Currently, new restrictions on CO2 emissions and energy consumption are being introduced worldwide. Many countries are implementing innovative solutions to reduce CO2 emissions, as well as material and energy consumption. The aim of this study is to investigate innovative geopolymer-based adhesives for the production of layered building envelopes. The adhesive is reinforced with composite ribs and contains natural fibers. The natural materials used to manufacture the samples are completely biodegradable. The geopolymer was alkali activated using 10 M sodium hydroxide solution and aqueous sodium silicate solution. The aim of this study is to compare and determine the thermal insulation properties of natural fiber-based materials such as coconut felt, jute felt, hemp felt, flax felt, hemp wool, flax-jute wool and to determine the effect of these materials on geopolymer composites. The composite material uses 4 layers of natural insulation and is reinforced with fiberglass ribs. The publication presents the results of physicochemical studies of geopolymer precursors and natural thermal insulation materials, studies of the thermophysical properties of fibers, felts, felt and wool, morphological studies of the structure and texture of fibers, as well as studies of the physical and thermophysical properties of finished multilayer partitions. The results show that these materials have great potential in the production of prefabricated structures and as structural insulation. The thermal conductivity of the composite made using four layers of natural fibers increased by 40% (the thermal conductivity decreased from 1.36 W/m × K to about 0.8 W/m × K). It is expected that the results of this study will find future application in the field of energy-efficient and low-carbon buildings.
Years of scientific research have confirmed that the main cause and driver of climate change is the greenhouse effect, which is a phenomenon in which the earth’s surface temperature rises due to greenhouse gases in the atmosphere. Greenhouse gases include methane, nitrous oxide, fluorinated greenhouse gases and carbon dioxide (CO2). Carbon dioxide is produced by human activities and is the main contributor to global warming1,2,3,4. Carbon dioxide concentrations are currently at their highest level in 14 million years and continue to increase steadily. Figure 1 shows the growth trend of global CO2 emissions from energy production and industry between 1900 and 20225,6,7. The graph shows that the growth of CO2 emissions is significant and new solutions are needed to reduce the impact of climate change. The new concept should contribute to climate protection and have a positive impact on the possibilities for adaptation and mitigation of climate change.
The construction industry is one of the key areas where a range of measures can be taken to reduce CO2 emissions. Buildings account for 30% of global final energy consumption and 26% of global energy-related emissions (8% of direct emissions from buildings and 18% of indirect emissions from electricity and heat production in buildings). According to the International Energy Agency (IEA), direct emissions from the building sector fell in 2022 compared to the previous year, although extreme temperatures led to an increase in heating-related emissions in some regions. Energy consumption in the building sector grew by around 1% in 2022. Many countries are developing increasingly stringent minimum performance standards and building energy codes. The use of increasingly efficient and renewable building technologies is accelerating, but the industry needs to change faster to achieve net zero emissions (NZE) by 2050. The current decade is critical to implement the measures needed to achieve the target of using zero-carbon technologies in all new buildings and 20% of existing buildings by 20308.
Examples of regulations aimed at reducing CO2 emissions include the REPowerEU scheme and Fit for 559,10. In the construction industry, the concept of sustainable construction is of vital importance11, with particular attention to the impact of materials and products on the environment. This requires not only a reduction in carbon emissions, but also a careful use of natural resources12,13,14. CO2 emissions are not only due to insufficient insulation and inefficiency in buildings, but also to the production of building materials. Existing environmental reports and scientific studies show that the entire construction industry produces 38% of global CO2 emissions. According to various estimates presented in the Climate Policy Report, the cement industry produces around 5–6% of global CO2 emissions. However, it is important to remember that when analysing the construction industry from this perspective, it is not only the cement industry that produces large volumes of CO2, but also the construction chemicals and insulation materials used in the construction of buildings.15,16 The International Energy Agency report notes that the direct CO2 intensity of cement production has remained stable over the past five years, but increased slightly (by 1%) in 2022. In contrast, to achieve net zero emissions (NZE) by 2050, the industry needs to achieve an average annual reduction in CO2 intensity of 4% by 2030. For example, reducing the clinker-to-cement ratio through the use of clinker substitutes, further improving energy efficiency, switching to low-carbon fuels, improving material efficiency and adopting innovative technologies such as carbon capture and storage (CCS) are considered as ways to achieve this goal17.
