A critical review of alkali-activated metakaolin/blast furnace slag-based cementitious materials: Reaction evolution and mechanism

1 Introduction

With the sustained expansion of world economy, the market demand for cement is gradually increasing. However, in the process of manufacturing cement, high-temperature decomposition of limestone releases a large amount of CO₂, which has a great impact on environment and is contrary to the current low-carbon and environmental protection concept that is actively advocated worldwide [1]. Therefore, the search for a green and high-performance alternative cementitious material is imminent.

Alkali-activated cementitious materials (AACMs) are a new category of cementitious materials generated by the reaction between alkali activators and silicate solid raw materials with volcanic ash activity or potential water-hardness, which have the characteristics of simple raw material handling, reduced energy consumption, low CO₂ emissions, and production costs compared with traditional silicate cement [2]. Metakaolin (MK) is one of the commonly used precursors for the preparation of AACMs and has attracted much attention. It is a highly active mineral additive generated by low-temperature calcination of ultra-fine kaolinite, which contains amorphous aluminum silicate and exhibits high volcanic ash activity [3]. Alkali-activated MK cementitious materials (AAMCMs) have remarkable advantages in terms of volumetric stability, high temperature and corrosion resistances, but they also suffer from the deficiencies of slow hydration rate, long setting-hardening time at ambient temperature and low early strength [4]. Meanwhile, another common precursor, blast furnace slag (BFS), is also attracting attention. BFS is a kind of waste discharged in the process of smelting iron, which can be classified into acidic, alkaline and neutral BFS according to the content of alkali oxides in chemical composition, and can be processed into engineering materials with different purposes [5]. Similarly, alkali-activated BFS cementitious materials (AABCMs) exhibit numerous advantages, including high early strength, excellent durability and low hydration heat. However, they also have their inherent limitations, such as excessive volumetric shrinkage, rapid setting-hardening and susceptibility to cracking [6].

Obviously, AACMs prepared from two single precursors show some deficiencies in terms of engineering properties, but there is a potential for these two materials to complement each other with respect to material properties. In view of this, scholars have tried to develop alkali-activated MK-BFS cementitious materials (AAM-BCMs) by reasonably blending these two precursors, and a series of studies have been conducted regarding reaction mechanism [6,7,8], workability [9,10,11], mechanical properties [12,13,14], shrinkage properties [15,16,17], and durability [18,19,20], and considerable achievements have been made. It has been shown that the reasonable blending of these two precursors not only contribute to regulate workability, enhance mechanical properties and durability, but also effectively solve the deficiency of volumetric shrinkage, compensate for the defects of a single precursor that is susceptible to cracking [21,22,23]. However, the exact reaction mechanism of AAM-BCMs has not been fully clarified currently, and the vast majority of existing studies still remain in the description of reaction process, and there is a lack of in-depth research on reaction mechanism that affects its final structure factors. Meanwhile, the differential recognition of micro-structure due to current level of understanding and incongruity of the relevant detection techniques leads to an as yet inconsistent interpretation of the reaction mechanism [24,25,26].

This article presents a comprehensive review of reaction evolution and mechanism of MK/BFS-based cementitious materials in the light of extensive literature. A systematic assembly and summary of existing reaction evolution and mechanisms is first carried out, followed by a comprehensive comparison and analysis. The reasons why the reaction mechanisms have not yet formed a unified understanding are elaborated, the synergistic mechanism of MK and BFS is also revealed, and the future mechanism research is prospected, with a view to providing theoretical support for the subsequent research on material properties of AAM-BCMs as well as their engineering applications.

2 Reaction mechanism of AAMCMs

MK is an amorphous alumina-silicate material formed by calcining kaolinite at 600–800°C to remove –OH, which is mainly an amorphous phase with a small amount of crystalline phase. MK is an ultrafine powder that typically has a blaine value of 1,000–1,500 m²·kg⁻¹ and a particle size range of 0.357–66.9 μm. Alkali-activated MK is mainly a chemical reaction process in which amorphous or semi-crystalline alumina-silicate raw materials are partially or completely converted into three-dimensional network structure in the presence of appropriate activators [4,27]. Scholars have performed a series of studies regarding reaction mechanism of AAMCMs and considerable research results have been obtained [27,28]. However, due to the relative complexity of polymerization reaction process in the system, multiple components, rapid rate of gel generation, almost simultaneous reaction of various phases as well as the limited availability of detection techniques, a unified understanding of reaction mechanism of AAMCMs has not yet been formed [29,30].

