The development of ultralightweight expanded perlite-based thermal insulation panel using alkali activator solution

Abstract

The International Energy Agency (IEA) states that global energy consumption will increase by 53% by 2030. Turkey has 70% of the world’s perlite reserves, and in order to reduce energy consumption a thermal insulation panel was developed in Turkey using different particle sizes of expanded perlite (EP). In this study, 0–1.18 mm (powder) and 0–3 mm (granular) EP particle sizes were selected, since they have the lowest thermal conductivity coefficients among all the particle sizes. In addition, an alkali activator solution was used as a binder in the mixtures. The alkaline activator solution was obtained by mixing sodium hydroxide solution (6, 8, 10, and 12 mol·L⁻¹) and sodium silicate (Module 3) at the different ratios of Na₂SiO₃ to NaOH of 1, 1.5, 2, and 2.5. This study aimed to experimentally determine the optimum binder and distribution ratio of EP, with the lowest coefficient of thermal conductivity and the lowest density. The lowest thermal conductivity and the lowest density were determined as 0.04919 W·m⁻¹·K⁻¹ and 133.267 kg/m³, respectively, in the sample prepared with 83.33% powder-size EP, 6 mol·L⁻¹ sodium hydroxide solution, and ratio of Na₂SiO₃ to NaOH of 1.5. The density, thermal conductivity, and compressive strength of the sample showed the same trends of behavior when the Na₂SiO₃ to NaOH ratio was increased. In addition, the highest compressive strength was measured in 12 mol·L⁻¹ NaOH concentration regardless of particle size. In conclusion, the study predicts that the EP-based thermal insulation panel can be used as an insulation material in the construction industry according to the TS825 Thermal Insulation Standard.

Graphical abstract

Keywords

expanded perlite / alkali activator / thermal insulation panel / thermal conductivity

1 Introduction

Factors that increase energy consumption worldwide rapidly include population growth, industrialization, and urbanization. In developed countries, the contribution of buildings to energy consumption varies between 20% and 40% [1]. Specifically, in Turkey energy consumption originates 37% from industry, 35% from buildings (heating, cooling, and ventilation), 18% from transportation, 6% from agriculture, and 4% from non-energy usage [2]. Various thermal insulation materials with a low thermal conductivity coefficient are used to conserve energy in buildings [3]. This study aimed to develop a thermal insulation panel with a low density, sufficient compressive strength, and thermal conductivity value (λ) lower than 65 mW·m⁻¹·K⁻¹, according to Turkish TS 825 standard [4]. Natural expanded perlite (EP) material was preferred due to its superior thermal and physical properties, absence of harmful effects, and the existence of 70% of the world’s reserves in Turkey [5].

Perlite is an acidic volcanic glass with a pearly shine when broken, and differences in terms of color and structure. Raw perlite, which varies from light gray to bright black, is a siliceous volcanic rock that expands up to 20 times its original volume with the effect of steam when rapidly heated from 700 to 1200 °C, thus creating EP [6–12]. Raw perlite is most commonly used in expanded form worldwide in different sectors including the construction industry. For example, in the USA 51% of EP is used in the construction industry, 15% as filler, 14% as horticultural aggregate, 10% for filtration, and 10% for other applications [11–15]. In the literature, EP has been frequently investigated because of its thermal and sound insulation properties and low-density.

Yalgın [16] investigated the usage areas of EP and stated that the low thermal insulation coefficient comes from the many air bubbles formed during the expansion. Likewise, low density was linearly proportional to the thermal conductivity behavior. When mixed with various binders, the samples had a dry density between 600– 900 kg/m³ and thermal conductivity between 0.186–0.256 mW·m⁻¹·K⁻¹. In the literature, EP is also used as a lightweight aggregate in mixtures because of its low density [14–25]. In those studies, EP was replaced with aggregates at replacement levels between 10% to 80% for different purposes. The thermal, mechanical, and physical properties of the samples were measured. Studies observed that the thermal performances were improved significantly, while the mechanical properties were adversely affected by the increase of EP ratio [14,15,17–19,23,25].

Furthermore, EP has been especially preferred in Refs. [26–31] for the development of thermal insulation panels. EP can be used as a panel by binding with alkali activator solutions and different raw materials. According to previous studies, panel properties can vary depending on the type of curing, curing temperature, amount and concentration of binders, type, and combination of binders. Tian et al. [13] investigated the factors limiting the usage of EP-based thermal insulation panels containing sodium silicate. The best thermal conductivity performance and high strength were obtained from the sample containing a 30% Na₂SiO₃ solution with a weight ratio of 2.5:1. In addition, it was determined that the optimum curing temperature was 105 °C for the panel. Swanepoel and Strydom [27] preferred sodium silicate and sodium hydroxide as an alkali activator solution. The samples were cured at different hours (6, 24, 48, and 72 h) and at different curing temperatures (40, 50, 60, and 70 °C). X-ray diffraction analysis showed that quartz was the main component and most of the geopolymer structure was amorphous and glass-like. In addition, the optimum curing temperature and optimum curing time were determined as 60 °C and 48 h, respectively, for these composites. Al-Bakri et al. [29] prepared alkali activator solutions by mixing sodium hydroxide with sodium silicate and potassium hydroxide with potassium silicate. From the study, investigating different NaOH concentrations, the highest compressive strength was obtained from a 2.5 ratio of Na₂SiO₃/NaOH. Usha et al. [30] investigated geopolymer binders as an alternative to Portland Cement. It has been observed that high compressive strength was obtained when geopolymer materials with high reactivity. In addition, the compressive strength improved with the increase of the ratio of sodium silicate to sodium hydroxide. Kawade et al. [31] used an alkali activator solution prepared with NaOH/Na₂SiO₃ at a 2.5 ratio. In the mixture, NaOH solution was used in concentrations of 12, 14, and 16 mol·L⁻¹. The test results showed that the compressive strength increased with the concentration. Azimi et al. [32] studied the geopolymer reaction process and the usage of geopolymer compounds in thermal insulation applications. Since NaOH solution is more alkaline compared to KOH solution at the same concentration, it was stated that the molar concentration of NaOH for alkaline solutions should be 8–16 mol·L⁻¹, and the ratio of sodium silicate to sodium hydroxide by weight should be between 1.75 and 3.

