Alkali-Activated Materials Reinforced via Fibrous Biochar: Modification Mechanisms, Environmental Benefits, and Challenges

Abstract

Alkali-activated materials, as a low-carbon cementitious material, are widely known for their excellent durability and mechanical properties. In recent years, the modification of alkali-activated materials using biochar has gradually attracted attention. Fibrous biochar has a highly porous structure and large specific surface area, which can effectively adsorb alkaline ions in alkali-activated materials, thereby improving their pore structure and density. Additionally, the surface of the biochar contains abundant functional groups and chemically reactive sites. These can interact with the active components in alkali-activated materials, forming stable composite phases. This interaction further enhances the material’s mechanical strength and durability. Moreover, the incorporation of biochar endows alkali-activated materials with special adsorption capabilities and environmental remediation functions. For instance, they can adsorb heavy metal ions and organic pollutants from water, offering significant environmental benefits. However, research on biochar-modified alkali-activated materials is still in the exploratory phase. There are several challenges, such as the unclear mechanisms of how biochar preparation conditions and performance parameters affect the modification outcomes, and the need for further investigation into the compatibility and long-term stability of biochar with alkali-activated materials. Future research should focus on these issues to promote the widespread application of biochar-modified alkali-activated materials.

1. Introduction

The construction industry is a major contributor to global carbon emissions, primarily due to the production of Portland cement [1]. Alkali-activated materials (AAMs) have emerged as sustainable alternatives, leveraging industrial byproducts like fly ash and slag to minimize environmental impacts [2]. However, AAMs face challenges such as shrinkage, inconsistent mechanical performance, and durability concerns, limiting their widespread application [1,3].

Simultaneously, biochar, a carbon-rich byproduct of biomass pyrolysis, offers potential benefits in construction materials due to its high surface area, porosity, and environmental sustainability [4]. The integration of biochar into AAMs has been proposed as a strategy to address performance limitations while contributing to carbon sequestration and waste valorization. Despite promising findings, the use of biochar in AAMs is not without challenges:

Material Variability: Biochar properties, such as surface area, pore structure, and chemical composition, vary significantly based on feedstock and pyrolysis conditions [5]. This inconsistency complicates its integration into AAMs and leads to unpredictable performance outcomes.

Interactions with Alkali-Activated Binders: Biochar influences the hydration kinetics, gel formation (C-S-H and N-A-S-H), and microstructure of AAMs [6]. While this can enhance properties like shrinkage resistance and durability, the exact mechanisms and optimal biochar characteristics are not fully understood.

Mechanical and Durability Performance: While biochar may improve compressive strength and reduce shrinkage, its effects on long-term durability (e.g., chloride resistance, carbonation, and sulfate attack) remain inconsistent across studies [7,8]. These uncertainties hinder its adoption in structural applications.

Economic and Environmental Trade-Offs: Although biochar contributes to carbon sequestration and the circular economy, its production can be energy-intensive and costly, potentially offsetting the sustainability benefits if not optimized [8].

Scalability and Standardization: The lack of standardized protocols for biochar production, characterization, and incorporation into AAMs limits its scalability and industrial adoption.

Thus, while biochar-enhanced AAMs show potential for addressing critical environmental and performance challenges in the construction sector, their practical implementation is constrained by unresolved scientific, technical, and economic issues. Further research is needed to optimize biochar properties, standardize its application in AAMs, and evaluate its long-term performance in real-world conditions.

2. Fundamentals of Alkali-Activated Materials (AAMs)

The composition and reaction mechanisms of AAMs are highly versatile, allowing tailored solutions for specific applications. By understanding the interaction between precursors, activators, and curing conditions, researchers aim to develop AAMs with superior performance and sustainability compared to traditional cementitious materials. Further exploration of these mechanisms is essential for overcoming challenges and enabling the widespread adoption of AAMs in construction.

2.1. Composition of Alkali-Activated Materials

Alkali-activated materials (AAMs) primarily consist of an aluminosilicate precursor, an alkali activator, and water, which together form a chemically bonded inorganic matrix [9]. The precursor materials, such as fly ash, ground granulated blast furnace slag (GGBS), or metakaolin, provide the silica (Si) and alumina (Al) necessary for the polymerization process [10]. The alkali activator, typically in the form of sodium or potassium silicates and hydroxides, initiates the dissolution of the precursor, leading to the formation of a three-dimensional geopolymeric network [11]. This network is responsible for the material’s mechanical strength and durability [12]. The water content facilitates the reaction process and influences the workability of the mixture. Additional components, such as fillers, fibers, and supplementary cementitious materials like biochar, can be incorporated to provide specific properties, such as thermal insulation, fire resistance, or shrinkage control [13,14]. The precise proportions and selection of these components depend on the desired performance characteristics and the application of the AAM.

2.2. Reaction Mechanisms of Alkali-Activated Materials

The reaction mechanism of alkali-activated materials (AAMs) involves a series of chemical processes leading to the formation of a geopolymeric binder. Initially, the alkali activator dissolves the aluminosilicate precursor, releasing silica (Si) and alumina (Al) species into the solution [15]. This dissolution phase is influenced by factors such as the precursor composition, particle size, and activator concentration. The dissolved species then undergo polymerization, where Si and Al tetrahedra link together through sharing oxygen atoms, forming a three-dimensional network [16]. Concurrently, alkali cations (e.g., Na⁺ or K⁺) stabilize the negatively charged polymer structure. This results in the development of a gel-like phase, often referred to as a N-A-S-H gel (sodium–alumino–silicate–hydrate) in fly ash-based systems or C-A-S-H gel (calcium–alumino–silicate–hydrate) in slag-based systems [2]. These gels harden over time, creating a dense, cohesive matrix. The overall reaction kinetics and resulting microstructure depend on factors like curing temperature, water-to-activator ratio, and the type of alkali activator used, ultimately determining the mechanical and durability properties of the AAM [9].

2.3. Factors Determining Reaction Mechanisms

The performance of alkali-activated materials (AAMs) is influenced by several key factors, including the type and composition of the aluminosilicate precursor [17], the concentration and type of alkali activator [18], the water-to-binder ratio [19], curing conditions [12], and the incorporation of supplementary materials [20]. The precursor, such as fly ash, slag, or metakaolin, determines the availability of silica and alumina for the geopolymerization process and affects the material’s strength and durability [1]. The activator type (e.g., sodium or potassium-based) and its concentration influence the dissolution and polymerization rates, while the water-to-binder ratio affects the workability and setting time. Curing conditions, including temperature and humidity, play a critical role in the reaction kinetics and the development of the microstructure [21]. Additionally, supplementary materials like biochar, fibers, or fillers can enhance specific properties such as thermal insulation, shrinkage resistance, or mechanical strength. These factors interact in complex ways, making the optimization of AAM formulations essential for achieving the desired performance characteristics for specific applications.

2.4. Challenges in AAM Development

The development of alkali-activated materials (AAMs) faces significant challenges related to shrinkage, workability, and long-term durability [16]. One major issue is the high shrinkage of AAMs during drying, which can lead to microcracking and reduced structural integrity [14]. This is attributed to the rapid hydration kinetics and high porosity of AAMs, causing a significant reduction in pore volume and internal stress development. Strategies to mitigate shrinkage include incorporating supplementary materials like fly ash or slag [11], which slow down hydration, or using external curing methods to maintain the moisture content. Workability is another critical challenge [22]. The high alkalinity of the activating solution often results in rapid stiffening and poor flowability, making AAMs difficult to handle and place, especially in large-scale construction [23]. To address this, superplasticizers and viscosity-modifying agents are commonly used to enhance flowability and extend workability. Optimizing the particle size distribution and activator concentration also helps improve the handling properties of AAMs. Long-term durability is a key concern, as AAMs may exhibit different behaviors under various environmental conditions. While they show excellent resistance to sulfate attack and carbonation, they can degrade under high humidity, freeze–thaw cycles, or chemical attack. The potential for alkali–silica reactions (ASR) also poses a risk. Comprehensive testing, including accelerated aging and long-term exposure tests, is essential to evaluate and enhance the durability of AAMs. The use of inhibitors or stabilizers can further mitigate the formation of expansive reaction products.

