# Thermal properties of recycled plastics waste/hemp shive composites

Lara Aldaou, Nabil Issaadi, Nordine Leklou, Ouali Amiri

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Lara Aldaou, Nabil Issaadi, Nordine Leklou, Ouali Amiri. Thermal properties of recycled plastics waste/hemp shive composites. Journal of Building Engineering, 2025, 108, pp.112885. 10.1016/j.jobe.2025.112885. hal-05502077

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# Thermal properties of recycled plastics waste/hemp shive composites

Lara Aldaou*, Nabil Issaadi, Nordine Leklou, Ouali Amiri

Nantes Université, Ecole Centrale Nantes, CNRS, GeM, UMR 6183, F-44600, Saint Nazaire, France

## ARTICLE INFO

**Keywords:**

Plastics waste

Thermal conductivity

Density

Composite materials

Hemp shive

## ABSTRACT

This research aims to investigate the feasibility of valorizing recycled plastic aggregates in hemp concrete mixtures such as high-density polyethylene (HDPE), polyvinyl chloride (PVC), Caou-tchouc Aggregates (CG), polyurethane (PUR), and Polypropylene-Polyethylene (PE/PP) by replacing hemp shives in hemp concrete with recycled plastic aggregate or by adding these types of plastics to the hemp concrete formula and to study the effects of replacing and adding per-centage on the thermal performance. The composite material presented in this work was made from a mixture of hemp shives, two types of binders, and five different plastic types in a replacing percentage by mass of 10 %, 30 %, 50 %, and 70 % and adding percentage by mass of 30 % and 50 % of the hemp shives mass. A wide set of basic physical and thermal properties of the developed material is determined and their assessment is made using a comparison with reference hemp concrete formula. The thermal conductivity increases with the increasing amount of plastic aggregates compared to the reference material in the dry state; however, it provides evidence of greatly improved thermal insulating capabilities in the wet state.

This study demonstrates that plastic waste may successfully be valorized and used as a substitute for vegetal fibers at some ratio either by adding them to bio-composite materials. These new composites would seem to offer a low-cost material with coherent thermal properties; They would also help to solve certain solid waste problems created by the production of plastics and save energy.

## 1. Introduction

Plastic pollution has become a significant environmental challenge, requiring urgent and effective solutions to mitigate its impact [1,2]. One promising approach is the development of building materials derived from agricultural byproducts [3–5] and industrial waste [6], which are gaining popularity in civil engineering and architectural applications. This effort aligns with the principles of sustainable development, a concept introduced in the 1987 Brundtland Report, which emphasizes the need for innovative materials that are recycled or environmentally friendly [7]. This concept allows economic development to be linked to social development while respecting the environment. Today, an average French person produces 67 kg of plastic waste per year, of which only 26.5 % is recycled [8]. The incorporation of this waste to create innovative and cleaner materials is therefore a necessity to revalue this waste due to its economic and environmental benefits [9]. The global issue of plastic pollution has reached critical proportions, necessitating immediate attention and innovative solutions. As our planet grapples with the devastating consequences of plastic waste, it is

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Lara.aldaou@univ-nantes.fr (L. Aldaou).</footnote>
imperative to explore sustainable alternatives that can mitigate its profound environmental impact. Within this context, the fields of civil engineering and architecture have increasingly turned to eco-friendly building materials derived from agricultural resources and industrial waste [10].

There are indeed many types of plastics, present in every corner of the building [11]. Current techniques make it possible to recycle the majority of these materials. Their recycling is a major challenge in the fight against global warming. For this, better recovery of plastics is essential to address other environmental problems, such as underwater pollution and the accumulation of waste in the ocean [12].

So, recycling plastic waste to create new materials, such as cement composites and fiber composites, seems to be one of the best options for plastic waste disposal due to its economic and environmental benefits [13–21]. On January 16, 2018, the European Commission published its first European strategy on plastics, as part of the transition to a circular economy. In addition, France is one of the largest emitters of greenhouse gases in Europe with 418 MT CO2eq, according to statistics carried out in 2021 [22]. To address this issue, the French government has set ambitious goals, including the recovery of 100 % of plastic packaging by 2025, as outlined in the Energy Transition Law for Green Growth. This commitment was further reinforced with the publication of the Circular Economy Roadmap (FREC) on April 23, 2018 [23].

Furthermore, In February 2021, the European Parliament endorsed the New Circular Economy Action Plan, urging the Commission to allocate funds for circular initiatives and propose binding EU targets for 2030 to reduce material footprints, consumption, and set specific recycled content targets [24]. These figures do not include the fossil energy needed to generate electricity. As a result, scientists are advising to take control of greenhouse gas emissions, low-carbon economy, development of renewable resources, and technological change [25]. Moreover, thermal insulation is crucial for lowering overall energy consumption in buildings and facilitating the utilization of unconventional renewable energy sources to support sustainable development to minimize energy losses and greenhouse gas emissions [26].

In essence, our research aspires to bridge the substantial gap between the urgent need for effective plastic waste management and the burgeoning demand for sustainable construction materials. By investigating various avenues for the valorization of these plastic wastes, we have chosen the most studied bio-based materials in the literature, which is hemp concrete, as the base for our study, we aim to contribute to the development of cleaner, environmentally responsible solutions that align with the principles of sustainable development. Through experimentation and analysis, we seek to unlock the thermal insulation potential of these materials, heralding a new era of construction practices that are both ecologically sound and economically viable meanwhile, most insulation materials nowadays have a negative impact on the environment [27].

