Development and Characteristics of Ultra-Lightweight Concrete Mixed with Polyvinyl Alcohol Solution and Perlite

Abstract

Polyvinyl alcohol (PVA) is a nontoxic, biodegradable, water-soluble polymer used in various fields. The use of PVA in concrete has been limited due to problems such as initial setting delay, early age strength reduction, and decreased mechanical and durability performance caused by the generation of microbubbles. This study aims to verify the performance and effectiveness of PVA as a foaming agent by reversely utilizing the microbubble generation phenomenon of PVA, which has been recognized as a problem. In addition, the production and characteristics of ultra-light foamed concrete were evaluated using perlite (PL). PVA solutions with concentrations of 2.5% and 5.0% were considered. PL ratios of 10% (P1), 20% (P2), 30% (P3), and 40% (P4) were applied to the mass of the PVA solution. The mass of PVA solution (s), OPC (c), and PL considered in the experiment were classified into s/c, s/PL, PL/c, and s/(c + PL), respectively, and the optimal mixing ratio was presented based on the experimental results. In addition, a new high-temperature-curing method was applied that combines high-temperature wet curing (60 °C, relative humidity 90% or higher) and high-temperature dry curing (105 ± 5 °C) to improve the strength reduction problem at the early age and to increase the pore expansion effect. In addition, thermal analysis (TG/DTA), X-ray diffeaction (XRD), and Fourier transform infrared spectroscopy (FT-IR) were applied to analyze the hydration reaction products. The formed pores were imaged from the cross-section of each sample using an optical microscope, and the pore size was measured using image-processing software. The experimental results proved that PVA is a sufficiently effective material as a foaming agent for manufacturing foamed concrete in a high-temperature-curing environment. PVA showed a synergistic effect in expanding pores and increasing porosity, and PL improved insulation and lightweight. The manufactured ultralight foam concrete had a density of less than 1.0 g/cm³, a strength of 1–6 MPa, and a thermal conductivity of 0.13–0.19 W/m∙K. The appropriate mixing ratios within the range considered in this study were s/PL ratio of 5–3.33, PL/c ratio of 0.2–0.45, and s/(c + PL) ratio of 0.77–1.25.

Introduction

Polyvinyl alcohol (PVA) is an environmentally friendly, water-soluble polymer used in a variety of fields, products, and manufacturing processes, including food, paper, adhesives, medical applications, and textile manufacturing (Nigto et al. 2022; Lu et al., 2018; Cao et al., 2023; Tarnowiecka-Kuca et al., 2023). The most common form of PVA used in construction is PVA fibers, which are utilized in the production of fiber-reinforced concrete (Tran et al., 2022; Yao et al., 2024; Zhao et al., 2023). However, unlike PVA fibers, experiments and studies on the properties of concrete or cement using PVA solutions are very poor. Recently, studies have been conducted on the use of PVA solutions to improve the properties of cement and concrete, and many researchers have investigated the characteristics of cement and concrete mixed with PVA.

Most of the previous studies on concrete or cement mixed with PVA have focused on basic research on fluidity (Allahverdi et al., 2010; Balaha et al., 2007; Liu et al., 2020; Ohama, 1998), setting (Singh & Rai, 2001), properties of PVA (Moukwa et al., 1993; Santos et al., 1999), and mechanical properties (Morlat et al., 2007; Thong et al., 2016). In addition, some have conducted and reported studies on the effects of PVA on hydration reaction and hydration reagents of cement (Georgescu et al., 2002; Nguyen et al., 2016; Xing & Wang, 2023). And there were some studies on bleeding (Kim et al., 1998; Kim et al., 1999), porosity (Dong et al., 2023), and durability (Kou & Poon, 2010; Wu et al., 2021). And some researchers reported experimental results on surface modification of recycled aggregates (Huiwen et al., 2006; Kou & Poon, 2010; Mannan et al., 2006). Despite many previous studies using PVA, concrete members or field construction cases using PVA are very rare. This is due to performance degradation factors known through previous studies on concrete using PVA. The known representative limiting factors can be summarized as follows:

First, the initial hydration reaction may be delayed, leading to reduced early strength. This occurs because PVA coats the surfaces of both hydrated and unhydrated cement particles, preventing water molecules from contacting the cement (Pique & Vazquez, 2013; Singh et al., 2003). Second, the addition of PVA generates numerous micropores within the matrix, which can improve fluidity due to the "ball-bearing" effect; however, this may significantly compromise mechanical performance (Kim & Robertson, 1997; Kim & Robertson, 1999). Despite the improvement in the fluidity, workability, and durability of concrete using PVA, the decrease in strength due to micropores limits its use as a structural material and raises concerns about long-term durability. Third, the multitude of variables involved in using PVA can negatively affect stable performance and production efficiency. Currently, the PVA content (dosage) used in cement and concrete is typically less than 3%. This limitation is due to the significant impact that the dosage of water-soluble PVA has on the initial properties and mechanical performance of cement and concrete. In addition, an increase in PVA dosage raises the viscosity of the cement paste (Nguyen et al., 2015), which affects the workability and fluidity of the mixture. Specifically, an increase in PVA dosage negatively impacts fluidity due to increased viscosity, complicating the separation of materials or the production of homogeneous concrete (Nguyen et al., 2015). The mechanical performance of cement or concrete mixed with PVA is influenced by curing conditions, the properties of PVA (such as hydrolysis and molecular weight), PVA dosage, and the water/cement (w/c) ratio; however, no clear trends or characteristics have been established (Kim & Robertson, 1998; Kim & Robertson, 1999; Knapen & Van Gemert, 2006). Furthermore, there is a lack of research investigating the long-term durability of cement and concrete modified with PVA.

Perlite (PL) is an amorphous aluminosilicate volcanic glass that expands to approximately 5–20 times its original volume when heated to temperatures between 850 and 1200 °C (Karakaş et al., 2023; Rashad, 2016). Expanded perlite has a bulk density ranging from 80 to 240 kg/m³, a high surface area due to its micropores, low acoustic transmittance, low thermal conductivity (Celik et al., 2016; Davraz et al., 2020; Karakaş et al., 2023), and chemical inertness, making it widely used in various fields. To date, many researchers have utilized perlite in the construction industry to develop lightweight structures or insulation materials. The microporous structure of perlite helps reduce the density of concrete, demonstrating that lightweight concrete can be achieved using perlite (Bakhshi et al., 2023; Davraz et al., 2020; Sengul et al., 2011). Furthermore, numerous experiments and studies have reported on the enhancement of insulation performance in concrete incorporating perlite (Demirboga & Gül, 2003; Ibrahim et al., 2020; Sengul et al., 2011). However, due to its lower strength when compared with conventional aggregates, the mechanical performance of concrete decreases as the replacement ratio of perlite increases (Alexa-Stratulat et al., 2024; Bakhshi et al., 2023; Karakaş et al., 2023). Some researchers have also noted improvements in compressive strength when incorporating perlite (Demirboğa et al., 2001). Perlite is gradually expanding its application in concrete as a construction material that can simultaneously enhance lightweightness and insulation.

