L Bouchra

C Szopa

A Buch

D Coscia

Cyril Pr

Szopa

Thermal stability of adsorbents used for gas chromatography in space exploration

Keywords:

For analytical purpose, thermal desorption is now used in gas chromatographs developed to analyse the chemical composition of planetary environments. Due to technical constraints, the thermal desorption cannot be as finely controlled as in the laboratory resulting in possible thermal alteration of the adsorbents used. For these reasons, the influence of heat on physical and chemical properties of various adsorbents, either used or that could be used in gas chromatographs for space exploration, is studied. If the adsorbents made of carbon molecular sieves and graphitised carbon black that were tested show a very high thermal stability up to 800°C, the porous polymers tested are highly degraded from a minimum temperature that depends on the nature of the polymer. Poly-2,6-diphenylphenylene oxide is shown to be the more thermally robust as it is degraded at higher temperatures, confirming it is currently the best choice for analysing organic molecules with a space instrument. Finally, the products of degradation of the porous polymers tested were analysed after heating the porous polymers at 400°C and 800°C. They were identified and listed as potential contaminants of analyses performed with this type of adsorbent. If the exposure to the higher temperature produces numerous organic compounds, mainly aromatic ones, a few ones are also detected at the lower temperature tested, meaning they should be considered as potential contaminants. Again poly-2,6-diphenylphenylene oxide should be preferred because it releases less organic compounds, the structure of which is completely specific to the adsorbent composition.

Highlights

Thermal evolution of adsorbents used for GC in space exploration is studied up to 800°C Graphitised carbon blacks and molecular sieves are thermally stable Porous polymers are drastically degraded by the heat Potential organic contaminants produced by porous polymers are tentatively identified and listed

1.

Introduction

Gas chromatography has been used for decades for space exploration [START_REF] Sternberg | Gas Chromatography in Space Exploration[END_REF]. Due to its robustness and relative technical simplicity, this instrumentation has been part of the scientific payload of several space probes that explored Venus [START_REF] Oyama | Laboratory Corroboration of the Pioneer Venus Gas Chromatograph Analyses[END_REF], Mars [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF][START_REF] Rushneck | Viking gas chromatograph-mass spectrometer[END_REF], comets [START_REF] Goesmann | COSAC, the cometary sampling and composition experiment on Philae[END_REF] and even Titan [START_REF] Niemann | The Gas Chromatograph Mass Spectrometer for the Huygens Probe[END_REF]. It is used for the analysis of the chemical composition of the atmospheres, and of condensed materials collected at the surface or in the atmospheres of these bodies. It is favoured to characterize the molecular composition of complex samples containing, or releasing, numerous gaseous compounds which cannot be directly identified with spectrometry alone (e.g. [START_REF] Freissinet | Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars[END_REF]). Through time, the complexity of space experiments increased to get more and more information from the analysed samples.

In particular, the preparation of samples (mainly the solid ones) has become more and more complex for gas chromatography in space exploration, to reach the ambitious scientific goals of the instruments, and to meet the technical constraints imposed by the flight of the space probes and the in situ operations. For the last chromatographs we built to explore the surface of Mars, this results in quite long times of transfer from the sampling system to the gas chromatographs. For this reason, our team transposed the thermal desorption technique, commonly used in the laboratory to analyse breath samples [START_REF] Xu | GC-Based Techniques for Breath Analysis: Current Status, Challenges, and Prospects[END_REF] or to monitor air composition [START_REF] Cecinato | Sampling and analytical methods for assessing the levels of organic pollutants in the atmosphere: PAH, phthalates and psychotropic substances: a short review[END_REF], to space application. During the sampling phase, it allows to trap the maximum amount of analytes (enrichment) and spatially focalize them for the transfer duration. During the injection phase, it allows to transfer the trapped chemical species into the chromatographic columns as fast as possible. The whole process is assumed to ensure to have the best possible separation of the analytes with the best detection sensitivity. We did it first successfully for the Sample Analysis at Mars (SAM) experiment onboard the Curiosity rover currently exploring the Mars surface [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF], and we will improve it for future missions, as Exomars 2020 [START_REF] Goesmann | The Mars Organic Molecule Analyzer (MOMA) Instrument: Characterization of Organic Material in Martian Sediments[END_REF].

In this frame, adsorbents of different natures are used, depending on the chemical species to be analysed. For example, in SAM, Carbosieve is used to trap the most volatile species and Tenax® (TA and GR) for the other volatile compounds. Some of these adsorbents have an upper temperature of thermal stability quite close to their usual recommended operational temperature (400°C as the limit temperature against about 300 °C for operating Tenax®). Moreover, as on Earth, despite their interest for trapping the analytes, adsorbent can be a source of organic molecules which are finally observed in the chromatograms with the analytes released by the sample (see [START_REF] Axel Clausen | Degradation products of Tenax TA formed during sampling and thermal desorption analysis: Indicators of reactive species indoors[END_REF] for an example). Then, the compounds released by the adsorbent can be contaminants of the analysis and it is important to limit their contribution, and to clearly identify them in order to discriminate them from the analytes present in the samples. The knowledge of the nature of these possible contaminants prior to the in situ measurement is of high importance because the operation limitations of space instruments usually do not allow to perform systematic blank runs to identify background contributions.

Usually, these by-products are produced through thermal decomposition or alteration of the adsorbent. For space exploration, the need for building light, sturdy and relatively simple instruments, added to the fact that environmental conditions of the instrument cannot be perfectly stabilized, results in a much less accurate thermal control of the desorption unit compared to laboratory instrumentation. As a consequence, some adsorbent present in the trap could be submitted to a thermal stress (e.g. maximum temperature overshoot) sufficient to lead to an alteration of the adsorbent. These are the main reasons why we performed a systematic study of the behaviour of various adsorbents of interest for space application, when submitted to temperatures up to 800°C. This article presents observed changes of physical and chemical features of different adsorbents commonly used in analytical chemistry when submitted to thermal stress, and the lessons learned for their use in space instrumentation specifically.

Material and methods

Adsorbents

Three main types of adsorbents are commonly used for thermal desorption injection with gas chromatography: i. carbon black for organic molecules of low to medium weight; ii. porous polymers for organic molecules of medium to heavy weights; iii. and molecular sieves rather devoted to small molecules and noble gases. In this work, we studied these all three types of adsorbent materials which are listed in Table 1. We studied the all three categories of adsorbents, and we selected at least two different adsorbents of each type to compare their behaviour when submitted to thermal treatment.

For graphitized carbon blacks (GCB), Carbopack B is the more commonly used today but we also tested Carbopack C. For porous polymer, Tenax® TA is a critical adsorbent for analysing the trapping and injection of organic molecules in the experiments developed to explore the surface of Mars [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF][START_REF] Goesmann | The Mars Organic Molecule Analyzer (MOMA) Instrument: Characterization of Organic Material in Martian Sediments[END_REF], and an adsorbent commonly used to monitor the Earth atmosphere composition [START_REF] Kumar | Volatile Organic Compounds: Sampling Methods and Their Worldwide Profile in Ambient Air[END_REF]. This is the reason why in addition to its nature, we also studied the influence of the size of Tenax® TA particles on its thermal behaviour. Tenax® GR is a mixture of Tenax® TA and GCB, and it is used for trapping lower molecular weight compounds. Both Tenax® adsorbents are used in the SAM experiment [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF]. Porapak Q is another type of porous polymer that could be used in a gas analyser onboard a future Russian landing probe to the moon surface named Luna Resours [START_REF] Hofer | Prototype of the gas chromatograph-mass spectrometer to investigate volatile species in the lunar soil for the Luna-Resurs mission[END_REF]. Finally, among the molecular sieves, we studied Carbosieve SIII which is also used in the SAM instrument on Mars, and Molsieve 5A which is of different chemical composition and could have an interest for future applications in space exploration, especially for analysing very volatile organic and inorganic molecules, as well as noble gases.

Adsorbent commercial name

Type of adsorbent

Density (g.mL -

Scanning Electron Microscopy

In order to physically characterize each adsorbent studied and their possible evolution when submitted to pyrolytic conditions, Scanning Electron Microscopy (SEM) was used. With this aim, a few particles of each adsorbent, either taken from their original vial, or collected in the pyrolysis cup after their heating, have been deposited on a double-sided tape composed of carbon to ensure the electric conduction required for performing SEM measurements. The adsorbents were analysed in low vacuum mode at a 0.45 torr pressure with an acceleration voltage of 15 kV. The particles of adsorbent were observed in an Environmental Scanning Electron Microscope (ESEM FEI Quanta 200).

Thermal Gravimetry Analysis and Evolved Gas Analysis

Thermal evolution study of most of the adsorbents selected were carried out using Thermal Gravimetry Analysis (TGA). Among the thermal analysis techniques, TGA is commonly used for characterizing the thermal properties of both inorganic and organic materials. It allows to measure the sample mass loss as a function of the sample temperature and time. TGA analyses were performed using a SENSYS evo (Setaram) TGA instrument, using a CALISTO software for data recording and treatment. The samples were placed in a 100 µL platinum crucible. The average sample mass put in the crucible was approximately 20 mg for each analysis. The samples were heated from the ambient temperature (∼20°C) to 1000°C at a 10°C.min -1 heating rate and in an argon (chemically inert) atmosphere to prevent any reactivity of the sample with the carrier gas. Once the maximum temperature was reached, the samples were kept for one hour at the final pyrolysis temperature.

In complement to TGA, Evolved Gas Analyses (EGA) were performed to follow the chemical species released by the adsorbents as a function of temperature throughout their heating. With this aim we used an EGA/PY-3030D micro-oven pyrolyser (Frontier Lab), installed on the split/splitless injector of a gas chromatograph (GC/Trace GC Ultra, Thermo Scientific) coupled to a quadrupole mass spectrometer (MS/ISQ LT, Thermo Scientific). A short-deactivated (Siltek™ process of silicon layer deposition, Restek) tube (2.5 m long, 0.10 mm internal diameter) made of stainless steel was installed between the injector and the MS. This geometry is used to ensure a fast transfer of the sample from the injector to the MS, and thus making the correlation between the sample temperature and the volatile species detected with MS easy. Moreover, the gas flow restriction induced by the capillary tube allows to maintain the pressure in the MS sufficiently low to operate it.

The main operating conditions of the GC-MS instrument were set as follows: the injector was used in the split mode at a temperature of 250°C; the column temperature was kept at 250°C and the carrier gas (Helium, 99.9999% purity, Air Liquide) flow rate was set constant to 0.5 mL.min -1 with a split flow rate of 10 mL.min -1 . The GC to MS transfer line was heated at 250°C as the electron ionization ion source. The ionization energy was 70 eV and the ions produced were scanned in the 12 to 650 m/z range.

