Thermal, Mechanical, and Microstructural Properties of Novel Light Expanded Clay Aggregate (LECA)-Based Geopolymer Concretes

Abstract

The construction sector’s reliance on traditional cement significantly contributes to CO₂ emissions, underscoring the urgent need for sustainable alternatives. This study investigates fine (0–4 mm), rounded, uncoated, porous-surfaced lightweight expanded clay aggregate (LECA)-based geopolymers, which combine the low-carbon benefits of geopolymers with LECA’s lightweight and insulating properties. Geopolymers were synthesized using lignite-rich fly ash with varying ratios of LECA to aggregate. Mechanical testing revealed that the reference mixture without LECA (REF-GEO) achieved the highest compressive strength of 37.89 ± 0.75 MPa and flexural strength of 7.62 ± 0.11 MPa, while complete substitution of the aggregate with LECA (LECA-100%) reduced the compressive strength to 17.31 ± 0.88 MPa and flexural strength to 3.41 ± 0.11 MPa. The density of the samples decreased from 2.06 g/cm³ for REF-GEO to 1.31 g/cm³ for LECA-100%, and thermal conductivity dropped significantly from 1.15 ± 0.07 W/mK to 0.38 ± 0.01 W/mK. Microstructural analysis using XRD and SEM-EDX highlighted changes in the material’s internal structure and the increase in porosity with higher LECA content. Water vapor permeability increases over time, particularly in samples with higher LECA content. These findings suggest that LECA-based geopolymers are suitable for low-load or non-structural elements. They are ideal for sustainable, energy-efficient construction that requires lightweight, insulating, and breathable materials.

1. Introduction

Recently, 151 nations, including members of the European Union, pledged to achieve net-zero CO₂ emissions by 2050 and the scale and significance of the construction and cement industry (CCI) in global infrastructure make it a key target for decarbonization [1]. However, reducing emissions in this sector presents unique challenges: the industry not only contributes substantially to global emissions, which are rapidly increasing, but the majority of these emissions come directly from the cement production process rather than from energy consumption [2].

Concrete remains one of the most widely used construction materials due to its strength, cost-effectiveness, and numerous benefits compared to alternatives. Unfortunately, its manufacturing process imposes significant environmental costs and high energy demands [3]. This underscores the urgent need to explore alternative solutions that align with sustainable building practices while being effective and environmentally responsible [4].

A promising option is the use of geopolymers, which are eco-friendly inorganic polymers made from aluminosilicate materials and characterized by an amorphous or semi-crystalline three-dimensional structure [5]. They are synthesized through a process called geopolymerization, involving the reaction of an alkaline activator with silicon (Si) and aluminum (Al) derived from raw materials under ambient conditions. This reaction entails dissolving Si and Al atoms from the source material, reorganizing their structure, and condensing them into a new solid matrix [6].

Common raw materials for geopolymers include clays and clay minerals such as kaolin and meta-kaolinite, industrial byproducts like fly ash and slag, and recycled materials. Among these, fly ash—a fine, glassy residue from coal combustion—is one of the most frequently used [7]. Fly ash is classified by its coal source, with lignite-based fly ash characterized by higher calcium content and self-cementing abilities, while anthracite-based fly ash is lower in calcium and primarily exhibits pozzolanic behavior. Recycling low-quality lignite-based fly ash into geopolymers is a promising solution, as geopolymers are sustainable, low-carbon construction materials. Compared to Portland cement, geopolymer binders significantly reduce environmental impact, emitting up to five times less CO₂ and requiring only a third of the energy. On a per-volume basis, geopolymer concrete lowers CO₂ emissions by roughly 65% and energy consumption by 52% when compared to Ordinary Portland Cement (OPC) concrete of similar strength. Recent advancements, such as using smaller amounts of binder and replacing sodium hydroxide with sodium carbonate as an activator, have further decreased emissions—by as much as 22% compared to earlier studies [8].

