Recent Advances in Fly Ash- and Slag-Based Geopolymer Cements

Abstract

This review study promotes the sustainability of civil infrastructure by advancing the materials science of alternative cementitious materials. Supported by extensive global research and industrial trials, geopolymer cement has emerged as a promising approach to reducing the ecological impact of ordinary Portland cement (OPC) due to its superior engineering properties and eco-friendly benefits from industrial waste utilization. Geopolymers are inorganic polymers formed by the polymerization of aluminosilicate precursors, such as fly ash (FA), slag, and metakaolin, in the presence of alkaline activating solutions. This work integrates findings across multiple domains, including precursor chemistry, microstructural evolution, mechanical and durability performance, and sustainability metrics like carbon footprint and energy consumption. A key contribution of this review is the comparative evaluation of FA-based and slag-based GPC systems against OPC concrete, emphasizing the factors influencing their mechanical and durability properties, while also distinguishing differences in environmental impact, microstructural development, and overall performance. The findings highlight that slag-based systems generally exhibit lower environmental impacts, especially in energy demand and emissions, while regional differences in precursor availability constrain how widely the LCA and economic results can be applied. Building on previous reviews that have considered these topics, this study jointly examines technical performance and sustainability indicators and identifies regional variations that influence feasibility. The synthesis provides a balanced, evidence-based assessment of the potential and limitations of GPC as a lower-carbon alternative to OPC, supporting efforts to reduce the climate impact of future concrete construction.

1. Introduction

Presently, concrete is the most consumed material in the world after water, and it is undoubtedly the most essential building and construction material [1,2]. Ordinary Portland Cement (OPC) concrete is utilized in construction twice as much as all other building materials combined, and this demand is evident in its global production, which exceeds 30 billion tons each year [1,2,3,4]. The demand for concrete will continue to rise with the continuous increase in the rate of industrialization and urbanization, resulting in a rise in the need for infrastructure [5,6]. Concrete is composed of cement (binder), sand (fine aggregate), crushed stone or gravel (coarse aggregate), water, as well as sometimes supplementary cementitious material (SCMs) and chemical admixtures [1,5]. Concrete exhibits outstanding engineering properties, and when it is properly designed, the material is versatile, durable, and economical, and provides certain environmental benefits. Its most remarkable characteristic is that, unlike any other available material, it is an engineered product that can be tailored to meet nearly any conceivable set of performance standards. This adaptability makes it a crucial component in modern construction and infrastructure [1]. However, due to its enormous consumption, concrete and cement are also significant contributors to environmental problems and climate change, responsible for more than 8–10% of global anthropogenic CO₂ emissions [7,8]. This high share stems mainly from the production of clinker, the key intermediate in cement manufacture, which releases approximately 0.85 tons of CO₂ per ton of clinker produced through both limestone calcination and fuel combustion. With an average clinker-to-cement ratio of 0.72–0.75, global cement production exceeding 4 BMT annually emits about 2 BMT of CO₂, as shown in Figure 1 [9,10,11], equivalent to roughly 250–300 kg of CO₂ per m³ of conventional concrete.

The manufacturing process of cement is highly energy-intensive, involving the acquisition, transportation, grinding, mixing, and heating of raw materials, such as limestone, clay, and shale, in a cement kiln at temperatures up to 1500 °C to produce clinker. The clinker is then crushed and blended to produce cement [12,13]. For instance, the CO₂ emissions result mainly from the decomposition of limestone (main reaction: CaCO₃ → CaO + CO₂) to produce CaO (the main content of clinker) and CO₂, and well as from the burning of fossil fuels during cement production to provide the thermal energy required (heated to around 1400–1500 °C) for calcination to occur. Moreover, cement and concrete production require overexploitation of quarries, which also has a significant impact on landscape pollution. Alongside environmental motivations, evolving policy frameworks and corporate sustainability commitments are further driving research into alternative cementitious systems. To contribute to sustainable development, immediate action is needed to minimize CO₂ emissions from cement manufacturing. Minimizing cement and concrete consumption is also paramount to achieving the net-zero emission scenario outlined in the International Energy Agency (IEA) report, which emphasizes decarbonization of the built environment as a critical global priority.

Consequently, cement scientists and manufacturers have begun to emphasize both improving the sustainability of OPC production and exploring alternative binders as complementary pathways toward achieving net-zero targets [14]. Some of the elucidated alternative binders to OPC technology include calcium aluminate cements, magnesium cements (CACs), calcium sulfoaluminate cements (CSA), supersulfated cements, the magnesium phosphate system, and geopolymer cement [15,16,17,18]. Compared to other binder technologies, geopolymer cement has a longer in-service track record and offers enormous potential for a sustainable future and climate change mitigation for structural engineering applications [19], supported by the extensive body of fundamental research relating gel chemistry and microstructure to their performance [20]. Aside from being a perfect replacement candidate, geopolymer cements are more durable, with excellent resistance to acid and sulfate attack, fire resistance, low thermal conductivity, extended service life, and suitability for sustainability, and they offer an eco-friendly disposal option for waste materials without the usual practice of landfill disposal.

Joseph Davidovits, a pioneering researcher in geopolymer concrete (GPC), developed this inorganic material in the 1970s using industrial waste products [21]. This process, known as geopolymerization, involves chemically activating aluminosilicate-rich industrial waste with an alkaline solution to form a durable and sustainable binder [22,23]. The reactive aluminosilicate precursors, such as fly ash (FA) [24,25], metallurgical (including blast furnace) slag [25,26], metakaolin (MK) [27,28], calcine clays, and/or volcanic ash [29], are commonly used in geopolymerization. The utilization of GPC as an alternative to OPC concrete has been reported to reduce CO₂ emissions by up to 80% [30,31,32]. Since their development, there has been a significant increase in their application as an alternative material in concrete structures. For example, the largest GPC project in the world is Brisbane West Well Camp Airport (BWWA), Australia, where 70,000 tons of GPC were used for the pavement and floor panels, making it the world’s first greenfield public airport [33].

However, these conventional precursors have shortcomings associated with them, and these shortcomings will continue to limit their usage for the foreseeable future. For instance, FA, a byproduct of coal combustion that accumulates at the top of boilers [34,35,36], is experiencing a decline in availability due to the decreasing reliance on coal-fired power generation, which has led to a reduction in coal combustion residuals production [37]. Despite the reduction in the availability of FA, demand continues to rise, thereby leading to a market imbalance and a continuous rise in cost [37]. Similarly, slag, which is a byproduct of the steel-making industry [38,39,40], is also experiencing a recent reduction in its production due to the priority given to the use of recycled steel for steel production over the traditional production of steel [41,42,43,44]. As a result, its decreasing availability has driven up costs, further limiting its use. Meanwhile, MK, which is produced through the thermal activation of kaolin clay, has been reported to have high strength and durability compared to conventional concrete [45,46,47]. However, despite its excellent mechanical performance, MK-based GPC is uneconomical when compared to its prototype [44,48]. The excessive cost of MK is attributed to the low rate of production, due to the limited number of manufacturing plants [44].

The challenges associated with conventional precursors in GPC demand the urgent development and adoption of new SCMs that can replace traditional precursors while enabling the production of net-zero GPC to align with the 2050 carbon neutrality targets. Over the past three decades, numerous review articles have examined the mechanisms, physicochemical characteristics, strength, and material properties of GPC. However, despite the increasing emphasis on sustainable construction materials over the past decade and the push toward 2050 carbon neutrality goals, there remains a significant gap in comprehensive reviews that analyze the sustainability of different GPC systems and provide recommendations for achieving net-zero emissions.

Research Significance and Contribution

Several reviews have advanced the understanding of GPC systems but remain limited in scope. Recently, Patil et al. (2023) provided a broad overview of SCMs, such as FA, slag, MK, and rice husk ash, and their influence on mechanical properties, but did not extend their discussion to sustainability indicators, such as CO₂ footprint or energy demand [49]. Venkatesan et al. (2024) offered a comprehensive analysis of Class F FA, alkaline activator molarity, and curing conditions on compressive strength, yet their review was narrowly restricted to siliceous fly ash-based systems, neglected durability properties and excluding other precursors and environmental dimensions [50]. El Alouani et al. (2024) [51] surveyed geopolymer synthesis, characterization, and their applications. Although their research was multidisciplinary, it did not address slag-based geopolymers or the role of GPC in advancing sustainability toward net-zero emissions [51]. Zhang (2024) concentrated on the durability performance of GPC, including acid, chloride, sulfate, and freeze–thaw resistance, and, while providing a critical durability perspective, did not consider recent development in life-cycle emissions or cost implications [52]. Similarly, Magotra and Jee (2024) highlighted the durability and microstructural features of FA-based GPC and presented its potential for reducing carbon emissions by up to 80%, but their scope was limited to FA systems, overlooking slag and other precursors, as well as regional variability in material availability and economic performance [53].

