Experimental investigation of the impact of wood sawdust incorporation on the physical and thermal properties of fired clay bricks

Abstract

Wood sawdust, a byproduct of the wood industry, presents significant environmental and economic challenges due to its disposal. This study investigates the effects of incorporating wood sawdust into fired clay bricks on their physical and thermal properties. Five different clay and wood sawdust mixtures, labeled S1, S2, S3, S4, and S5, were prepared with sawdust proportions of 0%, 2%, 4%, 6%, and 8%, respectively. Both the clay and wood sawdust were subjected to comprehensive physical and chemical testing. Three samples from each mix were analyzed for bulk density, porosity, and thermal properties. The results show that the inclusion of wood sawdust significantly impacted the physical, and thermal properties of the bricks. As the sawdust proportion increased, the bulk density of the bricks decreased, while porosity increased. Moreover, the thermal properties of the bricks were notably enhanced with the addition of wood sawdust, indicating its potential to improve the thermal insulation performance of fired clay bricks. These findings suggest that incorporating wood sawdust into clay bricks could offer a sustainable solution to both enhancing their thermal efficiency and addressing waste disposal challenges.

Introduction

In recent years, the world has experienced a significant surge in the generation of solid waste, driven by rapid industrialization, urbanization, and an increasing consumption-driven society. This surge in waste production has created numerous environmental and economic challenges, leading to widespread pollution, elevated greenhouse gas emissions, escalating waste management costs, and the depletion of valuable resources^1–7. These issues have highlighted the need for effective solutions to manage and repurpose waste, as well as the importance of transitioning towards more sustainable materials and practices. One of the most pressing global challenges today is the development of innovative, high-performance materials that can play a key role in creating positive-energy buildings, which not only meet the energy needs of their occupants but also contribute positively to the environment by generating more energy than they consume.

This shift towards more sustainable building materials is particularly critical, as traditional construction materials, particularly cement, are responsible for approximately 36% of global energy consumption and around 40% of CO₂ emissions^8–14. As such, the building and construction sector is one of the largest contributors to global environmental degradation, and the need for energy-efficient, low-emission alternatives has never been more urgent. Therefore, there is a growing demand for sustainable construction alternatives—materials and practices that significantly reduce the environmental impact of buildings throughout their lifecycle. These materials will be crucial in advancing the roadmap toward zero-net energy buildings, which aim to balance the energy consumed with the energy produced, minimizing or eliminating the environmental footprint of building construction and operation.

In this context, solid waste materials have emerged as promising additives in the production of building materials, particularly in efforts to create eco-friendly bricks and insulating materials. The incorporation of waste materials into construction products offers several key advantages: (i) it provides a way to replace environmentally harmful components like cement, which has a significant environmental footprint due to both the energy-intensive production process and the high levels of CO₂ emissions it generates; (ii) it helps to develop cost-effective, locally sourced construction materials, thereby reducing dependence on non-renewable resources, transportation costs, and supply chain complexities; and (iii) it enhances passive design strategies that improve hydrothermal comfort in buildings, contributing to improved energy efficiency and occupant well-being^15–22.

Other researchers have explored the incorporation of biomass materials into traditional mortars. For instance, Maria Stefanidou et al.²³ investigated the use of lavender fibers—a byproduct of lavender oil extraction—in lime-based mortars. Their study aimed to improve both the sustainability and performance of construction materials through the integration of natural fibers. The findings demonstrated that incorporating lavender waste into mortar formulations not only enhances thermal insulation properties but also offers a sustainable alternative by reducing agricultural waste and encouraging the use of renewable resources. This work underscores the potential of natural fiber additives to contribute to the development of environmentally friendly and energy-efficient building materials.

To promote sustainable development in construction, numerous studies have explored the potential of incorporating various types of solid waste materials into brick compositions. These materials have been investigated not only for their ability to enhance sustainability and reduce environmental impact but also for their potential to modify both the physical and mechanical properties of building materials like clay bricks. Among the most studied waste materials are wood sawdust, waste glass, optical fibers, chicken feather fibers, fly ash, limestone dust, rubber, polystyrene, and a range of other industrial by-products^8,24–37. These additives are explored for their potential to improve key attributes of bricks, such as strength, durability, thermal insulation, and fire resistance—properties that are critical to ensuring the reliability and performance of building materials in real-world applications.

Although clay bricks are one of the most widely used and efficient earth-based building materials, there remain several areas where they can be improved, particularly in terms of mechanical strength and unit weight. One significant issue with traditional fired clay bricks is that they are often heavy, which increases transportation costs and can also limit their use in certain applications. At the same time, while clay bricks are known for their good thermal insulation properties, there is still potential for improvement in their ability to retain and release heat, especially when considering the energy demands of modern buildings. In light of these challenges, wood sawdust has emerged as a particularly promising waste material to incorporate into clay brick compositions in order to address these issues.

Wood sawdust offers a number of potential benefits when used in brick manufacturing, including lowering the weight of bricks by increasing their porosity, improving thermal insulation properties, and offering a sustainable way to repurpose a widely available waste product. As a lightweight additive, wood sawdust can reduce the bulk density of fired clay bricks, making them easier to handle and transport. This also helps lower the overall energy requirements for construction and reduces the carbon footprint associated with brick production.

