S Guiheneuf
email: simon.guiheneuf@univ-ubs.fr

Damien Rangeard
email: damien.rangeard@insa-rennes.fr

Thibault Cusin

Florence Collet

Sylvie Prétot

Arnaud Perrot
email: arnaud.perrot@univ-ubs.fr

Simon Guihéneuf
email: simon.guiheneuf@insa-rennes.fr

S G Conceptualization

A P ; Validation

Writing-Original Draft

Effect of bio-stabilizers on capillary absorption and water vapour transfer into raw earth

Keywords: Earth-based building materials, Bio-based additions, Capillary absorption, Water vapour permeability, Mechanical strength

Introduction

Earthen construction, which is a global traditional building technique that has been used for thousands of years, has been, to some extent, neglected in the last century due to the development of the steel and concrete building industries, even if this vernacular technique has survived in both industrialised and developing countries. In the context of global warming that forces humans to reconsider the global economic growth paradigm, this construction technique is now regaining interest in the building industry, mostly because of its low environmental impact, its recyclability and its availability locally [START_REF] Azeredo | Applicability of rheometers to characterizing earth mortar behavior. Part I: experimental device and validation[END_REF][START_REF] Aubert | Towards a simple compressive strength test for earth bricks?[END_REF][START_REF] Bui | Compression behaviour of non-industrial materials in civil engineering by three scale experiments: the case of rammed earth[END_REF][START_REF] Moevus | Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase[END_REF][START_REF] Pacheco-Torgal | Earth construction: Lessons from the past for future eco-efficient construction[END_REF][START_REF] Arrigoni | Life cycle analysis of environmental impact vs. durability of stabilised rammed earth[END_REF][START_REF] Morel | Building houses with local materials: means to drastically reduce the environmental impact of construction[END_REF].

However, even if raw-earth seems to be a promising building material, it also presents some disadvantages, such as its significant variability and durability issues that limits the democratization of its use [START_REF] Houben | Building with Earth: Design and Technology of a Sustainable Architecture[END_REF]9]. Such a large variability of resources (using various processing routes) is also leading to unguaranteed minimal mechanical performances [10][START_REF] Bui | Effect of moisture content on the mechanical characteristics of rammed earth[END_REF][START_REF] Morel | Compressive strength testing of compressed earth blocks[END_REF][START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Guihéneuf | Cast, compaction, vibro-compaction or extrusion: processing methods for optimizing the mechanical strength of raw earth-based materials[END_REF][START_REF] Champiré | Impact of relative humidity on the mechanical behavior of compacted earth as a building material[END_REF]. Furthermore, the water sensitivity of earth, in both liquid and vapour phases, is often quoted as its main weakness related to durability because it can alter the material and strongly reduce its mechanical properties. On the contrary, these hygroscopic characteristics of rawearth are also strong assets, as this material is able to adsorb or release moisture from the atmosphere of a building and thus naturally regulate the indoor hygrothermal conditions [START_REF] Houben | Building with Earth: Design and Technology of a Sustainable Architecture[END_REF]9,[START_REF] Hall | Analysis of the hygrothermal functional properties of stabilised rammed earth materials[END_REF][START_REF] Allinson | Hygrothermal analysis of a stabilised rammed earth test building in the UK[END_REF][START_REF] Mcgregor | Conditions affecting the moisture buffering measurement performed on compressed earth blocks[END_REF][START_REF] Mcgregor | A review on the buffering capacity of earth building materials[END_REF]. Hence, it seems to be important to aim to keep the initial mechanical properties of this building material, by protecting it from water in a liquid and vapour phase without constraining the diffusion of water vapour.

Several options have been studied to optimize, preserve and maintain the mechanical properties and enhance the water durability of this material. One option is to stabilize it with hydraulic binders [START_REF] Walker | Properties of some cement stabilised compressed earth blocks and mortars[END_REF][START_REF] Reddy | Cement stabilised rammed earth. Part A: compaction characteristics and physical properties of compacted cement stabilised soils[END_REF][START_REF] Reddy | Cement stabilised rammed earth. Part B: compressive strength and stress-strain characteristics[END_REF][START_REF] Khelifi | Design of clay/cement mixtures for extruded building products[END_REF][START_REF] Tripura | Behavior of cement-stabilized rammed earth circular column under axial loading[END_REF][START_REF] Landrou | Development of a Self-Compacted Clay based Concrete, rheological, mechanical and environmental investigations[END_REF][START_REF] Khelifi | Mechanical enhancement of cementstabilized soil by flax fibre reinforcement and extrusion processing[END_REF] but it is commonly agreed that their use should be avoided to aim for the lowest embodied energy building material [START_REF] Van Damme | Earth concrete. Stabilization revisited[END_REF][START_REF] Marsh | Discussion of "Earth concrete. Stabilization revisited[END_REF][START_REF] Van Damme | Reply to the discussion of the paper "Earth Concrete. Stabilization Revisited[END_REF][START_REF] Ouedraogo | Is stabilization of earth bricks using low cement or lime contents relevant?[END_REF]. Another option is to deflocculate the clay particle's microstructure to optimize the rheology of the material and thus its final mechanical properties [START_REF] Moevus | Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase[END_REF][START_REF] Landrou | Development of a Self-Compacted Clay based Concrete, rheological, mechanical and environmental investigations[END_REF][START_REF] Perrot | Yield stress and bleeding of fresh cement pastes[END_REF][START_REF] Perrot | Linking rheological and geotechnical properties of kaolinite materials for earthen construction[END_REF][START_REF] Landrou | A fresh look at dense clay paste: Deflocculation and thixotropy mechanisms[END_REF] by using a dispersant such as Sodiumhexametaphosphate (Na-HMP) that acts as a superplasticizer on cement particles in concrete. Since these dispersants are chemical and industrial products, it seems wise to minimize their content in the mix.

Other options could have less impact on the environment. Thus, the optimization of the processing method could systematically improve the final mechanical properties of the material by increasing its dry density [START_REF] Guihéneuf | Cast, compaction, vibro-compaction or extrusion: processing methods for optimizing the mechanical strength of raw earth-based materials[END_REF][START_REF] Bruno | Briques de terre crue : procédure de compactage haute pression et influence sur les propriétés mécaniques[END_REF][START_REF] Bruno | Étude du comportement hygro-mécanique de la terre crue hypercompactée pour la construction durable[END_REF][36][START_REF] Bruno | Optimization of bricks production by earth hypercompaction prior to firing[END_REF]. In addition, to substitute the previously cited additions that increase the water durability and the mechanical properties of the material during its life, some bio-based additions could be used. From empirical knowledge, several bio-based additions have been used around the world to enhance earthen building materials [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][START_REF] Vissac | Argiles & biopolymères, les stabilisants naturels pour la construction en terre[END_REF]. The potential of bio-polymers to enhance mechanical properties, water durability and the rheological behaviour of raw earth building materials has also been highlighted in recent research works [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][40][START_REF] Galán-Marín | Clay-based composite stabilized with natural polymer and fibre[END_REF][START_REF] Anger | Approche granulaire et colloïdale du matériau terre pour la construction[END_REF][43][START_REF] Hafshejani | A novel bio-based deflocculant for bentonite drilling mud[END_REF][START_REF] Banakinao | Dataset of the use of tannin of néré (parkia-biglobosa) as a solution for the sustainability of the soil constructions in West Africa[END_REF][START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF][START_REF] Ouedraogo | Stabilisation de matériaux de construction durables et écologiques à base de terre crue par des liants organiques et/ou minéraux à faibles impacts environnementaux[END_REF][START_REF] Chang | Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries[END_REF].

Firstly, in this study, the impact of selected bio-based additions on the water durability of different earthen materials is assessed by using a capillary absorption test. Discussion is supported by apparent contact angle observations carried out on compacted earth samples. Then the effect of some of the most promising additions on the water vapour permeability of the material is investigated. The main aim of this study is to highlight that some bio-polymers and bio-based additions, by various mechanisms, can strongly enhance the water durability, and therefore the mechanical properties of earthen materials during in-service life without affecting the transfer of water vapour in their pores, which is one of their most useful physical properties.

Materials and methods

Materials

Tested earth materials

This study is part of a regional project in Brittany, France that aims to develop earthen construction on a wider scale. Three kinds of earth (K, R and S) representative of the variability of the local resources were studied. All of these resources have already been studied in previous works [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Guihéneuf | Cast, compaction, vibro-compaction or extrusion: processing methods for optimizing the mechanical strength of raw earth-based materials[END_REF][START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF].

Kaolin based soil: K

The kaolin based soil (K) is a model earth reconstituted from quarry materials and composed of kaolin coming from Ploemeur, with the size of the particles ranging from 0 to 10µm (d50 = 4µm), EN-196.1, standard sand (with particles having a diameter of up to 4 mm) and a finer river sand provided by the same supplier as the standard sand and having a particle size distribution ranging from 63 to 200 m. The kaolin has a specific gravity of 2.65 and a specific surface area of 10 m 2 /g. The final mix obtained using a Dreux and Gorisse method [START_REF]Nouveau guide du béton et de ses constituants -Georges Dreux[END_REF] in order to optimize the granular compacity is composed by mass of 17% of kaolin, 23% of river sand and 60% of standard sand. This soil K has a small methylene blue value of 0.2 and can be qualified as low active earth. The Plasticity Index of the soil K (<200 m) is about 13.4 with a liquid limit of 32% and a plastic limit of 18.6%. It has already been shown that with these types of low-active soils, the effect of a bio-based stabilizer is significant [START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF].

