Introduction

Foamed concrete (FC) is acknowledged as a high-performance insulating material for wall assemblies1,2, exhibiting a spectrum of favorable characteristics such as low density, thermal retention, thermal insulation, fire resistance, acoustic insulation, and frost resistance3,4,5. Its deployment contributes significantly to the enhancement of building energy efficiency. Foamed concrete is a new lightweight thermal insulation material with a large number of closed holes, which is mainly related to its preparation process6.

Foaming agent is crucial to the stability of foam7,8,9. This places higher demands on the raw materials used in the production of foaming agents. Protein raw materials are divided into plant protein and animal protein. The extraction of animal protein faces problems such as scarce raw material resources, complex processes, high costs, and environmental pollution10,11. However, plant protein foaming agents have not been widely promoted due to their poor foaming performance. To address the challenges faced by the aforementioned foaming agents, this article proposes the use of biomass materials with advantages such as wide sources, green environmental protection, and renewable raw materials to prepare high-performance plant protein foaming agents.

Foam is unstable from the point of view of thermodynamics. The mechanism of destroying the stability of foam mainly includes: The liquid in the foam grid structure is discharged under gravity (foam liquid separation); Gas exchange leads to changes in bubble size (bubble coarsening); The rupture of the liquid film leads to the merging of bubbles (bubble coalescence). These mechanisms of action are influenced by various factors such as surface/interface tension, bubble size, liquid fraction, and bubble viscoelasticity. Scholars usually add surfactants to protein foaming agent to improve the stability of foam. Maglad12 investigated how anionic foaming agents affect the distinct properties of foamed concrete. Fresh-state tests indicate that foamed concrete based on alpha olefin sulfonate (AOS) could be a preferred choice when shorter setting time, better spreadability, and higher density are required. Compared with lightweight foamed concrete (LWFC) based on sodium lauryl ether sulphate (SLES), sodium lauryl sulphate (SLS), and sodium alcohol ether sulphate (AES), the AOS-based foamed concrete shows lower capillary sorption, water absorption, and gas permeability. Sahu13 evaluated the relative performance of foam produced by several different synthetic surfactants. Experimental findings reveal that incorporating carboxymethyl cellulose sodium salt (CMC) into surfactant solutions leads to a marked increase in their viscosity. This viscosity enhancement, in turn, contributes to the refinement of foam microstructure and the optimization of air void distribution within concrete, ultimately resulting in a notable improvement in the compressive strength of foamed concrete.

However, adding surfactants to foaming agents can also have some negative effects. High-quality surfactants usually come at a relatively high price. Adding surfactants to the foaming agent will increase the cost of raw materials and reduce the market competitiveness of the product. In order to ensure that the surfactants are evenly dispersed in the foaming agent and can function properly, it may be necessary to adjust and optimize the production process, which will increase energy consumption and equipment investment. Some surfactants have poor biodegradability and are difficult to decompose in the natural environment, which may cause pollution to the soil, water bodies and other environments14,15. With the increasingly stringent environmental protection requirements, this may limit the use of foaming agents containing such surfactants. The most significant reason why surfactants are not suitable for protein foaming agents is that they may interact with protein molecules, thereby changing the structure and properties of the protein molecules, resulting in a deterioration of the foaming performance of the foaming agent and the stability of the foam16. In recent years, scholars17 have proposed a new method of using transition metals as complexing agents and mixing them with hydrolyzed proteins to prepare foaming solutions.

In this study, systematic experimental investigations were carried out to elucidate the influence of transition metal ion Fe(II) on the stability of aqueous foams and foamed concrete specimens fabricated via hydrolyzed pumpkin seed protein (HPSP)-based systems. The internal mechanism of the interaction between transition metal ions Fe (II) and HPSP to control the stability of foam was analyzed by means of integrated small angle X-ray scattering (SAXS). Moreover, by comparing the physicochemical properties of foams without Fe(II) and those with Fe(II) incorporation, a new protocol for constructing ultra-stable foam systems was put forward. Optical microscopy (OM) was used for morphological characterization of foam microstructures, while scanning electron microscopy (SEM) and X-ray computed tomography were applied to evaluate the microstructural features and performance-related characteristics of the resulting foam concrete.

