Statement of Novelty

  • A composite insulation material based on flax waste and liquid glass is proposed

  • Hygrothermal properties of the composite is competent among organic based insulations

  • Thermal conductivity of 0.0728 W m−1 K−1 was reached at 50% moisture while desorption

  • Surface crystallization and chemical spectrometry of the composite was demonstrated

Introduction

In the European Union, the number of new buildings is relatively low, and most buildings were built before the introduction of mandatory energy efficiency requirements such as Directive 2002/91/EU [1]. One of the most attractive alternatives for building owners is to invest in energy efficiency [2, 3]. The expected benefits for homeowners are therefore cost savings, lower energy consumption and reduced air pollution. From a technical point of view, there are several studies on energy savings in buildings based on new insulation envelopes with a low thermal conductivity coefficient of less than ≤ 0.015 W m−1 K−1 [4, 5], and taking into account the use of innovative materials such as mineral fiber insulation panels [6], vacuum insulation panels [7], thermochromic coverings [8] and alternative natural-based materials such as hempcrete [9]. However, such materials are expensive [10] and therefore inaccessible to the mass market. Additional pressure on property owners arose from the flare-up of the 2022 energy crisis in Europe due to the tightened political situation on the continent. As a result, prices for all types of energy sources have risen significantly on the European markets. In the first half of 2022, for example, electricity prices for private households in the EU increased the most in the Czech Republic (+ 61.8%) [11]. This means that the economic rationale for energy efficiency in buildings and structures has become more dramatic than ever.

Another, albeit old, issue is agricultural waste, in particular the problem of the efficient use of flax straw in agriculture. The two fiber crops grown in the European Union are flax and hemp, with flax grown on about 125,000 hectares and hemp on about 15,500 hectares in 2004 [12]. Due to its exceptional environmental flexibility, this plant is cultivated from the Mediterranean and North Africa to the Middle East, the Caucasus, India and Canada. The leaders in the cultivation of flax in Europe are France, Belgium and the Netherlands [13]. The Czech Republic ranks sixth in the cultivation and production of flax in the EU [14]. Every year, the cultivation of flax in Europe binds 250,000 tons of CO2. Flax is very sensitive to its immediate environment and has a natural positive effect on the soil. It improves the quality of the subsequent harvest by 20 to 30% [15]. There is currently a problem with recycling waste from industrial processing. The European Union generates about 1.3 billion tons of waste annually, of which 700 million tons is agricultural waste. There are problems with the disposal of this production waste that need to be solved effectively. An efficient solution is also needed for the issue of recycling these secondary production by-products [16]. For example, chopping flax straw (Linum usitatissimum L.) produces a waste product such as flax straw fibers—the woody part of the flax stalk, mainly in the form of small straws. Flax is cultivated for the oil from its seeds and the fibers from its stalks. The retting and decortication of the stalks results in three main fractions: long staple fibers, short (tow) fibers and woody core tissue. The woody tissue obtained from the flax harvest is called flax shives [17]. Flax strawis the main fiber flax by-products of fiber flax (about 50% of the flax biomass) [18]. In a flax spinning mill, 2.5–6 thousand tons of straw are produced annually, depending on capacity. The amount of straw per tons of raw material is up to 0.6 tons. Currently, flax straw has no purposeful use, so a significant part of it is landfilled [19].

At present, flax is not only used to produce cooking oil and fabrics for clothing and bedding. In line with ecological principles, the demand for new sources of biodegradable raw materials is increasing with the progress of civilization and the use of advanced technologies. Studies on flax as a potential source of natural, environmentally friendly industrial raw materials are underway [20].