Local soil extracted in Redon, Brittany (rammed earth): R

This studied material R is usually processed using a rammed earth technique. It is a clayey sand with 50% of its particles finer than 10 μm and 95% of its particles finer than 2 mm. The soil particles are a mix of quartz and various types of clay: kaolinite, illite and chlorite. The Plasticity Index of the soil R is about 14.5 with a liquid limit of 36.5% and a plastic limit of 22%. This soil has a methylene blue value of 0.7.

Local soil extracted in St Sulpice, Brittany (cob earth): S

The studied earth S is usually processed using the cob earth technique. It is a fine clayey soil with more than 70% of its particles finer than 20 μm. The soil particles are a mix of quartz and various types of clay: kaolinite, illite and some traces of vermiculite (swelling clay) as determined by XRD analysis. The Plasticity Index of the soil is about 16.5 with a liquid limit of 46% and a plastic limit of 29.5%. This soil has a methylene blue value of 1.1.

The particle size distribution (PSD) of the three tested soils obtained by laser diffraction (LD) and sieving or hydrometer analysis (HA) and sieving, are plotted in Fig. 1. It can be seen that the PSD slightly varies between methods. This is due to the fact that both methods assumed that the particles are spherical and that this assumption affects the results of each technique differently (sedimentation velocity for HA and light diffraction for LD) leading in both cases to slight measurement errors.

Bio-based additions

Several bio-based additions, the biopolymers, were also tested. After a review of previous studies and traditional recipes [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][40][START_REF] Galán-Marín | Clay-based composite stabilized with natural polymer and fibre[END_REF][START_REF] Anger | Approche granulaire et colloïdale du matériau terre pour la construction[END_REF][43][START_REF] Hafshejani | A novel bio-based deflocculant for bentonite drilling mud[END_REF][START_REF] Banakinao | Dataset of the use of tannin of néré (parkia-biglobosa) as a solution for the sustainability of the soil constructions in West Africa[END_REF][START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF][START_REF] Ouedraogo | Stabilisation de matériaux de construction durables et écologiques à base de terre crue par des liants organiques et/ou minéraux à faibles impacts environnementaux[END_REF][START_REF] Chang | Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries[END_REF] different bio-based additions, that can be classified into 4 different families were chosen. Their potential to enhance raw-earth based materials are described in the following subsections. As in the antecedent studies [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF], the dosages of the different additions were determined in order to optimize; the dry mechanical strengths of different earthen materials, their rheological behaviour, and their mechanical Polysaccharides are long chains of sugars that can react with water and form a gel.

Combined with earthen materials, they can form microscopic bonds between the clay layers [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF]. In this study Alginates (Alg), Xanthan Gum (Xan), Cellulosic Glue (CG) have been selected because of their availability locally.

The chains of alginate (Alg) can attach themselves and form a cross-linked isotropic insoluble gel [START_REF] Funami | Rheological properties of sodium alginate in an aqueous system during gelation in relation to supermolecular structures and Ca2+ binding[END_REF]. Alginates have already been tested in past studies as a stabilizer for raw earth material [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Galán-Marín | Clay-based composite stabilized with natural polymer and fibre[END_REF]43,[START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF]. In this study, the alginate used is a white powder of sodium alginic salt Cimalgin HS3 ® provided by Cimaprem (Redon, France). The chosen dosage of alginate is 1% of the mass of the dry earth.

Produced by fermentation of glucose or sucrose by the "Xanthomonas campestris" bacterium, Xanthan gum (Xan) is a highly stable branched heteropolysaccharide [START_REF] Chang | Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries[END_REF][START_REF] Cadmus | Biodegradation of Xanthan Gum by Bacillus sp[END_REF][START_REF] Becker | Xanthan gum biosynthesis and application: a biochemical /genetic perspective[END_REF]. This gum is mostly used for its unique rheological properties in the food industry and several other sectors. Recent studies have highlighted that Xanthan gum (Xan) could strongly increase the mechanical properties of soils and earthen materials [START_REF] Chang | Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries[END_REF][START_REF] Chang | Effects of Xanthan gum biopolymer on soil strengthening[END_REF]. In addition, these long macromolecules could adsorb onto the surface of clay particles and thus protect earthen materials. For this study, two dosages of xanthan gum, provided by the "wake-up-easy sportfood" company, were retained at 0.5% and 1% of the mass of the dry earth.

According to Anger et al. [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF], many different vegetal glues are used all around the world to enhance the properties of earthen constructions. Thus, a vegetal cellulosic glue addition (CG) was also tested. In this study, an industrial 100% vegetal wallpaper glue was used with a dosage of 6.25 grams per litre of mixing water.

Lipids: Linseed Oil (LO) and Vegetal-based Varnish (VV)

Lipids are the fat content of living systems. These biopolymers are insoluble in water and are used in earthen construction for their hydrophobic nature, notably in outside coatings of vernacular buildings [START_REF] Vissac | Argiles & biopolymères, les stabilisants naturels pour la construction en terre[END_REF].

Linseed oil (LO) is made up of 90% of polyunsaturated fatty acids that are part of the lipids. These fatty acids are amphiphilic with a polar head and a non-polar unbranched chain. During drying, the oil molecules oxidise and polymerise by creating a cross-linked network. This phenomenon, called siccativation, is very slow and can be catalysed by the presence of oxygen, the presence of metallic ions and the temperature or the light energy [START_REF] Schaich | Lipid Oxidation: Theoretical Aspects. In: Bailey's Industrial Oil and Fat Products[END_REF][56][START_REF] Dridi | Influence de la formulation sur l'oxydation des huiles végétales en émulsion eau-dans-huile[END_REF]. For this study, two doses of linseed oil mixed in the mass of the earthen material were chosen at 1% and 2% of the mass of the dry earth. The effect of a brush application of this oil on the surface of the earthen materials was also tested.

A vegetal-based varnish (VV) consisting of a vegetal oil emulsion without any volatile organic compounds or formaldehyde was also tested. It was coated with a brush on the surface of the different earthen materials.

Proteins: Casein (Cas)

The proteins are macromolecules of living systems that are usually amphiphilic.

Casein (Cas) has both hydrophilic and hydrophobic properties. It has been used

previously in vernacular raw earth construction as a coating or mixed up in the mass of the material to reinforce its water durability [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][START_REF] Vissac | Argiles & biopolymères, les stabilisants naturels pour la construction en terre[END_REF]. As in a previous study [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF], micellar casein powder was used at a dosage of 1% of the mass of the dry earth.

Tannins: Chestnut Tannins (Tan), Oak Seed Extract (OSE)

Tannins are complex polyphenols that can be encountered in most plants [START_REF] Hafshejani | A novel bio-based deflocculant for bentonite drilling mud[END_REF].

Tannins have been used since antiquity to make leather from hides. Also, in raw earth traditional construction, tannins are used to strengthen coatings of earthen walls in relation to their water durability [START_REF] Vissac | Argiles & biopolymères, les stabilisants naturels pour la construction en terre[END_REF][START_REF] Banakinao | Dataset of the use of tannin of néré (parkia-biglobosa) as a solution for the sustainability of the soil constructions in West Africa[END_REF]. According to other studies, under certain conditions, tannins can also act as a dispersant for clay particles [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][START_REF] Anger | Approche granulaire et colloïdale du matériau terre pour la construction[END_REF] and thus could increase the mechanical and water durability properties of earthen blocks.

In this study, the first type of tannins used were chestnut tannins (Tan) originally sold for wine making applications. Here, the tested dosage was 0.4% in the mass of the dry earth as in previous works [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF].

Also, Hafshejani et al. [START_REF] Hafshejani | A novel bio-based deflocculant for bentonite drilling mud[END_REF] have highlighted the fact that an Oak Seed Extract (OSE) could be used as a dispersant for bentonite drilling mud. In this study, the addition of an OSE powder sieved at 315 μm, was studied. In the case of the addition of OSE powder, the dosage was 0.8% in the mass of the dry soil.

Other materials

Sodium-hexametaphosphate (HMP)

As previously described, Hexametaphosphate (HMP,) when added at a small concentration, can be an efficient dispersant for clay particles [START_REF] Moevus | Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase[END_REF][START_REF] Landrou | Development of a Self-Compacted Clay based Concrete, rheological, mechanical and environmental investigations[END_REF][START_REF] Perrot | Linking rheological and geotechnical properties of kaolinite materials for earthen construction[END_REF][START_REF] Landrou | A fresh look at dense clay paste: Deflocculation and thixotropy mechanisms[END_REF]. The use of a dispersant can be efficient to minimize and optimize the porosity of earthen materials, and thus help to increase their mechanical and water durability properties. In order to highlight the effect of an efficient dispersant on the water durability of earthen materials, Hexametaphosphate (HMP) was used as it has been used in previous studies [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Guihéneuf | Cast, compaction, vibro-compaction or extrusion: processing methods for optimizing the mechanical strength of raw earth-based materials[END_REF][START_REF] Perrot | Linking rheological and geotechnical properties of kaolinite materials for earthen construction[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF]. Also, some of the bio-based additions could have a thickening effect on the rheological properties of the raw earth-based materials [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF]: thus, the addition of an efficient dispersant in the mix design is important. The dosage chosen here was 0.0425% in the mass of the dry soil. In this study, it is assumed that HMP acts only on clay particles to ease the sample's processing [START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF]. As a consequence, it is here considered as a first approximation that HMP does not interact with bio-based additions.

