Low-Carbon Insulating Geopolymer Binders: Thermal Properties

Abstract

In the context of the growing need to reduce greenhouse gas emissions and to develop sustainable solutions for the construction industry, foamed geopolymers represent a promising alternative to traditional binders and insulation materials. This study investigates the thermal properties of novel low-emission, insulating geopolymer binders made from fly ash with diatomite, chalcedonite, and wood wool aiming to assess their potential for use in thermal insulation systems in energy-efficient buildings. The stability of the foamed geopolymer structure is also assessed. Measurements of thermal conductivity, specific heat, microstructure, density, and compressive strength are presented. The findings indicate that the selected geopolymer formulations exhibit low thermal conductivity, high heat capacity and low density, making them competitive with conventional insulation materials—mainly load-bearing ones such as aerated concrete and wood wool insulation boards. Additionally, incorporating waste-derived materials reduces the production carbon footprint. The best results are represented by the composite incorporating all three additives (diatomite, chalcedonite, and wood wool), which achieved the lowest thermal conductivity (0.10154 W/m·K), relatively low density (415 kg/m³), and high specific heat (1.529 kJ/kg·K).

1. Introduction

Given the increasingly stringent requirements for the insulation/thermal modernisation of buildings and industrial installations, it is necessary to seek new economically and environmentally efficient material solutions [1,2,3,4,5]. For several decades, scientists have focused their attention on the production and testing of geopolymer materials, which are characterised not only by good strength properties, but also by excellent fire resistance [6,7,8,9,10]. These materials can successfully replace traditional building materials in many applications, ranging from structural applications to aesthetic and cladding applications, etc. [11,12,13,14].

Geopolymers can also foam and produce a highly porous structure with low density. They are therefore also a good material for use in the thermal insulation of buildings, etc. [15,16,17,18,19]. They have also been observed to have a better heat accumulation capacity than Portland cement-based binders and concretes. This gives us hope for real possibilities for their use on an industrial scale. In the case of foamed geopolymer materials, their good thermal resistance, insulating properties, heat storage capacity, and the fact that they can be produced sustainably suggest that their production will be possible soon [20,21,22,23,24]. However, it should be noted that these materials have been known for several decades, and the existing barriers to their implementation have not been overcome. One of the barriers is the difficulty in controlling the foaming process and maintaining the porous structure during curing. This raises problems with the repeatability of the results obtained. Settling geopolymer foams is a significant problem in the manufacturing process, as settling causes a change in their internal structure. They then form a more compact and dense structure, which negatively affects their performance properties, especially their insulating properties. The density of the material increases, and the number of pores, which are responsible for low thermal conductivity and good acoustic insulation, decreases significantly. As a result, the material, which was supposed to be light and well insulating, becomes heavier and less effective in performing its function [25,26,27,28]. To counteract this phenomenon, scientists have been developing and testing various types of additives and modifiers for many years, which are designed to stabilise the foam and prevent it from settling during the setting and hardening process. Such additives include mineral materials such as Portland cement, gypsum, and hydrated lime, which increase the viscosity of the mixture and help maintain the foam structure. Their use allows for partial inhibition of the sedimentation process and improvement of mechanical stability in the initial curing phase [29,30,31,32]. In addition to mineral stabilisers, chemical additives, in particular synthetic surfactants, are also increasingly used in industry. This group includes compounds that reduce the surface tension between liquid and gas, which promotes the formation of stable and homogeneous air bubbles in the foam structure. Examples of such substances include ethoxylated fatty alcohols, alkylbenzenesulfonates, and polyether silicone surfactants, which are exceptionally effective in stabilising foam structures. The use of these compounds allows for the production of more homogeneous foams with uniform pore distribution and better insulation parameters [33,34,35,36,37]. However, intensive research is still ongoing to improve both the chemical composition of geopolymer mixtures and the methods of introducing and activating stabilising additives to obtain a product with optimal performance, durability, and low environmental emissions.

In this study, geopolymer foams were modified with both mineral additives and fine wood wool. Recent studies confirm that ceramic additives similar to diatomite and chalcedonite can improve insulating properties of geopolimeric materials and actively influence the microstructure and porosity of geopolymer foams [38,39,40,41,42]. Diatomite contributes its intrinsic micro-porous structure, reduces density and thermal conductivity, and exhibits partial pozzolanic reactivity, indicating that it is not an inert filler when interacting with NaOH. Although the use of chalcedonite in geopolymer materials has not been previously investigated, some assumptions can be made based on the available literature on its related applications. Considering its structure and morphology, the effect of chalcedonite is likely to be slightly different, as it may stabilise the structure without actively enhancing foaming [43,44,45].

The role of natural fibres, including wood-based additives, in geopolymer foams is discussed in detail in a recent review by Walbrück et al. [46]. Wood fillers in geopolymer foams serve multiple functions that contribute to the performance and sustainability of the material. One of their primary roles is foam stabilisation. Due to their fibrous morphology and surface chemistry, wood fibres can act as physical barriers that hinder bubble coalescence during the foaming process, thereby promoting a more uniform pore distribution and enhancing foam stability. The presence of fibres helps to evenly distribute stresses during the geopolymer’s setting and drying process, which reduces the risk of cracking. In addition to stabilising the foam, wood fibres improve the thermal insulation properties of geopolymer foams. Their inherently low thermal conductivity and low density help reduce the overall heat transfer through the material, making them suitable for energy-efficient building applications. Furthermore, wood fibres exhibit good compatibility with the geopolymer matrix, which facilitates effective interfacial bonding. This compatibility is crucial for mechanical reinforcement, as it enables the fibres to bridge microcracks and distribute stress more evenly throughout the matrix. As a result, the incorporation of wood fibres can lead to an enhancement in flexural strength, contributing to the structural integrity of the foam without significantly compromising its insulating performance.

To the best of the authors’ knowledge, the use of wood wool as a fibrous additive specifically in geopolymer foams has not been previously reported in the literature. However, a related and particularly interesting context is provided by wood wool insulation boards, where wood wool is the main component, combined with mineral-based matrices. Wood wool cement boards (WWCBs), using ordinary Portland cement (OPC) as the binder, have become widely used in construction. Most recently, Koch et al. replaced OPC in such boards with a geopolymer binder, achieving, thanks to additional modifications, a density of 392 kg/m³ and a porosity of 76%, while meeting the prescribed minimum compressive strength (20 kPa) and bending strength (1700 kPa) requirements for WWCBs intended for use in thermal insulation of buildings [47]. This represents an interesting approach, which serves as one of the reference points in the discussion presented in this paper, where a different concept is explored. In Koch’s work, wood is the dominant component, whereas in the current article, the focus is on foamed geopolymer; both approaches have their advantages and drawbacks.

