1 Introduction

Portland Cement is the most extensively adopted artificial binder for concrete composite, and Ordinary Portland cement concrete (OPC) is the most widely used building material universally [1]. According to the Cement Industrial Report for 2022, global cement production was estimated to be about 4.16 billion metric tons [2]. The scale of cement industry is concerning given that the cement industry is one of the largest emitters of carbon dioxide (CO2) worldwide [3], accounting for around 8% of global greenhouse gas emissions, according to the International Energy Agency (IEA) [4]. The CO2 is primarily released from the calcination process during cement manufacture, which involves heating limestone to high temperatures to produce clinker, the main ingredient in cement. In addition, the cement industry is also dependent on fossil fuels for energy, which further contributes to its carbon footprint. The greenhouse gases (predominantly CO2) released from fossil fuel combustion needed for the cement production has become the chief anthropogenic contributor to global warming in the past few generations [5]. With increasing alarms over the environmental pollution caused by the production of OPC, there has been continuing research to find alternative concrete binders. Research has shown that geopolymers are a promising replacement for OPC [6].

The nomenclature "geopolymer" represents the artificial aluminosilicates with a structure similar to naturally formed zeolites [7, 8]. The material commonly used to produce geopolymer is a natural or artificial minerals enriched in amorphous SiO2 and Al2O3, such as metakaolin and low-calcium (class F) fly ash [9]. Geopolymers are produced through a process that starts from the activating of the reactive SiO2 and Al2O3 with alkaline activators [10], in which the glassy structures in the source material are reconected to very cemented, well-compacted composites [11, 12]. The reaction between the aluminosilicate materials and the alkaline solution results in the formation of a polymeric chain structure that replaces the traditional cementitious binding agents like calcium silicate hydrates (C-S–H) and calcium hydroxide (Ca(OH)2, CH) [13]. It has lower pH than OPC and is free of calcium hydroxide. The basic building blocks in this polymer chains are Si–O-Al bonds. These chains are interconnected by bridging oxygen atoms to form a three-dimensional, amorphous or non-crystalline geopolymer network structure [10, 14,15,16]. Meanwhile, due to the absence of water, the geopolymerization process lacks crystallinity [14].

Geopolymers possess numerous advantages as a concrete binder due to their distinct chemical and physical composition [16, 17]. For instance, the polymeric chain and amorphous structure of geopolymer produce strong adhesion with aggregates [18] and, thus consequently provide dense concrete with lower porosity that leads to superior tension and penertration resistance than traditional OPC [16]. According to current experimental results, the Geopolymer Concrete (GPC) showed comparable engineering properties to those of OPC. It has been shown in previous research that GPC outperforms OPC in terms of compressive strength [19], bending resistance [20], and bond with reinforcing components [21]. It is worth noting that the chemical structure of GPC grants it with plenty of advantages, such as the defense against acid [22] and sulfate attacks [23], as well as the chloride [24], conductivity [25] and thermal insulation [26], which indicates a great potential for application in a marine environment and, chemical and metallurgical industries [27].

The favorable mechanical strength and durability resistance mentioned previously have led to geopolymer being widely recognized as one of the most highly anticipated eco-friendly construction materials for the future. The reaction process [28], the chemical structures [29], the mechanical properties [20, 30], and the durability of geopolymers [27] have been through intense experimental and theoretical studies. Nevertheless, there is no consensus on the proper manufacturing process and bond evaluation methods between geopolymer and reinforcing bars.

The production of geopolymer differs from Portland cement, with the latter being a well-developed and mature technique supported by an entire industry. In contrast, the former is relatively new and has mainly been explored at the individual level, primarily within laboratory settings. The utilization of complex mineral sources and intricate curing procedures in geopolymer production has heightened the importance of quality control and the challenges of large-scale GPC manufacturing.

Bond, is the most crucial interaction between steel or other reinforcing bars and concrete [31]. It transfers the stress between the two materials, steel and the concrete (Fig. 1). Bond allows the two distinct materials to act together as a single unit [32]. An inadequate bond between reinforcement and concrete may lead to a slip of reinforcement and a reduction in the strength and stiffness of the structure. In extreme cases, it can even lead to structural failure. Bond mainly comes from chemical adhesion, friction, and mechanical interlock between reinforcement and concrete. The change in concrete type and composition can affect all three factors (listed in Table 1) [33].

Fig. 1
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Bond mechanism (a) chemical adhesion, b friction, and c mechanical interlock [34]

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Table 1 Composition of bond force
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From the analysis of the source of bond, the bond between reinforcement and concrete can be influenced by several factors, including the surface condition of the reinforcement, rib design, embedment length, concrete quality, curing conditions, and confinements. Numerous attempts have been made to enhance the bond between bars and concrete, with the most effective approach being the addition of fibers. Fiber-reinforced concrete (FRC) offers several benefits regarding bond with reinforcement. Firstly, increased interfacial bond: Fibers, such as steel or synthetic fibers, distributed throughout the concrete matrix can improve the bond between the reinforcement and concrete. The fibers act as additional points of anchorage and help in transferring stresses between the two materials [32]. Secondly, Crack control: FRC exhibits improved crack control compared to conventional concrete. The presence of fibers helps to distribute and control cracks that may form in the concrete, reducing the likelihood of cracks propagating and affecting the bond between reinforcement and concrete.

Understanding the bond between reinforcement and geopolymer concrete is crucial for ensuring the structural performance and integrity of geopolymer concrete structures. The bond directly affects the load transfer capacity [35], crack resistance [36], and overall strength of the reinforced geopolymer concrete elements [33]. The bond behaviour between reinforcement and geopolymer concrete plays a vital role in the design and optimization of reinforced geopolymer concrete structures. The bond strength, bond stress distribution, and development length allow engineers to accurately predict the behaviour of these structures under various loading conditions. As geopolymer concrete gains prominence as a sustainable construction material, it becomes essential to develop standardized testing methods, guidelines, and design codes. Studying the bond behaviour contributes is the precondition to the development of reliable standards and specifications for reinforced geopolymer concrete.

