Development of Unfired Clay Bricks with Alumina Waste from Liquid Nitrogen Production: A Sustainable Alternative for Construction Materials

Abstract

A major breakthrough in environmentally friendly building materials is the development of sustainable unfired clay bricks including alumina waste produced during liquid nitrogen generation. Though used extensively, conventional fired clay bricks require energy-intensive manufacturing techniques that produce significant amounts of CO₂ and aggravate environmental damage. By removing the need for high-temperature firing and allowing for the valorization of industrial byproducts including alumina waste and lateritic soil, unfired clay bricks offer a reasonable low-carbon alternative. High silica and alumina contents define the alumina waste, which shows pozzolanic reactivity, thus improving the physicomechanical performance of the bricks. With alumina waste substituting 0–8.57% of the cement content, seven different formulations showed improvements in compressive strength, reduced water absorption, and optimal thermal conductivity. Especially, the mechanical performance was much enhanced with alumina waste inclusion up to 30%, without sacrificing thermal insulation capacity or moisture resistance. Further supporting the environmental and financial sustainability of the suggested brick compositions is the economic viability of using industrial waste and regionally derived soils. A comparative analysis of the conventional fired bricks shows that the unfired substitutes have a much lower environmental impact and show better mechanical properties, including greater compressive strength and modulus of rupture. These results support the more general goals of circular economy systems and low-carbon urban development by highlighting the feasibility of including alumina waste and lateritic soil into sustainable building materials. Using such waste-derived inputs in building fits world initiatives to lower resource consumption, lower greenhouse gas emissions, and build strong infrastructure systems.

1. Introduction

With their application spanning millennia, bricks have been among the oldest and most flexible building materials; they are a basic building block for innumerable construction developments all around [1,2]. Among the several kinds of bricks, Thailand’s most often used fired clay brick is prized for its strength, cost, and attractive qualities. But the conventional technique for making fired clay brick uses a high−energy process, whereby the bricks must be fired at temperatures over several days [3,4]. In addition to the great energy consumption, this firing process causes environmental pollution in the form of particulate matter and smells. High temperatures, usually between 900 °C and 1200 °C, are needed for firing bricks in kilns, which consumes significant quantities of energy, mostly derived from fossil fuels [5]. This process contributes to air pollution and global warming by releasing carbon dioxide (CO₂) and other pollutants [6,7]. Actually, as one of the main industrial contributors to CO₂ emissions worldwide, the cement a major component of conventional brickmaking sector alone, accounts for almost 7% of total emissions [8,9,10]. Apart from greenhouse gas emissions, the firing process generates particulate matter that might compromise human health and worsen air quality. Furthermore, the long time needed for firing causes problems including shrinkage and warping of the bricks, which often results in uneven sizes that do not satisfy the intended specifications, therefore lowering the general consistency and quality of the resultant product. These production and environmental inefficiencies draw attention to the need for investigating sustainable and effective alternative brickmaking techniques.

Different research has looked at the possibility of manufacturing unfired bricks in order to solve the sustainability issues of conventional fired bricks, an environmentally friendly choice since unfired bricks, sometimes known as green bricks, do not undergo the energy-intensive firing process [11]. Using alternative raw materials, such as cement, soil, and other industrial byproducts [12,13,14], these bricks are made to be a durable and sustainable building material devoid of heating or burning. Consequently, unfired bricks present a great possibility to lower energy consumption and carbon emissions, therefore supporting more environmentally friendly building techniques. Moreover, they do not depend on the mining of natural clay, thus lowering the environmental impact connected with the procurement of raw materials. By encouraging waste recycling and, therefore, lowering the need for landfill disposal, the use of industrial waste including alumina waste helps to further the sustainability of unfired bricks [5,15].

Alumina waste, a byproduct of many industrial operations, including the generation of liquid nitrogen, presents one interesting substitute material for unfired brick manufacturing. Both silica (SiO₂) and alumina (Al₂O₃), both of which have pozzolanic qualities, abound in alumina waste. When combined with cement and water, pozzolanic materials chemically react to improve the strength and durability of the resulting composite material [11,16]. The capacity of alumina waste to react with calcium hydroxide (Ca(OH)₂ in the presence of water generates calcium silicate hydrates (C−S−H) and calcium aluminate hydrates (C−A−H), contributing to the strength and durability of the material [17]. Moreover, the high sodium oxide (Na₂O) content in alumina waste acts as an alkali activator, promoting the dissolution of amorphous aluminosilicates and accelerating the formation of binding gels, such as sodium aluminosilicate hydrate (N−A−S−H). These combined mechanisms improve the mechanical performance and durability of unfired bricks while partially replacing cement, thereby reducing clinker demand and associated CO₂ emissions. This dual mechanism results in a denser microstructure, lower water absorption, and improved durability. Thus, adding alumina waste into unfired bricks could offer a good approach to enhance the mechanical qualities of these bricks, as well as help waste recycling and lower the environmental effect of building components. Another useful resource is lateritic soil, common in tropical/subtropical areas and high in iron and aluminum oxides, which offers great plasticity [18]. Sivarajasingham et al. (1962) reported SiO₂/Al₂O₃ ratios between 1.33 and 2.0, suitable for brickmaking [19]. Based on tests like those by Anteneh & Mamuye (2019), clay–laterite bricks (5–20%) show a compressive strength of more than 10 MPa (megapascal), meeting Indian criteria [20,21]. Another study found that laterite and cement (1:3:7 ratio) CSEBs (compressed stabilized earth bricks) produced 28 days’ worth at 14.68 MPa [22]. Experiments combining laterite with pozzolana and cement also showed improved mechanical performance over mixes based just on cement. More recent research (2025) validates the optimal strength of clay mixtures using ~30% laterite.

Apart from alumina waste, lateritic soil has also been investigated as a possible component in unfired brick manufacturing. Rich in iron and aluminum oxides, lateritic soil, which is abundant in many tropical and subtropical areas, has the required plasticity and workability to be used in brickmaking [16]. Earlier research has shown that high-strength, durable bricks can be produced from laterite soil in combination with cement and other materials. Laterite soil’s inclusion in unfired bricks helps to increase the workability and density of the resultant product, thus enhancing its general performance. Alumina waste has been used partially in place of cement in concrete and other building materials according to several studies. Mola−Abasi et al. (2017), for instance, showed that adding alumina waste to cement-based mixtures increases the compressive strength and lowers the resulting material’s swelling [23]. Likewise, studies by Canpolat et al. (2004) revealed that alumina waste might be a good replacement for cement in concrete, improving the mechanical qualities of the material and lowering the environmental impact of cement manufacturing [24].

Furthermore, the economic feasibility of using industrial waste materials in construction is strongly influenced by fluctuations in raw material and energy prices. Over the past five years, the global price of ordinary Portland cement (OPC) has shown high volatility, rising by over 25% between 2020 and 2023 due to increased energy costs and supply chain disruptions [25]. In contrast, waste-derived materials such as alumina byproducts and locally sourced laterite soil are not subject to international market volatility, making them a more stable and predictable input for sustainable construction. In terms of policy alignment, the integration of alumina waste into brick production supports several regulatory and sustainability frameworks. For example, Thailand’s “National Roadmap on Waste Management (2016–2025)” encourages the valorization of industrial waste and promotes circular economy practices, aligning with Sustainable Development Goal 12 (Responsible Consumption and Production) [26]. Additionally, carbon pricing mechanisms, such as the voluntary carbon credit market under Thailand’s T-VER [27], provide financial incentives for reducing CO₂ emissions from construction materials. These policy mechanisms directly support the development of low-carbon alternatives such as the unfired bricks proposed in this study, which eliminate the need for high-temperature kiln firing and significantly reduce embodied carbon.