Hemp concrete serves as the foundational reference point for our research. Its documented benefits in thermal properties are well-established, making it an exemplary choice for sustainable construction. This natural composite material, derived from the inner woody fibers of the hemp plant combined with lime and water, boasts remarkable thermal performance attributes [28–32]. These include effective insulation and moisture regulation, which contribute to maintaining comfortable indoor environments while reducing energy consumption. The gradual absorption and release of moisture further enhance indoor air quality and prevent mold growth [33]. Such qualities firmly establish hemp concrete as a sustainable construction material, aligning seamlessly with the principles of eco-conscious building practices. By employing hemp concrete as our reference material, we aim to harness its proven advantages as a benchmark while exploring how the integration of recycled plastics can further enhance its properties. By doing so, we provide a comprehensive solution to the dual challenges of managing plastic waste and promoting sustainable construction.

The importance of addressing plastic waste as part of sustainable construction is underscored by the significant role it plays in solid waste composition. Within this rapidly evolving sector, the construction industry recognizes the potential for growth through the utilization of recycled plastics. These plastics encompass various types, omnipresent within the construction landscape, including Polyvinyl Chloride (PVC), constituting 50 % of plastics used in construction (found in applications such as tunnel membranes and swimming pool coverings), 5 % polyurethane foam (used for insulation), 15 % High-Density Polyethylene (HDPE) for pipes and conduits, and 20 % Polypropylene (PP), among others [34–36]. Although current techniques facilitate the recycling of a majority of these materials, their true value remains limited, especially within the context of France [8]. The recycling of plastics assumes significant importance in the context of mitigating global warming, as recycled plastic materials emit considerably fewer CO2 emissions than their non-recycled counterparts [37]. Therefore, valorizing plastic in concrete for construction materials has emerged as a promising avenue for sustainable development. Recent studies have shown encouraging results in this realm, indicating the potential for plastic waste to be transformed into valuable resources for the construction industry [38,39].

The primary focus of this research lies in exploring the feasibility of valorizing recycled plastic aggregates by either replacing them with traditional hemp fibers in hemp concrete or adding them to it. Our selection encompassed a range of plastic aggregates, including representatives from different polymer families such as thermoplastic elastomers and rigid plastics [40]. These aggregates include Polyethylene High-Density (HDPE), Polyvinyl Chloride (PVC), Caoutchouc (GC), Polyethylene/Polypropylene (PE/PP) and Polyurethane (PUR). This approach allowed us to comprehensively analyze the performance of each plastic type within the composite material. The central objective of this study is to scrutinize how varying percentages of plastic replacement influence the thermal performance of the resulting composite material. Specifically, this research delves into replacement percentages of 10 %, 30 %, 50 %, and 70 %, shedding light on the potential of these novel materials to offer sustainable, eco-conscious alternatives within the realm of construction. Additionally, the thermal behavior and long-term durability of construction materials are significantly affected by moisture absorption, which alters their water content. Hence, we are investigating the relationship between thermal conductivity and water content variation [41]. This involves conducting a series of laboratory experiments and analyses to assess both the thermal conductivity and water absorption characteristics of the composite materials.

Numerous studies have investigated the incorporation of recycled plastics into construction materials. For instance, Parra et al. [17]
<table><caption>**Table 1**
Main characteristics of the hemp shives.</caption><thead><tr><th>Characteristics of Hemp shives</th><th></th></tr></thead><tbody><tr><td>Skeleton density</td><td>1380 kg/m³</td></tr><tr><td>Particle density</td><td>256 kg/m³</td></tr><tr><td>Particle length</td><td>2-25 mm</td></tr><tr><td>Particle width</td><td>0.5-8 mm</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Samples of different types of plastic (a) HDPE (b) PVC (c) PE/PP (d) GC (e) PUR.</figcaption></figure>

and Patnaik et al. [18] analyzed the structural and thermal behavior of concrete and insulation materials with plastic and fiber waste. Other works, such as Liu et al. [13] and Lei et al. [21], have focused on mechanical properties of HDPE and PP composites. However, few studies have systematically assessed the thermal insulation properties of hemp concrete incorporating multiple types of recycled plastic, both as substituting and adding aggregates, under controlled proportions. This research aims to fill that gap by comparing five distinct plastic waste types and two binders, providing a unique insight into the trade-offs between thermal performance, density, and sustainability.

The novelty of this work lies in the comparative evaluation of both addition and substitution methods for integrating recycled plastic aggregates into hemp concrete. Moreover, by incorporating a diverse array of plastic types (HDPE, PVC, PE/PP, PUR, GC) and using two binders (cement and lime), this research provides a comprehensive understanding of how various plastic families affect the thermal performance of bio-based composites.

In this work composite based on recycled plastic aggregates and hemp shive were prepared by mixing procedure and the effect of adding/substituting and coupling these agents on the thermal performance of the resultant composite was investigated. The first results presented in this study focus on composites developed through the substitution of hemp shives with recycled plastic aggregates. The study first presents results related to the substitution method, followed by the addition method in Section 3.3.3, always maintaining a comparison with the substitution approach to highlight the differences in density and thermal conductivity.

