Additive manufacturing with geopolymer foams: A critical review of current progress
Article Category: Review Article
Published Online: Mar 31, 2025
Page range: 115 - 132
Received: Mar 13, 2025
Accepted: May 13, 2025
DOI: https://doi.org/10.2478/msp-2025-0013
Keywords
© 2025 Wei-Chien Wang, published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
The issues of increasing carbon dioxide emissions and energy consumption have attracted the attention of researchers [1]. Various factors in industry and construction worldwide cause these problems. In civil engineering, ordinary cement is responsible for the most substantial carbon dioxide emissions [2], as its production requires the burning of raw materials at extremely high temperatures exceeding 1,500°C [2]. Additionally, the highest energy consumption occurs during construction activities, particularly in transportation and the use of heavy equipment [3]. Researchers across various fields aim to reduce carbon dioxide emissions. Alkaline-activated materials (AAMs) were initially introduced in the 1940s by Kravinko, as explained in the Alves et al. papers [4], while geopolymer technology was proposed by Prof. Davidovich in the 1970s. Geopolymer, which is a subset of AAMs (specifically Si- and Al-based), serves as a viable alternative for achieving significant reductions in environmental impact [5]. The geopolymer process entails the utilization of materials with elevated alumina and silicate concentrations, which undergo a chemical reaction with an alkaline activator solution [6]. A number of aluminosilicate materials can be identified as potential sources, including fly ash [7], metakaolin [8], silica fume [9], and slag [10]. These materials have the capacity to replace ordinary cement in a number of applications. As these materials are by-products, they have the potential to reduce carbon dioxide emissions significantly.
The production of geopolymer foam, which is also referred to as foamed, aerated, cellular, or porous concrete, involves the incorporation of pores into the paste or mortar, resulting in a low-density structure [11]. The integration of porous structures can greatly improve the properties and functionalities of materials, particularly in insulation applications. These structures exhibit exceptional mechanical energy absorption capabilities [12], and the increased permeability of porous materials enables highly efficient filtration processes [13]. The intrinsic fire resistance of these materials offers a vital layer of safety, making them particularly suitable for environments where fire hazards are of paramount concern [14]. Furthermore, the durability of porous structures allows them to endure freeze–thaw cycles, preserving both structural integrity and longevity in extreme weather conditions [15]. Geopolymer foam can be an effective insulation material, eliminating the need for conventional insulation in existing structures. This feature enhances efficiency and offers a more sustainable construction alternative.
Geopolymer materials have been studied since the 1980s, while research on geopolymer foam only began in 2010, in accordance with the findings presented in the study of Dhasindrakrishna et al. [16]. Consequently, investigations into geopolymer foam, particularly for three-dimensional (3D) printing, remain limited, leading to challenges in scaling its use in both laboratory and field settings. Three-dimensional printing technology, otherwise known as additive manufacturing (AM), entails the creation of 3D objects from a digital file through the layering of materials [17]. Three-dimensional printing enables the creation of complex and customized designs that are often unachievable with traditional casting methods [18]. Implementing layer-by-layer control facilitates the accurate distribution of pores, thereby enhancing the insulation properties [19]. The capability of 3D printing to precisely control porosity and density facilitates the production of lightweight and robust materials [20]. Therefore, 3D printing facilitates precise, sustainable, and efficient production of geopolymer foams, positioning it as an optimal solution for advanced construction and insulation applications.
In reviewing the literature on geopolymer foam mortar for 3D printing applications, a variety of research methodologies were identified, primarily of an experimental nature. Most studies relied on laboratory-based testing to assess the mechanical properties of the geopolymer mixtures. As geopolymer foam acts as an insulation material by creating pores within the structures, key factors such as density, compressive strength, porosity, and thermal conductivity were analyzed. The mix designs generally included varying proportions of aluminosilicate materials in combination with alkaline activators and foam-generating agents. Additionally, several studies employed extrusion-based 3D printing setups to evaluate the methods and applications of the geopolymer structures. This review focuses on geopolymer foams and their applications in 3D printing, an increasingly significant area due to the lightweight, thermally insulating, and sustainable properties of the material for construction. This study was performed using various databases, including Scopus, Web of Science, ScienceDirect, SpringerLink, IEEE Xplore, and Google Scholar. Keywords used in the search included “geopolymer foam,” “3D printing,” “alkali-activated materials,” “porous geopolymer,” and “additive manufacturing,” with Boolean search combinations applied to refine the results, targeting academic researchers, engineers, and industry professionals engaged in advanced construction technologies, materials science, and sustainable infrastructure.