Materials and methods

Experimental raw materials

Pumpkin seeds were sourced from a local market in Zibo City, while analytical-grade sodium hydroxide and ferrous sulfate (FeSO₄) were procured from Shanghai McLean Chemical Co., Ltd. The cement employed in this experimental study was P.II 52.5 Portland cement, which exhibits a density of 3.57 g/cm³, a specific surface area of 361 m²/kg, and a specific strength of 59 MPa. Its comprehensive compositional details are provided in Table 1. For the plant protein foaming agents readily available on the market (PS), they were sourced from Zhicheng New Building Materials Inc. (China) at a price of 13 USD per liter, and the foaming agent solution used had a concentration of 4%.

Table 1 Cement composition (by weight, %).
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Experimental methods

Production of HPSP

Pumpkin seeds were first sterilized with steam for 20 min. After sterilization, they were dried at 40 ℃ for 18 h, then cooled to 20 ℃ and stored in a cool place for subsequent use.

The operation steps for extracting HPSP foaming agent are shown in Fig. 1, and the specific introduction refers to previous research18.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Schematic diagram of extraction method for HPSP foaming agent.

Single factor test

In order to explore suitable conditions for protein extraction, a single factor experiment was conducted to investigate the effects of three variables: pH value (9, 10, 11), hydrolysis temperature (40, 50, 60 ℃), and hydrolysis duration (2, 3, 4 h). The test was performed in a sequential manner, with each variable examined individually while keeping the other two parameters constant. Specifically, when assessing temperature and hydrolysis time, the pH value was fixed at a predetermined optimal level, and similar control strategies were applied for other variables.​.

For the pH effect assessment: Alkaline pH values (9, 10, 11) were tested to determine their influence on foaming ability, with hydrolysis temperature and duration maintained at 50 ℃ and 3 h, respectively.​.

For the temperature effect assessment: Hydrolysis temperatures of 40, 50, and 60 ℃ were evaluated under fixed conditions of pH 10.0 and hydrolysis duration of 3 h to quantify their impact on foaming ability.​.

For the hydrolysis time effect assessment: To verify the significance of hydrolysis duration on foaming ability, the test was conducted at a fixed pH of 10.0 and temperature of 50 ℃, with duration variables set as 2, 3, and 4 h.​.

Response surface methodology

Based on the results of the single factor experiment, the response surface methodology (RSM) was used again to optimize the three key experimental variables mentioned above19.

The ranges and selected levels are presented in Table 2. A central composite design (CCD) was employed to generate 20 experimental runs, each configured with distinct combinations of the three variables. All experimental data were subjected to statistical analysis using the STAT Ease software package (STAT Ease Inc., USA).

Table 2 Factors and levels in response surface methodology.
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Analysis methods

Taking the preparation of 400 mL foam solution of 1% HPSP and 0.2% FeSO4 (Fe-HPSP) binary composite system as an example, 400mL HPSP liquid was weighed in the container, 0.8 g FeSO4 white powder was weighed with weighing paper and added into the beaker, in which a magnetic rotor is put, and then the beaker was placed on a constant temperature magnetic stirrer for more than 1 h, and the solution obtained was used for testing various properties. Test method for obtaining foam volume by foaming with foaming agent solution, refer to previous literature20,21.

Production of foam and foamed concrete

Mix the two foaming agent solutions to be tested with water.After being placed in the foaming machine for standing, the foaming agents were subjected to an air pressure ranging from 0.4 to 0.6 MPa22. Determine the density of fresh foam in 1 L standard container.

The mix proportion of foamed concrete required in this experiment is adjusted according to the following formula23:

$${\rho _d} = {S_a}{m_c}$$
(1)
$${V_2} = K(V - {V_1}) = K[V - (\frac{{{m_c}}}{{{\rho _c}}} + \frac{{{m_w}}}{{{\rho _w}}})]$$
(2)

As presented in Table 3, the mix proportion design of foamed concrete is summarized as follows. FC-PS and FC-Fe-HPSP represent two distinct types of foamed concrete, corresponding to the respective foaming agents used. Firstly, mix the cement slurry thoroughly for half a minute at a speed of 100 revolutions per minute. Subsequently, the pre-prepared foam and remaining ingredients were added to the cement slurry sequentially. The mixture was then stirred at 60–120 revolutions per minute for 3 min to incorporate the foam into the cement matrix. To ensure the proper performance of the foamed concrete, the mixing duration must be strictly controlled to avoid excessive stirring, which could lead to foam defoaming, pore structure deterioration, and subsequent macroscopic performance issues24.

