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Analysis of Active Ion-Leaching Behavior and the Reaction Mechanism During Alkali Activation of Low-Calcium Fly Ash

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Abstract

The dissolution and release of active ions, such as Si4+, Al3+ and Ca2+, from fly ash directly affect the rate and extent of reaction product formation, which in turn affect the physical and mechanical properties of fly ash filling materials. In this study, low-calcium fly ash was soaked and activated in NaOH solutions with different concentrations (approximating the optimum dose range) for different lengths of time. The amounts of active ions leached and the changes in the mineral composition, chemical functional groups and surface morphology were tested and analyzed via ICP-OES, XRD, FTIR and SEM/EDS techniques. Based on these analyses, the reaction mechanism of alkali activation of low-calcium fly ash was further investigated. The results showed that the NaOH activation effect can significantly increase the amount of active ions leached from low-calcium fly ash. Notably, the amount of Si4+ and Al3+ leached clearly increased with increases in both NaOH concentration and soaking time. The plausible reaction mechanism is discussed in detail, which is that the alkali activator principally affected the surface of the vitreous particles of low-calcium fly ash and induced differing surface modifications in the dissolution stage, depolymerization stage, polycondensation and polymer gel stage and diffusion stage. It was observed that the progress of the reaction is controlled by dissolution in the early stages, whereas activation is governed by diffusion when the surfaces of the fly ash particles are covered by precipitates.

Introduction

Fly ash has been widely used as a building material and as a component in products such as cement, concrete products, construction products, bridge engineering products and geopolymers (Blissett and Rowson 2012; Çiçek and Cincin 2015; Kotwal et al. 2015; Kim et al. 2016; Roychand et al. 2016). In recent decades, fly ash has generally been used as a raw material in mine paste backfilling (MPB) in China. The addition of fly ash can enhance the performance of filling material, decrease engineering costs, improve filling material workability, and reduce pumping power requirements. This MPB technology provides a green solution for utilizing fly ash and avoids the negative impact of this material on the environment and ecosystems (Qian et al. 2007). However, when fly ash filling materials were introduced, the replacement of Portland cement with fly ash, especially in high volumes, was found to result in serious defects, such as low early strength and large deformations. These defects directly affect the ability of the material to control ground subsidence. Previous research has showed that this low early strength is due to the slow pozzolanic reaction activity of fly ash (Shi 1996). Therefore, increasing the pozzolanic activity of fly ash is helpful for improving the early physical and mechanical properties of fly ash filling material.

A number of approaches for accelerating the pozzolanic activity of fly ash have been investigated (Shi 2001; Guo et al. 2014; Nikolić et al. 2014). Many authors have found that the addition of chemical activators can effectively accelerate the pozzolanic reactivity of fly ash and increase the convenience of chemical activation, and such methods are also cost effective. These chemical activators include acid activators, alkaline activators, sulfate activators, and chloride activators (Shi and Day 1993; Palomo et al. 1999; Guerrero et al. 2000; Fernández-Jiménez and Palomo 2005; Fernandez-Jimenez et al. 2007; Rashad 2014; Zhang et al. 2016). The clear consensus is that fly ash is an acidic material containing acidic oxides, and because of its pozzolanic reactivity in alkaline medium, it most likely requires activation (Xie and Xi 2001). Therefore, alkaline activators have a more noticeable effect than other types of activators. Moreover, reports have indicated that among such alkaline activators, NaOH treatment has the best activation effect on fly ash (Bijen and Waltje 1989; Shi et al. 2005). At the same time, some authors noted that the dose of the alkali activator has a considerable influence on the material strength. No clear activation effect occurs when the dose is small, whereas the strength of the material is reduced when the dose is too large, because the material strength decreases as the amount of the alkali activator in the mixture increases; thus, an optimum dose exists (Purdon 1940; Tango 1992; Shi et al. 2005; Luan et al. 2010).

Alkali-activated fly ash promotes the release of active Si4+ and Al3+ ions via decomposition of the dense silicon–aluminum network structure (Ma et al. 2012; Khalifeh et al. 2015; Zhuang et al. 2016). The behavior of leached active ions in the pozzolanic reaction process and the formation of reaction products have important impacts. The leaching behavior of low-calcium fly ash in the presence of optimal doses of alkali activators in filling material has not yet been studied in detail. In addition, the leaching of ions and the subsequent formation of reaction products during this period of the development process is not well understood. The exploration of these problems is considered to be valuable in reducing the dose and cost of alkali activators and for promoting the widespread field application of alkali activators in MPB technology.