The stabilisation of the porous structure and the enhancement of the foaming process by mineral and organic additives, as mentioned above, are important not only for the insulating properties of these materials but also due to an additional, integrated function of porous geopolymer materials—their high capacity to adsorb CO₂ molecules. This ability has been demonstrated in numerous studies, showing performance comparable to standard adsorbent materials such as activated carbon or zeolites [48,49,50,51]. From the perspective of sustainable development of functional and structural materials, another advantage of geopolymer materials lies in their substantially lower carbon footprint compared to conventional binders. In the study by Turner and Collins [52], the CO₂-e emissions of geopolymer concrete were compared with those of 100% OPC-based concrete. It was found that the geopolymer concrete exhibited only about a 9% lower carbon footprint than the conventional OPC concrete, significantly less than previously anticipated and reported in earlier studies. However, most commonly, the researchers report reductions in the range of 26–80%, as highlighted in the recent work by Al-Fakih et al., particularly when accounting for the widespread use of waste-derived precursors for geopolymers at the same binder dosage as OPC [53].

This paper addresses the production of advanced functional porous structures—synthetic inorganic polymers based on fly ash—with a focus on their thermal insulation and heat accumulation capabilities. This research involved a multi-variant synthesis of geopolymer foams incorporating ceramic and organic additives: diatomite, chalcedonite, and wood wool.

The innovative aspect of this study lies in the deliberate selection and integration of these materials, each fulfilling specific functions within the composite. Chalcedonite, a rare cryptocrystalline silica from the only European mine located in Poland, and diatomite, a porous silicate also sourced from a unique Polish deposit, contribute to the structure and thermal performance. Fine wood wool simultaneously acts as a fibrous, porous, and binding filler that stabilises and strengthens the foam, also mitigating cracking during hardening. Based on the available literature, this is the first reported use of chalcedonite in geopolymer materials and the first purposeful application of wood wool as a multifunctional additive in geopolymer foams.

The first simultaneous use of a combination of chalcedonite, diatomite, wood wool, and fly ash, together with a small amount of clay cement and organic surfactant, to develop a lightweight, porous geopolymer binder with insulating properties, has not yet been described in the literature. While plant fibres, diatomite, and synthetic foaming additives have been analysed separately, such a synergistic combination of these components—especially in the context of thermally stable, low-emission structural and insulating binders—has not been reported before. Chalcedonite, as a local mineral waste from the quartz aggregate sector, does not appear in the literature as a component of geopolymer binder or as a mineral foam stabiliser. Its use in our work is an attempt to add value to waste without prior processing. Wood wool, although known for its use in WWCBs, was used by us as an organic stabiliser of the geopolymer foam structure, without cement binder as the main component. The literature is dominated by the use of cellulose, flax, hemp, or straw, while wood wool as a geopolymer foam stabiliser is, in our opinion, an original approach. Diatomite, in turn, has a dual function in our formulation: as a stabiliser of the system and as an additive improving thermal insulation—and its combination with organic and waste additives has not yet been widely described in the context of low-emission structural foams. In addition, the entire system has been designed based on locally available, low-processed waste and raw materials (including no grinding of chalcedonite or chemical activation of diatomite), which significantly reduces the carbon footprint of the material. In our opinion, this practical approach combines innovative aspects in terms of raw materials, technology, and the environment.

The composites were developed as part of a project investigating not only thermal insulation but also the CO₂ adsorption capacity of foamed geopolymers—an area of growing interest that remains insufficiently explored. Geopolymer technology enables the low-temperature synthesis of functional materials with high surface area; however, the foamed structure must be stabilised to achieve low densities and reliable performance. It is widely recognised that one of the main barriers to the broader implementation of foamed geopolymers is the lack of repeatability associated with their low structural stability. Therefore, the issue of stabilising the porous structure in the context of thermal properties and potential CO₂ sequestration constitutes an important focus of this study. At the same time, the problem of cracking and deformation during hardening was effectively addressed. The environmental and industrial relevance of this work is also significant: the materials show potential as low-carbon, sustainable alternatives to conventional insulators. They are produced from local and waste-derived materials, aligning with circular economy principles and strategies aimed at reducing greenhouse gas emissions. Thus, the goals set and achieved through the use of these additives were mainly

• High foaming (high porosity and low density);
• Improved insulating properties—thanks to additives with low thermal conductivity and the avoidance of heavy, inactive fillers;
• Maintaining adequate structural stability and durability;
• Compliance with sustainability principles.

By integrating thermal conductivity, foam stability, heat capacity, compressive strength, and microstructure analyses, this study provides a comprehensive understanding of the material’s behaviour and performance.

This article, therefore, offers a pioneering contribution by proposing a novel, environmentally responsible approach to designing durable, functional geopolymer foams through the synergistic use of chalcedonite, diatomite, and wood wool.

2. Materials

2.1. Co-Forming Materials of Geopolymer Foams

In this research, geopolymer foams based on various sources of waste and natural raw materials were developed, which highlights their low-emission and sustainable nature. The main base component was class F fly ash obtained from the combustion of hard coal at the Skawina Power Plant (CEZ Skawina S.A., Skawina, Poland). This ash is characterised by a high content of amorphous reactive phase and a favourable ratio of aluminium and silicon oxides, which makes it a suitable precursor in the synthesis of geopolymer materials. To enrich the formulation and improve the functional properties of the foams, two other waste mineral raw materials from Polish deposits were additionally used: diatomite from Jawornik Ruski, supplied by GÓRTECH (Specialised Mining Company GÓRTECH Ltd., Cracow, Poland), and chalcedonite from deposits in Inowłódź (Crusil sp. z o.o., Inowłódź, Poland). Diatomite, a light sedimentary rock with high porosity, served as an internal structure modifier. Chalcedonite, rich in silica, had a positive effect on the accumulation properties of the final product. In the particle size distribution analysis performed by laser diffraction, the D90 values were as follows: 32 µm for fly ash, 20 µm for diatomite, and 20 µm for chalcedonite.

Fine-grained quartz sand from the Świętochłowice Sand Pit (Świętochłowice, Poland) was used as an inorganic filler, which increased the density of the material and its volume stability. In addition, high-alumina cement with the trade name Górkal 70 (manufactured by Górka Cement Sp. z o.o., Trzebinia, Poland) was added to the mixture, acting as a hydraulic additive supporting the setting and hardening processes under conditions of elevated temperature and humidity. To obtain a closed, porous structure, the recipe used ash microspheres (TERMO-REX S.A., Jaworzno, Poland), which acted as a light filler, increasing the thermal insulation of the material. Constant amounts of cement (100 g), sand (80 g), and microspheres (160 g) were adopted based on publications on foamed geopolymers [18,22,24], which emphasised the importance of the balance between the total mass of fillers and the pore volume for maintaining foam stability. To stabilise the foam and improve the uniformity of pore distribution, an organic surfactant—syringaldehyde—supplied by Merck (Merck, Darmstadt, Germany) was also added to the mixture. This compound acted as a surfactant that supported the foaming process and prevented the coalescence of gas bubbles in the initial setting phase. The use of 5 g of organic stabiliser (syringaldehyde) was based on our own experience and that of other researchers using surfactants in similar ranges [33,34,36].