Lightweight aggregates (LWAs), defined by international standards like EN 13055 and ASTM C330M, are essential for creating lightweight concrete (LWC). This has already been proven multiple times by studies that investigated the properties of LECA and its role as an additive to concrete for enhancing mostly insulating [9], acoustic [10], and self-healing properties [11] or in combination with other alternative materials like Rubcrete [12] and reinforcing fiber [13]. These lightweight aggregates can be of natural origin (e.g., perlite [14]), byproducts of industrial processes (e.g., fly ash and slag), or derived from recycled materials (e.g., ceramic tiles [15]). Infra-lightweight concretes (ILCs), characterized by dry densities under 800 kg/m³ and superior thermal insulation, represent a cutting-edge development in sustainable construction [16].

Lightweight expanded clay aggregate (LECA) is a lightweight material made from clay that expands when subjected to high temperatures ranging from 1050 °C to 1250 °C [17]. It is one of the most used lightweight aggregates in concrete due to its enhanced alkaline corrosion and frost resistance. Although research has shown that the incorporation of LECA into concrete mixes can lead to reduced mechanical properties in comparison with other LWC mixes [18], adding fiber has been found to compensate for the strength reduction with enhanced strength compared to normal concrete [19]. Despite this, the concrete composite enhanced with LECA demonstrates a higher strength-to-weight ratio than conventional concrete composites [20]. Lightweight concrete’s performance depends on various factors, including the choice of lightweight aggregate, the water–binder ratio, the properties of the binder, and, critically, the porosity of the aggregate. This porosity plays a pivotal role by enhancing the material’s ability to absorb energy effectively [21].

Lightweight geopolymer concrete is a low-density alternative to conventional LWC, offering easier handling and transportation during construction. Its production supports environmental sustainability by incorporating various byproducts and aggregate wastes [22]. However, the limited application of LWGPC may be attributed to insufficient understanding of the factors influencing its production [23].

While previous research has predominantly focused on the mechanical and durability properties of lightweight expanded clay aggregate (LECA) in conventional concrete, there has been limited exploration of its integration with fly ash-based geopolymers, especially concerning thermal properties and microstructural characteristics. This study aims to address this gap by investigating the synergy of fly ash geopolymers with varying LECA-to-aggregate ratios. It examines not only mechanical properties such as compressive and flexural strength but also critical thermal properties, including thermal conductivity, density, and water vapor permeability. Unlike prior research, this study incorporates detailed analyses of thermal conductivity and water vapor permeability alongside advanced microstructural characterization, offering novel insights into the performance of LECA-geopolymers for sustainable construction applications. Furthermore, comprehensive microstructural analyses using XRD and SEM-EDX provide valuable insights into the internal structure of the material, bridging existing knowledge gaps and advancing the potential applications of lightweight geopolymer concrete.

2. Materials and Sample Preparation

The formulation of LECA-based geopolymers involved a variety of materials like fly ash, sodium silicate, sodium hydroxide, LECA, and aggregate.

Lignite-based fly ash, serving as the primary binder, with a mean particle size distribution (d50) of 20.25 μm, was sourced from Šoštanje Power Plant, Trbovlje, Slovenia. It was formed as a byproduct during the combustion of lignite. Mineral impurities in the coal undergo thermal transformation, producing fine ash particles that are captured from flue gases using electrostatic precipitators or filters. The collected fly ash is then dried, sieved, and prepared for use.

Its particle size distribution parameters, shown in Figure 1, were determined using a Fritsch Analysette 22–28 laser granulometer, FRITSCH GmbH, Idar-Oberstein, Germany, with dynamic image analysis. A total of 2 g of material was placed in a small 50 mL Erlenmeyer flask and then first mixed with 1 drop of a wetting agent (Dusazin 901) and stirred with a glass rod. Water was added dropwise with constant stirring to obtain a homogeneous suspension of about 25 mL. The chemical composition and other parameters of the fly ash were determined using X-ray fluorescence (XRF) and are given in

Table 1. Chemical composition of fly ash determined by XRF analysis.

Parameter	Fly Ash % [m/m]
SiO2	38.46
Al2O3	22.61
Fe2O3	4.82
CaO	23.80
MgO	5.81
SO3	2.99
LOI (loss on ignition)	1.51

, in which there is a significantly high percentage of CaO.

