Jonathan Page
email: jonathan.page@univ-artois.fr.

Sofiane Amziane

Moussa Gomina

Chafika Djelal

Fabrice Audonnet

fibre coating for fibre-reinforced cementitious composite

Keywords:

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

Introduction

For decades, fibres have been used in cementitious materials to improve the tensile strength and post-cracking behaviour [START_REF] Rashiddadash | Experimental investigation on flexural toughness of hybrid fiber reinforced concrete (HFRC) containing metakaolin and pumice[END_REF][START_REF] Dawood | High strength characteristics of cement mortar reinforced with hybrid fibres[END_REF][START_REF] Page | Design and multi-physical properties of a new hybrid hemp-flax composite material[END_REF]. The most commonly used in cementitious composites are steel fibres, glass fibres and polypropylene fibres. However, all these reinforcement materials have the disadvantage of being derived from non-renewable resources. Economic issues related to the rising costs of fossil resources, their increasing scarcity, and environmental impacts inherent to their production therefore lead to explore other raw materials. This is how plant fibres are viewed as a solution for the future of the construction industry [START_REF] Onuaguluchi | Plant-based natural fibre reinforced cement composites: A review[END_REF][START_REF] Kicińska-Jakubowska | Review of Natural Fibers. Part I-Vegetable Fibers[END_REF][START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF][START_REF] Faruk | Progress Report on Natural Fiber Reinforced Composites[END_REF]. Among the potential plant fibres, flax stands out because of its excellent mechanical properties and low density [START_REF] Baley | Average tensile properties of French elementary flax fibers[END_REF][START_REF] Lefeuvre | Elementary flax fibre tensile properties: Correlation between stress-strain behaviour and fibre composition[END_REF].

Many research projects dealt with the incorporation of bio-based fibres into cementitious materials. They highlighted that in the fresh state, flax fibres affect the rheology of the mixture [START_REF] Chafei | Optimizing the formulation of flax fiberreinforced cement composites[END_REF][START_REF] Tung | Mechanical properties of flax fibre reinforced cement composites[END_REF][START_REF] Bentur | Fibre reinforced cementitious composites[END_REF][START_REF] Coutts | Flax fibres as a reinforcement in cement mortars[END_REF]. These rheological disturbances are mainly attributed to the high demand for water due to the high-water absorption capacity of flax fibres (up to 150 % of dry fibre mass) and their high specific surface area [START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF]. Numerous studies have shown that plant fibres have a considerable influence on the rheological behaviour of mortars and concretes [START_REF] Chafei | Optimizing the formulation of flax fiberreinforced cement composites[END_REF][START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF]. The use of a superplasticizer and a viscosity-modifying admixture significantly reduces these disturbances [START_REF] Hoang | Mechanical and microstructural characterization of flax fibre-reinforced cement composite[END_REF][START_REF] Page | Étude expérimentale des propriétés d'un béton de chanvre incorporant un adjuvant viscosant[END_REF]. In the hardened state, composites reinforced with plant fibres show an improvement of the flexural toughness or strength and increased ductility [START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF][START_REF] Merta | Fracture energy of natural fibre reinforced concrete[END_REF][START_REF] Sedan | Mechanical properties of hemp fibre reinforced cement: Influence of the fibre/matrix interaction[END_REF]. The transition from a brittle matrix to a ductile fibrereinforced composite having controlled post-peak behaviour is noted by several authors [START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF][START_REF] Silva | An experimental investigation of the fatigue behavior of sisal fibers[END_REF][START_REF] Silva | Physical and mechanical properties of durable sisal fiber-cement composites[END_REF].

However, this change in behaviour is not always accompanied by an improvement in flexural strength [START_REF] Kriker | Mechanical properties of date palm fibres and concrete reinforced with date palm fibres in hot-dry climate[END_REF].

Despite the advantageous physical properties obtained for cementitious composites reinforced with plant fibres, many studies highlighted problems at the fibre-matrix interface, sometimes manifested by a gap between these two components [START_REF] Tonoli | Eucalyptus pulp fibres as alternative reinforcement to engineered cement-based composites[END_REF][START_REF] Fonseca | Properties of an Amazonian vegetable fiber as a potential reinforcing material[END_REF][START_REF] Garikapati | Mechanical behavior of flax-lime concrete blocks made of waste flax shives and lime binder reinforced with jute fabric[END_REF]. This gap is often attributed to swelling of the fibres during the concrete mixing process (in the fresh state) and consecutive shrinkage during the curing of the material (in the hardened state). Moreover, these studies have shown that the hydrophilicity of plant fibres is responsible for these volume variations, which could cause this poor fibre/matrix interface [START_REF] Tonoli | Impact of bleaching pine fibre on the fibre/cement interface[END_REF].