Meanwhile, classifying the standard production procedure is essential as a prerequisite for any mechanical testing. Given the importance of intensive production and bond studies for engineering applications of GPC, the current lack of research in this area is a critical issue. To fill this gap, this paper summarizes and discusses current studies on the production process, mechanical properties, and bond performance of GPC. In summary, this paper aims to discuss the potential for geopolymer concrete to create a reliable and compatible dual system with reinforcing bars, which could greatly benefit the production and construction of engineering projects in light of global warming challenges.

2 Production and properties of geopolymer and GPC

In 1978 Davidovits invented a term ‘geopolymer’ to identify the unique artificial aluminosilicates with similar micro structures to the zeolites, a material widely founded in natural [37]. Since then, geopolymer binders, as burgeoning constituents within the field of inorganic polymer binders, have garnered attention within industrial construction systems and are widely regarded as prospective binding materials for the concrete industry.

As shown in Fig. 2 the process of polymerization that occurs in silicate/aluminate-rich materials under the exist of alkaline solutions is typically referred to as geopolymerization [37,38,39]. As discussed in the introduction, this reaction produced a tri-dimensional micro structurals on the backbone of Al and Si ions [40]. The function of the source material is to provide SiO2 and Al2O3, while the alkali activator's role is to break the Si–O and Al-O bonds and accelerate the formation of the tetrahedral framework in the geopolymer products [41]. Geopolymer binder differs from ordinary Portland cement in terms of its source materials, reaction mechanism, and resulting products. Therefore, the feedstock, mix design, manufacturing processes, and properties of GPC differ significantly from OPC.

Fig. 2
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Processes of geopolymerization from FA [39]

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2.1 Materials and synthetic theory

The alkali activator adopted in geopolymerization is typically a combination of alkali metal hydroxides (e.g. sodium hydroxide, potassium hydroxide) and/or alkali metal silicates (e.g. sodium silicate, potassium silicate) [42]. In laboratory settings, sodium and/or potassium silicate (Na2SiO3, Ka2SiO3) and sodium and/or potassium hydroxide (NaOH, KaOH) are typically pre-mixed before being added to the source material and casting process. This combination of alkali activators helps initiate the geopolymerization reaction and facilitate the formation of the polymeric gel matrix [43].

The raw materials used in the manufacturing of geopolymers could be rock-forming minerals metakaolin, melanite, kaolinite and clays [44] or industrial solid wastes, such as waste glass, red mud, rice-husk ash, and fly ash [45].

When it comes to the production materials and methods, there are, in fact, two established models for achieving alkaline-activated cement (Fig. 3) [45, 46]. When using source materials that mainly contain silicon and aluminum, such as low-calcium fly ash and metakaolin, and relying on a high concentrated alkaline solution to produce geopolymers, the primary chemical process is polymerization. As a result, this model is also referred to as the polymeric model [43] or the “low calcium” model. On the contrery, for the other model, the mineral materias are silicon and high calcium sources, such as Ground Granulated Blast Furnace Slags (GGBS) or high calcium-fly ash (class F FA). The activator in this model is low to mildly concentrated alkaline solutions. In the second model, there are a series of more complicated reactions, including both hydration and polycondensation, occurred. In this “high calcium” model, the primary reaction product is believed to be calcium silicate hydrate (C-S–H), rather than inorganic polymers as in the first model [30, 47, 48]. In the second model, the combination of high calcium and low calcium materials can be used to produce alkali-activated cement, as the reactions involved can accelerate the geopolymerization process [49, 50]. As this paper focuses on the research and development of geopolymers, the following sections will primarily describe the material and manufacturing processes used in the first model.

Fig. 3
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The two models of making alkali activated cement

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2.1.1 Metakaolin

Metakaolin is a type of clay mineral formed by the calcination of kaolin clay at temperatures between 500°C and 800°C [51]. The resulting metakaolin has a high content of reactive silica and alumina, which makes it an ideal source material for geopolymerization. Metakaolin-based geopolymers have been shown to exhibit high compressive strength, low shrinkage, and good durability, making them suitable for a wide range of applications in construction and infrastructure [26, 51, 52]. The alkali activator used in the production of metakaolin-based geopolymers is typically a combination of high concentrated alkali solution blended with sodium silicate and sodium hydroxide, although other combinations of alkali activators may also be used depending on the specific application requirements.

2.1.2 Low calcium fly ash

Fly ash is the main solid residues generated during coal combustion, which is collected by a dust collection system in the power generation or smelting industry. When released into the environment, fly ash can lead to various adverse effects [53]. Firstly, it contains toxic elements such as mercury, lead, and arsenic, which can contaminate soil and water sources, posing serious health risks to both humans and wildlife [54]. Inhalation of fly ash particles can also cause respiratory problems and lung diseases in individuals living nearby [55, 56]. Fly ash can be used in construction industry with a variety of applications. However, only a small portion of the total fly ash produced is currently being reused, while the majority is still disposed of in landfills or surface impoundments.

The challenging disposal rate of fly ash causes environmental and sociological problems in many countries worldwide. The disposal rate of fly ash varies depending on the region and the regulations in place. In the United States, a significant portion of fly ash is disposed of in landfills. According to the American Coal Ash Association, the disposal rate of fly ash in landfills in the US was approximately 32.9% in 2020 [57], maning more than 9.2 million tonnes of fly ash was discard carelessly. The waste disposal problem in developing countries is even more severe [58]. For instance, the fly production of India is more than 200 million tonnes annually, of which approximately 60 million is abandoned [59]. In China, despite a re-use rate of up to 65% for fly ash, the sheer magnitude of fly ash output reached a staggering 572 million tons in 2020. As a result, China is currently facing a even severe waste disposal problem compared to other countries [60].

