Impact of the loading of date palm fibers on the performances of mortars

Abstract The development of eco-friendly composites as insulating materials in buildings offers practical solutions to reduce energy consumption, which is why scientists have started in recent decades to search for more sustainable and eco-friendly materials. It is well known that building materials are among the most commonly used materials and have an obvious negative impact on the environment. Therefore, this article presents a study on the use of a new bio-composite material, composed of natural fibers of date palm, cement, and sand. The objective of this study is to assess the thermal insulation properties, as well as the water absorption and mechanical performance of this material for the construction of buildings. The percentage by weight of date palm fiber in the test samples varied from 0% to 30% for a mixture of two fiber sizes equal to 3 mm and 7 mm. The characteristics of these samples were determined experimentally in terms of compressive strength, as well as thermal conductivity. The results show that the introduction of date palm fibers in mortars causes a decrease in compressive strength and thermal conductivity. This study affirmed that the use of vegetable fibers in cementitious composites has a positive effect on the properties of insulations and on the costs.


Nomenclature
The composites used consisted of cement, sand and date palm fibers (DPF).Portland cement from Algeria was used, as well as sand from Bousaada-Algeria.This sand is coarser, with a maximum diameter reaching 5 mm with a widely distributed particle size distribution.Date palm fibers are used as inclusions.It was obtained from the oasis of Biskra-Algeria.Due to environmental influences, the palm fibers were contaminated with large amounts of sand and dust.The samples were washed with fresh water and manually disassembled into bundles of fibers.Before use, the DPF meshes were washed with high pressure water to remove polluting particles.Afterwards, these fibers were A huge demand for energy in a building is one of the several factors that have an impact on the environment and our life.One of the promising alternatives to this problem is thermal insulation of the building with the use of biomaterials from vegetal and agriculture waste.These biomaterials need low energy consumption for their production, reduce CO 2 emission, and are renewable materials.In general, previous researches have shown that the presence of natural fibers in concrete can improve crack resistance, toughness, and post-crack performance.On the other hand, the use of natural fibers in concrete leads to problems related to durability in an alkaline medium (Claramunt et al., 2011).Presently, great efforts have been directed towards the use of natural fibers, such as hemp, coir, sisal, jute, and date palm fibers, which are abundant and without risk to the health, as insulation in building materials.They provide cost-effective and high performance insulation, promoting sustainable development (Ramakrishna, 2005;Li, 2000).Natural fibers like date palm fibers (DPF) are a good candidate for the development of effective insulating green materials compared to other materials (Agoudjil et al., 2011).
Mixtures reinforced with date palm fibers are now considered to have good thermal and mechanical properties, besides being lightweight.Thus, they can well be used in building components as new composite materi-als to improve the energy efficiency of buildings (Benmansour et al., 2014;Chikhi et al., 2013;Haddadi, 2015).World production of the date palm is constantly increasing, which indicates the importance of date palm wood.After the date fruit harvesting, important quantities of date palm rachis, and leaflet wastes are accumulated every year.Algeria has more than 710,000 metric tons (Chandrasekaran; Bahkali, 2013).The use of DPF in building technologies can improve thermal insulation performance, especially in desert regions, which can help reduce energy consumption (Abani, et al., 2015).Similar to other plant fibers, such as alfa and hemp, the presence of DPF in concrete leads to a reduction in concrete shrinkage (Taheri;Salwa, 2013).Djoudi et al., (2012) concluded that a certain vibration is essential in order to obtain suitable workability and compaction of the date palm fiber concrete.The fiber content results in a reduction in compressive and flexural strengths.Concrete density and thermal conductivity decrease with increasing fiber content and length (Benaniba et al., 2020;Sethuraman et al., 2020).In this context, some published articles have focused on new eco-friendly materials reinforced by natural fibers, such as palm fibers, (Ali;Alabdulkarem, 2017) corncob, (Laborel et al., 2018, Chabriac et al., 2016) rice straw, (Bassyouni et al., 2015, Ismail et al., 2022), and sisal fibers (Li et al., 2000, Ohenoja et al., 2016;Sağlam et al., 2022).The potential of this raw material is based on its sustainability, low cost, availability, low density, great quantity, and low environmental impacts.(Senthamaraikannan et al. 2019 ;Berardi et al., 2016 ;Al-Oqla et al., 2014).Many researchers dealt with the use of palm tree by-products in construction materials due to their high thermal insulation properties.In addition, (Sun et al., 2022;Chikhi et al. 2013) studied the effect of the petiole and rachis of palm trees on thermal conductivity and compressive and flexural strengths of gypsum-based composite materials.The effect of the same fibers on thermal conductivity and compressive strength of mortar-based composite materials has been investigated by (Benmansour et al., 2014).In addition, Kriker et al., (2005) and Benaniba et al., (2020) have evaluated mechanical properties of reinforced concrete with Algerian DPF.Djoudi et al., (2014) have studied the effect of the addition of Algerian DPF on thermal and mechanical properties of plaster concrete.
In this study, an effort has been made to investigate the effect of the inclusion of DPF on the mechanical and thermal properties of bio-composite concrete.For this, two fiber lengths (3 and 7 mm) were chosen with contents of 3% to 15%.The results presented herein can open a production component of cement composites containing date palm fibers with good mechanical properties, low cost and reduced environmental impact.

