test conditions until it reaches a steady state. At this point, the thermal conductivity coefficient is measured for 30 min, taking two measurements per minute. With these results, the final value is extracted using the median. This test was conducted in the physics laboratory of the Escuela Técnica Superior de Edificación de Madrid.
For the capillary water absorption test, RILEM RC 25-PEM standard [40] indications were followed, positioning the 4 × 4 × 16 cm3 samples vertically on a 1 cm-high grid placed in a container. The container is filled with water until it reaches the top of the grid. After five minutes, water height is marked on the sample. From this point on, the height is marked every minute until 15 min. The final height reached shows the greater or lesser absorption of water by capillary action in the composites, reflecting the absorption rate.
Finally, the total water absorption coefficient is conducted according to UNE-EN 520 standard [41]. For this test, samples must be completely immersed in a container with water for a minimum period of two hours. Once this has been done, they are weighed repeatedly to obtain the total amount of water absorbed until the difference between two consecutive measurements is less than 0.1 g. Total water absorption percentage is determined by the following expression:
Total   Absorption = m f − m 0 m 0 × 100
where m 0 is the initial mass of the dried sample in grams and m f is the final mass of the sample after immersion in water.

2.3.2. Mechanical Characterization Tests

For mechanical characterisation, the following tests have been conducted: surface hardness, dynamic modulus of elasticity, flexural and compressive strength, and finally, scanning electron microscopy (SEM).
Surface hardness, which reflects the resistance of the developed composites to surface scratching, was determined with the help of a Shore C hardness tester [26]. For this purpose, 4 × 4 × 16 cm3 samples were used. Five measurements were taken on two faces.
For dynamic modulus of elasticity determination, a 50 kHz-frequency Matest C368 time ultrasonic machine was used. Once the ultrasound propagation speed waves in samples were obtained, it was determined by means of the following expression:
M O E = ρ ( 1 + μ ) ( 1 − 2 μ ) v u s 2 ( 1 − μ )
where MOE is the modulus of longitudinal elasticity, ρ is the density of the tested plaster composite, μ is the Poisson’s ratio of the material, and v u s   is the propagation velocity of the ultrasonic wave.
For flexural and compressive strength tests, samples of 4 × 4 × 16 cm3 were used, following UNE-EN-13279-2 standard [31] recommendations. This test was conducted using a hydraulic press model AUTOTEST 200-10SW from IBERTEST (Madrid, Spain) [45]. First, a three-point bending test was performed on samples using a loading speed of 10 N/s until break. Once the samples were rotated in two parts, they were assessed until failure in the compression modulus of the press, applying a loading speed of 20 N/s.
Finally, to complete the newly designed composite’s mechanical characterisation, a scanning electron microscopy (SEM) study was conducted. The objective of this SEM technique was to understand the mixture behaviour of the HDPE and binder. To conduct these microscopies, a Jeol JSM-820 microscope operating at 20 kV was used; samples were previously coated with a thin layer of gold with a metalliser to ensure the conductivity of the electrons [33].

2.3.3. Precast Tests

Finally, the technical feasibility of the developed gypsum composites for use in precast panels for ceilings was analysed. Simple bending and dynamic surface hardness tests were conducted on plates. A novel prototype construction system for its application was designed.
Bending test on plates were conducted using a Proeti, S.A. testing machine. Tests were conducted with 40 × 30 × 1.5 cm3 plates, analysing whether they exceeded the minimum load of 0.18 kN required by UNE-EN 14246: 2007 [43].
A dynamic surface hardness test was conducted by determining the footprint diameter a steel ball leaves on the plate surface when dropped from a given height. For this purpose, at least five measurements were taken. A Ø50 mm steel ball was dropped from 50 cm [43].
Finally, the design of a plate ceiling and a discussion of its implementation possibilities have been included to complement this subsection. The discussion and critical reviews are qualitatively based on the results obtained in the different tests using a rosette chart that shows these new composites’ possibilities and the potential markets where they can be used.