Another innovative element of this research was the use of fibrous wood material, commonly known as wood wool (supplied by Dach-Wkręt, Babice, Poland). It consisted of thin, elongated wood strands mechanically shredded from softwood, typically with a width of 5 mm. It was chopped to the nominal length of approximately 20 mm. Unlike isolated microscopic wood fibres, this material retains the macroscopic fibrous structure of wood, providing low bulk density and high porosity. It was free from chemical treatment and synthetic additives and was supplied in a dust-free form. Its presence was intended not only to improve the insulating parameters of the material but also to strengthen its structure and reduce shrinkage deformation. The use of 12 g of wood wool in each sample was due to limitations related to maintaining the homogeneity of the mixture while preserving the appropriate rheological properties. These proportions are similar to those used, for example, in the works [46,47,54], where natural fibres constitute a small but functional share of the composite.

To fully characterise the base materials used, their chemical composition was analysed by X-ray fluorescence (XRF) using a SHIMADZU EDX-7200 spectrometer (SHIMADZU Europa GmbH, Duisburg, Germany). The results of the analysis are summarised in

Table 1. Oxide analysis of base materials.

Precursor		Oxide Composition (wt.%)
	SiO2	Al2O3	Fe2O3	CaO	K2O	TiO2	SO3
Fly ash	59.27	30.35	3.68	2.24	2.57	0.66	0.33
Diatomite	78.99	15.90	2.62	0.28	1.38	0.35	0.40
Chalcedonite	94.35–99.54	0.40–3.69	0.12–0.49	0.01–0.10	0.06–0.42	0.06–0.42	-
Sand	98.99	-	0.18	0.17	0.33	-	0.13
Cement	-	82.86	-	17.08	-	-	-
Microspheres	55.59	38.05	2.41	0.80	2.15	0.78	-

. Data on the chemical composition of chalcedonite were obtained from information provided by the manufacturer [55]. On the other hand, the microstructure of selected waste raw materials is shown in Figure 1 to better illustrate their potential as functional components in the structure of geopolymer foams. The photo was taken with a JEOL IT200 scanning electron microscope, and the chalcedonite photos were taken with a JEOL JSM-5510LV (JEOL Ltd., Akishima, Tokyo, Japan) [55].

2.2. Manufacture of Geopolymer Foams

To produce geopolymer foams, an automated mixing process was employed using a GEOLAB M/LMB-s laboratory mixer (Warsaw, Poland), which is designed in accordance with applicable standards for the preparation of mortars and concrete mixes. The process started with a dry mixing stage, where class F fly ash, the main mineral component, was poured into the mixer, along with additives such as diatomite, chalcedonite, or wood wool, depending on the sample variant being tested, and other components responsible for the porous structure and stability of geopolymer foams. The entire mixture was mixed for 5 min at a constant speed of 50 rpm to achieve initial homogenisation of the material and an even distribution of the components throughout the entire volume of the mixture. The next step was to introduce an activating component into the liquid system—a previously prepared alkaline solution. It consisted of two components: a 10-molar sodium hydroxide (NaOH) solution, prepared by dissolving technical NaOH with a purity of over 99% (PCC Rokita SA, Brzeg Dolny, Poland) in distilled water, and an aqueous solution of R-145 sodium silicate (water glass) with a molar ratio of SiO₂/Na₂O of 2.5 and a density of approximately 1.45 g/cm³, supplied by ANSER Chemical Plant (Wiskitki, Poland). The choice of the 10M concentration was dictated by the authors’ earlier research and literature reports confirming that such high concentrations are necessary and have a positive effect on the structure of the obtained geopolymer foams [9,24,56,57,58]. The dosage of 10M NaOH and the proportions of the alkaline solution were selected on the basis of numerous studies [8,9], which indicate that a high concentration of the alkaline solution (8–12 M) promotes the intensification of the geopolymerisation process in fly ash systems. Both components were combined in a mass ratio of 1:2.5 (NaOH: Water glass), resulting in a stable, strongly alkaline, and highly reactive geopolymerisation activator. After introducing the alkaline solution into the dry mixture, the mixing process was continued for another 10 min. During this time, a homogeneous, plastic mass with appropriate rheological properties was obtained, enabling its further forming and foaming. To obtain a porous structure with a predominance of closed pores, 35% hydrogen peroxide was added to the finished mixture as a porogen. After dosing, the mixture was stirred for an additional 2 min, allowing for the even distribution of the foaming agent and the initiation of the oxygen release reaction, which was responsible for the development of the porous structure. The formed mass was immediately transferred to previously prepared laboratory moulds, which were secured with protective film. This film was suitably selected and used in a manner that did not hinder the free expansion of foam during the foaming process. The samples prepared in this way were placed in an SLW 750 chamber dryer (POL-EKO Perfect-Environment, Wodzisław Śląski, Poland), where they were heated at 75 °C for 24 h. This stage was crucial for the geopolymerisation process and for obtaining the initial structural strength of the material. After annealing, the samples were removed from the moulds and allowed to mature under laboratory conditions (room temperature, relative humidity approximately 50%) for 28 days. This seasoning time allowed for the complete consolidation of the geopolymer structure and the achievement of the mechanical and physicochemical properties necessary for further testing. All samples were in the form of 20 × 20 × 3 cm plates.

Table 2. Composition and determination of prepared compositions of foamed geopolymers.

Sample ID	Sand [g]	Microspheres [g]	Fly Ash [g]	Chalcedonite/ Diatomite [g]	Cement [g]	Stabiliser [g]	Wood Wool [g]	H2O2 [mL]	Alkaline Activator [mL]
Ref. F.A.	80	160	795	-	100	5 g	-	25	350
Ref. F.A + D	80	160	681	114	100	5 g	-	25	375
Ref. F.A + CH	80	160	681	114	100	5 g	-	25	375
Ref. F.A + CH + D	80	160	681	57 CH + 57 D	100	5 g	-	25	375
F.A. + WW	80	160	795	-	100	5 g	12	25	425
F.A. + D + WW	80	160	681	114	100	5 g	12	25	425
F.A. + CH + WW	80	160	681	114	100	5 g	12	25	375
F.A. + CH + D + WW	80	160	681	57 CH + 57 D	100	5 g	12	25	375

presents detailed designations of the tested samples along with their quantitative composition. Reference samples are marked as follows: “Ref. F.A.” represents fly ash, “D” denotes diatomite, “CH” stands for chalcedonite, and “WW” refers to wood wool.