As shown by Table 2 and Fig. 4, the chemical ingredients of low calcium fly ash is mainly oxides of silicon (SiO2), aluminium (Al2O3), iron (Fe2O3), and calcium (CaO), with magnesium(Mg), potassium (K), sodium (N), and sulphur (S) present in smaller amounts [61, 62]. The various types of fly ashes could be divided into two basic categories according to their calcium contents: ASTM class F fly ash (low-calcium fly ash, contains less than 10% of CaO) and ASTM class C fly ash (high-calcium fly ash, contains more than 10% of CaO), respectively [63]. The majority of class C fly ash consists of silica and alumina, accounting for 80% to 85% of the total weight [64]. The high amounts of amorphous silica and alumina composites make it a suitable industrial waste for manufacturing geopolymers [65]. Class F fly ash-based geopolymers are synthesized by mixing the fly ash with alkaline activators, which dissolve the glassy sphere shells of the fly ash and generate inorganic polymer structures [45]. The physic-chemical structure of alkaline-activated fly-ash geopolymers provides them with many outstanding binder characteristics.

Table 2 Chemical composition of class F fly ash determined by XRF [62]
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Fig. 4
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XRD results of FA1, FA2 and FA3 (with 20% Al2O3 as spike phase) [62]

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The relatively small particle size and spherical smooth surface of fly ash help it to act as a filler for voids in the composite [66]. The previous experimental research has illustrated that class F fly ash-based GPC can achieve superb engineering characteristics when compared to OPC. This is especially the case for class F fly ash-based GPC, which produces dense concrete with better tensile resistance [16], compressive strength under extreme temperature [42], bending resistance [67], and bond stiffness with reinforcing bars [68, 69]. In addition, the microstructure of fly ash-based geopolymers provides them with many outstanding resistances in severe environments, such as defense against acid attacks [70] and sulfate attacks [71], as well as chloride [72], and fire resistance [73, 74], offering immense potential for their use in marine, nuclear, and military industries [75].

In contrast to binders derived from natural aluminum–silicon materials, geopolymers created using class F fly ash exhibit significantly enhanced sustainability and energy efficiency. Geopolymer production plays a crucial role in transforming this waste by-product into a valuable construction material. Geopolymers derived from fly ash necessitate a reduced amount of sodium silicate solution, which is known to be a high-energy-consuming chemical [8, 45]. Thus, they have a lower environmental impact than a GPC made from natural minerals such as metakaolin [66]. As heat curing is required for fly ash geopolymer, it has the most potential to replace OPC as a dominant concrete binder in the pre-cast concrete industry. In addition to its significant contributions to waste disposal, CO2 emissions reduction, and energy consumption, class F fly ash geopolymer is favored in laboratories for its proving to be a relatively stable and repeatable mix in experiments, such as the pull-out bond test. This makes it an important choice, not only for its environmental benefits but also for its suitability in both industry and research applications.

2.1.3 Ground granulated blast furnace slag (GGBS) and class C fly ash

As previously discussed, Ground granulated blast furnace slag (GGBS) and Class C fly ash contains a high amount of CaO and is typically used to produce geopolymer products in conjunction with class F fly ash or other pozzolanic materials. The specific blend ratios are typically chosen based on the specific requirements of the concretes. As the hydration process in GGBS or class C fly ash does not require heating, alkali-activated high calcium minerals can achieve the designed strength under ambient curing conditions [76]. Therefore, adding a suitable amount of these minerals can aid in fabricating ambient curing specimens. Furthermore, the addition of GGBS or class C fly ash results in the rapid formation of calcium aluminate hydrate and other calcium compounds. These additions further enhance the geopolymerization process, leading to an increase in the early strength of alkali-activated cement under ambient curing conditions. This is attributed to the within a short period. This is attributed to the. Consequently, these samples are suitable for studies requiring rapid early strength development [77]. However, the chemical reaction responsible for generating calcium compounds also triggers a rapid setting process that renders fresh concrete unworkable within a matter of minutes [78]. High calcium additives could help to adjust the setting time of class F fly ash-based geopolymers [79]. Specifically, when it comes to the complex casting conditions such as in the bond tests or the densely reinforced member, it is worth noting that they typically require a longer handling period from the time of fresh concrete production and larger sample sizes. In these cased, low calcium material is a better source material, as it can provide comparable strength and satisfactory workability. On the contrary, in the case of urgent construction or repair, high calcium concrete is needed.

2.2 Mix design and manufacture

Compared to OPC, the synthesis of GPC is typically a more involved and complex process as shown in Fig. 5. Conducting experiments to evaluate the mechanical and bond properties of GPC can be challenging, especially when it comes to obtaining stable mixes for large samples with consistent parameters. GPC is sensitive to changes in mix proportions and materials than OPC, which make it difficult to achieve reliable and consistent results. In addition, the use of recycled or alternative materials in GPC introduced additional variability, which further complicated the process [69].

Fig. 5
figure 5

The synthesis process of GPC

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2.2.1 The properties of source materials

First, understanding the properties of the mineral materials is crucial in determining the appropriate mix design and producing consistent geopolymer products. Unlike Portland cement, which is produced using a standardized process, geopolymer binders rely on minimally processed natural materials or industrial by-products that are usually collected [80], whose chemical compositions vary widely. Therefore, it is necessary to examine the chemical properties of the source materials to provide data for the mix design and casting of GPC. Additionally, other factors, such as the type and concentration of alkali activators, the particle size and the use of superplasticizers, all together affect mix properties. Palomo et al. [81] conducted a study on the transformation of micro-structure of alkali-activated fly ash during the geopolymerization process and demonstrated that the activation rates and chemical structures are influenced by the particle size distribution and chemical compositions of the fly ash, the concentration of alkali and the total Si/Al ratio used in the reaction [82, 83].

