Microstructural Evolution and Mechanical Properties of Fly-Ash-Based Grouting Materials in Different Aqueous Environments

Abstract

Grouting is widely used in the treatment works of goaf, which can enhance the foundation bearing capacity, reduce deformation, and ensure the stability of the construction of goaf. As the goaf is located below the water table line, the mechanical properties and microscopic changes of the stone body in the water-rich environment have not been revealed, which leads to the effect of grouting treatment in water-rich goaf being difficult to achieve in terms of the expected goal. This paper used uniaxial compression, electron microscopy (SEM), and X-ray diffraction (XRD) to study the mechanical properties and microscopic changes of the nodular body under natural, pure water, and tap water curing and revealed the deterioration mechanism of the nodular body’s mechanical properties under water curing. The research results show that under identical material proportions and curing durations, compared to naturally cured specimens, the specimens cured in purified water and tap water exhibited a significant increase in the content of unreacted fly ash, a reduction in the amount of hydration products such as C-S-H gel and ettringite, and a looser microstructure, resulting in average decreases in uniaxial compressive strength of 35.7% and 49.9%, respectively. In addition, the presence of chloride ions and Friedel’s induced decalcification of the C-S-H gel under tap water curing conditions led to a significant deterioration in the physical strength of the grouted stones.

1. Introduction

For engineering projects that need to cross the goaf, grouting reinforcement is one of the common means of foundation treatment at present, and its main purpose is to fill the voids and enhance the strength of the foundation to ensure the safety and stability of the project [1,2,3]. However, the underground environment of the goaf is complex and often contains substantial amounts of mine water [4,5]. Mine water mainly originates from a variety of ways, such as continuous recharge of underground aquifers, infiltration of atmospheric precipitation, and water for mining production [6,7,8]. Therefore, the mechanical properties, strength, and modulus of elasticity of the cemented body formed by the grouting material after curing in the water-conservation environment play a decisive role in controlling the deformation of water-rich goaf [9,10].

Generally, the mechanical behavior of grouted materials is influenced by the water–solid rate, solid rate, and curing time [11,12]. Studies have shown that under the condition of anhydrous maintenance, the early strength development of grouted concretions lags due to insufficient hydration reaction, and the later strength depends on the secondary hydration reaction of fly ash and other admixtures, with the strength and elastic modulus of the concretions negatively correlated with the fly ash content and porosity [13,14,15,16]. Meanwhile, slow removal of free water induces the conversion of CaO-Al₂O₃-8H₂O to 3CaO-Al₂O₃-6H₂O, leading to the deterioration of the strength of calcium aluminate cement concretions [17].

However, the presence of water in the goaf has a significant deterioration effect on the mechanical properties of the grouted nodules [18,19], which will lead to a decrease in the compressive strength and an increase in the plastic deformation of the grouted nodules compared to the natural curing conditions [20,21]. Previous studies have shown that the formation of calcium hydroxide plays a key role in the early strength development of fly ash specimens under water-conditioned conditions [22]. Previous studies have shown that in the water-saturated condition, the tensile strength of fly ash specimens was enhanced despite the reduction in strength and brittleness [23]. Studies showed that water has a significant effect on the cement hydration process in mixed fly ash and cement specimens [24]. The researchers showed that the strength of cementitious specimens cured in water increased with time, but the rate of strength increase gradually slowed down [25]. The researchers found a similar pattern when cementitious specimens were cured in deionized water [26]. Previous generations investigated the effect of the water–cement ratio on the properties of cementitious materials in a humid environment and found that the plastic deformation of the specimens increased with increasing water–cement ratio while the strength decreased, a phenomenon related to the conversion of ettringite (AFt) to high-sulfur-type hydrated calcium sulfate-aluminate (AFm) [27,28]. For aluminum sulfate cement specimens, the strength of specimens cured in water is greater than that of specimens under natural curing because water curing promotes the hydration of aluminate sulfate cement more than natural curing [29]. The deterioration of the mechanical properties of grouted concretions is an almost inevitable phenomenon in water-rich environments. However, the existing studies mainly focus on the macroscopic mechanical properties of grouted stone bodies under the water environment condition, while fewer studies have been conducted to explain the mechanism of the microstructural evolution of grouted stone bodies on their mechanical properties under the water environment.