A systematic study on reaction mechanisms of AAMCMs was conducted and it was concluded that polymerization reaction followed the mechanism of dissolution–depolymerization–polycondensation, and that reaction process was approximated as four stages as dissolution, migration diffusion, condensation, and dehydration solidification [4,31,32,33], as shown in Figure 1. The surface of precursor gradually dissolved, and Al–O and Si–O bonds in amorphous SiO₂ and MK were fractured. Al monomer and Si monomer were generated, and Al and Si monomers continuously migrated and diffused into interstitial space of particles under the action of free water. Through dehydroxylation, Al and Si monomers underwent reorganization and combined with Na⁺ or K⁺ to produce oligomers, such as aluminosilicate and silicate. The decrease in concentration of Al and Si monomers in the system induced further dissolution of precursors, and the oligomers underwent a condensation reaction to form a poly-silica-aluminate gel phase. As polymerization reaction proceeded, the poly-silica-aluminate gel phase continued to dehydrate and condense, and ultimately solidified into a highly polymeric geopolymers consisting of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ with a three-dimensional network structure through alternating bonding of shared oxygen atoms.

Figure 1

Schematic diagram of reaction mechanism of AAMCMs.

Different investigations have concluded that covalent bonds such as Si–O–Si and Si–O–Al in silica-alumina-based materials were rapidly fractured in strong alkaline solutions to produce aluminum-rich gels with a low polymerization degree (Gel-1) [34,35]. As the polymerization reaction proceeded, the [SiO₄]⁴⁻ in the system gradually replaced [AlO₄]⁵⁻ in low polymerization degree Gel-1 and were successively converted into relatively stable silica-rich gel (Gel-2). Subsequently, Gel-2 further underwent a polycondensation reaction, ultimately generating a polyalumino-silicate gel with a three-dimensional network structure. However, the structural morphology of aluminum-rich Gel-1 and silica-rich Gel-2 generated in the polymerization reaction as well as the transformation mechanism between the two have not yet been studied clearly, mainly because of the rapid rate of Gel-1 and Gel-2 generation in geopolymerization reaction, and disintegration, reconstruction, and condensation of silica-aluminum-based materials are almost simultaneous and reaction kinetics are interdependent, which makes it impossible to conduct a targeted research on various phases individually [36].

Meanwhile, Van Deventer et al. [37,38,39,40] concluded that in a strong alkali solution, the aluminum silicate precursors firstly dissolved into aluminosilicate monomers and diffused into alkali solution to form aluminium and silicate monomers. A rapid polymerization reaction between some of silicate monomers generated further produced polymeric silicates. As reaction proceeded, the aluminate in the system reacted with silicate monomer to produce aluminosilicate oligomer (Si–O–Al structural unit). Subsequently, the formed oligomers continued to dehydrate and condensed to form nano-crystalline (or semi-crystalline) aluminosilicate nuclei and amorphous aluminosilicate polymers. Then, the aluminosilicate nuclei was further converted into zeolite phase crystals, while the amorphous aluminosilicate geopolymers were converted into aluminosilicate gels. The condensation reaction of aluminosilicate oxides with alkali-metal silicates produces different polymeric Si–O–Al bonds, as shown in Figure 2. It is noteworthy that Figure 1 emphasizes the dynamic process of silica–aluminum material reaction from monomer to gel. Differently, Figure 2 illustrates that the Si/Al ratio determines the connection mode of [SiO₄]⁴⁻ and [AlO₄]⁵⁻, thereby forming different polymer structures. The two diagrams are essentially representations of the same system at different scales.

Figure 2

Polyaluminosilicate polycondensation reaction.

Additionally, on the basis of the Faimon model [41], a mechanism model describing complex, multi-step chemical reaction sequence of geopolymers from the perspective of reaction kinetics was proposed [42,43], as shown in Figure 3. The arrow in the figure showed that amorphous aluminosilicate gels and nano-crystalline zeolite phase crystals could be inter-converted in the presence of an extended curing periods [44]. However, considering that the focus of kinetic model of polymerization reaction was the initial phase of reaction, the inter-conversion process was not considered in the model.

Figure 3

The proposed reaction sequence of geopolymerization.

Moreover, generation–development–evolution of hydration products in the matrix zone of K-PSDS type geopolymers was quantitatively traced by environmental scanning electron microscopy [45] and it was found that at the initial of geopolymerization reaction, the accumulation of MK particles in matrix zone was relatively loose, and some pores existed between particles. As reaction proceeded, the spongy colloid was gradually generated and wrapped in the surface layer of particles, the pore space between particles decreased, the micro-structure became dense, and crystalline products of regular shape were not observed. Compared with the matrix zone, the hydration products in the inter-facial zone of K-PSDS-type geopolymer exhibited many large voids in the early stage of geopolymerization reaction, and the particle distribution was not as tight and homogeneous as that of the matrix zone, but the difference in the microscopic morphology of hydration products in the late stage was marginal [46]. Additionally, infrared spectra analysis results showed that [AlO₄]⁵⁻ was bonded to [SiO₄]⁴⁻ chains during geopolymerization to form a three-dimensional network structure, which could be inter-converted between isoconstituent zeolite crystals and geopolymers under appropriate conditions [47]. Obviously, this finding was in line with the conclusions drawn by Provis and coworkers [42,43] and literature [44].