Introduction
The composites are obtained by mixing prompt Portland cement (CPJ-CEM II / A 32.5), sand and water for several concentrations by weight of DPF Mix equal to 6%, 12%, 18%, 24% and 30%.Composites are made by mixing fibers, cement and sand in a blender with a velocity equal 40 rpm.Dry mixing is necessary to homogenize the mixture.The three constituents Ce-ment, DPF and sand were then mixed.Water was added gradually and the mixing continued for 5 min.To reduce water loss by evaporation in the ambient air, the mixture was quickly poured into rectangular molds with a shape of 40 × 40 × 160 mm 3 .The mold was filled with material and left in the open air for almost 24 hours.Then, the samples were immersed in water at 21±2 °C for 28 days according to standard EN 196-1 (Dhakal et al., 2006).The samples were dried in the open air for 48 h in the molds and 28 days after demolding.The proportions of the materials used in the mixtures are presented in Table 1.Thus, MDP Mix (Mixture of sand, cement, DPF3 and DPF7 ) is designed as a mixture of sand, cement, DPF3 and DPF7.The SEM micrograph of a crude fiber shows the surface containing a large number of unfinished cultivated fibers that should be residual lignin and artificial impurities (sand and dust).A cross section of a single DPF reveals a large number of single hollow fibers collected and linked by a primary layer, as presented in Figure 2. first dried in the sun for two days and then in an oven at T=70°C until dry.Then, the fibers of diameter less than 0.8 mm were cut to the desired length.In this study, two fiber sample sizes equal to 7 mm (DPF7) and 3 mm (DPF3) are considered as shown in Figure 1.DPF Mix (Mixture of DPF3 and DPF7) is then designed as a mixture of DPF3 and DPF7.The compressive strength device is illustrated in Figure 3.At 28 days, the compressive strength was tested on a half-prism obtained after rupture of the samples according to standard EN 196-1 (Taoukil et al., 2011).The mechanical properties obtained from the tests are carried out on five test samples.The compressive strength of mortars is determined from the maximum load using the following equation:

Composite preparation
Figure 4 presents the water absorption of composites samples.It shows that the addition of fibers in the mortar produces an important increase in the water absorption.At first, it was noted that the water absorption coefficient of vegetable fibers was very high.They are able to absorb a mass of water greater than their own mass.
We can explain this by the fact that the DPF is a hygroscopic material.(Chikhi et al., 2013) studied the effect of water on gypsum composites filled with DFF and found that the water content in the composites strongly depends on the water absorption capacity of DFF.In addition, (Toukil et al., 2012;Kessal et al., 2022) investigated the effect of water on a sandy solution filled with wood wool and discussed similar behavior.They also found that the high hygroscopicity nature of wood wool is a disadvantage that results in high shrinkage and swelling of the composites.This can lead to damages that can affect the durability of building materials.