In this paper, the mechanical properties of the stone body under different ambient conditions were carried out to systematically evaluate the influence of maintenance conditions on the mechanical properties of the stone bodies. Through scanning electron microscope (SEM) and X-ray diffraction (XRD) tests, the strength evolution law of the nodular bodies in different water environments was comparatively analyzed from the perspectives of microstructures and hydration products to further reveal its deterioration mechanism under water-rich conditions. The results of the study can help to deeply understand the changes in mechanical behavior of grouting materials in complex water environments so as to provide a theoretical basis and practical guidance for grouting reinforcement projects in water-rich goaf.

2. Materials and Methods

2.1. Material Selection

This study utilized secondary fly ash, produced in Shijiazhuang Zhengyu Material Technology Company (Shijiazhuang, China), the chemical composition and performance indicators of which are detailed in

Table 1. Chemical composition and performance index of fly ash.

Basic Chemical Composition/%			Density/ (g/cm3)	Water Demand Rate/%	Burnout /%	Moisture Content/%
SO3	f-CaO	SiO2, Al2O3, Fe2O3
1.2	2.9	88	2.0	101	7	0.2

. Ordinary silicate cement (425 type) was selected, produced by Hebei Yanxin Building Materials Company (Cangzhou, China), and water glass, Na₂O-nSiO₂, produced by Shandong DuoFeng Huagong (Linyi, China) with a modulus of 2.4, was used. Curing was conducted using pure water and tap water, with the chlorine content of the tap water in Mentougou provided in

Table 2. Chlorine content of tap water in the Mentougou area.

Index	Unit of Measure	“Standards for Drinking Water Quality” GB 5749-2022 [31]	Mentougou
Free chlorine	mg/L	The contact time with water ≥30min, Factory water and terminal water limits ≤2, The factory water balance ≥0.3, The residual water at the end ≥0.05.	0.10~0.50
Permanganate index (O2)	mg/L	3	0.53~1.4

. Following the MOT JTG/T 3331-2024 (MOT 2024) [30], the cement–fly ash proportioning formula was calculated, with water glass mixing (SS) representing the percentage of water glass relative to the cement mass.

2.2. Experimental Design

In this experiment, the solid-to-solid rate (A) was established at four levels: 9:1, 8:2, 7:3, and 6:4. The water–solid rate (B) was set at four levels: 1:0.8, 1:1.0, 1:1.2, and 1:1.4. Water glass (SS) was varied across four levels: 0%, 1%, 2%, and 3%. The relationship between the one-way and orthogonal tests was plotted as shown in Figure 1. Three maintenance methods were employed: natural curing, pure water curing, and tap water curing. Natural curing refers to the curing of specimens at room temperature without water immersion after demolding. In contrast, pure water and tap water curing involve the immersion of specimens in purified water and tap water, respectively, at room temperature.

(a)

One-factor tests

Experimental and control groups were designed, with the one-way experimental framework summarized in

Table 3. One-way experimental program design.

Factors			Natural Curing	Water-Rich Curing
A	B	SS/%	Numbering	Numbering
9:1	1:0.8, 1:1.0, 1:1.2, 1:1.4	0	S1…S4	SW1…SW4
8:2	1:0.8, 1:1.0, 1:1.2, 1:1.4	0	S5…S8	SW5…SW8
7:3	1:0.8, 1:1.0, 1:1.2, 1:1.4	0	S9…S12	SW9…SW12, SWT9...SWT12
7:3	1:0.8, 1:1.0, 1:1.2, 1:1.4	3	SG9...SG12	SWG9...SWG12
6:4	1:0.8, 1:1.0, 1:1.2, 1:1.4	0	S13…S16	SW13…SW16

Notes: SW indicates pure water conditioning; SWT denotes tap water conditioning; SWG refers to pure water conditioning with added water glass.

. Curing durations were set at 3, 7, 14, and 28 days for natural curing, and at 14 and 28 days for curing in pure water and tap water.

(b)

Orthogonal test

In this test, the orthogonal test method was used to study the effects of various factors and test levels under the constraints of the test conditions, with the aim of determining the optimum proportion of cobble grouting materials in a water-rich environment. The test levels were designed with reference to previous studies [32], which provided valuable references for the experimental design, and to the Technical Specifications for Design and Construction of Highway Engineering in Mined-out Areas (MOT JTG/T 3331-2024, MOT 2024) [30], which stipulates that when grouting operations are carried out in non-water-rich perimeter rock, the content of fly ash should not be less than 70%, and water-solidification ratios should be within the range of 0.77 to 1. In order to improve the applicability of grouting material proportion in water-rich environments, a three-factor, four-level orthogonal test program was designed, as shown in

Table 4. Orthogonal test factor level table.

