Unfired Bricks from Wastes: A Review of Stabiliser Technologies, Performance Metrics, and Circular Economy Pathways

Abstract

Unfired bricks offer a sustainable alternative to traditional fired bricks by enabling the large-scale reuse of industrial, construction, and municipal wastes while significantly reducing energy consumption and greenhouse gas emissions. This review contributes to eliminating knowledge fragmentation by systematically organising stabiliser technologies, performance metrics, and sustainability indicators across a wide variety of unfired brick systems. It thus provides a coherent reference framework to support further development and industrial translation. Emphasis is placed on the role of stabilisers—including cement, lime, geopolymers, and microbial or bio-based stabilisers—in improving mechanical strength, moisture resistance, and durability. Performance data are analysed in relation to compressive strength, water absorption, drying shrinkage, thermal conductivity, and resistance to freeze–thaw and wet–dry cycles. The findings indicate that properly stabilised unfired bricks can achieve compressive strengths above 20 MPa and water absorption rates below 10%, with notable improvements in insulation and acoustic properties. Additionally, life-cycle comparisons reveal up to 90% reductions in CO₂ emissions and energy use relative to fired clay bricks. Despite technical and environmental advantages, broader adoption remains limited due to standardisation gaps and market unfamiliarity. The paper concludes by highlighting the importance of hybrid stabiliser systems, targeted certification frameworks, and waste valorisation policies to support the transition toward low-carbon, resource-efficient construction practices.

1. Introduction

Modern societies generate enormous amounts of solid waste from construction, industrial activities, and everyday municipal life. Globally, approximately 2.01 billion tons of solid waste are generated annually, and this number is projected to increase to around 3.40 billion tons by 2050 [1]. Construction and demolition (C&D) waste constitutes a significant portion (approximately 30–50%) of the total, resulting from building demolitions, excavations, and renovations [2]. Industrial processes also generate significant quantities of waste; for instance, coal combustion produces millions of tons of fly ash annually, while the iron and steel industries generate substantial amounts of slag and metallurgical residues [3,4]. Additionally, municipal solid waste (MSW), comprising household and commercial garbage such as organic materials, plastics, paper, and inert waste, is rapidly increasing due to urbanisation. Despite recycling efforts, considerable amounts of MSW still end up in landfills or incineration facilities, generating ash that requires further disposal [5].

Traditional waste disposal methods primarily involve landfilling and open dumping for municipal solid waste (MSW) and some industrial wastes, as well as on-site stockpiling for industrial by-products such as fly ash, red mud, and mine tailings. These practices raise serious environmental concerns, including consuming valuable land resources and potentially leaching contaminants into soil and groundwater. Organic waste decomposition in landfills releases methane, a potent greenhouse gas, while open dumping sites commonly contribute to air and water pollution [6]. Incineration reduces waste volumes and recovers some energy but produces air pollutants and toxic ash that must still be disposed of. Industrial waste, such as fly ash ponds and slag heaps, poses additional risks due to dust pollution and the leaching of heavy metals. Poorly managed mine tailings, for example, can release hazardous metals such as lead and zinc into surrounding ecosystems [7]. Furthermore, demolition debris in the construction sector is often landfilled or down-cycled as low-value fill, representing lost opportunities for material reuse [8].

The current waste management practices contribute to environmental pollution, resource depletion, and greenhouse gas emissions, hindering sustainability efforts. In response to these issues, researchers increasingly advocate adopting a “circular economy” approach, transforming solid waste into useful resources [9]. One promising pathway is integrating various wastes into construction materials, particularly bricks, to reduce landfill burden and conserve virgin raw materials. Brick production is an ideal target due to its large-scale global production, wide utilisation in construction, and potential to incorporate diverse mineral wastes.

Traditional fired clay bricks require high-quality clay and high-energy kiln-firing processes. In contrast, unfired bricks (non-fired, baking-free, or chemically bonded bricks) offer the flexibility to use a broader range of waste materials with significantly reduced energy demands [10]. This review specifically explores the potential of unfired bricks as a sustainable, large-scale waste utilisation strategy to alleviate landfill pressures, conserve natural resources, and minimise environmental impacts. These objectives are aligned with international sustainability goals, particularly the European Union’s Circular Economy Action Plan (2020) [11], which promotes recycled content in construction materials and sets higher recovery targets for construction and demolition waste to reduce environmental impact across the building sector.

However, despite growing interest and numerous studies on unfired bricks, the field remains fragmented and lacks a coherent framework for evaluating stabiliser technologies and material performance. Current research exhibits significant variation in raw materials, stabiliser types, testing methods, and performance metrics, making it difficult to benchmark or scale promising solutions. This review seeks to address this gap by systematically classifying stabilisers, consolidating performance data, and exploring the sustainability implications of unfired brick systems derived from waste.

Review Scope and Methodology

This review focuses on unfired brick technologies incorporating waste-derived stabilisers, including cement, lime, geopolymers, and bio-based systems. The literature was selected from peer-reviewed journal articles, conference proceedings, and technical reports published primarily in the last 10–15 years. The search included combinations of keywords such as “unfired bricks”, “waste stabilisers”, “compressive strength”, “geopolymers”, and “bio-stabilisers”. Emphasis was placed on studies reporting quantitative data on mechanical, physical, and environmental performance.

Figure 1 summarises the review structure and outlines the major stages in preparing and evaluating unfired bricks, including raw materials, stabilisation systems, forming and curing methods, performance evaluation, and sustainability assessment.