This work intends to investigate the possibilities of using alumina waste from liquid nitrogen generation in the development of unfired bricks in line with the increasing interest in sustainability and the use of alternative materials. The main goal is to evaluate the mechanical, thermal, and chemical characteristics of the produced bricks by substituting various amounts of alumina waste for different amounts of cement. This work will concentrate on the manufacturing of unfired bricks at several weight percentages using a mixture of lateritic soil, cement, and alumina waste. To assess their fit for building uses, the resultant bricks will be tested in a variety of ways, including compressive strength, water absorption, modulus of rupture, and thermal conductibility. The results of this study should provide an insightful analysis of the viability of using alumina waste and lateritic soil in unfired brick manufacturing, helping to create reasonably priced, environmentally friendly building materials.

2. Materials and Methods

The methodology employed in this study focuses on producing unfired clay brick composites using hydraulic cement (Figure 1a), laterite soil (Figure 1b), alumina waste (Figure 1c), and tap water as primary components. The laterite soil, sourced from a local compressed block factory in Mahasarakham Province, Thailand, exhibited a reddish−orange color. Alumina waste, obtained from a liquid nitrogen production company in Thailand, was invalid or expired for liquid nitrogen production. Hydraulic cement was purchased from a local supplier (Thai Watsadu Co., Ltd., Muang, Thailand). Prior to the brick preparation, all raw materials hydraulic cement, laterite soil, and alumina waste were dried at 105 °C for 24 h to remove moisture and, subsequently, sieved through a No. 16 mesh sieve (1.18 mm) to achieve a uniform particle size. The sieved materials were stored in airtight containers to maintain consistency and prevent moisture absorption.

Table 1. The fundamental properties of the laterite soil used in preparing the unfired brick composites.

Property	Experimental Result
Specific Gravity	2.64
Liquid Limit (%)	37.1
Plastic Limit (%)	20.9
Plasticity Index	16.2
Soil type (USCS Classification)	SC

USCS = Unified Soil Classification System.

provides the fundamental properties of the laterite soil used in production for unfired brick composites. These properties were critical in determining the soil’s suitability as a component in brick production. The soil’s specific gravity was measured at 2.64, indicating its density in relation to water. The Atterberg limits, which are essential in classifying soil plasticity, were also assessed, as follows: the liquid limit (LL) was 37.1%, the plastic limit (PL) was 20.9%, and the plasticity index (PI) was calculated to be 16.2. Based on the Unified Soil Classification System (USCS), the soil was categorized as silty clay (SC), highlighting its mix of sand and clay characteristics. These properties suggest that the soil possesses moderate plasticity, which can contribute to the workability and binding properties of the brick mixture. The combination of these factors, particularly the soil’s plasticity and specific gravity, provides insight into the material’s behavior in composite mixtures, impacting the mechanical and structural characteristics of the final brick product.

The preparation of unfired clay brick composites followed specific compositional ratios, as detailed in

Table 2. The preparation ratio of unfired clay brick composites.

No.	Code	Preparation Ratio by Weight (kg)
		Hydraulic Cement	Laterite Soil	Alumina Waste	Tap Water
1	HLA0 (1.0:6:0.0)	1 (14.286%)	6 (85.714%)	0.0 (0%)	0.7
2	HLA1 (0.9:6:0.1)	0.9 (13.858%)	6 (85.714%)	0.1 (0.428%)
3	HLA2 (0.8:6:0.2)	0.8 (11.429%)	6 (85.714%)	0.2 (2.857%)
4	HLA3 (0.7:6:0.3)	0.7 (10.000%)	6 (85.714%)	0.3 (4.286%)
5	HLA4 (0.6:6:0.4)	0.6 (8.572%)	6 (85.714%)	0.4 (5.714%)
6	HLA5 (0.5:6:0.5)	0.5 (7.143%)	6 (85.714%)	0.5 (7.143%)
7	HLA6 (0.4:6:0.6)	0.4 (5.715%)	6 (85.714%)	0.6 (8.571%)

. Seven sample formulations (HLA0 to HLA6) were prepared with varying percentages of hydraulic cement (strength grade: 32.5 MPa, ASTMC1157 TYPE GU) and alumina waste, while the proportion of laterite soil was maintained at 85.714% across all samples. The ratios of hydraulic cement ranged from 14.286% (HLA0) to 5.715% (HLA6), with alumina waste increasing incrementally from 0% to 8.571%. The amount of tap water was held constant at a ratio of 0.7 for all formulations to ensure consistency in the mixture.

The preparation process (Figure 2) began with accurately weighing the components according to the specified ratios using an electronic balance with an accuracy of ±0.01 g. The dry components laterite soil, hydraulic cement, and alumina waste were thoroughly mixed in a mechanical mixer to ensure homogeneity. Tap water gradually added to the dry mixture under continuous stirring until a uniform and workable paste was achieved. The preparation of brick specimens was conducted systematically to achieve the target density of 1700 kg m³ and ensure uniformity across all samples. Initially, the dry components hydraulic cement, soil, and alumina waste were thoroughly mixed to ensure homogeneity and prevent particle segregation. Following the dry mixing process, the predetermined amount of water gradually added while continuously mixing until a uniform and workable paste was obtained. The well−mixed material was then carefully poured into molds, specifically designed for brick production, with the weight of the mixture controlled and measured to ensure that the final bricks achieved the desired density. The bricks were produced in the following two specific dimensions: 65 mm × 140 mm × 40 mm in accordance with the Thai Industrial Standard (TIS 77-2545) and 100 mm × 100 mm × 100 mm. Larger bricks were set aside for thermal conductivity testing at 28 days; the smaller-sized bricks were tested for compressive strength at curing ages of 7, 14, 28, and 60 days; modulus of rupture; and water absorption.

The brick mixture was compacted using a manually operated hydraulic press, wherein the lever mechanism was fully engaged to achieve maximum compaction force. A controlled pressure of 10 MPa was applied, a level commonly employed in unfired brick studies to replicate field−scale mechanical compaction conditions [28]. Following compaction, the bricks were carefully demolded to avoid structural deformation and, subsequently, allowed to air-dry in a shaded area for 24 h at an ambient temperature. To ensure real-time control over brick density, each specimen was weighed before and after compaction, and its volume was calculated based on standardized mold dimensions. Bricks exhibiting a dry density deviation exceeding ±5% from the target value of 1.65 g/cm³ were either rejected or reprocessed to maintain consistency in material compaction and structural integrity. Following the initial resting period, the brick specimens underwent a curing process to promote hydration and enhance strength development. Each brick was first immersed in water for 2 min to ensure full moisture penetration, then allowed to drain off excess water. Subsequently, the specimens were individually sealed in plastic bags to maintain a stable internal moisture environment throughout the curing phase. To further ensure consistent humidity and prevent premature drying, a piece of water-saturated tissue paper was placed inside each bag, creating a nearly saturated atmosphere around the brick. This technique is widely adopted in cementitious materials research as a practical alternative to controlled curing chambers, effectively minimizing moisture loss due to evaporation while supporting ongoing hydration reactions. The sealed specimens were then stored under regulated conditions until the designated testing ages (0–28 days). This procedure guaranteed that for next investigations all bricks satisfied the necessary strength and durability requirements. At the specified testing intervals, this method guaranteed consistency in specimen preparation and allowed for accurate assessment of important physical and mechanical properties, including compressive strength, modulus of rupture, water absorption, and thermal conductivity.

2.1. Characterization of Raw Materials and Unfired Clay Brick Composites

2.1.1. Physical and Chemical Testing

To ascertain their morphological, mineralogical, chemical, and structural characteristics, the raw materials activated alumina waste from liquid nitrogen generation along with clay were methodically analyzed. The surface morphology and microstructural properties of the raw materials and the unfired clay brick composites were investigated using scanning electron microscopy (SEM, SEM−TM4000Plus, HITACHI, Tokyo, Japan ). The crystalline phases found were identified and quantified using X-ray diffraction (XRD, Bruke, D2 Phaser, Karlsruhe, Germany) analysis, so offering details on the mineralogical composition. By means of Fourier transform infrared spectroscopy (FTIR, Bruker (INVENIOR), the functional groups and chemical bonds were investigated, providing information on possible interactions between the clay matrix and the included activated alumina waste. X-ray fluorescence (XRF, AXIOS MAX) spectroscopy helped to ascertain the elemental composition of the samples. These characterization methods were carried out to gain thorough knowledge of the physical and chemical characteristics necessary for assessing the performance and qualities of the unfired clay brick composites.

