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# Physicomechanical and hygrothermal characterization of a sugarcane waste / spent coffee grounds composite for buildings

Tala Moussa<fnref n="1" />*, Chadi Maalouf<fnref n="1" />, Akram Ghanem<fnref n="2" />, Christophe Bliard<fnref n="3" />, Boussad Abbes<fnref n="4" />, Céline Badouard<fnref n="1" />,<fnref n="5" /> Mohammed Lachi<fnref n="1" />, Lina Bufalino<fnref n="6" />, Fabien Bogard<fnref n="1" />,<fnref n="5" />, Guillaume Polidori<fnref n="1" />

## ORCID of the author

Tala Moussa [0000-0002-3530-2514](https://orcid.org/0000-0002-3530-2514)

Chadi Maalouf [0000-0002-3530-2514](https://orcid.org/0000-0002-3530-2514)

Akram Ghanem [0000-0002-8145-6952](https://orcid.org/0000-0002-8145-6952)

Christophe Bliard [0000-0001-7614-1043](https://orcid.org/0000-0001-7614-1043)

Boussad Abbes [0000-0003-1192-6549](https://orcid.org/0000-0003-1192-6549)

Céline Badouard [0000-0001-5722-170X](https://orcid.org/0000-0001-5722-170X)

Mohammed Lachi [0000-0003-4953-8445](https://orcid.org/0000-0003-4953-8445)

Lina Bufalino [0000-0002-7688-3140](https://orcid.org/0000-0002-7688-3140)

Fabien Bogard [0000-0002-6001-0556](https://orcid.org/0000-0002-6001-0556)

Guillaume Polidori [0000-0002-6078-6397](https://orcid.org/0000-0002-6078-6397)

<footnote n="1">¹Institut de Thermique, Mécanique et Matériaux, ITheMM EA 7448, Université de Reims Champagne Ardenne, 51687 Reims cedex 2, France,</footnote>

<footnote n="2">²Ecole d'Ingénieurs des Sciences Aérospatiales (ELISA Aerospace), 48 Rue Raspail, 02100 Saint-Quentin, France,</footnote>

<footnote n="3">³Institut de Chimie Moléculaire de Reims, ICMR-UMR 7312 CNRS, Université de Reims Champagne Ardenne, 51687 Reims cedex 2, France,</footnote>

<footnote n="4">⁴Matériaux et Ingénierie Mécanique, MATIM, Université de Reims Champagne Ardenne, 51687 Reims cedex 2, France,</footnote>

<footnote n="5">⁵Pôle de Recherche Châlonnais, URCA, Châlons-en-Champagne, France,</footnote>

<footnote n="6">⁶Instituto de Ciências Agrárias (ICA), Universidade Federal Rural da Amazônia, 2150-Curio Utinga, Belém 66077, Brazil</footnote>

<footnote>*Correspondence: tala.moussa@univ-reims.fr Tel.: +33 03 26 91 86 75</footnote>
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**Abstract**

Sugar and coffee industries produce considerable amounts of sugarcane bagasse and spent coffee grounds as waste products, these products having very limited value - plant fertilizer for the former and livestock feed or energy source for the latter-. Their valorization potential as construction materials in the built environment is being investigated. In this work, a higher value use of these two agri-wastes is presented through the elaboration and characterization of an environmentally friendly composite material (Starch-bonded sugarcane Bagasse spent Coffee Composite SBCC) for building applications, based on starch-bonded spent coffee grounds reinforced with sugarcane bagasse fibers. The advantages of fiber reinforcement are evaluated by comparison with a previously studied composition solely based on starch and coffee grounds. Samples are characterized in terms of absolute and bulk density, sound absorption coefficient, compressive strength, as well as hygrothermal properties. Fiber incorporation led to a relatively lighter material than the non-reinforced ones, with a bulk density of 321 kg.m⁻³ while presenting increased mechanical properties, withstanding maximum compression stress of 2.8 MPa, and presenting Young's modulus of 3.15 MPa. Both thermal conductivity (0.088 W.m⁻¹.K⁻¹) and diffusivity showed a good thermal insulation capacity (0.2 m².s⁻¹) of the bagasse-reinforced composite. The potential of this new non-structural composite as an acoustic insulator is also assessed, results show that the addition of bagasse did not affect sound absorption compared to the previously studied composites without fibers. In light of the experimental characterization, this new composite would be suitable for use as a non-structural material in building applications.

**Keywords:** Spent coffee grounds; potato starch; sugarcane bagasse; bio-based composite; acoustical properties; mechanical properties; sorption isotherm.
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## Author Contributions

**Tala Moussa**: Conceptualization, sample preparation, formal analysis, data curation, writing -original draft.

**Chadi Maalouf**: Formal analysis, investigation, editing.

**Akram Ghanem**: Formal analysis, investigation, proofreading.

**Christophe Bliard**: Methodology, sample preparation, investigation, formal analysis, manuscript editing;

**Boussad Abbes**: Investigation, formal analysis, validation.

**Céline Badouard**: Investigation, data curation.

**Mohammed Lachi**: Formal analysis, investigation, **Lina Bufalino**: Formal analysis, investigation.

**Fabien Bogard**: Formal analysis, investigation. **Guillaume Polidori** Project administration, supervision, editing, review.

## Data Availability

The data that support the findings of this study are available from the authors.

