With a view to reinforcing the earth with a plant fiber as a bio-sourced material, the palm palm (Borassus aethiopum) from Mlomp is chosen ( Figure 1 ). The tree is abundant in the region and throughout the tropical zone of Africa, in Asia up to New Guinea. It has a single stem which forms the trunk with a diameter between 30 and 50 cm and can reach 19 to 30 m depending on the country and climate [23] . It is a versatile tree, from the roots to the leaves, everything is exploited [24] . The leaves are used to make basketwork, walls for buildings, furniture and roofs. For this study, the palm leaf is used as an additive to the various earth samples taken. To do this, the leaf was harvested and dried in the laboratory in the open air for 15 days. Then, it was crushed to obtain fine fibers ( Figure 2 ) to be mixed in proportions of 2%, 4% and 6% with the earth.

Drying took place around 31˚C and the grinding of the palm leaves to obtain fibers was done with a mini plant grinder. These conditions can be applied to other different environments.

In order to find the right construction land to sustainably develop buildings, analyzes and measurements were carried out on 3 sites in the commune of Mlomp. In fact, Momp has buildings of different colors due to the coloring of the construction earth. The 3 types of earth were extracted on site, for the 3 sites, around the houses ( Figure 3 ).

Site 1 appears blocky but very crumbly to the touch. The sample gives a very grainy appearance ( Figure 4(1) ). Site 2 presents with smaller fine grains ( Figure 4(2) ). Site 3 is presented with compact blocks ( Figure 4(3) ). To characterize these 3 types of raw earth, we will carry out the granulometric analysis, the Atterberg limits and the Proctor test.

Granulometric analysis is fundamental for the identification and classification of soils. Granulometry has a great influence on the geotechnical properties of earth [25] . The particle size tests are carried out on the raw earth of sites 1, 2 and 3 according to the standard NF P 94-056.

The Atterberg limits are carried out according to the standard NF P 94-051.

The Proctor test is carried out according to the standard NF P 94-093. The samples are compacted in a standardized mold ( Figure 5(1) ), using a standardized checker according to a well-defined process and conventional energy. We obtain earthen cylinders ( Figure 5(2) ).

Regarding the mechanical and thermal tests, the different samples of raw earth or mixed with crushed palm leaf fibers, for sites 1, 2 and 3, were made at the optimum water content. The same mixtures were used to make the samples for both the mechanical tests and the thermal tests. The percentage of material contained in each test sample is described in the table below ( Table 1 ).

Table 1. Table of composition of materials at the sample level for each site.

The 3 types of raw earth are mixed with crushed palm leaf fibers at percentages of 0%, 2%, 4% and 6% in order to test their mechanical and thermal performances. Greater length and thickness of the fibers lead to difficulties in homogeneity of the mixture and greater segregation of the composite material. It was observed during mixing that samples with fiber dimensions greater than 10 cm present transverse cracks which lead to segregation of the material and transverse cracks during drying.

Compressive and tensile strength tests were carried out in accordance with standards NF EN 13286-40 and NF EN 13286-41 July 2003. For this, molds of 10 × 10 × 5 ( Figure 6(1) ) are used to make test pieces in the form of test specimens of 10 cm × 10 cm × 5 cm ( Figure 7(1) ). For thermal tests, molds of 10 × 10 × 3 cm ( Figure 6(2) are used. All the test specimens obtained are then spread out to dry in the ambient air of the laboratory for a week before the start of the various tests ( Figure 7 ).

The samples obtained are presented in Figure 7 .

The mechanical tests were carried out with a “Multifunction Press” ( Figure 8 ). The sample to be tested is placed in the press and subjected to simple compression or tension until rupture as shown in Figure 8 .

The thermal characterization of the raw earth from Site 1, Site 2 and Site 3 as well as the raw earth with crushed palm leaf fibers mixtures (2%, 4% and 6%) is carried out. To evaluate the thermal properties of the materials, we determined the thermal conductivity (λ) and thermal effusivity (E). Indeed, as noted by [26] in almost all research programs on construction and insulation materials, importance is given to knowledge of these two parameters. The measurements were carried out using the asymmetric hot plane method which is a technique for measuring the conductivity and thermal effusivity of materials in transient conditions. The measurement time is short, however for heterogeneous materials containing inclusions, care must be taken to ensure that the thickness of the sample is equal to several times the size of these inclusions [27] .