For pyrolysis, we applied the following program of temperature to all the adsorbents: 280°C for 30 min, then heating at 10°C.min -1 up to 800°C that was maintained for 5 min. The total time of the sequence was 82 min during which the mass spectrometer recorded the signal. Knowing the program of temperature of the pyrolyser as a function of time, the EGA curves representing the MS response as a function of the sample temperature can have been simply obtained by converting the time of analysis in temperature. A mass of 5.0±0.1 mg of each adsorbent was pyrolyzed. It was chosen as a trade-off between the sensitivity of the method and the saturation of the MS signal.

Gas Chromatography-Mass Spectrometry analysis of the products of decomposition of the adsorbents

The goal of this measurement is the identification of the products of degradation of the adsorbents, if they exist, as they can be possible contaminants in the chromatograms. With this aim, the same experimental set-up as described for the EGA measurements was used. Only the deactivated tube was replaced by a MXT-5 (Restek) chromatographic column with the following geometry: 30 m long, 0.25 mm internal diameter and 0.25 µm stationary phase film thickness. This is a metallic column (Siltek™ process) with a stationary phase made of poly diphenyl (5%) dimethyl (95%) siloxane bonded to the deactivated inner wall of the column. The operating conditions were similar except the flow rate that was set to 1.0 mL.min -1 constant, and the program of temperature was 35°C for 2 min then heating at 10°C.min -1 up to 300°C maintained for 3 min. The MS was used in the full scan mode to detect all the ions in the 10-450 m/z range. The mass spectrum of the chemical species detected were compared to those of the National Institute of Standards and Technology (NIST) reference mass spectra library for identification. The less retained compounds (up to about 20 min retention) are identified with a significant degree of confidence. The more retained compounds have a more complex structure and usually a lower signal to noise ratio that increase the uncertainty of the mass spectra. The identification of the corresponding chemical species can be considered as essentially tentative. But the chemical structures given should be quite similar to the detected molecule's one.

As the adsorbents are heated too slowly to perform a brief injection of the gases released into the chromatographic column, a cryofocusing system (Frontier) was used to cool down the head of the column at the liquid dinitrogen temperature throughout the pyrolysis, and then to inject the species condensed in a small volume by stopping the cryocooling and heating the column. For each adsorbent studied, a specific program of temperature was used for pyrolysis. The different pyrolysis steps are summarized in Table 2 for each adsorbent. After each step, the molecular species released after reaching the final temperature were analysed with GC-MS. Prior to perform the pyrolysis itself, a first step consisted in desorbing the species trapped on the adsorbent by heating them at 280°C for 20 min, temperature to which the tested adsorbents are not decomposed, as observed on the EGA and TGA analyses (even if Porapak Q could be slightly degraded), and because 280°C is very close to the upper temperatures (300-350°C) usually used to clean the adsorbents in order to limit the possible contamination [START_REF] Sonnette | Tenax-TA Spiking Approach of Thermal Desorption Coupled to GC-MSMS for the Quantification of PAHs in Indoor Air and Dust[END_REF][START_REF] Chu | Comparison of Adsorption/Desorption of Volatile Organic Compounds (VOCs) on Electrospun Nanofibers with Tenax TA for Potential Application in Sampling[END_REF][START_REF] Glas | A novel approach to evaluation of adsorbents for sampling indoor volatile organic compounds associated with symptom reports[END_REF].

For this study, we focused only on the main materials that were observed to decompose thermally, meaning Tenax® TA (60-80 mesh) and Porapak Q. Figure 1 shows the chemical structure of these two organic polymers. Tenax® GR was not studied as it is composed in part of Tenax® TA which is the source of the mass loss observed using TGA. 5.0±0.1 mg of material was introduced in the pyrolyser for Tenax® TA (60-80 mesh) and Carbosieve SIII, and 1.0±0.1 mg of material for Porapak Q.

Adsorbent

Results and discussion

Thermal decomposition TGA analysis

The use of TGA allowed to follow the mass loss of each sample of adsorbent as a function of the temperature. The results obtained for all the tested adsorbents are presented in Figure 2.

On Figure 2, we can observe two general trends. First, there are adsorbents keeping their mass almost constant up to 1000°C, the maximum temperature. This behaviour is representative of materials which are very stable to the temperature. The corresponding adsorbents are the molecular sieves and the graphitized carbon blacks. The mass loss observed at the lowest temperatures (from the ambient to ≈200°C) for Molsieve 5A is attributed to the desorption of atmospheric gases which are efficiently trapped by this material. It is not observed for the carbon molecular sieves tested which are not efficient to trap these low mass species (see [START_REF] Aseev | Analysis of volatile lunar compounds: The study of gas retention time on Carbosieve SIII adsorbent with respect to temperature[END_REF] as an example for an application to space science). This is not fully surprising to observe such a stability as the materials produced to make these adsorbents are usually obtained by controlled pyrolytic processes occurring usually at temperatures up to 700-800°C. Now, when looking more carefully at the curves, we observe a small loss of mass for these materials (≈4% for Carbopack B, ≈5% for Carbosieve SIII, and ≈2% for Molsieve 5A), the loss occurring rather at the highest temperatures (typically above 800°C). This indicates a very limited decomposition that should impact the superficial part of the adsorbent only or this could be ascribed to moisture vaporization [START_REF] Cataldo | A study on the thermal stability to 1000°C of various carbon allotropes and carbonaceous matter both under nitrogen and in air[END_REF][START_REF] Cataldo | A study on the thermal stability of the. photopolymer, the ozopolymer, and the photochlorinated derivative of C60 fullerene[END_REF].

Then, we observe that the adsorbents made of porous polymers are resistant to the increase of temperature up to a threshold temperature for which they are subjects to a significant mass loss of tens of percent. This corresponds to a massive thermal decomposition that should be accompanied by an outgassing of volatile molecules. Once this large mass loss is finished, we observe at the higher temperatures that the adsorbents still lose mass but at a much lower rate. It is interesting to notice that the massive mass loss is observed at the same temperature for Tenax® TA and Tenax® GR, as the material decomposing for both adsorbents is the same, i.e. Tenax® TA. It shows that the presence of graphitized carbon black in Tenax® GR does not catalyse the thermal decomposition of Tenax® TA. Moreover, the mass loss of Tenax® GR (≈45%) is about 30% less important than for Tenax® TA (≈55%) as there is only 70%wt of Tenax® TA in Tenax® GR. The results obtained are therefore consistent with the composition of Tenax® GR. For Porapak Q, we observe that the temperature of massive mass loss is lower than for Tenax® TA (450°C instead of 550°C), in a range of temperatures similar to those observed in previous studies [START_REF] Caşcaval | The thermal degradation of copolymers of vinyltoluene and divinylbenzene[END_REF], and that the mass loss is much more significant (≈ 80%). As for Tenax®, after the massive mass loss, Porapak Q continues to lose mass but at a much lower rate, down to ≈95%. These observations demonstrate that Porapak Q is made of a material much more fragile to the heat than Tenax® TA.

Evolved Gas Analysis

On Figure 3 is plotted the evolution of total ion current of the mass spectrometer detector as a function of the temperature for each adsorbent. All the adsorbents were submitted to this diagnostic. Two main behaviours are observed.

First, all the adsorbents based on porous polymers show a significant outgassing observed when MS detector response significantly increase. For all three Tenax® adsorbents tested, this outgassing is observed from about 475°C to 800°C with a maximum observed at 570°C. The major part of the gases are shown to be released from 475°C to 677°C. The major ion observed throughout the outgassing has m/z=246, which is typical of the m/z value of the ion formed from 2,6-diphenyl-pphenylene oxide which is the monomer of the material composing Tenax®. For Porapak Q, the outgassing occurs at lower temperatures, basically from 300°C to about 750°C with a maximum gas release observed around 450°C. The major ions observed throughout the outgassing have m/z=117 and 132. These m/z values are typical from methylstyrene and ethylstyrene ions respectively. The peak observed is broader than those observed for Tenax® and most of the outgassing occurs from 350°C to 600°C. This may due to the fact that Porapak Q is made of a copolymer of ethylvinylbenzene/divinylbenzene, making its decomposition slower than for pure 2,6-diphenyl-pphenylene oxide polymer. Therefore, it can be concluded that porous polymers are not chemically stable when heated up to 800°C, resulting in a probable loss of their adsorption properties and a possible contamination of the analytical set up with the gaseous molecular species produced if the minimum temperature at which they decompose is reached. As Porapak Q releases gases at lower temperature than Tenax®, it means it is more fragile when submitted to thermal stress. This explains why its maximum operating temperature recommended by the suppliers is much lower for Porapak Q compared to Tenax® (Table 1). It can be noticed that these maximum operating temperatures are effectively lower than the minimum temperatures at which the porous polymers start to release gases. It must also be underlined that results obtained with EGA are in agreement with the TGA measurements. Now focusing on the different Tenax® adsorbents studied, we observe that the outgassing is similar for both adsorbents. The only difference observed is the peak area with more material outgassed for Tenax® TA (60-80 mesh) and the less observed for Tenax® GR. As the mass of adsorbent sampled is the same, it means there are chemical differences that should explain this observation. As said for interpreting TGA data, Tenax® GR contains 70% of Tenax® TA and 30% of graphitized carbon black, this last adsorbent being stable with the temperature. It thus results that the Tenax® GR sample has 30% less material that can decompose compared to pure Tenax® TA and this explains why the amount of gases observed to be released by Tenax® GR is the lowest one, the corresponding peak area being ≈77% of the pure Tenax® TA peak area. Concerning the two pure Tenax® TA samples, we observe that the adsorbent with bigger particles (60-80 mesh) releases more gases than the adsorbent with smaller particles (80-100 mesh). It thus indicates that there is a particle size effect in the release of products of degradation and bigger particles are more subject to decompose than smaller ones.

On Figure 3, the second behaviour observed is an absence of gases released by the molecular sieves and graphitized carbon blacks, whatever the temperature is. This confirms the results obtained with TGA that shows that these adsorbents are all thermally stable up to 800°C. The only exception to this rule is Carbopack C which shows a low outgassing around 550°C (major ions observed have m/z=91, 105 and 119, typical from alkylated aromatic species as toluene, or dimethylbenzene). This degassing may be due to the fact that the surface area is smaller for this adsorbent than for the other two.

Evolution of the adsorbent grains morphology

As we demonstrated the heating of adsorbents up to 800°C could have a significant effect on adsorbents, we tried to follow the physical evolution of the adsorbent particles with temperature. We limited this approach to Porapak Q, Tenax® TA and Carbosieve SIII as they are representative of the main behaviour observed for adsorbent to thermal stress and described above.

