Strength and Microstructural Characteristics of Activated Fly Ash–Cement Paste

Abstract

Incorporating high volumes of fly ash (FA) in filling materials reduces costs and carbon emissions, but low early strength limits its use. This study investigates the effects of sodium sulfate decahydrate (Na₂SO₄·10H₂O) and calcium hydroxide (Ca(OH)₂) as activators at concentrations of 0.5%, 1.0%, 1.5%, and 2.0% on the mechanical properties and microstructure of tailings–cement–fly ash composites. Compressive strength testing, X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used to evaluate performance at different curing stages. Results indicate that all activators enhance early strength, with 2.0% Ca(OH)₂ yielding the greatest improvement. Microstructural analysis showed that activators boost quartz reactivity and create denser structures. Na₂SO₄ promotes ettringite and gypsum formation, while Ca(OH)₂ increases alkalinity, enhancing gel formation from FA. These findings clarify how activators improve the performance of activated fly ash–cement paste (AFCP).

1. Introduction

At present, the rapid and stable development of human society requires a large amount of energy and mineral resources as its cornerstone [1,2]; however, mining operations generate substantial quantities of tailings, which can lead to surface subsidence and groundwater contamination [3,4]. In recent decades, cemented paste backfill (CPB) has become a preferred method for managing large volumes of tailings from metal mines due to its safety, environmental sustainability, efficiency, and economic advantages [5,6]. CPB involves mixing aggregates, such as tailings, binders, additives, and water, in specific proportions to create a paste that meets required technical specifications [7]. This mixture is then transported underground via a pipeline to fulfill its intended functions. However, the commonly used binder ordinary Portland cement (OPC) constitutes approximately 75% of the total CPB cost [8]. Moreover, OPC production can account for up to 70% of the lifecycle greenhouse gas emissions associated with CPB [9].

The search for alternatives to OPC has gained significant interest among researchers. Studies have demonstrated that replacing cement with copper tailings enhances the acid resistance and reduces the chloride ion permeability of mortar mixtures, making them suitable for use in corrosive environments [10]. Similarly, substituting 10–15% of the concrete content with copper tailings has been found to yield significant gains in strength. Research has shown that increasing the proportion of copper tailings improves the effective immobilization of leachable elements within the mixture, thereby promoting sustainable development [11]. Novel binders consisting of lime products and naturally occurring volcanic ash have also been explored, which effectively manage sulfate tailings and can reduce greenhouse gas emissions by up to 80% [12]. Additionally, the feasibility of using steel slag as a partial replacement for cement to improve the properties of ultra-fine tailing fills has been examined [13]. Fly ash (FA), a byproduct of coal combustion in thermal power plants, has also been widely utilized as an alternative cementitious material [14,15,16,17,18].

FA, a byproduct of coal combustion in thermal power plants, primarily consists of siliceous or calcareous materials [19]. According to a 2023 report by the National Geochemical Society in the United States, global coal-fired power plants generate an annual output of FA ranging from 6 to 8 billion tons [20]. In China, this figure approaches 900 million tons [21]. If not managed properly, this substantial volume of FA can contribute to atmospheric pollution and pose health risks due to the presence of toxic chemicals. However, when utilized in cement-based materials, FA can significantly enhance durability [22]. Over the past three decades, the use of FA as a supplementary cementitious material (SCM) has evolved from moderate cement replacement levels of 20–25% to much higher volumes [19]. Despite the successful integration of FA into cement mixtures, a notable drawback is the reduced early strength of fly ash–cement pastes compared to traditional cement pastes [23]. This reduction may fail to meet design strength requirements, causing delays in the stabilization of backfilling, which in turn affects subsequent filling progress and may not meet operational requirements for equipment, ultimately impacting mining cycles. Some studies have found that the addition of FA significantly reduces the compressive strength at 3 days, indicating that FA slows the early hydration reactions in fly ash–cement composites. Consequently, this necessitates a longer curing period to achieve the desired strength outcomes [24,25,26].

