Optimizing Mix Design for Alkali-Activated Concrete: A Comprehensive Review of Critical Selection Factors

Abstract

In the construction sector, cement and concrete are among the most widely utilized manufactured materials, yet their environmental impact remains a significant concern. The concrete industry is a major contributor to carbon dioxide emissions, accounting for over 8% of global greenhouse gas emissions annually. Several reports have estimated that between 1930 and 2013, a total of 4.5 gigatons of carbon was sequestered through the carbonation of cement-based materials. This process offset approximately 43% of the carbon dioxide (CO₂) emissions resulting from cement production during the same period, excluding emissions related to fossil fuel consumption in the manufacturing process. It is well established that producing one ton of cement results in approximately 0.60–0.98 tons of CO₂ emissions, coupled with substantial energy consumption. To mitigate these environmental effects, developing low-carbon or cement-free binders has become crucial. Alkali-activated binders (AABs), derived from industrial by-products or agricultural waste materials and activated with a low-molarity or one-part activator, are increasingly recommended as sustainable alternatives to reduce greenhouse gas emissions in the cement industry and minimize the consumption of natural resources. The production of alkali-activated concrete (AAC) involves several critical factors that significantly influence its mix design, fresh properties, and compressive strength (CS) performance. This study aims to provide a comprehensive review of the key factors affecting AAC’s mix design, workability, and CS characteristics. Firstly, the study discusses various methods employed for AAC mix design and the factors influencing these designs. Secondly, it examines the impact of binder type, source, chemical, mineralogical, and physical properties, as well as alkaline activator solutions, water content, and fillers on AAC’s workability, setting times, and strength development. Additionally, the study explores the correlation matrix and predictive performance models for fresh and strength properties. Lastly, the relationship between workability and CS is extensively analyzed. The review concludes by highlighting the existing challenges and prospects of AACs as sustainable construction materials to replace traditional cement and reduce carbon emissions.

1. Introduction

Cement and concrete are fundamental components in modern construction. Rapid global urban development projects have driven the rising demand for cement. Over the past 65 years, global cement consumption has surged nearly tenfold [1,2,3]. According to the U.S. Geological Survey’s Mineral Commodity Summaries (2021) [4], over 4.4 billion tons of ordinary Portland cement (OPC) are produced annually for use in the construction industry. Notably, China accounts for more than 57% of this total (approximately 2.5 billion tons), which is utilized in various infrastructure projects such as buildings, bridges, roads, and dams [5,6,7]. More than half of this OPC volume is designated for concrete production, while the remainder is used in mortar, plaster, and blocks [8,9]. Numerous studies in the literature [10,11,12,13,14] have highlighted that OPC remains the primary binding agent in concrete production despite its adverse environmental impact. The cement manufacturing process is a major contributor to carbon dioxide (CO₂) emissions, with approximately one ton of CO₂ released for every ton of OPC produced, reflecting a typical ratio of 1:0.60–0.98 [5,15,16]. The cement industry is estimated to account for roughly 8% of global anthropogenic greenhouse gas emissions. Consequently, OPC is considered environmentally detrimental due to its substantial contribution to CO₂ emissions and climate change. Moreover, the rapid expansion of global industrial and urban development has improved living standards but has also generated significant industrial and domestic waste. This growing concern has driven international efforts to improve waste management practices, particularly in recycling and resource recovery [17,18]. Consequently, researchers are actively investigating alternative eco-friendly construction materials, such as ‘green’ concrete and other products manufactured from recycled waste, to address environmental concerns and promote sustainability in the construction sector [19,20,21,22,23].

Alkali-activated concretes (AACs) are gaining increasing attention as a sustainable alternative to traditional cement concrete due to their potential to significantly reduce carbon dioxide emissions and reliance on natural resources [24,25,26]. By utilizing industrial by-products such as fly ash, slag, and other aluminosilicate-rich waste materials, AAC not only lowers the environmental footprint associated with cement production but also promotes circular economy practices. Its superior chemical resistance, high early strength, and long-term durability make AAC a promising material for eco-efficient construction, aligning with global efforts to achieve carbon neutrality in the built environment [27,28,29]. The use of AABs in producing AACs is increasingly encouraged due to their considerable environmental advantages, such as conserving natural resources, recycling industrial and agricultural waste, and mitigating landfill concerns [30,31]. Various by-products and waste materials with suitable chemical compositions, including calcium oxide (CaO), alumina-silicates (Al₂O₃–SiO₂), sodium oxide (Na₂O), and magnesium oxide (MgO), are extensively employed in AAB formulations. These materials include ground blast furnace slag (GBFS) [32], fly ash (FA) [33], waste tile ceramics (WTCPs) [34], palm oil fuel ash (POFA) [19], red mud (RMs) [35], metakaolin (MK) [36], waste glass (WGMs) [37], and rice husk ash (RHAs) [38]. Among these, GBFS is the most commonly utilized material in AAB production due to its high CaO content (≥40% of the total chemical composition) alongside acceptable levels of Al₂O₃–SiO₂ [39]. The presence of CaO plays a crucial role in the geopolymerization process, facilitating the formation of dense gels, promoting early strength development, and enabling curing at ambient temperatures [40,41,42].

In the production of AACs, FA is commonly utilized in combination with GBFS [43,44,45]. FA, an industrial by-product generated by thermal power plants during electricity production, is rich in amorphous alumina and silica. Its widespread availability worldwide makes it an appealing material for AAC synthesis [11,46,47]. Numerous studies have examined the characteristics of FA-based alkali-activated materials, revealing their promising potential as cementitious alternatives, particularly due to their impressive durability [48,49]. Research findings [19,50,51] have reported similar engineering properties of AACs that support their use in construction. However, challenges such as the requirement for high curing temperatures (40–85 °C), slow setting times, and low compressive strength hinder the widespread use of FA-based AACs. Therefore, incorporating high-aluminosilicate materials such as FA with materials high in calcium, such as GBFS, can significantly address several issues in the production of AACs and improve the both fresh and strength performance.

On the other hand, the properties of the alkaline activator solution (AAS)—such as molarity, modulus, type, and dosage—play a crucial role in determining AACs’ fresh and hardened performance. An optimal concentration of the AAS has been shown to improve the strength of AACs. However, exceeding this concentration leads to the deterioration of mechanical properties and structural integrity due to the presence of free OH⁻ ions in the alkali-activated matrix. Additionally, factors such as curing temperature and aging significantly affect the mechanical properties of AACs. However, their influence is only pronounced when the activator concentration is sufficient to drive the geopolymerization of aluminosilicates. Initially, a two-part AAS consisting of sodium or potassium hydroxides and sodium silicates with a high molarity (12–16 M) was widely used in AAC production [52]. It is well established that alkaline activator materials, such as sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃), are produced at a high cost and require significant energy consumption, leading to substantial carbon dioxide emissions. Moreover, these materials are classified as hazardous and pose risks to workers’ health due to their aggressive nature. Consequently, the extensive use of these substances significantly impacts the sustainability of concrete production and limits their application in the construction industry. Therefore, the use of a one-part or a low-molarity two-part AAS is highly recommended to promote more sustainable and environmentally friendly construction materials [41,52].

Numerous studies focus on optimizing the strength of alkali-activated materials and understanding the mechanisms of geopolymerization. Bernal et al. [53] analyzed the evolution of binder structure in sodium silicate-activated GBFS–MK blends to evaluate the influence of MK addition on the final strength of the binder. In studies [54,55], researchers examined the setting and hardening behavior of alkali-activated systems based on the Al₂O₃ to SiO₂ ratio, demonstrating that this ratio affects both the setting time and the ultimate strength of the material. Chindaprasirt et al. [56] investigated the impact of SiO₂ to Al₂O₃ and Na₂O to SiO₂ ratios on setting time, workability, and final strength, concluding that the optimal ratios for the binder used are 2.87 to 4.79 for SiO₂ to Al₂O₃ and 1.2 to 1.4 for SiO₂ to Na₂O. Bernal and Provis [57] explored the strength and durability of alkali-activated materials, highlighting recent advancements and future perspectives in the field.

In the investigation and selection of the optimal mix design for alkali-activated paste, mortar, and concrete, most studies have primarily focused on compressive strength (CS) [58,59]. For example, Kupaei et al. [60] conducted experiments on fly ash-based oil palm shell alkali-activated lightweight concrete, determining the optimal mix proportion solely based on CS; likewise, Hadi et al. [61] applied the Taguchi method to identify the optimal mix design, considering only factors influencing compressive strength. However, it is essential to account for setting time and workability in the mix design process for practical applications. To assess the workability of alkali-activated materials, the mini-slump test has been used for paste and mortar [62,63,64], while the slump test has been employed for AACs [42].

The optimization of mix design for AAC is essential to establish it as a viable alternative to traditional Portland cement concrete, particularly in balancing performance and environmental benefits. As AAC utilizes industrial by-products such as fly ash, slag, and waste glass, its optimized formulation can significantly reduce carbon emissions and reliance on natural resources [21,27]. However, achieving desired mechanical properties, durability, workability, and setting behavior requires a tailored mix design that considers the chemical composition, activator concentration, curing conditions, and material interactions. Therefore, mix design optimization plays a crucial role in maximizing the sustainability potential of AAC while ensuring its structural performance meets or exceeds conventional concrete standards.

This paper aims to comprehensively review AACs as environmentally friendly and sustainable construction materials, focusing on the factors influencing their mix design, fresh properties, and strength characteristics. Additionally, the study extensively evaluates the CS performance of the proposed concrete and systematically summarizes the factors affecting its performance. Various types of AACs are examined based on the type of AAS, preparation methods, binder sources, chemical composition, mix design, and curing regimes. Furthermore, the relationship between workability and CS performance of AACs is compared and analyzed. Figure 1 presents the flowchart of this review paper, outlining the key factors considered in assessing the performance of the proposed concrete.

2. Significance of Study

Despite extensive research, comprehensive and reliable information on the mix design process of AACs remains limited. The complexity of this process is influenced by various factors, including the ratios of aluminates, silicates, calcium, magnesium, and sodium hydroxides, as well as the concentration and modulus of the alkaline activator, the water-to-solids ratio, and the curing conditions. Consequently, trial-and-error methods are often required. Lloyd and Rangan [65] were among the first to propose a mix design approach for fly ash-based AACs, while Sreevidya et al. [66] introduced a design method that incorporates Indian standards for different grades of AACs. Several researchers have conducted optimization studies using methodologies such as the Taguchi method [61,67], particle packing fraction analysis [68,69], and response surface methodology [70,71] to assess the characteristics of AACs. These studies highlight the absence of a standardized design code for AACs, presenting an opportunity for further research into various optimization methodologies based on specific parameters.

Most studies have focused on AACs developed using a single precursor, revealing a gap in research on mix designs that incorporate multiple commonly used precursors. Further investigation is required to determine optimal mix proportions and understand various factors’ effects on AACs’ performance. This study aims to review and analyze existing research on AACs’ mix design, summarizing key findings to establish a comprehensive database for AACs’ mix design strategies. Specifically, it seeks to explore the use of different industrial and agricultural waste materials as precursors, activated with various alkaline solutions. Based on the insights gained from this review, it is possible to design high-performance AACs tailored to specific environmental conditions and construction applications.

3. Alkali-Activated Mix Design Methods

The global research community has undertaken extensive efforts to develop AAC mix designs using various industrial by-products rich in alumina silicates, such as MK, FA, RM, RHA, GBFS, and WTCP, among others [72,73,74,75,76,77]. Traditionally, most mix design procedures for producing AACs have relied on trial-and-error methods. Existing mix design methods for AACs rely heavily on extensive trial-and-error experimentation and are limited to specific precursor materials. Various approaches have been developed for designing cement-free paste, mortar, and concrete to optimize research efficiency, minimize costs, and reduce material consumption in these experimental processes. In study by Pattanayak et al. [75], a novel approach to the mix design of AAC_S using FA, GBFS, and silica fume (SF) as precursors, with varying molarities of alkali activators under ambient and oven curing conditions, was proposed. The mix design process for the proposed concrete is outlined in Figure 2, detailing the relevant standards and material quantities. In the initial step, the materials for producing AAC_S were selected, utilizing a ternary blend of FA, GBFS, and SF as precursors and a two-part alkaline activator solution composed of sodium hydroxide (NH) and sodium silicate (NS). In the subsequent phase (Step 2), various parameters were selected for the mix design, including aggregate gradation, concrete grade, precursor-to-solution ratio, and the NS-to-NH ratio. According to IS 10262:2019 [78], the characteristic strength for the mix proportion is then calculated. The material content required for preparing AAC specimens is determined in the third step. For instance, the total volume of aggregates (V_TA) is calculated using the absolute volume method, while the content of fine and coarse aggregates is determined using the following formula [79]:

$$V_{TA}=0.98-\left[\left({\displaystyle \frac{V_{x1}}{{SG}_{x1}}}+ {\displaystyle \frac{V_{x2}}{{SG}_{x2}}}+ {\displaystyle \frac{V_{NH}}{{SG}_{NH}}}+ {\displaystyle \frac{V_{NS}}{{SG}_{NS}}}\right)\times {10}^{-3}\right]$$

(1)

where V_x1→xn is the volume of binder’s materials (precursors), and SGx1→xn is the specific gravity of binder’s materials.

From Equation (1), VTA is calculated, and the mass of fine (RS) and coarse (CA) aggregates is determined as follows:

$$RS Mass=RS\% \times V_{TA} \times {SG}_{RS} \times {10}^3$$

(2)

$$CA Mass=CA\%\times V_{TA}\times {SG}_{CA}\times {10}^3$$

(3)

In the final step, the target compressive strength (TCS) of AACs was determined and assessed. If it achieves the required strength, then the process stops. Otherwise, return to the second step and repeat the parameter modification.

Based on the obtained results, the CS performance of the designed AACs is significantly influenced by the concentration of NH, the combination of precursors, and the curing conditions. Regarding the NH concentration, the trend in TCS values shows an increase as the solution molarity rises from 8 M to 10 M, 12 M, and 14 M. At 28 days of curing, specimens prepared with a 14 M NH solution achieved TCS values of 3.6 MPa, 83.9 MPa, and 11.2 MPa for FA, GBFS, and a blend of FA and SF, respectively. However, when the curing condition was changed from ambient to oven curing, the TCS of FA-based AACs increased to 12.8 MPa. Furthermore, incorporating SF into the FA matrix further increased TCS, exceeding 22.4 MPa.

In a study conducted by Ji et al. [80], the researchers introduced a machine learning (ML)-assisted mix design approach for AACs, aimed at substantially reducing experimental workload while accommodating a diverse range of precursor materials. Initially, a database was established to record the slump and CS of AACs (Figure 3), serving as the foundation for developing ML prediction models tailored to these parameters. The mix design process was then optimized using the particle swarm optimization (PSO) algorithm. The proposed mixture proportions were experimentally validated, and in cases where performance targets were not met, the results were utilized for model refinement through iterative adjustments until the desired outcomes were achieved. The findings highlight the strong predictive capabilities of the developed ML models, achieving an accuracy of 94% and a coefficient of determination of 0.95. The ML-guided approach successfully generated concrete mixtures attaining CS of 20, 40, and 60 MPa while simultaneously fulfilling slump requirements. Experimental validation of the target strength levels after iterative refinement yielded 25.2, 43.8, and 66.6 MPa results, respectively. These promising results underscore the effectiveness of the proposed optimized mix design method in achieving both strength and workability targets for AACs with minimal trial experiments.

In ref. [80], the dataset on the slump of AACs consists of 215 data groups, with 189 obtained from existing literature [81,82,83,84,85,86,87,88] and 26 were generated through the authors’ laboratory experiments. This dataset includes eight input variables: the weight of FA per cubic meter, the weight percentage of Na₂O content, the solution modulus (Ms = SiO₂/Na₂O molar ratio), the water-to-FA ratio, the content and ratio of fine and coarse aggregates to FA, the fineness modulus, and the maximum size of fine and coarse aggregates. The output variable is the AACs’ slump, which typically ranges from 0 to 270 mm.