Finally, the mixture was poured into 100 mm×100 mm×100 mm molds. After 24 h, the specimens were demolded and transferred to a standard curing chamber for a 28 d curing period. Place the standard cured foamed concrete sample in the oven for 24 h, set the oven temperature to 50 ℃, and then measure its density immediately. All experimental procedures were conducted in accordance with the Chinese standard “Foam Concrete” (JG/T 266–2011).

Table 3 Mix proportion design of foamed concrete.
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Analysis method

Characteristics of Fe-HPSP foaming agent solution

The content of 16 hydrolyzed amino acids in HPSP was analyzed by high-performance liquid chromatography (HPLC). The conformational changes of proteins caused by the coordination between HPSP and Fe(II) were observed by low-temperature transmission electron microscopy (Cryo-TEM).

Small-angle X-ray scattering (SAXS) experiments were conducted at 25 ℃ using a SAXS point 2.0 instrument (manufactured by Anton Paar, Austria), which is equipped with a Primux 100 micro X-ray source. The two-dimensional SAXS scattering map obtained above can be used to derive the correlation function between scattering intensity (I) and scattering vector (q). The effective range of the scattering vector q was 0.03–5 nm⁻¹, and the SAXS data were processed using SASFIT software for fitting analysis25.

Test on foam

Viscosity measurement: Foam viscosity was determined using a rotary viscometer (model NDJ-1) fitted with a No. 1 rotor, with all measurements carried out in a 0.5 L beaker.

Surface tension measurement: The surface tension was determined at 25 °C using a Klux K100 surface tension meter, with the Wilhelmy plate method (lifting ring technique) adopted for the test.

Foam stability: Foam stability was evaluated by measuring the weight loss of foam samples at specific time intervals (5, 10, 30, 60, 120, 180, 240, 360, 720, and 1440 min) after filling the foam into a 1 L container26.

Optical microscopy analysis: The thickness of the foam wall was characterized using an optical microscope (model DM 750).

Test on foamed concrete

Mechanical property testing: Firstly, the cured foamed concrete specimens are placed in a drying oven set at 60 °C and subjected to continuous drying until their mass stabilizes (i.e., no further changes in mass are observed). Subsequently, the compressive strength tests of these dried specimens are conducted in accordance with the standard GB/T 11,969 − 2008. During the testing process, a constant loading rate of 1 kN/s is strictly maintained to ensure the accuracy and consistency of the test results.

Water absorption performance testing: The water absorption characteristics of the specimens were evaluated following the guidelines specified in JGJ/T 341–2014. For this test, samples that had undergone a 2 d curing period were subjected to drying at a temperature of 60 ℃. This drying process was carried out until the samples reached a constant weight, allowing for the measurement and recording of their actual dry mass, denoted as md. The mw represents the mass of the sample after soaking in water for 24 h, and calculates the water absorption rate:

$$(m_{w}/m{d})\times100\%$$
(3)

Shrinkage detection: The shrinkage rate of the specimens was determined following the specifications of GB/T 11,969 − 2008. First, the specimens, whose initial lengths had been recorded, were placed in a standard curing chamber. Subsequently, their lengths were remeasured at preset time points, which included 3, 7, 14, 21, 28, 60, and 90 days. The specimens employed in this experiment had dimensions of 40 mm × 40 mm × 160 mm.

Uniformity: The uniformly mixed foamed concrete slurry was poured into a cylindrical mold with dimensions of 50 mm in diameter and 1000 mm in height. Following 28 d of standard curing, small cylindrical specimens (30 mm in height) were cut at 250 mm intervals along the length of the cured cylinder. These small cylinders were then placed in an oven and dried until their mass remained constant. Substitute the measured mass and volume of each sample into the following formula27 to calculate its density and evaluate its uniformity:

$$R=\frac{{\rho}_{x}}{{\rho}_{0}}$$
(4)

R denotes the density ratio; ρx stands for the density at a specific position x; and ρ0 represents the density at the top position of the specimen.