Numerous studies have sought to enhance the early strength of fly ash–cement through various methods, including adjusting curing temperatures [25], increasing FA fineness, and incorporating chemical activators [27]. Research indicates that chemical activation can effectively accelerate the pozzolanic reaction of FA, particularly with sulfates and alkalis [23]. Activators have been shown to significantly improve the initial strength of backfill, with Na₂SO₄ specifically noted for its positive impact on both early strength and steel fiber bonding in high-volume FA mortars [19]. Analyses utilizing TGA, XRD, and SEM have demonstrated that Na₂SO₄ effectively accelerates the consumption of Ca(OH)₂ and enhances ettringite formation while reducing pore size and total porosity. Investigations into alkali activation reveal that combining Na₂SO₄ with NaOH as a composite activator may lead to slightly lower strength compared to Na₂SO₄ alone, but combining it with Na₂SiO₃ can significantly enhance strength, with sodium silicate content capped at 2.5% [28]. Various analytical techniques have been employed to investigate the hydration effects of Na₂SO₄ and Ca(OH)₂ when incorporated into hybrid cements containing high FA content. It was found that increasing the dosage of this composite activator improves the early mechanical performance of high FA content cement mortars [29,30]. While early mechanical performance tends to decline with increased FA content, the addition of activators can significantly mitigate this decline. Despite extensive research on the effects of individual changes in activator type or dosage, there is a relative scarcity of studies examining the combined macroscopic and microscopic properties of fly ash–cement binary binder pastes with high FA content, particularly those involving combinations of multiple types and dosages of activators.

This study investigates the effects of varying proportions (0.5%, 1.0%, 1.5%, and 2.0%) of Na₂SO₄·10H₂O and Ca(OH)₂ activators, both individually and in combination, on the macro-mechanical properties and microstructure of tailings–cement–fly ash composite systems. The research focuses on the evolution of uniaxial compressive strength over curing periods of 3, 7, and 28 days. To analyze the composition and morphological changes of hydration products, XRD and SEM-EDS techniques were employed. This research establishes a clear link between compressive strength and corresponding microstructural characteristics, elucidating the activation mechanisms of fly ash during the hardening process. Furthermore, it provides valuable insights into how these mechanisms directly influence macro-mechanical properties, highlighting the critical role of activators in the composite system.

2. Materials and Methods

2.1. Materials

The raw materials used in this study for the preparation of activated FA–cement paste (AFCP) include tailings, FA, cement, activators, and water. Both tailings and FA were sourced from an iron ore mine in Gansu Province, China. According to the Chinese national standard GB175-2007 [31], P.O. 42.5 cement was utilized in this experiment. Two types of activators were employed: analytically pure reagents of Na₂SO₄·10H₂O and Ca(OH)₂. All pastes were prepared using tap water.

The densities of the tailings and FA measured using the water pycnometer test are 2.966 g/cm³ and 2.950 g/cm³, respectively. The main chemical compositions of the tailings, FA, and cement, as analyzed by X-ray fluorescence (Rigaku, Tokyo, Japan), are presented in

Table 1. Chemical compositions of mineral tailings, fly ash, and cement (wt%).

Compositions	Mineral Tailings	Fly Ash	Cement
SiO2	50.63	53.57	22.60
Al2O3	2.72	7.25	5.74
Fe2O3	25.54	8.43	3.48
K2O	-	3.88	0.79
MgO	5.62	3.46	2.01
CaO	9.39	9.37	60.56
SO3	1.92	6.55	2.79

. Figure 1 illustrates the mineral composition of FA, as assessed by XRD (Rigaku, Tokyo, Japan). Notably, compared to cement, FA contains a lower calcium content (less than 10%) and exceeds a 50% silicon content. The particle size distribution of tailings, FA and cement was analyzed using a TopSizer Laser Particle Size Analyzer (OMEC Instruments Co., Ltd., Zhuhai, China), as shown in Figure 2. The analysis revealed a similar particle size distribution in both the tailings and fly ash, with particle size parameters d₁₀, d₅₀, and d₉₀ measuring 1.48 μm, 17.39 μm, and 88.22 μm, respectively, indicating a broad range of particle sizes.