3. Results and Discussion

In this section, the results obtained are described. These discussions are based on the results obtained in other current research related to the work conducted.

3.1. Physical Characterization Results

Figure 3 shows the results for thermal conductivity and its relation to bulk density.
In Figure 3, it can be seen how incorporating HDPE residue as a partial replacement produces a reduction in both bulk density and thermal conductivity values compared to the P0.65 reference material. Thus, it can be observed that bulk density in the P0.65-10% sample has decreased by up to 7.35% compared to the reference material. A similar behaviour was observed by Romero-Gómez et al., by incorporating polypropylene residues in substitution percentages of 2.5–5–7.5–10% by weight [25]. In the same way, thermal conductivity with the largest amount of recycled raw material is 0.19 W/m·K, representing a 26.7% reduction compared to the P0.65 sample. This improvement in the thermal behaviour of plaster composites with the addition of plastic waste has already been observed by previous researchers, such as Pedreño-Rojas et al., who, by incorporating polycarbonate waste, managed to reduce gypsum composites conductivity by up to 32% [28], or Vidales-Barriguete et al., who incorporated shredded cable waste, obtaining conductivities close to 0.23 W/m·K [16]. Finally, it should be noted that, in all physical properties, a linear trend has been observed in bulk density and thermal conductivity decrease as the HDPE content in the plaster composites increased.
The results derived from the behaviour study of this material under water action are presented. It can be said that gypsum composites have a low durability against the action of water, as has been observed in previous research [46,47]. However, in recent decades, plastic residues have been used to reduce the water absorption capacity of these composites, improving traditional gypsum material performance [32]. For this reason, the water absorption capacity of the developed materials has been analysed. These results are presented in Figure 4.
Firstly, Figure 4 shows how capillary height reached at the end of the test decreases as the samples’ HDPE content increases. These results agree with those obtained by Romero-Gómez et al. when incorporating granular polypropylene residues in gypsum composites in percentages such as those used in this research [25]. The lowest height reached was obtained in the P0.65-10% composite, being 4.8 cm after 10 min, which represents a decrease of 41.5% compared to the P0.65 sample. On the other hand, total water absorption was also reduced, because of the hydrophobic behaviour of the recycled HDPE. This absorption percentage was reduced up to 23.1% compared to the reference plaster in composites with higher recycled raw material content. The observed behaviour followed a parabolic trend, with absorption capacity decrease being sharper after the addition of 6% HDPE waste. This decrease in absorption capacity has been observed by other researchers who have worked with the addition of recycled plastic aggregates, such as López-Zaldívar et al., who observed a reduction in water absorption in gypsum composites using recycled rubber aggregates [48].