In contrast to earlier reviews that primarily focused on isolated aspects such as mechanical performance or chemical mechanisms, this study integrates findings across multiple domains, including precursor chemistry, microstructural evolution, mechanical and durability performance, and sustainability metrics like carbon footprint and energy consumption. A key contribution of this review is the comparative evaluation of FA-based and slag-based GPC systems against OPC concrete, emphasizing the factors influencing their mechanical and durability properties, while also distinguishing differences in environmental impact, microstructural development, and overall performance. Moreover, this review also explores how FA- and slag-based GPC are utilized from a global perspective and how their regional variability may affect the achievement of net-zero emissions. Alongside this, a structured methodology was adopted to provide a comprehensive understanding of the sustainable benefits of different GPC systems in comparison with conventional OPC concrete.

This review comprises nine sections, including the introduction (Section 1). Section 2 discusses the methodology employed, and Section 3 synthesizes advances in reaction chemistry and mechanisms, highlighting knowledge gaps. Section 4 examines physico-chemical properties and microstructure. Section 5 reviews workability and mechanical performance (compressive, tensile, flexural, and modulus of elasticity). Section 6 evaluates durability against acids, chlorides, sulfates, and corrosion, and their interrelations. Section 7 appraises methodological quality and limitations. Section 8 analyzes sustainability (energy use, CO₂ emissions, economic viability, and regional cost variability), with comparisons of FA/slag and alternative precursors to conventional concrete. Section 9 concludes with key findings and future research directions toward net-zero geopolymer mixes.

2. Methodology

To systematically review the existing literature, this study followed the PRISMA protocol. The review process included the following steps: (1) defining the research question, (2) selecting authoritative databases and searching relevant literature, (3) screening for inclusion, (4) assessing the quality of the literature, and (5) review.

2.1. Research Questions

The following research questions were formulated to guide the systematic literature review process and set out the research objectives:

• What are the key physio-chemical mechanisms and microstructural characteristics of fly ash- and slag-based geopolymer binders, and how do they differ from OPC systems?
• How do the fresh and hardened properties of FA/slag-based geopolymers compare with OPC concrete?
• How do fly ash- and slag-based geopolymer concretes perform under major durability treats (i.e., acid, chloride, sulfate attack, and steel corrosion) relative to OPC concrete?
• From a cost and life-cycle sustainability perspective, under what regional and supply conditions can FA/slag-based geopolymer concretes be realistically adopted at a large scale as low-carbon alternatives to OPC?

2.2. Database Selection and Literature Search

In this study, the Web of Science database was selected due to its extensive and precise coverage of peer-reviewed publications. For the literature search, a total of 3106 publications were collated, of which 2728 were retained after removing review articles. To ensure quality, the review considered only publications in English with full-text availability. After screening based on titles and abstracts, the number was further reduced, and following assessment against the eligibility criteria, the set was narrowed again. The process is shown in Figure 2.

2.3. Literature Screening and Assessment

During screening and assessment, publications focused on other composite classes or approaches (e.g., OPC concrete merely citing geopolymer articles, computational tools, modeling) were excluded; only studies relevant to our research questions were selected. Overall, articles were included only if they characterized the following key aspects:

• Form of geopolymer materials (i.e., paste, mortar, and concrete);
• Type of pre-cursor material (i.e., fly ash, slag, fly ash + slag);
• Chemical and microstructural parameters based on XRF, XRD, FTIR, TGA, SEM, and TEM tool data;
• Physical properties of constituent materials based on density, water absorption, porosity;
• Fresh properties based on workability.
• Hardened properties based on compressive strength, tensile strength, flexural strength, modulus of elasticity.
• Durability properties based on resistance to acid, sulfate, chloride, and corrosion;
• Environmental impact based on regional variation.
• Cost of production based on regional variation.

3. Background and Mechanism of GPC

3.1. Background of GPC

The discovery of a new class of inorganic material, or GPC binder, cement, and concrete, has resulted in wide scientific interest and kaleidoscopic development of applications due to their high mechanical strength and up to 80% lower CO₂ emission compared to OPC concrete [54]. GPCs are inorganic polymers formed by the polymerization of aluminosilicate source material in the presence of alkaline activating solutions (i.e., activators such as alkali metal silicates, hydroxides, and carbonates) [20,55]. Among the various precursors used for GPC system production, two main reaction products, depending on precursors’ composition, are formed [56,57]. On the one hand, precursors rich in Si + Ca (e.g., blast furnace slag) form calcium-aluminate-silicate-hydrate (C-A-S-H) as the main reaction production in a low-alkaline solution [58]. On the other hand, precursors rich in Si + Al (e.g., metakaolin and fly ash) with low Ca content, produced inorganic polymers of sodium-aluminate-silicate-hydrate (N-A-S-H) [59]. The resulting products govern the properties of the cement from various perspectives, including mechanical strength, durability performance in acidic media, resistance to chloride penetration, sulfate attacks, and seawater. Another important constituent of geopolymerization is the activating solution, which consists of alkalis and sometimes reactive silica, like sodium hydroxide, sodium silicate, and potassium hydroxide, required to initiate the reaction process [60].

Geopolymers are a family of alumino-silicate binders, which are the result of an inorganic polycondensation reaction, a process known as geopolymerization. This reaction yields an amorphous to semicrystalline three-dimensional tecto-aluminosilicate frameworks with the general empirical formula [61] of M_n{−(SiO₂)_z–AlO₂−}_n. wH₂O, where M is the cation or positive ion, such as sodium (Na⁺) or potassium (K⁺), calcium (Ca²⁺) is required in the structure to balance the negative charge, n is a degree of polycondensation, and the subscript z defines the Si-to-Al molar ratio, which is a vital factor in determining the engineering properties of the resulting geopolymers. Such frameworks are regarded as poly(sialates), where sialate is an abbreviation for the silicon-oxo-aluminate building unit. The sialate network is a combination of silicate and aluminate ([SiO₄]⁴⁻ and [AlO₄]⁵⁻) tetrahedra, cross-linked by shared oxygen atoms [62]. Thus, poly(sialates) are chain and ring polymers with Si⁴⁺ and Al³⁺ in IV-fold coordination with oxygen, ranging from amorphous to semicrystalline, and are inaugurated as geopolymers [61]. Figure 3 describes the three main classes of polymeric chains commonly observed.

While these structural classifications are well established, the geopolymerization process itself is highly sensitive to raw material variability. The balance between precursor composition, activator chemistry, and curing conditions largely determines the rate of dissolution and condensation, the type of gel that forms, and the resulting microstructure [63]. For instance, low-calcium fly ash systems typically exhibit slower reactivity at ambient curing and often require elevated temperatures to achieve adequate strength, whereas calcium-rich slag systems react more rapidly under similar conditions [64]. These distinctions in reaction pathways underpin many of the performance outcomes discussed in later sections, making it clear that geopolymerization cannot be understood without reference to the precursor and processing context.

3.2. Mechanism of Geopolymerization

Understanding the key process in the reaction kinetics of geopolymers is essential for setting behavior, bulk microstructure, and property development of geopolymeric gels. Thus, Figure 4 presents a highly simplified reaction kinetic of geopolymerization (according to the main pioneers of the geopolymer system). Though the figure is presented linearly, these reaction mechanisms are largely coupled and occur concurrently, making the basic understanding of the system difficult and complex. Notably, the actual mechanism of setting and hardening of the GPC material is still not clear due to its rapidity. The most proposed and accepted mechanism consists of the chemical reactions that occur in three stages [57,65,66]: (a) dissolution of the aluminosilicate source through the action of hydroxide ion from alkali activators to form reactive precursors (monomers); (b) restructuring, or rearrangement, or condensation of precursor ions to produce oligomers; (c) gelation/polycondensation of oligomers to form polymeric structures.

The high pH of the alkali solution (such as sodium or potassium hydroxide solutions) initiates a dissolution reaction of the aluminosilicate material, leading to the formation of monomers of aluminate and silicate species. This reaction can be apportioned into two periods: (I) dissolution—hydrolysis, and (II) hydrolysis—polycondensation; which are believed to proceed almost simultaneously as the precursor material is mixed with the alkaline solution [67]. Notably, the dissolution of aluminosilicate particles at the surface has always been assumed to be the mechanism responsible for the conversion of solid particles during geopolymerization. As shown in Figure 3, water is usually consumed at this stage, followed by the release of water in other succeeding geopolymerization processes [68,69,70].

After dissolution, the aluminate and silicate species are integrated into the aqueous phase that may react with silicate ions present in the alkaline solution. The complex mixture of aluminate, silicate, and aluminosilicate species is thereby formed. At high pH, the amorphous aluminosilicates dissolve very rapidly, creating a supersaturated aluminosilicate solution. These highly saturated solutions result in the formation of a gel-like structure, and because of condensation, large networks are formed by oligomers/oligo-sialates in the aqueous system [67]. This process also results in the release of water (that was technically consumed during dissolution). As such, water plays an important role in the reaction medium that helps in the formation of a hydrated gel structure. This gel structure is termed bi-phasic, that is, water and aluminosilicate binder form the two phases. The conversion time between the aqueous supersaturated aluminosilicate solution and gel is hypothesized to be dependent on the source material composition, concentration of the alkali solution, and synthetic conditions [71,72]. The processes of rearrangement and reorganization of aluminosilicate bonds determine the microstructure features, pore size, and distribution of the materials, which in turn determine many chemical and physical properties of geopolymeric materials. This stage also leads to the dissolution of more precursor materials, and the solution becomes saturated.