This review examines the potential of geopolymer foam 3D printing in construction, particularly on insulation materials that facilitate sustainable building practices. Porous building materials with controlled densities, often in solid foam forms, are particularly suitable for versatile applications such as energy-efficient and lightweight construction [21]. Three-dimensional printing is increasingly recognized in construction, but there is a lack of knowledge regarding its processes and the material properties of geopolymer foam. Accordingly, this study aims to examine the subject of 3D printing in construction using geopolymer foam to collate pertinent research, identify challenges, and establish a foundation for future investigations.
Aluminosilicate material is a binding agent with a high alumina and silicate content, allowing it to react with alkaline activator solutions, converting into different oligomers that eventually polymerize into intricate structures [22]. Davidovits described the reaction process as the condensation of different tetrahedrally coordinated aluminosilicate units designated as polysialates [23]. These polysialates are composed of tetrahedral SiO₄ and AlO₄ units that are interconnected through shared oxygen atoms, resulting in polymer categorized into three forms: polysialate, polysialate siloxo, and polysialate disiloxo [24].
The aluminosilicate source material is typically composed of industrial waste, including fly ash [25], slag [10], and silica fume [26], as well as agricultural waste, such as sugarcane bagasse ash [27], rice husk ash [28], palm oil ash [29], and bamboo leaf ash [30]. Mine tailings, which are byproducts produced during mining operations, present an opportunity for utilization in the development of geopolymers. These materials comprise finely ground rock and chemical residues that remain after the ore has undergone processing [31]. In their research, Ahmari and Zhang examined the application of copper mine tailings in high SiO2 content sourced from a mining company located in Tucson, Arizona [32]. Furthermore, Burduhos Nergis et al. conducted a study that integrated barite mine tailings in high Fe2O3 content with coal ash and metakaolin, assessing their combined properties for potential applications [33]. These materials serve as a cementitious replacement for Portland cement and are recognized for their sustainability, as they produce significantly lower carbon dioxide emissions in comparison to conventional cement [34].
Fly ash is a solid by-product of coal combustion in power plant boilers and flue gases, comprising particles of very small size [25]. The composition of fly ash can differ significantly based on the origin of the coal being combusted. As amorphous and crystalline oxides or minerals, fly ash usually comprises SiO2, Al2O3, CaO, and Fe2O3. Classification of fly ash as either Class C or Class F is dependent on the calcium oxide levels present, as defined by the ASTM C 618 standard [35]. Class C fly ash contains SiO2, Al2O3, and Fe2O3 ranging from 50 to 70 wt%, with a CaO content exceeding 20 wt% [35]. Class C fly ash is primarily produced from burning lignite coal sources and contains high calcium [22]. Class F fly ash contains more than 70% SiO2, Al2O3, and Fe2O3 and a CaO content of less than 10 wt% [35]. Class F fly ash is produced from the combustion of anthracite or bituminous coal and exhibits a minimal calcium content [25]. Table 1 shows a comparison of the chemical content of fly ash with other industrial wastes. The higher the SiO2 and Al2O3 content in the fly ash, the greater its potential as an aluminosilicate source of geopolymer foam that can be activated with an alkaline solution. The production of fly ash-based geopolymer emits less CO2 than the production of OPC, which involves the high-temperature calcination and decomposition of a large amount of limestone [36].
Chemical contents of industrial waste as an aluminosilicate source.
| Chemical composition (%) | Fly ash | Slag | Silica fume |
|---|---|---|---|
| SiO2 | 52.31 | 39.8 | 92.05 |
| Al2O3 | 28.59 | 11.2 | 0.72 |
| Fe2O3 | 6.22 | 1.2 | 1.31 |
| CaO | 4.01 | 34.4 | 0.46 |
| MgO | 2.05 | 7.6 | — |
| K2O | 1.62 | — | 1.51 |
| SO3 | 1.79 | 0.46 | 0.41 |
| N2O | — | 0.2 | 0.45 |
| References | [37] | [10] | [38] |
Fly ash contributes to carbon dioxide emission reductions while providing additional advantages, including enhanced sustainability in construction, diminished dependence on virgin materials, lower global warming potential, significant life-cycle cost savings, effective recycling of industrial waste, extended service life, and reduced carbon emissions [39]. These benefits can be categorized as environmental, construction-related, and economic feasibility benefits. In recent years, numerous scientific progress has been made in the preparation of fly ash-based geopolymers, leading to a more comprehensive understanding of their performance. The mechanical properties of fly ash-based geopolymer foam are influenced by the type of fly ash, the ratio and type of alkaline activator, and the curing conditions. A well-balanced mix proportion in the fly ash-based geopolymer foam can yield favorable outcomes in terms of compressive strength, flexural strength, density, thermal conductivity, durability, and fire resistance. Abdollahnejad et al., in their research using fly ash in geopolymer foam, show a higher hydrogen peroxide and a higher activator and binder ratio, resulting in a lower compressive strength [40]. The density becomes lower when there is a lower activation/binder ratio and a higher foaming agent.