In the building industry, thermal insulation panels are selected according to properties such as thermal conductivity, water vapor diffusion, fire resistance, toxicity, water absorption ratio, structure type, the surface of the building, cost, region, or area. In addition, thermal insulation panels should be easily installed in buildings, be economical, and should not harm the natural environment. The main purpose of this study is to develop a competitive and applicable thermal insulation panel for the construction industry by using EP material with different particle sizes showing high thermal performance and the alkali activator solution.

2 Materials and method

2.1 Materials

In the perlite mines industry, micronized perlite is used for filtering purposes, cryogenic perlite for heat preservation of tanks in the transport of cryogenic gases, construction perlite for lightness and insulation properties, and agricultural perlite for infiltration. In order to develop thermal insulation panels, samples were taken from 4 different types of EP, which are currently used for different purposes (supplied by GENPER Company, Turkey), The four different types and particle sizes used Filter perlite (P02), Cryogenic perlite (P05), Construction perlite (P12), and Agricultural Perlite (P14) for this study. Their thermal conductivity coefficients were then determined. Samples were taken from 5 different parts from 2 different bags for each product. The material was then placed in the mold without being compressed, and the thermal conductivity coefficient value was determined. The lowest thermal conductivity coefficient was measured in the P05 and P12 types. Therefore, EP was used in two different particle sizes granular (0–3 mm) and powder (0–1.18 mm) to obtain a thermal insulation panel (Fig.1). The physical and chemical properties of different particle sizes of EP are given in Tab.1 and Tab.2. The properties were obtained from laboratory experiments and manufacturers [33]. Fig.2 shows the results for the lower and upper limit particle size distribution of granular and powder EP particles, the results for the mixture containing these two particles, and the results for the standard particle size distribution of fine aggregate according to the ASTM C33 standard. Moreover, gypsum plaster (ABS Company, Turkey) with A1 class of fire resistance, 50 shore D surface hardness, 2 d drying time, and 0.30 W·m⁻¹·K⁻¹ thermal conductivity coefficient was used in the mixtures to strengthen the panel surface. Alkali activator solution was used by mixing sodium silicate (Module 3) and sodium hydroxide to bind the EP particles in experimental studies. Sodium hydroxide solution (ALKİM Company, Turkey) was in powder form and 99% pure. Sodium silicate solution (ALKİM Company, Turkey) was in liquid form, with a pH of 11.2, and specific density of 1.4 g/cm³, and melting and boiling points of −0.6 and 100.6 ºC, respectively.

According to Fig.2, granular and powder EP have particle sizes such that the biggest EP particle is up to 4 mm. The mixture was evaluated as a fine aggregate. In addition, the most suitable granulometry among the distribution of fine aggregate limit according to the ASTM C33 standard was obtained from the granular EP sample [34].

According to Tab.1, the granular and powder particles of EP have A1 fire-resistance. One of the main reasons for using EP in the construction industry as a thermal insulation material is its fire resistance as well as its low thermal conductivity coefficient in different particle sizes. Structural deteriorations observed in EP samples are 885 and 900 °C for powder and granular particles. High deterioration temperatures will provide time for a building to be evacuated by maintaining the stability of the structure in case of a fire.

According to Tab.2, the chemical compositions of granular and powder EP samples are nearly the same and the components do not contain nitrate, sulfate, phosphorus, heavy metal, a radioactive element, and organic matter. Therefore, EP particles are very pure chemically. In addition, Al₂O₃ (13.8% by weight) and SiO₂ (73.50% by weight) in EP compounds indicate they can be activated to form alumina silicate binders with alkali hydroxides or silicates at high pH conditions.