In concrete research, workability is often evaluated based on whether the slump falls within a specific range rather than being defined by an exact value. In this study [80], the AACs’ slump is categorized into four levels: A, B, C, and D. The slump values for these classes are defined as follows: Class A (<60 mm), Class B (60–120 mm), Class C (120–180 mm), and Class D (>180 mm). This classification system provides a structured approach for characterizing slump levels using both empirical data and established guidelines [89], hereby improving the clarity and accuracy of the analysis. When the slump reaches Class D, the AACs meet the required workability criteria. This class represents the highest slump level, indicating the maximum workability range [89]. Based on the analysis of 215 AACs’ mixtures, the authors reported that 36%, 22.3%, 16.3%, and 25.4% of the mixtures fell into Classes A, B, C, and D, respectively.

The concrete strength dataset referenced in [80] comprises 795 data points sourced from existing literature. It includes 13 input features: FA content (ranging from 250 to 554), sodium oxide (Na₂O) content (0.05–0.30), modulus of silica (Ms) (0.25–2.08), water-to-FA ratio (0.21–0.54), fine aggregate-to-FA ratio (0.64–3.16), coarse aggregate-to-FA ratio (1.84–6.27), fineness (1.99–3.74), and maximum particle sizes of fine and coarse aggregates (7–25 mm). Additionally, it considers the basicity coefficient (0–0.30), hydration modulus (0.23–0.95), curing temperature (21.5–110 °C), curing duration (0–28 days), and the age of tested concrete specimens (1.17–480 days). The basicity coefficient [(CaO + MgO)/(SiO₂ + Al₂O₃)] and the hydration modulus [(CaO + MgO + Al₂O₃)/SiO₂] are derived from the chemical composition of FA as reported in the literature. The output feature is compressive strength (CS), which ranges from 1 to 107 MPa.

To standardize variations in specimen shapes, all strength values are normalized to those of a 100-mm cube by applying correction factors based on literature sources [90,91]: 1.1 for a 150-mm cube, 1.2 for a 100 mm × 200 mm cylinder, and 1.32 for a 150 mm × 300 mm cylinder. Analysis of the collected CS data reveals a distribution closely resembling a normal curve, with the majority of values concentrated between 20 and 60 MPa. Specifically, 76.7% of specimens exhibit CS within this range, while 8.5% fall below 20 MPa, and 14.8% exceed 60 MPa.

When utilizing the PSO algorithm for collaborative mix design based on two XGBoost models, the primary objective is to achieve TCS of 20, 40, and 60 MPa for the AACs while ensuring compliance with the slump criteria specified by class D standards. The mix design parameters must remain within the predefined minimum and maximum values. Additionally, certain parameters are fixed due to experimental constraints. The PSO algorithm aims to determine an optimal set of mix design parameters that minimizes a specified target value, defined as:

$$Target=\left|CS-TCS\right|+\left[\left|Slump-3\right| \times {10}^3 \right]+Penalty 1+Penalty 2$$

(4)

Note: Penalty 1 and Penalty 2 represent two distinct penalization terms.

The primary objective is to minimize the target value while ensuring that the design compressive strength closely aligns with the target, the slump value remains at 3, and both Penalty 1 and Penalty 2 are reduced to zero. In machine learning, classification labels are typically converted into numerical values. For slump classification, categories A, B, C, and D are represented by 0, 1, 2, and 3, respectively. If the slump deviates from 3, it is assigned a significantly large positive value.

Penalty 1 enforces volume constraints by assigning a value of 1000 when violated; otherwise, it is set to zero. This constraint is defined in Equation (5) due to the non-constant density of the alkaline activator solution, necessitating its formulation. Penalty 2 addresses alkali solubility constraints in experimental conditions, ensuring complete dissolution of alkalis for the preparation of the AAS. If solubility constraints are violated, the penalty is set to 1000; otherwise, it remains zero. The criterion for Penalty 2 is outlined in Equation (6), where the solubility of NH at 25 °C is 1000 g/L, meaning the mass of NH must be less than the mass of water to satisfy this constraint.

$${\displaystyle \frac{M_{FA}}{U_{FA}}}+{\displaystyle \frac{M_{AAS}}{U_{AAS-lower}}}+{\displaystyle \frac{M_{(RS+CA)}}{U_{(RS+CA)}}}>1>{\displaystyle \frac{M_{FA}}{U_{FA}}}+{\displaystyle \frac{M_{AAS}}{U_{AAS-upper}}}+{\displaystyle \frac{M_{(RS+CA)}}{U_{(RS+CA)}}}$$

(5)

M_i represents the quantity of the ith ingredient, while U_i denotes the density of the ith ingredient.

$$M_{NH }<M_{water}$$

(6)

In a study conducted by Hadi et al. [92] and illustrated in Figure 4, the effects of four key factors on the mix design of AACs were examined. These factors include binder content, the ratio of NS to NH, the ratio of AAS to binder, and the ratio of additional water to binder. The authors designed 28 mix compositions and extensively evaluated the impact of these four factors on workability, setting time, and CS. Unlike previous studies that analyzed these factors separately [93,94], this research investigated the combined influence of the solution-to-binder ratio and the NS-to-NH ratio. The GBFS content, used as a replacement for FA, was maintained at 40%, while the additional water-to-binder ratio was set at 0.15. The inclusion of additional water in the paste mix design was considered necessary because, in binary, ternary, and multi-binder systems with high GBFS content, extra water is required to extend the setting time and enhance workability [61,95,96,97]. The ratio of additional water to binder was found to affect the properties of alkali-activated materials significantly [96]. The authors concluded that these four factors substantially influenced the workability, setting time, and CS of the proposed AACs.

Gopalakrishna and Dinakar [74] developed a mix design methodology for FA-GBFS-based recycled aggregate AACs. Recognizing that variations in the physical properties of binder and aggregate materials can lead to inconsistent results, the authors introduced a new mix design approach. This method accounts for the specific gravity of raw materials to address the limitations of previous designs. Additionally, some processes either lack sufficient alkaline activator content to achieve the desired CS or contain an excess. While factors such as the properties of coarse and fine aggregates, the type of admixture, the proportions of fine and coarse aggregates, and the water-to-binder ratio also influence the final CS of the proposed concrete, when materials of similar quality are used, the water content plays a critical role in determining the ultimate CS of the product.

In this experiment, the authors maintained the GBFS content at 30% for all designed mixtures to balance strength development and setting time. When the GBFS content exceeds 30%, strength increases; however, setting time decreases, making the preparation of the proposed concrete significantly more challenging [74]. Similar studies using natural aggregates have reported the same findings [98], which were considered in the present investigation. As illustrated in Figure 5, the authors introduced a novel method for designing AACs using FA, GBFS, and recycled aggregate concrete. The GBFS content, used as a partial replacement for FA, was fixed at 30%, while the effects of varying recycled aggregate ratios (ranging from 0.3 to 0.8) on the workability and strength of the proposed concrete were evaluated. The primary objective of their study was to provide a suitable and straightforward mix design approach. Their findings indicate that the three most critical factors in AAC mix design technologies are the solution-to-binder ratio, solution absorption, and aggregate grading.

When formulating AAC mixtures, the proportion of water to binder solids is a critical factor, as it directly affects both the workability and strength of the mix [99]. To determine the specific quantities of the alkaline activator solution, binder, water content, and aggregates, researchers recommend using the following equations:

$$W_{NH}= W_{AAS} \times {(Z+1)}^{-1 }$$

(7)

$$W_{NS}=W_{NH}\times Z$$

(8)

$$W_{water}={[W}_{NS}\times (1-{NS}_{soild,\%})+W_{NH}\times (1-{NH}_{soild,\%}\left)\right]$$

(9)

The authors assumed that Z = NS-to-NH, and AAS = NS + NH.

To determine the total aggregate solids, it is recommended to use the following equations:

$$V_{total} = V_{binder}+ V_{agg}+ V_{AAS}+ V_{air}$$

(10)

V_Air = 2% of V_total (according to IS 10262-2009).

$$V_{agg}=0.98-\left[\left(\left({\displaystyle \frac{W_{binder}}{{SG}_{binder}}}\right)+\left({\displaystyle \frac{W_{NH}}{{SG}_{NH}}}\right)+\left({\displaystyle \frac{W_{NS}}{{SG}_{NS}}}\right)\right)\times {10}^{-3} \right]$$

For the calculation of fine and coarse aggregates, the sieve passing percentage and the specific gravity of each aggregate type were taken into account. The following equation can be used to determine the weight of each type:

$$W_{agg}=[\left(V_{agg} \times {SG}_{agg}\right)\times {10}^3$$

(11)

It has observed that the addition of water in the matrix of alkali-activated concrete has been shown to have a negative impact on the mechanical properties and porosity of the proposed concrete [82]. For this reason, many researchers recommend using the superplasticizer rather than adding water to enhance the workability of the proposed concrete [100]. In preparation for the fresh concrete, the following procedure of materials mixing, presented in Figure 5, was utilized and recommended. The authors’ target to achieve CS is arranged between 30 and 60 MPa after 28 days of curing at ambient temperatures. A strong relationship was found between strength development and ratios of alkaline solution to binder. Additionally, the interfacial transition zone (ITZ) of the proposed concrete was found to be weaker with a ratio of solution to binder of up to 0.70, compared to 0.30 and 0.40 ratios.

Hamdi et al. [92] developed twenty-eight paste mixtures to examine the influence of four key factors on the workability and strength performance of the pastes: GBFS content, the ratio of NS to NH, the proportion of AAS to binder, and the water-to-binder ratio. The authors established three criteria to determine the optimal mix design: (i) adequate workability; (ii) initial and final setting times that meet the required standards [89]; and (iii) the highest CS among the mixtures that satisfy the first two criteria (as shown in Figure 6). Based on test results for CS, setting time, and workability, the optimal mix design was identified as having a GBFS content of 40%, an AAS-to-binder ratio of 0.5, an NS-to-NH ratio of 2.0, and a water-to-binder ratio of 0.15. as Additionally, the authors [92] proposed a procedure for developing a predictive model for CS at 28 days, which can be divided into four steps. The initial step takes into account only a single factor. Subsequently, each successive step incorporates an additional factor. The final model integrates all four specified factors. The multivariable regression models were developed using the CFTOOL toolbox in MATLAB R2016b [101]. Initially, the GBFS content was examined. Based on the GBFS content and the 28-day CS results, the polynomial regression equation was formulated as follows:

$$f_1= F_1\left(C_G\right)=-0.01 \times {\left(C_G\right)}^2+1.3 \times C_G+12.4$$

(12)

where CG represents the value of GBFS content, F₁ is the polynomial regression equation that accounts for its influence, and f1 is the predicted 28-day CS based on its effect.

Next, the influence of AAS-to-binder ratios was examined. A polynomial regression equation was developed based on the mix proportions and the 28-day compressive strength of mixes, which were prepared with fixed contents of GBFS, NS-to-NH, and additional water-to-binder ratio while varying the AAS/binder ratio. The equation is expressed as follows:

$$f_2= F_2{\left(AAS:B\right)\times F}_1\left(C_G\right)=\left[-6.4 \times {\left(AAS:B\right)}^2+5.2 \times \left(AAS:B\right)\right]\times F_1\left(C_G\right)$$

(13)

where F₂ represents the polynomial regression equation accounting for the influence of AAS-to-binder, and f₂ denotes the predicted 28-day CS, considering the effects of GBFS content and AAS-to-binder. Additionally, when AAS-to-binder is equal to 0.5, the value of F₂ becomes 1, resulting in Equation (13) being equivalent to Equation (12). This indicates that Equation (13) can also be utilized to predict the test results estimated by Equation (12). Furthermore, the impact of NS-to-NH ratios was taken into account. A polynomial regression equation was developed based on the mix proportions and the observed 28-day compressive strength results. The equation is expressed as follows:

$$f_3= F_3\left(NS:NH\right) \times f_2 \times f_1$$

(14)

$$F_3=\left[-0.3\times {\left(NS:NH\right)}^3]+[1.4\times {\left(NS:NH\right)}^2\right]-\left[1.8\times \left(NS:NH\right)\right]+1.4375$$

(15)

where F₃ represents the polynomial regression equation accounting for the influence of the NS-to-NH ratio, and f₃ denotes the predicted 28-day CS considering the effects of GBFS content, AAS-to-binder ratio, and NS-to-NH ratio. Furthermore, when the NS-to-NH ratio is 2.5, the value of F₃ equals 1, making Equation (14) equivalent to Equation (13). This indicates that Equation (14) can also be utilized to predict the test results estimated by Equations (12) and (13). Lastly, the impact of the addition of the water-to-binder ratio was taken into account. Based on the mix proportions and the 28-day CS results, the following polynomial regression equation was developed:

$$f_4= F_4\left(W:B\right) {\times f_2\times f}_2\times f_1$$

(16)

$$F_4=\left[-22.22\times {\left(W:B\right)}^2]+[2.0\times \left(W:B\right) \right]+1.2$$

(17)

where F₄ represents the polynomial regression equation incorporating the effect of the AAS-to-binder ratio, and f₄ denotes the predicted 28-day CS, accounting for the influences of GBFS content, AAS-to-binder ratio, NS-to-NH ratio, and additional water-to-binder ratio. Furthermore, when the additional water ratio is set to 0.15, the value of F₄ equals 1, and Equation (16) becomes equivalent to Equation (14). This indicates that Equation (16) can also be utilized to predict the test results obtained from Equations (12)–(14). Consequently, Equation (16) serves as the final predictive model for the 28-day CS of the proposed paste. Equation (16) was applied to calculate all mix formulations to assess the model’s performance. The analysis revealed that the correlation coefficient between the predicted values from Equation (16) and the experimental results was 0.983, demonstrating that this mathematical model can predict the 28-day CS with a very high degree of accuracy due to the appropriate selection of equations.

Srinivasula et al. [98] suggested the following chart for free cement mix design (Figure 5). For this purpose, the effects of several factors, including binder content, AAS-to-binder content ratio, aggregates, and NH molarity, were considered. The authors found that the compressive strength is significantly affected by the specific gravity of raw materials. The study further demonstrates that medium to high compressive strengths, ranging from 32 to 66 MPa, can be achieved even under ambient temperature curing conditions.

Arpitha and Parthasarathy [102] proposed an improved methodology for mix design and performance evaluation of cement-free materials by integrating a hybrid Taguchi-based Grey Relational Analysis (GRA) with Principal Component Analysis (PCA). This approach optimizes both the fresh and hardened properties of alkali-activated materials simultaneously. The study involved a series of nine experiments conducted using the L9 orthogonal array, as recommended by the Taguchi method, with key variables including the molarity of NH, GBFS content, AAS-to-binder ratio, and NS-to-NH ratio, each examined at three distinct levels. The mix design was analyzed using Analysis of Variance (ANOVA), with a focus on strength, flowability, and setting time. Additionally, the optimized combination—14 M NH, 75% GBFS, an AAS-to-binder ratio of 0.50, and an NS-to-NH ratio of 2—was validated through confirmatory experiments. The findings demonstrated the effectiveness of the Taguchi-GRA-PCA method in optimizing the design of alkali-activated materials.

In a study conducted by Reddy et al. [98], the researchers developed an alkali-activated mix design using a binary blend of FA and GBFS. The mix design was formulated by integrating the American Concrete Institute (ACI) strength versus water-to-cement ratio curve for conventional concrete, the absolute volume method, and the combined grading concept. The proposed methodology is user-friendly and provides flexibility in selecting either the desired compressive strength or the specific alkaline activator content-to-binder solids ratio, and vice versa. The AACs achieved significantly high strength for a given AAS-to-binder ratio, which corresponds to the water-to-cement ratio in conventional concrete, ranging from 66 MPa at an AAS-to-binder ratio of 0.4 to 32 MPa at a ratio of 0.8. Additionally, an attempt was made to propose a modified strength versus AAS-to-binder ratio curve based on the experimental results.

