Enhancing the residual strength of clay bricks exposed to elevated temperatures using metal oxides, carbon black, and sodium hydroxide activation

1 Introduction

One of the most crucial challenges in the building industry today is energy conservation (Moujoud et al., 2023). According to reports, buildings use one-third of the world’s energy, yet because building materials have poor thermal insulation (Kazmi et al., 2018a), half of that energy is lost (Moujoud et al., 2023; Muheise-Araalia and Pavia, 2021). In fact, building materials have been extensively developed to enhance thermal insulation and maximize buildings’ energy efficiency (Moujoud et al., 2023). The use of inexpensive, lightweight, and environmentally friendly thermal insulating materials has lately gained popularity in the perspective of sustainability (Moujoud et al., 2023; Mendes et al., 2019). Because of its exceptional mechanical and physical qualities, particularly its strength, durability, and fire resistance (Moujoud et al., 2023; Kadhim et al., 2026; Revelo H et al., 2026), fired clay brick is one of the most popular building materials in the world (Mendes et al., 2019; Revelo H et al., 2026). In fact, a combination of clay, silica sand, feldspars, and water are the inexpensive and plentiful raw ingredients used in the straightforward manufacturing process of burnt clay brick (Topçu and Işikdaǧ, 2007; Muñoz et al., 2020). Approximately 1,391 billion bricks are produced worldwide each year, and this number is predicted to rise steadily as a result of the growing demand in the building industry (Muñoz et al., 2020; Gao et al., 2023; Statkauskas et al., 2023). Fired bricks actually lose a lot of heat through the walls of the house because of their low thermal conductivity value, which is typically 1.0 ± 0.4 W/(m.K) depending on the raw materials and the firing procedure (Moujoud et al., 2023; El Fgaier et al., 2015; Zhao Z. et al., 2026). The formation of a ceramic bond during fire, in which partial vitrification of clay particles produces interparticle necks that provide structural stability, is closely related to the mechanical behavior (Sukmak et al., 2013). Fluxing oxides influence the densification trajectory and final strength development by encouraging liquid-phase sintering at lower temperatures (Wang et al., 2021). The working conditions of clay bricks are determined by the pore structure, mineralogical stability, and resistance. Properly fired bricks have low water absorption, high resistance to freezing and thawing, and chemical stability (Mendes et al., 2019; Seddik Hassan et al., 2022; Riaz et al., 2026). However, bricks often contain free lime (CaO), which increases the risk of brick expansion due to portlandite (Ca(OH)₂) formation (Moujoud et al., 2023; Forget et al., 2015). Understanding the basic connections between composition, processing, and performance in clay brick technology is crucial because of the interaction between these durability variables and the inherent material qualities (Munir et al., 2018; El Hammouti et al., 2022).

The response of clay bricks to the action of high temperatures is of interest in questions of fire safety, technology of high-temperature processes, and the strength of bricks after fires (Zhu et al., 2025). The mechanical properties of clay bricks are subject to complex physicochemical processes when exposed to high temperatures (Zhu et al., 2025). The mechanical properties of clay bricks are subject to complex physicochemical processes when exposed to high temperatures (Zhu et al., 2025). Physically bound water evaporates at 200 °C, which leads to the fact that bricks either practically do not decrease in strength or even increase due to the polymerization of clay particles (Forget et al., 2015). At 400 °C, dihydroxylation of kaolinite occurs, which is accompanied by the removal of crystallization water and the formation of metakaolin (Forget et al., 2015; Zhu et al., 2025). As a result of this process, there is a decrease in mass, strength, and an increase in porosity, depending on the qualitative composition of the brick (Forget et al., 2015). At 600 °C, dihydroxylation of kaolinite is completed, and the polymorphic transition of quartz begins (Kaze et al., 2023). As a consequence of the polymorphic transition, the volume of the brick expands, and intra-grain cracks form, which reduce the strength of the brick (Kaze et al., 2023). At 800 °C, metakaolin is transformed into spinel and mullite, which are formed at 1,200 °C, thereby increasing the strength of the brick (Forget et al., 2015; Gavali et al., 2019).

Clay bricks exposed to this temperature can retain approximately 48%–91% of their original compressive strength (Mendes et al., 2019; Seddik Hassan et al., 2022). At 1,000 °C and Above, extensive vitrification and continued mullite crystallization initially improve ceramic bonding (Zhu et al., 2025; Abdellaoui et al., 2026). However, at higher temperatures, glass-phase migration, thermal incompatibility, and grain-boundary weakening significantly reduce mechanical strength (El Hammouti et al., 2022). Comparative studies show that clay bricks outperform other masonry materials under fire exposure, retaining up to 74.4% of their original strength after ISO 834 fire testing while exhibiting the lowest increase in porosity (Topçu and Işikdaǧ, 2007; Muñoz et al., 2020). Their superior thermal stability highlights the potential for further enhancement through advanced material modification techniques (Sukmak et al., 2013; Wang et al., 2021). Geopolymer brick, another name for alkali-activated brick, is created by mixing, molding, and curing at either room temperature or a higher temperature (Akintola et al., 2020). Alkali-activators like sodium hydroxide (NaOH) (Khairy, 2026), potassium hydroxide (KOH), lithium hydroxide (LiOH), and silicates of potassium and sodium are used to activate the brick’s source materials, which include silica, alumina, and fillers or one aggregates (Statkauskas et al., 2023; Gavali et al., 2019; Aurelie Tchouateu Kamwa et al., 2023). Industrial wastes that include significant amounts of silica and alumina, such as ashes (FA, RHA, SCBA, BA), GGBS, and surkhi (waste from burnt clay brick), serve as source materials for alkali activation (Muheise-Araalia and Pavia, 2021; Statkauskas et al., 2023).

Alkali activation is an effective method for producing cementitious materials from aluminosilicate precursors, with sodium hydroxide (NaOH) serving as the primary activator (Statkauskas et al., 2023; Gavali et al., 2019). Hydroxide ions dissolve aluminosilicate species, which subsequently polymerize to form binding gels (García-Díaz et al., 2024) (Zhou et al., 2024). In clay-based systems, reactivity depends mainly on mineral composition and thermal treatment, with heat-treated kaolinite (metakaolin) exhibiting the highest reactivity (Seco et al., 2026; Al Sayed and Abdelkhalik, 2026). Depending on precursor composition, alkali activation forms either N-A-S-H gel in low-calcium systems or C-A-S-H gel in calcium-rich materials (Cardoza and Colorado, 2024). Moreover, the structure and properties of alkali-activated binders were predominantly dependent on NaOH concentration, curing regime, and precursor characteristics (Aurelie Tchouateu Kamwa et al., 2023). While the increased concentration of activating solution improved the geo-polymerization degree, the excessive amount of alkali initiators could provoke the formation of surface layers detrimental to the curing process (Statkauskas et al., 2023) (Khairy, 2026).

The utilization of NaOH for brick manufacturing was confirmed to produce the material with satisfactory properties, which could be achieved at relatively low curing temperatures (Gavali et al., 2019; García-Díaz et al., 2024). However, the activity of clay mineral was dependent on its composition, making the addition of sodium silicate a necessary condition for some clay types (Ahmed et al., 2018). Previously conducted studies showed that the mechanical properties of activated clay bricks were benefited from the increased NaOH concentration and firing temperature, the optimal values of which were dependent on the mineral composition of raw materials (Statkauskas et al., 2023; Kaze et al., 2023; Seco et al., 2026).