## Funding

This research received no external funding

## Acknowledgments

The authors would like to thank the Pôle de Recherche Châlonnais for its material assistance, as well as eRcane from Réunion island for providing the bagasse fibers.

## Ethics declarations

**Conflicts of 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.

## Highlights

* Properties of starch-bonded sugarcane bagasse/spent coffee composite are investigated.

* Density, porosity, Young Modulus, and acoustic properties are measured.

* Both thermal conductivity and diffusivity are determined.

* Hygric behaviour is investigated through sorption isotherm.

## Graphical Abstract

<figure><img src="image_1.png" /></figure>
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Abbreviations

<table><tr><td>a</td><td>Thermal diffusivity (m<sup>2</sup>.s<sup>-1</sup>)</td></tr><tr><td>C</td><td>Parameter of GAB model (see Table 2)</td></tr><tr><td>E</td><td>Young's modulus (MPa)</td></tr><tr><td>E<sub>d</sub></td><td>Deviation modulus (%)</td></tr><tr><td>K</td><td>Parameter of GAB model (see Table 2)</td></tr><tr><td>m</td><td>Mass (kg)</td></tr><tr><td>m<sub>0</sub></td><td>Mass of dry sample (kg)</td></tr><tr><td>V</td><td>Volume (m<sup>3</sup>)</td></tr><tr><td>ρ<sub>abs</sub></td><td>Absolute density (kg.m<sup>-3</sup>)</td></tr><tr><td>M<sub>s</sub></td><td>Mass of the sample (g)</td></tr><tr><td>M<sub>1</sub></td><td>Mass of the pycnometer with cyclohexane and the sample (g)</td></tr><tr><td>M<sub>2</sub></td><td>Mass of the pycnometer filled with cyclohexane (g)</td></tr><tr><td>RMSE</td><td>Root mean square error (%)</td></tr><tr><td>SBCC</td><td>Starch-bonded sugarcane Bagasse spent Coffee Composite</td></tr><tr><td>SCG</td><td>Spent Coffee Grounds</td></tr><tr><td>S/EBP-40</td><td>Starch/Extruded sugar Beet Pulp</td></tr><tr><td>SSCC-40</td><td>Starch-bonded spent Coffee Composite (with 40% starch)</td></tr><tr><td>SSCC-100</td><td>Starch Composite (100% Starch)</td></tr><tr><td>w</td><td>Water content (%)</td></tr><tr><td>W<sub>m</sub></td><td>monolayer moisture content (kg/kg)</td></tr><tr><td>α<sub>w</sub></td><td>Water activity (%)</td></tr><tr><td>α<sub>wc</sub></td><td>The weighted sound absorption coefficient</td></tr><tr><td>λ</td><td>Thermal conductivity (W.m<sup>-1</sup>.K<sup>-1</sup>)</td></tr><tr><td>ρ<sub>bulk</sub></td><td>Bulk density (kg.m<sup>-3</sup>)</td></tr><tr><td>ε</td><td>Engineering strain (mm/mm)</td></tr><tr><td>σ</td><td>Engineering stress (MPa)</td></tr></table>
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# 1. Introduction

The fight against global warming is one of the crucial issues of most countries in the world nowadays. High-energy demand, population growth, and increasing greenhouse gas (GHG) emissions intensify the drastic impact on the world's climate, increasing the frequency and intensity of extreme weather events. High temperatures were recently recorded accompanied by torrential rains causing substantial human and economic damage. The International Energy Agency (IEA) reported that electricity demand increased by 2% in 2023 down from the 2.3% observed in 2022 or the 2.4% in the period of 2015-2019, where the major fraction is produced by fossil fuels pushing the GHG emissions to record levels, growing by 3.5% in 2021 and by 2.5% in 2022 [1]. According to the COP26 Climate Conference [2], the building sector is responsible for almost one-third of the total global final energy consumption and nearly 15% of direct CO₂ emissions [3]. Implementing sustainable construction methodologies minimizes environmental impact, fostering a greener industry [4, 5]. Striving for eco-friendly solutions through recycling underscores a commitment to environmental responsibility, advancing the principles of sustainability in the built environment. As a result, a substantial shift is bound to occur in our vision on the use of materials, energy consumption, and bioclimatic building design.

Regarding France, in the continuity of the energy transition for green growth, the French Ministry of Ecology introduced, in January 2022, the new Environmental Regulation RE2020 for new buildings that considers the GHG emissions over the entire life cycle of the building. Sustainable solutions have been proposed to reduce climate impact including the use of bio-based agricultural co-product materials such as hemp, beet pulp, cork, typha, straw, miscanthus, flax, etc. in the building sector [6–12]. These materials can sequestrate carbon from atmospheric CO₂. The low-value co-products mixed with binders can be used as insulators or non-load-bearing walls in construction [13–16]. On the other hand, the circular economy is a major consideration, where waste can be recycled and valorized locally, such as SCG transformed into compost and natural fertilizer for the soil or can also be recycled through a methanization process. Similarly, sugarcane bagasse can be recycled in packaging processes. Numerous studies have considered the use of household or agricultural waste as a Portland cement (PC) substitute - also known as green concrete [17] - due to its pozzolanic properties [18], others suggested their utilization in the elaboration of ceramic materials [19].

