Design of a Novel Hybrid Rice Straw/Husk Bio-Based Concrete Featuring Enhanced Mechanical and Hygrothermal Properties

In order to reduce the consumption of energy and the emissions of greenhouse gases and CO2 generated by the construction industry, bio-based concretes made of plant aggregates are increasingly used in the optimization of building envelopes, thanks to their good hygrothermal performances, their renewable origin, and biodegradability. The present study focuses on the effect of combining different proportions of rice straw (RS) with rice husks (RH) on mechanical and hygric, and thermal properties of straw/husk concrete. The experimental investigation seeks to evaluate the thermal conductivity, moisture buffer value (MBV) and mechanical compressive properties of these concretes. The results evidences clearly that it is interesting to associate these two residues. The thermal conductivity of the bio-based concretes slightly decreases with increasing rice straw content and rises almost linearly with concretes density. The MBV measurements reveal that rice straw confers to concretes an excellent moisture buffering capacity. Finally, the compression test results highlight that the addition of rice straw induces high deformability and enables concretes to store a high quantity of energy.


Introduction
Building sector is the largest energy consumer in Europe and the US accounting for more than 40% of the total energy consumption and responsible for emitting one-third (36%) of associated greenhouse gases, mainly because of heating, cooling, and air conditioning systems according to the European Commission [1,2]. Within this context, and in order to achieve significant energy savings and reduce CO 2 emissions, the European Commission has established a set of binding directives to promote more rational use of energy [3]. These policies aim to achieve at least 32.5% energy efficiency for 2030 and promote an energy-free building through the introduction of nearly zero-energy buildings (NZEBs) as the new building target, this latter, requires a zero or a low amount of energy for operations (i.e. for heating, cooling, lighting, and other appliances), mainly coming from renewable sources [4].
Enhancing the thermal insulation properties in building envelopes represents a key step in minimizing the energy demand for winter heating and summer cooling purposes of buildings [5]. Up to now, building envelopes are mainly realized with traditional insulation materials obtained from fossil energy such as expanded polystyrene or from mineral origin such as glass and rock wools. However, in the context of sustainability and environmental protection, alternative materials such as bio-aggregatebased concretes are developed by combining plant origin aggregates which are renewable and local resources with mineral binders, in order to replace conventional thermal insulation materials and reduce non-renewable resources consumption [6,7]. Indeed, researches, showed that bio-based concretes are highly suitable for building applications since they exhibit an excellent thermal performance and enable moisture management by adsorbing and desorbing water vapor [8,9]. Moreover, they allow to design a carbon negative building material due to carbon sequestration, thanks to the absorption of CO 2 present in the atmosphere through photosynthesis during the plant's growing stage, and to the absorption of high quantities of CO 2 during the carbonation of lime (curing process) [10][11][12]. In fact, according to Boutin et al. [13] and Ip and Miller [14], hemp concretes enables to store approximately 0.35-0.36 kg of CO 2 eq per square meter of wall built (with a thickness of 25-30 cm) over a year.
In this sense, several researches have explored the possibilities of the incorporation of other lignocellulosic aggregates in concretes to develop an alternative/similar concrete to lime hemp concrete. Indeed, bio-based concretes are currently manufactured from different vegetable aggregates, such as hemp hurds [15,16], sawdust of sunflower [17,18], flax shives [19], and rice husk [20][21][22]. Some advantages of these particles include their very low density, very low thermal conductivity, and high hygroscopic property. Additionally, they have a short growing period and an annual harvest.
Thus far, the widely studied bio-based lightweight concretes are hemp based concretes, however, in the previous study conducted by Chabannes et al. [20], rice husk concrete was found to have a very low thermal conductivity (0.10-0.13 W m − 1 K − 1 ) depending on aggregates to binder ratio, these values of the thermal conductivity coefficient are comparable to those of hemp concrete (0.09-0.13 W .m − 1 . K − 1 ). Add to that, Nozahic et al. [23], find that hemp and sunflower concretes show large similarities in terms of mechanical performance. In another study, Rahim et al. [24] compared hygric properties of lime concrete made of rape straw with hemp lime concrete, the results show that the MBV values for both concretes are higher than > 2.0 g/(m 2 .%RH), therefore, both concretes exhibit an excellent moisture buffer capacity.
In the case of straw based building materials, Belayachi et al. [25] investigated the use of wheat and barley straw in lime concretes for thermal insulation applications and studied the influence of the type of straw and straw to binder ratio (S/B) on the mechanical and thermal properties. The results indicated that concretes prepared with wheat straw have the highest compressive strength, while, the thermal conductivity of concretes decreased with increasing straw content. Moreover, Labat et al. [26], investigated the hygrothermal properties of straw-clay concretes and compared them to bio-based materials, results revealed that straw-clay concretes showed a high sorption capacity, a very high water vapor permeability and a low thermal conductivity (ranging from 0.071 to 0.120 W m − 1 K − 1 ) which is comparable with hemp concretes conductivity of equivalent density. Furthermore, Belhadj et al. [27,28], used barley straws fibers (0.08, 0.16 and 0.24 wt%) in order to lightweight sand concretes. While, Xin Chen et al. [29], utilized rice straw to prepare cemented tailings backfill, and studied the effect of the rice straw content and length on compressive strength and elastic modulus. Results highlight the improvement of the stiffness and ductility of the rice straw cemented tailings backfill with increasing fiber content and length.
However, there is a remarkable lack of research on the use of rice straw in bio-based concretes. For instance, rice straw is often burned in the field to remove rice residues left in the field after harvesting grains. Or burnt to produce ashes which could be used in lime and cement-based system due to its high amount of silica which might improve its mechanical properties [30]. Nonetheless, this practice has negative impacts on the environment, in fact, rice straw open field burning leads to air pollution and contributes to global warming through emissions of greenhouse gases [31][32][33].
To sum up, based on the findings reported on mechanical and hygrothermal properties of straw concretes, and on the negative impacts of burning straw on the environment, their use in insulation materials could be highly recommended in buildings for their ability to lower heat transfer and to moderate indoor humidity variations. Also, the use of rice straw in the preparation of concretes can be considered as an environmental protection measure.
A recent study conducted by Page et al. [34], on a hybrid hemp-flax concrete showed that the incorporation of the flax fibers increased the ductility and improved the compressive strength of hemp concrete. Furthermore, Abbas et al. [35] showed that hybrid composites made of sunflower pith and hemp shiv are more permeable than those made of sunflower pith, the authors reported a water vapor permeability of about 5.09 0.10 − 11 kg/(m s Pa) for hybrid concrete, 2.48 0.10 − 11 kg/(m s Pa) for sunflower pith concrete and 5.09 0.10 − 11 kg/(m s Pa) for hemp concrete. In addition, hybrid concretes presented a higher moisture buffering capacity (MBV of about 1.71 g/ (m 2 . %RH)) compared to hemp concretes (1.63 g/(m 2 .%RH)) and sunflower pith concretes (1.21 g/(m 2 .%RH)). However, it should be pointed out that hemp concretes were made with a higher aggregate/binder mass ratio (0.33) than sunflower pith concretes (0.10) and hybrid concretes (0.23). The study of Pachla et al. [36] is one of the first to combine rice husk and rice straw into cellular concrete, authors evaluated the effect of incorporating rice straw fibers with different length (1 cm, 2 cm, 3 cm) into cellular concrete using only rice husk. Straw fibers was added as a replacement for husk particles at three different percentages (5%, 10%, 15%), they show that the thermal conductivity was directly proportional to the straw fiber length, and that the introduction of straw fibers increased the three-point bending strength.
The association of two different biomass types seems to be more advantageous, that is why it was decided in this work to associate two residues of rice production to design a lightweight straw/husk hybrid concrete for insulation applications with density < 800 kg/m 3 . This paper therefore investigates through a multi-physical experimental characterization the influence of combining different proportions of rice straw (S) with rice husks (H) (66%, 50% and 33%) on thermal, hygric and mechanical performances of three different formulations with three different straw/husk ratios, and compares them with the pure rice husk and rice straw concretes. The variation of hygrothermal and mechanical properties as function of the straw/husk ratio was analyzed and discussed.