Microstructure and pore feature analysis: The surface morphology and pore size distribution of foamed concrete blocks were studied by Quanta 250 scanning electron microscope and X-ray computed tomography.

Results and discussion

Hydrolyzed Pumpkin Seed Protein (HPSP) Solution

Single factor test results

(a) The pH value.

As shown in Fig. 2 (a). As the pH value of pumpkin seeds increases, the foaming ability of pumpkin seed protein first increases, and then shows a decreasing trend after reaching pH 10.0. The change in pH value may have a certain impact on alkaline protease activity, so the optimal pH value for hydrolyzing pumpkin seed protein is 10.0.

(b) Temperature.

As shown in Fig. 2 (b). The foaming ability shows an upward trend with the increase of hydrolysis temperature before 50 ℃. As the hydrolysis temperature increases, the enzymatic activity of alkaline protease also gradually increases; When the hydrolysis temperature is 50 ℃, the foaming ability of pumpkin seed protein reaches its maximum, indicating that the enzymatic activity of alkaline protease is highest at 50 ℃; After the hydrolysis temperature exceeds 50 ℃, the foaming ability of pumpkin seed protein shows a decreasing trend with the increase of hydrolysis temperature, and the enzyme may begin to inactivate after reaching a certain temperature. It can be inferred that high or low temperatures can affect the enzymatic activity of alkaline proteases28. So alkaline protease reaches the optimal hydrolysis degree at a hydrolysis temperature of 50 ℃.

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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Single factor test results.

(c) Hydrolysis time.

As shown in Fig. 2(c). The foaming ability of pumpkin seed protein continuously increases and the rate is fast before 3 h of hydrolysis, but the hydrolysis rate decreases after 3 h. Perhaps the spatial structure of some proteins changed after hydrolysis for 3 h, which affected the foaming ability of pumpkin seed proteins. Considering actual production efficiency, a time of 3 h is optimal for hydrolyzing pumpkin seed protein.

Optimize extraction factors

(a) Publicity fitting and data analysis.

The evaluation of foaming agents involves two key aspects: foaming capacity and foam stability29. Prior research has indicated that protein-based foaming agents exhibit excellent foam stability but are deficient in foaming performance. Foam stability is primarily determined by two factors: the rigidity of the foam film and the liquid drainage time. The foam films formed by protein foaming agents possess high rigidity and are resistant to rupture, which can be attributed to the strong intermolecular interactions of their high-molecular-weight active components30. Based on this analysis, foaming capacity was selected as the core evaluation index in the present study.

As shown in Table 4, the experimental data of response surface analysis were designed using CCD. The measured foaming volume of foam ranges from 420 mL to 489.5 mL. Design Expert software was used to process 20 sets of designed experimental data, and the empirical relationship between the dependent variable Y (foaming performance of foam) and three key experimental variables (X1, X2 and X3 respectively corresponding to pH value, reaction temperature and reaction time) was obtained. This model can be used to predict the foaming performance of foaming agents.

$$\begin{aligned}&Y=453.65+9.66X_{1}+17.04X_{2}-12.25X_{3}+3.75X_{1}X_{2}\\&+2.62X_{1}X_{3}+0.6250X_{2}X_{3}+4.30X_{1}^{2}-6.04X_{2}^{2}-4.81X_{3}^{2}\end{aligned}$$
(5)

The response surface method was used to analyze the variance of the data, and the results are shown in Table 5. If the p-value of the model is less than 0.0001, it indicates that the model has high significance. The coefficient of certainty (R2) of the model is 0.9736, indicating that the observed value is highly consistent with the predicted value. The model exhibits a non-significant lack-of-fit with an F-value exceeding 0.05, coupled with a low variation coefficient (CV) of merely 1.08%. These results collectively validate the model’s high reliability. Consequently, when adjusting experimental variables, the model can be reliably applied to make reasonable predictions regarding the foaming characteristics of bubbles.

