These functional properties are the result of the vegetal aggregates employed in the formulation,whilst mechanical strength is provided by the presence of frame.Numerous studies have been carried out on defining and understanding the properties of the material,or optimising the formulations used.A number of recent studies have looked at the durability and long-term behaviour of these insulators,but the accelerated aging processes are much too short or overly severe in comparison to real-world conditions of use,such as the application of freeze–thaw cycles.However,lack of knowledge about the evolution of the properties of plant-based concretes over time is a barrier to the development of these materials.Indeed,the lack of guarantee as to the durability of the desired properties over a given period of time does not inspire confidence in users,architects or insurance companies.It is for these reasons that a study is conducted on the durability of hemp-based concretes,with the aim of identifying the mechanisms which cause the functional properties of the material to change.However,to date,there has been no study done of the change over time observed in the plant aggregates on their own,although it is these particles which are responsible for a large proportion of the properties for which the material is used.In order to understand the mechanisms of aging of plant-based concretes,it seems essential to understand the behaviour of the particles in bulk over time.With a similar chemical composition,made up of cellulose,hemicellulose and lignins for the most part,it is possible to draw inspiration from the numerous studies examining the durability of wood.
Durability studies on wood have produced standards which guarantee quality and users’ expectations over a given period of time,mobile vertical rack in set usage conditions and with an amount of maintenance.Depending on the application,specific wood species are chosen according to the applications and the conditions of use.Various aging factors are applied on wood in these studies,such as immersion in water for periods from 10 days at 60 and 90 °C to 8 and 2000 years in natural conditions,variations of relative humidity during 20 days to one year,exposure to UV aging until four weeks or natural aging without protection during several years.Therefore,depending on the study in question,the aging conditions are very different in terms of environment or exposure duration.Whatever the protocols used,the functional properties are monitored over time,namely mostly mechanical and/or hygroscopic properties,but also acoustic properties.Overall,aging leads to deterioration in the mechanical properties and to an increase in water sorption capacity.Other parameters which characterise the microstructure of the material are also studied,such as porosity,swelling,or variations in weight of the materials.The variations of functional and microstructural properties are often explained by a change in the chemical compositions of the wood,usually due to the degradation of the lignin or hemicellulose.These chemical degradations may result from photo-oxidation or thermo-oxidation reactions,but above all,from the action of micro-organisms which can grow on lignocellulosic materials in high humidity conditions.Indeed,the micro-organisms use the cellulosic materials as a nutrient,and they may,selectively or otherwise,attack pectin,lignins,hemicellulose or cellulose.These micro-organisms are present in the air and in water,but also in the plants as they grow.Indeed,in order to separate the fibres from the stalks more easily,the material may be retted for a time,enabling the micro-organisms to consume the pectin which binds the fibres together.Therefore,such retting impacts upon the properties of the materials.There are various tests of resistance to the growth of micro-organisms,placing the plant materials inoculated with moulds in conditions of high hygrometry,as for example 26 °C and 85 and 95% RH,or 20 °C and 70% RH.
The result of an attack by micro-organisms is a loss of mass of the materials,which can lead to structural alterations which modify the functional properties,such as mechanical properties.In view of these results,three environments are chosen in this study to assess the durability of hemp aggregates used for vegetal concretes.The second one is an accelerated aging imposing variations in humidity.The relative humidity varies from 40% RH to 98% at a fixed temperature of 30 °C.Indeed,in the normal conditions of use of hemp concrete,unlike wood,hemp shiv should never be exposed to direct UV light or to liquid water.This temperature favours the development of microorganisms,which has been identified as a significant degradation factor.Finally,a harsher,outdoor aging process is used.The objective of this article is to identify the mechanism by which hemp shiv ages in the presence of moisture.The potential evolution of the functional properties of the material is characterised and linked to the micro-structural changes occurring in the different environments.The acoustic properties are measured using a Kundt tube.Two types of properties are obtained: the acoustic absorption co-efficient α and the transmission loss TL.The acoustic absorption co-efficient is the proportion of sound energy absorbed by a material.It is expressed for values ranging between 1 and 0.The transmission loss represents the sound insulation provided by a material.3 measurements are taken for each batch of bulk shiv over a frequency range [250–2000 Hz].At A0,the batches contain 40.7 g of hemp shiv,to obtain a controlled density of 130 kg·m−3 in the volume of the sample chamber.The different batches are aged and reused for the acoustic measurements.The hydric behaviour is determined on the basis of the water vapour sorption isotherms using a DVS machine.A few granules representing a few tens of mg are suspended in a microbalance within a sealed thermostatically controlled chamber at 25 °C.The schedule for the DVS is set to start at 0% RH up to 95%.A given relative humidity is applied until the weight change of the sample is less than 0.0005%.min−1 or during 12 h if the weight is not stabilised.Sorption isotherms are produced by plotting mass change against relative humidity and illustrate the water vapour sorption capacity as a function of the relative humidity.The overall sorption behaviour across the whole range of relative humidity values is simulated using the GAB model,as described in an earlier study.