3. Inherent Characteristics of Fibrous Biochar

3.1. Production of Fibrous Biochar

Fibrous biochar is a carbon-rich material produced through the thermal decomposition of organic biomass, such as agricultural residues, forestry waste, or organic municipal waste, in an oxygen-limited environment, a process known as pyrolysis [24]. The pyrolysis process involves heating the biomass at temperatures typically ranging from 300 °C to 700 °C under limited oxygen conditions. The broad pyrolysis temperature range is primarily due to the diverse chemical compositions and structures of the fibrous biomass feedstocks employed. The complexity of these feedstocks necessitates a wide temperature span to accommodate their varying pyrolysis characteristics. Temperature significantly influences the biochar structure in several ways. With increasing temperature, the microstructure of fibrous biochar undergoes notable changes. At 300–400 °C, the biochar surface is relatively rough and retains some fibrous features of the feedstock, with incomplete carbonization. As the temperature rises to 500–600 °C, the surface becomes smoother and more compact, with initial pore development, and above 700 °C, a complex porous structure with well-defined pores and cracks emerges. From a crystalline structure perspective, at low temperatures, the carbon atoms in biochar are predominantly amorphous. However, as the temperature increases, the carbon atoms gradually arrange into more ordered structures, enhancing graphitization and leading to greater thermal and mechanical stability of the biochar. Biochar production has garnered significant attention for its potential in carbon sequestration, soil improvement, and waste management [25]. The current research focuses on optimizing production methods, such as pyrolysis, gasification, and hydrothermal carbonization, to enhance biochar’s properties and energy efficiency. Studies emphasize controlling parameters like temperature, feedstock type, and residence time to achieve desirable biochar characteristics tailored to specific applications [26].

Advancements have been made in integrating biochar production with renewable energy systems. For example, the use of pyrolysis gases and bio-oil as co-products contributes to the energy efficiency of the process, while circular economy approaches utilize waste biomass as feedstock. Researchers are exploring the modification of biochar with chemical and physical treatments to enhance its adsorption, catalytic, and nutrient-holding capacities, broadening its applicability in environmental remediation and industrial processes [27].

However, challenges remain in scaling up biochar production. Technical hurdles, such as process consistency and energy balance, must be addressed to make biochar production commercially viable. Furthermore, the environmental and economic sustainability of biochar systems requires deeper investigation, including lifecycle assessments, feedstock availability, and market dynamics. Regulatory frameworks and standards for biochar quality and application safety are also underdeveloped, posing barriers to widespread adoption.

Future research aims to bridge these gaps by developing integrated systems that combine biochar production with energy generation and other value-added processes. Innovations in reactor design, along with a better understanding of biochar’s interactions with ecosystems, will be critical to overcoming existing limitations and maximizing the benefits of this promising technology.

3.2. Physicochemical Properties of Biochar

Porosity is a key physical property of biochar, significantly influencing its adsorption capacity, water retention, and nutrient exchange capabilities [28]. The porosity of biochar, comprising micro-, meso-, and macropores, is largely determined by the feedstock composition and the pyrolysis conditions, particularly the temperature and heating rate. Higher pyrolysis temperatures typically increase porosity by driving off volatile organic compounds and leaving behind a more porous carbon structure. Lignocellulosic feedstocks, such as wood and crop residues, are often associated with biochar that has a well-developed pore network, suitable for applications like soil amendment and pollutant adsorption. Recent studies have highlighted the importance of mesoporosity for water and nutrient management in agricultural soils, while micropores are essential for gas adsorption and catalytic processes. To enhance porosity, researchers have explored chemical and physical activation methods, which significantly increase the surface area and pore volume, broadening biochar’s applicability in environmental and industrial sectors. However, the variability in porosity measurement techniques and the lack of standardized protocols present challenges for comparing biochar porosity across studies.

Thermal stability is a critical property of biochar, influencing its long-term performance in applications such as carbon sequestration and soil improvement [29]. The thermal stability of biochar is primarily determined by the degree of carbonization during production, which is influenced by pyrolysis temperature and feedstock type. Higher pyrolysis temperatures typically result in biochar with greater aromaticity and higher carbon content, both of which contribute to enhanced thermal stability. This stability is often assessed using techniques like thermogravimetric analysis (TGA) and is linked to biochar’s resistance to decomposition under high-temperature conditions or microbial activity in soils. Studies have shown that biochar derived from woody biomass generally exhibit higher thermal stability compared to those from herbaceous or animal-based feedstocks, due to their inherently higher lignin content. Modifications such as mineral doping and co-pyrolysis with other materials have been explored to further enhance biochar’s thermal resilience. However, the trade-offs between increasing thermal stability and retaining other functional properties, such as porosity or nutrient availability, remain a focus of ongoing research.

Apart the physical properties, biochar can be significantly various from the chemical perspective [30]. The chemical composition of biochar is a complex mixture of carbon-rich structures and mineral components, significantly influenced by the feedstock type and pyrolysis conditions. Biochar typically consists of aromatic carbon structures that confer chemical stability, along with functional groups containing oxygen, hydrogen, and nitrogen, which are important for reactivity and adsorption properties. Higher pyrolysis temperatures promote the formation of more condensed aromatic carbon structures, enhancing stability but reducing the abundance of oxygen-containing functional groups. Feedstock type plays a critical role in determining the elemental composition, with woody biomass producing biochar with a higher carbon content, while herbaceous and animal-based feedstocks contribute more ash and mineral components. Key inorganic constituents, such as calcium, magnesium, potassium, and phosphorus, are often retained in the biochar and contribute to its utility as a soil amendment. Recent research has focused on functionalizing biochar through chemical treatments to enhance its surface chemistry for applications like heavy metal adsorption and catalysis. However, the variability in feedstock and production methods poses challenges in standardizing the chemical characteristics of biochar for specific applications.

The surface condition of biochar, including its functional groups, roughness, and surface charge, is pivotal to its performance in various environmental and industrial applications [31]. Biochar surfaces typically feature oxygen-containing functional groups, such as hydroxyl, carboxyl, and carbonyl groups, which are critical for interactions with pollutants, nutrients, and microbes. These functional groups are influenced by feedstock type and pyrolysis temperature, with lower temperatures favoring a higher abundance of oxygenated groups and higher temperatures resulting in a more inert surface [32]. Surface roughness and morphology, determined by the inherent structure of the feedstock and pyrolysis process, affect biochar’s adsorption capacity and catalytic activity. Researchers have employed post-production treatments, such as acid or base activation and metal impregnation, to modify biochar surfaces, enhancing properties like adsorption efficiency and chemical reactivity. Advanced characterization techniques, including scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), have been instrumental in analyzing these surface features. However, variability in biochar surface conditions due to differences in production parameters highlights the need for standardized protocols to ensure consistent performance across applications.