Numerous experimental studies have been conducted to improve the performance of clay bricks, particularly in terms of energy efficiency, durability, and environmental sustainability^38–43. For example, Alabduljabbar et al.⁸ explored methods to reduce firing cycle times in the production of clay bricks, which would not only lower the energy consumption associated with brick firing but also reduce CO₂ emissions during the manufacturing process. Their findings showed that reducing the firing time could significantly enhance the mechanical properties of bricks, including their strength and durability, while improving the environmental performance of the bricks through lower energy use during production.

Other studies have focused on enhancing the fire resistance of construction materials. For instance, Shupu Wang et al.⁴⁴ investigated the potential of using a gypsum-wheat straw composite as a sheathing panel for improving the fire resistance of light wood frame walls. This composite material overcomes the limitations of conventional gypsum boards, which are often prone to cracking and compromising the structural integrity of walls under fire conditions. Similarly, Tianyi Wu et al.⁴⁵ developed a high-performance wheat straw-gypsum composite designed to address the issue of cracking and separation caused by water release when conventional gypsum boards are exposed to high temperatures.

Despite the significant advancements in the improvement of physical and mechanical properties of clay bricks, thermal insulation properties of modified bricks have received less attention. While the enhancement of strength and fire resistance are crucial, improving the thermal performance of building materials is essential to achieving energy efficiency goals in construction. Given the growing focus on energy-efficient buildings and the increasing demand for materials that can help regulate indoor temperatures while minimizing the carbon footprint of buildings, it is necessary to study how the incorporation of wood sawdust and other waste materials can enhance the thermal insulation properties of fired clay bricks.

Given these identified gaps, the primary objective of the present study is to investigate the effects of incorporating wood sawdust into clay bricks, with a particular focus on their thermal insulation properties. By using locally sourced materials, this research aims to promote sustainable, eco-friendly construction practices while addressing the increasing demand for building materials that not only improve energy efficiency but also reduce environmental impact. The study involves the comprehensive measurement of all relevant properties involved in the thermal transport mechanism of modified bricks, allowing for the identification of materials that can effectively contribute to better thermal performance in construction, while helping to reduce the overall environmental impact of the built environment.

Materials and methods

Experimental program

The experimental study is divided into three main parts:

1. Part 1: Characterization of Wood Sawdust and Clay. This part involves the physical and chemical characterization of the raw materials used in the study, namely wood sawdust and clay. The characterization tests include determining the moisture content, particle size distribution, chemical composition, and other relevant properties that influence the behavior of the raw materials during processing.

2. Part 2: Preparation of Fired Clay Bricks with Varying Proportions of Wood Sawdust. In this part, fired clay bricks are prepared by incorporating different proportions of wood sawdust into the clay mixture. The five different mixes include 0%, 2%, 4%, 6%, and 8% wood sawdust, and the bricks are then formed and subjected to the firing process at a predetermined temperature. The physical properties of these bricks are evaluated after firing.

3. Part 3: Evaluation of the Thermal Properties of Fired Clay Bricks. The final part focuses on testing the thermal properties of the fired clay bricks. This includes evaluating parameters such as thermal conductivity, thermal diffusivity, and specific heat capacity, in order to assess the potential of these bricks as thermal insulation materials.

Materials

Clay

The clay used in this study has a specific density of 2.650 g/cm³, indicating that it has a relatively dense structure compared to many other types of clay. This density is crucial for understanding the material’s overall behavior during the preparation and firing processes of the bricks. In addition, the Blaine specific surface area of the clay is measured at 28 m²/g, which is an important indicator of the clay’s fineness and surface area available for chemical reactions during the firing process. A higher surface area can lead to improved bonding and strength in the final fired product.

The chemical composition of the clay is summarized in Table 1, where it is evident that the predominant compound in the clay is silica (SiO₂), making up 55.08% of the total composition. Silica is a key component that contributes to the strength and durability of fired clay bricks. Additionally, a significant proportion of the clay is composed of alumina (Al₂O₃), which constitutes 14.14%. Alumina plays a critical role in the thermal properties of the material, particularly its resistance to high temperatures and its influence on the shrinkage behavior during firing. The presence of iron oxide (Fe₂O₃), which accounts for 6.07% of the clay, is noteworthy. Iron oxide is responsible for giving fired clay bricks their characteristic red color, a result of the oxidation process that occurs during firing at high temperatures.

Table 1.

Chemical composition of natural clay.

Component	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O	TiO2	LOI*
Percentage	55.08	14.14	6.07	6.86	2.91	1.18	2.68	0.62	11.01

*LOI represents the Loss On Ignition.

Further analysis reveals that the organic matter content of the clay averages around 1.2%, indicating that the clay is predominantly inorganic in nature. This low organic content is beneficial because organic matter can burn off during firing, potentially causing undesirable cracking or warping in the bricks. Additionally, the clay exhibits a high level of reactivity, as evidenced by its Methylene Blue Value (MBV) of 1.15. The MBV is a measure of the clay’s active nature, with higher values indicating a higher tendency for the clay to react with other materials, such as water, during the preparation and molding stages.