A practical solution to improve alternatives to cement, produce low-carbon binders and increase energy efficiency is to use materials with significantly lower CO2 emissions and recycle difficult-to-recycle waste from the energy industry. In addition, these materials can also provide effective thermal insulation through the use of renewable biomaterials. When we talk about geopolymer materials, we mean composite materials that are made from natural fibers or other renewable materials that are usually obtained from waste. There have been many studies on the use of natural materials in building composites. The use of biomass waste and plant fibers as fillers for traditional adhesives has demonstrated the environmental potential to reduce overall energy consumption and carbon emissions in the construction industry. For example, coffee grounds can be used and studies have shown that adding just 5% of coffee grounds (SCG) by weight can increase the thermal insulation performance of mortar by 58% and reduce its thermal diffusivity by 34%. Thus, SCG is a promising biowaste for the development of impact-resistant thermal insulation solutions on an industrial scale18. In addition, tea production waste is an attractive material, as studies have shown that the addition of ST (tea production waste) can significantly reduce the thermal conductivity (67%), thermal diffusivity (57%) and density (24%). The addition of up to 2.5 wt% ST can meet the structural requirements of lightweight concrete (> 3.5 MPa) and can be used for wall structures, while composites with 7.5 wt% ST can be used for thermal insulation purposes19. Another interesting example of a material is olive oil production waste. Studies have shown that the use of olive pomace can reduce the thermal conductivity of composite materials by 55.68% and increase the compressive strength by 81.7%20.
Studies conducted in Morocco have shown that the use of 4% straw by weight in plaster reduces its thermal conductivity by 68%. Energy simulations conducted in six climate zones of Morocco using the Type56a module of TRNSYS software concluded that the implementation of the proposed solution could reduce greenhouse gas emissions from buildings by 13% and reduce annual costs by approximately 13% across all climate zones of Morocco21. Studies were also conducted on the addition of Pennisetum sphaerocephalum (PS) fibers and their effect on the thermomechanical properties of adobe. The experiments showed that adobe with the addition of polystyrene fibers can be used as an environmentally friendly thermal mass component for modern buildings, contributing to thermal insulation22. The topic of using natural fibers in innovative geopolymer materials is well known and has been widely covered in the literature23,24,25,26.
Although the use of biomass-based waste or natural fibres has a positive effect on the performance of building materials, it is not without its drawbacks and limitations. Natural fibres in cementitious or geopolymer matrices can degrade significantly27,28,29. As described in numerous scientific papers30,31,32,33,34,35, the addition of fibres to adhesives may require appropriate treatment to enhance their durability or improve the bond quality between fibres and matrix. Cellulose is the compound that determines the strength of the fibre, while lignin and hemicellulose are responsible for its low durability, so it is usually necessary to treat it, usually with alkaline solutions, to improve the adhesion of the matrix to the fibre at the contact zone. For example, fibres such as açaí, jute, sisal, bamboo and kura have been shown to be useful as fillers in cementitious matrices after appropriate treatment36.
This paper summarizes the mechanism of alkaline degradation, influencing factors and approaches to improve PF in alkaline matrices. The advantages of PF are wide availability, low price, energy saving and environmental protection, but its structure and properties lead to degradation of the alkaline matrix. Increasing the durability of PF-reinforced geopolymers and slowing down the degradation of the matrix are the key to their large-scale application in construction. The main conclusions are as follows: the hygroscopicity of PF affects its mechanical properties, but this property can be used to improve the curing efficiency in the composite matrix, thereby increasing the strength of the matrix. A certain degree of alkaline degradation of PF is observed in the geopolymer matrix, and the degree of degradation is closely related to the alkalinity of the geopolymer. Under the action of calcium hydroxide, PF can cause degradation and mineralization of the cement matrix. The geopolymer matrix significantly slowed down the degradation of PF compared with the cement matrix. The most direct way to reduce the degradation of PF in the matrix is ​​to further reduce the pH value and the content of calcium hydroxide in the matrix. The alkaline degradation of phenolic resin (PR) in the matrix has a negative effect on the mechanical properties of composites, while the degradation effect of fibers can be mitigated by chemical modification. Nanomaterials can improve the microstructure of the composite matrix, accelerate the polymerization reaction of the matrix, increase the content of matrix gel, increase the density of the matrix, and improve the bond between the fiber and the matrix, thereby reducing the degradation rate of PF. In fact, the PF used in the matrix is ​​divided into recycled fiber and virgin fiber, straw fiber and coconut shell fiber are waste, and the full utilization of waste fibers has important environmental significance. In addition, replacing cement with geopolymer is an effective method to slow down the degradation of PF in the matrix. At the same time, low-alkali activators should be used in the geopolymer matrix. Currently, there are relatively few quantitative studies on the effect of geopolymer mixing ratio on fibre degradation, so this should be one of the future research directions37.