Plant Aggregates
The untreated rice husk and whole rice straw were collected from Aigues-Mortes in the Camargue province of France (harvested in 2021). The dried rice straw was carefully cut into particles of 4 and 6 cm lengths by manual cutter (scissors). It was not longitudinally cut to ensure the cylindrical shape and save its intra-granular porosity. In order to limit the scatter of the results, only the rice straw particles obtained from the upper part of the stems plant (10 cm under the inflorescence of the plant called as panicle) and consisting of stem and leaf blade were used in this study. The particles extracted from this sampling area exhibit lower diameter which increase from bottom to top. The designations of the rice husk and rice straw are respectively: RH and RS (Fig. 1).

Preparation and Manufacturing of Concretes
Two eco-aggregates derived from rice plant were used to manufacture the composite specimens and five different mixtures corresponding to different rice husk-rice straw mass 1 3 ratios different proportions of rice straw (RS) with rice husks (RH) (100%, 66%, 50%, and 33%) were prepared and investigated in order to determine the optimal composite.
All mixtures were performed with a binder-to-aggregate mass ratio (B/A) of 2, the mixing water-to-binder mass ratio (W m /B) was taken as 0.5, and the pre-wetting water (W p ) was calculated from the water test capacity of aggregates after 5 min. The mixing process was identical for all mixes; the mixing procedure can be divided into three steps: • Pre-wetting of natural particles for 5 min before incorporating the binder. • In second, the binder was added and combined with the particles for two minutes. • At last, the mixing water was added and all constituents were mixed for five minutes until complete homogeneity was obtained.
The fabrication of concrete has been performed manually by a molding method in three layers of the same weight and same height. Cylindrical specimens (of 11 cm in diameter and 22 cm in height) were manufactured by a manual compaction process with a steel device [20,23]. As each aggregate has a different density and water content, which depends on its composition and structure, the fresh apparent density of concretes could not be kept constant. Therefore, the proportions of the different constituents were calculated according to the target fresh density of the concretes. Furthermore, the total water amount was determined considering the sum of the water for binder hydration (W m ) and the amount of water used to saturate the bio-aggregates (Wp), which should be sufficient to obtain a good homogeneity without phase segregation of the mixture.
Specimens were left to cure in a conditioned room at 20 °C for three days, after that, they were demolded and stored in a conditioned room at 20 °C and 50%HR until testing, in order to ensure a complete and rapid carbonation of concrete [37]. The mix proportions and the fresh density of all samples are reported in Table 1.