2. Comparison of Fired and Unfired Brick Technologies

2.1. Raw Materials

Brick production has traditionally relied on firing clay masonry units in kilns at 900–1000 °C, a highly energy-intensive process that emits substantial pollutants [12]. Fired bricks typically use selected clay or shale as the primary raw material, sometimes with minor additives (sand, pigments, or pore formers). In contrast, unfired bricks can incorporate various raw materials, including low-grade or waste materials unsuitable for firing. Unfired brick formulations often combine soil or clay with industrial by-products like coal fly ash, ground granulated blast-furnace slag (GGBS), construction demolition aggregates, mine tailings, or incinerator ash [13]. These materials typically supply silica, alumina, calcium, and other mineral constituents that form cementitious bonds at ambient temperatures when activated. By allowing high replacement of natural clay with wastes (50% or more in many cases), unfired bricks directly address resource depletion and waste recycling goals. However, the brick’s performance may deteriorate if the waste replacement level is too high without proper stabilisation. Studies have found, for example, that in cementitious materials, replacing more than 50% of traditional aggregates or clay with crushed concrete or ceramic waste tends to reduce strength significantly unless additional stabilisers or strengthening measures are employed [10,13,14]. Unfired bricks thus enable the use of diverse raw materials but require carefully engineered mixtures to ensure quality.

2.2. Manufacturing Process

Fired brick production involves mining and preparing clay, molding green bricks, air-drying, and then firing in a kiln at a high temperature for several days. The kiln-firing step consumes the most energy, driving off volatile matter, burning out organics, and vitrifying the clay minerals to impart hardness [12]. Unfired brick production, on the other hand, forgoes the kiln step. Instead, unfired bricks gain strength after mixing and shaping through a curing process at ambient or mildly elevated temperatures. The bricks are usually molded by compaction (pressing) or the extrusion of a wet mixture into the desired shape [13]. Rather than sintering, strength is developed through chemical reactions of stabilisers, such as hydration, pozzolanic reactions, or other bonding mechanisms. Curing regimes for unfired bricks can vary: many are air-cured for 7–28 days; others may be steam-cured at 40–80 °C or even CO₂-cured in a controlled atmosphere to accelerate carbonation [15,16,17]. The manufacturing of unfired bricks is thus generally simpler, with lower heat requirements, though it may involve additional handling time for curing.

2.3. Energy Consumption and Emissions

A key advantage of unfired bricks is the drastic energy usage and emissions reduction. Firing bricks in kilns consumes substantial fuel, often coal or natural gas, to reach ~1000 °C, and this energy use translates to high carbon dioxide (CO₂) emissions. Life-cycle assessments indicate that conventional burnt clay bricks require approximately 2.0–3.0 MJ of energy per brick and emit around 0.2–0.3 kg of CO₂ per kilogram of brick [17,18]. Recent studies have quantified that unfired bricks can require 10–15 times less energy than fired clay bricks of equivalent size [17,18]. In one comparative analysis, unfired waste-based bricks consumed only ~10% of the energy of traditionally fired bricks. Consequently, the CO₂ emissions associated with production plummet. Singh et al. [17] reported just 43 g CO₂ emitted per unfired fly ash brick versus about 290 g CO₂ for a similar fired clay brick, an ~85% reduction in carbon footprint per brick. Other emissions are likewise reduced: fired kilns release CO₂, oxides, and particulate matter from fuel and clay impurities. Unfired bricks avoid these combustion-related pollutants. The only significant emissions for unfired bricks come from the cement or lime used (which carry embodied CO₂ from their manufacture); however, even for stabiliser production, the net GHG emissions are far lower [15].

2.4. Strength and Application

Fired clay bricks achieve high strength and durability through vitrification, making them suitable for structural masonry (typical compressive strength 17.2 MPa for standard bricks according to the ASTM C62-13a [19]). When properly stabilised, unfired bricks can attain comparable strength ranges, though the mechanism is different (cementitious binding instead of ceramic bonding) [13]. In terms of applications, there is a broad overlap and some distinctions. Fired bricks have long been used in load-bearing walls and exterior facades due to their strength and water resistance. Unfired bricks can also be used in structural walls, provided they meet the required strength class and are protected from excessive moisture. Many unfired bricks (e.g., compressed stabilised earth blocks) are used in low-rise construction, infill masonry, partition walls, and rural building projects [13]. In general practice, unfired clay masonry units are expected to achieve moderate compressive strength to ensure adequate structural performance. They are typically avoided in high-rise load-bearing structures or highly aggressive environments unless additional protective treatments or enhancements are employed. Another application advantage is that unfired bricks, given their higher porosity, often have better insulation properties (lower thermal conductivity) than dense-fired bricks [20]. Figure 2 shows the difference in CO₂ emissions and energy use between fired and non-fired bricks.

3. Use of Stabiliser in Unfired Brick Production

3.1. Cement

Portland cement significantly enhances unfired clay bricks’ strength and water resistance, even at modest dosages (~5–10% by weight) [10,22]. A cement addition in this range can yield several-fold strength increases (e.g., ~6–8 MPa at approximately 12% cement) and significantly enhance durability [10,22]. Cement-stabilized bricks easily meet structural strength requirements and resist softening when wet [17]. However, cement production is carbon-intensive; hence, partial replacement with lower-CO₂ stabilisers, such as lime, is often sought [17]. Still, cement remains a primary stabiliser due to its rapid strength gain and durability (often optimised in content or combined with other stabilisers) [17].