According to Chen et al. [24], CaO influences the geopolymerization process by enhancing the dissolution of the glassy phase in fly ash, promoting interactions with the alkali activator, and increasing the degree of polymerization of geopolymer gels. At optimal levels (11%), it improves the microstructure by forming calcium silicate hydrate (C-S-H) gel and ettringite, which enhance flexural and compressive strength while reducing drying shrinkage by compensating for shrinkage with slight expansion. However, excessive CaO reduces strength and increases water immersion expansion due to the overproduction of ettringite, highlighting the importance of balance in its content [24].

Sodium hydroxide (NaOH, 98% purity) at a concentration of 10 molar and sodium silicate (Na₂SiO₃) from Samson Kamnik d.o.o, Kamnik, Slovenia, were utilized as an alkali activator and a binder to initiate the geopolymerization process. Specifications for Na₂SiO₃ are listed in

Table 2. Na₂SiO₃ specifications.

Characteristics	Unit	Value
SiO2	(%)	25.4–29.2
Na2O	(%)	8.2–8.6
Dry substances	(%)	33.6–37.8
Density (20 °C)	(g/cm3)	1.353–1.378
Viscosity (20 °C)	(mPas)	30–100
pH (20 °C, 5% solution)		11–12
Molar ratio		3.2–3.5
Typical properties		Viscous liquid, almost colorless, free of visible impurities

, and for NaOH, they are listed in

Table 3. NaOH specifications.

Characteristics	Unit	Value
Chemical formula		NaOH
CAS No.		1310-73-2
Molecular mass	(g/mol)	39.99
NaOH content		98%
Density	(g/cm3)	2.13
Melting point	(°C)	318
Solubility in water (20 °C)	g/100 g water	111

.

The LECA grains were sourced from Laterlite Plus SAM d.o.o, Ljubljana, Slovenia (0–4 mm, 440 kg/m³), and standard sand aggregate (0.08–2 mm) was purchased from Normensand GmbH, Germany. The LECA and aggregate properties are presented in

Table 4. The LECA and aggregate properties.

Property	LECA	Standard Sand
Grain Size Distribution	0–4 mm	0.08–2.00 mm
Bulk Density (kg/m3)	760	Not specified (high, compact sand)
Crushing Resistance (N/mm2)	≥12.0	Not specified
Thermal Conductivity (W/mK)	0.125	Not applicable
Moisture Content	Not specified	≤0.2%
Fire Resistance	Euro Class A1 (non-combustible)	Not applicable

.

Mix Design of LECA Geopolymers

To achieve the optimal compressive strength of the lignite-based fly ash geopolymer, the mix proportions and activator ratios were carefully adjusted, following the guidelines provided by Chindaprasirt et al. [25]. The activator ratio (Na₂SiO₃/NaOH) was established at 1.50, while the aggregate-to-fly ash ratio was set at 2.00. The sand-to-geopolymer paste ratio, consisting of Na₂SiO₃, 10 M NaOH, and fly ash, was kept at 1:1. The exact proportion of aggregate by mass was approximately 54.6% of the total composite mass.

Five sample sets were prepared, each with varying proportions of LECA and aggregate. LECA, consisting of particles up to 4 mm, replaced the equivalent particle size fraction of sand in the mixture. The reference sample (REF-GEO) contained 100% aggregate and 0% LECA by mass. Each subsequent sample included a 25% mass reduction in aggregate and a 25% mass increase in LECA. The final sample contained 0% aggregate and 100% LECA by mass. First, NaOH and fly ash were mixed for 2 min. Sodium silicate was then added, and the mixture was stirred for an additional minute. Finally, the aggregate and LECA were incorporated, and the mixture was stirred until homogeneous. A minimum flow of 110 ± 5% was held to ensure the geopolymer’s workability and ease of placement into the molds. In the case of the LECA addition, this was achieved by incorporating a superplasticizer MELAPRET AAS 100 (produced by Melamin Chemical Factory d.d. Kočevje., Slovenia); a sulphonated melamine-based product was used for the geopolymers due to its compatibility with alkaline environments and ability to improve workability by effectively dispersing aluminosilicate particles.