The introduction of metal oxides into the ceramic mass is one of the traditional ways to improve the technological properties and the characteristics of the finished ceramic material (Alshammari et al., 2026). Each metal oxide, when added to the ceramic composition, has its own effect on the composition and structure of the finished material and affects the sintering process (Kaze et al., 2023; Thejas and Hossiney, 2022). For example, iron oxide is one of the main components of many types of natural clay and plays an important role in firing, acting as a source of liquid phase and forming hematite crystals (Mendes et al., 2019) (Al Sayed and Abdelkhalik, 2026). The addition of Fe₂O₃ in an amount up to 5% reduces the porosity of fired bricks and increases their strength; however, an excess of this oxide has a detrimental effect on the properties of bricks (Thejas and Hossiney, 2022; Alshammari et al., 2026). On the other hand, the introduction of iron oxide allows for the reduction of the firing temperature and the improvement of technological properties of clay bricks such as density and thermal stability (Gao et al., 2023). The addition of MgO is known to increase the thermal stability of clay and improve its consistency, especially when a high percentage of fireclay is used (Cui et al., 2026). The addition of MgO is known to increase the thermal stability of clay and improve its consistency, especially when a high percentage of fireclay is used (Cui et al., 2026). Moreover, MgO increases the ceramic density and contributes to the formation of cordierite, which is beneficial for both thermal and chemical stability. In addition, it has a positive effect on the plasticity and workability of the ceramic mass, reduces its porosity, and increases its resistance to thermal shocks. Furthermore, MgO was proved to be a good binder for unfired bricks (Cui et al., 2026; Bian et al., 2026). However, the excess of MgO can lead to the formation of low-melting oxides that reduce the high-temperature properties of ceramic materials (Bian et al., 2026).

Al _**2** O _**3** significantly improves the refractoriness, mechanical strength, and thermal stability of clay bricks (Alshammari et al., 2026). Higher alumina contents increase load-bearing capacity at elevated temperatures and enhance resistance to thermal shock (Alshammari et al., 2026). Fireclay bricks containing 36%–48% Al₂O₃ exhibit excellent stability up to 1750 °C (Alshammari et al., 2026). Nevertheless, excessive alumina may reduce alkali resistance and increase shrinkage, while chamotte addition helps minimize cracking and improve dimensional stability (Riaz et al., 2026; Aurelie Tchouateu Kamwa et al., 2023). Calcium oxide acts as a fluxing agent that promotes densification through liquid-phase formation during firing (Alshammari et al., 2026). Its content, together with firing temperature and silica, strongly influences compressive strength and water absorption (Alshammari et al., 2026; Adazabra et al., 2023a). CaO also contributes to the formation of calcium aluminosilicate phases, such as anorthite and gehlenite, which improve brick durability (Alshammari et al., 2026) (Wei et al., 2024). However, excessive free lime may cause expansion and durability problems after hydration (Muheise-Araalia and Pavia, 2021). The enhancing combination of metal oxide addition and alkali activation represents a promising but insufficiently explored approach for enhancing the high-temperature performance of clay bricks. While each technique has been investigated independently, the interactive effects between metal oxide doping and alkali activation on the thermal stability and residual mechanical properties of clay bricks remain poorly understood. The enhancing mechanisms that may operate in combined metal oxide–alkali activation systems are multifaceted.

Metal oxide addition can affect the reactivity of clay towards alkali activation, participate in the gel formation process at the molecular level, and alter the physicochemical properties of the formed gel. It is supposed that the introduction of metal oxides into the system could change the nature of bonds in the gel structure, which would improve the thermal stability of the interfacial layer. Specifically, the presence of iron, magnesium, or calcium ions in the interfacial gel region can favor the crystallization of certain minerals during the heating stage. These minerals can strengthen the interfacial layer and improve its durability. Another proposed mechanism of action of metal oxide addition concerns the optimization of porosity in the interfacial layer. The reduction of large pores can be achieved by filling part of the brick pores with the interfacial gel or its filler particles. In addition, metal oxide addition could improve the mechanical properties of the interfacial gel itself. A strong interfacial layer can be formed if the interfacial gel interacts with bricks both at the molecular level (through the formation of chemical bonds) and at the structural level (through the interlocking of particles). Previous studies have primarily focused on understanding the structural changes in metal-oxide-doped bricks or alkali-activated bricks with the addition of various oxides. Metal oxide addition was mainly studied in the context of alkali-activated geopolymers. In that field of research, the effect of different oxides on the setting time, strength, and thermal stability of geopolymers was studied; however, the effect of metal oxide addition on the residual strength of clay bricks was poorly considered.

1.1 Research significance

Although considerable effort has been made to improve the characteristics of clay brick by using either oxide-doped additions or alkaline activation, the combined contribution of the last two was insufficiently studied. In particular, little is known about the performance of the clay bricks modified with oxide additives and simultaneously subjected to chemical activation by an alkaline solution at increased temperatures.

For example, the mechanical properties of chemically activated clay bricks were mostly studied at room temperature, while they should be used at much higher temperatures, e.g., in case of fires. This work proposes to dope clay by industrially available oxide-doped additions (like Al₂O₃, Fe₂O₃, etc.,) and activate the brick by sodium hydroxide aqueous solution. We propose to prepare such bricks in order to demonstrate the possibility to design a material with strength comparable to that of traditionally fired bricks but manufactured without firing at very high pressures and temperatures. As a result, it will be possible to both use locally available raw materials (clay) and reduce the negative impact of brick manufacturing on the environment due to the elimination of the firing stage. Besides, we will characterize the bricks’ performance in terms of mechanical strength, porosity, and phase transformations prior to and following the thermal treatment at temperatures up to 800 °C or higher. The information will allow us to estimate the temperatures at which the brick strength will start to decrease or, on the contrary, increase due to thermal treatment. These data are valuable since, to the best of our knowledge, the mechanical properties of the clay bricks subjected to chemical activation by alkaline solutions with multiple additives were rarely studied at high temperatures. We expect that our findings will contribute to the understanding of how the clay bricks will perform in case of fires at temperatures of over 400 °C (when the traditionally fired bricks have already significantly deteriorated). It will also allow for designing heat-resistant structures such as heat-exchangers or their linings, which can also be made from alkali-activated bricks.

2 Materials and methods

2.1 Alkali-activated brick development

One of the most common enterprises is the production of clay bricks, which mostly use natural raw resources that are accessible locally. In construction, this kind of brick is frequently utilized. According to Figure 1, the manufacturing process begins with extracting clay from quarries, then purifying it to remove impurities and large stones. It is then mixed with several additives to improve its workability and firing properties.

The traditional optimum mixture used in most brick factories consists of clay at a rate of 80%–85% by dry weight, with fine sand added at a rate of 10%–15% to control shrinkage and reduce cracking during drying and firing, the used clay chemical composition is presented in Table 1. In some modern factories, fillers or improvers such as fly ash, quarry dust, or ceramic waste are used, usually at rates ranging from 5% to 20%, to enhance mechanical properties and reduce the consumption of natural clay.

TABLE 1. Chemical compositions of used clay.

Also, geopolymerized clay brick, another name for alkali-activated brick, is created by mixing, molding, and curing at room temperature or at a higher temperature. Alkali-activators including sodium hydroxide (NaOH) and Sodium metasilicate (Na₂SiO₃) are used to activate the brick’s source materials, which include silica, alumina, and fillers or fine aggregates. Industrial wastes that include significant amounts of silica and alumina, such as ashes (FA) or surkhi (waste from burnt clay brick), serve as source materials for alkali activation.

NaOH combinations were utilized as simple activators. The molarity of the NaOH with Na₂SiO₃ were one of the key variables investigated in this analysis. There was only one focus used. Fixations of NaOH at a molarity of 8 (M) were reviewed. After the NaOH preparations were completed, they were left to stand for a full day before being mixed with further combination ingredients. Before being used in the geopolymerization cycle, the combination of sodium hydroxide configurations was allowed to represent 1 day. The chunk of NaOH had a molecular weight (M.W.) of 40 g/mol, a melting point of 318 °C, a thickness of 2.13 g/cm³, and a purity of 96%. Na₂SiO₃ sol. was acquired from a local manufacturer. It is composed of 17% Na₂O, 32% SiO₂, and 51% H₂O with a density of 1.63 kg/m³ and silica modulus (i.e., SiO₂ to Na₂O ratio) of 1.80. For instance, the Morgan Specialty Synthetic Compounds Organization in Al Obour, Egypt, sells a 50 kg bundle of sodium silicate basic.

