Thermophysical characterization of a new clay-based construction material from the Atlas region

As part of the valorization of local materials and the search for energy efficiency in the building sector, a thermophysical study was carried out on a new clay-based composite material from the Atlas region reinforced by peanut shells, which constitute an abundant ecological and untapped waste. In this study, a series of samples (100×100×26 mm3) of this composite are elaborated at different mass fractions of peanut shells. The thermal parameters are estimated by metrological methods such as the steady-state hot plane method and the flash method. The results showed, in addition to the lightness offered by peanut shells to these composites, that the thermal conductivity and thermal diffusivity vary respectively from values 0.623 W.m−1.K−1 and 4.8 m2.s−1 for raw clay to the values 0.323 W.m−1.K−1 and 3.5 m2.s−1 for the mass fraction of peanut shells to ϖ = 9%, which give this material the quality of the insulating walls.


Introduction
The building industry remains the largest consumer of energy and among the factors that can reduce this consumption is thermal insulation, therefore our contribution is to develop new building materials with improved thermal performance while giving value to local materials. A lot of research has been carried out on local materials such as cement stabilized clay studied by H. Ezbakhe et al [1]. El Bakkouri et al [2] presented a thermomechanical study of lightweight concrete with cork or with olive pomace. Khabbazi et al [3] carried out an experimental study of the thermal and mechanical properties of a new insulating material derived from cork and cement mortar. Elhamdouni et al [4] have demonstrated the effect of alfa fibres on the thermophysical characterization of a clay extracted from northern Morocco. Mounir et al [5] have worked on the thermal inertia and thermal properties of clay-plastic composites. The same authors [6] carried out an experimental study of the thermal properties of clay and sheep's wool samples, the results showed that the addition of sheep's wool gives the clay a 45% lightness factors. Cherki et al [7] studied the thermal behavior of an ecological composite material based on gypsum plaster reinforced with cork grains, they found that this composite is thermally better than plaster without additives. Lamrani et al [8] highlighted the effect of olive pomace on an extracted clay from southern Morocco. In another work, the same authors studied the thermal performance of a new consolidated plaster-based material with peanut shells, their results showed that the thermal conductivity is reduced from 0.301 W.m -1 .K -1 for plaster without additives to 0.141 W.m -1 .K -1 for plaster with a mass fraction of 20% of the peanut shells, thus proving the possibility to use this new material as a false ceiling [9] . In this work we are interested in developing and characterizing a new material based on clay and peanut shells which is a renewable natural product and eco-friendly. We have highlighted the influence of the mass proportions of additives on the density and thermophysical properties of the materials produced. Among the metrological methods used to identify thermophysical parameters, we chose the steadystate hot-plane method developed by Jannot et al [10] to estimate the thermal conductivity of the material and the Flash method to estimate thermal diffusivity [11] [12]. Many authors have already used these methods to characterize the thermophysical properties of heterogeneous composite materials, so these methods have shown each time their robustness for insulating materials.

Sample preparation
The materials used are: Raw clay: it is a local natural material coming from the region of the atlas, it can constitute the binding phase of the composite Peanut shells: it is an abundant, ecological and untapped waste. The dispersed particles of the peanut shells used have sizes between 2,5 mm and 5 mm. The SEM image of a particle of peanut shells (figure 2) shows a microporous morphology that justifies a possibility of air containment improving the thermal insulation of the material. The samples prepared are composed of a clay matrix containing peanut shells. We found, after a few tests, that the mixing rate that allows the clay paste to be prepared with a normal consistency is with and are respectively the quantity of water and the quantity of clay.
Samples are prepared by incorporating the peanut shells into the clay paste and then poured into molds. After 24 hours they are removed from the molds and left in the open air for 4 days. To remove moisture from the pores of each of these samples, they are dried in a vacuum draying oven at a temperature of 60 degrees. After drying they are weighed and stored in plastic bags.
The macroscopic aspect of samples with different mass contents of peanut shells is illustrated in

Hot plate method in steady state:
This method makes it possible to characterize the thermal conductivity of the samples [10] , its description is given in figure 4, where a heating element is sandwiched between the sample and an insulating foam, in order to maximize the flow of heat through the sample, the thermocouples are positioned to measure the temperatures T 1 and T 0 at the centers of the upper and lower faces of the sample and the temperature T 2 at the center of the heating element , the role of the two aluminium blocks, which has a fairly high thermal conductivity, is to reach steady state after a reasonable time. We can write: with: is the total flow emitted by the heating element by joule effect, 4 and are successively the thermal conductivity and the thickness of the insulating foam. is the thickness of the sample, and are successively the electrical resistance and the surface of the heating element traversed by an electric current I under the effect of a voltage imposed on its terminals.
The combination of these equations leads to the expression of the thermal conductivity of our sample:

Theoretical models of equivalent thermal conductivity of a two-phase composite material
Theoretical models have been developed in the literature for predicting the equivalent thermal conductivity of a composite material consisting of two phases: continuous phase and dispersed phase [13][14] [15]. The expressions of the equivalent thermal conductivity for each model are given by the following relationships: -Series model: -Maxwell model: with: and are respectively the volume content and the thermal conductivity of the dispersed phase and is the thermal conductivity of the continuous phase

Flash method
This method is used to estimate the thermal diffusivity of solids [11][12], its principle is described in Figure 5: The Laplace transform of the temperature elevation on the underside of the sample is determined using quadruples formalism [16] by the expression: With: : The Levenberg-Maraquart algorithm [18][15] is used to estimate parameters reducing the quadratic error between the experimental temperature recorded at the center of the sample and theoretical expression given by the complete model these two algorithms are programmed in MATLAB

Results and discussion
In order to take account of the measurement error, three measurement tests are carried out for each sample and the arithmetic mean of these three tests is adopted.

Density of the samples
Knowing the dimensions and dry mass of the samples, we determined the average density of each sample (Table 1) with a relative error less than 0.3%.
The apparent density of the composite diminishes with increasing fraction of peanut shell which makes it possible to offer lightness to the material produced. On the other hand, the use of the law of mixtures for a two-component medium allows the volume fraction y of the peanut shells in each sample to be deduced:

Thermal conductivity
Using the steady-state asymmetric hot plane method, three measurement tests are carried out to statistically validate the results. Figure 6 and Table 2 show the variation in the average thermal conductivity of the samples as a function of the shells content. From these results, it can be noted that: -The measurement error does not exceed 3%, which shows the robustness of the method used -The average thermal conductivity of the samples decreases with increasing shell content, making the processed material more and more thermally insulating.

Comparison of conductivity with theoretical models of equivalent thermal conductivity
The figure 7 shows the plot of the curves obtained by applying the theoretical models: series, parallel, Beck, Maxwell and Woodside as well as the experimental curve of thermal conductivity obtained by the hot-steady-state method. When observing the curves in the figure, we can already see that the experimental results lie between the lower limit (series model) and the upper limit (parallel model) of thermal conductivity. It is clear that the prepared samples are not represented by the maxwell model because the peanut shells are not spherical, whereas the geometric woodside model is the closest to representing these samples, reflecting the geometric mean of a random distribution of additives in the clay matrix.

Practical interest of elaborate composites
On the basis of the experimental results obtained, the following two elements can be introduced to value each composite in comparison with the raw clay: -Lightness factor of the composite compared to the raw clay: -Energy saving that can be obtained using two walls of the same thickness and subjected to the same temperature gradient, one consisting of the composite and the other consisting of pure clay: The preceding expressions are applied to evaluate the results obtained of the factor of lightness and energy saving for the different elaborated samples compared to clay only (figure 8). We notice that when we increase the percentages of peanut shells, these two parameters increase and the energy gain approaches 50% for a mass fraction of 9%.

Thermal diffusivity
The flash method allows to identify the parameters: Thermal diffusivity a, thermal flux  0 and the heat exchange convection coefficient h , by minimizing the quadratic error between the experimental curve and the theoretical curve ( Figure 9) given by the complete model and programmed under MATLAB. The results of thermal diffusivities obtained are summarized in Table 3, the measurement uncertainty of the thermal diffusivity is about 3%. The residue, defined as the difference between the two curves, is almost flattened as shown in Figure 9, which confirms that until the date t = 1400 s the 1D model is still valid. Figure 10 illustrates the reduced sensitivity curves for the three parameters to be estimated. Analysis of these curves shows that thermal diffusivity can be reliably estimated at a time around 300 s, while the other two parameters can only be estimated at longer times.

Conclusion
This study underlined that the thermal insulation quality of clay, as a traditional building material, is significantly improved by the addition of peanut shells, thus offering better energy efficiency. However, this work has focused on the influence of additive content, which opens up a possibility to study the impact of the internal structure of the composite (the shape of the additives and their dispersion in the binder) on its thermal behavior. It will also be interesting to carry out an in-depth study on the optimization of the mechanical and acoustic properties involved in the choice of building materials.