Enhancing the properties and environmental performance of fired clay bricks through lignocellulosic additives an investigation of oil palm mesocarp fibre (OPMF) as a sustainable alternative

Abstract

This article investigates the role of lignocellulosic additives, specifically Oil Palm Mesocarp Fibre (OPMF), in modulating the properties and environmental performance of fired clay bricks. OPMF, a readily available material containing high levels of lignocellulose, possesses desirable properties for use in brick making. Clay soil was substituted with OPMF at varying levels (0%, 1%, 5%, and 10%), and bricks were fired at 1050 °C with a controlled heating rate of 1 °C/min. A systematic experiment was employed, combining physical-mechanical properties (density, shrinkage, water absorption, porosity, compressive strength, and thermal conductivity), microstructural analysis (XRD, SEM-EDX, TGA-DTA), and and leaching analysis (TCLP and SPLP tests) were evaluated. The results revealed that incorporating 10% OPMF significantly reduced the mechanical properties, with compressive strength decreasing from 24.6 MPa to 4.1 MPa. This decrease was attributed to increased firing shrinkage (0.7 to 2.5%) and porosity (13 to 25%). However, OPMF addition resulted in a lighter brick (density decreased from 1799 to 1563 kg/m³) and enhanced thermal conductivity (improved from 0.54 to 0.34 W/m.K), primarily due to the increased porosity. Leaching tests confirmed that heavy metals (Pb, Cr, and Zn) were successfully immobilized, meeting USEPA and WHO guidelines. This indicates that heavy metals were effectively immobilised and encapsulated within the clay matrix, forming a stable mineral phase. These findings suggest that incorporating up to 5% OPMF offers a sustainable approach to brickmaking, promoting circular economy while reducing environmental impact.

Introduction

The rapid growth of urbanization and industrialization has driven a surge in demand for construction materials, accelerating resource depletion and creating substantial environmental strain. Studies indicate that the construction sector accounts for 40% of global raw material consumption (including bricks, sand, gravel, stone, timber, and steel), 40% of worldwide waste generation (primarily from construction and demolition activities), and 33% of global greenhouse gas emissions, largely due to cement and steel production^1–3. India, for example, the second-largest clay brick producer has depleted over 300 millimetres of fertile topsoil in six decades through its brick industry, which operates approximately 100,000 kilns producing 250 billion bricks annually while consuming 35 million metric tonnes of coal⁴. This linear economic model of “extract, produce, use, and discard” in brick manufacturing is increasingly unsustainable, exacerbating natural resource exhaustion and generating massive waste streams^4,5.

Sustainable construction practices prioritize systems that enhance resource efficiency, minimize waste, and mitigate environmental harm by shifting from linear to circular economic models^5,6. This involves utilizing renewable or recyclable materials, designing structures for longevity and adaptability, and implementing robust waste management strategies^4,6. To address the carbon footprint of conventional brick production, researchers are increasingly turning to recycled waste materials. Agricultural byproducts like fruit peel waste⁷, sugarcane bagasse⁸, hazelnut shells⁹, oat husk, barley husk, and middlings¹⁰, sawdust, grape seeds, and cherry seeds¹¹.

Among the various agro-industrial residues, oil palm mesocarp fibre (OPMF) has gained increasing attention due to its lignocellulosic composition and favourable attributes including low density, thermal insulation potential, and renewability. Malaysia, Indonesia, and Thailand collectively generate several million tonnes of OPMF annually as by-products of palm oil extraction, making it regionally abundant and globally relevant for sustainable construction applications. The specific selection of OPMF over other agricultural residues is justified by its unique technical profile; its high cellulose-to-lignin ratio and robust fibrous structure significantly influence the capillary network and pore morphology during the firing process. Furthermore, utilizing OPMF addresses a critical waste management crisis in palm-oil-producing regions, where the accumulation of mesocarp fibre often leads to uncontrolled open-air burning or landfilling. The fibre typically comprises 35–45% cellulose, 25–30% hemicellulose, and 15–20% lignin, accompanied by minor inorganic impurities such as silica and trace metals arising from processing operations and soil uptake. OPMF is inherently heterogeneous in size and morphology, generally occurring as coarse fibrous residues that necessitate appropriate pre-treatment to achieve compatibility within ceramic matrices¹².

However, its integration into fired clay bricks remains underexplored despite its potential to advance sustainable construction. While OPMF has been studied in composite materials¹, road paving¹³, cement mortar^14,15, concrete^16,17, cement sand bricks^18,19, unreinforced and fibre adobe brick²⁰ and reinforcement in composites²¹, existing research often overlooks critical factors such as scalability, environmental trade-offs, material compatibility, and long-term performance. These recent applications^15–21 highlight OPMF’s versatility and favorable physicochemical properties, while also underscoring the research gap concerning its integration into fired clay bricks.

To address this gap, the present study investigates OPMF from a new perspective, focusing on its role as a thermally reactive additive rather than as a passive reinforcing material. This distinction is critical, as the thermal decomposition of lignocellulosic matter contributes directly to pore generation, reduced density, and altered firing energy requirements. In this context, prior works focusing on OPMF as non-decomposing reinforcement are briefly acknowledged, whereas this study emphasizes its thermally reactive role in ceramic matrices. Moreover, a broader review of fired clay bricks incorporating agricultural wastes such as rice husk, sawdust, and sugarcane bagasse demonstrates that factors including particle size, organic composition (cellulose, lignin, hemicellulose ratios), and firing temperature critically govern the resultant brick density, strength, and porosity. By situating this study within that wider context, the novelty and sustainability potential of OPMF are highlighted as an innovative approach toward eco-friendly brick production.

Ezugwu et al.²² investigated the effects of hydrothermal treatment of plant fibres for cement composites. They found that plant fibres improved the fibre properties and increased the performance of the composites. However, the study lack in analysis of economic feasibility or interfacial bonding dynamics. Meanwhile, Rafidah et al.¹³ investigated the potential of oil palm fibre (OPF) as a sustainable reinforcement material for road construction. OPF, a by-product of palm oil production, proves to be a promising alternative to conventional synthetic fibres due to its high content of cellulose, hemicellulose and lignin. However, the study is mainly based on laboratory experiments and lacks data from large-scale field trials. Bisong et al.¹⁴ and Fokam et al.¹⁵ investigated the use of OPMF to reinforce cement mortar and improve crack resistance in structures built with cement mortar. They also simulated crack formation and growth up to fracture. However, their model needs to be validated against experimental data to ensure its accuracy and reliability.

Meanwhile, Amartey et al.¹⁶ and Mydin¹⁷ investigated the potential OPMF as an additive in foamed concrete, which could improve the mechanical properties of concrete. However, the environmental impact has not yet been thoroughly investigated. Nafu et al.¹⁸ and Sali & Deraman¹⁹ conducted a comprehensive study on the influence of OPMF on the thermal properties of cement sand bricks. While their methodology was thorough, the potential for fungal growth on the bricks was not investigated. This omission is worrying as fungal growth could affect aesthetics and potentially pose health risks. The study by Eslami et al.²⁰ investigated the short- and long-term properties of adobe bricks containing palm fibres with different weight ratios. They acknowledge that the expressions developed may not be directly applicable to materials from other regions. However, it would be helpful to provide more details about the characteristics of the local materials used and to conduct comparative studies with other regions to assess the generalisability of the results. Graupner et al.²¹ investigated fibre bundles from toddy palm fruit fibre as reinforcement in PLA composites. Although they investigated this novel material, the study lacked a detailed analysis of the crucial interfacial bonding between the fiber and the matrix, which affects the load transfer and overall performance of the composite.