Hydrated Lime (HLime)

One of the aims of this study is to highlight the fact that the use of a mineral binder is not the only option to protect raw earth materials from water. Therefore, for comparison purposes, tests were also conducted on earth samples stabilized with hydrated lime at a content of 5% in the mass of the dry soil.

Methods

Capillary water absorption tests

As several studies have already reviewed [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][START_REF] Vissac | Argiles & biopolymères, les stabilisants naturels pour la construction en terre[END_REF], some bio-based additions can have the ability to reduce the water sensitivity of earthen materials. In order to highlight this property, a capillary absorption test was developed, inspired by the AFNOR NF EN 13057 [START_REF] Afnor | NF EN 13057 -Produits et systèmes pour la protection et la réparation des structures en béton -Méthodes d'essai -Détermination de l'absorption capillaire[END_REF] French standard and the AFNOR XP P 13-901 [59]

French standard for cement stabilized compressed earth blocks. The atmosphere in the test room was maintained at a temperature of 20°C and a relative humidity of 50%. The water level in the water bath remained constant during the test. The test stopped when the water content of the samples no longer evolved.

As already described earlier, the sample weight change versus the elapsed time t was followed. By plotting the cumulative absorption inflow φ (kg/m 2 , weight of absorbed water by unit area) versus the square root of time (t 0.5 ), an absorption curve with two phases was obtained (Fig. 2). During the first phase, the absorption kinetic was mainly governed by the capillary forces: the mass of absorbed water in the sample quickly increased. Then, the second phase started when the water level reached the top of the sample. During this phase, the diffusion of the entrapped air in the sample's pores governed this kinetic [START_REF] Janssen | A Confrontation of Two Concepts for the Description of the Over-capillary Moisture Range: Air Entrapment versus Low Capillarity[END_REF]. In Fig. 2, the transition in the second phase is visible at t 0.5 =50 s 0.5 .

Following these observations, two different criteria were used to describe the water capillary absorption of the tested samples. First, a capillary absorption coefficient Acap (g.m -2 .s -0.5 ) was determined to highlight the capillary absorption velocity during the first phase. In order to take the variation of the dry density of the samples into account, the value of Acap/n was considered, where "n" is the apparent porosity of the sample. Then, the capillary moisture content wcap, that is the moisture content of the sample when the second phase of the absorption curve started, was determined. In order to determine the Acap value, different methods that stem from several studies [59-62] can be used: Hall's method, the 1-tangent method and the fix-time method.

More than 80 capillary absorption tests were conducted on different formulations of different earths. As described earlier, the absorption cumulative inflow φ (kg/m 2 ) versus the square root of time (t 0.5 ) was obtained. These curves highlighted the capillary absorption pattern of each sample. During the data analysis of these tests, three distinct absorption kinetics were observed. The three types of pattern are plotted in Fig. 2. Here it is important to find the most efficient and accurate method to derive the capillary absorption coefficient from each type of pattern. For a better illustration, the dimensionless cumulative inflows (inflow divided by maximal value for each case) are plotted. On the first pattern, phases 1 and 2 are not clearly distinct. On the second pattern, the two phases are clearly distinct. In both the first and second patterns (most of the time) the shape of the first phase is slightly concave. On the third pattern, the two phases of absorption are distinct but the shape of the first phase is convex.

Curve pattern n°1

Curve pattern n°2

Curve pattern n°3

The 1-tangent method consists in performing a linear regression on the linear part of the first phase of absorption. This method consists in the estimation of the slope of this tangent and is based on the diffusivity equation [START_REF] Wilson | British standard and RILEM water absorption tests: A critical evaluation[END_REF][START_REF] Philip | Theory of Infiltration[END_REF]. A simplified expression of this equation is shown in Eq. ( 1).

m t -m 0 A =A cap ×t 0.5 +k 1 [START_REF] Azeredo | Applicability of rheometers to characterizing earth mortar behavior. Part I: experimental device and validation[END_REF] In this experiment, this method was chosen because it seems to be the most versatile for the data's interpretation. This method seems to provide accurate Acap values for the second and the third type of patterns, but approximate results [START_REF] Feng | Hygric properties of porous building materials (III): Impact factors and data processing methods of the capillary absorption test[END_REF] for the first type of pattern. However, the obtained Acap values for the first type of pattern can remain consistent if the time range used for the computation is carefully chosen.

This 1-tangent method efficiently describe the material's average behaviour during the first absorption phase and can be used for each type of pattern.

In order to take into account density variation between samples, Acap values were divided by the apparent porosity (n) of the tested samples before comparison. In each case, wcap was also determined at the end of the second phase, in order to take into account of the different capillary absorption behaviours.

Contact angle measurements

Each impact of a bio-based addition on the water durability of different earthen materials was induced by a protection mechanism at the microscopic scale of the material. In the case of some identified efficient bio-based additions, contact angle measurements of water droplets on the surface of different formulations of the soil S were conducted, in order to highlight those mechanisms. Soil S was chosen because it presents the highest clay content, the highest water affinity and is supposed to react strongly with water droplets. Thus, if an addition is able to limit the interactions between clays and water, it would be easily noticeable for the soil S. Hence, the contact angle of a sessile droplet of water on the surface of earthen materials was studied using a DSA30 drop shape analyser (Krüss GmBH). Tests were performed on dry samples of the soil S combined with selected additions in a room at a temperature of 20°C and a relative humidity of 50%. Sessile droplets of 15 µL of water were created using a vertical syringe (needle with an outer diameter of 1.835 mm). The contact angle was thus measured from a shape analysis based on the Young-Laplace equation at the interface between the water droplet and the material. Such a measurement is usually performed on a nonporous surface in order to obtain surface properties of the solid. In this study, the observation of the kinetic of infiltration of a single droplet obtained evidence of potential infiltration resistance, for example due to hydrophobic properties, provided by a bio-stabiliser. Tests were conducted on samples having the same densities, that had been processed by extrusion, the different earth mix being at a same given plastic consistency. To obtain this constant consistency, the water content of each mix was slightly modified, since each addition is modifying the rheological behaviour of the mortar at the fresh state. The evolution of the contact angle versus time was followed for different formulations of soil S. The analysis of the different contact angles obtained could be a strong indicator of the change in infiltration behaviour caused by the addition of the bio-based stabiliser that strengthens earthen materials in relation to their water durability.

Water vapour permeability tests

Some bio-based additions used to limit the water capillary absorption and protect the materials may also limit the water vapour transfer. The water vapour permeability of a material characterises its ability to transfer moisture under a vapour pressure gradient. It includes vapour transfer by diffusion, vapour transfer by effusion and liquid transfer (linked to capillary condensation) [START_REF] Collet | Comparison of the hygric behaviour of three hemp concretes[END_REF][START_REF] Collet | CARACTERISATION HYDRIQUE ET THERMIQUE DE MATERIAUX DE GENIE CIVIL A FAIBLES IMPACTS ENVIRONNEMENTAUX[END_REF][START_REF] Mazhoud | Hygric and thermal properties of hemplime plasters[END_REF]. In this study, the water vapour permeability was measured following the recommendations of the cup method international standard [68] described in the Collet et al. study of hemp concretes [START_REF] Collet | Comparison of the hygric behaviour of three hemp concretes[END_REF]. The test was performed under isothermal conditions (23°C) for a given set of relative humidity (0/50 %RH).

Before the measurement, the samples were firstly dried in an oven at 60°C and then stabilised at (23°C, 50%RH). The stabilisation was reached when the variation of mass was lower than 0.01% between three consecutive weighing's at a 24 hours interval. Five samples were tested for each formulation.

Four formulations were tested in order to study the effect of additions on water vapour permeability. The selected soil was chosen according to its strength and its ability to be shaped into cylinders of 2cm in height and 11 cm in diameter, representative of the material, that were cast at the same "suited for casting" consistency. The chosen additions were selected according to their potential to strongly reduce the water sensitivity of earthen materials. Thus, the selected formulations were: Soil S without additions, Soil S-HMP, Soil S-HMP-Xan(0.5%)

and Soil S-HMP-LO(2%).

Compressive strength tests

Finally, in order to highlight the fact that an increase of water durability leads to an increase of the in situ mechanical properties of earthen materials, the compressive strength of several cast samples of chosen formulations of the soil S, were put into a wet chamber (20°C, 98%HR) for different durations. The samples were cast at the same "suited for casting" consistency in order to obtain similar densities. Compressive strengths at the dry state were also measured for all of the formulations. The compressive tests were carried out with a sample of an aspect ratio following the adopted procedure of previous studies [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Guihéneuf | Cast, compaction, vibro-compaction or extrusion: processing methods for optimizing the mechanical strength of raw earth-based materials[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF].

Bio-based stabilizers influence on capillary water absorption of raw-earth based materials: different mechanisms

First, as some of the studied bio-based additions can enhance the water durability of earthen materials, several capillary absorption tests, previously described, were conducted on different formulations of the three studied soils. In the following subsections, an analysis of these tests is conducted, focusing on the impact of the bio-based additions on these absorption behaviours and on the distinct mechanisms involved in the capillary absorption limitation.

Capillary absorption tests: results and bio-polymer effects

Various capillary absorption behaviours for different earths

Firstly, the results obtained for the three different soils without any addition highlighted their variability of behaviours regarding the capillary absorption due to their various compositions. The clay content, the particle size distribution and the nature of the clays of each tested soil leads to distinct Acap and wcap values that could help to refine the understanding of earthen material behaviours.