Each variant of the developed geopolymer mixture used a different amount of alkaline activator, stemming from variations in the chemical and physical properties of the additives employed. Additives such as diatomite, chalcedonite, and wood wool differ not only in their porous structure and hygroscopic properties but also mainly in their mineral composition and reactivity in a strongly alkaline environment. Each material interacts uniquely with the alkaline solution, affecting both liquid absorption and the reactivity of silica and aluminosilicate compounds. For example, diatomite, characterised by a high specific surface area and abundant amorphous silica, displays high reactivity in alkaline solutions, which can cause rapid gelation during mixing. Consequently, the amount of activator was minimally increased to prevent excess thickening and to maintain the desired consistency and workability. Chalcedonite, a mineral with lower reactivity and a more crystalline structure, also required more activator to effectively induce geopolymerisation and ensure proper bonding. Regarding wood wool, which does not participate directly in chemical reactions but has a high liquid absorption capacity, the proportions of liquid components were adjusted to compensate for water loss and preserve the mixture’s plasticity. The differing doses of alkaline activator among the variants aimed to tailor the reaction conditions to the specific properties of each additive, ultimately ensuring process reliability, producing a homogeneous microstructure, and achieving optimal thermal properties in the final geopolymer foams.

Figure 2 displays the appearance of foamed geopolymer samples immediately after demoulding, that is, in the raw state without further processing. The characteristic features of the material’s surface after the annealing process, such as irregularities, air bubbles, and the natural porous structure of the foams, are visible. Figure 3 shows the same types of specimens after mechanical processing, which involved precisely cutting them to the required dimensions. These prepared samples were intended for further laboratory tests, particularly measurements of thermal properties. The machining aimed to standardise the geometry of the samples and ensure proper contact with the measuring plates of the device used to analyse thermal insulation properties.

No visible cracking or surface deformation was observed during the curing process, indicating good volumetric stability of the obtained composites. Quantitative analysis of shrinkage and mass loss was not feasible at this stage of the study and will be included in future research.

3. Methods

3.1. Mechanical Properties of Geopolymer Foams

Compressive strength tests were performed using a universal testing machine MTS Criterion 43 (Eden Prairie, MN, USA), equipped with TestSuites 1.0 software and having a measuring range of up to 30 kN. This device is designed to perform mechanical tests on building materials such as cement mortars and geopolymer concretes. The test procedure was carried out in accordance with the requirements of European standard EN 196-1—‘Methods of testing cement—Part 1: Determination of strength’, in particular according to the provisions of Section 9.2 relating to compressive strength tests [59]. Cubic samples measuring 25 × 25 × 25 mm were used for the tests. The samples were placed in the load chamber in such a way that the compressed surfaces were positioned exactly perpendicular to the direction of the force. The load was applied gradually and evenly until the sample was completely destroyed (fractured). Based on the measured maximum force (F_c), the compressive strength (R_c) was determined according to Formula (1).

$$R_c={\displaystyle \frac{F_c}{A}}\left[\mathrm{M}\mathrm{P}\mathrm{a}\right]$$

(1)

where

R_c—compressive strength [MPa];

A—sample cross-sectional area [mm²];

F_c—maximum load [N].

All measurements were performed in five replicates for each type of mixture. The results obtained are presented as average values. The collected data played an important role in the further analysis of the effectiveness of geopolymer foams, especially in terms of their use as lightweight, durable materials with insulating properties.

3.2. Thermal Properties of Geopolymer Foams

The thermal properties of the tested foamed geopolymer materials were measured using a technologically advanced HFM 446 Lambda plate apparatus manufactured by Netzsch (Selb, Germany), designed for the precise determination of the thermal insulation parameters of various building materials, including geopolymer composites. This device operates based on the two heating plates method (the so-called hot and cold plate method), meeting the requirements of some international standards, such as ASTM C1784 [60], ASTM C518 [61], ISO 8301 [62], EN 12664 [63], and other recognised thermal metrology standards. The HFM 446 device has a very wide measurement range—from 0.007 to 2.0 W/m·K, which allows for the analysis of both very light foams and denser composites. High measurement accuracy (±1–2%), repeatability (±0.25%), and reproducibility (±0.5%) ensure the reliability and consistency of the results obtained. The use of Peltier modules enables precise control and stabilisation of the measurement temperature. Thermal conductivity and thermal resistance tests were carried out in the range of 0–20 °C, which corresponds to the actual operating conditions for insulation materials used in construction. As part of the same test procedure, the HFM 446 apparatus (Netzsch, Selb, Germany) was also used to measure the specific heat (Cp) at 27.5–32.5 °C and the volume density of the samples. This allowed for a more complete thermophysical characterisation of the materials, which is important when assessing their potential use in insulation systems, especially in the context of efficient heat storage and transfer. The test samples were prepared in the form of plates with dimensions adapted to the measuring chamber of the device, ensuring good contact with the heating and cooling plates and uniform temperature distribution during the measurement. The mass of the samples was determined with high accuracy using a RADWAG PS 200/2000.R2 analytical laboratory balance (Radom, Poland) with an accuracy of 0.01 g, and the geometric dimensions were determined using a precision calliper with a resolution of 0.01 mm. Based on the collected data, the thermal conductivity coefficient (λ), thermal resistance (R), specific heat (Cp), and density were calculated, which enabled a comprehensive assessment of the materials in terms of their functionality as modern, low-emission thermal insulators. The results obtained served as a starting point for further analysis of the material’s suitability, particularly in lightweight construction systems requiring high insulation and accumulation properties.

3.3. Morphology of Geopolymer Foams

A microscopic analysis of foamed geopolymer materials was performed using a Keyence VHX-7000 digital optical microscope (KEYENCE INTERNATIONAL (BELGIUM) NV/SA, Mechelen, Belgium)—a modern tool for high-resolution observation and documentation of the surface structure of engineering materials. The microscopic studies aimed to perform a detailed assessment of the porous microstructure of geopolymer samples, which directly affects their thermal insulation properties. Before the analysis, each sample was carefully prepared—the observation surface was cleaned and mechanically levelled to enable optimal focus and avoid interference resulting from topographical irregularities. Thanks to its advanced optical system and automatic focus adjustment, the Keyence VHX-7000 microscope allowed for sharp and detailed images even at high magnification, without the need for additional sample preparation. Various magnification levels were used for the research to enable both general observation of pore distribution and morphology, as well as detailed analysis of local structural defects, stabilising filler distribution, and pore wall characteristics. All micrographs were taken in reflected light mode, using ring illumination and an intelligent HDR function that automatically selected exposure parameters for maximum contrast resolution and detail clarity. Thanks to the integrated Depth Composition function, it was possible to obtain a complete representation of the three-dimensional surface, even in cases of highly developed foam topography. These observations provided essential data for assessing the quality of the foams, including their degree of foaming, pore uniformity, and the presence of any defects, such as microcracks, discontinuities, or particle aggregates.