After gelation, and as the linkage of the gel network grows, the system continues to reticulate and reorganize, resulting in the three-dimensional aluminosilicate network commonly regarded as geopolymers—GPC solidification [23,57,66]. The dissolution/alkalination of the aluminosilicate source material and formation of the polymetric species phase are regarded as the nucleation stage, which explains the first two steps proposed by Glukhovsky [73]. This phase is dependent on the thermodynamic and kinetic parameters (such as synthesis of temperature and mixing). While the gel formation, polycondensation, and reorganization phase, in which the nuclei reach a critical size, and crystals begin to mature, is regarded as the growth phase. Although water is not incorporated into the final aluminosilicate network, it plays a critical role throughout the reaction. During the dissolution stage, water facilitates ion transport and enhances the mobility of silicate and aluminate species, while during polycondensation and gelation, water is progressively expelled. The amount and distribution of residual water strongly influence pore structure, strength development, and durability, with excess water often linked to higher porosity and insufficient water associated with incomplete dissolution [74]. This complexity continues to fuel ongoing debate in the literature regarding the precise role of water in geopolymerization.

4. Physio-Chemical Characterization of GPC

4.1. Chemical Properties of FA-Based GPC

While the chemical and microstructural analysis of GPC has been extensively studied, this section uniquely addresses the chemical properties of FA-based GPC in direct comparison to slag-based systems, a gap not previously explored. Various advanced characterization techniques, including X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), have been employed to assess the chemical composition, mineralogy, and microstructural behavior, as well as to predict geopolymerization kinetics and reaction products. This section provides a comprehensive review of these methodologies, highlighting their role in advancing the understanding of GPC chemistry. For instance, XRF is utilized to determine the elemental composition of raw materials, while XRD identifies the crystalline phases present in the GPC reaction products. FTIR provides molecular-level insights by distinguishing different bond types within the GPC network, facilitating both qualitative and quantitative structural analysis. SEM, coupled with energy-dispersive spectroscopy (EDS), enables the examination of micro-to-nanoscale morphology, elemental distribution, and structural characteristics of the synthesized GPC matrix. TEM offers high-resolution phase and elemental analysis, diffraction studies, and real-time observation of dynamic behaviors at the atomic scale. TGA is employed to monitor mass decomposition as a function of temperature and time, providing critical insights into the pozzolanic reactivity and thermal stability of GPC reaction products.

4.1.1. X-Ray Fluorescence (XRF)

XRF determines the elemental composition of GPC reactants, providing quantitative data on the oxide contents of aluminosilicate source materials. Recent studies have extensively utilized XRF to analyze the chemical compositions of various precursors, offering insights into their suitability for dissolution and geopolymerization processes [75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99]. Figure 5 illustrates the average percentage of oxide composition of various FA-based GPC. The main pozzolanic oxides present are silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃), and the minor oxides are iron oxide (Fe₂O₃), magnesium oxide (MgO), calcium oxide (CaO), potassium oxide (k₂O), sodium oxide (Na₂O), titanium oxide (TiO₂), and sulfur oxide (SO₃), etc. Notably, the combined pozzolanic oxides (SiO₂, Al₂O₃, and Fe₂O₃) in all reviewed studies exceed 50%, classifying them as pozzolanic materials per ASTM C618 and playing a crucial role in geopolymerization reactions. However, the reaction products in a GPC system vary based on the precursor type. FA- and MK-based GPCs, rich in SiO₂ and Al₂O₃, primarily form N-A-S-H gels, contributing to high strength, whereas slag-based GPCs, with higher CaO content, predominantly generate calcium–silicate–hydrate (C-S-H) phases, influencing mechanical and durability properties [100].

In reference to Figure 5, the mean compositions of SiO₂, Al₂O₃, and Fe₂O₃ are 52.86%, 26.89%, and 6.91%, with standard deviations of 5.63%, 5.19%, and 2.69%, respectively. The minor oxides, including MgO, CaO, K₂O, Na₂O, and SO₃, have mean compositions of 1.85%, 4.27%, 1.43%, 0.47%, and 0.63%, with standard deviations of 1.61%, 3.24%, 0.93%, 0.35%, and 0.50%, respectively, generally ranging between 0.30% and 4%. According to the reviewed studies, the loss on ignition (LOI) varies between 0.04% and 3.89%, primarily due to inefficient and uncontrolled calcination of FA production, which significantly influences geopolymerization and strength development [101]. If the FA has high LOI (unburned carbon content), it will absorb the activator solution, resulting in poor workability of the concrete, a decrease in compressive strength, and ultimately higher cost of production [102].

4.1.2. X-Ray Diffraction (XRD)

XRD is another crucial technique for analyzing GPCs, offering insights into phase composition, microstructural evolution, and material reactivity during geopolymerization. This method is fundamental in distinguishing between amorphous and crystalline phases, enabling a deeper understanding of the transformation mechanisms occurring throughout the synthesis process [75,76,85,103,104,105]. The intensities and diffraction patterns are usually evaluated within the diffraction angle (2Ɵ) between 10° and 80°, allowing for the identification and quantification of both amorphous and crystalline phases and providing insights into the degree of reaction and the nature of the GPC matrix [76,85,103,105].

For a typical FA-based GPC system, XRD analysis usually shows a broad hump in 2Ɵ values between 15° and 35°. The broad hump represents the highly reactive amorphous phase, which is the glassy and disordered aluminosilicate material in the FA that react with alkaline activators during geopolymerization to form the N-A-S-H geopolymeric gel [75,82,85,106]. It also revealed that the major identified crystalline phases are quartz, mullite, and sometimes hematite, which are represented by sharp peaks superimposed on the amorphous hump [60,104,107]. These components originate from the raw FA and do not fully dissolve during geopolymerization, persisting in the final hardened GPC matrix.

To compare the composition of GPC derived from different precursor sources, Figure 6 presents a representative XRD spectrum of FA-based, FA-slag-based, and slag-based GPC matrices, highlighting their distinct mineral compositions [104]. The results confirm that the crystalline phases in FA-based GPC are predominantly represented by sharp peaks of quartz and mullite (alumina silicate), indicative of its high aluminosilicate content [103,107]. In contrast, the slag-based GPC exhibits an amorphous hump at around 28–31° and 10–20°, dominated by high vitreous calcite content, reflecting its high calcium content [108,109]. Consequently, C-A-S-H gels are usually present within the GPC matrix containing slag content [75,110]. The blended FA–slag GPC generally reflects the combined characteristics of both precursors, showing intermediate amorphous features associated with simultaneous N-A-S-H and C-A-S-H gel formation [104]. Quantitative phase analysis was conducted through Rietveld refinement using AUTOQUAN version 2.70, and it revealed that the amorphous content increased from approximately 80% in 100% FA mixes to about 85–86% in slag-rich GPCs, while crystalline phases such as quartz and mullite decreased correspondingly. The higher amorphous fraction in the slag-based matrix indicates greater reactivity and earlier strength gain from rapid C-A-S-H gel formation, thus exhibiting denser microstructures and better mechanical properties [104]. As such, XRD analysis is crucial in assessing the extent of geopolymerization, determining precursor material reactivity, and characterizing the composition of the final GPC product.

4.1.3. Fourier-Transform Infrared Spectroscopy (FTIR)

Among the diverse characterization techniques employed to analyze bond structures in a geopolymer system at the molecular level, only a select few are unique and have been found effective. FTIR is one of the most prominent methods, offering both qualitative and quantitative insights. It has been extensively utilized to investigate the structural evolution and mineralogical composition of FA-based GPC systems [75,80,82,85,111,112,113]. The technique provides insight into the molecular structure and bonding by correlating specific wavenumbers, which correspond to distinct molecular bonds and structures within the materials, to vibrational modes associated with them, and the wavenumber ranges from 4000 to 400 cm⁻¹ [110,114,115].

For a typical FA-based GPC system, FTIR spectra show the formation of bands between 1000 and 1200 cm⁻¹, which are attributed to the asymmetric stretching vibrations of Si-O-T (where T could be Si or Al), and this confirms the development of aluminosilicate frameworks [110,111,112,114,116]. It also reveals the carbonate bands around 1400–1600 cm⁻¹ which are linked to the CO₃^2- group stretching vibrations, indicating carbonation due to the presence of unreacted particles and porous nature [110,114]. FTIR analysis also shows the location of broad bands around 3400–3500 and 1600–1700 cm⁻¹, corresponding to O–H stretching and H–O–H bending of adsorbed H₂O molecules, respectively [114,116,117,118]. Other prominent bands revealed by FTIR include the symmetric stretching and bending of Si-O-Si/Al-O-Si (600–800 cm⁻¹) and Si-O bending (400–600 cm⁻¹) [119,120,121].