3.2. Mechanical Characterization Results

The results obtained from the mechanical characterization tests are shown below. Figure 5 shows the tests performed.
Figure 6 shows the results obtained for the flexural strength test and the dynamic Young’s modulus.
As can be seen in Figure 6, there is a progressive decrease in flexural strength as the amount of recycled material added increases. Thus, a substitution of up to 10% of the original plaster material by HDPE waste has produced in the composites a decrease in flexural strength of approximately 34.0% compared to the P0.65 sample. This decrease in strength has followed a linear behaviour in the tested composites, although, for all the substitution percentages analysed, the minimum strength set by the standard of 1 MPa has been exceeded. This decrease in flexural strength is also reflected in the decrease in dynamic modulus of elasticity, as has been observed in previous studies using recycled raw materials [49]. In any case, the mechanical flexural strength values obtained show the possibility of applying these gypsum composites to produce new prefabricated products in construction. Other authors, who have used plastic waste from CDs and DVDs [15], or shredded extruded polystyrene waste [50], observed a similar decrease in flexural strength as the amount of added plastic material increased. Figure 7 shows the results obtained for compressive strength and surface hardness.
Surface hardness represents resistance to surface scratching, which is quite important when defining their future applications. Figure 7 shows how surface hardness decreases as the percentage of HDPE incorporated increases. The most unfavourable decrease occurred for the P0.65-10% sample, being 14.3% lower than the P0.65 reference sample. Authors such as Del Rio et al. have found a progressive decrease in surface hardness as recycled material content in gypsum composites increases, which agrees with the results obtained [51]. Compressive strength has also experienced a linear decrease as a higher percentage of HDPE aggregates is introduced into gypsum matrix. Pedreño-Rojas et al. observed similar behaviour when incorporating polycarbonate waste into the gypsum composite matrix [52]. This reduction in compressive strength is attributed to a higher porosity [53]. In this sense, P0.65-10% has experienced a decrease in strength of 24.2% compared to the reference sample. However, as occurred in the flexural strength test, all composites exceeded the minimum compressive strength of 2 MPa.
To complete the mechanical characterisation, a scanning electron microscopy study was conducted. This SEM analysis allows one to determine the mixture and connection characteristics between added HDPE plastic waste and the plaster matrix. The images obtained for the P0.65-6% sample are shown in Figure 8.
In Figure 8, the characteristic dihydrate crystals formed in the matrix can be seen. These acicular crystals are generated during the setting process [54]. On the other hand, some pores generated in the matrix because of the water evaporation from the mixing process can be observed in Figure 8a. Regarding the matrix–residue interface, an irregular adhesion between the HDPE and the plaster material is observed. It is possible to see the cracks that may cause plastic aggregate disintegration [25]. This irregular cohesion is related to the lower compressive strength in those samples containing this recycled raw material, as observed by other authors [55]. Finally, Figure 8b shows some dihydrate crystals formed on the HDPE residue surface.

3.3. Precast Samples Results

The final section of the experimental campaign includes a study conducted on prefabricated plates for ceilings. The aim is to evaluate the newly developed composites’ behaviour under the stresses they will be under when applied. The results for plate flexural strength and impact hardness are shown in Figure 9.
Figure 9 shows how the flexural strength in the plates decreases as the HDPE waste content increases, in accordance with the tests performed on standardised RILEM samples shown in Figure 6. UNE-EN 14246 standard requires a minimum strength of 0.18 kN to guarantee the optimum performance of these products [43]. It can be observed in the tests conducted on 4 × 4 × 16 cm3 samples that all composites exceeded the standard requirements. However, when used in 40 × 30 × 1.5 cm3 format, the P0.65-10% composites did not exceed the minimum value required by the standards (0.16 kN < 0.18 kN). This is a limit for the incorporation of these wastes in plaster materials, since, as in other mechanical properties, a progressive decrease in flexural strength is experienced as the HDPE aggregate content increases. This decrease in strength has been observed in previous research [57] and it has been necessary to incorporate reinforcing fibres to increase the ductility of the composites produced.
Figure 9 also shows that the diameter of the footprint after the dynamic hardness test is also reduced as the HDPE waste content increases. This is a positive effect, as it implies that the incorporation of this recycled plastic material allows the reduction of the impact caused by the 50 mm steel ball. The P0.65-8% sample, which contains the most HDPE waste amount suitable for precast elaboration, reduced the footprint by up to 12.3% compared to P0.65. This effect has been observed in previous research that has used waste plastics to elaborate prefabricated panels [58].
The plaster material’s main advantage is its versatility and application possibilities [57]. It is a workable material with an aesthetic finish, which makes it an optimal solution for creating different configurations in building [59]. The results obtained in the designed materials in this research makes it possible to create specific configurations that provide other benefits, such as aesthetic vision and the possibility of combining them with other elements [60].
Thus, the design and development of a new ceiling panel with the materials studied as the main element is proposed. Figure 10 shows an axonometric view of the proposed system, with a deeper development of the anchoring system and possible configurations in Figure 11a,b, respectively.
The system shown in Figure 10 and Figure 11 allows the user an adjustable adaptation, with this system being easy to execute, showing a typical application for the composites developed in this work. However, the possibilities for adaptation are multiple and can be explored to develop new sustainable construction systems, elaborated under circular economy criteria. In this sense, this proposal is in line with the recommendations of the European Green Deal [61], trying to expose novel solutions that are aligned with the European objectives for the sustainable development of the construction industry.