Figure 4 shows images obtained before and after heating Porapak Q adsorbent particles up to 800°C. It can be observed that the particle size drastically decreased from ∼230 µm down to ∼80 µm, but the particles keep a roughly spherical shape. This size decrease is compatible with the significant mass loss of material observed both with TGA and EGA measurements. Looking at a given particle, the soft and homogenous spherical shape observed before thermal treatment, changed to a multifaces carved surface. If we have no information about surface roughness of the particles with SEM, the decrease of the size of the particle should drastically limit its specific surface, and then its adsorption power. As an organic polymer, Porapak Q material should have significantly graphitized at the temperatures used for the pyrolysis, resulting in a modification of the adsorption properties.

Figure 5 shows images obtained before and after heating Tenax® TA (60-80 mesh) adsorbent particles to 800°C. As for Porapak Q, the particle size drastically decreased by a factor of about 2, but the particles roughly kept their initial shape. This size decrease is compatible with the significant mass loss of material observed with TGA and EGA measurements. Looking at a given particle, the surface globally looks like similar before and after the heating. But looking closer to the surface, it seems that the highly porous and sinuous surface observed initially, was flattened by the thermal stress. This could indicate a decrease of the porosity of the adsorbent which could induce a loss of adsorption efficiency. Similarly, to Porapak Q, the decrease of the size of the particles, and the likely graphitization of the polymer should drastically limit its adsorption properties.

Figure 6 shows images obtained before and after heating Carbosieve SIII adsorbent particles to 800°C. In this case, the particles keep their original size and shape after heating them up to 800°C. This is consistent with the absence of mass loss observed with TGA and EGA measurements. The only change observed is related to the surface roughness of the particles. Indeed, if initially the particle surface is flat and soft, after heating up to 800°C, we observe a fluffier material. This shows that if Carbosieve SIII is globally thermally resistant, its structure is modified by the thermal treatment. And this change should modify its adsorption properties as the manufacturer recommend not to use it at temperatures higher than 400°C (Table 1).

Volatile products of degradation

The programs of temperatures used to heat the different adsorbents were defined from the results obtained with TGA and EGA analyses. The different ranges of temperature defined correspond to different steps of mass loss from the adsorbents observed with TGA, which may correspond to steps of decomposition releasing different volatile molecules. By sharing the pyrolysis in steps, we wanted to test this assumption.

Figure 7 shows the chromatograms obtained when analysing the gases released by Carbosieve SIII after the pyrolysis. It can be observed that only a few compounds are detected. If we except the peaks attributed to column bleeding, only methanol, water and carbon dioxide are detected. Methanol and water are mostly observed at the lowest temperatures the adsorbent is heated, meaning these chemical species should be mainly desorbed from the adsorbent, and that they are not products of decomposition of Carbosieve SIII. Water comes from the atmosphere whereas methanol probably comes from residual solvent used to clean the material used to prepare the sample for pyrolysis. Carbon dioxide is observed on both chromatograms. This is compatible with background CO2 present in the carrier gas which is trapped by the cold trap. This analysis confirms that Carbosieve SIII does not decompose into volatile species when heated to high temperatures, as already shown by TGA.

Figure 8 shows the chromatograms obtained after analysing the products of pyrolysis of Tenax® TA (60-80 mesh) for the two steps of the pyrolysis it was submitted to. Most of the chemical species identified in these chromatograms are listed in Table 3.

For the lower pyrolysis temperatures range (from 280 to 400°C), water and carbon dioxide mainly coming from the air present in the background of the carrier gas are detected. In addition to products of decomposition of the column's stationary phase, a few polyaromatic compounds are observed late in the chromatogram. These polyaromatic molecules tentatively identified from their mass spectra, have a structure which is compatible with products of degradation from Tenax® TA because they include aromatic and cyclic structures that bear oxygen atoms. In spite of the detection of these degradation products, we can notice that they are limited in number, and that they have a really characteristic molecular structure compared to usual organic molecules targeted by the chromatographs used for space exploration. This demonstrates that Tenax® TA is quite thermally stable up to 400°C, confirming the observations done with TGA and EGA analyses. This also in accordance with the maximum recommended operating temperature given by the supplier (350°C), which should be the temperature around which these compounds should start to be released by Tenax® TA.

For the higher pyrolysis temperature range (Figure 8b), numerous organic compounds are detected in addition to air and products of degradation of the column stationary phase (Table 3). Most of these compounds are monoaromatic or polyaromatic compounds bearing oxygen or not. The two compounds the more retained detected after the low range of temperature pyrolysis are not present in this chromatogram. This is likely due to the higher temperatures used in this case that should prevent the formation of these species, or that could degrade them in lighter ones. The most abundant compounds detected in the chromatogram is the alcohol derived from 2,6-diphenyl-pphenylene oxide (peak #14 in Figure b). This observation is in agreement with the TGA measurements. But many other molecules are observed in various abundances, with molecular structures which can be easily derived from the decomposition or rearrangement of 2,6diphenylphenylene oxide. Then, the detected molecules must be released by Tenax® TA. This experiment shows the dramatic effect of heating Tenax® TA at temperatures higher than 400°C because the number and the amount of degradation compounds produced is significant. In addition to the certain alteration of its adsorption properties, this result shows that this adsorbent must not be heated at temperatures higher than 400°C because it can produce numerous organic molecules that can interfere with the detection and identification of molecules present in the analysed sample. Indeed, many of the degradation products detected (like benzene or phenol) are not strictly characteristic of the adsorbent because these chemical species can be found in many environments, including planetary ones [START_REF] Vinatier | Study of Titan's fall southern stratospheric polar cloud composition with Cassini/CIRS: Detection of benzene ice[END_REF]. Then their presence in the chromatograms measured in situ would lead to confusing situations for the identification of molecules indigenous to the probed environment.

As a conclusion, this study shows that Tenax® TA is quite stable when heated up to about 400°C, releasing only a low amount of polyaromatic molecules very specific to the polymer, but Tenax® TA is significantly degraded when heating it up 800°C, leading to the massive release of numerous molecules susceptible to contaminate the analysis. Figure 9 shows the chromatograms obtained after analysing the products of pyrolysis of Porapak Q for the two steps of the pyrolysis it was submitted to. Most of the chemical species identified in these chromatograms are listed in Table 4.

For the lower pyrolysis temperatures range (from 280 to 400°C, Figure 9a), water and carbon dioxide mainly coming from the air present in the background of the carrier gas are detected. In addition to products of decomposition of the column's stationary phase, several aromatic and cyclic organic molecules are observed in the chromatogram. The detected molecules are mostly monoaromatic molecules which are either ramified or associated to a cyclic structure (aromatic or not). Such molecular structures are compatible with the pyrolysis of the ethylvinylbenzene-divinylbenzene copolymer that leads to the production of a monoaromatic motifs, combined with products of the alteration or modification of the linear carbon chain of the polymer. Then, for this organic polymer, the number and the amount of compounds released at relatively low temperature is significant. Moreover, the nature of the compounds, even if they are specific to the copolymer, can be difficult to discriminate from organic molecules indigenous to samples that can be analysed in planetary environments, like on Titan that is a medium rich in hydrocarbons (see [START_REF] Anderson | Organic Ices in Titan's Stratosphere[END_REF][START_REF] Cable | Titan Tholins: Simulating Titan Organic Chemistry in the Cassini-Huygens Era[END_REF][START_REF] Morisson | Titan's organic aerosols: Molecular composition and structure of laboratory analogues inferred from pyrolysis gas chromatography mass spectrometry analysis[END_REF] for instance). Therefore, Porapak Q is an adsorbent relatively thermally weak at 400°C, producing several organic molecules that can interfere with the analysis of a sample. This explains why its maximum recommended operating temperature given by the supplier (250°C) is lower than for Tenax® TA.

For the higher pyrolysis temperature range (Figure 9b), numerous organic compounds are detected in addition to air and products of degradation of the column stationary phase (Table 4). As for low temperature pyrolysis, the majority of the detected compounds are monoaromatic alkylated hydrocarbons. A few biphenylic hydrocarbons are probably present in the last part of the chromatogram, but the uncertainty in the attribution of the chemical species from comparison with the NIST mass spectra library prevents their strict identification, as numerous peaks in this region of the chromatogram (for retention times longer ∼than 30 min). These molecular groups being characteristic from those of ethylvinylbenzene-divinylbenzene, the detected molecules must be released by Porapak Q. As for Tenax® TA, this experiment shows the dramatic effect of heating this adsorbent to temperatures higher than 400°C because the number and the amount of degradation compounds produced is significant. In addition to the certain alteration of its adsorption properties, this result shows that this adsorbent must not be heated at temperatures higher than 400°C because it can produce numerous organic molecules that can interfere with the detection and identification of molecules present in the analysed sample. It can also be noticed that the number of species detected when pyrolyzing Porapak Q to 800°C is significantly higher than for Tenax® TA, showing that ethylvinylbenzene-divinylbenzene can produce a larger variety of molecules compared to 2,6diphenylphenylene oxide.

As a conclusion, this study shows that Porapak Q is relatively unstable when thermally stressed. Its use requires to limit strictly its heating to relatively low temperatures (250°C from the supplier) whether it should start to produce many organic molecules susceptible to contaminate the analysis.

Conclusion

Among the adsorbents used, or that could be potentially used, in the development of instruments for space exploration, this study shows that carbon molecular sieves and graphitized carbon blacks are mechanically and chemically inert materials when submitted to temperatures usually used for thermal desorption (∼300°C) and pyrolysis (800°C). Without presuming about the effect of thermal treatment on the adsorption properties of these materials, this result proves that these adsorbents can be used without any risk of contamination or degradation of the analytical system. Concerning the porous polymers tested, both are fragile materials when submitted to temperatures higher than ∼300°C but Tenax® TA is more robust as a significant degradation is observed only from 600°C. As these adsorbents have similar adsorption properties, this means that Tenax® TA should be preferred as it is more able to resist to local or global overheats that can occur in space instrumentation. Moreover, Porapak Q produces more organic degradation products than Tenax® TA, which should be difficult to discriminate from analytes because they are mostly common monoaromatic hydrocarbons. Tenax® TA produces volatile degradation products which can be more easily identified due to their specific molecular structure, at least at reasonable temperatures relevant to thermal desorption (∼300°C). These results therefore strengthen the choice of adsorbents that were, or are still used for thermal desorption in space instrumentation, and they will be useful for the development of new instruments, and for the interpretation of the data collected in the planetary environments explored.