To compare the composition of GPC derived from different precursor sources, Figure 7 shows the FTIR analysis of FA-based, slag-FA-based, and slag-based GPC matrices, showing various bands with different broadness and intensities [122]. The Si-O-T band (around 1000 cm⁻¹) in FA-based GPC is broader and less intense compared to the slag-based system. This is due to the partial dissolution of FA particles, the amorphous structure, and the slower geopolymerization process [82,112,123]. The carbonate band around 1400 cm⁻¹ is more pronounced in the slag-based system due to the higher calcium and magnesium content, and is present in others owing to the porosity of FA particles [124,125,126]. The O–H stretching (~3400 cm⁻¹) and H–O–H bending (~1600 cm⁻¹) bands are present across all GPC samples, indicating the presence of water. However, these bands appear broader in the FA-based system, suggesting higher porosity compared to the slag-based system, which exhibits a more compact and denser microstructure [117,127,128]. Therefore, FTIR is crucial in the study of chemical properties of different GPC systems, as it provides valuable insight into the microstructure, bonding, and degree of polymerization.

4.1.4. Scanning Electron Microscope (SEM)

SEM analysis provides critical insights into the microstructure of cementitious materials, including GPC, by revealing morphology, particle size, and surface texture, which are key factors influencing material performance. FA-based GPC particles are widely reported to exhibit a predominantly spherical shape with smooth surfaces, which enhances their reactivity and facilitates efficient geopolymerization upon activation with an alkaline solution [106,114,129,130,131,132,133,134,135,136,137]. For GPC mixes made with FA, the spherical nature of the particles aids in creating a firm microstructure, which lowers voids and improves the material properties, and the smoothness of the particles encourages the creation of geopolymeric gels, thus improving the durability and strength of the GPC matrix [138,139]. Many studies also revealed that SEM analysis shows the dispersion of unreacted FA particles in the GPC matrix [60,137,140]. The unreacted particles and cracks are often seen in poorly cured samples, and this can lead to a reduction in the long-term durability performance of the GPC [140]. However, well-reacted FA particles have a more compact microstructure with less porosity, which improves their mechanical performance [141]. Several studies combine SEM with Energy Dispersive X-ray Spectroscopy (EDS) to verify the existence of the aluminosilicate gels and track their formation by detecting changes in chemical composition over time [75,114,130,135,137,142,143]. The microstructural changes identified by SEM-EDS, such as the development of these gels and the quantity of the unreacted particles, are strongly linked to their durability and mechanical properties [144,145]. Essential elements such as Si, Al, Na, and trace amounts of Ca are also confirmed by EDS analysis, which reflects the composition of FA and the products of geopolymerization [142,146].

Typical SEM micrographs of the FA-based (100% FA) and blended FA-slag-based (60% FA–40% slag) GPC matrices from [147] are shown in Figure 8. In the FA-based GPC (Figure 8a), a significant number of unreacted and semi-reacted FA particles are visible, along with the formation of N-A-S-H gel surrounding the particles. This results in a relatively porous microstructure that can reduce early-age strength and durability performance. In contrast, the blended FA-slag sample (Figure 8b) shows the presence of C-A-S-H gel, which forms a denser and more cohesive matrix [148,149]. The angular morphology of the slag particles promotes faster geopolymerization, improving matrix compaction and reducing porosity. However, the higher calcium content and faster reaction rate can also lead to microcracking and shrinkage, which may compromise long-term dimensional stability [148,150,151]. These microstructural differences explain the generally higher mechanical and durability properties of blended FA-Slag GPC compared to 100% FA-based mixes.

4.1.5. Transmission Electron Microscope (TEM)

TEM experiments are a powerful tool that allow for imaging material nano- and microstructure and characterizing material chemistry, revealing details on the distribution and interaction between the crystalline and amorphous phases at high resolution [75,85,110,138,152,153,154]. It operates by capturing and producing high-resolution images that reveal the internal microstructure of the material, after a highly focused electron beam is directed through the thin GPC sample [107,155]. Recent advances in data collection technology for TEM have enabled high-volume and high-resolution data collection at a microsecond frame rate. Thus, this makes TEM the perfect technique for real-time acquisition and microstructural characterization during GPC synthesis (during active dissolution, particle-to-gel conversion, and the formation of hydration products). Previous research that incorporates TEM hypothesized that the key processes occurring in the transformation of the solid aluminosilicate source into the synthetic alkali aluminosilicate are somehow mechanistic and proceed linearly [71,72].

Studies revealed that TEM analysis of FA-based GPC showed the significant presence of nanocrystalline phases—mullite and quart—and amorphous aluminosilicate gels, which are essential in the development of strength and durability of the GPC [59,75,110]. It also shows the transformation of spherical FA particles to homogeneous N-A-S-H gel during geopolymerization by revealing how these particles diffuse to form a compact and dense structure, in which a few weakly crystalline particles due to unreacted FA particles are created together with the GPC gel product [75,110,152]. For example, Figure 9 shows TEM of the FA-based GPC matrix, which shows the amorphous GPC gel formed after the activation of the aluminosilicate and the formation of crystalline materials, which could be quartz or mullite [110,152]. TEM reveals nanoscale details that are important for further understanding and for optimizing these materials for a variety of applications, making it an indispensable tool.

4.1.6. Thermogravimetric Analysis (TGA)

TGA is a useful technique for examining the thermal stability and decomposition behavior of the GPC matrix as a function of weight loss with changes in temperature [115,116,118,156,157,158,159,160,161,162,163]. The rate of weight loss with the corresponding microstructural composition in the FA-based GPC system is mainly studied using TGA within the range of 0–1000°, but most of the weight loss occurs by 700°, and the GPC would be in a stable thermal state [118,164,165,166,167].

Studies revealed that weight loss in FA-based geopolymer typically occurs in three stages [116,160,161,162,168]. The greatest weight loss is usually from the first stage (<200 °C), which is due to the loss of evaporable and residual water within the GPC matrix that is not chemically bound [156,164,167]. The second stage of weight loss occurs between 200 and 400 °C, which is ascribed to the release of chemically bound water from the N-A-S-H gel and the process is known as the dehydration of the hydroxyl group (dihydroxylation) [114,165,169].

The third stage is the carbonate decomposition, which occurs between 400 and 700 °C and usually results in minimal loss in weight [106,166,170]. Öz, Doğan-Sağlamtimur et al., and Aziz, Al Bakri Abdullah et al., presented the TGA thermogram of FA-based and slag-based GPC matrices from [171] and [172], respectively, as shown in Figure 10. It was reported that FA-based GPC mix exhibited lower overall weight loss (about 8%), which could be due to its stable aluminosilicate structure that can better withstand thermal and vapor release during decomposition, compared to the slag-based mix, which experienced about 20% loss in weight at 1000 °C, owing to its richness in calcium compounds that are less thermally stable [112,173,174].

Across the different techniques, consistent patterns emerge between FA- and slag-based systems. XRD and FTIR highlight the dominance of N-A-S-H gels in fly ash mixes and the greater presence of C-A-S-H phases in slag-rich mixes, findings that SEM and TEM confirm through differences in porosity and matrix density. TGA further supports these distinctions by showing lower weight loss and greater thermal stability in FA-based systems compared with the higher mass loss observed in calcium-rich slag mixes. Collectively, these results explain why slag systems often exhibit higher early strength and denser microstructures, while FA systems tend to develop more slowly but offer superior long-term stability.

4.2. Physical Properties of FA-Based GPC

Similar to the chemical analysis of GPC, the evaluation of its physical properties, those measurable without altering the material’s composition, is critical to the performance and durability of GPC, essentially, the net-zero GPC mix. Density, water absorption, porosity, and shrinkage are common tests employed to evaluate the physical properties of GPC [14,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210]. Notably, these properties are influenced by a range of factors, including aggregate characteristics (texture, moisture, shape, and distribution), the ratio of alkaline activators to binder (A/B), sodium hydroxide molarity (SH molarity), the ratio of sodium silicate to sodium hydroxide (SS/SH), curing conditions, the fineness and chemical composition of FA, as well as the incorporation of additives or fibers.

According to research, aggregates have significant impacts on the physical properties of any GPC or mortar, affecting its density, porosity, water absorption, and others. Dense aggregates improve GPC density and enhance thermal insulation [211,212]. Well-graded aggregates minimize porosity while poorly graded or irregularly shaped aggregates increase porosity, which in turn leads to weaker strength [24,212]. Beyond aggregates, which have been extensively studied, this section also predicts the main controlling factor in a GPC system, enabling optimized mix designs. Figure 11 shows the analysis of factors (aggregates excluded) affecting the physical properties of FA-based GPC. The figure demonstrates that the SS/SH ratio is the most influential factor across studies, followed by SH molarity and curing conditions. This underscores the critical role of alkaline activator composition and concentration in determining the physical properties of FA-based GPC systems. SS/SH directly determines the silica content available for gel formation during geopolymerization, and higher SS/SH tends to produce a denser GPC matrix with reduced porosity and absorption [185,191]. Optimizing the SS/SH ratio, which typically ranges between 2.0 and 2.5 depending on precursor chemistry, activator modulus, and related processing conditions, is essential for achieving desirable physical properties [14,213]. Additionally, SH molarity controls the alkalinity of the activating solution and determines the rate of dissolution of the aluminosilicate from FA. Higher SH molarity improves dissolution, thereby enhancing geopolymerization, but may lead to brittleness and shrinkage if excessively high [182,214]. Elevated temperature curing also accelerates geopolymerization, improving density and reducing porosity; however, excessive curing may result in shrinkage [177,190]. Other factors, including the chemical compositions and particle sizes of the aluminosilicate source, also influence density and porosity. Higher surface area and amorphous content increase reactivity, which results in better geopolymerization, reducing porosity and enhancing density [208,209,210].