The mixture was poured into a 40 × 40 × 160 mm mold and vibrated to remove air bubbles. The molded geopolymers were wrapped in plastic bags and cured in an oven at 45 °C for 48 h. For each set of geopolymers, at least 9 samples were made, which were then carefully examined. Figure 2 shows fractured GEO-REF and LECA 100% samples after 2 days of curing at 45 °C.

3. Characterization Methods

The prepared geopolymers were tested for compressive and tensile strength, density, thermal conductivity, and microstructural properties using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD).

3.1. Compression and Flexural Tests

After the curing process, the sample prisms underwent testing for both compressive and flexural strength. Compressive strength tests were performed using the Zwick Roell Z600, ZwickRoell GmbH & Co. KG, Ulm, Germany apparatus. The compressive strength (σ_c) was determined using the following equation:

$${\sigma }_{\mathrm{c}}={\displaystyle \frac{{\mathrm{F}}_{\mathrm{c}}}{{\mathrm{A}}_0}}$$

(1)

where F_c (N) represents the force applied perpendicular to the largest surface area of the specimen, and A₀ (mm²) is the surface area of the specimen prior to loading.

For the three-point flexural strength test (σ_f), the following equation was used:

$${\sigma }_{\mathrm{f}}={\displaystyle \frac{{\mathrm{F}}_{\mathrm{f}}·1.5·\mathrm{l}}{{\mathrm{b}}^3}}$$

(2)

where F_f (N) is the applied force, l is the span between the support points, and b is the width of the prism (mm). The compressive and flexural strength of geopolymers was evaluated over 28 days, with measurements taken on the 2nd, 7th, and 28th days.

3.2. Density

Density measurements were taken to assess how the incorporation of LECA affected the weight of the materials and their corresponding strength. Density was calculated, following Equation (3):

$$\rho ={\displaystyle \frac{\mathrm{m}}{\mathrm{V}}}$$

(3)

where m is the mass of the sample and V represents the volume of the molds (40 × 40 × 160 mm). The measurements were taken on the 2nd, 7th, and 28th day.

3.3. Thermal Conductivity

Thermal conductivity was measured using the Hot Disk^® TPS 1000, Gothenburg, Sweden, to determine the insulating capabilities of the material. It uses a spiral sensor made of a fine wire that serves as both a heat source and a temperature sensor. This sensor is placed in contact with the sample material. The sensor is typically a flat, circular foil. When the instrument is turned on, a small electrical current passes through the spiral sensor. This generates a controlled amount of heat. As the sensor heats up, it transfers this heat to the surrounding material (the geopolymer). As the material absorbs the heat, the temperature of the sensor and the surrounding material starts to increase. The Hot Disk^® system continuously monitors temperature changes at the sensor location. Since the sensor is in direct contact with the sample, the rate of temperature rise depends on the thermal properties of the material, particularly its thermal conductivity (λ) [26]. The thermal conductivity is calculated using the following equation:

$$\lambda ={\displaystyle \frac{\mathrm{Q}}{4\pi \mathrm{r}\Delta \mathrm{T}}}$$

(4)

where λ is the thermal conductivity (W/m·K), Q is the heat input from the sensor (W), r is the radius of the sensor (m), and ΔT is the temperature increase over time (K).

Five measurements were conducted for each sample, and the results were presented as their average.

3.4. X-Ray Diffraction

X-ray powder diffraction is a quick, non-destructive method that requires little preparation for both qualitative and quantitative examination of pure and multi-component materials. Diffraction patterns are created when an X-ray beam hits a crystalline sample, exposing the material’s chemical and structural characteristics [27].

A silicone monocrystalline container was coated with the sample powders. An EMPYREAN X-ray diffractometer was used to make the XRD measurement with the following parameters: 5° < 2θ < 80°, step size 0.026°, and time 300 s/step. The EVA program (Bruker version 7.0),) was used to do a phase analysis of the XRD data, and the TOPAS software (Bruker) was used to calculate the phase quantities while accounting for the structural aspects of the phases that were discovered.

3.5. Scanning Electron Microscopy Coupled with Energy-Dispersive X-Ray Spectroscopy (SEM-EDX)

By analyzing X-ray emissions specific to each element in the sample, SEM-EDX, which combines scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), offers information on the sample’s chemical composition. Phases in concrete and blended-cement pastes can be distinguished with this technique [28].