The calcium carbonate (CaCO₃) content in the clay is 6.5%, which classifies it as non-calcareous. This is significant because non-calcareous clays generally do not exhibit significant swelling behavior when mixed with water, which is important in preventing undesirable deformations or cracking when the clay is processed into brick pastes. The stability of the clay during processing is a key factor in ensuring high-quality final products.

In terms of physical properties, the clay exhibits a plasticity index of 27, a liquidity index of 1, and a consistency index of 2%. These values indicate that the clay is highly plastic, meaning it has a relatively high ability to retain its shape when molded. A plasticity index of 27 suggests that the clay is suitable for brick production because it can be easily worked and shaped without cracking. The liquidity index and consistency index indicate that the clay can form a smooth, workable paste without becoming too runny or too stiff during the mixing process.

The plastic limit of the clay is approximately 25%, which is the water content at which the clay transitions from a plastic to a semi-solid state. This value is important for estimating the amount of water required to form bricks. Proper control of water content during the mixing process is crucial for ensuring that the brick paste has the right consistency and can be molded efficiently without issues such as cracking or poor compaction.

The particle size distribution of the clay, as shown in Fig. 1, reveals that the material consists of 10% clay, 40% fine silt, 20% medium silt, 12% coarse silt, and 18% sand. The sand content in particular is notable, as it indicates that the clay already contains a sufficient amount of coarse particles, making the addition of extra sand as a degreaser unnecessary. This is important because sand, when added in excess, can affect the workability of the clay and may alter the final texture and strength of the bricks.

Fig. 1.

Particle size distribution curves of soil.

Sawdust identification

The wood sawdust used in this study was sourced from local wood processing plants, ensuring that the material was readily available and representative of regional wood waste. The sawdust is derived from redwood, a species renowned for its remarkable size, often considered a giant among softwood trees. Redwoods are also noted for their exceptional resistance to weathering, making them a durable and long-lasting material in various environmental conditions. As a result, redwood is classified as a lightweight wood, which contributes to the potential benefits of incorporating it into building materials such as fired clay bricks.

In this research, we intentionally used only one species of wood sawdust to maintain consistency and control over the experimental variables. Different wood species vary significantly in terms of their chemical composition, density, fiber structure, and combustion behavior—all of which can influence the physical and thermal properties of the final fired brick product. By selecting a single wood species, we ensured that any observed changes in brick properties were due to the proportion of sawdust added rather than variability in the raw material itself. This approach allowed for more accurate assessment of the relationship between sawdust content and the performance of the bricks, while also laying the groundwork for future studies that may explore comparative effects of different biomass types.

The chemical composition of the wood sawdust reveals that it is predominantly composed of 50% carbon, which plays a key role in the material’s combustibility and energy content, 44% oxygen, and 6% hydrogen. These proportions are important as they indicate the sawdust’s organic nature and provide insights into its behavior during thermal treatments. Notably, the combustion temperatures of redwood sawdust typically range from 300 °C to 500 °C, with these temperatures reflecting the conditions under which the organic components of the wood will combust and the material will undergo structural changes.

In this study, dry wood sawdust was used as an additive in the clay brick formulation to ensure consistency, accuracy, and reliability in the experimental process. Moisture content in raw materials can significantly affect the mixing ratio, drying behavior, and firing performance of clay bricks. By using dry sawdust, we eliminated the variability introduced by water content, allowing for better control over the mix proportions and reducing the risk of unexpected shrinkage or cracking during drying and firing. Moreover, dry sawdust ensures a more predictable decomposition during the firing process, contributing to the formation of uniform pores that enhance the thermal insulation properties of the bricks. This approach supports the objective of producing consistent, sustainable, and thermally efficient building materials.

In terms of physical properties, the specific density and bulk density of the wood sawdust are measured at 0.50 g/cm³ and 0.16 g/cm³, respectively. The specific density indicates the inherent mass of the wood material per unit volume, while the bulk density provides insight into how much air space exists within the sawdust particles. These densities are critical for understanding the behavior of the sawdust when mixed with other materials, such as clay, and for predicting the overall performance of the resulting brick samples in terms of weight and porosity.

Methods

Preparation of clay and wood sawdust brick samples

Five distinct mixes, designated as S1, S2, S3, S4, and S5, were prepared with varying proportions of wood sawdust, specifically 0%, 2%, 4%, 6%, and 8% by mass of the clay. These proportions represent the mass fraction of wood sawdust relative to the total mass of clay, with each mix serving as a different experimental condition for investigating the impact of wood sawdust incorporation on the physical and thermal properties of the resulting fired clay bricks.

For each mix, plate-shaped brick samples were fabricated, each with dimensions of 270 × 270 × 40 mm. These dimensions were selected to ensure consistency in the size and shape of the samples, facilitating uniform testing and comparison of the physical and thermal properties across the different mixes. The specific compositions of the clay and wood sawdust mixtures, including the exact percentages of clay and wood sawdust in each mix, are provided in Table 2.

Table 2.

Mix composition of brick samples.