4. Preparation of Alkali-Activated Specimens

The authors in [103] prepared alkali-activated specimens following the procedure outlined in Figure 7. The mixtures were designed using AAC-fine powder (RCFP), with varying percentages of GBFS replacing the RCFP to assess the impact of Al/Si, Ca/Si, and Na/Al ratios on the specimens’ performance. By incorporating GBFS at different levels (0–50%), these ratios were modified from 0.31, 0.60, and 0.93 to 0.37, 0.89, and 0.89, respectively. In all mixtures, the authors maintained a fixed NH molarity, NH content, NS content, and NS-to-NH ratio at 14 M, 0.016, 0.024, and 1.5, respectively. The AAS was prepared by dissolving flake NH in a pre-measured volume of water at a specified ratio, stirring thoroughly to achieve a 14 mol/L NH solution. The NS was then added to the solution, mixed thoroughly, and sealed with plastic wrap before being cooled to room temperature for later use. The RCFP and GBFS were combined and mixed in a planetary cement mixer for three minutes. The AAS was then gradually introduced into the mixer and stirred for an additional five minutes to produce the alkali-activated paste. The paste’s fluidity and setting time were first evaluated before being poured into molds. The samples were subsequently cured for 24 h at 20 °C and 95% relative humidity. After demolding, they were stored under the same curing conditions until reaching the required age for testing.

As mentioned previously, the addition of water to the matrix of alkali-activated concrete has been found to adversely affect its mechanical properties and increase its porosity [82]. Consequently, numerous researchers recommend the use of superplasticizers instead of additional water to improve workability [100]. For preparing fresh concrete, the mixing procedure outlined in Figure 8 was employed and is recommended. The target compressive strength was set between 30 and 60 MPa after 28 days of curing at ambient temperature. A strong correlation was observed between strength development and the ratio of alkaline solution to binder. Furthermore, the interfacial transition zone (ITZ) was found to be weaker when the solution-to-binder ratio reached 0.70, compared to ratios of 0.30 and 0.40.

In a study conducted by Sheng et al. [104], the authors proposed a two-stage process for preparing fresh concrete mixtures, as illustrated in Figure 9. In the first stage, FA, GBFS, and sand were dry-mixed at a stirring speed of 50 rpm to achieve a homogeneous blend. Subsequently, water was added, and the mixture was stirred at the same speed for an additional three minutes. In the second stage, the precursor slurry was transferred to an innovative extrusion device using a concrete pump, while sodium silicate powder was conveyed via a screw conveyor. Both materials were then thoroughly mixed within the extrusion device.

This experiment evaluated the effects of three factors on 3D-printed alkali-activated mortars: the ratio of Na₂O to the binary binder (FA + GBFS), stirring time, and stirring speed. Four Na₂O-to-binder ratios—3%, 4%, 5%, and 6%—were examined. Additionally, four stirring durations were tested and set at 45, 60, 75, and 90 s, while stirring speeds of 60, 120, 180, and 240 rpm were considered for mortar preparation. Other parameters were kept constant, including the GBFS-to-FA ratio (0.70:0.30), the sand-to-binary binder ratio (1.20), the water-to-binary binder ratio (0.36), and the NS molarity (SiO₂/Na₂O = 0.95). The findings indicated that the dynamic yield stress, flowability, plastic viscosity, and both initial and final setting times of the prepared mortar were influenced by the selected factors. Moreover, an increasing content of NS powder, along with higher mixing speeds and prolonged mixing times, led to a decreasing trend in performance.

In a study conducted by Abed et al. [105], a mechanochemically alkali-activated grout was utilized to activate GBFS, FA, NH, and NS through dry grinding in a ball mill for two hours. Following this process, water was the sole additive required to initiate the geopolymerization reaction. For comparison, a conventionally alkali-activated grout was also examined. A total of twenty-four GBFS and FA mixtures were prepared with varying GBFS-to-FA ratios (0/100, 50/50, 75/25, and 100/0) at three different NH molarities (1.25, 2.5, and 3.75) to evaluate the rheological characteristics, setting time, bleeding, and unconfined CS of both grouts.

The experimental findings indicated that the mechanochemical activation method reduced the rheological characteristics and fresh properties, including setting time and bleeding, of the cement-free grout compared to the conventional activation process. Additionally, the CS of mechanochemically activated grout was higher than that of conventionally activated grout. Furthermore, both the GBFS content and NH concentration had a significant impact on the rheological, fresh, and mechanical properties of all cement-free grouts, irrespective of the activation method. An increase in molar concentration and GBFS content led to substantial improvements in rheological characteristics and mechanical properties, while simultaneously resulting in a significant reduction in bleeding capacity and setting time.

5. Geopolymerization Mechanism of AACs

AABs have emerged as promising alternatives to conventional Portland cement, offering both environmental and performance advantages. These binders are primarily formed through the reaction of aluminosilicate-rich materials with alkaline activator solutions, resulting in the formation of a hardened matrix with desirable mechanical and durability properties. Common precursor materials include industrial by-products such as FA, GBFS, WTCPs, and calcined clays like MK [36]. Each of these materials contributes differently to the binder system: FA, rich in reactive silica and alumina, promotes long-term strength development; GBFS, with its high calcium content, enhances early strength and contributes to a denser matrix; and MK provides a highly reactive aluminosilicate phase that supports rapid geopolymerization.

The activation process typically employs alkaline solutions such as sodium hydroxide and sodium silicate, which play crucial roles in dissolving the solid precursors and facilitating the polymerization of silicate and aluminate species [106]. Sodium hydroxide primarily assists in breaking down the aluminosilicate structures of the raw materials, while sodium silicate supplies soluble silica that contributes to the development of the geopolymeric gel network (Figure 10). The synergy between the precursors and activators significantly influences the reaction kinetics, microstructure evolution, and final performance of the binder [107]. Understanding the interplay among these components is essential for tailoring alkali-activated systems to meet the mechanical, chemical, and durability requirements of various construction applications.

6. Workability Performance

The workability of concrete is a crucial factor as it determines the ease with which the material can be handled and placed, its ability to fill molds effectively, and the overall quality and strength of the final structure. Adequate workability ensures uniform compaction without segregation, resulting in a durable and aesthetically pleasing construction. In the field of alkali-activated technology, numerous studies [92,97,105] have reported that the workability of alkali-activated materials—measured through flow/slump, viscosity, and initial and final setting times—is significantly affected by various factors. These include the binder source and its chemical and physical properties, the molarity of NH, NS-to-NH, and modulus of AAS, the types and content of fillers, aggregate size, water content, the addition of admixtures, and the ratios of AAS to binder and binder to filler. This section provides a comprehensive review of the impact of these factors on the workability performance of alkali-activated materials.

6.1. Effect of Binder Source, Chemical, and Physical Properties

The presence of aluminosilicate, calcium, and magnesium plays a crucial role in the composition of alkali-activated materials. While the polymerization process of these materials remains not fully understood, research indicates that their workability is significantly affected by the chemical composition of the source materials [92,93]. FA has been widely recognized as a suitable precursor for alkali-activated materials due to its abundant availability and appropriate silica and alumina content [93]. However, a major challenge is that alkali-activated materials based on FA’ class F require high-temperature curing, typically between 40 °C and 80 °C. To enable curing at ambient temperature, GBFS is commonly incorporated [108,109]. The addition of GBFS substantially increases the CaO content in alkali-activated materials, which has been found to significantly influence their properties. As the CaO content rises, the viscosity of alkali-activated materials increases, while the setting time decreases [92,93].

GBFS, a byproduct of industrial processes, has recently been utilized to enhance the properties of FA and other high-ASs waste-based alkali-activated materials [93]. In alkali-activated GBFS, the primary reaction product is calcium silicate hydrate (C–S–H), whereas in FA-based systems, it is an amorphous hydrated alkali ASs [110]. While alkali-activated GBFS demonstrates high strength, challenges such as rapid setting, limited workability, and significant drying shrinkage have been reported [111]. Incorporating GBFS into FA-based alkali-activated material improves workability, extends setting time, and enhances strength while also reducing the solution demand.

In a study by [105], the authors utilized low-calcium FA to produce both mechanochemically and conventionally alkali-activated grouts. The disposal of coal FA powder in landfills reduces the availability of valuable fertile land; therefore, its appropriate disposal and utilization are crucial. FA-based alkali-activated grout is characterized by high workability and low shrinkage [39]. However, the broader application of FA in alkali-activated synthesis is hindered by its low reactivity and limited strength gain when cured at room temperature [112,113].

To address the low reactivity of FA in the authors’ experiment [105], GBFS was incorporated into the FA-based alkali-activated grout due to its lower solution and water demand, as well as its high early mechanical strength [114]. It is well established that alkali-activated binders composed solely of GBFS exhibit certain challenges, including poor workability, high viscosity, rapid setting, and significant shrinkage [36,115,116,117,118,119]. Previous research has shown that combining GBFS and FA in alkali-activated grout synthesis results in higher reactivity, improved CS, and reduced shrinkage compared to grouts made entirely from either GBFS or FA [72,120].

In their study, the authors employed a two-part AAS using NS and NH in a 0.50 NS-to-NH ratio to activate the binder. The NH solution was prepared at three different molarities: 1.25 M, 2.5 M, and 3.75 M. The results indicated that the activation method, GBFS content, and NH molarity significantly influenced yield stress, plastic viscosity, setting time, and bleeding capacity. The findings further revealed that mechanochemically alkali-activated grout exhibited lower yield stress and plastic viscosity compared to conventionally alkali-activated grout. Additionally, the mechanochemically alkali-activated grout had 24.5% and 20% shorter initial and final setting times, respectively, compared to the conventionally alkali-activated grout. Moreover, the setting time decreased as GBFS content and NH molarity increased.

Sheng et al. [104] evaluated the viscosity, dynamic yield stress, and static yield stress (pre-activation) of the precursor slurry to assess the pumpability of AACs. The flocculation of the precursor slurry results from the mutual attractive forces between precursor particles and the higher specific surface energy of fine particles [121]. Figure 11 illustrates the variation in static yield stress over time, which determines the pressure required for pumping initiation. Notably, the static yield stress of the precursor slurry exhibited minimal change compared to that of the AACs precursor. Additionally, both dynamic yield stress and plastic viscosity increased over time. The dynamic yield stress plays a crucial role in the extrudability of 3D-printed alkali-activated concrete [122]. Continuous flocculation and hydration reactions contribute to the formation of reaction by-products, which strengthen particle bonding and progressively increase the dynamic yield stress over time [123]. The dynamic yield stress can be enhanced by increasing the Na₂O-to-binder ratio, extending the mixing time, and raising the mixing speed. Furthermore, the authors reported that higher mixing time and speed improve the mixing efficiency of AACs. Overall, the development of yield strength and elastic modulus is primarily influenced by the Na₂O-to-binder ratio, followed by mixing time.

Hadi et al. [92] investigated the impact of replacing FA with GBFS on the workability of alkali-activated mixtures. The study involved twelve alkali-activated paste mixtures designed by the authors, considering the AAS-to-binder ratio, NS-to-NH ratio, and additional water-to-binder ratio for all mixes. The test results for setting time and mini-slump base areas are presented in Figure 12. As shown in Figure 12a, the initial and final setting times of alkali-activated pastes were significantly affected by the GBFS content, exhibiting a decreasing trend as the GBFS proportion increased. In mixtures without GBFS, the initial and final setting times exceeded 20 and 25 h, respectively. However, with the incorporation of 10%, 20%, 30%, and 40% GBFS, these times were substantially reduced by 73%, 83%, 90%, and 95%, respectively. The trend observed for the final setting time was similar to that of the initial setting time. Additionally, the gap between initial and final setting times narrowed as GBFS content increased. These findings confirm that increasing GBFS content significantly accelerates the setting of alkali-activated pastes [34,93]. Furthermore, alkali-activated mixtures containing 20–40% GBFS complied with AS 3972 requirements for both initial (≥45 min) and final (≤360 min) setting times, similar to those of cement.

Figure 12b presents the effect of GBFS content on mini-slump test results for alkali-activated pastes, along with data from three OPC mixtures. The results indicate that the mini-slump base area of alkali-activated pastes decreased significantly as GBFS content increased. The relationship between mini-slump base area and time appeared nearly linear. Additionally, the difference in base area between 15 min and 60 min became more pronounced with higher GBFS content. In other words, as the GBFS content increased, the mini-slump base area declined more rapidly. This phenomenon is attributed to the acceleration of polymerization processes with higher GBFS content [42]. Hadi et al. [92] further observed that within 60 min, all alkali-activated paste mixtures exhibited better workability than OPC pastes. However, in all OPC paste mixtures, the base area remained stable between 15 min and 60 min.

Huo et al. [103] examined the relationship between the initial constituent molar ratios and the physical–mechanical properties of alkali-activated materials composed of recycled concrete fine powder (RCFP) and GBFS. In their study, the authors utilized RCFP and GBFS as binders to synthesize alkali-activated materials and investigated their effects on performance. Building upon previous findings, they explored the quantitative relationships between initial molar ratios (Si/Al, Na/Al, and Si/Ca) and the macro-properties of the alkali-activated materials, while also analyzing the underlying mechanisms of influence. The results indicate that incorporating GBFS significantly reduced setting time and workability. Moreover, GBFS enhanced the degree of geopolymerization and facilitated the formation of mixed gelling products, including C-(A)-S-H, N-A-S-H, and (C, N)-A-S-H, leading to a more refined internal matrix structure. The study [103] further revealed that the relationships between initial constituent molar ratios and macro-properties were complex, involving multiple models. The initial and final setting times exhibited nonlinear correlations with initial molar ratios, whereas strong linear correlations were observed between initial molar ratios and flowability [103]. Additionally, a nonlinear relationship was identified between setting time and fluidity. These findings contribute to a deeper understanding of the interactions between composition, properties, and structure in alkali-activated materials, offering valuable insights for optimizing their composition design.

In previous studies [124,125,126,127,128,129,130], the variation values for the initial and final setting times in this and related studies were evaluated, as influenced by the initial Si/Al, Na/Al, and Ca/Si molar ratios. A significant nonlinear correlation was identified between the setting times and these initial constituent molar ratios. This relationship is effectively represented by nonlinear equations, with the results exhibiting a strong degree of fit. The influence of initial molar ratios on setting time can be categorized into two stages: (1) Within a specific range of Si/Al, Na/Al, and Ca/Si ratios, an increase in the initial molar ratios leads to a prolongation of both the initial and final setting times, thereby extending the overall condensation process and causing a “delayed setting” effect. (2) When the initial molar ratio surpasses a certain threshold, the setting times decrease as the initial composition molar ratios continue to rise. The inflection point for these molar ratios varies depending on the material composition within the reaction system.

In one case, when the Si/Al ratio is low (<2), the system predominantly involves reactions between silicate and aluminate monomers, resulting in a faster setting and hardening time. As the initial Si/Al ratio increases, the concentration of dissolved silicate monomers in the reaction system continues to rise. These monomers first undergo condensation reactions to form oligomeric silicates, which subsequently polymerize with aluminates to generate three-dimensional aluminosilicate polymers. Since the condensation rate between silicate-silicate species is lower than that between silicate-aluminate species, the setting time of the system is extended as the initial Si/Al ratio increases [55,131,132]. Conversely, in another case, an increase in the initial Si/Al ratio leads to a reduction in setting time. The literature attributes this to the higher concentration of dissolved silicate species, which enhances the condensation reactions between silicate and aluminate species, thereby accelerating the setting process [133]. However, this explanation appears to contradict the mechanism described in the first scenario, necessitating further investigation in future research. Moreover, the setting behavior of alkali-activated materials is influenced by multiple factors, including the chemical composition of precursors, the type and concentration of the alkali activator, the alkali-to-solid ratio, curing temperature, and the liquid-to-solid ratio.