  ) 10°C.min -1 up to 300°C (5 min) 10°C.min -1 up to 800°C (3 min) Tenax® TA (60-80 mesh) 280°C (5 min) 10°C.min -1 up to 400°C (3 min) 10°C.min -1 up to 800°C (3 min) Porapak Q 280°C (5 min) 10°C.min -1 up to 400°C (5 min) 10°C.min -1 up to 800°C (3 min)

Figure 1 FigureFigure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9

 1456789 Figure1

1 ) Specific area (m 2 .g -1 ) Particle size (mesh) Maximum operating temperature (°C)

	Carbopack C	Graphitized	0.64	10	80-100	400
		carbon black
	Carbopack B	Graphitized	0.38	100	60-80	400
		carbon black
	Tenax® TA	Porous polymer	0.25	35	60-80	350
	Tenax® TA	Porous polymer	0.29	35	80-100	350
	Tenax® GR	Porous polymer	0.40	25	n.a.***	350
	Porapak Q	Porous polymer	0.66	550	50-80	250
	Carbosieve SIII* Molecular sieve	0.72	975	n.a.	400
	Molsieve 5A** Molecular sieve	0.34	n.a.	80-120	300

Table 1 :

 1 Adsorbents used in this study and their main characteristics. Data from suppliers (Supelco, Sigma-Aldrich).

	*Carbosieve SIII is a graphitized carbon black but its specific area makes places it in the molecular sieves category;
	**Molsieve 5A is a zeolite carbon molecular sieve; ***Not available.

Table 2 :

 2 Pyrolysis steps used to characterize the nature of the molecules released by the adsorbents when they decompose into volatile molecules. For Tenax® TA and Porapak™ Q were first heated at 280°C for desorbing the species trapped on the adsorbents, before starting the pyrolysis.

280-400°C first pyrolysis step (desorption) 400-800°C second pyrolysis step (degradation)

			19.4 43.3	ethenylmethylbenzene isomer Triphenylphenol msse 322 proeminente
			19.6 44.4	ethenylmethylbenzene isomer Triphenylphenol
			19.8 48.6	propynylbenzene isomer Triphenylphenol
			19.9 50.6	methylphenol isomer Triphenylphenol
			20.2	acetophenone
			20.9	methylbenzofuran isomer
			21.2	* Stationary phase fragment
			21.7	methylindene isomer
			21.8	4-methylbenzylvinylether
			21.9	methylindene isomer
			22.3	7 Naphtalene
			22.7	3-phenyl-3buten-2-one
			22.8	2,3 dihydro-3-methyl Inden-1-one
			23.7	* Stationary phase fragment
			23.9	methyl naphthalene isomer
			24.3	methyl naphthalene isomer
			25.0	4-phenyl 3-butene-2-one
			25.3/B	8 biphenyl
			25.8	diphenylmethane
			26.6	methylbiphenyl isomer
			26.7	methylbiphenyl isomer
			27.1	9 hydroxybiphenyl isomer
			27.2	dibenzofuran
			27.9	methylphenylphenol isomer
			27.95	phenalene isomer
			28.0	4-phenyl-4-cyclopentene-1,3-dione
	Retention time (min) 3.1 3.7 13.7 18.4 21.2 23.7 26.0 28.0 29.5 34.4	Compounds name 1 CO2 2 H2O * Stationary phase fragment Stationary phase fragment Stationary phase fragment Stationary phase fragment Stationary phase fragment Stationary phase fragment 1,1,3,3 tetramethylbutyl phen-2-ol b m-tertphenyl-2-ol	Retention 28.2 time (min) 3.3 29.3/B 5.0/B a 30.3 6.8/B 30.8 11.4/B 30.9 13.8 31.0 15.2 31.1 15.7 32.2 16.2 33.7 17.0 33.9/B 35.3/B 17.6 35.6	Identified compounds name methylphenylphenol isomer 1 10 hydroxybiphenyl CO2 2 phenanthrene H2O 3 11 biphenyldiol isomer benzene 4 9-vinylanthracene toluene 2-dibenzofuranol * Stationary phase fragment 5 terphenyl isomer ethylbenzene 12 phenylnaphtalene isomer ethynylbenzene 6 phenylmethyl naphthalene isomer styrene 13 tertphenyl isomer methylethyl-benzene isomer 14 m-tertphenyl-2-ol propenylbenzene phenyldibenzofuran isomer
	40.3/B	3 not identified	17.8 35.7	propylbenzene phenyldibenzofuran isomer 15
	49.3/B	4 not identified	18.2 37.5/B	benzaldehyde tertphenylol isomer 16
			18.4 38.1/B	* Stationary phase fragment 17 4,4 hydroxyphenoxybiphenyl
			18.5 38.8/B	phenol not identified 18
			18.6 39.4	propenyl-2-benzene 9-phenyl-9-fluorenol
			18.9 41.4	benzofuran terphenyl-phenyl isomer
			42.1	4-triphenylmethylphenol
			10

Table 3 :

 3 Retention time and name of each compound released by Tenax® TA (60-80 mesh) after each heating step applied to the adsorbent. Indices refer to the peak attribution given in Figure8. a B is indicated when the corresponding peak is broad in the chromatogram . b Chemical species written in italics are tentatively identified only, with NIST match score typically

	below 800.

Table 4 :

 4 Retention time and name for each compound released by Porapak Q after each heating step applied to the adsorbent. Indices refer to the peak attribution given in Figure9. a B is indicated when the corresponding peak is broad in the chromatogram . b Chemical species written in italics are tentatively identified only, with NIST match score typically below 800.

	449
	450
	280-400°C first pyrolysis step (desorption)	400-800°C second pyrolysis step (degradation)
	Retention	Identified compounds name	Retention	Identified compounds name
	time		time
	(min)		(min)
	3.1	1 CO2	3.2	1 CO2
	3.8	2 H2O	3.4	butene isomer
	13.6	* Stationary phase fragment	4.1/B a	2 H2O
	16.0	3 styrene	5.4	isobutyronitrile
	17.9	ethylmethylbenzene isomer	6.2	3 benzene
	18.4	Stationary phase fragment	11.1	4 toluene
	18.7	4 propenyl-2-benzene	13.7	* Stationary phase fragment
	20.5/B	5 ethenylethylbenzene isomer	15.1	ethylbenzene
	20,7/B	5 ethenylethylbenzene isomer
	21.0	diethenylbenzene isomer	15.5/B	5 xylene isomer
	21.3	diethenylbenzene isomer	16.3/B	6 styrene
	21.8	methylbutenylbenzene b	17.9/B	7 ethylmethylbenzene isomer
	22.2	6 not identified	18.5	* Stationary phase fragment
	22.4	not identified	18.9	8 propenyl-2-benzene
	22.7	not identified	19.4	methylmethylethylbenzene isomer
	23.4	ethylphenylethanone isomer	19.8	diethylbenzene isomer
	23.7	7 phenylbutenone isomer	20.0	diethylbenzene isomer
	24.0	phenylbutenone isomer	20.8/B	9 ethenylethylbenzene isomer
	32.2	not identified	21.4	diethenylbenzene isomer
	32.4	not identified	21.5	methylbutenylbenzene
	32.6	not identified	21.9	ethenyltrimethylbenzene isomer
			22.1	not identified
			22.2	ethenyltrimethylbenzene isomer
			22.3	not identified
			22.5	10 not identified
			22.8	ethylindene
			23.1	methyleneethylmethylbenzene isomer
			23.4	ethylmethylethylbenzene isomer
			23.6	bismethylethenylbenzene isomer
			23.8/B	11 phenylbutenone isomer
			24.1/B	12 phenylbutenone isomer
			24.9	13 methylethenylphenylethanone
				isomer
			25.1	phenylbutenone isomer
			25.2	not attributed
			25.3	methylethenylphenylethanone isomer
			25.4	vinylbenzoic acid isomer
			25.9	biphenyl

Acknowledgements

The authors acknowledge the funding support of the French national space agency, the Centre National d'Etudes Spatiales (CNES) (µ-GC Research and Technology program grant). They also thank Nathalie Ruscassier and Jamila El Bekri from CentraleSupelec, for their help to acquire images with Scanning Electron Microscopy, and TGA measurements.


Karim Touati

Daniel Levacher

Nassim Sebaibi

Farjallah Alassaad

Impact of phase change materials on lightened earth hygroscopic, thermal and mechanical properties

Keywords: building insulation, heat storage, light earth, phase change materials, thermal properties

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Face to global warming and its consequences, governmental authorities and NGOs have adopted regulations to reduce energy consumption and consequently CO2 emissions. It is known that construction field is one of the major sectors emitting greenhouse gases. It should be noted that building engineering accounts for 40 % of the world's energy consumption, a large part of that consumption being devoted to heating [START_REF] Cao | Building energy-consumption status worldwide and the state-of-the-art technologies for zero-energy buildings during the past decade[END_REF]. Specifically in France, the tertiary sector, which includes the residential house is responsible for around 45 % of total energy consumption and 21 % of CO2 emissions. Heating accounts for about 60 % of the total energy consumption in housing [START_REF] Hache | Beyond average energy consumption in the French residential housing market: A household classification approach[END_REF].

As part of the global energy transition, several research paths are explored to reduce the energy consumption and carbon footprint of buildings without degrading the occupants' comfort. Among the investigated solutions, heat storage represents a very promising technology which is capable to improve the energy efficiency of buildings and their technical installations (heating, air conditioning and domestic hot water). Moreover, heat storage is also investigated to compensate for the intermittency of renewable energies (thermal and hybrid solar systems, wind farming, etc.). Depending on the targeted application, latent heat storage, sensible heat storage or thermochemical storage will be selected.

For housing, some researchers try to improve the thermal performance of the walls without disturbing the building envelope by using active heat storage system [START_REF] Ango | Contribution au stockage d'énergie thermique en bâtiment : développement d'un système actif à matériaux à changement de phase, phdthesis[END_REF] or adding insulation layers [START_REF] Zach | Development of Thermal Insulating Materials on Natural Base for Thermal Insulation Systems[END_REF]. Active systems are those where the fluid circulation in the components is driven by a mechanical system (fan, pump, etc.) with the aim of replacing free convection by forced convection. This active action allows the capacity to store and/or withdraw energy depending on demand.

Otherwise, many research investigations are performed to improve the walls themselves. New innovative solutions begun to break through in the world of improving building walls, mostly made of concrete. These consist in adding to the cement matrix components that can improve the thermal behavior as the increase of the voids in the concrete by foam [START_REF] Ruiwen | Properties of high-strength foam concrete[END_REF] or the addition or substitution of light aggregates [START_REF] Yun | Evaluation of thermal conductivity for thermally insulated concretes[END_REF][START_REF] Nguyen | Influence of the volume fraction and the nature of fine lightweight aggregates on the thermal and mechanical properties of structural concrete[END_REF][START_REF] Chidighikaobi | Thermal effect on the flexural strength of expanded clay lightweight basalt fiber reinforced concrete[END_REF]. The porosity provided by the foam or lightweight aggregate decreases the thermal conductivity of the concrete according to their addition (dosage) into the mixture.