Similar polymerization mechanism studies have been conducted [48,49,50,51,52,53,54,55], and it was concluded that the vitreous phase in the material components under the action of strong-alkali was first dissolved, some of Al–O and Si–O bonds were fractured, and Al and Si oligomers were gradually formed. As geopolymerization reaction proceeded, the generated oligomers were successively converted into zeolite-like precursors, and the generated precursors were dehydrated to eventually form amorphous phase materials. As shown in Figure 4, the polymerization process of aluminosilicate polymers mainly involved the dissolution of MK in alkali solution, the formation and diffusive migration of silica–aluminum monomers, monomer condensation, and paste curing. Notably, Figure 3 standardizes the nomenclature to distinguish between the different structural phases, revealing the reaction sequence of the geopolymer, and Figure 4 explains the causal chain of the geopolymer formation process.

Figure 4

Schematic diagram of geopolymerization process of AMCMs.

Further investigation of the reaction products of alkali-activated aluminosilicates and their micro-structures revealed that the alkali activator first induced the breakage of Si–O and Al–O bonds in feedstock, and the dissolved silica–aluminum tetrahedra diffused from the surface of particles to interior, resulting in the disintegration of feedstock [49]. Subsequently, the dissolved tetrahedra combine with alkali metal ions to form a temporarily stable transition phase. Finally, as the transition phase became saturated, the silica–aluminum tetrahedra shed free water through a polymerization reaction to produce final product N–A–S–H gels.

On the basis of local charge model, the morphology and hydrolysis mechanism of aluminum components in geopolymers were studied [56,57,58,59] and the mechanism of promoting polymerization reaction was explored by calculating their partial charges of aluminum and silicate components, concluding that under the same pH condition, the partial positive charge of Al atoms was higher than that of Si atoms, and that [AlO₄]⁵⁻ was more capable of attracting other negatively charged groups, and the polycondensation reaction of aluminate appeared to take place more readily. Besides, Cui and coworkers [60,61,62] regarded that polymerization mechanism could be referred to zeolite molecular sieve synthesis mechanism, if the crystallization degree was low, an amorphous structure was generated in the system, and it was concluded that dissolution–reorganization–condensation was basic course of geopolymerization reaction, whereas due to a low crystallinity degree, it was difficult to accurately detect their structural morphology currently, and the specific reaction modes in the geopolymerization process needed to be further explored.

The early processes of geopolymerization reactions were studied using MAS-NMR and classified into accelerated, deceleration, and stabilization phases [63]. A large number of low-polymerized N–A–S–H gels were generated, and these low-polymerized gels gradually formed network structure based on chemical bonds of Si–O–Al and Si–O–Si through mutual dehydration polymerization reaction. Gradually, a small amount of unreacted MK was encapsulated by reaction products, reducing the geopolymerization reaction rate. Eventually, a three-dimensional network structure with Si–O–Al as the skeleton was formed during the stabilization phase.

Differently, some findings divided the hydration process of AAMCMs into initial, induced, accelerated, decelerated, and stabilized stages [64]. The low-level polymerization was conducted shortly after the surface dissolution of MK, the heat of adsorption occurring in the initial stage and the heat of low-level polymerization in the induced stage were released simultaneously, and therefore, the difference between the initial and induced stages could not be seen from exothermic curves of hydration, and there was no troughs during induction period, as in the case of silicate cements. Its hydration products consisted of silica–aluminium gels and silica–aluminium network without any crystal structure.

In summary, scholars have conducted a large number of polymerization reaction mechanism studies on AAMCMs. However, the exact mechanism of geopolymerization has not been fully clarified, and the majority of existing studies have only limited to the description of reaction process, which is generally divided into several stages, such as dissolution, diffusion, and polycondensation. Currently, there is a lack of in-depth research on the mechanism of factors affecting the final structure of geopolymers. With the existence of different coordination structures in geopolymers, it is not clear how to regulate the ratio between the coordination modes and the mechanism of their inter-conversion and evolution. The specific role of water molecules in the system throughout the geopolymerization process is unclear. An in-depth study of the charge density of the cations and the ionic size of the cations in geopolymer structure is needed to elucidate mechanisms regulating micro-structure. To address these issues, the whole polymerization reaction process should be analyzed more from a microscopic point of view to achieve an effective regulation of final products structure.

3 Hydration reaction mechanism of AABCMs

The main chemical compositions of BFS are CaO, SiO₂, and Al₂O₃, which generally account for more than 85% of total content, and also contain small amounts of MgO, Fe₂O₃, and sulfides [65,66]. The BFS is mainly composed of vitreous phases, with only a small amount of enstatite, gehlenite, and diopside. BFS is an irregular angular particle, which has a Blaine value (≥400 m²·kg⁻¹) and a particle size distribution of 0.431–76.0 μm.