Chemical Composition Analysis of Plant Particles
Composition analysis were performed using a successive solvent extraction procedure according to the following ASTM standards: alpha-cellulose (ASTM D1103-60), lipophilic extractives (ASTM D1107-56) and holocellulose (ASTM D 1104-56) used in previous studies [38][39][40]. It can be divided into three steps: 1st step: determination of lipophilic extractives (waxes, fats, resins and some gums) using Soxhlet extractor. A mixture of toluene/ethanol (2:1 w/w) was used as solvent and the extraction process continued for 8 h.
2nd step: holocellulose extraction was carried out using a fraction of the previously extracted sample. Water, acetic acid and sodium chlorite (NaClO 2 ) was used for the extraction process. Lignin ratio was calculated from holocellulose residue.
3rd step: α-cellulose was finally obtained by hemicelluloses solubilization in sodium hydroxide solution (NaOH) and acetic acid. Finally, inorganic components were determined according to (ASTM D 1102-84) method. where dried particles were ignited in the oven at 600 °C for 8 h. The heating was repeated until all the carbon is eliminated. The resulting holocellulose,  α-cellulose, and ashes were weighed using the IR balance (Precisa XM66) at 105 °C.

Morphological Investigations
Plant aggregate microstructure was carried out by means of an environmental scanning electron microscope (ESEM FEI Quanta 200) equipped with a XMAX 80 mm 2 Oxford Instrument detector used to determine the silica location on the particles. A layer of carbon was deposed on the surface of particles in order to avoid any degradation during analysis. Samples were examined under high vacuum with an accelerating voltage of 10 kV and a working distance between 10 and 20 mm.

Particles Size Distribution
Granulometric analysis of vegetal aggregates was performed using an image analysis processing according to RILEM TC 236-BBM [41] group recommendations using ImageJ software. The same approach was used by several authors [42,43], which is suitable for determining the lignocellulosic particle aggregate. It yields to accurate information such as the width (W), length (L), aspect ratio (L/W), and the equivalent area diameter that can be calculated according to Eq. (1).

Water Sensitivity of Particles
The water absorption ability of rice plant aggregates was measured according to the recommendation of the RILEM TC 236-BBM [41]. Dried particles (48 h at 105 °C) were placed in a metallic permeable spherical strainer and immersed in tape water until a complete wetting. This was repeated at different immersion times (after 1, 2, 5, 10, 30 min and 48 h). Then, particles were centrifuged using a spinner in order to extract the water excess present between the particles and on particles' surface. The water uptake was measured at given time intervals up to a maximum of 48 h. The absorption coefficient WA (water absorption ratio) was estimated and used to determine the amount of pre-wetting water (W p ). Tests were repeated 3 times (with three different samples of particles). The water absorption coefficient (WA) of aggregates was calculated using the Eq. (2).
where WA (t) is the water absorption ratio at time t, m(t) [g] is the soaked aggregate mass at time t, and mi[g] is the initial mass of dry aggregates.

Density Measurements and Porosity Estimation
Density Measurements The bulk density of natural particles is measured using protocols set by RILEM TC 236-BBM [41], The mass of aggregates was measured with an analytical balance with a readability of 0.01 g, and the corresponding volume was measured by filling a cylindrical mold of 11 cm in diameter and 22 cm in height with particles without compaction, Thus, the density is the ratio of the mass to the volume occupied by particles. Prior to measurements, particles were dried at 100 °C until a constant mass is reached and then cooled in a desiccator until room temperature (23 °C, 50%RH). Measurements were repeated five times. The absolute density excluding pores was measured with a helium pycnometer (AccuPyc 1330, Micromeritics, Norcross, GA, USA).
The Apparent particle density was calculated from the absolute density and the water absorption capacity of particles after saturation (W max ) according to Eq. (3).
where t is the particles absolute density and 0 is the open porosity in the particle obtained with the expression Eq. (4).
where t is the known absolute density, w is the density of water and W max is the water absorption capacity of particles after saturation (48 h). W max was taken as 110% for rice husk and 350% for rice straw. These values were obtained from the water sorption tests (see Sect. 3

.3.4).
Furthermore, the apparent density of concretes was measured based on the weight and the volume of the samples, three replicate samples were used for the test.

Porosity Estimation of Concretes
Knowing the densities of the different constituents, as well as their weight, the volumes occupied by each constituent were calculated and thus volume occupied by voids is concluded. According to Chabannes et al. [44], the inter-particles and total porosities within bio-based concretes can be calculated according to the Eqs. (5) and (6).
The total porosity (including micro-porosity of the binder and capillary voids within rice plant particles) was deduced using the absolute density of lime and straw particles knowing the volume occupied by each component in the mold.
On the other hand, inter-granular porosity can be deduced with the total volume of the specimen which is known using this time the apparent density of binder paste and the apparent density of a single particle. M lime , M RH , M RS is the mass of lime, rice husk and rice straw introduced in the specimen, where t is the particles absolute density, ap is the apparent density of the constituent abs is the absolute density of the binder and V S is the volume of the specimen. The apparent density of the binder paste was taken as 840.3 ± 31.8 kg/m 3 according to Chabannes et al. [20].