4. Results

4.1. Mechanical Properties of Geopolymer Foams

Table 3. Mechanical parameters of prepared compositions of foamed geopolymers.

Sample ID	Compressive Strength [MPa]
Ref. F.A.	1.86
Ref. F.A + D	1.56
Ref. F.A + CH	0.80
Ref. F.A + CH + D	1.58
F.A. + WW	2.00
F.A. + D + WW	0.90
F.A. + CH + WW	1.26
F.A. + CH + D + WW	1.50

presents the compressive strength results obtained for individual test samples. Based on these data, a graph was prepared, which is shown in Figure 4 and illustrates a comparison of the strength of individual mixtures.

Based on the data presented in Table 3 and Figure 4, a clear difference in the compressive strength of individual mixtures can be observed. Ref. F.A. sample achieved a strength of 1.86 MPa, which is the reference point for the other variants. The highest value was obtained for the sample containing fly ash with the addition of wood wool (F.A. + WW). The introduction of other additives—diatomite, chalcedonite, and their combinations—without wood wool generally had a negative effect on compressive strength, particularly in the case of chalcedonite alone. Meanwhile, for diatomite alone and for the diatomite–chalcedonite combination, the strength values were similar to each other and not as markedly reduced compared to the reference as in the FA + CH sample. Importantly, the inclusion of wood wool in this mixture significantly improved its mechanical properties. The opposite effect was observed for the F.A. + D + WW composition, where a decrease in strength was recorded compared to the material with diatomite alone. The addition of wood wool generally had a positive effect on the compressive strength of the tested mixtures. However, the effect of individual components is neither linear nor unambiguous, which underscores the need for further research into their mutual interactions.

4.2. Thermal Properties of Geopolymer Foams

Table 4. Thermophysical parameters of prepared compositions of foamed geopolymers.

Sample ID	Density [kg/m3]	Thermal Conductivity at 0–20 °C [W/m × K]	Thermal Resistance [m2 × K/W]	Specific Heat at 27.5–32.5 °C [kJ/kg × K]
Ref. F.A.	395.700	0.10288 ± 0.02	0.2537	1.438 ± 0.07
Ref. F.A + D	429.628	0.10506 ± 0.01	0.2474	1.405 ± 0.04
Ref. F.A + CH	402.740	0.12043 ± 0.01	0.2203	1.839 ± 0.04
Ref. F.A + CH + D	395.606	0.10265 ± 0.03	0.2547	1.511 ± 0.06
F.A. + WW	636.274	0.13448 ± 0.02	0.1367	1.456 ± 0.03
F.A. + D + WW	433.024	0.10642 ± 0.02	0.2106	1.627 ± 0.04
F.A. + CH + WW	369.069	0.11412 ± 0.01	0.2352	1.968 ± 0.05
F.A. + CH + D + WW	415.139	0.10154 ± 0.03	0.2529	1.529 ± 0.03

summarises the thermophysical parameters of the developed foamed geopolymer compositions. Key material properties such as thermal conductivity coefficient (λ), thermal resistance (R), specific heat (Cp), and bulk density of the tested samples are included. These data allow a comprehensive evaluation of the thermal insulation efficiency of the different formulation variants and provide a basis for comparisons between the functional additives used. Figure 5 presents a graph of the dependence of thermal conductivity coefficient (λ) on volume density. Analysis of this dependence allows the identification of structural trends, indicating how the density of the material affects its insulating properties.

The analysis of the data presented in Table 4 allows for a detailed assessment of the thermophysical properties of the tested foamed geopolymer compositions. The volume density of the tested samples ranged from 369.1 to 636.3 kg/m³. The highest density was observed in the sample containing fly ash and wood wool (636.3 kg/m³), which may indicate limited foaming efficiency for this material configuration. In turn, the lowest density (369.1 kg/m³) was observed in the sample with the addition of chalcedonite and wood wool, indicating a synergistic effect of both components on increasing porosity and reducing the composite’s weight.

The thermal conductivity coefficient (λ) of the obtained materials ranged from 0.1015 to 0.1345 W/m·K, which confirms their satisfactory insulating properties. The lowest λ values were observed for samples containing both mineral additives (chalcedonite and/or diatomite) and wood wool, especially in the case of the F.A. + CH + D + WW system, where the thermal conductivity coefficient was only 0.1015 W/m·K. Equally favourable results were obtained for the F.A. + CH + D sample (0.1027 W/m·K), which confirms the positive effect of these two mineral additives on improving the thermal insulation properties of the composite. The highest λ values, i.e., the worst insulation parameters, were observed for the sample containing only fly ash and wood wool (0.1345 W/m·K), which may be related to the greater heterogeneity of the structure and lower matrix cohesion. Thermal resistance (R), which is a function of thermal conductivity and sample thickness, also confirms the beneficial effect of mineral and organic additives on the insulating properties of the material. The highest thermal resistance values—0.2547 m²·K/W—were observed for the reference sample F.A. + CH + D and the F.A. + WW system, indicating that these materials most effectively limited heat flow. High R values were also achieved for compositions containing the entire combination of additives: F.A. + CH + D + WW (0.2529 m²·K/W), indicating that the optimal combination of components enables effective insulation without compromising the material’s structure. Specific heat (Cp), as a measure of a material’s ability to accumulate thermal energy, showed varied values ranging from 1.405 to 1.968 J/g·K. The highest Cp value—1.968 J/g·K—was observed for the F.A. + CH + WW composite. This may indicate an increased ability of this system to store heat, which may be desirable in some applications (e.g., fire protection systems or components exposed to variable temperature conditions). In contrast, the lowest Cp value (1.405 J/g·K) was observed for the reference system F.A. + D, which suggests a limited ability of this composite to absorb thermal energy. The data presented indicate that the introduction of mineral (chalcedonite and diatomite) and organic (wood wool) additives has a positive effect on reducing the thermal conductivity coefficient and increasing the thermal resistance of geopolymer materials.

The graph showing the dependence of the thermal conductivity coefficient on the volume density of the tested samples (Figure 5) demonstrates a clear correlation between the two parameters. As the density of the foamed composites increases, there is a tendency for the value of the thermal conductivity coefficient to increase. This means that samples with lower density, and therefore more porous, have better insulating properties. This is consistent with the mechanism of heat conduction in porous materials, where a greater amount of enclosed air (with very low thermal conductivity) in the structure effectively limits the transport of thermal energy. On the other hand, the higher density of the samples, resulting from a smaller number of pores or their poorer distribution, is associated with a higher proportion of the solid phase, which has a higher thermal conductivity than air, leading to an increase in λ. Of the samples analysed, those containing wood wool additives in combination with chalcedonite and/or diatomite allowed the lowest density and thermal conductivity values, confirming the favourable effect of the combination of texture and lightweight additives. On the other hand, the sample containing only fly ash and wood wool (without mineral additives) had the highest density and the highest thermal conductivity coefficient, indicating that the use of wood filler alone is not sufficient to provide an effective porous structure.