Different views have been proposed by scholars regarding the structural composition of BFS, mainly including continuous network vitreous structure [67], vitreous split-phase structure [68,69], and tetrahedral polymeric state distribution structure [70]. All three structures recognize that the basic structural units of BFS are [SiO₄]⁴⁻ and [AlO₄]⁵⁻. These tetrahedra interact with each other through “bridging oxygen” (oxygen atoms connecting two tetrahedra) or “non-bridging oxygen” (oxygen atoms connecting only one tetrahedron), forming the foundation of structural stability. Differently, the continuous network vitreous structure emphasizes overall orderliness, the vitreous split-phase structure focuses on phase-region differences, and the tetrahedral polymeric state distribution structure concentrates on the polymerization state of local units [67,68,69,70]. AABCMs are formulated from BFS supplemented with an appropriate amount of alkali content, featuring the merits of high early strength, chemical corrosion resistance, and low carbon. Scholars have performed a series of studies focusing on the hydration process and reaction mechanism, achieving fruitful research results and laying a satisfactory foundation for engineering applications [67,68,69,70].

The reaction process of AABCMs was different from that of ordinary Portland cement (OPC). The setting and hardening of OPC was a reaction process between cement clinker, with water as reactant, whereas the setting and hardening process of AABCMs was a reaction process between alkali and BFS active material, with water mainly acting as a mediator. However, its reaction required an activator to be involved, and currently the commonly used activators are NaOH and Na₂SiO₃. As shown in Figure 5(a), the activation of BFS with NaOH as activator could be divided into three stages as follows:

1. The initial reactive ions had a large unsaturation degree, and the main reactive ions were rapidly released in the presence of OH⁻, accompanied by exotherm. During this period, the concentration of reactive ions in solution gradually increased, and the unsaturation degree decreased.

2. The ions in contact with each other became substantially more, and C–S–H gel was gradually formed on the surface of BFS particles and Na⁺ was replaced with Ca²⁺ in the alkali activator to form C–S–H gel. As reaction proceeded, the Al tetrahedra gradually took over the bridging positions of the Si tetrahedra and Na⁺ was absorbed into the chain along with Al³⁺ to neutralize the charge imbalance, and C–S–H gel began to transform into C–(A)–S–H gel.

3. The C–(A)–S–H layer on the surface of BFS particles continued to grow until a reaction product layer was formed. OH⁻ crossed through the product layer at a lower rate and the reaction gradually terminated [71,72,73].

Figure 5

Reaction process of alkali-activated BFS (a) NaOH as an activator and (b) Na₂SiO₃ as an activator.

Differently, the activation of BFS with Na₂SiO₃ as activator could be divided into four stages as follows:

1. Initially, the unsaturation degree of Si was quite low, while the unsaturation degree of other ions was high, resulting in the rapid dissolution of other ions;

2. An Si layer was formed on the surface of BFS particles and began to grow due to much faster dissolution of other ions;

3. Previously released Ca²⁺ and Al³⁺ reacted with soluble Si in solution and generated C–(A)–S–H gels;

4. Reaction products gradually accumulated to form shells not only on the surface of BFS particles, but also in solution, and progressively entered stabilization phase, as shown in Figure 5(b) [72,74,75].

A similar study was carried out, which divided the hydration process of AABCMs into three stages as BFS dispersion and structural disintegration, condensation structure formation and development, and crystal structure formation. At the early stage of the reaction, the system first generated –Si–O– and –Si–OH, and it was believed that Na⁺ and OH⁻ mainly played a catalytic role in the generated C–S–H. Subsequently, the system entered the second stage, and the disintegrated Si–O–Si– polymerized again to generate unstable five-coordinated silica-centred ions and generated hydrides with different C/S ratios according to different ratios of alkali to SiO₂. Eventually, the colloidal particles formed crystals with solid phase generated in the previous stage, contributing to the formation of slurry structure [76].

Subsequently, the results of Mozagawa and Deja [77] found that under alkali activation conditions, some of Si⁴⁺ in the dissolved [Si₂O₇]⁶⁻ units were replaced by Al³⁺, leading to a pronounced amorphous phase. The substitution of Al for Si in the [SiO₄]⁴⁻ at the C–S–H bridging oxygen position contributed to the generation of the main hydration product as tetrahedral chain structure C–A–S–H. Meanwhile, the reaction system was accompanied by the generation of [AlO₄]⁵⁻, the content of which was related to the activator type and hydration process. Similar studies further confirmed that Si in C–S–H gels could be substituted by Al, and it was also found that Na⁺ could replace Ca²⁺ in C–S–H gels, and that the amount of Al substitution in the system, as well as the chain length of the generated C–A–S–H chain structure, were dependent on activator used [78,79,80].