While numerous studies have explored OPMF in composite materials, cement mortars, and concrete primarily emphasizing its reinforcing and tensile enhancement roles the present study diverges by examining OPMF as a pore-forming additive in fired clay bricks. This distinction is critical, as the thermal decomposition of lignocellulosic matter in OPMF contributes directly to pore generation, reduced density, and altered firing energy requirements. Consequently, previous works emphasizing OPMF as non-decomposing reinforcement are briefly acknowledged for context, while the current research focuses on its thermally reactive role in ceramic matrices.

Oil Palm Mesocarp Fibre (OPMF) was chosen as a suitable additive owing to its abundance as an agro-industrial by-product, its favorable physico-mechanical properties, and its sustainability potential. Generated in significant quantities during palm oil extraction, OPMF is lightweight, biodegradable, and composed of cellulose, hemicellulose, and lignin, which make it an effective natural reinforcement material for composites. Its inclusion in gap-graded asphalt mixtures provides additional tensile strength, enhances toughness, and improves resistance to deformation under sustained loading. Moreover, the utilization of OPMF addresses waste management challenges in palm oil-producing regions while reducing reliance on costly synthetic polymers, thereby aligning with sustainable construction practices and the broader goals of the UN Sustainable Development Goals (SDGs).

While the use of OPMF has been explored in various applications, there is currently limited published research on its use as a replacement for clay in brick manufacture. However, other agricultural waste materials are widely used as substitutes for clay in brick production^7–11,23,24. Therefore, OPMF’s untapped potential could further advance circularity in construction. Although OPMF is still at an early stage of research and development.

This study hypothesizes that incorporating OPMF into fired clay bricks can reduce density and improve thermal insulation while maintaining acceptable mechanical performance. It is further anticipated that the firing process will immobilize heavy metals within the ceramic matrix, ensuring compliance with international environmental safety standards. These performance enhancements underscore the potential of OPMF-modified bricks as a sustainable alternative to conventional clay products.

Although OPMF is an abundant and low-cost by-product of the palm oil industry, large-scale utilization may introduce practical challenges particularly in fibre collection, pre-treatment, and achieving uniform blending with clay. These factors highlight the need for future techno-economic assessments to evaluate the feasibility of industrial-scale implementation.

Within this context, the present work investigates the effects of varying OPMF contents on the physical, mechanical, thermal, and environmental (leaching) performance of fired clay bricks. The study is limited to laboratory-scale experimentation, with the intention of generating foundational data that can inform broader evaluations of scalability, sustainability, and practical adoption in the construction sector.

Materials and methods

Materials

The materials used in this study were clay soil from a brick factory in Yong Peng, Johor, and OPMF from a palm oil mill in Kluang, Johor. OPMF usually contains fruit fibre, crushed kernels and shells. The raw OPMF comprised heterogeneous fruit fibres with minor crushed kernel and shell residues. Prior to use, the fibre was washed, oven-dried at 105 ± 2 °C for 24 h, and shredded using a rotary cutter to improve uniformity and remove coarse contaminants. The shredded OPMF was then sieved to pass a 2 mm aperture. Microscopic examination (500× optical magnification) indicated that the processed fibres exhibited an average length of 1 to 3 mm and diameter of 0.20 to 0.45 mm, confirming a relatively uniform size distribution. Minor inorganic residues such as silica bodies and traces of soil minerals were observed, consistent with previous reports on OPMF composition. This processed fraction (< 2 mm) was subsequently incorporated into the clay matrix as the pore-forming additive. The application of the procedure described above was in accordance with standard laboratory methods ASTM International D2216²⁵ and British Standard 1377-2²⁶, which are designed to prevent natural moisture content from affecting the accuracy of the characterization of engineering behaviour²⁷. The adoption of ASTM protocols was ensure the methodological reliability, international comparability, and reproducibility of findings. These standards are widely recognized in construction material research for evaluating mechanical, physical, and environmental properties of bricks and related composites. By employing ASTM methods, the study not only maintains scientific rigor but also ensures that the results are benchmarked against globally accepted testing practices, thereby strengthening the credibility and applicability of the findings.

However, the limitation of the study is that higher oven-drying temperatures may induce partial decomposition of organic constituents in the raw materials, potentially altering their original composition and influencing subsequent characterization outcomes. Oven-dry samples at 105 ± 5 °C for 24 h, cool in a desiccator, and process immediately; avoid higher temperatures or extended drying durations to minimise thermal decomposition of organic components.

In the next step, the dried raw materials were processed into fine particles. The clay soil was ground, crushed and sieved through a 500 μm nominal sieve (Fig. 1a), while the OPMF were shredded and ground to ensure a uniform fiber length (Fig. 1b). According to Kadir & Sarani²⁸, there is no standard particle size for waste materials, and they suggested replicating the standard size of clay soil with a maximum of 2 mm. Unlike other waste materials that are in powder form, uneven fiber length can lead to entanglement during mixing, which is the main reason for weak bonding between the soil and the fiber and can weaken the mechanical properties of the manufactured brick^29,30. Therefore, shredding the fibres to a uniform length is essential to achieve the desired mechanical properties of the final product.

Fig. 1.

Grounded raw materials pass through (a) 500 μm nominal sieve for clay soil and (b) Shredded OPMF after sieving through a 2 mm mesh.

Methods

Chemical composition and geotechnical properties of raw materials

The clay soil was characterized using X-ray fluorescence (XRF) and X-ray diffraction (XRD) to determine its elemental composition and mineral phases. For XRD analysis, sieved samples (< 63 μm) were scanned with a Bruker D8 device (Cu Kα radiation, 10–90° range, 0.02° step angle). Due to the organic nature of OPMF, XRF was unsuitable; instead, energy-dispersive X-ray spectroscopy (EDX) was employed for elemental analysis.

The lignocellulosic profile (cellulose, hemicellulose, lignin) of OPMF was determined using a Technical Association of the Pulp and Paper Industry (TAPPI) method. According to TAPPI Method T264 cm-07³¹, the OPMF samples underwent a 6-hour Soxhlet extraction process before being analyzed. Subsequently, the Krusher-Hoffner method was used to determine cellulose content and the chlorination method was used to analyze hemicellulose content. The lignin content was determined using the TAPPI Method T222³². Finally, the ash content was analyzed using TAPPI Method T211³³.

Geotechnical properties of clay and OPMF were assessed using Atterberg Limits, particle size distribution, particle density in accordance with British Standard 1377−2²⁶. Additionally, a Standard Proctor Test was performed following British Standard 1377-4³⁴ to determine optimal moisture content.

The ignition loss (LOI) test for clay and OPMF were conducted according to British Standard 1377-2²⁶ using pre-dried samples to isolate the mass loss associated with organic matter and chemically bound volatiles. The samples were oven-dried at 105 ± 2 °C for 24 h prior to ignition to remove free and adsorbed moisture. The ignition temperature of 440 °C was selected following the British Standard procedure for organic-content estimation, which differs from high-temperature LOI (approximately 950 °C) used for mineral decomposition analysis. The selection of firing temperatures between 800 °C and 1100 °C is critical, as this range ensures the complete thermal decomposition of OPMF’s lignocellulosic components to create controlled micro-porosity while facilitating the vitrification of the clay matrix for structural integrity. This temperature rationale is fundamental to balancing the weight-reduction benefits of the fiber with the necessary immobilization of trace elements through high-temperature ceramic bonding.

Design of mix proportion and brick specimen preparation

Four brick mixtures were formulated with OPMF was incorporated at substitution levels of 1%, 5%, and 10% by weight of clay soil, while all other constituents and water content were kept constant across mixes (Table 1). Proportions were selected based on preliminary trials and literature indicating that waste additions more than 10% often degrade brick performance.