The concentration of Na₂O plays a crucial role in determining the feasibility, reaction rate, and stability of products in geopolymerization. Consequently, the initial Na/Al and Na/Si molar ratios are key factors in designing geopolymer compositions. In [103], the initial Na/Al ratio was selected. An increase in the Na/Al ratio, corresponding to a higher Na₂O content, significantly accelerates geopolymerization, thereby reducing the setting time [134,135]. In systems where calcium is present, the reaction mechanism varies depending on the alkali concentration. At low alkali concentrations (i.e., a low Na/Al ratio), calcium ion dissolution is the dominant process. The dissolved Ca²⁺ ions can directly participate in the reaction, leading to a relatively rapid setting time. However, as the alkaline content increases, the high concentrations of Na⁺ and OH⁻ ions enhance the dissolution of the silica-alumina precursor. As a result, the reaction is primarily governed by geopolymerization, which progresses at a slower rate than calcium hydration, ultimately leading to an extended coagulation time [136].

The incorporation of an appropriate amount of active calcium significantly influences AAC performance. An increase in the initial Ca/Si ratio generally reduces the setting time due to several contributing factors. First, calcium compounds exhibit greater solubility than silicate and aluminate components, leading to the formation of C-(A)-S-H gels. These gels provide additional nucleation sites and accelerate the gelation process of the alkali-activated paste [137]. Second, as a charge-balancing ion, Ca²⁺ demonstrates superior electrostatic interactions and charge-neutralization capacity compared to Na⁺, which enhances the formation of silicate and aluminate polymerization gels, further reducing setting time. However, in certain cases, an increase in the initial Ca/Si ratio unexpectedly leads to prolonged setting times. Research suggests that this anomaly is primarily due to differences in the physical properties of the materials rather than direct modifications in the Ca/Si ratio [128].

The flow diameter exhibited a significant decline when the incorporation of GBFS exceeded 30%, indicating a reduction in flowability. This variation in fluidity is primarily attributed to the distinct physical and chemical characteristics of RCFP and GBFS [138]. Compared to GBFS, RCFP particles are relatively larger and predominantly composed of crystalline minerals. When mixed with an alkali activator, the geopolymerization process occurs gradually at ambient temperature, resulting in a highly fluid slurry. In contrast, GBFS possesses high reactivity and a large specific surface area, facilitating rapid alkali activation reactions. As documented in previous studies, this reduces the flowability of the alkali-activated paste [139].

According to the findings in [103], the initial Si/Al, Na/Al, and Ca/Si molar ratios significantly influence workability performance. The results indicate a strong linear correlation between flowability and these molar ratios. The effect of initial molar ratios on flowability can be classified into two distinct trends. The first trend shows a continuous increase in flowability as the Si/Al, Na/Al, and Ca/Si ratios rise. Conversely, the second trend demonstrates a decrease in flowability with increasing Si/Al, Na/Al, and Ca/Si ratios. Furthermore, previous studies [127,130,133,138,140,141] widely evaluated the variation in flowability and final setting time of the alkali-activated system. The correlation between these parameters can be categorized into two types. In the first type, there is a positive correlation, where an increase in setting time corresponds to an enhancement in fluidity. The second type exhibits an inverse relationship, where an extended setting time leads to a decrease in flowability (as shown in

Table 1. Effect type and chemical composition of binder source on workability performance of AACs.

Refs	Source	Elements, Weight %			Workability Performance
		CaO	SiO2	Al2O3
[108,142,143,144,145,146,147]	FA class F	1.46–7.94	49.55–60.5	21.5–29.8	Longer initial and final setting times with higher workability.
[143,145,148,149]	FA class C	14.1–74.9	12.6–51.80	8.47–20.5	Significantly reduce the setting times and workability.
[40,42,108,146,147,150]	GBFS	35.6–51.8	26.87–40.60	10.90–13.3	Very fast setting times, high viscosity, and lower flowability.
[36,148,151,152,153,154,155,156]	MK	0.02– 0.61	49.6–59.5	35.1–44.1	Enhance the setting time and workability.
[34,157,158,159,160]	WTCPs	0.02–6.01	60.6–72.60	10.3–26.90	Higher setting time and excellent workability performance.
[19,108,111,161,162]	POFA	10.2–11.8	45.9–64.20	0.64–4.25	Delay the setting time and improve the workability.
[156,163,164]	RHAs	0.50–0.99	86.2–96.5	0.10–0.60	Accelerated setting, increasing viscosity, and reducing workability.
[154,165,166]	RMs	0.96–9.14	21.6–51.93	17.39–30.1	Accelerated or delayed setting, depending on other factors, such as the activator.
[37,41,167,168]	WGMs	1.75–11.2	69.14–70.65	1.49–13.06	A high dosage of waste glass reduces workability.
[145,149,152,155,156]	SF	0.1–0.14	78.02–97.30	0.06–10.6	The high surface area and reactivity of SF particles lead to an accelerated geopolymerization process, increased the viscosity, and reduced the setting time and workability.

).

6.2. Binder-Solution Effect

In this section, the influence of the AAS-to-binder ratio on the properties of alkali-activated materials is reviewed. Most published studies indicate that the workability of alkali-activated materials is significantly affected by the AAS-to-binder ratio. According to reference [92], the effect of the AAS-to-binder ratio on the setting times and flowability of alkali-activated materials has been investigated. The findings reveal that both the initial and final setting times are strongly influenced by the AAS-to-binder ratio, with a nearly linear relationship observed. As the AAS-to-binder ratio increases, both the initial and final setting times are extended considerably, with the final setting time increasing by approximately 75% compared to the initial setting time. This phenomenon is attributed to the excess alkaline solution, which raises the water content in the mix and subsequently hinders geopolymerization [169]. Furthermore, the results of mini-slump tests demonstrate a strong dependence on the AAS-to-binder ratio. It is well established that a higher dosage of AAS in the alkali-activated matrix increases the water content, thereby enhancing flowability. However, for all mixtures, mini-slump values exhibit a decreasing trend over time. This reduction in flowability is attributed to the formation of additional gels, which increase the viscosity of the mixture, ultimately leading to a loss of fluidity.

In reference [92], the author observed that the initial and final setting times were significantly affected by the AAS-to-binder ratio and the NS-to-NH ratio. The relationship between the AAS-to-binder ratio and the setting times was found to be nearly linear. As the AAS-to-binder ratio increased, both the initial and final setting times increased substantially, with the final setting time rising by approximately 75% compared to the initial setting time. Conversely, an increase in the NS-to-NH ratio resulted in a decrease in both the initial and final setting times. This effect can be attributed to the acceleration of polymerization processes due to the higher concentration of soluble silica.

Regarding the results of mini-slump tests, the workability of alkali-activated pastes was notably influenced by the AAS-to-binder and NS-to-NH ratios. It was observed that as the NS-to-NH ratio increased, the mini-slump base area decreased to some extent. This phenomenon occurs because the viscosity of NS solution is considerably higher than that of water and NH solution [93], and the viscosity increases with a higher NS-to-NH ratio. Conversely, an increase in the AAS-to-binder ratio led to a significant increase in the mini-slump base area, as the higher liquid content improved the workability of the alkali-activated pastes.

6.3. Solution Molarity Effect

In [105], the authors observed that an increase in molar concentration significantly enhanced the initial apparent viscosity, yield stress, and plastic viscosity of alkali-activated grouts, irrespective of the activation method. The setting time was notably influenced by the activation method, slag GBFS content, and molarity. The results indicated that the alkali-activated grout exhibited 24.5% and 20% shorter initial and final setting times, respectively, compared to conventionally alkali-activated grout. Additionally, the setting time decreased as NH molarity and GBFS content increased. The bleeding capacity of mechanochemically alkali-activated grout was 34% lower than that of conventionally alkali-activated grout, suggesting that the mechanochemical activation mechanism reduced particle size and increased surface area. Furthermore, regardless of the activation method, the bleeding capacity of all mixtures decreased with increasing slag content and molar concentration.

Pattanayak et al. [75] investigated the impact of NH molarity on different types of binders. Figure 13 presents the results of the workability test for the AAC mix, demonstrating a declining trend in workability as the NH molarity increases, a pattern also observed by other researchers [90]. Specifically, the slump value for the GBFS mix at 8 M was recorded at 22.4 mm, decreasing to 21.6 mm, 21 mm, and 19 mm at 10 M, 12 M, and 14 M, respectively, indicating a reduction in workability with increasing NH concentration. However, for the same NH molarity, the FA mixture exhibited higher slump values compared to the GBFS mixture. A similar trend was observed for the FA-SF-based AAC mix, where the slump values progressively declined with increasing NH molarity from 8 M to 14 M.

In contrast, an increase in GBFS content results in reduced workability due to the angular shape of GBFS particles and the enhanced reactivity associated with its high calcium content [93]. Conversely, FA particles, characterized by their spherical shape and lower calcium content, improve workability. However, the amorphous structure and large surface area of FA increase the demand for liquid solution compared to conventional concrete, as supported by Lim and Pham [170], underscoring the influence of precursor material properties on slump values. When SF partially replaces FA in the mix, the slump values further decrease due to the increased surface area and finer particle size of silica fume [171]. Additionally, as the NH molarity rises from 8 M to 12 M, the increased concentration enhances cohesion, leading to a reduction in the workability of AAC [172].

In a study conducted by Kumar and Reddy [173], the authors examined the impact of varying NH molarity (8, 10, 12, 14, and 16 M) on the workability performance of self-compacting AACs. The findings revealed a significant reduction in workability as NH molarity increased. Specifically, for slump flow, increasing the NH molarity from 8 M to 16 M resulted in a decrease in value from 720 mm to 690 mm, respectively. A similar trend was observed in the L-box test, where the value declined from 0.95 to 0.84. This reduction in workability is attributed to the higher viscosity of the AAS (NH and NS) compared to water. As the NH molarity increased, so did the viscosity of the NH solution. Furthermore, the reduction in available water due to higher NH molarity further diminished the workability of the concrete. Higher concentrations of NH solution accelerate the polymerization process, leading to a faster setting and hardening rate, thereby reducing workability [174]. Additionally, this effect may be due to the cohesive and sticky nature of fly ash-based self-compacting AACs, as the fine FA particles interact with the highly viscous AAS. Similar findings have been reported by previous researchers [175,176].

6.4. NS-to-NH Effect

The study in [92] examined the effects of the NS-to-NH mass ratio at different AAS-to-binder ratios on the properties of alkali-activated pastes. The findings indicated that both the initial and final setting times were significantly influenced by the NS-to-NH ratio across various AAS-to-binder ratios. Specifically, an increase in the NS-to-NH ratio resulted in a decrease in setting times due to the acceleration of polymerization processes caused by the higher concentration of soluble silica [106]. Furthermore, all mixes complied with the final setting time requirements specified in AS 3972 [177] for comparison.

The results also showed that the mini-slump test outcomes were strongly affected by both the AAS-to-binder and NS-to-NH ratios. Over a 30-min period, pastes prepared with NS-to-NH ratios of 2.0 and 2.5 became too stiff to be cast into a mini-slump cone. As the time extended to 45 and 60 min, the mini-slump values of the mixes significantly decreased. Additionally, an increase in the NS-to-NH ratio led to a reduction in the mini-slump base area, which can be attributed to the higher viscosity of NS solution compared to water and NH solution [106]. The viscosity increased further with a higher NS-to-NH ratio. Moreover, the results demonstrated that an increase in the AAS-to-binder ratio led to a significant expansion of the mini-slump base area. This is because a higher liquid content improves the workability of the proposed pastes [106].

6.5. Addition of Water, Superplasticizer, and Filler Content Effect

Although certain studies have demonstrated that the addition of extra water negatively impacts the CS of AACs [61,92,96], it is sometimes necessary to enhance workability and extend setting times [96], particularly in AACs containing GBFS. According to the findings in [92], both the initial and final setting times of alkali-activated pastes increase as the additional water-to-binder ratio rises. A similar trend was observed in workability, where the influence of additional water was assessed using mini-slump tests. The results indicate that a higher amount of additional water leads to a larger mini-slump base area.

According to the study in [74], the authors stated that achieving a total aggregate grading proportion that aligns with the Deutsches Institut für Normung (DIN) grading curve is essential, and both fine and coarse aggregate grading play a crucial role in producing well-packed and dense concrete. The efficient packing of various aggregate fractions, including fine and coarse aggregates passing through 4.75 mm, 12 mm, and 20 mm sieves, significantly impacts the workability of conventional concrete, particularly in recycled aggregate AACs. Preliminary research indicates that AAC mixtures with a high ratio of recycled aggregate to fine aggregate tend to experience segregation, whereas mixtures with an excessively low recycled aggregate-to-fine aggregate ratio are prone to bleeding. Consequently, a mixed grading approach was adopted—consisting of 40% sand, 43% aggregates within the 6–12 mm range, and 17% aggregates within the 12–20 mm range—to mitigate segregation and bleeding while enhancing workability. Ultimately, the proportioning of individual aggregates was determined so that the overall grading conformed to the DIN B grading curve.

Revise the AAC solution in recycled aggregate-based concrete (RA-AAC). Based on preliminary studies, concrete workability for a non-air-entrained mix typically falls within a slump range of 75–100 mm for moderate-grade concrete. Achieving this level of workability generally requires a water content of 200 kg/m³ when the maximum nominal aggregate size is 20 mm [100]. Given that AAC is the most costly component in the production of RA-AAC, maintaining the solution content at 200 kg/m³ is deemed appropriate. This amount not only supports improved workability but also contributes to reducing overall production costs. Superplasticizer (SP) was added where necessary. Consequently, the total AAC content in the RA-AAC mixture was kept within the recommended water content range, as illustrated in Figure 14 [178].

6.6. Correlation Matrix and Predictive Performance

In a study conducted by Haodong Ji et al. [80], the researchers constructed a correlation matrix to analyze the relationships between features in the slump dataset. The results, presented in Figure 15, indicate that certain features exhibit correlations. For example, Coarse/FA and Fine/FA show a negative correlation with FA. However, some feature pairs, despite their statistical correlation, are not empirically related. Notably, the strongest correlation among these pairs is observed between the fineness modulus of fine aggregate (Fs) and Water/FA. Yet, Water/FA represents a concrete mix design parameter, whereas Fs. refers to the fineness modulus of sand. These features are independent and do not share a direct relationship.

7. Compressive Strength Performance

The CS is widely recognized as the primary criterion for selecting concrete in construction applications. In the case of alkali-activated paste, mortar, and concrete, numerous factors significantly influence strength performance at both early and later stages. These factors include the type, chemical composition, and physical and mineral properties of the binder; the type, content, and modulus of the alkaline activator solution; the water and superplasticizer content; and the type, physical properties, and content of fine and coarse fillers. This section provides a comprehensive review and discussion of the factors affecting the compressive strength of the proposed alkali-activated concretes.

7.1. Effect of Binder Source and Chemical and Physical Properties

The characteristics of the binder, including its chemical, physical, and mineral properties, as well as the ratios of Ca/Al, Ca/Si, and Si/Al, play a crucial role in determining the designed CS of the proposed alkali-activated materials [179,180]. The relationships between CS and initial constituent molar ratios in similar studies are reported in Table 2. As shown in the table, the influence of initial molar ratios on CS varies across different reaction systems. Huo et al. [103] investigated the effect of binder chemical composition—specifically, Si/Al, Na/Al, and Ca/Si ratios—on the strength performance of AACs. Figure 16 presents the CS of RCFP-GBFS binary alkali-activated materials as a function of these initial molar ratios. The results indicate that within a specific range, an increase in the initial Si/Al or Na/Al ratio corresponds to a decrease in CS, whereas a higher Ca/Si ratio leads to an increase in CS. Moreover, CS exhibits a linear relationship with the initial Si/Al, Ca/Si, and Na/Al ratios, with correlation coefficients exceeding 90%.