Slag is defined as a waste product produced in metallurgical or specialized processes, including smelting or refining of ferrous or non-ferrous metals [41]. In the study of Ismail et al. [22], slag is classified into three categories: ferrous, non-ferrous, and non-metallurgical. The most prevalent are ferrous slag, including blast furnace and steel slag. Figure 1 illustrates the production of blast furnace slag during the melting and reduction of iron ore in a blast furnace. Blast furnace slag mainly comprises SiO2, Al2O3, and CaO, following the ASTM C989 [42]. Steel slag is formed when hot metal is transformed into crude steel or scrap metal is melted in various furnaces. The primary components of steel slag are Fe2O3 and CaO, with lower amounts of SiO2 and Al2O3. Blast furnace slag or ground granulated blast furnace slag (GGBS) is commonly used as a raw material in geopolymer production due to its elevated aluminosilicate content that can react with alkaline solutions, as shown in Table 1.
Process of producing the slag. Adapted from Hanani Ismail et al. paper, licensed under CC BY-NC 4.0
Silica fume is a waste product generated by electric arc furnaces during the production of ferrosilicon or silicon metal [38]. Silica fume is a well-documented pozzolanic reactive cementitious material [43]. Silica fume consists of a high proportion of minute spherical particles containing over 90% silicon dioxide [38], as shown in Table 1. Furthermore, the high SiO2 content of silica fumes and their ability to serve as an aluminosilicate source can also be exploited as a foaming agent in geopolymer foam. A previous study demonstrated that the use of silica fume resulted in improvements to the mechanical, physical, and durability properties [44]. As highlighted by Jena et al. in their paper presented at Sustainable Construction and Building Materials [44], demonstrated the effect of silica fume in fly ash-based geopolymers, which can reduce the density, increase compressive strength, and provide a good range of ultrasonic pulse velocities. This result correlates with the silica fume-based geopolymer foam study of Shakouri et al. [45].
The activator solution serves as the primary component in the geopolymer foam, facilitating the dissolution of aluminosilicate precursors. Activators are divided into two principal categories: alkali activators and acidic activators. The division is based on the pH of the solution, which determines whether the precursor can be activated under acidic or alkaline conditions. The primary role of the activator solution is to neutralize the negative charges of aluminate and silicate, thereby facilitating the formation of crystalline structures within the geopolymer matrix. Accordingly, the ratio between the type of the activator solution and the activator solution/binder ratio influences the physical and chemical properties of geopolymer foam.
The alkaline activator solutions commonly utilized in the geopolymer field are sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). One mix proportion allows for the utilization of two distinct types of alkaline activator solutions, namely hydroxide and silicate. Malkawi et al. [46] conducted experiments using the combination of sodium hydroxide and sodium silicate, with the objective of influencing the workability, setting time, and compressive strength. It was observed that the addition of NaOH resulted in a reduction in workability and an extended setting time, whereas the incorporation of Na2SiO3 led to an enhancement in the compressive strength at a later age. In another study [47], the combination of NaOH and KOH (potassium hydroxide) with Na2SiO3 and fly ash were used as precursors. The variation between KOH and Na2SiO3 results in higher compressive strength at a later age, while the opposite result was shown for NaOH and Na2SiO3. This is because the reaction between K+ is slower than Na+ ions in condensation. The combination of Na2SiO3 and Na2CO3 (sodium carbonate) was utilized by Ma et al. [48]. Their findings revealed that Na2CO3 markedly influenced the accelerated final setting time; however, it exhibited a comparatively weaker compressive strength than Na2SiO3. Interestingly, sodium aluminate (NaAlO2) can also be used as an alkaline activator solution in geopolymers. By varying the percentage of NaAlO2 and Na2O (sodium oxide), Chen et al. investigated the properties of alkali-activated slag [49]. An increase in the percentage of NaAlO2 results in a prolongation of the setting time, whereas a reduction in the percentage of Na2O has the opposite effect. NaAlO2 yielded denser pore structures, resulting in a higher compressive strength.