The capillary absorption curves of each of the three different earthen materials are plotted in Fig. 3. The Acap (determined with the 1-tangent method) and wcap values, as well as the apparent porosity value n, of each material are shown in Table 1. of the first phase also highlights a faster but constant capillary absorption (Higher Acap) that leads to a stable second phase that displays a higher wcap value.

Soil

Secondly, the finest soil S has a higher clay content than the soil R, and its clayey part is composed of kaolinite, illite and some traces of vermiculite (swelling clay) that presents a high specific area. Again, the capillary absorption curve displays two distinct main phases. However, during the first phase, the absorption flow was initially quite slow and then a strong acceleration of the absorption was easily noticeable (Pattern n°3). This is probably due to the nature of the fine fraction of the soil S, these clay particles being more active and reacting more strongly with the initially adsorbed water after a first period of absorption. Vermiculite clays are expanding clays that are composed of thin layers linked by exchangeable cations (Mg 2+ ) that present high hydration capacities [START_REF] Anger | Approche granulaire et colloïdale du matériau terre pour la construction[END_REF]69]. In this case, during the first phase of absorption it is likely that the first period of slower absorption is linked to the only "common" capillary forces as already observed for soils K and R.

Then, when the acceleration is noticeable on the absorption curve, it is likely that the absorption is driven by both the capillary forces and the hydration of the interlayer cations. That could explain the acceleration of capillary absorption during the first phase, with the highest value of Acap. The soil S is also the finest of the soils tested, and it is likely that its pores are very small in a strongly ramified porosity. That could also explain the higher velocity of the capillary absorption in the first phase, in view of the compositions and behaviours of the two other soils (K and R, coarser soils, with a lower porosity likely to be less ramified). It would be interesting to link those results with estimations of the mean size of the pores of each soil because capillary forces are driving the absorption phenomenon [START_REF] Fabbri | Analysis of the water absorption test to assess the intrinsic permeability of earthen materials[END_REF]. Note that the properties of the porous network can be linked to the clay content, the nature of the clayey part and the specific area of each soil and can partly explain the absorption kinetic ranking between tested soils [10, 71, 72].

Finally, these capillary absorption curves are linked to two main physical processes: the capillary absorption in a porous construction material directly linked with the porosity of the material, and the adsorption of water on the surface of the clay particles that could catalyse this capillary absorption of water, and affect this phenomenon.

Effect of bio-based additions on capillary absorption

As stated previously, the impact of several additions on the capillary absorption behaviours of the studied soils were followed. For each combination (Soil + addition), at least 2 samples were studied, and the mean values of wcap and Acap balanced by their apparent porosities were obtained and plotted in Fig. 4. In this values obtained for the soils combined with HMP only, since in almost all of the studied combinations HMP was added. HMP was added in the mix for most of the studied bio-based additions in order to ease the fabrication of the samples and to optimise their densities as in previous studies [START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF]. It also appears that HMP seems to have a limited effect on the capillary absorption properties of the three studied soils. If some interactions between biological stabilisers and HMP may have occurred, they have not been taken into account in this study. HMP was just supposed to act as a dispersant for the clay particles and to help to densify each sample. Thus, for a combination of a soil with HMP and a given bio-based addition, it was considered that the change observed in the capillary absorption properties could be attributed to the effect of the bio stabiliser. The wcap and Acap/n values obtained for the soils without any addition are also plotted in this Figure.

First, it is easily noticeable that each addition seems to have the same effect on the capillary absorption behaviours of the three soils. Apart from a few exceptions, for each of the studied combinations, the Acap/n and wcap values are the highest for soil S and the lowest for soil K. The previous observations and conclusions are thus borne out. It also appears that the effect of the bio-based additions that are slowing the water capillary absorption are more noticeable on soil S. For these additions (Xan, VV, LO, CG, OSE, Tan) an average of a 61% reduction of the Acap/n value is visible for soil S, when the average reduction is about 58% for soil K and 52% for soil R. Since the bio-based additions are probably interacting first and foremost with the clayey parts of the soils, their influence should be more visible on the finest soils that present the highest clay content and specific areas.

"Soils+HLime", "Soils with no additions" and "Soil R+Cas" present higher Acap/n values higher than the reference soil (Soil+HMP). It can be noted that these formulations are presenting the lowest dry densities, potentially showing the porosity effect on capillary absorption and the Acap/n value. Thus, other tests will be conducted in order to better understand the influence of the dry density of a given material on its capillary absorption behaviour in further studies.

For some additions that are thickening the material (Linseed Oil, Caseine, Xanthan gum, Alginate, Cellulosic glue), a variation in the final dry density cannot be avoided. The analysis of these Acap/n values is thus relevant to highlight the effect of the selected bio-based additions on the capillary absorption behaviour of earthen materials. The wcap values are also giving some hints on those capillary absorption behaviours. For all of the tested samples, the wcap values were obtained after 24 hours of the experiment, except for the addition of xanthan gum: in these cases, the wcap was never reached (no mass stabilisation) and the given values were calculated after 4 months of the experiment. absorption for all of the samples (Acap/n values are divided by a factor ranging from 3 to 5). However, and despite a reduction of the wcap values, it also appeared that as soon as the absorbed water had crossed the varnish layer, the sample started to swell and crack. The vegetal varnish could thus increase the water durability of earthen materials until a certain limit.

For the addition of linseed oil (LO 1% or 2% in the mass of the material, or

LOSurf on the surface of the samples), all of the obtained Acap/n values are at least halved (Fig. 4). For all the tested proportions and cases, this observation can be made. This strong reduction of the Acap/n values is probably linked to the highly hydrophobic character of dried linseed oil that repels water from the material. The wcap values decrease by around 30% for all the tested proportions and cases.

However, throughout the duration of the tests, the samples that stabilized with linseed oil in the mass (1% or 2%) do not display any signs of swelling and maintain a strong apparent cohesion, visible after 72 hours of testing. These observations cannot be made with the addition of linseed oil on the surface of the materials: after 24 hours of testing, a strong swelling of the samples is visible and their cohesions are highly jeopardized, such as observed for the reference samples (Soils+HMP). For the addition of linseed oil in the mass, it resulted as if the linseed oil had covered the surface of the pores of the material and thus could protect the clayey binder of the material from interlayer water infiltration. That could also explain the low wcap values and the lack of swelling of the materials stabilized with linseed oil in the mass.

Then, the addition of different contents of Xanthan gum was also tested and this bio-based addition seemed to be the strongest water capillary absorption reducer for earthen materials. In all of the different cases, the Acap/n values were divided by a factor ranging from 5 to 10. The absorption kinetic in the first phase was strongly reduced in this way. In addition, the water capillary absorption was so much reduced, that the transition of this phenomenon in phase 2 was never reached, even after 4 months of the experiment. Because of phase 2 never having being reached, the wcap value cannot be precisely determined. The calculated "wcap" values after 4 months were still lower than the references values.

Furthermore, after 4 months of testing, the samples did not display any signs of swelling and maintained a strong apparent cohesion. It is very likely that the Xanthan gum was forming microscopic bonds between the clay layers [START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF] in the material, creating another network of links between the particles of the soil.

However, the risk of fungal proliferation in this kind of material exposed to constant moisture should be taken into consideration [START_REF] Simons | Susceptibility of earth-based construction materials to fungal proliferation laboratory and in situ assessment[END_REF].

Finally, the addition of Hydrated Lime in the materials was tested and it appeared that this addition did not reduce the Acap/n values. However, all of the tested samples of earthen materials combined with hydrated lime presented wcap values divided by 1.5. In addition, during all the experiments, the samples did not display any signs of swelling and kept their initial shapes. As the effect of lime on the soil's microstructure depends on their different natures [START_REF] Ouedraogo | Is stabilization of earth bricks using low cement or lime contents relevant?[END_REF], the carbonation of lime created a rigid skeleton in the material [START_REF] Reddy | Stabilised soil blocks for structural masonry in earth construction[END_REF]. Hence, the protected clays could avoid to adsorb water on their surface and that could prevent the earthen material from swelling.

Insight provided by water droplet penetration: highlighting hydrophobic properties

For some of the tested bio-based additions (Xan, LO, LOSurf and VV), a strong reduction of the water capillary absorption has been observed and those additions seem to be promising in order to protect earthen materials from the effects of liquid water. However, the protection mechanisms at the microscale seem to be quite different depending on the type of bio-based addition.

First, there is likely to be a difference between the coating additions (VV, LOSurf) and the mass treatments (LO in the mass, Xan) that would seem to imply distinct absorption behaviours. For the mass treatments, the Acap/n values are strongly reduced (divided by a factor ranging from 2 to 10) and the wcap values are slightly reduced for the 1% linseed oil addition, further reduced for the 2% linseed oil additions and strongly reduced for all of the xanthan gum additions.

Furthermore, all of these mass treatments imply that the raw earth samples do not display any signs of swelling and maintain a strong apparent cohesion. In the case of the coating additions, if the Acap/n values are strongly reduced (divided by a factor ranging from 2 to 5) and the wcap values are slightly reduced, but after a certain time of water capillary absorption, the protected earth samples start to swell and crack. There is thus an apparent difference between coating additions and mass treatments. The mass treatment appears to provide a global protection that prevents interlayer adsorption of water between the clay particles of the soil while the coating only seems to create a protective layer on the surface of the sample that stays effective until a certain point. Consequently, in the case of a coating, as soon as the water has crossed this protective layer, the capillary absorption behaviour becomes similar to the behaviour of an unprotected earth sample.