The influence of initial molar ratios on CS can be categorized into three patterns: (i) In some reaction systems, CS increases almost linearly with higher initial molar ratios [181]; (ii) in other systems, CS and the initial molar ratios demonstrate a negative linear correlation; (iii) the most common trend is an initial increase in CS followed by a decline as the molar ratios continue to rise. Each reaction system has an optimal initial molar ratio, although the exact values vary. The first two patterns are generally limited to minor variations in initial molar ratios, whereas the third trend—characterized by an increasing then decreasing relationship—is widely accepted and observed in numerous studies. Integrating the findings from the first two patterns, CS and initial molar ratios exhibit a linear correlation in both the increasing and decreasing phases.

Table 2. The optimal molar ratio of initial materials for different alkali-activated systems.

Refs.	Reaction System		Molar Ratio			Workability and Strength Performance
	Binder	AAS	Si/Al	Na/Al	Ca/Si
[139]	FA + GBFS	NS-NH	1.67	0.40	0.78	The authors reported that the CS was positively influenced by increasing the molar ratios of Si/Al, Na/Al, and Ca/Si, and with increasing these molar ratios to 1.67, 0.4, and 0.78, specimens achieved CS higher than 45 MPa.
[182]	MK + SF	NS-NH	1.90	0.73	-	The CS of prepared specimens was found to be significantly influenced by molar ratios of Si/Al and Na/Al. A positive effect was observed when the molar ratio of Si/Al increased from 1.62 to 1.9, and CS values increased from 8 MPa to 22 MPa; then dropped to 20 MPa with the molar ratio rising to 1.95. For the Na/Al molar ratio, the increase in the ratio from 0.43 to 0.73 led to an increase in the CS from 8 MPa to 32 MPa. However, a high loss (˃70%) in CS was observed when the molar ratio increased to 0.93, and the specimens showed lower performance.
[183]	RM + coal MK	NS-NH	-	1	-	From the reported results, CS values are significantly influenced by the Na/Al molar ratio. Increasing the molar ratio from 0.75 to 1.0 results in an excellent improvement in strength performance. However, increasing the molar ratio up to 1.0 (1.3) causes a drop in CS value, and the specimens lost more than 25% compared to the optimum molar ratio.
[140]	OPC kiln dust + RHA + SF	NS-NH	-	-	0.52	The flowability and both initial and final setting times are negatively influenced by increasing the molar ratio of Ca/Si, and the workability trend decreases with the increasing molar ratio. For strength performance, with the increase in the molar ratio of Ca/Si from 0.15 to 0.52, the CS was enhanced by more than 20%. However, increasing the molar concentration to 0.8 slightly leads to a drop in the CS value. A significant drop in CS was observed when the molar ratio reached 1.8, and specimens lost more than 40% of CS.
[55]	MK	NS-NH	1.91	-	-	The results show that increasing or reducing the Si/Al molar ratio to 2.5 or 1.25, respectively, results in a loss of CS of more than 6%.
[184]	MK + Meta-halloysite	NS-NH	1.45	0.92	-	The authors reported that increasing the molar ratios of Si/Al and Na/Al, from 1 and 0.60 to 1.45 and 0.92, respectively, positively enhanced the CS value from 30 MPa to more than 60 MPa.
[185]	FA + GBFS + Steel slag	NS-NH	1.85	0.40	0.58	The CS of prepared specimens was significantly influenced by the molar ratios of Si/Al, Na/Al, and Ca/Si. By reducing the molar ratio of Si/Al from 1.85 to 1.5, the CS dropped from 40 MPa to 28 MPa. Similarly, the reduction in Na/Al and Ca/Si molar values from 0.40 to 0.30 and 0.55 to 0.42 resulted in a significant loss in CS (˃22%).
[186]	Volcanic ash + MK	NS-NH	1.90	0.61	-	With the increase in the molar ratios of Si/Al and Na/Al to 2.30 and 0.75, respectively, more than 24% of CS was lost.
[187]	Fused volcanic ash + MK	NS-NH	1.85	0.84	-	Increasing the molar ratio of Na/Al from 0.84 to 1.15 results in a loss of CS of more than 20%.
[188]	Lithium slag + FA + SF	NS-NH	1.31	0.29	-	Significant loss of CS was found when the Na/Al molar ratio increased from 0.29 to 0.48.
[189]	FA + Steel slag	NS-NH	1.62	0.31	0.42	Increasing the molar ratio of Na/Al from 0.31 to more than 0.40 leads to a loss of more than 15% of CS.
[190]	Mine tailings slag + Calcium carbide residue	Soda residue + NH	2.78	1.11	1.02	The results obtained from this study show that the Si/Al molar ratio slightly influenced the CS of specimens. However, the most significant and largest drop in strength was observed when the molar ratios of Na/Al and Ca/Si increased to 1.26 and 1.7, respectively.
[191]	FA + Recycled fine powder	NS-NH	3.53	0.71	-	The results of CS slightly dropped when the Si/Al molar ratio increased up to 3.53. However, the specimens lost more than 20% of CS when the molar ratio of Na/Al increased from 0.71 to 0.80.
[192]	MK	NS-NH	1.90	-	-	Increasing the Si/Al molar content from 1.0 to 1.9 significantly enhanced the CS from 20 MPa to 79 MPa. However, an increase in silica content beyond that resulted in a drop in strength to 64 MPa.
[193]	Recycled concrete fine powder + Slag + Nano-SiO2	NS-NH	2.77	-	0.86	It is observed that the CS is slightly influenced and drops with increasing silica content.
[194]	MK	NS-NH	1.50	-	-	A significant drop in CS (from 80 MPa to 42 MPa) occurred with increasing silica content up to the optimum level.
[195]	FA + OPC	NS-NH	2.97	0.87	0.83	CS was found to be enhanced with increasing the molar ratio of Ca/Si using GBFS as a source of calcium materials in the geopolymerization process.
[196]	FA + GBFS	NS-NH	-	-	0.17	The CS was slightly influenced by increasing the molar ratio of Ca/Si from 0.17 to 0.20.
[197]	MK + RHA + Chicken eggshell	NS-NH	-	-	0.40	Increasing the Ca/Si molar ratio to 0.40 resulted in a slight loss of CS, and a significant drop in strength was observed when the molar ratio reached 1.0.
[198]	FA + Calcium Aluminate Cement	NS-NH	-	-	0.43	CS significantly dropped with increasing the molar ratio of Ca/Si up to 0.43.
[199]	Iron ore tailings + lime + GBFS	NS	-	-	0.35	Increasing the Ca/Si ratio up to 0.35 resulted in a decrease in the strength values.
[103]	RCFP + GBFS	NS-NH	2.68	0.89	0.89	Flowability and initial and final setting times tend to decrease with an increasing Ca/Si molar ratio and with a reduction in the Na/Si and Si/Al molar ratios. CS tends to increase with the increasing molar ratio of Ca/Si.

Table 2. The optimal molar ratio of initial materials for different alkali-activated systems.

Refs.	Reaction System		Molar Ratio			Workability and Strength Performance
	Binder	AAS	Si/Al	Na/Al	Ca/Si
[139]	FA + GBFS	NS-NH	1.67	0.40	0.78	The authors reported that the CS was positively influenced by increasing the molar ratios of Si/Al, Na/Al, and Ca/Si, and with increasing these molar ratios to 1.67, 0.4, and 0.78, specimens achieved CS higher than 45 MPa.
[182]	MK + SF	NS-NH	1.90	0.73	-	The CS of prepared specimens was found to be significantly influenced by molar ratios of Si/Al and Na/Al. A positive effect was observed when the molar ratio of Si/Al increased from 1.62 to 1.9, and CS values increased from 8 MPa to 22 MPa; then dropped to 20 MPa with the molar ratio rising to 1.95. For the Na/Al molar ratio, the increase in the ratio from 0.43 to 0.73 led to an increase in the CS from 8 MPa to 32 MPa. However, a high loss (˃70%) in CS was observed when the molar ratio increased to 0.93, and the specimens showed lower performance.
[183]	RM + coal MK	NS-NH	-	1	-	From the reported results, CS values are significantly influenced by the Na/Al molar ratio. Increasing the molar ratio from 0.75 to 1.0 results in an excellent improvement in strength performance. However, increasing the molar ratio up to 1.0 (1.3) causes a drop in CS value, and the specimens lost more than 25% compared to the optimum molar ratio.
[140]	OPC kiln dust + RHA + SF	NS-NH	-	-	0.52	The flowability and both initial and final setting times are negatively influenced by increasing the molar ratio of Ca/Si, and the workability trend decreases with the increasing molar ratio. For strength performance, with the increase in the molar ratio of Ca/Si from 0.15 to 0.52, the CS was enhanced by more than 20%. However, increasing the molar concentration to 0.8 slightly leads to a drop in the CS value. A significant drop in CS was observed when the molar ratio reached 1.8, and specimens lost more than 40% of CS.
[55]	MK	NS-NH	1.91	-	-	The results show that increasing or reducing the Si/Al molar ratio to 2.5 or 1.25, respectively, results in a loss of CS of more than 6%.
[184]	MK + Meta-halloysite	NS-NH	1.45	0.92	-	The authors reported that increasing the molar ratios of Si/Al and Na/Al, from 1 and 0.60 to 1.45 and 0.92, respectively, positively enhanced the CS value from 30 MPa to more than 60 MPa.
[185]	FA + GBFS + Steel slag	NS-NH	1.85	0.40	0.58	The CS of prepared specimens was significantly influenced by the molar ratios of Si/Al, Na/Al, and Ca/Si. By reducing the molar ratio of Si/Al from 1.85 to 1.5, the CS dropped from 40 MPa to 28 MPa. Similarly, the reduction in Na/Al and Ca/Si molar values from 0.40 to 0.30 and 0.55 to 0.42 resulted in a significant loss in CS (˃22%).
[186]	Volcanic ash + MK	NS-NH	1.90	0.61	-	With the increase in the molar ratios of Si/Al and Na/Al to 2.30 and 0.75, respectively, more than 24% of CS was lost.
[187]	Fused volcanic ash + MK	NS-NH	1.85	0.84	-	Increasing the molar ratio of Na/Al from 0.84 to 1.15 results in a loss of CS of more than 20%.
[188]	Lithium slag + FA + SF	NS-NH	1.31	0.29	-	Significant loss of CS was found when the Na/Al molar ratio increased from 0.29 to 0.48.
[189]	FA + Steel slag	NS-NH	1.62	0.31	0.42	Increasing the molar ratio of Na/Al from 0.31 to more than 0.40 leads to a loss of more than 15% of CS.
[190]	Mine tailings slag + Calcium carbide residue	Soda residue + NH	2.78	1.11	1.02	The results obtained from this study show that the Si/Al molar ratio slightly influenced the CS of specimens. However, the most significant and largest drop in strength was observed when the molar ratios of Na/Al and Ca/Si increased to 1.26 and 1.7, respectively.
[191]	FA + Recycled fine powder	NS-NH	3.53	0.71	-	The results of CS slightly dropped when the Si/Al molar ratio increased up to 3.53. However, the specimens lost more than 20% of CS when the molar ratio of Na/Al increased from 0.71 to 0.80.
[192]	MK	NS-NH	1.90	-	-	Increasing the Si/Al molar content from 1.0 to 1.9 significantly enhanced the CS from 20 MPa to 79 MPa. However, an increase in silica content beyond that resulted in a drop in strength to 64 MPa.
[193]	Recycled concrete fine powder + Slag + Nano-SiO2	NS-NH	2.77	-	0.86	It is observed that the CS is slightly influenced and drops with increasing silica content.
[194]	MK	NS-NH	1.50	-	-	A significant drop in CS (from 80 MPa to 42 MPa) occurred with increasing silica content up to the optimum level.
[195]	FA + OPC	NS-NH	2.97	0.87	0.83	CS was found to be enhanced with increasing the molar ratio of Ca/Si using GBFS as a source of calcium materials in the geopolymerization process.
[196]	FA + GBFS	NS-NH	-	-	0.17	The CS was slightly influenced by increasing the molar ratio of Ca/Si from 0.17 to 0.20.
[197]	MK + RHA + Chicken eggshell	NS-NH	-	-	0.40	Increasing the Ca/Si molar ratio to 0.40 resulted in a slight loss of CS, and a significant drop in strength was observed when the molar ratio reached 1.0.
[198]	FA + Calcium Aluminate Cement	NS-NH	-	-	0.43	CS significantly dropped with increasing the molar ratio of Ca/Si up to 0.43.
[199]	Iron ore tailings + lime + GBFS	NS	-	-	0.35	Increasing the Ca/Si ratio up to 0.35 resulted in a decrease in the strength values.
[103]	RCFP + GBFS	NS-NH	2.68	0.89	0.89	Flowability and initial and final setting times tend to decrease with an increasing Ca/Si molar ratio and with a reduction in the Na/Si and Si/Al molar ratios. CS tends to increase with the increasing molar ratio of Ca/Si.

The mechanical properties of AACs are closely associated with the structural characteristics of the final products, which are significantly influenced by the initial Si/Al ratio. At lower Si/Al ratios, polycondensation primarily occurs between Si-O and Al-O bonds, forming long-chain, single silicon-aluminum-type gel products (PS, -Si-O-Al-) [200]. As the Si/Al ratio increases, the concentration of soluble silica rises, initially facilitating the polymerization of silicate species. These silicate polymers subsequently condense with Al-O to form a three-dimensional rigid network structure, comprising poly sialate-siloxo (PSS), poly sialate-disiloxo (PSDS), or poly sialate-multisiloxo (PSMS), which enhances CS [201]. A higher soluble silica content also strengthens the interparticle connections, providing nucleation sites for gel formation [202,203]. Consequently, increasing the initial Si/Al ratio can improve CS. However, exceeding the optimal Si/Al ratio may adversely affect CS, as an excessive Si/Al ratio can hinder precursor dissolution and release in an alkaline environment, leading to a substantial accumulation of unreacted material within the system [95,203,204]. Moreover, an increased alkaline activator may result in a higher initial Si/Al ratio. Excessive addition of NS can raise the system’s viscosity, impeding ion transport and polymerization reactions [203,205].

The initial Na/Al ratio variation primarily results from changes in precursor and alkali content. An increase in the Na/Al ratio enhances system alkalinity, facilitating the effective reaction of precursors and improving CS [182]. Additionally, a higher concentration of Na⁺ ions promotes electrostatic attraction and charge neutralization, further accelerating geopolymerization [206]. However, several adverse effects may occur when the initial Na/Al ratio exceeds the optimal value. First, an excessively alkaline environment can lead to the dissociation and depolymerization of reaction products. Second, an excess of Na⁺ ions may adsorb onto particle surfaces and interact with Si-OH and Al-OH groups, weakening the bonding strength between gels and unreacted particles, ultimately reducing CS [207]. Moreover, surplus Na+ can react with atmospheric CO₂, leading to carbonation and alkali leaching, which negatively impact CS improvement.

The initial calcium-to-silicon (Ca/Si) ratio plays a pivotal role in determining the structure and composition of the final product, which subsequently affects the CS of the AACs. Numerous studies have demonstrated that incorporating an appropriate amount of calcium into the geopolymerization system can significantly enhance the material’s performance. The primary functions of calcium within this system include the following: (i) calcium acts as a cation during geopolymerization, balancing charges and forming calcium-alumino-silicate-hydrate (C-(A)-S-H) gels, which coexist with alkali-activated gels to improve matrix compactness [208]; (ii) as the initial Ca/Si ratio increases, additional calcium can serve as nucleation sites for precipitate formation, promoting collisions and coagulation between alkali ions and solid particles, thereby increasing the degree of geopolymerization [72,209,210]. However, when the Ca/Si ratio exceeds the optimal level, CS decreases, likely due to excessive calcium ions reacting with hydroxide (OH⁻) to form calcium hydroxide (CH) precipitates. These precipitates increase the viscosity of the reaction system, hindering mass transfer. Additionally, the deposition of CH on the surface of ASs particles impedes their dissolution and disrupts geopolymerization, ultimately reducing CS. Based on existing research, as summarized in Table 2, the optimal initial Si/Al ratio for achieving maximum CS generally ranges from 1.3 to 3.5, the optimal Na/Al ratio falls between 0.3 and 1.1, and the ideal Ca/Si ratio lies within 0.4 to 1.0. Variations in these optimal values across different reaction systems can be attributed to differences in particle size, specific surface area, and the active component content of the precursors.