While not as prevalent in the field of geopolymer research as alkaline activators, aluminosilicate precursors can be activated using acidic activator solutions. Le-Ping et al. [50] showed that acid-based geopolymer has more resistance to higher temperatures than alkali-based geopolymer, with a maximum temperature of 1,450°C. Phosphoric acid (H3PO4) is a widely used acidic activator solution for geopolymer. Zhang et al. [51] noted that acid-based geopolymer exhibited enhanced bonding, leading to augmented compressive strength and elevated thermal stability. It can be observed that an increase in phosphoric acid concentration results in an elevated heat output, which can be attributed to the accelerated rate of hydration observed in the H3PO4-based geopolymer paste, as documented in the study of Pu et al. [52]. Lin et al. [53] investigated the characteristics of metakaolin geopolymer activated by phosphoric acid, which showed that metakaolin needs a longer time for hardening. The researchers used two curing steps to avoid thermal cracking and fast hardening – the higher phosphoric acid molar ratio resulted in higher compressive strength with secondary cured at 60°C.
The incorporation of a foaming agent is a requisite component in the geopolymer foam production process, owing to the necessity of creating bubble pores within the geopolymer matrix. There are two types of geopolymer foaming methods. The first is the chemical foaming agent, which functions by undergoing a chemical reaction that produces gas, thereby leading to foam formation. These agents undergo decomposition when heated or when mixed with alkaline solutions, generating gas bubbles in the matrix [54]. Chemical agents frequently precipitate a chemical reaction or thermal decomposition, producing gaseous products like hydrogen or oxygen and the formation of a porous structure [20], which has been presented in the review article by Kočí and Černý [55]. Liu et al. [11] indicated that the pores in the GFC support both internal gas pressure and the confinement pressure from the chemical foaming agent. This confinement pressure is primarily influenced by viscous resistance due to slurry movement, along with surface tension and the weight of the slurry [56]. The second type is the physical foaming agent, which typically consists of volatile substances that evaporate and expand under the application of heat or pressure, thereby leading to foam formation without the involvement of a chemical reaction [55]. This article provides a more in-depth analysis of the chemical foaming agent foam, which is widely used in the geopolymer industry due to its rapid and straightforward bubble formation within the matrix.
Hydrogen peroxide (H2O2) and metal powder (aluminum or zinc) are the foaming agents commonly used for geopolymer foam as shown in Figure 2. Furthermore, lesser-known substances such as silicon powder and various silicon-containing compounds [57], including FeSi alloys and silicon carbide (SiC) [58], can be utilized as foaming agents due to their capacity to generate hydrogen in alkaline environments. H2O2 easily decomposes into water and oxygen, as shown in equation (1), and then releasing oxygen gas creates empty air called a bubble or closed pore. The percentage of H2O2 in geopolymer foam influenced the porosities and densities, affecting the compressive strength and thermal conductivity. In the research using bagasse ash and metakaolin foamed with H2O2, Pantongsuk et al. [59] found that an increase in H2O2 concentration resulted in a corresponding increase in porosity. Using 1–1.5 wt% H2O2 yields low thermal conductivity. This finding was consistent with the observations made by Huang et al. [60] regarding the fly ash-based geopolymer foam, which exhibited a higher percentage of H2O2, increased porosity, decreased compressive strength, and decreased thermal conductivity. Furthermore, Yan et al. [61] observed that a higher concentration of H2O2 reduced the compressive strength in metakaolin-based geopolymer foam. The properties of hydrogen peroxide geopolymer foam are affected not only by H2O2 but also by the type of the starting material, the activator solution used, and the mixture ratios:
Aluminum is a recognized substance that has the capacity to split water, react with water under alkaline conditions, and release hydrogen gas and soluble aluminate ions [62]. The reaction is shown in equation (2). In the investigation of the effect of an alumina foaming agent on the nanostructural fly ash geopolymer properties, Hajimohammadi et al. [63] found that the agent impeded the development of strength due to the reduction of reactions and gel formation from the fly ash. The concentration of alumina powder has a direct proportional effect on the density and compressive strength, with higher concentrations resulting in lower strengths [64]. A comparative analysis of the efficiency of H2O2 and alumina powder as foaming agents in foam geopolymer [65] revealed that H2O2 exhibited superior performance compared to Al powder. This can be attributed to its favorable insulation parameters, which reduce density and thermal conductivity, as shown in Figure 1. This porous material is highly appropriate for application as an insulation material.
Geopolymer foam using foaming agent: (a) hydrogen peroxide and (b) aluminum powder (results from personal data).