The principle consists of sending thermal excitation to the sample using a heating resistor and recording the temperature rise of the probe [28] . This method consists of applying a constant heat flow using a heating resistor on one side of the sample to be characterized and recording the evolution of the temperature T(t) at the center of this same resistance in which a thermocouple was placed. Figure 9 shows thermal conductivity and thermal effusivity measuring device.

Figure 10 shows the schematic diagram of the asymmetric hot plane method.

For more details, this method was also used by [15] , by [29] in their article for the valorization of millet stem fiber reinforced with gum arabic, for the thermal insulation of buildings, by [30] in the study of the thermal properties of typha-gum arabic and typha-starch, by [31] for the thermochemical study and mechanical properties of laterite bricks stabilised with cements and [17] .

The results of granulometric tests are recorded in the figure below ( Figure 11 ).

The distribution of particle size classes is shown in Table 2 .

The grain size curve of the raw lands of Mlomp ( Figure 11 ) shows that they have a fairly tight grain size with the presence of a higher percentage of fines for Site 2 and Site 3. This shows that we know of clayey lands at Site 2 and Site 3. However, Site 1 retains powdery characteristics and is non-plastic ( Table 2 ).

Atterberg limits are geotechnical parameters intended to identify a soil and to characterize its state of plasticity using its plasticity index. The Atterberg boundaries were created only on the lands of Site 2 and Site 3. Site 1 does not contain sufficient clay fraction. The results were entered in Table 2 .

Table 2. Physical characteristics of Mlomp earths.

The results of the Proctor tests on the raw earth samples from the 3 sites are presented in Figure 12 .

The compaction characteristics show that the raw lands of Site 2 and Site 3 have the highest densities. This is explained by the presence of fine clay elements which increases cohesion, thus facilitating the increase in density. We also note lower water contents on Site 2 and Site 3. This can also explain the older and more resistant nature of the constructions at site 2 and site 3, and the greater fragility of the buildings in site 1.

The results of the Proctor tests on samples of raw earth from sites 1, 2 and 3 of Mlomp mixed with 2, 4 and 6% of palm tree fibers are shown in Figures 13-15 .

The results of the Proctor test ( Figures 13-15 ) show that, for all 3 sites, the addition of crushed palm leaf fibers at 2, 4 and 6% gradually decreases as the density increases and increases optimal water content. This reduction in density is explained by a non-homogeneous distribution of the fibers and its powdery nature which reduces the cohesion as well as the density of the earth. In addition, the increase in water content is due to the absorbent nature of the fibers. The increase in water content is explained by the absorbent nature of the palm tree, which facilitates its ability to regulate humidity.

The compressive and tensile strengths of raw earth test specimen and those of raw earth mixed with crushed palm leaf fibers (2%, 4% and 6%) are shown in Figures 16-18 .

The study of the variation in compressive and tensile strengths of the earth from sites 1, 2 and 3 of Mlomp ( Figures 16-18 ) generally shows a decrease in tensile strength and an increase in the compressive strength when the proportion of fibers increases to 6%. For the Mlomp earths, a drop in resistance of compression at 2% of the palm tree fiber mixture was observed. This is due to the low resistance of the palm tree which behaves in the form of a plate and which reduces the cohesion and shear resistance of the mixture. At high percentages, the palm tree occupies a significant volume in the mixture which promotes adhesion between the fibers and increases strength. The compressive strength at Site 3 keeps very similar values because for the raw earth, the results show 0.28 N/mm², the 2% mixture, 0.27 N/mm² and the 4% also, 0.27 N/mm².

This compressive strength reaches its maximum at 6% palm wood for Sites 2 and 3. This variation in resistance is due to a segregation of the palm tree in the mixture ( Figure 19 ) and the random distribution of the palm tree in the mixtures.

For tensile strength, the result obtained from the raw earth sample from site 1 (0.2 N/mm²) is lower than that from sites 2 and 3, respectively 0.24 N/mm² and 0.26 N/mm² ( Figures 16-18 ). This means that the raw earth from site 3 works better in traction than the other two. The mixtures of earth and leaf fibers for site 1 present better tensile results than for sites 2 and 3. But overall, for the earth samples with mixtures of palma leaf fibers, we don’t note improvement for tensile strength. The more crushed palm leaf fibers in the mixture, the lower the tensile strength for all three sites. The addition of palm leaf fibers to the various samples of raw earth does not offer sufficient mechanical strength like concrete to be used as load-bearing structures.