Table 4 CCD experimental data.
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In addition, the p value represents the significance of each independent variable. The smaller the p-value, the more significant the corresponding independent variable31. According to the F-value, the order of influence of various factors on protein foaming ability is hydrolysis temperature (X2) > hydrolysis time (X3) > pH (X1).

Table 5 Outcomes of the analysis of variance.
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(b) Response surface experimental analysis.

Contour plots (as shown in Fig. 3) were generated through the fitting of experimental data, which vividly illustrate the correlation between independent variables and dependent variables.

Figure 3 (a) depicts the impacts of reaction temperature and pH on foaming properties. As illustrated in the figure, foaming performance shows a steady upward trend with the increase of both reaction temperature and pH value. Moreover, the elevation of pH leads to a rise in the ion concentration of the solution, which in turn provides a favorable ionic environment for the formation and stabilization of foam. Given that protein molecules possess amphiphilic characteristics, they get adsorbed onto the liquid membrane upon foam formation, thereby rendering the liquid film surface either positively or negatively charged. The introduction of inorganic ions further induces the liquid film surface to carry charges of the same polarity.

When the liquid film is impacted, the electrostatic repulsion force prevents the drainage of the liquid film, so as to avoid the rapid rupture of foam after it is generated and extend the stability time32. From the contour map, it can be seen that the foaming ability is highest (> 480 ml) when the temperature is between 55 ℃ − 60 ℃ and the pH value is around 11. Conditions beyond this optimal range will reduce the foaming ability of proteins, as the Maillard reaction may occur. The protein content decreases, but the production of degradable melanin decreases, resulting in reduced foaming ability33. This change affects the quality of the products and reduces their foaming ability. During the experiment, it was continuously observed that the color of the solution had significantly darkened. In addition, the contour map shows partially diagonally distributed circular ridges, indicating a slight interdependence between reaction temperature and pH value34. Consistent with this finding, the p-value of 0.0534 presented in Table 5 further corroborates that the interactive effect between reaction temperature and time is likely to have no significant impact on foaming properties.

Figure 3 (c) illustrates the effect of reaction time and pH value on foaming properties. When the reaction time is low, the foam property increases almost linearly with the increase of pH value. However, if the reaction time is greater than 3 h, increasing the pH value actually deteriorates the effect. In addition, when the pH value is greater than 10, increasing the reaction time will reduce foaming ability. This may be because high alkaline environments can cause protein denaturation35, making it less likely for protein particles to dissolve in water. Due to this reason, foaming agents become unfavorable for foaming.

As shown in Fig. 3 (e), a longer reaction time is not conducive to foaming. When the reaction temperature is constant, the higher the reaction temperature, the better the foaming properties. This phenomenon can be attributed to the elevation of protein content alongside the moderate degradation of partial protein molecules36. Specifically, the hot alkaline hydrolysis approach accelerates the decomposition of organic substances through heating, while the addition of alkali diminishes the high-temperature resistance of cells37. Higher levels of cell destruction indicate that more protein will be released into the solution and more foam will be produced. In addition, proteins are degraded when exposed to high pH or high temperatures. In the process of partial protein degradation, the breaking of intra- and intermolecular bonds leads to the exposure of more previously concealed hydrophobic amino acids to aqueous solvents. This kind of structural alteration usually enhances the flexibility and surface hydrophobicity of the degraded proteins, consequently boosting their foaming capacity38.

In summary, shorter reaction time, higher reaction temperature, and elevated pH value exert a significant promotional effect on cell lysis and protein degradation, which in turn enhances the foaming properties of the product. Nevertheless, when process parameters exceed a specific threshold, issues such as excessive degradation and Maillard reaction may arise, potentially leading to a decline in foaming performance.

Fig. 3
Fig. 3The alternative text for this image may have been generated using AI.
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Response surface curves (up) and contour plots (down) : (a) and (b) fixed X3 at 3 h; (c) and (d) fixed X2 at 50℃; (e) and (f) fixed X1 at 10.