2.2. Preparation of AFCP Specimens and Curing Conditions

The slurry concentration used at the mine was 74%, with a cement-to-tailings ratio of 1:4, yielding effective results. To reduce cement costs while enhancing the early performance of the AFCP, a fly ash-to-cement ratio of 6:4 was adopted. Activators (A) were added at 0.5%, 1.0%, 1.5%, and 2.0% of the mass of the binding materials, as detailed in

Table 2. Experiment schemes.

Samples	FA/C	W/C	T/C	A/(C+FA)
				A-I/(C+FA)	A-II/(C+FA)
A0	1.5	4.39	10	0	0
A1	1.5	4.39	10	0.5	0
A2	1.5	4.39	10	1	0
A3	1.5	4.39	10	1.5	0
A4	1.5	4.39	10	2	0
B1	1.5	4.39	10	0	0.5
B2	1.5	4.39	10	0	1
B3	1.5	4.39	10	0	1.5
B4	1.5	4.39	10	0	2
C1	1.5	4.39	10	0.25	0.25
C2	1.5	4.39	10	0.5	0.5
C3	1.5	4.39	10	0.75	0.75
C4	1.5	4.39	10	1	1

A-I and A-II in Table 2 refer to Na₂SO₄·10H₂O and Ca(OH)₂, respectively.

.

All dry materials, including tailings (T), FA, and cement (C), were initially combined at low speed in a steel mixing drum for 3 min to minimize powder loss to the air. Following this, tap water (W) and the activator solution were slowly added within 3 min, taking care to prevent splashes during mixing. The mixture was then gradually stirred at high speed for 5 min. The fresh slurry was poured into steel cubic molds (70.7 mm × 70.7 mm × 70.7 mm) in two stages. After each pour, a rod was used to tap the mold 10–20 times to compact the slurry and eliminate air bubbles, and this process was repeated as necessary. The molds were initially cured in the air and covered with plastic sheets to prevent moisture evaporation. After 24 h, all prepared paste samples were demolded, labeled, and cured for the required duration in a constant temperature chamber (20 ± 1 °C) with a relative humidity of 95 ± 1%.

2.3. Analysis Methods and Apparatus

The samples were maintained at standard curing periods of 3, 7, and 28 days. Subsequently, mechanical performance tests and microstructural analyses were conducted. Uniaxial compressive strength was assessed under various dosages of the three different activator combinations following the guidelines specified in the Chinese national standard GB/T 17671-1999 for the strength testing of cement paste [32]. For each experimental group, three specimens were tested, and the average value was calculated as the final compressive strength.

The specimens exhibiting the best and worst strength performance for each activator were selected for microstructural analysis at 3, 7, and 28 days. The filled specimens were crushed, and small pieces from their centers were selected. These pieces were soaked in anhydrous ethanol for 24 h to halt the hydration reaction, then dried in a 60 °C oven until a constant weight was achieved. Smaller sections, approximately 5 mm × 5 mm, were cut to ensure flatness. The surface of these sections was coated with carbon for SEM-EDS analysis (SEM: Carl Zeiss AG, Oberkochen, Germany; EDS: Bruker, Berlin, Germany). The remaining samples were ground and sieved through a 75 µm standard sieve designated for X-ray diffraction (XRD) analysis.

For XRD, a SmartLab 3 kW X-ray diffractometer (Rigaku, Tokyo, Japan) was used, featuring a Cu target, a goniometer radius of 185 mm, and a scanning range of 0° to 152°. The SEM analysis employed the ZEISS EVO 18 scanning electron microscope, with magnification ranging from 5 to 1,000,000× and a resolution of up to 3072 × 2304 pixels.

3. Results and Discussion

3.1. Compressive Strength Test

3.1.1. The Effect of Na₂SO₄ on the Strength of Fly Ash–Cement Paste

Figure 3 illustrates the uniaxial compressive strength (UCS) of fly ash–cement paste blocks with varying additions of Na₂SO₄, tested at curing ages of 3, 7, and 28 days. Generally, the strength increases with longer curing periods, with more pronounced improvements observed from days 3 to 7 compared to the increment from days 7 to 28. The different amounts of Na₂SO₄ exhibited distinct effects on strength development over time. For instance, sample A1 (0.5% Na₂SO₄) achieved a strength of 0.89 MPa after 3 days, while sample A2 (1.0% Na₂SO₄) attained its highest strength of 0.92 MPa at the same age. In contrast, samples A3 (1.5% Na₂SO₄) and A4 (2.0% Na₂SO₄) demonstrated strengths comparable to the control sample A0, though they were only slightly higher.