Then, by focusing on the two most effective mass treatments (LO in the mass and Xan), and more precisely on their interaction with liquid water, it seems that there are strong differences in their implied mechanisms that protect the microstructure of earthen materials.

Contact angle measurements were conducted for the soil S combined with the above-mentioned additions and on soil S without additions. Other tests were also conducted on this soil combined with other additions (Tan, Cas or Alg). For all of the studied combinations, HMP was added in the mix, except for the additions of tannins (Tan 0.4%) and for the reference (Soil S without addition). In Fig. 5, pictures of the contact angle of a 15 µL sessile droplet of water on the surface of the different tested samples after 0.01 seconds of contact are shown. In Fig. 6 the evolution of the mean contact angles versus the time are plotted. For the addition of linseed oil in the mass, the contact angle of the sessile droplet of water is much larger than for the other studied samples and remains at the surface for a much longer period of time. In this case, the water droplet is not absorbed before 30 seconds, whereas it is absorbed within a second for an unprotected sample.

Considering these observations, it can be concluded that the linseed oil mass However, for the different additions of xanthan gum (0.5% in the mass of the dry soil or 1%) there is no incidence on the measured contact angles, the water being absorbed within a second in the porosity of the samples in the same way as in the unstabilized samples. In this case, the mechanism that is limiting the capillary absorption is different. Some previous studies have observed several samples of different soils stabilized with xanthan gum by means of a scanning electron microscope (SEM) [START_REF] Chang | Effects of Xanthan gum biopolymer on soil strengthening[END_REF][START_REF] Chen | The drying effect on xanthan gum biopolymer treated sandy soil shear strength[END_REF], and have highlighted the fact that the xanthan gum is reinforcing the soil microstructure by creating a Xanthan gum-fine soil network via electrostatic and hydrogen bonding. Xanthan gum is not hydrophobic, but it can be bonded with the clayey parts of the studied soil and thus create another bonding network between the particles of the soil. This allows the sessile droplet of water to be absorbed in the material within a second (Fig. 5 and Fig. 6).

However, this contact angle measurement is mostly evidencing the hydrophobic properties of the additions, even if the overall macroscopic behaviour of the sample remains hydrophilic (contact angle lower than 90°) due to the porous nature of the material. Also, it is not directly correlated to the capillary absorption, because the absorption of the sessile droplet is also affected by a gravity effect and surface roughness. Finally, these two efficient mass treatments seem to strongly improve the durability of earthen materials regarding the water capillary absorption by significantly modifying their microstructure. That being said, it would be unfortunate to redesign the raw earth material regarding its water durability to such a point that it could affect the transfer of water vapour in its pores, which is one of its most interesting properties. Thus, it is relevant to focus on the water vapour permeability of a given soil combined with linseed oil and xanthan gum in its mass and to compare the obtained results to the water vapour permeability properties of this material without addition and combined with HMP. These results could give some indications about the ability of these stabilized materials to transmit water vapour, to regulate the indoor humidity of buildings and to improve ambient comfort conditions.

Water vapour permeability of selected stabilized materials

This section details the results of the water vapour permeability and water vapour resistance factor studies performed on four formulations: Soil S without addition, Soil S + HMP, Soil S + HMP + LO(2%) and Soil S + HMP + Xan(0.5%).

Fig. 7 illustrates an example of mass uptake Δm along time. This kinetic is representative of the results obtained on all specimens. The steady state was reached after 8 days and the slope was calculated from linear regression between day 8 and day 21. The correlation coefficient is very high (> 0.999). Table 2 shows the average water vapour permeability calculated from the 5 samples of each formulation. Table 2 Water vapour permeability mean results for four formulations of soil S

The measured water vapour permeability and water vapour resistance factor are of the same order of magnitude for all of the tested formulations, around 9.5.10 -12 kg/(m.s.Pa) and 21 respectively. Thus, these earthen samples present good moisture transfer capacities for structural materials [START_REF] Collet | CARACTERISATION HYDRIQUE ET THERMIQUE DE MATERIAUX DE GENIE CIVIL A FAIBLES IMPACTS ENVIRONNEMENTAUX[END_REF][START_REF]NF EN ISO 10456 -Building materials and products -Hygrothermal properties -Tabulated design values and procedures for determining declared and design thermal values[END_REF]. Furthermore, for a given formulation, the density influences the water vapour resistance factor as it affects the porosity. So, the variation of the water vapour resistance factor is plotted versus density in Fig. 8 for all samples of all the formulations. As expected, the water vapour resistance factor increases with density. Results show that Soil S, Soil S + HMP and Soil S + HMP + Xan(0.5%) presents similar relationships between water vapour diffusion resistance and dry density with measurements almost grouped on a same line. They are slightly greater for the Soil S + HMP + LO(2%) formulation along the investigated density range. The addition of HMP does not influence the transmission of water vapour in the soil S.

The addition of linseed oil in the mass of the material produces an increase in the water vapour resistance factor along the density range, by 25 % in average. In fact, the linseed oil coats and partially fills the pores. In addition, it is highly hydrophobic so it reduces the transfer of water vapour.

The addition of xanthan gum does not affect the water vapour resistance for a given density of soil. However, as it induces lower density, it leads to lower resistance.

Effect of water uptake on the mechanical behaviour of earth -A practical discussion

It is known that the mechanical properties of the earthen materials are decreasing with their moisture content [10, [START_REF] Bui | Effect of moisture content on the mechanical characteristics of rammed earth[END_REF][START_REF] Champiré | Impact of relative humidity on the mechanical behavior of compacted earth as a building material[END_REF] and several studies have already highlighted the fact that bio-based additions could increase the mechanical properties of earthen materials [START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Anger | PaTerre+: Interactions argiles/biopolymères , Patrimoine Architectural en terre et stabilisants naturels d'origine animale et végétale[END_REF][40][START_REF] Galán-Marín | Clay-based composite stabilized with natural polymer and fibre[END_REF][START_REF] Anger | Approche granulaire et colloïdale du matériau terre pour la construction[END_REF][43][START_REF] Hafshejani | A novel bio-based deflocculant for bentonite drilling mud[END_REF][START_REF] Banakinao | Dataset of the use of tannin of néré (parkia-biglobosa) as a solution for the sustainability of the soil constructions in West Africa[END_REF][START_REF] Menasria | USING ALGINATE BIOPOLYMER TO ENHANCE THE MECHANICAL PROPERTIES OF EARTH-BASED MATERIALS[END_REF][START_REF] Perrot | Strategies for optimizing the mechanical strengths of raw earth-based mortars[END_REF][START_REF] Ouedraogo | Stabilisation de matériaux de construction durables et écologiques à base de terre crue par des liants organiques et/ou minéraux à faibles impacts environnementaux[END_REF][START_REF] Chang | Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries[END_REF]. In this study, it has been highlighted that two bio-based mass treatments (Linseed Oil in the mass and Xanthan Gum) of earthen materials could limit their capillary absorption of water, increasing their durability regarding water without affecting the transfer of water vapour in their pores. Four formulations of the soil S were thus studied: Soil S+HMP, Soil S+HMP+LO(1%), Soil S+HMP+LO(2%), Soil S+HMP+Xan(O.5%). The evolution of the compressive strength of these formulations versus their moisture content is plotted in Fig. 9. The evolution of the moisture content of these formulations versus the time they have spent in the wet chamber is plotted in Fig. 10. Once again, these results highlight the two different protecting mechanisms of linseed oil and xanthan gum. In Fig. 9 it is clear that the xanthan gum is limiting the decrease of the compressive strength of the earth samples while their moisture content is increasing. However, in Fig. 10 it is also noticeable that the moisture content of the samples stabilized with xanthan gum is increasing at the same rate as the moisture content of the unstabilized samples, after having being put in wet chamber. These observations could confirm that xanthan gum is probably creating a bonding network of gel gluing the soil particles, as has been highlighted in previous studies [START_REF] Chang | Effects of Xanthan gum biopolymer on soil strengthening[END_REF][START_REF] Chen | The drying effect on xanthan gum biopolymer treated sandy soil shear strength[END_REF], increasing the sample's in-situ mechanical properties.

The hydrophilic properties of xanthan gum are well known and this gum is reacting with water to create a network of gel that strengthens the soil when drying [START_REF] Chang | Effects of Xanthan gum biopolymer on soil strengthening[END_REF][START_REF] Chen | The drying effect on xanthan gum biopolymer treated sandy soil shear strength[END_REF]. It is also justifiable to question whether xanthan gum is still hydrophilic after bonding with the soil's particles or not. As previously stated, in Fig. 9 it also seems that the moisture content of the samples stabilized with xanthan gum is increasing at the same rate as the moisture content of unstabilized samples, after having being put in wet chamber. Therefore, it can be assumed that this stabilisation method is not absorbing an additional amount of water after drying. This potential change of hydrophilic nature still has to be thoroughly studied. On a wider scale it still seems that the addition of xanthan gum in an earthen sample could help to increase its water durability, leaving the clay particles available for water adsorption without in any way affecting its affinity with water vapour and its hygric properties. At first sight, by focusing again on Fig. 9, the addition of linseed oil at different given contents does not seem to limit the decrease of the mechanical properties of the tested earth samples while their moisture is increasing. However, in Fig. 10 it is clearly noticeable that these additions allow the earth samples to decrease their adsorption of ambient humidity and thus to present a lower moisture content increase. These observations are also confirming the foregoing conclusions about the effect of linseed oil on earthen materials. If linseed oil is not creating a new network of bonds in the material that provides better mechanical strengths, it is coating all of the pores of the material, limiting the interactions of the dry earthen material clay particles with the ambient humidity by its highly hydrophobic properties. Thus, the increase of the moisture content of the material is slowed.