## 2. Materials and methods

### 2.1. Materials

#### 2.1.1. Hemp shiv

The hemp shiv is a part of the hemp stem. More precisely, it is the woody part of this stem. The hemp shiv used in our material is a commercial shiv, obtained after mechanical defibration, dust removal and grinding of the stem. This hemp shives were provided by the CHANVRIBAT company. Its properties are detailed in Table 1.
<table><caption>**Table 2**

Characteristics of the plastic waste aggregates used in this study.</caption><thead><tr><td>Plastic waste aggregates</td><td>Particles size (mm)</td><td>Apparent Volumic Mass (kg/m³)</td><td>Real Volumic Mass (kg/m³)</td><td>Water absorption (%)</td></tr></thead><tbody><tr><td>PVC</td><td>4-0.08</td><td>640</td><td>1175</td><td>3.9</td></tr><tr><td>PE/PP</td><td>5-0.08</td><td>385</td><td>918</td><td>25.92</td></tr><tr><td>Caoutchouc</td><td>2-0.08</td><td>574</td><td>1192</td><td>4.8</td></tr><tr><td>HDPE</td><td>8-1.18</td><td>455</td><td>945</td><td>3.65</td></tr><tr><td>PUR</td><td>4-0.08</td><td>65</td><td>240</td><td>53</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Size grading of the plastic aggregates used in this study.</figcaption></figure>

<table><caption>**Table 3**

Cement chemical composition.</caption><thead><tr><td>Components</td><td>Mass%</td></tr></thead><tbody><tr><td>Calcium oxide (CaO)</td><td>45.00</td></tr><tr><td>Silica(SiO<sub>2</sub>)</td><td>33.00</td></tr><tr><td>Alumina (Al<sub>2</sub>O<sub>3</sub>)</td><td>9.5</td></tr><tr><td>Ferric oxide (Fe<sub>2</sub>O<sub>3</sub>)</td><td>0.88</td></tr><tr><td>Potassium oxide (K<sub>2</sub>O)</td><td>0.56</td></tr><tr><td>Magnesium oxide (MgO)</td><td>5.8</td></tr><tr><td>Sodium oxide (Na<sub>2</sub>O)</td><td>0.22</td></tr><tr><td>Sulphuric anhydride (SO<sub>3</sub>)</td><td>2.7</td></tr><tr><td>Loss on ignition</td><td>1.4</td></tr></tbody></table>

### 2.1.2. Plastics waste aggregates

The recycled plastic aggregates used in this study were provided by specialized industrial partners: WESTPLAST (HDPE, PVC, and PE/PP) (Fig. 1(a), (b) and (c)), HXA (recycled Caoutchouc rubber aggregates, GC(Fig. 1(d))), and EFFICITE (recycled polyurethane, PUR) (Fig. 1(e)). These suppliers carried out the initial sorting of plastic waste by polymer type using a combination of mechanical and manual methods, including flotation and density separation. Each fraction was then visually inspected to ensure consistency and homogeneity.

The physical properties of the plastic aggregates namely particle size range, apparent and real volumic mass, and water absorption were determined through a series of laboratory experiments. The testing procedures were performed in accordance with standard methods apparent and real volumic mass were determined using a precision pycnometer method ISO 12154:2014 [42]. Water absorption capacity was measured. These physical properties of the plastic aggregates are detailed in Table 2, while their particle size distribution curves are illustrated in Fig. 2. Particle size analysis was performed in accordance with the standard sieving method NF EN 933-1 [43], using a series of mesh sizes: 0.08 mm, 0.425 mm, 0.85 mm, 1.18 mm, 1.7 mm, 2.0 mm, 3.35 mm, 5.0 mm, and 8.0 mm.

### 2.1.3. Binders

#### 2.1.3.1. Cement.
The cement used in this study is CEMIII/C 32,5N SR PM NF. Its density is 2,94 g/cm³ and its specific area is 4647 cm²/g. It is constituted of 85 % slag and 15 % clinker. This cement exhibits a significantly lower carbon footprint compared to
<table><caption>**Table 4**
Lime composition.</caption><thead><tr><th>Lime Composition</th></tr></thead><tbody><tr><td>Hydrated lime 75 % that contains 98 % of Ca(OH)<sub>2</sub></td></tr><tr><td>Hydraulic binder 15 %</td></tr><tr><td>Pozzolanic binder 10 %</td></tr></tbody></table>

<table><caption>**Table 5**
Mix proportions of composite with incorporated plastic waste for the formula with substitution percentage.</caption><thead><tr><th>Mix</th><th>Mass (g)<br>Binder</th><th>Hemp</th><th>Water</th><th>Plastic waste</th></tr></thead><tbody><tr><td>Ref</td><td>205</td><td>105</td><td>330</td><td>0</td></tr><tr><td>10 %</td><td>205</td><td>94.5</td><td>330</td><td>10.5</td></tr><tr><td>30 %</td><td>205</td><td>73.5</td><td>330</td><td>31.5</td></tr><tr><td>50 %</td><td>205</td><td>52.5</td><td>330</td><td>52.5</td></tr><tr><td>70 %</td><td>205</td><td>26.5</td><td>330</td><td>78.7</td></tr></tbody></table>

<table><caption>**Table 6**
Mix proportions of composite with incorporated plastic waste for the formula with adding percentage.</caption><thead><tr><th>Mix</th><th>Mass (g)<br>Binder</th><th>Hemp</th><th>Water</th><th>Plastic waste</th></tr></thead><tbody><tr><td>Ref</td><td>205</td><td>105</td><td>330</td><td>0</td></tr><tr><td>30 %</td><td>205</td><td>105</td><td>330</td><td>31.5</td></tr><tr><td>50 %</td><td>205</td><td>105</td><td>330</td><td>52.5</td></tr></tbody></table>

<table><caption>**Table 7**
Formulations nomenclature.</caption><thead><tr><th>Nomination</th><th>Meaning</th></tr></thead><tbody><tr><td>F</td><td>Formula</td></tr><tr><td>C or ch</td><td>With Cement or with lime</td></tr><tr><td>PVC</td><td>With polyvinyl chloride aggregates</td></tr><tr><td>HDPE</td><td>With high-density polyethylene aggregates</td></tr><tr><td>PE/PP</td><td>With Polypropylene-Polyethylene aggregates</td></tr><tr><td>GC</td><td>With Caoutchouc Aggregates</td></tr><tr><td>PUR</td><td>With Polyurethane aggregates</td></tr><tr><td>10/30/50/70</td><td>10 % of substitutions/30 % of substitutions/50 % of substitutions/70 % of substitutions</td></tr><tr><td>30A or 50A</td><td>30 % of adding/50 % of adding</td></tr><tr><td>SUB</td><td>Substitutions</td></tr><tr><td>AJ</td><td>Adding</td></tr></tbody></table>

traditional Portland cement [44]. Its chemical composition is given in Table 3.