Thus, the present study evaluated the amount of active ions, such as Si4+, Al3+ and Ca2+, leached from low-calcium fly ash mixed with 5 different concentrations of NaOH solution (approximating the optimum dose range) and the corresponding variation trends over time. Moreover, we studied the changes in the mineral composition and surface morphology of the fly ash residues during alkali activation, and the results provide insights into the reaction processes and micro-mechanisms of alkali activation of low-calcium fly ash.

Experimental Procedures

Materials

The fly ash samples were collected from the Xingneng power plant (Shanxi Province, China). The specific gravity of the fly ash was 2.1 g cm−3, and its specific surface area was 0.293 m2 g−1. Particle size analysis of the fly ash samples was conducted using a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., Worcestershire, UK). The particle size distribution of the fly ash samples (Fig. 1) ranged from 0.286 to 457.1 μm, with the 0–1 μm fraction accounting for 1.17% of the sample, the 1–10 μm fraction accounting for 11.93%, the 10–100 μm fraction accounting for 59.88%, and the 100–460 μm fraction accounting for 27.02%.

Fig. 1
figure1

Particle size distribution of the raw fly ash.

The chemical components of the fly ash were determined using X-ray fluorescence spectrometry (XRF S8 TIGER, Bruker AXS Gmbh, Germany), and the fly ash was found to primarily consist of SiO2 (58.14%), Al2O3 (30.03%), Fe2O3 (3.80%), CaO (3.58%) and MgO (2.53%). The major oxides SiO2, Al2O3 and Fe2O3 presented a total content of over 70 wt%, and the CaO content of less than 10% indicated that these fly ash samples should be classified as Class F according to the definition in ASTM C 618. Figure 2 shows that the major mineralogical composition of the fly ash samples, determined via X-ray diffraction (XRD, Rigaku D/Max2500, Japan) analysis, consisted of quartz, mullite, and a noncrystalline vitreous phase.

Fig. 2
figure2

X-ray diffraction pattern (XRD) of the raw fly ash.

The chemical reagent NaOH (Tianjin Chemical Works, > 96 wt%) was used as the alkaline activator. Deionized water was employed in the experiment.

Sample Preparation and Test Plan

  1. (1)

    The fly ash samples collected at the power plant were placed in a 110 °C electric oven and dried for 12 h to a constant weight. These samples then were divided into 30 subsamples of 100 g each, which were weighed on an electronic balance with a precision of 0.0001 g.

  2. (2)

    We prepared five 2400-mL NaOH solutions with concentrations of 0, 0.25, 0.75, 1.25 and 2 mol L−1, (corresponding to NaOH doses in the filling material of 0, 1, 3, 5, and 8%, respectively, calculated based on the mass of water in the mixture), which were labeled A, B, C, D, and E, respectively. Then, each solution was divided into 6 portions of 400 mL, for a total of 30 aliquots that were stored separately in 1 L polyethylene bottles marked A1,…, E6.

  3. (3)

    The fly ash samples from step (1) were successively transferred to polyethylene bottles, which were shaken for 5 min, sealed, and placed in a 25 °C water bath to maintain a constant temperature. The soaking times were 3, 6, 12, 24, 48 and 72 h, thereby generating the desired 30-sample test scheme (Table 1).

    Table 1 Ion leaching test scheme for alkali-activated low-calcium fly ash with different NaOH concentrations and soaking times.
  4. (4)

    The supernatants of the sample solutions were filtered through membrane filters (0.8 μm) to remove the undissolved fly ash. A total of 1 mL of each leaching solution was collected using pipettes with a 0.01 mL precision and diluted 1000-fold using deionized water in a stepwise manner to meet the test sample concentration requirements (< 40 ppm) of the ICP-OES instrument. The diluted solution was allowed to rest for 2 days in sealed polyethylene bottles, and no precipitates were found in any of the diluted solutions before analysis. Each sample was then prepared for the analysis of the Si4+, Al3+ and Ca2+ ion concentrations.

  5. (5)

    The fly ash residues (including unreacted or undissolved particles, particles under alkaline attack and reaction products) were collected after different soaking times and then washed three times with deionized water to remove the surface NaOH solution. Then, these residues were dried for 48 h in a 50 °C electric oven to a constant weight in preparation for analysis of the mineralogy, chemical functional groups and surface morphology.

Ion Leaching Test

ICP-OES (Thermo iCAP 6300, USA) was employed to determine the concentrations of Si4+, Al3+ and Ca2+ in the leaching solutions described above after dilution and the measurement results of the sample have been investigated by the calibration curve method. The test included the following steps.