Stabilizing agents play a pivotal role in regulating the porosity of geopolymer foam, which is contingent upon the principles of modifying material properties. Related to the porosity, stabilizers facilitate the formation of a homogeneous and tailored pore structure. The formation of uniform and even pore sizes has been demonstrated to result in a reduction in density and an increase in compressive strength. The stabilizing agent may be derived from a variety of sources, including protein [66], vegetable oil (such as sunflower oil, canola oil, and olive oil) [67], and commercial stabilizers such as sodium dodecyl sulfate (SDS) [68], sodium dodecyl benzene sulfonate [69], and Sika Lightcrete [70]. Guo et al. [71] used polyvinyl alcohol (PVA) as a nanoparticle stabilizer in foamed concrete. A higher concentration of PVA has been demonstrated to reduce thermal conductivity while simultaneously enhancing flexural and compressive strength, as evidenced by Tiyasangthong et al. [72]. The effects of using PVA as a stabilizing agent on the pore structure are shown in Figure 3. The application of SDS as a stabilizer and H2O2 as a foaming agent is demonstrated by Korat and Ducman [68], which affects the pore structure of the geopolymer matrix. The generated pores were spherical and uniform in nature, with the high SDS concentration resulting in the formation of macropores within the matrix. The use of vegetable oil as a stabilizing agent is shown in Figure 4; the formation of a uniform porous structure and achieving optimal properties are notable [67]. The use of a stabilizing agent can enhance structural integrity through improved pore uniformity.
Microscopic results of samples using 2 wt% H2O2 with the variation of PVA (results from personal data).
Microscopic results of samples using 2 wt% H2O2 with the variation of vegetable oil (results from personal data).
To enhance the lightweight properties of geopolymer foam materials, lightweight aggregates and fibers were utilized as additive materials. This approach aims to optimize the overall performance and structural integrity of the geopolymer matrix. The principal advantage of utilizing lightweight aggregate lies in its capacity to reduce the density of the matrix structure. This reduction significantly impacts the thermal properties of insulation, largely due to the presence of microstructural voids [73]. Liu et al., in their research, evaluated that the use of oil palm shells as lightweight aggregate can reduce the thermal conductivity by 22 and 48% than conventional wall materials [74]. In the research conducted by Pasupathy et al., expanded perlite (EP) was employed as a lightweight aggregate in the formulation of geopolymer foam structures [75]. The findings suggest that an increase in the concentration of EP results in a reduction of both the density and thermal conductivity of these structures, highlighting the potential of EP in enhancing the properties of geopolymer foams. Besides, fibers were utilized as reinforcement in the geopolymer foam matrix to improve the properties, especially mechanical, and prevent cracking due to shrinkage and thermal stress, according to Wang et al. [76]. They conducted a study that utilized basalt fiber as a reinforcement in geopolymer foam. The findings indicated that increasing the amount of basalt fiber leads to a reduction in both fluidity and thermal conductivity. However, when exposed to high temperatures between 600 and 1,000°C, thermal conductivity increased, while linear shrinkage decreased [76]. Zhang et al. employed kenaf fibers [77], whereas Wang et al. used polypropylene fibers [78] as reinforcement in their respective studies. Their research demonstrates that the inclusion of these fibers significantly improves the compressive strength and thermal conductivity of the material. This enhancement is attributed to the bubble segmentation observed within the matrix structure. The assessment of additive types of materials should be undertaken regarding the prevailing structural conditions and specific objectives.
The geopolymerization process is a complex chemical reaction involving the dissolution of silicate and aluminate species from the solid aluminosilicate source in an alkaline solution [25]. These aluminosilicates are then condensed with silicate species from the alkali silicate-activating solution, resulting in the formation of a gel, the solidification of the gel, and the subsequent hardening of the material [79]. This process is presented by Xu and Van Deventer [80]. As illustrated in Figure 5, geopolymer foam starts from the dissolution of aluminosilicate with an alkaline solution. The dissolution process initiates nucleation, wherein small nuclei are formed from dissolved aluminum and silica ions that subsequently aggregate to create blocks. This is followed by oligomerization, a critical phase that facilitates the development of a more intricate structure. During oligomerization, the small nuclei grow through linking, resulting in the formation of oligomers. Subsequent to oligomerization is the polymerization stage, characterized by the continued reaction and bonding of oligomers to form elongated chains and a 3D network. Introducing a foaming agent, such as hydrogen peroxide, along with an optional surfactant, augments the slurry’s volume by generating bubbles that contribute to the formation of geopolymer foam. The generated foam on the structure can reduce the thermal conductivity of the structure [81].
The geopolymerization foam process (results from personal data).
The role of geopolymer foam materials in determining the bulk density, porosity, morphology, mechanical properties, and thermal conductivity of porous geopolymers is of crucial importance, particularly in high-performance applications [82]. These versatile materials play a crucial role in various industries, serving as catalyst supports that enhance reactions and as membranes for accurate separation [83]. They effectively filter out impurities and capture unwanted particles, all while providing essential thermal insulation that boosts energy efficiency [84]. Opting for these materials guarantees superior performance and reliability. The geopolymerization reaction includes a gelling phase that helps retain the shape of the wet material, making it crucial for AM [85].