3.2. Lime

Lime (calcium hydroxide) reacts with clay over time to form cementitious compounds via pozzolanic action [13,17]. After ~28 days of curing, lime-stabilized bricks typically achieve a compressive strength of several MPa [10,13]. However, lime is less reactive than cement; hence, roughly double the lime content is needed to match the strength of a given cement content [10]. Due to the cemented matrix, lime-stabilized bricks retain more strength when wet than unstabilised clay (though less than cement-stabilized bricks) [23]. Lime also has a lower carbon footprint than cement (lower kiln energy and CO₂ uptake by carbonation) [17]. It is often combined with additives such as slag or fly ash to enhance early strength while maintaining sustainability [10,17].

3.3. Fly Ash and Pozzolanic Stabiliser

Fly ash—a by-product of coal power—is a widely studied partial stabiliser for unfired bricks. When added to a clay mix (typically with a small amount of lime or cement as an activator), fly ash can form an additional cementitious gel, increasing strength up to an optimal dosage [3,13]. Beyond that optimum, excess fly ash acts as an inert filler and can reduce strength [10,13]. For example, approximately 20% fly ash substitution in a soil mix increased the 14-day strength to ~5–7 MPa; however, higher replacement levels caused the strength to drop [13]. Including FA in the order of 10–20% (with a small addition of lime or cement) typically yields bricks that meet the required strength standards and exhibit improved resistance to water damage [10,22]. With intensive activation, fly ash can even serve as the primary stabiliser; bricks made of ~90% fly ash, supplemented with a small amount of lime and steam-cured, have achieved a compressive strength of ~50 MPa [3]. Other waste pozzolans (e.g., slag or metakaolin) similarly enhance the strength and density of unfired bricks [10,13]. In summary, utilising FA and related industrial wastes enhances brick performance and recycles materials, thereby supporting sustainability goals [10,13].

3.4. Geopolymers

Geopolymer stabilisers, alkali-activated aluminosilicates, can produce high-strength, unfired bricks with significantly lower emissions than Portland cement. These systems activate materials such as fly ash, calcined clay, or slag with alkaline solutions to form a hardened inorganic polymer stabiliser [3,13]. Geopolymer-stabilized bricks have achieved compressive strengths of approximately 10–20 MPa under ambient or mild heat curing, comparable to or exceeding those of conventionally fired clay or cement-stabilized bricks [17]. Such bricks also retain most strength when soaked (e.g., more than 70% of their dry strength) [24]. Because geopolymers utilise industrial waste and reduce the need for cement clinker, their production can reduce CO₂ emissions by approximately 25–50% compared to traditional bricks [17]. Some geopolymer mixes benefit from initial curing at a moderate temperature (~40–60 °C) to accelerate strength gain, though newer formulations can cure at room temperature [10]. Field trials confirm that geopolymer bricks can meet structural requirements, demonstrating their promise as a sustainable masonry stabiliser [17].

3.5. MICP and Bio-Stabilisers

Microbially induced calcite precipitation (MICP) is a biological method for stabilising unfired bricks. Urease-producing bacteria introduced with urea and calcium into the mix precipitate calcium carbonate (limestone) in the soil, which binds the particles together [25]. This “bio-stabiliser” can achieve a compressive strength of several without any Portland cement; lab experiments have reached ~1–2 MPa after several days of MICP curing [25]. MICP is a very low-carbon process (occurring at room temperature), but it is slow, and ensuring uniform bonding throughout the brick is challenging, which currently limits its strength potential [25]. Research continues to accelerate calcite formation (e.g., using enzymes) and enhance the strength of these “bio-stabilised bricks” [25].

Natural organic additives have also been tested as stabilisers. Small bitumen, plant resins, or polysaccharide gums can bind soil particles and reduce moisture penetration in unfired bricks [25]. To improve this, researchers have explored chemically cross-linking the biopolymers or combining them with small amounts of a mineral stabiliser to enhance their water resistance [8]. Currently, purely bio-based stabilisers are primarily suitable for non-structural applications.

3.6. Others

Beyond the above categories, various other stabilisers have been investigated. One example is gypsum, which provides quick setting and shrinkage reduction. Adding a few percent of gypsum to a clay mix provides early strength gains and significantly reduces shrinkage. However, gypsum alone is water-soluble and offers limited strength; hence, gypsum-stabilized bricks may soften if wet [10,13]. Another alternative is reactive magnesium oxide (MgO), which hardens by hydration and carbonation. Bricks stabilised with reactive MgO cement have achieved compressive strengths around 7–10 MPa under ambient curing, with significantly higher strengths (up to 57 MPa) achievable under accelerated carbonation due to the formation of stable Mg(OH)₂ and carbonate phases [26]. Thus, MgO can approach cement performance with a lower carbon footprint, though it requires ample curing time and CO₂ exposure to reach full strength.

Various industrial residues (e.g., cement kiln dust) and small polymer additives (e.g., PVA emulsions) have also been used to partially stabilise bricks, though their contributions to strength are relatively modest [10]. In practice, the best results often come from hybrid approaches: Combining cement, lime, and pozzolans (and sometimes a bit of gypsum) in one mix can outperform any single stabiliser, and adding fibers (such as straw, coir, or polypropylene) helps control shrinkage cracking [10,13]. Each of these “other” additives has particular benefits and limitations. Gypsum provides rapid set but poor water durability, bitumen improves waterproofing but not strength, MgO offers strength and carbon sequestration but requires long curing, polymers add ductility but increase cost, etc. This diversity of stabilisers allows unfired bricks to be tailored to specific needs, underscoring the technology’s versatility in sustainable construction [10]. The diverse effects of these stabilisers on engineering performance are summarised in

Table 1. Engineering Properties of Stabilised Unfired Bricks with Different Stabilisers.