The raw materials used in this study consisted of clay, sand, and fly ash. The reference mixture (Control) was formulated with the following composition: clay (82 wt%), sand (13 wt%), and fly ash (5 wt%). All percentages are expressed as weight fractions of the total dry solids. The oxides and carbon black were incorporated as mass replacements of clay, maintaining constant total solid mass. The oxides and carbon black were incorporated in two different methods: First, it was added as mass replacements of clay, maintaining constant total solid mass. Second, it was used as an additive to the mixture. This replacement/Additive approach was selected to evaluate the effect of each additive while maintaining consistency in total solids loading across mixtures. In this investigation, a local supplier called “Sika” provided Class F fly ash (FA_F), which was used as a pozzolanic powder. The specific gravity was 2.13 g/cm³, the bulk density was 1.06 g/cm³, the specific surface area was 6,347 cm²/g, and the soundness was 1.0 mm. FA_F had a specific gravity of 2.36, was gray in color, and was significantly lighter than PC. Table 2 lists the chemical compositions of used oxides and fly ash Class F.

TABLE 2. Chemical compositions of used oxides and fly ash Class F.

2.2 Metal oxides

The raw materials were characterized using X-ray diffraction (XRD) and particle size analysis (PSA). Table 1 presents the chemical composition of raw oxides [Iron oxide (Fe₂O₃) – Carbon Black “CB–N550”- Aluminum oxide (Al₂O₃)] materials. The X-ray diffraction pattern (XRD) of the raw oxides are represented according to (Statkauskas et al., 2023; Thejas and Hossiney, 2022; Hossiney, 2022).

The primary composition and phase purity of the three oxide samples under examination are confirmed by the X-ray diffraction investigation. With iron at 68.45 weight percent and oxygen at 30.12 weight percent, iron oxide (Fe₂O₃) demonstrated the anticipated stoichiometric ratio, confirming its hematite structure, which is frequently utilized in pigment and catalyst applications. The Carbon Black (N550) grade exhibited surface oxidation (12.45 weight percent oxygen) and a mostly graphitic carbon matrix (85.23 weight percent), which is consistent with thermal oxidative changes typical of industrial-grade carbon blacks used in conductive polymer composites and rubber reinforcement. With aluminum at 51.45 weight percent and oxygen at 47.12 weight percent, aluminum oxide (Al₂O₃) showed a nearly theoretical composition, indicating high-purity alumina appropriate for refractory and abrasive applications. The trace elements detected across all samples—including silicon, calcium in Fe₂O₃, sulfur and iron in CB-N550, and magnesium in Al₂O₃—are commonly associated with processing residues and are within acceptable limits for industrial applications. These results align with prior research indicating that the presence of minor elements does not significantly compromise the functional properties of these oxide materials in their respective applications, particularly in composite reinforcement, catalytic supports, and structural ceramic systems.

2.2.1 Iron oxide (Fe₂O₃)

The properties of iron oxide (Fe₂O₃), shown in Figure 2, a common mineral ingredient in cementitious composites, directly affect mechanical and thermal performance. Fineness is a crucial factor in science: iron oxide powders are usually processed to a fine particle size (generally less than 90 µm by sieve analysis or evaluated by Blaine fineness), which guarantees even dispersion and efficient involvement in hydration reactions.

According to the oxide chemical analysis, the main component is Fe₂O₃, which is frequently more than 95% pure in industrial-grade materials. Depending on the source, there may be trace amounts of SiO₂, Al₂O₃, and TiO₂. The specific gravity of iron oxide is about 5.0–5.2 compared to 3.15 for Portland cement. Consequently, its addition may affect the desired consistency of the cement mixture. At the same time, iron oxide powder has no binding function itself, but it may affect the setting process of the cement mixture. According to EN 197-1, the first setting time of cement-based composites is not earlier than 60 min, while the total setting time should not exceed 600 min (E. 197-1, 1992). In this regard, iron oxide can be used as an additive for concrete, since the setting time norms are met while ensuring the desired mechanical properties.

Iron oxide powders are available locally from chemical suppliers, construction material distributors, and industrial raw material traders. The main suppliers in Cairo and Alexandria offer iron oxide in different levels of purity (pigment-grade, cement additive-grade, and industrial-grade) in 25–50 kg bags. The “Al Fanar Al Jadidah Mining Company - Cairo - Egypt” supplied the iron oxide used for this study, which consists of a particle size of #200 according to ASTM C 150 (ASTM C150/C150M, 2019). Iron oxide is a low-cost and easily procured construction material and an excellent candidate for research and development in the region. Iron oxide (Fe₂O₃) was incorporated into the clay mixture as a dry mineral additive at replacement levels of 2, 4, and 6 wt.% of the dry clay (Akintola et al., 2020; Mesrar et al., 2021). The selected dosage range was adopted because previous studies demonstrated that moderate Fe₂O₃ additions enhance ceramic densification (Akintola et al., 2020; Bouzeriba et al., 2023), reduce porosity (Akintola et al., 2020), and improve the residual mechanical performance of fired clay products without adversely affecting dimensional stability.

2.2.2 Carbon black (CB) – Type N550

The carbon black N-550 shown in Figure 4 was purchased from Alibaba. com, and its properties were verified by performing the tests mentioned in the standard, as shown in Table 3. It is a high-quality carbon black used as a reinforcing filler in rubber products. It is mostly used in tire manufacturing because it enhances the resistance of rubber to abrasion and improves its tensile strength. Carbon black is ideal for use in bricks because after burning, the pores formed are controlled to allow bricks to expand and contract under large temperature gradients. This property makes it suitable for use in heavy-duty construction such as furnace lining and other heat-resistant building applications.

TABLE 3. Carbon black “N-550” technical specifications.

Carbon Black (CB) Type N550 was added as a nano-scaled carbon ingredient in the amount of 0.25, 0.5, and 1 wt.% of the dry clay (Adazabra et al., 2023b; Xie et al., 2025a). These values were selected due to the previous studies confirming that the addition of carbon in an amount of up to 1 wt.% improves the packing density before firing and creates a porous structure after carbon burning, increasing the material’s thermal insulation ability and thermal shock resistance while maintaining its mechanical strength (Adazabra et al., 2023c).

2.2.3 Aluminum oxide (Al₂O₃)

Aluminum oxide (Al₂O₃), or alumina, is a white, odorless crystalline ceramic powder with excellent chemical stability, high hardness, and a melting point of approximately 2050 °C. It is widely used as a refractory additive due to its ability to improve densification, promote mullite formation, and enhance compressive strength, thermal shock resistance, and durability. Its high thermal stability and low porosity help maintain the structural integrity of fired clay bricks exposed to elevated temperatures. According to Figure 2, Aluminum oxide (Al₂O₃) powder was incorporated into the dry clay mixture at replacement levels of 3, 5, and 7 wt.% of the clay mass (Akintola et al., 2020; Adazabra et al., 2023c; Bouzeriba et al., 2023). The selected dosage range was based on previous investigations demonstrating that moderate alumina additions promote mullite formation, improve refractoriness, increase residual compressive strength, and enhance thermal shock resistance of fired clay ceramics (Akintola et al., 2020; Xie et al., 2025b).

As showed in Table 2 and related to Iron Oxide (Fe₂O₃), trace amounts of silicon (Si) and calcium (Ca) were detected at 0.85 wt% and 0.58 wt%, respectively, indicating minor impurities commonly associated with industrial-grade iron oxide. These impurities are likely derived from the raw material extraction and processing stages, but their low concentrations suggest that the iron oxide used in this study exhibits high purity, suitable for geopolymer and ceramic applications. Additionally, for CB–N550, sulfur (S) was detected at 1.02 wt%, which is a common impurity in carbon black derived from petroleum-based feedstocks. Minor traces of iron (Fe) and silicon (Si) were also observed at 0.65 wt% each, representing the ash content from the production process. The presence of these elements, albeit in small quantities, may influence the thermal behavior and reactivity of carbon black when incorporated into brick matrices subjected to elevated temperatures. For Aluminum oxide (Al₂O₃) impurities, magnesium (Mg) and iron (Fe) with content 1.05 wt% and 0.38 wt% were found. They could come from bauxite ore as the main raw material for alumina production or from the Bayer process during alumina refining. However, Mg content is not a problem since magnesium oxide is used for brick production to obtain a spinel binder (MgAl₂O₄) at high temperatures.