2.1.2. Mechanical and Thermal Testing

Several necessary techniques are used in testing unfired clay bricks to assess their mechanical and thermal characteristics. Applying a progressively increasing load on a compression testing machine such as an Instron Universal Testing Machine (Chunyen, CY100) until the brick fails measures the compressive strength, and division of the maximum load by the cross-sectional area of the brick yields the strength. Weighing the brick dry, submerging it in water for 24 h, and then re-weighing it helps one find the water absorption percentage. Usually, one does this using a submersion tank and a digital scale. The brick is set on two supports, and a load is applied at the center until it fractures for the modulus of rupture (MOR). Measuring with a flexural testing machine such as the Chunyen (CY100, TIS 243-2520), the bending strength is computed using the load at rupture, span, and dimensions. Using a thermal conductivity analyzer such as a C−Therm, which heats one side of the brick and gauges the temperature gradient to ascertain its heat conductivity, allows one to evaluate the thermal conductivity. These tests taken together offer comprehensive knowledge of the strength, durability, and thermal efficiency of the brick.

2.2. Utilization of Unfired Clay Brick Composites Compared with Traditional Unfired Clay Bricks

2.2.1. Masonry Construction Using Unfired Clay Brick Composites

Unfired clay brick composites incorporating activated alumina waste were used to construct a masonry wall measuring 0.4 m × 0.6 m. Prior to construction, the bricks were fully soaked in water to ensure saturation, preventing moisture absorption from the mortar during laying. A cement mortar was prepared using a cement-to-sand ratio of 1:5, following the manufacturer’s recommendation. The bricks were laid using standard masonry techniques to evaluate their bonding performance with the cement mortar. The structural integrity of the wall was assessed through visual inspection and observation over a curing period of 30 days. The focus was on monitoring the adhesion strength, visible cracking, and overall mechanical stability compared with conventional fired bricks.

2.2.2. Plastering with Cement Mortar

Following the completion of the masonry, the wall surface was plastered using the same cement mortar mix. The plastering process was conducted to evaluate the adhesion of the cement layer to the surface of the unfired clay bricks. The plastered wall was subjected to curing under ambient conditions for 30 days. After curing, the wall was inspected for signs of detachment, slippage, cracking, or other failures that could indicate inadequate bonding. This testing aimed to validate the practical application of the unfired clay brick composites in conventional construction practices, particularly in terms of the masonry and plastering performance.

3. Result and Discussion

Figure 3 shows the hydraulic cement (Figure 3a), alumina waste (Figure 3b), and laterite soil (Figure 3c), illustrating the physical qualities of the raw materials used in the unfired brick composite’s preparation. A finely ground gray powder, hydraulic cement is absolutely vital as a binding and stabilizing agent. Mixing it with water causes hydration reactions that produce solid compounds, improving the mechanical strength and durability of the composites. Rich in iron and aluminum oxides, laterite soil, which has a reddish-orange hue, offers a composite bulk. Although it may compromise water resistance, its natural plasticity and workability make it the perfect base material; its porosity makes breathability possible. A sustainable filler, alumina waste is a light beige powder produced as a byproduct from liquid nitrogen generation; its small particles not only increase brick density but also help to lower voids, therefore improving both the mechanical and physical characteristics. These materials, taken together, produce a reasonably priced, environmentally friendly composite, in which the cement guarantees stabilization, the laterite soil serves as the main structural component, and the alumina waste encourages waste use while enhancing the brick performance. As demonstrated in Figure 3d, unique identifiers (HLA1, HLA2, HLA3, HLA4, HLA5, and HLA6) label the unfired clay bricks, implying that they reflect varying compositions or formulations of the components. The smooth and homogeneous surface of these bricks suggests well−mixed materials with few defects. The types and ratio of the hydraulic cement, alumina waste, and laterite soil powders used in their manufacture most certainly affect the consistent orange hue of the bricks. Each brick is appropriate for different uses depending on the particular formulations identified by the labels that might affect different properties, including strength, porosity, or thermal resistance. These bricks probably belong for use in construction or another industry where the material’s qualities, such as color, insulation, or durability, are crucial.

3.1. SEM Analysis

The figure presents a series of scanning electron microscope (SEM) images showcasing the surface morphology and microstructural characteristics of various samples. These images provide insight into the particle size, shape, texture, and extent of the particle agglomeration or bonding. Each micrograph was taken at high magnification, as indicated by the scale bars at the bottom of each image. Figure 4a displays relatively coarse and angular particles with visible porous surfaces and agglomerations. The large, irregular particles suggest raw or minimally processed materials, likely laterite soil or alumina waste, which are known for their rough, granular structures. The presence of voids between particles indicates poor compaction and limited cohesion, typical of raw materials before hydration or mixing with cementitious binders. Figure 4b highlights samples with more fragmented and finer particles, indicating mechanical or chemical breakdown processes. The smaller particle sizes and increased surface roughness could enhance the reactivity of the material during cementitious reactions. The visible microcracks and intergranular voids suggest that the material may have undergone partial hydration or early-stage binding reactions. Figure 4c, the microstructure reveals more compacted particles with reduced pore spaces, signifying improved particle packing, likely due to the addition of hydraulic cement. The formation of cementitious products such as calcium silicate hydrates (C−S−H) and calcium aluminate hydrates (C−A−H) may contribute to filling the voids and binding the particles together. The presence of some microcracks could be due to drying shrinkage or thermal stresses during sample preparation. Figure 4d displays a denser matrix with well-bonded particles, indicative of advanced hydration and cementation. The surface appears rough, with a network of fine particles embedded within a cementitious matrix. The reduced porosity suggests increased strength and durability, potentially corresponding to a mixture containing alumina waste, which enhances the formation of pozzolanic compounds. The tightly packed structure and absence of large cracks imply that alumina addition contributed to a denser and more cohesive microstructure. Overall, the progression from loose, coarse particles to a dense, compacted matrix illustrates the impact of hydration and alumina addition on microstructural development. The improved particle packing, reduced porosity, and formation of cementitious phases contribute to enhanced mechanical properties, making the alumina-modified lateritic soil–cement composite a promising material for construction.

3.2. XRD Analysis

The X−ray diffraction (XRD) pattern shown in Figure 5 and

Table 3. XRD pattern interpretation summary.

Sample	Main Phases Identified	Key Peaks (2θ)	Phase Characteristics	Key Miller Indices (hkl)	Main Phases Identified (Full Names)	Key References for XRD Phases
Pure Soil	Quartz, Kaolinite, Illite	~20.8°, ~26.6°, ~36.5°	Natural clay minerals, moderate crystallinity	(001), (101), (112)	Silicon Dioxide (SiO2), Kaolinite (Al2Si2O5(OH)4), Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)]	JCPDS 46-1045 (Quartz); [29]
Pure Cement	C3S, C2S, C3 A, C4AF, Quartz	~29°, ~32°, ~34°, ~52°, ~26.6°	Typical cement phases, low α-Al2O3 presence	(012), (020), (141), (211), (101)	Tricalcium Silicate (Ca3SiO5), Dicalcium Silicate (Ca2SiO4), Tricalcium Aluminate (Ca3Al2O6), Tetracalcium Aluminoferrite (Ca4Al2Fe2O10)	JCPDS 49-0442 (C3S) [30]
Pure Alumina	α-Al2O3 (Corundum)	~35.2°, ~43.4°, ~52.6°	High crystallinity, sharp intense peaks	(104), (113), (024)	Alpha-Aluminum Oxide (Corundum)	JCPDS 10-0173 [31]
20% Alumina	Quartz, Kaolinite, Illite, α-Al2O3	~20.8°, ~26.6°, ~35.2°, ~43.4°, ~52.6°	Hybrid profile, partial incorporation of α-Al2O3	(001), (101), (104), (113), (024)	Silicon Dioxide (SiO2 Kaolinite (Al2Si2O5(OH)4), Illite(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)], α-Al2O3 (Corundum)	JCPDS 10-0173 [29,30]

