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Article

Influence of Fly Ash on the Macro Properties and Mineral Crystal Characteristics of Alkali-Activated Slag Grouting Materials

by
Guodong Huang
1,*,
Jiahao Xu
1,
Jun Qi
1,
Fengan Zhang
2 and
Baoxuan Dou
1
1
School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan 232001, China
2
Special Equipment Inspection Institute of Anhui, Hefei 230051, China
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(11), 999; https://doi.org/10.3390/cryst15110999
Submission received: 28 October 2025 / Revised: 17 November 2025 / Accepted: 18 November 2025 / Published: 20 November 2025
(This article belongs to the Section Crystal Engineering)
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Abstract

Alkali-activated slag grouting materials exhibited excellent mechanical properties but still faced technical challenges such as insufficient fluidity and overly rapid setting. To enhance their workability, this study introduced fly ash as a modifying component, leveraging its morphological and activity effects to systematically investigate the composite influence on fluidity, setting time, and compressive strength. The mechanism was further elucidated through microstructural analysis of the mineral crystallization characteristics of polycondensation products. The results indicated that with increasing fly ash content, the fluidity of the grouting material continuously improved, and both the initial and final setting times were significantly prolonged, albeit at the expense of a gradual decline in compressive strength. At a 20% fly ash content, the fluidity spread increased to 292 mm, the initial and final setting times were extended to 70 min and 103 min, respectively, while the 1 d and 28 d compressive strengths reached 11.8 MPa and 48.1 MPa, achieving an optimal overall performance that met practical grouting requirements. Microscopic analysis revealed that fly ash enhanced the rheological properties and delayed the setting process through the “ball-bearing effect” and its low early-age reactivity. However, as the fly ash content rose, the active calcium content in the system continuously decreased, inhibiting the formation and development of key mineral crystals such as calcium silicate and calcium aluminosilicate, thereby leading to the reduction in compressive strength.

1. Introduction

Alkali-activated slag grouting materials (AASGM) have garnered considerable interest in the field of civil engineering in recent years as a sustainable alternative to conventional cementitious systems [1]. These materials are primarily composed of industrial by-products such as Granulated Blast Furnace Slag (GBFS is a sandy, granular byproduct of the iron making process in a blast furnace. It is formed when the molten slag, which is composed of gangue from iron ore, ash from fuel, and fluxes fused at high temperatures, is rapidly quenched with water) and steel slag [2,3,4], which are activated using alkaline agents like sodium silicate or sodium hydroxide to form an inorganic polymer with a three-dimensional network structure [5]. This class of grouting materials exhibits a number of compelling advantages. From an environmental perspective, they contribute significantly to solid waste utilization and demonstrate a markedly lower carbon footprint and energy consumption compared to traditional Portland cement. Mechanically, they are characterized by rapid setting and early-age strength development, achieving compressive strengths exceeding 20 MPa within 5 h, while also developing high long-term strength due to their dense microstructure [6]. In terms of durability, they display superior resistance to acid and sulfate attack, lower drying shrinkage, and enhanced water stability relative to conventional cement-based materials. Additionally, they offer excellent impermeability and economic benefits, owing to the wide availability and low cost of the raw materials [7].
However, the limitations of these materials cannot be overlooked. A major challenge lies in controlling their workability: the freshly mixed slurry often exhibits insufficient fluidity (with flow spread below 250 mm) and excessively rapid setting (initial setting within 30 min), coupled with pronounced time-dependent rheological behavior [8]. These characteristics lead to a sharp increase in pumping resistance, frequently causing blockages at pipeline bends or constrictions, which results in construction interruptions and equipment wear [9]. Moreover, the short setting window severely limits the available operation time, as rheological properties deteriorate rapidly, amplifying the risk of pipe blockage and making the grouting process difficult to control [10]. The combination of poor fluidity and fast setting considerably restricts the effective penetration and diffusion of the grout through geological formations, preventing adequate filling of fine fractures in rock and soil or structural voids [11]. This often results in discontinuous and incomplete grout veins, leaving significant ungrouted zones [12]. Furthermore, the premature loss of fluidity impairs proper wetting and encapsulation at rough interfaces, significantly reducing the bond strength between the grout and surrounding rock or existing concrete [13]. Macroscopically, these issues lead to compromised integrity, density, and uniformity of the grouted body, undermining the intended functions of reinforcement, seepage prevention, and void filling [14]. Consequently, the overall project quality and long-term safety are jeopardized, while substantial additional costs are often incurred for troubleshooting and secondary grouting operations [15].
By incorporating fly ash as a modifying component, this study achieves remarkable improvement in slurry fluidity and precise control over setting behavior while preserving the system’s excellent early-age strength development and long-term mechanical performance [16]. The modification strategy leverages the unique physical morphology and low early-stage chemical reactivity of fly ash to overcome critical technological barriers in engineering applications [16]. This research systematically elucidates how variations in the fly ash-to-slag ratio regulate the calcium-to-silicon ratio, a fundamental chemical parameter that governs the formation and transformation of distinct mineral crystalline phases. Furthermore, it clarifies the consequent evolution in microscopic architecture and interfacial characteristics, thereby establishing a scientific basis for precision design of grouting material performance.