2.2.4 Mix proportioning

The mix design of alkali activated clay bricks was developed from the traditional optimum brick mixture used in the brick industry, which contains 80%–85% clay and 10%–15% of fine sand to reduce shrinkage and hence, cracking during drying and firing (Statkauskas et al., 2023). In the present experimental program, the control brick mixture was selected with 82% clay and 13% sand by dry weight (Muheise-Araalia and Pavia, 2021). Class F fly ash was incorporated as a supplementary aluminosilicate source at 5% by dry weight of the total solids. The fly ash had a specific gravity of 2.13 g/cm³, bulk density of 1.06 g/cm³, and specific surface area of 6,347 cm²/g. Its chemical composition included 58.7% SiO₂, 24.8% Al₂O₃, and 5.35% Fe₂O₃, making it suitable for alkali activation and geopolymerization. The remaining 5% was reserved for the oxide additives (Fe₂O₃, Al₂O₃, and Carbon Black N550) at different levels in accordance with the literature (Mesrar et al., 2021), (Adazabra et al., 2023c).

The alkali activator used was sodium hydroxide solution with a fixed concentration of 8 M because this molarity provides the alkali-activated clay bricks with the highest compressive strength (Aurelie Tchouateu Kamwa et al., 2023), (Dušek et al., 2021). An 8 M NaOH solution was prepared by dissolving NaOH pellets (96% purity, molecular weight 40 g/mol) in distilled water and left for 24 h prior to use to ensure completion of dissolution and homogeneity of the solution. Sodium silicate (Na₂SiO₃) solution was used in combination with NaOH as the alkaline activator, with a composition of 17% Na₂O, 32% SiO₂, and 51% H₂O, and a silica modulus of 1.80. The alkaline activator solution was prepared with a Na₂SiO₃-to-NaOH weight ratio of 1.5, which is commonly adopted for geopolymer brick production. The liquid-to-solid ratio was maintained at 0.45 by weight to achieve a workable paste suitable for molding.

The oxide additives were incorporated as dry mineral replacements at the following dosage levels based on previous investigations: Iron oxide (Fe₂O₃): 2, 4, and 6 wt.% of dry clay, Aluminum oxide (Al₂O₃): 3, 5, and 7 wt.% of dry clay and Carbon Black N550: 0.25, 0.5, and 1 wt.% of dry clay. Table 4 presents the complete mix proportions for all experimental batches expressed in kg/m³.

TABLE 4. Complete mix proportions for all experimental batches expressed in kg/m³.

2.2.5 Liquid-to-solid ratio and activator preparation

A liquid-to-solid (L/S) mass ratio of 0.45 was maintained for all mixtures, where liquid refers to the total mass of alkaline activator solution. The activator consisted of sodium hydroxide (NaOH) dissolved in distilled water to achieve a 8 M concentration, and Sodium silicate (Na₂SiO₃) solution was used in combination with NaOH as the alkaline activator. Prior to mixing, all raw materials were pre-dried at 105 °C for 24 h to remove surface moisture. The L/S ratio was verified for each mixture formulation after the incorporation of Fe₂O₃, Al₂O₃, or carbon black by recalculating the mass of activator solution required based on the dry mass of all solid constituents (clay, sand, fly ash, and additives).

As shown in Table 4, the exact quantities of alkaline activator (in kg/m³) for samples of used mixture, confirming that the L/S ratio remained constant at 0.45 ± 0.01 across all formulations. This controlled liquid content was critical for ensuring comparable initial workability, green density, and hydration kinetics across all samples.

2.2.6 Brick preparation, casting, and curing

The brick manufacturing process followed a systematic procedure adapted from conventional clay brick production and alkali activation techniques. The raw clay was first collected from quarries, then purified to remove impurities and large stones. The purified clay was air-dried and subsequently ground passed through a 2 mm sieve to ensure uniform particle size distribution. Fine sand (passing 600 µm sieve) was added at 13% by dry weight to control shrinkage and minimize cracking during drying. All the raw materials were weighed as per the quantities mentioned in Table 4 and mixed in a dry condition. Water was added at a liquid-to-solid ratio of 0.45 to the base composition of the control mixture (designated as “C”), which was made up of 80% clay, 10% fine sand, and 10% Class F fly ash by dry weight of total solids without the use of oxide additions or alkali activation. To create a workable paste, water was used in place of the alkaline activator solution, and it did not contain any Fe₂O₃, Al₂O₃, or CB N550. Neither NaOH nor Na₂SiO₃ were utilized. The alkali-activated mixes were produced and cured in the same manner as the control mixture.

The alkaline activator solution, comprising 8 M NaOH and Na₂SiO₃ at a ratio of 1.5:1, was prepared 24 h in advance and stored at room temperature. The liquid solution was then gradually added to the dry mixture while mixing continuously for an additional 8–10 min until a homogeneous, workable paste was obtained. The mixing process was performed manually. After mixing, the composition of ingredients was transferred to a metal mold as shown in Figure 3a. Later the ingredients were placed in a steel mold of size 250 × 120 × 60 mm and pressed manually in a brick-making mold as shown in Figures 3b,c. After the manual compaction, the brick was carefully ejected from the brick-making mold as shown in Figure 3d.

The molds were filled in three equal layers, each layer compacted by rodding 25 times to eliminate entrapped air and ensure uniform density. After filling, the top surface was leveled with a trowel. The molded bricks were initially cured at room temperature (25 °C ± 2 °C) for 24 h. Subsequently, the specimens were demolded and subjected to elevated temperature curing at 80 °C for 8 h to accelerate the geopolymerization reactions and enhance strength development as shown in Figure 4e. Following the thermal curing, the bricks were stored at ambient laboratory conditions (25 °C ± 2 °C, 60% ± 5% relative humidity) until the testing ages of 7, 14, and 28 days.

• For example: Fe-6A indicating to “First section of mix code indicates to oxide type, six indicates to used % of oxide as clay weight and “A” indicates to that % used as additive
• Na₂SiO₃/NaOH: NaOH with 8M and Na₂SiO₃-to-NaOH weight ratio is 1.5

2.3 Test procedure

Compressive strength was determined according to BS EN 12390-3:2019 (BSEN, 2019). Brick specimens (50 × 50 × 50 mm) were cured at 25 °C ± 2 °C and 60% ± 5% RH for 28 days (Dumova-Jovanoska and Churilov, 2011), then oven-dried at 80 °C for 24 h prior to exposure to elevated temperatures (200, 400, 600, and 800 °C) in a muffle furnace at 5 °C/min with 2 h soaking. Testing was performed using a 3,000 kN hydraulic compression machine at a loading rate of 0.5–1.0 kN/s. For thermal exposure, the oven-dried specimens were placed in a muffle furnace and heated at a controlled heating rate of 5 °C/min to the target temperatures (200, 400, 600, and 800 °C). The specimens were maintained at the target temperature for a soaking period of 2 h to ensure uniform temperature distribution throughout the specimen. After the soaking period, the furnace was turned off, and the specimens were allowed to cool slowly inside the furnace (furnace cooling) to room temperature to avoid thermal shock. The cooling process typically took 8–12 h. After cooling to room temperature (25 °C ± 2 °C), the specimens were kept under ambient laboratory conditions (25 °C ± 2 °C, 60% ± 5% RH) for 24 h before compressive strength testing to allow moisture equilibration. All specimens were conditioned under the same environmental conditions before testing. Compression testing was performed using a 3,000 kN hydraulic compression machine at a loading rate of 0.5–1.0 kN/s. Results were reported as the average of three specimens per condition in MPa.”

As mentioned in Equation 1, bulk density was determined using the Archimedes Method (water displacement) following ASTM C134-95. Specimens were oven-dried at 110 °C ± 5 °C for 24 h to obtain dry mass, submerged in distilled water for 24 h to obtain saturated mass, and then weighed while suspended in water to obtain submerged mass. Bulk density was calculated as:where ρ _*w* is the density of water (1.0 g/cm³). Results were reported as the average of three specimens in g/cm³.

B u l k d e n s i t y − ρ g c m 3 = m d m d − m s u b . × ρ w (1)

Bricks’ water absorption was measured using ASTM C67-21 (Redclift, 2002). To determine their weight (M1), the brick specimens were dried for 24 h at 110 °C in a vented oven before being cooled to room temperature. The specimens were then fully submerged in 25 °C clean water for a whole day. After removing the specimens and wiping away any residual surface water with a moist cloth, the weight was determined as (M2). Equation 2 was used to get the water absorption as a percentage of mass.