All samples demonstrated significant increases in strength at 7 days compared to 3 days. Sample A1 exhibited the highest increase of 82.02%, reaching a peak strength of 1.62 MPa. In contrast, sample A2 showed a lower increase of 66.30% but still achieved a commendable final strength of 1.53 MPa, largely due to the additive’s impact on its 3-day strength. Although samples A3 and A4 also experienced significant increases, they did not attain the desired 7-day strength levels.

After 7 days, the overall strength development of the samples slowed, with a noticeable reduction in the rate of increase compared to earlier periods. Sample A4 achieved a peak increase of 37.69%, while sample A3 reached a respectable increase of 32.6%. Conversely, samples A1 and A2, which initially performed well, exhibited a slower hardening rate after 7 days, with increases below 20%. These findings indicate that the Na₂SO₄ activator enhances early strength development; however, its impact on mid-term strength growth is minimal. Furthermore, there exists an optimal dosage of Na₂SO₄ that effectively accelerates early strength.

The results conclusively demonstrate that specimens infused with Na₂SO₄ activators consistently outperformed the control group in terms of strength. However, as the dosage of Na₂SO₄ increased from 0.5% to 2.0%, an initial surge in strength was observed at 3, 7, and 28 days, followed by a gradual decline. This trend aligns with previous observations [23]. Notably, samples A1 and A2 demonstrated significant strength increases compared to sample A0, while samples A3 and A4 showed minimal variations across all curing periods. Overall, the optimal performance of FA activation occurred with a Na₂SO₄ content of 0.5%, while a dosage of 1.5% resulted in the weakest effect.

3.1.2. The Effect of Ca(OH)₂ on the Strength of Fly Ash–Cement Paste

Figure 4 illustrates the effect of the Ca(OH)₂ activator on the UCS of fly ash–cement specimens. The results indicated a positive correlation between UCS and the content of Ca(OH)₂. Sample B4 recorded the highest strengths at days 3, 7, and 28, measuring 0.99 MPa, 1.892 MPa, and 3.045 MPa, respectively. Notably, sample B4 exhibited a significant strength increase of 91.11% from days 3 to 7, surpassing the 28-day strength of sample A0 at 7 days, and continued to grow at a high rate, achieving an additional 60.94% increase from days 7 to 28. In contrast, despite the incorporation of 0.5% Ca(OH)₂, sample B1 demonstrated strength variations similar to those of sample A0.

Compared to the control group, the strength of the test blocks increased significantly with higher amounts of Ca(OH)₂ at various curing ages. Sample B4 showed the optimal performance, with strength increases of 36.36%, 49.33%, and 76.21% at days 3, 7, and 28, respectively. This suggests that alkaline activation had a substantial impact on the strength of the test blocks, particularly by enhancing early strength and notably improving medium-term strength. The likely explanation for this enhancement is that the addition of calcium hydroxide increases the alkalinity of the slurry, facilitating the depolymerization of glassy components in FA and promoting reactions between active components within the glass phase and the hydration products of cement [33,34]. Consequently, a Ca(OH)₂ dosage of 2% yielded the best activation effect on FA, while a dosage of 0.5% resulted in the weakest effect.

3.1.3. The Effect of Composite Activator on the Strength of Fly Ash–Cement Paste

The preceding sections analyzed the effects of individually adding Na₂SO₄ or Ca(OH)₂ on the UCS of the test specimens. Figure 5 illustrates the impact of the composite activator, which was created by mixing these components in a 1:1 ratio. The results closely mirror the trend observed in group B, with optimal performance noted at an activator dosage of 2.0%. The maximum strengths at days 3, 7, and 28 reached 0.92 MPa, 1.646 MPa, and 2.364 MPa, respectively. Additionally, the strengths of the four samples in group C aged 3 days were comparable. Between days 3 and 7, the rate of strength development ranked as follows: C4 > A0 > C3 > C1 > C2. From days 7 to 28, the progression was ranked as follows: C3 > C2 > C4 > C1 > A0. This indicates that the addition of the composite activator significantly enhanced the mid- to long-term strength development of the specimens.