2. Experimental Materials and Mix Proportions

2.1. Experimental Materials

The binder materials consisted of GBFS and fly ash (FA). The GBFS, classified as Grade S95, was supplied by an energy mixture in Huainan, with its properties conforming to the Chinese national standard (Ground granulated blast furnace slag used for cement, mortar and concrete, GB/T 18046-2017) [17]. The specific surface area of GBFS is 415 m2/kg, and the residue of the 45 micron square pore sieve is less than 9%. The Class II FA, obtained from a power generation company in Anhui Province, complied with the requirements specified in (Fly ash used for cement and concrete, GB/T 1596-2017) [18]. The specific surface area of FA is 308 m2/kg, and the residue of the 45 micron square pore sieve is less than 18%. The chemical compositions of both GBFS and FA were determined through X-ray fluorescence (XRF) spectroscopy, with the results summarized in Table 1. The alkaline activator system comprised commercial-grade sodium hydroxide (NaOH) with a purity greater than 99% and liquid sodium silicate (Na2SiO3) with a chemical composition of 9.68% Na2O, 25.26% SiO2, and 65.02% H2O by mass. The modulus of liquid sodium silicate after mixing reached 1.0 (Na2O:SiO2). Throughout the experimental program, tap water was utilized as the mixing water.

2.2. Mix Proportion Design

The experimental mix proportions are presented in Table 2. G10 formulated with 100% GBFS as the binder, served as the control mixture. Under the condition of a constant alkaline activator content, the FA content from G9F1 to G4F6 was systematically increased from 10% to 60%, replacing an equivalent mass of GBFS. This design aimed to comprehensively investigate the influence of FA on the fluidity, setting behavior, and mechanical properties of AASGM.

3. Experimental Program

3.1. Specimen Preparation and Curing

The preparation and curing procedures for AASGM followed the Chinese standard (Methods of Testing Cements-Determination of Strength, GB/T 17671-2021) [19]. Using G9F1 as an example, NaOH was first dissolved in water according to the proportions listed in Table 2, and the solution was poured into a JJ-5-type cement mortar mixer. Pre-blended GBFS and FA were then introduced into the mixer. The mixture was stirred at low speed for 30 s, followed by high-speed mixing for another 30 s, which yielded a homogeneous alkali-activated grout. The fresh mixture was cast into triple-gang molds measuring 40 mm × 40 mm × 160 mm. After molding, the specimens were immediately transferred to a standard curing chamber that was maintained at 20 ± 1 °C and a relative humidity of ≥98%. Demolding was performed after 24 h, and the specimens were subsequently cured until the designated testing ages.

3.2. Fluidity, Setting Behavior and Compressive Strength Testing

The fluidity of AASGM was evaluated using the method in accordance with the Chinese standard (Test method for fluidity of cement mortar, GB/T 2419-2005) [20]. Setting behavior was assessed following (Test methods for water requirement of standard consistency, setting time and soundness of the Portland cement, GB/T 1346-2024) [21], with timing commencing from the initial contact between the activator and the binder materials. Compressive strength tests were performed in compliance with (Methods of Testing Cements-Determination of strength, GB/T 17671-2021) [19] using a YAW-300D fully automatic cement mortar testing machine (Jinan Liling Testing Machine Co., Ltd., Jinan, China). A liquid-to-solid ratio of 0.6 was maintained consistently throughout all tests.

3.3. Microscopic Analysis

Microstructural analysis of AASGM was conducted using a D8 Advance X-ray diffractometer (XRD, Bruker, Berlin, Germany) and a VERTEX 80v Fourier-transform infrared spectroscopy system (FT-IR, Bruker, Berlin, Germany). These techniques were employed to systematically investigate mineral crystallization behavior and structural evolution, with particular focus on crystal nucleation and growth mechanisms as well as time-dependent correlations between microstructure and performance. XRD analysis was performed over a 2θ range of 5° to 70° with a step size of 0.02° and a counting time of 1 s per step, ensuring both high resolution and adequate signal-to-noise ratio. FT-IR spectra were collected within the wavenumber range of 4000 cm−1 to 400 cm−1. Paste samples were precisely crushed and further refined using a planetary ball mill to achieve a powder fineness exceeding 300 mesh.

4. Experimental Results and Analysis

4.1. Liquidity Analysis

4.1.1. G10 Liquidity Analysis

Figure 1 presents the fluidity analysis of FA modified AASGM. The G10 exhibited a flow diameter of 242 mm, which meets the fluidity requirements for compaction grouting (180–260 mm) and fracture grouting (240–300 mm). However, this value falls below the minimum thresholds for permeation grouting (280–350 mm), dynamic water sealing grouting (260–320 mm), and large-volume filling grouting (300–400+ mm) [22]. Therefore, enhancing the fluidity of AASGM is essential to broaden its engineering applicability and process adaptability, thereby fully leveraging its early-age and high-strength characteristics.

