INVESTIGATION ON THE USE OF THE CEMENT MORTAR CONTAINING BANANA FIBERS AS THERMAL INSULATOR IN BUILDING

* Sibiath O. G. Osseni 1 , Clement Ahouannou 1 , Emile A. Sanya 1 and Yves Jannot 2,3 . 1. Laboratory of Applied Energetics and Mechanics (LEMA), Polytechnic School of Abomey-Calavi / University of Abomey-Calavi, 01 BP. 2009 Cotonou (Benin). 2. University of Lorraine, LEMTA, UMR 7563, Vandœuvre-les-Nancy, F-54500, France. 3. CNRS, LEMTA, UMR 7563, Vandœuvre-les-Nancy, F-54500, France. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

For composite materials development, a growing interest is also found in the plant fibers as reinforcements for their availability in terms of their quantity, their annual regularity and their no conflicting usage. Among the most used fibers include flax fibers, hemp, jute, cane, bamboo, raffia, cotton, kapoc, coconut, abaca, sisal, curauá, alfa, banana ..., the majority has been physico-chemically, morphologically, mechanically and thermally studied and characterized (Ardanuy, 2015). Toledo (1999), Wang et al. (2007), Tung et al. (2012) and Mostafa et al. (2015) showed that the addition of the fibers (coconut, sisal, flax, and banana) significantly improves the mechanical properties of the cementitious composites, like flexural strength, Compressive strength, ductility, which can be used for the production of structural elements in building. Note that the fibers have previously undergone various treatments, in order to ensure the durability of the materials.
In this study, we have selected the banana trunk fibers as reinforcement element to the usual cement matrix because it is a renewable resource, locally available in Benin and there is a few works reported in the literature on composites made of cement mortar-banana trunk fibers. The overall objective is the valorization of Benin natural resources through the development of new composite materials banana fibers-cement mortar which can be used as thermal insulator in building. The thermal characterization will then allow us to assess the effect of fibers incorporation in the matrix mortar.

Materials and methods:-
Materials:-Banana trunk:-Banana plant is from the hot, humid tropical jungles of Southeast Asia. From the Musaceae family, banana would be the largest herbaceous flowering plant. Its trunk consists of a single branch which, towards the end of its life, will generate a regimen of fruits called "bananas". It has a buzzing rhizome that gives rise to leaves with sheaths. The stipe or pseudo-trunk has a large diameter up to 20 cm and a height, of up to 8 m, depending on the variety. The true stem of the banana plant does not exceed the ground level: the rhizome, from which are born (up) auxiliary buds that give discharges, ensuring the sustainability of the species and (down) of roots which penetrate into the ground (Fig  1). The banana production continues, according to FAOSTAT in 2014, to grow both in Benin and in the rest of West Africa. A banana "tree" or plant often produces only one regimen of fruits at a time, hence the importance of the cut, after harvest, to make place for a next one. This raw material is then always locally available.

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Fibers extraction and processing:-The banana tree trunks are cut after collecting the banana stems and detached in scales. The fibers extraction is done manually using a dull knife which serves to scrape the scales surface. All fibers obtained are soaked in hot water and dried after cooling in order to remove surface impurities. They are also rinsed with tap water and dried at 60 °C five times to reduce their water absorption capacity. The fibers (Fig 2) are then cut at an average length of 30 mm using scissors.

Composites formulation:-
The mortar samples consist of CPJ 35 cement giving a compressive strength at 28 days equal to 35 MPa, the lagoon sand with grains diameter less than 4 mm and the cement is dosed at 250 kg m -3 of the mixture. The ratio W/C is equal to 0.7 to allow composites molding.
The weighed and pre-wetted fibers are thoroughly drained and added gradually to the mortar in the proportions of 1%, 2% and 3% by substitution to the equivalent percentages in mass of cement and sand mixture. Beyond 3%, we get a little malleable mixture and difficult to mold (Fig 3). All samples are molded into the dimensions: 10 × 10 × 3 cm 3 (Table 1, Fig 4). They are dried in the open air for 48 h, oven dried for 24 h at 103 ° C ± 2°C then packed in zip bags for three days before testing. This set up condition allows homogenizing the moisture in the material.   (1)

Determination of the water absorption:-
Operations are performed in the following order:  immersion in a water tank at 28 ± 2 ° C for 72 hours. Before weighing ( ), the sample is wiped with a soft cloth to rid the superficial water  drying for 72 h in a ventilated oven whose temperature is maintained at 103 ± 2 ° C.
The weight is considered constant when two successive weighing 24 h do not give a greater than 0.1% difference. The water absorption is expressed in percent of the dry mass and is calculated by the following relationship: = − × 100. (2)