Level	Factors
	A	B	SS/%
1	9:1	1:0.8	0
2	8:2	1:1.0	1
3	7:3	1:1.2	2
4	6:4	1:1.4	3

. Natural and water-rich curing conditions were applied, with maintenance periods set at 3, 7, 14, and 28 days for natural curing and 14 and 28 days for water-rich curing.

(c)

Microscopic tests

All test samples were uniformly extracted from the center of each specimen, and mineral composition analysis was performed using X-ray diffraction (XRD) with a continuous scanning range of 5° to 75° at a scanning speed of 20°/min. The Hitachi SU5000 scanning electron microscope produced by Hitachi High-Tech Co., Ltd. in Tokyo, Japan was used to observe the samples of 10 mm × 10 mm. The designs for SEM and XRD tests are presented in

Table 5. Design of SEM and XRD experiments.

Type of Curing	Natural Curing	Pure Water Curing	Tap Water Curing
Curing time/day	3, 28	28	28

.

2.3. Experimental Procedure

According to the standard [30], samples were cast using molds measuring 70.7 mm × 70.7 mm × 70.7 mm. The weighted fly ash and cement were mixed and stirred for 5 min, followed by the addition of water and an additional 5 min of stirring. The slurry was immediately poured into the molds until the surface was level with the mold edges. After pouring, specimens for water-rich curing were immediately submerged in water, ensuring the water level was 1 cm above the specimens. For natural curing, specimens were uniformly wrapped in cling film post-demolding. Under water-rich curing conditions, specimens continued to be immersed in water after demolding. All specimens underwent uniaxial compression testing at specified curing ages. The specimen fabrication, curing, and testing process (Figure 2) is shown.

3. Results and Discussion

3.1. Deformation and Failure Characteristics

Figure 3a,b display representative axial stress–strain curves under varying curing conditions, considering a solid rate of 2.33 and a water–solid rate of 0.83. The results indicate that the natural, pure water, and tap water curing conditions exhibited distinct mechanical behaviors: (1) Under natural curing, stress increased almost linearly with strain until reaching peak stress, followed by a sudden drop, indicating brittle failure. In contrast, pure water and tap water curing showed minimal stress variation with strain before the peak, with a slight stress decrease post-peak, suggesting plastic damage. Notably, specimens under tap water conditioning exhibited greater plastic deformation. (2) At 28 days of curing, peak stress increased by 52.7% for natural curing, 58.4% for pure water curing, and 63.8% for tap water curing. Water-rich conditions notably accelerated hydration in the later stages, leading to faster strength gains.

Figure 4a–f illustrate damage patterns under various curing conditions and ages; the red dashed lines indicate cracks of width ≥1mm, while the green dashed lines represent cracks <1mm. Observations include the following: (1) The specimen was brittle under natural conservation; the surface was peeled off, and with the increase in conservation time, the crack openness decreased. (2) The pure water conservation surface was broken, and the crack opening degree was large but not spalling, indicating strong adhesion; large plastic deformation of tap water-conditioned specimens, the surface was more complete without spalling, and there was strong adhesion. (3) Throughout the deformation-to-failure process, the specimens showed noticeable localized deformation along with mid-section expansion under pure water and tap water curing. (4) The specimens cured with tap water exhibited the highest peak axial strain, which was 60% higher than that of the naturally cured specimens and 150% higher than that of the deionized water-cured specimens (Figure 3a,b). This significant deformation correlated with the deterioration of the pore structure due to ion erosion.

