# Effect of recycled waste cardboard chips on the physico-thermal-hygric properties of hemp-lime biocomposites

Akli Younsi, Armelle Nouviaire, Rafik Belarbi

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Akli Younsi, Armelle Nouviaire, Rafik Belarbi. Effect of recycled waste cardboard chips on the physico-thermal-hygric properties of hemp-lime biocomposites. Results in engineering, 2026, 29, pp.109773. 10.1016/j.rineng.2026.109773. hal-05531606

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Research paper

# Effect of recycled waste cardboard chips on the physico-thermal-hygric properties of hemp-lime biocomposites

Akli Younsi<sup>a,*</sup> ◐, Armelle Nouviaire<sup>a</sup>, Rafik Belarbi<sup>a,b</sup>

## ARTICLE INFO

**Keywords:**

Recycled waste cardboard chips

Hemp-lime biocomposites

Sustainable building insulation materials

Density

Porosity

Thermal conductivity

Vapor permeability

## ABSTRACT

Buildings are major contributors to global energy consumption and environmental impacts, highlighting the critical need for sustainable and energy-efficient insulation materials. Biocomposites incorporating recycled cardboard have emerged as promising solutions for such applications; however, their production methods are often labor-intensive, energy-demanding, and time-consuming. To address these limitations, the present study explores the feasibility of using recycled waste cardboard chips (RWCC) as a partial or complete replacement for conventional hemp shives in hemp-lime biocomposites. A simplified, low-energy, and rapid processing methodology was developed in which untreated RWCC, cut directly from post-consumer cardboard, were incorporated into hemp-lime matrices without any pre-treatment. The RWCC were mixed using a rapid blending procedure and lightly compacted, thereby eliminating the need for grinding, crushing, soaking, defibrating, extended compaction, or thermal curing. Substitution ratios ranging from 0 to 100 wt% were investigated to assess physical, thermal, and hygric properties. The resulting biocomposites exhibited low densities (514–682 kg/m³) and high porosities (68.7–75.5%), leading to low thermal conductivity (0.0775–0.0919 W/(m·K)), high heat capacity (972–1036 J/(kg·K)), low thermal diffusivity (1.25–1.51 × 10⁻⁷ m²/s), and low thermal effusivity (200–248 J/(m²·K·s¹/²)), thereby enhancing both insulation performance and thermal inertia. Adequate vapor permeability (1.24–2.12 × 10⁻¹¹ kg/(m·s·Pa)) and moisture buffer capacity (0.66–1.59 g/(m²·%RH)) were also observed, supporting healthy indoor environments. Overall, the results demonstrate the feasibility of valorizing untreated RWCC in hemp-lime matrices, achieving properties comparable to those of established bio-based insulation materials while significantly reducing processing intensity.

## 1. Introduction

The 2024/25 Global Status Report for Buildings and Construction indicates that, in 2023, buildings accounted for 32% of global energy demand and 34% of global CO₂ emissions [1]. Reducing energy consumption in the building sector is therefore critical to lowering overall energy demand and advancing environmental sustainability, particularly through reductions in heating and cooling requirements [2]. This goal can be effectively achieved by improving thermal insulation, notably through the use of bio-based materials [3–5], such as hemp-lime biocomposites [6,7]. These materials provide excellent thermal [8–12] and acoustic insulation performance [13], while also offering additional benefits, including effective moisture regulation [9–11,14,15] and resistance to fire [8], pests, and mold [6,7]. Moreover, hemp-lime biocomposites are widely regarded as carbon-neutral and potentially

carbon-negative, primarily due to the substantial amount of CO₂ absorbed by hemp during its growth phase [6,7,16].

In addition to improving building energy efficiency through enhanced thermal insulation, environmental sustainability can be further promoted by integrating innovative recycling practices into insulation materials. The World Bank's 2018 report on solid waste management indicated that paper and cardboard accounted for approximately 17% of global solid waste, representing the second-largest waste fraction after food and green waste. The report also highlighted substantial variability in the proportion of recyclable materials, including paper, cardboard, plastic, metal, and glass, across income groups, ranging from approximately 16% of the municipal solid waste stream in low-income countries to nearly 50% in high-income countries [17].

Given the increasing emphasis on recycling [1,17], recent studies

<footnote><sup>a</sup> LaSIE UMR CNRS 7356, La Rochelle University, Avenue Michel Crépeau, 17042 La Rochelle Cedex 1, France</footnote>

<footnote><sup>b</sup> Department of Architecture, Canadian University Dubai, City Walk, Dubai P.O. Box 415053, United Arab Emirates</footnote>

<footnote>* Corresponding author at: La Rochelle University, LaSIE UMR CNRS 7356, Avenue Michel Crépeau, 17042 La Rochelle Cedex 1, France.
E-mail addresses: akli.younsi@univ-lr.fr (A. Younsi), armelle.nouviaire@univ-lr.fr (A. Nouviaire), rafik.belarbi@univ-lr.fr (R. Belarbi).</footnote>
have explored the incorporation of recycled waste cardboard, primarily composed of cellulose fibers derived from wood pulp [18], into building insulation materials. The following paragraphs present a selection of key works in chronological order, with particular emphasis on findings most relevant to the present study:

- Sair et al [19] investigated biocomposites prepared from dried, precut, and crushed cardboard and cork fibers, combined with natural gypsum and water. The materials were mixed for 10 minutes and compacted under a compression load for 48 h. The resulting thermal conductivity ranged from 0.062 to 0.076 W/(m·K), and the compressive strength, measured on 40 × 40 × 160 mm samples, ranged from 0.2 to 0.3 MPa.

- Benallel et al [20] explored biocomposites composed of cardboard paste as a binder and five different plant fibers (alfa, reed, fig branches, and olive leaves) as reinforcement. Each formulation was mixed in a blender. The cardboard paste was prepared by cutting and crushing cardboard packaging into small pieces, followed by wetting with water for 30 minutes to form a viscous mixture. The resulting biocomposites presented apparent bulk densities between 176 and 205 kg/m³ and thermal conductivity values ranging from 0.072 to 0.116 W/(m·K).

- Mathews et al [2] studied biocomposites composed of cardboard aggregates, three different binders (corn starch, lime, and clay), water, and boric acid as a flame-retardant additive. The bio-composites were compacted under a 30 kg load. Cardboard aggregates were prepared by shredding cardboard boxes into thin strips, followed by grinding until 60% of particles were smaller than 4.75 mm. The resulting materials exhibited apparent bulk densities ranging from 406 to 892 kg/m³, thermal conductivity from 0.066 to 0.128 W/(m·K), and compressive strength, measured on 200 × 200 × 40 mm samples, from 6.2 to 15 MPa.

- As a subsequent contribution, Benallel et al [21] examined bio-composites prepared using the same cardboard paste as a binder and date palm fibers as reinforcement. The materials were blended and manually compacted using a flat surface. The resulting bio-composites demonstrated apparent bulk densities ranging from 199 to 313 kg/m³, thermal conductivity between 0.058 and 0.081 W/(m·K), thermal diffusivity from $1.53 \times 10^{-7}$ to $2.62 \times 10^{-7}$ m²/s, and specific heat capacity from 1331 to 1679 J/(kg·K).

- Mazaherifar et al [18] investigated biocomposites composed of cardboard fibers, commercial corn starch, water, and sodium bicarbonate. The cardboard fibers were prepared by manually tearing the cardboard into small pieces, soaking them in water for several hours, and then defibrating them in a blender. Each biocomposite was mixed and subsequently heated at 105°C for 24 h. The resulting materials possessed apparent bulk densities ranging from 237 to 284 kg/m³, total porosity between 76% and 80%, and thermal conductivity values of 0.048–0.049 W/(m·K).

Although the studies summarized above demonstrate the potential of recycled waste cardboard for building insulation materials, they also reveal significant limitations in current fabrication methods. Processing recycled cardboard-based biocomposites typically involves multiple labor-intensive, energy-demanding, and time-consuming steps, including shredding [2], grinding [2], crushing [19–21], prolonged soaking [18], wetting [20,21], defibrating [18], extended mixing [19–21], prolonged high-load compaction [2,19], and extended thermal curing at elevated temperatures [18,19]. These processes increase costs and environmental impacts, further limiting efficiency and scalability.

These cumulative limitations highlight a significant and unresolved gap in the literature regarding the production of recycled waste cardboard-based biocomposites. While previous studies have demonstrated the feasibility and performance of these materials, they have predominantly relied on complex, labor-intensive, energy-intensive and time-consuming fabrication processes. Consequently, it remains

insufficiently explored whether recycled cardboard can be effectively valorized using minimally processed aggregates without compromising thermal, hygric, or mechanical performance. This lack of systematic investigation risks constraining the practical implementation, scalability, and broader adoption of recycled waste cardboard-based insulation materials in sustainable building applications.

To address these challenges, the present study investigated the incorporation of recycled waste cardboard chips (RWCC) as a partial or complete replacement for conventional hemp shives in hemp-lime bio-composites. The novelty of the work lies in a simplified, energy-efficient, and rapid processing methodology, coupled with a reproducible mix-design procedure that enables straightforward production of the bio-composites. Untreated RWCC, cut directly from post-consumer corrugated cardboard, were incorporated into hemp-lime matrices without any pre-treatment. The RWCC were mixed using a rapid blending procedure and lightly compacted, thereby eliminating the need for grinding, crushing, soaking, defibrating, extended compaction, or thermal curing. RWCC substitution ratios from 0 to 100 wt% were systematically studied to evaluate their effects on key physico-thermal-hygric properties, including density, porosity, thermal conductivity, heat capacity, thermal diffusivity, thermal effusivity, vapor permeability, and moisture buffer value. The primary objective was to determine the influence of RWCC substitution ratio on these properties and to assess whether this simplified production approach could achieve performance comparable to that of conventional bio-based insulation materials.

## 2. Materials and methods

### 2.1. Materials

This section is structured as follows. It begins with a description of the raw materials, including the binder and aggregates. The preparation of the biocomposites is then presented in chronological order. First, the governing equations for the mix design are introduced, followed by a description of the mix design parameters and the resulting calculated proportions. The mixing procedure is subsequently outlined, and the fresh densities are measured to verify consistency with theoretical values. The molding and conditioning procedures are then described. Finally, the compressive behavior of the hardened biocomposites is evaluated to confirm that they possess sufficient mechanical capacity to support their own weight.

#### 2.1.1. Raw materials

The five biocomposites examined in this study were prepared using the following constituents. The binder consisted of a gray formulated lime-cement blend, designated FL A 3.5 CL45, K40, L15, in accordance with the EN 459–1 standard [22]. The blend comprised 45 wt% air calcium lime (CL), 40 wt% clinker (K), and 15 wt% limestone (L), and exhibited a true density of $\rho_L = 2440 \pm 2$ kg/m³. Two types of aggregates were employed, as illustrated in Fig. 1: commercially available hemp shives and recycled waste cardboard chips (RWCC), which were produced by cutting clean, dry, post-consumer corrugated cardboard with a thickness of 5 mm into small strips of the same thickness using a manual paper trimmer, without any additional treatment.