This study aims to improve the weaknesses reported by previous researchers on concrete using PVA and to suggest new utilization methods. Here, two weaknesses are utilized and improved. The first is the numerous micropores generated by PVA, and the second is the setting delay and strength reduction at early age. The first is to maximize the micropores generated by PVA, which were reported as weaknesses, and to verify the effectiveness and performance of PVA as a foaming agent. This is to utilize the microbubbles generated by PVA, which were previously recognized as weaknesses, as a foaming agent from a reverse perspective. The second weakness, the setting delay and strength reduction at early age, is the application of a new curing method. In this study, a high-temperature-curing method is applied to improve the initial hydration reaction delay. This new high-temperature-curing method promotes the hydration reaction by sequentially performing high-temperature wet curing (60 °C, relative humidity ≥ 90%) and high-temperature dry curing (105 ± 5 °C). The newly introduced high-temperature-curing method is expected to have a synergistic effect of shortening the setting time and improving the early-age strength through the promotion of hydration reaction, while simultaneously performing the expansion effect of micropores. In addition, to further improve the lightness along with the foaming effect by PVA, an ultra-light foam concrete was manufactured using PL as a lightweight aggregate, and its characteristics were investigated. To this end, 2.5% and 5.0% PVA solution concentrations and various OPC-PL mixing ratios were selected, and the average pore diameter, compressive strength, dry density, insulation properties, and hydration reaction substances were analyzed.

Materials, and Methods

Materials

The components of the OPC and PL used in the experiment were analyzed using XRF, and the results are presented in Table 1. The density of PL is 0.11 g/cm³, thermal conductivity is 0.03–0.05 W/mK, the particle size is 10–100 μm, and the average diameter is 50 μm. The PVA used in the experiment was a product of Kuraray (Japan), and the detailed properties were provided by the manufacturer and are shown in Table 2.

Table 1. Chemical composition of ordinary Portland cement (OPC) and perlite (PL)

	Chemical composition (%)
	SiO2	Al2O	Fe2O	MgO	CaO	K2O	SO3	Na2O
OPC	22.63	5.81	2.54	3.02	62.19	0.67	2.8	0.13
PL	76.85	14.06	0.93	0.21	1.37	2.65	–	3.49

Table 2. Properties of polyvinyl alcohol (PVA)

Viscosity (mPa·s)	Hydrolysis (mol%)	Molecular weight	pH
4.6–5.4	86.5–89.0	24,500	7.0

Methods

Before determining the final mixing ratio, preliminary experiments were conducted on various dosages of PVA solution and OPC–PL mixing ratios to establish a suitable range and identify influencing factors. To manufacture ultra-lightweight foamed concrete, the amount of PVA, foaming method, and curing conditions must be designed appropriately. Therefore, the following three factors were considered crucial when designing the mixing ratio: (i) PVA solution dosage to maximize the foaming effect of PVA, (ii) selection of the OPC-PL ratio range with consideration for lightweight properties, and (iii) curing temperature and method to enhance the initial hydration reaction and early strength.

Preliminary experiments were conducted based on these three considerations, and the mixing conditions or ranges were determined as follows: when PVA is dissolved in water, it forms a viscous aqueous solution. In most studies involving cement or concrete with PVA, the PVA content is typically less than 3.0% (Tone et al. 2016; Wu et al., 2021). In this study, based on previous research, two concentrations of PVA—2.5% and 5.0%—were selected. During the preliminary experiments, a material separation phenomenon was observed in some mixtures when the PVA solution concentration was below 2.0%, causing some PL to float to the surface. However, at PVA solution concentrations of 2.5% or higher, material separation of PL due to viscosity was not observed. Therefore, 2.5% and 5.0% were chosen to prevent PL separation and to maximize pore generation and expansion effects under high-temperature-curing conditions.

The minimum amount of OPC was considered to achieve ultra-lightweight characteristics. For this purpose, the ratios (s/c) of PVA solution to cement were selected as 1.0, 1.5, and 2.0. The density of OPC is approximately 28.6 times greater than that of PL. Consequently, increasing the amount of OPC raises the weight of the specimen and reduces its lightweightness, so a minimum amount was designed.

The curing conditions involved two environments. The first stage of high-temperature curing uses a high-temperature wet-curing machine (60 °C, RH ≥ 90%) in a constant temperature and humidity chamber, maintained for 12 h. This is followed by curing in a high-temperature dry-curing machine (105 ± 5 °C) in a dryer for an additional 12 h. The first stage, high-temperature wet curing, promotes the hydration reaction of OPC and facilitates the expansion of micropores generated by PVA. The second stage of high-temperature dry curing aims to evaporate moisture, thereby fixing the pores in the matrix and hardening the PVA.

The detailed mixing ratios are shown in Table 3. In this table, four ratios were considered to analyze the properties of ultra-lightweight foamed concrete according to the mixing ratio: (i) weight ratio of PVA solution to OPC (s/c), (ii) weight ratio of PVA solution to PL (s/PL), (iii) weight ratio of the combined weights of OPC and PL to the weight of the PVA solution (s/(c + PL)), and (iv) weight ratio of PL to OPC (PL/c). An appropriate range for each ratio was selected through preliminary experiments prior to the main experiment. The PL was designed to increase relative to the OPC ratio to identify trends based on changes in the PL amount. This change in the PL amount affects the values of the s/PL, s/(c + PL), and PL/c ratios.

Table 3. Detailed mixing ratios

Level	s/ca	Dosage of PVA sol. (%)	PVA solution (g)	OPC (g)	PL (g)	s/PLb	s/(c + PL)c	PL/cd
2S10P1	1.0	2.5	1000	1000	100	10.00	0.91	0.10
2S10P2					200	5.00	0.83	0.20
2S10P3					300	3.33	0.77	0.30
2S10P4					400	2.50	0.71	0.40
2S15P1	1.5	2.5	1000	666.6	100	10.00	1.30	0.15
2S15P2					200	5.00	1.15	0.30
2S15P3					300	3.33	1.03	0.45
2S15P4					400	2.50	0.94	0.60
2S20P1	2.0	2.5	1000	500	100	10.00	1.67	0.20
2S20P2					200	5.00	1.43	0.40
2S20P3					300	3.33	1.25	0.60
2S20P4					400	2.50	1.11	0.80
5S10P1	1.0	5.0	1000	1000	100	10.00	0.91	0.10
5S10P2					200	5.00	0.83	0.20
5S10P3					300	3.33	0.77	0.30
5S10P4					400	2.50	0.71	0.40
5S15P1	1.5	5.0	1000	666.6	100	10.00	1.30	0.15
5S15P2					200	5.00	1.15	0.30
5S15P3					300	3.33	1.03	0.45
5S15P4					400	2.50	0.94	0.60
5S20P1	2.0	5.0	1000	500	100	10.00	1.67	0.20
5S20P2					200	5.00	1.43	0.40
5S20P3					300	3.33	1.25	0.60
5S20P4					400	2.50	1.11	0.80