4.1.2. Analysis of the Impact of FA on Liquidity

The incorporation of 10% FA in G9F1 resulted in a notable increase in flow spread to 269 mm, representing an 11.2% enhancement compared to the G10. This improvement indicates that FA effectively enhances fluidity, enabling the mixture to meet the fluidity requirements for dynamic water sealing grouting and approach the lower threshold for permeation grouting. However, this value remains substantially below the high fluidity range required for large-volume filling grouting, thus limiting its applicability in such engineering scenarios [23]. The spherical glassy microbeads characteristic of FA contribute to a “ball-bearing effect”, which reduces internal friction among solid particles and disrupts flocculated structures, thereby significantly improving the rheological performance of the grout [24]. Concurrently, the relatively slow dissolution rate of FA in alkaline environments during early stages allows its particles to adsorb free water, forming a lubricating water film that further facilitates particle sliding. The combined action of the physical morphology of FA and its interfacial behavior in alkaline media constitutes the primary mechanism responsible for the observed fluidity enhancement [24].
With increasing FA content, the flow spread of G8F2 and G7F3 rose to 292 mm and 310 mm, respectively, maintaining an upward trend, though the rate of increase declined to 8.6% and 6.2%. This indicates that the “ball-bearing effect” and interfacial adsorption of FA further contributed to fluidity enhancement, allowing the material to meet the performance requirements for large-volume filling grouting [22]. However, when the FA content exceeded 30%, the fluidity from G6F4 to G4F6 continued to increase only marginally with the growth rate falling below 3%. This can be attributed to the particle packing state approaching saturation within the system. Beyond this point, additional FA no longer significantly improves the particle size distribution, but due to its higher specific surface area, leads to an increased water demand. Moreover, an excess of spherical particles induces more frequent interparticle collisions under shear, which consequently elevates flow resistance [23]. In summary, while an appropriate amount of FA significantly enhances the fluidity of grouting materials, the extent of improvement follows a pattern of initial marked increase followed by gradual attenuation. Controlling the FA content within 30% provides an optimal balance between engineering applicability and material economy.

4.2. Analysis of Setting Law

4.2.1. Analysis of G10 Setting Law

Figure 2 presents the setting law analysis of FA modified AASGM. The G10 exhibited initial and final setting times of 37 min and 51 min, respectively. Research indicates that, with the exception of dynamic water sealing grouting which requires an initial setting time of less than 10 min, other grouting techniques-including permeation, fracture, and compaction grouting-generally demand an initial setting time within the range of 45 to 120 min (approximately 1–2 h) [9]. Large-volume filling grouting further requires a setting time exceeding 2 h. This recommended range ensures a practical balance between construction operability and early-age strength development. However, the rapid setting characteristics of G10 fail to meet the requirements of the aforementioned conventional grouting techniques, rendering it unsuitable for achieving adequate diffusion and filling or for rapid water sealing applications. Consequently, systematic regulation and optimization of the setting behavior of AASGM are imperative.

4.2.2. The Influence of FA on the Setting Law

With 10% FA incorporation, G9F1 demonstrated extended initial and final setting times of 51 min and 72 min, representing increases of 37.8% and 41.2%, respectively, compared to the G10. This result confirms that FA significantly retards the setting process, substantially extending the available operation time and enhancing the suitability of AASGM for engineering applications requiring prolonged penetration or large-volume placement. When the FA content increased to 20%, the G8F2 exhibited further extended setting times of 70 min for initial set and 103 min for final set, corresponding to additional retardation of 37.3% to 43.1% compared to the G9F1. This progressive delay indicates that higher FA contents continue to retard the setting process, with the retardation effect becoming more pronounced. The low early-age reactivity of FA enables it to function primarily as a physical filler during the initial reaction stage, not only improving fluidity but also forming a physical barrier layer on the surface of GBFS particles that effectively impedes direct contact between GBFS and the alkaline solution [24]. This spatial steric effect significantly slows the initial dissolution rate of GBFS particles, providing an extended induction period for the setting process [25]. More importantly, as a low-calcium, high-silica-alumina material, FA incorporation substantially reduces the overall calcium-to-silicon ratio in the system, thereby inhibiting the rapid nucleation and precipitation of high-calcium C-A-S-H gels while promoting the formation of slower-forming low-calcium N-A-S-H gel systems [26]. Simultaneously, the stable glassy network structure of FA results in slow dissolution kinetics in alkaline environments during early stages, preventing it from rapidly releasing active Ca2+ and Al3+ like GBFS to participate in gel network formation, thus effectively prolonging the induction period of the setting process [27].
With continuously increasing FA content, G7F3 to G4F6 exhibited a progressive reduction in reactive components, leading to further retardation of the setting process. The extent of this retardation effect became progressively more pronounced. This regulatory effect enabled the initial setting time of AASGM to gradually meet the requirements of various grouting techniques, thereby providing the necessary time window for long-distance penetration and complete filling of fine fractures in complex geological conditions, while effectively avoiding engineering issues such as pipeline blockage or insufficient diffusion caused by premature setting. In summary, FA effectively modulates the setting characteristics of AASGM, with its retarding effect intensifying as incorporation levels rise. This modification transforms AASGM from initially failing to meet basic grouting requirements to progressively achieving setting performance adaptable to diverse construction processes.