The literature has long argued for the expediency of replacing wood in construction with various agricultural wastes, especially hemp, flax and wheat due to their 100% bio-based particleboard, thermal conductivity, thermal performance, sorption and mechanical properties [21]. Some researchers have analyzed these same agricultural by-products in terms of acoustic properties and have concluded that they exhibit excellent acoustic absorption, reverberation characteristics with appropriate material thickness which is sufficient for green building applications and meet various certifications such as LEED [22]. Flax straw fibers can be used in their raw form as loose thermal insulation of partitions in a timber frame structure. In the past, shives dry mixed with lime or clay was used as insulation for ceilings [23] and even in structural concrete blocks [24, 25]. Due to their low added value and easy availability, flax straw are also a real potential reinforcing ingredient in bio-composites [26]. The composition mixtures tested were possible to utilize as thermal insulation of partitions, structural elements or as finishing materials. For example, available research of Benmahiddine et al. [27] demonstrated a bio-based material with an average compressive strength of 0.8 MPa that was produced by flax concrete. It had a mass composition of 14.5% flax straw fibers in bulk, 35% binder (traditional PF70), and 1.45 water-to-binder ratio what allow utilizing this material as filler and insulation in buildings.

The number of agricultural byproducts produced by the flax scutching business is growing along with the areas under flax production. If so, flax straw fibers might even replace the popular wood flour used in wood plastic composites, and their addition to wood flour might help keep the cost of such materials down along with waste recycling. In fact, flax is an annual plant that grows by temporarily storing carbon dioxide [28]. A significant yield and the specific properties of straw and flax fires make it possible to orient the use of these agricultural wastes in the production of thermal insulation products. Based on the research [29] reducing the thermal conductivity of thermal insulation materials made of flax fibers at higher relative air humidity is made possible by the introduction of modified liquid glass, which lowers the absorption humidity. Some studies found that straw-glass composites have excellent water and fire resistance [46, 47]. The thermal conductivity coefficient reached no more than 0.052 W m−1 K−1 at a density of 200 kg m−3 and compressive strength at 10% deformation of at least 0.35 MPa [47]. In particular interest was the source where peat moss and liquid glass-based thermal insulating slabs have been created [30]. It has a density of between 155 and 170 kg m−3, and its thermal conductivity value is 0.03–0.04 W m−1 K−1. However, the material significantly shrinks and deforms during the drying process in an insulation slab made of moss. It was suggested that milled straw be added to the moss in the amount of 20–30% by weight to solve this issue according to authors.

Thus, the first and the main objective of the study is to find a compositional ratio of a binder (liquid glass) for a flax straw waste to obtain an insulating composite with competitive density value [kg m−3] and thermal conductivity coefficient λ [W m−1 K−1] among similar organic-based insulating materials. The secondary objective of current investigation is to determine such hygrothermal and physicochemical properties of the found insulating composite as: compressive strength at 10%-deformation [MPa]; Sorption moisture Wc [%] and thermal conductivity coefficient λ [W m−1 K−1] during sorption and desorption processes at 20–50–80–95–80–50–20 relative humidity in the camera [%]; comparison of obtained characteristics of cubic and panel samples; Heat transfer resistance for wall enclosing structures of 10 cm thickness [m2 K W−1]; Scanning electron microscope inspection of the surface topography and crystallization uniformity within the VF-parameter of 1–2 mm, 100–200 µm and < 70 µm; as well as the sum spectrum of chemical elements by weight in % on a surface of examined composite based on BSE-analysis.

Materials and Methods

Prior to use in the tests, flax straw fibers were purchased from a local farmer in the Czech Republic in a 10-kg bag and then ground by hand to a maximum particle size of 25 mm. It was stored and dried for 6 months under natural indoor conditions at constant temperature and humidity parameters (T = 18–25 °C, RH = 50–55%). Flax fiber with a size of 12–25 mm were used for the further experiments. Previous research [44] discovered that the average apparent density of flax straw is 120–135 kg m−3, the porosity is 85% and the thermal conductivity coefficient ranges between 0.048 and 0.085 W m−1 K−1 at volumetric humidity 0–10%. According to another study [45] the water absorption capacity of flax at 24 h is 350 ± 11%. The shavings were sieved through a sieve with a mesh size of 10 mm to separate inclusions of straw dust and a very fine fraction of up to 12 mm, taking into account the diagonal of the mesh.