Water absorption by plant fibres is therefore an important issue limiting their use in composite materials. Surface treatment may be a solution to limit or eliminate water absorption by these fibres [START_REF] Valadez-Gonzalez | Effect of fiber surface treatment on the fiber-matrix bond strength of natural fiber reinforced composites[END_REF][START_REF] Nozahic | Influence of sunflower aggregates surface treatments on physical properties and adhesion with a mineral binder[END_REF]. Given the problems encountered when using plant particles with a cementitious matrix, many studies have been conducted to modify some of their characteristics. Most of the treatments tested had the objective of limiting the hydrophilic behaviour of the fibres while others aimed to modify their surface composition. Chemical treatments are the most common experienced. They consist of prolonged immersion of the fibres in water-based solution [START_REF] Chafei | Effect of flax fibers treatments on the rheological and the mechanical behavior of a cement composite[END_REF][START_REF] Olorunnisola | Effects of pre-treatment of rattan (Laccosperma secundiflorum) on the hydration of Portland cement and the development of a new compatibility index[END_REF][START_REF] Sellami | Improvement of mechanical properties of green concrete by treatment of the vegetals fibers[END_REF][START_REF] Le Troedec | Influence of various chemical treatments on the composition and structure of hemp fibres[END_REF] or in an alkaline solution (sodium or calcium hydroxide, etc.) [START_REF] Nozahic | Influence of sunflower aggregates surface treatments on physical properties and adhesion with a mineral binder[END_REF][START_REF] Le Troedec | Influence of various chemical treatments on the composition and structure of hemp fibres[END_REF][START_REF] Sedan | Effect of calcium rich and alkaline solutions on the chemical behaviour of hemp fibres[END_REF][START_REF] Toledo Filho | Durability of alkali-sensitive sisal and coconut fibres in cement mortar composites[END_REF] or esterification [START_REF] Hill | A study of the potential of acetylation to improve the properties of plant fibres[END_REF][START_REF] Khazma | Optimization of flax shivecementitious composites: Impact of different aggregate treatments using linseed oil[END_REF]. Chemical treatments generally dissolve potentially troublesome plant extractibles and modify the chemical composition of the fibre surface. In addition, these treatments have significant environmental impacts (use of strongly acidic or basic chemicals, water consumption, need for temperature cure, etc.) [START_REF] Li | Chemical Treatments of Natural Fiber for Use in Natural Fiber-Reinforced Composites: A Review[END_REF]. Thermal treatments are regularly applied on wood; they have therefore been experienced on plant fibres [START_REF] Bilba | Silane treatment of bagasse fiber for reinforcement of cementitious composites[END_REF][START_REF] Carada | Effects of heat treatment on the mechanical properties of kenaf fiber[END_REF]. In much the same way as chemical treatments, thermal treatments eliminate hemicelluloses and other extractibles, and limit the volume variations of the fibre. However, this treatment remains difficult to master and requires heating for a long period. A mineral coating can be applied to the lignocellulosic fibres or aggregates using a hydraulic binder (cement, lime, etc.) [START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF][START_REF] Ferraz | Compatibility of pretreated coir fibres (Cocos nucifera L.) with Portland cement to produce mineral composites[END_REF][START_REF] Monreal | Effects of aggregate coating on the hygral properties of lignocellulosic composites[END_REF][START_REF] Chafei | Dependence of the properties of cementitious composites on the nature of the hydraulic binder coating the reinforcing flax fibers[END_REF]. Mineral coatings seem to reduce the hydrophilicity of the fibres, they chemically isolate the fibre from the matrix and limit the volume variations. However, they cause a considerable mass increase. Moreover, the long-term mechanical performances are degraded by the mineralisation of the fibres in contact with the binder used for the coating [START_REF] Toledo Filho | Durability of alkali-sensitive sisal and coconut fibres in cement mortar composites[END_REF][START_REF] Ramakrishna | Evaluation of durability of natural fibre reinforced cement mortar composite-A new approach[END_REF]. Still with the aim of limiting the plant fibre or aggregate's water absorption, another solution is to apply an organic coating (polymer): paraffin wax [START_REF] Juarez | Performance of "Agave lecheguilla" natural fiber in portland cement composites exposed to severe environment conditions[END_REF], acrylic polymer [START_REF] Chafei | Effect of flax fibers treatments on the rheological and the mechanical behavior of a cement composite[END_REF][START_REF] Collet | Comparison of the hygric behaviour of three hemp concretes[END_REF][START_REF] Khazma | Impact of aggregate coating with a PEC elastomer on properties of lightweight flax shive concrete[END_REF], or linseed oil [START_REF] Nozahic | Influence of sunflower aggregates surface treatments on physical properties and adhesion with a mineral binder[END_REF][START_REF] Khazma | Optimization of flax shivecementitious composites: Impact of different aggregate treatments using linseed oil[END_REF][START_REF] Monreal | Effects of aggregate coating on the hygral properties of lignocellulosic composites[END_REF]. These treatments significantly reduce the water absorption of the aggregates and improve the composite properties.

Nevertheless, the surface created could modify the adhesion with the matrix. But, depending on the nature of the polymer, the environmental impact may also be significant. From this point of view, linseed oil appears as the most eco-friendly polymer.

Linseed oil (LO) is a drying oil susceptible to auto-oxidation and polymerization resulting in tough elastic films upon exposure to air [START_REF] Sharma | Addition polymers from natural oils-A review[END_REF]. Linseed oil is composed of three fatty acids with long unsaturated aliphatic chains (oleic, linoleic and linolenic acids) and saturated chains (stearic and palmitic acids). The polymerization reaction is a three-step radical chain reaction (induction, propagation and termination). Both linoleic and linolenic acids rapidly oxidize, since their unsaturated bonds are close to one another. The reaction rate is improved with an increase in temperature and depends on the linoleic fatty acid to linolenic fatty acid ratio [START_REF] Stenberg | A study of the drying of linseed oils with different fatty acid patterns using RTIR-spectroscopy and chemiluminescence (CL)[END_REF]. Moreover, the heat liberated during the oxidation-polymerization steps improves even more the reaction kinetics, eventually leading to spontaneous combustion of the linseed oil [START_REF] Abraham | A solution to spontaneous combustion in linseed oil formulations[END_REF]. For this reason, it is recommended not to use linseed oil at a temperature above 60°C.

From the literature review, linseed oil stands out because of its bio-based origin and its efficiency in reducing the water absorption of plant aggregates. Thus, this work investigates a way to improve the properties of flax-fibre-reinforced mortar using linseed oil as coating treatment applied to flax fibres. The first part of this study concerns the optimisation of the linseed oil-to-fibre ratio in order to avoid agglomeration of the fibres or overconsumption of oil. The second part focuses on the effects of this treatment on the bio-composite properties in the fresh and hardened states.

Experimental program

Materials

The hydraulic binder was developed from two commercial powders: CEM I cement (67% in mass), metakaolin (33% in mass). The cement used is a CEM I 52.5 R ordinary Portland cement (OPC) in accordance with EN 197-1 standard. The compressive strength on standardized mortar at 28 days is 72.0 MPa, the Blaine fineness is 4200 cm 2 .g -1 , and the density about 3.13 g.cm -3 . Metakaolin was also used as a mineral addition of Portland cement to develop the binder. The metakaolin used is in accordance with NF P18-513 standard. This powder has a Blaine fineness of 4270 cm 2 .g -1 , and the density about 2.57 g.cm -3 . In addition to cement and metakaolin, a highly pure limestone was used (total carbonates equal to 98.7%), in accordance with the NF P 18-508 standard. The Blaine fineness of the limestone filler is 5440 cm 2 .g -1 , and its density is 2.70 g.cm -3 . These mineral additions were used to reduce the negative effect of the alkaline medium produced by OPC, and the crystallization of portlandite into the fibres [START_REF] Claramunt | Natural fiber nonwoven reinforced cement composites as sustainable materials for building envelopes[END_REF][START_REF] Pereira | Use of highly reactive rice husk ash in the production of cement matrix reinforced with green coconut fiber[END_REF][START_REF] Mármol | Mechanical and physical performance of low alkalinity cementitious composites reinforced with recycled cellulosic fibres pulp from cement kraft bags[END_REF].