illustrates the crystalline phases present in the various material compositions, including laterite soil, hydraulic cement, alumina waste, and mixtures containing 20% alumina. The XRD patterns of the four samples demonstrate clear distinctions in phase composition and crystallinity, enabling a structural interpretation of phase interactions in alumina-based composites. Pure alumina (α−Al₂O₃) exhibited intense and sharp diffraction peaks, particularly at 2θ values of ~35.2°, ~43.4°, and ~52.6°, which correspond to the (104), (113), and (024) planes of the corundum. The well-defined nature of these reflections confirms the high purity and crystallinity of the α-alumina. The absence of amorphous halos and peak broadening suggests a large crystallite size and minimal lattice strain. In contrast, pure soil is dominated by mineral phases such as quartz (notably at ~26.6°) and clay minerals (e.g., kaolinite and illite) with peaks around ~20.8°, reflecting a more heterogeneous, less crystalline composition typical of natural geomaterials. The peaks are narrower than amorphous backgrounds but broader than those of pure alumina, indicating moderate crystallinity. The XRD pattern of pure cement reveals the characteristic phases of Portland cement, such as C₃S (alite), C₂S (belite), C₃A, and C₄AF appearing across 2θ values of ~29°, ~32°, ~34°, and ~52°. Notably, α−alumina peaks are absent, confirming no contribution from corundum. A residual quartz peak is observed, likely due to added silica or fillers in the raw mix. The 20% alumina mixed sample displays a hybrid profile. Peaks from quartz (~26.6°) and clay minerals (~20.8°) persist, affirming the soil–cement matrix contribution. Meanwhile, the emergence of distinct α-alumina peaks at ~35.2°, ~43.4°, and ~52.6° indicates the partial incorporation of crystalline alumina. However, the lower intensity of these peaks compared with the pure α-Al₂O₃ sample suggests limited dispersion or incomplete phase transformation at this doping level. No significant peak shifting was observed, implying minimal lattice substitution or interphase diffusion. In conclusion, the combined XRD analysis reveals the distinct phase composition of each component and the transformation pathway upon alumina addition. Pure alumina is highly crystalline with dominant α−Al₂O₃ phases. Pure soil consists mainly of quartz and clay minerals, while cement contributes silicate and aluminate phases without alumina-specific peaks. The 20% alumina mixture demonstrates a successful integration of crystalline α-Al₂O₃ into a cement−soil matrix, as evidenced by the presence of characteristic alumina peaks. This supports the feasibility of using alumina as a functional additive for modifying mineral-based matrices. However, the incomplete dominance of α−Al₂O₃ peaks suggests that higher alumina content or enhanced dispersion/sintering processes may be necessary to fully develop the corundum structure within the blended system.

3.3. ATR-FTIR Analysis

The ATR-FTIR spectra presented in Figure 6 and

Table 4. The key functional groups and their associated absorption peaks from the ATR–FTIR spectra of materials used for the preparation unfired clay brick.

Raw Material	Functional Group	Absorption Peak (cm–1)	Vibration Type
Hydraulic cement	C–Br or C–I stretching	527, 458	Stretching vibrations
Laterite soil	C–Br stretching	688	Stretching vibrations
	C=C bending, alkene monosubstituted	997, 908	Bending vibrations
	S=O stretching (sulfonyl chloride)	1407	Stretching vibrations
Alumina waste	S=O stretching (sulfoxide)	1033	Stretching vibrations
	C=C bending, alkene monosubstituted	997, 908	Bending vibrations
	C=C bending, trisubstituted	807	Bending vibrations
Unfired clay brick	C–Br or C–I stretching (from hydraulic cement)	527, 458	Stretching vibrations
	C–Br stretching (from laterite soil)	688	Stretching vibrations
	N–O stretching (from nitro compound)	1554	Stretching vibrations
	S=O stretching (from sulfonyl chloride)	1407	Stretching vibrations
	C=C bending, alkene monosubstituted (from alumina waste)	997, 908	Bending vibrations

highlights the chemical interactions and functional groups of the raw materials hydraulic cement, laterite soil, and alumina waste as well as the final unfired clay brick composite. The spectrum for hydraulic cement shows significant absorption peaks at 527 cm⁻¹ and 458 cm⁻¹, which are attributed to C–Br or C–I stretching vibrations, indicating the presence of silicate phases crucial for binding and strength. For laterite soil, characteristic peaks appear at 688 cm⁻¹, corresponding to C-Br stretching vibrations, while the bands at 908 cm⁻¹ and 997 cm⁻¹ represent C=C bending vibrations of alkenes, reflecting the soil’s iron-rich and mineral composition. Additionally, a strong peak at 1407 cm⁻¹ corresponds to S=O stretching, suggesting sulfur-containing compounds. In the alumina waste spectrum, peaks at 1033 cm⁻¹ signify S=O stretching vibrations, and bands at 997 cm⁻¹ and 908 cm⁻¹ indicate C=C bending vibrations. A medium-intensity peak at 807 cm⁻¹ aligns with trisubstituted alkenes, pointing to alumina-related oxides and reactive compounds. The spectrum of the final unfired clay brick composite integrates key features from all three raw materials, reflecting their chemical synergy. Peaks at 688 cm⁻¹ (halo compounds) and 1554 cm⁻¹ (N–O stretching vibrations) highlight contributions from laterite soil and alumina waste, while absorption bands at 527 cm⁻¹ and 458 cm⁻¹ confirm the influence of hydraulic cement, signifying the presence of silicate phases. Additionally, the S=O stretching observed at 1407 cm⁻¹ further validates the role of sulfur-rich compounds in the composite. The overlapping absorption bands in the composite spectrum confirm the effective interaction of hydraulic cement as a stabilizer, laterite soil as the structural component, and alumina waste as a filler. This combination not only enhances the mechanical strength and density of the unfired clay bricks but also reflects an eco-friendly approach to utilizing industrial waste.

3.4. XRF Analysis

Table 5. Chemical composition of the hydraulic cement, laterite soil, and alumina waste analyzed by XRF.

Component	Weight (%)
	Hydraulic Cement	Laterite Soil	Alumina
SiO2	23.942	79.029	57.917
Al2O3	4.446	17.761	27.793
CaO	59.317	0.058	0.679
Fe2O3	3.220	2.187	0.980
K2O	0.410	0.092	0.159
SO3	5.179	-	0.208
MgO	2.687	-	-
Na2O	-	-	12.058

and Figure 7 presents the chemical composition of the hydraulic cement, laterite soil, and alumina waste as analyzed using X–ray fluorescence (XRF). These data highlight significant differences in the composition of these materials, reflecting their unique chemical characteristics and suitability for unfired clay brick production.

Hydraulic cement is primarily composed of calcium oxide (CaO), accounting for 59.317% by weight, which is a defining characteristic of cement and essential for its hydraulic properties. It also contains notable amounts of silicon dioxide (SiO₂) (23.942%) and smaller proportions of aluminum oxide (Al₂O₃) (4.446%), which contribute to its strength development during hydration. Additionally, cement includes minor constituents, such as sulfur trioxide (SO₃) (5.179%), magnesium oxide (MgO) (2.687%), and iron oxide (Fe₂O₃) (3.220%), which play roles in its performance and setting behavior, as indicated in

Table 6. Roles and effect of MgO, SO₃, and Na₂O in hydraulic cement and alumina waste.