In order to comply with thermal and environmental regulations a lot of investigations have been undertaken, at housing full scale and especially their walls, in order to find an innovative solution limiting energy consumption.

Long known earth building techniques (rammed earth, cob, lightened earth, etc.), have been revisited for a better understanding of their thermal, hygroscopic and mechanical behavior [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF][START_REF] Laborel-Préneron | Plant aggregates and fibers in earth construction materials: A review[END_REF][START_REF] Holzhueter | The Potential for Light Straw Clay Construction in Japan: An Examination of the Building Method and Thermal Performance[END_REF][START_REF] Phung | Formulation et caractérisation d'un composite terre-fibres végétales: la bauge[END_REF][START_REF] Hamard | Rediscovering of vernacular adaptative construction strategies for sustainable modern building : application to cob and rammed earth[END_REF][START_REF] Goodhew | Building with earth: How we are working to revive an ancient, sustainable building technique[END_REF].

Building with earth-based materials represents a real alternative. Build with these kinds of materials can bring many benefits, mainly the renewability of the main components and the low carbon footprint. Earth is a natural resource that is often widely available. Almost all mineral soils that contain clay can be used for construction [START_REF] Houben | Traité de construction en terre[END_REF]. Usually, these latter do not require any treatment or transformation and are enough locally abundant allowing the reduction of emissions due to transportation.

Nowadays, the interesting properties of earth i.e. raw soil, mainly consist in its capacity to regulate the building hygrometry and having a high thermal mass, at low economic and environmental cost. As a raw material, without additives or cooking, earth is an excellent humidity regulator due to its absorption and evaporation capacity. It captures or releases humidity from the air according to the ambient humidity, thus promoting a healthy indoor environment [START_REF] Zhang | Effect of hygrothermal behaviour of earth brick on indoor environment in a desert climate[END_REF][START_REF] Cagnon | Hygrothermal properties of earth bricks[END_REF][START_REF] Medjelekh | Characterization of hygrothermal transfers in the unfired earth[END_REF][START_REF] Touré | Mechanical and hygrothermal properties of compressed stabilized earth bricks (CSEB)[END_REF][START_REF] Giuffrida | Hygrothermal Properties of Raw Earth Materials: A Literature Review[END_REF][START_REF] Anger | Construire en terre, une autre voie pour loger la planète[END_REF]. This natural humidity regulation is a quality that does not found with conventional building materials such as concrete.

Consequently, earth construction investigation had a renewed interest these last years with many research projects acted and in progress. Hereafter, some of them dealing with earth construction (light earth, rammed earth, cob, etc.) are reported with their main objectives:

-ECO-TERRA is a research project that aims to acquire scientific and technical knowledge necessary to develop light earth construction materials for efficient green buildings. To enhance resources value from different territories, studies were carried out to understand the links between constituents variability and physical properties of earth-based mixtures [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF]. -PRIMATERRE research project concerns "the renovating and building sustainably: the challenge of raw materials". It mainly focuses on proposing means of measuring and guaranteeing the hygrothermal, mechanical and seismic performances of earth materials. The case of rammed earth was particularly studied. Development of recommendation guides and educational courses is also targeted [START_REF] Nabouch | Seismic Assessment of Rammed Earth Walls Using Pushover Tests[END_REF].

-

CobBauge is a European research project focusing on developing, testing and establishing an innovative low carbon technology using local soil and vegetal fibers. This technology, based on mixtures of soil in a plastic state and plant fibers, aims the development of a structural and insulating material. Formulation studies have been carried out to determine optimal earth / fiber mixtures. These optimal mixtures are adopted to produce two levels building that meets the French and UK standards. In this project, walls are made up of two layers, namely a structural (cob) and a thermal layer (light earth). Two experimental buildings are under construction, in France (Manche department) and in England (Devon), to assess the site behavior of buildings (housings) using the developed optimal earth / fiber mixtures [START_REF] Goodhew | Building with earth: How we are working to revive an ancient, sustainable building technique[END_REF][START_REF] Phung | Hygrothermal Behaviour of Cob Material[END_REF].

In order to improve earth-based materials thermal properties, many studies have been implemented. As example, the impact of density on the hygrothermal properties of earthen-based materials has attracted a particular interest [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF][START_REF] Phung | Formulation et caractérisation d'un composite terre-fibres végétales: la bauge[END_REF][START_REF] Cagnon | Hygrothermal properties of earth bricks[END_REF][START_REF] Medjelekh | Characterization of hygrothermal transfers in the unfired earth[END_REF][START_REF] Niang | Hygrothermal performance of various Typha-clay composite[END_REF][START_REF] Labat | From the experimental characterization of the hygrothermal properties of straw-clay mixtures to the numerical assessment of their buffering potential[END_REF].

More, these recent years new method targeting improvement of the thermal behavior of buildings (or housings) has appeared. It consists of incorporating phase change materials (PCM) into the construction walls. The PCM principle in the building engineering can be clearly explained considering a front wall in which PCMs are used. When the outside temperature exceeds the PCMs melting temperature range, this latter absorb heat by changing from a solid to a liquid state, thus delaying the heat flow inside the building (or housing). Then, the stored heat is released when temperature drops below the PCM solidification temperature range. This provides more homogeneous indoor temperatures, which tend to be close to the melting point of the PCMs. This technology has the interesting feature of shifting the building heating and cooling load from peak to off-peak periods of electricity consumption [START_REF] Soares | Review of passive PCM latent heat thermal energy storage systems towards buildings' energy efficiency[END_REF]. Furthermore, most of these materials are based on non-renewable resources with a high environmental footprint [START_REF] Serrano | Stabilized rammed earth incorporating PCM: Optimization and improvement of thermal properties and Life Cycle Assessment[END_REF]. For building applications, PCMs are introduced mainly under microcapsules form. They are constituted in a paraffinic core and a polymeric shell [28]. Paraffin is considered as an inherently biodegradable material [START_REF]PureTemp -Global Authority on Phase Change Material[END_REF] but the polymeric shell is a non-biodegradable substance. Nevertheless, readily biodegradable and biobased microencapsulated PCMs start reaching the market [28,[START_REF]PureTemp -Global Authority on Phase Change Material[END_REF]. Also, recent research is focusing on these aspects [START_REF] Prajapati | Biodegradable Polymeric Solid Framework-Based Organic Phase-Change Materials for Thermal Energy Storage[END_REF][START_REF] Boussaba | Elaboration and properties of a composite bio-based PCM for an application in building envelopes[END_REF][START_REF] Németh | Fully bio-originated latent heat storing calcium alginate microcapsules with high coconut oil loading[END_REF][START_REF] Boussaba | vegetable fat: A low-cost bio-based phase change material for thermal energy storage in buildings[END_REF][START_REF] Boussaba | Investigation and properties of a novel composite bio-PCM to reduce summer energy consumptions in buildings of hot and dry climates[END_REF]. Accordingly, biobased PCMs cores [START_REF] Boussaba | Elaboration and properties of a composite bio-based PCM for an application in building envelopes[END_REF][START_REF] Németh | Fully bio-originated latent heat storing calcium alginate microcapsules with high coconut oil loading[END_REF] and shells [START_REF] Prajapati | Biodegradable Polymeric Solid Framework-Based Organic Phase-Change Materials for Thermal Energy Storage[END_REF][START_REF] Boussaba | Elaboration and properties of a composite bio-based PCM for an application in building envelopes[END_REF][START_REF] Fang | Form-stable phase change material embedded with chitosan-derived carbon aerogel[END_REF][START_REF] Feczkó | Microencapsulation of n-hexadecane phase change material by ethyl cellulose polymer[END_REF]] are being developed.

Several studies have examined the use of many PCM types adapted to the building engineering. Types concerned various aspects i.e. free, micro or macro encapsulated. The incorporation of PCMs in mortar or concrete gave promising results by reducing their thermal conductivity and increasing their thermal mass [START_REF] Cunha | Effect of temperature on mortars with incorporation of phase change materials[END_REF][START_REF] Ling | Use of phase change materials for thermal energy storage in concrete: An overview[END_REF]. However, the incorporation of PCMs into structural concrete/mortar degrades its mechanical performance [START_REF] Cunha | Effect of temperature on mortars with incorporation of phase change materials[END_REF][START_REF] Dakhli | The Effect of Phase Change Materials on the Physical, Thermal and Mechanical Properties of Cement[END_REF][START_REF] Djamai | Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars[END_REF][START_REF] Drissi | Etude de l'impact des Matériaux à Changement de Phase (MCP) sur l'hydratation des bétons-MCP, Conférence: journées scientifiques Franco-maghrébines-CMC[END_REF][START_REF] Kong | Thermal and mechanical properties of mortar incorporated with paraffin/palm oil fuel ash composite[END_REF][START_REF] Cunha | Mortars based in different binders with incorporation of phasechange materials: Physical and mechanical properties[END_REF]. In spite of the numerous studies carried out in this field, performances of these materials are not completely controlled.

Present study aims to combine the suitable hygrometric and thermal conductivity of lightened earth, with the PCMs appropriate heat storage capacity. Indeed, when compared to other earth-based materials, lightened earth presents a low density. Incorporation of PCM into this latter could improve its thermal buffering capacity. First, microencapsulated PCMs will be introduced into soil-fiber mixtures. Mixtures containing 0 %, 2 %, 5 %, 10 % and 20 % of PCM will be formulated. Then, mechanical, thermal (thermal conductivity and specific heat capacity) and hygroscopic behaviors (sorption/desorption capacity, water vapor permeability) of the soil-fiber mixtures will be analyzed for the different PCMs dosages.

Materials

Earth material or soil-fiber mixture

The chosen soil-fiber mixture was investigated in CobBauge research project for which different formulations were studied. Various soil and fiber types i.e. combination of 12 soils and 6 fiber types (wheat straw, hemp straw, hemp seed, flax straw, flax shives and reed), were mixed, tested and discussed. In present study, the soil-fiber mixture that gave the best thermal properties was selected [44]. So, the mixture composed of silty-clay soil and reed was chosen.

The soil classification is made using conventional geotechnical characterization and regarding applicable standards. The grain size distribution was conducted differently according to the fine (laser granulometry with LS 13 320 from Beckman Coulter) and coarse particles (>80 μm) following the standard XP P94-041 [START_REF]Soil : investigation and testing. Granulometric description[END_REF]. In addition, soil plasticity is determined through the Atterberg limits in accordance to the standard NF P94-051 [START_REF]Soil : inverstigation and testing. Determination of Atterberg's limits. Liquid limit test using cassagrande apparatus[END_REF]. The clay activity is determined via the methylene blue value (MBV) according to the standard NF P94-068 [START_REF]Soils : investigation and testing. Measuring of the methylene blue adsorption capacity of à rocky soil. Determination of the methylene blue of a soil by means of the stain test[END_REF]. Atterberg limits allow the determination of the soil liquidity limit (LL), plasticity limit (PL) and plasticity index (PI). The methylene blue value and granulometry allow the soil classification as sandy, silty or clayey.