Verification and minor adjustments

The conclusion drawn from the above model prediction is that the maximum dependent variable value (488 mL) can be achieved when the independent variables are set as X1 = 11.502, X2 = 55.167℃, and X3 = 1.576 h. To enhance experimental operability, change the reaction conditions to X1 = 11.5, X2 = 55℃, and X3 = 1.5 h.

According to the conclusion drawn from the model, the average value obtained from three repeated foaming experiments is 486 ml, indicating that the model’s prediction conclusion is relatively correct.

Fe-HPSP foaming agent solution

HPLC

The formation of foam depends on the ability of protein to quickly absorb and expand on the gas-liquid interface, while the stability of foam depends on the formation of a flexible but corrosion resistant membrane, which can reduce the permeability of gas and inhibit the coalescence of bubbles. Therefore, the stability of foam depends on the structure and composition of protein. Bruna and other scholars analyzed the influence of protein amino acid types on the stability of foam by using biscinchonic acid determination method and high performance liquid chromatography. The research results show that aspartic acid has a negative impact on the stability of foam, while the presence of histidine and complexine has a positive contribution to the stability of foam39. As an amphiphilic molecule, the hydrophobic groups (isoleucine, leucine and phenylalanine) in the protein can also make it spontaneously adsorbed on the air-water interface. The formation of the protein adsorption layer leads to the reduction of the interfacial tension, which is conducive to the formation and stability of foam17.

Fig. 4
Fig. 4The alternative text for this image may have been generated using AI.
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High performance liquid chromatography of HPSP.

The content of 16 hydrolyzed amino acids in HPSP was determined by high-performance liquid chromatography (HPLC). Figure 4 shows the high-performance liquid chromatogram of HPSP. From the figure, it can be observed that the content of 1#-aspartic acid, which is unfavorable to the stability of foam, is relatively small. In addition, the content of hydrophobic amino acids, such as 13#-isoleucine, which is favorable to the stability of foam, is relatively high.

Cryo-TEM

This section is based on the hydrophobic amino acids rich in HPSP, which can coordinate with metal cations in the form of anions to produce particles with partial wettability (protein metal ion complexes). Their amphiphilicity can stabilize the gas-liquid interface, thereby improving the foaming ability and stability of the base liquid17.

Fig. 5
Fig. 5The alternative text for this image may have been generated using AI.
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Cryo TEM images.

Figure 5 (a, b) shows the low-temperature transmission electron microscopy (Cryo- TEM) images of HPSP, Cryo-TEM images indicate that HPSP molecules have a spherical structure, with most HPSP molecules smaller than 10 nm in size. Transition metal ion Fe (II) has the ability to coordinate with functional groups on HPSP side chains. Figure 3 (c, d) shows that the coordination between HPSP and Fe (II) causes a change in protein conformation. After adding 0.2% FeSO4, it can be observed that HPSP molecules aggregate in the bulk phase, this is related to the formation of HPSP and Fe (II) complexes.

SAXS

Conduct small angle X-ray scattering (SAXS) experiments to gain a deeper understanding of the interaction between metal ions and HPSP. Figure 6 shows the variation of scattering intensity (I) with scattering vector (q) obtained in 1% HPSP solutions containing different concentrations of FeSO4.

As the concentration of Fe (II) increases, the I (q) of the solution continues to increase at low q (q < 0.1 nm− 1), indicating the formation of HPSP and Fe (II) complexes. The slope of the SAXS curve continues to slowly decrease with the increase of q, indicating that the HPSP and Fe (II) complex has multi-level structural characteristics. The Beaucage model is often applied to systems with different scale structures or multi-level structures17. This model can fit the Guinier and Porod regions of the SAXS curve, smoothly transition between them, and obtain the particle’s radius of rotation and Porod index. The scattering intensity is given by the following fitting formula:

$$\begin{gathered} I(q)=G\exp \left( { - \frac{{\mathop q\nolimits^{2} \mathop R\nolimits_{b}^{2} }}{3}} \right)+B\exp \left( { - \frac{{\mathop q\nolimits^{2} \mathop R\nolimits_{{sub}}^{2} }}{3}} \right){\left( {\frac{{{{[erf(q{R_b}/\sqrt 6 )]}^3}}}{q}} \right)^p} \hfill \\ +{G_s}\exp \left( { - \frac{{\mathop q\nolimits^{2} \mathop R\nolimits_{s}^{2} }}{3}} \right)+{B_s}{\left( {\frac{{{{[erf(q{R_s}/\sqrt 6 )]}^3}}}{q}} \right)^{{p_s}}} \hfill \\ \end{gathered}$$
(6)

Fitting parameters in the formula: Guinier scale factor G, radius of gyration R, and Porod index P.