Mechanical Proprieties
Mechanical compression tests on concrete specimens were performed using an electromechanical testing machine (MTS Criterion) with a 50 kN capacity. The tests were conducted following the protocol described by Niyigena et al. [42], with a loading rate of 5 mm/min. Cycles of loading/disloading were applied for 1%, 2% and 3% strain. The tests were conducted on cylindrical specimens 11 × 22 cm 3 (11 cm in diameter and 22 cm in height) after 60 days of curing (after reaching a complete hygric stabilization) in a climate-controlled room at 20 °C and 50 relative humidity. For each mixture at least three specimens were tested. The Young's modulus is the mean value of the strongest increase in the strength/strain ratio recorded at each loading stage [42]. The compressive strength (σ max ) or the stress at 5% and 30% strain (σ 5%, σ 30% ) was determined depending on the stress-strain behavior of concrete and used to compare the concretes.

Thermal Properties
Thermal conductivity of concrete was measured by means of a conductivity meter (FP2C-NeoTIM) using a hot wire probe method (NF EN ISO 8894). The heating time was taken as 400s and the electrical power was 0.2 W. Prior to testing, specimens were dried at 100 °C then cooled for about one night at room temperature (20 °C,50%HR). Tests were conducted at least six times for each formulation under the same conditions (20 °C, 50%HR).

The Moisture Buffer Value
The moisture buffer value was measured according NORD-TEST project method [45]. To this end cylindrical samples of 10 cm in diameter and 10 cm in height were sealed on all but one side with an aluminum foil tape, and were exposed to daily cyclic relative humidity variations: 8 h exposure to 75% relative humidity followed by 16 h at low relative humidity exposure to 33% relative humidity at a constant temperature of 23 °C using a WeissTechnik WKL150/10 climate test chamber. Then, specimens were weighed until the change in mass between the last three consecutive cycles was less than 5% of discrepancies. The MBV (moisture buffer value) was calculated by Eq. (7).
where m, is the average between the weight gain/loss during absorption/drying cycles [kg]. RH high/low are the high (75%) and low (33%) relative humidity [%]. and A is the exposed surface area [m 2 ].

SEM Observations of Concretes
For each sample, interfacial interactions between lime and aggregate were analyzed by ESEM (FEI Quanta 200). Microstructural analysis of the concretes was also conducted on compression tests specimens in order to highlight the structure of concretes such as the arrangement of vegetable particles and porosity in concretes.

Characterization of Rice Plant Particles
Morphology Figure 2, shows the cross-sections of rice husk (RH) and rice straw (RS) with different magnifications. Micrographs revealed a honeycombing and porous structure for rice straw (Fig. 2a). From the outside to the inside, it is formed of an epidermis (Ep), which has a concentrated layer of silica and other inorganic substances, xylem that contain vessels (Ve) and serve as capillaries for moisture sorption (water transport), phloem (PL) containing parenchyma cells (PC) which transports sugars, proteins and minerals, a pith characterized by largest parenchyma cells, and a lumen in the center. Although, SEM observations showed that rice husk is composed of an external epidermis characterized by a very particular morphology (convex-concave surface) rich in amorphous silica and an internal epidermis containing less silica (Fig. 2b). Between those components we can also observe a region of vascular bundles and parenchymal cells.
The interior of the rice husk is made of cortex containing bundles of fibers, phloem and xylem. It can be seen that RH contains small pores along the cross-section with less variability in size, with a size range of 5-10 μm. However, these results are lower than those mentioned by Chabannes et al. [20,44]. While, RS is characterized by a very porous structure with a random pore size distribution, the numbers of cell sizes and diameter continuously increase more and more towards the lumen of the straw particle. The average pore size for RS is in the range of 20-60 μm, and the average pore area varies from 250 to 950 µm 2 . Table 2. shows the biochemical composition of rice plant particles obtained by the solvent extraction procedure. Rice husk (RH) and rice straw (RS) vary in chemical composition, however, the main components of the rice plant particles are cellulose, hemicelluloses, lignin, lipophilic extractives and ashes. Rice husk (RH) contains about 36% cellulose, 22% hemicelluloses, 17% lignin, 8% lipophilic extractives and about 16% ash, which consists mainly of silica (45%). Compared to rice husk (RH), rice straw (RS) is higher in holocellulose (i.e. cellulose, hemicelluloses) content, this component is considered as very hydrophilic polysaccharide due to its high content of hydroxyl group.
It should be kept in mind that the concentrations of these components depend strongly on the location of a particle on plant. Indeed, Jin et al. [51], divided rice straw into fractions by morphological character (internodes, nodes, leaves sheath and blade) and investigated the chemical composition of different fractions of whole straw. Results reveal that leaves have larger cellulose fraction contents than either internodes or nodes.

Size and Shape Distribution
The morphological characteristics of rice straw particles (RS) and rice husk (RH) were compared based on image analysis. Results show that rice husk width varies from 0.5 to 3.2 mm and the maximum length is about 9.1 mm. In the case of rice straw particles, the width distribution varies from 1 to 11 mm and the length can reach up to 6 cm. Moreover, the average equivalent diameter is 3.5 mm for rice husk, and 15 mm for rice straw (Fig. 3).
In addition, the aspect ratio defined as the length on width ratio (L/W) is higher for rice straw aggregates (AR = 10) and shows a higher standard deviation than rice husk (AR = 5) which is linked to the heterogeneity of particles obtained by the manual cutting process (Fig. 4). Indeed, the particle size distribution (PSD) mainly depends on the production process (milling) and the quality of separation. Based on these results, it can be seen that rice husk has a near-spherical shape whereas rice straw has an elongated cylindrical shape. These characteristics must affect the microstructure (particles arrangement, density, and porosity) of concerts and thus its mechanical and hygrothermal proprieties.