Table 1.

Proposed mixtures used for making brick.

Mixture identification (g)	Clay soil (g)	OPMF (g)	Water content (%)
CB-0	2800	0	17.0
OPMFB-1	2790	10	16.8
OPMFB-5	2720	80	17.7
OPMFB-10	2640	160	18.2

Previous studies reported that incorporating lignocellulosic fibres beyond 10% tends to adversely affect the workability and mechanical integrity of composites due to excessive porosity and weak interfacial bonding. On this basis, the current study adopted 0–10% OPMF substitution levels to explore the optimum balance between enhancing thermal performance and maintaining structural integrity³⁵.

The brick production process began with mixing clay soil, OPMF (0%, 1%, 5%, or 10% by mass), and water adjusted to optimal moisture levels (16.8 to 18.2%) based on Standard Proctor compaction results. The homogeneous mixture was mechanically blended, moulded into standard-sized bricks (215 × 102.5 × 65 mm) under 2000 psi pressure, and air-dried for 24 h to reduce moisture by 2 to 4%. Subsequent oven drying at 105 °C (24 h) and firing in a laboratory furnace at 1050 °C (1 °C/min heating rate and 2-hour soaking time) ensured vitrification and solidification, followed by controlled cooling to minimize shrinkage.

Physical and mechanical properties of brick

The physical and mechanical properties of the bricks were evaluated through a series of standardized tests. The physical properties of the bricks were determined by measuring and evaluating their shape, size, colour, shrinkage, and density. These parameters provide information about the overall appearance and quality of the bricks. The structural properties of the bricks were evaluated by measuring their compressive strength in accordance with British Standard EN 772-1³⁶, a crucial factor in determining their load-bearing capacity.

The durability of the bricks was evaluated based on water absorption and porosity in accordance with British Standard EN 772-7³⁷ and ASTM C62-23³⁸. These tests provide insights into the bricks’ ability to withstand moisture and external conditions. Finally, the thermal conductivity was studied in accordance with ISO 8301³⁹ for measuring the heat transfer properties of materials. This parameter is crucial for understanding the thermal behaviour of buildings and structures, as it influences heat gain and loss through the walls.

To ensure the reproducibility and statistical reliability of the experimental results, each test was conducted in triplicate (n = 3) for every OPMF substitution level (0%, 1%, 5%, and 10%). This replication approach ensures that the measured values accurately represent the average behaviour of the materials while maintaining experimental consistency and comparability across all testing conditions.

Microstructure and thermogravimetric analysis

The microstructural and morphological properties of the bricks were investigated using XRD, SEM-EDX and digital images. These microscopic images of the raw materials and the brick samples were taken with the Hitachi HORIBA Integrated Analysis System (SEM/EDX Series). The porosity size of the bricks was then determined using an Olympus SZH10 zoom stereo microscope. To analyse the particle size and porosity of the bricks, the images were analysed using the Fiji ImageJ software (version 1.52 g) developed by the National Institute of Health (NIH)⁴⁰.

The changes in material mass and phase transitions were investigated using thermogravimetric and differential thermal analyses (TGA-DTA). These complementary techniques provide valuable insights into the thermal behaviour of the bricks. TGA measures the change in mass of the sample with increasing temperature, while DTA measures the temperature difference between the sample and a reference. In this study, the heating rate was set at 1 °C/min and the maximum temperature to 1050 °C. The TGA-DTA was performed using a Perkin Elmer Pyris Diamond TG/DTA with a temperature range of 30 to 1200 °C as the maximum temperature.

Leaching test

Two analytical methods were used to assess the potential release of harmful elements and pollutants from the bricks produced: Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP). TCLP is a standardised method defined by the US Environmental Protection Agency (EPA) Method 1311⁴¹, simulates the leaching of contaminants in a landfill environment. It is applicable to liquid, solid, and multiphase samples and assesses the mobility of both organic and inorganic compounds. Meanwhile, the SPLP method, described in EPA Method 1312⁴², utilizes a leaching fluid resembling acid rain to determine the potential release of contaminants under acidic conditions. The concentrations of leached elements and contaminants were analysed using an inductively coupled plasma mass spectrometer (ICPMS), model Perkin Elmer ELAN 9000. This advanced analytical technique combines the sensitivity and specificity of mass spectrometry (MS) with the high-temperature ionisation capabilities of an inductively coupled plasma source. This enables the detection and quantification of even traces of heavy metals in the leachates.

Results and discussion

Characterization of raw materials

The chemical composition of the clay soil used in this study was determined using X-ray fluorescence (XRF) analysis. Table 2 shows the results aligns with recommended thresholds for brick production, exhibiting adequate silica (55.7%), alumina (24.4%), and iron oxide (4.46%)⁴³-⁴⁴. Its low calcium oxide (CaO: 0.25%) and alkali oxide (Na₂O: 0.3%, K₂O: 2.24%) content classify it as non-calcareous, minimizing risks of efflorescence and thermal instability. Specifically, PbO, P₂O₅, and ZnO were either below the detection limits of the XRF equipment or not detected in quantifiable amounts during analysis as previously reported in Šveda & Sokolář⁴⁵. The negligible loss on ignition (LOI: 1.9%) confirms limited organic or clay mineral content, ensuring suitability for high-temperature firing without excessive mass loss or structural defects.

Table 2.

Chemical composition of clay soil.

Oxide	SiO2	Al2O3	Na2O	K2O	Fe2O3	CaO	MgO	PbO	P2O5	ZnO	TiO2	MnO	LOI
Content (%)	55.7	24.4	0.3	2.24	4.46	0.25	1.2	-	-	-	0.94	0.04	1.92

The OPMF exhibited a lignocellulosic profile dominated by cellulose (40.2%), lignin (13%), and hemicellulose (9.8%), with an ash content of 9.3% (Table 3). These values, although slightly lower than those reported in previous studies, confirm its high organic content, further supported by a loss on ignition (LOI) of 13.71%^46,47. Elevated LOI may increase porosity, shrinkage, and black core formation during rapid firing, however, such effects can be mitigated through controlled combustion at a slow firing rate of 1 °C/min⁴³. Notably, OPMF’s organic content offers advantages: reduced firing energy demands due to self-combustibility and enhanced post-firing porosity for improved thermal insulation. The substantial organic fraction of OPMF contributes beneficially by lowering firing energy requirements through self-combustion and enhancing post-firing porosity, thereby improving thermal insulation. The remaining minor fraction consists of extractives (fats, waxes, resins, and soluble organics) and inorganic residues, primarily silica and soil-derived minerals, as confirmed by SEM–EDX analysis (Fig. 3).

Table 3.

Fibre content of OPMF.

Chemical composition	Cellulose	Hemicellulose	Lignin	LOI
Raw OPMF (wt%)	40.2	9.8	13	13.71

Fig. 3.

SEM image of OPMF at 500× magnification; inset: typical EDX spectrum showing elemental constituents.

The XRD pattern of the raw clay soil is presented in Fig. 2a. The crystalline phases identified quartz (SiO₂) as the primary crystalline structure, characterized by prominent scanning degrees at 20.8° and 26.6° 2θ. Minor minerals present include muscovite [KAl₂(AlSi₃O₁₀)(F, OH)₂], kaolinite [Al₂(Si₂O₅)(OH)₄], and hematite (Fe₂O₃). Muscovite and kaolinite are the original clay minerals responsible for the soil’s plasticity and firing reactivity⁴³. The presence of quartz confirms the necessary skeletal structure, enhancing strength and load-bearing capacity. The hematite (Fe₂O₃), observed as a stable iron-oxide phase, is responsible for the natural coloration of the clay and significantly influences the final red color of the fired bricks. These crystalline spectra verify the high content of SiO₂ (55.7%), Al₂O₃ (24.4%), and Fe₂O₃ (4.46%) reported by XRF (Table 2).