This paper investigates multilayer geopolymer composites containing natural fiber insulation mats by analyzing the advantages and limitations of adding fibers to geopolymer matrices, and to develop an environmentally friendly innovative solution in the field of construction and insulation for the construction industry. By using layered composite materials in which the fiber layers are in the form of relatively thick aggregates, it is possible to reduce the degradation of natural fibers due to the fact that the matrix is ​​only in contact with the surface layer of the used mat. In this case, if degradation occurs, only a small part of the used fibers will degrade. The use of natural fibers, a renewable material, can benefit the environment, since these materials can replace the commonly used plastic insulation or mineral wool. The tests carried out have shown that the thermal insulation performance of such mats and felts made of natural fibers does not differ from that of commonly used materials (the thermal conductivity ranges are the same). Furthermore, the fabrication of multilayer composites provides the additional benefit of improved insulating properties due to the presence of multiple resistances at the boundaries between different phases/materials, which is confirmed by studies by other authors38,39.
Multilayer geopolymer materials based on artificial raw materials and natural fibers, if implemented, will be the answer to the existing problems of the modern world, namely the reduction of carbon dioxide emissions and, accordingly, the improvement of construction standards. The multilayer structure is characterized by an outer shell consisting of several high-strength outer layers and one or more low-density inner layers40. Geopolymers are the best alternative to currently popular materials based on Portland cement, which is confirmed by numerous publications. The geopolymer production process is more economical and environmentally friendly due to two aspects. It is estimated that the production of geopolymers can reduce carbon dioxide and other greenhouse gas emissions by up to 10 times and consume 4-8 times less electricity compared to traditional building materials using Portland cement. Geopolymers also have a smaller carbon footprint and fit into the circular economy. The properties of these materials are similar to those of commercially available products, but they are more environmentally friendly and energy efficient in terms of climate protection. The use of natural fibres is another environmentally friendly factor, as fibres are a readily available, renewable material that performs as well as or better than existing building insulation solutions41,42,43.
The objective of this publication is to conduct fundamental research on natural plant-based materials and fly ash as the main raw material for the production of multilayer geopolymer composites. In addition, the finished prototypes of the materials are presented, as well as their main physical and thermal properties, which are crucial for the optimization of new solutions. It is important to note that this is the first article in a series and will cover the basics of the research. Due to the breadth of the topic, it will be covered in subsequent publications. The matrix material of the new composite material is a geopolymer obtained by alkaline activation of aluminosilicates (synthesized), and natural plant fibers are used as the material for the insulating layer. The prototype material was manufactured using a casting method, in which two fibers were placed inside the material in the form of a fibrous mat. The new proposed construction solution is undoubtedly innovative and is more energy efficient and environmentally friendly than the traditional building materials used so far. Multilayer geopolymer composites are a type of material that can be produced in a factory-made process. The innovation of this solution lies not only in the use of fibers in several layers of aggregated geopolymers, but also in the reinforcement of all layers with elastic glass fiber bonds to prevent possible decomposition. This new solution offers many advantages, such as:
To understand the methodological procedures outlined in this paper, Figure 2 below provides an experimental flow chart of the steps performed in the methodology and research section.