4.3. Analysis of Compressive Strength

4.3.1. G10 Compressive Strength Analysis

Figure 3 presents the compressive strength of FA modified AASGM. The G10 achieved a compressive strength of 12.9 MPa after 1 d, which subsequently increased to 29.2 MPa at 3 d, 36.7 MPa at 7 d, and reached 48.5 MPa by 28 d. This progressive strength development clearly demonstrates the distinctive early-age and high-strength characteristics of the AASGM. The 1 d strength exceeding 10 MPa satisfies the early-stage mechanical requirements for most grouting applications, with the exception of emergency water-plugging scenarios and high-demand structural reinforcement projects. Given this robust mechanical performance foundation, the primary focus for enhancing the engineering applicability of AASGM should now shift to the systematic improvement of its key fresh properties, particularly fluidity and setting time.

4.3.2. The Influence of FA on Compressive Strength

The G9F1 incorporating 10% FA demonstrated compressive strengths of 12.5 MPa at 1 d, 28.6 MPa at 3 d, 35.2 MPa at 7 d, and 50.8 MPa at 28 d. While the 1 d, 3 d, and 7 d strengths showed minor reductions of 1–3% compared to the G10, the 28 d strength exhibited a 4.7% improvement. Although rapid early-age strength development remains crucial for AASGM performance, the incorporation of 10% FA did not significantly compromise early-age properties, with all values remaining fully adequate for grouting applications. More importantly, the 10% FA addition contributed to an 11.2% enhancement in fluidity and a 37.8% extension of initial setting time. This marginal reduction in early strength can therefore be considered an acceptable compromise given the substantial improvements achieved in workability and construction process adaptability.
When the FA content increased to 20%, the G8F2 achieved compressive strength of 11.8 MPa at 1 d, 27.4 MPa at 3 d, 35.2 MPa at 7 d, and 48.1 MPa at 28 d. These values represent a reduction of 5–8% compared to the G9F1, indicating a continued decline in compressive strength with increasing FA content, accompanied by an expanding reduction margin. FA particles typically exhibit a dense, glassy microsphere morphology characterized by a highly polymerized and stable aluminosilicate glassy network. This structural configuration results in a significantly slower dissolution rate under ambient alkali-activated conditions compared to GBFS. The alkali-activated reaction requires high-concentration OH- to attack and break chemical bonds, leading to depolymerization and the release of reactive silicate and aluminate species, which subsequently undergo polycondensation into gel phases [25]. However, the stable Si-O-Si and Si-O-Al bonds in the FA glassy phase demonstrate stronger resistance to alkaline attack. Their sluggish dissolution kinetics lead to an insufficient concentration of silicate, aluminate monomers, and oligomers available for gel formation during the early hydration stage, thereby delaying the nucleation and growth of gel products [26]. This mechanism represents the primary cause for the reduced early-age compressive strength observed in the G8F2.
During the early stage, insufficiently reacted FA particles primarily function as physical fillers. Their smooth surface morphology and low early-age chemical reactivity hinder the formation of strong interfacial chemical bonds with the gel system, thereby limiting their contribution to mechanical strength [27]. Consequently, in the initial polycondensation phase, the limited amount of gel products formed must encapsulate a substantial volume of unreacted FA particles, resulting in underdeveloped matrix microstructures. This characteristic directly manifests as delayed mechanical property development at the macroscopic level, particularly in terms of early-age strength evolution [23]. Nevertheless, the spherical particle morphology of FA significantly enhances the rheological behavior of the fresh mixture, while its low early-age reactivity effectively prolongs the initial setting time [26]. Taking G8F2 as an example, compared to the G10, it achieved a 20.7% improvement in fluidity and an 89.2% extension of initial setting time, while experiencing only an approximate 10% reduction in compressive strength. This performance profile indicates that the AASGM has transitioned from initially failing to meet grouting requirements to fully accommodating practical engineering demands, while retaining its fundamental advantages of early-age and high strength. Comprehensive analysis confirms that a GBFS to FA ratio of 8:2 achieves an optimal balance between workability and mechanical performance, representing the most favorable overall characteristics.
A progressive reduction in compressive strength was observed with increasing FA content, exhibiting a systematic and increasingly pronounced decline. The G7F3 and G6F4 demonstrated compressive strength decreases of approximately 20–30% relative to the G10. Despite this reduction, the strength values remain within acceptable limits for the majority of grouting applications. Concurrently, these mixtures displayed enhanced fluidity and extended initial setting times, affording a broader operational window for effective penetration and complete filling in complex geological formations. In contrast, the G5F5 and G4F6 underwent a substantial decrease in compressive strength, with reductions reaching 50–80%. This pronounced deterioration is attributed to the high FA content significantly diminishing the proportion of reactive components available for the alkali-activated process. The resultant insufficient formation of condensation products gels, coupled with the development of a poorly structured microstructure, ultimately prevents the establishment of a robust three-dimensional network framework [25]. Although G5F5 and G4F6 possess excellent fluidity and delayed setting characteristics, their inadequate mechanical performance renders them unsuitable for conventional grouting and reinforcement purposes, restricting their application to large-volume filling operations where minimal strength is required. In summary, the influence of FA content on AASGM performance follows a distinct threshold behavior. Maintaining FA content below 20% enables an optimal compromise between compressive strength and workability. Exceeding this threshold initiates strength reductions of 20–30%, necessitating careful evaluation based on specific project requirements. Further increasing FA content beyond 40% results in severe mechanical property degradation, limiting material utilization to scenarios with minimal strength demands. Consequently, precise regulation of FA dosage is imperative in engineering practice to achieve targeted performance characteristics.