In ref. [102], the authors reported that the GBFS emerged as the most influential factor in deciding the CS of AACs, whereas the AAS-to-binder ratio primarily governed flowability. GBFS content held a predominant percentage contribution of 95%, primarily dictating CS, with other properties playing a marginal role. Additionally, the AAS-to-binder ratio emerged as the secondary influential factor for setting time, contributing 10.34% and tempering the dominance of GBFS to some extent.

Utilizing both the Taguchi-GRA-PCA method and the Taguchi-GRA-equal weight-based method, the concurrent optimization of all response variables led to an identical optimized combination as follows: a 14 M NH solution, 75% GBFS, an AAS-to-binder ratio of 0.5, and an NS-to-NH ratio of 2. However, GBFS was the most prominent and influential factor in the PCA-based method, while the equal weight-based method had the AAS-to-binder ratio as the vital component impacting it.

The study in [92] examined the impact of GBFS and various factors, such as the solution-to-binder ratio and additional water, on the strength performance of AACs. Mix designs were developed with progressively increasing GBFS content to evaluate its influence on the properties of AACs. All mixtures were prepared with consistent AAS-to-binder, NS-to-NH, and additional water-to-binder ratios. The results of CS testing indicated that mixes without slag exhibited slow strength development under ambient conditions, with a 28-day compressive strength of only 12.4 MPa. However, the incorporation of GBFS significantly enhanced CS. At 28 days, alkali-activated mixes containing 10%, 20%, 30%, and 40% GBFS, as a proportion of the total binder, demonstrated CS increases of 98%, 169%, 253%, and 293%, respectively, compared to the 0% GBFS mix. This trend aligns with findings from other studies [61,93]. The observed increase in strength is attributed to the denser microstructure of AACs with higher GBFS content, resulting from the enhanced formation of C-S-H gel [113].

7.2. Effect of Solution on Binder Ratio

In this section, the influence of the AAS-to-binder ratio on the properties of AACs, as well as its impact on determining the optimal NS-to-NH ratio, was examined. For mixtures with identical NS-to-NH ratios but varying AAS-to-binder ratios, it was observed that an increase in the AAS-to-binder ratio led to a significant decline in compressive strength. The optimal NS-to-NH ratio was found to be 2.0 for all AAS-to-binder ratios. The addition of excess NS hinders water evaporation and structure formation, thereby affecting the material properties [94]. Studies on the influence of oxide ratios indicate that CS improves with an increasing SiO₂/Al₂O₃ ratio and decreasing Na₂O/Al₂O₃ and H₂O/Na₂O ratios [168,169]. Further investigations into the effects of silica modulus (Ms = SiO₂/Na₂O), water-to-binder ratio, and Na₂O content have identified compositions achieving CS of up to 40 MPa at lower proportions [211,212]. These variables have also been analyzed to develop predictive models for assessing CS [213].

For a given binder material, the ratios of SiO₂/Al₂O₃, Na₂O/Al₂O₃, and H₂O/Na₂O tend to increase with higher activator proportions. A minimum amount of activator material is essential to initiate the polymerization reaction. This study focuses on developing AACs with lower 4 M and 6 M NH molarities. The findings suggest low-molarity AACs can significantly enhance sustainability by reducing alkaline activator consumption. However, this approach challenges the conventional understanding that higher molarity improves strength. If successful, lowering the molarity of NH could provide additional benefits, such as reduced costs and enhanced handling efficiency.

7.3. Effect of Activator Toxicity and Cost

The impact of the toxicity and cost of alkaline activator solutions on AAC mix design and its practical applications is both significant and complex. The most effective alkaline activators are alkali hydroxides and alkali silicates. However, commonly used options such as sodium NH and NS are expensive, environmentally harmful, highly caustic, and pose serious health and safety risks. These substances can endanger worker safety, raise environmental concerns, and complicate regulatory compliance. Their high alkalinity (pH > 12) is known to cause chemical burns, eye injuries, and respiratory problems, necessitating strict safety protocols, including specialized handling training and emergency response procedures. Additionally, their high toxicity may restrict usage in regions with stringent environmental and occupational safety regulations, resulting in increased expenses for permits, environmental assessments, and waste management.

Sodium/potassium hydroxide and silicate solutions are significantly more expensive than traditional cementitious materials. Their high toxicity and cost present challenges for on-site mixing, hindering widespread use in conventional construction. These materials are more practical in precast applications, where controlled conditions can be maintained. Such limitations affect the overall economic viability of AACs, necessitating regulated environments for both mixing and storage. Additionally, the bulk and weight of these solutions contribute to elevated transportation costs, making AACs less appropriate for low-cost housing or use in developing regions, where health and financial constraints are critical concerns. However, AACs show greater potential in high-performance infrastructure projects—such as bridges or wastewater systems—where the enhanced durability can justify the higher investment.

Commercially produced NH and NS are among the primary contributors to greenhouse gas emissions and the overall cost associated with geopolymer materials [214,215]. Moreover, these alkaline activators are highly toxic and corrosive, posing serious health risks. Several studies have proposed alternative approaches to mitigate the adverse effects of alkaline solutions. These include the use of low-molarity activators [42], one-part activators [216,217], low-modulus activators [218], and activators derived from industrial or agricultural waste [214]. These alternatives have demonstrated mechanical performance comparable to, or even exceeding, that of geopolymers made with conventional activators, thereby supporting their potential use in structural applications. Waste-derived activators, such as biomass ash and waste glass, have been shown to significantly reduce the environmental impact of alkali-activated materials. However, comparing the CO₂ emissions of geopolymer and OPC concrete remains a complex issue due to variations in mix design and material proportions required to achieve similar performance. Davidovits [219] initially estimated that geopolymers could reduce CO₂ emissions by up to 80% compared to OPC. Subsequent research by Fawer [220], Habert et al. [221], and Turner and Collins [222], however, reported that alkali-activated materials offer only a marginal reduction in CO₂ emissions—approximately 9–10%—when compared to equivalent OPC binders. These later studies also accounted for emissions resulting from the production of industrial waste materials used as precursors. Notably, the largest share of emissions in AAC stems from the environmental impact of sodium silicate production.

Mellado et al. [223] report that the production of 1 kg of NaOH results in 1.12 kg of CO₂ emissions, while the production of 1 kg of alkali silicate solution generates 1.2 kg of CO₂. Furthermore, environmental concerns such as ozone depletion and human toxicity are associated with the manufacturing of AACs. Despite these issues, AACs are generally regarded as having a lower environmental impact than OPC, although this view often overlooks the ongoing debate surrounding the CO₂ emissions of geopolymers. The literature highlights that a major obstacle to the wider adoption of AAC is its production cost, which is frequently comparable to or higher than that of OPC. Therefore, to enhance the feasibility of AACs, it is essential to reduce production costs while preserving their high performance and environmental benefits.

In recent years, extensive research has been carried out on the utilization of silica-rich materials as both precursors and activators in geopolymers, including fly ash, blast furnace slag, silica fume, rice husk ash, and waste glass [224,225,226]. FA and GBFS are considered less suitable for silica extraction due to their relatively low amorphous silica content, typically ranging between 40% and 50%. In contrast, materials such as diatomaceous earth [227], silica fume [228], waste glass [229], and nano silica [230] contain significantly higher levels of silica, which enhance the cross-linking of aluminosilicates to form a three-dimensional network structure. The dissolution behavior of silica is influenced by its structural form, with amorphous silica dissolving more readily than crystalline silica [231]. Additionally, the alkalinity of the activating solution plays a crucial role in the dissolution process. However, beyond a certain threshold, increasing alkalinity can negatively affect strength development [232]. Alternative silica sources have shown the capability to produce sodium silicate solutions with comparable performance to conventional formulations. Nevertheless, the use of waste-derived activators has been associated with delayed setting times [233]. Depending on the extraction process employed, these alternative activators can also contribute to a reduction in CO₂ emissions of up to 50% when compared to commercially available sodium silicate [224].

In summary, the toxicity and high cost of alkaline activator solutions present significant barriers to the broader adoption of geopolymer concrete, having a substantial impact on mix design and binder selection. Although geopolymers demonstrate excellent mechanical strength and durability, addressing issues related to activators remains crucial. Continued research into environmentally friendly, cost-effective, and less hazardous activators is essential to enhance the scalability and sustainability of geopolymer technology.

7.4. Effect of Solution Type and Molarity

A combination of NS solution and NH solution is commonly employed as the alkaline activator in alkali-activated materials to activate the selected binders. Research has demonstrated that the mass ratio of NS to NH solution significantly influences the properties of the activator in alkali-activated materials. However, reported optimal NS-to-NH ratios vary among studies, likely due to differences in the composition of the AAS and binder used, as well as variations in the AAS-to-binder mass ratio. Nath and Sarker [93] determined that an NS-to-NH ratio of 1.5 yielded the highest CS in AAC tests, with an AAS-to-binder ratio of 0.45. In contrast, Morsy et al. [94] conducted a series of AAC tests with an AAS-to-binder ratio of 0.4 and reported an optimal NS-to-NH ratio of 2.0. Similarly, Bakri [234] performed tests on FA-based AAC, where specimens were cured at an elevated temperature. The findings indicated that the optimal NS-to-NH ratio varied depending on changes in the AAS-to-binder ratio. Therefore, the ideal NS-to-NH ratio for achieving maximum CS is not fixed but rather depends on the specific AAS, binder, and their corresponding AAS-to-binder mass ratios, necessitating further investigation for different material compositions.

In a study conducted by Reddy et al. [98], the researchers developed an alkali-activated mix design utilizing a binary blend of FA and GBFS. The mix design was formulated by incorporating the ACI strength versus water-to-cement ratio curve for conventional concrete, the absolute volume method, and the combined grading concept. The proposed methodology is user-friendly and provides flexibility in selecting the desired CS or a specific ratio of AAS-to-binder solids, and vice versa. The results indicate that the CS of AACs, for a given AAS-to-binder ratio that parallels the water-to-cement ratio in conventional concrete, is significantly high, ranging from 66 MPa to 32 MPa as the AAS-to-binder ratio varies from 0.4 to 0.8. Additionally, the study proposes a modified strength versus AAS-to-binder ratio curve based on the experimental findings.

The influence of solution molarity on the CS of self-compacting AAC was analyzed by Kumar and Reddy [173]. The results of the effect of NH solution molarity on the CS of FA-based self-compacting AAC after 7 and 28 days are presented in Figure 17. The findings indicate that the molarity of the NH solution significantly affects the strength development of FA-based AACs. Specifically, the concrete exhibits greater hardness as the NH molarity increases from 8 M to 16 M. At 28 days, the mixture with 16 M demonstrated the highest CS, reaching 34.9 MPa. Compared to the 8 M mix, the CS increased by 10.95%, 24.08%, 30.65%, and 35.58% for mixes with 10 M, 12 M, 14 M, and 16 M, respectively. This improvement is likely attributed to the enhanced dissolution rate of aluminosilicate minerals at higher NH concentrations, which promotes the geopolymerization process [52].

FA-based self-compacting AAC blends, formulated with NH solution concentrations ranging from 8 M to 16 M, exhibited the desired fresh properties in compliance with EFNARC guidelines [235]. The molarity of the NH solution significantly influenced the strength of fly ash-based self-compacting AACs cured at room temperature. Specifically, the CS increased by approximately 35.58% as the NH concentration rose from 8 M to 16 M.

7.5. Effect of Solution Modulus (NS to NH) and Curing Methods

In a study by Kumar et al. [236], the researchers examined the potential of low-molarity AAC for structural use. A total of 35 mix compositions were prepared using 4 M and 6 M NH solutions, achieving CS ranging from 10 to 40 MPa. Among these, three mixes exhibiting CS of 30 ± 2 MPa were selected for further evaluation of their tensile strength, elastic modulus, and plastic strain. The findings demonstrate that, despite similar CS, variations in mix design significantly affect the hardened properties, suggesting the feasibility of multi-parameter optimization. The influence of curing conditions reveals a low rate of reactivity at ambient temperatures, with notable improvements observed at higher curing temperatures. The ability to tailor low-molarity AAC beyond CS presents opportunities to improve its sustainability, cost-effectiveness, and overall structural performance.

Recent research has demonstrated that the mechanical strength of AAC improves with increased dosages of activator solutions and reduced water content [237,238,239]. Among the various binder materials, low-calcium FA, high-calcium FA, and GBFS have shown greater strength potential compared to other AS sources [240]. NH and NS are commonly used as primary activator materials [241]. Investigations into curing conditions indicate that while elevated temperature curing facilitates early strength development, ambient curing conditions often result in a higher rate of strength gain over time [242]. For elevated temperature curing, the curing temperature and duration have typically ranged from 40 to 80 °C and 24 to 72 h, respectively [243]. Reported studies have used NH solutions with molarities between 10 and 18, and NS-to-NH ratios ranging from 1.5 to 4.0 [244]. Higher molarity and elevated activator ratios generally lead to an increased total dosage of activators, which can negatively impact the fresh and hardened characteristics of AACs [245]. To improve workability and strength, high dosages of naphthalene-based superplasticizers are frequently used [246]. Depending on the specific mix composition, the CS of high-molarity AAC typically falls between 10 and 50 MPa [247]. However, some studies have reported CS exceeding 70 MPa through optimized blending of binder materials [248,249,250]. Additionally, pumice-based AAC subjected to elevated temperatures between 100 and 800 °C has been found to retain more residual strength than conventional concrete under similar conditions [251].

As indicated in [236], the CS of AACs prepared using 4 M and 6 M alkaline solutions, with varying water-to-binder and NS-to-NH ratios, is presented in Figure 18. The figure shows that CS increases with a higher NS-to-NH ratio and a lower water-to-binder ratio. Moreover, the impact of the NS-to-NH ratio on strength variation is more significant than that of the water-to-binder ratio. Notably, the increase in CS due to a higher NS-to-NH ratio is more pronounced at lower water-to-binder ratios, with the rate of increase diminishing as the water-to-binder ratio rises. For specimens prepared with the 6 M solution, compressive strength ranges from 38.02 MPa to 9.81 MPa, while for the 4 M specimens, it ranges from 32.37 MPa to 7.13 MPa. The strength distribution, as well as the comparison between Figure 18a,b, indicates that multiple mix designs can produce similar compressive strength values in AACs. The study highlights the potential for developing low-molarity AACs with CS values ranging from 10 to 40 MPa.

In a separate study [163], the objective was to investigate the behavior of FA-based self-compacting AACs in both fresh and hardened states under varying NH molarities. The NH concentration ranged from 8 M to 16 M, increasing in increments of 2 M. The ratio of NS to NH was maintained at a constant value of 2. The AAS-to-binder ratio was fixed at 0.45 throughout the geopolymerization process. All specimens were cured under ambient conditions prior to testing. The results demonstrate that increasing the NH molarity from 8 M to 16 M improves the CS of the proposed concrete.

In terms of CS, it improves with increased soluble silicate content in the AAC [252], as these silicates actively contribute to the polycondensation process [129]. However, when the NS concentration exceeds 16 M, the geopolymerization reaction slows down [253], as the excess silicate content disrupts the formation of the geopolymeric network and reduces the final strength [254]. Skvara et al. [255] found that the modulus (SiO₂/Na₂O ratio) of the AAC solution plays a key role in determining the properties of FA-activated systems. In fact, research confirms that modulus is a crucial parameter influencing geopolymer strength development [240]. Previous studies used an NS-to-NH ratio of 2.5, with modulus values ranging from 1.72 to 2.37 [66]. In contrast, the present study reports a higher modulus value of 2.99.