Sample Designation	Sawdust Mass / Clay Mass (%)	Water Mass / Clay Mass (%)
S1	0	15
S2	2	18
S3	4	22
S4	6	25
S5	8	28

It is important to note that the water content for each mixture was determined through a series of successive trials aimed at achieving the ideal mixture consistency. The goal was to ensure that the clay and sawdust blend could be adequately molded without excess water. The trials involved gradually adding water and testing the consistency of the mixture until a smooth, homogeneous paste was formed that exhibited no excess moisture at the base of the metal mold. This step is crucial, as excess water could affect the final texture, density, and overall quality of the bricks.

.

The steps for preparing the clay and wood sawdust mixture are as follows:

1. Mixing the Clay and Wood Sawdust: First, the clay and wood sawdust are thoroughly mixed together, as shown in Fig. 2a.

2. Adding Water: Next, the appropriate amount of water is added to the mixture. The mixing continues until a homogeneous paste is obtained, as depicted in Fig. 2b.

3. Molding the Brick Samples: Finally, the mixture is placed into a metal mold to form the brick samples. The molding process ensures a smooth texture for the brick, as illustrated in Fig. 2c and d.

Fig. 2.

Steps of preparing the sawdust and clay brick sample.

Once the brick samples are carefully removed from the metallic mold, they undergo a controlled drying process in a laboratory oven to completely eliminate the moisture content retained within the clay and wood sawdust mixture. This step is essential in the manufacturing cycle, as any residual water within the bricks can cause the formation of internal steam pockets during firing, leading to cracking, warping, or even structural failure of the bricks. The purpose of this pre-firing drying phase is, therefore, to ensure dimensional stability and structural integrity, while minimizing defects caused by thermal stress during the firing stage.

As illustrated in Fig. 3, the drying process is performed over a period of 4 h, following a gradual temperature increase. This staged approach allows for progressive evaporation of the water, avoiding rapid moisture loss that could cause surface tension imbalances and cracking. The drying schedule is carefully controlled as follows:

Fig. 3.

Brick samples drying program.

• 0.5 h at 40 °C: A gentle initial stage to begin surface moisture evaporation without inducing thermal shock.

• 1.5 h at 60 °C: A moderate increase in temperature to promote deeper moisture migration from the interior toward the surface.

• 1 h at 80 °C: Continued evaporation at a higher temperature to remove most of the bound water.

• 1 h at 110 °C: Final drying phase to eliminate any remaining moisture, ensuring the brick is fully dried and thermally stable before firing.

This stepwise thermal treatment ensures that the brick samples reach a uniform moisture content of near zero before firing, which is a critical prerequisite for producing high-quality fired clay bricks. Proper drying not only improves the mechanical strength and dimensional accuracy of the bricks but also contributes significantly to the thermal and physical performance of the final product.

After the drying process is successfully completed and all residual moisture has been eliminated, the brick samples are subjected to the firing stage, which is one of the most critical phases in the brick manufacturing process. Firing is conducted in a laboratory-controlled oven capable of reaching temperatures up to 1200 °C. The purpose of this stage is to induce mineral transformations, promote sintering of the clay particles, and burn out the organic matter, such as the incorporated wood sawdust. These changes result in the development of the final mechanical strength, dimensional stability, and thermal performance of the bricks.

The total firing cycle lasts 23 h and is carefully divided into three key stages to allow for gradual heating and cooling, minimizing internal thermal stress and ensuring uniform structural transformation throughout each brick. The firing schedule, as illustrated in Fig. 4, is as follows:

Fig. 4.

Brick samples firing program.

• 9.5 h of gradual heating from 70 °C to 850 °C: During this stage, the temperature is increased progressively. This slow rise allows for the controlled removal of any remaining volatile substances and facilitates the thermal decomposition of wood sawdust, which typically occurs between 300 °C and 500 °C. This stage also initiates the sintering of clay minerals, helping the particles to bond without creating thermal shock.

• 4 h at a constant temperature of 850 °C: This plateau stage is maintained to allow complete sintering and oxidation of residual organic materials, ensuring full development of the ceramic matrix. The selected firing temperature of 850 °C is sufficient to achieve a good balance between mechanical strength and thermal insulation properties, especially given the presence of pore-forming wood sawdust in the mix.

• 9.5 h of controlled cooling from 850 °C back to 70 °C: A gradual cooling phase is essential to prevent the formation of microcracks or warping due to rapid temperature drops. This stage allows the internal structure of the bricks to stabilize and prevents damage that could compromise their structural and thermal performance.

This meticulously designed firing cycle ensures that the bricks undergo the necessary physical and chemical transformations, such as dehydroxylation, oxidation of iron oxides, and vitrification of certain mineral phases, which collectively contribute to the durability and functional efficiency of the final product. In particular, the burnout of wood sawdust during firing creates a network of fine pores within the brick matrix, enhancing thermal insulation and reducing density, making the bricks suitable for lightweight construction and energy-efficient building applications.