The characterization test results are gathered in Table 1 and shown in Figure 1. To be able to adjust the water content in mixtures, the reed fibers water absorption capacity is performed according to RILEM protocol [START_REF] Amziane | Recommendation of the RILEM TC 236-BBM: characterisation testing of hemp shiv to determine the initial water content, water absorption, dry density, particle size distribution and thermal conductivity[END_REF]. The raw fibers were cut but left in their natural raw shape, they were not crushed. Reed fibers length is ranged from 4 to 6 cm.

Then, soil, fiber and PCM Nextek 24D absolute density was determined by using a Helium pycnometer (Accupyc II 1340) and given in the Table 2.

Phase Change Materials (PCMs)

The selection of the PCM to be incorporated in soil-fiber mixture was based on several criteria, namely: the phase change latent heat, chemical properties and especially the melting temperature. The selected melting temperature must take into consideration the expected thermal comfort and the typical climatic conditions of the building (or housing) location [START_REF] Kheradmand | Experimental and numerical studies of hybrid PCM embedded in plastering mortar for enhanced thermal behaviour of buildings[END_REF]. The average temperature in France varies between 0 and 30 °C during the year [50]. Thus to satisfy the climate conditions encountered in France, the initial selection is limited to PCM having a melting temperature ranging from 5 to 25 °C. Microencapsulated PCMs are preferable compared to bulk PCMs, as they prevent leakage during solid-liquid phase transitions and also offer a larger heat transfer surface per unit volume [START_REF] Park | Magnetic nanoparticleembedded PCM nanocapsules based on paraffin core and polyurea shell[END_REF]. Then, to ensure the human comfortable environment, the PCM phase change temperature should be ranged approximately between 16 and 25 °C [START_REF] Sarı | Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage[END_REF].

According to the above-mentioned criteria and the commercially available PCMs, Nextek 24D demonstrated favorable thermal properties. Nextek 24D is composed by a paraffinic core and a polymeric shell. The Nextek 24D is encapsulated in patented robust capsules that offer great thermal stability and high dispersibility [28]. So, in present study, Nextek 24D was adopted for the soil-fiber mixture series tested and analyzed. The characteristics given by providers are checked before starting any mixture. The verified properties are: density (helium pycnometer), particle size distribution (laser particle size), phase change latent heat and specific heat capacity (Differential Scanning Calorimeter DSC at 10K/min). The different results are given in Table 3.

Samples preparation

First, the soil is crushed in order to obtain particles having a maximum diameter of 2 mm. Then, it is dried in a ventilated oven at 40 °C for 2 weeks. The fibers are hand cut at 5 ± 1 cm length. Then, they were covered and stored at room temperature, i.e. 20 °C.

The different soil-fiber-PCM mixtures weight ratios are: water/soil ratio is equal to 1 and fiber/soil ratio is equal to 0.25; PCM/(soil+fiber) vary from 2 % to 20 %, see Table 4. Ratios used correspond to desired soil-fiber formulation.

Table 4: Lightened earth formulation (in wt%). Fiber and water percentages are function of soil amount. The PCM percentage is function of (soil+fiber) dry mass.

SFi: S = Soil, F = Fiber, and i = PCM content. The sample preparation method is the same for all the different mixtures. In order to ensure a homogeneous distribution, soil and PCM with their corresponding mass are first mixed manually at dry state within a mixing tank. So, the necessary water is added and the mixing is allowed to continue until the soil mixture reaches a liquid state. The fibers are then added gradually. The mixing procedure is carried out within a mixer (PROVITEQ Concrete Mixer 65 L) for 120 s. At the end, the soil-fiber mixture is bagged and stored at room temperature i.e. 20 °C. The next day the mixture is manually compacted with the help of a wooden cleat in previously oiled molds.

Formulation

Knowing molds volume, mixtures appropriate weights were calculated, in order to obtain a same density. To get a cylindrical sample for compressive testing of approximately 700 kg/m 3 dry density, 1.46 kg of dry mixture is needed. Considering the water in the formulation, the necessary mass to be filled in the mold is 2.628 kg. It should be noted that fiber length, samples dimension and compaction direction promoted a preferred orientation of the fibers over the sample width, see Figure 2. Then, the molds are left during one more day at room temperature and then placed in an oven at 40 °C. After two days of drying in the oven, the cylindrical (diameter Ø11 cm x height 22 cm) molds are completely removed. The prismatic molds (22 cm x 22 cm x 4 cm) are partially removed (every two days, two sides at a time) while keeping the samples at 40 °C. The drying process takes almost 3 weeks. The previously dried samples are put at a room conditions of 20 °C and 50 % RH. Characterization of the prepared samples begin once equilibrium is reached (when difference in mass between two daily weightings is less than 0.1 %). This equilibrium is reached usually after 1 week at room conditions. Table 5 summarizes the different tests performed, met standards, size and number of the tested samples.

Experimental methods

Hygroscopic behavior

It is well known that lightened earth, as all the hygroscopic materials, has a real potential in improving thermal and indoor humidity comfort. More, these materials significantly impact the buildings (or housings) indoor air quality and energy consumption. In this section, the lightened earth hygroscopic properties are presented with the description of methodologies used to study these latter.

To investigate solids interaction with vapor, the Dynamic Vapor Sorption (DVS) technique is usually used. In present study, sorption isotherms of the raw materials (soil, fiber, PCM) and the mixtures are studied according to the standard ISO 12571 [START_REF]Hygrothermal performance of building materials and products -Determination of hygroscopic sorption properties[END_REF]. To do so, the ProUmid SPSx-1μ sorption/desorption system was used. This equipment allows a precise measurement of the samples mass and sorption kinetics through a precision balance and a careful control of temperature and humidity. Before adsorption analysis begins, the samples are first dried to reach a constant weight (weight change is less than 0.1 %). After that, the samples were exposed to an atmosphere relative humidity ranging from 10 % to 90 % in 5 steps, while maintaining the test temperature at 23 °C.

Then, to study the ability of a material to allow a vapor to pass through, the water vapor permeability should be known. This parameter represents the ratio of the quantity of water vapor passing through a material per unit: of thickness, of time, and of vapor pressure difference existing on each side of the material. In present study, the water vapor permeability of the different samples was measured according to the standard ISO 12572 [START_REF]Hygrothermal performance of building materials and products -Determination of water vapour transmission properties -Cup method[END_REF]. This is achieved by following the mass of a sample submitted to a moisture gradient. The dry cup method requires to create a moisture gradient from inside of the cup (≈ 0 % RH) to outside (≈ 50 % RH) through the sample.

Measurements of water vapor permeability using the dry cup method provide information on the material behavior when moisture transfer is dominated by vapor diffusion. The flow of vapor through the material is obtained from weighing the sample cup assembly. At steady state, the water vapor flow (G) through the sample is given by the slope of the regression line of the sample cup assembly mass vs time. This is achieved after eliminating the test initial non-linear phase.

Thus, flux density (or transmission rate) of the vapor (g) is calculated by the formula in equation ( 1), where A is the area of the exposed specimen.

=

In Equation ( 2), it can be seen that water vapor resistance (Z) can be calculated from water vapor permeance (W). This latter depends, among others, on the difference water vapor partial pressure between sample sides (ΔPv).

= 1 = × ∆ (2)
Then, from Equation (3), water vapor resistance of the air layer present in the measuring cup ( ) is achieved by knowing the air layer thickness (da) and air water vapor permeability at atmospheric pressure ( ) which is equal to 2×10 -10 kg.m -1 .s -1 .Pa -1 .

=

The corrected water vapor permeance Wc is obtained using equation ( 4).

= 1 - (4)
Then, the sample water vapor permeability ( ) can be determined using equation [START_REF] Ruiwen | Properties of high-strength foam concrete[END_REF]. is the sample thickness.

= × (5)
Finally, sample water vapor resistance factor (µ) is found by the following equation ( 6).

= (6)

Moisture levels necessary to perform the tests are generated by silica gel (≈ 0 % RH), magnesium nitrate (≈ 50 % RH). All samples were upstream conditioned at a temperature of 23 °C and a relative humidity of 50 %.

The determination of moisture sorption isotherms and water vapor permeability allow evaluation of the PCM impact on soil-fiber mixtures hygroscopic behavior.

Thermal behavior

To use a given material as thermal insulator and/or heat storer in buildings (or housings) envelope, some thermal properties should be known. Sometimes, these properties must satisfy threshold values that can be defined in standards. In present study, we are concerned in two thermal properties: thermal conductivity and specific heat capacity. Methodologies used to obtain these parameters are described hereafter.

Thermal conductivity is a physical property that characterizes the ability of a material to conduct heat from a hot to a cold point. The suitable methodology to determine this property depends on the nature, the shape and the size of the samples to investigate. For building materials, thermal conductivity measurements are usually performed with a heat flow meter HFM 436 Lambda. This technique consists in creating a temperature gradient between two plates through the investigated material. By means of two heat flow sensors in the plates, heat flow entering and leaving the material is measured. When the equilibrium state is reached and the heat flow is constant, then the material thermal conductivity is calculated using Fourier's law [START_REF] Gooch | Fourier's Law of Heat Conduction[END_REF], assuming that the sample dimensions are known. In present study, thermal measurements were carried out on 22 x 22 x 4 cm 3 prismatic samples at different mean temperatures: 14, 24 and 34 °C. The temperature difference between the two sides of the samples is fixed at a value of 10 °C for each measurement.

Specific heat capacity (Cp) is another physical property that characterizes the ability of a material to store heat.

The measured parameter represents the quantity of heat required to raise the temperature of one kilogram of a material by one Kelvin. To determine this parameter, Differential Scanning Calorimetry (DSC) technique is usually used. This technique is especially relevant when investigations are run on small samples (weighing up to few grams).

In present work, Cp measurements are performed with a DSC calorimeter according to the standard ISO 11357-4 [START_REF]Plastiques -Analyse calorimétrique différentielle (DSC) -Partie 4 : Détermination de la capacité thermique massique[END_REF]. During tests, temperature was increased from -20 °C to 60 °C continuously. Heating rate was 10 °C.min -1 . This rate can affect the Cp absolute values, but it remains relevant in the case of comparative study.

Mechanical behavior

To analyze the effect of PCM incorporation on the mechanical strength of the soil-fiber mixtures, unconfined compressive strength (UCS) tests have been performed. The test recommendations stipulated in the standard EN 1015-11 [START_REF]Methods of test for mortar for masonry -Part 11 : determination of flexural and compressive strength of hardened mortar[END_REF] were followed. The UCS strength were obtained using INSTRON SCHENCK press machine with a capacity of 100 kN.