Moreover, the extent of silicon and aluminum leaching from the source material is strongly affected by the NH concentration in the AAC solution [256]. When a higher NS-to-NH ratio is applied, the strength tends to decrease because the NS solution contributes only about 10% Na₂O—lower than in previous studies. Additionally, increasing the NH concentration generally enhances CS [257], as it accelerates the dissolution of raw materials and raises the solubility of silicates and aluminates [253]. Nonetheless, excessive NH can lead to premature precipitation of aluminosilicate gel, limiting the formation of other geopolymeric components and ultimately reducing strength [258]. In this study, the addition of extra NH reduced strength in specimens with low NS-to-NH ratios. Experimental results suggest that an NS-to-NH ratio of 1.5 and an NH concentration of 16 M yield superior workability and CS.

In [102], authors evaluated the average signal-to-noise (S/N) ratios for different NS-to-NH ratios, indicating that the CS does not follow a consistent trend with increasing NS-to-NH ratios. Previous research has suggested that higher NS-to-NH ratios increase the concentration of [SiO₄]⁴⁻ ions, which accelerates polymerization and enhances strength [27,73]. In contrast, Dave and Bhogayata [259] reported that increasing this ratio actually led to reduced strength. Similarly, Nath et al. [93] observed that while the seven-day strength of FA-GBFS-based AACs remained relatively unchanged with higher NS-to-NH ratios, the strength at later ages declined. The sodium content also influences the strength development in AAC in the mixture, as Na⁺ ions are essential for charge balance and the formation of the geopolymeric network. Although increasing the NS-to-NH ratio raises the sodium content, it may also inhibit water evaporation and disrupt the development of the alkali-activated structure, ultimately reducing CS [82]. In the present study, variations in the NS-to-NH ratio had a limited effect on strength, but the highest CS was achieved at a ratio of 1.5.

7.6. Effect of the Addition of Water, Superplasticizer, and Filler Content

It is well established that the water-to-binder ratio or additional water is inversely related to the development of AACs’ strength at both early and later stages. This relationship remains valid and is acknowledged by all major construction codes. In AAC mixes, workability presented challenges, particularly at higher binder contents. Where necessary, an appropriate amount of superplasticizer was added to ensure the mix remained workable. Compared to the concrete matrix, this inverse relationship also holds true for high-strength aggregates. The strength of AACs is significantly influenced by the size, total content, and ratio of aggregates to binder, as well as the fine-to-coarse aggregates ratio and type of aggregates. However, due to the presence of adhered concrete, recycled aggregate (RA) tends to exhibit a slightly porous structure, leading to a weaker bond within the concrete. In fact, RA features two ITZs, which contribute to its overall porosity and reduced strength. Various treatment techniques for RA have been documented in the literature, including the application of paraffin-based sprays [260]; immersion in pozzolanic slurry [261]; use of polymer emulsions [262]; microbial-induced treatment [263]; and surface pre-treatment with a combination of cement slurry and silica [264]. As a binder in concrete, recycled aggregate alkali-activated concrete (RA-AAC) utilizes industrial by-products such as FA and GBFS, derived from thermal power and steel manufacturing processes.

The study adopts the CS versus water-to-binder ratio principle, commonly used in conventional concrete mix design per ACI 211.1 [178], as the basis for developing recycled aggregate-based AAC using FA and GBFS. In the proposed methodology, the water-to-binder ratio is replaced by the alkali-activated solution-to-binder ratio. Proposed AAC mixes were prepared at various AAS-to-binder ratios, and their 28-day CSs were measured under ambient curing. The resulting CS vs. AAS-to-binder curve was then compared with the standard ACI CS vs. W/C curve using average strength values. This substitution enables the integration of the AAS-to-binder ratio into the mix design methodology. When the primary focus is achieving a 28-day CS target, the W/C ratio (as shown in Figure 19) can be effectively replaced with the AAS-to-binder ratio.

To align the total aggregate grading with the DIN B grading curve, as previously discussed, it is essential to ensure appropriate grading of both fine and coarse aggregates to produce dense and well-compacted concrete. The efficient packing of various aggregate fractions—including fine and coarse aggregates passing through 4.75 mm, 12 mm, and 20 mm sieves—is critical in determining the workability of conventional concrete, and more significantly, recycled aggregate-based AAC. Preliminary studies indicate that AAC mixes with a high ratio of RA to fine aggregate tend to segregate, whereas mixes with an excessively low RA to fine aggregate ratio are prone to bleeding. Consequently, as depicted in Figure 20, a blended grading approach was adopted to minimize both segregation and bleeding, while also enhancing workability. Subsequently, the individual aggregate proportions were selected so that the combined grading aligns with the DIN B grading curve.

In ref. [178], mix design charts were developed using a preliminary methodology to achieve high-performance RA-AAC, as shown in Figure 21. The methodology was further validated through CS tests on the designed mix proportions. The mix design charts were created with a binder composition of 70% FA and 30% GBFS for all the AAS-to-binder mixes studied. The alkaline solution was fixed at 200 kg/m³ for all the AAC mixes examined. After selecting the concrete for final analysis, a commonly used grading was adopted, which included 40% fine aggregate in the combined grading. The binder content for these AACs varied from 250 to 666 kg/m³. The 28-day CSs of these concretes ranged from 30 to 60 MPa, with nearly all mixes achieving their design strengths. This confirms the feasibility of developing AAC mix proportions that can meet the expected performance criteria, based on the information provided in these design charts.

7.7. Bond Zone and Aggregates Content

The AAB is a suitable material for use in recycled aggregate concretes (RAC) to improve mechanical properties, bond strength, and the interfacial transition zone (ITZ) between the paste and aggregates, which are typically weaker in comparison to natural aggregate concretes (NAC) [74,109,265,266]. Colangelo et al. [267] suggested that, to reduce the environmental impact of conventional OPC concrete, the use of alternative aggregates and binders should be promoted as a more eco-friendly option. Villoria Saez and Osmani [268] reported that the highly porous paste or mortar attached to the recycled aggregates (RA) can absorb the alkaline solution, triggering geopolymerization within the original ITZ. This process improves the RA content and enhances the toughness of AACs. In a study by [269], the authors examined the CS of AAC using RA and natural aggregates (NA) at room temperature, finding that RA-based AAC outperformed NA-based concrete significantly.

7.8. Effect of Curing Methods

In a study conducted by Pattanayak et al. [75], the researchers extensively examined the impact of different curing methods on the mix design and strength performance of AACs. Specifically, two curing regimes were considered: ambient curing (AC) and oven curing (OC), applied for durations of 7 and 28 days. After 28 days of curing, the highest CS recorded for the control mix was 48.53 MPa. Under AC conditions, the maximum CS values for FA-based AAC, GBFS-based AAC, and a combination of FA and SF-based AAC at NH concentrations of 8 M, 10 M, 12 M, and 14 M were as follows: 2.06 MPa, 2.96 MPa, 3.20 MPa, and 3.63 MPa for FA-AAC; 62.1 MPa, 64.8 MPa, 83.7 MPa, and 83.93 MPa for GBFS-AAC; and 6.4 MPa, 7.026 MPa, 9 MPa, and 11 MPa for FA–SF–AAC, respectively. These results indicate that, under AC conditions, CS increased for all types of AAC as the concentration of NH increased.

In OC conditions, the CS values for FA-AAC were 4.49 MPa, 5.606 MPa, 8.84 MPa, and 12.79 MPa at NH concentrations of 8 M, 10 M, 12 M, and 14 M, respectively. FA–SF–AAC demonstrated even higher CS values of 10.696 MPa, 13.233 MPa, 17.00 MPa, and 22.36 MPa for the same NH concentrations. Notably, GBFS-AAC exhibited the highest CS among the mixtures, achieving 63.97 MPa, 67.30 MPa, 85.84 MPa, and 87.53 MPa at 8 M, 10 M, 12 M, and 14 M NH concentrations, respectively. The superior performance of GBFS-AAC, particularly the strength of 87.53 MPa at 14 M NH under OC conditions, is attributed to its higher calcium oxide content. This promotes the formation of tobermorite, accelerating early strength development compared to other mineral additives [172,270].

The results demonstrate that increasing the NH concentration from 8 M to 14 M leads to reduced workability but enhanced CS in both curing regimes. This improvement is due to increased leaching of alumina and silica, which promotes the geopolymerization process, thereby strengthening the AAC. Additionally, the findings suggest that incorporating GBFS and SF offers a more effective alternative to FA in AAC formulations. This is due to the formation of stronger hydration gels and improved microstructure, resulting from better particle packing and higher calcium oxide content [270].

Figure 22 illustrates the CS of AAC specimens subjected to various curing conditions, including ambient temperature, steam, and oven curing, as reported in [236]. Under ambient conditions (Figure 22a), the polymerization reaction responsible for strength development in AAC progresses slowly. This is evident from the fact that the CSs at 7 and 14 days account for only 5.65% and 28.62%, respectively, of the 90-day strength. In contrast, elevated temperature curing significantly accelerates the polymerization process, enabling greater strength development within shorter curing durations (Figure 22b). Specifically, oven curing for up to three days markedly improves the rate of strength gain. However, extending the curing period beyond three days may lead to a decline in strength development.

The influence of curing temperature is depicted in Figure 22c. Temperatures up to 80 °C enhance CS, whereas higher temperatures result in a decline. A notable strength reduction at 105 °C may be attributed to the rapid loss of moisture, leading to the formation of voids or microcracks within the specimen. Figure 22d compares oven and steam curing at 60 °C for three days. Oven curing yields higher CS and is therefore recommended for AAC. The reduced strength observed in steam-cured specimens could be due to the deep penetration of water vapor, which, under loading, may generate internal pore pressure and cause premature failure.

7.9. Correlation Matrix and Predictive Performance

Numerous studies in the literature have assessed the predictive accuracy and influencing factors related to the CS of AACs. In reference [179], the authors utilized the XGBoost algorithm to construct predictive models for the CS of AACs. Additionally, the particle swarm optimization (PSO) algorithm was applied to refine the mix design of AAC to meet specific CS requirements. Figure 22 presents both the predictive performance and the Taylor diagram for the XGBoost model across the full dataset. As illustrated in Figure 23a, predictions that align more closely with the 45-degree diagonal line indicate higher accuracy. The model’s performance, evaluated using the coefficient of determination (R²), achieved a value of 0.92, demonstrating a high level of predictive accuracy. Figure 23b displays the Taylor diagram for the XGBoost model, which provides a visual summary of multiple statistical measures, such as standard deviation, root mean square error (RMSE), and correlation coefficient, allowing for comprehensive analysis and comparison with actual data. The diagram reveals that the model performs exceptionally well, with a standard deviation nearly matching the observed CS, a correlation coefficient exceeding 0.95, and an RMSE below 5.0 MPa. These results confirm the model’s reliability and robustness in predicting CS under various conditions.

Figure 23c indicates that most features exhibit weak correlations, with correlation coefficients below 0.5. Regarding CS, the features FA, hydration modulus (HM), and curing time (CrTime) show positive correlations, suggesting that increases in these variables are associated with higher compressive strength. In contrast, Water-to-FA and Fine-to-FA demonstrate negative correlations, indicating that higher values correspond to lower compressive strength. The Pearson correlation coefficient assesses the linear relationships between the features and the target variable.

Figure 23d illustrates the relative importance of each feature in predicting CS using the XGBoost model. Among the features, the authors [179] reported that the FA demonstrates the highest contribution, as its silica and alumina content play a key role in strength development through the geopolymerization process in AACs [271]. This is followed by the influence of HM, CrTime, and curing temperature (CrTem). In contrast, features such as the maximum size of coarse aggregate (MaxS) and the fineness modulus of fine aggregate (Fs) show relatively weaker associations with CS, indicating a lesser effect on strength variability within the analyzed dataset. The authors of [179] found that the performance of the XGBoost-based machine learning model has been assessed across the complete dataset. Nonetheless, its ability to generalize to unseen data has not been confirmed. Iterative mix design, informed by experimental data, will be essential to evaluate and validate the model’s generalization performance, thereby supplementing the findings from cross-validation.

The results presented above demonstrate that the iterative mix design method guided by machine learning (ML) achieves superior performance. This approach not only satisfies the strength requirements but also meets the workability criteria. It is important to note that the designed AAC mix for each iteration is validated through a minimal number of laboratory tests, ensuring the practicality and validity of the mix design. Further examination of the iterative process indicates that most deviations from the design objectives arise from the strength not meeting the target requirements, likely due to the limited size of the strength database, which leads to model inaccuracies. Therefore, ongoing efforts to collect extensive and continuous multinational databases are essential in future research to improve the generalization capability of ML models for AAC mix design. As decision-makers increasingly face the challenge of balancing multiple objectives and meeting various design specifications, the demand for AAC mixtures that are cost-effective, high-strength, workable, durable, and environmentally sustainable is growing. This highlights the need for multi-objective optimization in AAC mix design. Additionally, efforts should be directed at enhancing prediction accuracy by integrating other ML techniques, such as feature engineering and model comparison, which will further improve the performance of the ML model and facilitate more robust and precise optimization of AAC mix designs.

Recent studies [272,273] have employed thermodynamic simulation-based analytical methods to support the comprehensive management of phase assemblages in alkali-activated materials and to inform the design of suitable precursors. In the work by Xiao et al. [273], the stability domains and quantities of simulated phases were mapped using ternary contour diagrams of the SiO₂–CaO–Al₂O₃ system. This analysis revealed the overall relationships among precursor chemical compositions, phase assemblages, and pore solution pH. As shown in Figure 24, the analytical outcomes align well with experimental data, confirming that the primary precipitation regions of C-(N-)A-S-H occur at CaO/SiO₂ ratios around 1.0 and in areas with relatively low aluminum content. Conversely, N-A-S-H phases are predominant in calcium-deficient regions. Strätlingite is primarily found at intermediate concentrations of silicon, aluminum, and calcium, while katoite and AFm phases tend to form when Al₂O₃/CaO is approximately 3.0 under silicon-deficient conditions. The results further indicate that the precipitation regions of a given phase may extend across a range of pH values, suggesting the potential for simultaneous pH control and stabilization of phase precipitation levels.

7.10. Relationship Between the Workability and Compressive Strength

The relationship between workability and CS is crucial in AAC technology because both properties significantly influence the performance, durability, and structural integrity of proposed AACs. The relationship between workability and CS is a balancing act. Good workability ensures proper placement and compaction, which are essential to achieve the full CS of the prepared AACs. Managing this balance, especially through the AAS-to-binder ratio, additional water, and admixtures, is crucial to producing high-quality concrete. Hadi et al. [92] investigated the optimal mix design of AACs and examined the relationship between workability and CS. In their study, three criteria were proposed to define the optimal mix design for alkali-activated pastes: (1) the workability, as measured by the mini-slump test, of the alkali-activated paste should be equal to or better than that of OPC paste 60 min after mixing; (2) the initial and final setting times of the alkali-activated paste must meet the requirements for Type alkali-activated cement as specified in AS 3792; and (3) given that the first two criteria are satisfied, the CS of the alkali-activated paste should be maximized.

In reference to [179], data visualization plays a vital role in deepening the understanding of the alkali-activated slump and CS datasets. It is evident that most features follow a normal distribution and cover a wide range of values. From the obtained results, the authors observed that no strong correlation exists between these features and slump. However, for certain features, such as the water-to-fine aggregate (FA) ratio, a potential trend can be observed, where an increase in water-to-FA appears to correspond with an increase in slump. The relationship between input features and CS suggests that CS tends to increase with an increase in FA and decline with an increase in water-to-FA. Nevertheless, this visual correlation might not be entirely reliable. Consequently, performing a correlation analysis between the input features and the output feature, such as using the Pearson correlation coefficient, is essential to gain a clearer understanding of their relationships.