3.4. Studied Properties Qualitative Discussion

A qualitative classification of the different lightweight composites developed has been included. To this end, they have been classified according to the results obtained, thus constructing the graph shown in Figure 12, aiming to determine the possible application fields of each material included in this research.
If only the mechanical characterisation tests are considered, Figure 12 shows a progressive decrease in these properties as the recycled content increases. However, composites with higher HDPE residue content showed better properties for building applications. It should be highlighted that the P0.65-10% composite would be optimal for use in situations requiring higher thermal insulation; however, its flexural strength did not exceed the limits of the standard. Regarding this point, it should be taken into consideration that prefabricated plasterboards for false ceilings usually have an external paper sheet and sandwich-type configurations, which gives them greater mechanical resistance [62]. For this reason, it is proposed as future research work to explore the possibilities of the application of these lightened plaster materials by incorporating plastic waste, with and without external reinforcement, analysing their properties and contrasting the results with the requirements of current regulations.

4. Conclusions

In this work, a feasible method for the reincorporation of HDPE waste aggregates in the design of new sustainable building materials has been presented. To this end, the design of different plaster composites has been proposed, with a progressive substitution of the original raw material by this recycled plastic material. The results show that it is possible to apply these composite materials to prefabricated building material development. The most relevant conclusions can be listed as follows:
  • Progressive HDPE waste incorporation allows a reduction in both thermal conductivity and final bulk density. In this sense, reductions of up to 26.7% and 7.35% have been achieved for each of these properties, respectively.
  • A decrease in the water absorption capacity of the gypsum composites is observed as the amount of added plastic residue increases. In this sense, the P0.65-10% sample managed to reduce the height reached by the water after the capillary test by up to 23.1%.
  • Regarding mechanical strength, a linear decrease in flexural and compressive strength, as well as surface hardness, was observed as more HDPE waste was added. Nevertheless, all the tested composites exceeded the minimum strength values required by UNE-EN 13279-2. However, after SEM analysis, irregular adhesion between the HDPE aggregate and the plaster matrix was revealed, as the recycled materials used did not undergo any prior surface treatment.
  • Finally, in the plates study, all samples exceeded the flexural strength limit of 0.18 kN, except for the composite with 10% HDPE residue added. These results highlight the importance of conducting full-scale tests to better understand the performance of the materials developed.
The main limitations of this research, which in turn serve to propose future lines of work, are twofold. Firstly, it would be interesting to conduct an economic study together with a carbon footprint analysis of the developed products, which would allow us to delve into the possibilities of the commercial exploitation of these gypsum materials produced under circular economy criteria. On the other hand, this work has not considered fire resistance and sound absorption tests, which would be interesting to set in accordance with European regulations, which would allow us to define these materials’ application possibilities.