The Quanta 200 3D Scanning Electron Microscopy – Secondary Electron Imaging (Sem-SEI) device (FEI Company, Hillsboro, OR, USA) equipped with FEI Sirion 400 NC energy-dispersive X-ray spectroscopy (EDX), (FEI Company, Hillsboro, OR, USA). at various magnifications was used to characterize the sample surface’s elements and morphology.

3.6. Water Vapor Permeability

Water vapor permeability tests were performed to evaluate the material’s ability to manage moisture. The ASTM E96 desiccant method was employed to assess the water vapor permeability, a standard method for testing a material’s vapor transmission efficiency [29]. The principle of the method is shown in Figure 3.

Each sample was placed in a container filled with a drying agent (silica gel, SiO₂) that effectively absorbs moisture from the surroundings, maintaining an internal relative humidity close to zero.

The sample was securely positioned on top of the container, with the edges sealed using silicone to prevent moisture ingress through the joints, ensuring that vapor transmission occurred solely through the sample. All five samples were tested in triplicate to ensure reliability. The containers were weighed at specific time intervals to monitor the mass change. The increase in mass within the container resulted from water vapor passing through the sample and being absorbed by the silica gel. This change in mass can be used to calculate the rate of water vapor transmission:

$$\mathrm{P}={\displaystyle \frac{\Delta \mathrm{m}\cdot \mathrm{d}}{\mathrm{A}\cdot \mathrm{t}\cdot \Delta \mathrm{p}}}$$

(5)

where P is the water vapor permeability (g·m/m²·h·Pa), Δm is the change in mass due to water vapor transmission (g), d is the thickness of the material (m), A is the exposed area of the sample (m²), t is the time over which the mass change is measured (h), and Δp is the partial pressure difference across the material (Pa).

4. Results and Discussion

4.1. Compressive and Flexural Strengths

The compressive and flexural strengths of the LECA geopolymers were evaluated over 28 days, with measurements taken on the 2nd, 7th, and 28th days. Fly ash-based geopolymers exhibit exceptional performance in compression strength tests, usually better results than traditional concrete [30]. The compressive strength of fly ash-based geopolymer concrete is influenced by factors such as the sand-to-aggregate ratio, alkali solutions, calcium content, additives, and curing conditions [31]. Chindaprasirt et al. report the compressive strength of fly ash-based geopolymer to be up to almost 35 MPa [19]. The results, presented in Figure 4, indicate that the strength of the material decreased as the proportion of LECA increased.

While the reference mixture (100% fly ash, REF-GEO) demonstrated the highest compressive (37.89 ± 0.75 MPa) and flexural strength (7.62 ± 0.11 MPa), the samples with higher LECA content showed reduced mechanical performance. Substitution of LECA with aggregate at rates up to 50% still results in high compressive strength values of 30.59 ± 0.84 MPa and flexural strength values above 5.57 ± 0.25 MPa. However, with a 75% substitution of LECA for the aggregate, the values decrease to 21.28 ± 0.78 MPa for compressive strength and 4.38 ± 0.08 MPa for flexural strength. The weakest was the LECA 100% sample with a maximal compressive strength of 17.31 ± 0.88 MPa and a maximal flexural strength of 3.41 ± 0.11 MPa. This reduction in strength is attributed to the lightweight nature of LECA, which introduces more porosity into the material. However, despite the decrease in strength, the LECA-containing geopolymers still displayed adequate performance in comparison to traditional concrete with LECA. Murugan et al. report the compressive strength of traditional concrete with 20% LECA to be 13.89 MPa and the flexural strength to be 4.6 MPa [9].

