The wett ability of the surface is controlled by the surface chemical composition as well as by the morphology of the mic-structure.Surfaces with a similar chemical composition may have different wett ability behaviour due to the surface topology.In this study,the surface of hemp shiv underwent micro-structural changes via deposition of an organo-functionalised silica coating.Ethanol diluted sol series enhanced the surface roughness of the hemp shiv.At higher HDTMS loading,undiluted coatings lowered the surface roughness of the shiv which could explain the reason for lower contact angles compared to diluted coatings.Sol A-1 and sol A-5 have HDTMS molecules that are not fully hydrolysed and being deposited onto the membrane as a flat thick film as seen in Fig.6b.The reduced surface roughness can be attributed to the extra HDTMS molecules on the coated surface.Since an organic-inorganic hybrid coating was used,the ratio of TEOS: HDTMS was critical to control the roughness of the coatings resulting in variable water repellent properties of the coated hemp shiv.Most of the coatings enhanced the surface roughness except sol A-1 and sol A-5.These coatings had smooth surfaces with cracks which could explain the lower contact angles even though it had the highest loading of hydrophobic agent.It was observed that the TEOS: HDTMS molar ratio in the coating formulations affected the hydrophobicity of the coated hemp shiv.From Fig.3 it can be seen that varying the concentration of HDTMS in the formulations affects the water contact angle.When TEOS: HDTMS was 1: 0.01 corresponding to 0.5 wt% HDTMS,the contact angle was below 100◦ which suggests the concentration of the hydrophobic agent was too low to provide sufficient level of hydrophobicity.
The best results were obtained with TEOS: HDTMS ratio 1: 0.02 with contact angles up to 118◦.However,when the TEOS: HDTMS ratio was increased to 1: 0.06,the hydrophobicity was decreased for the undiluted sol coatings.These results can be explained by the combined effect of surface roughness and energy.TEOS is hydrophilic whereas HDTMS is hydrophobic and changing their molar ratio can affect the surface roughness and energy ofthe coated material.Increasing the HDTMS concentration would reduce the surface energy.However,best trimming trays the surface roughness can be reduced if the HDTMS concentration is high enough as the extra silane fills the inter-particle gap.Similar results have been reported in different coating systems.Although sol B-7 coating enhanced the surface roughness,it had developed cracks which lowered the water contact angle to 98◦.The presence of surface cracks arising as a result of shrinkage after drying the coated shiv is a significant factor to be considered when hydrophobic properties are concerned.The hydrophobicity of modified hemp shiv can be compromised as the water molecules can penetrate through the cracked coating wetting the bulk of the material over time.Therefore,sol-gel coatings chemically modified the surface of hemp shiv which overall improved the hydrophobicity of the material.The high water repellence can be attributed to the long alkyl chains of HDTMS that provide high hydrophobicity.The chemical composition of hemp shiv is mainly composed of cellulose,hemicellulose and lignin,which altogether contain a large percentage of oxidized carbon in their structure.Hydroxyl groups are known to contribute towards majority of the carbon-oxygen bonds in bio-based materials.The XPS data confirmed that the sol-gel deposition on hemp shiv significantly altered the surface chemistry.The surface carbon content of the coated hemp shiv decreased by 41.28%.On the other hand,the oxygen content increased by 26.51%.
This change in C/O ratio and increase in surface oxygen concentration can be attributed to O CH3 bonds present in the polysiloxane coating on the surface of the sol-gel coated hemp shiv.Moreover,the decrease in the surface carbon concentration of the sol-gel coated shiv can be attributed to the masking effect of the polysiloxane coating which reduces the detectability of surface cellulose and hemicellulose.The C1s high resolution XPS spectra indicate that the surface has been modified by the silica based coating that led to disappearance of C3 and C4 components of the C1s peaks.A shift in the binding energy of C2 component was observed along with the decrease in the intensity ofthe C2 componentfor the sol-gel coated sample.This shift indicates the presence of a carbon atom linked to an oxygen and silicon atom.It has also been shown that curing above room temperature drives the dehydration reaction at the adsorption sites between hydroxyl groups of the cellulose and the silanols forming Si O C bonds.These bonds are formed by the linkage between polysilanol network with the cellulose hydroxyl groups via polycondensation as illustrated in Fig.9.The increase in the intensity of C1 component for sol-gel coated sample from 48.01% to 91.09% indicates the presence of C H and C C bonds from the HDTMS hydrocarbon chain.Hemp concretes are the most widely used biobased concretes in France.As an example,VICAT,one of the European leaders in the production of building materials,has developed a construction system composed of hemp shiv and Natural Prompt Cement.Although construction rules exist for these materials,their growth is still limited by the lack of knowledge and high variability of the performances of biobased concretes,especially for their mechanical properties.In addition,even if it is not the main performance expected,they still require sufficient mechanical resistance to be unmolded,transported onsite,and handled.Several parameters,such as setting delays of the binder or potential degradation of plant particles have been identified to explain their mechanical weakness.