Density and porosity measurements

The density and porosity of the various clay and wood sawdust brick samples were measured using the prepared plate samples, following the guidelines outlined in the standard NF ISO-5017⁴⁹. To determine the density (denoted as ρ), each fired brick sample was carefully weighed. The density was then calculated using the equation:

1

Where M₁ and V are respectively the weight and the volume of the dry sample. The volume V was determined by water displacement, ensuring accurate measurement of the brick’s volume. This procedure allowed for the precise determination of the brick’s density, which is a critical property for evaluating the suitability of the bricks in construction applications.

Similarly, the porosity of the samples (denoted as η), was determined based on the relation between the sample’s volume and the volume of voids present within the brick structure. The measured porosity provides valuable insight into the brick’s internal structure, which is important for assessing its insulation and strength characteristics.

The porosity was determined as follow: First, weighting the dry sample. Second, weighting the sample after eliminating the air by using a desiccator during 72 h. Finally, weighting the sample after it was being saturated in water.

The porosity, η, of brick sample was determined using the following equation:

2

Where M₂ and M₃ are respectively the sample weight after elimination air and the weight of the saturated sample.

Thermal conductivity measurement

All prepared clay and wood sawdust brick samples were rigorously tested to evaluate their thermal conductivity using a specialized thermal box apparatus, which is designed to simulate heat flow through building materials under controlled laboratory conditions. This testing was conducted in full accordance with the NF EN ISO 8990⁵⁰ standard, which outlines the steady-state method for determining thermal transmission properties through a calibrated and reproducible setup.

The thermal box apparatus functions by maintaining a controlled temperature gradient across the two opposite surfaces of each brick sample. One face of the sample is exposed to a hot chamber, while the opposite face is maintained at a lower reference temperature. The apparatus is equipped with heat flux sensors and thermocouples to continuously measure both the heat transfer rate and the surface temperatures during the experiment. By applying a steady-state condition—where the temperature difference and heat flow reach equilibrium—the thermal conductivity (denoted as λ) of each sample can be accurately calculated using Fourier’s law of heat conduction.

This method is particularly suitable for materials like modified clay bricks, as it provides a precise measurement of their ability to resist heat flow—a critical property in evaluating their performance as thermal insulating materials in building envelopes. The inclusion of wood sawdust in the brick matrix is expected to influence this property by introducing micro-pores through combustion during firing, thereby enhancing the insulating capability of the bricks.

The NF EN ISO 8990 standard ensures that the test is carried out under standardized and repeatable conditions, minimizing variability and allowing for meaningful comparisons between different compositions (S1 to S5). Through this method, the study effectively quantifies the impact of increasing wood sawdust content on the thermal efficiency of fired clay bricks, offering valuable insight for their application in energy-efficient and sustainable construction

The thermal conductivity, denoted λ of each sample was given by using the following relation:

3

Where:

• λ is the thermal conductivity in W/m.K.

• e is the specimen thickness in m;

• U is the electric tension in V;

• S is the plate sample section in m²;

• T₁, T₂, T_B and T_a are the temperatures measured using platinum temperature sensors in K;

• R is the heater in Ω;

• C is the overall heat transfer coefficient.

Thermal diffusivity measurement

The thermal diffusivity of each clay and wood sawdust brick sample was evaluated using a testing setup similar to that employed for the thermal conductivity measurements. While both properties are closely related, thermal diffusivity provides additional insight into the material’s ability to respond to temperature changes over time, which is crucial for assessing the material’s behavior in dynamic thermal environments.

In this test, the thermal diffusivity was determined by monitoring the temperature variation over time at the non-irradiated (cold) surface of each brick sample, following the application of a steady heat flux on the opposite (hot) face. By measuring how quickly heat is transmitted through the sample and how the temperature evolves on the unheated side, the thermal diffusivity coefficient (denoted as α) was calculated. This coefficient reflects the material’s capacity to conduct thermal energy relative to its ability to store.

In this study, the thermal diffusivity of each brick sample was calculated using the Degiovanni model, which is a widely accepted method for determining thermal diffusivity in materials. The relationship used to calculate the thermal diffusivity is given by the following equation:

4

Where α_1/2, α_1/3 and α_2/3 are the three Degiovanni model relations⁵¹ given as follows:

5

6

7

Where e represents the specimen thickness, and t1/3 and t2/3 refer to specific time intervals during the heating process. The times t1/3, t1/2, t2/3, and t5/6 represent the partial times at which the temperature reaches 1/3, 1/2, 2/3, and 5/6 of its maximum value, respectively, during the thermal transient experiment⁵¹.

The measured values of α provide valuable insight into the material’s ability to conduct and store heat, which is critical for assessing the thermal efficiency of building materials like fired clay bricks. Materials with lower thermal diffusivity are typically better insulators because they resist the flow of heat, helping to maintain stable internal temperatures in buildings.

Thus, by using the Degiovanni model and these relationships, we can accurately quantify the thermal diffusivity of the brick samples and understand how variations in their composition, such as the inclusion of wood sawdust, may affect their thermal performance.

Specific heat capacity measurement

The specific heat capacity, denoted as C_p, is a fundamental thermophysical property of a material that represents the amount of thermal energy required to raise the temperature of one kilogram of the material by one degree Celsius (J/kg·°C). It plays a crucial role in determining how much thermal energy a material can store, which is directly related to its thermal performance in construction applications.