Except the special heat curing process, GPC mix require similar consideration to that has long been used for OPC, which is a compromise between the compressive strength and workability. Except that the strength of GPC depends on the relative proportions of the alkaline liquid to solid minerals, rather than the water to cement ratio [84]. In general terms, the compressive strength of GPC grows with the increase of the alkaline liquid to solid binder ratio and lowers with an increase in the water to alkaline liquid or solid binder ratio. On the other hand, the workability increases with an increase in the water to alkaline liquid or solid binder ratio [85]. Moreover, similar to OPC, the strength and workability of fly ash-based Geopolymer Concrete (GPC) are greatly influenced by the quality and grading of aggregates. According to Hardjito, the coarse and fine aggregates also make up around 75% to 80% of the GPC's mass [66].

Besides, the strength of GPC is closely related to the molar ratio of Si to Al present in the alkaline solution. Increasing the amount of silicate ions in the alkali activator has been shown to substantially improve the final strength of the hardened products [86]. Lee and Van Deventer [87] investigated the Interfacial Transition Zone (ITZ) between the binder and aggregates in hardened GPC and discovered that the presence of soluble silicates in the activating solution can enhance the interfacial bond quality between the binder and aggregates. By increasing the soluble silicate dosage, a denser binder and more stable aggregate/binder interfaces were formed. Hence, sodium silicate and other silicate solutions are frequently incorporated to enhance the ultimate durability of GPC [42, 88, 89].

2.2.2 The curing regimes

Besides, the final compressive strengths fly ash-based geopolymer binder also depend on the curing regimes significantly [83]. Within a range, increasing the curing temperature and duration contributes to the growth of compressive strength of fly ash-based geopolymer concrete [82, 83]. The findings of Sindhunata et al. [90] indicate that the formation of amorphous products can be enhanced under a higher curing temperature.

Extensive studies have been undertaken to establish the most suitable curing temperature and duration for GPC. Memon et al. [91] reported that 70 °C cured GPC specimens exhibited the best compressive strengths compared to those heated at 60°C, 80°C, and 90°C for the same duration. Hardjito et al. [87] conducted a study to examine the variations in compressive strengths of GPC cylinders cured at temperatures at 30°C, 60°C, 80°C, 100°C and 120°C for 6 to 108 h. They suggested that a suitable duration for heat curing is 24 h. Fernandez-Jimenez et al. [92] found that GPC samples heated at 85°C for 20 h after casting exhibited high compressive strength. Satpute Manesh et al. [93] conducted a study on GPC samples heated at different temperatures ranging from 30°C to 120°C for varying periods (6, 12, 16, 20, 24 and 72 h). The results indicated that significant growth in compressive strength occurred only in the initial stages, with increases after 16 h of heating being insignificant. Therefore, they suggested minimize the 24 h’s heating duration to 16 h to reduce the energy consumption during the curing process. On the contrary, there are some researches that proved longer heating durations lead to higher strength [94]. For example, Fig. 6 shows that the sample heated for 72 h had a denser micro morphology, more geopolymer products, and higher compressive strength [94]. According to the above research, there is no agreement on the curing temperature and duration for GPC. It has been suggested that the curing temperature range is up to 90°C and the curing duration is 16–48 h [82, 83].

Fig. 6
figure 6

The influence of curing durations of the strength development and micro morphology of class F fly ash GPC

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Additionally, it has been pointed out that a pre-24 h’ of 'resting time' at ambient temperature before heat curing improved the final strength of GPC samples [95]. Although Bakharev suggested that samples activated with a sodium silicate solution are suitable for 6 h of heat curing, rather than 24 h Bakharev [96]. Nevertheless, regarding the curing duration, further research is needed to reach a uniform criterion.

2.2.3 The additives

Lloyd and Rangan [97] reported that, similar to OPC, the mobility of fresh GPC increased with the total water content in the mixtures. Chindaprasirt et al. [98] suggested that commercial polycarboxylate superplasticizers can also improve the workability of high-calcium fly ash geopolymers, however, the rheology study of low-calcium fly ash geopolymers showed different resuls [99] Cui suggested that the naphthalene plasticizer is a better choice for GPC and the optimal dosage of superplasticizer is 2–6% of the binder content and, a higher dosage did not significantly contribute to workability [100].

According to the findings of current research, it is clear that while mix design and casting of GPC may not be particularly challenging, ensuring quality control of this material demands significant expertise. Working with an unfamiliar material that requires the use of collected minerals, high-alkali liquids, minimal water, and precise curing techniques can make testing with GPC mix a daunting task. As such, the primary challenge in conducting mechanical and bond tests on GPC is to establish a stable mix and reliable manufacturing procedure. To ensure uniform requirements in the manufacturing of test samples, such as in bond tests, it is strongly recommended to maintain a constant mix design and curing regimes.

2.3 Engineeringproperties

The mechanical properties of GPC products, including compressive strength, indirect tensile strength, modulus of elasticity, and Poisson’s ratio are the basic index that evaluate the quality of mixes [101].

2.3.1 Compressive strength

First, geopolymer products can achieve the designed strength much earlier than cement composites [42, 102, 103]. As a result, the compressive strength of GPC is often characterized by the 7-day strength rather than the conventional 28-day strength [104,105,106]. Investigative studies have delved into understanding the influence of diverse parameters on the compressive strength of geopolymer concrete, including the selection and proportion of source materials (e.g., fly ash, slag, red mud [107] and metakaolin) [79], the concentration and type of alkali activators [30], curing methods [83], and the incorporation of various additives (such as fibers [108] and nano-materials [109]) to enhance performance. The compressive strength of GPC mix could exceed 200 MPa easily with proper design and curing [110]. Second, the compressive behaviour and failure criteria of fly ash-based GPC are resembled that of OPC [42]. The compressive stress–strain relations of GPC under compression demonstrates a top strain range of 0.0024 to 0.0026 [89] and the constitutive model derived from Collins et al.’s research [111] on OPC could be employed to estimate the stress–strain behaviour of GPC under compression [112,113,114].