Using sample A0 as a reference, the strength of the test blocks increased with the addition of more activator. Specifically, at day 3, samples C1 and C2 exhibited growth rates of 11.57% and 19.97%, respectively, but showed only modest increases of 1.34% and 2.53% at day 7. In contrast, samples C3 and C4 demonstrated significant strength enhancements at all ages, with sample C4 achieving the highest growth rate of 36.81% at day 28. This trend can be attributed to the increase in slurry pH resulting from the addition of alkali activators, which accelerates the depolymerization of the glass phase in FA. This process releases active components that react with the hydration products of cement, facilitating the condensation of oligomeric active components into polymers [35]. Additionally, appropriate amounts of sulfates catalyze the transformation of hydration products into more stable forms, such as ettringite, thereby enhancing the overall stability of the system [36]. Consequently, optimal activation of FA occurs with a Na₂SO₄ to Ca(OH)₂ composite activator ratio of at 1:1 and a dosage of 2.0%, while a dosage of 0.5% results in less effective activation.

3.2. Microscopic Test

3.2.1. Microscopic Analysis of Filling Bodies Activated by Na₂SO₄

Based on the compressive strength results, the best and worst samples, A1 and A3, were selected, respectively, for microscopic analysis, comparing them with the control sample A0 to identify significant differences. Figure 6 presents the XRD analysis of these samples at days 3, 7, and 28. The XRD analysis revealed the presence of gypsum (CaSO₄·2H₂O), hydrated calcium aluminate (C-A-H), calcium hydroxide (Ca(OH)₂), mullite, and quartz (SiO₂), as well as unidentified phases, such as ettringite (AFt) and calcium silicate hydrate (C-S-H).

The early diffraction peaks of Ca(OH)₂ were weaker in sample A0, while SiO₂ peaks were more pronounced, indicating a limited hydration of the fly ash–cement mixture. In contrast, with the addition of the activator to samples A1 and A3, the SiO₂ peaks diminished, and the peaks for Ca(OH)₂ and C-A-H intensified, suggesting enhanced hydration rates. The early hydration products primarily arise from cement reactions, where C-S-H and Ca(OH)₂ form from the hydration of tricalcium silicate. Although the hydration of dicalcium silicate is similar to that of tricalcium silicate, it occurs more slowly, resulting in a lower yield of Ca(OH)₂ but with larger crystal formations. The two reactions are described as follows [37]:

$$3\mathrm{CaO}\cdot {\mathrm{SiO}}_2+n{\mathrm{H}}_2\mathrm{O}\to x\mathrm{CaO}\cdot {\mathrm{SiO}}_2\cdot y{\mathrm{H}}_2\mathrm{O}+(3-x)\mathrm{Ca}{(\mathrm{OH})}_2$$

$$2\mathrm{CaO}\cdot {\mathrm{SiO}}_2+n{\mathrm{H}}_2\mathrm{O}\to x\mathrm{CaO}\cdot {\mathrm{SiO}}_2\cdot y{\mathrm{H}}_2\mathrm{O}+(2-x)\mathrm{Ca}{(\mathrm{OH})}_2$$

During the hydration process of cement, the hydration of tricalcium aluminate (C₃A) also occurs. This reaction rapidly releases heat and initially produces C-A-H, which ultimately transforms into a more stable form known as hydrated garnet (C₃AH₆). The final hydration products of C₃A, in the presence of CaSO₄·2H₂O, depend on the amount of CaSO₄·2H₂O added [38]. Initially, the reaction with SO₄²⁻ forms ettringite (AFt), which contributes positively to the strength of the filling material. As the Na₂SO₄ content increases, the concentration of SO₄²⁻ also rises, leading to the formation of more AFt. While this initially enhances strength, excessive AFt can induce expansion effects that result in cracking and damage to the filling material, ultimately causing a reduction in strength [39]. If CaSO₄·2H₂O is depleted before the complete hydration of C₃A, monosulfoaluminate hydrate (AFm) will form through the reaction of AFt with the remaining C₃A.