Multilayer geopolymer composites were manufactured using fly ash from the Skawina Thermal Power Plant in Poland. The fly ash used is class F and has the certificate number 1488-CPR-0166/W. According to the above document, the main parameters of this man-made waste are also provided, including losses during firing divided into categories A and B, grinding fineness – category H (declared value – 20%), particle density of the material – 2210 kg/m3. This ash has a high content of SiO2 (silicon dioxide) and more than half is Al2O3 (aluminum oxide). The second base material for the composite material was sand from the Skawina sand and gravel plant in Poland. Both materials were added in a 50/50 ratio by weight. The geopolymer polycondensation reaction was initiated by adding 10 mol/l of alkaline sodium silicate and a solution of sodium liquid glass. The sodium silicate R-145 used has a molar modulus of 2.5 and a density of about 1.45 g/cm3.
To provide the final structure with stability and strength, and to maintain the multilayer composite material in a single structure without delamination under the influence of external disturbances, composite ribs (4 mm and 8 mm in diameter) covered with fiberglass and impregnated with epoxy resin (TROKOTEX, Poland) were used. The physical and chemical properties of these tendons are given in Table 144.
Several natural fibres were tested for key parameters before two composite materials for the core were selected. A total of 9 natural fibres were tested. The test materials were two types of cellulose (PRO AGRO by VestaEco, Poland), one type of hay and wood chips (supplier – Dach-Wkręt, Poland), one type of coconut fibre from Sri Lanka (manufacturer – PROMAT, Poland), another type of flax and hemp fibre, and three types of hemp fibre (Double Raw Fiberworks, Poland). In addition to the natural fibres, natural mats, felt and wool were also tested. The coconut mats were purchased from the wholesale supplier of upholstery materials Akces (Strazw, Poland). The remaining felt and wool were purchased from Double Raw Fiberworks (Krakow, Poland). Nine different fibre types were selected to compare the most important physical and thermal parameters, as well as the structure and texture of the components. Most of these fibers are widely available in the market, while hay and wood chips were chosen because they had never been used for insulation of building envelopes before. As for mats, felts and wool, materials containing the above fibers were chosen because the properties and characteristics of these fibers are different in nature, and the properties of finished products widely sold based on these fibers are also different. We conducted a market research to select and compare various standard fibers, mats, wool and felts in terms of availability, thermal conductivity, environmental performance and price.
Since the newly developed materials are intended to be environmentally friendly and energy efficient, they were also analyzed from this point of view. Table 2 shows the data on embodied energy (EE) and embodied carbon (EC). These values ​​were determined using the life cycle analysis (LCA) method. Life cycle analysis (LCA) is a method used to assess the environmental impact of a product throughout its entire life cycle, including raw material extraction and processing, production, distribution, use, recycling and final disposal. In conventional buildings, the operational energy consumption is close to the total energy consumption, and the embodied energy consumption is relatively low; In low-energy buildings, the embodied energy consumption accounts for a large proportion of the total energy consumption; in passive buildings, the operational energy consumption and the embodied energy consumption are equal; Finally, autonomous or energy-efficient buildings do not consume energy during operation, and the total energy consumption taken into account in the life cycle assessment is embodied energy consumption (total energy consumption is higher than that of passive houses)45,46,47.
The density of the finished composite plate is calculated geometrically based on the mass and volume of the sample. The density of a regular shaped object is calculated as follows:
For a heat-conducting cube in a steady state, the amount of heat transferred depends on the material and is proportional to the cross-sectional area of ​​the cube, the temperature difference, and the time of the heat flow. This amount is determined by the following formula:
Where Q is the amount of heat flowing through the object (J), t is the flow time (s), d is the thickness of the partition (m), S is the cross-sectional area of ​​the object (m2), and \(\Delta \text{T}\) is the temperature difference in the direction of thermal conductivity (K).