2.1.3.2. Lime "Tradical PF70". The binder used for our material is the Tradical PF70. This binder is mainly composed of hydrated lime. Its precise composition is detailed in Table 4 and its density is 0.65 g/cm³.

It is a commercial binder specially designed to make hemp concrete. The hemp concrete obtained can then be used for different applications depending on the formulation chosen: wall, lining, screed, roofing. Several formulations are given by the manufacturer in its technical sheet that enables to obtain the best final properties of hemp concrete for each targeted application [45].

## 2.2. Methods

The material developed is based on the formulation of hemp concrete [29], in which hemp shives are replaced by plastic aggregates or added to it. However, given the diversity of plastics and binders that can be used in this composition, a selection of materials has been made. In this study, we have limited our choice to two binders: lime and CEM III/C. We also selected five types of plastic: PVC, HDPE, PE/PP, PUR, and GC. For this purpose, we prepared different composites using each type of plastic and binder. Reference samples without plastic aggregates were also prepared. The mass proportions of hemp shive substitution were 10%, 30%, 50% and 70%. The substitution and addition percentages mentioned in this section are calculated relative to the mass of hemp shives in the reference mix, and not to the total mass of the composite. The proportions of each ingredient are summarized in Table 5. This
<figure><img src="image_5.png" /><figcaption>Fig. 3. Example of different type of formulations with PVC aggregates.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Working diagram.</figcaption></figure>

identification was crucial in guiding further studies towards formulations with optimal thermal and compacity properties. The second type of formulation involves the addition of plastics at 30 % and 50 % (Table 6). In order to illustrate the identification of our formulas, here is an example of their name meaning (e.g., FCPVC30: hemp formulation with 30 % substitution by PVC plastic mixed with cement). The formulation nomenclature is listed in Table 7 and Fig. 3 illustrates an example of the specimen. In order to present in a structured way, the different steps undertaken during this research, a succinct working diagram is presented in Fig. 4. The first results presented in this study focus on the substitution method, where hemp shives are progressively replaced by recycled plastic aggregates. Subsequently, the addition method is introduced in Section 3.3.3. In this second part, the results obtained with added plastics are systematically compared with those obtained through substitution, in order to highlight the differences between the two approaches.

### 2.2.1. Preparation of plastic/hemp composite and curing

The hemp concrete formulation studied corresponds to the "wall" application according to the binder's technical specifications [45]. Samples were cast in the form of rectangular prisms measuring 40 × 40 × 160 mm³ and stored for 72 h in a room at 50 ± 5 % relative humidity and 23 ± 2 °C. The samples were then demolded and left to cure for 28 days [3].

Sample drying is a crucial preparatory step before measuring thermal conductivity. The moisture content of samples affects their heat conduction properties due to sorption/desorption phenomena in response to relative humidity. To ensure accurate measurements, drying is conducted according to standard NF P 75-101 [46]. To protect the materials from thermal shock and possible deformation caused by plant particles such as hemp residue, it is preferable to maintain a temperature of 50 °C.

### 2.2.2. Workability

A material's workability is an essential characteristic that influences its ease of processing and handling. To assess this property, a commonly used measurement is made using a maniabilimeter following the by the norme NF P18-452 [47]. The maniabilimeter is a standardized device used to quantify the fluidity and flowability of a material, particularly concrete mixes, mortars or other cementitious pastes. It measures the distance traveled by a sample subjected to a certain level of energy when placed on a vibrating table.

### 2.2.3. Density

Density represents the relationship between the mass and volume of the specimen. Mass measurements were conducted using a precise analytical balance with a readability of 0.01 g and a linearity of 0.01 g. For each dimension (length, width, height), the average was derived from four values measured with a high-precision electronic caliper accurate to 0.01 mm. The density measurement was performed on prismatic specimens (160 × 40 × 40 mm³), which were oven-dried at 50 °C until reaching a constant mass. A mass variation of less than 0.1 % between two consecutive weighings, taken at 24-h intervals, was considered indicative of mass stability
<figure><img src="image_7.png" /><figcaption>Fig. 5. Equipment used for the experimental characterization: Hotdisk TPS 1500.</figcaption></figure>

<table><caption>**Table 8**
Workability of formulations made with Lime.</caption><thead><tr><th rowspan="2">Substitution percentage</th><th colspan="5">Flow time</th></tr><tr><th>PE/PP</th><th>GC</th><th>HDPE</th><th>PVC</th><th>PUR</th></tr></thead><tbody><tr><td>Ref</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>10 %</td><td>&gt;2,5min</td><td>&gt;2,5min</td><td>&gt;2,5min</td><td>&gt;2,5min</td><td>&gt;2,5min</td></tr><tr><td>30 %</td><td>30 s</td><td>24 s</td><td>11,39 s</td><td>29 s</td><td>&gt;2,5min</td></tr><tr><td>50 %</td><td>18,86 s</td><td>2,3 s</td><td>10 s</td><td>2,6 s</td><td>30 s</td></tr><tr><td>70 %</td><td>1,7 s</td><td>1,5 s</td><td>3,13 s</td><td>1 s</td><td>15s</td></tr></tbody></table>

[46].