Fig. 6
Fig. 6The alternative text for this image may have been generated using AI.
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The variation of scattering intensity I of HPSP-Fe(II) complex system with scattering vector q.

As shown in Fig. 6, it can be observed that using the Beaucage model can fit the scatter data of SAXS well. The fitting parameters of the model are presented in Table 6. The parameter P reflects the distribution characteristics of the aggregates. In the absence of Fe(II) addition, the fitted P value is less than 3, which indicates that the system exhibits mass fractal properties. Conversely, under conditions of high Fe(II) concentrations, the P value falls within the range of 3–4, signifying that the system displays surface fractal characteristics. Rb, Rs, and Rsub are the rotational radii Rb of the large-scale (protein tertiary structure), mesoscale (protein secondary structure), and small-scale (protein primary structure) molecules in solution, respectively. We also noticed that the parameter Rb increased and the parameter Rs decreased. This once again fully confirms that the addition of transition metal ions to the opal solution promotes the formation of complexes. Furthermore, an increase in Ns = G/Gs indicates that the addition of Fe (II) can significantly promote the aggregation of HPSP molecules.

Table 6 Beausage model fitting parameters.
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In summary, the addition of Fe (II) resulted in the formation of HPSP and Fe (II) complexes in the solution. The addition of Fe (II) promotes the exposure of hydrophobic groups in proteins and increases the size of aggregates. In the following, we will prove that protein metal ion binary complexes can significantly improve the performance of foam.

Fe-HPSP foaming agent solution

Density, viscosity and surface tension of foam

As shown in Table 5, the density of Fe-HPSP was 27.78% higher than that of control group, while its foam viscosity increased by 34.29%. The surface tension of Fe-HPSP decreased by 15.53% compared to control group. These data collectively indicate that the comprehensive characteristics of Fe-HPSP foam are superior to those of control group foam40.

Table 5 Comprehensive characteristics of the foams.
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Drainage rate and micro morphology of foam

The research shows that about 25% of foam liquid of PS is discharged in the first 5 min, while only 9% of Fe-HPSP foam is discharged, as shown in Fig. 7(a). Based on previous studies, it can be concluded that the foam characteristics of the control group are weaker than that of Fe -HPSP41.

Fig. 7
Fig. 7The alternative text for this image may have been generated using AI.
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Drainage and Morphology of foams.

As shown in Fig. 7(b) and (c), the liquid film thickness increased by 42.37 μm after the addition of transition metal ions into the foam. This may be because the coordination between transition metal ions and HPSP causes the aggregation of protein molecules in the body phase, which increases the thickness and strength of foam, thus slowing down the drainage time of foam. In short, when transition metal ions are used as foam stabilizers, the overall performance of foam is improved.

Effect of foamed concrete

Physical property

Figure 8 (a) illustrates both the control group and the experimental group exhibited an increase in compressive strength over time. After 28 d of standard maintenance, the compressive strength of the control group reached 6.15 MPa, whereas that of the experimental group rose to 8.67 MPa. These results confirm that the incorporation of transition metal ions enhances the compressive strength of foamed concrete.

Figure 8 (b) shows the variation pattern of drying shrinkage rate for two groups of specimens. After 90 days, the drying shrinkage rate of the control group was measured at 2.37 × 10³, whereas that of the experimental group, owing to the introduction of transition metal ions, was reduced to 1.34 × 10³.

Figure 8 (c) shows the variation pattern of water absorption rates for two groups of specimens. The control group exhibited a water absorption rate of 23.3%, in contrast to the experimental group, which only absorbed 5.4% of water. This indicates that the incorporation of transition metal ions significantly enhances the water resistance of foam concrete (i.e., reduces its water absorption). Kearsley42 previously pointed out that half of the water absorption capacity of foamed concrete can be attributed to its matrix structure. Accordingly, transition metal ions are capable of filling the capillary pores within foam concrete, thereby exerting a mitigating effect on its water absorption26.