Water absorption tests of MDP Mix
(2) The thermal conductivity of the test pieces was measured under laboratory conditions of 50% RH and 20°C by using the hot-wire method after drying of samples for 28 days.This transient method is the classic method of measuring the thermal conductivity of insulating materials.The method involves placing a thermal shock probe sandwiched between the two samples to be characterized (Dupain et al., 2009).The probe locally produces a weak heating of the material to a few degrees above ambient temperature.Evaluation of thermal conductivity and volume heat capacity is based on periodically recording of the sample temperature as a function of time.By mathematical processing of this signal integrated in the software provided, the thermal conductivity is identified.

Thermal conductivity of composites
(1) This test consists of drying the samples in an oven for 24 hours at a temperature of 70 ± 2°C to ensure a complete loss of moisture, and then immediately weighed to the nearest 1mg.This weight is designated as ms by using a Sartorius balance with a PT150 mod-el.Then, the samples were immersed in water at room temperature for different periods equal to 5 minutes.After that, they were removed and the water droplets on their surfaces were cleaned with a dry system.Before returning the samples to the water for saturation, we have measured the new weight designed by m(t).Thus, the percentage of water absorption in the samples was calculated by the difference in mass between the samples immersed in water and dry samples using the following equation (Boumhaout et al., 2015):

Water absorption of DPF
Figure 6 shows the relationship between the thermal conductivity and the density of the composites.From these results, it can be seen that the thermal conductivity of composites increases when the density increases.Indeed, there has been observed a direct relationship between the den-sity of the composites and thermal conductivity.An increase in air voids leads to a decrease in the density of composites, which leads to a decrease in resistance and a decrease in thermal conductivity.The relationship between conductivity and density is always tested on materials with a min-eral matrix and plant fibers according to Alrim et al., (1999); Khedari et al., (2004), andBelkadi et al., (2022) where they used a clay matrix and Portland cement, respectively.Thus, it is important to preserve the fact that the influence of the concentration of DPF has a stronger effect on the

Effect of the density on thermal conductivity
Figure 5 shows the evolution of the thermal conductivity as a function of fiber content.From these results, it has been noted that the addition of the DPF into the mortar matrix reduces the thermal conductivity of the composite.This decrease is expected because date palm fibers have a lower thermal conductivity than the mortar matrix.In addition, it is interesting to remark that the fibers embedded in the composite tend to generate porosity and air in the matrix and reduce the density.Therefore, the first benefit of using natural fibers is to lighten the composite.The thermal conductivity of porous materials is determined by the voids in the samples.These voids occur due to the packing of the fibers.In general, short fibers are harder to align and pack denser than larger fibers (Khedari et al., 2001;Chiker et al., 2021;Dridi et al., 2022).Thus, for given fiber content, a mortar filled with short fibers leads to an increase in the number of voids, which leads to low thermal conductivity and lower density of the composites.For ϕ > 24%, it is clear that the influence of the fiber content on the thermal conductivity of the composite is negligible.Thereby, according to the obtained results, the decrease of thermal conductivity is about 87% for the samples MDP Mix , for the fibers content of 30%.This decrease is due to the significant difference between the thermal conductivity of the fiber and matrix materials used and the insulating nature of the fibers, which have a thermal conductivity of 0.083 W m -1 K -1 (Ledhem et al., 2000;Al Rim et al., 1999).
Indeed, it has been observed that beyond approximately 24% of date palm fibers loading, the decrease of the thermal conductivity of composite was relatively low.For this reason, it has proven useful to limit the fiber content around this value.In fact, from the results achieved in this study, we can clearly assume that the addition of date palm fibers could significantly improve the thermal insulation of composites with a decrease in effective thermal conductivity.The values range between 6.83 MPa and 28.88 MPa, which represent the lowest and the highest MDP Mix compressive strength having a fiber content of 30% and 6%, respectively.Indeed, it has been noted that the compressive strength decreases with the increase of date palm fiber content.In fact, for 6% and 30% of date palm loading, the compressive strength of composites decreases by about 18% and 80.51%, compared to the mortar without fibers 0%.This behavior is due to the increase of fiber content that leads to a low density of the samples.In fact, at high content, if the fiber is stiff, the packing of the fiber becomes difficult, and voids are introduced into the composite.A relationship between date palm fiber content and compressive strength appears clearly.The higher the date palm fiber content, the lower the compressive strength.This relationship is consistent with several studies performed on concrete reinforced with vegetal fibers.Thus, the use of vegetal fibers does not increase the compressive strength of concrete, as they increase the volume of voids and decrease the compactness of the composite (Bahloul et al., 2009;De Pellegrin et al., 2021).Theoretically, the addition of DPF to a cement matrix will reduce the compressive strength of the composite.Therefore, the increase in the porosity of the composite material as a result of the addition of fibers is the main factor responsible for the decrease in compressive strength.These results are in accordance with literature (Aruniit et al., 2011;Belkadi et al., 2018).