Fig. 2.

(a) XRD pattern of clay soil, (b) XRD pattern of OPMF.

The XRD analysis of raw OPMF, depicted in Fig. 2b, highlights the presence of multiple mineral phases. Quartz (SiO₂) emerges as the dominant phase, characterized by prominent peaks at 20.9° and 26.7° 2θ. Minor constituents, including kaolinite [Al₂(Si₂O₅)(OH)₅], cristobalite (SiO₂), dolomite [CaMg(CO₃)₂], and magnetite (Fe₃O₄), were also detected, with distinct peaks at 12.3°, 21.9°, 45.8°, and 59.9° 2θ, respectively. The predominance of quartz is linked to silica bodies on the fiber surface, likely introduced during the preparation of raw material from untreated OPMF. These findings align with prior studies on the mineralogical composition of natural fibres and their derived products^48,49. The detection of kaolinite is attributed to surface impurities, while the presence of dolomite likely originates from the calcareous source of OPMF. The identification of these phases reinforces the heterogeneous nature of OPMF and its mineralogical complexity.

Microstructure and thermogravimetric of raw materials

Figure 3 presents an SEM image of clay soil at 500× magnification. The SEM analysis revealed that the clay particles possess an irregular shape and a rough, angular texture. As illustrated in the micrograph, the particles are not uniform, with dimensions ranging from approximately 0.002 mm to 0.055 mm. This morphology is typical of natural clay fines and contributes to the material’s bulk properties. Meanwhile, the EDX analysis confirmed the presence of major elements consistent with a typical clay mineral composition, Carbon (C), Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), and Potassium (K). The dominance of Si, Al, and O aligns with the primary components of aluminosilicate clay minerals, and the presence of Fe and K is often associated with accessory minerals or impurities. These elemental findings were consistent with the chemical composition determined by X-ray Fluorescence (XRF) in Table 2.

Figure 3 presents an SEM image of untreated OPMF at 500× magnification. The image highlights the fiber’s rough surface morphology and porous internal structure, aligning with observations reported by Chieng et al.⁴⁹. Notably, silica deposits are visible on the OPMF surface, likely resulting from the untreated of the raw material. EDX spectra were collected from three randomly selected surface areas (approximately 100 × 100 μm each) to obtain a representative qualitative profile of the main elements. The analysis identified carbon and oxygen as major constituents, with minor signals of silica, aluminium, iron, magnesium, and calcium corresponding to mineral residues. These data are interpreted semi-quantitatively due to the inherent uncertainties in measuring light elements such as carbon and oxygen. The results confirm the presence of mineral impurities on the fibre surface, consistent with the silica-body deposits observed under SEM.

The TGA–DTA test was conducted in ambient air (static atmosphere) without a controlled gas flow to replicate the oxidative environment typical of brick firing. The thermal behavior of the clay soil (CB-0%) is presented in the TGA-DTA curve (Fig. 4a), recorded at a heating rate of 1 °C/min. The total mass loss observed up to 685.9 °C is approximately 1.1 mg. Assuming an initial sample mass of 21.6 mg (the starting weight loss value), this total mass loss represents approximately 5.1% of the sample mass. The first stage, spanning from 20 °C to 261.9 °C is characterized by an initial shallow mass loss of 0.4 mg. This mass reduction is linked to an endothermic activity at the start of the curve (near 24 °C), representing the removal of hygroscopic and loosely bound water from the clay body.

Fig. 4.

(a) TGA-DTA of clay soil; (b) TGA-DTA of OPMF.

The second and most significant mass loss phase occurs between 261.9 °C and 685.9 °C, recording a mass reduction of 0.7 mg. This stage features a notable exothermic reaction peaking at 404.1 °C. This exothermic reaction is fundamentally related to the oxidation and burnout of the limited organic matter present in the clay soil, followed by the endothermic dehydroxylation of clay minerals (such as kaolinite), a process that is often concurrent or overlapping. The exothermic nature in this range aligns with the oxidation of organic constituents, which can release pollutants like carbon monoxide and carbon dioxide.

Beyond 685.9 °C, the TGA curve stabilizes, indicating minimal further mass loss. However, the DTA curve continues to show an upward (exothermic) trend. This high-temperature region (near 800 °C) typically involves overlapping exothermic and endothermic reactions that facilitate the structural rearrangement and crystallization of new phases such as alumina or mullite precursors and the onset of vitrification (near 900 °C). This thermal profile is consistent with the mineralogical nature of the clay soil¹⁰.

The TGA-DTA curve for OPMF (Fig. 4b) revealed a total mass loss of 5.0 mg, equivalent to 73% decomposition of the material. Composed primarily of cellulose, hemicellulose, and lignin, OPMF displayed an initial endothermic mass loss of 0.96 mg at 46.3 °C, attributed to moisture evaporation. The major mass loss of 4.04 mg occurred across a broad temperature span of 118.7 to 682.1 °C, driven by the overlapping thermal degradation of the lignocellulosic constituents and extractives^47,50. The exothermic shoulder starting around 200 °C is typically linked to the decomposition of the less stable hemicellulose (approximately 200–315 °C). The most significant weight loss phase, peaking around 306.9 °C, primarily corresponds to the rapid decomposition of cellulose (approximately 315–400 °C). Finally, the gradual mass loss extending up to 682.1 °C is characteristic of the slow, wide-range decomposition and oxidation of lignin and residual carbonaceous char. This high-temperature, gradual decomposition of lignin explains why the major degradation phase spans well beyond 400 °C, indicating that complete combustion of all organic components does not occur below 400 °C but continues up to approximately 700 °C under these oxidative conditions.

The heating rate was maintained at 1 °C/min to enable gradual decomposition and phase transition observation. Under these conditions, partial oxidation and mineral stability led to a higher residual mass fraction (approximately 27%) compared with the ash content (approximately 9%) obtained from the TAPPI T211 method, which involves complete combustion in forced air at 525 °C. The higher residue in TGA–DTA therefore represents a combination of mineral oxides (mainly SiO₂ and Al₂O₃) and partially oxidised carbonaceous char, consistent with the high silica and metal-oxide content confirmed by XRD and SEM–EDX analyses.

Geotechnical properties of raw materials

The geotechnical characteristics of the raw materials are summarized in Table 4. The particle size distribution of the clay soil used in this study indicates that it consists of 57% sand (0.075 to 4.75 mm) and 43% fine-grained particles (< 0.075 mm). Conducting a particle size distribution analysis is crucial, as it directly influences the workability of the moulding process and the quality of the final brick products. According to the Unified Soil Classification System (USCS), these proportions classify the soil as silty clay with low plasticity. The obtained distribution falls within the optimal range recommended by Mueller et al.⁴³ and ILO⁴⁴, which suggest that the ideal soil for brick production should contain 20 to 75% sand and 20 to 50% fine-grained material. The relatively high sand content in this study enhances the mechanical strength of the bricks by improving load-bearing capacity and facilitating the moulding process, as sand prevents excessive adhesion of soil to the mould. Meanwhile, the fine-grained fraction contributes to structural stability by increasing internal cohesion and acting as a binder between the sand and clay particles during firing, thereby improving the overall strength and integrity of the fired bricks. However, excessive clay content should be avoided, as it can reduce workability and increase firing shrinkage, potentially causing deformation in the finished products.

Table 4.

Geotechnical properties of raw clay soil.