Moreover,on existing buildings,problems of powdering of the materials in the core have been observed even when using binder/vegetal couples validated by the construction rules.Currently,while cementitious binders are well characterized,the physico-chemical mechanisms that affect the setting of the binder in biobased concretes have not yet been well understood so far.The modification of the hydration of the binder is related to the extraction of compounds from the plant aggregates which are not chemically inert,contrary to the mineral aggregates of conventional concretes.As a consequence,empirical testing is currently used to assess the compatibility of specific biomass particles and binders couples.These tests validate the performance of a biomass batch and binder couple at a given time,but do not consider the important variability of plant chemical composition and micro-structure.The latter depends for example on plant species,genetic material and environment of its growth,but also on their transformation process.This lack of knowledge about the robustness and performance of plant-based concretes is the main obstacle to their development.Hemp crete offers a means of achieving the rules set by the French national environmental regulations,which applies since 2022,but also of improving technical aspects of the concrete used in construction today.When dry,hemp has a very low bulk density of around 110 kg.m− 3,meaning that hemp-based concrete is lightweight.It is an effective insulation material with good hygrothermal and acoustic properties.It has low thermal conductivity due to its porosity and low density.That porosity and the hydrophilicity of hemp create a buffering effect that regulates the humidity within the concrete.Hemp crete is also interesting from an ecological point of view,as it requires no pesticides and cleans the soil,leaving it in a better state than before.Furthermore,it reduces the distance and time of transport from the source to the building site since it uses materials cultivated locally.Finally,the plant particles in the hemp concrete act as a CO2 trap during the lifetime of a building.
Existing studies on bio-based concretes formulated with mineral binders and vegetal aggregates from different origins and chemical compositions showed that the properties of materials are disparate.As an example,for identical formulations,the use of nine types of hemp shiv from different sources leads to differences in compression strength by a factor of 5,while the impact on thermal conductivity is smaller.These results are attributed to the particle size distribution of the bio-resources,their water absorption and chemical composition and to the lixiviation of vegetal compounds.About impact of vegetal components on the hydration of binders,some studies highlight the modification of the mechanisms and kinetics of hydration of mineral binders by vegetal components.Indeed,during the processing of plant-based concretes,vegetal compounds,such as polysaccharides and polyphenols extracted in the fresh concrete mixture,interact with the mineral phases.Some of these extracted compounds can slow down or even completely inhibit the setting of the binders.These results,observed with various vegetal concrete formulations containing wood,hemp,miscanthus,corn or sunflower,depend on the selected plant-mineral binder couple.A correlation can be drawn between the biochemical nature of raw plant materials,including the plant cell wall quality,the composition of the leachates,and the mechanical properties of bio-based concretes.On a different scale of analysis,an interfacial transition zone is observed around the vegetal particles.The ITZ is a gradual transition region where the composition and micro-structure of the binder matrix are affected by the presence of the aggregate particles.This impacts on the physical properties,mechanical properties,functional properties and durability of vegetal concretes.In this study,a newly implemented protocol is set up using a hemp pellet put into a cement paste.The halo of less hydrated material created around this pellet is monitored using image analysis.Different analysis methods have been used to characterize the hemp crushed for the pellet and to analyze the ITZ and the hydrated matrix.The cement used to make up the cement paste is a Portland cement CEM I 52.2 R,trimming tray kept at a temperature of 20 ◦C in a sealed plastic bag.To reduce bleeding in the cement paste,KELCO-CRETE DG-F from BASF was used.This is a water-soluble biopolymer used in hydraulic cementious systems as a viscosity modifying agent.The characterizations of hemp were performed on sample dried at 50 ◦C for at least 2 days,in accordance with the RILEM recommendations.The hemp used for the experiments has a bulk density of 116 kg.m− 3.Its particle size distribution is extremely heterogeneous and the initial rate of absorption is 135 %.
A ThermoFisher Scientific iS50 device with Attenuated Total Reflection mode was used for Fourier transform infrared spectroscopy measurements.The spectra were acquired from 500 to 4000 cm− 1,with a digital step of 0.5 cm− 1 and on average,with 32 successive scans.Spectra were corrected between 1900 and 2400 cm− 1 to remove CO2 contribution,with an automatic baseline.The data were processed using OMNIC® software.It was decided to differentiate the internal face from the external face of the shiv to see its influence.X-ray diffraction analysis was performed on a Brüker D8 Advance device directly on a pellet of constant size,which was made of 0.1 g of hemp ground and compressed at a pressure of 10 tons.After drying at 50 ◦C,the pellet was removed from the oven and tested as soon as possible to reduce the impact of water absorption due to air exposure,which can affect the results.The data were analyzed using Specta Software.In order to study the ITZ between the hemp and the binder,the experiments were conducted on hemp pellets in contact with cement paste.The protocol was adapted and improved to tackle the issues encountered in previous studies like robustness,repeatability and accuracy,to monitor the formation of the ITZ from the moment the binder is finished being poured.The compositions of the cement pastes used in this study are presented in Table 1.The overnight data acquisition showed the formation of the ITZ.First,the pellet absorbs the water in the cement paste.Then it begins to dry,and the ITZ forms from its outer edges after approximately 5 h.The cement paste slowly becomes lighter in the vicinity of the pellet.The extracts of hemp seem to be released as soon as the pellet is placed into the cement paste,and percolate around the pellet over a distance of few millimeters.Then,the paste begins to set from the edges of the mold to the pellet,and the halo appears due to the presence of chemical components which delay or interfere with the setting.FTIR spectra of cement show that while similarities can be found between the ITZ and the hydrated matrix in terms of the micro-structural composition,like the presence of portlandite and O–H bending in cement hydrates,certain components are also in common with the anhydrous cement,like the presence of gypsum,alite,belite,and anhydrite.FTIR mapping shows the evolution of the area under the band relative to portlandite at 3640 cm− 1,with a base line from 3660 to 3600 cm− 1.Portlandite is chosen because it partially reflects the hydration of the cement.The color scale of the mapping ranges from blue,showing a very small vibration band area,through green,to red,highlighting a larger vibration band area.The lower part of the map corresponds to the area near the hemp pellet,as evidenced by the observation.