### 2.2.4. Thermal analysis

The measurement of thermal conductivity is done using the Hot Disk method according to the ISO 22007-2:2022 [48]. This method is based on the principles of the two-sensor thermal probe technique. This experimental approach is employed for the rapid determination of a material's thermal conductivity.

The Hot Disk TPS 1500 apparatus comprises a flat circular sensor, also known as a disk, which is brought into direct contact with the material sample. The disk is equipped with two integrated temperature sensors, placed at a fixed distance from each other. A sinusoidal electrical current is applied to the disk, resulting in heat generation. The temperature sensors measure temperature differences induced by the heat flow through the sample.

By utilizing temperature data and the geometric characteristics of the disk, the software associated with the Hot Disk TPS 1500 (Fig. 5) calculates the thermal Properties of the sample by leveraging mathematical advancements introduced by Gustavson et al. [49]. Through these calculations, thermal conductivity, thermal diffusivity, and heat capacity can be accurately determined. The initial result for heat capacity is provided in MJ/(m³.K) and is subsequently converted to the standard heat capacity unit, J/(kg. K), by dividing the result by the calculated sample density.

### 2.2.5. Thermal conductivity at different water content

The evolution of the thermal conductivity as a function of water content was examined for composite formulations comprising polyvinyl chloride (PVC), polyurethane (PUR), and polyethylene propylene (PEPP) at two varying concentrations 30 % and 50 % through both addition and substitution methodologies. First, a subsequent evaluation was conducted to assess the thermal characteristics of these composite materials in a dry state (0 % water content). To ensure consistent drying and stabilization, the specimens were dried in an oven set at 50 °C, adhering to the guidelines outlined in standard NF P 75-101 [46]. Following this phase, a controlled amount of water was introduced to the specimens by pulverization, simulating real-world environmental conditions in accordance with the ISO 10051:1996 standard [50-52]. This step was instrumental in examining the effects of water content on thermal conductivity. The specimens were then encapsulated in plastic film and maintained at a controlled temperature for three days. Upon completion of the conditioning period, the specimens were meticulously extracted from the film, and their mass was accurately measured to determine the absorbed water content. Subsequent thermal conductivity assessments were conducted using the hot disk method immediately after each moisture conditioning phase to detect any changes in thermal performance attributed to water absorption. This procedure was repeated four times, with each cycle involving the addition of water, followed by thermal conductivity testing, three days of plastic film encapsulation, and subsequent water content measurement. The collected data underwent rigorous analysis to determine the effects of incorporating recycled plastic aggregates and to ascertain the influence of the binder on the thermal properties of the composites under wet conditions.
<table><caption>**Table 9**
Workability of formulations made with cement.</caption><thead><tr><th rowspan="2">Substitution percentage</th><th colspan="5">Flow time</th></tr><tr><th>PE/PP</th><th>GC</th><th>HDPE</th><th>PVC</th><th>PUR</th></tr></thead><tbody><tr><td>Ref</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>10 %</td><td>12 s</td><td>10 s</td><td>&gt;2,5min</td><td>5,7 s</td><td>3,46 s</td></tr><tr><td>30 %</td><td>1,53 s</td><td>2,1 s</td><td>1,5 s</td><td>1,66 s</td><td>1,4 s</td></tr><tr><td>50 %</td><td>3,28 s</td><td>1,2 s</td><td>0,4 s</td><td>3,9 s</td><td>0,6 s</td></tr><tr><td>70 %</td><td>1 s</td><td>0,8 s</td><td>&lt;0,1 s</td><td>&lt;0,1 s</td><td>&lt;0,1 s</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Density in dry conditions for specimen with (a) PUR particles (b) PVC particles (c) HDPE particles (d) GC particles (e) PE/PP particles.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. Heat capacity for different values of density.</figcaption></figure>

# 3. Results and discussions

## 3.1. Workability

Upon analyzing the data presented in Tables 8 and 9 a distinction between formulations containing cement and those containing lime becomes evident. Cement-based formulations exhibit shorter setting times compared to lime-based formulations, leading to deduce that cement-based samples are more workable than their lime-based counterparts. This observation finds an explanation in lime's high adhesion capacity, contributing to a slower setting process. Furthermore, it is noteworthy that the formulation containing polyurethane (PUR) exhibited the longest flow time among the formulations. This phenomenon may be linked to the initial water absorption characteristics for each plastic, as indicated in Table 1. Additionally, for formulations involving the incorporation of plastic by adding process, it was observed that all formulations had a flow time exceeding 2,5 min.

## 3.2. Density of composites obtained by the substitution method

Fig. 6 present the bulk density values at the dry state of composite materials. In the case of the reference formulation devoid of plastic, the obtained results exhibit a correlation with findings from alternative research [53]. The composite formulations exhibit bulk density values ranging from 412 to 570 kg/m3 for those incorporating lime and from 379 to 642 kg/m3 for those incorporating cement. The results indicate an upward trend in apparent density with an increasing percentage of substitutions. Particularly notable are the formulations featuring 70 % PE/PP and rubber substitutions, which exhibit elevated bulk density values. This phenomenon appears to be correlated with the lower density of hemp particles (Table 1) relative to the higher density of plastic particles (Table 2).