Depending on the kind of porosity,it may be open and accessible for measurement,open but not accessible to measurement,or closed.With a pore size in the millimetre range,the interparticle porosity,represented in grey in Fig.3,is always open and accessible.On the other hand,the intraparticle porosity may contain closed pores and pores not accessible to the measurement.For simplicity’s sake,no distinction is drawn between intraparticle porosity which is closed or open but not accessible; both are denoted by Φintra_closed.This category is randomly represented in dark green in the diagram.The open and accessible intraparticle porosity is called Φintra_opened.By combining different techniques,it is possible to quantify the different volume fractions corresponding to the vegetal part or to each type of porosity.In all cases,these volume fractions are expressed as a function of a volume of bulk shiv.The bulk density ρ of the hemp shiv is the ratio of the mass of a batch of bulk hemp shiv to the volume it occupies in unstressed conditions.The density of the plant cell wall ρcell wall is an intrinsic value of the material,which depends on its chemical composition.In this case,the vegetal cell wall is expected not to contain any porosity.The volume corresponding to the vegetal cell walls of the material is measured by introducing a fluid into the pores in the material.Knowing the mass of the sample introduced,the density of the vegetal part can then be calculated.To measure this density,the hemp shiv is ground to a powder of grain size less than 500 μm in order to eliminate the porosities of the vegetal cell walls.The volume of the powder is then measured using a helium pycnometer.New energy storage systems have been continuously developed for replacement of fossil fuel to meet global energy requirements in the last decades.Multifunctional materials with an integration of structural and non-structural functions create new areas in the advanced energy storage system,such as structural super capacitor,which can maintain its capacitive function under a mechanical loading.At least two components of structural super capacitor are required to satisfy structural function such as structural electrode and structural electrolyte or separator.Maybe the structural electrolyte is more challenging,because that it must combine high ionic conductivity with good mechanical strength.However,it is generally known thationic conductivity and mechanical performance have an inverse relationship,because ionic conductivity requires pore structure for ion migration but strength needs compact structure.
A bicontinuous structure of electrolyte is designed to keep a good balance of performance,where one phase provides ionic conduction and the other phase is responsible for mechanical properties.The earliest structural electrolyte is polymer resin as structural component and ionic liquid based electrolyte as ionic conductive phase because polymer-based electrolyte has good mechanical properties,long lifetime,short charging time and wide potential window.Generally,polymer resin may contain of epoxy resin with more than two oxirane groups,polyethylene oxide or polyethylene glycon,while ionic liquid phase can combine several types of cations and anions.Shirshova et al.prepared ionic liquid-epoxy resin composites to obtain a structural electrolyte with ionic conductivity of 0.8 mS·cm−1 and a Young’s modulus of 0.2 GPa.Cole et al. reported polymer electrolyte-based stretchable super capacitors with a specific capacitance of 5–10 F·g−1 and mechanical stress of 0–3 MPa in suit microtensile characterization.Greenhalgh et al.fabricated structural super capacitors with epoxy which exhibited a compressive strength of 19.44 MPa and a specific capacitance of 4.5 m F·g−1.A novel structural super capacitor based on ionic liquid-polyester resin had a specific capacitance of 2.48 F·g−1,an equivalent series resistance Rs of 370 Ω and an in-plane shear strength of 102.4 MPa.However,the polymer electrolytes tend to have a low specific capacitance,and as the electrochemical property improves,the mechanical performance typically decreases,vice versa.Thus such polymer electrolyte may not well solve the challenge of balancing the electrochemical and mechanical properties of structural super capacitor.Recently,another kind of structural electrolytes were reported for building applications,cement as structural component and aqueous electrolyte as second phase.Normally cement as gel materials has high mechanical strength as well as prosperous pore structure.The pore size of pore structure can be divided into b20 nm,20–100 nm,100–200 nm and N200 nm,vertical grow rack and the amount of macropores in cement is N90% of total pores.As hydrated radius of aqueous electrolyte is generally smaller than 0.5 nm,such pore structure in cement is large enough to be filled with aqueous electrolyte to form structural electrolyte as well as to provide channels for ions passing,which is similar to the effect of pore size on the performance of electrodes.Zhang et al.reported a cement-based structural electrolyte with a specific capacitance of 10 F·g−1 and a compressive strength of 9.85 MPa.
Subsequently,Xu et al.fabricated a structural electrolyte based on geopolymer with a compressive strength of 33.85 MPa and a specific capacitance of 33.4 F·g−1.Ma et al.also reported a combining magnesium phosphate electrolyte with a compressive strength of 24.5 MPa and a specific capacitance of 40.9 F·g−1.Therefore,combining cement with aqueous electrolyte is a promising approach to develop structural electrolyte due to its low cost,structural integrity,chemical stability and good compatibility with structural component.Thus pore structure has an important role in the performance of cement-based structural electrolyte.The porosity and pore inter connectivity are likely to play critical roles in characterizing the pore structure of the structural electrolyte.Hemp fiber has been chosen to alter pore structure of cement based structural electrolyte in this work.There are two reasons for choosing the hemp fiber as pore forming agent.The first reason is that unique interconnected partially graphitic carbon nanosheets with significant volume fraction of mesoporosity has been created from hemp fiber,which exhibits excellent electrochemical performance in super capacitor and sodium ion battery.The other reason is that a large content of hemp fiber often leads to considerable number of macropores,which can result in forming interconnected pore structure in cement.Thus the effect of pore structure of structural electrolyte on the performance of structural super capacitor was investigated by adding various volume fractions of hemp fiber in this work.Typical SEM image of the structural electrolyte containing 15 vol% of hemp fiber is shown in Fig.3a and the corresponding EDX analysis result is listed in Fig.3d.It is clearly seen that hemp fiber is nonuniformly distributed in the matrix due to the large fiber content and non-homogeneous mix.Due to the presence of hemp fiber seriously interfering with image processing,careful selections of parts of SEM images without hemp fiber were used to measure the two parameters of pore structure,including porosity and pore connectivity,like Fig.3b.Image analysis technique is employed to process the SEM images to quantitatively characterize pore structure of the structural electrolyte with hemp fiber,like Fig.3c.