Even if the interactions between the material and the ambient humidity are limited, the material stabilised with linseed oil is still able to store moisture and is open to water vapour transfer. However, it would be interesting to determine its moisture buffering capacity.

Conclusion

The aim of this study was to highlight that some selected bio-based additions are able to enhance the water durability of earthen materials. For mass treatments such as linseed oil or xanthan gum and coatings with linseed oil or vegetal-based varnish, the water capillary absorption of earthen structural materials is strongly reduced. By limiting this capillary absorption, the mechanical properties of the tested samples can be maintained when exposed to in-situ conditions. Hence, their water durability was enhanced. Several patterns of capillary absorption were observed for the different studied soils and additions, enabling a better characterization and understanding of the studied soils as well as an identification of distinct protection mechanisms regarding the water durability of the different bio-based additions. Furthermore, for the most efficient additions, the water vapour permeability of stabilized earthen samples was monitored. The results

show that these additions can enhance the water durability of earthen materials without affecting the transfer of water vapour in their pores and thus their hygric properties. These results are promising and xanthan gum or linseed oil, combined with efficient processing methods, can be good alternatives to hydraulic binders in

Fig. 1

 1 Fig. 1 Particle size distribution of soils K, R and S

  strengths while their water content increases. The tested dosages were also determined through preliminary tests in order to choose the range of biopolymer content that provides significant improvement.Polysaccharides: Alginates (Alg), Xanthan Gum (Xan), Cellulosic Glue (CG)

20 *

 20 40*40mm3 cast samples of earth-based material, mixed with the previously listed different bio-based additions were placed on: a filter paper, a nylon grid, on a water-soaked sponge and in a flush water bath. All of the samples were cast at the same consistency showing the same plastic behaviour. To obtain such targeted consistency, the water content of each mix (Earth+addition) was adapted, depending on the impact of the bio-based addition on the earth's rheology. The sample weight variation was recorded during the experiment. The aim here was to highlight the effect of bio-based additions on the liquid water capillary absorption of earth-based samples using a simple laboratory test. The tests were conducted on dried (after one week in a 50°C heat chamber) samples. Hence, the initial water content of the tested samples was zero asHall [60] suggests. The test started as soon as the dry sample was placed on the filter paper/nylon grid/sponge device.

Fig. 2

 2 Fig.2 Different patterns of capillary absorption

Fig. 3

 3 Fig. 3 Capillary absorption behaviours of soils K, R and S The soil K displays two slightly different absorption phases (Pattern n°1/Pattern n°2), and a rather slow capillary absorption (low Acap) with a low wcap. In this soil, the low content of clay particles (approximately 10% of particles finer than 2 m) and their limited specific area (pure kaolinite) can explain the low values of Acap and wcap.The soil R has a rather low clay content and its clayey part is composed of kaolinite, chlorite and illite that present higher specific areas than kaolinite. These types of clay are relatively stable clays. The absorption cure of the soil R highlights two clearly distinct capillary absorption phases (Pattern n°2), the shape

Figure, the wcap

  Figure, the wcap and Acap values of each combination are compared to reference

Fig. 4

 4 Fig.[START_REF] Moevus | Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase[END_REF] ACap/n values and wCap values for several combinations of soils K, R and S For the addition of OSE and Chestnut tannins, the water capillary absorption seems to be slowed (Acap/n value is decreased by 20% in comparison with the reference value Acap/n obtained with only HMP), but their effect on the water capillary absorption behaviours of earthen materials appears to be quite low. The same observations can be made for the additions of casein, alginates and cellulosic glue.Concerning the addition of a Vegetal Varnish on the surface of the earthen materials, at first sight, it caused a strong reduction of the water capillary

Fig. 5 Fig. 6

 56 Fig. 5 Pictures of contact angles at t=0.01s for a 15 µL sessile droplet of water for different formulations of Soil S

Fig. 7

 7 Fig. 7 Mass uptake versus time during water vapour permeability testexample of soil S

Fig. 8

 8 Fig. 8 Evolution of water vapour diffusion factors resistance factors (µ) versus the dry densities for four formulations of soil S Finally, the addition of xanthan gum in the mass of earthen materials protects earth samples from the absorption of liquid without affecting their water vapour permeability while the use of linseed oil reduces it.In order to complete this study on moisture transfer, it would be interesting to characterise the ability to adsorb and store moisture from the atmosphere, with the measurement of a Moisture Buffer Value (MBV)[77]. This ability widely impacts indoor comfort.

Fig. 9 Fig. 10

 910 Fig. 9 Evolution of compressive strength versus the moisture content for four mixes of soil S

Table 1

 1 Capillary absorption results for soils K, R and S.

		K	R	S
	Acap (g.m -2 .s 0.5 )	26	167	231
	Wcap (kg/m 3 )	285	513.7	665
	Apparent porosity n	0.26	0.32	0.36
	Acap/n (g.m -2 .s 0.5 )	100	514	647

months values

Acknowledgements

The authors would like to thank Patrice Estellé, Associate professor at LGCGM, for his support and advice for the contact angles measurements.

Availability of data and material

The data that support the findings of this study are available from the corresponding author, S. G., upon reasonable request.

Declarations Funding

The authors would like to thank ECOMATERRE who funded a part of this study, and for its work in structuring the regional research program on earth construction.

Conflicts of interest

The authors declare no conflict of interest. order to stabilize earth-based building materials and produce raw earth blocks that achieve compliance with modern standards regarding strength and durability.


Julio Perez-Carbajo

José B Parra

Conchi O Ania

Patrick J Merkling

Sofia Calero

Molecular Sieves for the Separation of Hydrogen Isotopes

Stable molecular hydrogen isotopes, D 2 and T 2 , are both scarce and essential in several energy, industrial and large-scale, fundamental research applications. Due to the chemical similarity of these isotopes, their extraction and purification from hydrogen has relied for decades on expensive and energy-demanding processes. However, factoring in the phenomenon of quantum sieving could provide a new route for these separations. In this work, we have explored how to separate hydrogen isotopes by adsorption taking these quantum effects into account. To this end, we have conducted adsorption measurements to test our deuterium model, and performed a widespread computational screening over 210 pure-silica zeolites for D 2 :H 2 and T 2 :H 2 separations. Based on low-coverage adsorption properties, a reduced set of zeolites have been singled out and their performance in terms of adsorption capacity, selectivity and dynamic behavior have been assessed. Overall, the BCT-type zeolite clearly stands out for highly selective separations of both D 2 and T 2 over H 2 , achieving the highest reported selectivities at cryogenic temperatures. We also identified other interesting zeolites for the separation of hydrogen isotopes that offer an alternative way to tackle similar isotopic separations by an aimed selection or design of porous materials.

Introduction

Separation of hydrogen isotopes is one of the most challenging current research areas, especially from an industrial point of view. The natural abundances of non-synthetic hydrogen isotopes are about 156 ppm for deuterium (D) and only traces for tritium (T). Despite their scarcity, they both turn out to be crucial in several technological and industrial fields. Deuterium is employed in isotopic tracing for medical treatment and detection, nuclear magnetic resonance and neutron scattering techniques, and in development of deuterated drugs. Tritium has uses in the armament industry and in analytical chemistry. They are both used as raw materials in tokamak-type nuclear fusion reactors, where deuterium is also a key component for moderating neutrons. In fact, the yield of hydrogen isotopes for nuclear feedstock is under 10%, so recovering them from waste is crucial to increase the efficiency and reduce nuclear residues. Additionally, even if hydrogen is the desired product, extraction of deuterium from hydrogen bulk might be economically attractive, while for the case of tritium its environmental impact may make this operation a mandatory requirement.

Chemical similarities between H 2 , D 2 , and T 2 have been traditionally considered a major obstacle for the separation by molecular sieving methods. Other methods have therefore been used to carry out that separation: Cryogenic distillation, proton exchange reactions, thermal diffusion, centrifugation, electrolysis, or chromatography, among others. These techniques are quite energy consuming (the first three enumerated), or have a low yield (the last three mentioned). Thus, a cost-effective method with a high separation throughput would be desirable. In the mid-nineties, Beenakker et al. published a study on quantum sieving, an effect arising when the difference between the diameters of the pore and of the molecule approach the de Broglie wavelength. Under these conditions, similar molecules with different masses present different adsorption behaviors that can lead to heavier molecules in confined channels experiencing less repulsive interaction energies than lighter ones; this also affects diffusion properties in favor of the heavier isotope. That finding helped understanding the separation of isotopes in nanoporous materials. From then on, several works have explored hydrogen isotope separation in carbon nanotubes, zeolites, and metal-organic frameworks (MOFs). Although single-walled carbon nanotubes and zeolites were the first materials to be investigated, main research efforts in the last years have been focused in MOFs. Admitting that to exploit flexibility and breathing effects in MOFs is achieving promising results, zeolites, advantageously with respect to MOFs, are known for their thermal stability, economical production and scalability. In this sense, to the best of our knowledge, most studies on zeolites have only examined a few specific structures, so a widespread screening is lacking. This work performs a computational investigation on 210 zeolites to find the best candidates for the separation of D 2 or T 2 from H 2 at temperatures running from cryogenic up to 200 K, and in the 10 -1 -10 5 kPa pressure range. To that end, we have validated and used a Lennard-Jones (L-J) potential with Feynman-Hibbs quantum corrections for H 2 , and then derived the corresponding L-J parameters for the isotopes. D 2 interactions with pure silica zeolites have been validated with experimental adsorption isotherms at cryogenic temperatures. This study comprises an exhaustive comparison of hydrogen isotopes adsorption characteristics at low loadings, and then both dynamic and static analysis for selected zeolites to recommend the structures and working conditions leading to a highly-selective hydrogen isotope separation.