4.4. XRD Analysis

4.4.1. XRD Analysis of G10

The XRD pattern of G10 at 1 d (Figure 4a) exhibits a broad and diffuse diffraction hump within the 20–40° (2θ) range, which is attributed to calcium (alumino) silicate minerals including tobermorite (16.3°, 29.1°, 37.8°), hillebrandite (25.2°, 30.5°, 50.7°), gehlenite (24°, 31.5°, 61.2°), and gismondine (18.3°, 28°, 33.6°). The diffuse nature of this hump indicates that these phases predominantly exist in an amorphous state characterized by short-range order and long-range disorder, reflecting a highly disordered microstructure of the reaction system at early ages [24]. This result confirms that under high-alkalinity activation, reactive Ca, Si, and Al components from GBFS rapidly dissolve and depolymerize, subsequently undergoing polycondensation to form calcium silicate and calcium aluminosilicate minerals [27]. The rapid formation and accumulation of these phases are critical for achieving the fast setting, hardening, and early-age strength development observed in the G10. The XRD pattern of G10 at 28 d demonstrated markedly enhanced intensity and sharper diffraction profiles for characteristic peaks corresponding to tobermorite, hillebrandite, gehlenite, and gismondine. This indicates a progressive improvement in crystallinity and more complete crystal development of the calcium silicate and calcium aluminosilicate phases during the prolonged curing period. The observed phase evolution, accompanied by further formation of crystalline products, underpins the continuous and substantial development of compressive strength in mixture G10 at later ages by providing a robust and well-structured microscopic framework [22].

4.4.2. FA Regulated XRD Analysis

The XRD pattern of the G9F1 at 1 d (Figure 4b) clearly reveals the directed regulatory effect of FA on phase evolution during the polycondensation process in the alkali-activated system. Compared to the G10, the characteristic diffraction peaks representing key strength contributing phases including tobermorite, hillebrandite, gehlenite, and gismondine exhibited no significant reduction in intensity. This observation indicates that the incorporation of 10% FA did not suppress the formation or degree of polymerization of the primary mineral phases, thereby explaining the maintained early-age compressive strength. However, new characteristic peaks absent in the G10 pattern emerged in the G9F1 diffractogram, identified as quartz (20.9°, 36.5°, 68.2°), calcite (39.4°, 43.2°, 48.6°), and mullite (26.3°, 60.8°). These crystalline phases represent inherent mineral components of FA, which remain chemically stable and exhibit low reactivity in early-stage alkaline environments, thus rarely participating in dissolution polycondensation processes [27]. Their presence in the XRD pattern directly confirms that most FA particles remained largely unreacted at 1 d, primarily functioning as physical fillers within the system. The low early-age chemical activity of FA contributed to retarding the overall reaction progress, providing a reasonable explanation for the extended setting time [28]. Meanwhile, the unreacted FA particles acted as micro-aggregates, forming a composite structure with the gel matrix that synergistically optimized the overall material performance without significantly compromising early-age strength development [29]. The XRD pattern of the G9F1 at 28 d displayed a significant increase in the intensity of characteristic diffraction peaks corresponding to key mineral phases including tobermorite, hillebrandite, gehlenite, and gismondine, while concurrently showing a marked reduction in the intensity of the mullite peak inherent to FA. This phase evolution indicates that the reactive components in FA actively participated in the alkali-activated polycondensation process during the extended curing period, effectively promoting the continuous formation and crystalline development of calcium silicate and calcium aluminosilicate minerals [26]. Thereby, this microstructural development established a solid foundation for the sustained growth of later-age compressive strength in the G9F1.
The XRD pattern of the G8F2 at 1 d (Figure 4b) reveals a discernible inhibitory effect of FA on the phase evolution of AASGM. Compared to the G9F1, the diffuse hump intensity within the 20–40° range, which corresponds to calcium (alumino) silicate hydrates, was notably attenuated in the G8F2. Furthermore, characteristic diffraction peaks of phases such as tobermorite, hillebrandite, gehlenite, and gismondine exhibited pronounced broadening, indicating reduced crystallinity and diminished polymerization degree of the polycondensed products. This phenomenon primarily stems from the replacement of GBFS by FA, which significantly decreased the total reactive components particularly calcium content in the system. The resulting imbalance in the molar ratio between silicate/aluminate sources and calcium sources available for polycondensation in the alkaline environment led to lowered overall reactivity and reduced efficiency of minerals formation [26]. These microstructural changes provide a rational explanation, from the perspective of phase composition, for the progressive decline in compressive strength and the extended setting time observed with increasing FA content [27]. Additionally, the enhanced intensity of diffraction peaks corresponding to inert crystalline phases such as quartz, mullite, and calcite in the G8F2 XRD pattern further confirms the residual accumulation of low-reactivity components from FA, offering direct evidence for the described reaction inhibition mechanism. The XRD pattern of the G8F2 at 28 d showed increased intensity in the diffuse diffraction hump associated with calcium (alumino) silicate polycondensation products compared to its early-age counterpart, yet this intensity remained markedly lower than that observed in the G9F1 at the same curing age. This result clearly demonstrates that increasing the FA content to 20% significantly inhibits the formation of silicate and aluminosilicate crystalline phases during the later stages of polycondensation. Furthermore, the elevated FA content altered the chemical environment and nucleation kinetics of the mineral phase, leading to reduced crystallinity and diminished structural ordering of the primary polycondensation products, such as silicate and aluminosilicate hydrates [30]. This microstructural degradation constitutes the fundamental reason for the lower 28 d compressive strength of the G8F2.
The XRD pattern of the G4F6 at 1 d (Figure 4d) reveals substantial alterations in the early-stage phase evolution of the AASGM. The characteristic diffraction peaks corresponding to tobermorite and hillebrandite completely disappeared, while the peak intensities of gehlenite and gismondine showed continued significant reduction compared to the G8F2 at the same curing period. Concurrently, the diffraction peaks of quartz, mullite, and calcite exhibited progressively enhanced intensities. These observations collectively indicate that the high FA incorporation of 60% creates a severely calcium-deficient environment within the polycondensation system, primarily due to the proportional reduction in GBFS content [29]. This calcium deficiency primarily affects the polycondensation formation of calcium silicate minerals, preventing the crystallization of tobermorite and hillebrandite, while simultaneously severely impeding the crystallization of calcium aluminosilicate minerals, resulting in diminished peak intensities for gehlenite and gismondine. Consequently, the reduced formation of both calcium silicate and calcium aluminosilicate crystalline phases directly accounts for the dramatic decline in the 1 d compressive strength of the G4F6. This microstructural evidence further elucidates the mechanism behind the continuous extension of initial setting time with increasing FA content, as the deficient calcium availability slows down the overall reaction kinetics and gel network development [30]. Even after extended curing to 28 d, the XRD pattern of the G4F6 displayed no significant enhancement in the diffraction peaks of calcium (alumino) silicate hydrates. This persistent suppression of crystalline phase formation demonstrates that the polycondensation process for both calcium silicate and calcium aluminosilicate minerals remains severely constrained in high-FA systems [31]. The chronically calcium-deficient condition, combined with the limited long-term reactivity of FA particles, fundamentally restricts the progressive development of strength-bearing phases [25]. This microstructural stagnation provides the fundamental explanation for the negligible strength gain observed in the G4F6 between 1 and 28 d, highlighting the critical influence of FA content on both the short-term and long-term performance evolution of AASGM.