Porosity measurement

Global porosity is an important physical parameter in the characterization of a material. It is measured by immersing small samples in a non-wetting oil, in this work disaromatized petroleum is used. The tests are carried out according to the standard NF ISO 5017 [START_REF]Produits réfractaires façonnés denses -Détermination de la masse volumique apparente, de la porosité ouverte et de la porosité totale[END_REF].

The samples are previously saturated under vacuum in a desiccator for at least 24 hours which allows the liquid to replace the air in the pores without interacting with the sample.

Later, they are weighed under oil and then weighed in air. The samples were dried in the oven at 105± 5 °C until a constant mass is reached, i.e. until two successive weighing, before and after a stay of 24 hours in the oven, do not differ by more than 0.05 %, considered as the dry mass. This makes it possible to obtain the volume of the voids initially filled with oil, by means of a mass difference between the saturated and the dry states. Hence the accessible porosity p0 is given by:

= ! -"!# ! -$ % (7)
where:

dry is the mass of dry specimen. -is the mass of saturated specimen in air. oil is the mass of saturated specimen in oil.

Water vapor permeability

Results of the measurements performed on the different studied mixtures are reported in Table 7. From this table, first we can notice that soil-fiber mixtures present a water vapor permeability 7-8 times larger than the one of a standard cement mortar [START_REF] Issaadi | Moisture characterization of cementitious material properties: Assessment of water vapor sorption isotherm and permeability variation with ages[END_REF]. The value of the water vapor resistance factor of the formulation without PCM (SF0) is relatively close to the one reported by Phung (µ ≈ 9 for 3 % reed) [START_REF] Phung | Formulation et caractérisation d'un composite terre-fibres végétales: la bauge[END_REF]. Afterwards, these results show that PCM addition decreases water vapor permeability of the soil-fiber mixtures. The diminution is more pronounced as the PCM rate is high. As for the sorption/desorption test, the diminution of the water vapor permeability with PCM content is due the decrease of porosity and also the hydrophobic nature of the PCM. The addition of PCM has the same effect on the ability of the mixtures to fix or to let pass moisture through it. Even though the decreases are in the range of few percent, the hygroscopic character of such materials is still conserved. Whether sorption or water vapor permeability, the soil-fiber-PCM mixtures remain more hygroscopic than structural cob [START_REF] Collet-Foucault | Caractérisation hydrique et thermique de matériaux de génie civil à faibles impacts environnementaux[END_REF] and much more hygroscopic than cement mortar.

Thermal conductivity

The thermal conductivity (λ) of soil-fiber-PCM mixtures was measured at three different temperatures (14 °C, 24 °C and 34 °C). Temperatures are chosen so as to investigate thermal conductivity in the PCM phase change zone (24 °C) and outside the PCM phase change zone (14 °C and 34 °C). At a temperature of 14 °C, the PCM is in a solid state and at a temperature of 34 °C, in a liquid state. Taking a look at Table 8 and Figure 6, first, it can be seen that thermal conductivity increases with temperature for the sample without PCM and for the samples containing PCMs when temperature is outside the phase change zone. This evolution was expected as it is well known that thermal conductivity of insulating materials rises with temperature [START_REF] Rohsenow | Handbook of Heat Transfer[END_REF].

RH [%] Mass change [%] t [h] SF0 SF2 SF5 SF10 SF20 Mortar RH
Then, it can be observed that the addition of PCMs decreases thermal conductivity of soil-fiber mixtures on the whole temperature scale. However, this decrease is not always proportional with the incorporated PCM amount. When the PCM is completely solid or liquid (at 14 °C or 34 °C), thermal conductivity tends to stabilize and the decrease in λ becomes negligible beyond 5 % of PCM. Otherwise, in the transition zone (24 °C), thermal conductivity decreases continuously until the quantity of PCM approach 10 %. Beyond this PCM content, the diminution of λ slows down.

Thus, this overall decrease in thermal conductivity can be due to the PCM low thermal conductivity compared the one of lightened earth i.e. soil-fiber mixture, and probably to the disorder brought by the PCM incorporation in this mixture. In addition, the diminution is more significant in the phase change zone. At 24 °C, SF20 thermal conductivity is approximately 14 % less than the one of SF0. This diminution can be due to disorder increase inside the microcapsules (coexistence of solid and liquid phases).

More PCM quantity is high more the decrease in thermal conductivity is significant until a kind of saturation at approximately 5 % or 10 % PCM. This indicates that a formulation may have an optimum of PCM amount to add.

The value of this optimum should be defined regarding the material's different properties and nature. As illustration, Dakhli et al. [START_REF] Dakhli | The Effect of Phase Change Materials on the Physical, Thermal and Mechanical Properties of Cement[END_REF] reported an optimum value of approximately 20 % PCM for a standard cement mortar. Finally, it can be confirmed that addition of Nextek 24D PCM improves the insulating properties of soilfiber mixture depending on its incorporation level and temperature.

Specific heat capacity

As this study deals with multicomponent mixtures, the specific heat capacity can be calculated using a mixing law.

To do so, & of the different constitutive components in a raw state is determined by the DSC technique. To get the mixtures specific heat capacity, Equation ( 8) is applied at each measuring temperature [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF].

& = . & + . & + . & (8)
In this equation, & and are respectively dry specific heat capacity and mass fraction of the component in the mixture. Indexes ), * and & represent soil, fiber and phase change material. The obtained results are presented in Figure 7. The constitutive materials mass fractions are calculated at the laboratory ambient environment (≈ 50 % RH and 20 °C).

Figure 7 represents the evolution of mixtures calculated specific heat capacity as function of temperature and PCM content. From a first sight on the different curves, it can be noticed that & increases with temperature. The increase slope becomes steeper when temperature exceeds 20 °C, this is particularly visible on the mixture without PCM.

Regarding the PCM effect, from Figure 7 it can be assumed that, in general, addition of Nextek 24D increases & value of the soil-fiber mixtures. & value increases linearly with PCM content. Two separate parts can be identified: Outside and within the phase change zone. Before phase change, specific heat capacity is 4 %, 11 %, 21 %, and 38 % larger than the one of the control sample for PCM content of 2 %, 5 %, 10 % and 20 %, respectively. After phase change, & evolution presents the same allure than before the transition zone. In general, the specific heat capacity increases as we approach the phase change temperature. At the pic value, mixtures & values are 45 %, 108 %, 207 %, and 380 % larger than the one of the control sample for PCM content of 2 %, 5 %, 10 % and 20 %, respectively. In fact, at this level, specific heat capacity increases from 576 J.kg -1 .K -1 for the sample without PCM, to around 2762 J.kg -1 .K -1 for the mixture containing 20 % PCM. It should be noticed that in the phase change range, the specific heat capacities represent a combination of two contributions: latent heat of phase change and sensible heat.

Mechanical behavior

To understand the impact of PCM introduction on the mechanical resistance of soil-fiber mixtures, the stress-strain relationships are reported on Figure 8. From this figure, it can be noticed that stress-strain curves in compression tend to have a degree 2 polynomial form. Since studied materials are ductile, no particle breakage was observed during the mechanical test. Consequently, the maximum stress is not a relevant indicator to study the samples compressive strength [START_REF] Laborel-Préneron | Plant aggregates and fibers in earth construction materials: A review[END_REF][START_REF] Flament | Valorisation des fines de lavage de granulats : application à la construction en terre crue[END_REF][START_REF] Cérézo | Propriétés mécaniques, thermiques et acoustiques d'un matériau à base de particules végétales : approche expérimentale et modélisation théorique[END_REF][START_REF] Simons | Development of bio-based earth products for healthy and sustainable buildings: characterization of microbiological, mechanical and hygrothermal properties[END_REF]. At the best of our knowledge, no standard sets a strain value at which compressive strength should be studied. Consequently, Unconfined Compressive Strength (UCS) is determined at a strain of 2 %. UCS at chosen strain of the studied mixtures presents similarity to those already observed on other bio-based mixtures [44].

Also, the addition of PCM has almost no effect on mechanical strength of the soil-fiber mixtures. As it can be observed on Figure 8, stress-strain graphs corresponding to the different mixtures are very close with an evident shape similarity. This may be explained by the high porosity in such type of mixtures. Thus, adding PCM of a few tens of micrometers size in high porous materials do not influence their mechanical behavior, see Table 9. This is not the case on the compressive strength that can be observed in cementitious matrices [START_REF] Dakhli | The Effect of Phase Change Materials on the Physical, Thermal and Mechanical Properties of Cement[END_REF][START_REF] Jayalath | Properties of cementitious mortar and concrete containing micro-encapsulated phase change materials[END_REF]. In these latter, PCM incorporation in mortar or concrete will increase porosity and consequently degrade the mechanical resistance.

Considering that the primary function these mixtures i.e. soil-fiber-PCM in building, is purely thermal. Then, given that these materials are self-supporting. It can be assumed that the obtained weak compressive strength does not constitute a limitation for the use of these mixtures in building (or housing) envelopes.

Literature comparison

To be able to evaluate performances of the developed mixtures, their hygrothermal behavior is compared to literature. In Table 10, hygrothermal properties for different earth-based materials are reported: lightened earth with different densities, cob, earth bricks, etc. It should be noted that reported properties have been obtained in experimental conditions (temperature and relative humidity) which are sometimes different. This fact was not considered in present comparisons.

From the lightened earth section in Table 10, it can be seen that mixtures investigated in present study are hygroscopically and thermally (thermal conductivity) less than those in other works This may mainly be due to the differences in the used materials and preparation/conditioning methods. In addition, differences in density can be observed. In fact, materials in present study are denser when compared to those reported in refs [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF][START_REF] Niang | Hygrothermal performance of various Typha-clay composite[END_REF][START_REF] Labat | From the experimental characterization of the hygrothermal properties of straw-clay mixtures to the numerical assessment of their buffering potential[END_REF]. This difference in density can explain, in part, the disparities between here-obtained results and those reported in literature. Indeed, this parameter is of a crucial importance. Niang et al. [START_REF] Niang | Hygrothermal performance of various Typha-clay composite[END_REF] reported that thermal conductivity increases from 0.065 to 0.112 W/(m.K) when lightened earth density increases from 323 to 586 kg/m 3 . This increase in density also affects the lightened earth hygroscopic properties. In fact, maximum moisture sorption decreases, and water vapor resistance factor increases. Moreover, Colinart et al. [START_REF] Colinart | Hygrothermal properties of light-earth building materials[END_REF] reported that thermal conductivity clearly increases with density. However, no effect of density on water vapor resistance factor was clearly observed.