3D printed elements (results from personal data).
Bedarf et al. [21] concluded that foam 3D printing technologies can be classified into three methods, including material, material with air, and binder, according to the extruding process. This method entails pushing material through a nozzle and layering it repeatedly to form a solidified 3D object. Its roots can be traced back to a technique known as fused deposition modeling. The spray method utilizes compressed air to distribute an atomized medium in small droplets over a specific area. It is widely used in the construction sector for applications that require the coverage of larger surfaces, including painting, applying coatings, and spraying concrete or polyurethane insulation foam. The final method is binder jetting, a particle-bed 3D printing technique. In this process, bulk particles are uniformly distributed across the print bed and are bonded together using a binder, allowing for the creation of the printed object layer by layer. The extruding method is the most effective technique for producing geopolymer foam in 3D printing due to its simplicity and precise control over the resulting shape via the nozzle. The reference for the fundamental shapes is those used by Bedarf et al. in their experiments [19]. These shapes include direction-parallel, direction-parallel cross, direction-continuous spiral, and double-wall contour-parallel shell configurations. The printability of geopolymer foam is influenced by its extrudability, setting time, and interlayer bonding, which are essential for ensuring structural integrity and reducing defects [86].
In the application, the geometry of the samples was designed using specialized software. The study conducted by Ziejewska et al. employed Blender software for design, which was then printed using the ATMAT Galaxy 3D printer based in Kraków, Poland [90]. Zoude et al. investigated a 3D printing technique for geopolymer foam through direct ink writing [91]. The steps are as follows: after incorporating a foaming agent, the mixture was allowed to rest for 20 min and subsequently sealed for 1 h at room temperature before being printed with a Robocasting device. Interestingly, there is a study that discusses AM by using lunar regolith-based geopolymer [92]. Ulubeyli discussed the application of swarm ground robots in 3D printing processes, highlighting the capability to operate without direct human intervention [93]. The concept of a robotic swarm involves multiple autonomous robots collaborating to produce a 3D print, with each robot assigned a specific task, thus enhancing efficiency through feature decentralization and task specialization.
As previously stated, geopolymer foam exhibits superior insulation performance attributable to its porous morphology. This section presents an examination of the ways in which the intrinsic properties of geopolymer foam affect its insulation effectiveness.
The pore structure of geopolymer foam, which incorporates a foaming agent, plays a significant role in determining its strength properties. Notably, there is a correlation between the density of geopolymer foam and its compressive strength. Table 2 describes the impact of concentration and various foaming agents on geopolymer foam’s compressive strength and density, as reported in the previous literature. Increased concentrations of H2O2 lead to a decline in compressive strength, likely due to a reduction in density caused by heightened porosity. When alumina is used as a foaming agent, a similar trend is observed, with higher density correlating with improved compressive strength. These results indicate that a high concentration of foaming agents will create a more porous structure, which correlates with reduced density and strength. As observed by Dhasindrakrishna et al. [16], a reduction in density can enhance thermal insulation and sound absorption, although this may lead to a decline in strength and durability.
Effect of foaming agents on the compressive strength and density.
| Foaming agent | Compressive strength and density | Ref. |
|---|---|---|
| H2O2 | Increasing H2O2 concentration from 1 to 3.5% in geopolymer foam leads to a decrease in compressive strength (44.81–3.2 MPa) and density (1,021–142 kg/m3) | [94] |
| Using 0.1–1% H2O2 reduces the compressive strength from 5.9 to 0.26 MPa and the apparent density from 1,100 to 230 kg/m3 | [95] | |
| Al powder | The use of Al powder with increasing percentages of 0.01–0.15% reduces the compressive strength (42.0–4.6 MPa) and density (1,830–1,031 kg/m3) | [64] |
| 1.5–5.0% of Al powder decreases the compressive strength from 4.35 to 0.9 MPa and density from 1,309 to 403 kg/m3 | [96] |
The characteristics of geopolymer foam are significantly shaped by the porosity generated within the matrix due to the incorporation of foaming agents. Table 3 shows the materials affecting the porosity of geopolymer foam structures. The addition of H2O2 not only increases the overall porosity but also enhances the concentration of connected pores throughout the material [97]. The interconnected pore structure plays a crucial role in the foam’s performance. However, it is important to note that, as outlined earlier, an increase in porosity can decrease both the material’s density and its compressive strength, a relationship further described in Table 2. The pore size and open porosity of geopolymer foams are primarily influenced by their capacity to resist destabilizing processes, such as coalescence and ripening [98]. The stabilizing agent influences the uniformity of the pore structure and percentage of porosity. Different stabilizers generate distinct pore patterns. Specifically, Tween 80 enhances porosity [97], while olive oil reduces it [67]. Besides the foaming agent and stabilizing agent, the higher ratio of the alkaline solution affected the increase of geopolymer foam porosity [99]. This phenomenon is particularly noticeable in chemical foaming processes conducted at elevated alkali concentrations and curing temperatures, as it leads to an accelerated rate of foam generation [100]. The percentage of porosity significantly influences the insulation properties of materials, a relationship that can be substantiated through thermal conductivity testing, which serves as one of the standard evaluation methods for insulation effectiveness.