In addition, chemical admixtures were used for mortars reinforced with flax fibres. A high range water reducer superplasticizer based on polycarboxylates was added with a ratio of 0.30% relative to the mass of binder (dry extract) to improve workability in the fresh state. In addition, a viscositymodifying agent (VMA) based on a bio-polymer of high-molecular weight was added to the mortars with a ratio of 0.02% relative to the mass of binder (dry extract) to prevent segregation between the paste and the aggregates.

An alluvial sand with a grain size of 0/2 mm was used. This sand has a specific gravity of 2.66 g.cm -3 , and an absorption coefficient of 0.15%. The flax fibres used were harvested in Normandy (France) in 2014, cut at a constant length of 12 mm and provided by Vandecandelaere Company (Depestele Group, Caen -France), which is specialized in flax farming and scutching. These raw flax fibres have a real density of 1.52 g.cm -3 , and a mean diameter of (14.7±3.0) µm. The water absorption of the flax fibres after immersion for 24 h is (132±5) %. This value was used to correct the effective water content for the mix design of the mortars. A flax fiber can be described as a 10-20 μm wide cylindrical composite with concentric cell walls which differ in terms of chemical composition and morphology. The polymers involved in the composition of fibers are cellulose (64-74 %), hemicelluloses (11-21 %), lignin (2-3 %) and pectin (1-3 %) [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Bismarck | Surface characterization of flax, hemp and cellulose fibers; surface properties and the water uptake behavior[END_REF][START_REF] Tserki | Novel biodegradable composites based on treated lignocellulosic waste flour as filler. Part I. Surface chemical modification and characterization of waste flour[END_REF][START_REF] Khalil | Acetylated plant fiber-reinforced polyester composites: A study of mechanical, hygrothermal, and aging characteristics[END_REF]. The tensile strength of flax fibres is (1254 ± 456) MPa. The mechanical tests were performed on 50 samples of elementary fibers, using the protocol described in the French standard NF T25-501-2.

Linseed oil treatment

Linseed oil (LO) is a drying oil susceptible to auto-oxidation and polymerization resulting in tough elastic films upon exposure to air [START_REF] Sharma | Addition polymers from natural oils-A review[END_REF]. Linseed oil is composed of three fatty acids with long unsaturated aliphatic chains (oleic, linoleic and linolenic acids) and saturated chains (stearic and palmitic acids). The polymerization reaction is a three-step radical chain reaction (induction, propagation and termination). Both linoleate and linolenate acids rapidly oxidize, since their unsaturated bonds are close to one another. The reaction rate is improved with an increase in temperature and depends on the linoleic fatty acid to linolenic fatty acid ratio [START_REF] Stenberg | A study of the drying of linseed oils with different fatty acid patterns using RTIR-spectroscopy and chemiluminescence (CL)[END_REF]. Moreover, the heat liberated during the oxidation-polymerization steps improves even more the reaction kinetics, eventually leading to spontaneous combustion of the linseed oil [START_REF] Abraham | A solution to spontaneous combustion in linseed oil formulations[END_REF]. For this reason, it is recommended not to use linseed oil at a temperature above 60°C.

The linseed oil used in this work is a boiled linseed oil with an accelerated oxidation, supplied by Natura ® laboratories. It was extracted from farmed flax, harvested and produced in Normandy. It was obtained by cold extraction without any solvent, filtered at 1 µm, and then heated at 150 °C. This linseed oil has a density of 0.93.

For carrying out the treatment, the flax fibres were previously dried at 50±1°C and then introduced into a planetary mixer. The mixer is then started at a speed of 30 rpm and after 30 s the linseed oil is introduced for a period of 2 min 30 s. Different oil / fibre mass ratio were experienced: 0.25, 0.5, 1.0, 1.5 and 2.0. Mixing was maintained for 2 min 30 s more to ensure a homogeneous fibre coating. The fibres thus treated are placed in a ventilated oven at 50±0.1°C for 14 days.

Preparation of cement composite

The respective proportions of the mortar's components are detailed in Table 1. For all compositions, the effective water on binder (CEM I cement + Metakaolin) ratio was set at 0.50 and the sand on powders (binder + limestone) ratio was set at 1.0.

Flax fibres were added with different proportions: 1.0, 2.0 and 3.0 %vol. to the total batch volume.

An additional amount of water was added to take into account the water absorption and flax fibres.

These quantities are given in Table 1.

After mixing, the mortars were cast in two layers into prismatic moulds (40x40x160 mm). To expel the air from the mixture, the moulds were subjected to 60 hits on a vibrating table. The test pieces were demoulded 24 h after manufacturing, and then placed in a climatic chamber at (20±1) °C and (65±5) %RH.

Testing methods for flax fibre

Real density

The real density of raw and treated flax fibres was measured using a helium pycnometer, on an oven-dried fibres sample of about 1.00 gram; six samples were tested. Experiments were performed on a Micromeritics AccuPyc II 1340.

Water absorption

Due to their nature and chemical composition, flax fibres are very susceptible to water absorption [START_REF] Célino | The hygroscopic behavior of plant fibers: a review[END_REF]. The flax fibres were heated to (50±0.1) °C in a ventilated oven for 48 h prior to the test. The water absorption kinetic was obtained by placing 3.00 g of flax fibres in a microperforated bag, then immersing the bag in water for different times (from 1 min to 72 h). The experiment was repeated six times for each duration of soaking. After immersion, the bags were placed in a centrifuge operating at 500 rpm for 30 s to remove the excess water on the surface of the fibres.