After researching the properties of natural materials, two composite materials were selected for the filling: coconut mat and flax hemp felt1. These two materials were chosen because they have similar density and thermal conductivity and are widely available in the market. Coconut mats are commonly used in mattresses, but there are no reports of this material being used as insulation for building envelopes, so we wanted to use it in a completely different industry. Another material has similar properties to coconut mats, so we wanted to compare the performance of these two materials in multi-layer partitions. As mentioned above, the multi-layer geopolymer composites are made from fly ash and sand. Reinforcement was also added to improve the strength of the finished precast prototype. Nine 15 cm long steel rods (4 and 8 mm in diameter) were added to each geopolymer composite, each with four layers of natural material. The two types of reinforcement were used to compare the effect on the thermal insulation properties of the finished composite. Since the mats and felts vary in thickness and weight, components of different weights are added to the composite. Both materials have similar density, but they are very different physically and visually. Table 3 shows the properties of the finished multilayer composite (dimensions 15 x 15 x 15 cm). The table indicates the weight and weight fraction of each component. In the rest of the article, we will compare the properties of the composites depending on the added additives and the total weight fraction of the components (natural insulation and reinforcement). The table below shows that the amount of added additives is very small compared to the reference composite. On the other hand, Fig. 3 shows a model of the multilayer geopolymer composite. The reinforcement bars are marked in red.
Model of multilayer geopolymer composite: (a) composite material, (b) natural insulation layer, (c) geopolymer layer.
The X-ray fluorescence analysis of the chemical composition of oxides was performed on substrates such as fly ash and sand. The X-ray fluorescence analysis of oxides was performed using a SHIMADZU EDX-7200 X-ray fluorescence spectrometer from Duisburg, Germany. The tests were performed in air using a bracket and a polyester film designed for bulk materials. The results are shown in Tables 4 and 5.
The raw materials used in the production of geopolymers are rich in silicon and aluminum oxides. The above analysis of the chemical composition shows that both fly ash and sand contain large amounts of these oxides. Fly ash typically contains 50–60% by weight of SiO2 and about half as much Al2O3. Sand usually has a higher SiO2 content (about 85–95 wt%), followed by Al2O3, which is about 4 wt%. According to the generally accepted definition, geopolymers are inorganic, amorphous, synthetic aluminosilicate polymers formed by the synthesis of silicon (Si) and aluminum (Al) and minerals of geological origin. Thus, both materials correspond to the concept of geopolymer materials.
The particle size analysis of both base materials was performed using an Anton Paar PSA 1190LD particle size analyzer (Anton Paar GmbH, Graz, Austria) in combination with the Kalliope Professional software (version 2.22.1). The particle size analysis of fly ash was performed using the wet method (water was used as a dispersant), whereas the particle size analysis of sand was performed using the dry method due to its very large particle size. For materials with small particle sizes, wet analysis should be used; for materials with large particle sizes, dry analysis is recommended. The particle size measurements were performed five times for each material, after which particle size distribution graphs and cumulative curves were plotted. The measurement results are shown in Figure 4.
Substrate particle size analysis: (a) fly ash particle size distribution, (b) fly ash accumulation curve, (c) sand particle size distribution, (d) sand accumulation curve.
Fly ash is generally a very fine material, as confirmed by particle size analysis. The average particle size of this material was approximately 13 microns. Sand is generally coarser, as confirmed by this analysis. The average particle size of sand is 220 microns. After the addition of the alkaline activator, the two substances reacted well with each other and demonstrated the ability to form a coherent structure similar to concrete and Portland cement mortar.
For natural fibers, density measurements and thermal conductivity tests were performed on a HFM Lambda 446 plate apparatus (NETZSCH Pumpen & Systeme GmbH, Waldkraiburg, Germany), and SEM images were obtained on a JEOL IT 2000 microscope (JEOL, Akishima, Tokyo, Japan), while macroscopic images were obtained on a Keyence VHX-7000 digital optical microscope (KEYENCE INTERNATIONAL, Mechelen, Belgium). To observe the microstructure, the fibers were attached to special carbon disks, placed on a metal table and then on a stand. A special carbon glue EM-Tec C33 was also used for better adhesion to the fibers and to improve the conductivity of the material. Before testing, a conductive gold layer was deposited on the fiber surface using a DII-29030SCTR Smart Coater vacuum sputtering system (JEOL Ltd., Massachusetts, USA). The authors analyzed only the morphology of the samples; The chemical composition of the fibers was not studied. Table 6 shows the results of density and thermal conductivity measurements (average of 3 measurements), and Figures 5 and 6 show microscopic and macroscopic photographs of the fibers. Microscopic images were taken at 100x magnification, and macroscopic images at 20x magnification.