4.3. Morphology of Geopolymer Foams

Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13 show the microstructure of all the prepared foamed geopolymer samples. The images were captured using a digital optical microscope at various magnification levels, allowing for a detailed examination of the internal porous structure of the materials. Varying the magnification scale made it possible to capture both the general arrangement of the pores and the phase distribution of the components, as well as finer details such as the morphology of the pore walls, the presence of wood wool, or the nature of the mineral additives.

The analysis revealed significant differences between the material variants in terms of pore size distribution and their homogeneity. The microstructure of the reference sample, designated F.A. and made solely from fly ash without functional additives, was heterogeneous. It showed both large pores exceeding 1 mm in diameter and areas of considerable density, almost devoid of voids. Such an uneven, porous structure resulted from an unstable foaming process, which, in the absence of stabilising additives, led to local foam collapse and irregular areas of compaction. This microstructure causes not only reduced repeatability of material parameters but also worsened thermal conductivity and lower insulating efficiency. Conversely, samples containing lignocellulosic fibres, such as wood wool (F.A. + WW variant), exhibited a markedly different microstructure. The addition of wood filler in the geopolymer matrix had a stabilising effect on the foaming process. The resulting structure was highly homogeneous, with fine, regularly spaced pores evenly distributed throughout the material. Wood wool improved pore distribution and bolstered the structural integrity of the composite, reducing micro-cracks and levelling shrinkage stresses during curing.

Another variant tested involved samples with diatomite (F.A. + D), a natural silica material with high microporosity and pozzolanic properties. Microscopic analysis showed improved porosity compared to the base sample. The structure contained finer pores, although their distribution was less homogeneous than in wood-containing samples. Diatomite also served as a binding agent in the geopolymer matrix, influencing the rate and course of geopolymerisation and consequently affecting pore distribution and foam stability. Samples with chalcedonite (F.A. + CH), a silica rock with a dense, compact structure, displayed a distinctly different microstructure. Instead of the open porous structure typical of geopolymer foams, a more compact microstructure was observed, with fewer pores and a smaller void volume. Chalcedonite did not promote the foaming process; rather, its presence contributed to mechanical stabilisation, limiting deformation during setting.

The most promising results were achieved with samples containing a combination of all three additives: diatomite, chalcedonite, and wood wool (variant F.A. + CH + D + WW). These samples exhibited an exceptionally homogeneous microstructure, with fine, regularly distributed pores throughout. No local structural defects were observed, and phases were evenly dispersed.

Numerous microscopic images were taken during the experiment to document the microstructure of various geopolymer variants. This report presents selected, representative images that illustrate typical morphological features. It is important to note that conclusions regarding additive influence on homogeneity, distribution, and pore size were based on a comprehensive analysis of all microscopic data collected, although, for editorial reasons, not all images are included.

At the current stage of the research, pore size distribution analysis using mercury intrusion porosimetry (MIP) was not conducted. The assessment of porosity was performed indirectly through microstructural observations, bulk density measurements, and thermal conductivity evaluation. Future studies will include quantitative pore analysis (e.g., MIP or SEM image analysis) to further investigate the relationship between microstructure and physical performance of the composites.

5. Discussion

Recent studies emphasise that the development of sustainable, energy-efficient insulation materials remains a significant challenge, despite the availability of numerous solutions [64,65,66,67]. Conventional materials such as polymer foams and mineral wool offer excellent thermal insulation but rely on non-renewable resources, are difficult to recycle, or have high embodied energy [64,66]. Biopolymer-based aerogels and other novel materials have been proposed to address these drawbacks, yet they often suffer from high cost or insufficient mechanical stability [65]. As an alternative, geopolymers offer a combination of strength, stiffness, non-combustibility, relatively low thermal conductivity, refractory properties, and the potential to incorporate industrial by-products or renewable fillers [68,69]. Nevertheless, their density and thermal conductivity remain significantly higher than those of conventional insulation materials, which motivates ongoing research into optimising their composition and structure [56,57,68,69]. Based on the obtained results of compressive strength, density, thermal conductivity, and thermal resistance, as well as microscopic observations, the selected tested composites can be considered as potential competitors to currently used insulation materials in certain application areas. Before moving on to these considerations, the key mechanical and thermal properties of the materials are first discussed.

The analysis of mechanical properties, combined with microscopic observations, reveals that the compressive strength of foamed geopolymer composites is strongly influenced by the microstructure shaped by the applied additives. The highest strength (2.0 MPa) was achieved for the F.A. + WW sample, which exhibited a homogeneous microstructure with fine, evenly distributed pores and no significant defects. This suggests that wood wool stabilises the foam, bridges cracks, and reduces shrinkage stresses, which is consistent with the literature and with findings reported for natural fibres [46]. At the same time, it should be noted that this sample exhibited a clearly higher density compared to the reference sample and the other compositions.

In contrast, the F.A. + CH sample showed the lowest strength (0.8 MPa). Although chalcedonite appeared to stabilise the mineral matrix, it disrupted the foaming process, leading to heterogeneous compaction and poor mechanical performance. The addition of wood wool to this system (F.A. + CH + WW) partly compensated for these adverse effects, improving the structure’s homogeneity and increasing strength to 1.26 MPa.

The addition of diatomite alone (F.A. + D) moderately reduced strength compared to the reference, but improved porosity compared to F.A., though the pore distribution remained less homogeneous than with wood wool. Interestingly, combining diatomite with wood wool (F.A. + D + WW) led to a notable decrease in strength, suggesting unfavourable interactions between these two additives.

The most promising results were obtained for the F.A. + CH + D + WW sample, which combined all three additives. This system showed a highly homogeneous microstructure with fine, evenly distributed pores and no significant defects. The synergistic effect of the additives—with wood filler stabilising the foam, diatomite enhancing porosity, and chalcedonite providing mineral mass stability—resulted in an optimised structure that translated into improved mechanical and insulating properties.

These observations indicate that the effects of individual additives are not simply additive and depend on their mutual interactions and proportions. In particular, wood wool shows potential as a beneficial component, but its performance strongly depends on the type of accompanying mineral fillers. Further research is needed to elucidate the microstructural mechanisms underlying these interactions and to optimise the composition for improved mechanical performance.

Although numerically the differences in thermal conductivity between the material variants are not large, the microstructural and physicochemical analyses carried out showed clear qualitative differences in terms of structural stability and ability to conduct and store heat. The differences in thermal conductivity values between the samples ranged from 0.10154 to 0.13448 W/m·K. Measurable trends were observed, indicating the effect of the various additives on the porosity, foam homogeneity, and curing stability of the material. Despite the small range of differences, the obtained results are repeatable and may be of significant importance in the application context, especially in the case of passive insulation. The observed correlation between density and thermal conductivity is consistent with the literature, which confirms that in porous geopolymer materials, thermal conductivity increases with increasing density due to a higher proportion of the solid phase and reduced air content [15,16,17,18,19,20,21,22,23,24].