Effect of agent material on the porosity of geopolymer foam.
| Material agent | Effect on porosity | Ref. |
|---|---|---|
| Foaming agent | The addition of aluminum powder at concentrations of 0.07–0.2% increased porosity from 47.9 to 58.4%, while H2O2 at 0.5–2% enhanced porosity from 37.9 to 44.9% | [101] |
| Increasing the H2O2 concentration from 1 to 3.5% in geopolymer foam increased the porosity from 32.3 to 63.1% | [94] | |
| Stabilizing agent | Tween 80 as a surfactant, with a concentration ranging from 1 to 5%, increased the porosity from 38 to 86% | [97] |
| Using olive oil as a stabilizing agent at concentrations ranging from 1.25 to 15% with the same H2O2 concentration reduced the porosity from 75.1 to 72.3% | [67] | |
| Alkaline solution | Increased ratio of NaOH to Na2SiO3 enhanced the porosity from 55.6 to 66.3% while maintaining the same foaming agent concentration | [99] |
| Increasing the Na2O ratio from 4 to 7% in an alkaline solution increased the porosity of the structure from about 61.6 to 68% | [11] |
The thermal conductivity of structures is significantly influenced by their porosity. Geopolymer foam, as an insulation material, demonstrates a lower thermal conductivity compared to traditional concrete and mortar structures. Xu et al. found that an increase in porosity is linked to a decrease in thermal conductivity, which decreased significantly from 0.6462 W/(m K) to 0.1825 W/(m K) [94]. The decrease in thermal conductivity is due to the formation of unconnected pores, which enhances porosity and significantly lowers thermal conductivity. According to the findings presented by Shen et al., an increase in the porosity of the geopolymer structure reduces the thermal conductivity [102]. Otherwise, in materials characterized by high density, an increase in compressive strength is frequently accompanied by an elevation in thermal conductivity, according to Jaya et al. [97]. Zhang et al. found that geopolymer foam is an effective insulation material due to its low thermal conductivity and outstanding fire resistance properties in structural applications [84]. Considering the impressive properties of geopolymers, insulating materials derived from them present a highly promising option for effective insulation solutions [97]. Table 4 provides a comprehensive comparison of the relationships among the various properties, offering valuable insights for analysis.
Reference relationships among density, compressive strength, porosity, and thermal conductivity of geopolymer foam.
| Precursor | FA | SA | Density (kg/m3) | Compressive strength (MPa) | Porosity (%) | Thermal conductivity (W/(m k)) | Ref. |
|---|---|---|---|---|---|---|---|
| Metakaolin | H2O2 | Olive oil | 320–610 | 0.2–3.9 | 75–87 | 0.147 | [103] |
| Metakaolin | Na2O2 | SDBS | 300–460 | 0.6–1.6 | 72–81 | 0.085–0.115 | [104] |
| Fly ash | H2O2 | — | 240–340 | 0.60–0.38 | 79–81 | 0.09–0.07 | [105] |
| Metakaolin | H2O2 | — | 470–1,210 | 0.37–6.00 | 36–86 | 0.11–0.30 | [97] |
| FA, GGBS | SDS | RCA | 824–838 | 1.3–1.8 | 42–71 | 0.270–0.360 | [106] |
| Metakaolin | SLES | — | 690–1,060 | 4.7–14.8 | 56–72 | 0.197–0.364 | [107] |
| FA, POFA | Sika AER | — | 1,300–1,700 | 8–13 | 25–40 | 0.47–0.50 | [74] |
| FA | H2O2 | CSFS | 310–380 | 1.45–1.60 | 70–82 | 0.095–0.139 | [108] |
| FA | H2O2 | Calcium stearate | 150–240 | 0.45–0.75 | 88.94–91.94 | 0.0485–0.0594 | [109] |
| FA | H2O2 | Calcium stearate | 200–300 | 1.06–2.84 | 59–83 | 0.05223–0.0711 | [110] |
| HCFA | Natural protein | — | 844–2,100 | 2.7–57.8 | 2.82–49.42 | 0.13–1.62 | [111] |
| Metakaolin | H2O2 | Vegetable oil | 370–740 | 0.3–11.6 | 66–83 | 0.11–0.17 | [67] |
| Metakaolin | H2O2 | Tween 80 | 300–750 | 0.3–9.4 | 67–86 | 0.289–0.091 | [112] |
Geopolymer foam in 3D printing presents several challenges and limitations throughout the process. A key factor is the time required for implementation, as 3D printing involves the gradual construction of shapes layer by layer. As a result, workability and setting time are critical considerations [85]. Stability is crucial during the printing process. The layer-by-layer assembly can lead to the collapse of pores due to their weight, vibrations, and the mixing action within the machine. Therefore, it is essential to carefully select materials and adhere to appropriate ratios. The chosen materials must retain their shape and strength throughout the printing process while also possessing sufficient binding properties to ensure successful outcomes. Achieving consistent thickness and strength in the final