Compound	Component	Role and Effect
MgO	Hydraulic Cement (2.687%)	Magnesium oxide (MgO) plays a dual role in cement chemistry. At moderate levels, it contributes to the formation of magnesium silicate hydrates (M–S–H), which improve long-term strength and dimensional stability. However, excessive MgO may cause delayed expansion due to the formation of periclase. In this study, the MgO level is well within limits, contributing to enhanced durability without compromising dimensional stability.
SO3	Cement (5.179%) Alumina Waste (0.208%)	Sulfur trioxide (SO3) regulates the setting time and controls the early strength gain of cement. In cement, SO3 exists mainly in the form of calcium sulfoaluminates (ettringite), which prevent flash setting and promote early hydration. The minor SO3 in alumina waste enhances sulfate availability without overwhelming the system. Thus, SO3 helps maintain appropriate rheological and setting behavior, especially in unfired bricks cured under ambient conditions.
Na2O	Alumina Waste (12.058%)	Sodium oxide (Na2O) acts as a strong alkali activator. In the presence of reactive silica and alumina, Na2O can promote alkali-activated (geopolymeric) reactions, enhancing early-stage pozzolanic activity and strength development. The high Na2O content in alumina waste improves the reactivity of amorphous aluminosilicates, boosting the formation of cementitious gels (N–A–S–H type). Its presence compensates for the lower CaO in waste-based formulations, enhancing overall binding mechanisms.

. Laterite soil, on the other hand, is highly rich in silicon dioxide (SiO₂), constituting 79.029% of its total weight, indicating its silicate−rich nature, which is typical of lateritic soils. It also has a significant amount of aluminum oxide (Al₂O₃) (17.761%), suggesting its potential pozzolanic activity and compatibility with cementitious materials. The soil contains negligible calcium oxide (CaO) (0.058%) and trace amounts of iron oxide (Fe₂O₃) (2.187%) and potassium oxide (K₂O) (0.092%), reinforcing its low lime content and siliceous nature. In other words, this chemical profile emphasizes the need for external calcium and alkali sources provided by cement and alumina waste to drive pozzolanic and hydration reactions. The high silica and alumina contents of the soil (SiO₂: 79.029%; Al₂O₃: 17.761%) makes it an ideal pozzolanic partner, but its low inherent reactivity necessitates the activation role played by Na₂O and the stabilization functions of CaO and MgO. Alumina waste exhibits a distinct composition (Figure 7), with silicon dioxide (SiO₂) at 57.917% and a high concentration of aluminum oxide (Al₂O₃) at 27.793%. These two oxides suggest that the material could enhance pozzolanic reactions when combined with cement. Additionally, alumina waste contains a notable quantity of sodium oxide (Na₂O) (12.058%), which is not observed in the other materials. Minor components include calcium oxide (CaO) (0.679%), iron oxide (Fe₂O₃) (0.980%), potassium oxide (K₂O) (0.159%), and sulfur trioxide (SO₃) (0.208%). It can be seen that the presence of minor oxides, such as MgO (2.687%), SO₃ (5.179% in cement and 0.208% in alumina), and Na₂O (12.058% in alumina) plays a synergistic role in enhancing the brick performance. MgO contributes to long-term dimensional stability through M-S-H phase formation. SO₃, as a setting regulator, aids early hydration and improves workability, particularly crucial in unfired systems. Na₂O acts as an alkali activator, facilitating the dissolution of amorphous silica and alumina to form binding gels, thereby densifying the composite matrix. Their roles are substantiated by FTIR and XRD analyses, which show characteristic sulfate, silicate, and aluminate bands and reflections.

Overall, the chemical composition analysis, as shown in Figure 7, indicates that laterite soil and alumina can complement the cement matrix due to their high silicate and alumina contents. The alumina-rich alumina waste, in particular, could enhance the pozzolanic reaction, contributing to the strength and durability of the unfired clay brick composites. The hydraulic cement, with its dominant calcium oxide content, serves as the primary binder, while laterite soil and alumina waste act as supplementary materials that potentially improve the brick’s physical and mechanical properties. This combination highlights the synergistic use of locally available materials to develop sustainable and cost-effective building components.

3.5. Compressive Strength

The compressive strength test results for brick specimens measuring 65 mm × 140 mm × 40 mm were assessed at the ages of 7, 14, 28, and 60 days. The results, averaged from seven specimens, are presented in Figure 8. The findings indicate that as the cement replacement with alumina waste increased up to 20% (HLA2), the compressive strength showed a corresponding increase. However, beyond a 20% replacement, the compressive strength tended to decrease. The 20% replacement level consistently exhibited the highest compressive strength across all testing ages. Notably, the substitution of cement with alumina waste at 10%, 20%, and 30% led to an enhancement in the compressive strength of the clay bricks, with the strength increasing over time due to the hydration reaction between cement and water. The mixes that replaced cement with alumina waste at 10%, 20%, 30%, and 40% demonstrated significant improvements in compressive strength between the ages of 28 and 60 days. Specifically, the HLA2 mix achieved the highest strength gains. The observed strength development in the HAL2 mixture (20% alumina waste replacement) can be attributed to the pozzolanic reaction between the reactive silica and alumina present in the alumina waste and the calcium hydroxide (Ca(OH)₂) released during cement hydration. This optimal balance facilitates the formation of additional calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H) phases, contributing to a denser microstructure and enhanced mechanical properties. The SEM analysis (Figure 4d) supports this by revealing a compact matrix with minimal porosity, while the XRD results (Figure 5) confirm the partial incorporation of crystalline α−Al₂O₃ into the cement–soil system, alongside the persistent presence of quartz and clay minerals. Complementary FTIR spectra further validate the formation of silicate and aluminate bonding environments indicative of pozzolanic activity. Together, these findings demonstrate that the synergistic mineralogical transformations and microstructural densification induced by alumina waste are directly responsible for the enhanced compressive strength (177.4 ksc, kilogram−force per square centimeter) and modulus of rupture (33.3 ksc) observed in HAL2. This integrated microstructural–mechanical interpretation highlights the critical role of alumina waste in improving the performance of unfired clay brick composites. However, at higher replacement levels, the excess alumina waste exceeds the available calcium hydroxide (Ca(OH)₂) from the cement hydration, limiting the extent of pozzolanic reactions. This imbalance results in insufficient formation of binding phases such as C–S–H and C–A–H, thereby weakening the overall binder matrix and reducing the mechanical strength of the composites.

When comparing the compressive strength of the brick specimens with the standards set forth by TIS 77-2565 [32], it was found that all mixes and test ages met the requirements for quality class C. Over 28 days, the mixes with 10% (HAL1) and 20% (HAL2) alumina waste replacement also met the criteria for quality class B. Furthermore, by the age of 60 days, the mixes with 10% (HAL1), 20% (HAL2), and 30% (HAL3) alumina waste replacement satisfied the requirements for quality class A.

3.6. Water Absorption

Figure 9 presents the average water absorption of seven clay brick specimens tested at 28 days of age. The data reveal that replacing cement with 10% to 30% alumina waste results in a reduction in water absorption. Specifically, the mixes with 20% and 30% alumina waste demonstrated lower water absorption compared with the control mix without alumina waste. This behavior aligns with findings by Sing et al. (2016) and Mehta & Monteiro (2014), who reported that partial replacement of cement with pozzolanic materials can reduce porosity and enhance the microstructure, thereby lowering the water absorption [33,34]. However, when cement is replaced with 40% to 60% alumina waste, the water absorption tends to increase with a higher alumina waste content. Despite this trend, the water absorption values across all seven mixes are relatively similar. Therefore, it can be concluded that substituting cement with up to 30% alumina waste can effectively reduce the water absorption, while higher replacement levels (above 40%) cause only a slight increase in the water absorption. The observed water absorption values are consistent with the compressive strength and modulus of the rupture characteristics. In general, mixes with a higher compressive strength and modulus of rupture tend to exhibit lower water absorption [35]. According to the TIS 77−2565 standards, all tested mixes met the water absorption requirements for quality classes A and B, demonstrating that alumina waste can be utilized as a sustainable alternative binder without compromising performance [32].