Methodology

Experimental procedures

Experimental gas adsorption isotherms on pure silica zeolites (MFI and ITQ-29) at cryogenic temperatures were performed in a volumetric analyser provided with a turbomolecular vacuum pump and three pressure transducers (0.13, 1.33 and 133 kPa, uncertainty within 0.15% of each reading). The volumetric analyser was coupled to a helium cryocooler (Gifford-McMahon) that allows a fine temperature control between 25-325 K with a stability of ±0.1 K. Isotherms were recorded in the pressure range between 10 -2 and 120 kPa. Before the analysis, zeolites were outgassed under dynamic vacuum at 623 K (1K/min) overnight. All gases were supplied by Air Products at an ultrahigh purity (i.e., 99.995%). MFI and ITQ-29 (LTA topology) pure silica zeolites were supplied by the Institute of Chemical Technology (ITQ), being both structures nearly completely pure SiO 2 crystals.

Computational methods, models, and force field

Energies between adsorbates themselves and with zeolites are dominated by electrostatic and van der Waals interactions. Electrostatic interactions are modeled by Coulombic potentials, using the Ewald summation to compute the long-range terms. On the other hand, Lennard-Jones (L-J) potentials are used to describe the van der Waals interactions. Quantum corrections have been added to Lennard-Jones potentials via an effective potential based on the Feynman-Hibbs variational approach to reproduce the quantum behavior of hydrogen isotopes at cryogenic temperatures. This approach has been repeatedly reported as an effective way to study the adsorption of quantum H 2 , D 2 , and T 2 in zeolites and other nanoporous materials. Lennard-Jones potential with Feynman-Hibbs corrections is cut and shifted to zero at a cutoff radius of 12 Å.

Zeolite lattices are considered rigid and only formed by interconnected SiO 4 tetrahedra. The coordinates of the framework atoms are taken from IZA database except for ISW, ITE, ITW, LTA, and MWW whose pure silica atom coordinates have been provided by ITQ: ITQ-7, ITQ-3, ITQ-12, ITQ-29, and ITQ-1, respectively. Static point charges for all of them (q Si = +0.786 e - and q O = -0.393 e -) stem from Garcia-Sanchez et al.. Lennard-Jones interactions between framework atoms need not be considered in a rigid framework. where $U_{\rm{L-J}}$ is the classical Lennard-Jones potential, prime and double prime symbols refer to first-and second-derivative, 𝑟 𝑖𝑗 is the distance between two interacting particles, 𝜇 𝑖𝑗 is the reduced mass of 𝑖 and 𝑗 particles given by 𝜇 𝑖𝑗 -1 = 𝑀 𝑖 -1 + 𝑀 𝑗 -1 (being 𝑀 the molecular mass), ℏ is the reduced Planck constant, 𝑘 𝐵 is the Boltzmann constant, and 𝑇 is the temperature.

Widom test-particle method, through Monte Carlo simulations using the so called Canonical ensemble, was used to determine Henry coefficients (K H ) of adsorbates from 25 K to 200 K. K H is related to the excess free energy of an adsorbed molecule by the following expression:

where 𝑇 is the temperature, 𝑅 the gas constant, 𝜌 𝑓 the density of the framework, ⟨𝑊⟩ the Rosenbluth factor of the single chain molecule and ⟨𝑊 𝑖𝑔 ⟩ the Rosenbluth factor of the molecule in the ideal gas. Given that K H provides information about adsorption at infinite dilution, selectivity at zero loading, S 0 ij = K i H / K j H , establishes a measure of preferential adsorption of adsorbate 𝑖 over 𝑗.

Adsorption isotherms were computed by Monte Carlo simulations in the Grand Canonical ensemble (𝜇𝑉𝑇), in which the chemical potential 𝜇 is directly related to fugacity and thereby to the pressure through the fugacity coefficient. Constant values were set for temperature and pressure in simulations, spanning the ranges T K and, at least, P [10 -1 -10 5 ] kPa. Both pure component and 1:1 mixture simulations were carried out for selected zeolites. Adsorption selectivity (𝑆 𝑖𝑗 ) highlights the preferential adsorption of one component (𝑖) over another (𝑗) from their molar fractions in the adsorbed phase (𝜃) and in the bulk phase (𝑥), according to:

𝑆 𝑖𝑗 = 𝜃 𝑖 ⋅ 𝑥 𝑗 𝜃 𝑗 ⋅ 𝑥 𝑖
Molecular dynamics (MD) simulations were performed to obtain self-diffusion coefficients (D) of the adsorbates. They were determined from the slope of the mean-square displacements once the adsorbed species reached the diffusive regime inside the structure. MD simulations were performed in the Canonical ensemble (NVT), integrating Newton's laws of motion using a velocity-Verlet algorithm with an integration time step of 0.5 fs and simulated times upwards of 325 ns. A Nosé-Hoover chain thermostat was used to ensure the average temperature was constant. The number of adsorbates was set to 1.

All the aforementioned techniques were simulated using RASPA molecular simulation software.

Results and discussion

Deuterium/Hydrogen separation

To determine the operational properties and performance of 1:1 hydrogen isotope separation using zeolites, we have carried out a thorough study on both equilibrium and time-dependent conditions. To that end, molecular simulations have proven to provide deep physical insights from which macroscopic behavior of adsorbates is inferred for these confined systems.

Additionally, simulation methods allow to span the operation conditions further than experiments, which are restricted by economical considerations and number of trials. However, accurate models and force fields are completely necessary to obtain useful simulation results to target further experimental research. Therefore, we first validated the hydrogen model taken from literature and also the derived model for deuterium used in this work by comparing the computed and experimental adsorption isotherms of two well-known pure silica zeolites (MFI and LTA) at 77.3 K and 90.2 K. Both models reproduce precisely the experimental adsorption isotherms in LTA throughout the whole pressure range, as shown in Figure 1. Computed adsorption isotherms also reproduce the experimental data in MFI, although the loading of the two molecules is slightly underestimated at 77.3 K and 5-10 kPa. Nevertheless, deviations affect both hydrogen and deuterium in much the same way, meaning selectivity determinations are reliable, and differences are not substantial either.

We have computed and analyzed Henry coefficients for H 2 and D 2 as a function of temperature in the range of 25-200 K for 210 zeolites. As a general trend, K H D2 and K H H2 are found to decrease with temperature and the heavier D 2 is more strongly adsorbed than H 2 , therefore S 0 D2H2 >1. This trend is declining as temperature increases (Figure 2). Figure 2 shows the five most selective zeolites at each temperature; as seen, zeolite BCT stands out by far as the best candidate (i.e., highest S 0 D2H2 ) to perform a separation at low loading over the whole temperature range. It is especially selective up to 50-60 K. The second most selective zeolite depended on the temperature: AVL up to 50 K and MVY from 60-200K. Other zeolite structures in the top five at some temperatures are: AHT, ANA, BPH, EUO, IWW, LTJ, MSE, NSI, OSI, SAO, SBS, SBT, SSF, and SZR. All the computed S 0 D2H2 values are summarized in Table S1 in the Supporting Information.

Figure 2: Selectivity at zero loading of deuterium over hydrogen as a function of temperature. Detailed graphics show close-up for the 25-50 K (bottom-left) and 60-200 K (bottom-right) temperature ranges, excluding BCT zeolite to ease the view.

Given the superior selectivity values of zeolites BCT, AVL, and MVY for the separation of deuterium from hydrogen, these zeolites were investigated in more detail. Zeolite BCT has a tetragonal structure with a one-dimensional pore system, formed by parallel 8-membered-ring channels along the 𝑧-axial direction with transversal side-pockets on alternating sides. The orthorhombic form of zeolite MVY also has a one-dimensional, sinusoidal channel system, comprised of 10-membered-ring cavities along the 𝑥-axial direction. On the other hand, zeolite AVL is a two-dimensional structure, with a trigonal crystalline system consisting of cylindricallike cages connected by six 8-membered-ring windows (three on each end of the cylinder) and lentil-like cages with three 8-membered-ring windows (Figure 3).

Figure 3: Energy surface areas and lattice atoms (yellow for Si and red for O) for BCT (top), AVL (bottom-left), and MVY (bottom-right) zeolites.

Individual channels or pore systems are highlighted in blue, green, and orange, respectively, to ease the view. In AVL, bright-green is for lentil-like cages and dark-green for cylindrical cages.