  1. (1)

    A total of 6 mL was collected from each of the 30 leaching solutions after dilution and was divided into 3 aliquots of 2 mL each. Each of the 3 aliquots was placed in a separate 100 mL polyethylene volumetric flask.

  2. (2)

    Then, 0.1 mL of 36% HNO3 was added to two of the volumetric flasks from Step 1. Into one of these two flasks, 5 mL of an aluminum standard solution (0.1 mg mL−1), 2 mL of a calcium standard solution (1 mg mL−1) and 2 mL of a silicon standard solution (1 mg mL−1) were successively added. This solution was subsequently diluted with secondary deionized water to a constant volume. Into the second flask, 10 mL of the aluminum standard solution (0.1 mg mL−1), 4 mL of the calcium standard solution (1 mg mL−1) and 4 mL of the silicon standard solution (1 mg mL−1) were successively added, and this solution was then diluted with secondary deionized water to a constant volume to prepare the standard solution.

  3. (3)

    In the remaining polyethylene volumetric flasks from Step 1, 0.1 mL of 36% HNO3 was added, and the solution was then diluted to a constant volume with deionized water to prepare the sample solution.

  4. (4)

    The instrument automatically adjusts to the initialized state. The three elements to be tested (Si, Al and Ca) and the corresponding spectral lines (251.612 nm for Si, 309.271 nm for Al and 393.366 nm for Ca) were selected from iTEVA software to begin the test. The standard solutions were tested first, and the three standard curves for Si, Al and Ca were drawn with the concentration as the abscissa and the intensity of the emission spectrum as the ordinate. Then, the sample solutions were run to determine the intensity of the emission spectra of the three elements on their own spectral lines and the corresponding absolute values of the abscissa on the standard curves, which were the concentrations of Si4+, Al3+ and Ca2+ in the solutions. Each sample was tested in parallel three times under repeatable conditions and the coefficients of variation (CV) for the ion concentration measurements were less than 5%. The average value was taken as the result for comparison with the precision requirements.

  5. (5)

    The leached amounts of Si4+, Al3+ and Ca2+ were calculated according to Eq. (1):

$$ W = \frac{C \times V \times d}{M} $$
(1)

where W is the amount of leached Si4+, Al3+ or Ca2+ (mg g−1); C is the average Si4+, Al3+, or Ca2+ concentration of the sample solution (mg L−1); V is the volume of the leaching liquid (L); d is the dilution ratio; and M is the dose of fly ash (g).

The increase in the rate of leaching of the active ions under the alkali activation effect at different soaking times was calculated relative to that without alkali activation, according to Eq. (2):

$$ {\text{G}} = \frac{{{\text{W}}_{NaOH} - {\text{W}}_{water} }}{{{\text{W}}_{water} }} \times 100{\text{\% }} $$
(2)

where G is the percentage of the increase in the leaching of Si4+, Al3+ or Ca2+ (%); WNaOH represents the amount of leached Si4+, Al3+ and Ca2+ in the presence of NaOH solutions of different concentrations (Groups B, C, D and E; mg g−1); and Wwater represents the amount of leached Si4+, Al3+ and Ca2+ in deionized water (Group A; mg g−1).

Test of the pH Value of the Leaching Solution

A pH meter (PHS-3C Rex, China) was used to determine the pH value of the leaching solution at 25 °C. Prior to these measurements, the electrode was calibrated with a standard solution of borax (pH 9.18) and a mixed standard solution of potassium dihydrogenphosphate and disodium hydrogen phosphate (pH 6.86). The reported results are the average of three tests.

XRD Measurement

An XRD instrument (Rigaku D/Max2500, Japan) was used to determine the mineralogy of the fly ash residues. Under the test conditions, Cu was employed as the target, and Kα was used as the radiation, and the test specimens were step-scanned at 2θ values from 10 to 60°, with 0.05° steps and a rate of 0.5 s per step.

FTIR Measurement

A Fourier transform infrared spectroscopy instrument (FTIR, Thermo Fisher Nicolet iS5, USA) was used to characterize the chemical functional groups of the fly ash residues. FTIR is a key technique for the analysis of alkali-activated materials, particularly for low-calcium systems, in which it can probe the connectivity within Si–O–(Si, Al) frameworks via shifts in the peak corresponding to the asymmetric stretch of that bond (Provis et al. 2015). Before FTIR measurement, the pellet was dried for 12 h in a vacuum oven to minimize the effect of water on the spectrum. The FTIR spectra were determined by the potassium bromide (KBr) pellet method (1:100 wt% for the finely ground samples and KBr). The spectra were collected in the 400−4000 cm−1 wavenumber region with 32 scans at a resolution of 4 cm−1.