4.2. Density and Thermal Conductivity

The inclusion of LECA in the geopolymer mixtures resulted in significant changes to both the density and thermal conductivity. As shown in Figure 5, as the LECA content increased, the density of the geopolymer decreased, making the material much lighter. The GEO-REF sample had the highest density at 2.06 g/cm³. The lowest density was recorded for the LECA 100% sample, at 1.31 g/cm³. Conventional concrete has, reportedly, a density of 2.5 g/cm³, and traditional concrete with 20% LECA has a density of 1.9 g/cm³ [9]. This is particularly advantageous in construction applications where reducing the weight of structural components can lead to cost savings and improved handling like highly stressed facades of office buildings with many wide window and door openings [16]. Compared to the data in Figure 4 where the strength of the samples generally increases with time, the density decreases. The decrease in density can be attributed to water evaporation, shrinkage, and the reduction in voids during hydration, where water content and volume decrease over time, while the increase in strength happens as the hydration products continue to form and bind the particles together, improving the overall material structure and mechanical properties.

In recent years, the thermal insulation of buildings has gained considerable focus because of its potential for long-term energy savings [32]. Thermal conductivity tests revealed, parallel with density, that geopolymers with higher LECA content exhibited better insulation properties. The REF-GEO sample had 1.15 W/mK as the maximal thermal conductivity and the lowest thermal conductivity was exhibited by the LECA 100% at 0.38 W/mK. The thermal conductivity of traditional concrete typically ranges between 2 and 1.2 W/mK [33]. Araujo et al. managed to decrease conductivity by 73% when using LECA and fly ash to replace Portland cement [34].

4.3. SEM-EDX

The microstructural analysis of the LECA geopolymers provided further insight into how the material’s internal structure changes with the addition of LECA.

For better understanding, prior to measurements of the LECA geopolymer, XRD and SEM measurements were conducted for LECA grains. X-ray diffraction (XRD) was used to identify the crystalline phases present in the LECA material. Figure 6 and Figure 7 present XRD and SEM analyses of lightweight expanded clay aggregate (LECA, also known as glinopor), illustrating its chemical composition and surface morphology. The XRD spectrum shows the crystalline phases present, with peaks corresponding to specific compounds, where the pie chart quantifies the material’s composition as 45.6% calcium carbonate, 29.2% sodium calcium aluminum silicate (N-C-A-S), 17.8% magnesium silicate, 4.7% iron oxide, and 2.7% silicon dioxide. These components contribute to the material’s properties, including its porosity and thermal insulation. The SEM micrographs, taken at magnifications of 1000×, 5000×, and 10,000×, reveal the surface morphology, highlighting a highly porous structure with interconnected voids. At lower magnifications, the coarse texture and larger pores are visible, while higher magnifications emphasize the roughness and finer details of the material’s microstructure. The porosity, as seen in the SEM images, correlates with the chemical phases identified in the XRD analysis, particularly the dominance of calcium carbonate and silicates, which influence both the mechanical strength and thermal properties.

The set of SEM images in Figure 8 represents the inner microstructure of the geopolymer sample without the addition of LECA, showing a more compact and dense structure and minimal porosity at varying magnifications of 500×, 1000×, and 10,000×. At lower magnifications, the matrix surface is dominated by visible cracks and fractured regions, while higher magnifications reveal a granular texture with scattered particulates and rough surfaces, which reflects the dense and homogeneous nature of the material.

In contrast, the second set of SEM images, shown in Figure 9, corresponds to the inner geopolymer sample matrix containing LECA. At 500×, 1000×, and 5000× magnifications, these images reveal a significantly rougher and more porous structure with interconnected voids and uneven surfaces, indicating the influence of the lightweight aggregate. At lower magnifications, a clear transition between the dense and porous zones is visible, while at higher magnifications, the interconnected pores and smooth walls of the porous regions are evident. The pores appear filled, while the LECA material grains do not seem altered or to have reacted in any way.

Figure 10 presents a SEM-EDX analysis of the geopolymer with LECA, identifying the elemental composition. The bar chart shows high concentrations of carbon and oxygen, followed by silicon, aluminum, and smaller amounts of sodium, magnesium, calcium, and iron. The high carbon percentage may originate from organic impurities on the surface of the sample that were not completely removed during sample preparation (e.g., oils or residues). Before pouring the fresh mixture into the molds, the molds are coated with oil to facilitate demolding and to prevent damage to the shape of the samples.

The elemental composition analysis further confirmed the distribution of LECA within the matrix, contributing to its lightweight and insulating properties.