2.3.2 Tensile strength

The tensile strength of GPC is mainly examined through the splitting tensile strength test [115, 116]. The experimental results shows that the factors influencing the tensile strength of GPC are similar to those affecting its compressive strength [102, 104]. Specially, the strong adhesion with steel fibers enables the steel fiber reinforced GPC to achieve a high tensile strain capacity over 7.5% [117]. Several studies have highlighted the significant influence of the geopolymerization process on enhancing the tensile strength of the material [30, 62, 76, 79, 106, 118, 119]. Cui [62] incorporated the available experimental data to establish correlations between compressive strength and splitting tensile strength of GPC specimens. Figure 7 clearly shows that when the GPC and OPC have no significant difference in compressive strength, GPC exhibits significantly higher splitting tensile strength. The superior tensile strength of GPC enables its potential application in a wide range of structural elements.

Fig. 7
figure 7

The correlation between fc and ft GPC [62]

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In particular, the tensile strength and bond strength with reinforcing bars are positively correlated for geopolymer concrete. When an axial pull-out force is applied to a ribbed reinforcing bar embedded in concrete, the angled ribs impose radial forces outward on the surrounding concrete. This radial component tries to split the concrete matrix perpendicular to the bar axis. The tensile strength of the concrete resists these radial forces and prevents splitting cracks from developing. The superior tensile strength exhibited by GPC compared to conventional concrete indicates it can achieve stronger bond performance.

2.3.3 Yong’s modulus

In a separate set of experiments, Hardjito observed that the Yong’s modulus of elasticity of fly ash-based GPC specimens demonstrated an upward trend with increasing compressive strengths. The Yong’s modulus of GPC ranged from 23 to 31 GPa when the compressive strengths ranging from 44 to 89 MPa, which were comparatively lower than that exhibited by OPC with similar compressive strengths [118]. Sofi et al. [120] also found that the modulus of elasticity of GPC under compression is much smaller than that of OPC. From the above studies, it could be concluded that, the elastic modulus of GPC is lower than that of OPC. The experimental values in the current studies are all below to the prediction values obtained from the existing standard [121]. In line with this finding, when comparing the compressive strength and elastic modulus, of GPC and corresponding OPC specimens, Cui [62] concluded, based on the statistical test results, that when GPC and OPC exhibit similar compressive and tensile strength, the elastic modulus of GPC is significantly lower than that of OPC (Fig. 8).

Fig. 8
figure 8

The correlation between fc and Ec of GPC [62]

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2.3.4 Poisson's ratio

The experimental values of Poisson's ratio for GPC mixes showed an overall increase with the rise in compressive strength. These values ranged from 0.23 to 0.26 for compressive strengths of 35.2 MPa to 44.4 MPa, which were slightly higher than the commonly cited values for OPC with similar strength (typically ranging from 0.11 to 0.21).

The research presented has demonstrated that GPC is comparable or even superior to OPC in terms of early compressive strength and tensile strength. This offers improved engineering properties and expanded application potential for GPC. However, the mechanical performance of GPC is highly dependent on the manufacturing process used. A uniform production standard is sorely needed for this innovative concrete type to reach its full potential.

2.4 Durability

Geopolymer products are prized for their excellent volume stability and remarkable resistance to alkali-aggregate reaction [122]. Talling and Brandstetr [123] observed that alkaline-activated concrete was completely free from the alkali-aggregate reaction. Wallah [124] compared the drying shrinkage of GPC and OPC. Their findings showed that the heat-cured fly ash-based GPC exhibited very low drying shrinkage. The experimental shrinkage strain of GPC after one year was five to seven times smaller than the prediction value calculated using the empirical equation for the shrinkage of OPC. These studies clearly demonstrate the superior volume stability of geopolymers compared to OPC.

Meanwhile, the covalent bond in the micro framework of geopolymer provides it with outstanding chemical resistance. It is effective in resisting acid [70], sulphate attacks [125], and chloride penetration [126]. It is evident in immersion tests that geopolymers have exceptional acid resistance; the loss of mass is low when the residual compressive strength is high [127]. As observed by Davidovits et al. [15], the mass losses of GPC after four weeks immersion in a 5% sulphuric acid solution is less than 2%. Song et al. [128] also conducted acid immersion tests and illustrated that GPC remained structurally intact even after eight weeks in a 10% sulphuric acid solution, with only minor localized cracks observed. In contrast, OPC exhibited exposed aggregates within four weeks in the same acidic concentration, indicating significant durability degradation.

Moreover, steel bar reinforced GPC exhibits excellent corrosion resistance due to its superior resistance to chloride penetration [129, 130], which is known to be a major cause of reinforcement corrosion in traditional concrete structures. Kupwade-Patil and Allouche [72] followed the corrosion development of steel in low-calcium fly ash-based GPC and OPC under cyclic wet and dry NaCl solution. After 12 months, only minimal corrosion was observed on some GPC specimens on the interface between rebar and GPC matrix. In contrast, a lot of orange gross corrosion products were observed on most of the OPC specimens, indicating quite significant corrosion.

GPC has demonstrated impressive mechanical performance and exceptional stability in aggressive environments, making them a highly attractive option for various construction applications, such as transportation infrastructure, petroleum and chemical industry, and offshore engineering. However, to use them in reinforced structures, except the standardrizing the manufactruting process, the bond between this new type of concrete and reinforcement must be established and verified.

3 Bond properties between steel bar and geopolymer concrete

The bond between steel reinforcement and GPC has garnered increasing research attention in recent years due to the potential advantages of using geopolymer concrete in construction. The bond is actively studied by conducting standard bond tests commonly used in reinforced OPC research. Key characteristics examined include bond strength, bond stress distribution, the interfacial transition zone, and bond-slip curves, which represent targeted test data from bond assessments. Researchers also utilize bond testing to evaluate factors influencing bond performance in geopolymer concretes.