Coffee is one of the most consumed beverages in the world after water, where more than 175 million 60 kilograms bags were consumed in 2021-2022 [20] resulting in a large amount of Spent Coffee Grounds (SCG) waste (about two billion tons of agro waste in 2020) [21]. This SCG solid residue, remaining after brewing the coffee grains has a high moisture content and is very prone to mold growth.

SCG contain high amounts of chemical compounds including lignin, lipids, cellulose and other polysaccharides. Some articles describe the extraction of amino acids, fatty acids, polyphenols, and minerals from SCG [22] and their transformation into value-added products.

In terms of waste-to-energy routes and in line with sustainability, and environmental policies, various uses of coffee products and by-products have been studied, and different ways of valorization have been suggested. Most of them include soil conditioners, agricultural fertilizers, nutrient fungi growth substrate and feeding of ruminants [23], dye sorbents [24], gas odor absorbent [25], biofuel [26–28], firelighters [29] and briquettes [30] prepared from a mix of SCG and other raw materials such as wood (pine, larch or spruce sawdust, or pellets) [24, 27, 28]; the high calorific values being 22 to 24 MJ.kg⁻¹ [34]. Other studies have investigated its valorization in the
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production of polymers and composites precursors [35–37], green phase change material [38], or as a 3D printing material [39].

Few papers have reported the use of SCG in the construction domain when most initiatives encourage the introduction of biodegradable materials with low CO₂ impact as alternatives to traditional construction materials.
Roychand *et al.* [40] reported that SCG pyrolysis at 350 °C led to a significant improvement in the compressive strength (about 29.3%) of the composite concrete blended with coffee biochar.

In red ceramics fabrication, SCG have been used to produce a porous structure with better thermal insulation properties [41, 42]. Busch *et al.* [43] studied the feasibility of the valorization of SCG to produce dense/porous bi-layered red floor tiles.

Lachheb *et al.* [44] identified the thermal properties of composite plaster using SCG with different weight percentages (2%, 4%, and 6%). It was found that a 40% drop in thermal conductivity (from 0.5 to 0.31 W.m⁻¹K⁻¹ was observed by adding 6 wt% SCG to the plaster-based material. SCG were used in a novel bio-composite (lime-based mortar) with an optimum percentage of 10% for thermo-plastering applications in construction [45]. Results showed a drop in bulk density (15.4%), an acceptable drop in compressive strength (from 2.8 to 2.5 MPa for samples up to 5% SCG), and a 47% drop in thermal conductivity (from 0.58 to 0.31W m⁻¹·K⁻¹).

Yun *et al.* [46] investigated the acoustical performance of SCG panels. Their work revealed that due to high compressibility and microscopic layered structure offering a large specific surface area allowing the absorption of transmitted sound, SCG is a viable addition to prefabricated noise barriers [47].

Moussa et al. [48] presented a previous study on the properties of a spent coffee grounds/potato starch bio-based composite. Thermal conductivity was found at about 0.093 W.m⁻¹·K⁻¹ with a Young Modulus of 6.1 MPa, and moisture content between 6 and 8% for respectively 30% and 60% relative humidity, which corresponds to the human comfort zone.

Brazil is the world's largest cane sugar-producing country with more than 640 Mton/year [49]. The fibrous residue left after processing sugarcane is called bagasse [50] and constitutes about 30% of the cane weight [51]. Bagasse is mostly burned to provide the energy necessary for the refineries or sugarcane mills to cogenerate heat and electricity [52]. The use of sugarcane bagasse in the building construction sector has become increasingly popular in recent years. As a byproduct of the sugar industry, it offers a sustainable and environmentally friendly alternative to traditional building materials. It is sometimes integrated into clay bricks, adobes, masonry, and compressed earth blocks [53, 54]. One of its main advantages is the durability. Due to its high lignin content, [55] it is resistant to fungi, insects, and fire, making it a reliable option for building structures [56, 57]. It also has excellent insulation properties. Some authors have also added bagasse fiber to reinforce or manufacture gypsum board and cement or mixed it with polymer resins and natural glues [54, 58–60]. Their work suggests that fresh water washing and slaked lime pretreatment of bagasse provide good long-term physical and mechanical properties to these composites and keep insects away as the remaining sugar is removed from the bagasse after treatment. The use of bagasse in construction also contributes to the reduction of waste and promotes sustainable practices [61].

Few studies have been conducted to characterize SCG and their processability when mixed with a vegetal or mineral binder and fibers. In this focus, authors present a novel investigation which concerns the simultaneous valorization of two waste products from the agri-food industry ie sugarcane bagasse and spent coffee grounds which responds to an environmental preoccupation in tropical regions such as Brazil. The novelty lies in the association of these two types of co-products which can be found on a large scale. This work solves some problems
encountered in the previous investigation carried out using SCG and potato starch only [48] and aims to assess the
feasibility of the use of such fully biobased composites in the field of construction instead of fossil non-recyclable
ones such as polystyrene. The fabrication of a starch-bonded SCG composite material with sugarcane bagasse and
the physicochemical properties of the resulting composite are discussed thoroughly.

**2. Materials and Methods**

2.1. *Materials*

The potato starch used in the preparation of the samples was purchased from *Roquette* (Lestrem, France) it is 99% pure. Glycerol was purchased from *Aldrich* (Saint Quentin Fallavier, France). Sugarcane bagasse was supplied by the *eRcane* manufactory (La Reunion, France). SCG were collected from different restaurants in the city of Reims, France. The spent coffee grounds were dried right after collection in a climatic chamber at 60°C and stored in dry conditions to avoid mold growth.