Mortars

Ref.

Mortar

Raw Fibres Mortars Linseed Oil (ratio = 0.25) treated Fibres Mortars Linseed Oil (ratio = 0.5) treated Fibres Mortars The water absorption capacity of these fibres was determined gravimetrically using Eq. 1. This testing method is derived from the recommendations of the RILEM Technical Committee 236 [START_REF] Amziane | Recommendation of the RILEM TC 236-BBM : characterisation testing of hemp shiv to determine the initial water content, water absorption, dry density, particle size distribution and thermal conductivity[END_REF].

Fibre content (%) 0 % 1 % 2 % 3 % 1 % 2 % 3 % 1 % 2 % 3 % Sand 0/2 (
The experiment was repeated six times. The result is expressed by:

= - × 100
Eq. 1

with WA(t) being the water absorption ratio at time t, m(t) the soaked fibres' mass at time t, and m0

the initial dry mass.

ATR-FTIR

To obtain information on the surface chemistry of flax fibres, attenuated total reflectance (ATR)

was used in conjunction with infrared spectroscopy (FTIR). This technique enables direct analysis of particles to be carried out. It gives information on chemical groups and bonding by studying vibratory frequencies in the mid-infrared domain (4000-400 cm -1 ). The experiments were performed on a Nicolet Magna IR 600. All background and sample acquisitions were acquired in the transmission mode at 4 cm -1 resolution, with accumulation of 50 scans, at room temperature.

Testing methods for mortar properties

Workability

The workability of the different mortars was assessed with the flow table, in accordance with EN 1015-3. The flow value is measured by the mean diameter (in two perpendicular directions) of a sample of the fresh mortar which was placed on the flow table tamper by means of a truncated conical mould, and given 15 vertical hits by raising the flow table and allowing it to fall freely from a height of 10 mm.

Air content

The air content of the fresh mixes was evaluated with a 1 L pressure gauge according to the EN 1015-7 standard.

Compressive and 3-point bending tests

The mechanical properties of the mortars were determined by implementing compressive and three-point bending tests in accordance with the EN 196-1 standard after 28 days of curing. For each mix, three specimens were tested. The compression test was implemented at a loading rate of 2.4 kN.s -1 and the bending test at 50 N.s -1 .

Thermal conductivity

The thermal conductivity of the mortar specimens was measured with a conductimeter using the transient hot-wire method (EN ISO 8894). A heat flow by Joule effect is generated, and the temperature variation over time was measured using a thermocouple. The heating element was associated with the thermocouple in the hot-wire probe. The rise in temperature measured by the sensor is limited to 20°C and the measuring time is dependent on the material, here fixed at 120 s.

Measurements were taken by positioning the thermal probe between two specimens of the same mix (Fig. 1) and were conducted three times after 28 days of curing. The specimens were oven-dried before thermal measurements.

Water accessible porosity

The water accessible porosity of the hardened mortar specimens was calculated through vacuum saturation and with a hydrostatic balance according to the NF P18-459 standard at 28 days curing.

For each mix, three specimens were tested. The saturated surface dry state of the specimens was obtained through vacuum saturation. Initially, the specimens were placed in a vacuum chamber in the presence of silica gel in order to decrease the contact angle between the water and the pores and dry the specimens. After 4 h, the water was introduced into the chamber to immerse the specimens, and the vacuum was maintained for an additional 44 h. The porosity was obtained from the mass of the saturated surface dry specimens measured in air (mair) and in water (mwater), and oven-dried at 50°C (mdry), using Eq. 2.

= --× 100 Eq. 2

Water absorption by capillary action

The porosity and permeability of materials induce water to rise through capillary networks. This capillary rise leads to the deposition of hygroscopic salts in the material pores and causes the proliferation of fungi and molds. The capillary water absorption of a material defines the water quantity that can be absorbed by the specimen when one of its sides is in prolonged contact with water. This coefficient of capillary water absorption Cw (kg.m -2 .min -0.5 ) was determined as described in the EN 1015-18 standard. The procedure given in this standard for determining this coefficient involves several stages. First, samples were oven-dried until a constant dry mass was obtained. The samples were then immersed in water to a depth varying from 5 to 10 mm. Two masses M10 and M90

were then recorded, respectively, at 10 and 90 min after water immersion. The capillary coefficient was then calculated by means of Eq. 3. Three measurements were carried out for each mix, after 28 days of curing.

= 0.1 × -Eq. 3

Results and discussion

Optimisation of linseed oil treatment

Previous studies using linseed oil for the treatment of plant particles made use of variable oil/particle ratios. For example, Monreal el al. chose ratios of 0.33, 0.5 and 1 [START_REF] Monreal | Effects of aggregate coating on the hygral properties of lignocellulosic composites[END_REF]. Nozahic et al.

adopted an oil / sunflower aggregates ratio of 0.5 [START_REF] Nozahic | Influence of sunflower aggregates surface treatments on physical properties and adhesion with a mineral binder[END_REF] while Khazma chose oil / flax shives ratios of 1, 2 and 3 [START_REF] Khazma | Optimization of flax shivecementitious composites: Impact of different aggregate treatments using linseed oil[END_REF], and Sellami used an oil/fibre ratio of 0.78 [START_REF] Sellami | Improvement of mechanical properties of green concrete by treatment of the vegetals fibers[END_REF]. A wide range of ratios has therefore been explored, depending on the type of the plant particles. Thus, a wide range of ratios has been tested, depending on the type of the plant particles. Therefore, the literature review was not useful in determining an adequate ratio for the treatment of flax fibres. Hence, in order to optimize the mass quantity of linseed oil relative to the flax fibres, five different LO/fibre ratios were investigated: 0.25, 0.5, 1, 1.5 and 2.