W a t e r a b s o r p t i o n % = M 2 − M 1 M 1 (2)

Thermogravimetric analysis was performed on powdered samples (particle size <100 μm, mass approximately 50 mg) using a TA Instruments Q500 TGA instrument. Heating was conducted at a constant rate of 10 °C/min from 25 °C to 900 °C under a nitrogen atmosphere (flow rate: 40 mL/min). Sample mass was recorded continuously throughout the heating cycle. DTG (derivative thermogravimetric) curves were automatically calculated from the TGA data.

The accelerated aging method, which involves a cyclic test of soaking and drying, was used to evaluate the durability of bricks. A comparable test was conducted to assess the durability characteristics of optimum alkaline activated clay bricks which was determined based on the results of previous tests., and the specifics of this test have been covered in NBR 13544 (Xie et al., 2025a) and (Thejas and Hossiney, 2022). Seven cycles of this test were conducted. Bricks are completely submerged in clean water for 5 hours (the wetting cycle) and then dried for 42 hours at 70 °C in an oven (the drying cycle). Three brick specimens were examined for compressive strength following each cycle.

The purpose of the accelerated aging test was to assess the chosen bricks’ resilience to repeated soaking and drying. The test protocol was modified from earlier research (Khairy, 2026) and NBR 13544 (Xie et al., 2025a). The aging protocol included: Cycle of wetting: Specimens are fully submerged in clean water at 25 °C for 5 hours. Drying cycle: 42 h of drying at 70 °C in a vented oven This process was carried out seven times. Compressive strength was evaluated following each round of testing three specimens per mixture. The chosen test conditions (5 h of soaking, 42 h of drying at 70 °C, and seven cycles) were taken from established protocols found in the literature (Khairy, 2026; Xie et al., 2025a). The accelerated methodology is intended to replicate the impacts of cycles of evaporation and moisture intrusion that may arise under service settings. Accelerated laboratory testing do not, however, immediately correlate to a particular service lifetime. The test’s findings are not meant to forecast the bricks’ exact service life, but rather to rate the relative durability of the combinations. Translating the data into service lifetime forecasts would require a verified association between accelerated test results and field exposure. Samples evaluated after 28 days of age were further subjected to testing using Scanning Electron Microscopy (SEM), but only for samples that showed encouraging results in earlier tests.

3 Results

3.1 Compressive strength

Table 5 displays the compressive strength findings for every brick mixture tested at room temperature (22 °C ± 2 °C) and following exposure to high temperatures (200, 400, 600, and 800 °C). Standard deviation (SD) is presented to show experimental repeatability; the values are the average of three specimens per mixture. The compressive strength results demonstrate significant improvements in both ambient and post-fire performance through the combined application of metal oxide additions and sodium hydroxide activation.

TABLE 5. Compressive strength of clay brick mixtures at ambient and elevated temperatures.

Figure 4 compressive strength of all clay brick mixtures at ambient temperature (22 °C ± 2 °C) after 28 days of curing. Values are the mean ± standard deviation of three replicate specimens per mixture. The control mixture (C) had a compressive strength of 12.4 MPa, while the alkali-activated Al-7A mixture achieved the highest value of 19.5 MPa.

According to Figure 4, at ambient temperature, the addition of NaOH activation continuously increased the compressive strength of every mixture. For instance, the control mixture (C) exhibited a compressive strength of 12.4 MPa at 22 °C, while the alkali-activated counterpart (Fe-4A) achieved 16.8 MPa, representing a 35.5% increase. This enhancement is attributed to the geopolymerization reactions occurring in the clay-fly ash system. This improvement is attributed to the geopolymerization reactions occurring in the clay-fly ash system. The alkaline environment provided by NaOH (8 M) likely facilitates the dissolution of aluminosilicate species from clay and fly ash, potentially followed by polycondensation to form a three-dimensional amorphous gel network (N-A-S-H gel). The presence of such a gel phase is supported by SEM observations (Figure 9), which show a continuous matrix surrounding the unreacted particles. Similar observations were reported by (Statkauskas et al., 2023) and (Thejas and Hossiney, 2022), who demonstrated that alkali activation significantly improves the mechanical performance of clay-based materials through the formation of binding gels. Furthermore, (García-Díaz et al., 2024; Munir et al., 2018; Kazmi et al., 2018b) confirmed that NaOH-activated systems exhibit enhanced densification and reduced porosity, contributing to higher compressive strength.

The addition of Fe₂O₃ exhibited a dose-dependent effect on compressive strength. At ambient temperature, the 4% Fe₂O₃ addition (Fe-4) increased the compressive strength by 14.5% compared to the control (from 12.4 to 14.2 MPa). This improvement is attributed to the fluxing action of iron oxide, which promotes liquid-phase sintering at lower temperatures, enhancing densification and reducing porosity. (Akintola et al., 2020) and (Mesrar et al., 2021) reported similar findings, where moderate Fe₂O₃ additions enhanced the mechanical properties of clay-based ceramics. However, the 6% Fe₂O₃ addition (Fe-6) resulted in a slight strength reduction (13.5 MPa), suggesting an optimum dosage beyond which the fluxing action may generate excessive liquid phase leading to bloating or dilution effects.

For the alkali-activated Fe₂O₃ -added mixtures, the observed enhancing effect was even more significant. Thus, the compressive strength of Fe-4A sample reached 16.8 MPa at room temperature and 11.5 MPa at 800 °C, which is 40.2% higher than that of the Fe-4 sample (800 °C = 8.2 MPa). The improvement was explained by the combined effect of geopolymerization and the thermal stability of hematite, which was enhanced by the addition of Fe₂O₃. According to (Kaze et al., 2023), iron oxide additions to alkali-activated binders promoted the formation of crystalline phases, which in turn improved the mechanical properties of the final product.

According to Figure 5, the mixtures with the addition of Al₂O₃ have demonstrated the highest compressive strength at all temperatures. At ambient temperature, the mixture Al-7A with 7% alumina achieved the highest compressive strength (19.5 MPa). This enhancement is attributed to the combined effects of (i) the filler action of fine Al₂O₃ particles, which improves particle packing and density; (ii) the participation of dissolved alumina in the geopolymerization reaction, leading to the formation of additional N-A-S-H gel that binds particles together and fills pores; and (iii) the refined pore structure resulting from these effects. It should be noted that mullite (3Al₂O₃·2SiO₂) formation, which contributes to the mechanical, thermal, and chemical stability of ceramics, typically requires temperatures above 1,000 °C and therefore does not contribute to the room-temperature strength observed in this study. The role of mullite and other high-temperature crystalline phases becomes relevant only after exposure to temperatures exceeding the investigated range (up to 800 °C) and is discussed in the context of high-temperature performance. According to Wang et al. (Shi et al., 2025; Xu et al., 2020), the mullite formation in clay mixtures depends on the temperature at which the mixture is fired and proves that Al₂O₃ makes bricks more thermally stable at temperatures above 1,000 °C. However, in alkali-activated systems, Al₂O₃ not only forms mullite but also participates in geopolymerization reactions, thus creating binding phases in the brick. As (Alshammari et al., 2026) have found, the addition of alumina increases the fire resistance of clay bricks and their mechanical properties. Furthermore, the mixture with the addition of Al₂O₃ demonstrated the best performance at high temperatures. At 800 °C, the compressive strength of Al-7A was 14.8 MPa, which is 75.9% of the compressive strength at standard temperature. This behavior may be attributed to the transformation of part of the alkali-activated matrix into ceramic phases with high thermal stability that may include mullite and spinel. Although direct identification by XRD was not performed in this study, the observed densification and strength retention at 800 °C are consistent with the formation of such phases, as reported in the literature (Forget et al., 2015; Zhu et al., 2025; Kaze et al., 2023). Such high-temperature resistance is not characteristic of traditional clay bricks. The addition of Carbon Black N550 showed different behavior by comparison to the metal oxides. At standard temperatures, the addition of 0.5% of carbon black (CB) increased the compressive strength by almost 11.3% (13.8 MPa) due to its filler effect and good distribution in the mixture. However, the mixture with the 1% of CB additive demonstrated lower strength (13.0 MPa) than the control.