Thermal properties measurement:-
The thermal properties, such as thermal effusivity, the volumetric heat capacity and thermal conductivity, are determined by the hot plate method with two temperature measurements (Figs 5 and 6).   The temperatures of the two aluminium blocks are equal and identical. A good thermal contact is achieved because the components of the hot plate (insulating blocks, aluminum blocks, heater and thermocouple) are correctly set on the tray of the gallows by a clamping device with a moderate holding pressure provided by a threaded metal pin.
Three-dimensional modeling of hot plate with two temperature measurements:-The transfer in the sample can be modeled in 3D as followed ( Fig 7
The analytical solution of equation (3) with initial and boundary conditions, equations (4 to 10), by the variables separation method and the Laplace transform is complex. But the numerical method, based on Comsol simulations of the device and the quadrupole modeling, is the most used.
Quadrupole modeling:- Fig 6 defined the used notations in the model, as the Laplace transform of the temperature and the Laplace transform of the heat flux density, then the location of the two thermocouples in the experimental device. It is considered that the transverse dimensions of the heater is large compared to the sample thickness. The heat transfer is therefore unidirectional in the central area of the device.
The thermal contact resistances, at the interfaces of heating element/polystyrene and sample/polystyrene, have been neglected because of the contact with a soft and flexible thermal insulator. Considering that the heating element is enough thin such that we can apply the quadrupole formalism (Maillet, 2000) which gives: The thermal balance between the heater and the unheated face of the sample can be written where: 1 Laplace transform of the temperature in the unheated face; ∅ 1 Laplace transform of the heat flow density on the unheated face of the sample setting: 1149 The temperatures 0 and 1 in equations (13) and (15) are obtained in the Laplace space. The inverse Laplace transformation is realized by using the De Hoog algorithm (De Hoog, 1982) to calculate the temperature of the heated face 0 and the temperature of the unheated face 1 .
The principle of this method consists to estimate the value of the parameters and , which is the product of the density (kg m -3 ) and the specific heat, of the sample that minimize the sum of the quadratic error Ψ = − 2 =1 between the experimental and the theoretical temperature curves calculated from equations (13) and (15).

Estimation method of the thermal parameters:-
The estimation errors on the two thermal parameters of the samples to be characterized are due to the uncertainties on the "known parameters", the noise on the temperature measurements and an error model when a 1D model is used to represent a 3D heat transfer.
The following method has been used to evaluate the two last estimation errors as a function of time interval, for one surface temperature measurement method on one hand, and for two surfaces temperature measurement method on the other, considering two structure materials, respectively: a) cellular concrete (low density and conductivity) and b) concrete (high density and conductivity) which thermal properties are: (  a 3D simulation of 0 and 1 is realized using COMSOL software and considering a constant time step of 0.1 s  a random noise with a standard deviation value = 0.1 °C is added to each signal  these simulated and noised curves 0 and 1 are used as experimental curves to estimate the thermal parameters by three different methods :  the relative deviations of the estimated values from the initial values used in COMSOL simulations are calculated. The best estimation method will be the one which gives the lower relative error. Fig 8 shows that the best method to estimate is the "1T E lam" method enabling a precise estimation on the larger time interval. Then, the best way to estimate the other parameters is to estimate the thermal diffusivity by the "1T E + 2T a" method enabling a precise estimation of until 800 s. The other parameters and can then be calculated from the determined values of and . a) For cellular concrete ("1T E") , estimation of and using 0 ("1T E lam"), estimation of and using 0 and 1 ("2T E lam"), estimation of using 0 and 1 with known by "1T E" method ("1T E + 2T a").

Results and Discussions:-
After calibrating the device, the estimated surface of the heating element was 0.0105 m 2 and its estimated thermal capacity was 428.6 (J K -1 ). From the different measurements, we plotted the graphs of the figures 9 to 12.    Density and thermal properties:-It could be noted that the density, the thermal effusivity and the thermal conductivity decreased when the percentage of incorporated fibers increased, giving respective relative deviations of 25.54%, 56.09% and 65.68% between the thermal conductivity of the reference sample, which was equal to 1.14 W m -1 K -1 , and that of the composites. An analysis of the results on these figures allowed concluding that the cement mortars containing banana trunk fibers were more insulating than conventional mortar.
Water absorption:- Fig. 12 showed that the water absorption was substantially linear and increased respectively of about 1.5, 2 and 2.5 times for composites containing 1%, 2% and 3% of fibers in relation to the reference. Similarly, the thermal effusivity and conductivity decreased when the absorption rate increases. Depending on the climate, these composites would be used alone in construction or set in sandwich with a less hygroscopic and insulating material to reduce their ability to absorb water for example and to guarantee their durability. These results were interesting because in a tropical climate, low heat conductivity materials and low specific heat were searched for thermal insulation in building. These materials could be used, for example, to fill a carrier structure, to achieve the insulating boards, for cladding or for internal partitions valuing agricultural waste while reducing the quantities of used cement and sand.

Conclusion:-
One have formulated and made composites containing banana trunk fibers previously washed with hot water and dried. Cement were dosed at 250 kg m -3 of mortar with a ratio W / C equal to 0.7. By using the hot plate method, one had shown a strong relationship between the fibers proportion and the thermal effusivity and conductivity for new synthesized composite materials. Then, these properties made to these materials, the candidates for thermal insulators in building. The water absorption was also function of the fibers proportion. A further study on mechanical properties and microstructure of the fibers and the composites would allow defining their optimal use conditions.