In the context of this study, the specific heat capacity is not measured directly but is instead calculated indirectly using the values of other thermophysical properties—namely, thermal conductivity (λ), thermal diffusivity (α), and bulk density (ρ). The interrelationship among these parameters is governed by the following expression:

8

Results and discussion

The average results obtained from the experimental tests conducted on all clay and wood sawdust-fired brick samples are comprehensively summarized in Table 3. These results represent the mean values calculated from multiple tests performed on each mix composition (S1 to S5), ensuring a reliable and statistically sound evaluation of the material properties. The data in Table 3 include key physical and thermal performance indicators such as bulk density, total porosity, thermal conductivity (λ), thermal diffusivity (α), and specific heat capacity (C_p).

Table 3.

Experimental results.

Samples	S1	S2	S3	S4	S5
Density [Kg/m3]	2090	1910	1745	1562	1398
Porosity [%]	18	29	33	37	42
Thermal conductivity [W/m K]	0.55	0.51	0.49	0.45	0.41
Thermal diffusivity [mm2 /s]	0.572	0.513	0.492	0.458	0.427
Specific heat capacity [J/kg.C]	879	1035	1097	1215	1317

By comparing these average values across different wood sawdust proportions, the table provides clear insight into the influence of increasing wood sawdust content on the overall performance of the fired clay bricks. These results serve as the foundation for analyzing trends, drawing conclusions about the suitability of wood sawdust as a sustainable additive, and recommending its application in lightweight and thermally efficient building materials.

Effect of sawdust ratio on the porosity of fired bricks

The results of the total porosity tests for the clay and wood sawdust-fired bricks are summarized in Table 3 and graphically illustrated in Fig. 5. These results clearly demonstrate a progressive increase in porosity as the percentage of wood sawdust in the mix increases. Specifically, the total porosity rises from 18% in the control brick sample without sawdust (Mix S1) to a significantly higher value of 42% in the sample containing 8% wood sawdust by mass (Mix S5).

Fig. 5.

Effect of wood sawdust incorporation on total porosity of fired clay bricks.

This pronounced increase in porosity can be directly attributed to the combustion of wood sawdust particles during the firing process. As the bricks are subjected to high temperatures, the organic content in the sawdust burns off, leaving behind voids and micro-pores within the brick matrix. The more sawdust that is incorporated, the greater the number of pores formed during firing, resulting in a more porous internal structure.

The formation of these pores not only affects the internal architecture of the bricks but also plays a significant role in determining their mechanical and thermal performance. While increased porosity tends to reduce density and mechanical strength, it also contributes to enhanced thermal insulation, as the air trapped within the pores acts as a natural thermal barrier.

From these observations, it can be concluded that the incorporation of wood sawdust into clay bricks has a significant impact on their total porosity. This characteristic can be strategically utilized in the development of lightweight and thermally insulating building materials, particularly suited for energy-efficient and environmentally friendly construction applications.

Effect of sawdust ratio on the density of fired bricks

The bulk density values of the fired clay and wood sawdust brick samples are reported in Table 3, while Fig. 6 illustrates the variation in density as a function of the sawdust content incorporated into each mix. As a main conclusion: the bulk density of fired clay brick is inversely proportional to wood sawdust proportion. The experimental results reveal a notable and consistent reduction in the density of the bricks as the proportion of wood sawdust increases.

Fig. 6.

Effect of wood sawdust incorporation on bulk density of fired clay bricks.

For instance, the control sample (Mix S1), which contains no wood sawdust, exhibits the highest density at approximately 2090 kg/m³. In contrast, the sample containing 8% wood sawdust by mass (Mix S5) shows a significantly reduced density of about 1398 kg/m³. This represents a reduction of nearly 33%, clearly demonstrating the impact of sawdust addition on the overall mass and compactness of the bricks.

This decrease in bulk density can be directly linked to the increase in porosity, which occurs as a result of the thermal decomposition and combustion of the wood sawdust during the firing process. As the organic sawdust particles are burned off, they leave behind voids and interconnected pore networks, reducing the solid volume of the brick while increasing the proportion of air-filled spaces. Since air has a much lower density than the solid clay matrix, the overall bulk density of the material decreases significantly.

From a practical perspective, the reduction in density has important engineering and environmental implications. First, lower-density bricks are easier to handle, transport, and install, leading to potential cost savings in logistics and labor. Second, reduced density generally correlates with lower thermal conductivity, making these bricks more suitable for thermal insulation applications.

As a main conclusions :

• The bulk density of fired clay bricks is inversely proportional to the percentage of wood sawdust incorporated. As the sawdust content increases, the density consistently decreases.

• When the sawdust content exceeds 4% (as seen in samples S4 and S5), the bricks can be classified as lightweight bricks, which are highly desirable in modern construction for reducing structural load and improving energy efficiency.

• Thus, the incorporation of wood sawdust into clay bricks not only promotes waste valorization but also leads to the development of lightweight, thermally efficient, and eco-friendly building materials.

• Red clay brick, with incorporation wood sawdust proportion higher than 4%, can be considerate as lightweight bricks.