## 2.2. Granulometry

The granulometry of SCG was identical to the one previously published by the authors [48] and presented in Fig.1a. he distribution of the bagasse fibers was performed using the Keyence digital microscope VHX970F. The bagasse fiber distribution is very heterogeneous with a fiber length between 5 mm and 5 cm with a ratio Length/Diameter (L/D) between 5 and 50, the average being around 20 (Fig. 1).

<figure><img src="image_2.png" /><figcaption>**Fig. 1 :** (a): Grading curve of SCG from [48], (b) Sugarcane bagasse fibers.</figcaption></figure>

2.3. *Starch-bonded sugarcane Bagasse spent Coffee Composite (SBCC) preparation*

In a previous work, various compositions of Starch-SCG composite (SSCC) were tested [48]. Results showed that
the compositions with 40% starch (SSCC- 40) showed better results in terms of friability and adherence between
starch and coffee. In the preparation of the Starch-Bonded sugarcane bagasse spent Coffee Composite (SBCC), a
formulation was prepared by mixing 350g SCG with 140g starch, 28g glycerol, and 35g bagasse in 1400g H₂O.
Glycerol was added to help with the cohesion of the composite. The percentages of starch and bagasse are
calculated as a ratio of SCG and not the total sample weight. The optimal amount of bagasse was found to be 10%
of the SCG mass. With more than 45g bagasse (corresponding to 13% in weight), the composition was very
complicated to mix efficiently. The mixture was then heated to 100°C with constant and vigorous mechanical stirring until the mixture turned into a very viscous paste. The obtained paste was shaped in prepared molds with different dimensions: 100x100x100 mm³ cubes and 100x25x25 mm cuboids. The samples were compacted at 44 kPa and then cooled to 60°C before being frozen to -30°C. The frozen samples were pulled out of the molds and freeze-dried for several days (about one week). Interestingly enough, the dry SBCC samples with bagasse did not display the shrinkage previously observed with SSCC.

## 2.4. Mechanical testing

The Instron 33R 4204 testing machine, equipped with a 50 kN load cell, was used for compression tests. The tests were performed in the same direction as the specimen compaction, according to the ASTM D1621 standard for determining the compressive properties of rigid cellular materials [62] under indoor conditions of 23°C room temperature and 50% relative humidity with a rate of 10 mm/min., Pure starch aerogel samples SSCC-100 prepared from 140g starch in 1L H₂O in identical conditions presented dimensions 85x85x85 mm³ after isotropic shrinkage, SSCC-40 compositions samples with dimensions 40x40x40 mm³, and the sugarcane bagasse fiber-based composite samples SBCC with dimensions 100x100x100 mm³ were tested to determine Young's modulus. The measurements were repeated three times to ensure reproducibility. The Instron 33R 4204 testing machine was equipped with Digital Image Correlation (DIC) measurement setup. To measure the deformation during the compression tests, images were captured using two Charge-Coupled Device (CCD) cameras equipped with lenses of 35 mm focal length. Before testing, the sample surface was sprayed with a fine black-and-white acrylic resin-based spray creating stochastic black-and-white contrast patterns for the subsequent DIC procedure. Sets of digital grayscale pictures of the surface contrast were taken every second during testing and served as input to calculate the displacement field.

The apparent Young's modulus was determined by using Hooke's law:

$$ \sigma = E \varepsilon \qquad (1) $$

where σ is the engineering stress (MPa), E is Young's modulus (MPa), and ε is the engineering strain (mm/mm). Typically, the apparent Young's modulus is calculated as the slope of the first linear part of the compression curve σ = f (ε). For each composition, the mean value of three samples was obtained. The apparent Young's modulus E was calculated as the mean value of three samples for each composition.

## 2.5. Thermal conductivity

The device used for the measurements of thermal conductivity, thermal diffusivity, and volumetric heat capacity is ISOMET 2114 (Applied Precision, Ltd.)[47]. Equipped with a needle as a thermal measurement probe, this device uses the heat generated in the electrical resistor to record temperatures periodically as a function of time. This enables thermophysical characteristics (thermal diffusivity, specific heat capacity, and conductivity) to be determined with an accuracy of close to 5%. The measurements were repeated three times to ensure reproducibility using three samples.
## 2.6. Acoustic absorption coefficient

The sound absorption coefficient of the composite material is determined using a B&K 4206 Kundt tube (with two fixed microphones). Two BK 4187 quarter-inch microphones, a BK 2735 power amplifier, a Type 3160-A-042 analyzer, and a power amplifier complete the analysis devices. Sound absorption and surface impedance measurements were carried out according to EN ISO 10534-2 [63]. The wide frequency range (50-6400 Hz) was achieved by using two different tubes of 10 and 2.9 cm. This range is used to measure the sound absorption coefficient of 4 cm thick coffee grounds-bagasse-starch, coffee grounds-starch, and starch samples. Measurements were taken on both sides of three circular cylindrical samples and averaged to ensure reproducibility.