^a s/c: Weight ratio of OPC to PVA solution (PVA sol/OPC)

^b s/PL: Weight ratio of PL to PVA solution (PVA sol/PL)

^c s/(c + PL): Ratio of the sum of the weights of PL and OPC to the weight of the PVA solution (PVA Sol./(OPC + PL))

^d PL/c: Weight ratio of PL to OPC (PL/OPC)

In Table 3, the labels of the samples are indicated as follows: The first number “2” signifies that the concentration of the PVA solution is 2.5%, while “5” denotes a concentration of 5.0%. Next, “S10” indicates that the s/c ratio is 1.0, “S15” indicates that the s/c ratio is 1.5, and “S20” indicates that the s/c ratio is 2.0. Furthermore, “P1” indicates that the amount of PL is 100 g, “P2” denotes 200 g, “P3” signifies 300 g, and “P4” represents 400 g.

For the PVA solution, distilled water was heated to a high temperature of 80–100 °C, followed by the addition of PVA particles according to the selected dosage, and slow stirring for 1–2 h with a magnetic stirrer. A thermostat was set to maintain the temperature of the solution at 80–90 °C. The PVA solution, in which the PVA particles were completely dissolved, was left to stand at room temperature for about 6 h. The OPC and PL were weighed according to the mixing ratio and added to the mixer, which was then cooled to room temperature and mixed. Mixing was performed using a device specified in ASTM C305 at low speed (140 ± 5 r/min) for 60 s, paused for 30 s, and then mixed at medium speed (285 ± 10 r/min) for 120 s. After pausing midway for 30 s during mixing, the paste collected on the sides of the bowl was scraped back into the batch. After mixing, the sample was poured into a 50 mm cubic mold, the surface was flattened, and it was placed in a high-temperature wet-curing machine (60 °C, RH ≥ 90%). After 12 h, the samples were removed from the high-temperature wet-curing machine and placed in a high-temperature dry-curing machine (105 ± 5 °C) for an additional 12 h. Thereafter, the samples were stored in an environment with room temperature, similar to the conditions used in the laboratory.

In this study, the effect of expanding the pores generated by PVA was investigated by applying a high-temperature dry-curing process. An optical method was employed to observe the distribution, shape, and diameter of various large pores. This method was also applied in previous studies using PVA (Cao et al., 2023). First, images of the macropores on each sample surface were acquired using an optical microscope and then binarized using Photoshop image-processing software. Subsequently, the images were analyzed after binarization using Image-J software to obtain the average pore diameter of the macropores distributed on the sample surface. For the analysis of hydration reaction materials, the crushed sample pieces after measuring the compressive strength were immersed in isopropyl alcohol for 24 h and then dried in a vacuum desiccator for 48 h. The dried crushed pieces were ground into a fine powder of less than 75 μm using a mortar and pestle, and then XRD, TG/DAT and FT-IR were measured. Hydration oxide analysis was performed to analyze the type and amount of hydration reactants according to the amount of OPC and dosage of the PVA solution. X-ray diffraction (XRD) was conducted using an X'Pert3 from Malvern Panalytical, with Cu-Kα radiation (λ = 1.54443 Å), a 40-kV voltage, a 0.017° (2θ) step size, and a measurement range of 5°–60°. Thermogravimetric analysis (TGA) and differential thermal analysis (TG/DTA) were performed in a nitrogen gas environment using a Discovery TGA 55 from TA Instruments in a temperature range of 30–800 °C at a rate of 10 °C/min. To determine changes in the hydration products, Fourier-transform infrared spectroscopy (FT-IR) was performed using an IS50 instrument (Thermo Fisher Scientific Nicolet). The FT-IR spectra were measured in the wavenumber range of 400–4000 cm⁻¹. The thermal conductivities of the test specimens were measured using MP2 equipment, which can measure values ranging from 0.03 to 5.0 W/m∙K using a Transient Plane Source sensor (TPS-4) that meets the standards of ASTM D7984.Dry density was calculated using the following equation:$${\rho }_d=\frac{M_d}{V},$$where ${\rho }_d$ is the dry density, $M_d$ is the mass after drying in a dryer at 105 ± 5 °C for 24 h, and V is the volume of the test specimen.

Results and Discussion

Macroporosity

Figure 1 presents an photograph of the mixtures shown in Table 3. The samples measure 50 mm × 50 mm × 50 mm. The foaming characteristics of each sample indicated that the number and size of pores increased as the amount of PL increased in the two types of PVA solution concentrations. Specifically, the pore diameter was observed to gradually decrease in the order of P4 > P3 > P2 > P1. In addition, at each s/c ratio, the P1 mixtures (2S10P1, 2S15P1, 2S20P1, 5S10P1, 5S15P1, and 5S20P1) exhibited material separation, with light PL floated to the top of the sample, while heavy OPC settled to the bottom. Notably, as the s/c ratio increased from 1.0 to 2.0, the precipitation of OPC became more pronounced, leading to the lower OPC being divided into two parts: that is, most of the pores are observed in the upper part where the PL is located. This phenomenon is thought to occur because the PL was not uniformly distributed within the matrix of the sample due to the smaller amount of PL used. Considering the amount of PVA solution and OPC considered in this study, it is judged that the development of an optimal mixing method or mixing device that does not cause material separation is necessary for the production of ultra-light foam concrete with various foaming effects and performances. This study is in the initial stage of examining the performance and effect of PVA as a foaming agent, but in follow-up studies, research is needed to improve mixing homogeneity.

[See PDF for image]

Fig. 1

Images of Series 1 samples: a 2S10P1, b 2S10P2, c 2S10P3, d 2S10P4, e 2S15P1, f 2S15P2, g 2S15P3, h 2S15P4, i 2S20P1, j 2S20P2, k 2S20P3, l 2S20P4, m 5S10P1, n 5S10P2, o 5S10P3, p 5S10P4, q 5S15P1, r 5S15P2, s 5S15P3, t 5S15P4, u 5S20P1, v 5S20P2, w 5S20P3, and x 5S20P4

Furthermore, the P4 samples, which contained the largest amount of PL at each s/c ratio, exhibited irregular pore sizes and distorted pore shapes rather than circular ones. This is believed to result from the excessive amount of PL during high-temperature curing, which interferes with the expansion and movement of pores, thereby deforming their shapes. In contrast, the P2 and P3 samples demonstrated a homogeneous pore distribution with pores of similar sizes. Therefore, regardless of the s/c ratio in the two types of PVA solution concentrations, when the s/PL ratio is 5.0 and 3.33, the s/(c + PL) ratio ranges from 0.77 to 1.43, and the PL/c ratio falls within the range of 0.2 to 0.6, the pore presentation shape and distribution are considered appropriate.