3.1 Bond test

These tests are used to evaluate the bond performance of reinforcements in GPC and identify the factors affecting the bond behaviour. As shown by Table 3, there are three types of bond tests: the direct pull-out (lollipop) test (Fig. 9), the beam test (Fig. 10), and the beam-end (ASTM A944) pull-out test (Fig. 11). The direct pull-out test has a straightforward mechanism for measuring bond by pulling the reinforcement bar directly out of a concrete cylinder or prism. This method is the most convenient and direct way to test the bond between the bar and concrete behavior and has been conducted to study the bond of reinforced GPC [131]. The beam test involves testing a reinforced concrete beam by applying a concentrated load to the mid-span and measuring the resulting bond stress distribution along the reinforcement bar. This method provides a realistic representation of the bond behaviour in real structures but requires a larger and more complex test setup. The beam-end pull-out test, which is a modified version of the beam test, involves using a beam specimen sized half along the longitudinal axis and testing the pull-out force of reinforcement. This method is simpler and easier to perform than the full beam test and also accurately represents the bond behaviour of flexural members. The two pull-out test methods have become the most commonly used techniques for assessing the bond-slip behavior of GPC due to their convenience, intuitive setup, and ability to easily obtain accurate results.

Table 3 Comparison of common test methods
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Fig. 9
figure 9

Direct pull-out test

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Fig. 10
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Beam test

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Fig. 11
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Beam-end test

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3.2 Bond failure criteria

There are three main bond failure modes in the pull-out tests—bar failure, concrete splitting, and pullout [132]. Bar failure occurs when the reinforcement yields excessively before the interface debonding, indicating the bond strength exceeded the bar's capacity. This is more likely with higher grade, more ductile bars. Concrete splitting happens when radial forces from the ribbed reinforcing bar overcome the concrete's tensile strength, causing cracks perpendicular to the bar. Concrete splitting failure is governed by the surface deformations of the reinforcing bars and the tensile strength of concrete; higher ribs or more pronounced surface protrusions generate larger radial forces that can split the surrounding concrete with low tensile strength. The considerably high tensile strength of GPC provides better splitting resistance with ribbed bars [133]. Pullout failure involves the bar debonding and sliding out without bar yielding or concrete splitting, signaling poor chemical/mechanical interlock at the interface. This usually occurs with smooth bars in low strength concrete. Pullout is also more likely with insufficient embedment length. Identifying the failure mode helps optimize design and avoid premature bond failures [134].

3.3 Bond strength and bond stress distribution

Bond stress is commonly determined as the shear stress, calculated by dividing the maximum pull-out force by the contact area between the bar and concrete. The highest bond stress calculated is called bond strength, which is an important bond characteristic. This definition is in accordance with standards such as ASTM D7913/D7913M-14 [135] or ACI 440.3R-04 [136]. However, this way of calculation is based on the assumption that the bond stresses developed along the embedded length are constant, while, in reality, the bond stress changes dramatically from the loaded end to the free end. Cui [69] attached eight strain gauges along the bond length of steel bars embedded in geopolymer concrete to measure the longitudinal variation in bond stress in GPC beams during beam-end pull-out tests. The experimental results demonstrated that GPC exhibits a unique bond stress distribution pattern that differs significantly from that of OPC. The variation arises from the geopolymer binder's strong adhesion and dense interfacial transition zone (ITZ), resulting in a higher initial bond stiffness that significantly improves crack restraint in reinforced structures [36]. The bond stress distribution in Fig. 12 showed that the strong adhesion between GPC and steel bars prevents debonding to a greater extent than in OPC. As seen in Fig. 5, this leads to a more even stress transfer from the loaded-end to the free-end. Research on the bond stress distribution of GPC is scarce, and experiments on the bond strength of reinforced GPC components have commonly assumed a uniform distribution of bond stress.

Fig. 12
figure 12

Bond stress distribution of GPC (up) and OPC (down) [69]

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When ribbed bars are used, it has been reported that the bond strength of GPC is marginally higher than OPC under comparable conditions. Sofi et al. [120] used 27 beam-end specimens and 58 ‘lolipop’ specimens to evaluate the bonding performance of GPC with reinforcement in the pull-out tests. The results showed that the bonding performance of GPC mixes was superior to OPC. Sarker [21] employed the beam-end pull-out test to study bond strength and found that GPC had higher bond strength than OPC. This finding was consistent with the results of Zheng and Xiao [137], Abdulrahman [106], Castel and Foster [138], Bilek et al. [139] and Zhang [140] who observed higher bond strength between ribbed steel bars and GPC than between steel bars and OPC at both medium and extremely high temperatures. Sofi [120] believed that the superior bond strength of reinforced GPC allows a shorter bond length than the lengths recommended by standard design equations (AS 3600 [121], EC2 [141], and ACI-318 [142]). When plain bars are used, the bond strength in GPC is twice that in OPC due to the robust adhesion provided by geopolymer binders, as discovered by Cui [68].

3.4 Interfacial transition zone (ITZ)

The ITZ is the region between the steel bar and the surrounding concrete material where the porosity and other properties are different from the bulk. In the case of OPC, the ITZ between the aggregates/steel bar and the surrounding binder is typically characterized by a porous and weak structure, which can result in lower bond strength between the steel bar and the surrounding concrete [143]. This is because the weak ITZ can allow for the formation of cracks, which can lead to debonding.

In contrast, the ITZ between the steel bar and GPC is denser and strongly bonded, due to the nature of the geopolymer binder [18, 144]. Geopolymer concrete has a different chemical composition [119, 145] and microstructure [144] than OPC, which leads to a stronger bond between the steel bar and the surrounding concrete. As seen from Fig. 13 [146], there are no crystalline or other weak layers observed in the ITZ between the geopolymer and steel bar. The strong ITZ in geopolymer concrete can limit the formation and propagation of cracks, resulting in a higher bond strength between the steel bar and the surrounding concrete.