Table 1 summarizes the measured properties of the aggregates. Thermal conductivity and specific heat capacity were measured in the bulk state using the methods detailed in Sections 2.2.3 and 2.2.4, respectively. Bulk density was determined using a cylindrical mold with dimensions of Ø75 × 55 mm, while true density was assessed in the fresh state following the procedure described in Section 2.1.2. Particle size characteristics were derived from image analysis-based particle size distribution measurements, conducted in accordance with the recommendations of RILEM TC 236-BBM [23] using the open-source software ImageJ [10,23]. For this purpose, hemp shives and RWCC particles were manually dispersed on a flat, non-reflective surface (320 × 320 mm) to
<figure><img src="image_2.png" /><figcaption>Fig. 1. Commercial hemp shives (left) and RWCC (right).</figcaption></figure>

<caption>**Table 1**</caption>

Measured thermal, density, and particle size properties of commercial hemp shives and RWCC.

<table><thead><tr><th></th><th>Hemp shives</th><th>RWCC</th></tr></thead><tbody><tr><td><strong>Thermal properties (bulk state)</strong></td><td></td><td></td></tr><tr><td>Thermal conductivity (W/(m·K))</td><td>0.0555 ± 0.0025</td><td>0.0654 ± 0.0023</td></tr><tr><td>Specific heat capacity (J/(kg·K))</td><td>1244 ± 5</td><td>1298 ± 1</td></tr><tr><td><strong>Density characteristics (kg/m³)</strong></td><td></td><td></td></tr><tr><td>Bulk density</td><td>138 ± 8</td><td>80 ± 4</td></tr><tr><td>True density in fresh state</td><td>367 ± 5</td><td>1394 ± 4</td></tr><tr><td><strong>Particle size characteristics</strong></td><td></td><td></td></tr><tr><td><strong>Length (mm)</strong></td><td></td><td></td></tr><tr><td>Range</td><td>2 - 19</td><td>15 - 29</td></tr><tr><td>Dominant value</td><td>4</td><td>22</td></tr><tr><td>d<sub>10</sub></td><td>3</td><td>16</td></tr><tr><td>d<sub>50</sub></td><td>6</td><td>20</td></tr><tr><td>d<sub>90</sub></td><td>15</td><td>26</td></tr><tr><td><strong>Width (mm)</strong></td><td></td><td></td></tr><tr><td>Range</td><td>1 - 6</td><td>4 - 11</td></tr><tr><td>Dominant value</td><td>2</td><td>6</td></tr><tr><td>d<sub>10</sub></td><td>1</td><td>4</td></tr><tr><td>d<sub>50</sub></td><td>2</td><td>6</td></tr><tr><td>d<sub>90</sub></td><td>4</td><td>9</td></tr><tr><td><strong>Thickness (mm)</strong></td><td></td><td></td></tr><tr><td>In dry state</td><td>1 - 6</td><td>5</td></tr><tr><td>After few minutes in wet state</td><td>1 - 6</td><td>1</td></tr><tr><td><strong>Aspect ratios (–)</strong></td><td></td><td></td></tr><tr><td>Dominant length / Dominant width</td><td>2</td><td>3.7</td></tr><tr><td>Dominant length / Thickness in dry state</td><td>0.7 - 4</td><td>4.4</td></tr><tr><td>Dominant length / Thickness in wet state</td><td>0.7 - 4</td><td>22</td></tr><tr><td>Dominant width / Thickness in dry state</td><td>0.4 - 2</td><td>1.2</td></tr><tr><td>Dominant width / Thickness in wet state</td><td>0.4 - 2</td><td>6</td></tr></tbody></table>

prevent particle overlap and ensure a representative particle population. High-resolution images were captured under uniform lighting conditions and processed by conversion to 8-bit grayscale followed by thresholding to isolate individual particles. The projected length and width of each particle were then measured to obtain the corresponding particle size distributions (Fig. 2).

The particle size distributions shown in Fig. 2 and the corresponding properties summarized in Table 1 reveal clear differences in morphology and aspect ratio between the two aggregates. Hemp shives are generally shorter and narrower, with dominant lengths of approximately 4 mm and widths of about 2 mm, resulting in moderately elongated particles. In contrast, RWCC particles are significantly longer and wider, with dominant lengths of approximately 22 mm and widths of about 6 mm, and they present more uniform length distribution. Moreover, RWCC particles undergo a pronounced reduction in thickness upon wetting. While their thickness in the dry state is approximately 5 mm, it decreases

<figure><img src="image_3.png" /><figcaption>Fig. 2. Particle length and width distributions of hemp shives and RWCC, determined by image analysis.</figcaption></figure>

to about 1 mm after a few minutes in the wet state. This dimensional change leads to a substantial increase in the dominant length-to-thickness ratio, reaching values as high as 22, compared to values not exceeding 4 for hemp shives. As a result, RWCC particles present a higher aspect ratio in the wet state, which may significantly influence the behavior and performance of biocomposites incorporating these particles.

### 2.1.2. Biocomposites

The biocomposites were prepared in accordance with the procedure described below. To ensure reproducibility, the mix design was established such that the total volume of all constituents was equal to 1 m³. The biocomposite formulations were determined using Eqs. (1)–(4).

$$ \frac{L}{\rho_L} + \frac{W}{\rho_W} + \frac{H}{\rho_H} + \frac{C}{\rho_C} = 1 \text{ m}^3 \quad (1) $$

$$ \frac{W}{L} = x \quad (2) $$

$$ \frac{H}{L} = (1 - i) \times y \quad (3) $$

$$ \frac{C}{L} = i \times y \quad (4) $$

where: L, W, H, and C (kg) represent the masses per cubic meter of the biocomposite for grey formulated lime, tap water, commercial hemp
shives, and RWCC, respectively, while $ρ_L$, $ρ_W$, $ρ_H$, and $ρ_C$ (kg/m³) denote their corresponding true densities.

Because the aggregates, particularly RWCC, undergo morphological changes upon contact with water during mixing (Table 1), conventional methods for determining true density, such as helium pycnometry, may not provide representative values. Consequently, the true densities of the aggregates, $ρ_H$ and $ρ_C$, were determined under actual mixing conditions, namely in the fresh state. Measurements were conducted by weighing freshly cast, lightly hand-compacted mixtures placed in a cylindrical mold with dimensions of Ø75 × 55 mm, corresponding to a volume of 242.9 cm³, under controlled environmental conditions of 23 ± 3°C and 55 ± 5% RH, as described in Section 2.2.1. For each test, the total mixture volume was fixed to match the mold volume, and the amount of the component under investigation was adjusted until the mold was completely filled without underfilling or overflow. The mass of the filled mold was then measured, and the true density of the component was derived directly from the measured quantities. To determine the true density of hemp $ρ_H$, mixtures consisting of lime, water, and hemp shives were prepared according to Eq. (1), using a total component volume of 242.9 cm³ instead of 1 m³. The following parameters were fixed: $ρ_L = 2440$ kg/m³, $ρ_W = 1000$ kg/m³, C = 0, x = 1.47, y = 0.47, and i = 0. $ρ_H$ was varied within a range of 200 to 500 kg/m³, and repeated measurements were performed until consistent mold filling was achieved. This procedure yielded $ρ_H = 367 ± 5$ kg/m³. The same experimental approach was applied to determine the true density of RWCC $ρ_C$. In this case, mixtures were prepared using lime, water, and RWCC, with H = 0 and i = 1, while maintaining the same total volume and fixed parameters $ρ_L$, $ρ_W$, x, and y. $ρ_C$ was varied within a range of 1200 to 1500 kg/m³, and repeated measurements resulted in $ρ_C = 1394 ± 4$ kg/m³.

The variable “i” represents the substitution ratio of commercial hemp shives with RWCC and is defined as the mass fraction of RWCC relative to the total mass of hemp shives and RWCC, such that $i = \frac{C}{H+C}$. Values of “i” were set to 0, 0.3, 0.5, 0.7, and 1, corresponding to biocomposites M0, M30, M50, M70, and M100, respectively. The mix design procedure began with the definition of a reference biocomposite, M0, representative of a typical hemp based industrial wall insulation material, from which all subsequent formulations were derived by varying “i”. The composition of M0, for which C = 0 and i = 0, consisted of 34 wt% lime, 50 wt% water, and 16 wt% hemp shives, corresponding to a water-to-lime ratio $\frac{W}{L} = 1.47$ and a hemp-to-lime ratio $\frac{H}{L} = 0.47$. These ratios correspond to x = 1.47 and y = 0.47. To isolate the effect of partial or full substitution of hemp shives with RWCC, the parameters x and y were kept constant for all formulations. Consequently, all biocomposites contained 34 wt% lime, 50 wt% water, and 16 wt% aggregates. Substitution was applied exclusively within the aggregate fraction by varying “i”, with formulation M100, for which H = 0 and i = 1, consisting entirely of RWCC as aggregates.

The mix proportions of the five biocomposites were calculated by solving Eqs. (1)–(4) with the substitution ratio “i” set to 0, 0.3, 0.5, 0.7, and 1. The resulting compositions are summarized in Table 2, with “LM” and “A” denoting the lime matrix and aggregates, respectively. To further quantify the material composition, the volume content of each component was determined by dividing its mass by its corresponding true density. Based on these calculations, the volume content of the lime matrix (vol% LM) was found to increase by 9.7%, 17.5%, 26.4%, and 42.5% for biocomposites M30, M50, M70, and M100 respectively, compared to M0. Conversely, the volume content of the aggregates (vol% A) decreased by 14.8%, 26.4%, 38.9%, and 62.6% for the same biocomposites. These changes in vol% LM and vol% A are expected to significantly influence the physical, thermal, and hygric properties of the biocomposites.