3.3. Applications of Biochar in Construction

Biochar has emerged as a promising material in construction due to its lightweight, porous structure and environmental benefits [33,34]. Its incorporation into construction materials, such as concrete, mortar, and bricks, has been explored to improve its thermal insulation capacity, reduce its weight, and enhance its mechanical properties, as depicted in Figure 1, which illustrates the production and testing of bio-based concrete composites using biomaterials as an additive [1,35]. Studies have shown that biochar can increase the water retention and workability of cementitious materials while reducing shrinkage and cracking during curing [36]. Additionally, biochar’s ability to sequester carbon and its use as a partial replacement for sand or cement contribute to reducing the carbon footprint of construction projects. Functionalized biochar has also been investigated for its potential to enhance the durability and resistance of construction materials to environmental degradation. Challenges remain in achieving optimal compatibility between biochar and construction matrices, as excessive incorporation can negatively affect the material’s strength. Ongoing research focuses on optimizing biochar production and processing to ensure consistent quality and maximize its benefits in sustainable construction practices.

4. Pivotal Factors of Biochar-Reinforced AAMs

Biochar has been investigated for years as a source of lightweight aggregates in cement concrete [38,39,40,41]. It shows multiple benefits, such as boosting mechanical properties and enhancing environmental resistance [42,43,44]. The high surface area and porous structure of the biochar are beneficial to promote the microstructure evolution of AACMs by providing long-standing pores as buffering zones and thus promoting strong bonding with the alkali-activated matrix [45,46,47]. It was reported that biochar could improve the water retention and workability of AACMs, making them more suitable for applications requiring controlled setting times and reduced shrinkage [48]. Additionally, functionalized biochar, modified through chemical or thermal treatments, has been explored to further enhance its reactivity and compatibility with the alkaline environment of AACMs [49,50]. Even though the virtues of biochar have been well-recognized, several main challenges remain to ensure the enhancement of the overall performances of AAMs. Most importantly, as an additive in alkali-activated cementitious materials (AACMs), the understanding of its reinforcement effects remains unclear. The main distinction between cement concrete and AAMs is far more than the alkali impact; the element interactions between the AAMs and the biochar need to be systematically investigated. Further deep insights need to be disclosed to leverage its environmental benefits and its functional performance in AACMs.

4.1. Incorporation Techniques

Direct mixing of biochar with alkali-activated materials (AAMs) is the simplest and most commonly employed method. This approach involves mixing untreated/treated biochar with AAM directly, allowing its natural properties, such as porosity and high surface area, to enhance the performance of the materials. Direct mixing can improve the water retention capacity, reduce shrinkage, and enhance the thermal insulation properties of AAMs [51]. The mixing parameters are critical to fabricate high-performance biochar reinforced AAMs [52]. However, untreated biochar may exhibit poor compatibility with the alkaline environment of AAMs, potentially resulting in uneven dispersion and mechanical strength reduction at higher inclusion levels [53]. As a result, in the past decade, researchers strove to investigate the main determinants of biochar, such as the effects of surface treatment approaches, particle size, dosage, and curing conditions, on the performance of directly mixed AAMs to optimize their incorporation. In summary, while direct mixing offers a straightforward method for incorporating biochar into AAMs, it requires careful optimization of the biochar properties and mixing parameters to achieve the desired performance. Future research should focus on developing standardized surface treatment protocols and exploring the effects of different biochar types and sources on the overall performance of biochar-reinforced AAMs.

4.2. Interfacial Chemistry

Surface treatment of biochar is a widely explored method to mitigate the compatibility challenge when incorporated into AAMs. Treatment techniques, such as acid or alkaline washing [54], steam activation [32], and plasma treatment [55], have been implemented to modify the surface chemistry, thus increasing its reactivity, and improving the bonding ability with the alkaline binder [56]. These surface treatment methods introduce or expose functional groups like hydroxyl [57], carboxyl [58], or carbonyl [59], which enhance the interaction between biochar and the alkali activator, leading to the reinforcement of the mechanical properties [45], workability [60], and durability [61] of the AAMs. Additionally, surface treatment can benefit the integration of biochar and the AAM matrix, thus reducing the water demand and promoting more efficient hydration [62]. It was found that acid-treated biochar can improve the compressive strength and reduce porosity in biochar–AAM composites [56].

Apart from surface treatment of biochar, functionalization through nanotechnology also enhances its utility in AAMs by tailoring its physical and chemical properties for specific applications [28]. Functionalization techniques involve introducing nanoparticles [63], doping with metals [64], or coating biochar surfaces with polymers [56] to boost its reactivity, durability, or specific functional capabilities. Previous studies claimed that nano-silica or metal oxides can improve the pozzolanic activity and bonding within AAMs, resulting in the enhancement of mechanical properties and environmental resistance [65]. One study provides valuable insights into the influence of nanoceramic-plated waste carbon fibers on the performance of alkali-activated mortars. This research demonstrated that the application of nanoclays can significantly enhance the hydrophilicity of carbon fibers, thereby improving their deagglomeration and dispersion within the AAM matrix. The results of the study indicate that the incorporation of nanoclay-treated fibers leads to a notable increase in the mechanical strength of the mortars, with a maximum increase in flexural strength of 19% for a fiber content of 0.25 wt.% [66]. This approach not only enhances the mechanical properties but also aligns with the goal of developing sustainable construction materials by utilizing waste carbon fibers and minimizing the environmental impact. Functionalized biochar also shows promise in specialized aims, such as self-healing concrete [67] and fire-resistant construction [68].

Although these surface treatment and functionalization methods have distinctive benefits, they will inevitably add extra manufacturing steps and increase the production cost of the biochar preparation process. Most importantly, these surface treatment methods and functionalization processes will produce secondary pollution due to using various chemicals. As a result, ongoing research is dedicated to optimizing treatment methods for scale production and environmentally friendly treatments in sustainable construction applications.

4.3. Particle Size and Distribution

The particle size and distribution of biochar play a vital role in determining the performance of biochar-reinforced AAMs. Particle size reduction provides a more contact area for interactions between the biochar and AAM matrix, leading to strengthening the bonding [69]. Prior works indicate that fine biochar particles can enhance the reactivity of the AAM matrix, potentially increasing its strength and durability [45]. This is attributed to the high specific surface area and porous structure of fine biochar particles, which facilitate better dispersion within the matrix and promote more effective interactions with the geopolymer binder. However, large biochar particles harm the workability of the AAMs [70] due to their larger pore structure, which allows them to absorb a substantial amount of water from the geopolymer mixture.

4.4. Curing Conditions

The curing condition is another important factor that determines the performance of biochar-mixed AAMs. It has been proven that the curing temperature, humidity, and curing time significantly affect the microstructure, strength development, and durability of biochar-AAM composites. An increase in curing temperature tends to accelerate the alkali activation process and enhance the formation of the binder phases, leading to improved early-age strength [71], while an excessive increase in the curing temperature might cause the biochar to lose its beneficial water retention properties, potentially affecting its long-term performance [72]. For alkali-activated binder specimens composed of 20% bagasse ash (BA) and 80% slag, those cured under ambient conditions exhibited higher compressive strength than those subjected to thermal curing at the same mix ratio and curing duration. This difference is attributed to the significant water loss experienced by the specimens during thermal curing at 65 °C, which in turn reduces the hydration degree of slag.

Apart from curing temperature, curing humidity is essential to optimize the hydration process. High humidity favors the hydration of the binder and is beneficial to mitigate the agglomeration of biochar in the matrix [61]. Previous studies have demonstrated that the internal curing effect of biochar can significantly enhance the compressive strength of magnesium oxychloride cement (MOC) under low humidity curing conditions. For instance, the addition of 5% biochar increased the compressive strength of MOC by 14.1% after 28 days of curing compared to pure MOC. Moreover, under low humidity conditions, the high water absorption capacity and porous structure of biochar can effectively reduce water penetration, thereby improving the durability of MOC. This phenomenon is attributed to the ability of biochar’s porous structure to absorb and retain substantial amounts of water, which delays water evaporation and provides a sustained supply of moisture for the hydration process, promoting the formation of hydration products and ultimately enhancing the mechanical properties and durability of MOC.