2.2 Mixture proportions

In this study, 64 mixtures were prepared. In order to determine the effects of EP particle size on the thermal conductivity performance of the panels, granular and powder particle sizes were used in 1:5, 1:2, 1:1, and 2:1 ratios. Sodium silicate (Module 3) and sodium hydroxide (6, 8, 10, and 12 mol·L⁻¹) were used to determine the effect on thermal behavior of using alkali activator solution at different concentrations and ratios. In addition, the Na₂SiO₃/NaOH ratios were 0.5, 1, 2, 2.5 ratios, following experience from the literature [32,35–38]. In the preliminary studies, when this ratio is lower than 1, the alkali activator was insufficient to bind EP materials and almost all the samples cracked after curing and most of them were broken. Therefore, this ratio was not preferred. Although use of a ratio higher than 2.5 improved the mechanical performance of the panel, this ratio was not preferred because it increased the density of the panel and the thermal conductivity coefficient was measured at more than 65 mW·m⁻¹·K⁻¹. For this reason, Na₂SiO₃/NaOH ratio was used in 1, 1.5, 2, and 2.5 ratios to obtain the best thermal conductivity performance. The mixing ratios of the study are given in Fig.3. The mixtures were divided into 4 different groups according to the NaOH concentration. Each group consisted of 16 samples with 4 different ratios of granular and powder EP and 4 different ratios of Na₂SiO₃/NaOH. While preparing the mixtures, 6 L of EP were used for each sample. However, gypsum plaster was kept at a constant 0.15 times by weight of the total EP in all mixtures. The alkali activator solution was determined according to the amount of mixing water in the mixture. The mixing water was kept constant at 0.80 times by weight of the total EP in all mixtures. Apart from this, curing has been shown in the literature to have an important role in panel performance. Although different curing temperatures were applied in these studies, 60 and 105 °C were commonly preferred [27,30,39–42]. In preliminary studies in the present work, the panels cracked and became unusable after a short time due to rapid evaporation at 105 °C. Therefore, the curing temperature of 60 °C was preferred. In addition, the curing time was set at 48 h.

2.3 Preparation of expanded perlite-based thermal insulation panels

The preparation steps of the panels are as follows. 1) A dry mix was prepared by adding 6 L of granular and powder EP and gypsum plaster at the ratios described in Section 2.2.2) Sodium hydroxide solution was prepared according to the amount of mixing water determined for the alkali activator mixture. Sodium silicate solution was added and they were mixed in a mechanical mixer at 400 rpm for 10 minutes until a homogeneous solution was obtained. 3) The prepared solution was added to the dry mixture slowly and mixed manually for 20 minutes without damaging the GP particles. 4) The mortar mixture was poured into a rectangular wooden mold of 300 mm × 300 mm × 50 mm in three stages. At each stage, the mixture was compacted by hand. A total of 4.5 liters of the mixture was added to the mold for each sample. 5) 1000 N force was applied to the mortar mixture in the mold. 6) After molding, the mold was kept in laboratory conditions for 48 h, and then cured in an oven at 60 °C for 48 h. While the samples were kept in the oven, they were placed in the vertical orientation in order to effectively transmit the oven temperature from the panel surface to the matrix. 7) After curing, the panels were removed from the mold and labelled according to their groups.

3 Experiments

The thermal, mechanical, and physical performances of the panels were determined and their usability as thermal insulation panels were investigated.

3.1 Measurement of density of expanded perlite-based panels

In order to measure the density of the panels, the thickness, width, length, and weight of the panels were measured after the curing process according to TS EN 822 [43] and TS EN 823 [44] standards. The density ‘ρ’ was calculated for each sample with the following equation, where m is sample weight, in kg; V is the volume of the sample, in m³; ρ is the density of the sample is given, in kg/m³.

V=\left(thickness\right)\ast \left(width\right)\ast \left(length\right),

\rho =\frac{m}{V}.

3.2 Measurement of thermal conductivity coefficient of expanded perlite-based panels

In order to determine the thermal performance of EP-based thermal insulation panels, the thermal conductivity coefficient was determined by the Hot Plate method. The principle of measurement is to position a sample between a hot and a cold plate, and to measure the heat flow [45]. During thermal conductivity experiments, the upper plate was set to 30 °C and the lower plate to 10 °C. The thermal conductivity coefficient of panels was measured with a LINSEIS HFM300 device. The technical specifications of the device are given in Tab.3.

3.3 Measurement of mechanical properties of expanded perlite-based panels

In order to determine the mechanical performance of EP-based thermal insulation panels, the compressive and flexural strengths were measured according to the TS EN 826 standard [46]. After the thermal conductivity coefficient was measured, the panels were divided into 3 parts to measure the mechanical properties. After the flexural strength test applied to 3 pieces of 160 mm × 160 mm × 40 mm size, the compressive strength test applied to 6 pieces of 40 mm × 40 mm size.

4 Results and discussion

In this study, a total of 64 panels which consisted of granular (0–3 mm) and powder (0–1.18 mm) EP-based samples which were prepared in different proportions were investigated as thermal insulation panels for the construction industry. The density, thermal conductivity coefficient, and mechanical properties of the panels were tested.

4.1 Density of expanded perlite-based panels

The dimensions of the panel removed from the 300 mm × 300 mm × 50 mm mold should be 240 mm × 240 mm × 50 mm due to the mold edge thickness. However, there were dimensional differences in the length, width, and thickness values of the panels during placing the mortar mixture and after the curing process. The average length, width, and thickness values of the panel groups prepared using different concentrations of NaOH solution are given in Tab.4.

After the curing process, the dimensions of the panels removed from the mold should be 240 mm × 240 mm × 50 mm. However, the mean width, length, and thickness values of the groups were reduced by 3.594%, 3.104%, and 1.3%, respectively. The highest losses were measured from the width and length of the panels. Apart from that, no significant change was observed between the dimensions of the panels with the increase of the NaOH concentration. Therefore, it is thought that the deteriorations are caused by the preparation and curing process rather than the proportions of the mixtures. Dimensional differences can be occurred as a result of improper placement of mortar, damage to the sample removed from the mold, and volume reduction or shrinkage after curing. After the curing, the dimensions and weight of the EP-based panels were measured and densities were determined. Depending on dimensions, different densities were observed. The effect of sodium hydroxide concentration and alkali activator ratio on the density in mixtures containing granular and powder-based EP in different ratios 2:1, 1:1, 1:2, and 1:5 is shown in Fig.4.