4.5. FT-IR Analysis

4.5.1. FT-IR Analysis of G10

The FT-IR spectrum of the G10 at 1 d (Figure 5a) reveals the evolution of key chemical bonds during the early-stage polymerization process of the AASGM. The strong absorption band at 998 cm−1 is attributed to the asymmetric stretching vibration of Si-O-Si bridges, confirming that the silicate tetrahedra in the C-S-H gel system have undergone significant polycondensation, transitioning from isolated monomers or oligomers into chain-like or layered structures with higher polymerization degrees [32]. This finding correlates well with the detection of calcium silicate crystalline phases in the XRD analysis, collectively demonstrating the formation of an early-stage gel system. The absorption band observed at 740 cm−1 originates from Al-O bond vibrations, indicating the successful incorporation of aluminate tetrahedra into the gel framework. This result shows strong consistency with the identification of gehlenite and gismondine crystalline phases in the XRD patterns, jointly verifying the formation of calcium aluminate phases and their structural role in constructing the three-dimensional network. The co-occurrence of Si-O-Si and Al-O vibrational features systematically demonstrates that under high alkalinity activation, the glassy structure of GBFS undergoes rapid dissociation, releasing highly reactive calcium, silicate, and aluminate [33]. These species subsequently reconstruct through polycondensation reactions, progressively building a three-dimensional aluminosilicate network structure dominated by C-A-S-H gel [34]. This structural evolution process not only represents the micro-mechanism behind the rapid setting of the G10 but also constitutes the fundamental reason for its remarkable early-age mechanical performance [28]. Additionally, the absorption band at 1414 cm−1 corresponds to the symmetric stretching vibration of CO32−, indicating the presence of carbonation products within the system. The H-O-H bending vibration at 1662 cm−1 represents physically adsorbed water in gel pores, while the broad band at 3540 cm−1 is associated with the oriented arrangement of OH− from the alkaline activator and the stretching vibrations of hydroxyl groups on gel surfaces. These features further reveal the rich interfacial chemical environment and the presence of unreacted alkaline components within the system.