5. Fresh and Hardened Properties of FA-Based GPC

This section reviews the fresh and hardened properties of FA-based GPC, including workability, compressive strength, tensile strength, and modulus of elasticity. It provides a detailed analysis of the testing procedures and identifies the key factors influencing both the fresh-state behavior and mechanical performance of the system. While numerous review studies and independent research have extensively examined the fresh and hardened properties of GPC mixtures, this section provides a comprehensive analysis of the key factors within the GPC binder that influence these properties—an aspect that remains underexplored in the existing literature. The comparative results presented in this section were derived through a quantitative frequency-based synthesis rather than a formal meta-analysis. Due to inconsistencies in mix designs and reporting formats across studies, statistical pooling was not feasible. Instead, factors influencing each property were identified within individual studies, and their frequency of dominance was counted across all studies and converted into percentages to show how often each parameter was reported as the most influential. This approach provides a clear overview of prevailing trends in the properties despite methodological variations.

5.1. Workability

The workability of FA-based GPC refers to its ease of placement, compaction, and finishing while maintaining cohesion and preventing segregation. Studies have shown that several factors influence the workability of FA-based GPC, including the ratio of alkaline activators to binder (A/B), sodium hydroxide molarity (SH molarity), sodium silicate-to-sodium hydroxide ratio (SS/SH), admixtures such as superplasticizers, and the fineness and chemical composition of FA. Additionally, parameters such as solid-to-liquid ratio, water-to-binder ratio, and the incorporation of slag also play significant roles in determining the rheological behavior of the system [14,25,91,201,213,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244].

FA-based GPC generally has better workability than slag-based GPC due to the spherical shape of fly ash particles, which reduces friction and enhances flowability [130,243]. In contrast, the irregular and angular particles in slag-based GPC increase water demand, leading to rapid stiffening [135,245]. Additionally, FA has a slower reaction rate, allowing for extended workability and easier handling, whereas slag is highly reactive, accelerating setting times and consequently reducing workability over time [239,246].

Figure 12 presents a synthesis of factors influencing workability based on the analysis of 35 studies. SH molarity was the most frequently investigated parameter (n = 17), followed by SS/SH ratio (n = 16) and A/B ratio (n = 15). However, when evaluating relative impact, the A/B ratio emerged as the most influential factor. Among the studies that assessed at least two of these three parameters (SH molarity, SS/SH ratio, and A/B ratio), 71.4% identified A/B as having the greatest effect on workability, while 28.6% favored SH molarity, and none identified SS/SH as the dominant influence. This finding highlights the critical role of A/B ratio in determining workability, as it directly controls the amount of liquid available in the mix. A higher A/B ratio generally results in improved flow due to increased activator volume, but beyond a certain threshold, it may cause segregation or bleeding [14,240]. SH molarity also significantly impacts workability. Lower molarities increase the water content in the activator solution, reducing viscosity and enhancing flow. Higher molarities, in contrast, accelerate dissolution and early geopolymerization, increasing stiffness and reducing workability [213,217,220,223,232]. While the SS/SH ratio affects the viscosity of the activator solution and the mix’s consistency, no study in this review identified it as the most decisive factor for workability. Nevertheless, it remains an important secondary parameter influencing fresh mix behavior [14,201,213,229].

5.2. Hardened Properties

Compressive strength, tensile strength and modulus of elasticity are significant engineering properties, of FA-based GPC systems, which have been widely studied, and numerous studies have shown that factors like A/B ratio, SH molarity, SS/SH ratio, additives (slag content, nano-silica), curing conditions, and many others (aggregate presence and extra water) have an influence on them [14,60,81,88,105,200,213,215,217,223,229,232,236,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267].

FA-based GPC generally exhibits lower compressive strength compared to slag-based GPC due to its lower calcium content, slower reaction rate, and more porous microstructure [243,266]. Slag contains a higher amount of calcium (CaO), which promotes the formation of C-S-H and C-A-S-H gels, significantly enhancing strength [243]. In contrast, FA is primarily aluminosilicate-based and relies on alkali activation to form N-A-S-H gel, which provides strength but is generally weaker than C-S-H [246,268]. Additionally, slag-based geopolymer develops a denser and more compact microstructure due to increased C-S-H gel formation, while FA-based GPC tends to be more porous, which can slightly reduce its compressive strength [112,245].

Among the 35 studies reviewed for the compressive strength of FA-based GPC, many investigated multiple parameters simultaneously. Figure 13 presents a synthesis of the number of studies considering each factor, alongside the proportion of studies identifying a particular factor as the most influential on compressive strength. SH molarity was the most frequently studied parameter (n = 23), followed by SS/SH ratio (n = 20) and A/B (n = 15). However, when evaluating relative impact, the A/B ratio emerged as the dominant factor, with 70.6% of the studies that considered at least two of the three factors (SH molarity, SS/SH ratio, and A/B ratio) identifying it as the most influential. SH molarity was dominant in 23.5% of studies, while SS/SH ratio was most influential in only 5.9% of the studies. An increase in the A/B ratio generally enhances compressive strength; however, exceeding the optimal threshold may lead to a reduction in strength. This reduction is due to an excessive concentration of the alkaline activator, which disrupts the formation of a stable GPC matrix, compromising structural integrity [239,240,263].

Ghafoor et al. [14] reported that increasing the A/B for fly ash-based GPC from 0.4 to 0.5 resulted in a reduction in compressive strength for cases composed of SH molarity 8 M and 10 M. A similar trend was also found by Arafa et al. [217], where around 10% reduction was observed when the A/B ratio was increased to 0.5 for a 10 M SH-based GPC compared to an A/B 0.4 mixture. However, literature indicates that the optimum A/B is dependent on the type of precursor used; more specifically, the particle size distribution of the precursor is the influencing factor [221]. The development of compressive strength of GPC depends on the dissolution of silicon and aluminum particles in the geopolymerization process, and the reaction is dependent on the molarity of the alkaline activator (e.g., SH). To understand the effect of the concentration of the activator, several studies have been conducted, and it was observed that increasing the molarity of the activator (e.g., NaOH) positively influences strength development [14,217]. However, there is also an optimum level of 14 M reported by some researchers, where they speculated that at higher molarity, there might be a congestion of hydroxide ions (OH⁻) in the matrix [14]. Alongside A/B and SH molarity, another influencing factor for GPC is the SS/SH ratio. The SS supplies the Si4⁺ ions into the mix to accelerate the reaction and help in building long, stable polymeric chains. Similar to the A/B ratio, increasing the SS/SH ratio also adversely affects the compressive strength of GPC beyond a certain point. However, the optimum SS/SH ratio is system-dependent.

The curing conditions, which include temperature and environment (ambient curing, steam curing, oven curing), also have a crucial impact on the development of FA-based GPC strength. The geopolymerization process is accelerated by high curing temperatures, which produce denser and more compact materials, and result in higher early strength of GPC mixes’ compressive strength [258,259,262]. However, the required strength could also be reached with ambient curing, but it takes longer than heat curing [247,254,255].

Compressive strength is considered the most substantial mechanical property of concrete also the most crucial parameter to initiate any construction project, and it is observed that other mechanical properties are primarily dependent on it. Typically, the tensile strength of GPC follows a similar trend to its compressive strength, where an increase in compressive strength is accompanied by an enhancement in tensile strength [269,270,271]. Similar to compressive strength, the tensile property of the GPC system also depends on A/B ratio, SH molarity, SS/SH ratio, curing conditions, and the incorporation of additives [24,60,84,86,91,142,150,200,201,202,227,237,238,240,242,254,272,273,274,275,276,277,278,279,280,281,282,283,284]. However, tensile behavior is also uniquely influenced by the quality of the matrix–aggregate interface and the binder’s ability to transfer stress across microcracks. Fiber incorporation (such as steel, polypropylene, or basalt) enhances this behavior by bridging cracks, delaying their propagation, and improving ductility and energy absorption [272,273,274,276]. The inclusion of additives and reinforcing fibers also contributes to the tensile strength properties by modifying the microstructure, thereby improving the tensile strength, which in turn improves cracking behavior, ductility, and tensile strength of FA-based GPC [86,272,273,281,285].