## 3.3. Thermal properties

### 3.3.1. Thermal capacity

The initial dry thermal capacity of the samples is depicted in Fig. 7. The heat capacity falls within the range of 515 J/kg.K to 1434 J/kg.K, with a median value of 852 J/kg.K and an average of 857 J/kg.K. There is a slight tendency for this value to increase with higher density. Nevertheless, there is notable variability among samples with similar density, indicating potential limitations in the precision of the characterization method specific to our study. This observation aligns with findings in Poupard et al., 2023 [3] and Gourlay et al., 2017 [54], where considerable deviation in specific heat capacity measurements for hemp concrete was noted using the Hot Disk method.

### 3.3.2. Thermal conductivity of composites obtained by the substitution method

Fig. 8 provides a comprehensive visualization of the intricate relationship between the thermal conductivity of composites and the degree of substitution for plastic aggregates in a dry state. Our meticulous analysis reveals a consistent and noteworthy trend: as the substitution rate increases, there is a discernible elevation in thermal conductivity. This phenomenon is applicable across both types of binders examined and extends to all plastic variants scrutinized in our study. A particularly intriguing revelation arises when focusing on formulations incorporating Polyurethane (PUR) (Fig. 8a) the thermal conductivity within this subset exhibits a remarkable range, falling between 0.14 and 0.19 W/mK. This finding is of paramount significance, as it positions PUR-based composites within a thermal conductivity spectrum closely aligned with that of the reference samples. This alignment underscores the potential of these formulations to mirror the thermal properties of established materials, a crucial consideration in evaluating their suitability for diverse applications. Some fluctuations observed in Fig. 8 can be explained by the combined influence of the type of plastic used and the
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Thermal conductivity of composites obtained by the substitution method in dry conditions for specimen with (a) PUR particles (b) PVC particles (c) HDPE particles (d) GC particles (e) PE/PP particles.</figcaption></figure>

percentage of substitution. In several cases, we noticed that increasing the substitution percentage did not significantly affect thermal conductivity. This is particularly true for PUR-based formulations, where thermal conductivity remained relatively stable even at higher substitution rates. This behavior is due to the intrinsically low thermal conductivity of PUR, which dominates the composite's thermal response regardless of its quantity. For other plastics, such as PVC or PE/PP, conductivity values may vary more with substitution level, depending on their density and interaction with the matrix. These results highlight that the nature of the plastic plays a more critical role than the percentage alone. Moreover, the natural variability of hemp-based composites can cause slight differences in thermal conductivity measurements, as observed in previous studies [3,53]. Therefore, the observed fluctuations are expected and do not compromise the reliability or the general trends of the results.

Furthermore, our investigation delves into the impact of density on thermal conductivity [53]. The observed positive correlation elucidates that an increase in density corresponds to a proportional rise in thermal conductivity. This observation holds critical implications for understanding how variations in material density can directly influence the overall thermal insulation potential of the
<figure><img src="image_11.png" /><figcaption>Fig. 9. Thermal conductivity vs density of the different materials of the literature compared to the studied materials.</figcaption></figure>

<table><caption>**Table 10**
Formulations mixed with Cement.</caption><thead><tr><th colspan="5">ADDING</th><th colspan="5">SUBSTITUTION</th></tr><tr><th>Formulation</th><th>λ (W/m K)</th><th>std</th><th>ρ (kg/m³)</th><th>std</th><th>Formulation</th><th>λ (W/m K)</th><th>std</th><th>ρ (kg/m³)</th><th>std</th></tr></thead><tbody><tr><td>FC0</td><td>0.140</td><td>0.015</td><td>378.8</td><td>2.2</td><td>FC0</td><td>0.140</td><td>0.015</td><td>378.8</td><td>2.2</td></tr><tr><td>FCPUR30A</td><td>0.122</td><td>0.004</td><td>344.7</td><td>7.9</td><td>FCPUR30</td><td>0.157</td><td>0.014</td><td>419.1</td><td>5.8</td></tr><tr><td>FCPUR50A</td><td>0.124</td><td>0.004</td><td>386.9</td><td>0.3</td><td>FCPUR50</td><td>0.157</td><td>0.011</td><td>416.7</td><td>6.0</td></tr><tr><td>FCPVC30A</td><td>0.149</td><td>0.007</td><td>412.3</td><td>4.2</td><td>FCPVC30</td><td>0.170</td><td>0.002</td><td>497.5</td><td>8.5</td></tr><tr><td>FCPVC50A</td><td>0.150</td><td>0.010</td><td>421.8</td><td>9.2</td><td>FCPVC50</td><td>0.191</td><td>0.006</td><td>573.6</td><td>10.5</td></tr><tr><td>FCPEPP30A</td><td>0.139</td><td>0.011</td><td>434.2</td><td>5.6</td><td>FCPEPP30</td><td>0.173</td><td>0.020</td><td>530.4</td><td>8.6</td></tr><tr><td>FCPEPP50A</td><td>0.149</td><td>0.004</td><td>390.8</td><td>6.0</td><td>FCPEPP50</td><td>0.197</td><td>0.009</td><td>571.0</td><td>9.5</td></tr><tr><td>FCHDPE30A</td><td>0.139</td><td>0.001</td><td>405.0</td><td>6.8</td><td>FCHDPE30</td><td>0.152</td><td>0.021</td><td>433.3</td><td>6.6</td></tr><tr><td>FCHDPE50A</td><td>0.165</td><td>0.012</td><td>400.4</td><td>9.0</td><td>FCHDPE50</td><td>0.205</td><td>0.003</td><td>552.7</td><td>8.3</td></tr><tr><td>FCGC30A</td><td>0.143</td><td>0.008</td><td>418.0</td><td>3.7</td><td>FCGC30</td><td>0.179</td><td>0.027</td><td>451.8</td><td>9.0</td></tr><tr><td>FCGC50A</td><td>0.146</td><td>0.005</td><td>424.5</td><td>7.8</td><td>FCGC50</td><td>0.201</td><td>0.003</td><td>575.2</td><td>9.7</td></tr></tbody></table>

studied composites. These findings are presented in the Appendix (Figure A1 and Figure A2).