The mixing of the biocomposites was performed at 23 ± 3°C and 55 ± 5% RH using a 20-liter electric mixer. First, the grey formulated lime was combined with tap water and mixed for 1–2 minutes until a

<table><caption>**Table 2**

Calculated mix proportions of the biocomposites.</caption><thead><tr><th></th><th>M0</th><th>M30</th><th>M50</th><th>M70</th><th>M100</th></tr></thead><tbody><tr><td colspan="6"><strong>Mass per 1 m³ of biocomposite (kg)</strong></td></tr><tr><td>Lime (L)</td><td>316</td><td>347</td><td>372</td><td>400</td><td>451</td></tr><tr><td>Water (W)</td><td>465</td><td>511</td><td>547</td><td>588</td><td>663</td></tr><tr><td>Hemp shives (H)</td><td>149</td><td>114</td><td>87</td><td>56</td><td>0</td></tr><tr><td>RWCC (C)</td><td>0</td><td>49</td><td>87</td><td>132</td><td>212</td></tr><tr><td>L + W + H + C</td><td>930</td><td>1021</td><td>1093</td><td>1176</td><td>1326</td></tr><tr><td colspan="6"><strong>Mix design ratios (–)</strong></td></tr><tr><td>i = C / (H + C)</td><td>0</td><td>0.3</td><td>0.5</td><td>0.7</td><td>1</td></tr><tr><td>W/L</td><td>1.47</td><td>1.47</td><td>1.47</td><td>1.47</td><td>1.47</td></tr><tr><td>H/L</td><td>0.47</td><td>0.33</td><td>0.23</td><td>0.14</td><td>0</td></tr><tr><td>C/L</td><td>0</td><td>0.14</td><td>0.23</td><td>0.33</td><td>0.47</td></tr><tr><td colspan="6"><strong>Mass content (wt%)</strong></td></tr><tr><td>wt% L</td><td>34</td><td>34</td><td>34</td><td>34</td><td>34</td></tr><tr><td>wt% W</td><td>50</td><td>50</td><td>50</td><td>50</td><td>50</td></tr><tr><td>wt% H</td><td>16</td><td>11.2</td><td>8</td><td>4.8</td><td>0</td></tr><tr><td>wt% C</td><td>0</td><td>4.8</td><td>8</td><td>11.2</td><td>16</td></tr><tr><td>wt% LM = wt% L + wt% W</td><td>84</td><td>84</td><td>84</td><td>84</td><td>84</td></tr><tr><td>wt% A = wt% H + wt% C</td><td>16</td><td>16</td><td>16</td><td>16</td><td>16</td></tr><tr><td colspan="6"><strong>Volume content (vol%)</strong></td></tr><tr><td>vol% L</td><td>13</td><td>14.2</td><td>15.2</td><td>16.4</td><td>18.5</td></tr><tr><td>vol% W</td><td>46.5</td><td>51.1</td><td>54.7</td><td>58.8</td><td>66.3</td></tr><tr><td>vol% H</td><td>40.6</td><td>31.1</td><td>23.7</td><td>15.3</td><td>0</td></tr><tr><td>vol% C</td><td>0</td><td>3.5</td><td>6.2</td><td>9.5</td><td>15.2</td></tr><tr><td>vol% LM = vol% L + vol% W</td><td>59.5</td><td>65.3</td><td>69.9</td><td>75.2</td><td>84.8</td></tr><tr><td>vol% A = vol% H + vol% C</td><td>40.6</td><td>34.6</td><td>29.9</td><td>24.8</td><td>15.2</td></tr></tbody></table>

homogeneous lime paste was obtained. Next, the dry aggregates (commercial hemp shives and/or RWCC) were gradually added and mixed for 2–3 minutes until they were uniformly coated with the lime paste.

To verify that the measured fresh bulk density was consistent with the theoretical value corresponding to the sum of L, W, H, and C for 1 m³ of biocomposite (Table 2), and thereby to validate the measured true densities of the individual components, particularly those of the aggregates $ρ_H$ and $ρ_C$, the fresh bulk densities of the biocomposites were measured. This was achieved by weighing freshly cast and lightly hand-compacted mixtures placed in a cylindrical mold with dimensions of Ø75 × 55 mm under controlled environmental conditions of 23 ± 3°C and 55 ± 5% RH, as described in Section 2.2.1. For each biocomposite formulation, the measurements were carried out three times. The relationship between the measured fresh bulk density, as defined by Eq. (5), and the corresponding theoretical value is shown in Fig. 3. A good agreement is observed, with a maximum deviation of approximately 1.8%, thereby confirming the accuracy of the measured true densities of

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Relationship between the measured fresh bulk density and the theoretical value calculated from the combined components (L + W + H + C) for 1 m³ of biocomposite.</figcaption></figure>
the components. The small discrepancies are mainly attributed to minor variations in the true densities of the aggregates, estimated at ± 5 kg/m³ for hemp shives and ± 4 kg/m³ for RWCC, as well as to variability associated with the hand compaction procedure.

Immediately after mixing and verifying the fresh bulk density, the biocomposites were cast and lightly hand-compacted in layers into molds of three different dimensions: 155 × 155 × 40 mm, 100 × 100 × 50 mm, and Ø80 × 20 mm. Fig. 4 shows the appearance of the freshly prepared biocomposites. The adopted mix design methodology and selected parameters ensured that the workability of biocomposites M30, M50, M70, and M100 remained comparable to that of the reference mixture M0, which represents a typical industrial wall insulation material. This behavior is attributed to the proportional increase in the masses of both lime (L) and water (W) accompanying the increase in the mass of RWCC (C), as reported in Table 2. The resulting increase in the mass of lime paste (L + W) effectively compensated for the higher water absorption capacity of RWCC compared with hemp shives, thereby maintaining consistent workability across all formulations. Consequently, despite the progressive replacement of hemp shives with RWCC, the fresh-state handling behavior of the biocomposites was preserved, demonstrating the effectiveness of the mix design strategy. It should be noted, however, that this assessment of workability is qualitative and based on visual inspection during casting and hand compaction. The apparent consistency in fresh-state behavior would benefit from quantitative evaluation, for instance using flow or slump tests, to enable more rigorous interpretation.

After molding, all specimens (155 × 155 × 40 mm, 100 × 100 × 50

mm, and Ø80 × 20 mm) were stored in a controlled environment at 23 ± 3°C and 55 ± 5 % RH for an initial curing period of two weeks. The specimens were then demolded and maintained under the same conditions for an additional two weeks. Subsequently, they were oven-dried at 50 ± 5°C until mass stabilization was achieved, defined as a relative mass variation of less than 0.05% over a 24 h period, which resulted in a drying process lasting more than six weeks. Consequently, all specimens were at least ten weeks old at the start of the characterization phase.

Before the characterization phase, the compressive behavior of the biocomposites was assessed solely to estimate its order of magnitude. Since these materials are primarily intended for building envelope insulation, where mechanical performance is not a critical factor, the assessment aimed to verify that the biocomposites possessed sufficient mechanical capacity to support at least their own weight [9,10].

Testing was conducted using a Zwick/Roell Z100® machine, following the EN 826 standard [24], with adjustments made to accommodate the sample dimensions. Three dry samples (40 × 40 × 40 mm) were tested along the casting direction at a loading rate of 10 N/s. These samples were prepared by saw-cutting 155 × 155 × 40 mm specimens from each biocomposite. Because the samples exhibited progressive densification rather than abrupt failure, the compressive stress was recorded at 80% of the maximum load to provide a preliminary estimate of the mechanical response of the biocomposites. This approach likely underestimates the true mechanical capacity and limits direct comparison with literature values, where compressive strength is typically reported at failure.

The maximum stress values measured at 80% of the ultimate load

<figure><img src="image_5.png" /><figcaption>Fig. 4. Freshly prepared biocomposites immediately after molding.</figcaption></figure>
were 0.97, 0.88, 0.83, 0.70, and 0.85 MPa for biocomposites M0, M30, M50, M70, and M100, respectively. Compared to the reference M0, which contains only commercial hemp shives as aggregates, the maximum stress decreased by approximately 9.3%, 14.4%, 27.8%, and 12.4% for M30, M50, M70, and M100, respectively. This reduction is mainly related to the shape, size (Table 1 and Fig. 2), and orientation of RWCC particles, which promote misalignment and heterogeneous stress distribution, resulting in less efficient load transfer under compression compared with hemp shives. In biocomposites containing both hemp shives and RWCC (M30, M50, and M70), the coexistence of aggregates with contrasting geometries and markedly different aspect ratios (Table 1) further disrupts particle alignment, reduces interlocking, and weakens bonding with the lime matrix, leading to lower stress. In contrast, in M100, where RWCC constitute the entire aggregate fraction, the elongated particles can align and interlock more effectively, forming a continuous network that enhances load transfer and partially compensates for the lower mechanical contribution of individual particles. This explains why M100 achieves higher stress than M50 and M70.

A comparison with published studies, where compressive strength is generally reported at failure, indicates that the stress values measured at 80% of the ultimate load (0.70–0.88 MPa) are lower than the compressive strengths reported for recycled waste cardboard-based biocomposites (6.2–15 MPa) [2] and argan nut shell-plaster biocomposites (2.29–2.52 MPa) [25]. By contrast, the measured stress values are comparable to the compressive strengths of flax-lime biocomposites (0.53–1.19 MPa) [10] and hemp-based biocomposites (0.49–1.05 MPa) [26], and exceed those reported for hemp-lime biocomposites (0.03–0.11 MPa) [9] and recycled waste cardboard-gypsum biocomposites (0.17–0.30 MPa) [19]. Overall, these results indicate that, regardless of RWCC content, the developed biocomposites possess adequate mechanical capacity to support their own weight, confirming their suitability for applications where mechanical performance is secondary to thermal insulation and sustainability.

## 2.2. Methods

The characterization of the biocomposites included analyses of their physical, porosity-related, thermal, and hygric properties.

### 2.2.1. Fresh density measurement

The fresh bulk density was measured by weighing freshly cast, and hand-compacted biocomposite mixtures placed in a Ø75 × 55 mm mold. Measurements were carried out in triplicate for each mixture at 23 ± 3°C and 55 ± 5% RH.

The fresh bulk density, $ρ_f$ (kg/m³), was calculated using Eq. (5).

$$ρ_f = \frac{m_f}{V_m} \qquad (5)$$

where: $m_f$ (kg) is the mass of the fresh biocomposite mixture, and $V_m$ (m³) is the apparent volume of the mold.

### 2.2.2. Vacuum water saturation test

The dry and saturated bulk densities, total porosity, and saturated water content were assessed using the commonly employed water saturation under vacuum method, in accordance with the NF P18–459 standard [27]. Dry samples were placed in a desiccator between two perforated porcelain plates and subjected to a pressure below 25 mbar for at least 4 ± 0.5 h. The samples were then saturated with tap water and fully immersed after 15 min, while the reduced pressure was maintained for 44 ± 1 h. Measurements were conducted on four 40 × 40 × 40 mm samples, obtained by saw-cutting dry 155 × 155 × 40 mm specimens from each biocomposite.

The dry bulk density, $ρ_{dry}$ (kg/m³), the saturated bulk density, $ρ_{sat}$ (kg/m³), the total porosity, $ε$ (%), and the saturated water content, $ω_{sat}$ (%), were determined using Eqs. (6)–(9), respectively.

$$ρ_{dry} = \frac{m_{dry}}{V_s} \qquad (6)$$

$$ρ_{sat} = \frac{m_{sat}}{V_s} \qquad (7)$$

$$ε = \frac{ρ_{sat} - ρ_{dry}}{ρ_w} \times 100 \qquad (8)$$

$$ω_{sat} = \left( \frac{ρ_{sat}}{ρ_{dry}} - 1 \right) \times 100 \qquad (9)$$

where: $m_{dry}$ (kg) designates the dry mass of the sample after oven-drying at 50 ± 5°C, $m_{sat}$ (kg) denotes the saturated mass of the sample after water saturation under vacuum, $V_s$ (m³) represents the apparent volume of the sample measured by hydrostatic weighing, and $ρ_w$ (kg/m³) is the water density.