## 2.7. Sorption isotherm

The sorption isotherm or the hygroscopic curve describes the material's water absorption in various relative humidity environment at equilibrium [64]. It can be measured using the continuous or discontinuous method [65]. In this work, the discontinuous method was adopted. The sorption isotherms were determined according to the European standard EN NF ISO 12571 [66] where the amount of adsorbed water vapor is recorded at successive stages at increasing specific pressure in other terms increasing relative humidities

Three cubic samples of dimensions 100x100x100 mm³ were prepared and dried for one week in an oven at 55°C until the dry state was attained. Then, they were introduced in the Binder MK720 climatic chamber at 23°C, and the relative humidity was fixed at 30, 50, 70, 80, and 92 %. For each level, the samples were repeatedly weighed until equilibrium. Equilibrium was reached once the mass of the sample showed a variation of less than 1% of the total mass, on three successive measurements at 24-hour intervals. The moisture content by mass w was calculated according to the following equation:

$$w = \frac{m - m_0}{m_0} \qquad (2)$$

where m, m₀, and w are respectively the mass of the wet sample, the mass of the dry sample, and the sorbed water mass.

After being exposed for about one month to 92% relative humidity, mold growth developed and was visible on the surface of the samples. Experimental results were fitted with GAB [67–69] model which is the most used for building materials based on the theory of physical adsorption, for which the equation is shown in Table 1.

<table><caption>**Table 1.** GAB model and parameters used for sorption isotherms.</caption><thead><tr><th>Model</th><th>Equation</th><th>Parameters</th></tr></thead><tbody><tr><td>GAB</td><td>W = <div style="display: inline-block; vertical-align: middle; text-align: center;"><div style="border-bottom: 1px solid black; padding: 0 5px;">W<sub>m</sub>KCa<sub>w</sub></div><div style="padding: 0 5px;">(1 - Ka<sub>w</sub>)[1 - Ka<sub>w</sub> + CKa<sub>w</sub>]</div></div></td><td>K, C, a<sub>w</sub></td></tr></tbody></table>

Where W is the equilibrium moisture content (in %), W_m the moisture content (in %), a_w the water activity (relative humidity %), C, and K the parameters related to the monolayer and multilayer properties respectively.

Experimental values were correlated using the least squares method. In addition, the coefficient of determination of the curve, R² was calculated to evaluate the quality of the fit of experimental results. The mean relative percentage deviation modulus E_d and the percentage RMSE were as well calculated using the following equations:
$$E_d = \frac{100}{N} \sum_{i=1}^{n} \frac{|m_{exp}^i - m_{mod}^i|}{m_{exp}^i} \qquad (3)$$

$$RMSE = \sqrt{\frac{\sum_{i=1}^{n} (m_{exp}^i - m_{mod}^i)^2}{N}} \qquad (4)$$

where $m_{exp}$ and $m_{mod}$ are the experimental and predicted mass values, respectively, and N is the number of values (N=5) corresponding to the measurements with different relative humidity levels of 20%, 40%, 60%, 80%, and 92%. An RMSE value below 10% is required to ensure the reliability of the measurements compared to the analytical values.

# 3. Results and discussion

## 3.1. Density and Porosity

The bulk density of the samples is defined as the ratio of the total specimen mass $m$ (kg) to the bulk volume $V$, ($m^3$) which includes the volume of the solid, the volume of the pores, and the volume of the space between the grains. It is calculated using the following formula:

$$\rho_{bulk} = \frac{m}{V} \qquad (5)$$

The absolute density of SBCC was determined using the cyclohexane-pycnometer method [10]. It was calculated using the following equation:

$$\rho_{abs} = \rho_{c} \frac{M_{s}}{M_{s} - (M_{1} - M_{2})} \qquad (6)$$

where $\rho_{abs}$ is the absolute density (kg.m⁻³), $\rho_c$ is the density of the cyclohexane, $M_s$ is the mass of the sample, $M_1$ is the mass of the pycnometer with cyclohexane and the sample, $M_2$ is the mass of the pycnometer filled with cyclohexane. The values are presented in **Table 2**.

<table><caption>**Table 2** Absolute density of SBCC</caption><thead><tr><th>Sample</th><th>ρ<sub>abs</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>c</sub> (kg/m<sup>3</sup>)</th><th>M<sub>s</sub> (g)</th><th>M<sub>1</sub> (g)</th><th>M<sub>2</sub> (g)</th></tr></thead><tbody><tr><td>SBCC</td><td>1350</td><td>778</td><td>15.372 ± 0.002</td><td>223.793 +/- 0.002</td><td>217.277 +/- 0.002</td></tr></tbody></table>

The porosity was calculated as:

$$Porosity = 1 - \frac{\rho_{bulk}}{\rho_{abs}} \qquad (7)$$

In the experiments, the porosity was found to be 0.762 which is comparable to the porosity starch-beet-root-bulb composite (0.721-0.778)[70].
## 3.2. Mechanical properties

While knowledge of mechanical properties is not a priority for building materials applied as non-load-bearing structures, it is necessary to ensure that they will support their own weight and the weight of the elements above them without collapsing. The stress-strain average curves of the mechanical compression tests are presented for three samples of each composition: SBCC, SSCC-100 (starch only), and SSCC-40 (Fig. 2). The previous study had shown that the compression of SSCC-100 exhibited a hyper-elastic foam-like behavior divided into three stages: linear elastic stage at low stresses controlled by cell wall bending, followed by a second regime corresponding to the collapse of the cells, truncated by a regime of densification in which the stress rises steeply [71]; whereas SSCC-40, displayed linear elasticity at low stresses, ended by a brittle fracture due to the weakest link mechanism between starch and SCG. By adding sugarcane bagasse fibers a hyper-elastic foam-like behavior is observed with the SBCC sample. At low stresses a linear elastic stage controlled by cell wall bending as in SSCC-100 material is observed because of similar starch foam structure. The effect of sugarcane bagasse fibers is clearly observed for the second and third stages showing the reinforcement action of the fibers. The new material (SBCC) withstands a higher load, compared to starch only, and to a similar composition but without fibers.