Fig. 13
figure 13

The ITZ of geopolymer with steel [145]

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3.5 Bond-slip curves

Correlations between bond and slip are one of the most significant characteristics of bond behaviour. The bond-slip curves derived from pull-out tests serve a dual purpose by effectively demonstrating the fluctuations in bond stress with the increasing relative slippage between concrete and steel bars, as well as revealing the initial bond stiffness, bond strength, and residual bond stress. The bond-slip curves vary from the failure criteria. Figure 14 shows the theoretical bond-slip curves pointed out by Eligehausen [147]. Parameters in this bond-slip model include the maximum bond strength, initial stiffness, shape of the ascending branch, and slope of the descending branch. By adjusting these, the model has been calibrated to fit experimental bond-slip data for many kinds of OPC concrete [148].

Fig. 14
figure 14

Theoretical bond-slip relationship curve by Eligehausen [147]

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However, in reality, it is difficult to obtain a complete bond-slip curve demonstrating all phases of the Eligehausen model from a single pull-out test, especially the descending branch, which is affected by continued damage accumulation that is challenging to capture in lab. Actually, in bond tests with steel reinforced OPC and GPC, usually only the ascending branch of the curve is obtained. The different initial stiffnesses shown in their bond-slip curves are then regarded as the primary distinguishing bonding behaviors between reinforced GPC and OPC. The initial bond stiffness represents the stiffness of the bond between the reinforcing bar and the surrounding concrete at very low slip levels. Higher initial stiffness means more bond stress for a given slip. This allows better transfer of forces between the bar and concrete, enabling the bar to resist microcracks more effectively. With higher stiffness, smaller slips are needed to develop significant bond stress. This reduces the width and propagation of cracks by allowing load transfer across cracks [36].

From Fig. 15, it is evident that under equivalent loads, no matter reinforced with plain or ribbed steel bar, GPC consistently exhibits smaller slip values than OPC, illustrating superior initial bond stiffness. This superior initial stiffness is the primary difference in the bond-slip curves between ribbed bar reinforced GPC and OPC [79, 106].

Fig. 15
figure 15

Bond-slip curves of GPC and OPC [68]

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3.6 Factors affecting the bond behaviour of steel reinforcing GPC

Previous research, studies, incorporating a substantial volume of experimental data from bond tests conducted on OPC, have revealed the influence of numerous factors on bond behaviour, encompassing both steel reinforcement and concrete-related aspects [33]. These factors can be categorized into three main groups: the features of the reinforced concrete (RC) structure, the properties of the reinforcement, and the characteristics of the concrete [149]. In the case of GPC, bond behaviour is also influenced by these traditional factors (Fig. 16). Some main factors include the level of confinement, bar diameter, surface characteristics of the bar, embedment length, presence and positioning of ribs, concrete consistency, strength, depth of the concrete cover, and the relative position of the bar relative to the concrete placement. These factors play a significant role in determining the bond strength between the reinforcement and the surrounding concrete [150, 151]. The results from pull-out tests indicate that the bond strength of GPC increases with the strength of the concrete (both compressive and tensile), concrete cover, and surface toughness of the bar, while decreasing with an increase in bar diameter [132, 133, 137, 139, 150,151,152].

Fig. 16
figure 16

The traditional and special factors that influence the bond performance of reinforced GPC

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Besides the conventional factors, GPC exhibits distinctive bond behaviour due to the unique characteristics of the geopolymer binder (Fig. 16). Fernandez-Jimenez [102] conducted a study comparing the bond strength of GPC mixes with varying proportions of Na2SiO3 in the alkali-activated solution. The results indicate that a higher SiO2 to Na2O leads to a greater bond strength. Additionally, the Si/Al ratio in the geopolymer binder is another factor that influences bond performance [82]. This ratio is directly linked to the reaction rate and porosity of the geopolymerization products [14], ultimately affecting the ITZ and initial bond stiffness. The ITZ between OPC and steel bar is weak due to the presence of a large amount of crystalline material. Compared to OPC, the ITZ between GPC and steel lacks crystalline content, which is one of the reasons why the distribution of the bond stress in GPC differs from that in OPC [69].

4 Bond between geopolymer and FRP

The excellent durability of GPC makes it a suitable construction material for marine environments. However, while GPC offers better protection against the corrosion of steel bars, it is still susceptible to damage in severe environments. The emergence of Fiber Reinforced Polymer (FRP) bars effectively solves the problem caused by erosion ions in corrosion problem [153, 154].

4.1 FRP bars

FRP bars are prepared by bonding continuous long fibers to a matrix material (such as synthetic resin) and then pulling them into the designed shape. As shown in Fig. 17, common types include Carbon Fiber Reinforced Polymer (CFRP) bars, Glass Fiber Reinforced Polymer (GFRP) bars, Aramid Fiber Reinforced Polymer (AFRP) bars, and Basalt Fiber Reinforced Polymer (BFRP) bars [154,155,156]. The manufacturing processes include conventional extrusion, injection extrusion, weaving extrusion, and winding extrusion [157, 158]. FRP bars have benefits of lightweight, high strength, strong designability, high safety, and strong corrosion resistance [134]. Compared with ordinary steel bars, FRP bars has great corrosion resistance and do not require extra protection (such as rust inhibitors) when used in severe environment, reducing costs and avoiding a series of obstacles caused by the corrosion of steel.