4.5.2. FA Regulated FT-IR Analysis

The FT-IR spectrum of G9F1 (Figure 5b) at 1 d showed no reduction in intensity of the Si-O-Si vibrational band at 968 cm−1, indicating that the 10% FA addition did not inhibit the formation of C-S-H gel. However, the full width at half maximum of this band increased, suggesting a higher proportion of layered C-S-H gel formation. The intensity of the Al-O vibrational signal at 766 cm−1 showed no significant decrease, demonstrating that the formation amount of C-A-S-H gel remained unaffected by FA incorporation. These observations indicate that the addition of 10% FA did not substantially alter the calcium-to-silicon ratio of the polycondensation system, allowing efficient progression of the polycondensation reaction and formation of polymerization products [31]. Nevertheless, a distinct absorption band emerged at 462 cm−1, corresponding to the asymmetric stretching vibration of Si-O bonds, which aligns with the characteristic quartz peak identified in the XRD pattern. This band was not detected in the G10. Furthermore, a significantly enhanced CO32− vibrational mode appeared at 1424 cm−1, indicating increased carbonate content. Both characteristic absorptions originated from the inherent properties of FA, confirming that FA introduction incorporated inert components into the polycondensation system [35]. This phenomenon not only explains the slightly lower compressive strength of G9F1 compared to G10 but also provides evidence for its extended setting time.
As shown in Figure 5c, the FT-IR spectrum of the G8F2 at 1 d exhibits a characteristic peak at 980 cm−1, attributed to the asymmetric stretching vibration of Si-O-Si bonds in silicate tetrahedra. This peak demonstrates reduced intensity, a shift toward higher wavenumbers, and a noticeable narrowing of the full width at half maximum. These changes indicate an alteration in the polymerization state of the silicate tetrahedra, where the formation of highly polymerized chain-like or layered structures is suppressed, and the overall degree of silicate network polymerization in the gel products declines [34]. The absorption peak at 778 cm−1, assigned to Al-O vibrations in aluminate tetrahedra, displays a similar shift to higher wavenumbers and peak narrowing. This suggests modifications in the coordination environment of aluminum within the gel structure, implying that the incorporation level of aluminum in C-A-S-H gel or its isomorphous substitution mode is significantly influenced [32]. The observed changes in both Si-O-Si and Al-O vibrational features indicate a decline in the formation efficiency and structural development of key gel products such as C-S-H and C-A-S-H, providing an explanation at the chemical bond level for the continued reduction in compressive strength of the G8F2 [30]. From the perspective of reactant composition, the increased FA content leads to a proportional decrease in GBFS proportion. Since the active calcium source in the mixed system primarily originates from GBFS (refer to Table 1, where the CaO content in GBFS is significantly higher than in FA), the relative deficiency of calcium directly affects the stoichiometric ratio and nucleation driving force for the formation of calcium silicate and calcium aluminosilicate products [33]. This results in a reduction in the number of active units participating in effective polycondensation reactions, thereby decreasing the overall reaction rate and cumulative product formation [30]. Furthermore, the vibrational peaks at 460 cm−1, corresponding to Si-O bonds in quartz, and at 1426 cm−1, associated with O-C-O stretching in carbonate groups, are significantly enhanced in the G8F2 group. Both vibrations originate from the inherent crystalline and carbonate components in FA. The increased intensity of these signals further confirms the accumulation of low-reactivity FA components within the system, providing microscopic evidence that supports the mechanistic interpretation of disrupted calcium-silica-alumina reaction equilibrium and suppressed polycondensation processes [34]. Thus, these spectral observations furnish comprehensive evidence for the causal relationship between increased FA content and the corresponding changes in macroscopic properties.
With continuously increasing FA content, the FT-IR spectrum of the G4F6 (Figure 5d) exhibits a progressive reduction in the characteristic peak intensities corresponding to the Si-O-Si asymmetric stretching vibration in silicate tetrahedra and the Al-O vibrational mode in aluminate units. Concurrently, the vibrational signals of the Si–O band (around 460 cm−1, associated with quartz) and the O-C-O symmetric stretching (near 1426 cm−1, attributed to carbonates) show further enhancement. These spectral changes are directly attributable to the increased incorporation of FA, whose inherent crystalline and carbonate phases act largely as inert fillers in the reaction system [35]. The continuous attenuation of the Si-O-Si and Al-O vibrational intensities provides clear evidence that elevated FA content significantly impedes the formation and structural development of calcium silicate and calcium aluminosilicate gels [36]. This inhibition arises from a critically reduced calcium availability, due to the proportional decrease in GBFS, which disrupts the stoichiometric balance required for effective polycondensation [27]. As a result, the nucleation and growth of strength-contributing gel phases are substantially suppressed, leading to a poorly developed and under-connected microstructure [37]. The concurrent enhancement of inert-phase vibrational signals further confirms the accumulation of unreacted or weakly reacted FA particles, which not only fail to contribute to mechanical strength but also physically disrupt the continuity of the gel network [38]. This microstructural deterioration, driven by both chemical dilution and physical interference, fundamentally explains the sharp and continuous decline in compressive strength observed in mixtures with high FA content [39].