SEM Measurement

Scanning electron microscopy (SEM, JEOL JSM-6700F, Japan), coupled with energy dispersive spectrometry (EDS, Oxford Instruments IET-200, UK), was employed to determine the surface morphology of the fly ash residues. The samples were sputter coated with a gold–palladium mixture before the assessment to reduce the interference of the negative charge during imaging.

Results and Analysis

Ion Leaching Behavior

Leaching Behavior of Si4+

Figure 3a presents the trend of the amount of Si4+ leached over time from fly ash in NaOH solutions of different concentrations. According to the figure, in deionized water, the amount of Si4+ leached reached 3.8 mg g−1 at 3 h then remained at 3.84 mg g−1 between 6 and 72 h. In the NaOH solution with a concentration of 0.25 mol L−1, the amount of leached Si4+ increased from 3.96 mg g−1 at 3 h to 4.16, 4.32, 4.60, 4.76 and 4.84 mg g−1 at 6, 12, 24, 48 and 72 h, respectively. In the NaOH solutions with concentrations of 0.75, 1.25 and 2 mol L−1, the amount of leached Si4+ increased from 4.08, 4.27 and 4.32 mg g−1 at 3 h to 5.36, 6.76 and 7.96 mg g−1 at 72 h, respectively. The results of fitting the test data showed that in the NaOH solutions with different concentrations, the amount of Si4+ leached increased according to a logarithmic curve over an extended time intervals. For a given time interval, the amount of Si4+ leached increased linearly with increases in the concentration of the NaOH solution, as shown in Fig. 3b.

Fig. 3
figure3

Curves of the amount of leached Si4+ showing variations with a time and b NaOH concentration.

Leaching Behavior of Al3+

Figure 4a shows the trend of the amount of Al3+ leached over time from fly ash in NaOH solutions of different concentrations. The figure shows that in deionized water, the amount of leached Al3+ increased from 0.48 mg g−1 at 3 h to 0.85 mg g−1 at 6 h and then remained at 0.92 mg g−1 between 12 and 72 h. In the NaOH solution with a concentration of 0.25 mol L−1, the amount of leached Al3+ increased from 0.61 mg g−1 at 3 h to 1.26, 1.48, 1.71, 1.95 and 2.03 mg g−1 at 6, 12, 24, 48 and 72 h, respectively. In the NaOH solutions with concentrations of 0.75, 1.25 and 2 mol L−1, the amount of leached Al3+ increased from 1.55, 2.02 and 2.27 mg g−1 at 3 h to 2.49, 3.30 and 4.00 mg g−1 at 72 h, respectively. The results of fitting the test data showed that in NaOH solutions with different concentrations, the amount of leached Al3+ increased according to a logarithmic curve over extended time intervals. For a given time interval, the amount of leached Al3+ increased linearly with increases in the concentration of the NaOH solution, as shown in Fig. 4b.

Fig. 4
figure4

Curves of the amount of leached Al3+ showing variations with a time and b NaOH concentration.

Leaching Behavior of Ca2+

Figure 5a shows the trend of the amount of Ca2+ leached over time from fly ash in NaOH solutions of different concentrations. According to the figure, in deionized water, the amount of leached Ca2+ increased slowly, from 0.62 mg g−1 at 3 h to 0.68 mg g−1 at 24 h, then remained at 0.68 mg g−1 between 24 and 72 h. In the NaOH solutions with concentrations of 0.25, 0.75, 1.25 and 2 mol L−1, the amount of leached Ca2+ was 0.032 mg g−1 at 3 h and increased to 0.052, 0.048, 0.048 and 0.044 mg g−1 at 72 h, respectively. Figure 5b shows that the amount of leached Ca2+ in water was higher than in the NaOH solutions, because the leached Ca2+ reacted with OH to generate a Ca(OH)2 precipitate (Provis et al. 2009). Because the concentration of OH in the water was much lower than in the NaOH solutions, only a limited amount of Ca(OH)2 precipitate formed in the water. As a result, the amount of leached Ca2+ in the water was much greater than that in the NaOH solutions. Ca2+ was detected in the NaOH solutions because Ca(OH)2 is slightly soluble in water and a small amount of calcium was released secondarily (Lee and Deventer 2002).