Liquid glass as a potentially promising binder has a relatively low density, high strength and minimal water absorption [46, 47]. Moreover, curing in the liquid glass composite during the molding process prevents the restoration of viscoelastic deformation. The value of the restored elastic compression deformation (under a specific pressure of 0.02 MPa) is only 5%. This is a favorable fact for the technology of molding, including the operation of pressing before curing the material [46]. Thus, sodium silicate or water (liquid) glass, chemical formula Na2O(SiO2) is the binder for the proposed organic insulating composite in the form of a liquid solution with a density of 1.30 g cm−3. Liquid glass is an inorganic but mineral of natural origin with excellent binding properties and sufficient flowability for mixing with a base flax straw. It also acts as a mineralizer for the filler. Liquid glass provides non-combustibility and bio stability of the material. For these reasons, we consider it one of the most suitable and complementary binders for combination with flax straw in the current study.

In the Table 1, we have established the following successive tasks and suitable methods to fulfil them.

Table 1 Tasks and the main methods for investigating the properties of the composite
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Composite Material

The amount of liquid glass in the mixture was varied in order to achieve complete wetting of the surface of the filler fibers and thus improve the quality of adhesion of the fibers to the binder. At this stage of the experiments, the optimum proportion of aggregate was selected by the experimental method. Compositions based on flax fires and a complex binder of liquid glass was developed to determine the efficiency of the aggregate with a binder. Based on the research results, the amount of binder in the mixture was changed at each new stage of the experiment to determine the optimal content.

The optimal mixing ratio of plant filler and binder was set according to the average value of the density and compressive strength at 10% deformation. The dosage of liquid glass was varied in order to find the optimal amount of binder in the experimental compositions. The lower amount limit of the binder was to find due to the necessity of complete coating of filler surface and the upper amount limit was set according to the average value of the density and compressive strength at 10% deformation of the material samples. The experimental 3 compositions were prepared according to the regulation EN 826 (2013) [31]. At first the particulate ingredients were thoroughly mixed for 2 min then the binder was added and the whole mixture was mixed for 2–3 min until homogeneous. The formation of samples was produced into the 100 × 100 × 100 mm metal forms by manual compression. The mixture is fed into the forms in 3 layers. Samples were kept in forms during 24 h under normal conditions. After removal the samples were dried to constant weight under the temperature of 60–70 °C. The bulk density was determined by calculating the ratio of the sample mass after drying to a constant weight to volume ratio of the sample. The samples were adjusted to the destruction of 0.4-ton press with the speed of lifting load 4 kg per second. The compressive strength at 10% deformation was determined by ratio of breaking load to the cross sectional area of the sample. The compressive strength test was performed in referring to European standard EN 826:2013 “Thermal insulating products for building applications—determination of compression behavior” [31]. The compressive strength was provided using automatic compression tester for cubes and cylinders CONTROLS, model 50–C99/B (for cubes up to 200 mm, and capacity from 0.5 to 500 kN). The composite content will be determined by applying the rule of greatest increment in resistance to deformation relative to the increase in the proportion of the binder component. This can be represented in particular as:

$$\left( {\frac{{y_{i + 1} - { }y_{i} }}{{y_{i} }} \times 100} \right) > \left( {\frac{{x_{i + 1} - { }x_{i} }}{{x_{i} }} \times 100} \right) \to y_{i + 1} ,\, i = \left[ {1.6} \right]$$
(1)

where, yi is a mass fraction of liquid glass Na2O(SiO2) in composition at sample number i; xi is a compressive strength in MPa at sample number i, when i = [1, 6] is a sample number from 1 to 6.