Microscopic images of fibers: (a) coconut fiber, (b) hay, (c) wood chips, (d) PRO AGRO cellulose, (e) VestaEco CELL cellulose, (f) flax-hemp fiber, (g) hemp fiber 1, (h) hemp fiber 2, (i) hemp fiber 3.
Macroscopic images of fibers: (a) coconut fiber, (b) hay, (c) wood chips, (d) PRO AGRO cellulose, (e) VestaEco CELL cellulose, (f) flax-hemp fiber, (g) hemp fiber 1, (h) hemp fiber 2, (i) hemp fiber 3.
Tests of the density and thermal conductivity of natural materials showed that wood chips have the lowest density, since they are the lightest. For comparison, PRO AGRO cellulose has the highest density. As for thermal conductivity, flax-hemp fibers had the lowest thermal conductivity, and hemp fiber 1 had the highest. All natural fibers have a density of 34–52 kg/m³ and a thermal conductivity of 0.039–0.052 W/m*K. A study of the structure and textural morphology of natural fibers using scanning and optical microscopy showed that different fibers differ significantly from each other. Coconut fiber has a spongy structure, excellent moisture absorption, air permeability and durability. Hay and chips have a uniform ribbon structure. The structure of cellulose fibers varies greatly in shape and arrangement. Flax fibers have an uneven, highly porous structure. On the other hand, hemp fiber has a cell structure with open pores. They absorb moisture and release it again after drying. Macroscopic images are used to demonstrate the appearance of the fibers. SEM images show the microstructure of the fibers, and digital optical microscope images show what the fibers actually look like at low magnification.
In addition to natural fibers, we also tested mats, felts, and wool made from the above fibers. The results show the test results for 10 different materials. Since each material is used for multi-layer partitions, the tests were conducted with 1, 2, and 3 layers of material. The density and thermal conductivity of these materials are listed in Table 7 (again, the average of three measurements is shown).
Tests of the density and thermal conductivity of the mats, felt and wool showed that the unpressed hemp wool has the lowest density due to the very large pores present in the material. The coir mat, on the other hand, has the highest density, being the heaviest of all the ingredients. In terms of thermal conductivity, the lowest results were found in the unpressed flax wool, while the highest were found in the unpressed hemp wool. All mats, felt and wool have a density of 14–127 kg/m3 and a thermal conductivity coefficient of 0.031–0.047 W/m*K. Two materials were selected for further testing: the flax hemp mat1 and the coir mat. Both materials have similar density and thermal conductivity.
The density of the board was calculated three times and an average value was calculated based on the results of the three measurements. The results are shown in Table 8. The volume of the boards was constant (15 × 15 × 5 cm), but the weight varied slightly between the samples. The dimensions of the samples were measured using a laboratory caliper with an accuracy of 0.01 mm, and the weight of the samples was measured using a RADWAG PS 200/2000.R2 precision laboratory analytical balance with an accuracy of 0.01 g.
Each component will result in a decrease in density as the added additives reduce the proportion of the base material. The greatest decrease in density was found in panels containing 4 coconut shell felts and 8 mm steel rods (22%), i.e. 6% of the weight of the ingredients (natural insulation and steel rods). For a similar variant but with a smaller rod diameter, the density decreased by 19%, or 4% of the component weight. The density of a sheet containing 4 flax pile and 4 mm steel rods decreased by 5% or 2% of the component weight, while the density of the same sample containing only 8 mm steel rods decreased by 13% or 3% of the component weight. In each multi-layer slab, the density decrease was greater after the addition of 8 mm reinforcement, as the increase in the reinforcement area resulted in a decrease in the proportion of geopolymer mortar. Coconut mats are much higher than flax mats, so their density decreases even more as the geopolymer content decreases.