Diffusion coefficients for single molecules have been evaluated with a twofold goal: determine the capacity of adsorbates to access adsorption centers inside zeolites and search for conditions under which a noticeable difference between D D2 and D H2 is reached, which would open up the possibility of performing a quantum kinetic sieving of the isotopes. Results are presented in Figure 4. Diffusion was computed in the temperature ranges in which zero-loading selectivities stood out: The whole range for BCT, 25-50 K for AVL, and 60-200 K for MVY. However, it is worth noting that diffusion coefficients for zeolite MVY at 80, 70, and 60 K could not be accurately determined after simulating 500 ns molecular dynamics, and were of no practical use. For the same reason, diffusion coefficients in BCT are not considered below 40 K. Regarding kinetic sieving, diffusion coefficients for D 2 and H 2 are quite similar in AVL and MVY, and also for most of the analyzed temperatures in BCT. Although diffusion coefficients are quite low for zeolite BCT, a priori a kinetic-based separation at 40-50 K would be possible.

Figure 4: Self-diffusion coefficients of D 2 (diamonds) and H 2 (circles) in BCT (blue tones), AVL (green tones), and MVY (orangy-red tones) at the temperatures at which they have high selectivity (see text) at zero loading.

After considering diffusion results, acceptable operation ranges to perform the D 2 /H 2 separation were restricted to 25-50 K in AVL, 40-200 K in BCT, and 90-200 K in MVY. Pure compound as well as 1:1 mixture adsorption isotherms were simulated at the temperatures of choice spanning a pressure range of at least [10 -1 -10 4 ] kPa. For all three structures, in pure component adsorption isotherms (Figure S1), deuterium starts to be adsorbed at lower pressure than hydrogen, although this difference diminishes when temperature increases. Additionally, the location of the adsorption sites within the pores of the zeolites is shown for pure component adsorptions in Figures S2-S4 for BCT, AVL and MVY, respectively. Both deuterium and hydrogen adsorb at the same sites, and their loading dependency is also similar due to the competition for the same sites. Paradoxically, adsorbates' positions at high loading tend to be better defined as the adsorption sites are narrowed down by the steric interactions between adsorbates. The pure-component data suggest the separation capability of the zeolites but, rather than predicting mixture separation from pure compound isotherms as it is commonly reported in the literature, we simulated 1:1 D 2 /H 2 mixture adsorption isotherms to calculate S D2H2 selectivities.

Figures 5,S5, and S6 contain the adsorption isotherms of the components of the mixture and also the temperature-and pressure-dependent adsorption selectivities in zeolites BCT, AVL, and MVY, respectively. In view of the adsorption selectivities exhibited by BCT (Figure 5, bottom), it must be noted that they are nearly parallel (in the log-log scale) over the whole pressure range at different temperatures. Additionally, the pressure dependence is rather small, selectivities remain within 0.8-1.3 times the selectivity at 100 kPa, so that an increase in pressure to ensure a high loading is not detrimental to separation. Selectivities between 80 and 128 depending on pressure are reached at 40 K, which, to the best of our knowledge, are the highest reported in the literature. It is especially advisable to maintain the pressure at 100 kPa or more to obtain deuterium loadings of 3 mol/kg or more. Under these cryogenic conditions, temperature control is important: a 20 K increase leads to selectivity decreases by one order of magnitude. Still, BCT achieves relevant separation ratios of ~ 8-9 at 60 K. Further rises in temperature lead to more moderate decreases in selectivity, but then the absolute values are not very high. Zeolite AVL also achieves high D 2 /H 2 adsorption selectivities at low temperatures (Figure S5). As in zeolite BCT, selectivity decreases with temperature but, in this case, its increase with pressure is much larger, reaching a remarkable value of S D2H2 =22 at 25 K and 10 5 kPa while adsorbing 14 mol/kg of molecular deuterium vs. barely 0.6 mol/kg of molecular hydrogen. It is also worth noting that S D2H2 =15 at 25 K and atmospheric pressure, with 13 mol/kg deuterium adsorbed. Regarding zeolite MVY, selectivities remain poor, S D2H2 <2, remaining even under 1.5 for most of the operation conditions considered acceptable (Figure S6)

Tritium/Hydrogen separation

Following the same methodology used for D 2 and H 2 , T 2 /H 2 separation at low coverage has been screened. As seen in Figure 6, BCT zeolite clearly stands out from the rest of zeolites, exhibiting the largest selectivity at all the temperatures simulated. Zeolites EUO, SAO, and OSI are second best at some temperature within the 25-80 K temperature range, for which these adsorption selectivities remain greater or equal to two. Given the prevalence of BCT for the separation of hydrogen isotopes, this zeolite was selected to further simulate the selectivity and adsorption performance for an equimolar T 2 /H 2 mixture. Based on the diffusion coefficients obtained for D 2 and H 2 , the adsorption of the equimolar mixture adsorption was computed at temperatures above 40 K (Figure 7). Although a similar shape of the adsorption isotherms/isobars is obtained compared with the deuterium case, it is worth noting that tritium loading is higher under the same (T,P) conditions while hydrogen is slightly less adsorbed, and therefore adsorption selectivity values rise up to S T2H2 =915 under optimal conditions (T=40 K and P=10 5 kPa).

Conclusions

We have screened various zeolite structures and operating conditions to select the best performing materials for the separation of hydrogen isotopes (D 2 /H 2 and T 2 /H 2 mixtures). To this end, we have proposed a deuterium model developed from an existing hydrogen model that incorporates quantum corrections. The model has been validated vs experimental hydrogen and deuterium adsorption isotherms in two pure silica zeolites (MFI and LTA). At low temperatures, these quantum corrections are responsible for the stronger adsorption of deuterium over hydrogen, as expressed by the adsorption selectivity. The adsorption selectivity at low loadings was found to be a good indicator for the adsorption selectivity at higher loadings, and much more temperature-sensitive than pressure-dependent. We have identified several zeolites to perform hydrogen isotope separation with a high selectivity. Notably, zeolite BCT exhibits the highest adsorption selectivity for D 2 /H 2 separation at low temperature. For instance, at 40 K and high loadings D 2 is ca. 80 to 130 times more adsorbed than H 2 , depending on pressure. This zeolite also presents a high deuterium selectivity (ca. S D2/H2 of 20) at 50 K and atmospheric pressure. We have also explored whether if in addition to the quantum sieving responsible for enhancing the adsorption of the heavier isotope, quantum kinetic sieving effect -a diffusion-driven separation due to differences in the energy barriers-could be used advantageously in this system. Data showed that the quantum kinetic sieving effect was small in the temperatures of interest. Alternatively to BCT, zeolite AVL showed a high loading capacity between 25-45 K, being able to separate deuterium from hydrogen reasonably well. Regarding T 2 /H 2 separation, zeolite BCT has proven to have an extremely high selectivity, favoring the heavier isotope.

In sum, we have demonstrated that hydrogen isotope separation is feasible using zeolites at cryogenic temperatures and, especially, zeolite BCT stands out for this task. Besides, this study provides new perspectives to tackle other isotope mixture separations such as H 2 /HD (hydrogen deuteride) or T 2 /D 2 , by selecting or designing highly selective and efficient porous materials.

  Hydrogen is modeled as a single, uncharged Lennard-Jones center, taken from van den Berg et al., and modified by Deeg et al. to make it a Feynman-Hibbs potential. The Feynman-Hibbs parameters defining the interaction with zeolites have also been developed by Deeg. et al. Likewise, deuterium and tritium inherit the same model characteristics but updating their mass in relation to hydrogen molecule, so self-and cross-interaction Lennard-Jones parameters remain unaltered for them. Molecular weight of hydrogen isotopes are taken from NIST: M H2 = 2.01588 u, M D2 = 4.02820 u, and M T2 = 6.03209 u. These increases in mass directly affect the effective radius of the molecules as a consequence of the Feynman-Hibbs effective potential expression:

Figure 1 :

 1 Figure 1: Experimental (open symbols) and simulated (filled symbols) adsorption isotherms of D 2 (green triangles and diamonds) and H 2 (blue circles and squares) in LTA (top) and MFI (bottom) at 77.3 K and 90.2 K.

Figure 5 :

 5 Figure 5: Adsorption loading (top) of deuterium (black lines grid) and hydrogen (red lines grid) from a 1:1 mixture as a function of pressure and temperature; grid cell colors match loading color-box. Adsorption selectivity (bottom) of deuterium over hydrogen as a function of pressure and temperature (color code assigned univocally for each temperature). Dashed lines and small symbols apply to selectivities at which the associated loading of D 2 is less than 0.1 mol/kg.

Figure 6 :

 6 Figure 6: Selectivity at zero loading of tritium over hydrogen as a function of temperature. Detailed graphics show close-up for the 25-50 K (bottom-left) and 60-200 K (bottom-right) temperature ranges, excluding BCT zeolite to ease the view.

Figure 7 :

 7 Figure 7: Adsorption loading (top) of tritium (black lines grid) and hydrogen (red lines grid) from a 1:1 mixture as a function of pressure and temperature; grid cell colors match loading color-box. Adsorption selectivity (bottom) of deuterium over hydrogen as a function of pressure and temperature (color code assigned univocally for each temperature). Dashed lines and small symbols apply to selectivities at which the associated loading of T 2 is less than 0.1 mol/kg.

This work was supported by the European Research Council through an ERC Starting Grant (ERC2011-StG-279520-RASPA). We thank C3UPO for the HPC support.

Tables with adsorption selectivities for D 2 /H 2 and T 2 /H 2 are provided, as well as pure component adsorption isotherms and preferential sites of adsorption maps for D 2 and H 2 in BCT, AVL and MVY zeolites.

Supporting Information Available

Tables with adsorption selectivities for D2/H2 and T2/H2 are provided, as well as pure component adsorption isotherms and preferential sites of adsorption maps for D2 and H2 in BCT, AVL and MVY zeolites.