3.7. Modulus of Rupture Results

The results of the modulus of rupture (MOR) tests for the various brick mixtures at 28 days are presented in Figure 10. The data indicates that the mixtures containing 10% (HAL1) to 40% (HAL4) alumina waste as a partial replacement for cement exhibit higher MOR values compared with the control mix without alumina waste. Conversely, mixtures with 50% (HAL5) to 60% (HAL6) alumina waste replacement show slightly lower MOR values than the control [36,37]. The trend in MOR closely aligns with the compressive strength results, where mixtures with higher compressive strength also tend to have higher MOR values. The modulus of rupture represents the material’s flexural strength or resistance to fracture when subjected to bending forces or impact. Therefore, incorporating 10% (HAL1) to 40% (HAL4) alumina waste as a cement replacement enhances the flexural strength and reduces the likelihood of fracture in the unburnt clay bricks [38]. These findings demonstrate that partial replacement of cement with alumina waste can improve the structural integrity of the bricks by increasing their resistance to breaking under load. The bar graph in Figure 10 illustrates the MOR results of the brick samples at 28 days for different mixtures (HAL0 through HAL6), where the percentage of cement replaced with alumina waste ranges from 0% (HAL0) to 60% (HAL60). From the graph, it is evident that the MOR values increase with alumina waste content up to 30%, after which they begin to decline. Specifically, HAL1, HAL2, and HAL3 exhibit MOR values of 33.17, 33.25, and 34.55 ksc, respectively, indicating significant improvement over the control mixture (HAL0) with an MOR of 29.22 ksc. These results align with previous studies that suggest alumina waste’s pozzolanic properties enhance mechanical strength by promoting secondary hydration reactions [39], which improve the microstructure of the matrix. Beyond 30% (HAL3) alumina waste replacement, however, the MOR values begin to decrease. HAL4, HAL5, and HAL6 show a noticeable reduction in the MOR values at 30.71, 27.28, and 26.44 ksc, respectively. This decline could be attributed to an excessive replacement of cement with alumina waste, which dilutes the amount of calcium hydroxide available for the pozzolanic reaction, resulting in a weaker binder matrix. Similar trends have been reported in the literature, where alumina waste replacement beyond 30% (HAL3) diminishes the mechanical properties due to the reduction in the cementitious phase [40]. The modulus of the rupture’s consistency with the compressive strength trends further confirms that alumina waste contributes to both compressive and flexural strength at moderate replacement levels (10–30%). The slight error bars, particularly in HAL3, reflect minimal variability in the MOR values, reinforcing the robustness of the test results. In conclusion, the use of alumina wastes up to a 30% replacement improves the modulus of rupture, consistent with prior research, while higher replacement levels diminish the mechanical benefits due to a reduced cementitious matrix.

3.8. Thermal Conductivity

Figure 11 presents the thermal conductivity values at 28 days of age for clay bricks, showing that mixtures containing 10% to 40% alumina waste, used as a partial replacement for cement, exhibit higher thermal conductivities than those without alumina waste. Consequently, bricks containing alumina waste demonstrate reduced thermal insulation properties. This outcome is consistent with findings by Demirboğa and Gül (2003), who observed that denser matrices resulting from certain pozzolanic replacements may lead to increased thermal conductivity due to reduced air voids [41]. However, the differences in thermal conductivity among the various mixtures are not substantial, and the trend in thermal conductivity closely mirrors that of compressive strength and modulus of rupture [42]. The graph further illustrates the thermal conductivity of various alumina waste–cement mixtures (HAL0 to HAL6), both before and after adjustment. HAL0, with no alumina waste, serves as the baseline with moderate conductivity, while increasing the alumina waste content from HAL1 to HAL3 causes a rise in thermal conductivity, with HAL3 showing a marked increase after adjustment. HAL4, containing 40% alumina waste, exhibits the highest thermal conductivity, indicating the most significant reduction in insulation properties. Interestingly, as the alumina waste content increases to HAL5 and HAL6, the thermal conductivity decreases, suggesting that higher alumina waste levels may help partially restore insulating capacity. This observation aligns with research, which highlights how excessive replacement levels can disrupt packing density and reduce heat conduction [43]. These trends highlight the complex relationship between alumina waste content and thermal performance, where moderate alumina waste content increases conductivity but excessive amounts reduce this effect. The adjustments made post−test refine the results, potentially accounting for experimental variations. This behavior suggests the need to optimize alumina waste content for specific applications, especially when considering thermal conductivity alongside other mechanical properties, such as compressive strength.

3.9. Comparison of Properties Between Unfired Clay Bricks Obtained and Traditional Clay Bricks

Table 7. Comparison of the properties of unfired activated alumina-waste-blended clay bricks and traditional fired clay bricks.

Property	Unfired Clay Bricks	Traditional Clay Bricks	TIS 77-2565 [32]
Compressive Strength (ksc)	177.4 (HAL2)	26.4 to 141.2	173 (Quality Class B)
	117.3 (HAL6)		102 (Quality Class C)
Water Absorption %	11.9 (HAL2)	13.5 to 17.7	22 (Quality Class B)
	13.4 (HAL6)
Modulus of Rupture (ksc)	33.3 (HAL2)	5.5 to 11.5	Not specified
	26.5 (HAL6)
Size Deviation (mm)	None	1.0 to 9.5	±2 to ±5, Depending on size
Density (g/cm3)	1.700	1.255 to 1.389	Not specified
Thermal Conductivity	1.17 (HAL2)	1.15	Not specified
	0.94 (HAL6)

illustrates the comparison of properties between unfired clay bricks and traditional clay bricks, which reveals significant differences in their performance metrics, particularly in the context of sustainable construction materials. The unfired clay bricks demonstrate compressive strengths of 177.4 ksc for the HAL2 mix and 117.3 ksc for the HAL6 mix, which are considerably higher than the traditional fired clay bricks, which range from 26.4 to 141.22 ksc according to TIS 77-2565 standards [32]. This higher compressive strength indicates that unfired bricks may be more suitable for load-bearing applications, likely due to the enhancement provided by the activated alumina waste. Additionally, the water absorption rates for unfired clay bricks are lower, at 11.9% (HAL2) and 13.4% (HAL6), compared with traditional bricks, which absorb between 13.5% and 17.72%. This suggests that unfired bricks offer better resistance to moisture ingress, critical for durability. The modulus of rupture is also significantly higher for unfired bricks, recorded at 33.3 ksc for the HAL2 mix and 26.5 ksc for the HAL6 mix, contrasting sharply with traditional bricks that range from 5.5 to 11.53 ksc. This increased flexural strength enhances their suitability for structural applications. Furthermore, unfired bricks show no size deviation, while traditional bricks exhibit a deviation of 1.0 to 9.52 mm, indicating better manufacturing precision in unfired bricks. In terms of density, unfired bricks have a density of 1.700 g/cm³ compared with the 1.255 to 1.3893 g/cm³ range of traditional bricks, correlating with improved mechanical properties. Lastly, thermal conductivity values for unfired bricks (1.17 for HAL2 and 0.94 for HAL6) are comparable to traditional bricks (1.154), suggesting effective thermal insulation. However, while it is generally true that denser materials tend to exhibit higher thermal conductivity due to increased particle contact and reduced porosity, the observed case of traditional clay bricks showing higher thermal conductivity despite their lower bulk density can be attributed to several microstructural and compositional factor such as firing-Induced mineralogical transformation, pore morphology and connectivity, matrix composition and bonding and effect of alumina waste and additives. These considerations explain why traditional fired bricks, though less dense, can have higher thermal conductivity compared with the alumina-modified unfired bricks in this study. Similar findings were reported by Demirboğa and Gül (2003) [41], who showed that the thermal behavior of construction materials is not solely dependent on density but also on phase composition and pore connectivity. Overall, the superior properties of unfired activated alumina-waste-blended clay bricks position them as a sustainable alternative in construction, offering enhanced performance while contributing to waste minimization efforts in the industry. Future research should focus on their long-term durability and environmental impacts in various construction contexts.