Author Contributions

Conceptualization, D.F.; methodology, B.C.-P. and D.F.; software, D.F. and M.Á.; validation, D.F., M.Á. and A.Z.-B.; formal analysis, D.F. and M.Á.; investigation, A.Z.-B. and B.C.-P.; resources, D.F.; data curation, D.F.; writing—original draft preparation, D.F. and M.Á.; writing—review and editing, A.Z.-B.; visualization, A.Z.-B.; supervision, D.F.; project administration, D.F.; funding acquisition, D.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank Almudena Ochoa Mendoza and Evangelina Atanes-Sánchez from Escuela Técnica Superior de Ingeniería y Diseño Industrial de Madrid for supplying the recycled HDPE aggregates uses in this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Mechanical strength and bulk density of gypsum composites with plastic waste: Reference is a plaster with a water/gypsum ratio of 0.7 by weight [24]; PP is polypropylene waste from coffee capsules [25]; EPS and XPS are expanded and extruded polystyrene waste, respectively [18]; Pipe foam insulation shredded in average diameter of 2.5 mm [26]; Plastic cable is shredded waste from low voltage cable sheaths [27]; Polycarbonate [28]; CD and DVD are shredded and screened waste [15]; LDPE is recycled low density polyethylene [29]; ELT-rubber is granulated rubber from disused tyres of average diameter 2.5 mm [30]; SAP (superabsorbent polymer) is 15 g additions of sodium polyacrylate [24].
Figure 1. Mechanical strength and bulk density of gypsum composites with plastic waste: Reference is a plaster with a water/gypsum ratio of 0.7 by weight [24]; PP is polypropylene waste from coffee capsules [25]; EPS and XPS are expanded and extruded polystyrene waste, respectively [18]; Pipe foam insulation shredded in average diameter of 2.5 mm [26]; Plastic cable is shredded waste from low voltage cable sheaths [27]; Polycarbonate [28]; CD and DVD are shredded and screened waste [15]; LDPE is recycled low density polyethylene [29]; ELT-rubber is granulated rubber from disused tyres of average diameter 2.5 mm [30]; SAP (superabsorbent polymer) is 15 g additions of sodium polyacrylate [24].
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Figure 2. Raw materials used: (a) HDPE; (b) Plaster (E-35).
Figure 2. Raw materials used: (a) HDPE; (b) Plaster (E-35).
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Figure 3. Results for thermal conductivity and bulk density.
Figure 3. Results for thermal conductivity and bulk density.
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Figure 4. Results of the hygroscopic properties analysed: total water absorption test and capillary water absorption test.
Figure 4. Results of the hygroscopic properties analysed: total water absorption test and capillary water absorption test.
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Figure 5. Mechanical characterisation tests: (a) determination of dynamic modulus of elasticity, (b) Shore C surface hardness, (c) flexural strength, and (d) compressive strength.
Figure 5. Mechanical characterisation tests: (a) determination of dynamic modulus of elasticity, (b) Shore C surface hardness, (c) flexural strength, and (d) compressive strength.
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Figure 6. Flexural strength and dynamic modulus of elasticity by ultrasonics.
Figure 6. Flexural strength and dynamic modulus of elasticity by ultrasonics.
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Figure 7. Compressive strength and Shore C surface hardness test results.
Figure 7. Compressive strength and Shore C surface hardness test results.
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Figure 8. Images obtained by SEM technique on sample P0.65-6%: (a) 500× magnifications; (b) 1000× magnifications.
Figure 8. Images obtained by SEM technique on sample P0.65-6%: (a) 500× magnifications; (b) 1000× magnifications.
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Figure 9. Results for plate bending strength and impact toughness [56].
Figure 9. Results for plate bending strength and impact toughness [56].
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Figure 10. Plate designed axonometric view.