3.2. Uniaxial Compressive Strength (UCS)

(1)

The influence of solid rate and water solid rate on UCS

When the water-solid rate was held constant, UCS decreased as the solid rate increased. Conversely, when the solid rate was constant, UCS decreased with an increase in the water-solid rate. Owing to the high fly ash content and its limited participation in early-stage hydration, the formation of C-S-H gel within the specimen after 28 days of curing was insufficient, and its structure remained loose, resulting in a lower uniaxial compressive strength [33]. As illustrated in Figure 5a,b, the strength of the grouting stones at all maintenance ages diminished with increasing solid and water–solid rates. As shown in Figure 5a, with the increase in the X value, indicating a higher proportion of fly ash content, the uniaxial compressive strength of the specimens gradually decreased; conversely, a decrease in the X value, reflecting an increase in the proportion of cement content, resulted in an enhancement of the uniaxial compressive strength. As illustrated in Figure 5b, the increase in X led to a reduction in the solid phase content, thereby causing a further decrease in the uniaxial compressive strength. The fitting of the data indicated an exponential decrease in UCS with both the solid rate and the water–solid rate. At a solid rate of 9, the UCS at each maintenance age did not exceed 0.5 MPa, and the daily growth rate of strength improved as the solid rate decreased. Specifically, for a solid rate of 9:1, the UCS did not surpass 0.25 MPa during the maintenance period, which was insufficient to meet the requirement of exceeding 2 MPa in strength. The daily growth increased with the decrease in the solid rate.

(2)

The influence of water glass on UCS

The incorporation of water glass accelerated the setting speed of the slurry and reacted with components in the fly ash cement to generate a gel, thereby enhancing early strength. In the early stages of curing, the strength of the grouting stone body with a low water–solid rate and the addition of water glass was significantly enhanced. As depicted in Figure 6a–d, the addition of 3% water glass significantly improved the strength at 3 days and 7 days of curing, achieving a maximum enhancement of 67.8% and a minimum enhancement of 4%. However, at 14 days and 28 days of curing, the presence of water glass exhibited a weakening effect on strength, with a maximum decrease of 22.8% and a minimum decrease of 5.7%. This may be attributed to the products formed by sodium silicate with fly ash and cement, which could hinder the hydration reaction during the later stages of curing.

(3)

The influence of conservation methods on UCS

Figure 7a shows the strength residual rates (SRR) of specimens with solid rates of 9, 4, 2.33, and 1.5, and water–solid rates of 1.25, 1.0, 0.83, and 0.71 for natural curing, and pure water curing for 14 and 28 days (SRR = UCS of specimens under water-rich curing/UCS of specimens under natural curing). As shown in Figure 7a, uniaxial compressive strength was not measured for the specimens cured in pure water for 14 days due to the difficulty of demolding when the solid rate was 9 and the water-solid rates were 1.25 and 1.0. At a solid rate of 9, the SSR of the specimens was negatively correlated with the water–solid rate and positively correlated with the curing time. At a solid rate of 9, the SSR of specimens cured for 14 days was lower than that of 28 days. For specimens with solid rates of 4 and 2.33, the SSR of curing for 14 days was lower than that of curing for 28 days at water–solid rates of 1.25 and 1.0, and the SSR of curing for 14 days was higher than that of curing for 28 days at water–solid rates of 0.83 and 0.71. For specimens with a solid rate of 1.5, the SSR of curing for 28 days was higher than that of curing for 14 days at water–solid rates of 1.25 and 1.0. For the water–solid rates of 1.25, 0.83, and 0.71, the SSR of 14 days of maintenance was higher than that of 28 days, while the SSR of 14 days of maintenance was lower than that of 28 days at a water–solid rate of 1.0. The preliminary strength of the specimens increased more slowly when the fly ash was mixed with a larger amount of fly ash, but with the prolongation of the maintenance time, the water-rich environment provided sufficient moisture for the secondary hydration reaction, which promoted the fly ash’s gradually participate in the hydration reaction, which in turn accelerated the strength of the specimen. The smaller the proportion of initial water, the smaller the effect of water curing on the strength of the stone body.

As shown in Figure 7b, the addition of water glass can improve the strength of specimens cured with pure water for 14 days. At water–solid rates of 1.25 and 1.0, the strengths of the specimens cured in pure water for 14 and 28 days were increased compared with those without water glass addition, and water glass showed a significant procoagulant effect. However, with the decrease in the water–solid rate and the prolongation of the curing time, the effect of water glass on strength enhancement gradually weakened, and even appeared to have a negative effect on strength. Under tap water conditioning, the strength of specimens with all rates and curing times was lower than that of pure-water-cured specimens. At 14 days of curing, the UCS of the tap-water-cured specimens deteriorated by an average of 10.41% relative to the UCS of the pure-water-cured specimens, and the strength deteriorated by a maximum of 16.23%. At 28 days of curing, the average deterioration of UCS of tap-water-cured specimens was 9.108%, and the maximum deterioration of strength was 15.98% compared to that of pure-water-cured specimens. The effect of tap water conditioning on the strength of the specimens decreased with the increase in the conditioning time. Under pure water curing conditions, adding 3% water glass specimen compared with the specimen without adding water glass, the average strength of the specimen was increased by 12.2% at 14 d; when the specimen was conditioned for 28 d, the addition of water glass both increased and decreased the strength of the specimen, and it could be seen that under the condition of pure water conditioning, the effect of water glass on the strength of the specimen increased with the growth of time becoming smaller, and eventually it became inhibitory to the strength.