Sample Formation Technique

The compositions of lightweight flax straw insulation was formed in the following order. First, the straw component was carefully mixed for 2 min, then liquid glass Na2O(SiO2) was added in the form of a liquid solution as a binder for a lightweight composite and the whole mixture was stirred 2–3 min until a homogeneous mass was obtained. The formation of samples was produced into the 100 × 100 × 100 mm metal forms by manual compression. The mixture is fed into the forms in 3 layers. The drying of the samples was based on the Suggested Protocols for Drying Samples to Constant Weight [32] and was proceeding in two stages: (a) during natural indoor drying for 48 h with relative air humidity 50% and ambient temperature of 20 °C in the form with the following demolding and (b) putting the samples in the drying camera for 48 h under the temperature of 60 °C and air humidity of 50% till the stable weight achieved or equilibrium moisture content achieved. Equilibrium moisture content is moisture content of a porous material in equilibrium with the environment and the relative humidity of the ambient air, at a specified temperature [33]. Drying to constant weight means that the drying process should be continued until two successive weightings, the second of which is carried out after an additional drying period corresponding to the nature and amount of the residue, differ by more than 0.50 mg per 1 g of substance taken [34]. That corresponds to 0.05% of the maximum possible change in weight when reaching a constant mass of material. For example, for a sample weighing 500 grams, the change in weight after drying should not exceed 0.25 grams.

Studying Hygrothermal Properties

The planned experiment on the stage of scientific research refers to laboratory experiments, the task of which is to study the general patterns of the sorption process—the increasing of the humidity of the material under investigation due to the absorption of water vapor from the surrounding air.

The degree of sorption humidification of building materials is expressed by the humidity content (%). Certain parameters of the air environment correspond to a certain degree of sorption humidity of the material, which is established as a result of achieving a dynamic equilibrium between the air and the material. When the relative humidity of the air rises or when the temperature decreases, the sorption humidity of the material rises.

The relationship between the relative humidity of the air and the sorption humidity of the material at constant temperature is expressed by sorption isotherms, which are used in calculating the moisture regime of the enclosing structure.

Each material is characterized by an individual family of isothermal curves of sorption humidification. Since a single functional dependence of sorption humidification of such composite samples has not been studied before, their sorption isotherms were constructed on the basis of experimental data and used in calculating the moisture regime of enclosing structures.

To determine the sorption humidity of the materials it was used:

  • Laboratory scales KERN 572 (max. measurement mass m = 3010 g; deviations d = 0.01 g);

  • Climate chamber BINDER, model KBF 240.

These experimental studies are realized on the basis of the laboratory research.

The procedure for conducting an experiment to determine the sorption humidity of building materials is regulated by ISO 12571:2013 [33] and includes several stages under the humidity of environment (20%, 50%, 80% and 95%) during absorption and desorption processes at the constant temperature of 20 °C. The expected required characteristics are to be studied to build the sorption isotherm of material what is essential to assist the quality of the flax straw insulation.

The sorption moisture of a sample of material Wc (%) is calculated by the formula:

$$\left| {{\text{W}}_{{\text{c}}} } \right| = \left| {\frac{m1 - m2}{{m2}} \times 100} \right|$$
(2)

where m1—the mass of the sample after sorption, g; m2—mass of sample after drying to constant weight, g.

The results of the material test show the dependence of the sorption moisture of the material on the relative humidity of air at temperature of 20 °C [35].

Experiments were conducted in laboratory conditions using ISOMET 2114 (Thermal properties analyzer) and are associated with monotonous measurements that were repeated sequentially with a frequency of one measurement a week during 8 consecutive months of 2021.

ISOMET 2114 is a hand-held measuring instrument for direct measurement of heat transfer properties of a wide range of isotropic materials including cellular insulating materials, plastics, glasses and minerals [35]. It is equipped with two types of measurement probes: needle probes for soft materials, surface probes for hard materials. It applies a dynamic measurement method, which enables reducing the measurement time in comparison with steady state measurement methods. The appliance conforms to the Low Voltage Directive: 2006/95/EC of 2006, EMC Directive 2004/108/EC of 2004, safety product standard EN 61010-1 of 2003 and EMC EN 61326 of 1997. According to User’s guide of ISOMET 2114, it has accuracy 5% of reading + 0.001 W m−1 K−1 for thermal conductivity measurements in a range of 0.015–0.70 W m−1 K−1, 10% of reading in a range of 0.70–6.0 W m−1 K−1 and measurement reproducibility 3% of reading + 0.001 W m−1 K−1. The accuracy and reproducibility performances are within the statistically acceptable range of the applied measurements of the study and do not sufficiently affect the achieved values and core statements of the research.