Observation of the polymerized linseed oil treatment

The absorbance of raw flax fibres, LO-treated flax fibres, and a thin film of polymerized linseed oil by means of ATR-FTIR are shown in Fig. 2. Table 2 summarizes the major bands observed in the FTIR spectrum of flax fibres and linseed oil, and their attachments to chemical group molecules. Flax fibre is rich in polysaccharides, with the ⎯OH hydroxyl group stretching leading to a large peak between 3125 and 3600 cm -1 [START_REF] Le Troedec | Influence of various chemical treatments on the composition and structure of hemp fibres[END_REF][START_REF] Célino | Qualitative and quantitative assessment of water sorption in natural fibres using ATR-FTIR spectroscopy[END_REF]. Out-of-plane cellulose-water linkage between 700 and 650 cm - 1 and cellulose β-links (900 cm -1 ) are also noted for raw flax fibres [START_REF] Célino | Qualitative and quantitative assessment of water sorption in natural fibres using ATR-FTIR spectroscopy[END_REF][START_REF] Abidi | Changes in the cell wall and cellulose content of developing cotton fibers investigated by FTIR spectroscopy[END_REF]. They are characteristic of plant fibres. Hydroxyl groups are able to form hydrogen bonds with water, which is responsible for the significant water absorption capacity of plant fibres. Stretching vibrations of the CH2 methylene group (2920 and 2850 cm -1 ) on the surface are also identified. This is characteristic of the presence of waxes and fats on the flax fibre surface. Peak characteristics from lignocellulosic compounds retain the same absorbance level as raw surfaces. The main difference is a major increase in CH2 methylene group stretching peaks. It indicates waxy or oily deposition on the surface by polymerized linseed oil.

Linseed oil oxidation can be verified from the triacylglycerol ester-bonding peak at 1740 cm -1 and from the triglycerides ester linkage peak at 1160 cm -1 [START_REF] De Viguerie | The drying of linseed oil investigated by Fourier transform infrared spectroscopy: Historical recipes and influence of lead compounds[END_REF]. The dotted spectrum corresponds to the spectrum of fibres treated with linseed oil (2), subtracted from the spectrum of raw fibres (1). There is a very close correspondence with the spectrum of polymerized linseed oil (3). The FTIR spectrum information does not enables us to draw conclusions about the homogeneity of the surface coating. Table 2. Main infrared absorption peaks and band for flax fibre and linseed oil between 4000 and 400 cm -1

Environmental scanning electron microscope (SEM) observations of raw and LO-treated flax fibres (ratio = 0.5) are given in Fig. 3. Fig. 3a shows several individual raw fibres naturally grouped in bundles. On the Fig. 3b, the polymer film formed by the deposition of linseed oil gives a glossy appearance and seems to coat all the fibres within the bundles. This will decrease the specific surface area of the fibres and limit their separation into single fibres.

Mass monitoring

After treatment, the flax fibres were placed in an oven at 50 °C for 2 weeks. A portion of the treated fibres was placed into metal cups so as to track the evolution of the mass of the samples over time. Fig. 4 shows the mass gains of the treated flax fibres relative to the initial mass of oil, as a function of the time after treatment, for each LO/fibre ratio. For all samples, linseed oil mass increased between 7 % and 14 %. The loss of mass in the first day at the very beginning of the treatment corresponds to the departure of the moisture naturally present in the fibres. Low LO/fibres ratio led to a significant and faster oil mass increase. This mass increase is the result of the oxidation of linseed oil. The polymerization of this boiled linseed oil seems to occur between 2 and 3 days after application of the treatment. This is in keeping with the results obtained by Lazko et al. who observed a mass increase of linseed oil of between 10 and 15% [START_REF] Lazko | Biocomposites based on flax short fibres and linseed oil[END_REF].

Density

The linseed oil coating treatment of fibres reduces the overall density of the treated fibres (Fig. 5).

The specific density of the raw flax fibres is (1.52±0.01) g.cm -3 . As a result of the treatment, the density of fibre + LO coating is (1.21±0.01) g.cm -3 for a LO/fibres ratio of 2. This is explained by the low density of the linseed oil, equal to 0.93 g.cm -3 . Thus, when the LO/fibres ratio increase, the proportion of oil is greater, resulting in a decrease in the treated fibre densities. Also note the very small standard deviations, indicated by the error bars in Fig. 5. This suggests good homogeneity of the application of the polymeric coating.

Water absorption

The main purpose of the linseed oil treatment is to reduce the water absorption of the flax fibres.

The linseed oil (LO) / fibres ratio will therefore be optimised mainly according to this criterion.

Water absorption measurements were therefore carried out on these treated fibres for 60 min of immersion. The results of these water absorption tests are presented in terms of mass ratios, based on the mass of flax fibres (Fig. 6). This type of representation was chosen to highlight the absorption results with respect to the linseed oil present on the treated fibres. Indeed, it is considered that linseed oil does not absorb water; it is therefore obvious that the absorption of the treated flax fibres is reduced in comparison with raw fibres. It is then necessary to calculate the ratio of the mass of water absorbed to the mass of raw fibres (masswater,abs./massraw,fibres), without the linseed oil mass. This allow to assess whether linseed oil is really effective in reducing this absorption. The mass of linseed oil in the treated fibres is known from the masses of oil and fibers introduced into the mixer. This curve (in black on the graph) is used to determine the optimal ratio corresponding to the minimum mass of linseed oil.

Logically, the raw fibres are those that absorb the largest amount of water: 115±4% after soaking for 60 min. It is also noteworthy that the higher the LO/fibres ratio, the lower is the amount of absorbed water.

However, it is important to compare the mass of absorbed water and the amount of raw fibre present in the sample to identify the most effective LO/fibres ratio. Therefore, the curve (relative to the right Y-axis in Fig. 4) shows the mass of absorbed water relative to the mass of raw fibres. Thus, the LO/fibres ratios of 0.25 and 0.5 seem to be the most effective in decreasing the water absorption.

These two ratios will therefore be employed for the rest of this study.

The water absorption kinetics of raw and treated flax fibres are presented in Fig. 7. Two distinct stages are evidenced for all fibres. First, a rapid mass increase of the fibres is observed for a short time. At the beginning of the water sorption, this process is driven by capillary forces. In less than 5 min, the raw fibres absorbed about 50 % of their maximum water retention capacity. A second absorption stage shows a less steep raise toward the saturation level. In the second stage, the absorption is driven by water diffusion. The highly hydrophilicity of plant fibres, due to the nature of their components, explains the strong dependency of their morphology and their physical properties on the relative humidity [START_REF] Stamboulis | Effects of environmental conditions on mechanical and physical properties of flax fibers[END_REF][START_REF] Thuault | Effects of the hygrothermal environment on the mechanical properties of flax fibres[END_REF]. This essential characteristic also gives rise to fierce competition between the flax fibres and binder in terms of water demand, which starts with concrete mixing.