In addition, the pore sizes of the samples with a PVA solution concentration of 5.0% were found to be larger than those of the 2.5% samples. This increase in pore size is attributed to the greater number of micropores generated with a higher amount of PVA, which expand into larger pores with greater diameters during the high-temperature-curing process.

It was confirmed that the PVA aqueous solution can serve as an effective foaming agent when PL is used as an aggregate and produced in a high-temperature-curing environment. Moreover, the shape and size of the pores were found to vary depending on the concentration of the PVA solution and the amounts and ratios of PL and OPC. This indicates the potential usability and effectiveness of PVA aqueous solution as a foaming agent.

Average Pore Diameter

Figure 2 presents a schematic of the process for acquiring images of micropores on each sample surface using an optical microscope, followed by binarization with Photoshop image-processing software. The images were analyzed post-binarization using Image-J software to obtain the average pore diameter of the macroscopic pores distributed on the sample surface.

[See PDF for image]

Fig. 2

Surface and binarized schematic images: a 2S10P2, b 5S10P3, c 2S15P4, d 5S15P4

Figure 3 displays the average pore diameter measured through image analysis of the binarized images. The changes in average pore diameter in relation to variations in the s/c ratio and the amount of PL are illustrated in Fig. 4a, b. In these figures, the change in average diameter with respect to the s/c ratio did not exhibit a specific trend for the P1 sample, which contained the smallest amount of PL. This lack of a clear pattern is believed to be due to the material separation observed in the P1 samples, as shown in Fig. 1.

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Fig. 3

Average pore sizes of samples: a 2.5% PVA solution, b 5.0% PVA solution

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Fig. 4

Comparison and trends of average pore diameters according to mixing ratio, a PL/c ratios, b s/(c + PL) ratios, c s/PL ratios

For the P2 to P4 samples, the average pore diameter increased with an increase in the s/c ratio. The reaction of OPC with water results in the formation of hydration products, leading to a rapid decrease in fluidity. This setting and curing process impedes the expansion of pores under high-temperature-curing conditions. Thus, regarding pore expansion, a smaller amount of OPC enhances the expansion effect. Consequently, the increase in the s/c ratio promotes the pore expansion effect, as a lower OPC amount is associated with the same quantity of PVA solution, leading to an increase in the average pore diameter. Notably, the P4 samples exhibited the largest average pore diameter due to having the smallest OPC amount and the largest PL amount in the mixture. However, as illustrated in Fig. 1, the pore shapes in the P4 samples were not spherical but rather severely distorted and irregular. This resulted in the largest measurement error for the average pore diameter in the P4 samples among both the 2.5% and 5.0% PVA solution concentration samples.

The samples with a 5.0% PVA solution demonstrated larger average pore diameters compared to the 2.5% samples. Excluding the P1 samples where material separation occurred, the 5.0% samples exhibited average pore diameters approximately 2.0–9.4% larger than those of the 2.5% samples. This increase is attributed to the greater generation of bubbles as the PVA concentration rises (Topic et al., 2015). It is inferred that these numerous pores rapidly coalesce into larger pores by absorbing adjacent pores during the expansion process in a high-temperature-curing environment.

In Fig. 3, at 2.5 and 5.0% PVA concentrations, the deviation of the average pore diameter values of the P4 samples with the highest amount of PL was the largest. Next, the deviation values decreased in the order of P1 and P2, and P3 showed the smallest deviation value. It is judged that the large deviation of the average pore diameter of P4 is due to the decrease in resistance to the expansion of bubbles due to the relatively large proportion of PL, which caused excessive pore expansion. As already mentioned in the explanation of Fig. 1 in the previous section, the pore sizes of the P4 samples were non-uniform regardless of the PVA concentration, and the shapes were not circular but rather distorted and irregular. Therefore, it is judged that when the amount of PL exceeds a certain limit, it causes an uneven negative effect on the formation and size distribution of pores. In contrast, P1 is the sample with the relatively smallest amount of PL. Since the amount of PL, which is lighter than OPC, is small, most of the pores that occurred are observed in the upper part of the sample, and almost none are observed in the lower part. This can be confirmed through the typical appearance of the P1 samples in Fig. 1. Therefore, it is judged that similar pore sizes are distributed homogeneously in the PL ratios of P2 and P3. As a result, the PL mixing ratio with a small deviation in the average pore diameter in the two types of PVA concentrations in Fig. 3 was achieved.

Figure 4 illustrates the relationship between the average pore diameter and the three mixing ratios (s/PL, s/(c + PL), and PL/c) detailed in the mixing ratio of Table 3. Figure 4a presents the relationship between the average pore diameter and the PL/c ratio. For all three types of s/c ratios, a larger pore diameter was measured with a PVA solution dosage of 5.0% compared to 2.5%. In addition, as the PL/c ratio increased, the average pore diameter also increased. This trend was consistent across all s/c ratios, indicating that the increase in the amount of PL, which has a lower density than OPC, significantly affects the pore expansion effect. Notably, the PL/c ratios for the samples 2S10P1 and 5S10P1 were 0.1, for 2S15P1 and 5S15P1 were 0.15, and for 2S20P1 and 5S20P1 were 0.2, which is consistent with the material separation observed in these samples, as shown in Fig. 1. Furthermore, the PL/c ratios for 2S10P4 and 5S10P4 were 0.4, for 2S15P4 and 5S15P4 were 0.6, and for 2S20P4 and 5S20P4 were 0.8, indicating that the pores exhibited irregular and twisted shapes rather than the circular pore shapes illustrated in Fig. 1. Therefore, it is considered advantageous to design the mix so that the PL/c ratio is maintained within the range of 0.2 to 0.6 to form pores with a spherical shape, minimal differences in pore diameters, and a similar, homogeneous size.

Figure 4b depicts the change in pore diameter according to the s/(c + PL) ratio. Consequently, an increase in the amount of OPC and PL interferes with the pore expansion effect, leading to a decrease in the average pore diameter. The range of s/(c + PL) ratios for only P2 and P3 samples is 0.77–1.43, excluding the samples with material segregation and irregular and distorted pores (P1 and P4). However, this range still includes the mixing ratios of some P1 and P4 samples. Therefore, the s/(c + PL) ratio does not provide sufficient information for selecting the optimal mix of ultra-lightweight foam concrete. It is deemed necessary to consider the remaining mixing ratios—s/c, PL/c, and s/PL—simultaneously.