Fig. 8
Fig. 8The alternative text for this image may have been generated using AI.
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Physical properties of foamed concrete.

Figure 8 (c) shows the variation pattern of consistency characteristics for two groups of specimens. As depicted in the figure, the density of both sets of samples decreases with increasing experimental height. Meanwhile, the bottom density of the control group samples was 1.28 times that of the top density, while the experimental group had a density of 1.04 times. Furthermore, the ratio of bottom densities between the control group and the experimental group was determined to be 1.23. In summary, the incorporation of transition metal ions contributes to enhancing the uniformity of foam concrete.

SEM images

The microstructure of the sample is shown in Fig. 9. Before adding transition metal ions, there were many cracks in the pores of the sample. Following the addition of transition metal ions, the pores within the samples appear smoother and more intact. She et al.26 showed that the higher the hydration degree of foam concrete sample, the more uniform the distribution of its hydration products, and the more complete the foam wall. So that adding transition metal ions is beneficial for improving the integrity of foam.

Fig. 9
Fig. 9The alternative text for this image may have been generated using AI.
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Microstructure of FC-PS (left) and FC-Fe-HPSP (right).

Pore characteristics

As shown in Fig. 10(a-d), the two-dimensional and three-dimensional images of the sample are scanned by a scanner. The pore size distribution pattern of the sample is shown in Fig. 10(e-f). As noted in previous research43,44, the pore size distribution of foamed concrete follows a logarithmic pattern.

The proportion of aperture sizes in the range of 100–500 μm has increased from 7.14% to 73.89%. According to previous research results, it is speculated that this may be because the transition metal ions are uniformly adsorbed around the foam, thus destroying the foam aggregation, and making the foam size more uniform26.

Porosity is an important index for studying foamed concrete materials. Under the condition of identical density for foam concrete, its porosity is generally regarded as comparable. Consequently, even with the same density, foam concrete may exhibit varying properties due to the combined influence of pore structure and matrix microstructure45. After the incorporation of transition metal ions, the pore size distribution of the samples becomes narrower, the pore size itself is reduced, and the strength of the pore walls is enhanced. These changes indicate that transition metal ions play a beneficial role in improving the performance of the samples46.

Fig. 10
Fig. 10The alternative text for this image may have been generated using AI.
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Aperture distributions of FC-PS and FC-Fe-HPSP.

Conclusions

In this study, a new idea to prepare foaming agent is proposed, that is, transition metal ions are used as plant protein foam stabilizers. The specific experimental results are as follows:

  1. 1.

    Pumpkin seeds were selected as the plant protein source. To maximize foaming performance, the response surface methodology (RSM), a novel optimization approach, was employed to adjust key process parameters in the hot-alkali experiment. The results indicated that the optimal reaction conditions were a pH of 11.5, a temperature of 55 ℃, and a reaction duration of 1.5 h. Under these parameters, the HPSP foaming agent achieved the highest foaming capacity, reaching 488 mL.

  2. 2.

    In order to prepare super stable protein foam, transition metal ions Fe(II) are used as foam stabilizers. The addition of Fe(II) resulted in the formation of HPSP and Fe(II) complexes in the solution, and promotes the exposure of hydrophobic groups in proteins and increases the size of aggregates. So as to make the protein foam more stable.

  3. 3.

    Compared with commercially available plant protein foaming agents (PS), the self-developed Fe-HPSP foaming agent exhibits more favorable properties, including higher density, viscosity, and stability. Additionally, optical microscope (OM) observations revealed that the incorporation of transition metal ions into HPSP resulted in an increase in liquid film thickness by 42.37 μm.

  4. 4.

    X-CT analysis results show that the self-developed Fe-HPSP foamed concrete has pore size distribution characteristics, which is more conducive to improving the characteristics of foam concrete.

In conclusion, the self-developed Fe-HPSP foaming agent boasts advantages such as high performance and environmental friendliness. As a result, it holds significant potential for widespread promotion and application.