Stabiliser System	Compressive Strength (MPa)	Flexural Strength (MPa)	Water Absorption (%)	Thermal Conductivity (W/m·K)	Drying Shrinkage (%)	Freeze–Thaw Strength Loss (%)	References
Cement-based	~2.5–7.4 MPa (with 4–10% cement)–higher with more cement	~1.2 MPa (with 10% cement)	~10% (low if well-compacted)	0.40–1.20 W/m·K (range for CSEBs)	~0.1% (typical allowable limit)	~6% mass loss after standard F-T cycles (significantly lower than unstabilized soil)	[27,28,29,30]
Geopolymer-based	~14 MPa (fly ash geopolymer brick, ambient cured)	~3.4 MPa (for ~18 MPa compressive brick)	~8–12% (generally low)	0.42–0.46 W/m·K (typical range)	–n/a (no notable drying shrinkage reported)	~30% strength loss after 150 F-T cycles (excellent durability vs. OPC concrete)	[31,32,33]
Lime-based	~16.5 MPa (28-day, 10% lime stabilization)	~0.5–1.0 MPa (lower than cement-stabilized)	~12–18% (higher porosity than cement)	~0.5–0.8 W/m·K (estimated, slightly lower than cement)	~0.2% (reduces raw clay shrinkage markedly)	Moderately durable; e.g., mass loss ~7.7% (lime) vs. 5.6% (cement) after drying; less freeze–thaw resistance than OPC-stabilized	[34,35]
Bio-stabiliser/MICP	~9 MPa (partially saturated microbial CaCO3 curing)	–n/a (brittle, not typically reported)	~5–10% (MICP reduces pore absorption)	–n/a (assumed similar to base soil)	–n/a (minimal drying shrinkage)	–n/a	[36,37,38]
Ceramic waste-based	~25 MPa (alkali-activated ceramic waste stabiliser)	–n/a	~10% (comparable to clay brick)	~0.45 W/m·K (comparable to geopolymer brick)	–n/a	High durability	[32,39]

. To further enhance clarity, Figure 3 visually compares compressive strength and water absorption across the stabiliser systems, as summarised in Table 1. Additionally, Figure 4 highlights the performance contrast between fired bricks and stabilised unfired bricks, consolidating values reported throughout the manuscript.

4. Engineering Properties of Unfired Bricks

Most of the compressive strength and water absorption values reported in this review are based on tests conducted according to established standards such as ASTM C67 [40], IS 3495 [41], and EN 772 [42], as cited in the respective studies. These standards ensure a degree of methodological consistency and enable cross-study comparisons. While some of the original studies report statistical indicators such as standard deviation or error margins, many do not. Therefore, the performance values presented here are primarily based on directly reported values and should be interpreted without assuming consistent statistical comparability.

The engineering performance of unfired bricks determines their suitability for structural and non-structural applications. Key performance parameters include compressive strength, water absorption, drying shrinkage, and durability under environmental stressors (e.g., freeze–thaw or wet–dry cycling). Various factors influence these properties, including the raw material composition, the stabiliser system and content, compaction pressure, curing regime, and ambient conditions. Recent studies suggest that both curing temperature and time can noticeably affect the strength of geopolymer-stabilised unfired bricks. For example, Verma et al. [43] achieved a peak strength of 53.46 MPa by curing at 120 °C for 24 h. In contrast, Tian et al. [44] reported that 80 °C gave the best results, while higher temperatures actually led to a drop in strength, likely due to damage to the gel structure. These findings point to the importance of fine-tuning both the thermal conditions and the choice of raw materials since different geopolymer systems may respond quite differently. In particular, various industrial by-products or waste materials have been widely studied to enhance unfired brick properties [10,12,22]. This section reviews representative data from the literature on each parameter to illustrate the quality and variability of unfired brick performance.

4.1. Compressive Strength

Compressive strength is one of the most critical performance metrics for masonry units as it reflects the load-bearing capacity of the bricks. Unfired bricks can achieve a wide range of compressive strengths depending on the stabilisation method and curing conditions. Cement-stabilized unfired bricks typically reach a compressive strength of about 5–10 MPa after 28 days of ambient curing, whereas alternative stabilisers, such as lime–fly ash or geopolymer systems, can attain even higher strengths under suitable conditions [10,22,24]. For instance, Preethi and Venkatarama [24] achieved over 20 MPa using alkali-activated (geopolymer) unfired bricks cured at room temperature. Fly ash–lime–gypsum bricks usually develop strengths in the range of 7–15 MPa, depending on the reactivity of the fly ash used [10]. At the lower end of the spectrum, bio-mediated bricks produced via microbially induced calcite precipitation (MICP) typically yield only about 1–4 MPa [25]. Recent advances, however, have achieved compressive strengths exceeding 21 MPa by modifying brick matrices with polymer latex admixtures [12].

Apart from the stabiliser type, various mix design and processing factors critically influence compressive strength. The water-to-stabiliser ratio and particle size distribution are especially important because they affect the packing density and stabiliser hydration. An optimal moisture content exists for molding that maximises density and strength. Excessive water increases porosity and weakens the brick, whereas insufficient water impedes stabiliser hydration and reduces compaction efficiency [10]. Proper aggregate gradation (a well-graded mix of particle sizes) improves packing efficiency and, thus, strength. However, overly fine soil can cause significant shrinkage upon drying, leading to microcracks that reduce long-term strength.