3.10. Masonry and Plastering a Wall with Cement Testing

The experimental construction using unfired clay bricks was carried out to evaluate their practical performance in masonry and plastering applications. A wall measuring 0.4 m × 0.6 m was built and plastered using a cement mortar specifically formulated for such purposes. The mortar was prepared according to the manufacturer’s instructions, using a cement to sand ratio of 1:5 by volume. Prior to construction, the bricks were presoaked in clean water for approximately 30 min to achieve full saturation, a standard practice to prevent excessive moisture absorption from the mortar and to ensure proper adhesion during bricklaying. The experiment found that, like with conventional fired bricks, the adhesion between the unburnt bricks and the cement mortar was strong and constant. Often a cause of concern in brick and mortar construction, the cement also adhered well to the surface of the unburned bricks during the plastering process, showing no evidence of detachment or slippage. No obvious cracks or structural flaws were seen after letting the structure cure for thirty days, indicating the mechanical stability and dependability of these unfired bricks in useful application. This outcome is important in verifying, under normal building conditions, the structural integrity of unfired clay bricks. The presoaking of the bricks before masonry is one important component of the study. Saturation prevents the bricks from absorbing too much moisture from the mortar, thus preserving the bond and preventing weak joints. The successful adhesion of mortar and the absence of cracks post−curing highlight the viability of unburnt bricks for building uses, implying they are a sustainable alternative to conventional fired bricks. Regarding their mechanical characteristics, the unfired bricks provide an equivalent performance in terms of adhesion and structural stability, so not less than their fired counterparts.

Through the partial cement replacement of activated alumina waste, the experimental use of unfired clay bricks as shown in Figure 12 shows notable improvements in construction material performance. It is clear from comparing the compressive strength and water absorption qualities based on the TIS 77-2565 standard [32] that activated alumina waste improves the structural and durability qualities of the bricks. Specifically, optimizing brick performance by substituting activated alumina waste at 30%, 40%, and 60% for varying quality grades (A, B, and C). Cement is replaced with A material with pozzolanic qualities and activated alumina waste enhances the microstructure of the bricks, thus producing a better compressive strength, reduced water absorption, and higher modulus of rupture than conventional fired bricks. Long-term durability and environmental stress resistance depend on this performance increase. While the initial performance of the alumina-based unfired bricks is promising; long−term durability under environmental exposure should be further studied in future work. Similar valorization of waste materials has shown potential for maintaining the structural integrity and environmental safety over time [44]. Although unburnt bricks made from alumina waste show a somewhat lower density, their thermal conductivity is still similar to that of fired bricks, thus preserving sufficient insulation qualities. Apart from these technical advantages, the application of activated alumina waste presents major financial and environmental ones. Unburnt brick manufacturing reduces carbon emissions and costs by lowering the demand for cement and doing away with the energy-intensive firing method. The experiment shows, generally, the possibilities of alumina-waste-modified unburnt bricks as a sustainable and effective substitute for conventional materials, providing better performance while lowering the environmental impact of construction. It is clear that using activated alumina in brick manufacturing not only improves the physical qualities but also lessens the general environmental effect. Often derived from waste products, activated alumina offers an economically feasible solution by lowering the demand for cement, which is connected with significant CO₂ emissions. Furthermore, reducing the carbon footprint is the manufacturing of unburnable bricks, which replaces the necessity of energy-intensive firing techniques. Recent studies confirm these findings, demonstrating how materials like alumina waste might improve masonry building performance even more [45,46].

3.11. Cost Analysis for Unfired Clay Brick Production

Table 8. Comparing the cost analysis between unfired clay bricks (using alumina waste) and commercial fired clay bricks in terms of material costs, energy consumption, production process, environmental costs, durability, and overall cost.

Cost Factor	Unfired Clay Bricks	Commercial Fired Clay Bricks
Material Cost	Lower due to alumina waste (byproduct) and laterite soil	Higher, relies on raw clay and energy intensive materials
Energy Consumption	Significantly reduced (no firing required)	High (firing process requires significant energy)
Production Process	Simpler, no need for high-temperature furnace, reduces labor and overhead costs	More complex, requires specialized equipment (kilns) and skilled labor
Labor Costs	Lower due to simplified production process	Higher due to more involved production process
Environmental Impact	Reduced CO2 emissions, uses waste materials (alumina waste), and sustainable materials (laterite soil)	High CO2 emissions, fuel use for firing, and does not utilize industrial waste products
Durability	Higher compressive strength, lower water absorption, and improved modulus of rupture	Good compressive strength but higher water absorption, leading to potential long-term maintenance issues
Overall Cost per Unit	Likely lower per unit due to reduced material and energy costs	Higher per unit due to higher material, energy, and labor costs
Economic Benefits and Scale	Significant advantages at scale, reduced costs for large-scale production, long-term economic benefits through durability and lower maintenance	Higher ongoing material and energy costs, but established mass production infrastructure helps maintain market position
Market Pricing	Lower cost compared to traditional fired bricks	Higher cost due to energy, material, and labor
Material Cost	THB 2 to 4 (USD ≈0.06 to USD 0.12)	THB 15 to 30 (USD ≈0.45 to USD 0.90)
Energy Cost	THB 0 (USD ≈0.00)	THB 5 to 10 (USD ≈0.15 to USD 0.30)
Labor and Production Cost	THB 2 to 5 (USD ≈0.06 to USD 0.15)	THB 5 to 8 (USD ≈0.15 to USD 0.24)
Environmental Cost	THB 1 to 2 (USD ≈0.03 to USD 0.06)	THB 3 to 5 (USD ≈0.09 to USD 0.15)
Total Price per Brick	THB 5 to 11 (USD ≈0.15 to USD 0.33)	THB 28 to 53 (USD ≈0.84 to USD 1.60)

Note: 1. THB = Thai baht; $ = USD. 2. The environmental costs shown in Table 6 were calculated based on CO₂ emissions per brick and the carbon price. Unfired bricks emit ~0.1–0.2 kg CO₂/brick, while fired bricks emit ~0.5–1.2 kg CO₂/brick. Using a carbon price of 0.5 THB/kg CO₂, the estimated cost is THB 0.05–0.10 for unfired bricks and THB 0.25–0.60 for fired bricks. These values were rounded and adjusted to account for other environmental impacts, resulting in a final cost of THB 1–2 for unfired bricks and THB 3–5 for fired bricks. 3. In the experimental construction, the same mortar mix (1:5 cement-to-sand ratio) was used for both brick types, ensuring consistency in cost comparison. While higher-strength bricks may permit or require stronger mortar in certain structural applications, no increase in mortar grade or cement content was applied in this study.

shows that unfired clay bricks have noticeably lower material and energy costs. While conventional fired clay bricks cost between THB 28 and 53 (USD ≈0.84 to USD 1.60), the total cost per brick runs from THB 5 to 11 (USD ≈0.15 to USD 0.33). This price difference is mostly related to the use of waste materials like alumina waste and laterite soil in unfired clay bricks, which not only lower material costs but also offer environmental advantages including lowered CO₂ emissions. Furthermore, unlike conventional fired bricks, which require a lot of energy to be fired at high temperatures, unfired clay bricks consume very little. While traditional fired bricks require more skilled labor and specialized equipment like kilns, thus increasing costs, labor and production costs are also lower for unfired clay bricks because their manufacturing technique is simpler. With better compressive strength and lower water absorption than conventional fired bricks, unfired clay bricks are more durable and can help to save long-term costs due to less maintenance. All things considered, unfired clay bricks offer a sustainable and reasonably priced substitute for conventional fired clay bricks, therefore saving both money and the environment.