Atterberg limits	Liquid limit (LL) (%)	Plastic limit (PL) (%)	Plasticity index (PI) (%)	Degree of plasticity	Type of soil
Clay soil	29.9	14.6	15.3	Low plasticity	Silty clay

Standard proctor test	CB-0%	OPMFB-1%		OPMFB-5%	OPMFB-10%
*OMC (%)	17	16.8		17.7	18.2
**MDD (g/cm³)	1.75	1.74		1.71	1.69

Specific gravity		Clay soil		OPMF
		2.56		1.31

Particle size distribution of soil		Sand content (0.075–4.75 mm) (%)		Fine grained (< 0.075 mm) (%)
		57		43

*OMC: Optimum Moisture Content.

**MDD: Maximum Dry Density.

The Atterberg Limit Test was conducted to evaluate the plastic limit (PL) and liquid limit (LL) of the clay soil. The clay soil exhibited a PL of 14.6%, an LL of 29.9%, and a plasticity index (PI) of 15.3%, indicating low plasticity and classifying it as silty clay. These values align with established standards from Mueller et al.⁴³ and ILO⁴⁴. The proportions of sand and fine-grained soil in the mixture are critical for ensuring brick strength, stability, ease of molding, and effective firing.

Optimal water content prior to brick production is essential for achieving uniform consistency and homogeneity in the mixture. As shown in Table 4, the incorporation of OPMF, a lignocellulosic material, correlates directly with the required optimum moisture content (OMC). Higher OPMF content necessitates increased OMC. For instance, mixes with 1% (OPMFB-1%), 5% (OPMFB-5%), and 10% (OPMFB-10%) OPMF require OMC values of 16.8%, 17.7%, and 18.2%, respectively. Thus, precise adjustment of OPMF content and corresponding water content is vital to maintain workability and structural uniformity during molding.

Particle density analysis revealed a marked difference between clay soil (2.56 g/cm³) and OPMF (1.31 g/cm³). The lower density of OPMF, attributed to its porous structure and lignocellulosic composition⁴⁷, complicates its integration with alumina during mixing. Additionally, the fibrous nature of OPMF may weaken interparticle bonding, potentially compromising brick strength if not adequately addressed in the mixing process.

Appearances of manufactured bricks

The visual quality of bricks significantly influences both aesthetics and product performance. As illustrated in Fig. 5, the CB-0% and OPMFB 1–10% bricks produced in this study exhibit uniform rectangular surface, texture and uniform size. These attributes demonstrate the precision of the manufacturing process in achieving targeted specifications, aligning with findings from earlier studies by Gosselain⁵¹ and Grim & Johns⁵², which validate the methodology employed here.

Fig. 5.

Appearance of fired clay bricks incorporating different OPMF waste contents (0–10%).

Post-firing observations also revealed minimal surface cracks or deformities, underscoring the structural resilience of the bricks and their ability to endure thermal stresses during production. This durability is essential for ensuring long-term performance of the bricks in various applications. However, comparative analysis highlighted increased porosity in OPMFB bricks relative to CB-0%. While moderate porosity can enhance thermal insulation properties⁵³, excessive voids may weaken structural integrity and compromise load-bearing capacity. To address this, further optimization of OPMFB brick formulations is recommended to balance porosity with mechanical strength and compliance with industry standards.

The color of bricks, a crucial factor in visual appeal, is primarily determined by the chemical composition of raw materials, particularly iron oxide (Fe₂O₃) content. As visually confirmed in Fig. 5, the OPMFB bricks exhibit a progressively deeper reddish hue compared to the lighter-toned CB-0% (0%) brick, with the effect being most pronounced at the 10% substitution level. This color distinction is directly linked to the formation of hematite (Fe₂O₃) during the high-temperature firing process 1050 °C. The XRD analysis confirms the persistence of hematite in both the control and OPMFB-5% bricks, a mineral phase known to impart the characteristic red color to fired clay products⁵³. The intensified red color in OPMFB bricks is hypothesized to result from a more complete oxidation of the iron compounds in the clay matrix. This enhanced oxidation is facilitated by the combustion of the OPMF organic matter during firing, which generates heat and supplies oxygen, thereby promoting the conversion of all iron compounds to the red, stable hematite (Fe₂O₃). Beyond visual appeal, color serves as an indicator of compositional characteristics, offering insights into material properties and firing behavior.

Properties of manufactured bricks

A detailed comparative evaluation of CB-0% and OPMFB brick properties is presented in Table 5, encompassing critical parameters such as firing shrinkage, dry density, water absorption, compressive strength, thermal conductivity, and porosity. This in-depth assessment provides valuable insight into the performance and properties of both brick types.

Table 5.

Summary of test results on CB-0% and OPMFB.

Properties	Bricks
	CB-0%	OPMFB-1%	OPMFB-5%	OPMFB-10%
Firing shrinkage (%)	0.3	0.8	1.3	1.6
Dry density (kg/m3)	1799	1741	1708	1563
Water absorption (%)	3	11	17	20
Compressive strength (MPa)	24.6	19.3	15.4	4.1
Thermal conductivity (W/m.K)	0.54	0.46	0.38	0.34
Porosity (%)	13	14	20	25

Firing shrinkage, a critical indicator of brick durability and structural strength, was analyzed for CB-0% and OPMFB bricks (Fig. 6). CB-0% bricks demonstrated minimal shrinkage (0.3%), whereas OPMFB bricks containing 1 to 10% waste materials showed increased shrinkage, ranging from 0.8% to 1.6%. The low shrinkage in CB-0% bricks is linked to their fine-grained clay soil, which reduces water demand during mixing, thereby limiting shrinkage. Optimal moisture content in the mixture is vital for minimizing shrinkage, as excess water during production correlates with higher shrinkage rates. These findings underscore the need to meticulously regulate clay composition and water content to manage shrinkage during manufacturing¹¹. In OPMFB bricks, elevated shrinkage is partly driven by the lignocellulosic nature of OPMF, which contains high levels of cellulose, hemicellulose, and lignin. During firing, the thermal decomposition of all organic matter (cellulose, hemicellulose, lignin, and extractives) releases volatile gases, forming internal voids and thereby amplifying shrinkage¹⁰.

Fig. 6.

Firing shrinkage of manufactured brick at different waste content.

While observed shrinkage values (0.3 to 1.6%) remain below typical industry thresholds (2.5 to 4%), acceptable limits may vary based on application-specific requirements^54,55. For instance, bricks used in high-temperature environments may necessitate stricter shrinkage control. Although gradual, uniform drying and firing protocols can mitigate stress and excessive shrinkage, these methods may prolong production timelines and raise costs, a significant challenge for large-scale operations. Notably, incorporating OPMF into fired bricks, combined with appropriate alumina additions, can counteract shrinkage despite its lignocellulosic properties, highlighting opportunities for sustainable material integration^4,10,47.

Figure 7 reveals a distinct inverse relationship between OPMF content and the dry density of bricks. Increasing the OPMF percentage from 1% to 10% resulted in a marked decline in density, from 1741 kg/m³ to 1563 kg/m³, compared to the CB-0% brick, which exhibited the highest density (1799 kg/m³). This trend is largely driven by the lignocellulosic composition of OPMF. During firing, the organic matter of cellulose, hemicellulose, and lignin decomposes at high temperatures, generating macro- and micropores within the brick matrix and thereby reducing overall density^47,56,57.

Fig. 7.

Dry density of manufactured brick at different waste content.