According to Fig.4, the densities of the EP-based panels varied between 133.267 and 276.161 kg/m³. Extruded polystyrene foam (XPS), Expanded polystyrene foam (EPS), glass wool, rock wool, polyurethane, wood panel, foam glass, phenol foam, and cork panels are used in the construction industry for thermal insulation. Densities of these vary between 13–570 kg/m³ [47–51]. Compared to other insulation materials, the densities of the EP-based insulation panels are within this range, and the low density can reduce the dead load of a structure and have the potential to be applied easily to buildings. In addition, regardless of the particle size distributions of the mixtures, the density increased at the same ratio as the binder ratio (Na₂SiO₃/NaOH) increased.

The densities decreased when the binder ratio was 1.5 in Group A panels with only 6 mol·L⁻¹ NaOH concentration for each group. The lowest density was measured as 133.267 kg/m³ in the A10 sample prepared with a 1:5 ratio of granular/powder EP, 6 mol·L⁻¹ sodium hydroxide concentration, and a binder ratio of 1.5. Regardless of particle size distribution ratio, the highest densities were measured from Group D panels prepared with a binder ratio of 2.5, 12 mol·L⁻¹ NaOH concentration. According to the figure, the densities among Group D panels increased with the decrease of granular EP ratio in the mixture. The increases were 20.43%, 22.11%, 32.05%, and 11.39% in different particle size distribution ratios, 2:1, 1:1, 1:2, 1:5, respectively. With the higher powder-size EP ratio in the mixture, the density of panels prepared with 8 mol·L⁻¹ NaOH solution had a higher density than those prepared with 10 mol·L⁻¹ NaOH solution. In contrast, the highest density was measured as 276.161 kg/m³ in the D8 sample prepared with a 1:2 ratio of granular/powder EP, 12 mol·L⁻¹ sodium hydroxide concentration, and a binder ratio of 2.5. The reason for this was either that the powder-size EP particles filled the voids in the matrix more than the granular-size EP particles, or the powder-size EP particles reacted more easily with the alkaline activator binder to form a larger amount of aluminate in the matrix. In addition, density increases that occurred with the increase of the binder ratio were due to the specific density of the sodium silicate.

4.2 Thermal conductivity coefficient of expanded perlite-based panels

The thermal conductivity coefficient must be lower than 65 mW·m⁻¹·K⁻¹, in accordance with the TS 825 Thermal Insulation Requirements for Buildings, for panels to be used on the exterior for thermal insulation [4]. The coefficient of thermal conductivity of panels was measured and the effect of sodium hydroxide concentration and alkali activator binder ratio on the thermal performance prepared with granular/powder ratios of 2:1, 1:1, 1:2, and 1:5 are shown in Fig.5.

As shown in Fig.5, the thermal conductivity coefficients of the panels were between 49.19 mW·m⁻¹·K⁻¹ and 66.97 mW·m⁻¹·K⁻¹. These results showed that EP-based panels can be used as thermal insulation panels in the construction industry regardless of the particle size distribution ratio in the mixture, satisfying TS 825 Standard. The thermal conductivity coefficients of granular and powder EP particles without binder solution were determined as 47.982 mW·m⁻¹·K⁻¹ and 43.321 mW·m⁻¹·K⁻¹, respectively. In the literature, the densities of the mixtures increased and the compressive strengths improved positively with the increase of Na₂SiO₃/NaOH ratio and NaOH concentration in mixtures using EP [13,27–31]. At the same time, it was observed in previous studies that thermal conductivity coefficient and compressive strength decreased with increased usage of EP [32,35,36]. This study aimed to develop a thermal insulation panel only using EP particles as raw material, unlike previous studies in the literature. Alkaline activator solution was used to bind the EP particles and to provide sufficient compressive strength to the panels. For this reason, it was expected that the density would increase with the increase of sodium silicate ratio in the mixture, and that the thermal performance of the EP particles would be adversely affected. In order that an EP-based panel can be competitive with other insulation materials it should have a low thermal conductivity coefficient, low density, and sufficient compressive strength performance. Therefore, the most important purpose was to obtain the panel with the lowest thermal conductivity by determining the optimum NaOH concentration, binder ratio, and distribution of different particle sizes of EP.