4.4. Water Vapor Permeability Measurements

The permeability tests revealed that as the LECA content increased, the material became more permeable to water vapor. This characteristic is particularly beneficial for construction materials used in environments where moisture regulation is important, such as in buildings requiring breathability to prevent condensation and mold growth and in coastal engineering [35,36]. The graph in Figure 11 illustrates that increasing LECA content significantly enhances water vapor permeability in geopolymer materials, with LECA-rich samples (LECA-25%, LECA-50%, and LECA-75%) showing much higher permeability compared to the REF-GEO sample. The maximal measured mass of transmitted water vapor was 22.15 g for the LECA-100% sample on the 34th day. Permeability increases over time, with substantial rises between 3, 20, and 34 days, particularly in samples with higher LECA content.

The REF-GEO sample remains minimally permeable throughout, highlighting the impact of LECA on moisture regulation. These findings confirm LECA’s role in improving breathability, making it a prospect for construction applications requiring both moisture control and thermal insulation, especially in humid or coastal environments [37].

Since vapor permeability is directly proportional to the mass of water vapor according to Equation (5), the values of vapor permeability could also be calculated from the given masses of water vapor.

5. Discussion and Comparison with the Current Literature

This chapter presents a discussion of the obtained results, comparing them with recent studies on LECA-based and other geopolymer systems. It focuses on thermal, mechanical, and microstructural properties to emphasize the contributions and novelties of this work

The compressive strength of the 50% LECA geopolymer concrete in our study (30.59 ± 0.84 MPa) is comparable to results reported by Priyanka et al. [38], where a compressive strength of 32.5 N/mm² was achieved with 50% LECA and similar mix proportions, emphasizing the viability of the chosen mix design. The flexural strength of 50% LECA (5.57 ± 0.25 MPa) in our study surpasses the values observed by Mahmoud et al. [10], who reported a flexural strength of 3.15 MPa for a similar mix, suggesting the influence of curing conditions and the concentration of NaOH in enhancing mechanical performance.

In contrast, the compressive strength of the 100% LECA mix in our study (17.31 ± 0.88 MPa) aligns with the range of 15.49 MPa reported by Khan et. al. [39]. However, it is lower than the 36.4 MPa reported by [38]. This discrepancy may arise from variations in LECA properties, mix proportions, and curing conditions, as well as the interfacial transition zone effects highlighted by Ouda et al. [40]. These findings underscore the inherent limitations of LECA due to its porous structure, reduced crushing strength, and weaker adhesion with the geopolymer matrix.

Thermal conductivity values in our study also align with the literature trends. The lowest thermal conductivity of 0.38 W/mK for 100% LECA in our results is significantly lower than the 0.621 W/mK reported by [40] for similar LECA proportions, indicating the effectiveness of LECA’s porous nature in improving thermal insulation properties.

The observed water vapor transmission mass (22.15 g at 34 days for 100% LECA) and the high water absorption capacity are consistent with [10,41] which attribute these characteristics to LECA’s high porosity. This property, while beneficial for thermal insulation, compromises mechanical strength, as discussed.

6. Conclusions

In conclusion, this study demonstrates the potential of lightweight expanded clay aggregate (LECA) geopolymers as sustainable construction materials. By incorporating LECA into geopolymer formulations, the resulting materials exhibit reduced density and improved thermal insulation, making them ideal for lightweight and energy-efficient applications. While the addition of LECA decreases mechanical strength due to increased porosity, it enhances key properties such as water vapor permeability and thermal performance. These attributes are particularly advantageous for construction in environments requiring breathability, moisture control, and thermal regulation, such as humid climates and coastal regions.

The results of the microstructural analysis further confirm the role of LECA in modifying the material’s internal structure and enhancing its insulating and lightweight properties. The trade-off between strength and functionality suggests that LECA geopolymers are most suitable for low-load or non-structural elements, particularly in thermally sensitive applications such as insulation panels and lightweight building components.

This research highlights the viability of LECA-based geopolymers as a sustainable alternative to traditional building materials, offering environmental benefits through reduced carbon emissions and improved performance in specific applications. Future studies could further optimize the balance between strength and functionality to expand their use in diverse construction contexts.