The dry density of all tested bricks (1563 to 1799 kg/m³) fell below the conventional benchmark of 2000 kg/m³ for standard bricks⁵⁸. This deviation arises from factors such as raw material composition such as soil type, OPMF incorporation and firing conditions^4,10,59. While the addition of OPMF lowers density, it offers practical benefits, including enhanced mixture workability, reduced material weight, lower transportation costs, and improved thermal insulation properties⁴. However, the implications of reduced density must align with application-specific needs. For instance, lower density may compromise load-bearing capacity in structural applications but prove advantageous in lightweight construction, insulation systems, or scenarios prioritizing weight reduction. Balancing these trade-offs is essential to optimize material performance for desired use cases.

The findings presented in Fig. 8 demonstrate a progressive decline in the compressive strength of bricks with increasing proportions of oil palm mesocarp fiber (OPMF). Bricks manufactured without OPMF exhibited a compressive strength of 24.6 MPa, whereas incorporating 1 to 10% OPMF resulted in a reduction to values ranging from 19.3 to 4.1 MPa. This trend is likely due to the decomposition of organic constituents during firing, which compromises the bonding between clay and waste material and induces pore formation, thereby weakening structural integrity^4,59. The clay soil matrix, which provides the primary structural bonding, contains SiO₂ and Al₂O₃ (Table 2) at levels that align with brick production standards. The resultant reduction in strength is directly related to the pore-forming role of the OPMF’s high organic content (high LOI), which reduces the overall densification and structural integrity of the ceramic body. Additionally, elevating firing temperatures improved compressive strength, likely by promoting densification and microstructural refinement. OPMF into composites typically lowers compressive and tensile strengths due to weak fibre–matrix bonding, while flexural strength may improve at low fibre contents before declining beyond the optimum level. The addition of OPMF also reduces density, yielding lighter composites. These outcomes reflect both the structural limitations and the sustainability benefits of OPMF, especially where weight reduction is desirable^4,43,60. OPMF into composites typically lowers compressive and tensile strengths due to weak fibre–matrix bonding, while flexural strength may improve at low fibre contents before declining beyond the optimum level. The addition of OPMF also reduces density, yielding lighter composites. These outcomes reflect both the structural limitations and the sustainability benefits of OPMF, especially where weight reduction is desirable.

Fig. 8.

Compressive strength of manufactured brick at different waste content.

The experimental bricks met industry requirements for multiple applications: load-bearing walls (≥ 7 MPa for Type 1, ≥ 14 MPa for Type 2), non-load-bearing partitions (≥ 1.4 MPa), and internal load-bearing walls (≥ 5.2 MPa)^38,61. The emphasis is placed on the fact that while the bricks meet minimum requirements for non-load-bearing applications 1.4 MPa for non-load-bearing partitions, 5.2 MPa for internal load-bearing walls, the 4.1 MPa strength at 10% substitution is significantly lower than typical structural standards. This result is contrasted with literature that often reports a maximum substitution of 3 to 7.5% to maintain structural integrity. This strengthens the conclusion that 10% OPMF is only suitable for non-load-bearing applications^10,56,62. This contrast underscores OPMF’s distinct properties and its role in preserving structural performance. However, further evaluation of other material characteristics remains essential when integrating higher organic waste volumes.

Water absorption and porosity, critical determinants of brick durability, are inherently linked to their porous structure, which governs moisture exchange with mortar^43,63. For optimal bonding and quality, bricks should exhibit low water absorption. However, introducing OPMF significantly increased water absorption rates. As illustrated in Fig. 9, bricks containing 1 to 10% OPMF displayed water absorption values of 11 to 20%, far exceeding the 3% observed in control bricks. This rise is attributed to pore formation caused by the combustion of organic matter during sintering, a phenomenon well-documented in prior research^56,57. Beyond OPMF content and raw material composition, factors such as firing temperature, duration, waste type and quantity, residual ash or carbon content, and testing methodology further influence water absorption^43,57,62. In this study, the heightened absorption primarily stems from OPMF’s lignocellulosic nature, which amplifies porosity.

Fig. 9.

Water absorption and porosity of manufactured brick at different waste content.

While British Standards classify Engineering Bricks A and B by water absorption thresholds of ≤ 4.5% and ≤ 7.0%, respectively³⁶, the bricks here surpassed these limits, rendering them unsuitable for high-load scenarios. However, their absorption rates align with ASTM standards for moderately weather-resistant bricks (≤ 22%)³⁸, underscoring the need to minimize water penetration to enhance durability.

Porosity trends mirrored absorption behavior, with control bricks (CB-0%) exhibiting 13% porosity, rising to 15 to 25% for 1 to 10% OPMF incorporations (Fig. 9). These values are notably lower than the 20 to 40% reported by Arslan et al.⁵⁴ in cellulose-fiber-enhanced bricks, highlighting the material-specific impact of waste type on porosity. High-temperature firing, which combusts organic additives, generated both open and closed pores, corroborating earlier studies on pore formation mechanisms^57,62. While organic waste can act as a pore-forming agent while reducing density and thermal conductivity, excessive incorporation may also risks compromising strength and absorption properties^43,56. Therefore, it is important to strike a balance between the positive pore-forming effects of organic waste and maintaining the strength and water absorption capacity of bricks. This balance can be achieved by carefully selecting the organic waste and optimising the firing parameters.

The relationship between water absorption and porosity, clarifying that higher porosity contributes to improved thermal insulation but also increases water uptake, which restricts durability under heavy-duty or external exposure conditions. To address the practical implications, we have specified the realistic applications of OPMF-incorporated bricks. While the water absorption values (11 to 20%) exceed the limits for engineering bricks, these bricks are suitable for non-load bearing applications such as interior partition walls, thermal insulation layers, and affordable housing in rural or low-rise construction, where reduced density and improved thermal comfort are beneficial.

Thermal conductivity of manufactured bricks

Thermal performance critically influences the suitability of construction materials in diverse thermal environments. As shown in Fig. 10, bricks without OPMF (CB-0%) exhibited a thermal conductivity of 0.54 W/m.K, while those incorporating 1 to 10% OPMF demonstrated a progressive reduction to 0.46–0.34 W/m.K. This marked decrease highlights OPMF’s potential as an effective thermal insulator, aligning with findings by Maafa et al.⁷ and Arslan et al.⁵⁶, who reported comparable improvements using cellulose-rich biomass waste. The observed improvement in thermal insulation is directly related to the high cellulose content in OPMF, which introduces porosity into the brick matrix. This relationship is reinforced by porosity data from Sect. 3.5, where 10% OPMF bricks achieved 25% porosity, which the highest among tested compositions, correlating with their lowest thermal conductivity (0.34 W/m.K). The synergy between cellulose-induced porosity and thermal performance underscores OPMF’s strategic role in optimizing insulation. Notably, the inverse trend between OPMF content and thermal conductivity emphasizes the importance of balancing additive levels to preserve structural strength while maximizing energy efficiency.

Fig. 10.

Thermal conductivity of manufactured brick at different waste content.

The incorporation of OPMF produced both favourable and unfavourable effects on brick performance, and these contrasting trends closely align with outcomes reported in previous research on biomass-incorporated fired clay products. The reduction in density (1799–1563 kg/m³) and thermal conductivity (0.54 to 0.34 W/m·K) observed in this study is consistent with reported improvements in insulation when cellulose-rich agricultural wastes are used as pore-forming additives. Studies by Maafa et al.⁷ and Kizinievič et al.¹⁰ similarly attributed enhanced thermal performance to increased porosity arising from the combustion of organic components. In this regard, the stronger performance of OPMF-incorporated bricks in enhancing thermal insulation falls within the upper range of improvements documented for other agro-waste additives.