Effect of sawdust ratio on the thermal conductivity of fired bricks

The thermal conductivity values of the various clay and wood sawdust-fired brick samples are presented in Table 3, while the corresponding variation as a function of the wood sawdust content is shown in Fig. 7. The results reveal a notable decline in thermal conductivity across all tested mixtures as the percentage of sawdust incorporated into the bricks increases.

Fig. 7.

Effect of wood sawdust incorporation on thermal conductivity of fired clay bricks.

For example, the reference sample (Mix S1)—which contains 0% wood sawdust—exhibits the highest thermal conductivity, measured at approximately 0.55 W/m·K. In contrast, the brick sample with the highest wood sawdust content (Mix S5, 8%) shows a significantly reduced thermal conductivity of around 0.41 W/m·K. This indicates a reduction of over 25%, underscoring the beneficial effect of sawdust incorporation on the thermal performance of the bricks.

The observed decrease in thermal conductivity can be primarily attributed to the increased porosity of the bricks caused by the combustion of wood sawdust during the firing process. As the sawdust burns out, it leaves behind a network of pores filled with air, which has a very low thermal conductivity—approximately 0.02 W/m·K. Since thermal conduction in solids is generally higher than in gases, the presence of more air-filled voids significantly impedes heat flow through the material.

Furthermore, the irregular pore structure introduced by the burned-out sawdust acts as a barrier to direct heat transmission, increasing the path length and resistance to thermal energy transfer. This effect becomes more pronounced with higher sawdust content, resulting in further improvement in insulation performance.

As a main conclusions :

• The thermal conductivity of fired clay bricks decreases significantly with the increase in wood sawdust content. This trend is consistent across all tested mixes.

• The reduction in thermal conductivity is primarily due to the increased air porosity, which replaces denser solid clay with low-conductivity air.

• As a result, clay bricks incorporating sawdust—especially at proportions of 4% and above—can be effectively considered as thermal insulation materials.

• These findings confirm the potential of using wood sawdust as a sustainable additive to enhance the energy efficiency of building envelopes by improving the thermal insulation properties of clay bricks.

In summary, the inclusion of wood sawdust not only contributes to waste valorization but also leads to the production of eco-friendly bricks with superior thermal performance, making them suitable for applications in energy-efficient construction.

Effect of sawdust ratio on the thermal diffusivity of fired bricks

The results of thermal diffusivity measurements for the fired clay bricks incorporating various proportions of wood sawdust are presented in Table 3, with the variation trend illustrated in Fig. 8. The experimental data clearly show that thermal diffusivity decreases progressively with increasing wood sawdust content.

Fig. 8.

Effect of wood sawdust incorporation on thermal diffusivity of fired clay bricks.

For instance, the reference brick sample (Mix S1), made entirely of clay without sawdust, exhibits the highest thermal diffusivity of approximately 0.572 mm²/s. In contrast, the sample containing 8% sawdust by mass (Mix S5) records a significantly lower thermal diffusivity of about 0.427 mm²/s, representing a reduction of over 25%.

This reduction in thermal diffusivity can be attributed to a combination of interrelated physical and thermal changes in the brick’s structure due to sawdust incorporation:

• Decrease in bulk density: As sawdust content increases, the brick becomes less dense due to higher porosity. A lower density means that the material contains more voids and less solid mass to transmit heat.

• Reduction in thermal conductivity: As previously discussed, the thermal conductivity of bricks decreases with higher sawdust content.

• It follows that a decrease in both λ and ρ contributes significantly to the reduction in α.

• Increased air content: As sawdust burns during firing, it leaves behind air-filled pores, which further hinders the rate at which heat is conducted and diffused through the material. Air has a much lower capacity for thermal transfer than solids, thus contributing to the overall reduction in thermal diffusivity.

As an illustration of this trend, thermal diffusivity drops from 0.513 mm²/s in bricks with 29% porosity to 0.452 mm²/s in bricks with 47% porosity, showing a clear relationship between porosity and thermal behavior.

As a main conclusions:

• Thermal diffusivity decreases significantly as wood sawdust content increases. This suggests that bricks with higher sawdust content can delay the transfer of heat through the material.

• Lower thermal diffusivity enhances thermal insulation, as materials with low diffusivity absorb and transmit heat more slowly, contributing to greater thermal stability within buildings. In practical terms, more energy (or time) is required for heat to penetrate the full thickness of the material.

• There is a direct correlation between reductions in thermal conductivity and reductions in thermal diffusivity. For example, incorporating 6% wood sawdust (Mix S4) results in an 18% reduction in thermal conductivity and a corresponding 20% reduction in thermal diffusivity when compared to the reference bricks.

These results support the potential of sawdust-clay bricks as effective thermal insulation materials, particularly suited for passive building designs aiming to reduce energy consumption and improve indoor comfort.

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Effect of sawdust ratio on the specific heat capacity of fired bricks

The results of the specific heat capacity measurements for the different fired bricks, with varying wood sawdust content, are presented in Table 3. Figure 9 illustrates how the specific heat capacity of the tested samples changes as a function of the proportion of wood sawdust incorporated into the bricks. The experimental results indicate a significant increase in specific heat capacity with higher wood sawdust content in all tested mixtures.