The modulus of elasticity (MOE) is another important parameter in assessing the deformation behavior of FA-based GPC, and studies have shown that similar factors such as A/B ratio, SH molarity, SS/SH ratio, curing conditions, additives, and the inclusion of fibers also influence its performance [14,43,142,237,238,240,241,242,282,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303]. Additives enhance the MoE of FA-based GPC by increasing gel density, stiffness, and improving microstructural integrity through the formation of denser N–A–S–H networks [287,297,298,299]. In contrast to tensile strength, MoE is strongly affected by aggregate stiffness, gel composition, and porosity. Higher degrees of N–A–S–H polymerization and reduced pore volume yield greater stiffness, whereas elevated porosity and weak interfacial transition zones reduce the modulus. Thus, while compressive strength remains a general indicator, tensile and elastic behaviors are independently controlled by microstructural cohesion, gel chemistry, and fiber–matrix synergy.

In general, optimal workability of the GPC matrix can be achieved without compromising strength by maintaining an A/B ratio between 0.4 and 0.6, SH molarity between 10 and 14 M, and an SS/SH ratio of 1.5 to 2.0. Curing conditions also play a role in the strength development of FA-based GPCs, particularly in precast applications. Early curing at temperatures between 40 °C and 60 °C for 24–48 h, typically followed by ambient curing, maximizes strength and MoE; however, this approach demands significant energy, potentially increasing the carbon footprint. Conversely, ambient temperature curing is a more sustainable and practical alternative for in situ applications, but it results in lower early-age strength and a longer time to achieve the desired strength and MoE. Enhancing early-age strength can be accomplished by incorporating additives that accelerate geopolymerization and extending the curing period, allowing for competitive strength and MoE with minimal energy consumption. In large-scale applications, however, maintaining uniform temperature and moisture during this hybrid curing process remains a practical challenge.

6. Durability Properties of FA-Based Geopolymer

This section reviews the durability-related properties of FA-based GPC, including acid attack, chloride attack, sulfate attack, and chloride-induced corrosion [304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339]. This section presents a detailed analysis of the effective testing methodologies currently employed to assess the long-term durability, performance, and serviceability of GPC structures. Additionally, a comparative evaluation of the durability properties of FA-based and slag-based GPCs against OPC concrete is conducted. A summary of strategies for optimizing GPC durability is also provided. Building on the evaluation of fresh and hardened properties, this section offers a comprehensive assessment of the critical factors within the GPC binder that influence durability, including chemical composition, SS/SH ratio, curing conditions, and the incorporation of additives. Despite the growing body of research on GPC, the impact of these parameters on long-term durability remains insufficiently explored, making this review a significant contribution. Figure 14 represents the performance matrix for GPC and OPC concrete against several aggressive agents.

6.1. Resistance to Acid Attack

The GPC system has shown remarkable potential for resistance against acid attack due to its chemical composition and microstructure. The ASTM C1898 test method is preferentially used to assess the acid resistance of concrete and mortar by immersing specimens in acidic solutions and measuring mass loss, strength reduction, and surface deterioration over time, with lower degradation indicating higher durability [340]. The low calcium content in the FA-based GPC system results in lower quantities of gypsum and ettringite formation, which tends to deteriorate under acidic attack [316,317,321]. In contrast to OPC concrete, which exhibits significant weight loss and strength reduction in acidic environments, studies have shown that the GPC matrix maintains its structural integrity under such conditions [307,314,315]. Similarly, slag-based GPC showed resistance to chloride attack but was slightly inferior to FA-based GPC due to the presence of calcium phases that form gypsum and ettringite, which may weaken the structure under acidic conditions. Additionally, the incorporation of additives (including nanosilica and silica fume) improved the resistance to acid attack due to their ability to modify the geopolymer matrix to a denser one, thereby minimizing degradation [74,135,318]. Higher molarity of SH between 10 and 12 M creates a denser geopolymer matrix with better resistance to acid attack [306,313]. Likewise, elevated temperature curing also enhances geopolymerization, resulting in reduced porosity and improved acid resistance [319].

6.2. Resistance to Chloride Attack

Another significant long-term property of GPC in chloride-rich environments is the material’s resistance to chloride attacks. This is typically assessed using the Rapid Chloride Permeability Test (RCPT), which quantifies the electrical charge passing through a 50 mm thick, 100 mm diameter concrete specimen over a 6 h period under 60 V DC voltage. The measured charge, expressed in coulombs, serves as an indicator of the material’s resistance to chloride ion penetration, with lower values signifying higher durability. This test is crucial for evaluating the long-term performance of concrete in chloride-rich environments [341].

FA-based GPC has significantly less permeability compared to OPC concrete due to the dense microstructure of its geopolymer matrix that reduces the ingress of harmful ions into it [307,311,342]. Similarly, slag-based GPC showed excellent resistance to chloride attack due to a denser microstructure and lower permeability, resulting from the formation of C-A-S-H. Moreover, the chemical bonding in the GPC structure adds to its durability regarding chloride-induced degradation, thus making it fit for marine and coastal applications as well [305,326]. Studies have shown that FA-based GPC performs better in a chloride environment, since the degrading effect observed in OPC concrete is not recorded for it [327,343]. SS/SH plays an important role in the formation of denser geopolymer matrices, and the inclusion of additives (such as slag, nano-titanium dioxide) modifies the microstructure, which makes it highly resistant to chloride attack [135,308,325].

6.3. Resistance to Sulfate Attack

The GPC system also demonstrates good resistance to sulfate attack due to its aluminosilicate network, which is less reactive to sulfate ions. ASTM C1012 [344] is usually used to assess the sulfate resistance of cementitious materials by measuring the length expansion of mortar or concrete bars immersed in a sodium sulfate (Na₂SO₄) solution over an extended period (e.g., 6 months to 1 year). Lower expansion indicates higher sulfate resistance, making this test critical for evaluating the durability of concrete in sulfate-rich environments.

The polymeric N-A-S-H gel structure in FA-based GPC minimizes the formation of expansive products like ettringite, reducing sulfate-induced deterioration and enhancing durability in sulfate-rich environments [317,320,328]. Indeed, in the literature, it is reported that FA-based GPC exposed to sulfate solutions showed only a slight reduction in strength after an extended exposure period, indicating its good performance in such conditions [321,330,345]. For slag-based GPC, studies show moderate resistance because sulfate ions react with calcium phases in slag, leading to the formation of expansive ettringite and gypsum. Additionally, C-A-S-H phases can interact with sulfates, forming expansive products, which may compromise durability. Allahverdi et al. [346] reported that slag-based GPC showed 0.03% expansion, whereas Saavedra et al. [323] reported an expansion of 0.0048% for FA/slag GPC, where FA was the dominant precursor. Notably, the addition of alternative materials, such as nanosilica, silica fume, and rice husk ash, significantly improve their sulfate attack resistant by reducing porosity and densifying the GPC matrix, thereby limiting the formation of expansive sulfate reaction products [322,323,324].

6.4. Resistance to Corrosion

Corrosion resistance is a critical concern for reinforced concrete structures, especially when exposed to harsh environmental conditions such as moisture, oxygen, carbon dioxide, chlorides, and sulfates. The Half-Cell Potential Test is among the most widely used non-destructive techniques for evaluating the likelihood of corrosion in reinforced concrete. This method measures the electrical potential of embedded steel using a copper–copper sulfate electrode, where more negative readings indicate an increased risk of corrosion, aiding in the early detection and mitigation of deterioration [347]. The absence of Portland cement in FA-based GPC and the formation of an alkali-activated aluminosilicate matrix influence the corrosion resistance properties, protecting embedded steel reinforcement [331,332]. In chloride-rich environments, GPC has demonstrated lower corrosion rates and prolonged durability than OPC concrete structures. A study by Reddy et al. (2013) assessed the resistance of GPC to chloride attacks and found that it exhibited excellent resistance to corrosion-induced cracking, with longer times to corrosion initiation compared to OPC concrete [333]. Similarly, different studies also found that FA-based GPC exhibited low chloride permeability, indicating reduced corrosion potential [335,338]. Slag-based GPC exhibits superior resistance due to its denser microstructure, which reduces the ingress of corrosive ions. The composition and type of activators used in GPC play a crucial role in its corrosion resistance [348,349]. Studies have indicated that SS/SH improves the binding properties of the GPC system, leading to better corrosion resistance [337]. Moreover, the incorporation of slag along with FA enhances the GPC matrix, reducing its porosity and improving durability [334].

Figure 14 and

Table 1. Comparison of durability properties among emerging GPC variants and OPC concrete with recommended optimization solutions.