However, the reduction in compressive strength with increasing OPMF content, from 24.6 MPa to 4.1 MPa, reflects a well-established trade-off between porosity generation and structural integrity. Comparable strength reductions have been reported in bricks incorporating sawdust, rice husk, and sugarcane bagasse, especially when waste additions exceed 5 to 7.5%, where excessive pore formation and weak fibre–matrix bonding become limiting factors. Notably, the strength values at 1% and 5% OPMF (19.3 and 15.4 MPa) remain competitive with or superior to values reported in several prior studies using similar biomass wastes, indicating that moderate OPMF incorporation can retain mechanical performance while still offering functional benefits.

In contrast, the higher water absorption (11 to 20%) and porosity (14 to 25%) observed in this study represent the weaker performance outcomes and correspond with the challenges frequently highlighted for biomass-modified bricks. While some studies achieved lower absorption rates through optimized firing or finer particle sizes, others reported similar or greater increases when using lignocellulosic materials with high volatile content. The findings here therefore fall squarely within the expected behaviour of organic waste–based fired bricks, although they emphasize the need for future optimization such as fibre pre-treatment, finer size control, or hybrid additives to improve moisture resistance.

Overall, the comparison with existing literature demonstrates that the performance trends observed in this study improved thermal efficiency and reduced density but reduced strength and higher absorption at elevated substitution levels are fully consistent with the established behaviour of biomass-modified fired clay bricks.

Microstructure analysis

The incorporation of OPMF significantly altered the bricks’ porosity and microstructure. As illustrated in Fig. 11a, the SEM image of OPMFB-5% at 5000x magnification displays a dense, interconnected pore network. Pore dimensions increased notably (from 0.05 to 0.50 mm with 5% OPMF addition), exhibiting variability in size and shape, including elongated forms. The increase in porosity, quantitatively confirmed by the water absorption test (Fig. 9), results from the combustion of the OPMF waste. Larger, irregular pores likely resulted from the volume occupied by the rapid decomposition of the bulk organic matter during firing, while smaller, elongated voids 0.34 to 1.09 μm in length, Fig. 11b) stem from the burnout of the individual OPMF fibers, retaining the residual shape of the original lignocellulosic particles (1 to 3 mm length, 0.20 to 0.45 mm diameter).

Fig. 11.

Microscopy image of OPMFB-5% under (a) Optical microscope 10x magnification and (b) SEM image 5000x magnification.

The enhanced water absorption (Sect. 3.5) strongly supports the morphological observation of an increase in open and interconnected porosity resulting from the OPMF burnout. In addition, quartz formation was evident in OPMFB-5%, marked by white spots similar to those in CB-0%. This aligns with studies identifying quartz as a common component of raw materials^64,65. The presence of these quartz particles in Fig. 11a likely originated from the unmelted crystalline silica in the raw clay and OPMF-10%, which is stable at the firing temperature of 1050 °C⁶⁵. Figure 11b highlights elongated, interconnected pores (0.34 to 1.09 μm), directly attributed to organic material combustion during firing. Surface cracks were also observed, potentially induced by thermal stresses.

Study on the occurrence of chemical and thermodynamic reaction during firing brick

X-ray diffraction (XRD) analysis of the CB-0% sample identified thermally stable minerals such as quartz (SiO₂), muscovite [KAl₂(AlSi₃O₁₀)(F, OH)₂], kaolinite [Al₂(Si₂O₅)(OH)₄], and hematite (Fe₂O₃), which persisted across the entire temperature range studied. These findings highlight the mineralogical resilience of the sample and underscore their relevance for diverse industrial applications.

Complementary TGA-DTA analysis of CB-0% revealed a cumulative mass loss of 0.52 mg during firing. Initial weight reduction (0.20 mg) at 38.6 °C corresponded to the evaporation of mechanically bound water. Exothermic decomposition of organic matter occurred between 200 and 400 °C, followed by endothermic dehydroxylation of clay minerals at 438.4 °C, contributing to structural mineral enhancement. A further mass loss of 0.32 mg between 173.3 and 646.9 °C coincided with oxidation and nascent phase formation, evidenced by a weak exothermic peak at 600 °C. Complete vitrification of the sample was confirmed at 901.8 °C.

In OPMFB-5% (Fig. 12), dominant phases included quartz, hematite, kaolinite, and muscovite, alongside newly formed jacobsite (MnFe₂O₄) and chromia (Cr₂O₃). Quartz and hematite exhibited no notable morphological or chemical changes, retaining stability up to the maximum temperature. Kaolinite underwent dehydroxylation near 500 °C, producing amorphous metakaolinite (Al₂O₃.2SiO₂). Hematite formation was linked to magnetite (Fe₃O₄) dissolution from Fe-rich muscovite, consistent with Monazam et al.⁶⁶, with subsequent oxidation converting residual magnetite to hematite. Muscovite peaks (19.9°, 29.8°, 37.8°, 61.6° 2θ) diminished progressively with heating, while jacobsite and chromia emerged at 800 °C.

Fig. 12.

XRD pattern of OPMFB-5%.

Jacobsite crystallization at 900 °C (42.1° 2θ) likely resulted from hematite reacting with manganese (II) oxide (identified via XRF in Table 2), aligning with Mao et al.⁶⁷, where it stabilized into a robust spinel structure. Chromia formation at 800 °C may stem from lower-temperature decomposition of chromium oxides, as proposed by Li et al.⁶⁸ and Wei et al.⁶⁹. Upon reaching the vitrification threshold, clay particles fused, creating a durable matrix upon cooling. The persistence of quartz at high temperatures further corroborated extensive vitrification.

As depicted in Fig. 13, the thermal profile of OPMFB-5% reveals a total weight loss of 0.95 mg, equivalent to approximately 9.5% of the sample mass during firing. The initial phase (10.1 to 165.9 °C) shows a 0.35 mg mass reduction attributed to the evaporation of mechanically bound water, marked by an endothermic peak at 41.1 °C. A subsequent phase exhibits a 0.60 mg loss driven by overlapping exothermic and endothermic processes. A pronounced exothermic peak at 219 °C signifies the initial and rapid decomposition of the organic matter (primarily hemicellulose and extractives) and mineral compounds, which aligns with the initial TGA mass loss of OPMF (Fig. 4). This major exothermic event confirms the complete degradation of the most volatile lignocellulosic components (hemicellulose and a portion of cellulose) below 310 °C. However, as the OPMF TGA shows oxidation of more stable components like lignin and residual carbonaceous char will extend to higher temperatures. This suggests residual clay breakdown likely continues at 454.3 °C (Fig. 13).

Fig. 13.

TGA-DTA of OPMFB-5%.

At 508.9 °C, a minor endothermic peak corresponds to kaolinite dehydroxylation, forming amorphous metakaolinite, alongside quartz inversion, a thermally induced structural rearrangement causing linear brick expansion. Between 700 and 900 °C, fluctuating exothermic reactions arise from crystallization of jacobsite (MnFe₂O₄) and chromia (Cr₂O₃), as observed in earlier XRD results. By 900 °C, organic residues are fully combusted, signaling brick maturity. At this stage, vitrification dominates, melting and fusing clay particles to form a dense, high-strength ceramic matrix upon cooling.

Leaching analysis of manufactured brick

Prior assessments of physical and mechanical properties revealed that incorporating up to 5% oil palm mesocarp fiber (OPMF) and firing at a controlled heating rate of 1 °C/min enhances brick performance. Consequently, a leaching analysis was conducted to evaluate the potential environmental risks of heavy metal release from these optimized OPMF bricks. Such testing is critical before integrating waste materials into construction products, as improper disposal practices may introduce heavy metals into raw materials, posing contamination risks. Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) tests were employed to simulate leaching behavior under varying environmental conditions and verify heavy metal immobilization.