Besides the curing temperature and humidity, elongating the curing time offers sufficient interaction between biochar and the alkali activator, improving the material’s cohesion and stability [73]. Recent research has shown that the unconfined compressive strength (UCS) improves with extended curing periods. This phenomenon can be caused by the enhanced bonding between the biochar and the soil matrix, as well as the formation of stable aggregates. For instance, the UCS of soil amended with 2% biochar increased by 7.19% after 30 days of curing compared to the initial state. This improvement is primarily due to the gradual manifestation of the internal curing effect of biochar over the curing period. The high porosity and water absorption capacity of biochar enable it to retain substantial amounts of moisture, which is gradually released during curing to provide a sustained supply of water for the hydration process, promoting the formation of hydration products and thereby enhancing the mechanical properties of the soil.

Researchers continue to explore the optimal curing conditions for biochar–AAMs to balance early-age strength with long-term durability, taking into consideration the unique properties of biochar and its interactions with the alkaline binder.

4.5. Water Absorption

Water absorption is a significant factor influencing the performance of biochar-mixed AAMs, as it affects both workability during mixing and the long-term durability of the material. Biochar, with its high porosity and hydrophilic surface, can absorb large amounts of water, which can alter the water demand and consistency of AAM mixtures. Studies have shown that moderate biochar incorporation can improve water retention, reducing shrinkage and enhancing internal curing by gradually releasing absorbed water during hydration, which benefits the strength development and stability of AAMs. However, an excessive biochar content may lead to an increase in water absorption, causing the mixture to become overly wet, which can compromise the material’s workability, setting time, and mechanical strength. Additionally, the water absorption characteristics of biochar-mixed AAMs are influenced by the particle size and surface treatment of the biochar, with smaller or modified particles generally enhancing the material’s performance.

Researchers continue to investigate the relationship between biochar water absorption and its effect on the overall properties of AAMs to optimize formulations for specific applications, balancing hydration and minimizing the negative effects of excessive water absorption.

The

Table 1. Impact of Biochar Dosage on Geopolymer Performance.

Biochar Dosage (%)	Compressive Strength	Durability	Workability	Thermal Conductivity
0–2	Slight to moderate increase	Improved	Slight improvement	Moderate reduction
2–10	Significant increase	Enhanced	Optimal	Substantial reduction
10–20	Tendency to decrease	Still improved	Reduced	Further reduction
>20	Significant decrease	May decline	Poor	Very low

below summarizes the effects of different biochar dosages on compressive strength, durability, and workability.

4.6. Regional Considerations for Biochar Feedstock Availability

The availability of biochar feedstock varies significantly by geography, which is a crucial factor to consider for the practical implementation of biochar-reinforced alkali-activated materials (AAMs). The types and quantities of biomass available for biochar production can influence the feasibility and sustainability of using biochar in AAMs in different regions [74]. Biochar feedstock can include agricultural residues, forestry waste, and organic municipal waste. The availability of these feedstocks is highly dependent on regional agricultural and forestry practices. For example, regions with significant agricultural activity, such as the Midwest in the United States or the Punjab region in India, have abundant crop residues like corn stover and rice husks [75]. In contrast, regions with dense forests, such as the Amazon basin or the Pacific Northwest in the United States, have a higher availability of forestry waste. The economic and environmental implications of biochar feedstock availability are significant. Regions with abundant biomass waste streams can benefit from lower feedstock costs and reduced waste management issues. For instance, using agricultural residues like rice husks in regions with intensive rice cultivation can provide a cost-effective and sustainable source of biochar feedstock [76]. This not only reduces waste disposal costs but also contributes to carbon sequestration and circular economy goals.

5. Reinforcement Mechanisms of Biochar Modified AAMs

5.1. Internal Curing Effect

Internal curing is one of the primary reinforcement mechanisms of biochar-mixed AAMs because it helps improve the hydration process, reduce shrinkage, and enhance long-term durability [62]. Figure 2 shows that, due to its high porosity and water retention properties, biochar acts as an internal reservoir that absorbs and maintains water or alkali solution during the mixing stage and gradually releases it during the curing process [50]. This process preserves the moisture levels within the AAM matrix, thus ensuring uniform hydration of the alkali activator and reducing cracks resulting from autogenous shrinkage [77]. A few studies have demonstrated that the internal curing effect can significantly improve the mechanical properties and durability of AAMs, especially during the early stages of curing, by providing sufficient alkali solution for activating precursor materials [78,79]. However, the effectiveness of internal curing depends on the amount and type of biochar used, as well as the curing conditions, such as temperature and humidity [61]. Further research needs to strive to optimize biochar content and curing parameters to maximize internal curing benefits in biochar-mixed AAMs.

The incorporation of biochar into alkali-activated materials (AAMs) has garnered significant attention in recent years, particularly concerning its role in internal curing and the enhancement of material properties. Internal curing involves the use of water reservoirs within the material to promote hydration, thereby improving strength and durability. A previous study [6] examined the effects of biochar addition on the properties of alkali-activated metakaolin pastes. The researchers found that incorporating small amounts of biochar (less than 2 wt%) increased its compressive strength by 15% after 28 days and reduced its water absorption by capillarity, potentially leading to improved durability. However, a higher biochar content was observed to decrease its mechanical properties while enhancing its dimensional stability and reducing efflorescence formation. Another study [80] examined the influence of biochar on the properties of alkali-activated slag pastes using two activator solutions, NaOH and Na₂CO₃. The findings highlighted that biochar incorporation could modify the microstructure and enhance the mechanical properties of the pastes, indicating its role in internal curing processes. Collectively, these studies underscore the potential of biochar as an internal curing agent in AAMs, contributing to enhanced mechanical properties, reduced shrinkage, and improved durability. The incorporation of biochar not only aids in internal curing but also offers environmental benefits by utilizing waste biomass, aligning with sustainable construction practices.

Incorporating biochar into alkali-activated materials (AAMs) for internal curing presents several challenges that require careful consideration:

Optimal Biochar Dosage: Determining the appropriate amount of biochar is crucial. Excessive biochar can adversely affect mechanical properties, while insufficient amounts may not provide adequate internal curing benefits. Studies [6] have shown that small amounts of biochar can enhance compressive strength, but a higher content may reduce mechanical properties.

Compatibility with Alkaline Activators: The interaction between biochar and alkaline activators in AAMs is complex. Biochar’s porous structure can absorb water, potentially affecting the hydration process. Research [81] indicates that biochar can influence the microstructure and mechanical properties of alkali-activated slag pastes, but the exact mechanisms are not fully understood.

Control of Autogenous Shrinkage: While internal curing aims to mitigate autogenous shrinkage, achieving effective control is challenging. The release rate of water from biochar must be carefully managed to prevent excessive shrinkage without compromising the material’s strength. Researchers [80] have explored the use of biochar to reduce autogenous shrinkage in alkali-activated slag binders, but optimal conditions are yet to be established.

Standardization and Quality Control: Variability in biochar properties, such as particle size, porosity, and surface chemistry, can lead to inconsistent performance in AAMs. Establishing standardized procedures for biochar preparation and incorporation is essential to ensure reliable outcomes. A systematic literature review [82] has highlighted the need for standardization in biochar-enhanced construction materials.