		Durability Properties				Remarks
		Acid Resistance	Chloride Resistance	Sulfate Resistance	Corrosion Resistance
Concrete Type	OPC Concrete	Low	Low	Low	Low	OPC concrete’s high calcium content reacts with acids and sulfates to form expansive compounds (gypsum and ettringite), leading to deterioration [315,317,328,336].
						OPC concrete’s high porosity and permeability allow chloride ions to penetrate easily, leading to accelerated corrosion of steel, followed by spalling and structural degradation [326,332,333].
	Slag-Based GPC	Moderate	Excellent	Moderate	Good	Calcium phases present in slag react with both acid and sulfate, generating expansive compounds, leading to deterioration of the structure [348,349,350,351].
						Due to the formation of C-A-S-H phases, slag-based GPC has a denser microstructure and lower permeability, resulting in excellent resistance to chloride penetration and corrosion of steel [339,348,349,352].
	FA-Based GPC	High	Good	High	Superior	Due to fly ash’s aluminosilicate framework and low calcium content, the formation of soluble or expansive compounds under acid and sulfate attack is low [317,321,328,330].
						Slight porosity in FA concrete can increase permeability, making it susceptible to chloride ingress and potential corrosion initiation of steel over time [307,311,353,354].
Recommended solution to durable GPC mix		Increase silica content by adding nano-silica or graphene for stronger N-A-S-H gel.	Use high SS content to improve chloride-binding capacity. Include silica-rich precursors like rice husk ash, silica fume.	Use aluminosilicate-rich precursors (high-Al2O3 FA, metakaolin).	Use alkaline activators with lower alkali content (lower SH molarity). —Add nano-silica or metakaolin to refine pore structure.

offers a comparative summary of the durability properties of OPC concrete, FA-based GPC, and slag-based GPC, highlighting the fundamental science behind these materials and tailored strategies for enhancing resilience in aggressive environments. Remarkably, GPC mixtures outperform OPC in resisting acid attack, chloride penetration, sulfate exposure, and corrosion, making them a more durable choice for harsh conditions. However, optimizing critical mix design parameters remains essential to unlocking their full potential and ensuring long-term structural integrity.

To provide a deeper understanding of the factors influencing GPC durability and highlight its potential for enhanced performance in aggressive environments, interrelationships among various durability performance properties were analyzed. Figure 15a,b illustrates the correlation between resistance to acid, sulfate, and chloride attacks in GPC, as observed and synthesized from experimental datasets in the literature. The trendline equations (y = 0.7883x + 23.745 and y = 0.2271x + 76.526) with R² values of 0.8833 and 0.774, respectively, demonstrate a good relationship and a strong positive correlation among durability properties. This suggests that when GPC shows increased resistance to one type of chemical attack, such as acid, its resistance to sulfate and chloride attacks also improves, though at varying levels. This interdependence arises from the chemical and microstructural evolution of geopolymer binders. A higher degree of geopolymerization produces a denser and more continuous reaction gel, typically N–A–S–H in low-calcium systems and C–A–S–H or hybrid N–(C)–A–S–H in calcium-rich systems, that fills pores and refines the pore structure [317,320,328]. The resulting compact matrix reduces permeability and ion diffusivity, thereby limiting the ingress of aggressive species and suppressing secondary reactions such as gypsum or ettringite formation. Consequently, a well-reacted, low-porosity matrix exhibits improved resistance to multiple deterioration mechanisms simultaneously. Ultimately, GPC exhibits a strong interdependence between acid, sulfate, and chloride resistance, making it highly suitable for infrastructure in aggressive environments.

However, it should be recognized that most durability investigations on geopolymer concretes have been conducted under accelerated laboratory conditions designed to simulate long-term degradation within shorter periods. While such methods are valuable for comparative evaluation, they often exaggerate the severity of exposure and may not fully represent in-service conditions. Therefore, translating laboratory durability results into reliable field predictions requires additional long-term studies and environment-specific validation.

7. Methodological Quality and Limitations of Reviewed Articles

The assessment of methodological quality of the reviewed articles is important to understand their reproducibility and broader applicability. For most of the investigations, which researched both the mechanical and durability parameters of geopolymer concrete, the studies were small in scale. Moreover, considerable variability was observed among reported results, particularly for mechanical properties such as compressive strength, due to differences in precursor chemistry, activator composition, curing temperature, and testing age. In addition, the results were generally presented as mean values without corresponding measures of variability, such as standard deviation or error ranges, which limits the statistical reliability and comparability of findings.

It has been reported that the strength and quality of GPC are dependent on precursors. Despite this, the majority of the studies relied on single-source fly ash, with chemical composition reported but limited mineralogical and microstructural verifications [14,213,217,237,238,241,250,321,325].

However, there are studies that compared the effects of precursor variability on both hardened and durability properties [91,93,201,222,227,229,239,240,251,252,256,263,291,311,314,328,348,353], but the absence of thorough characterization made the comparative conclusions unreliable. Alongside the lack of precursor variability, the scope of most of the articles was limited. For instance, most studies focused on early-age strength development, and very few examined the effect of long-term curing (56, 90, and 180 days) on mechanical properties [86,311,328,353]. Although the scope of the studies was limited to short-term analysis for mechanical properties, durability properties such as acid and sulfate resistance, chloride resistance were examined in depth for variables like precursor type, SH concentration, and SH/SS ratios.

Considering the methodological differences across studies, this review identifies prevailing trends and evidence-based patterns that provide valuable insights while recognizing system-specific variations. These findings emphasize the importance of improving experimental consistency across future studies, particularly in reporting mix proportions, activator compositions, and precursor characteristics. Enhancing methodological transparency and data comparability will enable more robust evaluations of mechanical and durability performance across different GPC systems.

8. Cost and Sustainability Analysis of GPC

This section examines the economic and environmental performance of GPC along with its variants, in comparison to the OPC concrete matrix. Life Cycle Assessment (LCA) is commonly employed to evaluate the environmental impacts of both OPC concrete and GPC variants, while life cycle cost analysis (LCCA) is used to assess the economic feasibility of GPC mixtures. Although numerous academic studies have explored the environmental impact of OPC concrete, the LCA and LCCA of GPC variants relative to OPC mixes remain underexplored. Although a comprehensive regional market analysis was beyond the scope of this review, it is important to note that precursor availability and logistics vary significantly across regions. For instance, fly ash remains more abundant in Asia due to continued coal combustion, whereas its supply is declining in the United States and parts of Europe following coal plant retirements. Slag availability is more stable in industrialized regions with active steel production. These regional variations influence both cost and sustainability metrics, highlighting the need for future LCAs and techno-economic studies to consider localized supply chain effects. From the case study analyses, the findings in this section emphasize the importance of selecting appropriate precursor materials and optimizing processing conditions, while also considering regional availability and logistics, to enhance the adoption of GPC as a viable alternative to OPC concrete.

8.1. Regional Variation in Production Cost of GPC and OPC Concrete

To understand the regional variation in the production cost of OPC concrete and GPC mixtures, a simplified cost analysis was performed. For consistency, 1 m³ of concrete with equivalent compressive strength ranging between 40 and 42 MPa was considered as the functional unit. Mixtures with equivalent compressive strength ranges are assumed to have similar functions and, thus, were compared together. However, the main purpose of this analysis was to showcase the regional variability of material costs and thus the variability in the production cost of GPC and OPC. The unit prices of the constituents of GPC and OPC for different regions were collected from the literature [355]. Subsequently, these values were multiplied by the amount of each constituent to obtain the total cost of each mixture and to estimate the regional variations. For this simplified cost analysis, only the prices of distinct constituent materials (i.e., OPC, FA, slag, SH, and SS) were considered as the main variables, while energy cost, labor cost, and transportation cost were not included. Figure 16 shows the regional price variation in OPC and GPC mixtures, and it is evident that, irrespective of the regional variation, GPC mixtures are almost one-and-a-half times as costly as their OPC-based counterparts on average for the markets of the USA and China. From the figure, it is also evident that, for FA-based GPC mixtures, the usage of sodium silicate is the main reason for making them the most costly mixtures. This cost disparity suggests that, while GPC presents significant environmental benefits, its higher production cost could be a major barrier to widespread adoption.

In addition to the above-mentioned simplified cost analysis based on material price, the overall cost associated with the production of 1 m³ of GPC and OPC concrete can be sensitive to the costs associated with transportation of raw materials and production of concrete, which can vary significantly by regional location. It has been reported in the literature that considering the long-distance import of GGBFS surpasses the environmental impacts related to OPC in 15 categories [356]. Therefore, it can be understood that, alongside environmental impacts, the cost of precursors can be influenced by location and transportation requirements.

8.2. Case Studies Sustainability Analysis

Table 2. Statistics of FA, slag, FA-GP, and slag-GP (USA) [355].

Annual Concrete Production (Mt/Year)	89
FA-Based GPC
FA requirement for 100% replacement of OPC (470 kg/m3)—(Mt/year)	42
Annual FA available for Use—Mt/year	156
Surplus (+)/Deficit (−)—Mt/year	+114
FA depletion time with 100% OPC replacement (years)	3214
FA Price/Ton (USD)	34
FA-based GPC cement Price/Ton (USD) [357]	100
FA-based GPC (Price/Ton) (USD) [357]	110–120
Slag-Based GPC
Slag requirement for 100% replacement of OPC (450 kg/m3)—(Mt/year)	40
Annual slag production—Mt/year	20
Annual slag production available for use (Mt/year)	0.98
Surplus (+)/deficit (−)—Mt/year	−39
Possible % replacement of OPC with slag-GPC at current supply	2%
Slag Price/Ton (USD)	16–72
Slag-based GPC cement price/Ton (USD) [357]
Slag-based GPC (price/Ton) (USD) [357]	130–140
% OPC Replacement Possible with current Na2SiO3 global market	11%
Current Market Price of Na2SiO3/ton (USD)	180–290
% OPC replacement possible with current NaOH global market	75%
Current market price of NaOH/ton (USD)	770–920
Cost of 1 ton OPC Concrete (USD) [357]	60–65

shows the data statistics, including consumption rate, price and global supply and demand of FA, slag, FA-based GPC, and slag-based GPC. The study shows that FA-based GPC presents the most promising alternative to OPC concrete, given its widespread availability and potential to eliminate a material that poses considerable environmental and health risks. Although the production rate of slag is lower than that of FA, with slag-based GPC replacing just 6.5% of annual OPC production. The analysis, however, indicates that slag is a more favorable option due to its cost-effectiveness, improved workability (minimizing segregation), and improvements in long-term performance. Notably, the high production cost of the activators (SS and SH) remains a challenge for these GPC mixes, requiring further technological advancements to reduce both activator costs and production energy.