Dry Densities and Porosities
The loose bulk density of the rice husk particles was in the range of 103 kg/m 3 which is 2 times greater than that of the rice straw particles (RS). Indeed, compared to rice straw, rice husk requires less volume at the same weight. This is surely linked to their boat-like shape, which must induce more voids (inter-granular porosity) between particles due to the imperfect rearrangement of these particles compared to rice straw that has a cylindrical shape. The very low bulk density of rice straw is justified by its higher intra-granular porosity (lumen). The loose bulk density of rice husk is close to the values reported in the literature [52], which is about 97 kg/m 3 . Nevertheless, the bulk density of rice straw can vary depending on the different forms it may take. Thus, for straw bale, the loose bulk density varies from 20 to 40 kg/ m 3 [53], while, it's about 61 kg/m 3 for chopped rice straw (of about 5 cm pieces) [54] . The absolute density measured with helium pycnometer was 1481 kg/m 3 for rice husk, this result meets the value found in the work of Chabannes et al. [52]. For the rice straw, the absolute density was 1373 kg/m 3 , compared to the absolute density of wheat straw and barley straw (ranging from 865 to 871 kg/m 3 ) reported by Bouasker et al. [55], the absolute density of rice straw is much higher.
The difference in absolute density between the two kinds of aggregates used in this study is explained by the different chemical composition, especially the content of ashes, which, are rich in inorganic elements (Si, Mg…) characterized by a high atomic weight, in fact, rice straw contains less ash than rice husk.
In addition, the apparent density of a single particle was calculated based on its absolute density and its open porosity. Results show that a single rice husk has an apparent particle density, which is almost 2 times higher than that of rice straw. This is due to its lower intra-granular porosity as shown in Table 3. This results in a lower internal porosity but a higher inter-granular porosity for rice husk. These findings are in agreement with SEM micrographs presented in Sect. (3.1.1), where a high rate of porosity and biggest pores were observed for rice straw particles.
Although RS have higher total porosity compared to RH (96% for RS and 93% for RH), this is related to the fact that rice straw has higher intra-granular porosity. It should be noted that the highly porous nature of rice straw and its low bulk density might play a very important role in improving the hygrothermal properties of rice straw concretes. The porosities measured in this study for rice husk are relatively close to those reported by Chabannes et al. [52].

Water Sensitivity of Particles
The kinetic curve of water absorption tests is plotted in Fig. 5. Two distinct stages can be observed. A first phase which lasts 5 min characterized by a linear curve where a rapid weight increase occurs (Fick laws diffusion). During this phase the intake of water results from the capillary action (capillarity forces) attributed to the higher open porosity observed for plant aggregates and causes an almost instantaneous mass increase. After 5 min of immersion, the water absorption coefficient shows a significant difference between rice husk (RH = 77%) and rice straw particles (RS = 187%), this is attributed to the largest pores in rice straw which seems to play a key role in their water absorption capacity by promoting water diffusion through plant cells, to their larger specific surface, and to their strong hydrophilic character provided by holocellulose (i.e. cellulose, and hemicelluloses).
A second absorption stage (non-Fickian diffusion), slow and continuous follows a logarithmic law occurs after five minutes of immersion up to saturation (48 h). As expected, both aggregates show a slower absorption kinetic at this stage. After 48 h of immersion, rice straw particles can  absorb up to 350% of its dry mass which is equivalent to 3,5 times its dry weight. Whereas, rice husk particles have only absorbed about 110% of water by mass (1,1 times its dry weight). This behavior is attributed to changes in the microstructure of aggregates due to the presence of internal stresses related to swelling phenomena [56]. Additionally, in the case of rice husk particles (RH) the water absorption appears to stabilize and reach saturation stage after 120 min. While rice straw particles keep (RS) absorbing a less important amount of water before reaching equilibrium after 4 h. This might depend on many parameters such as: the surface wettability of the vegetable particles, their chemical composition, the high porosity and the internal structure of these particles, which tends to reinforce the water absorption within rice straw particles (RS). In fact, the high internal porosity of the straw tends to enhance water diffusion within the aggregates, moreover, when considering the biochemical composition of aggregates, compared to rice husk, rice straw contains a high amount of hydrophilic components (i.e., cellulose and hemicelluloses) which contain hydroxyl reactive groups -OH and carboxyl groups -COOH. These groups can bound to free water molecules and then increase the hydrophilic character of straw particles.
It should be pointed out that the high water absorption during the first minutes might induce competition for water demand between binder and aggregates during the mixing process, which might impact binder hydration, hence, disrupting the setting of the concretes, therefore natural aggregates must be wetted before the implementation of the binder. Nevertheless, in the long term saturated aggregates can pump the absorbed water to the binder, which may be particularly beneficial to the hydration process of the binder. This characteristic might affect the thermal and hygric properties of concretes.