Thermal conductivity measurements were performed using a Lambda HFM 446 (Netzsch) plate meter. The device operates in accordance with ISO 8301, EN 12664, ASTM C1784, ASTM C518, etc. Temperature regulation and control is performed using a Peltier system. Thermal properties of the finished panels were measured using the described device based on the hot and cold plate method. For a more accurate performance characterization, thermal conductivity was tested in three temperature ranges: 0–20 °C, 20–40 °C and 30–50 °C. In practice, the material can operate at temperatures above 30 °C, so there are 3 different temperature ranges. Table 9 shows the average thermal conductivity obtained from the three tests.
As in previous studies, each component leads to a decrease in thermal conductivity, thereby improving the insulation characteristics. The greatest decrease in thermal conductivity (a decrease of 41%) was observed in the panel containing 4 coconut mats and 8 mm strips – 6% by weight of the composition (natural insulation and strips). Very similar results were obtained when adding 4 coconut mats (4 mm strips) to the composites – 4% by weight of the composition and 4 flax pile (8 mm strips) – 3% by weight of the composition. The lambda coefficient of the panels is 36% lower. Components with similar density and thermal conductivity were introduced into the geopolymer matrix, but the results showed that the thermal conductivity of the sample containing rods with a diameter of 8 mm was lower. The thermal conductivity of composite reinforcement bars is 0.35 W/m K, while the thermal conductivity of solid geopolymers is usually closer to 1–1.4 W/m K. The introduction of a component with low thermal conductivity and a larger surface area (8 mm) into the geopolymer matrix resulted in a better effect due to the reduction of the geopolymer proportion compared to 4 mm diameter reinforcement bars. Again, as with density, the larger surface area of ​​the coir mat results in a lower proportion of geopolymer, which increases the thermal insulation capacity of the structure. As for the difference between temperature ranges, it is about 0.01–0.02 W/m K.
This study analyzes the physicochemical properties of the materials used as precursors for the geopolymer matrix, as well as the physical and thermal properties of the natural materials used as the insulating layer. In addition, a qualitative analysis of the images of the natural materials was carried out using digital optical microscopy and scanning electron microscopy. After conducting the basic studies, prototypes of the finished multilayer composites were developed and tested for physical and insulating properties.
The prototype used composite reinforcement, namely glass fiber reinforcement impregnated with epoxy resin. These rods are an excellent alternative to steel reinforcement and prevent delamination of multilayer composite materials. This material was chosen because it has a number of key advantages. In combination with mats and felts made of natural fibers, applied in several layers, it is an attractive construction solution. The coefficient of thermal expansion of composite steel reinforcement is comparable to that of concrete, so concrete does not crack with temperature changes, which saves money on repairing cracks during the operation of the building. Fiberglass reinforcement is completely resistant to corrosion and does not change its properties even in highly corrosive and aggressive acidic and alkaline environments, so it does not require labor-intensive and expensive maintenance. These steel rods also have high frost resistance, which extends the service life of the material by 2-3 times44.
We also analyzed the data on the energy used to produce the materials and their CO2 emissions. Fly ash, sand, and geopolymers have significantly better environmental performance than traditional materials such as cement or concrete. The embodied energy of geopolymer is almost three times lower than that of concrete. Fly ash contains 46 times less energy than cement and emits 83 times less CO2. In terms of thermal insulation, polystyrene foam or polyurethane foam also performs worse than other materials or natural fibers. All natural materials tested outperformed traditionally used foams in environmental performance (by a factor of 5–10). 45,46,47
An analysis of the properties of natural materials showed that wood chips have the lowest density, since they are the lightest. For comparison, PRO AGRO cellulose has the highest density. Among all natural fibers, flax and hemp fibers have the lowest thermal conductivity, while hemp fiber has the highest thermal conductivity1. The thermal conductivity of all fibers was in the range of 0.039–0.052 W/m*K. An analysis of the results for mats, felt, and wool showed that hemp wool has the lowest density without pressure due to the presence of very large pores in the material. For comparison, coconut coir has the highest density, being the heaviest of all ingredients. Of the materials tested, flax wool showed the lowest thermal conductivity, while hemp wool showed the highest. All results range from 0.031 to 0.047 W/m*K. Several layers of material were tested to understand how the material would behave in a layered composite. The thickness of the material affects its insulation properties, so three-layer materials show worse results, but this does not prevent them from being a good replacement for existing insulation materials. Since all these materials are biodegradable and renewable, it can be concluded that they are a better addition to sandwich partitions than traditional insulation materials. Other researchers have obtained similar results for the density and conductivity of natural materials48,49,50.