Even though PCM introduction degrades lightened earth hygroscopic properties, as detailed in section 4.1 and 4.2, these properties remain comparable or better than those of the other earth-based materials considered as suitable to moderate buildings indoor humidity. In the case of water vapor permeability, here investigated materials behave as Cob or earth bricks, see Table 10. Regarding maximum sorption value, lightened earth-PCM mixtures are more efficient when compared to these latter. Otherwise, due to their high fiber content, soil-fiber-PCM mixtures present a lower thermal conductivity than Cob or earth bricks.

Finally, as expected, the main enhancement brought by PCMs is noticeable on the specific heat capacity of the lightened earth, near phase change temperature, see Table 10.

Other earthbased materials

Lightened earth

Earth-based materials

Conclusion

In this work, we were interested in the study of PCM effect on thermal, hygroscopic and mechanical properties of a lightened earth. Samples with different PCM content were elaborated. Thermal conductivity, specific heat capacity, sorption behavior, water vapor permeability and compressive strength were studied.

Regarding thermal properties, the study showed that mixtures thermal conductivity decreases when PCM content increases. This is supposed due to the weak thermal conductivity of the PCM and to the disorder increase when phase change takes place inside microcapsules. At 24 °C, the mixture with 20 % PCM was approximately 14 % less conductive than the one without PCM. Also, PCM incorporation showed an interesting effect on the lightened earth specific heat capacity. In fact, we saw that & increases linearly with PCM content. Near the phase-change temperature, mixture with 20 % PCM presented a & value 4 times larger than the one of the sample without PCM.

Regarding the hygroscopic properties, results indicate that PCM incorporation degrades the lightened earth hygroscopic properties. Indeed, soil-fiber mixtures sorption behavior and water vapor permeability are inversely proportional to the PCM content. The drop is supposed due to the PCM hydrophobic nature and porosity decrease which results in degradation of both properties. These two properties drop approximately in the same proportions, for a PCM content of 20 %, the decrease is about 20 %.

Looking changes in lightened earth hygrothermal behavior, a numerical study might be relevant to evaluate the PCMs real impact on indoor air and thermal comfort. Furthermore, lightened earth presented a weak mechanical resistance but sufficient to be self-supporting. The addition of PCM has almost no effect on mechanical strength of the soil-fiber mixtures.

Finally, it can be stated that incorporation of phase change materials in lightened earth improves their insulation level, their heat storage capacity and keep their compressive strength almost unchanged. From another side, PCM incorporation degrades the mixtures hygroscopic properties. However, whether it is sorption behavior or water vapor permeability, the soil-fiber-PCM mixtures remain comparable to other earth-based materials and much more hygroscopic than conventional building materials. Nextek 24D is composed by a paraffinic core and a polymeric shell. It is highly probable that its introduction into lightened earth will increase the environmental impact of the material. Consequently, the following question is worth asking: Does the reduction of the environmental impact due to energy savings generated by PCMs remain interesting in view of the impact generated by the production, transport, and disposal of this material?

In continuity of this work, simulations of energy saving and environmental impact due to PCM incorporation into lightened earth will be performed. The case of biobased PCMs will be carefully considered.

Figure 1 :

 1 Figure 1: Soil particle size distribution after crushing.

Ø11x22 3 Figure 2 :

 32 Figure 2: Samples used in thermal (22×22×4 cm3) and water vapor permeability (Ø15 cm x 6 cm) testing.

Figure 3 :

 3 Figure 3: Moisture sorption isotherm of the different soil-fiber-PCM mixtures and a standard cement mortar.

Figure 4 :Figure 5 :

 45 Figure 4: Moisture sorption isotherm of soil, fiber and PCM.

Figure 6 :

 6 Figure 6: Evolution of thermal conductivity of the soil-fiber mixtures as function of temperature and PCM content.

Figure 7 :

 7 Figure 7: Evolution of the lightened earth specific heat capacity as function of temperature and PCM content in heating.

Figure 8 :

 8 Figure 8: Stress-strain relationship for soil-fiber mixtures as function of PCM content.

Table 1 :

 1 Atterberg limits and methylene blue values of the crushed soil.

	Parameter	LL	PL	PI	MBV
		[%]	[%]	[%]	[g/100g]
	Soil	57.8	42.5	15.3	5.64

Table 2 :

 2 Absolute density measured by helium pycnometer.

	Material	Absolute Density
		[kg/m 3 ]
	Soil	2630
	Fiber	1020
	Nextek 24D	845

Table 3 :

 3 Physical properties of the studied PCM.

	PCM	Particle	Bulk	Melting temperature	Latent heat	Specific heat
		size [µm]	Density	[°C]			[J/g]	capacity
			[kg/m 3 ]					[J/(kg.K)]
	Nextek 24D	0-80	≈ 300-400	22,4*	24 ± 2**	168*	170**	12000*

* measured in the laboratory, ** Product data sheet

[28]

Table 5 :

 5

Standard, number and size of samples for testing.

Test Standard Sample dimensions [cm] Number of samples Moisture sorption isotherm

		ISO 12571 [53]	3x3x3	3
	Water vapor permeability	ISO 12572 [54]	Ø15x6	3
	Thermal conductivity	ISO 8301 [55]	22x22x4	3
	Compressive strength	EN 1015-11 [56]

Table 7 :

 7 Water vapor permeability measured on the different soil-fiber-PCM mixtures.

		SF0	SF2	SF5	SF10	SF20	Mortar [62]
	Water vapor permeability x10 -12 [kg.m -1 .s -1 .Pa -1 ]	25.631	24.997	23.468	22.227	21.305	3.160
	Water vapor resistance factor µ [-]	7.101	7.217	7.788	8.284	8.673	62.000
	Decrease in permeability [%]	0	2.5	8.4	13.3	16.9	-

Table 8 :

 8 Thermal conductivity of lightened earth with 2 %, 5 %, 10 % and 20 % of microencapsulated PCM at three different temperatures 14, 24 and 34 °C.

	Formulation	SF0	SF2	λ [W/(m.K)] SF5	SF10	SF20
		14	0.151 ± 0.007	0.149 ± 0.005	0.144 ± 0.010	0.144 ± 0.004	0.143 ± 0.005
	T [°C]	24	0.156 ± 0.008	0.144 ± 0.005	0.142 ± 0.007	0.135 ± 0.003	0.134 ± 0.004
		34	0.159 ± 0.007	0.153 ± 0.005	0.151 ± 0.008	0.151 ± 0.003	0.151 ± 0.003

Table 9 :

 9 Unconfined compressive strength parameters for soil-fiber mixtures as function of PCM content.

		1.2
		1
	Compressive stress [MPa]	0.4 0.6 0.8
		0.2
		0
		0	0.05	0.1	0.15	0.2	0.25
					Strain ε
			SF0	SF2	SF5		SF20
	Soil-fiber mixture	SF0		SF2	SF5	SF10	SF20
	Strain [mm/mm]	0.02		0.02	0.02	0.02	0.02
		UCS [MPa]		0.07		0.07	0.06	0.06	0.06
	Young's modulus [GPa]	1.347	1.153	0.841	1.010	1.368

Table 10 :

 10 Measured hygrothermal properties for different earth-based materials.

	Mix	Density [kg/m 3 ]	Sorption max [%]
	Earth + 25 % Reed + 0 -20 % PCM	650 -700	6 -7,7 at RH=90 %
	Earth + Straw	241 -531	12 at RH=93 %
	Earth + Typha Australis (20 %, 33 %)	323 -586	12,9 at RH=97 %	3,748 -7,057
	Earth + hemp shiv 34 % -67 %	200-350	5,3 -7,56 at RH=80 %	2.24 -4.14
	Unfired earth bricks	1761 -1797	5,3 at RH=95 %	-
	Extruded earth brick	1940 -2070	4 -6 at RH=97 %	7 -9
	Cob	1462 -2011	2,8 -4,1 at RH=90 %	7 -9,98

Acknowledgements

The authors wish to thank Jean-François Bardeau (IMMM, Le Mans Université) for the Cp measurement, Malo Le Guern (ESITC Caen) and François Streiff (PnrMCB) for the raw materials supply.

Results and discussion

Moisture sorption isotherm

The moisture sorption isotherms reported in Figure 3 show, for each investigated material, evolution of moisture content versus ambient air relative humidity at a constant temperature of 23 °C. According to Brunauer's proposal [START_REF] Andrade | Models of sorption isotherms for food: Uses and limitations, Vitae[END_REF], the sorption curves correspond to the type III and show hysteretic loops. Such kind of curves is typical of porous materials, as the investigated soil-fiber mixtures.

It is observed that the amount of absorbed water vapor from the ambience is inversely proportional to the percentage of PCM incorporated. For a better analysis of this decrease in the mixture's sorption/desorption values, the raw materials (soil, reed fiber and PCM) were characterized, see Figure 4. The results demonstrate that mixtures sorption/desorption behavior is largely governed by the presence of reed fibers. As PCM is a hydrophobic material, its addition into the mixtures implies a decrease in the amount of absorbed moisture in soil-fiber mixtures.

Soil-fiber mixtures slightly lose their hygroscopic performances with the addition of PCM. Nevertheless, these remains enough high compared to those of a standard cement mortar. The mixtures sorption/desorption capacity is 4-5 times larger than the one of standard cement mortar, see Figure 3. This mortar is prepared in the laboratory conforming to the standard NF EN 196-1 [START_REF]Methods of testing cement -Part 1 : determination of strength[END_REF]. In addition, for a sorption/desorption cycle, the soil-fiber-PCM mixtures retrieve their initial hygric state whilst a cement mortar retains a certain amount of moisture within it.

For all soil-fiber mixtures at each relative humidity, moisture content varies significantly within 24 hours of the change in relative humidity and then the phenomenon slows down to equilibrium. A better reactivity is observed both in mixtures sorption and desorption paths compared to standard cement mortar, see Figure 5. The rapid response of the earth-based material to a change in relative humidity is perhaps one of the strongest properties for moisture regulation in the interior of a building.

From Figure 3, it can be noted that the area of the zone located between sorption and desorption curves, hysteresis decreases slightly with PCM addition. The decrease in the mass difference value between different dosages is due to the component's nature, while hysteresis is probably due to the porosity of the mixtures. To verify this assumption, the mixtures overall porosity is measured by immersing small samples in a non-wetting oil. The results presented in Table 6 confirm the porous nature of the mixtures and point a decrease in porosity with addition of PCM. Looking its granulometry, we suppose that added PCM embeds and fills the mixtures pores. Furthermore, these porosity results can be correlated with those reported on the sorption/desorption curves. Indeed, the lower porosity acts to reduce areas of the hysteresis seen on Figure 3 and consequently leads to a reduction of the mixtures hygroscopicity.

When a material sorption ability is high that means it will prevent surface condensation pathologies (mildew). Consequently, that will increase moisture content inside the material leading to a degradation of its thermal insulation properties. In this context, knowing the material water vapor permeability becomes essential.