Compressive strength tests
thermal conductivity and density of the composites.In addition, the pres-ence of water reduces the insulating capacity of our composites.In this article, we are interested in the experimental study of the effect of the inclusion of DPF on the mechanical and thermal properties of bio-composite concrete.From the experimental results, we conclude that the use of the date palm fibers allows reinforcing concrete yields with the modification of some characteristics of the concrete:

Correlation between conductivity and water absorption
• The experimental investigations indicated that the increase of DPF content allows lightening of the mortar by decreasing its density and increases the insulating capacity of mortar by decreasing its thermal conductivity with about 87% for the MDP Mix samples with a fiber content of 30%.
• The resulting behavior shows a rapid increase in thermal conductivity with the absorption of water.
• The incorporation of DPF in the matrix does not improve in any case the mechanical compressive strength of concrete, where the values range between 6.83 MPa and 28.88 MPa, which represent the lowest and the highest MDP Mix compressive strength having a fiber content of 30% and 6%.
• Thus, the compressive strength is significantly reduced due to the inhomogeneous dispersion of the fibers, which can form clusters.
• Therefore, an increase in the porosity yields to the increase in the volume of voids, affecting the compactness of the composite.These outcomes will be considered in the future by using the industrial and agro-industrial materials for sustainable building construction.

Figure 2 -
Figure 2 -SEM micrographs of raw DPF (Cross-sectional view of the fiber).

Figure 3 -
Figure 3 -Compressive strength test device of the MDP Mix .

Figure 5 -
Figure 5 -Thermal conductivity as a function of concentration of DPF for composite MDP Mix .

Figure 4 -
Figure 4 -Water absorption as function of fibers content.

Figure 8
Figure 8 represents the correlation between resistance to conductivity and the density of mortars made with date palm

Figure 7
Figure7shows the variation of the compressive strength of the studied concrete samples as a function of the date palm fiber content after 28 days of hardening.The values range between 6.83 MPa and 28.88 MPa, which represent the lowest and the highest MDP Mix compressive strength having a fiber content of 30% and 6%, respectively.Indeed, it has been noted that the compressive strength decreases with the increase of date palm fiber content.In fact, for 6% and 30% of date palm loading, the compressive strength of composites decreases by

Figure 7 -
Figure 7 -Compressive strength as a function of the concentration of the fibers.

Figure 10
Figure 10 represents the correlation between the compressive strength versus the water absorption of mortars made with date

Figure 9 -
Figure 9 -Evolution of the conductivity versus the water absorption.

Figure 8 -
Figure 8 -Evolution of the conductivity versus the density.

Figure 9
Figure 9 represents the correlation between resistance to conductivity and the water absorption of mortars made with date palm fibers.A polynomial rela-

Table 1 -
Proportions of the materials used in mixtures.