Fig. 5
figure5

Curves of the amount of leached Ca2+ showing variations with a time and b NaOH concentration.

Analysis of the Differences in the Leaching Behaviors of Si4+, Al3+ and Ca2+

Although the alkali activation effect clearly increased the amounts of active ions leached from the low-calcium fly ash, the leaching trends for Si4+, Al3+ and Ca2+ were different.

  1. (1)

    The amounts of leached Si4+, Al3+ and Ca2+ were ranked as follows: Si4+>Al3+>Ca2+. Within 72 h, the maximum amount leached was 7.96 mg g−1 for Si4+, 4 mg g−1 for Al3+ and 0.04 mg g−1 for Ca2+, because the contents of silicon, aluminum and calcium in the fly ash were different. The content of SiO2 in the tested fly ash was 58.14%; the content of Al2O3 was 30.03%; and the content of CaO was only 3.58%.

  2. (2)

    The amount of leached Ca2+ decreased with increases in the concentration of the NaOH solution, whereas the amounts of leached Si4+ and Al3+ increased with increases in the concentration of the NaOH solution. Therefore, the trend for Ca2+ was different from those for Si4+ and Al3+, as Ca2+ generated a Ca(OH)2 precipitate in the NaOH solution, which decreased the concentration of free Ca2+ with increasing concentration of the NaOH solution.

  3. (3)

    Relative to that of the deionized water, the rates of increase in the leaching of Si4+, Al3+ and Ca2+ from the fly ash varied among the NaOH alkali solutions. Table 2 shows the percent increases in the leaching of Si4+, Al3+ and Ca2+ under the alkali activation effect after different soaking times relative to the condition without alkali activation. The results show that alkali activation increased the amount of leached Al3+ by 334.78% and that of leached Si4+ by 107.29%, whereas it produced a 92.53% decrease in the amount of leached Ca2+. These results can be explained as follows: in the aluminosilicate structure of fly ash, Si–O mainly occurs in a tetra-coordinated tetrahedral structure with a stable nature, and Al–O occurs in a multi-coordinated polyhedral structure, which means that the Al–O–Si bonds are relatively weak compared with the Si–O–Si bonds (Xiao and Lasaga 1994). When the fly ash is attacked by the alkali solution, the bond energy of the Al–O bond is lower than that of Si–O, thus increasing the solubility of Al compared with Si. The decrease in the amount of leached Ca2+ was caused by the generation of a Ca(OH)2 precipitate, thereby removing the Ca2+ from the NaOH solution.

    Table 2 Percent increases in the leaching of Si4+, Al3+ and Ca2+.

Changes in the pH of the Leaching Solution

Figure 6 shows that the pH of the leaching solution changed over time. The pH value increased from 7.01 to 9.73 when fly ash was added to deionized water, followed by a slow downward trend, and the pH reached 9.35 at 72 h. The early rapid dissolution of the alkaline oxides in the fly ash led to an increase in the OH concentration in the system, thus increasing the pH value of the deionized water. The subsequent decrease in the pH value was related to the decrease in the amount of alkaline oxides dissolved in the later period and the consumption of OH in the formation of intermediate aluminosilicate material. In the NaOH solution with a concentration of 0.25 mol L−1, the initial pH value was 12.9, and this pH changed to 12.75 after 72 h. At concentrations of 0.75, 1.25 and 2 mol L−1, the initial pH values were 13.46, 13.69, and 13.97, respectively, and remained stable throughout the test period.

Fig. 6
figure6

Variation in pH values in the leaching solution with time (25 °C).

Changes in the Mineral Composition of the Fly Ash Residues

The XRD test results for the mineral composition of the fly ash residues soaked in the 2 mol L−1 NaOH solution are shown in Fig. 7. The XRD pattern of the raw fly ash shows high intensities corresponding to quartz and mullite. After alkali activation, some peaks in the XRD patterns changed with respect to the soaking time. The mineral components in the system did not change significantly after soaking for 12 h. The characteristic peak intensities of quartz and mullite of the samples that soaked for 72 h were lower than those of the raw fly ash, indicating that the alkali solution caused the destruction and dissolution of the crystalline structures of the quartz and mullite in the fly ash. After 168 h, the characteristic diffraction peaks of quartz and mullite were significantly lower. The dispersed diffraction peaks of the noncrystalline-phase aluminosilicate glass in the raw fly ash became comparatively small after 672 h. Therefore, the destruction of the quartz and mullite crystalline structure continued under the action of alkali activation, and the noncrystalline-phase aluminosilicate glass was also attacked by the alkali solution. The XRD patterns of the samples show no significant formation of new peaks and no substantial crystallographic changes in the fly ash residues. In general, it is possible that the changes in the microstructure are related to amorphous sodium aluminosilicate hydrate (N–A–S–H) gel, which would be difficult to detect via XRD. Therefore, we performed FTIR and SEM analyses for further investigation.