<figure><img src="image_3.png" /><figcaption>**Fig. 2** Compression stress-strain curve for SSCC-100 (starch only), SSCC-40 samples, and the new composite with sugarcane bagasse SBCC</figcaption></figure>

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Images of a spray-coated sample with: crack propagation (left) and evolution of the major strain (right) at different loading stages for an SBCC sample.</figcaption></figure>

**Fig. 3** shows images of a spray-coated sample with the evolution of the major strain obtained by DIC at different loading stages for an SBCC sample composite showing strain localization response (Fig. 3 right) and crack propagation (Fig. 3 left) before separation by cracking of the sample. The compression modulus measures the stiffness of the material or the ability of the material to withstand changes in length when subjected to compressive loads. The higher the compression modulus, the stiffer the material. The different measurements are summarized in **Table 3**. For the SSCC-100 sample, the modulus is about 3.38 MPa, for sugarcane bagasse fiber-based composite SBCC, it is 3.15 MPa, while it increases to 6.09 MPa for the SSCC-40. To compare SBCC material to other common non-load-bearing agro-materials with similar starch content (40%), the specific modulus was calculated by dividing the Young's modulus by the bulk density. The results reported in Table 3 show that SBCC has almost the same specific modulus (0.0098 MPa/(kg.m⁻³)) as SSCC-40 (0.0104 MPa/(kg.m⁻³)) and higher than S/EBP-40 (Starch/Extruded sugar Beet Pulp) (0.0088 MPa/(kg.m⁻³)). The usefulness of specific modulus is to find materials that produce structures with minimal weight. Furthermore, the new composite withstands a higher compressive load (σ<sub>c</sub> = 2.8 MPa), compared to SSCC-40 (σ<sub>c</sub> = 1.43 MPa), and S/EBP-40 (σ<sub>c</sub> = 0.52 MPa) [70] thanks to the presence of bagasse fiber reinforcements.

<table><caption>**Table 3** Intrinsic mechanical properties of different agro-composites.</caption><thead><tr><th>Sample/Composition</th><th>Young's Modulus<br>E (MPa)</th><th>Specific Modulus<br>E/ρ (MPa/(kg.m<sup>-3</sup>))</th><th>Compressive<br>strength σ<sub>c</sub> (MPa)</th></tr></thead><tbody><tr><td>SSCC-100 <sup>[46]</sup></td><td>3.38 ± 0.50</td><td>0.0210</td><td>1.53 ± 0.05</td></tr><tr><td>SSCC-40 <sup>[46]</sup></td><td>6.10 ± 0.49</td><td>0.0104</td><td>1.43 ± 0.33</td></tr><tr><td>SBCC</td><td>3.15 ± 0.41</td><td>0.0098</td><td>2.82 ± 0.18</td></tr><tr><td>S/EBP-40 <sup>[65]*</sup></td><td>3.17 ± 0.40</td><td>0.0088</td><td>0.52 ± 0.05</td></tr></tbody></table>

<footnote>*Starch/Extruded sugar Beet Pulp</footnote>
### 3.3. Thermal conductivity

One of the most important thermophysical characteristics related to the effectiveness of the thermal insulation of a material is its thermal conductivity. The latter is defined by the heat flow through a material one meter thick and undergoing on both sides a temperature difference of one degree. The lower the conductivity, the better the thermal insulation. Mineral insulation materials such as glass wool, rock wool, or vermiculite, or synthetic insulation materials such as polystyrene or polyurethane are materials with high insulating power but unlike bio-sourced materials, they are also non-renewable, non-degradable, and not easily recyclable.

Cubic 100x100x100 mm SBCC specimens were studied. On each sample, the average is taken over a series of 10 successive measurements to deduce the final measurement value. The average value of the thermal conductivity obtained on these three samples was 0.088 W.m⁻¹.K⁻¹. This value of thermal conductivity remains very interesting for thermal insulation in buildings. Despite the presence of bagasse fibers, the thermal conductivity of the SBCC sample remained almost identical, even slightly lower than that of the sample without fibers (SSCC-40) whose measured thermal conductivity was 0.093 W.m⁻¹.K⁻¹. In this case, the thermal resistance remains practically the same. The greater the amount of bagasse fibers added to the material, the greater the decrease in thermal conductivity. This aspect is also observed for lightweight foamed concrete; the addition of polypropylene fibers decreases the thermal conductivity of the material [72]. In **Table 4**, the studied SBCC sample conductivity is compared to that of the other two composites previously studied to depict the effect of 10% sugarcane bagasse fiber addition.