Figure 4c illustrates the trend of the s/PL ratio and the average pore diameter. As the s/PL ratio increased, the average pore diameter decreased. This indicates that as the amount of PL increased, the average pore diameter also increased. The s/PL ratio for the samples exhibiting material separation was 10.0, while the samples with irregular and distorted pore shapes had a value of 2.5. It is concluded that an s/PL ratio in the range of 2.5 to 10.0 is appropriate for mixing, considering the distribution of pores and the homogeneity of pore sizes.

Dry Density

Figure 5 presents the results of dry density measurements. The dry density of the 5.0% PVA solution samples was lower than that of the 2.5% samples. This can be attributed to the larger average pore diameter observed in the 5.0% samples compared to the 2.5% samples, as discussed in the previous section. In addition, the 5.0% PVA solution generates more pores than the 2.5% solution (Topic et al., 2015), resulting in a relatively higher number of pores formed in the matrix, which significantly contributes to the reduction in density. This trend was consistently observed across the three s/c ratios.

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Fig. 5

Dry density

The dry density gradually decreased as the s/c ratio increased. In Table 3, an increase in the s/c ratio corresponds to a decrease in the amount of OPC for a constant amount of PVA solution. The density of OPC (3.15 g/cm³) is approximately 29 times greater than that of PL (0.11 g/cm³), indicating that an increase in the amount of OPC is a significant factor contributing to the increase in dry density. Therefore, it was determined that an increase in the s/c ratio results in a decrease in dry density, which can enhance lightweightness.

Regardless of the s/c ratio, an increase in the amount of PL led to a decrease in dry density. Consequently, the dry density of the P1 samples, which contained the smallest amount of PL, was the highest, while the dry density of the P4 samples, which had the largest amount of PL, was the lowest. This confirms that an increase in the amount of PL significantly impacts the reduction in dry density and is an essential material for achieving the lightweightness of the specimen.

As the amount of PL decreases in the order of P4 > P3 > P2 > P1, the deviation of the dry density value also decreases. This is related to the characteristics of the average pore diameter mentioned in Fig. 3. That is, as the average diameter increases, the dry density decreases. Therefore, the dry density of P4 has the smallest value. However, as the amount of PL increases, the pore size increases, and the shape of the pore is not spherical but distorted and irregular. Therefore, as the size and shape of the pores become heterogeneous, the internal space between the particles and the arrangement of the pores also become non-uniform. As a result, the dry density is affected by the shape, size, and distribution of the pores, and it is judged that as the amount of PL increases, the expansion of the pores, irregular shape, and extreme changes in pore size increase the deviation of the density value.

Figure 6 illustrates the effects of the PL/c, s/(c + PL), and s/PL ratios on dry density for three s/c ratios. Figure 6a shows the correlation between the PL/c ratio and dry density. For all three s/c ratios, dry density decreased as the PL/c ratio increased. In other words, when the amount of OPC remains constant, an increase in the amount of PL increase the PL/c ratio, resulting in a decrease in dry density and an improvement in lightweight. The slope of the graph for the samples with the highest amount of OPC (s/c = 1.0) was the steepest, indicating that in mixtures with a high OPC content, an increase in PL significantly and rapidly reduces dry density. In contrast, the samples with s/c ratios of 1.5 and 2.0 exhibited gentler and similar graph slopes compared to the s/c = 1.0 sample, despite the increase in PL amount.

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Fig. 6

Comparison and trends of dry density according to mixing ratio, a PL/c ratios, b s/(c + PL) ratios, c s/PL ratios

Figure 6b depicts the relationship between the s/(c + PL) ratio and dry density. As the s/(c + PL) ratio increased across the three s/c ratios, dry density also increased. When the amounts of PVA solution and OPC are constant, the s/(c + PL) ratio decreases as the amount of PL increases, which can be confirmed by the values shown in Table 3. The decrease in the s/(c + PL) ratio is attributed to the increase in PL amount, which enhances the density reduction effect and improves lightweight. In addition, the samples with the highest amount of OPC (s/c = 1.0) demonstrated the most significant dry density reduction effect in relation to the s/(c + PL) ratio, with a steep graph slope. As the amount of OPC gradually decreased and the s/c ratio increased, the dry density reduction effect due to the increase in the s/(c + PL) ratio slowed down, resulting in a gentler slope.

Figure 6c illustrates the effect of the s/PL ratio on dry density. As the amount of PL increased, the s/PL ratio decreased, and dry density also decreased. The amount of OPC influences the change in dry density according to the s/PL ratio. Specifically, the samples with the highest amount of OPC (s/c = 1.0) exhibited the largest change in dry density with increasing PL amount, resulting in the steepest graph slope. Conversely, the samples with s/c = 1.5 and 2.0 showed a decrease in dry density with increasing PL, with a gradual change in the slope of the graph.

Through the relationship between the three mixing ratios and dry density, it was demonstrated that the amount of OPC negatively affects lightweightness by increasing dry density. Conversely, an increase in the amount of PL significantly contributes to the reduction in dry density. Notably, the higher the OPC content in the mixture (e.g., s/c = 1.0), the steeper the change in dry density with respect to the PL amount. This indicates that both the concentration of the PVA solution, which affects pore formation, and the selection of OPC and PL ratios significantly influence the dry density values of ultra-lightweight foam concrete, highlighting the need for careful consideration in their selection.

Compressive Strength

Figure 7 displays the compressive strength measurements of the samples. Specifically, Fig. 7a shows the strength values at 1 day (d) and 28 days for samples mixed with the 2.5% PVA solution. As the amount of PL increased from P1 to P4, the compressive strength decreased for all s/c ratios. With an increase in the s/c ratio, which corresponds to a decrease in OPC content, the mechanical performance also declined. This is because OPC is the sole material that forms hydration products through hydration reactions, and the amount of OPC directly influences the quantity of hydration products and, consequently, the strength. Thus, a reduction in the amount of OPC significantly impacted the compressive strength. The compressive strength values at 1 d and 28 d exhibited small changes of less than approximately 5.0%, except for the P4 sample, which showed the lowest strength and standard deviation due to the irregular and distorted shape of the foamed pores and the large pore size.

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Fig. 7

Compressive strength of samples: a 2.5% PVA solution, b 5.0% PVA solution, c 2.5% and 5.0% PVA solutions at 1 d, and d 2.5% and 5.0% PVA solutions at 28 d

Figure 7b presents the compressive strength measurements for the 5.0% samples. This trend is similar to that of the 2.5% samples; as the s/c ratio and amount of PL increased, the compressive strength decreased. For the 5.0% samples, the difference in compressive strength values between the 1 d and 28 d samples was also small (≤ 5%). There are two possible reasons for the minimal difference in strength values between 1 and 28 d. First, the amount of OPC was relatively small. This mixture was designed to achieve ultra-lightweight through a high s/c ratio (≥ 1.0). A smaller amount of OPC implies limited production of hydration products. Second, high-temperature-curing promotes an early hydration reaction. The samples showed that the two-step high-temperature-curing process newly applied in this study accelerated the hydration reaction of OPC. It is inferred that the number of OPC particles undergoing rehydration or delayed hydration reactions was relatively small, despite the elapsed time up to 28 d.