The use of diatomite, a natural material characterised by high microporosity and pozzolanic activity, slightly reduced the thermal conductivity while improving pore homogeneity. Diatomite introduced additional micropores and enhanced bonding reactions within the geopolymer matrix, stabilising the foaming process. This effect was particularly pronounced when diatomite was combined with wood particles, yielding significantly lower λ values and confirming the synergistic nature of this combination. Chalcedonite, used alone, resulted in the highest recorded λ (0.12043 W/m·K), yet acted as a mineral foam stabiliser. Its presence reduced structural deformation and increased local material density, enhancing durability and heat capacity. Notably, chalcedonite exhibited the highest thermal energy storage capacity (up to 1.968 kJ/kg·K), a valuable property for passive thermal applications.

Wood particles, despite not significantly lowering the thermal conductivity when used alone (λ = 0.13448 W/m·K), played a crucial role as a foam stabiliser. Their beneficial effects were most apparent when combined with mineral additives. Wood wool reduced the risk of foam collapse, improved cohesion, limited cracking during drying and curing, and promoted a more uniform pore distribution. The most favourable technical parameters were observed in the composite containing all three additives—diatomite, chalcedonite, and wood particles, which achieved the lowest thermal conductivity (0.10154 W/m·K), low density (415 kg/m³), and high heat capacity (1.529 kJ/kg·K). This indicates that an appropriate combination of natural functional additives effectively controls both the porous microstructure and thermophysical properties of the material.

Mineral additives influence the structure of the forming N-A-S-H gel in geopolymers through pozzolanic reactions and potential Na⁺ ion exchange [23,69,70]. The amorphous silica present in diatomite promotes pozzolanic reactions, strengthening the geopolymer matrix. Chalcedonite, a cryptocrystalline form of SiO₂, can act as a selective adsorbent for alkaline ions (Na⁺), thus locally affecting pH and geopolymerization [71]. Lignocellulosic wood particles, with their capillary structure, facilitate moisture redistribution, limiting zones of reduced liquid phase availability and enabling more uniform curing. Additionally, the mechanical reinforcement provided by wood wool reduces foam collapse and preserves the porous structure during setting, enhancing thermal insulation. These multifaceted interactions confirm synergistic effects from the combined use of organic and mineral additives.

However, it should also be noted that while mineral additives may improve microstructural stability and reduce phase segregation, they may also counteract the foaming process by densifying the matrix. As demonstrated in the work of Wang et al. [72], hard mineral additives such as ground quartz can significantly influence the microstructure of geopolymers by reducing their porosity and increasing the material density. A similar effect can be observed when using chalcedonite—as a high-hardness silica-based additive—which may act antagonistically to the foaming process by densifying the matrix and limiting pore formation. Therefore, careful selection of proportions and combinations of organic and mineral additives is crucial.

Additionally, in the context of these findings, and consistent with the literature [73,74], it can be concluded that a proper combination of mineral additives and fibrous stabilisers significantly improves the resistance of geopolymer materials to temperature fluctuations, as well as to aggressive chemical environments.

Comparing the tested geopolymer composites (thermal conductivity: 0.1015–0.1345 W/m·K) with lightweight insulating geopolymers reported in the literature [75] shows that although the lowest reported λ values (~0.079 W/m·K) are lower than in the present study, the tested materials fall within the typical range for lightweight aerated concretes. The advantage of these composites lies in their low weight, volumetric stability, and fire resistance, as well as the possibility of CO₂ sequestration and manufacturing from industrial waste, which makes them a competitive and sustainable alternative for insulation applications. Further comparison with commercial insulation materials—expanded polystyrene (EPS), mineral wool, polyurethane foam (PUR), and aerated concrete—highlights both advantages and limitations. The tested geopolymers’ thermal conductivity (0.101–0.134 W/m·K) is higher than that of PUR (0.022–0.030 W/m·K), EPS (0.030–0.040 W/m·K), or mineral wool (0.035–0.045 W/m·K), yet comparable to aerated concrete (0.100–0.160 W/m·K). Assuming a 10 cm layer, the thermal resistance of the tested materials (0.137–0.255 m²·K/W) is also inferior to traditional insulators but similar to aerated concrete (approx. 0.6–1.0 m²·K/W), suggesting their suitability in structural layers or as insulation support. The density of the tested materials (369–636 kg/m³) exceeds that of typical insulation materials (EPS: 15–30 kg/m³, PUR: 30–50 kg/m³), but aligns with aerated concrete (400–600 kg/m³). This higher density translates to greater thermal mass and heat accumulation capacity, which is beneficial in some applications. Notably, specific heat values (1.405–1.968 kJ/kg·K) were higher than for mineral wool (0.8–1.0 kJ/kg·K) or aerated concrete (~1.1 kJ/kg·K), and comparable or better than EPS and PUR (~1.4–1.6 kJ/kg·K). The sample with F.A. + CH + WW (1.968 kJ/kg·K) exhibited high heat storage capacity, suggesting good thermal inertia and the potential to stabilise indoor temperatures. Thus, while the tested geopolymers do not match conventional insulation materials in terms of thermal conductivity and resistance, their favourable specific heat and durability make them promising as sustainable construction and thermal storage materials, especially in passive building contexts [76,77].

Another interesting comparison can be made with Koh et al. [47], who developed lightweight geopolymer composites composed primarily of wood wool bonded within the geopolymer matrix. Their materials exhibited somewhat lower thermal conductivity (0.070–0.085 W/m·K) and lower density (350–450 kg/m³) than those tested here, due to the large organic content. In contrast, the current study uses wood wool only as a minor additive within a foamed geopolymer matrix, yet this small fraction significantly influences properties. While Koh et al.’s composites may offer superior flexural strength, the foamed geopolymer materials presented here provide potentially greater durability and environmental stability, including improved resistance to insects and fire. Both materials share similar application potential, with the current foamed geopolymers representing a promising sustainable alternative to commonly known wood wool cement boards (WWCBs) with OPI binder.

Despite these promising results, scaling up the described solutions to industrial production entails several challenges. The inclusion of mineral additives requires precise control of their chemical variability. Additionally, foamed geopolymers are inherently difficult to control during the foaming process, requiring careful process monitoring to achieve repeatable results. Pilot-scale tests and confirmation of feasibility under industrial conditions must therefore precede any implementation.

Moreover, potential degradation factors—such as water absorption, freeze–thaw cycles, and structural erosion from long-term moisture exposure or atmospheric effects (e.g., carbonation and efflorescence)—must be taken into consideration. Further durability and ageing tests, including frost resistance, capillary absorption, salt resistance, and UV stability, are essential to fully confirm the suitability of these materials as outdoor building insulators.