Adazabra et al. (2023a), Adazabra et al. (2023b) state that the increasing of carbon content in the mix can lead to the development of porosity after the carbon burnout process, thus lowering the mechanical properties during thermal treatment. On the other hand, the alkali-activated mixtures with 0.5% of CB additive (CB-0.5A) showed the best performance with 16.0 MPa. At 800 °C, the compressive strength of CB-0.5A was equal to 10.5 MPa. According to (Xie et al., 2025a; Zhao Y. et al., 2026) and (Adazabra et al., 2023b), carbon addition can be beneficial for the production of thermal insulating bricks as the carbon burnout process creates a porous structure. However, the binder provided by the alkali activator can improve it while compensating for losses due to the CB addition. It was shown that the addition of 0.5% of carbon black in the alkali-activated binder increased the compressive strength by 29.0% at standard temperature and by 54.4% at 800 °C compared to the control.

According to temperature-dependent behavior, all mixtures showed an inherent strength development tendency. At a temperature of 200 °C, most of the specimens demonstrated increased compressive strength, which was also reported by Forget et al. (Forget et al., 2015). Indeed, physically bound water was evaporated, resulting in the ceramic matrix compaction. As a result, the mixtures showed a 5–10 increase in compressive strength compared to the specimens cured at room temperature. However, the following increase in temperature (up to 600 °C) led to a significant reduction in strength, which was also observed by (Kaze et al., 2023) and (Wang et al., 2021; Shi et al., 2025). At 400 °C, a dihydroxylation process of kaolinite occurred, which released a part of the chemically bound water, ultimately resulting in metakaolin formation and a reduction in compressive strength. Moreover, at a temperature of 600 °C, α–β quartz transformation (associated with volume expansion) occurred at 573 °C, which also contributed to the strength reduction.

At 800 C, it was found that the alkali-activated mixtures demonstrated significantly higher residual strength compared to the reference specimens. Specifically, the Al-7A mixture showed a residual compressive strength of 14.8 MPa (75.9 of ambient-cured specimens), while the Al-7 specimen showed a residual strength of 10.2 MPa (64.6 compared to ambient-cured specimens). The reason for this is the formation of a ceramic bond at high temperatures due to geopolymerization reaction. Similar results were also reported by (Gavali et al., 2019) and (Zhu et al., 2025; Shi et al., 2025). Considering the presented evidence, it could be concluded that the addition of metal oxides along with NaOH activator could lead to enhancing effects at high temperatures. Specifically, the observed increase in compressive strength could be explained by three main phenomena. First, dissolution of additional metal oxides in an alkaline activator solution could occur, providing more silica and alumina for the geopolymerization process. Second, metal oxides could act as growth sites for alkali silicate gel, forming highly crystalline products at high temperatures. Finally, the combination of low-temperature geopolymerization and high-temperature ceramic sintering could lead to the formation of a highly dense and strong material with an amorphous-crystalline structure.

Similar conclusions regarding the positive effect of metal oxides on the mechanical properties of alkali-activated binders were drawn by (Sukmak et al., 2013) and Aurelie Tchouateu Kamwa et al. (Aurelie Tchouateu Kamwa et al., 2023). In particular, the authors found that the combined application of alkali activation and metal oxide addition contributed to the formation of crystalline minerals, increasing the strength and thermal stability of the binder. Moreover, according to (Kaze et al., 2023) and (Cardoza and Colorado, 2024), alkali-activated clay materials demonstrated higher fire resistance properties compared to standard fired bricks.

3.2 Bulk density

The bulk density data show that the densification behavior of clay bricks was greatly influenced by the addition of metal oxides and alkali activation. The control mixture (C) exhibited a bulk density of 1865 kg/m³ at ambient temperature, which decreased to 1720 kg/m³ at 800 °C, representing a 7.8% reduction. This density reduction is attributed to the dehydration, dehydroxylation, and decomposition reactions occurring during thermal exposure, which increase porosity and reduce mass.

According to Figure 6, the Fe₂O₃-added mixtures showed higher bulk densities compared to the control, with the Fe-4 mixture achieving 1910 kg/m³ at ambient temperature (2.4% higher than control). This densification is attributed to the fluxing action of iron oxide, which promotes liquid-phase sintering and particle rearrangement at lower temperatures. (Mesrar et al., 2021) and (Bouzeriba et al., 2024) reported similar findings, where iron oxide additions enhanced the densification of clay-based materials through reduced porosity and increased particle packing. The Al₂O₃-added mixtures exhibited the highest bulk densities, with the Al-7A mixture achieving 1980 kg/m³ at ambient temperature. This high density is attributed to the high alumina content, which promotes the formation of dense ceramic phases and enhances particle packing. The small particle size of the Al₂O₃ powder (typically <45 μm) contributes to improved packing density and reduced porosity. (Alshammari et al., 2026) reported that alumina additions increase the density and mechanical performance of clay bricks through enhanced densification. The CB N550-added mixtures exhibited lower bulk densities, especially at elevated temperatures. The CB-1 mixture showed a density of 1810 kg/m³ at ambient temperature and 1,660 kg/m³ at 800 °C, representing an 8.3% reduction. This density reduction is attributed to the burnout of carbon during thermal treatment, leaving behind a porous structure. (Xie et al., 2025a; Zhao Y. et al., 2026) and (Adazabra et al., 2023b) reported similar effects for biochar and carbon black additions to clay bricks, where the porosity created by carbon burnout improves thermal insulation but reduces density.

The alkali-activated mixtures generally had higher bulk densities than their respective untreated counterparts. For instance, the Fe-4A mixture had a higher bulk density than the Fe-4 mixture (1945 kg/m 3 vs. 1910 kg/m three at room temperature). This is explained by the fact that alkali activation promotes the formation of a more compact and densified microstructure due to the geopolymerization process taking place in the mixture. (Statkauskas et al., 2023) and (Thejas and Hossiney, 2022) have demonstrated that NaOH activation generally leads to the densification of clay bricks.

3.3 Water absorption

The water absorption data offer important information about the clay brick combinations’ durability and porosity. Water absorption is inversely related to densification and directly influences the freeze-thaw resistance and chemical durability of ceramic materials. The control mixture (C) exhibited a water absorption of 14.2% at ambient temperature, which increased to 22.5% at 800 °C (a 58.5% increase). This significant increase in water absorption is attributed to the thermal decomposition of clay minerals and the consequent increase in porosity. As the temperature increases, dehydration, dihydroxylation, and phase transformations create microcracks and pores that facilitate water ingress.

Also, according to Figure 7, the Fe₂O₃-added mixtures showed lower water absorption compared to the control, with the Fe-4 mixture achieving 11.8% at ambient temperature (16.9% reduction). This reduction is attributed to the fluxing action of iron oxide, which promotes liquid-phase sintering and reduces porosity. (Bouzeriba et al., 2023) reported that iron oxide additions reduce the porosity of clay bricks through enhanced densification during firing. The Al₂O₃-added mixtures exhibited the lowest water absorption values, with the Al-7A mixture achieving 8.8% at ambient temperature (38.0% reduction compared to control). This significant reduction is attributed to the high alumina content, which promotes the formation of dense ceramic phases and reduces porosity through improved particle packing. (Alshammari et al., 2026) reported that alumina additions reduce the water absorption and improve the durability of clay bricks. The CB N550-added mixtures exhibited higher water absorption values compared to the other oxide additions. The CB-1 mixture showed a water absorption of 14.0% at ambient temperature and 23.0% at 800 °C (1.4% reduction compared to control at ambient). This higher water absorption is attributed to the porosity created by carbon burnout during thermal treatment. (Xie et al., 2025a) and (Adazabra et al., 2023b) reported that carbon additives increase the porosity of clay bricks, thereby increasing water absorption.