The measurement is based on the analysis of the temperature response of the material under investigation to heat flow impulses. The heat flux is excited by the electrical heating of a heating resistor inserted into the probe, which is in direct thermal contact with the sample under investigation. The evaluation of thermal conductivity and volumetric heat capacity is based on periodically sampled temperature records as a function of time, provided that heat propagation occurs in an unbounded medium [35]. The procedure for measuring thermal conductivity (λ) is shown in Figure 1.

Fig. 1
figure 1

Application of thermal properties analyzer ISOMET 2114 for samples P1, A1, A2, A3 (made by authors)

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Scanning Electron Microscope Analyses

Images of the composite surface were obtained using a scanning electron microscope (SEM) VEGA 3 SBU (TESCAN) with a secondary electron mode (SE) for topography surface inspection and energy dispersive X-ray spectroscopy (EDS) with a retractable Backscattered Electron (BSE) detector for sample composition information. SEM analysis was performed on the outer and inner surfaces of the flax straw particles with a size of 6.5 × 1 mm. The analysis was performed in the high vacuum pressure chamber (< 1×10−2 Pa). A 30 kV electron beam with a resolution of 3.0 nm (arbitrary unit, defined by Vega3 TESCAN) [36] and a working distance between 14.87 and 18.98 mm was used. The incident probe current, contrast and brightness levels were adjusted to obtain surface imaging (SE images) and dispersive spectroscopy (BSE images) with good quality for further processing. Each separate microscope image contains microscope mode information including detector mode, working distance in mm, view field in mm, scale, and zoom magnification.

Results

The flax fibers have a rough surface and platelet-shaped particles that provide fairly good adhesion to the binder and good compressive strength due to the dense structure of the material and the small size of the flax fibers. After the 10% compression test, the percentage of binder by weight of the flax in the samples is shown in Table 2. The compressive strength of the material depends on the adhesion of the filler to the matrix. It was found that the structure of the material with a large number of large pores negatively affects the strength of the material. More effective is the use of fine-grained, porous filler such as flax fibers. The resulting dependence of the compressive strength on the amount of complex binder in the composition is illustrated in Figure 2.

Table 2 The range of testing samples of compositions based on flax straw fibers
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Fig. 2
figure 2

Compressive strength of samples at 10%-deformation, MPa (made by authors)

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The lower limit of the binder consumption is due to the need to completely cover the aggregate surface, and the upper limit is due to the average density of the material (no more than 400 kg m−3).

This study proposes a basic composite composition in a ratio of 1:3 by weight of the components, that is, two parts of the binder to one part of the main, to create 3 cube-shaped samples with dimensions of 100 × 100 × 100 for further testing instructed by EN 12571:2013 [33]. Based on the resulting graph and Formula 1, the most acceptable ratio is 1:2.65 (the increase in compression strength + 33.3% while the mass fraction of binder was grown by 15.2%), in which the composite gained more strength with a slight increase in weight, however, for practical purposes the ratio was increased to a flat fractional value of 1:3. It was also decided to make one additional experimental sample in the form of a panel with a size of 200 × 250 × 35 mm and a composite mixture of 1:2, although the liquid glass mass fraction curve had a positive slope up to a ratio of 1.85 according to Figure 2 and Formula 1 (increase in both compressive strength by 11.1% and liquid glass mass fraction by + 8.8% respectively) However, as in the previous case, we align it to a ratio of 1 to 2. Thus, the mass fraction of liquid glass was only 2 times higher than the base material in order to also conduct two tests for sorption moisture and thermal conductivity, and the results obtained are compared. According to the recommendations of another study [46], the best straw-to-glass ratio is 1:2.25, however our current analysis reveals 1.85. We chose a roughly average ratio of 1:2 between these variables, taking into account, in addition, the ease of computation and practical implementation.