In order to support the sustainability claims presented in this work with quantitative data, an approximate estimation of the embodied CO₂ emissions associated with the production of raw materials was performed.

Table 5. Estimated CO₂ emissions related to material inputs (per 1 kg of dry mixture).

Component	Content in Mix [% wt.]	Emission Factor [kg CO2/kg]	Estimated CO2 Contribution [kg CO2/kg]
Fly ash (waste)	~60%	~0.02 (transport only)	0.012
Diatomite (natural)	~7%	0.10–0.15	0.009–0.011
Chalcedonite	~7%	~0.10	0.007
Wood wool	~1%	0.05–0.10	0.0005–0.001
High alumina cement	~8%	0.85–0.95	0.068–0.076
Microspheres	~12%	0.20–0.30 (incl. processing)	0.024–0.036
Sand	~6%	0.04–0.06	0.0025–0.0035
Surfactant, others	~<1%	1.00 (small mass)	<0.005
Total (estimate)	100%	–	~0.12–0.15 kg CO2/kg dry mix

presents the assumed emission factors and calculated values for the geopolymer formulation containing fly ash, diatomite, chalcedonite, wood wool, cement, and other additives. Based on typical unit data contained in the literature [25,52] referring to emissions related to the production of raw materials used, such as fly ash, clay cement, quartz sand, microspheres, and organic additives. In particular, approximate CO₂ emission factors were adopted at the following levels:

−

0.90 kg CO₂/kg for Portland cement (for comparison);

−

0.85–0.95 kg CO₂/kg for high alumina cement;

−

0.10–0.15 kg CO₂/kg for natural mineral additives (diatomite, chalcedonite);

−

~0.02 kg CO₂/kg for fly ash (as waste);

−

~0.20–0.30 kg CO₂/kg for microspheres (due to processing);

−

~0.05–0.10 kg CO₂/kg for organic additives (e.g., wood wool).

On this basis, it was estimated that the developed compositions emit approx. 0.12–0.15 kg CO₂ per 1 kg of dry material, which means a reduction in emissions of 80–85% compared to a typical cement-based system (~0.9 kg CO₂/kg). This result is consistent with literature data, which indicate a possible reduction in CO₂ emissions in the range of 30–80% depending on the composition and production conditions [13,23].

For comparison, traditional ordinary Portland cement (OPC) binder emits ~0.85–0.95 kg CO₂/kg, meaning that even partial replacement of cement with waste-based geopolymer matrices results in significant reduction. According to this estimation, the geopolymer formulations presented in this study generate up to 80–85% less embodied CO₂ than typical OPC-based binders per kg of dry material.

It is worth noting that this estimation does not include the operational energy savings due to improved thermal mass and insulation performance of the foamed composites. The high specific heat (up to 1.97 kJ/kg·K) and relatively low thermal conductivity (as low as 0.1015 W/m·K) suggest that such materials can also reduce building heating and cooling demands in passive systems. Future work will address full life cycle analysis (LCA), including CO₂ sequestration potential and durability under real conditions.

An approximate estimate of unit emissions for the most effective mixture (F.A. + CH + D + WW) was also carried out, assuming an average density of 415 kg/m³ and emissions of approx. 0.12–0.15 kg CO₂/kg dry mass. The total emissions amounted to ~50–62 kg CO₂/m³ of material. For comparison, Portland cement-based concrete (CEM I), with a typical density of ~2400 kg/m³ and emissions of 0.90 kg CO₂/kg, generates approx. 2160 kg CO₂/m³ [52], and PUR foam insulation boards generate approx. 80–150 kg CO₂/m³ (depending on the system and additives) [25]. WWCBs (wood wool cement boards) show emissions in the range of 100–250 kg CO₂/m³ in LCA analyses [47]. Therefore, per 1 m³ of material, our geopolymer binders allow for a reduction in emissions of ~70–75% compared to PUR, ~50–80% compared to WWCBs, and as much as ~97% compared to pure Portland cement. These results confirm the environmental potential of the materials, especially since they are produced using fly ash, local mineral additives, and clinker-free binders.

6. Conclusions

The goal of this research was to develop a lightweight, low-temperature geopolymer material with high thermal insulation properties, utilising low-cost, readily available natural and waste additives. This study focused on the effects of diatomite, chalcedonite, and wood wool additives on pore structure, thermal properties, bulk density, and compressive strength. The incorporation of porous mineral and organic additives influenced the pore structure and bulk density in a complex manner. These interactions between the additives affected thermal insulation properties variably, the material’s structural stability and durability, reflecting synergistic or antagonistic effects depending on the specific combination of additives, which in turn impacted the material’s structural stability and durability.

Impact of wood wool: The addition of wood wool increased density (up to 636 kg/m³ in the sample with wood wool alone), indicating a partial densification of the foam matrix. Although wood wool alone did not reduce thermal conductivity significantly (λ = 0.13448 W/m·K), its combination with other additives resulted in the lowest recorded value (0.10154 W/m·K). The filler acted effectively as foam stabilisers, promoting uniform pore distribution and limiting pore collapse, consistent with prior findings.

Impact of diatomite: Diatomite slightly increased density but contributed to a measurable decrease in thermal conductivity relative to ash-only samples, highlighting its porous microstructure and pozzolanic activity that aided foam stability. When combined with wood wool, diatomite further enhanced thermal insulation, demonstrating a clear synergistic effect.

The impact of chalcedonite: Chalcedonite alone increased thermal conductivity to the highest value observed (0.12043 W/m·K), likely due to a reduced capacity to form micropores. However, in combination with wood wool and diatomite, chalcedonite contributed positively by stabilising the foam structure without significantly increasing density. Moreover, chalcedonite notably improved heat capacity (up to 1.968 kJ/kg·K), an important attribute for passive thermal energy storage applications.

Combination of chalcedonite, diatomite, and wood wool: The ternary combination of chalcedonite, diatomite, and wood wool yielded the most promising results, achieving the lowest thermal conductivity (0.10154 W/m·K), moderate density (415 kg/m³), and high heat capacity (1.529 kJ/kg·K). This optimised composition benefits from enhanced structural stability, improved microporosity, and reduced cracking, rendering it a strong candidate for sustainable thermal insulation and passive heat storage in construction.

The resulting composites demonstrate potential as sustainable alternatives to conventional insulation materials such as lightweight aerated concrete and wood wool cement boards, offering promising thermal performance combined with enhanced fire resistance and durability. Future work should focus on process scale-up, long-term durability under environmental exposure, and further optimisation of additive proportions to maximise performance and applicability. Ongoing research is also investigating the ability of the developed materials to capture carbon dioxide, as part of a current research project.