Meanwhile, a comparable study was conducted [81], and it was found that BFS was a potentially hydrodynamically reactive mineral material with an amorphous to semi-crystalline three-dimensional aluminosilicate structure, where silica- and aluminium-rich feedstocks were activated by a mixture of alkaline solutions, and its dissolved [SiO₄]⁴⁻ and [AlO₄]⁵⁻ were combined to form monomers by sharing an oxygen atom. The monomers subsequently interacted to form oligomers, which further polymerized to form a three-dimensional network structure of aluminosilicate, as shown in Figure 6.

Figure 6

Hydration process of the BFS.

Apparently, it has been shown that the hydration process of AABCMs was similar to that of OPC, which could be classified into five stages as initial, induced, accelerated, decayed, and slow stages [82]. However, the hydration mechanism was different from that of OPC, a similar hypothesis could be used to explain its hydration process. The hydration calorimetric curve of reaction system showed two obvious exothermic peaks, in which the first exothermic peak was caused by reaction of silicate ions in water glass with Ca²⁺ in BFS to generate C–S–H gels, while the second exothermic peak was related to the formation of secondary C–S–H gels as well as other hydrates by silicate and alumina ions and Na⁺, Ca²⁺, and Mg²⁺ precipitated in the system. Differently, some scholars confirmed that OH⁻ first dissolved Ca component in BFS, which existed in the form of Ca²⁺ in the system [83]. The vitreous body of BFS began to decompose, and activator further penetrated into BFS particles, Si and Al components underwent dissolution, generating [SiO₄]⁴⁻ and [AlO₄]⁵⁻. However, this process was still dominated by condensation reaction of silicate ions, supplemented by depolymerization reaction, and the monomers produced by dissolution and silicate ions polymerized with each other to form dimers, polymers, and oligomeric gels. As reaction proceeded, the number of hydration products of BFS gradually increased, and the metal cations in solution further polymerized to generate C–(A)–S–H gels.

Meanwhile, the hydration mechanism of water glass-activated AABCMs was investigated by means of FTIR, DTA, and TMS-GC tests, and it was indicated that hydration process was depolymerization–polymerization process of silicate anions [84]. The polymerization state of silicate anions varied with hydration reaction, from monomer to dimer, then developed into oligomer, and gradually became polymer, the number of silicate anion monomers decreased, while the polymer gradually increased, the BFS vitreous gradually depolymerized, progressively forming silicate anions with a high polymerization degree. Additionally, Yuan and Gao [85] and Roy and Silsbee [86] concluded that BFS vitreous disintegrated the –Si–O–Al–O– network structure in the presence of OH⁻ and entered solution in the form of hydrated complex anions, causing the dissolution of BFS particles. The hydrolysis reaction of BFS was related to the form of binding of oxygen ions in solution, which generally existed in three forms: (1) O²⁻ bound to two Si⁴⁺, ≡ Si – Si ≡ HOH → 2 ≡ Si – OH . (2) O²⁻ bound to a Si⁴⁺ and a metal ion Me²⁺, ≡ Si – O – Me – O – Si ≡ + 2HOH → 2 ≡ Si – OH + Me 2 + + 2 OH − . (3) O²⁻ bound to two metal ions Me²⁺, the solubility of BFS vitreous Si – O – Me – O – Me – O – Si ≡ + 3HOH → 2 ≡ Si – OH + 2Me 2 + + 4OH − in the reaction process increased as pH of solution increased, and the amount of dissolution increased with the prolongation of reaction time. Similar studies had compared the silicate anion polymerization state of BFS cementitious materials activated by a small amount of Na₂SiO₃ or NaOH at different hydration ages and found that there was both condensation of silicate anions and depolymerization of polysilicate anions during the reaction process, and the whole reaction process was dominated by condensation reaction. Al³⁺ and Ca²⁺ condensed with oligosilicate anions to produce hydrated calcium aluminosilicate gel and needle C–S–H gel, prompting the condensation and hardening of reaction system and improving strength [87].