Panel is a more commonly used shape for the construction industry, especially in insulating materials and this experiment would give signals of how much the shape could behave in the same tests but with less binder. Thus we prepared 4 separate testing samples (Table 3).

Table 3 Composition of the mixture
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The raw material—flax straw, formation process and received composite—flax straw insulation, in a form of panel (Figure 3a, b) and in a form of cubes (Figure 4a, b).

Fig. 3
figure 3

Flax straw composite insulation in a form of panel (FSI P) (made by authors): a fresh mixture in a form; b dried P-sample; (made by authors)

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Fig. 4
figure 4

Flax straw composite insulation in a form of cubes (FSI A) (made by authors): a fresh mixture in the forms; b dried A-samples; (made by authors)

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Based on the dimensions of the FSI P and A samples, their volume were VP1 = 0.00175 m3 and VA1,A2,A3 = 0.000864 m3 relatively. Having the volume of the samples and given their masses immediately before the test, we calculated their density in kg m−3 (Table 4). The samples were compressed manually and therefore they had different densities despite on the same material composition and volume of cubic samples.

Table 4 Parameters of samples for test
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The aggregated measurement results of the laboratory study are shown in the Table 5.

Table 5 Initial and studying parameters of the test
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In the process of gaining moisture up to the fourth test stage of the experiment, the samples gained their constant mass and then started losing as a result of desorption from the fifth test stage to the seventh one. During each stage of the experiment, the shape of the samples remained unchanged, and the dimensions of the sides, i.e. width, length, and height in millimeters (mm) changed insignificantly ± 1% of their initial sizes, depending on sorption or desorption.

Graphs of sorption moisture or changes in moisture content in the samples were demonstrated in the Figure 5. Despite the fact that at the first test stage of the humidity regime of the climatic chamber, the internal humidity especially between A- samples differed by tens of times and between some of them by hundreds of times (Test stage 1: Wc1 = 0.003%, Wc1 = 0.098%, Wc1 = 0.25%), at the subsequent stages of the experiment it was relatively evened out and differed insignificantly (Test stage 7: Wc1 = 0.24%, Wc1 = 0.52%, Wc1 = 0.18%).

Fig. 5
figure 5

Sorption and desorption isotherms for samples (A1, A2, A3 and P1) at 20 °C

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The big difference after the first stage is explained by the overall high organic capacity of the organic matter, the unevenness of its structure, as well as the different weights due to manual tamping of the samples. However, the pattern of sorption moisture for all samples under all 7 humidity regimes of the climatic camera is quite similar.

On the basis of another measurement, a graph of the thermal conductivity for all four samples was plotted at the Figure 6 during all humidity regimes of the experiment meaning sorption and desorption stages at constant 20 °C. The behavior of the curves for all four samples was very similar and followed the same pattern. The thermal conductivity coefficient in all samples decreased as the samples dried and was fixed at regime of sorption at the humidity level of 80%, remained unchanged and steady at 95% humidity and during the fifth test stage of desorption.

Fig. 6
figure 6

The thermal conductivity coefficient of samples (P1, A1, A2 and A3) at sorption and desorption

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The coefficient fluctuated insufficiently within 0.14–0.15 W m−1 K−1 during 3–5 test stages. The thermal conductivity reached its minimum value in all four samples that reached a constant mass at a humidity level of the climatic chamber of 50% during desorption process. Notably that with further drying of the samples to the next humidity regime of 20%, a dynamic increase of the thermal conductivity was established.