### 2.2.3. Thermal conductivity determination

The dry thermal conductivity was determined under stationary conditions using the guarded hot plate method, according to the EN 12664 [28] and EN 12667 [29] standards. The λ–Meter EP500e® device was used, allowing heat transfer through a flat-surfaced specimen placed between two parallel isothermal hot and cold plates, creating a stable temperature gradient set at 15°C. Transversal heat dissipation was minimized by surrounding the specimen with guard rings, and mats were placed on each face of the specimen to account for surface roughness. Measurements were taken at 23°C on two dry 155 × 155 × 40 mm specimens of each biocomposite, alternating the faces between tests. Prior to testing, the specimens, which were already dry, were vacuum-dried at 23 ± 3°C in a desiccator to eliminate any residual moisture.

### 2.2.4. Specific heat capacity measurement

The dry specific heat capacity was measured using the continuous method, based on the EN 821–3 standard [30], with modifications to the heat change sequences. The Calvet Setaram BT 2.15® calorimeter, featuring 3D sensors that fully enclosed the sample to ensure accurate measurement of all emitted heat, was used. The calorimeter consisted of a reference cell, kept empty, and a measuring cell in which the sample was placed. Both cells were enclosed within the calorimeter block during the test. Heat exchange by the sample was measured using a 3D flow meter, which enabled the calculation of specific heat capacity. The heat change followed three sequences: (i) a constant temperature of 5°C for 4 h, (ii) a ramp from 5°C to 35°C at a rate of 0.1°C/min, and (iii) a constant temperature of 35°C for 5 h. Measurements were carried out on two dry 8 × 8 × 50 mm samples (2 ± 0.8 g), obtained by saw-cutting dry 155 × 155 × 40 mm specimens from each biocomposite.

### 2.2.5. Thermal diffusivity and effusivity calculations

The dry thermal diffusivity, $α$ (m²/s), and the dry thermal effusivity, $η$ (J/(m²·K·s¹/²)), were calculated using Eqs. (10) and (11), respectively.

$$α = \frac{λ}{ρ_{dry} \times c_p} \qquad (10)$$

$$η = \sqrt{λ \times ρ_{dry} \times c_p} \qquad (11)$$

where: $λ$ (W/(m·K)) and $c_p$ (J/(kg·K)) represent the dry thermal conductivity and the dry specific heat capacity, respectively.

### 2.2.6. Vapor permeability testing

The water vapor permeability and the water vapor resistance factor (also known as the water vapor diffusion resistance factor), defined as the ratio of the water vapor permeability of air to that of the biocomposite, were evaluated using the dry cup method in accordance with
the EN ISO 12572 standard [31]. The Gintronic GraviTest 6400® device, equipped with six cups in a controlled climatic chamber and a high-precision balance for mass monitoring, was used. A vapor pressure gradient was established under isothermal conditions at 23.2 ± 0.6°C between the interior of the cup and the climatic chamber, across both faces of the specimens, which were initially preconditioned at 22.7 ± 0.2°C and 59 ± 5% RH. This gradient was maintained by setting the RH inside the cup to approximately 3% using silica gel and the ambient RH in the chamber to 49.9 ± 0.8%. Mass changes were monitored until stabilization. Measurements were performed at an air velocity of 0.3 m/s on two Ø80 × 20 mm specimens from each biocomposite.

The water vapor resistance factor, also known as the water vapor diffusion resistance factor, μ (–), was calculated using Eq. (12).

$$ \mu = \frac{\delta_a}{\delta} \qquad (12) $$

where: δₐ (kg/(m·s·Pa)) and δ (kg/(m·s·Pa)) are the water vapor permeability of air and the water vapor permeability of the biocomposite, respectively.

### 2.2.7. Moisture buffering assessment

The moisture buffer value (MBV) was assessed under dynamic conditions using the Climats EXCAL 2211-HA® climatic chamber, following a procedure inspired by the NORDTEST protocol [32], with modifications to RH and air velocity settings. Specimens, sealed with aluminum adhesive on five sides and leaving one 100 × 100 mm side exposed, were initially preconditioned at 22.7 ± 0.2°C and 59 ± 5% RH. They were then subjected to 47 daily RH cycles over approximately seven weeks, consisting of 8 h at 73 ± 3% RH, followed by 16 h at 44 ± 4% RH, while maintaining a temperature of 23.2 ± 0.7°C, to replicate typical building conditions as closely as possible. The specimens were manually weighed at the end of each cycle during the final five cycles, outside the climatic chamber. Measurements were conducted at an air velocity of 0.51 ± 0.14 m/s on a single 100 × 100 × 50 mm specimen for each biocomposite.

The moisture buffer value, MBV (g/(m².%RH)), was calculated using Eq. (13).

$$ MBV = \frac{\Delta m}{A \times \Delta RH} \qquad (13) $$

where: Δm (g) is the moisture uptake or release during a RH cycle, A (m²) is the exposed specimen surface area, and ΔRH (%RH) is the difference between the highest and lowest RH applied during a RH cycle.

## 3. Results and discussion

The following sections present the physical, porosity-related, thermal, and hygric properties of the investigated biocomposites. To ensure a clear and comprehensive presentation of the experimental results, the data are reported in tabular form as mean values, standard deviations, and minimum and maximum values. This reporting approach captures both the central tendency and the full extent of the observed variability for each property, while accounting for uncertainties arising from measurement instrumentation, testing conditions, and the intrinsic heterogeneity of the materials [10]. The mean value represents the characteristic material response, whereas the minimum and maximum values define the observed bounds of variability, thereby providing a transparent assessment of data dispersion. In the figures, standard deviations are indicated by red error bars, facilitating visual interpretation of variability and supporting a robust evaluation of material performance.

### 3.1. Physical and porosity characteristics

Table 3 summarizes the measured values of fresh bulk density,

<caption>Table 3</caption>

Fresh bulk density of freshly cast and hand-compacted biocomposite mixtures, along with dry and saturated bulk densities, total porosity, and saturated water content, assessed on 40 × 40 × 40 mm samples using the vacuum water saturation test.

<table><thead><tr><th></th><th>M0</th><th>M30</th><th>M50</th><th>M70</th><th>M100</th></tr></thead><tbody><tr><td colspan="6"><strong>Fresh bulk density ρ<sub>f</sub> (kg/m<sup>3</sup>)</strong></td></tr><tr><td>Mean</td><td>914</td><td>1038</td><td>1104</td><td>1195</td><td>1322</td></tr><tr><td>Standard deviation</td><td>9</td><td>11</td><td>12</td><td>13</td><td>12</td></tr><tr><td>Minimum</td><td>906</td><td>1026</td><td>1090</td><td>1181</td><td>1310</td></tr><tr><td>Maximum</td><td>922</td><td>1051</td><td>1117</td><td>1210</td><td>1334</td></tr><tr><td colspan="6"><strong>Dry bulk density ρ<sub>dry</sub> (kg/m<sup>3</sup>)</strong></td></tr><tr><td>Mean</td><td>430</td><td>530</td><td>568</td><td>610</td><td>666</td></tr><tr><td>Standard deviation</td><td>9</td><td>11</td><td>20</td><td>15</td><td>14</td></tr><tr><td>Minimum</td><td>419</td><td>514</td><td>545</td><td>593</td><td>654</td></tr><tr><td>Maximum</td><td>435</td><td>540</td><td>592</td><td>627</td><td>682</td></tr><tr><td colspan="6"><strong>Saturated bulk density ρ<sub>sat</sub> (kg/m<sup>3</sup>)</strong></td></tr><tr><td>Mean</td><td>1193</td><td>1279</td><td>1301</td><td>1323</td><td>1356</td></tr><tr><td>Standard deviation</td><td>8</td><td>7</td><td>15</td><td>7</td><td>11</td></tr><tr><td>Minimum</td><td>1184</td><td>1269</td><td>1284</td><td>1314</td><td>1346</td></tr><tr><td>Maximum</td><td>1198</td><td>1285</td><td>1319</td><td>1329</td><td>1369</td></tr><tr><td colspan="6"><strong>Total porosity ε (%)</strong></td></tr><tr><td>Mean</td><td>76.3</td><td>74.9</td><td>73.4</td><td>71.3</td><td>69.0</td></tr><tr><td>Standard deviation</td><td>1.5</td><td>0.5</td><td>0.5</td><td>0.9</td><td>0.3</td></tr><tr><td>Minimum</td><td>74.9</td><td>74.5</td><td>72.8</td><td>70.0</td><td>68.7</td></tr><tr><td>Maximum</td><td>77.9</td><td>75.5</td><td>73.9</td><td>72.1</td><td>69.3</td></tr><tr><td colspan="6"><strong>Saturated water content ω<sub>sat</sub> (%)</strong></td></tr><tr><td>Mean</td><td>178</td><td>141</td><td>129</td><td>117</td><td>104</td></tr><tr><td>Standard deviation</td><td>7</td><td>4</td><td>5</td><td>4</td><td>3</td></tr><tr><td>Minimum</td><td>172</td><td>138</td><td>123</td><td>112</td><td>101</td></tr><tr><td>Maximum</td><td>186</td><td>147</td><td>135</td><td>122</td><td>106</td></tr></tbody></table>

determined from three mixtures per biocomposite, and of dry and saturated bulk densities, total porosity, and saturated water content, each determined from four samples per biocomposite.

### 3.1.1. Density

Compared to the reference biocomposite M0 (Table 3), which consists solely of hemp shives as aggregates, the fresh bulk density increases by approximately 13.6%, 20.8%, 30.7%, and 44.6% for biocomposites M30, M50, M70, and M100, respectively. Likewise, the dry bulk density rises by 23.3%, 32.1%, 41.9%, and 54.9%, while the saturated bulk density increases by 7.2%, 9.1%, 10.9%, and 13.7%.

Fig. 5 shows the evolution of fresh, dry, and saturated bulk densities as functions of the substitution ratio. All bulk densities increase linearly with increasing RWCC content, reflecting the higher true density of RWCC in the fresh state (ρ<sub>C</sub> ≈ 1394 kg/m³), which is approximately 3.8 times that of commercial hemp shives (ρ<sub>H</sub> ≈ 367 kg/m³). With the total component volume fixed at 1 m³ (Eq. (1)), a higher proportion of RWCC results in an increased overall mass (L + W + H + C), a larger lime

<figure><img src="image_6.png" /><figcaption>Fig. 5. Fresh, dry, and saturated bulk densities as functions of the substitution ratio.</figcaption></figure>
matrix volume content (vol% LM), and a reduced aggregate volume content (vol% A), as shown in Table 2. Consequently, bulk densities increase linearly with increasing “vol% LM” and the corresponding decrease in “vol% A”, primarily because the lime matrix is denser than the aggregates.