Besides, the strength evolution and hydration reaction mechanism of AABCMs were studied [88], and the types of hydration products, the hydration reaction degree, the polymerization degree of C–S–H gels, and the variation of Al/Si were investigated by means of compressive strength tests, XRD, SEM, and ²⁹Si NMR. When NaOH was used as alkali activator, the Si–O and Al–O bonds in the vitreous network structure were fractured, followed by reactions with Na⁺ and OH⁻ to form oligomers, such as –Si–O–Ca–OH, –Si–O–Na, Al ( OH ) 5 2 − , and Al(OH)⁻ ₄. Meanwhile, [SiO₄]⁴⁻ and [AlO₄]⁵⁻ monomers underwent a polycondensation reaction to produce Si–O–Si and Al–O–Si dimers. As reaction proceeded, the concentration of each ion increased, and the products gradually changed from a low polymerization degree to a high polymerization degree gel structure. When water glass was used as an alkali activator, hydrolysis of water glass generated water-containing silica gel and NaOH, and OH⁻ made a network structure on the surface of BFS vitreous body disintegrate and separate, continuously released Ca²⁺, Mg²⁺, etc., and constantly diffused to internal pores. The water-containing silica gel Si(OH)₄ then reacted with Ca²⁺ and OH⁻ in solution to generate C–S–H gels, and the consumption of OH⁻ accelerated hydrolysis of water glass. As hydration reaction proceeded, the vitreous was continuously hydrolyzed from outside to inside, releasing more Mg²⁺, Ca²⁺, and reactive SiO₂, which continued to react and produce C–S–H gels, and the dissolved Mg²⁺ and Ca²⁺ reacted with vitreous to form zeolite-like minerals [89,90].

Furthermore, it was found that the hydration process of alkali-activated BFS was roughly divided into three stages as hydrolysis and hardening of water glass, dispersion and dissolution of BFS, and hydration and hardening of BFS [68]. The protective film (silica–oxygen network structural layer) on the BFS surface was destroyed and OH⁻ entered into vitreous structure, prompting hydration reaction. Different conclusions suggested that the hydration process of alkali-activated BFS could be divided into five stages as initial hydration, transition, acceleration, deceleration, and decay [91]. In the transition stage, the water extended to the surface of BFS through the product layer of the initial hydration stage and hydrates on its surface, the system reaction was relatively gentle. As the hydration reaction continued, the BFS surface hydration gradually developed into structural space hydration, the amount of hydrated BFS gradually increased, and the system hydration entered the accelerated stage. When a fully hydrated layer appeared on the surface of BFS, it entered the deceleration stage, a large amount of BFS hydration occurred, the reaction zone extended to interior BFS, the water diffusion resistance increased, and the hydration entered decay stage.

On this basis, Yang et al. [92] further divided the hydration process of AABCMs into the following six stages:

1. Dissolution of BFS: the BFS particles dissociated Mg²⁺, Ca²⁺, and other ions under polarization of alkali metal ions and hydroxide ions.

2. Formation of loose hydration product layer: alkali metal ions penetrated the loose hydration product layer and continued to combine with unhydrated BFS.

3. Formation of hydration cover layer: the hydration products of the loose hydration layer continued to accumulate and form a cover layer.

4. Rupture of cover layer: the cover layer ruptured under the action of osmotic pressure and crystallization pressure, and the hydration reaction was accelerated.

5. Healing of cover layer: new hydration product precipitation occurred in the loose hydration layer, repairing the ruptured cover layer.

6. Formation of a dense cover layer: as the hydration products of the cover layer continued to increase, a dense cover layer was gradually formed, and alkali metal ions were unable to penetrate the cover layer and hydration process was slowed down. The final hydration process was controlled by the diffusion process that penetrated the dense cover layer. Additionally, the reaction process of BFS in different concentrations of sodium hydroxide solution was studied [93], as shown in Figure 7, and it was found that at lower sodium hydroxide concentrations, a ring of gel new-phase zone was first formed around BFS particles and then condensed and densified, with a wide range of product distribution. When the concentration of sodium hydroxide was relatively high, the BFS was rapidly depolymerized, resulting in the precipitation of hexagonal plate-like hydroxycalcite with the saturation index of calcium hydroxide greater than zero, and a layer of reaction products was rapidly deposited on the surface of BFS particles with a narrow product distribution. Notably, Figure 6 illustrates the molecular-scale chemical conversion process from feedstock to 3D network. However, Figure 7 illustrates the particle-scale spatial reaction process as a dissolution–precipitation reaction.

In summary, the reaction of AABCMs could be roughly divided into the stages of BFS disintegration, dehydration, and condensation and stable structure generation. Aluminosilicate was first dissolved from BFS. As dissolution reaction proceeded, the structure of BFS was destroyed, and the aluminosilicate generated by dissolution combined with alkali-metal ions to produce a low-polymerization state, stable intermediate-phase gel. As reaction proceeded, the intermediate phase material generated by reaction continued to react with the alkali activator to remove free water from the gel and polymerize. Subsequently, some of alkali-metal ions and dehydrated polymerized product further combined to achieve the role of balancing charge, while generating a stable structure.

Figure 7

Reaction process of slag.

Clearly, scholars have performed a series of studies focusing on the hydration process and reaction mechanism of AABCMs and achieved fruitful research results. However, due to the current level of understanding and incongruity of the relevant testing techniques, researchers hold different views on the reaction mechanism [94,95]. The main reason for this status quo is the different recognition of micro-structure of BFS, resulting in an as yet inconsistent interpretation of the alkali-activated mechanism.