The thermal diffusivity “a” characterizing the propagation velocity of a thermal wave in a considered material is given in **Table 4**. The measured value is 0.2 x 10⁻⁶ m².s⁻¹ which is about half the value obtained for both SSCC-40 and SCC-100 [48] but in the same order of magnitude as hemp concrete (0.227 x 10⁻⁶ m².s⁻¹) and cellular concrete (a = 0.274 x 10⁻⁶ m².s⁻¹) [73]. This low value will result in a slow evolution of the thermal wave crossing the material, which, in this case, will present better thermal inertia as well.

<table><caption>**Table 4** Intrinsic thermophysical properties of different agro-composites.</caption><thead><tr><th>Sample/Composition</th><th>Bulk Density<br>ρ (kg.m<sup>-3</sup>)</th><th>Conductivity<br>λ (W.m<sup>-1</sup>.K<sup>-1</sup>)</th><th>Diffusivity<br>a (m<sup>2</sup>.s<sup>-1</sup>) x 10<sup>-6</sup></th></tr></thead><tbody><tr><td>SSCC-100 <sup>[46]</sup></td><td>161 ± 23</td><td>0.066 ± 0.004</td><td>0.379 ± 0.097</td></tr><tr><td>SSCC-40 <sup>[46]</sup></td><td>588 ± 12</td><td>0.093 ± 0.006</td><td>0.438 ± 0.054</td></tr><tr><td>SBCC</td><td>321 ± 10</td><td>0.088 ± 0.013</td><td>0.200 ± 0.055</td></tr><tr><td>S/EBP-40 <sup>[65]*</sup></td><td>360 ± 14</td><td>0.076 ± 0.010</td><td>-</td></tr></tbody></table>

### 3.3. Acoustic absorption coefficient

The development of new materials for the construction industry must ensure that they are both sound-absorbing and heat-insulating. The acoustic insulation significantly reduces noise transmission, creating a quieter indoor environment and promoting better concentration, and overall well-being. Due to the acceptable value obtained for SBCC which is between the Rockwool and Polyurethane, SBCC can be considered as a good acoustic insulator. According to ISO 11654, weighted absorption coefficients (α<sub>wc</sub>) are calculated from practical sound absorption coefficients based on standard frequencies, and then compared to reference curves. Shifting the reference curve

<footnote>*Starch/Extruded sugar Beet Pulp</footnote>
forward by 0.05 increments until the sum of the values under the reference curve reaches 0.1 is the recommended procedure. The weighted sound absorption coefficient corresponds to the value of the reference curve at 500 Hz. The values are given in **Table 5** for the three studied samples (starch-bonded sugarcane bagasse spent coffee Composite (SBCC), starch-bonded spent coffee composite material (SSCC-40), and starch SSCC-100).

<table><caption>**Table 5** Acoustic indicators for the three studied samples and commercial products</caption><thead><tr><td>Sample</td><td>Composition</td><td>α<sub>wc</sub></td></tr></thead><tbody><tr><td>SBCC</td><td>SCG + 40%Starch+10%Bagasse</td><td>0.55</td></tr><tr><td>SSCC-40<sup>[46]</sup></td><td>SCG+ 40%Starch</td><td>0.56</td></tr><tr><td>SSCC-100<sup>[46]</sup></td><td>100% Starch</td><td>0.37</td></tr><tr><td>Rockwool<sup>[46]</sup></td><td></td><td>0.80</td></tr><tr><td>Polyurethane<sup>[46]</sup></td><td></td><td>0.35</td></tr></tbody></table>

When comparing these data, we notice that the α<sub>w</sub> value is identical for both composites SSCC-40 and SBCC. Both values are above 0.5, which means that the acoustic properties of this new composite are effective for use in the building sector like other commercial products, such as Rockwool and polyurethane.

### 3.4. Sorption isotherm

The sorption curve of SBCC samples is presented in **Fig. 4**. Experimental and numerical values of the GAB model are shown. The moisture content increases with increasing relative humidity. At low relative humidity, the moisture content increases linearly with increasing relative humidity. It reaches 13.35 % at 80 % RH after 29 days. In the last phase, the samples were introduced in a container equilibrated with a saturated potassium nitrate (KNO<sub>3</sub>) aqueous solution providing an environment of 92 % RH at room temperature. After 56 days, the moisture content reached 17.6 % for each sample, which is lower than the 21.52% observed in the SSCC-40 samples [48]. A sharp increase in the sorption behavior was observed. Based on the experimental results, the sorption curve is fitted using the GAB model. The numerical values are in good agreement with the experimental results with coefficients of determination of 0.997. The parameters are presented in **Table 6** and it appears that the *E* and *RMSE* values are less than 10 %, which means that the model is suitable [74]. *W*<sub>m</sub> increases up to 20%, which reflects that all the moisture content is in the monomolecular layer of the agro material. K and C are the fitted parameters, which depend on temperature with corresponding molar sorption enthalpies. Also, 0<K≤1 and 0<C<2, so it can be concluded that the sorption isotherm is of type III (isotherms without a point of inflection) [75].