Curing conditions are equally critical. Accelerated curing techniques, such as steam curing or CO₂ (carbonation) curing, can precipitate and permanently develop strength and achieve the final strength. For example, CO₂ curing of slag-rich unfired bricks yielded compressive strengths of 25–45 MPa within a few days [16]. The strength range achievable by different stabiliser systems under ambient curing is summarised in Figure 5.

In addition to mechanical strength, unfired bricks exhibit notable advantages in thermal and acoustic performance compared to conventionally fired clay bricks. Owing to their typically higher porosity, unfired bricks tend to have lower thermal conductivity—on the order of 0.3–0.6 W/(m·K)—than fired bricks, which are around 0.9–1.3 W/(m·K) [13,17]. This lower thermal conductivity translates to better insulation; for example, field tests have shown that buildings with unfired bricks maintain cooler indoor temperatures in hot climates, reducing mechanical cooling [12]. Incorporating fibers or lightweight aggregates into the brick mix can further decrease thermal conductivity. One study reported that adding straw fibers reduced the thermal conductivity of an unfired brick from approximately 0.57 to 0.14 W/(m·K) [12]. On the other hand, using higher proportions of cement or other dense stabilisers generally increases thermal conductivity due to the reduction in pore volume [12]. The internal porosity of unfired bricks enhances their acoustic properties by improving sound absorption. Porous unfired bricks have demonstrated sound absorption coefficients around 0.3–0.4 at mid-range frequencies, compared to only 0.02–0.05 for dense-fired bricks [12]. Unfired brick walls can significantly improve indoor acoustic comfort by dampening echoes and reverberation. Although their somewhat lower density may marginally reduce sound insulation (sound transmission loss) relative to denser fired bricks, this can be mitigated through design measures such as thicker or cavity walls. Properly constructed unfired brick walls have achieved sound transmission class (STC) ratings comparable to traditional fired-clay brick walls [45].

Figure 5. Compressive Strength Distribution of Unfired Bricks Stabilised with Different Stabiliser Systems [17,21,46,47,48,49].

Figure 5. Compressive Strength Distribution of Unfired Bricks Stabilised with Different Stabiliser Systems [17,21,46,47,48,49].

4.2. Water Absorption

Water absorption is a crucial indicator of a brick’s pore structure and moisture susceptibility, which correlates inversely with its durability. Mixes that include reactive industrial by-products, such as fly ash or ground granulated blast-furnace slag, tend to form a finer pore structure and exhibit lower water uptake due to ongoing pozzolanic reactions that fill pores [10,22,50]. In practice, ordinary Portland cement–stabilised unfired bricks typically exhibit water absorption by 10–20% by mass, whereas many geopolymer-stabilized unfired bricks can achieve absorption levels below 10% [24,50]. For example, Islam et al. [22] reported that incorporating 15–20% fly ash and a small amount of cement resulted in unfired bricks with a compressive strength over 5 MPa and water absorption rates around 7–10%, significantly lower than the 30–40% typically observed in unstabilized soil blocks. Polymeric additives are another effective way to reduce brick porosity: unfired bricks modified with a rubber latex additive showed water absorption of around 7.5% [12]. By contrast, unstabilised traditional earthen bricks or soil blocks can absorb 30–40% water due to their high open porosity, which makes them unsuitable for prolonged exterior exposure without protection [22].

Low water absorption is critical for frost resistance and general durability in humid environments. Incorporating pozzolanic or other reactive additives can further refine the pore network, thereby reducing capillary suction and enhancing long-term durability under wet conditions [50].

4.3. Drying Shrinkage and Dimensional Stability

In this review, dimensional stability is primarily assessed through drying shrinkage, but in the broader literature, it may also include warping or cracking, depending on the test method and observation scale.

Drying shrinkage is another critical parameter because it can lead to dimensional instability and cracking in masonry if not properly managed. When clay-rich bricks dry, the moisture loss causes them to contract; however, adding stabilisers and reinforcements can mitigate this effect. Cement and lime stabilisation markedly reduce shrinkage by binding soil particles together and reducing the amount of free water that can evaporate [51]. Pozzolanic additives, such as fly ash, also reduce shrinkage by forming additional cementitious bonds within the matrix, thereby restraining the clay’s tendency to contract [50]. Furthermore, including fibers has proven highly effective in mitigating drying shrinkage and associated cracking. For example, adding 1% areca nut fiber to an earthen brick mix reduced the linear drying shrinkage from 0.88% to 0.34%, and using a combination of 2.5% cement with 1% fiber further reduced the shrinkage to only 0.1% [52].

Table 2. Comparative Drying Shrinkage of Stabilised and Unstabilised Unfired Bricks at Different Curing Ages.

Brick Type/Mix	7-Day Shrinkage (%)	28-Day Shrinkage (%)	90-Day Shrinkage (%)	180-Day Shrinkage (%)	References
100% Cement (~10%)	0.2	0.3	0.35	0.4	[53,54]
50% cement + 50% Fly Ash	0.18	0.25	0.3	0.32	[55]
Lime-based (~10% Lime)	0.25	0.4	0.45	0.5	[35]
Geopolymer-based	0.15	0.22	0.25	0.27	[56]
1% Areca fiber +2.5% Cement	0.02	0.1	0.1	0.1	[52]

presents the drying shrinkage values of representative brick mixes at various curing stages, highlighting the influence of stabilisers and fibers.