The water-rich environment had a profound impact on strength; therefore, regression analysis was conducted on the UCS under natural and water-rich conditions as well as the number of curing days, as shown in Equation (1):

$${\sigma }_c=a+b{X}_1+c{X}_2$$

(1)

$${\sigma }_{\mathrm{c}}=0.128+0.573X_1-0.003X_2$$

(2)

Note: here, σ_c is the UCS (MPa) of the specimen under water-rich conditioning; X₁ is the UCS (MPa) of the specimen under natural conditioning; and X₂ is the number of days of conditioning.

In the regression analysis, the standard errors of all coefficients were less than 1. The coefficient for natural conditioning was positive, indicating a positive correlation between UCS under water-rich and natural conditions, while the coefficient for the number of curing days was negative, reflecting a negative correlation with strength.

Extreme deviation, variance, and matrix analyses were performed to evaluate the influence of variables A, B, and SS on the uniaxial compressive strength of grouting stones. The results indicated that the degree of impact on UCS from each factor ranked in the following order: A, B, SS. Furthermore, the extreme deviation analysis revealed that the optimal proportions for natural curing at 3 days, 7 days, and 14 days, as well as water-rich curing at 14 days, was A₄B₄SS₂. Conversely, the optimal proportions for natural curing at 28 days and water-rich curing at 28 days was A₄B₄SS₁.

Strength is a critical parameter in engineering construction. This study performed a regression analysis of the grouted stone body, considering varying solid rates, water–solid rates, water–glass admixture levels, and curing durations, to examine their influence on strength, as represented in Equation (3).

$${\sigma }_{\mathrm{c}}=a+bA+cB+dX+eS$$

(3)

$${\sigma }_{\mathrm{c}}=7.9399-6.278A-5.457B+0.044X-0.155S$$

(4)

Note: here, σ_c is UCS in MPa; A is fly ash to solid mass rate (solid rate); B is water to water–solid mass rate (water-solid rate); X is the number of days of maintenance; and S water glass mixing amount (0, 1, 2, 3, etc.).

In the regression analysis, the standard error of the coefficient for B was slightly greater than 1, while the standard errors of the other coefficients were all less than 1. The regression coefficients for A, B, and S were negative, indicating a negative correlation with strength, whereas the coefficient for X was positive, indicating a positive correlation. Among all variables, A had the largest absolute coefficient value, followed by B, S, and X. Notably, A, S, and X exhibited negative correlations with strength.

3.3. Elastic Modulus

The elastic modulus of specimens subjected to natural curing for 28 days was evaluated. As illustrated in Figure 8, the modulus of elasticity decreased with increasing values of A and B. When A was set at 9, the elastic modulus exhibited minimal variation with changes in B; however, a more pronounced change in elastic modulus was observed as A decreased. The maximum modulus of elasticity recorded was 238.6 MPa, while the minimum was 2.15 MPa, resulting in a difference of 236.45 MPa. The trend in modulus of elasticity correlated consistently with the changes in the UCS of the specimens.

Figure 9 illustrates the decay of the elastic modulus of the specimen with a solid ratio of 7:3 under tap water and pure water curing conditions. The percentages represent the ratio of the elastic modulus under each curing condition to that under natural curing, as defined by Equations (5) and (6). As shown in Figure 9a, the lowest modulus of elasticity was found for the tap-water-cured specimens at 14 days of curing, and the greatest loss of modulus of elasticity was 83.4% compared to the natural curing. The loss of elastic modulus of the pure-water-cured specimens was less compared to the tap water curing. Further, at 28 days of curing, the loss of elastic modulus decreased for both tap-water- and pure-water-cured specimens. The effect of water curing on the elastic modulus decay gradually diminished with the extension of curing time.

$$n_1={\displaystyle \frac{E_p}{E_c}}\times 100\%$$

(5)

$$n_2={\displaystyle \frac{E_t}{E_c}}\times 100\%$$

(6)

Note: The distribution of n₁ and n₂ represents the residual rate of modulus of elasticity of pure water curing; E_c, E_p, and E_t represent the modulus of elasticity of specimens cured naturally, cured in pure water, and cured in tap water, respectively.