Among the 64 panels, the lowest thermal conductivity coefficient was measured as 49.19 mW·m⁻¹·K⁻¹ in A10 sample prepared with a 1:5 ratio of granular/powder EP, 6 mol·L⁻¹ sodium hydroxide concentration, and a binder ratio of 1.5. Since the lowest thermal conductivity coefficient was obtained from the A10 sample among all panels, four more mixtures were prepared with the same mixing ratio in order to confirm the result. The average of the thermal conductivity coefficients of these panels was determined as 48.59 mW·m⁻¹·K⁻¹. For this reason, the alkali activator ratio was obtained while the ratio of Na₂SiO₃/NaOH is 1.5 with a granular/powder ratio of 1:5 and 6 mol·L⁻¹ NaOH concentration in mixtures. As shown in Fig.5, thermal conductivity coefficient decreased with the decrease of granular EP ratio for panels prepared with the same binder ratio and the same sodium hydroxide concentration. The reason for this decrease was that powder-EP has lower coefficient of conductivity than granular EP. In addition, the homogeneous distribution was affected by the higher amount of granular EP ratio in the matrix, permeability was increased and more voids occurred. This could also affect heat transfer by the panels. Besides, the thermal conductivity coefficient was reduced and showed insulation performance with the increase of powder EP ratio in the mixture. Here, the thermal performance can be improved by homogeneous distribution of two-particle sizes, decrease in the void ratio, and lower permeability in the matrix. In contrast, the highest thermal conductivity coefficient was measured as 66.97 mW·m⁻¹·K⁻¹ in D8 sample prepared with a 1:2 ratio of granular/powder EP, 12 mol·L⁻¹ sodium hydroxide concentration and a binder ratio of 2.5. Among Group D, all panels had the coefficient of thermal conductivity lower than the upper limit (65 mW·m⁻¹·K⁻¹) or insulation materials except the D4 and D8 samples. However, 8 and 10 mol·L⁻¹ concentrations showed similar thermal behavior. The results showed that the thermal conductivity coefficient increased significantly with the increase of NaOH concentration in the mixtures with addition of sodium silicate. With the increase of the binder ratio in the mixture, the density of the panels increased, as in the literature studies. For this reason, filling the voids with the binder solution can be shown as a factor increasing the density of the mixtures. At the same time, the presence of a high percentage of granular-EP caused an inhomogeneous distribution in the mixture, resulting in a higher void ratio. Two different conclusions are possible. Firstly, with the usage of 83.33% powder-EP, the number of voids that may occur in the matrix was minimized and thus a homogeneous distribution was achieved in the mixture with the lowest thermal conductivity coefficient. The lower conductivity of raw powder-EP was an indication that the mixtures prepared with this type showed better performance. The second possible reason is the effect of the amount and density of alkaline activator solution. Low NaOH concentration and low amount of Na₂SiO₃ in mixtures result in relatively low void ratios, and this should be expected to affect the density at a minimum level. Because the addition of an alkali activator in the mixture increases the conductivity, it should be expected that the lower thermal conductivity coefficient would be obtained from the sample containing the least amount of binder mixture. Such a sample contained 83.33% powder EP in the mixture, and the amount of binder didn’t adversely affect the thermal performance. Matching the thermal conductivity coefficients of raw powder and granular EP, the conductivity increased between 2.71% and 13% with the addition of binder solution to the mixture. From this, it is shown that when the mixtures were prepared with a 6 mol·L⁻¹ NaOH and Na₂SiO₃/NaOH ratio of 1.5, the thermal properties of the panel were not significantly affected and remained below the required upper limit. Although the binder solution negatively affects the thermal properties, it has been determined that effective thermal insulation panels can be produced with the usage of 1.5 the optimum ratio.

Fig.6 shows a linear relationship between the panel density and thermal conductivity coefficients. Apart from the mixtures with the lowest and highest thermal conductivity of the panels, the density of the panels varied between 160 and 220 kg/m³, and the thermal conductivity coefficient varied between 53 and 61 mW·m⁻¹·K⁻¹. The thermal conductivity coefficient and the density increased with the amount of sodium silicate, regardless of different EP particle size distribution ratios and of the NaOH concentration. Similar results were obtained in literature studies [52–54]. The main reason is that the specific density of sodium silicate is high and when the amount of sodium silicate in the mixture increases, the EP particles bind more easily.

4.3 Mechanical properties of expanded perlite-based panels

According to TS EN 826 standard, it is necessary to determine the mechanical properties of panels as well as their thermal properties, for use on exterior insulation. For this reason, flexural and compressive strength tests were measured to determine the mechanical properties of the panels. The effect of sodium hydroxide concentration and alkali activator binder ratio on the mechanical performance of panels prepared with granular/powder ratios of 2:1, 1:1, 1:2, and 1:5 are shown in Fig.7.

Fig.7 shows that while the lowest compressive strength was measured as 58.77 kPa in an A9 sample prepared with a 1:5 ratio of granular/powder GP, 6 mol·L⁻¹ sodium hydroxide concentration, and a binder ratio of 1.0, the highest compressive strength was measured as 147.749 kPa in D8 sample prepared with a 1:2 ratio of granular/powder GP, 12 mol·L⁻¹ sodium hydroxide concentration and a binder ratio of 2.5. In addition, the average compressive strengths between groups A, B, C, and D, which are grouped according to the NaOH concentration, were determined as 7.47, 7.95, 9.08, and 13.11 times the flexural strengths, respectively. The best mechanical properties were obtained from panels prepared with granular: powder EP in a 1:2 ratio. The results indicated that the usage of granular: powder EP of different ratios in the mixtures did not affect the compressive and flexural strengths of the panels. Beside the homogeneous dispersion of the EP in the matrix the mechanical properties of samples mainly depends on the concentration of NaOH and the amount of Na₂SiO₃ in the mixture. The compressive strength increases proportionally with the increase of NaOH concentration and binder ratio on each different graph of granular: powder EP ratio. In addition, the highest compressive strengths were obtained from 12 mol·L⁻¹ NaOH concentration with a binder ratio of 2.5. Similar results were obtained in literature studies [13,29–32,35–37]. Arifruzzaman and Kim [37] stated that perlite and sodium silicate binder-based composites were analyzed from three different perspectives i.e., manufacturing parameters (i.e., binder content, compaction pressure, and compaction ratio), properties (i.e., particle size, density, compressive strength, and compressive modulus), and volume fractions of constituents. In this study, compaction pressure and compaction ratio were constant for all mixtures. Other ratios were varied in order to obtain optimum ratios for best thermal performance. Erdoğan [36] observed that using only NaOH solution to bind particles gave low flexural and compressive strengths when cured at room temperature or elevated temperatures, due to the low solubility of silicon and aluminum from the perlite. Unlike the case concerning some other geopolymeric materials, increasing the concentration of the NaOH solution used did not consistently increase the strength of the specimens. But using NaOH and Na₂SiO₃ together, the compressive strengths of composites were significantly improved in Refs. [13,29–32,35–37]. As the Na⁺ ion ratio in the system increases with the increase in NaOH concentration, Na⁺ ions act as binders, providing the equilibrium of the charges and the formation of alumina silicate networks in the mixture. The geopolymerization is low due to the low concentration of NaOH and, hence, there is less leaching of silica and alumina from the source material [55]. In addition, with the increase of the Na₂SiO₃/NaOH ratio in the mixture, the SiO₂/Al₂O₃ ratio in the matrix increases by the same proportion. For this reason, more aluminosilicate gels are formed as a result of geopolymerization and their compressive strength improves positively. While amorphous compounds are formed with the increase of the SiO₂/Al₂O₃ ratio, they are more prone to form zeolites with a decrease in this ratio. The designed molar ratios were found to correlate with the phase and microstructure of reaction products related to their compressive properties [56]. Therefore, the NaOH concentration and the SiO₂/Al₂O₃ ratio, depending on the Na₂SiO₃ content, had greater effect than particle size distribution on the mechanical properties of the mixtures.