structure is one of the significant challenges [113]. Void formation between material layers can lead to high porosity and weak interlayer bonding, ultimately diminishing mechanical properties. Consequently, enhancing interlayer adhesion is necessary [114]. Moreover, there is currently a lack of established standards for the mechanical strength of materials used in 3D printing, often resulting in lower strength compared to conventional concrete. The design of materials is particularly sensitive to factors influencing stability and printability. Additionally, when using alkaline solutions, it is crucial to consider health, safety, and environmental concerns at the construction site.
Based on the challenges and limitations, the advanced material design should be further studied to improve the mechanical strength. The utilization of hydroxide and silicate as alkaline solutions may result in the production of less sustainable materials. Therefore, the exploration of alternative alkaline solutions warrants further investigation to enhance sustainability in material development [115]. Artificial intelligence and machine learning hold significant potential for optimizing printing parameters by conducting comprehensive analyses of variables such as temperature, layer height, printing speed, and material properties [113]. The future applications of this study may involve the incorporation of these materials as insulation within the building structure itself. This approach offers a cost-effective alternative to employing separate insulation materials after construction, thereby enhancing overall efficiency and sustainability in building design.
Geopolymer foam used in 3D printing is a lightweight and environmentally friendly material composed of aluminosilicate minerals activated by alkaline solutions. The incorporation of industrial waste materials, such as fly ash, slag, and silica fume, not only contributes to reducing carbon dioxide emissions but also enhances sustainability. Moreover, 3D printing technology can significantly decrease energy consumption and mitigate the environmental impact of construction activities. This material comprises precursors, which are aluminosilicate compounds combined with alkaline or acidic solutions and foaming agents. Optional stabilizers, fibers, and lightweight aggregates may also be included. The precursor materials, along with the alkaline or acidic solutions, are vital for the geopolymer’s formation. By adding a foaming agent, the mixture transforms into geopolymer foam, resulting in the generation of pores within the matrix structure. An increase in the foaming agent leads to larger pore sizes and greater porosity while concurrently reducing density, compressive strength, and thermal conductivity, as indicated in earlier studies. The low thermal conductivity of geopolymer foam makes it particularly suitable for insulation applications. When integrated with advanced 3D printing technology, geopolymer foam enables the creation of complex designs in architecture, infrastructure, and aerospace, all while minimizing material waste. In the 3D printing process, materials are deposited layer by layer in accordance with specified designs. Equipment such as robotic printers and direct ink systems is commonly employed in laboratory environments. The basic shapes and printing orientations – continuous and parallel – are determined by the specific requirements of the construction project. Nonetheless, challenges and limitations remain due to a lack of standardization within the industry. The properties of geopolymer foam after the 3D printing process, especially in terms of its microstructure and macrostructure, have a significant influence on its mechanical properties and durability. Future research should aim to explore these characteristics further, utilizing both existing methodologies and innovative approaches such as artificial intelligence and machine learning.
The authors acknowledge the National Science and Technology Council in Taiwan under Grant NSTC-111-2923-E-197-003-MY3.
Authors state no funding involved.
Wei-Chien Wang: Investigation, Methodology, Validation, Writing – Original Draft. Melati Sari Dewi: Investigation, Methodology, Formal analysis, Visualization, Writing – Original Draft, Writing – Review & Editing. Wei-Ting Lin: Conceptualization, Resources, Methodology, Supervision, Writing – Original Draft, Writing – Review & Editing. Marek Hebda: Data Curation, Methodology, Writing – Review & Editing.
Authors state no conflict of interest.