Fig. 17
figure 17

The different types of FRP bars [157]

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4.2 Factors influence the bond between FRP bars and concrete

The bond strength of FRP reinforced concrete is still mainly related to traditional factors such as the shape of the reinforcement and the strength of the concrete [159]. The surface treatment of FRP bar is an essential factor that affects the bond strength. Nanni et al. [155] found that bond strength between concrete and different types of FRP bars, such as smooth, sand-coated and ribbed FRP reinforcement varies greatly. The bond strength of smooth FRP bars comes from the chemical adhesion between the FRP and the concrete, and is the lowest. A relative slip will occur at relatively small stresses, and the bonding strength is rapidly lost without a significant attenuation zone [160]. The bond strength of surface-bonded sand-coated FRP bars is mainly from chemical adhesion and frictional force. This bond strength is higher than the former and can withstand larger tensile forces, and the bonding strength generally shows a significant attenuation with the detachment of sand particles [161]. The bond strength of ribbed reinforcement is significantly improved due to the strong interlocking force between the rib and the wrapped concrete, and the peak bonding stress depends on the concrete's tensile strength and the rib's bearing capacity [134]. It has been observed that an increase in surface roughness can enhance the bond strength, while hydrolysis reduces bond strength [134, 162].

4.3 Bond strength and bond-slip relations of FRP bars in GPC

4.3.1 The comparison of bond strength

The prevailing view is that the bond strength of FRP bar reinforced concrete is generally lower than that between steel bars and OPC [156]. The lower bond strength between FRP reinforcement and concrete compared to traditional steel rebar is attributed primarily to the smooth surface and inferior mechanical interlock of FRP bars, as well as the weaker chemical adhesion and susceptibility to long-term degradation of the FRP-concrete interface. Additional contributing factors are the lower modulus of elasticity of FRP materials and variability arising from their anisotropic properties and directional strengths. However, recent studies have demonstrated that geopolymer concrete (GPC) can achieve stronger bond strength with FRP reinforcement [163]. As a result, while FRP typically has a lower bond strength with concrete than steel, the bond strength of FRP reinforced GPC can be comparable to that of steel reinforced GPC [160, 164]. Romanazzi tested the bond strength of sand-coated GFRP bar reinforced GPC and OPC, respectively [165]. The bond strength between GFRP and GPC was found to be 55% higher than that of OPC, and this discrepancy was more significant than the differences observed in the bond strength between GPC and steel bar, which was 44%.

4.3.2 The bond slip relations

The failure types of FRP-reinforced geopolymer concrete (GPC) are similar to those of steel bar-reinforced GPC, except the high tensile strength of FRP bars makes bar failure far less likely to occur in pull-out tests [166]. Figure 18 shows typical bond-slip curves for pull-out and splitting failures obtained experimentally by the authors in a study on the bond behavior of ribbed GFRP bar-reinforced GPC. It is evident that the pull-out failure experienced a relatively long slip before failure, with two waves representing ascending and descending branches. The ribs on the surface of FRP bars are not as sharp as those on steel bars, preventing the concrete in front of the ribs from being completely crushed and split open. The accumulation of fine debris increases friction and mechanical interlock, leading to the second ascending portion of the curve. Thus shows a bond-slip relations that quite different from the Eligehausen model [147]. The slip preceding splitting failure was similar to those steel reinforced concrete, which is considerably small, and the curve only exhibited a straight ascending branch.

Fig. 18
figure 18

Typical bond-slip curves of ribbed GFRP bar reinforced GPC

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Figures 19 and 20 shows the bond-slip curves for ribbed CFRP [35]. and BFRP [167] bar reinforced GPC, respectively. Both curves were obtained from pull-out failure tests and exhibit patterns quite similar to Fig. 18. Despite the distinctly different fiber types and over 50% difference in tensile strength between the two bar types, their bond-slip curves remain remarkably similar in shape.

Fig. 19
figure 19

Bond-slip curves between CFRP bar and GPC [35]

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Fig. 20
figure 20

Bond-slip curves between BFRP bar and GPC [167]

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Okelo and Yuan [168] studied the bond-slip curves for sand-coated GFRP bar reinforced GPC. The sand coating appears to provide strong friction with the surrounding concrete, resulting in high initial stiffness in the bond-slip curves. However, debonding of the coated sand causes a steep drop in the curves, which differs markedly from the behavior of ribbed bars shown previously. These results indicate that bar surface treatment is a key factor influencing the bond-slip response of FRP-reinforced GPC.

When comparing bond-slip curves for FRP reinforcement inGPC versus OPC, Nanni et al. [169], Chaallal and Benmokrane [165], Telke [160] and Okelo and Yuan [168] have all reported similarity. The bond-slip curves for FRP reinforcement GPC follow the same overall pattern, except that they display a much larger residual bond stress after the peak point. This residual bond stress is attributed to the ductile behavior of FRP rebar.

5 Summary and future directions

This state-of-the-art review provides a comprehensive understanding of the production and bond properties of geopolymer concrete (GPC), including synthesis, manufacture, engineering properties, durability, ITZ characteristics, and bond behaviour with steel and FRP bars. Key findings include:

  1. (1)

    With proper mix design and manufacturing, geopolymer concrete (GPC) can demonstrate engineering properties comparable or even superior to OPC, along with high durability and chemical resistance. Standardizing the mix design and manufacturing processes based on selected mineral materials is crucial for mitigating inconsistencies in the characteristics of the final GPCproduct.

  2. (2)

    Bond strength of GPC is influenced by factors such as steel bar condition, alkali solution and GPC mix composition, curing conditions, and concrete age. However, there is a lack of quantitative research on the correlation between GPC mix composition and bond strength.

  3. (3)

    GPC exhibits excellent bond performance with steel and FRP reinforcement due to its dense microstructure and strong adhesion. Current bond strength equations, derived from OPC, tend to underestimate reinforced GPC bond strength. Acquiring more experimental data is necessary to improve accuracy and fill gaps in design standards.

  4. (4)

    Bond-slip relations of steel and FRP reinforced GPC are similar to OPC, except for higher initial bond stiffness. However, the bond stress distribution differs due to distinct ITZ structures.

Standardizing the production procedure and comprehending the bond behaviour of GPC is significant. Further research is needed to understand ITZ structures, bond deterioration, bond stress distribution, and more. More experimental data is required to establish comprehensive theories and effective standards for GPC-based structures.