The compressive strength of the materials used for thermal insulation in the construction industry varies between 0.1–0.4 MPa [47–51]. Although the alkali activator solution improved the mechanical properties of the panels positively, the highest compressive strengths of the panels were 147.749 kPa. Therefore, the mechanical properties of the panels prepared using only EP particles and binder solution had lower levels of compressive strength compared to other insulation materials. The reason for this is that EP has a very low unit weight and regardless of particle size is in the light aggregate class. However, in terms of increasing the usage of EP with high reserves and reducing the cost of the panel, EP-based thermal insulation panels have a performance that can be competitive with others in terms of both their thermal and physical properties.

The relationships between density, mechanical properties, and thermal conductivity coefficient of EP-based panels are shown in Fig.8. Fig.8 illustrates that, as the density increased, the compressive strengths increased exponentially. With the usage of granular and powder EP at the same ratio, the effect of sodium hydroxide concentration and binder ratio on compressive strengths was clearly seen. Although there are differences between all groups, the lowest compressive strength was measured at 6 mol·L⁻¹ sodium hydroxide concentration, and the highest compressive strength was measured at 12 mol·L⁻¹ sodium hydroxide concentration. However, while the increase in the binder ratio affected the compressive strength positively, thermal properties were affected negatively. The lowest compressive strengths were obtained with a binder ratio of 1.0 and the highest strengths were obtained at a ratio of 2.5. In addition, panels with high density also have high compressive strength. It was seen that compressive strengths improved with the increase of the binder ratio in the mixtures. However, there was an inverse relationship between the compressive strength and the thermal performance of the panels. The thermal conductivity coefficients of the panels with high compressive strength also increased, due to the increase in the amount of binder and the concentration of the NaOH solution. In addition, there was an inverse relationship between the compressive and flexural strengths of the panels. Here, panels with higher compressive strength had lower flexural strengths. In addition, when taking into account that the compressive strengths of thermal insulation panels vary between 0.1 and 0.4 MPa, it could be concluded that the compressive strength of the EP-based panels being between 0.05 and 0.15 MPa may be an obstacle to the use of EP in the construction industry. However, it is thought that there should be no problems since EP-based panels maintain their stability and will be applied to building surfaces which doesn’t have a bearing purpose. The fact that other insulation panels have below 1 MPa compressive strength and that there is no problem during the application of these also removes the negativity in this regard. In addition, EP-based panels can be easily shaped, which allows them to be applied by cutting.

5 Conclusions

In this study, the effects of EP particle size distribution, NaOH concentration, and Na₂SiO₃/NaOH ratio on the thermal performance of mixtures were investigated experimentally. Physical, mechanical, and thermal tests were applied to 64 mixtures. The conclusions of this study are as follows.

Granular and powder of EP have been preferred in different from the literature studies because of their low thermal conductivity coefficients and they were used different ratios to get homogeneous distribution in the mixture.

The density and compressive strength increased at the similar rate as the increase of sodium hydroxide concentration and the binder ratio. While the lowest density and lowest compressive strength were observed in granular: powder EP ratio of 1:5, the highest density and compressive strength were observed in granular: powder EP ratio of 1:2.

The thermal conductivity coefficient of panels varied between 49.19 and 66.97 mW·m⁻¹·K⁻¹. A10 panel had the best thermal performance and lowest density of 133.267 kg/m³. A10 was prepared with 1:5 granular: powder EP, 6 mol·L⁻¹ NaOH concentration, and binder ratio of 1.5. There were directly proportional relationships between thermal conductivity, density, and compressive strength.

Because of high natural abundance and reserves, low density, different particle sizes, low cost, and thermal conductivity coefficient lower than 65 mW·m⁻¹·K⁻¹, the EP-based panels can be used as thermal insulation materials in the construction industry.