Figure 10. Plate designed axonometric view.
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Figure 11. (a) Anchorage detail: 1, Plate; 2, Shaft; 3, Gear; 4, Anchorage system; 5, Plate fixing system; 6, Slab, 7, Rack; 8, Synthetic bar reinforcement. (b) Plate designed possible configurations, where the plate has a position with respect to the horizontal of (i) 0°; (ii) 30°; (iii) 45°, y; (iv) 90°.
Figure 11. (a) Anchorage detail: 1, Plate; 2, Shaft; 3, Gear; 4, Anchorage system; 5, Plate fixing system; 6, Slab, 7, Rack; 8, Synthetic bar reinforcement. (b) Plate designed possible configurations, where the plate has a position with respect to the horizontal of (i) 0°; (ii) 30°; (iii) 45°, y; (iv) 90°.
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Figure 12. Qualitative classification of the different compounds evaluated according to the results obtained in the tests conducted.
Figure 12. Qualitative classification of the different compounds evaluated according to the results obtained in the tests conducted.
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Table 1. Technical characteristics of the binder (Plaster type E-35).
Table 1. Technical characteristics of the binder (Plaster type E-35).
StandardThermal Conductivity (mW/m·K)Water Vapour Diffusivity (µ)Pureness Index (%)Flexural Strength (MPa)
UNE-EN 13279-13006>90>3
ColourFire Reaction According to CTE DB-SI [37]Granulometry (mm)Water/Plaster Rate by WeightpH
WhiteA10–0.2 mm0.7–0.8>6
Table 2. HDPE properties.
Table 2. HDPE properties.
AppearanceParticle Size (mm)Hardness (Units Shore D)Tensile Strength (MPa)Tensile Modulus (MPa)
Granulated1.47031.71.38
Elongation at Break (%)Bending Strength (MPa)Bending Modulus (GPa)Compressive Strength (MPa)Compressive Modulus (GPa)
40031.71.231.70.689
Table 3. Dosages used: quantities by weight and by volume.
Table 3. Dosages used: quantities by weight and by volume.
SampleWeight (g)Volume Percentage (%)
E-35WaterHDPEE-35WaterHDPE
P0.651000.0650.0—60.6039.40—
P0.65-2%980.2636.814.559.4038.602.00
P0.65-4%960.4623.629.058.2037.804.00
P0.65-6%940.6610.443.557.0037.006.00
P0.65-8%920.8597.258.055.8036.208.00
P0.65-10%901.0584.072.554.6035.4010.00
Table 4. Summary table with samples produced and list of tests and standards.
Table 4. Summary table with samples produced and list of tests and standards.
SeriesDimensionsTestsStandard
SERIES I4 × 4 × 16 cm3Shore C HardnessUNE-EN 13279-2 [31]
Flexural strength
Compressive strength
SERIES II4 × 4 × 16 cm3Capillarity water absorptionRILEM RC 25-PEM [40]
Total water absorptionUNE-EN 520 [41]
SERIES III24 × 24 × 2 cm3Thermal conductivityUNE-EN 1946-3 [42]
UNE-EN 14246 [43]
SERIES IV30 × 40 × 1.5 cm3Plates pure bending testUNE-EN 14246 [43]
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Álvarez, M.; Ferrández, D.; Zaragoza-Benzal, A.; Colorado-Pastor, B. Initiative to Increase the Circularity of HDPE Waste in the Construction Industry: A Physico-Mechanical Characterization of New Sustainable Gypsum Products. Appl. Sci. 2024, 14, 478. https://doi.org/10.3390/app14020478

AMA Style

Álvarez M, Ferrández D, Zaragoza-Benzal A, Colorado-Pastor B. Initiative to Increase the Circularity of HDPE Waste in the Construction Industry: A Physico-Mechanical Characterization of New Sustainable Gypsum Products. Applied Sciences. 2024; 14(2):478. https://doi.org/10.3390/app14020478

Chicago/Turabian Style

Álvarez, Manuel, Daniel Ferrández, Alicia Zaragoza-Benzal, and Bryan Colorado-Pastor. 2024. "Initiative to Increase the Circularity of HDPE Waste in the Construction Industry: A Physico-Mechanical Characterization of New Sustainable Gypsum Products" Applied Sciences 14, no. 2: 478. https://doi.org/10.3390/app14020478

APA Style

Álvarez, M., Ferrández, D., Zaragoza-Benzal, A., & Colorado-Pastor, B. (2024). Initiative to Increase the Circularity of HDPE Waste in the Construction Industry: A Physico-Mechanical Characterization of New Sustainable Gypsum Products. Applied Sciences, 14(2), 478. https://doi.org/10.3390/app14020478

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