A preliminary step involved identifying heavy metals in raw materials to compare concentrations pre- and post-incorporation into bricks (Table 6). The clay soil sample showed elevated lead (Pb: 0.586 ppm) and chromium (Cr: 0.690 ppm) levels, surpassing thresholds set by USEPA⁷¹ and WHO⁷⁰. However, zinc (Zn: 0.483 ppm) aligned with USEPA standards. Notably, while OPMF met USEPA guidelines, it exceeded WHO limits for Pb (2.790 ppm), Cr (0.586 ppm), and Zn (4.700 ppm). This accumulation in OPMF is linked to agricultural practices, particularly excessive fertilizer use, as noted by Manan et al.⁷² and Azura et al.⁷³. The latter further emphasizes that metals such as cadmium and zinc can persist in soil systems and subsequently be absorbed by plants via adsorption processes.

Table 6.

Heavy metal concentration of raw materials.

Chemical	Heavy metal concentration (ppm)		USEPA	WHO
	Clay soil	OPMF	(ppm)	(ppm)
Pb	0.586**	2.790**	5.0	0.11
Cr	0.690**	0.586**	5.0	0.05
Zn	0.483	4.700**	n.a	3.00
* Not complied USEPA [70]

**Not complied WHO⁷⁰.

All leachate concentrations were below the permissible thresholds set by USEPA (Pb and Cr: 5.0 ppm) and WHO (Pb: 0.11 ppm, Cr: 0.05 ppm, Zn: 3.0 ppm), underscoring that the immobilization of heavy metals in OPMF-incorporated bricks ensures compliance with international safety standards and minimizes risks of soil and groundwater contamination.

Using the TCLP method (Table 7), zinc (Zn) emerged as the primary metal leached from the control brick (CB-0%), with a concentration of 0.088 ppm, followed by lead (Pb: 0.001 ppm) and chromium (Cr: 0.003 ppm). Incorporating 5% OPMF reduced Pb and Cr leaching to 0.001 ppm and 0.007 ppm, respectively, though Zn leaching rose slightly to 0.118 ppm (a 34.09% increase). Similarly, SPLP tests revealed Zn as the most leachable metal from CB-0% (0.109 ppm), followed by Cr (0.008 ppm) and Pb (0.001 ppm). The addition of OPMF significantly lowered Pb and Cr leaching to 0.001 ppm and 0.007 ppm, respectively, while Zn leaching increased to 0.147 ppm.

Table 7.

Heavy metal analysis using TCLP method.

Chemical	TCLP method		SPLP method		USEPA	WHO
	CB-0%	OPMFB-5%	CB-0%	OPMFB-5%	(ppm)	(ppm)
Pb	0.001	0.001	0.001	0.001	5.0	0.11
Cr	0.003	0.007	0.008	0.007	5.0	0.05
Zn	0.088	0.118	0.109	0.147	n.a	3.00

*Not complied USEPA.

**Not complied WHO.

Both leaching methods confirmed that Pb levels remained within USEPA and WHO regulatory limits, indicating effective stabilization during firing. This is linked to hematite (Fe₂O₃) in the clay, formed via dehydroxylation and oxidation of iron oxides⁶⁸, which reacts with PbO to generate lead ferrite (Pb₂Fe₂O₅), encapsulating Pb in the brick matrix at high temperatures, as observed by Lu et al.⁷⁴. However, XRD analysis in this study did not detect Pb₂Fe₂O₅, potentially due to its thermal decomposition, as hypothesized in prior work⁷⁴. Chromium leaching also met regulatory standards, consistent with findings that firing converts CrO₃ to stable Cr₂O₃ at 900 °C, trapping Cr within the matrix⁶⁸. This conversion takes place at around 900 °C, whereby the Cr is trapped in the Cr₂O₃ structure and the Cr concentration in all brick samples is below the permissible limits. Zinc, though more prone to leaching, is hypothesized to form stable silicate or aluminate phases during vitrification, as reported by Li et al.⁶⁸.

Overall, heavy metal leaching from both CB-0% and OPMFB-5% bricks complied with toxicity and drinking water standards, with OPMF inclusion reducing metal concentrations in most cases. The firing process effectively immobilized metals by forming stable mineral phases upon reaching vitrification temperatures, aligning with previous studies^68,74. These results underscore the viability of OPMF integration in brick production, balancing enhanced material properties with minimal environmental risk.

Engineering relevance

The engineering relevance of incorporating OPMF into fired clay bricks. Specifically, OPMF contributes to reduced density and enhanced thermal insulation, which are advantageous for improving energy efficiency in buildings. However, the decline in compressive strength and increased water absorption at higher substitution levels restricts their use in structural or heavy-duty applications. Consequently, the engineering relevance of OPMF-incorporated bricks lies in their suitability for non-load bearing, partitioning, thermal insulation, and cost-effective construction materials, particularly in regions with abundant OPMF waste.

Conclusion

This study evaluated the feasibility of utilizing OPMF as a partial clay substitute in brick production. Key findings and implications are summarized as follows:

1. Enhanced Lightweight and Thermal Properties: Incorporating up to 5% OPMF significantly reduced brick density (from 1799 to 1708 kg/m³) and thermal conductivity (from 0.54 to 0.38 W/m.K), yielding lighter bricks with improved thermal insulation, which could enhance energy efficiency in buildings.

2. Porosity and Shrinkage Trends: OPMF addition increased porosity (13% to 25%), contributing to density reduction and thermal performance. While firing shrinkage rose slightly (0.3% to 1.6%), this can be mitigated through optimized raw material preparation.

3. Durability and Mechanical Trade-offs: Higher OPMF content led to elevated water absorption (3% to 20%), posing potential risks to long-term moisture resistance and structural integrity. Compressive strength also declined markedly (24.6 to 4.1 MPa), limiting load-bearing applications and necessitating careful suitability assessment.

4. Mineralogical Composition: The control brick (CB-0%) retained thermally stable minerals (quartz, muscovite, kaolinite, hematite). Introducing OPMF introduced jacobsite and chromia into the matrix, suggesting a complex, multifunctional material in the OPMFB-5% sample.

5. Environmental Safety: Leaching tests confirmed that lead (Pb) and chromium (Cr) concentrations in all bricks complied with USEPA and WHO standards, demonstrating effective immobilization during firing. Zinc (Zn), though the most leachable metal, remained within permissible drinking water limits.

In summary, integrating 5% OPMF offers a balanced compromise, enhancing lightweight and thermal properties while maintaining acceptable durability and mechanical performance. Although bricks with up to 5% OPMF substitution comply with ASTM thresholds, they fall short of stricter British Standards, particularly regarding strength and water absorption. Therefore, OPMF-incorporated bricks are best suited for non-load-bearing applications, thermal insulation, and affordable housing where energy efficiency and lightweight construction are prioritized. Future research should prioritize refining OPMF pre-treatment methods, optimizing firing protocols, and exploring complementary additives to counteract strength loss and moisture susceptibility. Such advancements could unlock the full potential of OPMF as a sustainable brick material, aligning eco-friendly waste valorization with construction innovation.

Author contributions

Amir Detho, Noor Amira Sarani: Conceptualization, Data curation, formal analysis, Investigation, Methodology, Validation, Visualization, Writing, Original draft preparation.Aeslina Abdul Kadir: Formal analysis, Data curation, Methodology, ConceptualizationMohd Fadhil Md Din, Nur Fatin Nabila Hissham: Writing-Reviewing, and EditingNur Fatihah Tajul Arifin, Mohd Khairolden Ghani, Hesham Hussein Rassem: Software, Resources, Formal analysis, Data curation.

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.