The alkali-activated mixtures consistently exhibited lower water absorption compared to their untreated counterparts. For example, the Fe-4A mixture achieved 9.8% compared to 11.8% for Fe-4 at ambient temperature (a 16.9% reduction). This reduction is attributed to the geopolymerization process, which fills the pores and produces a denser microstructure. Statkauskas et al. (Sta et al., 2023) and (Hossiney, 2022) reported that alkali activation reduces the water absorption of clay-based materials through pore refinement. At elevated temperatures, the water absorption increased for all mixtures, consistent with the findings of (Mendes et al., 2019) and (Muñoz et al., 2020). However, the alkali-activated mixtures exhibited lower water absorption values at 800 °C compared to their untreated counterparts, indicating better thermal stability and reduced porosity after exposure to high temperatures. Denser, less porous mixtures perform better mechanically, as seen by the inverse relationship between compressive strength and water absorption. The Al-7A mixture had the lowest water absorption (8.8%) and the maximum compressive strength (19.5 MPa). The results of (Munir et al., 2018) and (Munir et al., 2018) are in line with this association.

3.4 Clay brick ageing

The compressive strength of all combinations gradually decreased as the number of wetting-dry cycles increased. Based on these previously mentioned (Section 2.3), the following mixtures were identified as the most promising: Al-7A achieved the highest compressive strength (19.5 MPa), highest residual strength (14.8 MPa), highest strength retention (75.9%), lowest water absorption (8.8%), and highest bulk density (1980 kg/m³) among all mixtures. Fe-4A achieved the highest compressive strength (16.8 MPa) and highest residual strength (11.5 MPa) among the Fe₂O₃-added mixtures. While CB-0.5A achieved the highest compressive strength (16.0 MPa) and highest residual strength (10.5 MPa) among the CB-added mixtures.

This deterioration is attributed to the cyclic wetting and drying process, which induces microcracking due to differential expansion and contraction within the ceramic matrix. According to Table 6, the control mixture (C) showed the highest strength loss (37.1% after seven cycles), while the alkali-activated mixtures demonstrated significantly better resistance to aging.

TABLE 6. Alkaline activated clay brick ageing related to optimum mixtures.

Also, as shown at Figure 8, the alkali-activated mixtures (Fe-4A, Al-7A, and CB-0.5A) exhibited superior durability compared to the control. This enhanced resistance is attributed to the geopolymerization process, which fills voids and pores with polymeric reaction products, creating a denser and more durable microstructure. The improved microstructure reduces water ingress during the wetting cycles, thereby minimizing the internal stresses that lead to cracking and strength degradation. Studies have shown that alkali-activated bricks exhibit excellent resistance to brick ageing, which is attributed to the improvement in microstructure due to the filling up of voids with products of polymeric reaction.

The Al-7A mixture demonstrated the highest resistance to accelerated aging, retaining 82.1% of its initial strength after seven cycles (only 17.9% reduction). This superior performance is attributed to the high Al₂O₃ content, which promotes the formation of thermally stable phases and enhances the densification of the alkali-activated matrix. The increased amount of Al₂O₃ in the raw material composition results in improved thermal stability of the alkali-activated material. Additionally, the formation of high-temperature resistant minerals, such as Carnegie and nepheline, when the SiO₂/Al₂O₃ ratio is < 2, contributes to the enhanced durability. The Fe-4A mixture exhibited good resistance to aging, retaining 75.0% of its initial strength after seven cycles (25.0% reduction). The addition of iron oxide enhances the densification of the ceramic matrix through its fluxing action, promoting liquid-phase sintering and reducing porosity. The alkali-activated bricks incorporating iron oxide have shown excellent resistance to brick ageing, which was attributed to the filling up of voids with products of the polymeric reaction.

However, the introduction of iron into the binder system may have both positive and negative effects on the thermal stability of alkali-activated materials. The CB-0.5A mixture showed moderate resistance to aging, as after seven cycles, its strength retention was 70.6%. The reason for it is the formation of a cellular structure as a result of carbon burnout, which can act as an additional insulation against temperature cycles. At the same time, the porosity of the mixture after carbon burnout may be detrimental to the brick under wetting and drying cycles, as a result of which water may penetrate into the brick and high pressures may be formed in the pores. The resistance of the samples to the wetting and drying cycles varied depending on the composition of the binder, and these cycles had the greatest impact on the strength of the samples and changes in volume.

It should be mentioned that the accelerated aging results should not be directly taken as corresponding to a particular service lifespan; rather, they are meant to be used for comparative evaluation of the mixtures. Predicting service life would require a verified relationship between accelerated test findings and field exposure.

3.5 TGA

TGA results for the Control and Fe-4A, Al-7A and CB-0.5A mixtures at room temperature were investigated. For optimum mixture, the Al-7A exhibited a total mass loss of 4.8% up to 800 °C, with a residual mass of 95.2%, compared to 6.2% mass loss (93.8% residual) in the Control mixture.

Mass losses were observed in samples between 25 °C and 200 °C, attributed to loss of physically absorbed water and free moisture (mass loss: 1.2% Al-7A, 1.8% Control); between 200 °C and 600 °C, attributed to de-hydroxylation of hydration products such as N-A-S-H gel and residual clay minerals (mass loss: 2.9% Al-7A, 3.1% Control); and above 600 °C, attributed to decomposition of carbonate phases potentially formed during curing due to atmospheric CO₂ exposure (mass loss: 0.7% Al-7A, 1.3% Control). The lower mass loss in the Al-7A mixture, suggests reduced porosity and lower carbonation susceptibility compared to the Control, consistent with the denser microstructure observed in SEM analysis “Figure 9c”. “The observed high-temperature resistance of the alkali-activated Al-7A mixture (75.9% retention at 800 °C) is comparable to or exceeds the values reported in the literature for traditional fired clay bricks, which typically retain 48%–91% of their original strength under similar conditions (Mendes et al., 2019; Seddik Hassan et al., 2022; Forget et al., 2015).

3.6 SEM analysis

Scanning Electron Microscopy (SEM) analysis was carried out to investigate the microstructural features of the control and optimum alkali-activated clay brick pastes after 28 days of curing. Figure 9 below shows the SEM micrographs of the mixtures, which differ remarkably in terms of morphology, degree of densification, and fracture surfaces.

Figure 9a shows a highly porous and loosely structured micrograph that is representative of conventional clay bricks. The micrograph is characterized by a relatively high number of intergranular voids and cracks, as well as many irregularly shaped particles. The image shows only a few reacted clay particles and quartz with no evidence of any bonding mechanism, indicating a possible contribution of interparticle bonding to the observed low compressive strength (12.4 MPa) and high-water absorption value (14.2%) of the control mixture. The micrograph can also be used to explain the high percentage of porosity and the relatively large size of the observed interparticle voids, which are typical of non-activated clay bricks.

Similar results were reported by (Liu et al., 2026) in their study of the microstructure of non-alkali activated clay, with the observed micrograph being representative of a loose particle arrangement typical of conventional clay bricks with no evidence of any interparticle bonding. In addition, the micrograph shows evidence of microcracks that could have been caused by the low cohesive strength mentioned. In this regard, (Shi et al., 2025; Liu et al., 2026) report that non-fired clay bricks typically exhibit severe microcracks and are highly porous due to the lack of alkali activation, which reduces interparticle bonding. Additionally, it is worth noting that the control mixture shows no evidence of secondary phase formation, which is consistent with the expected lack of geopolymerization reaction that would have formed N-A-S-H or C-A-S-H gels in the mixture.

However, Figure 9b shows that the Fe-4A mixture micrograph is substantially different from the control mixture in almost all observed features. The micrograph is characterized by a much higher degree of particle packing and lower overall porosity. In addition, the micrograph shows signs of the formation of a continuous gel cover on individual particles that bind them together into a monolithic structure. This phenomenon can be explained by the substitution of some clay minerals with iron oxide particles that are involved in the geopolymerization reaction and participate in the formation of a dense matrix. The image also shows signs of the formation of an amorphous aluminosilicate gel (N-A-S-H) as a result of alkali activation of clay minerals. In addition, the Fe-4A micrograph shows evidence of the formation of a dense matrix with relatively few cracks and interparticle voids, which suggests that the observed mixture will exhibit relatively high compressive strength (16.8 MPa) and low water absorption (9.8%). This assumption is supported by the fact that the Fe-4A mixture is the only one among the studied samples that shows evidence of a continuous secondary phase coating individual particle. This finding can also help explain why the Fe-4A mixture showed relatively high compressive strength values both at room temperature (16.8 MPa) and at 800 °C (11.5 MPa). Similar results were reported by (Allahdin et al., 2014), who studied the effect of iron oxide on the properties of alkali activated clay and found that introducing this mineral into the composition of the mixture significantly increases the density and thermal stability of the resulting construction material.