Addressing these challenges necessitates a multidisciplinary approach, combining materials science, engineering, and environmental studies, to fully harness the benefits of biochar in alkali-activated materials. In summary, while biochar offers potential benefits for enhancing the properties of alkali-activated materials, its impact on workability is multifaceted and depends on factors such as water demand, particle size, surface chemistry, dosage, and curing conditions. Careful optimization of these parameters is essential to achieve a balance between improved material properties and acceptable workability in practical applications. In summary, internal curing with biochar offers significant advantages for enhancing the hydration process and reducing shrinkage in AAMs. However, the effectiveness of this mechanism varies with biochar properties and curing conditions. Future research should focus on developing standardized protocols for biochar incorporation and curing to ensure consistent performance across different applications.

Figure 2. Schematic diagram of the internal curing mechanism of biochar modified cement-based materials [83].

Figure 2. Schematic diagram of the internal curing mechanism of biochar modified cement-based materials [83].

5.2. Impact on Workability and Strength Development

The traditional perspective claims that the incorporation of biochar into alkali-activated materials (AAMs) can negatively affect their workability, particularly when high amounts of biochar are used as depicted in Figure 3. Biochar, with its porous and lightweight structure, tends to absorb water during the mixing process, which can increase the water demand of the mixture. This leads to a reduction in the flowability and ease of handling, making the material more difficult to mix, shape, and place. Additionally, biochar particles, especially those with larger sizes or an uneven distribution, can cause clumping or poor dispersion in the matrix, further exacerbating the workability issues. Studies have shown that excessive biochar content can result in a thick, less workable paste, potentially leading to challenges in achieving uniformity in the final material. The reduced workability can also affect the setting time and curing process, delaying the development of strength and durability. Therefore, careful optimization of the biochar dosage and particle size selection is necessary to mitigate these negative effects and ensure that biochar-mixed AAMs remain workable for practical construction applications. Research has shown that a low to moderate biochar content can enhance the workability [60], water retention [84], and internal curing properties of AAMs, reducing shrinkage and improving its long-term stability [85]. However, an excessive biochar dosage will lead to a negative impact on the mechanical properties due to the interfacial defects between biochar and the AAMs, leading to a reduction in overall cohesion and structural integrity [46]. Figure 4 illustrates that excessive incorporation of biochar can lead to weak interfaces between the biochar and the matrix, resulting in defect formation. Past research indicates that the optimal biochar dosage varies depending on the origin of the biochar [86], particle size [87], and the production conditions of both the biochar and AAMs [86]. It was believed that the optimal dosages of around 2–15% by weight can leverage the benefits and drawbacks of improving moisture retention and reducing shrinkage without compromising the mechanical properties [6]. Previous studies showed that incorporating a biochar dosage of less than 2% significantly enhances compressive strength and decreases water absorption, thereby increasing durability. However, a higher biochar content, while reducing compressive strength, enhances two-dimensional stability and improves durability by mitigating efflorescence formation. This suggests that biochar acts as a multifunctional additive, balancing mechanical properties and durability based on its dosage and interactions with the alkali-activated matrix.

At variance with this point, recent studies argued that incorporating biochar into AAMs is beneficial for its workability when provided in appropriate quantities [89]. Due to the innate porous structure, mixing in biochar can improve water retention, and thus enhance flowability [90]. Furthermore, the high specific surface area of biochar can improve the dispersion of the precursors, reducing clumping or particle aggregation and enhancing the hydration of the mix [91]. An interesting study stated that biochar can serve as a plasticizer and improve the workability of AAMs without significantly compromising their mechanical properties or durability [69]. Recent evidence demonstrates that by incorporating biochar into AAM, the porous structure of biochar enables it to reserve water and release the water during the hydration process. The capacity of water retention of biochar can improve the workability of AAM. Meanwhile, the gradual release of water can enhance the hydration process, leading to an improvement in mechanical and durability performance.

Similar to workability, it has been widely reported that biochar has a positive influence on strength development, particularly the early-age strength and long-term durability of AAMs [33,92,93]. The inherent porous structure leads to an internal curing process that contributes to enhancing the hydration of the AAM precursor, boosting the formation of binder cementitious phases and enhancing the overall performance [88]. Research indicates that an appropriate dosage of biochar can improve the compressive strength along with reducing shrinkage and mitigating cracking [62]. It has been noted that biochar could benefit alkali-activated materials as an internal curing agent. However, the presence of biochar will lead to a mechanical weak point due to its lower strength compared to the hydration products. Under external pressure, biomass materials are more prone to fracture than hydration products, thereby disrupting the matrix. At an optimal dosage, the internal curing mechanism of biochar dominates, manifesting as enhanced mechanical properties and improved durability.

However, similar to workability, excessive biochar content can negatively affect strength development [94]. Over-dosage addition of biochar will weaken the bonding between the AAM binder and biochar particles, reducing the overall cohesion and structural integrity of the AAM and leading to lower compressive and flexural strength [93]. Specifically, an inappropriate dosage harms the dispersion of biochar particles and damages the hydration of the AAM precursor [95]. It has been reported that the incorporation of rice husk ash (RHA) significantly enhances the mechanical properties of ground granulated blast furnace slag (GGBS)–based geopolymers. Experimental results indicate that when the RHA content is 15%, the compressive strength of geopolymer concrete reaches approximately 60 MPa within 3 days, comparable to that of conventional Portland cement concrete. However, when the RHA content exceeds 15%, the mechanical properties of the geopolymer begin to deteriorate. This decline is primarily attributed to the high specific surface area and porous structure of RHA. While an appropriate amount of RHA can improve the density of concrete by filling pores and refining the microstructure, excessive RHA introduces numerous micropores, which act as stress concentrators and ultimately weaken the overall strength of the concrete.

Continued research is focused on optimizing the biochar content and processing conditions to balance the positive effects on strength development while maintaining workability and durability in biochar-mixed AAMs.

5.3. Thermal Properties

Using biochar to enhance the thermal insulation properties of AAMs makes them promising for applications in energy-efficient or fire-resistant construction [51]. It can be observed from Figure 5 that an increase in biochar content leads to a corresponding decrease in thermal conductivity. The porous structure guarantees a low thermal conductivity and contributes significantly to reducing heat transfer, thereby improving its thermal insulation capacity. When mixed with the AAM matrix, biochar creates a network of air-filled pores, which decrease the overall density and serve as thermal barriers for the AAM matrix [90]. Studies have shown that a moderate biochar content can effectively enhance thermal resistance without significantly compromising mechanical strength or durability [96]. Additionally, the combination of thermal insulation with the inherent thermal stability of AAM creates a material suitable for use in extremely high-temperature environments [97]. Recent studies have demonstrated that adding biochar to AAMs can significantly boost their thermal insulation performance. The results showed that [98] biochar reduces the thermal conductivity of AAMs, enhancing their insulation efficiency. The enhancement of thermal insulation performance is primarily attributed to the porous structure and micropores of biochar, which efficiently trap heat and reduce thermal conduction. A study [99] revealed that the incorporation of biochar significantly improved the thermal insulation properties of gypsum-based composite materials, extending the insulation duration and slowing the rate of temperature decrease.

Ongoing research needs to focus on exploring the roles of biochar particle size, surface modification, and interactions with the binder matrix to further enhance the thermal insulation properties of biochar-enhanced AAMs.

5.4. Noise Damping

Recent research shows that biochar, with its porous nature, can remarkably enhance the sound-absorption properties of AAMs. Studies have indicated that the pore structure of biochar can effectively absorb sound wave energy and reduce noise reflection. A previous study [100] found that the porous structure of biochar can effectively absorb sound energy and reduce noise reflection. In addition, experiments have demonstrated that adjusting the amount and particle size of biochar can optimize the sound-absorption properties of AAMs, enabling them to deliver outstanding performance across different frequency bands. These studies have offered a theoretical basis for the application of biochar in construction sound-absorption materials.