Thus, raw flax fibres can absorb water up to 150% of their dry mass because of their hydrophilicity [START_REF] Chafei | Effect of flax fibers treatments on the rheological and the mechanical behavior of a cement composite[END_REF]. As Fig. 7 shows, the linseed oil treatments (0.25 and 0.5 LO/fibres ratios) greatly reduce water absorption by the fibres. The slopes at very short times confirm that the hydrophilicity of fibres is reduced by the linseed oil treatment, which is hydrophobic. The maximum water absorption after immersion for 24 h WA( 24) is reduced to (83±5) % of the dry mass for the 0.25 LO/fibres ratio and (73±6) % for fibres coated with linseed oil with a ratio of 0.5.

Comparison of the properties of cement composites reinforced with raw and linseed oil treated flax fibres

Linseed oil treatment significantly decreased the water absorption of the fibre, especially for 0.25 and 0.5 ratios. Mortars incorporating raw and treated fibres (with these two ratios) were manufactured and characterised in both fresh and hardened states. Three volume ratios of flax fibres were used: 1.0, 2.0 and 3.0 %vol. (Table 1).

Air content

The incorporation of flax fibres into mortar mixes increases the air content due to entrainment of additional air (Fig. 8). The reference mortar has the lowest air content, equal to (1.7±0.1) %. As expected, increasing the fibre content increases the amount of trapped air, whichever the fibre state (raw or treated). This increase in air content was also underlined by other authors [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF]. It is mainly assigned to the low ability of fibres to compact. Indeed, the fibres have a significant impact on the granular compactness of concrete mixtures [START_REF] Page | Multi-physical properties of a structural concrete incorporating short flax fibers[END_REF]. For mortars reinforced with 1.0 %vol. of fibres, the linseed oil treatment does not seem to have a significant impact on the air content, which is between (2.1±0.2) % and (2.4±0.2) %, depending on the fibre state. For mixtures with 2.0 %vol. of fibres, the linseed oil treatment (especially with the 0.5-LO/fibres ratio) results in a noticeable increase in air content compared to raw fibres. The effect of the linseed oil treatment on air content is even more pronounced for mixtures containing 3.0 %vol.

fibres. The air proportion represents (3.7±0.5) %, (4.3±0.7) % and (6.5±0.3) % with raw fibres, and with 0.25 and 0.5 LO/fibres ratio, respectively. Thus, the linseed oil treatment increases the amount of entrapped air in fresh mixtures. A previous study had also shown that linseed oil treatment has the effect of increasing the air proportion in plant fibre reinforced cementitious composites [START_REF] Page | Influence of different surface treatments on the water absorption capacity of flax fibres: Rheology of fresh reinforced-mortars and mechanical properties in the hardened state[END_REF]. This could be explained by the coating characteristics specific to linseed oil. Indeed, linseed oil sometimes seems to coat individual flax fibres and sometimes bundles of fibres. In the latter case, air could be entrapped in fibre bundles coated with linseed oil. Once these fibres are mixed with the mortar, the air remains entrapped, increasing the air content of the material.

Consistency of fresh mixes

Fig. 9 shows the flow diameters measured for the flow table test. Firstly, the reference mortar has a plastic consistency, with a flow of (18.0±0.4) cm. Note that this mortar does not contain a superplasticizer. In contrast, the mortars with flax fibres all contain a superplasticizer and a viscosity-modifying admixture to overcome the problems encountered with the use of flax fibres in cementitious composites [START_REF] Page | Multi-physical properties of a structural concrete incorporating short flax fibers[END_REF]. If chemical admixtures are added to the reference mortar mixture (in the same proportion that flax-fibre reinforced mortars), the consistency is very fluid, with a flow diameter of (29.8±0.2) cm. Moreover, despite the use of admixtures, the raw fibre-reinforced mortars have lower diameter flows than the reference mortar (with and without plasticizer): (17.8±0.3) cm with 1.0 %vol. of raw flax fibres, (13.7±0.6) cm with 2.0 %vol. and (11.4±0.6) cm with 3.0 %vol. Thus, as the fibre content is increased, the flow decreases, reflecting a reduction of workability. This trend has also been highlighted by other studies [START_REF] Tung | Mechanical properties of flax fibre reinforced cement composites[END_REF][START_REF] Page | Multi-physical properties of a structural concrete incorporating short flax fibers[END_REF]. The linseed oil treatment of flax fibres significantly enhances the workability of fresh fibre-reinforced mortars. Indeed, with the LO-0.5 treated fibres, the measured diameters are 69% higher, on average, than raw-fibre-reinforced mortars. With these treated fibres, the diameter is always greater than 18 cm, even with 3.0 %vol. of fibres in the mixture. With 1.0 %vol., the measured flow is near 30 cm, i.e. a very fluid consistency.

With the LO-0.25 treated fibres, the workability of the mortars is also improved compared to rawfibre-mortars, but to a lesser extent. For 1.0 %vol. fibres, the diameter is greatly increased, up to (25.9±0.6) cm -i.e. 45% higher than raw-fibre-reinforced mortar. On the other hand, the improvement is less significant with 3.0 %vol. of fibres, with an improvement of 19%. The significant reduction of the water absorption of flax fibres due the linseed oil treatment can account for these enhancements in workability. Moreover, the presence of the coating can also explain this improvement. Indeed, the linseed oil treatment has the effect of increasing the apparent cross-sectional area (due to coating) of the fibrous reinforcement, and therefore their specific surface area is reduced. Thus, the demand for cement paste to meet the workability criteria is greatly reduced.