In addition to composition, the curing regime is important in controlling shrinkage. A slow, controlled drying process helps prevent large moisture gradients within the brick, thereby avoiding differential shrinkage that can lead to warping or cracking [51]. Extended curing periods, which involve keeping bricks in a humid or covered environment for longer before full drying, also allow for further hydration or pozzolanic reactions, solidifying the matrix and reducing the remaining shrinkage potential [51]. Another effective strategy is to adjust the mix by tempering clay with sand or other non-plastic aggregates. Clay-rich mixes that incorporate a proportion of sand exhibit minimal shrinkage (often <0.1% linear shrinkage after 28 days) because the sand particles act as a dimensional stabiliser [51]. Thus, unfired bricks can be produced with negligible drying shrinkage through a proper mix design (utilising stabilisers, fibers, and non-plastic aggregates) and effective curing practices. This ensures the dimensional stability of the bricks over time, helping to maintain the integrity of walls built with them, as the risk of shrinkage cracking is minimised.

4.4. Thermal and Acoustic Properties

4.4.1. Thermal Conductivity

Generally, a lower bulk density is associated with lower thermal conductivity in earthen materials [57]. Stabilising unfired bricks with cement alone increases the density (and thus λ), but this effect can be offset by partially replacing some of the cement with low-density additives. Still, when a portion of the cement was replaced by rice husk ash (a lightweight waste additive), the thermal conductivity dropped to approximately 0.64 W/(m·K) [58]. Geopolymer-based unfired bricks also show favorable thermal behavior. A dense geopolymer brick mix was measured to have λ ≈ 0.59 W/(m·K), and by incorporating. Industrial waste (alumina waste) in the geopolymer, λ, was further reduced to ~0.28 W/(m·K) [59]. Many fly ash–based unfired bricks naturally achieve low thermal conductivities, typically in the 0.25–0.39 W/(m·K) [60]. In one optimised formulation, a geopolymer brick with a bulk density of ~1.3 g/cm³ achieved a thermal conductivity of 0.32 W/(m·K), roughly 50% lower than a traditional fired clay brick [59]. Overall, using lightweight aggregates, fibers, or porous fillers appropriately, unfired bricks commonly attain thermal conductivity values in the 0.2–0.6 W/(m·K) range, clearly outperforming conventional fired bricks in insulating capability [61].

4.4.2. Acoustic Properties

The acoustic insulation provided by brick masonry generally follows the mass law, which states that heavier, denser walls offer better sound reduction. Because unfired bricks are often slightly less dense than fired bricks (due to higher porosity or the inclusion of lightweight additives), a single-wythe wall of unfired masonry may have a modestly lower sound transmission loss compared to the same thickness of fired clay masonry. However, practical design measures can compensate for this difference to ensure adequate acoustic performance. For instance, fiber-reinforced earthen bricks have achieved airborne sound reduction levels of around 45 dB in laboratory tests, which meets typical building standards for airborne sound insulation (e.g., ASTM or Turkish standards) [45]. In these cases, including fibers and other additives improved thermal performance and did not compromise the wall’s ability to achieve sound insulation levels. Moreover, using mineral aggregates that enhance damping—such as basaltic pumice—within unfired bricks has been shown to improve sound insulation performance further [45]. Generally, a sufficiently thick wall constructed with unfired bricks and finished with suitable plaster or render can achieve sound insulation levels comparable to those of a standard fired-brick wall of equal thickness. Suppose higher acoustic performance is required (for instance, in party walls or external walls in noisy environments). In that case, design solutions such as double-layer (cavity) walls may be considered to improve sound attenuation. Thus, with proper design considerations, non-fired brick technology can deliver notable thermal insulation advantages without compromising acoustic comfort in buildings.

4.5. Durability: Freeze–Thaw and Wet–Dry Resistance

Durability is crucial for bricks intended for exterior exposure under freeze–thaw and wet–dry cycles. Repeated freezing and thawing or moisture wetting and drying can degrade porous materials over time; therefore, unfired bricks must be formulated to resist these cyclic stresses. Using stabilising stabilisers, such as cement, lime, or geopolymer, significantly enhances the durability of unfired bricks by refining the pore structure and reducing water penetration [23]. A refined (small and discontinuous) pore structure means less water can enter the brick, and any entering water exerts a less disruptive force upon freezing. Empirical studies have demonstrated the efficacy of stabilisers in this regard. Figure 6 illustrates the strength loss of unfired bricks with and without stabilisers after 15 freeze–thaw cycles. For example, Nikvar-Hassani et al. [62] found that geopolymer-stabilized unfired bricks retained over 90% of their original compressive strength after 50 freeze–thaw cycles. In contrast, unstabilised earthen blocks lost more than 30% of their strength under the same conditions.

In addition, lime-stabilized unfired bricks have been noted to undergo only minor degradation under cyclic wetting and drying; in some cases, they even exhibited slight strength gains due to continued pozzolanic reactions during the moisture cycling [63]. Thus, when properly designed with suitable stabilisers and reinforcements, unfired bricks are durable against freeze–thaw and wet–dry weathering. These findings suggest that unfired bricks can withstand harsh climatic conditions, making them a viable and durable material choice for sustainable construction in various environments. A broader comparison of stabiliser performance across durability and other engineering properties is summarised in Figure 7.

Figure 6. Freeze–Thaw Durability of Stabilised and Unstabilised Unfired Bricks [47,64].

Figure 6. Freeze–Thaw Durability of Stabilised and Unstabilised Unfired Bricks [47,64].

5. Sustainability and Circular Economy Potential

Recent studies indicate that geopolymer-based bricks can significantly lower CO₂ emissions—by approximately 43% to 63.5%—compared to ordinary Portland cement (OPC), with reported emissions ranging between 56 and 661 kg CO₂/m³, depending on the formulation and process design [65,66]. For reference, traditional fired clay bricks emit around 428 kg CO₂ per 1000 bricks [66]. In contrast, unfired bricks that incorporate waste-based stabilisers and microbial stabilisers such as MICP have shown notable potential to further reduce both carbon emissions and embodied energy [67].