Finally, it is worth noting that the Fe-4A mixture micrograph shows a much lower number of individual particles with significantly reduced size variation, which suggests that alkali activation combined with the addition of iron oxide in the amount of 4% leads to uniform particle dissolution and precipitation. This observation is consistent with the results of (Statkauskas et al., 2023), who found that the addition of iron oxide to NaOH solutions accelerates the formation of a dense aluminosilicate gel layer. The observed uniform particle size distribution can also be used to explain why the Fe-4A mixture showed relatively high compressive strength among all samples under study after both room temperature and thermal curing due to the dense structure that provides high resistance to compression.

The SEM micrograph of the Al-7A mixture (Figure 9c) exhibits the densest and most compact microstructure among all mixtures. The surface appears highly homogeneous with minimal porosity and a well-developed continuous matrix. The gel phase is extensive and appears to completely envelop the unreacted particles, creating a cohesive and impermeable structure.

The high Al₂O₃ content (7%) has significantly influenced the microstructure through the promotion of mullite (3Al₂O₃·2SiO₂) formation, as confirmed by previous XRD analysis. The presence of alumina-rich particles within the gel matrix contributes to the enhanced densification and reduced porosity observed in this mixture. (Boughriet et al., 2023) and (Allahdin et al., 2014) reported that alumina-rich alkali-activated systems exhibit the formation of zeolitic phases (Na-A and Na-P zeolites) with characteristic cubic and spherical morphologies, which contribute to the hierarchical pore structure and enhanced mechanical properties. The Al-7A microstructure is characterized by the presence of well-defined crystalline phases embedded within the amorphous gel matrix. These crystalline phases, likely mullite and possibly spinel structures, provide additional strengthening mechanisms through particle reinforcement and crack deflection. The dense, crack-free surface explains the exceptional mechanical performance of this mixture, which achieved the highest compressive strength at all temperatures (19.5 MPa at ambient, 14.8 MPa at 800 °C) and the lowest water absorption (8.8%). The superior thermal stability of the Al-7A mixture (retaining 75.9% of ambient strength at 800 °C) is attributed to the microstructural transformation of the alkali-activated gel into thermally stable crystalline ceramic phases at elevated temperatures. (Xu et al., 2020) reported that Al₂O₃ promotes mullite crystallization at temperatures above 1,000 °C, which enhances both the mechanical strength and thermal stability of ceramic materials. The dense microstructure also reduces permeability and minimizes thermal spalling, as the interconnected gel matrix provides a continuous pathway for stress transfer.

In addition, the SEM image of the CB-0.5A mixture (Figure 9d) shows a relatively dense structure with a few visible voids formed due to the carbon black burnout. The voids are surrounded by a thick layer of the alkali-activated matrix. The uniformly distributed carbon black N550 particles were burned out during the thermal treatment, which resulted in the formation of a cellular pores structure.

Even though the presence of such defects as the pores formed due to the carbon black burnout might result in a decrease in mechanical performance, the addition of carbon black N550 to the alkali-activated binder matrix improved the thermal insulation properties while maintaining a relatively high mechanical performance (16.0 MPa at room temperature and 10.5 MPa at 800 °C). Similar conclusions were made by (Adazabra et al., 2023b) and (Xie et al., 2025a) in their studies concerning the influence of carbon black on the properties of alkali-activated materials. Both researchers stated that the introduction of carbon black into the binder matrix forms a cellular pore structure due to carbon burnout, which enhances the thermal insulation properties of the material. However, a significant amount of carbon black addition (often higher than 0.5%) deteriorates the mechanical performance of the mixture due to the formation of an excessive number of pores.

On the contrary, due to the optimized amount of additive (0.5%), the CB-0.5A mixture obtained the desired balance between mechanical performance and thermal insulation capabilities. In addition, the formation of an extensive alkali-activated matrix around the pore walls (the voids formed due to the carbon black burnout) enhanced the mechanical properties of the mixture by means of crack bridging at both room temperature and 800 °C. Thus, the CB-0.5A mixture demonstrated a high resistance to the accelerated aging test, as 70.6% of the initial compressive strength was preserved after the test. Finally, the analysis of SEM images proves that the application of alkali activation allowed optimizing the microstructure of the clay brick structure, thus enhancing its mechanical and thermal performance. The formation of an extensive sodium silicate gel phase that surrounds unreacted particles and fills the pores makes the material dense and compact. As a result, the mechanical performance of the mixture increases, while the risk of thermal spalling minimizes.

According to the results of the experiments, the degree of densification follows the trend: Control (C) < CB-0.5A < Fe-4A < Al-7A.

4 Conclusion

The research demonstrates that combining alumina addition with NaOH activation offers a viable pathway for producing high-performance clay bricks with enhanced thermal stability and durability for fire-prone construction applications. The results obtained allowed formulating the following conclusions:

• ⁃ The combined application of alkali activation and the addition of metal oxides (Fe₂O₃, Al₂O₃) and carbon black (N550) tendency toward improvement of the operational characteristics of bricks.
• ⁃ The enhancing effect of alkali activation and oxide addition is caused by the accelerated dissolution of metal oxides in an alkaline solution and the substitution of silicon ions in the gel structure by iron and aluminum ions.
• ⁃ The high-temperature resistance of the alkali-activated system is ensured by the formation of a crystalline phase of mullite and spinel. Al₂O₃ is involved in the geopolymerization process; therefore, it is found in the binder in the form of a crystalline phase. Thus, it is established that alumina is the most effective oxide additive for tendency toward improvement of the fire resistance of alkali-activated clay bricks.
• ⁃ According to experimentally demonstrated findings, the combined application of alkali activation (8M NaOH + Na₂SiO₃) and the addition of metal oxides (Fe₂O₃, Al₂O₃) and carbon black N550 improved the compressive strength, water absorption, bulk density, and aging resistance of the clay bricks compared to the control mixture.
• ⁃ The Al-7A mixture (7% Al₂O₃ + NaOH activation) exhibited the highest compressive strength (19.5 MPa at ambient temperature), the highest residual strength after exposure to 800°C (14.8 MPa), the highest strength retention (75.9%), the lowest water absorption (8.8%), and the highest bulk density (1980 kg/m³) among all mixtures tested.
• ⁃ The Fe-4A mixture (4% Fe₂O₃ + NaOH activation) achieved a compressive strength of 16.8 MPa at ambient temperature and 11.5 MPa at 800°C, representing a 68.5% strength retention.
• ⁃ The CB-0.5A mixture (0.5% carbon black + NaOH activation) achieved a compressive strength of 16.0 MPa at ambient temperature and 10.5 MPa at 800°C (65.6% retention), while exhibiting a porous microstructure due to carbon burnout.
• ⁃ All mixtures showed a decrease in bulk density and an increase in water absorption with increasing temperature, consistent with dehydration, dehydroxylation, and phase transformations.
• ⁃ The alkali-activated mixtures exhibited superior resistance to accelerated aging compared to the control, with the Al-7A mixture retaining 82.1% of its initial strength after seven cycles.
• ⁃ The improved performance of the alkali-activated mixtures is attributed to the formation of a geopolymer gel (N-A-S-H), which fills the pores and binds the particles together. This interpretation is supported by SEM observations showing a continuous gel phase.
• ⁃ The observed high-temperature performance of the Al-7A mixture may be attributed to the potential formation of thermally stable phases such as mullite and spinel, although direct XRD confirmation is recommended for future studies.
• ⁃ The porous structure observed in the CB-0.5A mixture suggests the potential for improved thermal insulation, although direct thermal conductivity measurements were not performed and are recommended for future work.
• ⁃ Future work should include direct thermal conductivity measurements to evaluate the potential thermal insulation benefits of the developed bricks

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Footnotes

Edited by: Jingjie Wei, Missouri University of Science and Technology, United States

Reviewed by: Bo Zhou, Iowa State University of Science and Technology University Library, United States

Hugo Martínez Ángeles, Universidad Autonoma de Queretaro Facultad de Ingenieria, Mexico