1) Results

Thermal conductivity λ corresponds to the heat flow that passes through a material. It defines the ability of a material to conduct heat or insulate it. The lower the thermal conductivity, the more insulating the material. It is expressed by the unit (W m⁻¹ K⁻¹). The results obtained from the thermal conductivity measurements are presented in Figure 20 .

For site 1, the curve shows that the value of thermal conductivity decreases regularly, from raw earth (1.05 W m⁻¹ K⁻¹) to the mixture of 6% crushed palma leaf fibers (0.51 W m⁻¹ K⁻¹). For site 2, the curve shows the same trend as site 1, the thermal conductivity becomes lower with the addition of crushed palm leaf fibers (raw earth: 0.96 W m⁻¹ K⁻¹ and earth at 6% of fibers: 0.49 W m⁻¹ K⁻¹). The trend observed on sites 1 and 2 is also valid for site 3 with the thermal conductivity which drops to 0.44 W m⁻¹ K⁻¹ with the addition of 6% fibers, while the raw earth is at 0.92 W m⁻¹ K⁻¹. Site 3 has lower thermal conductivities than the other two regardless of the mixture used, even if there is a slight variation depending on the sites. These mixtures have a thermal conductivity much lower than that of concrete which is around 2 W m⁻¹ K⁻¹.

2) Discussion

In each of these three sites, the substitution of soil by fibers reduces the thermal properties of raw earth. This is in perfect correlation with the work of [32] in 2020 which also shows the use of clay and sisal fibers to develop materials for construction. The impact of adding sisal fibers on the thermal properties of these materials was studied. This study showed that the addition of sisal fibers significantly reduces the thermal conductivity of composites. In 2019, Lamrani et al. [33] developed construction materials based on clay, date palm fibers, straw fibers and olive waste. The impact of these fibers on the thermal properties of pure clay was evaluated. The results showed that the addition of these fibers improves the thermal properties of clay composites.

We note that at the 3 sites, the more crushed palm leaf fibers we add, the more the thermal conductivity decreases ( Figure 20 ). This can be explained by the fact that a dense material is replaced by organic matter, a less dense material as evidenced by the conclusions of the mechanical tests on the mixture samples. This makes the material more porous. And the more the thermal conductivity of a material decreases, the more insulating this material is.

1) Results

Thermal effusivity E describes the rate at which a material absorbs and releases heat. The lower the thermal effusivity, the faster the material will heat up with less energy. It is expressed in (W/(Km²)s^1/2. The results obtained from the thermal effusivity measurements are presented in Figure 21 .

We note on the thermal effusivity variation curves ( Figure 21 ) that for all sites, the trend is downward with the addition of crushed palm leaf fibers. For site 1, the curve shows that the thermal effusivity value declines regularly from the raw earth (1221 (W/(Km²) s^1/2) to the mixture of 6% of crushed palma leaf fibers (700 (W/(Km²) s^1/2). For site 2, the curve shows the same trend as site 1. The thermal effusivity becomes lower with the addition of crushed palm leaf fibers (raw earth: 1163 (W/(Km²) s^1/2) and earth at 6% fiber: (701 W/(Km²) s^1/2). The trend observed for sites 1 and 2 is also valid for site 3 with the thermal effusivity which drops to 650 (W/(Km²) s^1/2) with the addition of 6% fibers, while for the raw earth it is 1098 (W/(Km²) s^1/2). Site 3 has lower thermal effusivity values than the other 2 sites compared to the different mixtures ( Figure 21 ).

2) Discussion

At the three sites, the more crushed palm leaf fibers we add, the more the effusivity decreases. This can be explained by the fact that the quantity of earth is replaced by organic matter, a more porous and less dense material. As fibers are added, this contributes to a reduction in thermal effusivity. From the thermal conductivity and effusivity data, we can conclude that the addition of crushed palm leaf fibers improves the thermal comfort of earth walls. With these results, we can say that the raw earth material mixed with crushed palma (Borassus aethiopum) leaf fibers can be an alternative to reduce the energy consumption of buildings. Its use in buildings can contribute to reducing the number of air conditioning hours, and therefore fossil energy consumption, thanks to improving the thermal performance of the envelopes.