Fig. 9 presents the thermal conductivity and density of the materials under investigation alongside comparative data from existing literature and known materials [55–63]. The thermal conductivity values of the materials studied were found to be in line with those reported in the literature, and lower than certain notable studies such as Durian, Concrete and clay, chrome shavings/buffing dust waste, clay bricks, cigarette butts, and some rubber composites [57–61]. Despite this, the observed thermal conductivity of the studied materials does not reach the levels observed in lightweight conventional insulation materials like EPS, XPS, and fiberglass. However, it is important to consider that these conventional insulation materials can contribute to environmental pollution [27].

In summary, our comprehensive analysis unequivocally establishes that the hemp-based materials derived from our research exhibit commendable thermal insulation properties. This discovery positions them as a compelling and environmentally sustainable alternative to conventional materials that often necessitate high energy consumption during their manufacturing processes. As the scientific community continues to navigate the landscape of sustainable construction materials, our findings advocate for the integration of these composites into mainstream construction practices, offering a promising pathway towards enhanced energy efficiency and reduced environmental impact. Additionally, it is worth noting that these results also point to an interesting avenue for the valorization of plastic waste.
<table><caption>**Table 11**
Formulations mixed with Lime.</caption><thead><tr><th colspan="5">ADDING</th><th colspan="5">SUBSTITUTION</th></tr><tr><th>Formulation</th><th>λ (W/m K)</th><th>std</th><th>ρ (kg/m³)</th><th>std</th><th>Formulation</th><th>λ (W/m K)</th><th>std</th><th>ρ (kg/m³)</th><th>std</th></tr></thead><tbody><tr><td>Fch0</td><td>0.12</td><td>0.014</td><td>412.3</td><td>6.4</td><td>Fch0</td><td>0.12</td><td>0.014</td><td>412.3</td><td>6.4</td></tr><tr><td>FchPUR30A</td><td>0.136</td><td>0.004</td><td>379.1</td><td>4.3</td><td>FchPUR30</td><td>0.152</td><td>0.025</td><td>433.7</td><td>5.1</td></tr><tr><td>FchPUR50A</td><td>0.130</td><td>0.006</td><td>329.9</td><td>3.4</td><td>FchPUR50</td><td>0.161</td><td>0.017</td><td>476.6</td><td>6.8</td></tr><tr><td>FchPVC30A</td><td>0.141</td><td>0.002</td><td>434.0</td><td>11.1</td><td>FchPVC30</td><td>0.173</td><td>0.012</td><td>395.4</td><td>5.5</td></tr><tr><td>FchPVC50A</td><td>0.149</td><td>0.005</td><td>429.0</td><td>2.0</td><td>FchPVC50</td><td>0.180</td><td>0.017</td><td>484.5</td><td>5.8</td></tr><tr><td>FchPEPP30A</td><td>0.147</td><td>0.012</td><td>449.0</td><td>8.1</td><td>FchPEPP30</td><td>0.153</td><td>0.020</td><td>371.5</td><td>9.2</td></tr><tr><td>FchPEPP50A</td><td>0.151</td><td>0.008</td><td>461.4</td><td>4.4</td><td>FchPEPP50</td><td>0.208</td><td>0.019</td><td>548.8</td><td>7.5</td></tr><tr><td>FchHDPE30A</td><td>0.153</td><td>0.012</td><td>437.8</td><td>3.8</td><td>FchHDPE30</td><td>0.175</td><td>0.005</td><td>400.0</td><td>8.8</td></tr><tr><td>FchHDPE50A</td><td>0.161</td><td>0.008</td><td>438.3</td><td>9.5</td><td>FchHDPE50</td><td>0.197</td><td>0.009</td><td>476.8</td><td>1.8</td></tr><tr><td>FchGC30A</td><td>0.151</td><td>0.010</td><td>441.2</td><td>6.9</td><td>FchGC30</td><td>0.185</td><td>0.004</td><td>352.1</td><td>3.2</td></tr><tr><td>FchGC50A</td><td>0.151</td><td>0.008</td><td>448.5</td><td>5.8</td><td>FchGC50</td><td>0.220</td><td>0.025</td><td>548.7</td><td>10.1</td></tr></tbody></table>

### 3.3.3. Density and thermal conductivity of composites produced by the addition method: comparison with the substitution method

In conjunction with the thermal conductivity analysis depicted in Tables 10 and 11, a comprehensive comparative study was conducted. These tables present a meticulous examination of the differences between formulations incorporating added plastic and those where plastic substitutes hemp shives. Notably, our findings illuminate a distinctive trend: Formulations with added plastic consistently exhibit lower thermal conductivity compared to those in which substitution is applied.

Table 10 illustrates thermal conductivity and density values for formulations containing 30 % and 50 % plastic aggregates, employing both adding and substituting methods. A comparative analysis is conducted with the reference mix (FC0), which includes cement and five types of plastics. Similarly, in Table 11, thermal conductivity and density values are presented for formulations with 30 % and 50 % plastic aggregates, utilizing adding and substituting methods. The study again includes a comparison with the reference mix (Fch0), this time incorporating lime and five types of plastics.

In the case of the adding method, the formulations exhibit lower thermal conductivity values when using both lime and cement as binders, compared to the substitution method. This difference may be attributed to the less pronounced increase in density observed with the adding method.