Fig. 9.

Effect of wood sawdust incorporation on specific heat capacity of fired clay bricks.

For example, the reference brick (Mix S1), which contains no wood sawdust, has a specific heat capacity of approximately 879 J/kg·K. In contrast, the brick incorporating 8% wood sawdust (Mix S5) exhibits a much higher specific heat capacity of around 1317 J/kg·K, representing an improvement of approximately 50%. This increase can be attributed primarily to the changes in the internal structure of the brick caused by the incorporation of wood sawdust.

The increase in specific heat capacity when wood sawdust is added can be understood through the following factors:

1. Increased Air Content: Wood sawdust, when added to the clay mix and subsequently burned off during firing, creates numerous air-filled pores within the brick. The presence of these voids increases the porosity of the brick, which plays a critical role in its thermal properties. Air, which has a very low thermal conductivity (around 0.02 W/m·K), acts as an insulating medium, significantly reducing the rate of heat transfer through the material.

2. Thermal Conduction in Porous Materials: In porous materials like these fired bricks, the heat transfer occurs through two phases:

   • Solid Phase (the clay matrix), which conducts heat more effectively than air.
   • Interconnected Air Pores, which contribute to the material’s ability to store thermal energy. Since air has low thermal conductivity, it resists heat flow and thus requires more energy to raise the material’s temperature.

   As the porosity increases (due to the incorporation of wood sawdust), the conduction surface of the solid phase decreases, while the surface area of air pockets increases, which in turn increases the material’s specific heat capacity. This is because it takes more energy to raise the temperature of a material with more air voids and less solid material. Essentially, more energy is required to increase the temperature of a more porous brick.

3. Impact of Porosity on Heat Flux: The heat flux in porous materials is directly proportional to the surface area available for heat conduction. In highly porous materials, a larger fraction of the surface area consists of air-filled pores, which leads to a reduced heat flux for the same thermal gradient. Consequently, bricks with more porosity require more energy (in the form of heat) to raise their temperature, resulting in an increased specific heat capacity.

As a main conclusions;

• Specific Heat Capacity Increases with Sawdust Incorporation: The incorporation of wood sawdust significantly improves the specific heat capacity of fired clay bricks. The more sawdust incorporated, the higher the specific heat capacity. This is a direct consequence of increased porosity and the creation of air voids that store heat.

• Reduction in Thermal Conductivity Leads to Higher Specific Heat: There is a reverse relationship between thermal conductivity and specific heat capacity. As the incorporation of wood sawdust decreases the thermal conductivity of the bricks (as shown in Sect. 3.3), it simultaneously leads to an increase in specific heat capacity. For instance, 6% sawdust (Specimen S4) resulted in an 18% reduction in thermal conductivity and a 38% increase in specific heat capacity when compared to the reference brick.

• Thermal Insulation and Heat Storage: Bricks with more than 2% wood sawdust exhibit thermal insulating properties and demonstrate a capacity to store heat. This enhances their thermal performance, making them suitable for use in applications requiring efficient heat storage, such as in passive heating systems or energy-efficient buildings.

As a Practical Implications: The increase in specific heat capacity with the addition of wood sawdust makes these bricks highly suitable for thermal insulation applications. By improving the ability of the material to store heat, these bricks can help in maintaining steady indoor temperatures by storing heat during the day and releasing it at night, thus reducing the need for mechanical heating or cooling systems. This not only contributes to energy efficiency but also supports the development of sustainable, eco-friendly building materials.

Conclusions

The present study demonstrates that wood sawdust can be effectively used as an additive in fired clay brick production. The incorporation of sawdust results in lower bulk density, increased porosity, and improved thermal properties, which can contribute to both the sustainability of the brick industry and the recycling of wood waste. Below are the main obtained results:

• The total porosity of fired clay bricks increases with the proportion of wood sawdust. This is attributed to the formation of pores as the sawdust burns out during the firing process, leaving behind voids.

• The bulk density of the bricks decreases as the sawdust content increases, demonstrating an inverse relationship. Bricks with more than 4% sawdust can be classified as lightweight bricks, which may reduce structural load and improve handling.

• The thermal conductivity of the fired clay bricks significantly decreases with the incorporation of wood sawdust. This enhancement makes the bricks suitable for use as thermal insulation materials, contributing to energy-efficient building design.

• Thermal diffusivity decreases as the proportion of wood sawdust increases, indicating improved thermal insulation. Lower thermal diffusivity means that the material is slower to change temperature, which is desirable in climate-responsive construction.

• The observed reduction in thermal conductivity directly contributes to a reduction in thermal diffusivity, reinforcing the insulating characteristics of the material.

• The specific heat capacity of the bricks increases with the addition of wood sawdust. This suggests an improved ability to store thermal energy, enhancing the brick’s performance in regulating indoor temperatures.

Author contributions

O.B prepare the paper.

Funding

The authors extend their appreciation to Prince Sattam bin Abdulaziz University for finding this research work through the project number (PSAU/2024/01/29147).

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.