6. Environmental Resistance

AAMs reinforced by biochar have demonstrated outstanding environmental resistance [101] as shown in Figure 6 [102]. The addition of biochar can enhance resistance to various environmental stressors, including chemical attacks [103], freeze–thaw cycles [88], and carbonation [104], although its effects are highly dependent on factors such as biochar type, dosage, and surface treatment [52,93,105].

AAMs exhibit high chemical resistance due to their dense microstructure and low permeability, and biochar incorporation further improves these properties under certain conditions. For instance, the porous structure of biochar can absorb harmful ions like chlorides and sulfates, reducing their penetration into the matrix and mitigating their detrimental effects [106]. Surface-modified biochar, in particular, has been shown to enhance binding with the geopolymer matrix, limiting pathways for aggressive agents [107]. A previous study found that pyrolysis temperature significantly affects the surface properties and pore structure of biomass materials. This research investigated the surface properties of lignocellulosic materials at different pyrolysis temperatures and found that the specific surface area and porosity of biochar were significantly increased in the temperature range of 400–700 °C. This enhancement further improves the internal curing function of biochar in geopolymers, thereby increasing the compressive strength and durability of the geopolymer.

Besides enhancing chemical resistance, biochar also demonstrates benefits in mitigating freeze–thaw damage and reducing shrinkage-related cracking of AAMs due to the internal curing mechanism [79]. It acts as a reservoir to retain water, thus promoting curing, minimizing microcracks, and enhancing structural integrity [108]. Most importantly, the innate porous structure of biochar allows it to release the internal pressure resulting from ice formation during freezing cycles. It was reported that biochar-modified AAMs can significantly enhance freeze–thaw resistance, making these composites suitable for use in cold climates provided an appropriate size selection and good dispersion of biochar particles is utilized [109]. It has been demonstrated that low incorporation levels of biomass materials (<5%) do not significantly affect the mechanical properties of the matrix, whereas high levels (>20%) result in a substantial decrease in mechanical performance. However, a high biomass content significantly enhances the resistance to freeze–thaw cycles. This improvement is attributed to the increased porosity provided by the biomass materials, which allows for greater space for the freezing and thawing of water, thereby reducing crack initiation.

Similar to freeze–thaw damage mitigation, biochar can further improve the carbonation resistance of AAMs by enhancing moisture retention and reducing capillary pores [79]. The porous structure of biochar provides a buffering capacity that stabilizes the AAM matrix against pH changes induced by CO₂ ingress [106]. Functionalized or pre-treated biochar is often employed to maximize its beneficial effects on carbonation resistance. A recent study demonstrated that commercial coconut shell activated carbon impregnated with alkaline NaOH can significantly enhance CO₂ adsorption capacity in a fixed-bed column system. The optimal modification conditions were found to be a 32% NaOH concentration with a 3 h dwelling time, achieving a maximum CO₂ adsorption capacity of 27.10 mg/g at 35 °C. This suggests that NaOH-modified activated carbon is a highly effective adsorbent for CO₂ capture, with potential applications in carbon capture and storage technologies. However, improper incorporation of biochar, such as poor dispersion or excessive amounts, can increase carbonation susceptibility by introducing voids into the matrix.

7. Microstructural and Chemical Analysis

7.1. Microstructural Evolution of Biochar-Modified AAM

The microstructural evolution of biochar-modified AAM involves complex interactions between biochar particles and the AAM matrix [110]. This evolution process significantly impacts the material’s mechanical properties, durability, and overall performance [48]. As the precursors undergo alkali activation, the dissolution of the aluminosilicates forms a gel-like geopolymeric binder phase that encapsulates the biochar particles, leading to an interconnected microstructure [69]. Understanding these interactions is essential for optimizing the overall performance of biochar-modified AAMs.

As the precursors undergo alkali activation, the dissolved silica and alumina from the precursor form a gel-like amorphous structure, often referred to as N(K)-A-S-H (sodium–alumino–silicate–hydrate) or C-A-S-H (calcium–alumino–silicate–hydrate), in certain systems [111,112,113]. In biochar-modified AAMs, biochar particles are embedded within these N(K/C/M)-A(F)-S-H phases, contributing to the overall structure by reducing porosity and enhancing internal curing. The incorporation of an optimal amount of biochar in alkali-activated materials (AAMs) significantly enhances the mechanical properties and durability of the composites. Specifically, biochar particles embedded within the N(K/C/M)-A(F)-S-H phases contribute to a denser microstructure by reducing porosity and facilitating internal curing. This improvement not only increases the compressive strength but also enhances the resistance to sulfate attack, thereby improving the overall durability of the material. Additionally, the high specific surface area and porosity of biochar improve its water retention capacity, which is crucial for the hydration process and mitigates issues such as shrinkage and cracking [62]. It is evident from Figure 7 that the addition of biochar can effectively reduce the formation of cracks. The alkali activation process can also lead to partial carbonation or hydrolysis at the biochar surface, further promoting stronger chemical bonding with the geopolymer gel [60]. These interactions help maintain the structural integrity and improve its mechanical properties, such as compressive strength and resistance to shrinkage [114]. In this study, the role of biochar in GPC was extensively examined through partial replacement of fly ash at various ratios (80G20F0BC, 80G15F5BC, 80G10F10BC, 80G5F15BC, 80G0F20BC). Alkaline activators, including sodium silicate and sodium hydroxide, were employed to facilitate the geopolymerization process. Concrete specimens were subjected to curing for 7, 14, and 28 days at room temperature. The results revealed a significant correlation between biochar content and enhanced mechanical properties, with an optimal replacement level of 15% fly ash by biochar. Compressive, split tensile, and flexural strengths consistently improved across all tested ratios. This study underscores the potential of biochar as a sustainable alternative to fly ash in GPC, strengthening the matrix while supporting environmentally responsible concrete production. It highlights the synergistic relationship between enhanced mechanical properties and innovative material design, paving the way for future advancements in sustainable construction materials.

The interface between biochar and the alkali-activated matrix plays a critical role in the microstructural development of biochar-modified AAMs. The biochar particles, due to their porous and hydrophobic nature, tend to resist interactions with the matrix unless modified or treated. This resistance can result in weak interfacial bonding, leading to poor dispersion of biochar in the matrix and the formation of weak zones that may adversely affect the mechanical properties. However, surface treatments or functionalization of biochar, such as acid treatment or coating with silanes, can improve the compatibility of biochar with the alkali-activated binder, promoting better adhesion and reducing the risk of interfacial voids. Enhanced bonding at the interface leads to a more uniform distribution of biochar, which contributes to a denser, more stable microstructure.

The morphology of the amorphous binder phases in biochar-modified AAMs is often altered by the introduction of biochar. Typically, the alkali-activated binder phases are three-dimensional networks of aluminosilicate chains [115]. Combined with biochar, the microstructure evolution of the aluminosilicate network will be altered because the porous and particulate characteristics of the biochar particles provide additional space to alleviate the crystallization internal stress and reserve long-term alkali environment [116]. Previous studies have demonstrated that the morphology at the interfacial zone between the AAM binder phase and the biochar particles was significantly distinct from the AAM matrix [95]. For instance, the microstructure properties of geopolymer concrete (GPC) incorporating rice husk ash (RHA) at different levels (10%, 15%, and 20%) significantly influenced its microstructure and mechanical properties. At 10% RHA, the fractured surfaces of GPC exhibited a dense microstructure with a small amount of pore spaces, voids, and cracks. When the RHA content was increased to 15%, the microstructure became more compact and denser than the 10% RHA mixture, attributed to the coexistence of calcium hydration products within the polymerization product NASH. However, further increasing the RHA content to 20% led to an excess of silica, which hindered the polymerization process. In addition, the interface transition zone (ITZ) between biochar particles and the cementitious matrix has been identified as a critical region that significantly affects the overall performance of the composite. The ITZ between biochar and the matrix is consistently distinguishable across all biochar dosages.