Porosity

As it can be seen in Fig. 10, the addition of flax fibres increases the porosity of mortar, as reported for raw flax fibre reinforced concrete [START_REF] Page | Design and multi-physical properties of a new hybrid hemp-flax composite material[END_REF]. The increase in porosity correlates strongly to the air content of fresh mortars. This suggests that the increase of porosity is mainly due to the increase of the entrained air previously observed on mortars. A previous study had highlighted that the increased air content is probably caused by the decrease of the granular compactness induced by the addition of fibres into the mixture [START_REF] Page | Multi-physical properties of a structural concrete incorporating short flax fibers[END_REF]. The porosity raises from (13.9±0.2) %, for the reference mortar up to (19.9±0.2) % for mortar reinforced with 3.0% of linseed oil treated flax fibres (ratio = 0.5). The treatment of flax fibres with linseed oil induces a higher porosity in the composite in comparison with raw flax fibres, especially with LO/fibre ratio of 0.5. For the LO-0.25 treatment, the increase in porosity ranges from 3% to 9%, depending on fibre content. Obviously, the higher the fibre content is, the more the linseed oil increases the porosity. Coating with linseed oil results in higher porosity for LO/fibre ratio of 0.5; the relative increase in porosity ranges from 9% to 22%. As the mechanical properties of brittle materials are governed by the porosity, significant reductions in mechanical performances of these mortars are expected due to the high porosity.

Mechanical properties

The results of the compressive tests carried out on the various mortars are presented in Fig. 11.

Firstly, the reference mortar has a compressive strength of (39.6±1.7) MPa. For most fibre-reinforced mortars, a decrease in compressive strength is noted in comparison with the reference mortar (0 %vol. of flax fibres). For 1.0 %vol. of fibres, the compressive strengths of the flax-fibre-reinforced mortars are quite the same as the reference mortar. The compressive strengths then decrease significantly when the fibre content is raised. The values are between 31.7 and 33.9 MPa with 2.0 %vol. of flax fibres, and between 23.1 and 26.8 MPa with 3.0 %vol. of fibres. Linseed oil treatment appears to depress the compressive strength. Indeed, the strengths of the mortars reinforced with treated fibres are lower than those of the mortar with raw fibres, and in particular the linseed oil treatment with 0.5-ratio. A trend seems to be emerging with the fibre content. It appears that increasing the fibre content results in a decrease in compressive strength. The reduction of the compressive strength was highlighted by other authors on bio-fibre cementitious composites [START_REF] Page | Design and multi-physical properties of a new hybrid hemp-flax composite material[END_REF][START_REF] Kriker | Mechanical properties of date palm fibres and concrete reinforced with date palm fibres in hot-dry climate[END_REF]. This is attributed to the increased air content in the concrete, which reduces its compactness, and thus the compressive strength. Indeed, it was previously noted (Fig. 8) that the air content increases with fibre content. Other authors have also noted decreases in compressive strength with the use of linseed oil as a treatment for plant aggregates [START_REF] Nozahic | Influence of sunflower aggregates surface treatments on physical properties and adhesion with a mineral binder[END_REF]. These authors have observed that treating the plant particles with linseed oil created a lower quality interface between the matrix and the plant aggregates, which explains the observed decrease in compressive strength. One can think that during a compression test, the coalescence of the interfacial cracks quickly leads to the ruin of the sample (low value of the tensile strength). For a compressive resistance practically unchanged compared to the unreinforced mortar which has a porosity of 13.9%, with 1.0 %vol. of fibers the porosity increases according to the treatment applied: 5% for the raw fibers, 8% for LO-0.25 and 14% for LO-0.5. The same trend is observed for the reinforcement rates of 2.0 %vol. and 3.0 %vol. However, the relative increase in porosity is greater with higher fibre contents. are much lower, with values close to that of the reference mortar, ranging from (6.2±0.4 MPa) to (6.3±0.4 MPa). In the latter case then, the flax fibres do not seem to have any beneficial effect on the mortar's strength. With fibre contents of 1.0 %vol. and 2.0 %vol., at which flax fibres seem to significantly improve the composite flexural strength, coating with linseed oil can significantly improve the mortar's strength in comparison to raw flax fibres. However, both linseed oil treatments have similar results. The use of LO/fibres ratio of 0.5 instead of 0.25 does not seem to have a significant effect on the mortar' strength. Similar results were obtained in another study dealing with linseed oil-coated diss fibres, with significantly improved flexural strengths [START_REF] Sellami | Improvement of mechanical properties of green concrete by treatment of the vegetals fibers[END_REF].

Thermal conductivity

The thermal conductivity of mortars was measured in the dry state (drying at 60°C in an oven for 24 h, prior to the test). Fig. 14 presents the variation of thermal conductivity as a function of the dry density of mortars. A quasi-linear relationship seems to fairly describe the thermal conductivity as a function of the mortar density. This relationship has already been highlighted by several authors who investigated the thermal characteristics of bio-based building materials [START_REF] Collet | Thermal conductivity of hemp concretes: Variation with formulation, density and water content[END_REF]. Note that this relationship can be considered linear in the domain of interest to us here, but this is not true in general. The reference mortar exhibits the highest thermal conductivity, with values between 2.05 and 2.37 W.m -1 .K -1 . The increase in fibre content leads to a decrease of the thermal conductivity. Thus, the mortars incorporating 1.0 %vol. of flax fibres show thermal conductivities slightly lower than the reference mortar, ranging from 1.75 to 2.08 W.m -1 .K -1 . On the other hand, mortars with 3.0 %vol.

fibres have the best thermal characteristics, with conductivity between 1.23 and 1.68 W.m -1 .K -1 . Thus, flax fibres can slightly improve the thermal insulation properties of the material. In addition, for equal fibre content, mortars with linseed oil-treated flax fibres have lower thermal conductivities than those with raw fibres. Linseed oil therefore is beneficial to this characteristic, because it allows reducing the density of the material, which leads to a decrease in thermal conductivity.