The physical properties of the composites were studied, showing that each component resulted in a decrease in density, as the added additives reduced the proportion of the matrix. The greatest decrease in density was found in panels containing 4 coconut felts and 8 mm steel rods (22% decrease). The composite achieved its best results with the inclusion of 6% by weight of ingredients (natural insulation and reinforcement). For a similar variant, but with a smaller diameter steel rod, the component was introduced in an amount of 4% by weight, which resulted in a 19% decrease in density. The density of the sheet containing 4 flax mats and 4 mm steel rods decreased by 5% (2% decrease in composition by weight), while the density of the same sample containing only 8 mm steel rods decreased by 13% (3% decrease in composition by weight). In each sandwich slab, the density reduction was greater after the addition of 8 mm diameter steel rods, as the increased area of ​​the steel rods resulted in a decrease in the proportion of geopolymer mortar. Coir felt is thicker than flax felt and displaces more geopolymer, resulting in a greater density reduction. The reduction in density after the introduction of low density components is a natural physical phenomenon and has been demonstrated many times51,52.
Thermal analysis of the newly formed composites showed that the maximum decrease in thermal conductivity (41%) was observed in the panel containing 4 coconut shell mats and 8 mm strips, i.e. with the introduction of 6 wt.% of the component. Very similar results were obtained when adding 4 pieces of coconut shell (4 mm strips), i.e. 4% by weight of the component, and 4 pieces of flax felt (8 mm strips), i.e. 3% by weight of the component, to the composite. The lambda coefficient for these panels decreased by 36%. Components with similar density and thermal conductivity were introduced into the geopolymer matrix, but the results showed that the thermal conductivity of the 8 mm thick strip sample was lower, since the larger area of ​​the strip reduced the weight fraction of the geopolymer. Again, as with density, the larger surface area of ​​the coconut mat results in a smaller proportion of geopolymer, which improves the thermal insulation properties of the structure. The decrease in thermal conductivity after the introduction of low-density components is a natural physical phenomenon, which has been confirmed by many authors53,54.
The results of this study confirm the findings of other authors that geopolymers are ideal materials for the production of multilayer structural and insulating composites55,56. The research results and concepts presented in this paper make an innovative contribution to the development of sustainable buildings. Until now, only a few authors have proposed similar solutions. Research on this topic will be continued, and work is currently underway to develop technologies for the production of geopolymer composites for use in construction.
In the next work, we will conduct research to determine the influence of the distribution and orientation of natural materials in the geopolymer matrix. The key issue is the optimization of the production process and the parameters of the composite materials. The main goal of the authors was to create a material with the highest possible durability, which could be used for decades without the need for renewal and repair. This study will compare how the distribution and shape of the fibers affect the insulation and strength parameters. In addition, future work should focus on two key issues:
Create solutions that are easy to use. The applicability of the research being conducted must be demonstrated in a number of ways. The solutions created must be reproducible, and the planned products for the building partitions must be prefabricated. The production methods for such composite materials can be technically challenging due to the large number of layers.
The effect of the number of layers on the thermal conductivity of the entire partition has been studied. It is necessary to confirm and experimentally demonstrate that multilayer insulation is superior to single-layer insulation when using the same volume or weight of insulation in the form of aggregated natural fibers.
In further research, we will develop an optimal insulation material that can not only replace insulation in buildings, but can also be used to build houses and building structures that do not require insulation at a later stage. The target materials will be tested for the following parameters: thermal conductivity, thermal permeability coefficient, strength, flammability, frost resistance, sound insulation and scope of application.
Based on the results presented in this article, several conclusions can be drawn that summarize the research work:
An analysis of energy and carbon dioxide data for the materials showed that fly ash, sand and geopolymers had significantly better environmental performance than traditional materials such as cement or concrete. In terms of thermal insulation, polystyrene foam or polyurethane also performed worse than other materials or natural fibers.