4 Reaction mechanism of AAM-BCMs

Recently, scholars have tried to prepare AAM-BCMs by rationally blending two precursors of MK and BFS, with a view to realizing the advantages of complementary material properties. Yip et al. [96,97] carried out a study on the reaction mechanism of AAM-BCMs and revealed that both C–S–H gels and N–A–S–H gels existed in the reaction products, and whether the two gels existed simultaneously or not depended on alkalinity of activator as well as the MK-to-BFS ratio. In low alkalinity system, both gels were produced simultaneously, whereas in high concentration NaOH solution, the system mainly generated N–A–S–H gels accompanied by a small amount of calcium precipitate attached to the gel surface.

The simultaneous generation of C–S–H and N–A–S–H gels contributed to material strength, and the gels filled pore structure as well as connected voids between unreacted particles and hydration products. Ca²⁺ would not act as a balancing charge like Na⁺ and it would preferentially form Ca(OH)₂ and C–S–H gels. High alkalinity would lead to precipitation of Ca²⁺ as Ca(OH)₂, and only a small amount of Ca²⁺ could participate in the formation of C–S–H gels. At lower alkalinity, the Ca²⁺ solubility increased and C–S–H gels increased, but the decrease of OH⁻ resulted in slow dissolution of Al and Si in MK, and the initial dissolved Al was not enough to form N–A–S–H gels [97].

Subsequently, a similar study was conducted and confirmed that the hydration products of AAM-BCMs consisted individually and partially crystallized calcium- silicate and aluminosilicate phases as well as a small amount of zeolite phase structure. In the samples synthesized from low-magnesium BFS, a hydrotalcite-type phase was not observed, and secondary reaction products were found in samples with a water-glass modulus of 2.0 [98]. Further, to investigate compatibility of C–S–H and N–A–S–H gels, a mixture was synthesized using sol-gel method [99], and it was found that the pH level played a decisive role in synthesis of both C–S–H and N–A–S–H gels. The C–S–H gel phase was mainly formed at pH >11, whereas the combined synthesis results of both gels were inconclusive.

Besides, the existing studies have shown that the alkali equivalent of BFS was generally 4–8% and that of MK was 8–15%. Since the two materials have different alkali requirements [100], the alkali equivalent considerably affected the generation of hydration products of AAM-BCMs. Additionally, the effect of different NaOH concentrations on AAM-BCMs was investigated [101], and it was found that high NaOH concentration accelerated polymerization reaction, mainly stemming from the fact that high alkaline environment increased the dissolution of raw materials, especially MK. Similarly, it was found that the system would not produce enough C–S–H gels and mainly form N–A–S–H gels at high sodium hydroxide concentrations (i.e., 10 M or >10 M). When the sodium hydroxide concentration was ≤5 M, the low alkali concentration hindered the dissolution of MK and there was not enough Al to form N–A–S–H gels, with C–S–H gels as main products [102].

Meanwhile, the reaction mechanism of AAM-BCMs was explored using XRD, FTIR, IR, SEM, and CA testing techniques [103,104,105], as shown in Figure 8. The results showed that the incorporation of BFS could effectively promote properties at ambient temperature, and a high-strength structure of Si–O–Al network structure coexisting with C–S–H and C–A–H gels could be obtained. However, the excessive BFS content might cause the structural cracking and produce CaCO₃ crystals attached to structure, as shown in Figure 9, which was unfavorable to strength development.

Figure 8

FTIR photographs of AAM-BCMs with different BFS contents (a) low modulus conditions and (b) high modulus conditions.

Figure 9

Variation curve of compressive strength of geopolymer with BFS dosage and activator modulus (a) 7 days and (b) 28 days.

Similar to the conclusions reached by Yip et al. [96,97], it was also found that hydration reaction and geopolymerization reaction occurred simultaneously in the system, and the reaction products consisted of two-phases of amorphous low-calcium C–(A)–S–H and N–A–S–H gels. Both gels filled each other and compact internal structure, thus improving compressive strength and volumetric stability [106]. Likewise, the effect of calcium content on physical phase composition and micro-structure of AAM-BCMs by adjusting BFS-to-MK ratios [107], as shown in Figure 10. Similar conclusions were also reached and it was observed that AAM-BCMs were mainly composed of vitreous phase and amorphous phase. As CaO content increased, the quartz phase decreased, and when CaO content reached 10%, micro-cracks began to appear, and plagioclase (rock-forming mineralogy) was generated. Prolonging reaction time or increasing temperature was conductive to the formation of zeolite crystalline phase, decreasing water glass modulus or increasing molar ratio of Si/Al was also beneficial for the formation of stable square zeolite crystalline phases [108].

Figure 10

SEM photographs of AAM-BCMs with different CaO contents (a) 0% and (b) 5%.