3.4. Microscopic Experiment

3.4.1. XRD

As depicted in Figure 10a, the physical phase analysis diagrams for the grouted stone body specimens at 3 days and 28 days of natural curing, as well as at 28 days of pure water curing and 28 days of tap water curing, revealed insights. From Figure 10a, it can be seen that under the same condition of maintenance, C-S-H gel, ettringite, and calcium hydroxide corresponded to the increase in XRD peak intensity and the appearance of new peaks with the rise of time. Conversely, silica corresponded to the decrease in XRD peak intensity and the disappearance of peaks. Notably, variations in XRD peak intensity were more pronounced under different conditioning environments; the XRD peak intensities of C-S-H gel, ettringite, and calcium oxide significantly diminished. Figure 10b highlights the emergence of new compounds such as calcium chloride and Friedl’s salt under tap water conditions. Friedl’s salt may interfere with the hydration reactions in the later stages of curing, potentially leading to decreased intensity [34]. Ions present in tap water can disrupt normal hydration reactions, resulting in lower concentrations of strength-related products and ultimately leading to a decrease in UCS.

3.4.2. SEM

Previous research indicates that fly ash is an inert filler during the pre-hydration period [35]. As observed in Figure 11a,b, at 3 days, fly ash was densely packed with a smooth surface. As curing time increased, some fly ash particles engaged in secondary hydration reactions, while others become encapsulated by other hydration products. C-S-H gels were divided into types I and II. At 3 days, the C-S-H gel primarily consisted of type I, characterized by a sporadic net-like structure. By 28 days of natural curing, the C-S-H gel integrated with other hydration products to form a denser structure. AFt crystals filled the pore structures, establishing a skeletal framework among the hydration products [36]. After 3 days of natural curing, Ca(OH)₂ and needle-like AFt crystals were dispersed, lacking a compact structure, which contributed to low pre-strength. As seen in Figure 11b, the quantity of C-S-H gel increased. In contrast, Figure 11c,d reveal a reduction in C-S-H quantities and an abundance of unreacted fly ash, resulting in a less compact structure and, consequently, reduced strength. Pore shapes exhibited diversity, with denser pore distribution observed at 3 days of natural curing and 28 days of water-enriched curing. In 28 days of natural curing, hydration products occupied the pores, resulting in reduced pore size and number.

4. Conclusions

The mechanical properties of the stone bodies were evaluated under natural, pure water, and tap water conservation conditions through laboratory testing, shedding light on the deterioration mechanisms of stone bodies in water-rich environments. The key findings are as follows:

• The naturally conditioned specimens showed linear deformation before peak stress, and the modulus of elasticity was up to 238.6 MPa. Brittle damage occurred after reaching the peak, and the stress plummeted by 30% in a very short period of time; the pure-water- and tap-water-conditioned specimens showed a slow change of the stress with the strain before peak stress, and plastic damage occurred, and with the specimen’s residual strength remaining at around 45% of the peak strength; the tap-water-conditioned specimens showed the highest peak axial strain, which was 60% higher than that of the naturally cured specimens and 150% higher than that of the pure-water-cured specimens.
• The UCS of the stone body was closely related to the solid rate, water–solid rate, water–glass dosage, and maintenance time, and the solid rate was the main control factor. For the treatment of water-rich goaf, the water–solid rate should not be higher than 0.83; for the fly ash and silicate cement grouting materials, the fly ash dosage should not be higher than 80% in order to meet the strength requirements of the treatment project of water-rich goaf.
• Under natural conservation conditions, with the growth of conservation time, the surface of fly ash particles was rough and gradually involved in the secondary hydration reaction, and the C-S-H gel moved gradually from (I) type low density accumulation to (II) type high density accumulation; conversely, under the conservation of pure water and tap water, the fly ash particles indicated a smoother surface, the distribution of the C-S-H (II) gel was less, and the structure was not dense.
• With tap water curing, chloride and Friedl’s appeared, leading to decalcification of the C-S-H gel; under the attack of ions, the pore size became larger, and the porosity increased, resulting in lower strength and higher strain in the specimens.