Figure 7c compares the compressive strength values of the 2.5 and 5.0% samples at 1 d, while Fig. 7d compares the strength values at 28 d. At both time points, the 2.5% samples exhibited higher strength values than the 5.0% samples. As shown in Fig. 5, the dry density of the 5.0% samples was lower than that of the 2.5% samples, indicating a greater number of pores in the 5.0% samples. This observation aligns with the analysis of dry density results in Figs. 5 and 6, which demonstrate that the dry density of the 5.0% samples was significantly lower than that of the 2.5% samples. Consequently, the compressive strength of the 5.0% samples was lower than that of the 2.5% samples. As the PVA dosage increased, the strength decreased, consistent with previous reports on PVA-modified cement (Xing et al., 2023). In other words, the pores formed by PVA reduce mechanical performance, and an increase in the size or number of pores leads to lower compressive strength (Morlat et al., 2007; Topic et al., 2015).

As shown in Fig. 7, the deviation value for the compressive strength of the P4 samples was the largest regardless of the concentration of PVA. While P1, P2, and P3 showed similar deviation values for the compressive strength, P4 showed the largest deviation value. This is related to the change in pore size in Figs. 3 and 5. That is, P4, which had the largest pore size, irregular shape, and lowest dry density, had the lowest mechanical performance. As a result, it is judged that the change in strength was caused by the presence of large pores formed by the irregular size distribution of pores and excessive expansion, which increased the deviation value of the compressive strength. Therefore, the amount of PL needs to be limited to an appropriate range considering the pore size, shape, and distribution due to expansion.

The compressive strength results shown in Fig. 7 indicate that mechanical performance decreases as the PVA dosage increases. There are two possible explanations for this decline in strength with increasing PVA dosage. The first is that polymer materials such as PVA coat the surfaces of OPC particles and interfere with the hydration reaction (Pique & Vazquez, 2013; Singh et al., 2003; Topic et al., 2015). However, these hydration delay factors are judged to have a considerably small effect due to the new high-temperature-curing method applied in this study. Rather, it is judged that they may be due to the insufficient hydration reaction material due to the relatively small amount of OPC for the high s/c ratio. The second reason is the formation of a loose matrix due to the generation of pores by PVA (Lu et al., 2018; Kim & Robertson, 1999; Kim & Robertson, 1997; Viswanath & Thachil, 2008). And this second factor due to pores is thought to be the main cause of the decrease in strength.

Figure 8 illustrates the relationships between the compressive strength values of 2.5% and 5.0% PVA samples with three different s/c ratios, as well as the PL/c, s/(c + PL), and s/PL ratios.

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Fig. 8

Comparison and trends of compressive strength according to mixing ratio, a PL/c ratios, b s/(c + PL) ratios, c s/PL ratios

First, Fig. 8a examines the relationship between strength and the PL/c ratio. Regardless of the s/c ratio and PVA dosage, compressive strength exhibited a decreasing trend as the PL/c ratio increased. In all samples, an increase in the amount of PL resulted in a reduction in compressive strength. When the PL/c ratio exceeded 0.4, the compressive strength values for both the 2.5% and 5.0% PVA dosages were very similar. However, when the PL/c ratio was less than 0.4, the difference in compressive strength values between the 2.5% and 5.0% PVA dosages became more pronounced. Furthermore, when the s/c ratio was 1.0, the difference in strength values based on PVA dosage was minimal and similar. In contrast, at s/c ratios of 1.5 and 2.0, the difference in compressive strength values according to the PVA dosage was clearly observed. Therefore, as the amount of OPC decreases—as the s/c ratio increases—the difference in the compressive strength value according to the PVA concentration gradually decreases. This phenomenon means that the greatest influence on the mechanical performance is related to the amount of OPC that forms the hydration reaction material, and at high s/c ratios where the amount of OPC decreases, the change in strength becomes smaller even if the concentration of PVA increases as a result. It is judged that the effect of the amount of OPC on the mechanical properties is greater than that of the change in PVA concentration.

Figure 8b shows the relationship between the total binder amount (c + PL) and the mass ratio of the PVA solution (s/(c + PL)) in relation to compressive strength. Regardless of the s/c ratio and PVA dosage, compressive strength increased as the s/(c + PL) ratio increased. This implies that compressive strength improves as the amount of PL decreases. Similarly, in Fig. 8c, compressive strength also increased with an increasing s/PL ratio. In other words, a decrease in the PL amount under consistent mixing conditions positively influences the improvement of mechanical performance.

Hydration products (XRD)

Figure 9 presents the XRD results of the hydration reactants. Analysis of the hydration reactants in the samples revealed that the main hydration products included calcium carbonate (Cc), portlandite (Ch), belite (B), alite (A), gypsum (G), quartz (Q), monosulfoaluminate (Ms), and ettringite (E). The peak heights of each hydration reactant varied slightly depending on the dosage of the PVA solution and the amounts of OPC and PL, but no new hydration reactants were detected. In other words, the dosage of the PVA solution shown in Fig. 9 did not influence the formation of new hydration products.

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Fig. 9

XRD results for hydrated material analysis of samples, a 2.5% PVA solution, s/c = 1.5, b 5.0% PVA solution, s/c = 1.0, c 2.5% PVA solution, s/c = 2.0, d 5.0% PVA solution, s/c = 1.5, e 2.5% PVA solution, s/c = 2.0, and f 5.0% PVA solution, s/c = 2.0 (Cc: calcium carbonate; Ch: portlandite; B: belite; A: alite; G: gypsum; Q: quartz; Ms: monosulfoaluminate; E: ettringite)

In Fig. 9, the intensity of the ettringite peak, which is a representative hydration product of OPC, is very weak. This is attributed to the hindrance of the hydration reaction of the OPC particles by the PVA. Previous studies have reported similar findings regarding the delay and reduction in the formation of OPC hydration products caused by PVA (Georgescu et al., 2002; Nguyen et al., 2016). Another analysis is the very small amount of OPC. In this study, high-temperature-curing method was applied to improve the hydration delay by PVA. Nevertheless, the weak intensity of the Ettringite peak may be due to the very small amount of OPC used to maintain a high s/c ratio considering the lightweightness.