In addition, the author research group [109] conducted a study on alkali-activated MK-BFS inorganic adhesive for FRP-based structural rehabilitation. The geopolymerization and dissolution–polymerization reactions occur simultaneously and the reaction process could be roughly divided into dissolution-diffusion, physical phase equilibrium, gel generation, reconfiguration arrangement and polymerization-hardening. In comparison, the drying shrinkage of MK-BFS composite system is usually significantly lower than that of the single AABCMs system, which greatly reduces the risk of cracking due to shrinkage of the matrix, and improves volumetric stability and durability. Additionally, compared with single AAMCMs system, the two gel phases (i.e., C–(A)–S–H and N–A–S–H) formed by the compounding of BFS and MK fill each other, effectively densifying the internal structure. Evidently, the combination of the two precursors effectively overcomes the deficiencies of a single system and significantly improves the overall engineering properties through synergistic effect. The specific reaction process and mechanism are shown in Figure 11.

Figure 11

Reaction mechanism of alkali-activated MK-BFS inorganic adhesives.

In summary, different from the reaction mechanism of a single MK or BFS precursor in the presence of alkali activation, the reaction mechanism of AAM-BCMs combines some characteristics of each of the two precursors. Under different reaction conditions, the dissolution reaction occurs first, and both dissolution products contain [SiO₄]⁴⁻ and [AlO₄]⁵⁻. However, the dissolution product of BFS contains Ca²⁺, which first dissolved and then reacts with [SiO₄]⁴⁻ and [AlO₄]⁵⁻ to form C–S–H gels, and then further react to generate C–(A)–S–H gels. A condensation reaction also occurs between [SiO₄]⁴⁻ and [AlO₄]⁵⁻, combining with Na⁺ to form N–A–S–H gels. Clearly, compared with the AABCMs and AAMCMs, the current research on the reaction mechanism of AAM-BCMs is relatively limited, and the understanding of products generated by reaction system is insufficiently. Meanwhile, due to the multiple components of composite system, the variability of activators, the simultaneous occurrence of different reactions, as well as limitations of existing detection techniques, the current research on reaction mechanism of AAM-BCMs has not yet formed a unified understanding.

5 Conclusions

The reaction mechanism of MK/BFS-based cementitious materials was systematically sorted out and summarized, the mechanism of synergistic interaction between MK and BFS was revealed, the reasons why the existing research on reaction mechanism has not yet formed a unified understanding were explained, and future direction of mechanism research was envisioned. Conclusions are as follows:

1. Currently, scholars have carried out a series of research studies focusing on the reaction mechanism of AAMCMs and achieved considerable research results. The aluminosilicate in MK first dissolves to form [SiO₄]⁴⁻ and [AlO₄]⁵⁻, and then polymerizes to form gels with different polymerization degrees, which further polymerize with each other and remove free water, and ultimately forming a stable three-dimensional network structure. However, the exact mechanism of geopolymerization reaction has not been fully clarified currently, and the majority of existing studies have still stayed on the description of reaction evolution, which is generally divided into several stages, such as dissolution, diffusion, and polycondensation.

2. The reaction of AABCMs can be roughly divided into the stages of disintegration of BFS, dehydration and condensation and generation of stabilizing structures. The vitreous network structure composed of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ in BFS is gradually depolymerized, and the aluminosilicate generated by dissolution combines with alkali-metal ions to generate low-polymerization and stable intermediate-phase gels. Subsequently, a further reaction removes free water from the gels and polymerization occurs, and some alkali-metal ions are further bound to the dehydrated polymerized product to achieve a balanced charge while generating a stable structure.

3. The reaction mechanism of AAM-BCMs has the characteristics of both precursors. The dissolution–polymerization and geopolymerization reactions occur simultaneously, and the reaction process can be approximated to be divided into dissolution and diffusion, phase equilibrium, gel generation, reconstruction, and hardening of polymerization. The C–(A)–S–H and N–A–S–H gels generated simultaneously not only facilitate the filling of pores, the connection of unreacted particles, but also optimize pore size and enhance strength.

4. Currently, the reaction mechanism of alkali-activated MK/BFS-based cementitious materials has not yet formed a unified understanding, which stems from a number of factors, such as the complexity of the reaction process, the multiple components, the rapid rate of gel generation, the almost simultaneous reaction of various phases, as well as the limited availability of detection techniques. The current mechanistic analysis is largely based on existing studies and speculates that the processes of alkali activation reaction may be inaccurate, which needs to be demonstrated and analyzed by other approaches.

5. Future research needs to break through the limitations of single-component or static analysis, focus on the core directions such as multi-phase synergy, dynamic evolution, and environmental response and combine advanced characterization technologies and computational simulations to build a cross-scale mechanism framework of atom-microregion-macro. Alkali-activated MK/BFS-based cementitious materials are expected to realize revolutionary applications in low-carbon building materials, environmental remediation, and structural rehabilitation through mechanism innovation-driven material design.