The comparative environmental evaluation of OPC and GPC produced from FA and slag, based on recent studies [358,359,360], was investigated.

Table 3. Environmental Impact Indicators for FA-GPC, slag-GPC, and OPC concrete (functional unit 1 m³).

Cradle to Gate	Sample Types	Freshwater Toxicity (Kg 1,4-DB eq)	Energy Requirement (MJ/m3)	Climate Change (Kg.CO2-Eq)	Human Toxicity (kg 1,4 DB-Eq)	Ozone Depletion (Kg CFC-11-Eq)	Acidification (kg SO2 eq)	References
	OPC concrete	17.76	3550	1162	392	0.000039	4	[358,359]
	FA-based GPC	12.41–15.10	2400–2660	646–774	285–344	0.0000255	2.57–3.07	[358,359]
	Change (FA GPC–OPC)	(−2.66)–(−5.35)	(−890)–(−1550)	(−388)–(516)	(−48)–(−107)	−0.0000125	(−0.93)–(−1.43)
	OPC CML Method	139.53	4662.7	742.88	187	2.33 × 10−13	1.65	[360]
	Slag-based GPC CML Method	100	1780.29	172.37	150.26	4.54 × 10−13	0.93	[360]
	Change (GPC–OPC)	−39.53	−2882.4	−570.51	−36.74	+2.21 × 10−13	−0.72

presents the sustainability analysis and environmental impact, comparing FA-GPC, slag-GPC, and OPC. The functional unit is defined as 1 m³ of concrete with specified compressive strength. All analyses conducted in each study used cradle-to-gate LCA methodology, considering freshwater and toxicity, climate change, ozone depletion, acidification, energy consumption, and GHG emissions for all phases. As shown in Table 3, the analysis indicates that slag-based GPC has lower environmental impacts than FA-based GPC. A negative percent change signifies a reduction in economic, social, or environmental burdens. For instance, the energy consumption and CO₂ emissions for slag-based GPC are 33% and 31% lower than those of FA-based GPC and 62% and 77% lower than OPC mixes. This is because the use of slag reduces reliance on sodium silicate, further decreasing the energy demand for GPC production. In GPC mixes, the higher global warming potential and CO₂ emissions compared to OPC are primarily due to the high energy demand of sodium silicate production (the commonplace activator). This solution has major environmental impacts (such as terrestrial ecotoxicity, ozone layer depletion, and human, freshwater, and marine toxicity) on their long-term sustainability benefit [361]. This activating solution also limits its sustainability benefits due to the required high curing heat, difficulties in mixing and field handling, viscous nature, and high cost [362,363]. In a previous study, it was reported that the burden of sodium silicate ranges between 55 and 74% in all impact categories considered in an LCA study of geopolymer blocks, and that the indirect use of SS and the direct use of SH are the main contributors to environmental burdens [364]. In the same study, it was discussed that the intensity of the environmental burden depends on the production process of the activators. The production of NaOH is considered the most energy- and cost-intensive process, and this arguably controls the LCA of the geopolymer concrete system. The other constituent materials affect the environment in their individual ways, while NaOH’s production affects most of them [364]. Arguably, incorporating slag cement can mitigate this impact by reducing the required amount of sodium silicate, thereby lowering overall energy consumption and emissions. Another approach to overcome the impacts related to activator utilization is the development and use of waste-based solid activators in one-part (“just add water”) GPC, which represents a promising alternative, as it can reduce embodied carbon, simplify handling, and enhance practical applicability for large-scale construction.

9. Conclusions and Future Research

9.1. Conclusions

While numerous review papers have been conducted on GPC, there remains a lack of systematic studies comparing GPC made from different precursors, particularly FA and slag. Additionally, recent studies over the past decade have largely overlooked a comprehensive approach that not only examines the engineering properties of GPC but also considers its sustainability aspects. To address these gaps, this review critically evaluates all key branches of the GPC system, including physico-chemical properties, mechanical performance, durability, and sustainability considerations. This study aims to provide a foundation for developing or proposing a net-zero GPC, ensuring both structural efficiency and environmental viability. The key findings from this study are summarized as follows:

• GPC is synthesized by activating aluminosilicate precursors to produce N-A-S-H and C-A-S-H gels, which govern strength and durability. Low-Ca precursors such as FA predominantly form N-A-S-H, while high-Ca precursors like slag promote C-A-S-H gel formation, resulting in a denser microstructure and higher strength. However, this can also increase the risk of cracking due to the rapid geopolymerization process.
• Achieving optimal physical properties for net-zero GPC requires careful mix design, with particular attention to the SS/SH ratio. Characterization studies confirm that FA-based GPC primarily forms N-A-S-H gels, while slag-based GPC produces C-A-S-H phases, impacting strength and durability. These differences influence the reaction kinetics, gel structure, and thermal stability of the resulting matrix.
• The workability and mechanical properties of GPC, including compressive strength, tensile strength, and MOE, are significantly influenced by the A/B ratio. While high-Ca precursors improve mechanical strength due to the denser nature of C-A-S-H gels, they also accelerate setting time, reducing workability. Blending high- and low-Ca precursors and the use of admixtures can enhance workability without compromising strength, making GPC suitable for both precast and in situ applications.
• GPC exhibits superior resistance to acids, chlorides, sulfates, and corrosion compared to OPC concrete, owing to its aluminosilicate-based matrix that limits the formation of expansive degradation products. The durability of GPC is closely tied to its microstructural composition, and blending high- and low-Ca precursors can enhance its performance in aggressive environments. This strategic approach not only improves long-term durability but also supports the goal of achieving net-zero carbon GPC.
• The analysis, while assessing global feasibility, identifies FA-based GPC as the most viable alternative to OPC concrete due to its abundant global supply and potential to mitigate environmental and human health risks. However, sustainability assessments indicate that slag-based GPC exhibits lower overall environmental impacts compared to FA-based GPC. Using slag reduces reliance on SS, further decreasing the energy demand for GPC production.
• The review indicates that, based on precursor availability, curing techniques, and exposure environment, mass-level production of geopolymer concrete is feasible. However, adoption should be gradual—aligned with project-specific needs and progressing toward performance-based applications. In the meantime, alternative precursor materials are needed to address shortages of conventional precursors, and parallel research on new activators should continue.

9.2. Future Research

As GPC continues to emerge as a viable net-zero carbon alternative to OPC, several challenges must be addressed to enhance its sustainability, performance, and large-scale adoption. Future research should focus on optimizing material formulations, improving mechanical and durability properties, and refining sustainability strategies to further reduce the carbon footprint of GPC. The following research directions are essential in achieving these goals:

• With the declining availability of conventional precursors like FA and slag, future research should focus on identifying and optimizing alternative SCMs to ensure long-term sustainability. Potential sources such as agricultural residues, waste glass, and industrial byproducts offer promising low-carbon alternatives due to their abundance and reactivity. Investigating their geopolymerization kinetics, compatibility with different activators, and long-term performance is critical for developing sustainable, widely available, and cost-effective GPC binders that support carbon neutrality in construction.
• Optimizing GPC mix design through precise control of A/B and SS/SH ratios for given precursors is crucial to achieving net-zero carbon goals while ensuring high-performance standards. Future studies should focus on a systematic approach that tailors A/B and SS/SH ratios based on the specific precursors used to enhance workability, strength, and durability.
• While GPC is a more sustainable alternative to OPC, its feasibility for large-scale OPC replacement remains limited in many regions worldwide. Thus, future research should focus on minimizing the use of SH and SS in GPC activators due to their high cost and limited sustainability. The development and use of waste-based solid activators in one-part (“just add water”) GPC represents a promising alternative, as it can reduce embodied carbon, simplify handling, and enhance practical applicability for large-scale construction.
• Future research should explore emerging innovations such as CO₂ curing and AI-driven mix design. CO₂ curing has shown potential to enhance early-age strength, densify the GPC matrix, and reduce overall carbon emissions by promoting controlled carbonation. Similarly, artificial intelligence and machine learning techniques can be leveraged to optimize mix design by predicting the performance of GPC based on input variables such as precursor type, activator ratio, and curing conditions. These tools can minimize experimental trial-and-error and accelerate the development of high-performance, net-zero GPC systems.