Although the dry bulk density of the RWCC-based biocomposites (514–682 kg/m³) is higher than that of lightweight hemp-lime bio-composites (141–498 kg/m³) [9,14,15], recycled waste cardboard-based biocomposites (176–313 kg/m³) [18,20,21], and hemp-based biocomposites (175–240 kg/m³) [26], it is comparable to that of flax-lime biocomposites (461–665 kg/m³) [10,12] and certain recycled waste cardboard-based biocomposites (518–583 kg/m³) [2], and remains lower than that of hemp-lime plasters (733–887 kg/m³) [11] and argan nut shell-plaster biocomposites (1217–1249 kg/m³) [25]. Overall, regardless of RWCC content, the developed biocomposites possess sufficiently low bulk density, which is expected to enable moderate thermal conductivity and favorable hygric behavior, thereby supporting enhanced building energy efficiency and improved indoor air quality.

### 3.1.2. Porosity

Compared to M0 (Table 3), the total porosity decreases by approximately 1.8%, 3.8%, 6.6%, and 9.9% for M30, M50, M70, and M100, respectively. Similarly, the saturated water content decreases by approximately 20.8%, 27.5%, 34.3%, and 41.6%.

Fig. 6 illustrates the evolution of total porosity and saturated water content with the substitution ratio. Both total porosity and saturated water content decrease linearly as the proportion of RWCC increases. According to Eqs. (8) and (9), this reduction occurs because the dry bulk density ($\rho_{dry}$) increases at a higher rate (slope = 231 kg/m³) than the saturated bulk density ($\rho_{sat}$) (slope = 156 kg/m³), as shown in Fig. 5. It is important to note that bulk density, total porosity, and saturated water content are inherently linked and follow a linear relationship. Consequently, for the same reasons that bulk densities increase with a higher proportion of RWCC (Section 3.1.1), both total porosity and saturated water content decrease as “vol% LM” rises and “vol% A” decreases (Table 2), reflecting the lower density and higher porosity of aggregates compared to the lime matrix.

While the total porosity of the RWCC-based biocomposites (68.7–75.5%) is slightly lower than that reported for recycled waste cardboard-corn starch biocomposites (76–80%) [18] and certain hemp-lime biocomposites (74.2–79.5%) [15], it is comparable to those of flax-lime biocomposites (71–76%) [10,12] and other hemp-lime biocomposites (72–79%) [12,14], and exceeds that of hemp-lime plasters (66–72%) [11]. Across all RWCC contents, the developed biocomposites demonstrate sufficiently high total porosity, expected to support moderate thermal conductivity and effective moisture

regulation, thereby promoting enhanced building energy performance and healthier indoor environments.

## 3.2. Thermal properties

Table 4 depicts the measured dry thermal conductivities and specific heat capacities, along with the corresponding calculated dry thermal diffusivities and effusivities, each determined from two specimens per biocomposite.

### 3.2.1. Conductivity

When compared to the reference biocomposite M0 (Table 4), consisting exclusively of hemp shives as aggregates, the dry thermal conductivity increases by approximately 11.8%, 12.7%, 16.7%, and 26.8% for biocomposites M30, M50, M70, and M100, respectively.

Fig. 7 displays the evolution of dry thermal conductivity as a function of the substitution ratio. A linear increase in thermal conductivity with increasing RWCC content is observed, in agreement with the simultaneous increase in dry bulk density (Fig. 5) and decrease in total porosity (Fig. 6). The relationships between thermal conductivity and these two parameters are expressed in Eqs. (14) and (15).

$$ \lambda = 7.4 \times 10^{-5} \times \rho_{\text{dry}} + 0.037 (R^2 = 0.96) \quad (14) $$

$$ \lambda = -2.2 \times 10^{-3} \times \varepsilon + 0.24 (R^2 = 0.91) \quad (15) $$

These equations indicate that thermal conductivity, $\lambda$ (W/(m·K)), increases linearly with increasing dry bulk density, $\rho_{dry}$ (kg/m³), and decreases linearly with increasing total porosity, $\varepsilon$ (%), highlighting the influence of the biocomposite microstructure on heat transfer. The increase in thermal conductivity is primarily attributed to microstructural changes induced by increasing RWCC content. As the substitution ratio increases, the lime matrix volume content (vol% LM) increases, while the aggregate volume content (vol% A) decreases, as reported in Table 2. The lime matrix is denser, less porous, and significantly more thermally conductive ($\lambda_{LM} = 0.193$ W/(m·K) [33]) than air ($\lambda_{air} = 0.026$ W/(m·K)) or the aggregates ($\lambda_{H} = 0.0555$ W/(m·K) and $\lambda_{C} = 0.0654$ W/(m·K) for hemp shives and RWCC, respectively). Consequently, as air-filled pores are progressively replaced by the conductive lime matrix, the continuity of thermally conductive pathways increases, thereby enhancing heat transfer throughout the biocomposites. In addition, RWCC particles

<caption>**Table 4**</caption>

Dry thermal conductivity determined at 23°C on dry 155 × 155 × 40 mm specimens using the guarded hot plate method, along with dry specific heat capacity at 23°C, measured between 5°C and 35°C for dry 8 × 8 × 50 mm samples, and the corresponding calculated dry thermal diffusivity and effusivity at 23°C.

<table><thead><tr><th></th><th>M0</th><th>M30</th><th>M50</th><th>M70</th><th>M100</th></tr></thead><tbody><tr><td>Dry thermal conductivity λ (W/(m·K))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>0.0694</td><td>0.0776</td><td>0.0782</td><td>0.0810</td><td>0.0880</td></tr><tr><td>Standard deviation</td><td>0.0036</td><td>0.0001</td><td>0.0001</td><td>0.0001</td><td>0.0044</td></tr><tr><td>Minimum</td><td>0.0647</td><td>0.0775</td><td>0.0781</td><td>0.0809</td><td>0.0827</td></tr><tr><td>Maximum</td><td>0.0746</td><td>0.0776</td><td>0.0783</td><td>0.0811</td><td>0.0919</td></tr><tr><td>Dry specific heat capacity c<sub>p</sub> (J/(kg·K))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>1013</td><td>1001</td><td>1033</td><td>1035</td><td>976</td></tr><tr><td>Standard deviation</td><td>1</td><td>1</td><td>1</td><td>2</td><td>6</td></tr><tr><td>Minimum</td><td>1012</td><td>1000</td><td>1032</td><td>1033</td><td>972</td></tr><tr><td>Maximum</td><td>1014</td><td>1002</td><td>1034</td><td>1036</td><td>980</td></tr><tr><td>Dry thermal diffusivity α (×10<sup>-7</sup> m<sup>2</sup>/s)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>1.59</td><td>1.46</td><td>1.33</td><td>1.28</td><td>1.35</td></tr><tr><td>Standard deviation</td><td>0.11</td><td>0.06</td><td>0.08</td><td>0.05</td><td>0.06</td></tr><tr><td>Minimum</td><td>1.53</td><td>1.43</td><td>1.28</td><td>1.25</td><td>1.30</td></tr><tr><td>Maximum</td><td>1.69</td><td>1.51</td><td>1.39</td><td>1.32</td><td>1.38</td></tr><tr><td>Dry thermal effusivity η (J/(m<sup>2</sup>·K·s<sup>1/2</sup>))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>174</td><td>203</td><td>214</td><td>226</td><td>239</td></tr><tr><td>Standard deviation</td><td>9</td><td>3</td><td>5</td><td>4</td><td>11</td></tr><tr><td>Minimum</td><td>166</td><td>200</td><td>210</td><td>223</td><td>229</td></tr><tr><td>Maximum</td><td>181</td><td>205</td><td>219</td><td>230</td><td>248</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Total porosity and saturated water content as functions of the substitution ratio.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 7. Measured and theoretical dry thermal conductivity as a function of the substitution ratio.</figcaption></figure>

exhibit an intrinsically higher thermal conductivity, approximately 17.8% greater than that of hemp shives, which further contributes to the overall increase in thermal conductivity. It should be noted, however, that the thermal conductivity of the aggregates was measured in their bulk state, independently of the lime matrix, and therefore does not account for their effective intrinsic conductivity within the bio-composite material.

To better understand the contribution of each biocomposite constituent to the overall thermal conductivity, a theoretical estimation was performed using a simplified parallel-type weighted model [34]. In this approach, the thermal conductivity of the solid constituents, namely lime matrix, hemp shives, and RWCC, is combined according to their volume fractions, while the air-filled porosity is treated separately. The model assumes that heat flows preferentially through the solid phases in parallel, with the air fraction proportionally reducing the effective conductivity. It neglects complex microstructural effects, such as interfacial thermal resistance or preferential pathways, and therefore provides a first-order estimate of the thermal behavior of the biocomposites. Despite its simplicity, this approach is widely used in the literature and allows a practical assessment of the relative influence of each constituent on the overall thermal conductivity. The effective thermal conductivity is expressed using Eq. (16).

$$ \lambda_{\text{th}} = (v_{\text{LM}} \times \lambda_{\text{LM}} + v_{\text{H}} \times \lambda_{\text{H}} + v_{\text{C}} \times \lambda_{\text{C}}) \times \left(1 - \frac{\varepsilon}{100}\right) + \lambda_{\text{air}} \times \frac{\varepsilon}{100} \quad (16) $$

where: $v_{LM} = \frac{\text{vol}\% \text{LM}}{100}$, $v_{H} = \frac{\text{vol}\% \text{H}}{100}$, and $v_{C} = \frac{\text{vol}\% \text{C}}{100}$ (–) are the volume fractions of the lime matrix, hemp shives, and RWCC, respectively (Table 2).

Using Eq. (16), the theoretical thermal conductivities for mixtures M0, M30, M50, M70, and M100 were calculated as 0.0524, 0.0580, 0.0626, 0.0685, and 0.0769 W/(m·K), respectively, and are plotted in Fig. 7. These values are systematically lower than the experimentally measured conductivities (Table 4), with deviations of approximately 32%, 34%, 25%, 18%, and 14%.

The underestimation of thermal conductivity by the theoretical model can be attributed to several interrelated factors, reflecting the limitations of this simplified approach and its inherent assumptions, as previously noted. In addition, the model is highly sensitive to the thermal conductivity of the lime matrix ($\lambda_{LM}$), which was adopted from literature data on a lime matrix based on a comparable binder rather than measured experimentally in the present study. The thermal conductivities of the aggregates ($\lambda_{H}$ and $\lambda_{C}$) were determined in their bulk state and may not fully reflect their behavior within the hydrated lime matrix. During mixing and curing, these aggregates, particularly RWCC, undergo morphological changes that can increase their intrinsic thermal conductivity. Furthermore, the model is highly simplified and does not

account for complex microstructural features or the potential effects of carbonation. Collectively, these factors underscore the conservative nature of the approach and highlight the importance of combining experimental measurements with modeling to accurately characterize the thermal behavior of heterogeneous biocomposites.