As the curing period progresses, FA continuously participates in hydration reactions, leading to a decline in the diffraction peaks of SiO₂ crystals. This indicates that, over time, the SiO₂ crystals engage in hydration reactions and are consumed, resulting in the formation of more hydration products, such as C-S-H. The diffraction peaks of Ca(OH)₂ also decreased with curing time, and this reduction is more pronounced with the addition of activators. This suggests that the secondary hydration of FA consumes some Ca(OH)₂, and the activator enhances the hydration of FA to some extent. However, an increase in the activator content does not always yield better results; for example, in sample A3, the rate of Ca(OH)₂ consumption declined.

This indicates that, when the binder is composed of cement and FA, the hydration of cement clinker releases Ca(OH)₂, creating favorable conditions for the pozzolanic reaction of FA. Meanwhile, the pozzolanic reaction consumes Ca(OH)₂, facilitating further hydration of the cement clinker. Additionally, the micro-aggregate effect of FA enhances the particle structure of the fly ash–cement system, thereby improving the hydration efficiency of the system [40].

The hydration process in the fly ash–cement system initiates with the hydration of clinker minerals, followed by a secondary hydration reaction between Ca(OH)₂ and FA. The surface of FA adsorbs and reacts with Ca(OH)₂ produced during the hydration of cement clinker, leading to the formation of C-A-H and C-S-H. In the presence of gypsum and lime, the crystallization of AFt occurs. Initially, most hydration products appear in gel form, but over time, they progressively transform into fibrous crystals of increasing quantity. Eventually, these two forms interlock, forming a complex network structure.

The secondary hydration process of FA is as follows [41]:

$$x\mathrm{Ca}{(\mathrm{OH})}_2+{\mathrm{SiO}}_2+(n-1){\mathrm{H}}_2\mathrm{O}\to x\mathrm{CaO}\cdot {\mathrm{SiO}}_2\cdot n{\mathrm{H}}_2\mathrm{O}$$

$$3\mathrm{Ca}{(\mathrm{OH})}_2+{\mathrm{Al}}_2{\mathrm{O}}_3+2{\mathrm{SiO}}_2+m{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 2{\mathrm{SiO}}_2\cdot n{\mathrm{H}}_2\mathrm{O}$$

XRD analysis offers insights into the mineral phases present in various samples throughout the hydration process. To further investigate the differences in mechanical properties among the samples, SEM-EDS was employed to observe the micro-morphology of hydration products at different curing ages. Figure 7 presents the SEM-EDS images of samples A0, A1, and A3 after curing for 3, 7, and 28 days.

The SEM-EDS analysis reveals notable differences in the microstructure of samples with varying activator contents. Sample A0 features larger, elongated hydration products, along with significant pores. In contrast, samples A1 and A3 show a reduction in elongated structures and an increase in fibrous, needle-like products, which are particularly pronounced in sample A1. This change is attributed to the uniform distribution of the C-S-H gel and some AFt within the filler matrix, which contributes to reduced porosity. This factor also explains the enhanced strength observed in sample A1.

Based on the analysis, the hydration reaction model for fly ash–cement can be described as follows: Following the addition of water, minerals such as C₃A and C₃S from OPC clinker rapidly dissolve. C₃A reacts with gypsum to form AFt, whereas C3S hydrates into C-S-H and Ca(OH)₂, both of which become integral components of the fly ash–cement hydration system. When Na₂SO₄ is used as an activator, SO₄²⁻ reacts with dissolved Ca²⁺ and AlO₂⁻ in the liquid phase to form AFt [42]. The chemical reaction can be represented as:

$${\mathrm{AlO}}_2^{-}+{\mathrm{Ca}}^{2+}+{\mathrm{OH}}^{-}+{\mathrm{SO}}_4^{2-}\to 3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 3{\mathrm{CaSO}}_4\cdot 32{\mathrm{H}}_2\mathrm{O}$$