Fig. 7
figure7

XRD patterns of alkali-activated low-calcium fly ash treated with 2 M NaOH for 12, 72, 168 and 672 h.

Changes in the Functional Groups of the Fly Ash Residues

The FTIR spectra for the fly ash residues with different soaking times and different NaOH concentrations are shown in Fig. 8a, b, respectively. Both sets of FTIR spectra feature similar profiles and marked wavenumbers. A broad peak between 3432 and 3472 cm−1 is observed due to strong O–H stretching vibrations. Absorption bands at 1624–1634 cm−1 are assigned to the bending vibration of the O–H groups of the hydrated reaction products. Small bands were observed between 1400 and 1420 cm−1 and are associated with the stretching mode of O–C–O bonds in carbonate groups (Nadziri et al. 2017). The main binding gel of the NaOH-activated low-calcium fly ash is characterized by a peak in a characteristic region (< 1300 cm−1). The peak at a wavenumber of 1052 cm−1 in the raw fly ash shifts to a final position at approximately 1006 cm−1 in conjunction with a soaking duration of 336 h and approximately 1018 cm−1 in conjunction with NaOH solution concentration of 2 M, indicating a progressive reaction that is rich in silica gel. This shift is consistent with the formation of a sodium aluminosilicate-type gel (Zhang et al. 2012a; Vargas et al. 2014; Ismail et al. 2014; Nadziri et al. 2017). Alkali activation of low-calcium fly ash promotes the formation of more cross-linking in an aluminosilicate-type gel, which can be seen from the vibration in the band between 400 and 800 cm−1. Slight shifts occur in the bending vibrations of Si–O–Si from 776 to 792 cm−1, of Al–O–Al from 549 to 552 cm−1 and of Si–O–Si and O–Si–O from 460 to 425 cm−1 (Zhang et al. 2012b; Ismail et al. 2014). These results are consistent with the formation of an aluminosilicate gel containing Ca2+, i.e., a C/N–A–S–H gel (I. Garcia-Lodeiro et al. 2011). As shown in Fig. 7a, b, significant differences exist among the FTIR spectrum changes near the 450 cm−1 wavelength with increasing soaking time and NaOH solution concentration. The FTIR results verify that the soaking times and alkali solution concentrations strongly influence the formation of hydration products during the alkali activation of low-calcium fly ash.

Fig. 8
figure8

FTIR spectra of alkali-activated low-calcium fly ash for different a soaking durations and b NaOH concentrations.

Changes in the Surface Morphology of the Fly Ash Residues

Figure 9 shows the test results from the SEM analysis of the changes in the morphological characteristics of the fly ash residues with different soaking times in a 2 mol L−1 NaOH solution. Figure 9 shows that a number of morphologies may coexist in the fly ash residues, including unreacted or undissolved ash particles, particles under alkaline attack, and precipitates (including reaction products). Among these morphologies, a significant number of unreacted or undissolved ash particles indicates a slower rate and lower proportion of reaction in the system.

Fig. 9
figure9

SEM micrographs of alkali-activated low-calcium fly ash treated with 2 M NaOH for different soaking times: a raw fly ash, b 3 h, c 72 h, d EDS spectrum of precipitates, e hollow sphere, and f 672 h.

The raw fly ash mainly consists of spherical particles of different sizes, and the surfaces of the particles are relatively smooth and exhibit good sphericity, with small amounts of tiny particles, such as debris and agglomerates, being attached to the surface (see Fig. 9a). Most of these tiny particles consist of materials such as soluble substances, quartz, mullite, and unburned coal (Brueggen et al. 2010). After soaking for 3 h, the surface of the particles gradually changed from smooth to rough, and some randomly scattered precipitates with a flaky structure (probably Ca(OH)2) appeared on the surface of the particles (see Fig. 9b). After soaking for 72 h, the undissolved particle surface became rougher, and precipitates were continuously formed and precipitated in a disorderly manner on the particle surface (see Fig. 9c). The results of the EDS analysis showed that the main components of the precipitates were Si, Al, Na, and Ca (see Fig. 9d). Figure 9e shows that the hollow spheres of fly ash were filled with precipitates. After soaking for 672 h, Fig. 9f clearly shows that the surfaces of the undissolved fly ash particles were covered with a dense layer of precipitates, such as Ca(OH)2 and zeolite. This image provides a reminder that the precipitation of the reaction products occurs continuously and that a layer of precipitates covering the undissolved particles would inhibit the activation of these particles. Additionally, some cracks in the layer can be observed in Fig. 9f, which might be due to mechanical damage during sample preparation for SEM observation or could be caused by increases in the osmotic pressure of the interactions at phase boundaries, or could also be caused by internal moisture loss of hardened layer induced to drying shrinkage (Yan and Sagoe-Crentsil 2016).