5. Conclusions

This study systematically investigated the effects of FA content on the fluidity, setting behavior, mechanical properties, and mineral crystallization characteristics of AASGM. By combining XRD and FT-IR analyses, the role of FA in enhancing fluidity, delaying setting time, and promoting cooperative polycondensation reactions was elucidated. The main conclusions are summarized as follows:
(1) The fluidity of AASGM increased continuously with rising FA content, although the rate of improvement gradually diminished. At an FA content of 20%, the mixture achieved a flow spread of 292 mm, representing a 20.7% enhancement, which met the fluidity requirements for various grouting applications.
(2) Both the initial and final setting times exhibited progressive extension with increasing FA content, with the retardation effect becoming increasingly pronounced. At an FA incorporation level of 20%, the initial and final setting times were extended to 70 min and 103 min, representing an extension of 89.2% and 102%, respectively, thereby fulfilling the setting time requirements for grouting operations.
(3) The compressive strength of AASGM exhibits a progressive decline with increasing FA content, accompanied by a gradually expanding reduction margin. At a FA content of 20%, the system achieves an optimal balance among compressive strength, fluidity, and setting time. The compressive strength development follows this trajectory, 11.8 MPa (1 d), 27.4 MPa (3 d), 32.3 MPa (7 d), and 48.1 MPa (28 d), fully meeting the strength requirements for various grouting applications at different curing ages. Microstructural analysis reveals that increased FA content reduces the total calcium content in the system, thereby retarding the kinetics of the polycondensation reaction and decreasing the total formation of calcium silicate and calcium aluminosilicate gels. However, this mechanism simultaneously contributes to the regulation of setting behavior, enabling AASGM to maintain appropriate strength development while significantly improving workability. This study provides both theoretical foundation and practical pathway for performance optimization of AASGM through compositional design.
(4) The FA content was identified as a key governing parameter for application suitability. A dosage between 10% and 20% enabled a range of techniques (compaction, fracture, and permeation grouting), whereas concentrations above 30% resulted in properties exclusively tailored for large-volume filling. This differential suitability underscores the necessity of meticulous formulation design, coupled with rigorous tracking of shrinkage behavior to mitigate potential cracking.

Author Contributions

Investigation, B.D.; funding acquisition, J.Q.; formal analysis, F.Z.; data curation, J.X.; writing review and editing, G.H. All authors have read and agreed to the published version of the manuscript.

Funding

The research described in this paper was financially supported by Anhui Provincial Natura Science Foundation “Design and Collaborative Enhancement lechanism of Cement lmproved Coal Gangue Alkali Activated Double Cementitious Material” (2308085ME184); 2023 Wuhu Science and Technology Bureau Applied Basic Research Project “Designand Collaborative Enhancement Mechanism of Cement Improved Waste Incineration Bottom Ash Alkali Activated Double Cementitious Materials” (No. 2023jc01); Anhui Provincial Department of Education’s 2023 New Era Education Quality Project “Wuhu QiuhuaThermal Insulation Materials Co., Ltd. Graduate Enterprise Workstation” (2023qygzz021).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 15 00999 g001
Figure 1. Analysis of the influence of FA on the flowability.
Figure 1. Analysis of the influence of FA on the flowability.
Crystals 15 00999 g001
Crystals 15 00999 g002
Figure 2. Analysis of the influence of FA on setting law.
Figure 2. Analysis of the influence of FA on setting law.
Crystals 15 00999 g002
Crystals 15 00999 g003
Figure 3. Analysis of the effect of FA on compressive strength.
Figure 3. Analysis of the effect of FA on compressive strength.
Crystals 15 00999 g003
Crystals 15 00999 g004aCrystals 15 00999 g004b
Figure 4. Analysis of the Effect of FA on XRD of AASGM.
Figure 4. Analysis of the Effect of FA on XRD of AASGM.
Crystals 15 00999 g004aCrystals 15 00999 g004b
Crystals 15 00999 g005aCrystals 15 00999 g005b
Figure 5. Analysis of the Effect of FA on XRD of AASGM.
Figure 5. Analysis of the Effect of FA on XRD of AASGM.
Crystals 15 00999 g005aCrystals 15 00999 g005b
Table 1. Chemical composition of raw materials %.
Table 1. Chemical composition of raw materials %.
SiO2Al2O3Fe2O3CaOMgONa2OK2OSO3OthersLoss
GBFS32.4820.820.8234.556.161.150.650.132.061.18
FA55.3230.345.891.871.360.820.360.052.641.35
Table 2. Mix proportion/g.
Table 2. Mix proportion/g.
GBFSFANaOHLiquid Sodium SilicateWaterLiquid-Solid Ratio
G1010000452004700.6
G9F1900100452004700.6
G8F2800200452004700.6
G7F3700300452004700.6
G6F4600400452004700.6
G5F5500500452004700.6
G4F6400600452004700.6
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Huang, G.; Xu, J.; Qi, J.; Zhang, F.; Dou, B. Influence of Fly Ash on the Macro Properties and Mineral Crystal Characteristics of Alkali-Activated Slag Grouting Materials. Crystals 2025, 15, 999. https://doi.org/10.3390/cryst15110999

AMA Style

Huang G, Xu J, Qi J, Zhang F, Dou B. Influence of Fly Ash on the Macro Properties and Mineral Crystal Characteristics of Alkali-Activated Slag Grouting Materials. Crystals. 2025; 15(11):999. https://doi.org/10.3390/cryst15110999

Chicago/Turabian Style

Huang, Guodong, Jiahao Xu, Jun Qi, Fengan Zhang, and Baoxuan Dou. 2025. "Influence of Fly Ash on the Macro Properties and Mineral Crystal Characteristics of Alkali-Activated Slag Grouting Materials" Crystals 15, no. 11: 999. https://doi.org/10.3390/cryst15110999

APA Style

Huang, G., Xu, J., Qi, J., Zhang, F., & Dou, B. (2025). Influence of Fly Ash on the Macro Properties and Mineral Crystal Characteristics of Alkali-Activated Slag Grouting Materials. Crystals, 15(11), 999. https://doi.org/10.3390/cryst15110999

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