Partially hydrated calcium aluminate can also react with CaSO₄·2H₂O to form AFt, as represented by the following reaction equation:

$$3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 6{\mathrm{H}}_2\mathrm{O}+3\left({\mathrm{CaSO}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}\right)+2{0\mathrm{H}}_2\mathrm{O}\to 3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 3{\mathrm{CaSO}}_4\cdot 32{\mathrm{H}}_2\mathrm{O}$$

AFt forms a fibrous coating on the surface of FA particles. Its lower density compared to C-S-H allows for the diffusion of Ca²⁺ into the particles. Additionally, the SO₄²⁻ from Na₂SO₄ can displace some SO₄²⁻ from the C-S-H gel and react with Ca²⁺ to generate more C-S-H, thereby enhancing the activity of the FA. Furthermore, the CaSO₄ and AFt produced by SO₄²⁻ can expand, filling the pores in hydration products and increasing the density of the hardened material, which compensates for shrinkage.

Given its solubility in water, Na₂SO₄ can react with Ca(OH)₂ in the system to produce highly dispersible CaSO₄, which promotes the formation of AFt more effectively than externally mixed gypsum. Moreover, Na₂SO₄ can also react with Ca(OH)₂ to generate NaOH, increasing the system’s alkalinity and facilitating the pozzolanic reaction, ultimately enhancing the early strength of the fly ash–cement filler.

3.2.2. Microscopic Analysis of Filling Bodies Activated by Ca(OH)₂

Samples B4 and B1 were selected for XRD and SEM-EDS analysis because they performed the best and worst, respectively. Figure 8 presents the XRD results for both sample groups at different curing ages. A comparison of Figure 9a–c reveals minimal changes in the primary components of the hydration products across the samples; however, notable differences in the degree and quantity of products formed are evident.

The peak intensity of SiO₂ in sample B1 is significantly higher than in sample B4, indicating that a larger amount of Ca(OH)₂ accelerates the hydration reaction of cement, leading to greater consumption of SiO₂. As curing progresses, a distinct difference in peak intensity between the two samples becomes apparent, suggesting that the increased Ca(OH)₂ content enhances the system’s alkalinity, thereby stimulating the reactivity of FA. This facilitates the participation of active SiO₂ and Al₂O₃ from the glassy phase in the hydration reaction of cement, highlighting an improvement in the activation efficiency of Ca(OH)₂ over the curing period.

Ultimately, a comparison of the effects of curing time and calcium hydroxide content reveals that the latter has a more significant impact on the mineral composition of the samples.

Figure 9 illustrates the SEM-EDS images of samples with added Ca(OH)₂ at curing ages of 3 d, 7 d, and 28 d. The SEM image of sample B1 shows a higher presence of strip-like minerals and a significant porosity. In contrast, as the amount of activator increases in sample B4, more needle-like AFt is generated, resulting in a denser structure with reduced porosity. Furthermore, as the curing period progresses, the additional Ca(OH)₂ facilitates the hydration of FA, leading to the formation of AFt and other phases, enhancing the overall structural integrity of the material.

During the hydration process, FA supplies active reactants, including SiO₂, Al₂O₃, and oligomers, such as [SiO₄] and [AlO₄], which contribute to the formation of gel products like C-S-H and C-A-H, thereby enhancing early strength. The addition of Ca(OH)₂ increases the alkalinity of the slurry, affecting the glassy phase by altering the bond properties of Si-O-Si, Al-O-Al, and Si-O-Al. In the Si-O-Si bridging oxygen structure, the 2p electrons of oxygen overlap with the 3s and 3p orbitals of silicon to form σ bonds. Interaction with OH⁻ ions induces hydrogen bonding with the oxygen atoms in the silicate structure, influencing the stability of Si-O bonds and facilitating their cleavage. Under the influence of OH⁻, [SiO₄]⁴⁻ undergoes the processes of depolymerization and polymerization, reducing the concentration of monomers while increasing the amount of higher polymers. Dimeric and other oligomeric forms experience a cycle of monomer polymerization to oligomer formation, followed by depolymerization back to monomers or lower oligomers, representing a polycondensation reaction. Consequently, in the fly ash–cement gel system, alkalinity plays a crucial role in cleaving Si-O and Al-O bonds. Higher OH⁻ concentrations favor the leaching of reactive components from the FA, with increased pH values corresponding to a greater extent of reaction. The process of action is as follows [43]:

$$-\mathrm{Si}-\mathrm{O}-\mathrm{Si}\stackrel{{\mathrm{OH}}^{-}}{\to }-\mathrm{Si}-\mathrm{O}+-\mathrm{Si}-\mathrm{OH}$$

$$-\mathrm{Si}-\mathrm{O}-{+\mathrm{OH}}^{-}\to -\mathrm{O}-\mathrm{Si}-\mathrm{OH}$$