Reaction Mechanism During Alkali Activation of Low-Calcium Fly Ash

Based on the above test and analytical results, the reaction mechanism during the alkali activation of low-calcium fly ash can be further interpreted. It was divided into four stages, as shown in Fig. 10, and is described below.

Fig. 10
figure10

Schematic diagram of the reaction mechanism during alkali activation of low-calcium fly ash: a dissolution stage, b depolymerization stage, c polycondensation and polymer gel stage, and d diffusion stage.

Dissolution Stage

The tiny soluble particles on the surface of the fly ash particles dissolved upon contact with the alkali/water solution and released a series of active ions, such as Si4+, Al3+, Ca2+, and Mg2+ (Fig. 10a). The chemical reaction equations (Eqs. (3)–(8)) for the dissolution of the soluble solid particles at 25 °C are shown in Table 3, and the molar Gibbs free energy of the chemical reactions can be calculated using the standard molar Gibbs function. The calculation results showed that the molar Gibbs free energies of all of the above reactions were less than zero, indicating that the chemical reactions proceeded spontaneously. Moreover, the amount of tiny fly ash particles less than 1 μm in size (which accounted for approximately 1.17% of the total) that dissolved increased with increases in both the soaking time and NaOH concentration. Despite the relatively low percentage of these particles, the amounts were sufficient to significantly increase the ion concentrations in the solutions (Garcialodeiro et al. 2016). Consequently, an increased amount of tiny fly ash particles in the system increases the likelihood of achieving a better alkali activation effect, especially in the early period.

Table 3 Reaction equations for the soluble particles and their molar Gibbs free energy values (Zhang 2008; Liang 2007).

Depolymerization Stage

Figure 10b shows the depolymerization process of the fly ash particles under alkaline attack (note that in this figure, the aluminosilicate vitreous chain structure resulting from alkali attack may have been slightly exaggerated for effect). The addition of the alkali metal hydroxides results in a change in the ionic strength of the solution, which causes a redistribution of the electronic density around the chain structure (Glukhovsky 1965). The OH anions react with the surface of the fly ash particles via chemical hydration reactions, which causes the aluminosilicate network structure to break down and different complexes to form, such as \( {\text{Al}}\left( {{\text{OH}}} \right)_{4}^{ - } \) or \( {\text{Al}}\left( {{\text{OH}}} \right)_{6}^{{3 - }} \) and \( {\text{SiO}}\left( {{\text{OH}}} \right)_{3}^{ - } \), \( {\text{SiO}}\left( {{\text{OH}}} \right)_{2}^{{2 - }} \) or \( {\text{SiO}}_{3} \left( {{\text{OH}}} \right)^{{3 - }} \). The actual complex that forms depends on the alkali concentration in the solution (Swaddle et al. 1994; Xu and Deventer 2000). Furthermore, the alkali metal cation Na+ is dispersed throughout the aluminosilicate network structure. When the aluminosilicate network structure is depolymerized, Na+ neutralizes the excess charge via a physical electrostatic reaction, thereby generating Si–O–Na+ bonds and hindering the reversibility of the reaction (Shi et al. 2011).

Polycondensation and Polymer Gel Stage

After depolymerization occurs and unstable ionic monomers form, the active Al3+ and Si4+ ions react under catalysis by OH to form nuclei in silicate and aluminate species consisting of SiO4 and AlO4 tetrahedra. Polycondensation between the silicate and aluminate species occurs more readily because of the characteristic high activity of [Al(OH)4], as shown in the following reaction:

$$ \left[ {{\text{SiO}}_{2} \left( {{\text{OH}}} \right)_{2} } \right]^{{2 - }} + \left[ {{\text{Al}}\left( {{\text{OH}}} \right)_{4} } \right]^{ - } \to {\text{ }}\left[ {\left( {{\text{OH}}} \right)_{3} {\text{Al}} - {\text{O}} - {\text{SiO}}_{2} \left( {{\text{OH}}} \right)} \right]^{{3 - }} + {\text{H}}_{{\text{2}}} {\text{O}} $$
(9)
$$ \left[ {{\text{SiO}}\left( {{\text{OH}}} \right)_{3} } \right]^{ - } + \left[ {{\text{Al}}\left( {{\text{OH}}} \right)_{4} } \right]^{ - } \to {\text{ }}\left[ {\left( {{\text{OH}}} \right)_{3} {\text{Al}} - {\text{O}} - {\text{SiO}}\left( {{\text{OH}}} \right)_{2} } \right]^{{2 - }} + {\text{H}}_{{\text{2}}} {\text{O}} $$
(10)

The reaction described by Eq. (9) tends to form dimers and trimers, whereas that described by Eq. (10) tends to form larger oligomers and polymers. The above reactions show that aluminate and silicate species are linked to each other by the attraction between the hydroxyl groups of the aluminate and silicate monomers, thereby forming an intermediate complex. The two hydroxyl groups in the intermediate complex subsequently condense to form an aluminosilicate species by releasing an H2O molecule. This type of polycondensation reaction is usually a nucleophilic substitution reaction (Weng and Sagoe-Crentsil 2007). Thereafter, the aluminosilicate oligomers and the alkali metal cation Na+ are further polycondensed via the action of a coordination bond or an electrostatic bond to form aluminosilicate gel, which accumulates gradually. As shown in Fig. 10c, the precipitates (e.g., Ca(OH)2, aluminosilicate gel, etc.) precipitated on the surface of the fly ash particles are disordered after their formation.

Diffusion Stage

The precipitates eventually cover the surface of the undissolved particles, thereby hindering contact between the fly ash and the alkaline solution. This situation is unfavorable for the continued alkaline activation of the fly ash and gradually reduces the rate of alkali activation. Once the particles are completely covered by the precipitates, alkali attack occurs only via diffusion. Figure 10d illustrates the mechanism of the bidirectional diffusion of alkali-activated fly ash. Figure 10d clearly shows that Na+ and OH continue to attack the fly ash along the cracks and pores in the precipitate layer, while ionic silicon and aluminum monomers flow outward along these cracks and pores.

The mechanism of alkali activation of low-calcium fly ash is a consequence of a chemical reaction, which the reaction yields an inorganic polymeric-like material; bonded covalently and possessing favorable chemical and physical properties (Khalifeh et al. 2016). The results show that the progress of the reaction is controlled by dissolution in the early stages, whereas activation is governed by diffusion when the surfaces of the fly ash particles are covered by precipitates.

Conclusions

The NaOH concentration and soaking time have clear effects on the amount of active ions leached from low-calcium fly ash. At different NaOH concentrations, the amounts of leached Si4+ and Al3+ ions increased according to a logarithmic curve over an extended time period, whereas for a given time interval, the amounts leached increased linearly with increasing concentration of the NaOH solution. Alkali activation increased the amounts of leached Al3+ and Si4+ by 334.78 and 107.29%, respectively, relative to the condition without alkali activation. Ca2+ generates a Ca(OH)2 precipitate in the NaOH solution, which explains why the amount of leached Ca2+ is high in water and low in the NaOH solution.

OH forms different complexes via chemical hydration reactions with fly ash, and aluminosilicate gel is formed after the polycondensation reaction with alkali metal cations, such as Na+, via the action of a coordination bond or an electrostatic bond when low-calcium fly ash is soaked in an NaOH solution.

The alkali activator mainly affected the surface of the low-calcium fly ash particles and induced surface modifications that caused changes in the dissolution stage, depolymerization stage, polycondensation and polymer gel stage and diffusion stage. The effect of activation was related to the concentration of the alkaline solution, with higher concentrations leading to more significant effects. In addition, the characteristics of the fly ash, such as its fineness, vitreous content, mineral composition and chemical composition, also alter the effect of alkali activation.

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Authors’ contributions

BY conceived and designed the study, participated in discussing the results, drafted and revised the manuscript. TK contributed significantly to analysis and manuscript preparation. JK conducted the sample preparation and test work. YC acquired data and performed the data analyses. All authors read and approved the final manuscript.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (Grant Nos. 51174141 and 50974093). The authors would like to express their sincere thanks to the editors and anonymous reviewers for their valuable comments and insightful feedback.

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Correspondence to Tianhe Kang.

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Keywords

  • low-calcium fly ash
  • alkali activation
  • leaching behavior
  • reaction mechanism