There is no doubt that the incorporation of biochar will lead to a porosity increase in the geopolymeric binder phase [107]. A few studies claimed that this increase in porosity could be beneficial by improving the thermal insulation properties, but it will inevitably reduce the mechanical properties of the AAM [29]. Specifically, a study on porous fly ash-based geopolymers utilized fly ash and sodium water glass as raw materials, with H₂O₂ as a foaming agent to introduce porosity. The investigation focused on the effects of varying amounts of sodium water glass (60, 80, and 100 g) and H₂O₂ (4, 6, and 8 g) on the properties of geopolymer mortars cured at two different temperatures (55 °C and 85 °C). The results indicated that the optimal combination of sodium water glass (80 g), H₂O₂ (6 g), and curing temperature (55 °C) yielded a material with a porosity of 79.9%, thermal conductivity of 0.0744 W/m·K, and compressive strength of 0.82 MPa. This optimized geopolymer mortar showed promise as a thermal insulation material, particularly in applications where reduced mechanical demands are balanced by significant environmental and economic benefits. An opposite point of view argued that if the biochar particles were mixed in appropriate amounts and well-dispersed within the matrix, it could reduce the capillary porosity of the geopolymer, leading to a more dense microstructure that improves strength and durability [117]. In this study, novel bio-composites were prepared using biochar and natural inorganic clay (NIC) to evaluate the applicability of biochar to buildings. After preparing biochar from rice husk, coconut shell, and bamboo, these were mixed into NIC at four ratios to form a board, and their morphological, thermal, and moisture performance were analyzed. It was shown that the maximum rate of decrease of thermal conductivity was 67.21% due to biochar. Dynamic heat transfer analysis confirmed that the bio-composite was less sensitive to thermal changes due to the low thermal conductivity of biochar. Meanwhile, the bio-composite maintained reliable mechanical properties.

In the long term, the microstructure of biochar-modified AAMs affects their mechanical properties [78] and durability [109]. The improved bonding between biochar and the geopolymeric matrix enhances the resistance to chemical attack [33], carbonation [118], and thermal degradation [119]. Biochar’s porous nature and surface characteristics also contribute to its potential as an adsorbent, helping mitigate the ingress of harmful ions or gases into the material [95]. Over time, the interconnected network of biochar particles and geopolymer gel helps to preserve the integrity of the material, reducing the risk of cracking or degradation [85]. However, variations in biochar type, treatment, and dosage can influence its long-term performance, highlighting the need for tailored mix designs.

7.2. Physical and Chemical Interactions

The combination of biochar with AAMs significantly governs the hydration process and binder phase morphology during the geopolymerization process. Biochar interacts with both the alkali activators and the aluminosilicate precursors, which leads to changes in the microstructure and the overall properties of the final materials. These interactions affect the formation, distribution, and characteristics of the amorphous binder phases that are responsible for the binding and strength of AAMs.

Biochar has evident impacts on the hydration kinetics of the AAMs. It absorbs and retains the alkali solution in its pores, thus retarding the hydration reaction rate [46]. This delayed activation might slow down the formation of the early-stage binder phases and lead to a curing time extension [78]. Consistent with this observation, a recent study on biochar-modified geopolymers revealed that the addition of biochar significantly impacts the rheological and mechanical behavior of the material. Specifically, biochar increases the viscosity and reduces the flowability of the geopolymer paste due to its interaction with the alkali activator. This interaction not only delays the initial setting time but also results in a more compact and interconnected microstructure, as confirmed by microstructural analysis. Mechanically, biochar incorporation enhances the compressive and flexural strength of geopolymers, particularly at later curing stages, thereby improving long-term durability and strength development. Meanwhile, this early-stage reaction delay is accompanied by water retention that provides the internal curing conditions, maintaining moisture levels during the curing process and ensuring a more complete geopolymerization reaction over time [102]. It has previously been observed that the amount of rice husk ash accounted for a high proportion (50–80%), and the influence of rice husk ash content on compressive strength, flexural strength, water absorption, and moisture absorption coefficient was studied. The test results showed that, considering both compressive and flexural strength, a blend of 60% rice husk ash and 40% slag (R6S4) provides the best mechanical properties. The water absorption capacity of rice husk ash plays a crucial role in internal curing. In the later stages, the release of absorbed moisture enhances the degree of polymerization, thereby increasing strength. Meanwhile, the altered pore structure reduces the water evaporation rate, preventing free water molecules from migrating to the outer surface and thus inhibiting efflorescence. Specifically, the effect of biochar on hydration kinetics depends on its dispersion, dosage, particle size, surface properties, and the water-to-binder ratio, making it crucial to achieve optimized results in terms of hydration products and gel morphology [96].

Biochar also impacts the growth of the hydration products in AAMs, primarily by influencing the dissolution of aluminosilicate precursors during alkali activation. Some researchers believe that the presence of biochar provides additional nucleation sites that can affect the formation of the geopolymeric gel [114]. When biochar is introduced into the AAM matrix, its surface functional groups, such as hydroxyl and carboxyl groups, may interact with the silica and alumina ions released from the precursor material. This interaction may help stabilize the formation of the N-A-S-H gel, potentially increasing the amount of gel produced [46]. However, the interaction between biochar and the precursor material can be complex; if the biochar is not properly dispersed or treated, it may disrupt the formation of the gel network, leading to less efficient geopolymerization and reduced binder strength.

8. Environmental and Economic Perspectives

8.1. Environmental Perspective

Biochar is produced from the pyrolysis (even a simple burning process) of organic biomass. Biochar-modified AAMs offer an effective solution for waste management. Biomass residues from agricultural or forestry industries, often being discarded or burned, can be converted into biochar, thus reducing waste and promoting a circular economy. By utilizing these waste materials, biochar not only addresses waste disposal issues but also reduces the consumption of natural resources, contributing to the sustainable use of materials in the construction industry. Additionally, biochar’s potential to improve the recyclability and reusability of AAMs aligns with sustainability goals, supporting the development of greener and more resource-efficient construction materials.

Biochar has been considered a valuable resource as an additive for reducing the environmental footprint of construction materials. The integration of biochar into AAMs brings significant environmental benefits, particularly in terms of sustainability and waste management [120]. The use of biochar in AAMs not only helps carbon capture, as biochar itself stores carbon for long periods, but also reduces the consumption of cementitious materials, which is a major source of CO₂ emissions in the construction industry.

Furthermore, biochar has the potential to enhance the environmental resilience of AAMs. The porous structure of biochar also contributes to reducing the density of AAM concrete, which in turn lowers energy consumption during transportation and construction [68].

Moreover, biochar can adsorb and immobilize toxic elements and mitigate the leaching of pollutants, including heavy metals and organic contaminants [121]. This enhanced resilience can ensure longer-lasting infrastructure with fewer repair or maintenance activities, ultimately decreasing the overall environmental impact of construction materials [122].

8.2. Economical Perspective

From an economic standpoint, the use of biochar in alkali-activated materials can offer substantial cost savings in the long run, particularly in terms of material production, energy consumption, and maintenance costs. While the initial cost of biochar may vary depending on its source and production method, its incorporation into AAMs can reduce the overall reliance on costly Portland cement. Cement production is energy-intensive and a major contributor to the cost of concrete; thus, using biochar as a partial substitute can reduce material costs, especially when large quantities of biochar are available locally from agricultural or industrial waste. This shift from conventional cement-based systems to biochar-modified AAMs could make construction projects more affordable, especially in regions with abundant biomass resources.