Dry Densities
Once samples were removed from the mold, they were placed in an environment with stable conditions of temperature and relative humidity (20 °C, 50% RH) for curing. The variation of the specimen density with different RH/ RS ratios was measured every 48 h, for 60 days. The dry densities after 2 months of setting are highlighted in Table 4. It can be observed that the apparent density of concretes decrease with increasing rice straw content, the highest density is obtained for concretes performed only with rice husk, it is about 635 kg/m 3 , which is significantly higher compared to the value obtained for concretes performed only with rice straw (446 kg/m 3 ). However, it should be noticed, that the fresh density of RH concrete was higher. This can be partially explained by the important porosity and the greater amount of water absorbed by straw particles. The difference in initial water content between rice husk and rice straw particles might confirm these findings. In fact, as the water content of RS was more important than in RH particles, rice straw concretes require a higher quantity of water than rice husk concretes, hence, higher evaporation of the initial water was observed in the case of rice straw concretes after hygric stabilization.
Additionally, it appears that the dry density of concretes is also depending on the particles densities as well as the granular arrangement generated by mixing rice husk and rice straw particles, which might improve the compactness of concrete by reducing the inter-granular porosity within concretes and thus lead to the decrease of final density of concretes. In fact, the bulk density of rice husk particles is two time higher than that of rice straw particles, which results in a higher volume occupied by RS particles, compared to that of RH for the same mass.

Porosities Within Concretes
Concretes are characterized by three different porosities: inter-granular porosity, which corresponds to the air gaps, left between particles, intra-granular porosity, which corresponds to the pores within the particles and inside the binder paste, and the total porosity, which corresponds to the sum of both types of porosity and constituted from open and closed pores.
Porosities within concretes are plotted in Fig. 6. Results revealed that the total porosity increase by increasing rice straw content, which is due to the high degree of intragranular porosity of rice straw as explained previously (see Sect. 3.1.4). For instance, the total porosities of the two reference concretes (100%RH and 100%RS) are 80% for RS concrete, while, it is about 70% for RH concrete.
However, the inter-granular porosity of RH concretes is higher than that of RS concretes. This can be explained by the higher apparent density of the rice husk particles, their boat-like shape, and their random orientation within concretes, which tends to create several voids between particles. In contrast, the elongated shape of rice straw particles tends to induce a preferential orientation of these particles within concretes, which tend to reduce the intergranular porosity. Furthermore, the lowest inter-granular porosity was observed for 50%RS combination; this might be related to the granular arrangement of these particles in the concrete. It seems that the addition of rice husk particle leads to the filling of voids by favoring the particles physical entanglements within the concrete.

Microstructure observations at the particle /matrix interface
The scanning electronic microscopic images collected from the examination of the cross section of concretes are shown in Fig. 7. Results evidence the porous structure of concretes and highlight the pore's size diameter (voids) created between particles (inter-particles porosity). In fact, micrographs revealed that RH concretes (Fig. 7a) has a greater apparent macro-porosity (interganular porosity) compared to rice straw concretes (Fig. 7e). This is in agreement with the results obtained in Fig. 6. Yet, more pores, with different diameter are visible in the case of RH concretes. It seems that the addition of rice straw particles leads to a reduction of the quantity and the size of pores present between particles (33%RS, (Fig. 7b), (50%RS, (Fig. 7c) and (66%RS, (Fig. 7d). This can be explained by the particular morphology of the straw particles 'elongated shape' and their preferentially orientation, in fact straw particles put themselves in the perpendicular direction to the applied compaction when the concretes are mechanically compacted during the manufacturing process (Fig. 7e). On the contrary, to rice husk particles which are randomly oriented to three dimensions due to their particular geometry and size (Fig. 7a). Furthermore, SEM observation reveals that rice husk particles were inserted between rice straw particles (filling the pores), this might improve the compactness of hybrid concretes. However, the SEM micrographs are not representative of the whole material, since the porosity varies as a function of the position and the orientation of the vegetable particles, thus, further analysis by an X-ray tomography would be necessary to better depict the effect of the combination of rice husk and straw on the microstructure of concretes and the granular stacking.
On the other hand, in Interfacial Transition Zone (ITZ) between lignocellulosic aggregates and the hardened lime, it appears that the rice husk particles are properly surrounded by the lime matrix compared to straw particles, the presence of a thick layer of binder is observed on the surface of rice husk. This is related to the higher roughness of rice husk particles surface compared to rice straw, which seems to have a smoother external surface [57]. Indeed, the external surface of the husk is characterized by the presence of a dome-shaped protrusions [58], which is expected to improve mechanical interlocking by the creation of anchorage points with the lime binder. This might ultimately enhance quality and strength of the interfacial adhesion and thus the mechanical properties of concretes.