Despite the dry thermal conductivity of the RWCC-based biocomposites (0.0775–0.0919 W/(m·K)) being higher than that reported for hemp-lime biocomposites (0.0535–0.0667 W/(m·K)) [9], hemp-based biocomposites (0.052–0.057 W/(m·K)) [26], and recycled waste cardboard-based biocomposites (0.048–0.076 W/(m·K)) [18,19], it remains comparable to that of flax-lime biocomposites (0.082–0.127 W/(m·K)) [10] and other recycled waste cardboard-based biocomposites (0.071–0.128 W/(m·K)) [2,20,21], and is significantly lower than that of hemp-lime plasters (0.1963–0.2364 W/(m·K)) [11] and argan nut shell-plaster biocomposites (0.237–0.353 W/(m·K)) [25]. Regardless of RWCC content, the developed biocomposites present sufficiently low thermal conductivity, highlighting their suitability as effective building insulation and their potential to enhance energy efficiency in construction.

### 3.2.2. Heat capacity

Compared with M0 (Table 4), the dry specific heat capacity decreases by approximately 1.2% for M30 and 3.7% for M100, while increases of 2.0% and 2.2% are observed for M50 and M70, respectively. Despite these small variations, the heat capacities of all biocomposites remain very similar. This consistency is attributed to the combined effect of the relatively small difference in heat capacity between hemp shives and RWCC, which is only 4.3% (Table 1), and the constant mass contents of the lime matrix (wt% LM = 84%) and aggregates (wt% A = 16%) across all biocomposites (Table 2). The minor variations in heat capacity observed among the RWCC-based biocomposites arise from the slight difference in heat capacity between the two aggregates and, more importantly, from the inherent difficulty of maintaining identical component proportions in the smaller 8 × 8 × 50 mm test samples compared with the original 155 × 155 × 40 mm specimens from which they were saw-cut (Section 2.2.4).

To isolate the effect of sample preparation issues from differences in aggregate heat capacities, a theoretical calculation of the dry heat capacity of each biocomposite was performed using the heat capacities of the individual aggregates (Table 1) and the lime matrix. In this approach, the contributions of both the lime matrix and the aggregates were combined according to their known mass fractions (Table 2), assuming perfect homogeneity. By comparing the theoretical heat capacities with the experimentally measured values (Table 4), any discrepancies can be attributed solely to preparation-related technical challenges, specifically small variations in component proportions that occur during sample cutting. The theoretical dry heat capacity of each biocomposite, $c_{p,th}$ (J/(kg·K)), was assessed using Eq. (17).

$$ c_{p,th} = \xi_{LM} \times c_{p,LM} + \xi_{H} \times c_{p,H} + \xi_{C} \times c_{p,C} \quad (17) $$

where: $\xi_{LM} = \frac{\text{wt}\% \text{LM}}{100}$, $\xi_{H} = \frac{\text{wt}\% \text{H}}{100}$, and $\xi_{C} = \frac{\text{wt}\% \text{C}}{100}$ (–) are the mass fractions of the lime matrix, hemp shives, and RWCC, respectively (Table 2), while $c_{p,LM} \approx 969$ J/(kg·K), $c_{p,H} \approx 1244$ J/(kg·K), and $c_{p,C} \approx 1298$ J/(kg·K) denote their corresponding heat capacities. $c_{p,LM}$ was determined from M0 by assuming that the hemp shives are sufficiently short and narrow (Table 1 and Fig. 2) to preserve component proportions after sample cutting. Using the measured heat capacity for M0 (Table 4) in Eq. (17), where $c_{p,LM}$ is the only unknown, its value was calculated to be 969 ± 2 J/(kg·K).

Using Eq. (17) again, the theoretical heat capacities for M30, M50, M70, and M100 were calculated as 1016, 1017, 1019, and 1022 J/(kg·K), respectively, corresponding to deviations from the measured values (Table 4) of only 1.5%, 1.6%, 1.6%, and 4.7%. The larger deviation observed for M100, compared with the other biocomposites,
highlights the difficulty of maintaining consistent component proportions in the RWCC-based samples and underscores the necessity of preserving the original mix proportions at the sample scale, for example by producing smaller, tailored batches for heat capacity measurements.

Even though the dry specific heat capacity of the RWCC-based bio-composites (972–1036 J/(kg·K)) is lower than that of recycled waste cardboard-date palm fiber biocomposites (1331–1679 J/(kg·K)) [21], it is comparable to that of hemp-lime plasters (996–1009 J/(kg·K)) [11], hemp-based biocomposites (1050 J/(kg·K)) [26], and argan nut shell-plaster biocomposites (1034–1075 J/(kg·K)) [25], and exceeds that of other hemp-lime biocomposites (672–957 J/(kg·K)) [9], certain hemp-based biocomposites (763–783 J/(kg·K)) [26], and some argan nut shell-plaster biocomposites (606–626 J/(kg·K)) [25]. Overall, across all RWCC contents, the developed biocomposites possess a relatively high specific heat capacity, expected to enable them to store thermal energy and enhance the thermal inertia of building envelopes.

### 3.2.3. Diffusivity and effusivity

Relative to M0 (Table 4), the dry thermal diffusivity decreases by approximately 8.2%, 16.4%, 19.5%, and 15.1% for M30, M50, M70, and M100, respectively, while the dry thermal effusivity increases by approximately 16.7%, 23.0%, 29.9%, and 37.4%.

Fig. 8 shows the variation in thermal diffusivity and effusivity with the substitution ratio. Diffusivity decreases almost linearly, while effusivity increases linearly as the proportion of RWCC rises. According to Eq. (10), the decrease in diffusivity results from the dry bulk density ($\rho_{dry}$) increasing at a higher rate (slope = 231 kg/m³) (Fig. 5) than the thermal conductivity ($\lambda$) (slope = 0.0172 W/(m·K)) (Fig. 7), while the specific heat capacity ($c_p$) remains nearly constant (Section 3.2.2). In a similar manner, Eq. (11) explains the increase in effusivity as a consequence of the simultaneous rise in both dry bulk density (Fig. 5) and thermal conductivity (Fig. 7), with $c_p$ again remaining unchanged (Section 3.2.2).

The dry thermal diffusivity of the RWCC-based biocomposites (1.25–1.51 × 10⁻⁷ m²/s) is comparable to that of recycled waste cardboard-date palm fiber biocomposites (1.53–1.74 × 10⁻⁷ m²/s) [21] and straw-lime biocomposites (1.74 ± 0.7 × 10⁻⁷ m²/s) [12], and lower than that of hemp-lime biocomposites (3.21–5.85 × 10⁻⁷ m²/s) [9], hemp-based biocomposites (2.3–3.5 × 10⁻⁷ m²/s) [11,12,26], flax-lime biocomposites (2.46 ± 0.1 × 10⁻⁷ m²/s) [12], argan nut shell-plaster biocomposites (1.98–4.64 × 10⁻⁷ m²/s) [25], and certain recycled waste cardboard-date palm fiber biocomposites (1.92–2.62 × 10⁻⁷ m²/s) [21]. In line with these observations on thermal diffusivity, the dry thermal effusivity (200–248 J/(m²·K·s¹/²)) is comparable to that of hemp-lime and straw-lime biocomposites (225–256 J/(m²·K·s¹/²)) [12], and lower than that of flax-based biocomposites (339 ± 14 J/(m²·K·s¹/²)) [12] and argan nut shell-plaster biocomposites (511–634

<figure><img src="image_9.png" /><figcaption>Fig. 8. Dry thermal diffusivity and effusivity as functions of the substitution ratio.</figcaption></figure>

$J/(m²·K·s¹/²))$ [25]. Across their full range of RWCC contents, the developed biocomposites exhibit relatively low thermal diffusivity and effusivity, indicating slow heat propagation and limited heat exchange with the environment. This behavior is expected to enhance thermal inertia, stabilize indoor temperatures, and improve the overall thermal performance of buildings.

### 3.3. Hygric properties

#### 3.3.1. Vapor permeability and resistance factor

Table 5 provides the measured water vapor permeabilities and corresponding water vapor resistance factors, each determined from two specimens per biocomposite. Compared to the reference biocomposite M0, which is composed solely of hemp shives as aggregates, the water vapor permeability decreases by approximately 10.1%, 24.6%, 28.9%, and 11.4% for biocomposites M30, M50, M70, and M100, respectively. In contrast, the water vapor resistance factor increases by approximately 10.8%, 36.5%, 44.7%, and 12.4% for M30, M50, M70, and M100, respectively.

Fig. 9 illustrates the relationships between vapor permeability and the vapor resistance factor as functions of the substitution ratio. Except for biocomposite M100, which consists solely of RWCC as aggregates, vapor permeability decreases linearly, while the vapor resistance factor increases linearly as the proportion of RWCC rises. These trends correspond to the simultaneous increase in lime matrix volume content (vol% LM) and decrease in aggregate volume content (vol% A), as reported in Table 2. The reduction in vapor permeability and the corresponding increase in vapor resistance factor result from microstructural densification, evidenced by the increase in bulk density (Fig. 5) and decrease in total porosity (Fig. 6) at higher RWCC proportions. In particular, the denser and less porous lime matrix contains fine pores that hinder water vapor diffusion, whereas the more porous and less dense aggregates feature larger pores that facilitate vapor transport, thereby explaining the observed behavior across the biocomposite series.

The behavior of biocomposite M100, which has the highest “vol% LM” and the lowest “vol% A” (Table 2), deviates from this general trend. Although M100 contains 42.5% more “vol% LM” and 62.6% less “vol% A” than M0, it does not exhibit the lowest vapor permeability nor the highest vapor resistance factor. This difference arises not only from the formation of larger pores and a more pronounced interfacial transition zone between the lime matrix and the aggregates, without a significant change in total porosity, but also from the geometry and aspect ratio of the RWCC particles (Table 1). Owing to their elongated shape, RWCC particles tend to form more continuous and connected pore networks, promoting preferential pathways for water vapor diffusion through the biocomposite. This effect is most evident in M100, which contains the highest RWCC volume content (vol% C) and exclusively RWCC as aggregates. In contrast, in M30, M50, and M70, where “vol% C” is lower and the hemp shive volume content (vol% H) is higher (Table 2), the effect is less pronounced. When combined with RWCC, hemp shives,

<caption>Table 5</caption>

Water vapor permeability and water vapor resistance factor evaluated at 23°C, with an air velocity of 0.3 m/s, on Ø80 × 20 mm specimens using the dry cup method (3% RH – 50% RH).