Inspection of the Topography and Composition

The topography surface of the studied material was acquired on the SE-mode of the SEM VEGA 3 SBU, TESCAN. Images of two composite formulations with two sides of flax straw particles were depicted in Appendix 1 and 2 by three level of magnification expressed in a view field parameter. Pictures are grouped by VF-parameter of 1–2 mm, 100–200 µm and < 70 µm for visual comparing all sides of two composites in an equivalent zooming. Within the VF-parameter of 1–2 mm in both composites of two sides, an inhomogeneous organic structure of the composite material is observed. In the 100–200 µm view field images, there is quite uniform scattering of glass crystals is noticeable, covering approximately 15–20% of the surface on the hemp particle. The images with a VF of < 70 µm clearly show the crystallization of the surface by Na2O(SiO2) substance. These crystals carry the stability and strengthening of the proposed material. To make sure that this visual crystal-like coating is precisely the added liquid glass, spectral illumination of chemical elements were made (Figure 7a, b) based on the example of the original SE-mode image (Figure 8), which was further divided into 7 random squares for spectrum BSE analysis.

Fig. 7
figure 7

a, b Spectral illumination of chemical elements on the surface of the composite sample A2

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Fig. 8
figure 8

a Scanned surface image by electronic microscope with 7 random squares for spectral analysis by EDS; b Enlarged scale of the material surface in the location of sectors 2 and 3

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The total chemical composition of the chemical elements in these 9 squares is collected and provided by the standard SEM microscope software AZtec (Figure 9). EDS targets analysis with performance which is defined by the number of x-ray counts that are collected. Thus, the figure shows the percentage range of the elements in the composite mass and visualized in the keV (kilo-electron-volt) accelerating voltage range used for EDS analysis. The number of electrons is counted in seconds per electron-volt., cps.e−1 V−1 (counts per second per electron-volt). By this measure there is possible to identify the quantity volume of elements on the composite surface of 2–5 µm in depth.

Fig. 9
figure 9

Map sum spectrum of chemical elements on a surface of studied composite based on BSE-analysis

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Herein, we can observe that the number of Silicon (Si) electrons which are reflected from the surface of the composite is the largest among other elements and natrium, which is a part of a liquid glass is on the third largest amount by element volume. That confirms the validity of a wide coverage of the surface of an organic flax particle with a liquid glass Na2O(SiO2). Based on the data from the software Aztec (Table 6), the content of silicon (silicium) element (Si), albite (Na) elements and oxygen compounds (O) in a weight proportion are 11.3%, 6.39% and 47.73% relatively, what is 65.42% in a weight of the composite. This confirms the composition of the particle of sample A, which was made in a ratio of 1:2 (flax straw-liquid glass (Na2O(SiO2)). It confirms that the weight of liquid glass is approximately 66% (2 mass fractions) and organic matter is 33% (1 mass fraction) in the studied composite. Since this is an organic material, the confirmed percentage of the mass fraction may fluctuate and in our sample it fluctuates within about 1% only.

Table 6 Content of elements by weight in all 7 sectors (Fig. 8a), %
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In frames of the main objective, there is possible to compare the found composite material with other organic-based insulating materials in the construction sector. The study of Balčiūnas et al. [37] stated that thermal conductivity of organic composites depends on density on 97% and the factors are demonstrated regressive relation. Such composites usually show thermal conductivity from 0.055 to 0.076 W m−1 K−1 within the density range between 210 and 410 kg m−3. According to the results of the investigation and related research, we compare the thermal conductivity coefficient by both absolute equivalent meanings of the density and by heat transfer resistance of wall enclosing structures of 10 cm thickness among suggested and existing organic-based insulation materials (Table 7).

Table 7 Thermal conductivity of the studied and existing insulation materials at atmospheric pressure and around 20 °C
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Thus, when comparing the proposed composite with other insulating materials of organic origin, the thermal conductivity index is comparable and is at equivalent value. The proposed composite of flax and sodium silicate may represent a competitive material on the market of organic insulation materials by both the coefficient of thermal conductivity which is average with 0.0728 [W m−1 K−1] within 0.04–0.102 [W m−1 K−1] of other comparable materials and the value of heat transfer resistance for wall enclosing structures of 10 cm thickness which is far from the worse level of 1.53 [m2 K W−1] between 1.27 and 2.66 [m2 K W−1] of similar organic insulations.