Mechanical Behavior of Concretes
Compressive tests were carried out on cylindrical specimens after 2 months of setting. The stress-strain curves obtained for the five studied mixtures are presented in Fig. 8. From the stress-strain curves of rice husk concrete (100%RH), it is evident that this latter maintains a brittle behavior with maximum stress which passes through a maximum value, thus making it easier to determine the compressive strength value. On the other hand, other concretes containing rice straw particles feature a radically different behavior, they show a compacting behavior, since the compression does not lead to the collapse of these concretes. These latter show a large deformation capacity. As a consequence, in order to compare these concretes and investigate the impact of rice straw content on mechanical characteristics, stress at 5% strain close to the end of the linear elastic area and 30% strain in the strain hardening area values have been selected. In addition, the young's modulus (E) was calculated on loading cycles which is significantly different from the apparent one determined by the slope of the linear part of the stressstrain curve.
Concretes containing rice straw particles show a compaction phenomenon which is typical of a ductile behavior, concretes mirror the mechanical behavior of foam materials. The stress-strain curve of these concretes is similar to the curves obtained by applying load on vegetal aggregates [59]. It can be divided into three main regions: a quasi-elastic linear stage where the binder supports the stresses, a quasielastic stage where the closing of the intergranular voids and the collapse of the hollow straw tubes occurs indicating that binder failure is becoming greater, and lastly a densification stage occurs as the concretes become more compact and harder to crush while absorbing a large amount of energy without total structural failure, at this stage the stress increases almost linearly with the increase of deformation due to great compressibility of the plant aggregates. Actually, the binder matrix is completely damaged and does not play any mechanical role at this stage. This is similar to results reported in the literature for some biobased concretes heavily compacted [60][61][62] or performed with a low concentration of binder [25,34,63,64].
Rice husk concretes (100%RH) show a very brittle behavior; the stress-strain curve shows abrupt failure at 0.33 MPa (median value). This compressive strength is obtained for 8% strain. Furthermore, rice husk concretes have the highest elastic modulus which is about 55.2 MPa (median value). On the opposite, concretes prepared with straw particles continue to be deformed while undergoing significant deformation (up to 50%). Results indicate that rice straw generates greater deformability and leads to the increase in ductility of concretes. This behavior might be attributed to compactness related to the packing of particles and the filling of inter-particles porosity under a high stress. Rice husk could be inserted between the large rice straw particles and thus increase the compactness of the concretes.
The variation of the compressive strength (Stress measured at 5% and 30% strain), and floating Young's modulus of the concrete as a function of rice straw content are shown in Fig. 9. It is clear that the elastic modulus and the resistance (stress at 5 and 30%) of concretes are directly proportional to the straw content, indeed the greater is the RS/RH ratio, the lower is the Young modulus and the compressive strength measured at 5% and 30% strain. In fact, concretes performed with a low amount of rice straw (33%RS) show the highest stress at both 5% and 30% strain, the corresponding stress values are 0.21 and 1.11 MPa, respectively. Moreover, It can also be observed that the addition of the rice straw particles reduces the stiffness of concretes. Indeed, the young modulus significantly decreased by about 45% when 33% by mass of rice straw was added.
This might be due to the higher water absorption of the rice straw particles RS which lead to a longer evaporation period and thus decrease the rate of carbonation reaction which generally occurs when the water excess is eliminated. A second hypothesis, would be that the high content of water extractives within rice straw particles trigger a delay of the hydration of the dicalcium silicate (C 2 S) present in hydraulic lime by retarding the apparition of calcium silicate hydrate (C-S-H) and results in short-term resistance [65,66]. More, the low resistance can be also linked to the quality of the interfacial adhesion between lime paste and vegetal particles which is primarily related to the size of particles and their topography. A better interfacial bonding occurs between the small particles (RH) and the lime matrix. In effect, it is well known that interface/interphase plays a crucial role in load transfer [17,22]. Within this study, it was observed that concretes containing rice straw particles cannot be crushed, but can only be compacted under loading which only causes the deterioration of the micro-tubes of straw particles and lead to further densification of the concretes (Fig. 10). It can be seen that these concretes tend to return to their initial shape after undergoing a significant deformation (end of compression test). This highly viscous behavior is attributed Young modulus (MPa) Fig. 9 Effect of rice straw on compression properties of concretes: a Stress at 5% strain, b Stress at 30% strain and, c Young's modulus to vegetal particles [67][68][69] and could be of interest during a seismic event, since it is able to undergo massive amounts of stress during an earthquake [60,69].

Thermal Conductivity
The thermal conductivity of concretes was measured at (20 °C; 50%RH) in order to characterize the thermal behavior of the concretes. At least six measurements were carried out for each formulation, thus results are presented with mean, minimum, and maximum values due to the dispersion of the values. Figure 11a reports the variation of thermal conductivity of dry concretes with different RH/RS ratio. As can be seen in Fig. 11a. The rice husk concretes (100%RH) present the highest thermal conductivity 0.115 W/(m K). These higher thermal conductivity values may be attributed to the higher density of the rice husk concretes. These results are in agreement with the values reported in the literature. Actually, In a previous work, Chabannes et al. [20] found that the thermal conductivity of rice husk and hemp concrete is about 0.119 W/(m K) and 0.108 W/(m K) when the densities are respectively 637 and 459 kg/m 3 . Furthermore, results underlines that thermal conductivity tends to decrease with the increase of the amount of rice straw particles. Indeed, the thermal conductivities of 33%RS, 66%RS and 100%RS were 0.101, 0.086 and 0.092 W/(m K), respectively. The density of these concrete, which decreases with the increase of the quantity of straw, can explain this statement. However, for the composition (50%RS) is not the same trend, it gave an average thermal conductivity of 0.111 W/(m K) despite its low density. This can be justified by the fact that the intra-granular porosity of this concrete is very low. This may indicate that conductivity is more impacted by porosity than density. We also note that the concretes performed with 66%RS and 100%RS can be qualified as insulating building materials according to French regulation (λ < 0.1 W m − 1 K − 1 ) [70,71].
In addition, it seems that density and thermal conductivity have a linear tendency. Results underline that thermal conductivity increases almost linearly with the decrease in density which depends strongly on rice straw content (Fig. 11b), Indeed, the lighter the concrete, the lower its thermal conductivity. This is attributed to the high porosity and lower bulk density of rice straw particles. It seems that the porosity (voids' ratio) within concretes plays a key role in thermal performances since pores can be filled with air and leads to lower thermal conductivity. The linear relationship between thermal conductivity and density was previously reported for hemp-concretes by Collet et al. [16] and Cérézo [72].