Thermal Analysis (TG/DTA)

Figure 10 shows the results of the thermal analysis for the detection of hydration reactants. A weight change near 100 °C indicates moisture evaporation (Michel et al., 2011; Zhang et al., 2019). Weight loss in the temperature ranges of 80–120 °C or 130–150 °C is attributed to the decomposition of ettringite (Michel et al., 2011; Carballosa et al., 2020). In the 100–200 °C range, weight loss due to gypsum decomposition was observed (Vassileva & Vassilev, 2005). Weight loss from the decomposition of monosulfoaluminate occurs in the temperature range of 150–200 °C (Ma et al., 2019). Thus, weight loss due to the decomposition of various hydration reactants was observed in the temperature range of 30–200 °C. The mass loss observed in the temperature range of 370–450 °C is due to the decomposition of portlandite (Zhang et al., 2019). At 600–800 °C, a weight change was detected owing to the decomposition of calcium carbonate (Shi et al. 2010).

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Fig. 10

Thermal analysis results for hydrated material analysis of samples: a 2.5% PVA solution, s/c = 1.0, b 5.0% PVA solution, s/c = 1.0, c 2.5% PVA solution, s/c = 1.5, d 5.0% PVA solution, s/c = 1.5, e 2.5% PVA solution, s/c = 2.0, and f 5.0% PVA solution, s/c = 2.0

The quantitative changes in the hydration reactants are compared in Fig. 10. At approximately 200 °C, the presence and effect of PVA can be confirmed through the change in the weight loss rate due to the dehydration of PVA (Wiśniewska et al., 2011; Budrugeac, 2008; Yu et al., 2005). In Fig. 10, it can be seen that the 5.0% PVA solution samples exhibit a larger peak at approximately 200 °C than the 2.5% samples. The weight loss rate at approximately 200 °C is not distinctly clear because it includes the effects of gypsum decomposition. Nevertheless, when comparing samples with the same OPC content in Fig. 10, the change in the weight loss rate at approximately 200 °C was clearly confirmed as the PVA dosage increased, thereby validating the effect of the PVA dosage.

Furthermore, in the 2.5% PVA solution samples in Fig. 10, as the s/c ratio increased, the weight loss due to the decomposition of portlandite observed in the temperature range of 370–450 °C gradually decreased. In other words, the amount of portlandite can be seen to be smaller in Fig. 10a (s/c = 1.0) > Fig. 10c (s/c = 1.5) > Fig. 10e (s/c = 2.0). The same decreasing trend was observed for the 5.0% PVA samples (Fig. 10b (s/c = 1.0) > Fig. 10d (s/c = 1.5) > Fig. 10f (s/c = 2.0)). This phenomenon occurred because the amount of OPC decreased as the s/c ratio increased. In other words, the higher the OPC content, the more portlandite remains in the sample, resulting in a larger peak value of the DTA curve (Cao et al., 2023).

The presence of portlandite in the 370–450 °C range, indicated by the large and clear peaks detected in Fig. 10, suggests that many OPC particles remained unhydrated due to the influence of PVA. This supports findings from several previous studies that PVA interferes with the hydration reaction of OPC particles by coating their surfaces and blocking contact with water (Cao et al., 2023; Thong et al., 2016; Viswanath & Thachil, 2008). As shown in Fig. 10, despite the dosage differences of 2.5% and 5.0%, the difference in the weight loss rate due to portlandite decomposition in the 370–450 °C range was similar at the same s/c ratio. Some previous studies have reported that the amount of portlandite decreases as the PVA dosage increases (Cao et al., 2023; Nguyen et al., 2016). However, no significant change in the amount of portlandite was observed between the 2.5% and 5.0% PVA concentrations in this study. This is because the amount of OPC used in this experiment was small, and most of the portlandite would have been consumed by the hydration reaction in the initial stage of hydration with the high-temperature-curing method.

FT-IR

Figure 11 show the Fourier-transform infrared spectroscopy (FT-IR) analysis results for samples. The peak at approximately 450 cm⁻¹ indicates characteristics of the in-plane Si–O bending vibration in the SiO₄ tetrahedron (Luo et al., 2022). The strong and clear band at 1425 cm⁻¹ and weak peak at approximately 871 cm⁻¹ are characteristic of the stretching vibrations of the O–C–O bonds of CO₃²⁻. This corresponded to the calcite vibration mode (Bayat et al., 2018; Mikhailova et al., 2019). The wide band observed at approximately 990 cm⁻¹ is characteristic of the asymmetric stretching vibration of Si–O–T (Si or Al) (Luo et al., 2022; Zhang et al., 2021). The gentle peak observed at 1115 cm⁻¹ corresponds to the stretching vibrations of S–O due to SO₂⁴⁻. This is due to the presence of ettringite (Horgnies et al., 2013; Zhang et al., 2021) and monosulfoaluminate (Horgnies et al., 2013; Pooni et al., 2020). In addition, the absorption bands in the 1100–1200 cm⁻¹ range are characteristic of the vibration of the SO₄²⁻ group present in gypsum (Choudhary et al., 2015). The 1600–1700 cm⁻¹ and 3100–3700 cm⁻¹ ranges showed a strengthening band caused by the O–H vibration of water (H₂O) (Lemougna et al., 2017; Luo et al., 2022). The prominent peak at 3640 cm⁻¹ is characteristic of the symmetric and asymmetric stretching vibrations of O–H (García Calvo et al., 2022). The O–H bond characteristics observed at 3640 cm⁻¹ indicated the presence of portlandite (Ca(OH)₂) (García Calvo et al., 2022; Ylmén & Jäglid, 2013), ettringite (Horgnies et al., 2013), and monocarboaluminate (Horgnies et al., 2013).

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Fig. 11

FT-IR analysis results for hydrated material analysis of samples, a 2.5% PVA solution, s/c = 1.0, b 5.0% PVA solution, s/c = 1.0, c 2.5% PVA solution, s/c = 1.5, d 5.0% PVA solution, s/c = 1.5, e 2.5% PVA solution, s/c = 2.0, and f 5.0% PVA solution, s/c = 2.0

The absorption peaks at 1640, 2860, and 2940 cm⁻¹ confirm the presence of PVA. 2910 cm⁻¹ represents the asymmetrical stretching of CH₂, 2860 cm⁻¹ represents the symmetric stretching of CH₂, and 1645 cm⁻¹ represents the H–O–H deformation, indicating the presence of PVA (Deng et al., 2014; Qua et al., 2009). It is difficult to state a clear trend because the 1640 cm⁻¹ peak overlapped with the O–H vibration of water (H₂O) in the 1600–1700 cm⁻¹ range. However, as the dosage of the PVA solution increased from 2.5% to 5.0%, the absorption peaks at 2860 cm⁻¹ and 2940 cm⁻¹ tended to increase. This is clear when comparing the corresponding peaks for the 2.5% dosage sample in Fig. 11a, c, e and the 5.0% dosage sample in Fig. 11b, d, f. As the s/c ratio increases, the amount of OPC decreases; thus, the height of the peak at 3640 cm⁻¹ gradually decreases. In other words, s/c = 1.0 (Fig. 11a, b) > s/c = 1.5 (Fig. 11c, d) > s/c = 2.0 (Fig. 11e, f); the peak decreases in this order.