<table><thead><tr><th></th><th>M0</th><th>M30</th><th>M50</th><th>M70</th><th>M100</th></tr></thead><tbody><tr><td>Water vapor permeability δ (×10<sup>-11</sup> kg/(m·s·Pa))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>2.28</td><td>2.05</td><td>1.72</td><td>1.62</td><td>2.02</td></tr><tr><td>Standard deviation</td><td>0.17</td><td>0.08</td><td>0.36</td><td>0.32</td><td>0.10</td></tr><tr><td>Minimum</td><td>2.07</td><td>1.94</td><td>1.31</td><td>1.24</td><td>1.91</td></tr><tr><td>Maximum</td><td>2.42</td><td>2.12</td><td>1.99</td><td>1.90</td><td>2.09</td></tr><tr><td>Water vapor resistance factor μ (-)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>8.55</td><td>9.47</td><td>11.67</td><td>12.37</td><td>9.61</td></tr><tr><td>Standard deviation</td><td>0.64</td><td>0.39</td><td>2.73</td><td>2.56</td><td>0.50</td></tr><tr><td>Minimum</td><td>8.01</td><td>9.15</td><td>9.74</td><td>10.20</td><td>9.30</td></tr><tr><td>Maximum</td><td>9.36</td><td>10.03</td><td>14.80</td><td>15.65</td><td>10.18</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Water vapor permeability and water vapor resistance factor as functions of the substitution ratio.</figcaption></figure>

which have a smaller width (Table 1 and Fig. 2) and a more irregular geometry than RWCC (Fig. 1), disrupt the continuity of these vapor pathways by filling void spaces more effectively and increasing pore tortuosity. As a result, the vapor transport pathways become less continuous, leading to lower vapor permeability and a more monotonic increase in the vapor resistance factor. It should be noted that these explanations remain hypotheses that require confirmation through detailed microstructural analyses, such as scanning electron microscopy, to fully characterize pore structure and aggregate-matrix interactions.

The water vapor permeability of the RWCC-based biocomposites (1.24–2.12 × 10⁻¹¹ kg/(m·s·Pa)) is lower than that of hemp-lime plasters (3–4 × 10⁻¹¹ kg/(m·s·Pa)) [11] and some hemp-based biocomposites (2.50–4.18 × 10⁻¹¹ kg/(m·s·Pa)) [15,26]. However, it remains comparable to other hemp-based biocomposites (1.88–2.22 × 10⁻¹¹ kg/(m·s·Pa)) [15,26], indicating that, regardless of RWCC content, the developed biocomposites allow sufficient moisture transport to prevent condensation and support healthy indoor environments.

### 3.3.2. Moisture buffer value (MBV)

Table 6 summarizes the obtained moisture buffer values and the corresponding hygric regulation classifications based on the NORDTEST method [32]. In this method, RH cycles consist of 8 h at 75% RH (73 ± 3% RH in this study) and 16 h at 33% RH (44 ± 4% RH in this study), maintained at 23°C with an air velocity of 0.1 ± 0.05 m/s (0.51 ± 0.14 m/s in this study).

**Table 6**

Moisture buffer value assessed on 100 × 100 × 50 mm specimens subjected to approximately seven weeks of daily RH cycles (8 h at 73 ± 3% RH and 16 h at 44 ± 4% RH), at 23.2 ± 0.7°C and an air velocity of 0.51 ± 0.14 m/s.

<table><thead><tr><th></th><th>M0</th><th>M30</th><th>M50</th><th>M70</th><th>M100</th></tr></thead><tbody><tr><td>MBV-Uptake (g/(m².%RH))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>1.73</td><td>1.10</td><td>0.86</td><td>0.97</td><td>1.43</td></tr><tr><td>Standard deviation</td><td>0.11</td><td>0.03</td><td>0.06</td><td>0.05</td><td>0.13</td></tr><tr><td>Minimum</td><td>1.59</td><td>1.07</td><td>0.79</td><td>0.93</td><td>1.28</td></tr><tr><td>Maximum</td><td>1.83</td><td>1.14</td><td>0.93</td><td>1.03</td><td>1.59</td></tr><tr><td>MBV-Release (g/(m².%RH))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>1.71</td><td>0.87</td><td>0.69</td><td>0.79</td><td>1.29</td></tr><tr><td>Standard deviation</td><td>0.09</td><td>0.10</td><td>0.03</td><td>0.07</td><td>0.08</td></tr><tr><td>Minimum</td><td>1.59</td><td>0.76</td><td>0.66</td><td>0.69</td><td>1.21</td></tr><tr><td>Maximum</td><td>1.79</td><td>0.97</td><td>0.72</td><td>0.86</td><td>1.38</td></tr><tr><td>MBV-Mean (g/(m².%RH))</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>1.72</td><td>0.98</td><td>0.78</td><td>0.88</td><td>1.36</td></tr><tr><td>Standard deviation</td><td>0.09</td><td>0.14</td><td>0.10</td><td>0.11</td><td>0.12</td></tr><tr><td>Minimum</td><td>1.59</td><td>0.76</td><td>0.66</td><td>0.69</td><td>1.21</td></tr><tr><td>Maximum</td><td>1.83</td><td>1.14</td><td>0.93</td><td>1.03</td><td>1.59</td></tr><tr><td>Hygric regulation classification</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>Good</td><td>Moderate</td><td>Moderate</td><td>Moderate</td><td>Good</td></tr><tr><td>Minimum</td><td>Good</td><td>Moderate</td><td>Moderate</td><td>Moderate</td><td>Good</td></tr><tr><td>Maximum</td><td>Good</td><td>Good</td><td>Moderate</td><td>Good</td><td>Good</td></tr></tbody></table>

Classification categories are defined as Negligible (0–0.2), Limited (0.2–0.5), Moderate (0.5–1), Good (1–2), and Excellent (2 and above). “MBV-Uptake” corresponds to the 8-hour moisture adsorption phase, “MBV-Release” to the 16-hour moisture desorption phase, and “MBV-Mean” represents the average of both. Each measurement was performed on one specimen per biocomposite. Compared to the reference biocomposite M0, “MBV-Uptake” decreases by approximately 36.4%, 50.3%, 43.9%, and 17.3% for biocomposites M30, M50, M70, and M100, respectively. Additionally, “MBV-Release” decreases by 49.1%, 59.6%, 53.8%, and 24.6%, while “MBV-Mean” decreases by 43.0%, 54.7%, 48.8%, and 20.9%.

Fig. 10 shows the relationship between the mean MBV and the substitution ratio. Except for biocomposite M100, the MBV decreases almost linearly as the proportion of RWCC increases. This trend, together with the behavior of M100, closely mirrors the pattern observed for water vapor permeability (Fig. 9). Specifically, lower water vapor permeability, associated with a denser and less porous microstructure as previously discussed (Section 3.3.1), restricts moisture diffusion through the material. This limitation reduces both moisture uptake and release rates, thereby lowering the MBV. Conversely, higher vapor permeability, as observed in M100, leads to a higher MBV despite its lower aggregate volume content (vol% A) (Table 2), which would normally be expected to reduce moisture storage capacity. These findings suggest that, under the conditions studied, MBV is primarily governed by moisture transport within the specimens rather than by their moisture storage capacity, since MBV depends on both the storage and transfer properties of the biocomposite.

The moisture buffer value of the RWCC-based biocomposites (0.66–1.59 g/(m².%RH)) is lower than that reported for hemp-lime biocomposites (1.91–2.53 g/(m².%RH)) [9,14], flax-lime biocomposites (2.03–2.82 g/(m².%RH)) [10], and hemp-based biocomposites (2.93–3.63 g/(m².%RH)) [26], but remains comparable to hemp-lime plasters (1.18–1.68 g/(m².%RH)) [11]. Based on the mean MBV values and the NORDTEST hygric regulation classification, biocomposites M0 and M100 are classified as good moisture regulators, whereas M30, M50, and M70 are mainly classified as moderate regulators, with some values reaching the good-regulator range. Although slightly reduced, the MBV of M30, M50, and M70 remains close to that of well-established good regulators such as hemp-lime plasters [11], indicating that these biocomposites can still effectively moderate indoor humidity fluctuations, particularly when applied over large surface areas. This reduction therefore represents a moderate decrease in hygric buffering capacity rather than a critical limitation, reflecting a practical trade-off associated with the partial substitution of natural resources while maintaining functional relevance for building applications. Overall, all developed biocomposites demonstrate adequate hygric performance, supporting their suitability for indoor applications that require effective moisture regulation.

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Mean moisture buffer value as a function of the substitution ratio.</figcaption></figure>
## 4. Summary and conclusions

Recycled cardboard-based biocomposites are emerging as sustainable and energy-efficient building insulation materials with significant potential. However, their production is often labor-intensive, energy-demanding, and time-consuming. To address this, the present study investigated the feasibility of valorizing recycled waste cardboard chips (RWCC) as partial or complete substitutes for conventional hemp shives in hemp-lime matrices. Untreated RWCC, cut directly from post-consumer cardboard, were incorporated into the hemp-lime matrices without pre-treatment using a simplified, low-energy, and rapid processing methodology. The RWCC were mixed through a rapid blending procedure and lightly compacted, eliminating the need for grinding, crushing, soaking, defibrating, extended compaction, or thermal curing. Substitution ratios ranging from 0 to 100 wt% were evaluated to characterize the physical, thermal, and hygric properties of the resulting biocomposites.

The results demonstrated that, regardless of RWCC content, the developed biocomposites presented low dry bulk density (514–682 kg/m³) and high total porosity (68.7–75.5%), leading to low dry thermal conductivity values (0.0775–0.0919 W/(m·K)). The materials also exhibited relatively high dry specific heat capacity (972–1036 J/(kg·K)) and low dry thermal diffusivity (1.25–1.51 × 10⁻⁷ m²/s) and thermal effusivity (200–248 J/(m²·K·s¹/²)). These characteristics enhance both insulation performance and thermal inertia. Adequate moisture-related behavior was also observed, with water vapor permeability values ranging from 1.24 to 2.12 × 10⁻¹¹ kg/(m·s·Pa) and moisture buffer values between 0.66 and 1.59 g/(m²·%RH), supporting healthy indoor environments.

A comparison with data reported in the literature indicates that RWCC-based hemp-lime biocomposites consistently fall within the performance ranges of established bio-based insulation materials. These results provide a useful framework for selecting appropriate RWCC substitution levels to balance functional performance and environmental considerations. Lower substitution levels (e.g., < 50 wt%) were associated with reduced density and lower thermal conductivity, whereas higher substitution levels (e.g., > 50 wt%) led to increased density and thermal conductivity while remaining compatible with insulation performance requirements.

From an environmental perspective, incorporating untreated RWCC through a simplified, rapid, low-energy production process offers clear benefits. This strategy combines the established advantages of hemp-lime biocomposites with the valorization of minimally processed recycled cardboard, reducing reliance on hemp as a natural resource and promoting circular material use. Although higher RWCC contents may slightly increase embodied energy per unit volume due to greater material density during transport, this impact can be minimized through on-site production and application.