References

[1] Pérez-Lombard L, Ortiz J, Pout C. A review on buildings energy consumption information. Energy and Building, 2008, 40(3): 394–398

[2] EnginS. Thermodynamic assessment of energy efficiency in the heating of buildings. Thesis for the Master’s Degree. Edirne: Trakya University, 2012 (in Turkish)

[3] Bayraktar D, Bayraktar E. To assess the applications of thermal insulation in existing buildings. The Journal of Graduate School of Natural and Applied Sciences of Mehmet Akif Ersoy University, 2016, 7(1): 59–66

[4] TS 825. Thermal Insulation Requirements in Buildings. Ankara: Turkish Standard, 2008, 1–8

[5] Balo F, Ucar A, Yücel H. Development of the insulation materials from coal fly ash, perlite, clay and linseed oil. Ceramics-Silikáty, 2010, 54(2): 182–191

[6] ArifuzzamanMKimH. Development of new perlite/sodium silicate composites. In: International Conference on Mechanical, Industrial and Energy Engineering. Bangladesh: ICMIEE-PI-140116, 2014

[7] İzmirlioğlu, A. Mining Special Expertise Commission Report. Ankara: State Planning Organization. 2007 (in Turkish)

[8] OrhunO. Perlite. Scientific Mining Journal. 1969, 8(4): 213−222 (in Turkish)

[9] Shastri D, Kim H. A new consolidation process for expanded perlite particles. Construction & Building Materials, 2014, 60: 1–7

[10] Skubic B, Lakner M, Plazl I. Microwave drying of expanded perlite insulation board. Industrial & Engineering Chemistry Research, 2012, 51(8): 3314–3321

[11] U.S.Geological Survey. Mineral Commodity Summaries 2021: Perlite. Reston: U.S. Geological Survey, 2021

[12] Çelik A, Kılıç A, Çakal G. Expanded perlite aggregate characterization for use as a lightweight construction raw material. Physicochemical Problems of Mineral Processing, 2013, 49(2): 689–700

[13] TianY LGuo X LWuD LSunS B. A Study of effect factors on sodium silicate based expanded perlite insulation board strength. Applied Mechanics and Materials, 2013, 405–408: 2771–2777

[14] BulgurcuH. Investigation of heat and mechanical properties for cement and plaster additive building materials which containing expanded perlite. In: 17th Turkish Society of Thermal Sciences and Technology Congress. Sivas, 2009 (in Turkish)

[15] GündüzLŞapçıNBekarM. A technical analysis on improving the technical properties of pumice concretes with expanded perlite aggregates. In: Proceedings of 4th International Aggregate Symposium & Exhibition Congress. İstanbul: Union of Chambers of Turkish Engineers and Architects (UCTEA), 2006, 1–12 (in Turkish)

[16] YalgınS. Usage of expanded perlite and its’ products in thermal insulation. Turkey Engineering News, 1985, 314: 3−7 (in Turkish)

[17] Demirboğa R, Gül R. The effects of expanded perlite aggregate, silica fume and fly ash on the thermal conductivity of lightweight concrete. Cement and Concrete Research, 2003, 33(5): 723–727

[18] Morsy M, Aglan H. Development and characterization of nanostructured-perlite-cementitious surface compounds. Journal of Materials Science, 2007, 42(24): 10188–10195

[19] Sengul O, Azizi S, Karaosmanoglu F, Tasdemir M. Effect of expanded perlite on the mechanical properties and thermal conductivity of lightweight concrete. Energy and Building, 2011, 43(2−3): 671–676

[20] Türkmen İ, Kantarcı A. Effects of expanded perlite aggregate and different curing conditions on the physical and mechanical properties of self-compacting concrete. Building and Environment, 2007, 42(6): 2378–2383

[21] Çeli̇kAKılıçAAkkurtF. Investigation of availability of expanded perlite in the production of construction material. Journal of the Faculty of Engineering and Architecture of Gazi University, 2014, 29(3): 451−458 (in Turkish)

[22] KaraÇKaracasuMCanbazM AkalınK. The evaluation of performance properties of asphalt concrete with expanded perlite. In: 1st International Workshop on Construction and Electricity Applications on Vocational Education Congress. Eskişehir: Electronic Journal of Occupational Improvement and Research, 2015, 133–142 (in Turkish)

[23] Topçu İ, Işıkdağ B. Manufacture of high heat conductivity resistant clay bricks containing perlite. Building and Environment, 2007, 42(10): 3540–3546

[24] Skubic B, Lakner M, Plazl I. Sintering behavior of expanded perlite thermal insulation board: Modeling and experiments. Industrial & Engineering Chemistry Research, 2013, 52(30): 10244–10249

[25] Zhang X, Bai C, Qiao Y, Wang X, Jia D, Li H, Colombo P. Porous geopolymer composites. Composites. Part A, Applied Science and Manufacturing, 2021, 150: 106629

[26] Argunhan Z, Oktay H, Dogmus R. Investigation of the thermal and acoustic performance of perlite based building materials. European Journal of Technic, 2016, 6(1): 26–36

[27] Swanepoel J, Strydom C. Utilisation of fly ash in a geopolymeric material. Applied Geochemistry, 2002, 17(8): 1143–1148

[28] Rattanasak U, Chindaprasirt P. Influence of NaOH solution on the synthesis of fly ash geopolymer. Minerals Engineering, 2009, 22(12): 1073–1078