Capillary water absorption

Some authors have highlighted that plant fibre-reinforced cement composites are vulnerable to weathering and water absorption [START_REF] Sheridan | The effect of long term weathering on hemp and rapeseed concrete[END_REF][START_REF] Tonoli | Effects of natural weathering on microstructure and mineral composition of cementitious roofing tiles reinforced with fique fibre[END_REF]. Therefore, the water absorption of bio-based cement composites is an important parameter for the durability of this material. The capillary water absorption kinetics for the reference mortar and raw fibre-reinforced mortars are illustrated in Fig. 15a, while those of treated fibre-reinforced mortars are presented in Fig. 15b. In addition, the capillary water absorption coefficients of all mixes are provided in Fig. 16. It can be seen that the reference mortar shows the slowest and lowest water absorption behaviour, with a coefficient of 194 g.m -2 .min -0.5 . The addition of flax fibres to the cement matrix leads to an increase in the water absorption capacity. Obviously, higher fibre content leads to higher water absorption. After 72 h (i.e. 65.7 min 0.5 in Fig. 15), the absorption of the 3.0 %vol. raw fibre mix is almost twice as high as that of the reference mortar, 9.90 kg.m -2 and 5.35 kg.m -2 . This increase can be attributed to two phenomena: the high-water absorption capacity of plant aggregates and the increase in the water accessible porosity of the composites. However, the absorption coefficients remain within acceptable values in comparison with other studies incorporating plant particles [START_REF] Djelal | Feasibility study of using poplar wastes as sand in cement mortars[END_REF]. This is due in particular to the relatively low fibre content of the composites investigated in this study. It seems that the linseed oil treatment reduces the capillary water absorption capacity of mortars (Fig. 15 and Fig. 16). This treatment decreases both the absorption kinetics and the maximum water retention, especially in mixes containing 2.0 %vol. and 3.0 %vol. of fibres. The use of a higher LO/fibre ratio further decreases the absorption capacity of the mortars. However, the gain between LO-0.25 and LO-0.5 ratios remains marginal.

Conclusion

This work has investigated a way to improve the physical properties of flax-fibre-reinforced mortars using a linseed oil coating. The linseed oil treatment has been optimized by using low linseed oil content while minimizing water absorption. The following main conclusions can be drawn:

• The water absorption of flax fibres disrupts the rheological behaviour of the cementitious composites in the fresh state and also affects the mechanical properties in the hardened state. The hydroscopic behaviour of raw and linseed oil coated flax fibres was studied to begin with. It appeared that the flax fibres very rapidly absorb a large amount of water (up to 130% by mass). The linseed oil treatment decreases the water absorption capacity. Thus, the LO/fibres ratios of 0.25 and 0.5 seem to be the most effective for decreasing the water absorption.

• Linseed oil treatments reduced the water absorption of fibres, resulting in a significant improvement in workability for composites made with these treated fibres. Furthermore, these coating treatments increased the fibres' diameter, leading to a decrease of the specific surface area and therefore the water demand of the flax fibres.

• The mechanical tests carried out on hardened mortars have shown a continuous decrease of the compressive strength as fibre content increases. The higher air content and porosity of mortars with high fibre content may explain this decreasing trend in strength. Decreases in compressive strength were also noted with the use of linseed oil.

• With fibre contents of 1.0 %vol. and 2.0 %vol., for which flax fibres seem to significantly improve the mortar's flexural strength, linseed oil coating can significantly improve the mechanical strengths in comparison with raw flax fibres.

• Flax fibres can slightly improve the thermal insulation properties of the material. In addition, for equal fibre content, mortars made with flax fibres treated with linseed oil have lower thermal conductivity than those made with raw fibres. Linseed oil is therefore beneficial to this characteristic, because it helps reducing the density of the material, which leads to a decrease in thermal conductivity.

This study has therefore highlighted that linseed oil could be an interesting treatment solution for composites reinforced with plant fibres. However, microstructural observations will have to be carried out in order to study the interfacial transition zone at the fibre-oil-matrix interface. Future work will investigate the possible chemical interactions between polymerized linseed oil and the cement matrix. In addition, longer term experiments would give additional information about the effectiveness of the treatment over time and therefore its durability.

Fig. 1 .

 1 Fig. 1. Thermal conductivity measurement with the hot-wire probe.

Fig. 2 .

 2 Fig. 2. ATR-FTIR analysis of raw and linseed oil treated flax fibres, and of polymerized linseed oil film.

Fig. 3 .

 3 Fig. 3. SEM observations of raw flax fibres (a) and flax fibres treated with linseed oil (ratio = 0.5) (b).

Fig. 4 .

 4 Fig. 4. Mass monitoring of fibres treated with linseed oil during the curing time.

Fig. 5 .

 5 Fig. 5. Density of the coated fibres as a function of the LO/fibre ratio.

Fig. 6 .

 6 Fig. 6. Absorbed water mass relative to the mass of treated fibres, according to the LO/fibres ratio after soaking for 60 min.

Fig. 7 .

 7 Fig. 7. Water absorption of raw and treated flax fibres.

Fig. 8 .

 8 Fig. 8. Air content of fresh mortars.

Fig. 9 .

 9 Fig. 9. Flow diameters measured with flow table for the different mortars.

Fig. 10 .

 10 Fig. 10. Porosity of composites as a function of the air content.

Fig. 11 .

 11 Fig. 11. Compressive strength of composites after curing for 28 days.

Fig. 12

 12 Fig. 12 illustrates the reduction of the compressive strength as a function of the porosity of mortars. For an addition of 1.0 %vol. of flax fibre, the strength decrease is very slight, even negligible.

Fig. 12 .

 12 Fig. 12. Compressive strength as a function of the composite porosity.

Fig. 13 .

 13 Fig. 13. Flexural strength of composites after curing for 28 days.

Fig. 14 .

 14 Fig. 14. Thermal conductivity of composites according to their dry densities.

Fig. 15 .

 15 Fig. 15. Capillary water absorption of mortars with raw flax fibres (a) and with linseed oil treated fibres (b).

Fig. 16 .

 16 Fig. 16. Coefficient of capillary water absorption of mortars with raw flax fibres and with linseed oil treated fibres.

Table 1 .

 1 Composition of cement composites

Acknowledgments

The authors would like to acknowledge Vicat for supplying the cement used in this study, Argeco for the Metakaolin and BASF for supplying the superplasticizer and VMA admixture. In addition, sincere thanks go to the Depestele Company for the supply of flax fibres.

Data Availability

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