It also can be noticed that the introduction of fibers induced a reduction in moisture content as compared to the SSCC-40 samples. This can be explained by the glycerol effect binding with bagasse fibers and reducing its interaction with water. In the range of thermal comfort zone 30-60 %RH, the moisture content of SSCC-40 is 6 to 8 % higher than SBCC which is between 2% and 7%.
<figure><img src="image_6.png" /><figcaption>Fig. 4 Sorption isotherm of SBCC samples with numerical GAB model.</figcaption></figure>

<table><caption>Table 6 Parameter values for the sorption isotherm model.</caption><thead><tr><th></th><th>C</th><th>K</th><th>W<sub>m</sub></th><th>E(%)</th><th>R<sup>2</sup></th><th>RMSE</th></tr></thead><tbody><tr><td>GAB</td><td>0.570</td><td>0.605</td><td>20.551</td><td>6.704</td><td>0.997</td><td>0.343</td></tr></tbody></table>

Considering the measured value of mechanical, thermal, and acoustic properties, SBCC can be used as a lightweight non-load bearing in constructions, as well as acoustic insulator like conventional material (Rockwool) and thermal insulator (as for hemp-based composites) [76].

## 4. Conclusions

This paper reports the results of an experimental study on a composite based on sugarcane bagasse and spent coffee grounds for building applications. Potato starch was used as a binder for the new material, constituting 40% of the total sample weight, while the sugar cane bagasse reinforcement constitutes 10%. Bagasse fibers substantially reduced shrinkage during the drying process, as compared to other studied bio-based composites. Physicochemical and hygrothermal properties, such as granulometry, density, porosity, compressive Young's modulus, thermal conductivity, thermal diffusivity, acoustic absorption coefficient, sound average absorption, and humidity sorption isotherm were measured and analyzed. The new elaborated composite (SBCC) was compared to other previously characterized compositions using similar methods, namely SSCC-40, a starch bonded spent coffee grounds composite with 40% starch, and SSCC-100, a pure starch formulation. The main drawback of the spent coffee grounds composite studied previously was its friability, whereas the new concept in which fibers are incorporated overcomes this drawback and maintains the structural integrity of the bricks without deteriorating the physicochemical properties. SBCC showed a porosity of 0.762, an absolute density of 1350 kg.m⁻³, and a bulk density of 321 kg.m⁻³, making it substantially lighter than the SSCC-40 composite whose bulk density was 588 kg.m⁻³ and with a porosity of 0.576. However, it remains less porous than the starch SSCC-100 with porosity 0.893. As far as the mechanical properties are concerned, the presence of bagasse fiber in the new composite gives it a higher
compressive strength ($\sigma_c$ = 2.8 MPa), compared to SSCC-40 ($\sigma_c$ = 1.4 MPa) and S/EBP-40 ($\sigma_c$ = 1.4 MPa) [68]. Besides, it has almost the same specific modulus as SSCC-40 and higher than S/EBP-40. This indicates that with the new composite we can produce lighter bricks with better compressive strength. However, this new bio-based composite cannot be used as a load-bearing in buildings. The incorporation of bagasse does not increase the thermal conductivity that was measured at 0.088 W.m⁻¹.K⁻¹. The thermal diffusivity, however, is less than half that of SSCC-40, giving SBCC a thermal advantage as a building material compared to the other compositions. Concerning acoustic properties, the weighted sound absorption coefficient ($\alpha_w$), which measures the weighted sound absorption regardless of frequency with classes going from A (maximum absorption) to F (reflection), for SBCC was calculated at 0.55, which is close to the one previously calculated for SSCC-40 ($\alpha_w$ = 0.56).

Concerning hygric behavior, it can be noticed that the introduction of fibers induces a reduction in absorbed moisture content as compared to the SSCC-40 samples. In the human comfort zone between 30% and 60%, the moisture content of SBCC is between 2 and 7% compared to SSCC-40 where values are between 6 and 8%. This reduces the risk of mold formation (at 80-85%) and fungus proliferation, which can delay the material's degradability over time. Mold in general may lead to the deterioration of insulation, studs, and drywall, and some types can be hazardous to human health and can cause chronic diseases.

The use of bagasse fiber in spent coffee grounds composites embodies several sustainable aspects. It repurposes agricultural waste, diverting both materials from landfills. In addition, the incorporation of these natural fibers significantly decreases reliance on synthetic materials, diminishing the carbon footprint associated with traditional composite production. This approach also promotes circular economy principles by transforming waste into value-added products, fostering a closed-loop system.

Bagasse fiber reinforcement in spent coffee grounds composites represents a sustainable approach to construction that effectively utilizes food waste. Indeed, bagasse fibers enhance the mechanical properties of the composite, providing increased strength, durability, and thermal stability. This can lead to the development of eco-friendly building materials that require fewer non-renewable resources and have a lower carbon footprint compared to solely relying on virgin materials. This minimizes the generation of greenhouse gases associated with food waste decomposition in landfills and contributes to the advancement of eco-friendly building practices.

In conclusion, bagasse fiber reinforcement in spent coffee grounds composites offers advantageous mechanical properties, good thermal capacities, satisfactory soundproofing performance, and predictable hygric behavior, making it a compelling choice for various applications. Moving forward, potential future research directions could explore the optimization of manufacturing processes to enhance scalability and cost-effectiveness. Additionally, lifecycle assessment and further investigations into the long-term durability and environmental impact evaluation of these materials would be valuable for assessing their suitability for widespread adoption in construction projects. Furthermore, exploring novel applications such as in prefabricated building components, structural elements, and sustainable interior finishes could unlock new opportunities for use in the built environment. Overall, this innovative approach presents a promising addition to sustainable construction and bio-based materials.
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