Overall, the developed biocomposites demonstrate substantial potential as sustainable building insulation materials, offering a well-balanced combination of physical, microstructural, thermal, and hygric properties. Incorporating untreated RWCC provides thermal efficiency and moisture regulation comparable to that of other bio-based insulation materials, while the simplified preparation process reduces energy consumption and resource use, supporting a more efficient and environmentally responsible approach to insulation production.

Despite these promising results, certain limitations were identified. Compressive stress was evaluated only at 80 % of the ultimate load, which may underestimate the true mechanical capacity of the materials, and the coexistence of aggregates with highly contrasting aspect ratios was found to adversely affect performance. Additional challenges were encountered in maintaining consistent component proportions in small-scale test specimens used for heat capacity measurements. Furthermore, the behavior of the biocomposite composed entirely of RWCC (M100) highlighted the influence of RWCC geometry, pore connectivity, and aggregate distribution on hygric properties, emphasizing the need for

more detailed microstructural analyses.

Future research should focus on optimizing compressive strength testing protocols for this class of biocomposites, using RWCC geometries and aspect ratios close to those of hemp shives to improve mechanical performance, and preserving original mix proportions at the specimen scale by producing smaller batches tailored to heat capacity measurements. Detailed microstructural analyses, such as scanning electron microscopy, are recommended to better characterize pore structure and aggregate-matrix interactions. Additional studies on RWCC-only bio-composites, considering RWCC content, particle size, aspect ratio, and orientation, along with investigations of acoustic insulation, fire resistance, pest and mold resistance, long-term durability under varied climatic conditions, and life cycle assessment to provide a quantitative evaluation of environmental impacts, are needed to further support mix-design optimization, enhance eco-efficiency, and confirm the suitability of these materials for broader sustainable building applications.

## Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the authors used ChatGPT to assist with grammar checking. The authors have reviewed and edited all content and take full responsibility for the accuracy and integrity of the publication.

### CRediT authorship contribution statement

**Akli Younsi:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Armelle Nouviaire:** Writing – review & editing, Visualization, Validation, Investigation, Formal analysis, Data curation. **Rafik Belarbi:** Writing – review & editing, Visualization, Validation, Resources.

## 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.

## Data availability

The manuscript comprehensively includes all data supporting the reported results.

## References

[1] United Nations Environment Programme, Global Alliance for Buildings and Construction, Not just another brick in the wall–The solutions exist. Scaling them will build on progress and cut emissions fast. Global Status Report for Buildings and Construction 2024/2025, United Nations Environment Programme, 2025. https://wedocs.unep.org/xmlui/handle/20.500.11822/47214.

[2] J.M. Mathews, B. Vivek, M. Charde, Thermal insulation panels for buildings using recycled cardboard: experimental characterization and optimum selection, Energy Build. 281 (2023) 112747, https://doi.org/10.1016/j.enbuild.2022.112747.

[3] M. Char, Y. Khrissi, A. Tilioua, Development of ecological thermal insulation materials based on renewable natural fiber waste and gum Arabic: experimental study and characterization for use in semi-arid climates, Results Eng. 27 (2025) 106510, https://doi.org/10.1016/j.rineng.2025.106510.

[4] M.N.A.M. Taib, A. Salleh, N.A.N. Jamaluddin, T. Rasheed, M.H. Hussin, N. M. Julkapli, V.S. Saji, T.A. Saleh, A review on recent developments of oil palm solid waste upcycling into green bio-based materials for sustainable building and construction applications, Results Eng. 27 (2025) 107049, https://doi.org/10.1016/j.rineng.2025.107049.

[5] R. Khargotra, T. Alam, K. Thu, V. Sebestyén, K. András, T. Singh, Experimental study of eco-friendly insulating materials for solar thermal collectors: a sustainable built environment, Results Eng. 21 (2024) 101681, https://doi.org/10.1016/j.rineng.2023.101681.

[6] N. Asghari, A.M. Memari, State of the art review of attributes and mechanical properties of hempcrete, Biomass 4 (2024) 65–91, https://doi.org/10.3390/biomass4010004.
[7] H. Ansari, M. Tabish, M.M. Zaheer, A comprehensive review on the properties of hemp incorporated concrete: an approach to low carbon footprint construction, Next Sustain. 5 (2025) 100075, https://doi.org/10.1016/j.nxsust.2024.100075.

[8] S. Pochwala, D. Makiola, S. Anweiler, M. Böhm, The heat conductivity properties of hemp-lime composite material used in single-family buildings, Materials 13 (2020) 1011, https://doi.org/10.3390/ma13041011.

[9] O. Mahmood, M. Kavgic, M. Noel, Hygrothermal and mechanical characterization of novel hemp-lime composites with enhanced consistency, Constr. Build. Mater. 450 (2024) 138720, https://doi.org/10.1016/j.conbuildmat.2024.138720.

[10] F. Benmahiddine, R. Cherif, F. Bennai, R. Belarbi, A. Tahakourt, K. Abahri, Effect of flax shives content and size on the hygrothermal and mechanical properties of flax concrete, Constr. Build. Mater. 262 (2020) 120077, https://doi.org/10.1016/j.conbuildmat.2020.120077.

[11] B. Mazhoud, F. Collet, S. Pretot, J. Chamoin, Hygric and thermal properties of hemp-lime plasters, Build. Environ. 96 (2016) 206-216, https://doi.org/10.1016/j.buildenv.2015.11.013.

[12] M. Rahim, O. Douzane, A.D. Tran Le, T. Langlet, Effect of moisture and temperature on thermal properties of three bio-based materials, Constr. Build. Mater. 111 (2016) 119-127, https://doi.org/10.1016/j.conbuildmat.2016.02.061.

[13] M. Degrave-Lemeurs, P. Glé, A. Hellouin de Menibus, Acoustical properties of hemp concretes for buildings thermal insulation: Application to clay and lime binders, Constr. Build. Mater. 160 (2018) 462-474, https://doi.org/10.1016/j.conbuildmat.2017.11.064.

[14] F. Collet, J. Chamoin, S. Pretot, C. Lanos, Comparison of the hygric behaviour of three hemp concretes, Energy Build. 62 (2013) 294-303, https://doi.org/10.1016/j.enbuild.2013.03.010.

[15] K. Pietrak, M. Krakowiak, Ł. Cieślikiewicz, P. Brzyski, P. Łapka, Magnesium-hemp concrete is less vapor-permeable than lime-hemp concrete while the cup method is still problematic, Build. Environ. 280 (2025) 113112, https://doi.org/10.1016/j.buildenv.2025.113112.

[16] S.S. Shanbhag, M.K. Dixit, P. Sideris, Examining the global warming potential of hempcrete in the United States: a cradle-to-gate life cycle assessment, Dev. Built Environ. 20 (2024) 100572, https://doi.org/10.1016/j.dibe.2024.100572.

[17] S. Kaza, L.C. Yao, P. Bhada-Tata, F. Van Woerden, What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050, World Bank, Washington, DC, 2018, https://doi.org/10.1596/978-1-4648-1329-0.

[18] M.H. Mazaherifar, C. Coşereanu, C.M. Timar, S.-V. Georgescu, Physical and mechanical properties of foam-type panels manufactured from recycled cardboard, Constr. Build. Mater. 411 (2024) 134685, https://doi.org/10.1016/j.conbuildmat.2023.134685.

[19] S. Sair, B. Mandili, M. Taqi, A. El Bouari, Development of a new eco-friendly composite material based on gypsum reinforced with a mixture of cork fibre and cardboard waste for building thermal insulation, Compos. Commun. 16 (2019) 20-24, https://doi.org/10.1016/j.coco.2019.08.010.

[20] A. Benallel, A. Tilioua, M. Ettakni, M. Ouakarrouch, M. Garoum, M. Ahmed Alaoui Hamdi, Design and thermophysical characterization of new thermal insulation panels based on cardboard waste and vegetable fibers, Sustain. Energy Technol. Assess. 48 (2021) 101639, https://doi.org/10.1016/j.seta.2021.101639.

[21] A. Benallel, A. Tilioua, M. Garoum, Development of thermal insulation panels bio-composite containing cardboard and date palm fibers, J. Clean. Prod. 434 (2024) 139995, https://doi.org/10.1016/j.jclepro.2023.139995.

[22] EN 459-1 Standard, Building lime-Part 1: Definitions, specifications and conformity criteria, Afnor EDITIONS, 2012.

[23] S. Amziane, F. Collet, M. Lawrence, C. Magniont, V. Picandet, M. Sonebi, Recommendation of the RILEM TC 236-BBM: characterisation testing of hemp shiv to determine the initial water content, water absorption, dry density, particle size distribution and thermal conductivity, Mater. Struct. 50 (2017) 167, https://doi.org/10.1617/s11527-017-1029-3.

[24] EN 826 Standard, Thermal insulating products for building applications-Determination of compression behavior, Afnor ED. (2013).

[25] S. Fekkar, M. Touil, A. Akouibaa, S. Mordane, M. Rahmoune, R. Saadani, Exploring the thermal and mechanical properties of plasters reinforced with argan nut shells: New composites for energy efficiency applications, Clean. Mater. 17 (2025) 100320, https://doi.org/10.1016/j.clema.2025.100320.

[26] A. Hussain, J. Calabria-Holley, M. Lawrence, Y. Jiang, Hygrothermal and mechanical characterisation of novel hemp shiv based thermal insulation composites, Constr. Build. Mater. 212 (2019) 561-568, https://doi.org/10.1016/j.conbuildmat.2019.04.029.

[27] P18-459 Standard, Concrete-Testing hardened concrete-Testing porosity and density, Afnor EDITIONS, 2022.

[28] EN 12664 Standard, Thermal performance of building materials and products-Determination of thermal resistance by means of guarded hot plate and heat flow meter methods-Dry and moist products of medium and low thermal resistance, Afnor ED. (2001).

[29] EN 12667 Standard, Thermal performance of building materials and products-Determination of thermal resistance by means of guarded hot plate and heat flow meter methods-Products of high and medium thermal resistance, Afnor ED. (2001).

[30] EN 821-3 Standard, Advanced technical ceramics-Monolithic ceramics-Thermophysical properties-Part 3: Determination of specific heat capacity, Afnor ED. (2005).

[31] EN ISO 12572 Standard, Hygrothermal performance of building materials and products-Determination of water vapor transmission properties-Cup method, Afnor ED. (2016).

[32] C. Rode, R. Peuhkuri, B. Time, K. Svennberg, T. Ojanen, Moisture buffer value of building materials, J. ASTM Int. 4 (2007) 1-12, https://doi.org/10.1520/JAI100369.

[33] Y. Zouaoui, F. Benmahiddine, A. Yahia, R. Belarbi, Hygrothermal and Mechanical behaviors of fiber mortar: comparative study between palm and hemp fibers, Energies 14 (2021) 7110, https://doi.org/10.3390/en14217110.

[34] C. Zhan, W. Cui, L. Li, A fractal model of effective thermal conductivity of porous materials considering tortuosity, Energies 16 (2023) 271, https://doi.org/10.3390/en16010271.