Unfired brick technology offers notable sustainability advantages over traditional fired clay masonry. By valorising waste materials, lowering energy consumption, and minimising greenhouse gas (GHG) emissions, unfired bricks exemplify the principles of a circular economy and contribute to global sustainability goals (e.g., the UN Sustainable Development Goals on sustainable cities, responsible production, and climate action) [9]. In essence, this approach transforms landfill-bound waste into durable construction products, aligning construction practices with resource efficiency and climate change mitigation [10]. As summarised in

Table 3. Environmental Impact and Resource Utilisation Comparison between Fired and Unfired Bricks.

Parameter	Fired Clay Brick	Unfired Brick	Improvement (Using Unfired)	Reference
CO2 Emissions (production of 1000 bricks)	~5907 kg CO2e	~0 kg CO2e (no kiln firing)	~5907 kg reduction (nearly 100% cut in production emissions)	[18]
Life-cycle CO2 Footprint (per brick)	290 g CO2e/brick	43.3 g CO2e/brick	85% lower net carbon footprint per brick	[17]
Embodied Energy (manufacturing 1000 bricks)	~5305 MJ	~0 MJ (sun-dried; no fossil fuel)	~5305 MJ saved (essentially eliminated firing energy)	[18]
SO2 Emissions (kg/1000 bricks)	0.5–5.9 kg SO2	~0 kg SO2	100% elimination of SO2 emissions	[68]
Particulate (PM) Emissions (kg/1000 bricks)	0.64–1.4 kg PM	~0 kg PM	100% elimination of PM2.5 and dust emissions	[68]
Waste Material Utilization	Mainly virgin clay (natural soil); typically <15% waste additives	Can incorporate ≥60% industrial wastes (fly ash, slag, etc.)	Much higher reuse of waste resources (diverts waste from landfill)	[46]

, non-fired bricks dramatically outperform fired bricks across key environmental indicators–from CO₂ and pollutant emissions to energy demand and raw material usage. As shown in Table 3, replacing fired bricks with unfired alternatives can reduce carbon emissions, energy consumption, and air pollutants by over 80% while enabling higher utilisation of industrial waste.

5.1. CO₂ Emission Reduction

Fired brick production is among the most carbon-intensive processes in the building sector, emitting approximately hundreds of kilograms of CO₂ per ton of bricks produced [15,17]. These emissions primarily stem from fuel combustion in kilns and the calcination of clay minerals at high temperatures. In contrast, unfired bricks eliminate the high-temperature firing step and rely on ambient curing or low-energy chemical binding, which dramatically reduces carbon emissions. Life-cycle analyses consistently show that replacing conventional fired units with unfired bricks can reduce CO₂ emissions by roughly 60–90% [17,69]. For instance, Singh et al. reported that fly ash–based unfired bricks emit only about 43 g CO₂ per brick versus 290 g CO₂ for an equivalent fired clay brick–an 85% reduction in carbon footprint [17]. Similarly, studies on geopolymer-stabilised bricks (using alkali-activated industrial wastes as stabilisers) have demonstrated CO₂ emissions on the order of 70–80% lower than traditional kiln-fired clay bricks [18,69]. Beyond merely avoiding emissions, some innovative unfired brick processes can even sequester CO₂. For example, bricks cured via carbonation can permanently capture carbon dioxide from industrial exhaust gases during the hardening process [16], effectively turning the bricks into net carbon sinks.

5.2. Energy Savings

Eliminating the firing stage also translates into substantial energy savings. Traditional fired bricks require approximately 2–4 MJ of thermal energy per brick, supplied by burning coal or other fuels, to reach kiln temperatures of 900–1000 °C [15]. Unfired bricks, in contrast, require only a small fraction of that energy (often less than 0.3 MJ per brick) for mixing and curing, as no high-temperature treatment is involved [17]. This represents energy reductions of 75–90%. In practice, the energy input for unfired brick production comes mainly from manufacturing stabilisers (e.g., cement or lime) and mechanical mixing, which are far less energy-intensive than continuous kiln firing. Avoiding kiln fuel combustion conserves energy and reduces fossil fuel consumption in brickmaking, which can generally lead to reduced emissions and potential cost benefits. Studies have noted that fly ash–based unfired bricks consume 10–15 times less energy in their life cycle than equivalent fired clay bricks [17].

5.3. Waste Utilization Efficiency

One of the most significant environmental benefits of unfired bricks is their ability to incorporate large quantities of industrial and municipal waste, thus advancing the circular use of materials. Whereas conventional fired bricks are typically made primarily from mined clay (with little to no recycled content), unfired bricks can be formulated with 50–90% waste materials by mass in their composition [10,69]. Common waste-derived ingredients include coal combustion residues (fly ash and bottom ash), metallurgical slags like ground granulated blast-furnace slag (GGBS), red mud from alumina production, construction and demolition (C&D) debris, and even processed municipal solid waste incineration ash or sludge. Utilising these materials in brick production diverts them from landfills, alleviating the burden of waste disposal. For example, the FaL-G class of unfired bricks, widely adopted in India, is known to use over 60% fly ash (along with lime and gypsum) in place of natural clay, thereby repurposing huge volumes of coal ash into construction products [10]. The large-scale manufacturing of fly ash bricks—spurred in part by regulations mandating the use of power-plant ash in building materials—has effectively turned a disposal liability into a valuable resource. These practices not only reduce landfill usage and the environmental pollution associated with waste dumps but also conserve natural resources by decreasing the demand for virgin clay, shale, and sand for brick production [9].