3.12. Carbon Release (CO₂ Emissions) and Social Return on Investment (SROI)

Especially in terms of carbon reduction and social return on investment (SROI), unfired clay bricks provide major environmental and social advantages. Unfired clay bricks have low carbon emissions, unlike conventional fired clay bricks, which require great energy consumption during the firing process. Whereas unfired bricks contribute only about 100 to 200 kg of CO₂ per 1000 bricks, the firing process of traditional bricks releases between 500 and 1200 kg of CO₂ per 1000 bricks, producing a carbon savings of 400 to 1100 kg of CO₂ per 1000 bricks. This makes unfired clay bricks a far more ecologically friendly choice, greatly lowering the carbon footprint related with brick manufacturing. In addition, the use of alumina waste and the elimination of the firing process result in a substantial reduction in CO₂ emissions compared with conventional fired clay bricks. This is consistent with previous LCA studies on unfired bricks incorporating industrial byproducts, which report reductions in embodied carbon by 40–60% due to the avoidance of high-temperature processing [47].

Unfired clay bricks have great social and financial return on investment. Driven by elements like reduced energy consumption, improved durability (which results in lower long-term maintenance costs), and the use of waste materials like alumina waste and laterite soil, every THB 1 (or USD 0.03) invested in manufacturing unfired clay bricks generates almost THB 2 (or USD 0.06) of social value. The SROI analysis shows that unfired clay bricks have a 2:1 return on investment, thus they are not only reasonably affordable but also quite helpful from social and environmental angles. With significant long-term advantages in terms of carbon reduction, durability, and cost savings, unfired clay bricks thus offer a sustainable and socially responsible substitute for conventional fired bricks.

Figure 13 illustrates the carbon emissions per 1000 bricks, clearly showing that unfired clay bricks have significantly lower emissions, with only around 200 kg of CO₂, compared with the 1000 kg of CO₂ emitted by traditional fired clay bricks. This highlights the environmental advantage of unfired clay bricks, as they contribute much less to carbon pollution. The second graph presents the SROI for both types of bricks, revealing that unfired clay bricks generate a higher social return, with an estimated THB 20 of social value per brick, while traditional fired clay bricks show no social value contribution. This analysis follows the principles outlined by Social Value International and aligns with prior applications of SROI in evaluating sustainable construction materials [48,49]. This comparison visually emphasizes the environmental and social benefits of using unfired clay bricks, making them a more sustainable and socially responsible option compared with traditional fired clay bricks.

3.13. Significance of the Research

A sustainable alternative to conventional fired bricks is proposed through the development of unfired clay bricks incorporating industrial alumina waste sourced from CO₂ adsorption systems in liquid nitrogen production as a partial substitute for hydraulic cement. Unlike conventional bricks, which require high-temperature firing and result in significant energy consumption and carbon emissions, the proposed unfired bricks can be produced under ambient conditions, substantially reducing their environmental footprint. The use of alumina waste, which is naturally rich in reactive silica and alumina, improves the mechanical properties of the bricks through pozzolanic reactions. The findings demonstrate that substituting 10–30% of cement with alumina waste enhances compressive strength and structural performance. Economically, utilizing local soils and industrial waste materials lowers production costs, making the bricks particularly suitable for affordable housing solutions. Furthermore, the bricks performed effectively in practical construction applications, such as masonry and plastering, showing comparable results to conventional bricks. By converting industrial byproducts into high-performance construction materials, this approach supports principles of the circular economy and contributes to broader goals of sustainable urban development and responsible resource utilization.

Table 9. Contribution of this research.

Aspect	This Study	Supporting Studies	References
Energy Use & Emissions	Avoids high−temperature firing; drastically reduces energy use and CO2 emissions	Reported high emissions from conventional brick and cement production	[9,50]
Material Substitution	Uses alumina waste (10–30%) as partial cement substitute in unfired clay bricks	Showed strength gains using industrial pozzolanic waste in cement composites	[51,52]
Mechanical Performance	Improved compressive strength and modulus of rupture with alumina waste	Similar strength improvements found using waste additives in bricks	[51,53]
Cost-Effectiveness	Reduces cost using local soils and waste; suitable for low-income housing	Supported low-cost potential of soil/clay-based construction materials	[54,55]
Practical Usability in Construction	Performed well in masonry and plastering, comparable to conventional bricks	Observed good field performance of unfired bricks made from waste materials	[53]
Circular Economy/Waste Valorization	Promotes reuse of industrial waste and reduces landfill burden	Emphasized the importance of waste valorization in sustainable construction	[56]
Alignment with Global Goals	Supports SDGs 11 & 12 through sustainable cities and responsible resource use	Linked such strategies to broader environmental and policy frameworks	[30,36]

presents the contribution of these findings.

Several recent investigations have examined the incorporation of industrial and agricultural waste in brick production, such as fly ash, marble dust, rice husk ash, and various slags, each offering distinct benefits and limitations. For instance, Yadav & Mehta (2023) [57] reported that unfired bricks incorporating fly ash (20–30%) achieved compressive strengths in the range of 9–11 MPa but required extended curing in controlled conditions to attain such strength. Similarly, Karim et al. (2021) [58] demonstrated that the use of marble waste in bricks improved compressive strength to ~8 MPa, though water absorption remained above 15%, limiting their application in moisture-sensitive environments. In contrast, the alumina-waste-based unfired bricks developed in our study achieved significantly higher compressive strengths of up to 177.4 ksc, with water absorption values as low as 11.9%, surpassing both the Thai Industrial Standard TIS 77-2565 [32]. and most comparable waste-based bricks. Moreover, this bricks demonstrated a modulus of rupture of 33.3 ksc, indicating superior flexural performance compared with fly ash (typically <15 ksc) or rice husk ash bricks (around 12–18 ksc) [57,59,60].

4. Conclusions

The development of unfired clay bricks incorporating industrial alumina waste as a partial cement replacement represents a significant advancement in sustainable construction materials. The research integrates principles of material recycling, low−carbon construction, and circular economy by utilizing alumina waste and lateritic soil, both locally available resources. The research involved the systematic formulation of seven brick mixes with varying alumina waste ratios (0–8.571%) and comprehensive testing of their physical, mechanical, thermal, and chemical properties. Experimental evaluation also included practical construction applications, cost analysis, and environmental performance (CO₂ emissions and SROI).

The study revealed several critical insights, as follows:

• Mechanical Performance: The unfired bricks demonstrated significant improvements in compressive strength, modulus of rupture, and dimensional stability at optimal alumina waste contents (especially at 20–30% replacement), outperforming traditional fired clay bricks. The enhancement was attributed to the formation of additional cementitious phases (C–S–H and C–A–H) supported by pozzolanic reactions.
• Durability Indicators: Water absorption was effectively reduced in bricks with 10–30% alumina waste, suggesting improved pore structure densification. This improvement enhances resistance to moisture ingress, contributing to longer service life.
• Thermal Behavior: Although thermal conductivity slightly increased with moderate alumina content, values remained comparable to conventional fired bricks, maintaining adequate insulation performance. Microstructural analysis confirmed that thermal behavior correlated more with pore morphology than bulk density alone.
• Chemical and Structural Interactions: XRD, FTIR, SEM, and XRF analyses confirmed the successful incorporation of alumina phases into the brick matrix, validating the synergistic interaction between waste alumina and cementitious components.
• Sustainability Metrics: The environmental and economic advantages were substantial. Unfired bricks avoided the high−energy kiln firing process, resulting in CO₂ savings up to 1101 kg per 1000 bricks, and achieved a social return on investment (SROI) of 2:1, indicating strong societal value per unit cost.
• Applicability: The constructed masonry wall and plastering trials confirmed that the unfired bricks exhibited reliable performance under standard construction conditions, with no observed cracking or adhesion failure after curing.

Overall, the findings support the viability of incorporating up to 30% alumina waste in unfired clay brick formulations for sustainable, high−performance, and low-carbon construction. The study not only demonstrates technical feasibility but also reinforces broader goals of resource efficiency, industrial symbiosis, and climate−resilient infrastructure.