The economic advantages of biochar-modified AAMs are also evident in terms of energy efficiency. The use of biochar, which is produced through pyrolysis at relatively low temperatures compared to cement clinker production, can reduce energy consumption in the material’s production. This shift could have a significant impact on reducing the carbon footprint and operational costs in the construction industry. Additionally, the long-term durability and resilience of biochar-modified AAMs contribute to economic savings by reducing the need for frequent repairs or replacements. With their enhanced resistance to environmental degradation, including improved fire resistance, thermal insulation, and weathering resistance, these materials can lead to lower lifecycle costs in infrastructure development and maintenance.

Moreover, biochar-modified AAMs can create new economic opportunities in the growing bioeconomy sector. As demand for sustainable building materials rises, the market for biochar production and utilization is also expanding. The development of biochar-modified AAMs can stimulate the demand for biochar, leading to growth in the agricultural and industrial sectors involved in biochar production. This could create new jobs and business opportunities in areas such as waste management, renewable energy, and construction, contributing to local economic growth and the creation of a circular economy. By fostering the use of waste biomass in construction materials, biochar-modified AAMs can generate economic value from otherwise underutilized resources.

8.3. Life Cycle Assessment

The processing of raw materials, especially the pyrolysis system, is the main source of greenhouse gas emissions during the biochar production process. The carbon emissions of biochar production are not comparable across different studies due to differences in system boundaries, functional units, and other parameters. However, overall, the impact of biochar production on climate change is negative, indicating its potential for carbon reduction. Research has shown that carbon capture and energy production in the production process of biochar, as well as the advantages brought by the utilization of byproducts, can offset the greenhouse gas emissions caused by biochar production itself [123,124,125]. The literature indicates that biochar, as a partial substitute for cement, can significantly reduce the carbon footprint of cementitious composites. Specifically, replacing 5% of cement with biochar can reduce the production of approximately 210 million tons of cement, process 610 million tons of waste, and cut carbon dioxide emissions by 1.18 billion tons [126]. Furthermore, research findings demonstrate that the incorporation of biochar can significantly enhance the carbon capture and sequestration capacity of cementitious composites. The application of biochar in alkali-activated systems significantly reduces carbon emissions through mechanisms such as decreasing emissions from cement production, thereby providing strong support for the low-carbon development of alkali-activated systems.

8.4. Challenges

Despite the promising environmental and economic benefits of biochar-modified AAMs, several challenges must be addressed before their widespread adoption. One of the primary challenges is the variability in biochar quality, which can depend on the feedstock, production process, and surface treatment. Inconsistent biochar quality may lead to variable performance in AAMs, affecting their mechanical properties, durability, and overall effectiveness. Standardization of biochar production and quality control measures will be essential to ensure consistent results across different applications.

Another challenge is the cost and scalability of biochar production. While biochar can be derived from waste biomass, the large-scale production of high-quality biochar may require substantial investments in pyrolysis equipment and infrastructure.

Table 2. Trade-offs between environmental gains and energy input in biochar production.

Factor	Environmental Gains	Energy Input
Carbon Sequestration	Biochar acts as a long-term carbon sink, reducing atmospheric CO2 levels.	Energy is required for the pyrolysis process to produce biochar.
Soil Improvement	Biochar enhances soil fertility and water retention, leading to better crop yields.	Higher pyrolysis temperatures increase energy consumption but improve biochar quality.
Pollutant Adsorption	Biochar can absorb heavy metal ions and organic pollutants from water, improving environmental quality.	Energy input is necessary for the pyrolysis process, which can vary based on feedstock and technology used.
Waste Management	Biochar production helps convert agricultural and municipal waste into a valuable product.	The energy efficiency of the pyrolysis process can be optimized to reduce overall energy consumption.

shows the relationship between energy consumption and environmental benefits of biochar production. To make biochar-modified AAMs economically competitive with conventional building materials, cost-effective production methods and efficient supply chains for biochar will need to be developed. Additionally, more research is needed to optimize the dosage and integration methods of biochar in AAMs to achieve the best performance and value, as well as to further explore the cost–benefit ratio in various construction applications.

In conclusion, the environmental and economic benefits of biochar-modified alkali-activated materials make them a promising alternative to traditional cement-based construction materials. These materials not only help reduce the environmental impact of construction by utilizing waste biomass and decreasing the carbon footprint but also offer potential cost savings through the reduction of material and energy costs. The development of biochar-modified AAMs aligns with global sustainability goals and could pave the way for more resilient, resource-efficient, and economically viable construction practices. However, challenges such as biochar quality variability and production scalability must be addressed to fully realize the potential of biochar in the construction industry.

9. Potential Research Directions and Future Trends

9.1. Optimization of Biochar Production for Consistent Quality

To fully harness the benefits of biochar in enhancing AAMs, optimizing biochar production is crucial. The properties of biochar, such as its pore structure, surface area, and chemical composition, are highly dependent on the production process, including feedstock type, pyrolysis temperature, and heating rate. Variations in these parameters can lead to inconsistent biochar quality, which in turn affects the performance of biochar-reinforced AAMs. Future research should focus on developing standardized biochar production protocols that can ensure consistent quality. This could involve exploring different feedstocks and optimizing pyrolysis conditions to produce biochar with the desired characteristics, such as high porosity and functional group content. Additionally, the development of scalable and cost-effective biochar production technologies is essential to make biochar a viable option for large-scale applications.

9.2. Surface Modification Techniques to Enhance Compatibility with AAMs

Enhancing the compatibility between biochar and AAMs is another important area for future research. The surface properties of biochar, such as its hydrophobicity and chemical reactivity, can significantly impact its interactions with the alkali-activated binder [107]. Surface modification techniques, such as chemical functionalization, coating, or plasma treatment, can be employed to tailor the surface properties of biochar to improve its compatibility with AAMs. For example, introducing hydrophilic functional groups or applying a thin layer of silica or alumina on the biochar surface can enhance its interaction with the alkali-activated matrix, leading to better dispersion and stronger interfacial bonding. Research in this area should aim to develop effective and scalable surface modification methods that can significantly improve the mechanical and durability properties of biochar-reinforced AAMs.

9.3. Long-Term Performance Studies Under Diverse Environmental Conditions

Investigating the long-term performance of biochar-reinforced AAMs across a range of environmental conditions is crucial for their practical implementation. Although initial short-term studies have demonstrated encouraging outcomes, the long-term stability of these materials in real-world settings is still not fully understood. Future research must prioritize extensive long-term performance assessments, encompassing exposure to fluctuating humidity levels, repeated freeze–thaw cycles, and various chemical attacks. These studies will aid in identifying potential degradation pathways and devising effective mitigation strategies.

10. Conclusions

This review comprehensively examines the modification mechanisms, environmental benefits, and challenges of biochar-reinforced AAMs. Research findings suggest that biochar, with its highly porous structure, large specific surface area, and abundant surface functional groups, can significantly enhance the pore structure and bonding strength of AAMs with its alkali-activated matrix. This leads to improved mechanical properties and durability of the materials. Additionally, biochar endows AAMs with special adsorption capabilities, such as the ability to adsorb heavy metal ions and organic pollutants, offering significant environmental benefits. However, research on biochar-modified AAMs is still in the exploratory phase. There are several challenges to be addressed, such as the unclear mechanisms of how biochar preparation conditions and performance parameters affect the modification outcomes, and the need for further investigation into the compatibility and long-term stability of biochar with AAMs. Future research should focus on optimizing biochar properties, standardizing its application in AAMs, and evaluating its long-term performance under real-world conditions to promote the widespread application of biochar-modified AAMs and contribute to the sustainable development of the construction industry.