[29] Al-Bakri A M, Kamarudin H, Bnhussain M, Rafiza A, Zarina Y. Effect of Na₂SiO₃/NaOH ratios and NaOH molarities on compressive strength of fly ash based geopolymer cement. Journal of the American Concrete Institute, 2012, 109(5): 503–508

[30] Usha S, Nair D, Vishnudas S. Geopolymer binder from industrial wastes: A review. International Journal of Civil Engineering and Technology, 2014, 5(12): 219–225

[31] Kawade U, Salunkhe P, Kurhade S. Fly ash based geopolymer concrete. International Journal of Innovative Research in Science, Engineering and Technology, 2014, 3(4): 135–138

[32] Azimi E, Mustafa Al Bakri Abdullah M, Ming L, Yong H, Hussin K, Aziz I. Processing and properties of geopolymers as thermal insulating materials: A review. Reviews on Advanced Materials Science, 2016, 44(3): 273–285

[33] GENPERCompany. The Technical Specifications of Expanded Perlite. İstanbul: GENPER Company, 2022

[34] ASTMC33. Standard Specification for Concrete Aggregates. West Conshohocken: ASTM International, 2001

[35] Lach M, Korniejenko K, Mikula J. Thermal insulation and thermally resistant materials made of geopolymer foams. In: Proceedings of International Conference on Ecology and New Building Materials and Products. Cerna Hora: Procedia Engineering, 2016, 410–416

[36] Erdoğan. Use of perlite to produce geopolymers. In: Proceedings of 31st Cement and Concrete Science Conference Novel Developments and Innovation in Cementitious Materials. London: Imperial College London, 2011

[37] Arifuzzaman M, Kim H S. Novel mechanical behaviour of perlite/sodium silicate composites. Construction & Building Materials, 2015, 93: 230–240

[38] Yahya Z, Abdullah M, Hussin K, Ismail K, Razak R, Sandu A. Effect of solids-to-liquids, Na₂SiO₃-to-NaOH and curing temperature on the palm oil boiler ash (Si + Ca) geopolymerisation system. Materials (Basel), 2015, 8(5): 2227–2242

[39] Zhao J, Tong L, Li B, Chen T, Wang C, Yang G, Zheng Y. Eco-friendly geopolymer materials: A review of performance improvement, potential application and sustainability assessment. Journal of Cleaner Production, 2021, 307: 127085

[40] Wang H, Li H, Yan F. Synthesis and mechanical properties of metakaolinite-based geopolymer. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, 2005, 268(1–3): 1–6

[41] ZeybekO. Fly ash based geobrick production. Thesis for the Master’s Degree. Eskişehir: Anadolu University, 2009 (in Turkish)

[42] Bai P. Processing, properties and applications of highly porous geopolymers: A review. Ceramics International, 2018, 44(14): 16103–16118

[43] TS EN 822. Thermal Insulating Products for Building Applications Determination of Length and Width. Ankara: Turkish Standard, 2013, 1–7

[44] TS EN 823. Thermal Insulating Products for Building Applications—Determination of Thickness. Ankara: Turkish Standard, 2013, 1–13

[45] LINSEISCompany. HFM (Heat Flow Meter). Selb, 2021

[46] TS EN 826. Thermal Insulating Products for Building Applications—Determination of Compression Behaviour. Ankara: Turkish Standard, 2008, 1–15

[47] CandanN. The discussion of the thermal insulation and its peculiarities. Thesis for the Master’s Degree. Sakarya: Sakarya University, 2007 (in Turkish)

[48] AkıncıH. Today’s thermal insulation materials applied, their properties, application techniques and cost analysts. Thesis for the Master’s Degree. Sakarya: Sakarya University, 2007 (in Turkish)

[49] YamanÖŞengül ÖSelçukHÇalıkuşOKaraİErdemŞ ÖzgürD. Thermal insulation and thermal insulation materials in buildings, Turkey Engineering News, 2015, 487: 62–75 (in Turkish)

[50] IsbilirD. Applications of thermal insulations in buildings and investigations of their problems. Thesis for the Master’s Degree. Konya: Selçuk University, 2009 (in Turkish)

[51] OzicI. Summary an experimental study on the thermal insulation of buildings. Thesis for the Master’s Degree. Denizli: Pamukkale University, 2013 (in Turkish)

[52] Tekin İ. Properties of NaOH activated geopolymer with marble, travertine and volcanic tuff wastes. Construction & Building Materials, 2016, 127: 607–617

[53] Görhan G, Kürklü G. The Influence of the NaOH solution on the properties of the fly ash-based geopolymer mortar cured at different temperatures. Composites. Part B, Engineering, 2014, 58: 371–377

[54] Somna K, Jaturapitakkul C, Kajitvichyanukul P, Chindaprasirt P. NaOH-activated ground fly ash geopolymer cured at ambient temperature. Fuel, 2011, 90(6): 2118–2124

[55] Mustafa Al Bakri A M, Kamarudin H, Bnhussain M, Rafiza A R, Zarina Y. Effect of Na₂SiO₃/NaOH ratios and naoh molarities on compressive strength of fly-ash-based geopolymer. ACI Materials Journal, 2012, 109(5): 503–508

[56] Juengsuwattananon K, Winnefeld F, Chindaprasirt P, Pimraksa K. Correlation between initial SiO₂/Al₂O₃, Na₂O/Al₂O₃, Na₂O/SiO₂ and H₂O/Na₂O ratios on phase and microstructure of reaction products of metakaolin-rice husk ash geopolymer. Construction & Building Materials, 2019, 226: 406–417

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