Moisture Buffer Value
The effect of the addition of rice straw particles on hygric properties of hybrid concretes has been assessed by Moisture Buffer Value (MBV) measurements. This hygric property allows to evaluate the concretes ability to absorb and release moisture under dynamic conditions (moderate ambient relative humidity variations). Figure 12a present the MBV values of the five studied concretes. From an overall observation of the moisture buffer values MBV, it can be deduced that the MBV value varies greatly depending on the samples composition (constituent ratio), which results in differences in density and porosity.
Results underline that increasing the percentage of straw enhances the moisture buffering capacity of concretes. Indeed, the increase in the RS/RH ratio leads to a higher MBV value and allows for reaching better hygroscopic performances. It is evident that the moisture buffering capacity of concretes strongly depends on the hygroscopic characteristics of rice straw, Indeed, the porous structure of rice straw characterized by large pores, and high interconnected open porosity allow high moisture transfer and storage within concretes. One can see from this Fig. 12a, that concretes performed with high content of rice straw (100%RS and 66%RS) exhibit a superior capacity to store or release the moisture from the surrounding air compared to other concretes, with moisture buffer values of 2.1 and 1.87g/(m 2 %RH) respectively. On the other hand, rice husk concretes (100%RH -reference concrete) present the lowest moisture buffer value which is about 1.2g/(m 2 %RH), This is linked the internal structure of a rice husk particles which is characterized by very small pores and low intragranular porosity that reduce the vapor permeability of concretes and thus its moisture buffering capacity. It should be noticed that the higher specific surface area of rice straw particles might lead to higher sorption of moisture and thus enhance the ability of concretes to moderate moisture.
Based on the classification established by the NOR-DTEST Project, the first noticed point is that rice straw concretes (100%RS) are considered as excellent hygric regulators that improves hygrothermal comfort and indoor air quality in building, whereas all other concretes demonstrate a good moisture buffering capacity. Comparing to literature, moisture buffer values obtained in this study are higher than those of hemp plaster which have an average MBV value of 1.23 g/(m 2 %RH) for an average density of 720 kg/m 3 and a total porosity of 72% [73], but the obtained values are lower than those met in literature for rape straw concrete (2.6 g/m 2 %RH) with a density of 490kg/m 3 [24], and flax concretes [2.03-2.81 g/(m 2 %RH)] with densities ranging from 470 to 665 kg/m 3 [74]. However, compared to conventional building materials, the studied concretes are better hygric regulators than ordinary concrete, gypsum, brick, and cellular concrete used also as a self-insulating material for building envelope [75].
As it can be seen in Fig. 12b, a high correlation between the increase of MBV and the increase of the total porosity of concretes was underlined. This correlation shows that both the internal structure of concretes and intra-granular porosity of particles and binder play a key role in the hygric performances of the materials. In fact, concretes might store water in their different pores, which range approximately from the µm-mm for intra-particle porosity to the mm-cm for inter-particles porosity

Conclusions
The presented work provides a sustainable use option for rice culture waste. By development of an environmentally friendly material made from rice plant by-products. Moreover, this work analyzed the mechanical and hygrothermal proprieties of a new hybrid concretes containing rice husk and rice straw particles with varied rice straw/rice husk ratio. Globally, concrete behavior is mainly governed by the quantity of rice straw particles constituting the material. However, the following conclusions can be drawn from the present work: • The physical characterization shows that rice straw particles have a porous structure and a higher aspect ratio (L/d) with a high degree of total and intra-granular porosity. When compared to rice husk particles. Also, chemical analysis revealed that rice straw contains high degree of cellulose and hemicelluloses. This gives them a high water absorption capacity (hydrophilic property). • SEM observation of concretes showed good adhesion between the plant aggregates and binder, which validated the compatibility of both materials. They confirm also the closing of the pores between particles by the insertion of rice husk particles between straw particles due to manual compaction. • The porosity of concretes was increased gradually when the proportion of straw was increased; on the other hand, the increase in the amount of rice straw particles leads to a reduction in concretes density. • Concerning the thermal analysis, the thermal conductivity was directly proportional to density and porosity. However, it was shown that porosity is the property that has a greater impact on thermal conductivity. Furthermore, we concluded that the higher the rice straw (RS) content, the lower the thermal conductivity of concretes. • Concerning the hygric properties, it was shown that MBV increases with increasing rice straw content in the material. Compared to other concretes, concrete performed with rice straw only appears as relevant and shows the highest MBV value of approximately 2.2 g/ m 2 . % RH. The MBV of concretes is governed by the voids presented in samples. However, the hygric properties of concrete should be more explored in detail. • The compressive strength and young modulus of concrete decrease with increasing the content of rice straw in the concretes. A high straw content induces an increase in porosity, which ultimately results in poor mechanical performance. Furthermore, results indicate that the deformation does not lead to the fracture of concretes containing rice straw, but induces a continuous increase of stress, these materials enable the storing of high quantity of energy.
However, more profound statistical analysis and research is necessary in order to study the possible correlation between the different properties of these bio-based materials so that they are applicable to more materials.