In ref. [102], the compressive strength of each of the nine trials was evaluated on the 7th and 28th days. The highest strengths were recorded in Trials 3, 6, and 9, which shared the same GBFS content (75%) but differed in other parameters. Trial 7 exhibited the lowest strength among all the mixes, with 25% GBFS, 14 M NH, AAS-to-binder ratio of 0.5, and NS-to-NH ratio of 2.0. The results were analyzed by averaging the Signal-to-Noise (S/N) ratio for each response parameter across trials with the same design variable. For example, Trials 1, 2, and 3 all used a 10 M NH solution, and the average S/N ratio for the 7-day CS of 10 M NH was derived from the 7-day CS of these three trials [274]. A response table for CS was created based on these averaged S/N ratios for further analysis. As shown by their mean S/N ratio in Figure 25a, an increase in GBFS concentration led to improved strength of the AACs. A higher GBFS concentration increases the overall calcium content in the mix, which enhances the solubility of raw materials within a given alkaline environment, thereby promoting geopolymerization (GPZ). This improvement in GPZ results in the formation of additional C-S-H, C-A-S-H, and N-A-S-H gels [102], which may contribute to the observed increase in strength. Previous studies have shown conflicting results regarding the effect of molarity on strength. In [82], it was proposed that increasing NH molarity accelerates GPZ by promoting the dissolution of precursor particles, thereby enhancing strength. However, some studies have reported a reduction in CS after exceeding the optimal NH concentration, due to the early precipitation of aluminosilicate gel, which inhibits further GPZ [275]. Furthermore, the alumino-silicate source is a key factor in determining the optimal NH concentration [276]. For the materials used in this study, a 10 M NH solution is required as the threshold dose, as indicated in Figure 25a. Increasing the molarity above 10 M significantly raises the concentration of OH⁻ groups, which may hinder the formation of C-S-H gel by inhibiting GPZ and immediately affecting the overall porosity and mean S/N ratio, consistent with the findings of Mijarsh et al. [277]. Although the S/N ratio shows slight improvement between 12 M and 14 M NH concentrations, the difference is minimal. Another important factor is the AAS-to-binder ratio, which is directly related to the total water content in the system [278]. The amount of water must be sufficient to provide adequate hardness while ensuring enough GPZ [279]. In this study, the highest strength was achieved at the lowest AAS-to-binder ratio of 0.4, as shown in Figure 25a. Increasing the liquid content results in a decrease in strength, as a larger volume of liquid reduces the contact between the alkaline solution and precursor particles [79]. The relationship between AAS-to-binder ratios and CS followed a similar trend to that observed in cement concrete, where the water-to-cement ratio affects CS, as demonstrated in the works of Parthasarathy et al. [79].

Table 3. Signal-to-noise (S/N) ratio analysis for compressive strength—combined response table and ANOVA summary [102].

Parameter		CS, Day	Molarity	% GBFS	A/B	NS:NH	Total	Error
Level 1		7	33.32	28.99	33.09	32.69	-	-
		28	33.38	29.12	33.33	32.92	-	-
Level 2		7	32.14	32.83	32.47	32.45	-	-
		28	32.36	32.91	32.90	32.74	-	-
Level 3		7	32.32	35.96	32.22	32.63	-	-
		28	32.68	36.39	32.19	32.75	-	-
Delta		7	1.18	6.97	0.88	0.24	-	-
		28	1.02	7.27	1.14	0.18	-	-
Rank		7	2	1	3	4	-	-
		28	3	1	2	4	-	-
DOF		7	2	2	2	2	8	0
		28	2	2	2	2	8	0
Sum of squares	SSm	7	2.43	73.02	1.22	0.09	76.77	-
		28	1.63	79.29	1.98	0.06	82.97	-
	SSt	7	76.77	-	-	-	-	-
		28	0.81	39.64	0.99	0.03	-	-
Mean sum of squares		7	1.22	36.51	0.61	0.05	-	-
		28
% Contribution		7	3.17	95.12	1.59	0.12	100	-
		28	1.96	95.57	2.40	0.07	100	-

presents the response table alongside the ANOVA results for compressive strength. In both the 7-day and 28-day analyses, the GBFS content demonstrated a dominant effect, accounting for 95.12% and 95.57% of the total variance, respectively. The AAS-to-binder ratio also had a notable influence, contributing 1.59% at 7 days and 2.40% at 28 days. Molarity exhibited a moderate effect, explaining 3.17% of the variance at 7 days and 1.96% at 28 days. In contrast, the NS-to-NH ratio had a minimal impact, with contributions of 0.12% at 7 days and 0.07% at 28 days.

In a previous study [102], a flowability test was employed to evaluate the workability of the mix, defined as the internal energy required for adequate compaction [75]. The highest and lowest flowability values were recorded in Trials 5 and 6, respectively, both of which involved the use of 12 M NH. An increase in the AAS-to-binder ratio corresponds to a rise in the total water content of the mix, thereby improving its workability. Consequently, mixes with lower AAS-to-binder ratios exhibited greater stiffness than those with higher ratios. As illustrated in Figure 25b, increasing the AAS-to-binder ratio led to improved flowability. The physical properties and composition of the precursors play a significant role in determining the flowability of AACs. According to Qiu et al. [280], a higher content of GBFS tends to reduce flowability due to the irregular shape of slag particles. In contrast, the spherical shape of FA particles enhances the mix’s mobility and flowability. In the current research, an increase in flowability was observed when the GBFS content rose from 25% to 50%, likely influenced by the AAS-to-binder ratio (Figure 25b). However, a notable decline in flowability occurred as GBFS content increased from 50% to 75%, suggesting that the high GBFS proportion began to dominate over the effect of the AAS-to-binder ratio. Therefore, the optimal flowability was achieved at 50% GBFS content with an AAS-to-binder ratio of 0.5 (Trial 5), while the lowest flowability corresponded to 75% GBFS content with an AAS-to-binder ratio of 0.4 (Trial 6).

In Figure 25b, the average signal-to-noise (S/N) ratio for flowability is presented as a function of molarity, highlighting the irregular pattern in performance. The reduction in flowability from the 10 M to the 12 M solution may be attributed to the rise in pH associated with increasing molarity, which in turn enhances the reaction rate and viscosity of the mixture [136]. Conversely, a notable increase in flowability is observed between the 12 M and 14 M solutions, potentially due to the elevated [OH⁻] concentration at higher molarities. This increase promotes dissolution and delays gelation, resulting in improved flowability [281]. It is also likely that the effect of the AAS-to-binder ratio became dominant once more. Previous studies have identified the varying influence of the NS-to-NH ratio on flowability. Malkawi et al. [136] reported that a higher NS-to-NH ratio increases the viscosity of the alkaline solution, given that NS is more viscous than NH, thus reducing the mixture’s flowability. In contrast, Deb et al. [282] observed a decline in flowability as the NS-to-NH ratio decreased. In alignment with the findings of Prusty and Pradhan [278], the present study revealed an inconsistent trend in flowability with respect to variations in the NS-to-NH ratio.

As illustrated in Figure 25c, setting time increased with higher NH molarity, aligning with previous studies [278,283]. This trend may be attributed to the availability of calcium ions at lower NH concentrations, which promotes the formation of C-S-H and C-A-H gels. In contrast, higher molarity increases the concentration of Na⁺ and OH⁻ ions, enhancing the leaching of aluminosilicate precursors but reducing calcium ion dissolution. Consequently, the slower GPZ process at higher molarities compared to calcium-based hydration reactions at lower molarities leads to extended setting times [284]. Notably, the increase in setting time was most significant between 10 M and 12 M, while the difference between 12 M and 14 M was minimal.

Furthermore, an increase in GBFS content consistently reduced both initial and final setting times, corroborating previous findings [285]. As reported by Lee and Deventer [286], the high availability of calcium in GBFS offers heterogeneous nucleation sites, which accelerate geopolymerization and shorten the setting time. GBFS reacts more rapidly with the alkaline activator than FA [287]. The AAS-to-binder ratio also significantly influences the setting time of FA-GBFS-based AACs [285]. Findings from Shi and Day [288] and Weng and Crentsil [289] suggest that reduced water content enhances condensation, leading to quicker setting. Accordingly, in the present study, an increased AAS-to-binder ratio, which raises total water content, likely diluted the alkaline activator, slowed the reaction, and extended the setting time [92].

The impact of the NS-to-NH ratio on setting time displayed a non-linear trend. Initially, the average signal-to-noise (S/N) ratio for setting time increased as the NS-to-NH ratio rose from 1.5 to 2.0, possibly due to the dominant influence of the AAS-to-binder ratio. However, when the NS-to-NH ratio increased further to 2.5, a marked decrease in the mean S/N ratio was observed. This reduction in setting time may be explained by the higher soluble silica content in the mixture, which alters the reaction kinetics and accelerates both crystallization and dissolution of the precursor particles during condensation [290].

8. Comparative Study

This section provides a critical comparison of various methods and parameters that influence the mix design of alkali-activated concrete, emphasizing key factors such as precursor materials (fly ash, slag, metakaolin), alkaline activators (NaOH, Na₂SiO₃), solution molarity, and mix proportions. Through a systematic review of recent research, the section demonstrates how changes in binder composition, activator concentration, and curing conditions notably impact the mechanical strength and workability of alkali-activated concrete. The analysis identifies the most effective combinations tailored to different performance criteria and environmental contexts, aiming to offer a comprehensive framework for optimizing mix design.

Table 4. Summary of factors affecting the mix design, workability, and compressive strength development of AACs.

Factors	Workability Performance	Strength Development
High-calcium materials, such as slag and Class C fly ash.	An increased content of high-calcium materials in the alkali-activated matrix accelerates the hydration process and raises the viscosity, which notably decreases flowability and shortens both the initial and final setting times.	Materials with high calcium content positively influenced the development of compressive strength, with strength values increasing as the calcium content rose under ambient curing temperatures.
Aluminosilicate materials, such as fly ash (Class F), metakaolin, waste glass, and ceramic.	Compared to high-calcium materials, aluminosilicate materials exhibited greater flowability as well as longer initial and final setting times.	Specimens of alkali-activated concrete containing high-aluminosilicate materials exhibited slower early strength development and required elevated curing temperatures.
Binder-to-solution ratio	The optimal binder-to-solution ratio depends on the type and the chemical and physical properties of the binder. However, the workability of concrete generally improves as the solution content increases.	Increasing the solution content in the matrix beyond the optimal level results in a reduction in strength performance.
Sodium hydroxide molarity	A high concentration of sodium hydroxide led to a significant reduction in both flowability and setting times.	Specimens prepared with molarities of 10, 12, and 14 demonstrated the highest strength performance compared to specimens prepared at other concentration levels.
Ratio of sodium silicate to sodium hydroxide	A high sodium silicate-to-sodium hydroxide ratio increases the solution’s viscosity, resulting in reduced flowability and shorter setting times.	The ratio of 2.5 between sodium silicate and sodium hydroxide is considered optimal for achieving maximum strength performance.
Effect of the addition of water and superplasticizer	The addition of water and superplasticizer significantly improved the workability performance.	Excess water adversely affects strength performance. However, the use of superplasticizers is strongly recommended to improve both workability and strength, rather than adding more water.

summarizes the primary factors affecting workability and compressive strength development in the designed alkali-activated concretes. This synthesis helps researchers and practitioners select suitable materials and proportions to achieve targeted performance while promoting sustainability.

9. Conclusions

The effect of several factors, such as binder and alkaline activator properties, mix design, and curing methods, on workability and strength performance of alkali-activated concrete (AAC) has been widely reviewed, and the following conclusions are drawn:

This study highlights the crucial role of rigorous mix design strategies and well-defined selection criteria in enhancing the performance of AAC. As a sustainable and high-performance alternative to traditional Portland cement, AAC offers significant environmental and mechanical advantages. However, its successful implementation in modern construction hinges on a deep understanding of the complex interactions between precursors, activators, additives, and curing conditions.

The chemical composition of the raw materials-based binder influences the proposed concrete workability. The literature indicates that increasing the CaO to SiO₂ and Al₂O₃ ratios negatively affects flowability, resulting in both reduced initial and final setting times. However, the increasing molar ratio of Ca/Si had a positive effect on the development of compressive strength.

A one-part alkaline activator was highly recommended as a suitable solution for producing eco-friendly alkali-activated concrete compared to traditional concrete. However, a two-part alkaline activator solution (Na₂SiO₃ and NaOH) is the most commonly used activator in previous studies to prepare alkali-activated concrete specimens. Therefore, the molarity of NaOH, solution modulus (SiO₂/Na₂O), and molar ratio of Na/Al significantly affect the fresh and hardened performance of the proposed concrete. Most previous studies have found that increasing the molarity of NaOH causes an increase in Na+ concentration, leading to a significant decrease in the workability of design concrete. Several studies have recommended a molarity of 10–12 M as the optimum for preparing alkali-activated concrete with acceptable workability and compressive strength. However, many researchers have successfully produced alkali-activated concrete using a low molarity of NaOH (1.5–4 M) to achieve acceptable workability, strength, and sustainability goals.

In preparing alkali-activated concrete, it is observed that the flowability and compressive strength are influenced by the ratio of alkaline activator solution to the binder’s solid content. The increasing content of alkaline activator solution leads to an increase in the water content in the matrix, resulting in high flowability and longer setting times. However, the compressive strength is negatively influenced if the alkaline activator solution-to-binder ratio increases or decreases below 0.40–0.50. The optimum ratio entirely depends on the type of binder and the properties of the alkaline activator solution, especially the NaOH molarity and solution modulus.

The relationship between workability and compressive strength is a balancing act. Good workability ensures proper placement and compaction, which are essential to achieving the full compressive strength of the proposed concrete. In designing alkali-activated concrete, the required flowability, setting time, and target compressive strength play significant roles in mix design and the selection of raw materials.

The content, physical properties, and type of fine and coarse aggregates directly influence both the workability and compressive strength of the proposed concrete. The optimum content of filler was found to highly depend on the source of both fine and coarse aggregates.

The curing method in preparing the proposed concrete specimens depended on the adopted binder, and the solid binder was selected. The compressive strength of alkali-activated concrete is influenced by the curing method that was adopted after casting the specimens. For concrete specimens prepared with high SiO₂ and Al₂O₃ content, such as fly ash and metakaolin, the oven curing method with a temperature range of 50–80 °C is widely recommended to achieve the target compressive strength. However, the ambient curing regime is adopted for specimens prepared with high molar ratios of Ca/Si, especially for concrete prepared with ground blast-furnace slag.

Ultimately, advancing the acceptance and widespread use of AAC requires continued innovation in material characterization, process optimization, and standardization. Through informed mix design and careful material selection, the full potential of AAC can be unlocked to meet the evolving demands of modern infrastructure while contributing to global sustainability goals.

10. Recommendations and Future Vision

The present study has introduced several promising directions for future exploration. Based on the findings, the following recommendations are proposed for further research:

Comprehensive studies should be conducted to evaluate the performance of various industrial by-products (such as fly ash, slag, rice husk ash, red mud, waste glass) as alkali-activated precursors. Advanced characterization techniques should be employed to optimize the chemistry and microstructure of AAC, thereby enhancing its durability and mechanical performance. Additionally, more long-term field studies and accelerated aging tests are recommended to evaluate AAC’s resistance to environmental exposures, such as carbonation, chloride ingress, freeze-thaw cycles, and sulfate attacks, in various climates.

Life cycle assessments comparing AAC and traditional OPC-based concrete must be expanded, considering not only carbon dioxide (CO₂) emissions but also resource consumption, embodied energy, and end-of-life scenarios to validate their true sustainability potential.

Research should focus on developing low-cost and less corrosive alkaline activator systems, including hybrid or one-part (just-add-water) free cement systems, to improve handling, safety, and economic viability.

It is advisable to target high-performance and niche applications—such as precast elements, marine structures, and fire-resistant panels—where AAC’s superior thermal and chemical resistance can offer a distinct advantage over conventional concrete.

The compatibility of AAC with 3D printing, smart sensors, and carbon capture technologies should be explored to support the transition toward smart and green construction practices.