### 3.3.4. Evolution of the thermal conductivity in the function of the water content

Fig. 10 illustrates the evolution of thermal conductivity based on the water content of various composite formulations. It becomes evident that the thermal conductivity naturally rises with increasing water content in the specimens. This is because the progressive saturation of the materials leads to the gradual displacement of air pores by water. It's worth noting that water has a significantly higher thermal conductivity (0.54 W/m.K) compared to air (0.02 W/m.K) at the same temperature [64]. Consequently, as the materials become more humidified, their thermal conductivity increases.

Therefore, despite the increase in water content, the addition of plastic materials helps to maintain the thermal conductivity of the composites relatively lower compared to the reference curve referring to the hemp concrete formula (Fch0) which is not observed in the cement-based formula (FC0). This phenomenon could be attributed to the physical properties of plastic, which contain fewer pores than hemp shives, and the ability of the hemp shives to absorb water. The presence of fewer pores in plastic reduces the pathways for heat transfer, contributing to a lower overall thermal conductivity. Additionally, hemp shives capacity to absorb water might counterbalance the increase in thermal conductivity expected with higher water content, resulting in a less pronounced effect on the overall thermal conductivity of the composite materials [65]. Additionally, the enhanced adherence between lime binders and particles, along with lime's inherent resistance to water, may further contribute to the observed decrease in thermal conductivity. Furthermore, it is noteworthy that the formulation incorporating 30 % plastic by adding with lime as binders exhibits a reduced thermal conductivity compared to other formulations. This observation highlights the role of lime in reducing thermal conductivity and suggests that the plastic type does not significantly influence thermal conductivity. In terms of moisture, cement demonstrates a lower sorption and desorption capacity compared to lime, explaining why the FC0 curves fall between the plastic formulations curves. This phenomenon underscores the role of plastic in reducing the permeability of the mix.

This observation suggests that incorporating plastic aggregates at this specific percentage has a beneficial effect on the thermal
<figure><img src="image_12.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>(f)</figcaption></figure>

(caption on next page)
<caption>Fig. 10. Variation of thermal conductivity of the composites materials according to water content.</caption>

performance of the composite material. This phenomenon underscores the potential of using recycled plastic in construction materials to enhance their thermal properties.

## 4. Conclusion

The analysis centered on investigating the influence of integrating plastic waste aggregates (PUR, PVC, HDPE, GC, and PE/PP) into hemp concrete to contribute to the valorization of plastic waste. The experimental assessment resulted in the following findings.

* The presence of plastic waste aggregates significantly influenced the workability of fresh mixtures. An increase in the substitution rate of hemp shive with plastic aggregates resulted in a decrease in flow time. However, it is noteworthy that the addition of plastic did not adversely affect the overall workability of the mixtures.

* The incorporation of plastic waste led to a noticeable increase in the bulk density of the hemp concrete reference. This phenomenon can be attributed to the characteristics of plastic waste particles, which, upon integration, contribute to a higher overall mass per unit volume in the formulation matrix.

* By increasing the amount of plastic aggregates (PVC, HDPE, GC, and PE/PP), the thermal conductivity at the dry state showed a noticeable increase compared to the reference materials, likely due to the heightened density of the mixture. However, the addition of 50 % PUR waste aggregates resulted in **only an 8.33 % increase** in thermal conductivity, from 0.12 W/mK to 0.13 W/mK. This minimal increase is attributed to the unique properties of PUR, a plastic widely used in insulation applications, which inherently has low thermal conductivity and thus aligns closely with the performance of traditional materials.

* In a comparative analysis, the observation arises that formulations incorporating additional elements or components demonstrate a superior ability to maintain thermal properties when contrasted with formulations relying on substitution strategies. This implies that the introduction of supplementary materials positively influences the preservation of thermal characteristics within the formulated mixture. Furthermore, attention was drawn to the inadequacies of the Hot Disk method in accurately assessing thermal capacity.

* The addition of 30 % plastic materials in hemp concrete formulations can help mitigate the increase in thermal conductivity typically associated with higher water content. As a result, the presence of recycled plastic aggregates contributes to maintaining relatively lower thermal conductivity level regarding the reference Hemp formula, thereby enhancing the energy efficiency and sustainability of the composite materials.

Based on the empirical findings, it is deducible that this composite featuring waste plastic aggregates, as delineated in this research, offer a prospective resolution with regard to plastic waste disposal and concurrent amelioration of buildings' energy efficiency. The intricate ensemble of essential physical, and thermal properties delineated during the experimental scrutiny of the formulated materials also holds substantial utility for incorporation in the design methodology and evaluative studies pertaining to service life.

### CRediT authorship contribution statement

**Lara Aldaou:** Writing – original draft, Visualization, Methodology, Formal analysis, Conceptualization. **Nabil Issaadi:** Validation, Supervision, Methodology, Conceptualization. **Nordine Leklou:** Supervision, Methodology, Funding acquisition. **Ouali Amiri:** Supervision, Methodology.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors would like to thank Nantes University for their financial support. We also express our gratitude to the company WESTPLAST for providing the plastic materials.

## Appendix A. Comparative Analysis of thermal conductivity and density
<figure><img src="image_18.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>(e)</figcaption></figure>

**Fig. 1A.** Evolution of the thermal conductivity and the density in function of the percentage of substitution (a) PUR (b) PVC (c) HDPE (d) GC and (e) PE/PP.
<figure><img src="image_23.png" /><figcaption>Fig. 2A. Correlation between the density and the thermal conductivity for Formulations mixed with (a) lime and (b) cement.</figcaption></figure>

Data availability

No data was used for the research described in the article.

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