Moreover, $-\mathrm{Si}-\mathrm{O}-\mathrm{Ca}-\mathrm{OH}+\mathrm{HO}-\mathrm{Si}-\mathrm{O}-\to -\mathrm{Si}-\mathrm{O}-\mathrm{Si}-{+\mathrm{Ca}\left(\mathrm{OH}\right)}_2$

When activated by OH⁻, the reactive SiO₂ and Al₂O₃ in the FA glass phase leach and interact with Ca²⁺ on particle surfaces, leading to the formation of a hydration layer that encases the FA particles. At this stage, the reactivity of the FA depends on the rate at which Ca²⁺ diffusion progresses through the hydration layer. The addition of Ca(OH)₂ accelerates the saturation and crystallization of Ca(OH)₂ in the liquid phase. The fine Ca(OH)₂ crystals can adsorb some of the hydration gel, resulting in the formation of hydration products on the surfaces of the FA particles. This process reduces the thickness of the hydration encapsulation around the FA particles, facilitating the inward diffusion of Ca²⁺ and promoting hydration reactions within the FA.

3.2.3. Microscopic Analysis of Filling Bodies Activated by the Composite Activators

The strongest and weakest samples, C1 and C4, were selected for XRD analysis, as illustrated in Figure 10. The variation trends across different curing ages are similar to those observed in Figure 8. In sample C4, the diffraction peaks of SiO₂ show a decline compared to those in C1. Initially, a certain amount of Ca(OH)₂ was generated; however, as the curing period progressed, FA continued to participate in the hydration process, consuming the Ca(OH)₂ and leading to a decrease in its content within the sample. The lower quantity of Ca(OH)₂ in the composite activator compared to the single Ca(OH)₂ activator results in decreased system alkalinity, which may partially reduce the activation of FA. Additionally, Na₂SO₄ in the composite activator also contributes to the hydration reaction. As the curing age and amount of activators increased, the production of gypsum and C-A-H also rose, with gypsum formation significantly enhancing the strength of the filling sample. In samples with the same dosages, the diffraction peaks of quartz crystals and calcium hydroxide decreased with extended curing age, indicating that SiO₂ and Ca(OH)₂ continuously engaged in hydration throughout the curing period.

Figure 11 presents the SEM-EDS images for samples C1 and C4 at 3 d, 7 d, and 28 d. Comparing the SEM images indicates that higher amounts of the activator and longer curing times lead to denser microstructures. Shorter-aged samples show some unreacted cement and FA particles. Over time, FA particles decrease in size, and the formation of needle-like ettringite increases, further enhancing the strength of the fill samples.

4. Conclusions

This study investigated the effects of Na₂SO₄·10H₂O and Ca(OH)₂ as activators on the mechanical properties of high-volume fly ash–cement composites under single and dual activation. Through mechanical strength testing, XRD analysis, and SEM-EDS characterization, the strength evolution, mineral phase transformation, and activation mechanisms were explored, leading to the following key conclusions:

(1)

Activators enhanced the strength of fly ash–cement composites compared to the control group, with their effectiveness ranking as Ca(OH)₂ > composite activator > Na₂SO₄. The efficacy of activators diminishes with curing time.

(2)

The activation mechanisms of Na₂SO₄ enhanced the fly ash–cement system by facilitating the formation of AFt, which promoted Ca²⁺ diffusion and reaction with SiO₂ and Al₂O₃.

(3)

Ca(OH)₂ increased alkalinity, aiding bond cleavage and the formation of reactive oligomers. These interactions led to the production of C-S-H and C-A-H gel products, significantly improving early strength.