6. Discussion

Unfired bricks demonstrate strong potential for addressing construction’s environmental, technical, and socioeconomic challenges. A consistent finding across studies is that compressive strength depends heavily on the type of stabiliser and the mix design. Geopolymer and cement-based bricks often achieve higher strengths (≥10 MPa), while lime–fly ash or bio-stabilisers like MICP generally result in lower strengths suitable for non-structural use [12,22,24,25,49,66]. Materials like GGBS can improve strength, but only when properly activated; otherwise, their performance is limited under ambient cur-ing [10,69].

Moisture resistance and thermal performance are also key considerations. To enhance insulation, many unfired bricks incorporate porous waste materials, such as fly ash or lightweight aggregates. Well-designed mixes can achieve water absorption rates below 10%, but higher porosity may compromise frost durability [12,52]. To manage this trade-off, additives such as nanomaterials or fibers have been introduced to refine the pore structure, helping to balance insulation with durability [52,70].

From an environmental standpoint, unfired bricks significantly reduce CO₂ emissions and energy consumption compared to fired bricks. Studies report reductions of 80–90%, largely due to eliminating kiln firing and using recycled industrial waste [17,18,63]. However, life-cycle assessments (LCA) reveal that environmental impacts remain sensitive to regional factors such as electricity mix and transportation distances. To ensure methodological consistency, most of the LCA data cited in this review are based on cradle-to-gate system boundaries, encompassing raw material extraction, processing, and brick formation [9].

Despite their advantages, unfired bricks face barriers to widespread adoption. Inconsistent raw material quality (e.g., fly ash, sludge) can affect performance, while the lack of universal codes and limited market familiarity have slowed uptake [10,71]. Overcoming these issues will require standardised testing, certification systems, and supportive policies. National regulations—such as India’s fly ash mandates and China’s CDW recycling incentives—have proven effective in encouraging sustainable brick use [9,10].

Unfired bricks offer clear sustainability and performance benefits, but their broader adoption depends on targeted research, technical standards, and institutional support.

While several previous reviews have addressed aspects of unfired bricks, they have typically been limited in scope or focused on specific material classes. For example, one study provided a comprehensive review of the applications of coal fly ash, including its use in bricks, but primarily dealt with recovery pathways and utilization rates across sectors without focusing on formulation–performance correlations specific to unfired bricks [3]. In contrast, another review centred on how various waste additives influence the thermal and mechanical properties of unfired clay bricks, categorising results by waste types but offering limited discussion on process integration, environmental implications, or stabiliser diversity [12]. Another industry-oriented review elaborated on production strategies and process parameters critical to scaling up unfired brick manufacturing but mainly focused on cementitious and alkali-activated systems [10]. Compared to these, the present review offers a broader synthesis across multiple stabiliser types—including lime-based, alkali-activated, microbial, and bio-based—and links formulation trends not only to strength and durability but also to carbon footprint, circular economy relevance, and practical scalability. This integrative approach aims to bridge the gap between technical performance and sustainability targets, which has been rarely explored in prior works.

In addition to unfired bricks, recent reviews on bioconcrete materials—which incorporate microorganisms for self-healing and enhanced durability—have demonstrated promising potential for sustainable construction. For instance, bioconcretes enable autonomous crack repair through microbially induced calcium carbonate precipitation, significantly improving strength and service life [72,73]. Other studies have highlighted enhanced durability and reduced permeability compared to conventional concrete, though challenges remain regarding microbial survivability and scale-up costs [74,75,76]. These insights align with the broader goals of sustainable material development and reinforce the need to explore bio-based solutions beyond traditional stabilisers.

Similarly, reviews on concrete incorporating plant-based or agricultural waste highlight its viability as a sustainable material stream. Studies have explored the partial replacement of cement and aggregates with residues such as rice husk ash, wood waste, bagasse, coconut shell, and waste paper [77,78]. These materials have been found to maintain acceptable mechanical and thermal properties while enhancing ductility, fire resistance, and durability under moderate conditions [79,80]. Although challenges remain, particularly those related to shrinkage, thermal degradation, and compatibility with stabilisers, their widespread availability and low cost make them attractive for broader eco-construction strategies.

7. Conclusions

Unfired bricks present a sustainable and technically viable alternative to conventional fired masonry units. These bricks support circular economy principles and significantly reduce environmental impacts by utilising construction and demolition waste, industrial by-products, and municipal residues. Compared to traditional fired bricks, unfired bricks offer:

• Up to 90% reductions in energy consumption and carbon emissions
• Effective waste valorisation, with up to 80–90% waste incorporation
• Comparable mechanical performance, especially when stabilised with cement, lime, or geopolymer stabilisers
• Enhanced insulation and moisture buffering properties, suited for both rural and urban construction

Technological advancements in alkali activation, microbial stabilisers, and CO₂ curing are continuing to expand the application potential of unfired bricks. Nonetheless, challenges related to standardisation, long-term durability, and public acceptance remain. To realise their full potential, future research should prioritise the development of hybrid stabilisers, in-situ testing protocols, and digital design tools that allow customised mix optimisation based on local materials.

With growing pressure to decarbonise construction and manage solid waste, unfired bricks stand as a compelling pathway toward greener, lower-impact infrastructure. Their scalability, material flexibility, and environmental advantages position them as key components of next-generation building technologies.