In current research, thermal conductivity coefficient reaches its minimum value not at the minimum value of the achieved constant mass per the same volume (density) of P and A samples. Other studies’ findings basically show a clear relationship between thermal conductivity and density and a high sensitivity of such materials to changes in humidity [48]. There was also observed a significant factor for the demonstrated thermal conductivity coefficient minimum value not being at the minimum value of the achieved constant mass per the same volume (density). In other studies [49, 50], the classical increase in thermal conductivity with density is observed, however it is also highlighted that the binder type, the fiber type deviate the value of thermal conductivity from the variation law.

The prospects for using such a material are not limited to only insulating properties. According to some literature [37, 42, 43], the mixes that contained a lot of flax straw fibers displayed well porous-like acoustic absorption. The composite also has good prospects for use in areas requiring high soundproofing characteristics. This application, however, requires additional research on the subject of acoustic characteristics.

Conclusions

  1. 1.

    The study proposed a natural inorganic binder (Na2O (SiO2), liquid glass) for mechanically prepared flax straw waste for creation a comparable and competitive organic-based insulating material for the construction sector. The optimal composition of such insulation material was demonstrated during the compression test. Its density, with the best thermal conductivity coefficient of 0.0728 W m−1 K−1 was 342.7 kg m−3 and achieved at 50% relative humidity condition while desorption process. During a comparative investigation of similar materials, the suggested composite was found to be one of the lightest in terms of the density among seven evaluated materials in the range of 224–620 kg m−3. When comparing the thermal conductivity coefficient within composites of 0.04–0.135 W m−1 K−1, it was on a par with such materials as hempcrete and calcium silicate with 0.06 and 0.065 W m−1 K−1, respectively. Heat transfer resistance for wall enclosing structures of 10 cm thickness [m2 K W−1] with a value of 1.53 [m2 K W−1] in the range of all materials 0.9–2.66 [m2 K W−1] was also a good average result. As a conclusion, one may claim that the study presented a competitive insulating material with highly competitive hygrothermal characteristics.

  2. 2.

    Despite the fact that the panel form had a less strong structure, i.e. the wall thickness of the sample was smaller than the cubed samples, and the ratio of liquid glass was one third less (ratio 1:2) in the panel sample than in cubic ones (ratio 1:3). It was found that the change in the structure of forms during absorption and desorption occurred approximately in the same ratio. For example, although the sample of a cubic form A1 achieved the lowest value of the thermal conductivity coefficient (λ = 0.0728 W m−1 K−1) during desorption at RH = 50%, the sample of a panel form P1 showed a very close value to this meaning of λ = 0.0787 W m−1 K−1. This opens up a wide range of possibilities for employing a panel-shaped composite for insulating reasons. It suggests that the shape of the composite does not affect the deformation of the material during the changes in humidity, and also this difference in the ratio of parts of liquid glass in the composite does not significantly affect. This produces a reasoned prerequisite for continuing the experiment with the composition of the material in the likely direction of reducing the part of liquid glass in it, as well as the search for more practical forms of insulating panels, even in the direction of reducing its thickness.

  3. 3.

    According to results, the average mass of A-samples per m3 of the same size is 353 kg, which is on average by 57.23 kg or 19.34% heavier than the P-sample. This difference is explained by the higher content of liquid glass in the A-samples which have a ratio of 1:3, while in the P-sample this ratio is only 1:2. However, it is worth to note that the difference between the A and P-samples in thermal conductivity and sorption moisture were not so significant, which means there is probably an opportunity for further reducing the mass of the insulating material without losing thermal and hygroscopic properties.

  4. 4.

    The investigation generally confirms the theory assuming that thermal conductivity has a directly proportional dependence on the achieved mass of samples or their density (kg m−3), i.e. the greater density of samples associated with absorption, the greater the thermal conductivity and vice versa.

  5. 5.

    The obtained data were initially analyzed in terms of the sorption and thermal characteristics of the proposed composite material based on flax straw as agricultural waste and can be used in practice for the design of effective environmentally friendly envelopes of buildings, particularly in agricultural areas. Furthermore, thermal insulating materials based on plant fibers are quite affordable and can alleviate the problem of agricultural waste disposal.