Thermal conductivity

Figure 12 presents the thermal conductivity measurement results of the samples. In Figs. 12, when the s/c ratio was 1.5 and 2.0, the P1 samples were separated into layers of OPC and PL due to differences in density, making thermal conductivity measurement impossible (see Figs. 1e, i, q, u).

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Fig. 12

Thermal conductivity results for samples

In Figs. 12, both the 2.5% and 5.0% PVA solution samples exhibited low thermal conductivities in the range of 0.13–0.19 W/m∙K for P2 and P3. The s/PL ratios for P2 and P3 were 5.0 and 3.33, respectively. The sample with the highest amount of PL, P4, showed higher thermal conductivity than P3 due to its relatively large pore size and less dense matrix. Consequently, sample P4 (s/PL = 2.5) exhibited high efficiency and performance regarding thermal conductivity. The thermal conductivity of the 5.0% sample was lower than that of the 2.5% sample. As indicated in Figs. 1 and 3, the pore size of the 5.0% sample was slightly larger than that of the 2.5% sample. However, as shown in Fig. 5, the number of pores in the 5.0% sample was greater than in the 2.5% sample, resulting in a lower dry density value for the 5.0% samples. It is concluded that the 5.0% PVA solution samples formed larger pore diameters and a greater number of pores in the matrix, which enhanced insulation performance. The increase in pore diameter and quantity due to the higher PVA dosage contributes to improved insulation and decreased dry density (Lu et al., 2018).

In addition, when s/c = 1.0, the thermal conductivity decreased in the order of P1 > P2 > P3, while for s/c ratios of 1.5 and 2.0, thermal conductivity decreased in the order of P2 > P3. This indicates that, when the amount of OPC is constant, thermal conductivity tends to decrease as the amount of PL increases. This is because PL itself possesses insulating properties due to its extremely fine pores; thus, an increase in PL content reduces thermal conductivity (Celik et al., 2016; Davraz et al., 2020; Karakaş et al., 2023).

The results of the thermal conductivity measurements examined the effects of PVA concentration, which directly impacts pore formation in the matrix, and the amount of PL, which has inherent insulating properties, on insulation performance. It was demonstrated that insulation was improved with higher PVA concentrations and increased PL amounts. However, when the PL amount is excessive (as seen in the P4 sample), the pores can connect to form passages, allowing heat transfer and thereby reducing the insulating effect. Therefore, in order to improve the insulation performance, it is important to select an appropriate mixing ratio of OPC and PL that does not cause material separation and does not form excessive amounts of pores.

Figure 13 illustrates the trends in thermal conductivity based on changes in the PL/c, s/(c + PL), and s/PL ratios of the samples. The thermal conductivity characteristics according to the three mixing ratios in Fig. 13 indicate that the sample where material separation occurred (P1) and the sample with irregular and distorted pore shapes (P4) exhibited high thermal conductivity values. The thermal conductivity of these samples could not be measured, or the values showed significant differences compared to the other samples. Therefore, the range affecting the thermal conductivity of the remaining samples (P2 and P3), excluding those two cases, was considered.

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Fig. 13

Comparison and trends of thermal conductivity according to mixing ratio, a PL/c ratios, b s/(c + PL) ratios, c s/PL ratios

The PL/c ratio in Fig. 13a ranged from approximately 0.2 to 0.45, the s/(c + PL) ratio in Fig. 13b ranged from 0.77 to 1.43, and the s/PL ratio in Fig. 13c ranged from 0.33 to 5.0. The thermal conductivity measurement values for the P2 and P3 samples at the three different s/c ratios were in the range of 0.13–0.19 W/m∙K. In addition, the 5.0% PVA solution samples exhibited lower thermal conductivity than the 2.5% samples.

The higher concentration of the PVA solution contributes to a decrease in dry density due to the larger average pore diameter and increased number of pores. Therefore, it can be concluded that the appropriate selection of PVA solution concentration and the amount of PL are important factors in reducing thermal conductivity by influencing pore structure and the distribution of PL particles.

Conclusion

The following conclusions were drawn from the experiments on the manufacture and properties of ultra-light foam concrete using PVA solution as mixing water and ordinary portland cement (OPC) and perlite (PL) as mixing water.

The PVA solution demonstrated its effectiveness and potential as a foaming agent for the manufacture of foam concrete in a high-temperature-curing environment. The changes in the characteristics of the pore shape and size were confirmed at two types of PVA concentrations considered in the experiment, 2.5% and 5.0%. The developed ultra-light foam concrete had excellent foaming characteristics, low dry density, and low thermal conductivity, but its mechanical performance was very low. In addition, PVA did not affect the change of hydration reaction substances or the formation of new hydration reaction substances. The high-temperature-curing method proposed in this study could simultaneously achieve the synergistic effect of promoting the expansion of microbubbles generated by PVA and the hydration reaction at the early age stage. However, there are concerns about the additional energy consumption and cost of the high-temperature-curing process. Therefore, it is judged that it is necessary to develop an energy-efficient-curing method or a new admixture while improving the foaming effect of PVA. In this experiment, the materials, mixing ratio, and PVA concentration considered showed the production and performance of ultra-light foam concrete with a dry density of less than 1.0 g/cm³, a compressive strength of 1–6 MPa, and a thermal conductivity of 0.13–0.19 W/m∙K. The performance of ultra-light foam concrete showed that the mixing ratio of PVA solution (s), OPC (c), and perlite (PL) significantly affected the characteristics and performance of ultra-light foam concrete. Limited to the experimental conditions and mixing ratios considered in this study, the optimal mixing ratios for the production of ultra-light foam concrete were s/PL ratio 5–3.33, PL/c ratio 0.2–0.45, and s/(c + PL) ratio 0.77–1.25.

The developed ultra-light foam concrete has somewhat lower mechanical performance than its insulation performance and structural density. Therefore, it may be recommended to be used in a limited manner as an insulating panel or lightweight filler. As research is conducted in the future to improve strength and further improve performance, it is expected that the application range will expand to a wider range of applications and structural members.

Author contributions

C. Kang: validation, formal analysis, investigation, supervision, methodology, experimentation, writing original/final draft. T. Kim: project administration, conceptualization, validation, formal analysis, experimentation, writing original/final draft and writing review/editing. Y. Park: conceptualization, formal analysis, validation. S.Hong: validation, methodology, investigation, formal analysis. All authors read and approved the final manuscript.

Funding

This research received no external funding.

Availability of data and materials

The data sets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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